# Advances in actinide thin films: synthesis, properties, and future
directions

Kevin D. Vallejo1,∗, Firoza Kabir1,2, Narayan Poudel1, Chris A. Marianetti3,
David H. Hurley1, Paul J. Simmonds4,5, Cody A. Dennett1, and Krzysztof
Gofryk1,† 1Condensed Matter and Materials Physics, Idaho National Laboratory,
Idaho Falls, ID 83415, USA  2Glenn T. Seaborg Institute, Idaho National
Laboratory, Idaho Falls, ID 83415, USA  3Department of Applied Physics and
Applied Mathematics, Columbia University, New York, NY 10027, USA  4Department
of Physics, Boise State University, Boise, ID 83725, USA  5Micron School of
Materials Science and Engineering, Boise State University, Boise, ID 83725,
USA

###### Abstract

Actinide-based compounds exhibit unique physics due to the presence of 5f
electrons, and serve in many cases as important technological materials.
Targeted thin film synthesis of actinide materials has been successful in
generating high-purity specimens in which to study individual physical
phenomena. These films have enabled the study of the unique electron
configuration, strong mass renormalization, and nuclear decay in actinide
metals and compounds. The growth of these films, as well as their
thermophysical, magnetic, and topological properties, have been studied in a
range of chemistries, albeit far fewer than most classes of thin film systems.
This relative scarcity is the result of limited source material availability
and safety constraints associated with the handling of radioactive materials.
Here, we review recent work on the synthesis and characterization of actinide-
based thin films in detail, describing both synthesis methods and modelling
techniques for these materials. We review reports on pyrometallurgical,
solution-based, and vapor deposition methods. We highlight the current state-
of-the-art in order to construct a path forward to higher quality actinide
thin films and heterostructure devices.

actinides, thin films, characterization, topological properties, thermal
properties, magnetic properties, strong-correlation modelling

††preprint: AIP/123-QED

##  I Introduction

Due to their strong electronic correlations and populations of dual-natured 5f
electrons, the actinide elements, with atomic numbers ranging from 89 to 103,
represent a rather poorly understood section of the periodic table. These
transition elements with partially filled 5f electronic subshells exhibit a
wide range of exotic properties. They have multiple oxidation states: in an
aqueous solution, Pu can exist in four different states simultaneously, for
example. Their 5f electrons can occupy itinerant or highly localized states
[1]. They also exhibit divergent magnetic properties, from nearly magnetic (Np
and Pu) to transition-metal-like behavior (Th, Pa, U) [2]. Technologically,
the most widely used actinide compound to date has been UO2, in its role as
the primary fuel for commercial nuclear energy production [3, 4].

Despite these unique physical properties, their scarcity, inherent
radioactivity, and challenges concerning their synthesis mean that actinide
materials are still some of the least understood elements. Transuranic
actinides and their compounds have been confined to specialized facilities
where materials control and accountability can be ensured. Their study and
understanding are further hindered by their low natural abundance and
typically short half-lives, which limit material supply [1]. Developing an
understanding of the fundamental properties and performance characteristics of
actinide compounds is hence a challenging but highly active area of research
[5]. Understanding the physics of these complex materials more deeply will
ensure their continued technological relevance, and open new avenues for
discovery in the domains of quantum science and strongly correlated systems.

One way to enable this expansion in our understanding is to minimize the
structural complexity in experimental specimens. In practice, this could
include removing crystallographic defects, grain boundaries, chemical
heterogeneities, and other imperfections during the synthesis process. To this
end, thin film synthesis techniques have been used to produce high-quality,
single crystalline specimens of various actinide compounds, including those
used in nuclear fuel to study their physical properties [6]. Such efforts also
aid comparison between experimental measurements and results from modeling and
simulation tools. The geometries of these thin-film samples also permit the
implementation of “microlaboratory” experimental designs, wherein chemical
reactions may be explored in detail [7]. It is important to note that due to
their scarcity, reactivity, radioactivity, and toxicity, high-purity actinide
samples in _any_ geometry have only recently become available to use as
targets or sources [8]. However, despite these constraints, researchers have
found ways to explore a range of physical, chemical, and solution-based
techniques capable of producing actinide-based films with varying degrees of
success and quality. Here we catalog the successes, and highlight some of the
unique property and performance data generated to date.

This review is organized as follows: in section II we describe the synthesis
methods used to create actinide-based thin films. Section III provides a
review of actinide thin films that have been successfully synthesized, along
with the techniques used for their structural, electronic, and surface
characterization. Section IV summarizes the current and theorized properties
of actinide thin films, and in section V we give an overview of the efforts
and challenges related to modelling these structures. Finally, in section VI
we explore the rich scientific and technological promise of combining
different actinide thin films into heterostructures, together with the
associated challenges for this next step in the use of these important
compounds.

##  II Synthesis Methods

Ever since the discovery of radiation and its properties, there has been a
need to refine and purify actinide materials. During the twentieth century,
the focus shifted from Ra to U in the more than 180 minerals containing these
elements [9]. Motivated by the applications of radioactive compounds,
technology evolved and reports began to emerge of the synthesis of actinide
single crystals and thin films. One of the earliest examples is from 1955,
when Pu and U were dissolved in a cellulose lacquer and painted on metallic
foils [10]. The intervening 70 years have seen synthesis techniques developed
that incorporate the refined safety measures necessary to handle these
materials.

Spirlet and Voigt summarized the first attempts at refining actinide metals
and compounds [8]. Their review article includes the metallothermic reduction
of oxides and halides, as well as vacuum melting, selective evaporation and
condensation, electrorefining, zone melting, and electrotransport (see [8] and
references therein). They highlight solution-based techniques as the most
economical approach for single crystal synthesis. In the 40 years since
Spirlet and Voigt’s review, growth of high quality single crystal actinides
has advanced enormously, and these samples are of increasing interest from a
fundamental scientific perspective. As safety standards evolve and more
institutions have access to actinide sources, here we discuss the different
approaches that will be instrumental in helping single-crystal actinides reach
their full potential. Figure 1 summarizes several of these techniques
including pyrometallurgical, solution-based, and different chemical, physical,
and pulsed vapor deposition techniques. Table 1 also lists these synthesis
methods, together with the different compounds they have been used to produce,
and substrates when available.

Figure 1: Schematic representation of the main synthesis techniques used for
actinide thin film fabrication. (a) Shows a crucible often used in
pyrometallurgical techniques where materials are evaporated from a solid
source onto a substrate above. (b) Shows the polymer assisted deposition
approach, where as a polymer solution evaporates, the film grows. (c) Depicts
a common approach to vapor deposition techniques, where ultra pure material
(either in elemental form or in compounds) is directed towards a substrate
where it can react with a plasma, other compounds, or simply adsorb on the
substrate. (d) Shows a representation of sputtering techniques, where a plasma
of ionized material is deposited onto a substrate, with the option to have an
overpressure of a different material, such as N or O. (e) Shows the
hydrothermal approach, where pressure and temperature gradients are used to
precipitate the desired material from a mineralized solution onto a substrate
where a thin film can grow.

###  II.1 Pyrometallurgical methods

In early attempts to fabricate bulk single crystals, actinide compounds such
as oxides or alloys were placed in crucibles. When subjected to extreme
temperatures and pressures, the required materials are evaporated from the
solid source onto a substrate (Figure 1(a)). The high melting points of
actinide and rare-earth materials used in this early attempts presented a
challenge for substrate and crucible material selection. Among the first
reports attempting to grow actinide thin films was _mineralization_. In this
approach, the crucible is placed on a rotating, water-cooled support,
connected to a DC power supply. A tungsten wire acts as a cathode, emitting
electrons that weld a lid onto the crucible. Then, by heating the crucible
very close to its melting temperature, single crystals can grow inside from
the source material over a period of weeks [2, 8, 11]. In this way, Ac2O3 was
chemically converted to a face-centered cubic (fcc) Ac metal powder, along
with the presence of actinium hydride discovered through X-ray diffraction
(XRD) [12]. Similar approaches have yielded samples of Th3N4 [13], UN [14],
U2N3 [14], USb [11], UTe [11], and U metal [15]. Different transuranic
elements can be synthesized into thin films by reducing their oxide compounds.
These materials are loaded into a Ta crucible, together with a reducing agent.
The crucible is then heated with direct current filaments in order to
evaporate the pure metal and condense it onto a cooler substrate outside of
the crucible [15]. Pyrolitic decomposition has been used to thermally break
down precursor molecules containing U and Am, and form thin films of these
active species [16]. In general, these early methods introduced large numbers
of impurities, and resulted in poor control over the crystal quality of each
sample. Nonetheless they helped researchers improve their understanding of
growth parameters and physical properties, paving the way for more innovative
approaches.

###  II.2 Solution-based methods

The levels of radioactivity in some actinide materials make it impractical to
handle of more than trace amounts in facilities not designed for this purpose.
Dissolving these trace amounts in solution helps minimize the amount of
radioactive elements involved in single-crystal growth (Figure 2(b)). One of
the most successful approaches to solution deposition to date is polymer
assisted deposition (PAD), where precursor molecules are made by binding the
metal ions of interest to a polymer. As this polymer solution evaporates, the
metal film grows. This technique allows researchers to bypass the need for
vacuum equipment by controlling the growth process through the viscosity and
homogeneity of the metal-bearing precursor polymers [17, 18, 19, 20].
Electrodeposition is another solution-based approach. Atoms of the required
metal are suspended in solution and mixed with a metallic stirrer that acts as
the anode, while a suitable metal plate onto which the film will grow is kept
near the cathode [21, 22, 23]. This approach has provided an opportunity to
study the oxidation states in uranium oxides [18], and to grow thin films of
uranium oxides on foils [24].

Other solution-based approaches include hydrothermal synthesis, in use since
1839 when the first barium and strontium carbonate crystals were formed [25].
Pressure and temperature gradients are used to precipitate the desired
material from a high purity feedstock (Figure 2(e)). A mineralized solution
(e.g., CsF in the case of UO2) is often used to help transport the nutrient to
the substrate where the thin film can crystallize and grow [26, 27]. The main
difference between conventional solution growth and hydrothermal synthesis is
the viscosity of the liquid. Several actinide compounds have been successfully
synthesized using solution approaches, including ThO2 [28, 29], UN2 [19], UO2
[5, 18, 19, 24, 26, 27, 30], U3O8 [19], UO3 [17], UC2 [19], NpO2 [19, 20], and
PuO2 [19].

A key challenge associated with solution-based growth approaches is presence
of trace amounts of solvent in the final samples. Although the quality of thin
films prepared in the last few years is remarkable, further reductions in
defect and impurity densities is still a goal for higher precision
measurements.

###  II.3 Physical and chemical vapor deposition methods

Several types of physical vapor deposition (PVD) have been used to synthesize
thin films of actinides and surrogate materials (Figure 2(c)). Among these,
sputtering and electron beam evaporation (EBE) have been most commonly used
due to their comparatively low cost and ease of use. Different types of
sputtering are used for the deposition of thin films depending on the physical
properties of the targets required: radio frequency (RF), direct current (DC),
high frequency (HF), ion beam (IB), and magnetron sputtering are among the
most common. Multiple types can be combined to produce enhanced results in
terms of faster deposition rates or more uniform films (Figure 2(d)). EBE is
commonly used for its ability to heat relatively small areas of a source to
very high temperatures, allowing for high precision in deposition rates.

Reactive PVD (RPVD) occurs when a chemical reaction occurs between the
deposited material and the atmosphere surrounding the substrate, often during
an annealing step. In contrast, pulsed laser deposition (PLD) is a form of
physical vapor deposition where a high power, low bandwidth laser is used to
melt, evaporate, and ionize material from the surface of a target [3].

Other techniques used mostly for rare-earth compounds include chemical vapor
deposition (CVD) [31, 32] and molecular beam epitaxy (MBE) [33, 34]. There
have been numerous successful research efforts to use rare-earths as
surrogates for actinide materials [35, 36, 37, 38, 39]. Similarities in their
densities, electronic structure, and high melting temperatures give
researchers an opportunity to develop equipment and growth regimes that can be
translated from the rare earths to synthesize actinide thin films.

##  III Actinide Thin Films and Characterization

Commercial applications of actinide-based compounds depend on a thorough
understanding of their properties. This knowledge in turn helps researchers
develop ways to synthesize higher quality samples. The accuracy of current
physical property measurements for actinide-based materials are limited by
defects such as grain boundaries and impurities in the samples. Initial
studies of the properties of these materials used polycrystalline samples or,
in the best cases, samples with large single crystal grains. Defects such as
grain boundaries act as scattering centers for thermal energy carriers
(electrons and phonons). Electron- and phonon-mediated conduction processes
dictate a material’s thermal and mechanical properties, and understanding
these is vital for their full characterization. By eliminating grain
boundaries, high quality, single crystalline samples enable researchers to
study these processes in the absence of major scattering mechanisms. In
addition, the use of new sample geometries and crystallographic orientations
continue to expose novel physics and materials properties. For example,
controlling dimensionality and strain in actinide heterojunctions will create
an opportunity to explore novel quantum phenomena. Here we discuss the
promise, challenges, and possible synthesis routes for actinide-bearing
heterostructures exhibiting complex electron correlations for functional and
energy materials.

###  III.1 Thin film synthesis

Thin films of actinium metal have been produced since 1953, while thin films
of metallic protactinium were synthesized through electrodeposition as early
as 1964 [23]. These metallic thin films were used for $\alpha$-spectrometry
and nuclear reaction experiments. Following this work, deposition of thin
films of protactinium pentoxide (Pa2O5) by electro-spraying films was achieved
[22].

Reports of thorium-based thin film properties remain relatively scarce due to
the absence of a 5$f$ electron orbital. Instead, most attention has focused on
thorium’s bulk properties as a candidate fuel for nuclear reactors. With this
objective in mind, films of ThO2 have been synthesized using sputtering [40,
41], photochemical deposition [28], and electron beam evaporation [42]. Uno et
al.used a nitridation approach to synthesize Th3N4 [43]. Gouder et al.prepared
thin films of ThN and Th3N4 using sputter deposition, where photoemission
spectroscopy demonstrated a high density of 6$d$ states in ThN and a non-
metallic character in Th3N4 [44].

UN thin films have been grown via reactive and DC magnetron sputtering using a
Nb buffer layer on the $(1\bar{1}02)$ facet of sapphire [45], as well as on
Si(111), polycrystalline Ta foil and glass substrates [46]. Black et al.used
the same set of substrates (Si(111), polycrystalline Ta foil, and glass) in
attempts to grow U2N3 [46]. Scott et al.synthesized UN2 on LAO(001) substrates
using polymer assisted deposition [19]. Long et al.also achieved the growth of
both of these compounds by nitriding U metal flakes [47]. Gouder et al.grew
this compound on CaF2 using DC magnetron sputtering [48]. Adamska et al.grew
UMo films on a Nb(110) buffer layer on a sapphire substrate as well as U-Zr,
U-Mo, and U-Nb [49]. The study by Black et al.also included thin films of UC2
grown on YSZ(110) and U3O8 in different crystal structures that depended on
the orientation of the sapphire substrates [46]. Thin films of uranium
oxicarbides and oxinitrides were prepared on Si(111) substrates by Eckle and
Gouder using reactive DC sputtering [50]. Two compounds of uranium oxide were
grown using PAD on LSAT(100) substrates: $\alpha$-UO3 and $\alpha$-U3O8 [17].

Several attempts at synthesizing UO2 thin films have been reported with
various results. The first reported attempt is by Bierlein and Mastel in 1960,
where they studied the effects of irradiation by growing UO2 on carbon
substrates [51]. Elbakhshwan and Heuser summarized the evolution of UO2 thin
film growth up to 2017 [52], and discusses the use of CVD and sputtering
techniques (see Table 1). Their contribution also includes growth on several
different substrates using magnetron sputtering. Further advances in this
field since then include reactive sputtering on LSAT(110) [53, 54], magnetron
sputtering on YSZ(100) [55], DC sputtering on SrTiO3(110) [56], PAD of UO2+x
by Zhang et al. [57], and spin coat combustion on aluminum sheets by Roach et
al. [58]. UxTh1-xO2 thin film synthesis was achieved by Cakir et al.to study
surface reduction using ice [59].

Synthesis of transuranic compound thin films has been historically challenging
due to the high toxicity and scarcity of material precursors [60]. An early
attempt to produce thin films of PuO2 is reported by Shaw et al. [61]. More
recently, Wilkerson et al.summarized the state-of-the-art and reported the
growth of thin films using PAD on YSZ substrates [62]. Roussel discussed the
sputtering of PuO2 and $\alpha$-Pu2O3 films used to study inverse
photoemission [63]. Subsequent attempts to grow bulk single crystals of these
oxides used solution based approaches [64, 65, 66]. Other Pu-based single
crystal compounds synthesized include PuAs, PuSb, and PuBi for studies of
anisotropic magnetization [2]. Mannix et al.used the same fabrication method
to grow NpS and PuSb single crystals to investigate their magnetic structure
[67]. Durakiewicz et al.investigated the spectral features of USb, NpSb, PuSb,
NpTe and PuTe using angle-resolved photoemission spectroscopy (ARPES) on
samples grown using a mineralization technique [11]. Gouder et al.grew thin
films of PuSe and PuSb for study using ultraviolet photoelectron spectroscopy
(UPS) [68], and thin films of PuSix (with $x$ varying from 4 to 0.5) [48]. A
particularly successful research program involved PAD of PuO2 and NpO2 with
high enough crystalline quality that they enabled studies of band dispersion
and the Fermi surface, and helped validate several characteristics used in
5$f$ electron modelling approaches [19]. NpO2 was grown on LAO(001) substrates
using a UO2 buffer layer. Elements beyond Np have scarcely been fabricated in
sufficient amounts to attempt high quality crystal growth. Through a method of
oxidation, Radchenko et al.managed to fabricate high quality bulk samples of
Am, Cm, Bk, and Cf on a La crucible [15].

Table 1: Actinide thin film material growth summary. Compound | Growth Method | Buffer Layer(s) - Substrate | Reference
---|---|---|---
Ac/AcH2 | Solid reaction |  | [12]
Pa | Solution | Zn, Al, Mn | [1]
ThO2 | RF sputtering | various | [40]
| RF sputtering | glass/Si | [41]
| Photochemical deposition | polyimide membrane | [28]
| EBE | poly-Ir | [42]
| Hydrothermal |  | [29]
ThN | metal reaction | N/A | [13]
| DC sputtering | quartz | [44]
Th3N4 | Nitridation | N/A | [43]
| DC sputtering | Si(111) | [44]
UN | Reactive sputtering | Si(111) | [46]
|  | poly-Ta foil |
|  | glass |
| DC magnetron sputtering | $\langle 011\rangle_{\text{Nb}}\|\langle 1\bar{1}02\rangle_{\text{Al}_{2}\text{O}_{3}}$ | [45]
| Reactive sputtering | Spectrosil quartz glass | [69]
| DC sputtering | quartz glass | [70]
| Cathode sputtering | Niobium | [71]
| Nitridation |  | [14]
UN1.85 | RF sputtering | Si | [72]
$\alpha$-U2N3 | Nitridation |  | [14]
| Cathode sputtering | Si(100) | [73]
$\beta$-U2N3 | Nitridation |  | [14]
U2N3 | Reactive sputtering | Si(111) | [46]
|  | poly-Ta foil |
|  | glass |
| DC magnetron sputtering |  CaF2 | [45]
UN2 | PAD | $\langle 111\rangle_{\text{UN}_{2}}\|\langle 001\rangle_{\text{LAO}}$ | [19]
UO2 | Reactive sputtering |  LAO (001) / CaF2 (100) | [74]
| Reactive sputtering | $\langle 001\rangle_{\text{UO}_{2}}\|\langle 011\rangle_{\text{LSAT}}$ | [75, 53]
| Magnetron sputtering | $\langle 001\rangle_{\text{UO}_{2}}\|\langle 001\rangle_{\text{YSZ}}$ | [54]
| Magnetron sputtering | $\langle 111\rangle_{\text{UO}_{2}}\|\langle 111\rangle_{\text{Si}}$ | [76]
| Sputtering | Mo | [77]
| Chemical vapor deposition |  UO2 on Si | [32]
| Pulsed laser deposition | $\langle 100\rangle_{\text{UO}_{2}}\|\langle 111\rangle_{\text{LSAT}}$ | [3]
| PAD | $\langle 001\rangle_{\text{UO}_{2}}\|\langle 011\rangle_{\text{LAO}}$ | [18, 19]
| sol-gel |  MgO, Al2O3 | [78]
| Solution/foil | Fe | [24]
| Solution Spin Coat | Al | [30]
| DC sputtering | $\langle 001\rangle_{\text{UO}_{2}}\|\langle 011\rangle_{\text{SrTiO}_{3}}$ | [55]
| DC sputtering | $\langle 111\rangle_{\text{UO}_{2}}\|\langle 111\rangle_{\text{YSZ}}$ | [56]
Table 2: Actinide thin film material growth summary (cont.) Compound | Growth Method | Buffer Layer(s) - Substrate | Reference
---|---|---|---
UO2 | Hydrothermal |  CaF2 | [26, 27]
| Hydrothermal |  CaF2,ThO2,UO2,YSZ | [5]
| CVD | Si | [31]
| Reactive Magnetron Sputtering |  TiO2, Al2O3, YSZ, ZnO, NdGaO3 | [52]
| PAD | Si | [57]
| Spin coat combustion | Al sheet | [58]
U-O-Pd | Sputter co-deposition | Si | [79]
U3O8 (P$\bar{6}$2m) | PAD |  Al2O3(001) | [19]
U3O8 (Cmmm) | PAD |  Al2O3(012) | [19]
$\alpha-$UO3 | PAD | LSAT(100) | [17]
$\alpha-$U3O8 | PAD | LSAT(100) | [17]
UCxOy | Reactive DC sputtering | Si(111) | [50]
UOxNy | Reactive DC sputtering | Si(111) | [50]
U3 | DC sputtering | Si(111) | [80]
U1-xMox | DC magnetron sputtering |  Nb(110)/Al2O3(11$\bar{2}$0) | [81]
UMo | DC magnetron sputtering |  Nb(110)/Al2O3(110) | [49]
USb | Mineralization | N/A | [11]
UTe | Mineralization | N/A | [11]
UC2 | PAD | $\langle 001\rangle_{\text{UC}_{2}}\|\langle 011\rangle_{\text{YSZ}}$ | [19]
U | Diode sputtering | poly-Al and Mg | [48]
| Oxide reduction | La crucible | [15]
| UHV Magnetron sputtering | $\langle 001\rangle_{\text{U}}\|\langle 011\rangle_{\text{Nb/W}}\|\langle 1\bar{1}02\rangle_{\text{\text{Al}${}_{2}$\text{O${}_{3}$}}}$ | [82, 83]
| Sputtering | Al and Mg | [6]
UH2 | Reactive sputtering | Si (001) | [84]
UxTh1-xO2 | DC sputtering | Si(111) | [59]
(UxPu1-x)O2 | DC sputtering | Si(111) | [59]
NpH2 | Solid-gas reaction |  | [85]
NpSb | Mineralization | N/A | [11]
NpTe | Mineralization | N/A | [11]
NpO2 | PAD |  UO2/LAO(001) | [19]
| PAD | YSZ(100) | [19, 20]
| DC sputtering | Si(111) | [59]
NpS | Mineralization | N/A | [67]
PuN | Reactive Sputtering | Si(111) | [86]
PuH2/PuD2 | Solid-gas reaction |  | [87]
PuSb | Mineralization | N/A | [2, 67]
PuAs | Mineralization | N/A | [2]
PuBi | Mineralization | N/A | [2]
PuCoGa5 | Flux growth | not specified | [88]
PuSb | Mineralization | N/A | [11]
PuTe | Mineralization | N/A | [11]
Table 3: Actinide thin film material growth summary. Compound | Growth Method | Buffer Layer(s) - Substrate | Reference
---|---|---|---
PuO2 | DC sputtering | Si (111) | [89]
| PAD | YSZ(100) | [19, 20, 62]
| Sputtering |  $\alpha$-Pu | [63]
$\alpha$-Pu2O3 | Sputtering |  $\alpha$-Pu | [63]
PuSix | DC sputtering | Mo (100) | [68]
Am | DC Sputtering | Mo(100) | [90]
AmN | DC Sputtering | Mo(100) | [90]
Am2O3 | DC Sputtering | Mo(100) | [90]
AmO2 | DC Sputtering | Mo(100) | [90]
AmSb | DC Sputtering | Mo(100) | [90]
Am2 | Solid-gas reaction |  | [91]
Am | Oxide reduction | La crucible | [15]
CmH2 | Solid-gas reaction |  | [92]
Cm | Oxide reduction | La crucible | [15]
Bk | Oxide reduction | La crucible | [15]
Cf | Oxide reduction | La crucible | [15]

###  III.2 Characterization of Thin Films

We can divide materials characterization techniques into categories that deal
with different properties of the thin films in question. Figure 2 illustrates
how one can use various techniques to explore structural, surface and
electronic properties together in concert.

We can obtain plan view or cross-sectional structural images of a thin film
using transmission electron microscopy (TEM), scanning electron microscopy
(SEM) (see Figure 2(c) and (h)), atomic force microscopy (AFM), and scanning
tunneling microscopy (STM). These can be used to determine the size of
crystallites, the density of defects such as dislocations twins and stacking
faults, and crystalline quality, in addition to quantifying the roughness of
the surfaces and the shape, size and areal density of any structures present
on them. “Indirect” structural characterization may include Rutherford back
scattering (RBS), XRD, and Raman spectroscopy (see Figure 2(a), (d), (e), (f),
(i), and (k)). These techniques can provide information that complements what
can be seen in microscopy, and help identify properties such as alloy
composition, lattice parameter, and in the case of ion-damaged samples,
penetration depth.

Figure 2: Characterization techniques applied to the most commonly studied
actinide thin films: UN and UO2. The data presented in each panel have been
digitally extracted from the relevant references and are shown for
illustrative purposes only. (a) RBS data from UN before and after radiation
exposure ([70]), (b) XPS data for a UN thin film ( [45]), (c) SEM micrograph
of a UN thin film ( [71]), (d) UN Raman spectrum [72], and (e) an XRD
diffraction pattern for UN [93]. The equivalent data for UO2 come from (f)
[94], (g) and (h) [76], (i) [3], (j) [19], and (k) from [76]. Note that ARPES
data do not yet exist for UN thin films, making initial electronic
characterization a near-term target for any new synthesis.

Several techniques have been used to measure the thermophysical properties of
bulk actinide materials [4]. However, thin films have been targeted in
specific cases as a means to increase measurement accuracy while using non-
destructive techniques. It is here where photothermal approaches offer a
viable option for studying samples with small physical dimensions without
damage to the sample or the need for destructive sample preparation approaches
[95]. Several comprehensive reviews have been recently published on the
thermophysical properties of actinide oxides [4], nitrides [96], and carbides
[97, 98] due to the importance of these compounds to nuclear science and
industry.

Electronic structure characterization includes angle-resolved photemission
spectroscopy (ARPES), X-ray photoemission spectroscopy (XPS), STM-based
scanning tunneling spectroscopy (STS), and photoluminescence spectroscopy.
These approaches can shed light on charge carrier behavior, and the electronic
states present in a given material. These characteristics are of particular
interest for actinide thin films due to the interesting physics governed by
their 5$f$ electron states. XPS allows one to determine the elements present
within a material, along with their oxidation and bonding state. XPS is
especially useful for samples with reduced dimensionality within which the
localization of 5f electrons occurs [99]. Gouder and Havela identified the
complications of applying XPS to the study of actinides: an asymmetrical tail
in metallic systems can often prove difficult to differentiate from scattering
of electrons onto the Fermi level. In addition, the anisotropic distribution
of atoms at different temperatures contributes to higher noise levels in the
XPS signal [99]. The team lead by Black et al.identified the 5$f$ states of UN
around 0.8 eV [46]. Havela et al.indicate that 5$f$ localization behavior can
be studied in PuSb (localized), PuSe (intermediately localized), and PuN
(potentially delocalized) [86]. In addition, Gouder et al.studied the
localization of 5f electrons in PuSix [80]. Durakiewicz et al.investigated the
systematic changes in the 5f binding energy peak between antimonide and
tellurides of U, Pu, and Np [11]. Joyce et al.studied the dual nature of 5f
electrons using photoemission of $\delta$-Pu, PuIn3, and PuCoGa5 [100, 101].
On the same line, Eloirdi et al.reported results from XPS studies of PuCoGa5
deposited using sputtering [102]. Long et al.studied the electronic structure
of $\alpha$-U2N3 and found hints of metallic bonding [73]. Pentavalent uranium
was found in U2O5 by Gouder et al.using photoemission studies [103].

The ARPES technique has been used to determine precise band structures and
Fermi surfaces of actinide-based single crystals [19, 88, 104, 105, 106, 107,
108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122,
123, 124, 125, 126, 127]. Largely focused on bulk actinide compounds, these
ARPES studies have created new opportunities for determining the origin of the
unusual physical properties of 5$f$-electron materials. Going forward, ARPES-
based studies of epitaxial actinide thin films could lead to the discovery of
a wide range of emerging electronic, magnetic, and structural phenomena
(Figure 2(j)). Interactions between the epitaxial thin film and the underlying
substrate can produce properties that are substantially different from those
of the bulk materials. For instance, it has been reported that one can obtain
an unusual hexagonal close-packed phase of uranium (hcp-U) by depositing U
thin films onto a W (110) substrate [128, 129]. Theoretical calculations
predict that the hcp-U phase has an electronic instability, that could lead to
a possible charge density wave or magnetic ordering [130]. Different
orientations of the $\alpha$-U orthorhombic phase can also be obtained by
depositing U onto a variety of buffer/seed layers on sapphire substrates [82,
131]. ARPES was used to obtain the band structure of $\alpha$-U single
crystals at 173 K [132]. The valence band structure of $\alpha$-U was studied
in details using ultraviolet photoemission spectroscopy and X-ray
photoemission spectroscopy [133],

For ordered overlayers of U metal grown on a W (110) substrate, band-like
properties of the U 5$f$ states were observed, which were proposed to arise
from direct $f$–$f$ electron interactions [128]. STM/STS results showed that
the U density of states close to the Fermi level is dominated by the 5$f$
states [129]. STS and ARPES showed both the Fermi surface topology and
electronic structure of ordered $\alpha$-U films [134]. ARPES studies of
epitaxial thin films of the oxides PuO2 and NpO2 [20, 135], and uranium
carbide (UC2), reveal the importance of the underlying substrate structure in
stabilizing the epitaxial film [136]. But in general, compared with bulk
actinide crystals, only a few ARPES studies have been performed on actinide
thin films. Therefore, the use of ARPES to measure novel actinide thin films
synthesized with higher quality will produce results that are critical to
understanding their reactivity, aging, nuclear fuel behavior, and
environmental fate, as well as the potentially rich physics of these exotic
5$f$ materials.

##  IV Thermophysical, Magnetic, and Topological Properties of Actinide Thin
Films

###  IV.1 Thermophysical Properties

Thermophysical properties play a central role in determining the
characteristics of actinide materials. Atomic structure dictates macroscopic
behavior, and it is hence important to understand the temperature dependence
of these properties. A key research driver in actinide compounds is the
qualification process for nuclear fuel materials. This process is heavily
reliant on characterizing thermophysical properties: heat capacity, thermal
conductivity, elastic constants, and thermal expansion [4, 137]. Heat capacity
has been measured in several important actinide dioxides, such as ThO2 [95],
UO2 [138], NpO2 [139], PuO2 [140]. UO2 and NpO2 show an interesting magnetic
alignment transition that creates anomalies in their heat capacity. Thermal
conductivities have been measured for ThO2 [141], UO2 [142], NpO2 [143], and
PuO2 [144]. The heat capacities of select actinide nitride systems have been
measured, including ThN [145], and more recently for the UN-ThN mixtures used
in nuclear fuels [96].

Among these properties, thermal diffusivity and conductivity have been
recently studied as important factors in determining the properties of a
nuclear fuel candidate material [4]. The qualification process requires that
intrinsic thermophysical properties be resolved independently of any
particular specimen geometry, without the competing influence of lattice
defects or other chemical or structural imperfections. Building a fundamental
understanding of intrinsic properties allows for the effects of these
additional complexities, for example the types of structural defects generated
under exposure to irradiation, to be systematically included in computational
frameworks for engineering-scale performance. Much success to date has been
found in the use of non-destructive photothermal techniques to measure these
intrinsic thermophysical properties [95]. Such methods broadly consider
microscale thermal and acoustic wave propagation, which can be measured via
surface deformation, refractive index gradient, acoustic
expansion/contraction, or changes in optical reflectivity (thermoreflectance).
This last approach can be further subdivided into frequency-domain
thermoreflectance, time-domain thermoreflectance, spatial-domain
thermoreflectance, and hybrid methods [137, 146]. With small sampling volumes,
these methods are broadly applicable to the study of thin films [147].
Although they have not yet been applied to actinide thin films, we anticipate
that these techniques will help to shed light on their thermophysical
properties in the future.

Heterostructure devices that integrate different functional layers can
encounter performance issues as a result of restricted heat dissipation across
the heterointerfaces (Kapitza resistance). This restriction in heat
dissipation correlates with factors such as interfacial roughness, disorder,
dislocations, and bonding. As a result, computational modelling of these
interfaces often fails to capture all of their vibrational properties, for
example changes in mass density, phonon mean free path, and carrier scattering
across the interface [148].

Figure 3: Properties of interest in select actinide thin films considered in
this review. Under thermophysical properties, measurements of heat capacity in
some actinide oxides exhibit an anomaly at certain phase transition
temperatures. Magnetic properties to consider include the quantum, quantum
anomalous, and quantum spin Hall effects. Two important topological properties
are superconductivity (present in the UPt3 compound, and topologically
protected states) together with topological insulators in two and three
dimensions, respectively.

###  IV.2 Magnetic properties

Different aspects of electron and phonon behavior have been carefully studied
under the influence of magnetic fields. Among these, transport and its
different manifestations enabled the observations of several noteworthy
phenomena. These physical properties are part of what makes actinides
interesting to researchers beyond their nuclear applications [149]. Most
transition metal elements have itinerant $d$ electrons, while lanthanide
elements exhibit mostly localized 4f electrons. These fact help us distinguish
characteristics related to the conduction band of a material and its magnetic
properties relating to the 4f electrons [149]. Due to the scarcity and high
cost of 227Ac, single crystal samples of this metal have been synthesized to
date, making it difficult to study its magnetic properties experimentally
[150]. A similar situation exists for the actinide elements after Californium.

Considering the intervening elements, Th is a paramagnetic metal with three
phases that are heavily influenced by the concentration of impurities present
[1]. Pa metal was synthesized relatively early on in the development of
actinide research, and it was hence possible to determine its phase transition
and superconducting behavior [1], as discussed below. The early actinide
metals (U, Np, Pu) show no sign of magnetic ordering mostly because the
bandwidth of the 5$f$ electrons in these elements is too wide to satisfy the
Stoner criterion [151]. U metal exhibits a weak Pauli-type paramagnetism, with
$\alpha$-U having a charge-density-wave anomaly around 43 K that has been
observed in magnetic susceptibility measurements [1]. Np exhibits spin-orbit
coupling in the same order as other crystal-related electric effects, leading
to its metallic form and paramagnetic behavior [1]. Elemental Pu can be found
in both $\alpha$\- and $\delta$-Pu phases. Recent studies point to a lack of
evidence favoring any ordered or disordered magnetism in $\delta$-Pu [152],
which preferentially forms a valence fluctuation ground state [153]. Curium
(Cm) is the first element in the actinide series that orders magnetically. Cm
is antiferromagnetic (AF) below 65 K in the double hexagonal close packed
(dhcp) phase, while in the fcc phase it is ferromagnetic (FM) below 205 K
[154]. Berkelium (Bk) also exists in these two crystallographic phases (dhcp
and fcc) and similarly orders either AF or FM. Californium (Cf) also exhibits
a large effective magnetic moment, similar to Cm and Bk, and likely has a FM
ground state [155].

A variety of thin films of actinide compounds have provided insight into the
magnetic properties of these systems, such as the magnetic structures of NpS
and PuSb [67], itinerant antiferromagnetism [69] and magneto-optical Kerr
effect [156] in UN , weak ferromagnetism [157] and antiferromagnetism [74] of
UO2, and the high pressure behavior of electrical resistance in NpAs and NpBi
[158]. Magnetic susceptibility measurements of transplutonium elements Am, Cm,
and Bk helped determine the effective magnetic moments and Curie-Weiss
constants for some of their compounds [159]. Magnetic properties have also
been extensively studied in these materials using computational methods ([160]
and references therein). Bright et al.studied the magnetic and electronic
structure of UN and U2N3 epitaxial films using x-ray scattering [161].
Recently, using high-field magnetization and magnetostriction measurements,
the presence of a metamagnetic transition and a tri-critical point was
observed in UN single crystals, leading to the development of a magnetic phase
diagram for this material [162, 163]. Using micro-structured single crystals,
Hamann et al.found strong reconstruction of the Fermi surface at the high-
field transition [164].

###  IV.3 Superconductivity

Certain actinide metals are known to exhibit superconductivity at low
temperature. these include Th at 1.4 K [165, 166], Pa at 1.4 K [167],
$\alpha$-U at 0.7 K and $\gamma$-U at 1.8 K [168, 169, 170, 171]. Am was
predicted to be a superconductor, although confirming this experimentally was
a challenge due to its large self-heating, scarcity, and radioactivity.
However, this prediction was confirmed by Smith and Haire; when its fcc phase
is stabilized by quenching, Am becomes superconducting at 1 K [172].
$\alpha$-Np and $\alpha$-Pu have no reported superconducting signature down to
0.5 K [173] and Cm, Bk, and Cf are not expected to show superconductivity
because of their magnetic ground-state [173]. There are several actinide
compounds that show heavy-fermion superconductivity, including UBe13, UPt3,
URu2Si2, UNi2Al3, UPd2Al3, NpPd5Al2, PuCoIn5, PuRhGa5, PuRhIn5 and PuCoGa5
[88, 174, 175, 176, 177, 178, 179]. Ott et al.showed that UB13 is
superconducting below 0.85 K [180]. Joyce et al.studied photoemission data
from PuCoGa5, PuIn3, and $\delta$-Pu metal. A comparison between their results
and model studies indicate characteristics of both itinerant and localized Pu
5$f$ electrons [100]. Furthermore, Joyce compared $T_{c}$ in PuCoGa5 against
UCoGa5 and found no evident characteristics of strongly correlated electrons
[101]. Thus, valence fluctuations have been proposed as an alternative
mechanism for the high-temperature superconductivity observed in PuCoGa5 [181,
182]. Recently, unconventional [183], spin-triplet [184] superconductivity
with multiple superconducting phases [185] has been reported in UTe2. In
addition, there are indications for spontaneously broken time-reversal
symmetry [186], and chiral Majorana edge and surface states [187], the nature
of which are not yet understood. Brun, Cren, and Roditchev summarized the
superconductivity of 2D materials as a strong case for epitaxial monolayer
materials growth [188].

###  IV.4 Topological Properties

The study of conducting states in topologically nontrivial materials is often
hindered by the presence of bulk conducting states [189]. High quality thin
films offer one way of minimizing bulk contributions by increasing a sample’s
surface area-to-volume ratio, simplifying measurements of these topological
effects. Due the nature of their electronic structure, actinide materials
represent a fascinating, albeit challenging area for topological discoveries.
Promising technological applications and insights into fundamental physics
continue to motivate investigations on the complex manifold of 5$f$, 6$p$,
6$d$, and 7$s$ orbital shells [190]. These interactions, present in an
analogous manner in the rare earths, are expected to give rise to a variety of
topological behaviors in different compounds and geometries.

Heavy fermion materials owe their particular properties to the competition
between Coulomb repulsion of localized $f$ electrons, and hybridization with
itinerant $d$ electrons. These electron correlations produce complex states
including Mott physics, superconductivity, and correlated magnetism [191].
Understanding these electronic systems could potentially help elucidate other
strongly correlated electron materials, for example high-temperature
superconductors, multiferroics, and magnetoresistive materials. Topological
insulators possess conducting surface states protected by the non-trivial
topological property of spin-momentum locking. Several quantum materials have
been proposed based on topological properties that involve quantum phenomena
such as Majorana fermions, the anomalous quantum Hall effect, with
applications in spintronics and fault tolerant quantum computation. Magnetic
topological semimetals have been synthesized using MBE [192, 193]. These
materials have a FM ground state that breaks time-reversal symmetry with a gap
in the topological surface states [194]. A powerful tool to study these
phenomena is density functional theory (DFT), e.g. its use in identifying
phases in UNiSn corresponding to a topological insulator and a Weyl semimetal
depending on magnetic ordering [195]. Rocksalt compounds of Pu and Am were
predicted to form a topologically insulating ground state due to their 5$f$
electron being right on the boundary between localized and itinerant, and
coupling between electronic and spin-orbit interactions gives rise to band gap
formation [196]. The predicted compounds that Zhang et al.used for this study
include americium monopnictides and plutonium monochalcogenide [196]. Using
nuclear magnetic resonance (NMR), Dioguadi et al.demonstrated that PuB4 is a
strong candidate for a topological insulator, which has also been suggested by
Choi et al. [197, 198]. Recent theoretical work suggests that the intermediate
valence PuB6 is a strong topological insulator with nontrivial
$\mathbf{Z}_{2}$ topological invariants [199], similar to SmB6 [200].

Overall, much work is still to be done in understanding fully the properties
of these actinide compounds, and high quality thin films promise to be the
missing piece to provide a window into their discovery.

##  V Challenges with Modelling 5f Electrons

DFT [201, 202] is normally the zeroth level of first-principles theory that is
applied to any material, given the relatively small computational cost for the
degree of complexity it can capture [203, 204]. DFT has found success across
the periodic table for calculating a myriad of materials properties [205, 206,
207, 208]. Unfortunately, all known implementations of DFT have well-known
limitations [209, 210, 211]. In particular, DFT sometimes _qualitatively_
breaks down for certain properties in materials bearing strongly correlated
electrons. For example, Mott insulators are often incorrectly predicted to be
metals by DFT. [212] Materials with $f$-electrons are ripe for strongly
correlated electron physics, due to the relatively narrow bands formed by the
$f$-orbitals and large on-site Coulomb repulsion. Other limitations for DFT
calculations, particularly in strongly correlated systems, include the
inability to properly capture finite temperatures and excited states [213].

Fortunately, dynamical mean-field theory (DMFT) [214] can overcome many of the
shortcomings of DFT. DMFT can also be applied to real materials when combined
with DFT to yield the DFT+DMFT formalism [210]. DMFT requires the solution of
a quantum impurity problem, which is associated with the correlated shell of
electrons (e.g. $f$-electrons). These solutions can be achieved in realistic
multiorbital problems using continuous time quantum Monte-Carlo (CTQMC) [215,
216, 217]. While DFT+DMFT(CTQMC) is far more computationally expensive than
DFT, current algorithms and computing power allow for calculations in
relatively complex systems bearing $d$ and $f$ electrons [218, 219, 220, 221,
222, 223]. An efficient, but far less accurate solution of the DMFT impurity
problem can be achieved with the Hartree-Fock method. This method yields the
better known DFT+$U$ approach [224, 210], for which the computational overhead
is negligible when compared to DFT. Together, DFT, DFT+DMFT, and DFT+$U$ form
a hierarchy of first-principles approaches that serve as the basis for
parameterizing multiscale models to generate information at longer length and
time scales [225].

Figure 4: (a) Comparison between the experimental thorium nitride lattice
parameter as a function of temperature, and values calculated using molecular
dynamics (MD) simulations [226]. Reproduced with permission from Elsevier. (b)
Electron density plots calculated on the (001) plane for UN and ThN [227].
Reproduced with permission from the American Physical Society. (c) Band
structure and energy spectrum of AmN, showing spin-orbit coupling interactions
and a Dirac surface cone. From [196]. Reproduced with permission of AAAS.

Compared with bulk materials, the surfaces and heterostructures associated
with thin films introduce additional complexity to modeling efforts due to
confinement effects, charge transfer, strain, and structural reconstruction.
When considering actinide thin films, the presence of $f$-electrons in these
systems contributes yet another layer of complexity. In actinide oxides, a
wide range of phenomena have been observed in thin film heterostructures that
are absent from their bulk constituents. These phenomena include
superconductivity [228, 229], room temperature ferromagnetism [230], metal-
insulator transitions [231], and orbital reconstruction [232], with the clear
potential for novel device physics [233]. DFT+$U$ and DFT+DMFT have already
been used to confirm emergent properties in heterostructures composed from
transition metal oxides [234, 235, 236]. However, similar calculations for
heterostructures built from $f$-electron materials will undoubtedly be more
challenging than those for $d$-electron materials. The DFT+DMFT computations
will be substantially more computationally expensive, and DFT+$U$ computations
will struggle with a more treacherous energy landscape. Nonetheless,
$f$-electron heterostructures are within reach of our present theories and
computational resources, and there has already been some progress at the level
of DFT. Gao and Asok used DFT to study the (001), [237] and (111) [238]
surfaces of Am. The surface properties of UN, a candidate nuclear fuel, have
been studied using DFT+$U$, with potential applications for understanding
corrosion characteristics [239]. In short, a combination of models based on
DFT, DFT+$U$, and DFT+DMFT are well poised to study heterostructures composed
from $f$-electron material thin films, and will lay the foundation for
understanding such materials.

Optimization of algorithms based on strongly correlated electron systems in
DFT will provide more accurate descriptions of physical systems, able to in
turn provide higher level modelling systems a path towards integration with
engineering-level modelling paradigms based on first principles.

##  VI Future Directions: Thin film heterostructure synthesis and challenges

Interfaces in semiconducting, metallic, and semi-metallic materials offer
possibilities for investigating electronic properties not typically present in
the bulk, or in layers of a single material. In non-actinide semiconductors,
heterostructures built by stacking different thin films have enabled
researchers to discover new physics. For example, thin films and other low
dimensional heterostructures allow to study the quantum behavior of charge
carriers whose de Broglie wavelength is equivalent to the size of the
potential well within which they are confined. A deeper understanding of how
we can manipulate charge carrier behavior using heterostructures led to the
design of entirely new electronic and photonic devices. Indeed, Kroemer
famously stated that in these heterostructures, the interface is the device
[240]. Extending heterostructure growth to actinide-based compounds therefore
promises to enable researchers to study these materials in new ways. For
example, low-dimensional heterostructures composed of actinide materials with
5$f$ electrons, grown with interfaces containing tailored defect densities,
represent a fertile area for the discovery of various emerging phenomena
[241]. Studies have predicted a range of exotic physics for such structures,
including highly correlated electron interactions [190, 242], topological
insulator behavior [198, 195], and confined 2D properties [243, 244].

As we have seen, the synthesis of high quality, atomically precise thin films
of a single actinide material is still the focus of ongoing research. As a
consequence, the ability to routinely and controllably integrate two or more
of these thin films to create actinide heterostructures remains largely an
area for future development. Before one can begin to study the characteristics
of these heterostructures, one must first be able to produce high purity films
with uncontaminated interfaces. Controlling interactions at the interface
between two films requires a consideration of atomic intermixing, surface
energies, charge transfer, and epitaxial nucleation and growth. These effects
will likely be a function of the environment within which a heterostructure is
synthesized [245]. Solution- and chemical reaction-based synthesis approaches,
for example, often leave residues of unwanted atomic species during
deposition. For this reason, physical deposition methods offer perhaps the
best route to achieving pristine thin film formation, and hence
heterostructure synthesis. Due to its rapid output and relatively low cost,
pulsed laser deposition offers a convenient test-bed for initial film and
substrate screening studies. After some of the film-substrate combinations
predicted by theory have been demonstrated experimentally, even higher quality
films can be grown using molecular beam epitaxy (MBE), which uses ultrahigh
purity elemental source materials rather than chemical precursors or
solutions. The advantages of MBE include exceptionally high material purity,
and exquisite control over layer thickness and interfacial abruptness, both of
which are prerequisites for growing functional heterostructures. To our
knowledge, MBE has not yet been used for the synthesis of actinide thin films
or heterostructures.

The epitaxial challenges for integrating different materials include lattice
constant mismatch, dissimilar crystal structures, thermal expansion
differences, and incompatible growth environments. Several strategies have
been devised to overcome strain-related issues arising from lattice mismatch.
These include the use of graded buffers [246, 247], interfacial misfit arrays
[248], self-assembled quantum dots [249], alloying [250], lattice constant
mismatch inversion [251], and strain-relieving superlattices [252]. To
accommodate different crystal structures, a common crystal lattice parameter
can be used to geometrically align the lattices between each layer. An example
is the differing crystal structures of ErAs (rocksalt) and InGaAs
(zincblende). A rotation of the ErAs unit cell with respect to that of the
InGaAs brings the two lattices into registry, enabling the growth of coherent
ErAs/InGaAs heterostructures [253]. Mismatch between thermal expansion
coefficients in different layers of the heterostructure can induce strain upon
sample cooling, which is then relaxed through defect formation, negatively
impacting crystal quality. Common solutions to this challenge include growth
of thin protective thermal layers that prevent these relaxation processes, and
use of thermally pre-stressed films that can accommodate the strain upon the
change in temperature [254]. It will be critical to consider all of these
challenges and solutions as researchers begin to explore the synthesis of
actinide-based thin film heterostructures.

Figure 5: Sketch of a proposed heterostructure consisting of alternating UN
and ThN thin films. In practice, to avoid strain relaxation, the maximum
allowed thickness of each layer must be calculated from the Matthews-Blakeslee
equation [255].

To fill this technological gap, the Idaho National Lab has recently
commissioned an MBE system designed specifically for the growth of actinide-
nitride compounds. This first-of-its-kind MBE system will permit feasibility
studies of the growth of various actinide-based thin films. Proof-of-concept
experiments will target the MBE growth of heterostructures based on UN and ThN
(Figure 5). These heterostructures will enable the study of the electronic
properties of these interesting 5$f$-electron heterointerfaces. Given the
lattice mismatch of $\sim 5.7\%$ between UN and ThN, the substrate material
must be chosen carefully to allow growth of layers with reasonable thickness,
while remaining below the Matthews-Blakeslee limit for strain relaxation
[255].

An alternative approach is to create heterostructures based on UxTh1-xN alloys
that have lower lattice mismatch to the binary end-point compounds. To this
end, Table 4 lists a “recipe book” of actinide nitrides and candidate
substrates with reasonably lattice-matched alloys. Indeed, by incorporating
suitable buffer layers, some of these compound/substrate combinations have
already been synthesized [45]. The hope is that by exploring the MBE growth of
materials in these actinide nitride systems, their thermophysical, magnetic,
and topological properties can be studied with high precision. Actinide-
nitride-based systems that show promising initial results will be screened for
emergent phenomena, with a view to opening a new approach to investigating the
interplay between electronic correlations and topology in epitaxial thin films
and heterostructures.

Table 4: Crystallographic properties of UN and ThN thin films and candidate substrates. Material | Group |  $a$ (Å) |  $c$ (Å) | Substrate | Substrate lattice const. (Å) | Strain
---|---|---|---|---|---|---
UN |  Fm$\bar{3}$m | 4.88 | 4.88 | $\langle 001\rangle_{\text{UN}}\|\langle 001\rangle_{\text{Al}_{2}\text{O}_{3}}$ | 4.785 | 1.96%
|  Fm$\bar{3}$m | 4.88 | 4.88 | $\langle 001\rangle_{\text{UN}}\|\langle 111\rangle_{\text{Si}}$ | 3.135 | relaxed
|  Fm$\bar{3}$m | 4.88 | 4.88 | $\langle 001\rangle_{\text{UN}}\|\langle 111\rangle_{\text{NaF}}$ | 4.62 | 5.84%
UN2 |  Fm$\bar{3}$m | 5.31 | 5.31 | $\langle 111\rangle_{\text{UN${}_{2}$}}\|\langle 001\rangle_{\text{LAO}}$ | 3.79 | [256]
|  Fm$\bar{3}$m | 5.31 | 5.31 | $\langle 100\rangle_{\text{UN${}_{2}$}}\|\langle 001\rangle_{\text{ZnS}}$ | 5.41 | 1.02%
|  Fm$\bar{3}$m | 5.31 | 5.31 | $\langle 100\rangle_{\text{UN${}_{2}$}}\|\langle 001\rangle_{\text{GaAs}}$ | 5.3565 | 0.7%
|  Fm$\bar{3}$m | 5.31 | 5.31 | $\langle 100\rangle_{\text{UN${}_{2}$}}\|\langle 001\rangle_{\text{Si}}$ | 5.43 | 2.26%
$\alpha$-U2N3 |  Ia$\bar{3}$ | 10.684 | 10.684 | $\langle 001\rangle_{\text{U}_{2}\text{N}_{3}}\|\langle 001\rangle_{\text{Y${}_{2}$O${}_{3}$}}$ | 10.64 | 0.75%
$\beta$-U2N3 |  Ia$\bar{3}$ | 3.69 | 5.826 | $\langle 001\rangle_{\text{U}_{2}\text{N}_{3}}\|\langle 001\rangle_{\text{CaF}_{2}}$ | 5.47 | 2.05% (2:1 relation)
ThN |  Fm$\bar{3}$m | 5.20 | 5.20 | $\langle 001\rangle_{\text{ThN}}\|\langle 001\rangle_{\text{CaF}_{2}}$ | 5.47 | 5.37%
|  Fm$\bar{3}$m | 5.20 | 5.20 | $\langle 001\rangle_{\text{ThN}}\|\langle 001\rangle_{\text{YSZ}}$ | 5.125 | 0.76%
|  Fm$\bar{3}$m | 5.20 | 5.20 | $\langle 001\rangle_{\text{ThN}}\|\langle 001\rangle_{\text{Si}}$ | 5.43 | 5.00%
Th2N3 |  C$\bar{3}$m | 3.875 | 6.175 | $\langle 001\rangle_{\text{Th}_{2}\text{N}_{3}}\|\langle 001\rangle_{\text{LAO}}$ | 3.79 | 2.19%
Th3N4 |  R$\bar{3}$m | 3.875 | 27.39 | $\langle 001\rangle_{\text{Th}_{3}\text{N}_{4}}\|\langle 001\rangle_{\text{LAO}}$ | 3.79 | 2.19%

##  VII Outlook

Recent years have seen significant advances in the field of actinide materials
synthesis. Ongoing improvements in materials growth techniques are beginning
to produce single-crystal samples of actinide metals and their compounds.
These efforts include the synthesis of both bulk crystals and thin films,
within which grain boundaries are eliminated and the density of other
crystallographic defects is reduced. Syntheses using a variety of methods have
been demonstrated important initial material compatibility criteria
(temperature ranges, lattice constants, etc.) for reliable thin film
synthesis. As a result, researchers can, for the first time, investigate
thermal and electrical transport properties of actinides in samples with
dramatically reduced phonon and electron scattering. In the near term,
concerted efforts are being made to integrate different thin film actinides
into heterostructures whose properties can be tuned through strain,
confinement and other interfacial effects. The expectation is that, as a
result, the subtle physical features of these materials will begin to emerge.
Exotic magnetic and topological properties are expected to arise in these
strongly correlated systems with 5f-shell electrons, leading to the discovery
of new physics.

## Acknowledgements

This work was supported through the INL Laboratory Directed Research &
Development Program under U.S. Department of Energy Idaho Operations Office
Contract DE-AC07-05ID14517. F.K. acknowledges the support of the Glenn T.
Seaborg Institute for Actinide Science at Idaho National Laboratory. C.A.M and
D.H.H. acknowledge support from the Center for Thermal Energy Transport under
Irradiation (TETI), an Energy Frontier Research Center funded by the US
Department of Energy, Office of Science, Office of Basic Energy Sciences. The
authors would like to thank A. Tiwari at the University of Utah for useful
discussions. K.D.V. acknowledges the support of his herds of guinea pigs and
hamster.

## References

## References

  * [1] L R Morss, N M Edelstein, J Fuger, and J J Katz.  The chemistry of the actinide and transactinide elements, volume 1.  Springer, 2006.
  * [2] K Mattenberger, O Vogt, J C Spirlet, and J Rebizant.  Anisotropic magnetization of PuAs, PuSb and PuBi single crystals.  Journal of magnetism and magnetic materials, 54:539–540, 1986.
  * [3] E Enriquez, G Wang, Y Sharma, I Sarpkaya, Q Wang, D Chen, N Winner, X Guo, J Dunwoody, J White, and et al. Structural and optical properties of phase-pure UO2, $\alpha$-U3O8, and $\alpha$-UO3 epitaxial thin films grown by pulsed laser deposition.  ACS Applied Materials & Interfaces, 12(31):35232–35241, 2020.
  * [4] D H Hurley, A El-Azab, M S Bryan, M W D Cooper, C A Dennett, K Gofryk, L He, M Khafizov, G H Lander, M E Manley, and et al. Thermal energy transport in oxide nuclear fuel.  Chemical Reviews, 122(3):3711–3762, 2022.
  * [5] K Rickert, M A Velez, T A Prusnick, D B Turner, M M Kimani, T Bowen, and J M Mann.  Hydrothermal crystal growth on non-native substrates: The case of UO2.  Crystal Growth & Design, 21(11):6289–6300, 2021.
  * [6] T Gouder.  Thin layers in actinide research.  Journal of alloys and compounds, 271:841–845, 1998.
  * [7] K Mudiyanselage, A K Burrell, S D Senanayake, and H Idriss.  XPS and NEXAFS study of the reactions of acetic acid and acetaldehyde over UO2(100) thin film.  Surface Science, 680:107–112, feb 2019.
  * [8] J C Spirlet and O Vogt.  Preparation of actinides and actinide compounds.  Journal of Magnetism and Magnetic Materials, 29(1-3):31–38, 1982\.
  * [9] C R Edwards and A J Oliver.  Uranium processing: a review of current methods and technology.  Jom, 52(9):12–20, 2000.
  * [10] K M Glover and P Borrell.  A method for the preparation of thin films of plutonium and uranium.  Journal of Nuclear Energy, 1(3-4):214–217, 1955.
  * [11] T Durakiewicz, J J Joyce, G H Lander, C G Olson, M T Butterfield, E Guziewicz, A J Arko, L Morales, J Rebizant, K Mattenberger, and et al. Electronic structure of actinide antimonides and tellurides from photoelectron spectroscopy.  Physical Review B, 70(20):205103, 2004.
  * [12] J D Farr, A L Giorgi, M C Bowman, and R K Money.  The crystal structure of actinium metal and actinium hydride.  Technical report, Los Alamos National Laboratory (LANL), Los Alamos, NM (United States), 1953.
  * [13] A L Bowman and G P Arnold.  The crystal structure of Th3N4.  Acta Crystallographica Section B: Structural Crystallography and Crystal Chemistry, 27(1):243–244, 1971.
  * [14] H Tagawa and N Masaki.  X-ray and density studies of nonstoichiometric uranium sesquinitride.  Journal of Inorganic and Nuclear Chemistry, 36(5):1099–1103, 1974\.
  * [15] V M Radchenko, M A Ryabinin, and V A Stupin.  Production and investigation of thin films of metal actinides (Pu, Am, Cm, Bk, Cf).  IOP Conference Series: Materials Science and Engineering, 9(1):012034, 2010.
  * [16] I D Danilin, V D Kartushin, N V Pilipenko, I D Abramycheva, and S P Vesnovskii.  Production of films of uranium and americium compounds by thermal decomposition of volatile $\beta$-diketonates.  Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 480(1):75–78, 2002.
  * [17] I Kruk, B L Scott, E B Watkins, and L E Wolfsberg.  Growth and characterization of uranium oxide thin films deposited by polymer assisted deposition.  Thin Solid Films, 735:138874, 2021.
  * [18] A K Burrell, T M McCleskey, P Shukla, H Wang, T Durakiewicz, D P Moore, C G Olson, J J Joyce, and Q Jia.  Controlling oxidation states in uranium oxides through epitaxial stabilization.  Advanced materials, 19(21):3559–3563, 2007.
  * [19] B L Scott, J J Joyce, T D Durakiewicz, R L Martin, T M McCleskey, E Bauer, H Luo, and Q Jia.  High quality epitaxial thin films of actinide oxides, carbides, and nitrides: Advancing understanding of electronic structure of f-element materials.  Coordination Chemistry Reviews, 266:137–154, 2014.
  * [20] M T McCleskey, E Bauer, Q Jia, A K Burrell, B L Scott, S D Conradson, A Mueller, L Roy, X Wen, G E Scuseria, and et al. Optical band gap of NpO2 and PuO2 from optical absorbance of epitaxial films.  Journal of Applied Physics, 113(1):013515, 2013.
  * [21] N Getoff and H Bildstein.  Molecular plating: VI. quantitative electrodeposition of americium.  Nuclear Instruments and Methods, 70(3):352–354, 1969.
  * [22] R A A Muzzarelli.  The preparation of thin films of tantalum and protactinium by electro spraying.  Inorganic and Nuclear Chemistry Letters, 2(1):1–3, 1966.
  * [23] H Shimojima and J Takagi.  Electrodeposition of protactinium.  Journal of Inorganic and Nuclear Chemistry, 26(2):253–255, 1964\.
  * [24] S R Qiu, C Amrhein, M L Hunt, R Pfeffer, B Yakshinskiy, L Zhang, T E Madey, and J A Yarmoff.  Characterization of uranium oxide thin films grown from solution onto Fe surfaces.  Applied surface science, 181(3-4):211–224, 2001.
  * [25] R A Laudise.  Hydrothermal synthesis of crystals.  50 years Progress in Crystal Growth, page 185, 2004.
  * [26] C Young, J Petrosky, J M Mann, E M Hunt, D Turner, and T Kelly.  The work function of hydrothermally synthesized UO2 and the implications for semiconductor device fabrication.  physica status solidi (RRL)–Rapid Research Letters, 10(9):687–690, 2016.
  * [27] C L Dugan, G G Peterson, A Mock, C Young, J M Mann, M Nastasi, M Schubert, L Wang, W Mei, I Tanabe, and et al. Electrical and material properties of hydrothermally grown single crystal (111) UO2.  The European Physical Journal B, 91(4):1–7, 2018.
  * [28] Y Huentupil, G Cabello-Guzmán, B Chornik, R Arancibia, and G E Buono-Core.  Photochemical deposition, characterization and optical properties of thin films of ThO2.  Polyhedron, 157:225–231, 2019.
  * [29] K Rickert, T A Prusnick, E Hunt, A French, D B Turner, C A Dennett, L Shao, and J M Mann.  Raman and photoluminescence evaluation of ion-induced damage uniformity in ThO2.  Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 515:69–79, 2022.
  * [30] A Majumdar, K V Manukyan, S Dede, J M Roach, D Robertson, P C Burns, and A Aprahamian.  Irradiation-driven restructuring of UO2 thin films: Amorphization and crystallization.  ACS Applied Materials & Interfaces, 13(29):35153–35164, 2021.
  * [31] M D Straub, J Leduc, M Frank, A Raauf, T D Lohrey, S G Minasian, S Mathur, and J Arnold.  Chemical vapor deposition of phase-pure uranium dioxide thin films from uranium (IV) amidate precursors.  Angewandte Chemie International Edition, 58(17):5749–5753, 2019\.
  * [32] A Raauf, J Leduc, M Frank, D Stadler, D Graf, M Wilhelm, M Grosch, and S Mathur.  Magnetic field-assisted chemical vapor deposition of UO2 thin films.  Inorganic Chemistry, 60(3):1915–1921, 2021.
  * [33] L Tarnawska, A Giussani, P Zaumseil, M A Schubert, R Paszkiewicz, O Brandt, P Storck, and T Schroeder.  Single crystalline Sc2O3/Y2O3 heterostructures as novel engineered buffer approach for GaN integration on Si (111).  Journal of Applied Physics, 108(6):063502, 2010.
  * [34] L-Å Ragnarsson, S Guha, M Copel, E Cartier, N A Bojarczuk, and J Karasinski.  Molecular-beam-deposited yttrium-oxide dielectrics in aluminum-gated metal–oxide–semiconductor field-effect transistors: Effective electron mobility.  Applied Physics Letters, 78(26):4169–4171, 2001.
  * [35] T Mori.  Thermoelectric and magnetic properties of rare earth borides: Boron cluster and layered compounds.  Journal of Solid State Chemistry, 275:70–82, 2019.
  * [36] I Ohkubo, T Aizawa, K Nakamura, and T Mori.  Control of competing thermodynamics and kinetics in vapor phase thin-film growth of nitrides and borides.  Frontiers in Chemistry, 9:528, 2021.
  * [37] Z Li, J Kim, N Kioussis, S Ning, H Su, T Iitaka, T Tohyama, X Yang, and J Zhang.  GdN thin film: Chern insulating state on square lattice.  Physical Review B, 92(20):201303, 2015.
  * [38] K H Goh, A S M A Haseeb, and Y H Wong.  Lanthanide rare earth oxide thin film as an alternative gate oxide.  Materials Science in Semiconductor Processing, 68:302–315, 2017\.
  * [39] M Leskelä, K Kukli, and M Ritala.  Rare-earth oxide thin films for gate dielectrics in microelectronics.  Journal of Alloys and Compounds, 418(1-2):27–34, 2006.
  * [40] J E Johnson, D D Allred, R S Turley, W R Evans, and R L Sandberg.  Thorium-based thin films as highly reflective mirrors in the EUV.  MRS Online Proceedings Library (OPL), 893, 2005.
  * [41] N F Brimhall, A B Grigg, R S Turley, and D D Allred.  Thorium dioxide thin films in the extreme ultraviolet.  In Advances in X-Ray/EUV Optics, Components, and Applications, volume 6317, page 631710. International Society for Optics and Photonics, 2006\.
  * [42] M Bagge-Hansen, R A Outlaw, K Seo, and D M Manos.  Morphology and crystallization of ThO2 thin films on polycrystalline Ir.  Thin solid films, 520(13):4249–4253, 2012.
  * [43] M Uno, M Katsura, and M Miyake.  Preparation of Th3N4 and its oxidation behaviour.  Journal of the Less Common Metals, 135(1):25–38, 1987.
  * [44] T Gouder, L Havela, L Black, F Wastin, J Rebizant, P Boulet, D Bouexiere, S Heathman, and M Idiri.  Synthesis and electronic properties of Th–N films.  Journal of alloys and compounds, 336(1-2):73–76, 2002.
  * [45] E L Bright, S Rennie, M Cattelan, N A Fox, D T Goddard, and R Springell.  Epitaxial UN and $\alpha$-U2N3 thin films.  Thin Solid Films, 661:71–77, 2018.
  * [46] L Black, F Miserque, T Gouder, L Havela, J Rebizant, and F Wastin.  Preparation and photoelectron spectroscopy study of UNx thin films.  Journal of alloys and compounds, 315(1-2):36–41, 2001.
  * [47] Z Long, Y Hu, L Chen, L Luo, K Liu, and X Lai.  Un2-x layer formed on uranium metal by glow plasma nitriding.  Journal of Alloys and Compounds, 620:289–293, 2015.
  * [48] T Gouder.  Electronic structure of uranium overlayers on magnesium and aluminium.  Surface science, 382(1-3):26–34, 1997.
  * [49] A. M. Adamska, Springell Scott, T. B., R., A. D. Waren, L. Pico, and O. Payton.  Growth and characterization of uranium-alloy thin films, Apr 2014.
  * [50] M Eckle and T Gouder.  Photoemission study of UNxOy and UCxOy in thin films.  Journal of alloys and compounds, 374(1-2):261–264, 2004.
  * [51] T K Bierlein and B Mastel.  Damage in UO2 films and particles during reactor irradiation.  Journal of Applied Physics, 31(12):2314–2315, 1960.
  * [52] M S Elbakhshwan and B J Heuser.  Structural and compositional characterization of single crystal uranium dioxide thin films deposited on different substrates.  Thin Solid Films, 636:658–663, 2017.
  * [53] Y A Teterin, A J Popel, K I Maslakov, A Y Teterin, K E Ivanov, S N Kalmykov, R Springell, T B Scott, and I Farnan.  XPS study of ion irradiated and unirradiated UO2 thin films.  Inorganic chemistry, 55(16):8059–8070, 2016.
  * [54] A J Popel, A M Adamska, P G Martin, O D Payton, G I Lampronti, L Picco, L Payne, R Springell, T B Scott, I Monnet, C Grygiel, and I Farnan.  Structural effects in UO2 thin films irradiated with U ions.  Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 386:8–15, 2016.
  * [55] S Rennie, E L Bright, J E Darnbrough, L Paolasini, A Bosak, A D Smith, N Mason, G H Lander, and R Springell.  Study of phonons in irradiated epitaxial thin films of UO2.  Physical Review B, 97(22):224303, 2018.
  * [56] K I Maslakov, Y A Teterin, A J Popel, A Y Teterin, K E Ivanov, S N Kalmykov, V G Petrov, R Springell, T B Scott, and I Farnan.  XPS study of the surface chemistry of UO2 (111) single crystal film.  Applied Surface Science, 433:582–588, 2018.
  * [57] D Zhang, H Dong, Y Lou, Y Zhong, F Li, X Fu, Y Zheng, and W Li.  Optical properties, band structures, and phase transition of UO2+x epitaxial films deposited by polymer-assisted deposition.  AIP Advances, 11(11):115107, 2021.
  * [58] J M Roach, K V Manukyan, A Majumdar, S Dede, A G Oliver, P C Burns, and A Aprahamian.  Hyperstoichiometric uranium dioxides: Rapid synthesis and irradiation-induced structural changes.  Inorganic Chemistry, 60(24):18938–18949, 2021.
  * [59] P Cakir, R Eloirdi, F Huber, R J M Konings, and T Gouder.  Surface reduction of neptunium dioxide and uranium mixed oxides with plutonium and thorium by photocatalytic reaction with ice.  The Journal of Physical Chemistry C, 119(3):1330–1337, 2015.
  * [60] N R Miljević and Z S Z̆unić.  Environmental and health impact assessment of ammunition containing transuranic elements.  In Environmental Consequences of War and Aftermath, pages 209–251. Springer, 2009.
  * [61] J Shaw, A Whittaker, L Polley, and G Wortley.  Preparation of thin uniform films of PuO2 suitable for fission fragment counting.  Technical report, United Kingdom Atomic Energy Authority, Sellafield, Cumb., England (United Kingdom), 1958.
  * [62] M P Wilkerson, J M Dorhout, K S Graham, J J Joyce, I I Kruk, J Majewski, D T Olive, A L Pugmire, B L Scott, J T Stritzinger, and et al. Structural properties, thicknesses, and qualities of plutonium oxide thin films prepared by polymer assisted deposition.  Surface Science, 701:121696, 2020.
  * [63] P Roussel.  Inverse photoemission measurements of plutonium metal and oxides.  Journal of Electron Spectroscopy and Related Phenomena, 246:147030, 2021.
  * [64] C B Finch and G W Clark.  High-temperature solution growth of single-crystal neptunium dioxide.  Journal of Crystal Growth, 6(3):245–248, 1970.
  * [65] C B Finch and G W Clark.  High-temperature solution growth of single-crystal plutonium dioxide.  Journal of Crystal Growth, 12(2):181–182, 1972.
  * [66] A Modin, Y Yun, M-T Suzuki, J Vegelius, L Werme, J Nordgren, P M Oppeneer, and S M Butorin.  Indication of single-crystal PuO2 oxidation from O 1s x-ray absorption spectra.  Physical Review B, 83(7):075113, 2011.
  * [67] D Mannix, S Langridge, G H Lander, J Rebizant, M J Longfield, W G Stirling, W J Nuttall, S Coburn, S Wasserman, and L Soderholm.  Experiments on transuranium compounds with x-ray resonant exchange scattering.  Physica B: Condensed Matter, 262(1-2):125–140, 1999.
  * [68] T Gouder, F Wastin, J Rebizant, and L Havela.  5f electron localization in PuSe and PuSb.  Physical review letters, 84(15):3378, 2000.
  * [69] D Rafaja, L Havela, R Kužel, F Wastin, E Colineau, and T Gouder.  Real structure and magnetic properties of UN thin films.  Journal of alloys and compounds, 386(1-2):87–95, 2005.
  * [70] N-T H Kim-Ngan, A G Balogh, L Havela, and T Gouder.  Ion beam mixing in uranium nitride thin films studied by rutherford backscattering spectroscopy.  Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 268(11-12):1875–1879, 2010.
  * [71] X Wang, Z Long, R Bin, R Yang, Q Pan, F Li, L Luo, Y Hu, and K Liu.  Study of the decomposition and phase transition of uranium nitride under UHV conditions via TDS, XRD, SEM, and XPS.  Inorganic chemistry, 55(21):10835–10838, 2016.
  * [72] L Luo, Q Pan, Y Hu, K Liu, and X Wang.  Insight into the initial oxidation of UN1.85 thin films.  Applied Surface Science, 525:146535, 2020.
  * [73] Z Long, L Luo, Y Lu, Y Hu, K Liu, and X Lai.  Study on the electronic structure of $\alpha$-U2N3 by XPS and first principles.  Journal of Alloys and Compounds, 664:745–749, 2016.
  * [74] Z Bao, R Springell, H C Walker, H Leiste, K Kuebel, R Prang, G Nisbet, S Langridge, RCC Ward, T Gouder, and et al. Antiferromagnetism in UO2 thin epitaxial films.  Physical Review B, 88(13):134426, 2013.
  * [75] A J Popel, V A Lebedev, P G Martin, A A Shiryaev, G I Lampronti, R Springell, S N Kalmykov, T B Scott, I Monnet, C Grygiel, and et al. Structural effects in UO2 thin films irradiated with fission-energy Xe ions.  Journal of Nuclear Materials, 482:210–217, 2016.
  * [76] Q Chen, X Lai, B Bai, and M Chu.  Structural characterization and optical properties of UO2 thin films by magnetron sputtering.  Applied surface science, 256(10):3047–3050, 2010.
  * [77] S D Senanayake, G I N Waterhouse, H Idriss, and T E Madey.  Coupling of carbon monoxide molecules over oxygen-defected UO2 (111) single crystal and thin film surfaces.  Langmuir, 21(24):11141–11145, 2005.
  * [78] T T Meek, B Von Roedern, P G Clem, and R J Hanrahan Jr.  Some optical properties of intrinsic and doped uo2 thin films.  Materials Letters, 59(8-9):1085–1088, 2005.
  * [79] S Stumpf, A Seibert, T Gouder, F Huber, T Wiss, J Römer, and M A Denecke.  Development of fuel-model interfaces: Characterization of Pd containing UO2 thin films.  Journal of nuclear materials, 397(1-3):19–26, 2010.
  * [80] T Gouder, R Eloirdi, F Wastin, E Colineau, J Rebizant, D Kolberg, and F Huber.  Electronic structure of UH3 thin films prepared by sputter deposition.  Physical Review B, 70(23):235108, 2004.
  * [81] D Chaney, A Castellano, A Bosak, J Bouchet, F Bottin, B Dorado, L Paolasini, S Rennie, Christopher Bell, R Springell, et al.  Tuneable correlated disorder in alloys.  Physical Review Materials, 5(3):035004, 2021.
  * [82] R C C Ward, R A Cowley, N Ling, W Goetze, G H Lander, and W G Stirling.  The structure of epitaxial layers of uranium.  Journal of Physics: Condensed Matter, 20(13):135003, 2008.
  * [83] R Springell, R C C Ward, J Bouchet, J Chivall, D Wermeille, P S Normile, S Langridge, S W Zochowski, and G H Lander.  Malleability of uranium: Manipulating the charge-density wave in epitaxial films.  Physical Review B, 89(24):245101, 2014.
  * [84] H Ladislav, M Paukov, M Dopita, L Horàk, D Drozdenko, M Diviš, I Turek, D Legut, L Kỳvala, T Gouder, and et al. Crystal structure and magnetic properties of uranium hydride UH2 stabilized as a thin film.  Inorganic chemistry, 57(23):14727–14732, 2018.
  * [85] R N R Mulford and T A Wiewandt.  The neptunium-hydrogen system.  The Journal of Physical Chemistry, 69(5):1641–1644, 1965.
  * [86] L Havela, F Wastin, J Rebizant, and T Gouder.  Photoelectron spectroscopy study of PuN.  Physical Review B, 68(8):085101, 2003.
  * [87] R N R Mulford and G E Sturdy.  The plutonium-hydrogen system. I. plutonium dihydride and dideuteride.  Journal of the American Chemical Society, 77(13):3449–3452, 1955\.
  * [88] J L Sarrao, L A Morales, J D Thompson, B L Scott, G R Stewart, F Wastin, J Rebizant, P Boulet, E Colineau, and G H Lander.  Plutonium-based superconductivity with a transition temperature above 18 K.  Nature, 420(6913):297–299, 2002.
  * [89] Alice Seibert, T. Gouder, and F. Huber.  Interaction of puo2 thin films with water.  Radiochimica Acta, 98(9-11):647–657, 2010.
  * [90] T Gouder, PM Oppeneer, F Huber, F Wastin, and J Rebizant.  Photoemission study of the electronic structure of am, amn, amsb, and am 2 o 3 films.  Physical Review B, 72(11):115122, 2005.
  * [91] W M Olson and R N R Mulford.  The americium—hydrogen system.  The Journal of Physical Chemistry, 70(9):2934–2937, 1966.
  * [92] B M Bansal and D Damien.  Curium hydride.  Inorganic and Nuclear Chemistry Letters, 6(7):603–606, 1970.
  * [93] L Lu, F Li, H Xiao, Y Hu, L Luo, B Bai, J Liu, and K Liu.  Thermal stability of uranium nitride and oxynitride films in an ultra-high vacuum environment.  Journal of Nuclear Materials, 509:408–416, 2018.
  * [94] A I Apostol, A Pantelica, I Ortega-Feliu, N Marginean, O Sima, M Straticiuc, M C Jimenez-Ramos, and V Fugaru.  Ion beam analysis of elemental signatures in uranium dioxide samples: importance for nuclear forensics.  Journal of Radioanalytical and Nuclear Chemistry, 311(2):1339–1346, 2017.
  * [95] C A Dennett, Z Hua, A Khanolkar, T Yao, P K Morgan, T A Prusnick, N Poudel, A French, K Gofryk, L He, et al.  The influence of lattice defects, recombination, and clustering on thermal transport in single crystal thorium dioxide.  APL Materials, 8(11):111103, 2020.
  * [96] S S Parker, S Newman, A J Fallgren, and J T White.  Thermophysical properties of mixtures of thorium and uranium nitride.  Journal of Materials, 73(11):3564–3575, 2021.
  * [97] H F Jackson, D D Jayaseelan, W E Lee, M J Reece, F Inam, D Manara, C P Casoni, F De Bruycker, and K Boboridis.  Laser melting of spark plasma-sintered zirconium carbide: thermophysical properties of a generation iv very high-temperature reactor material.  International Journal of Applied Ceramic Technology, 7(3):316–326, 2010.
  * [98] D P Daroca, S Jaroszewicz, A M Llois, and H O Mosca.  Phonon spectrum, mechanical and thermophysical properties of thorium carbide.  Journal of nuclear materials, 437(1-3):135–138, 2013.
  * [99] T Gouder and L Havela.  Examples of quantification in XPS on 5f materials.  Microchimica Acta, 138(3):207–215, 2002.
  * [100] J J Joyce, J M Wills, T Durakiewicz, M T Butterfield, E Guziewicz, D P Moore, J L Sarrao, L A Morales, A J Arko, O Eriksson, and et al. Dual nature of the 5f electrons in plutonium materials.  Physica B: Condensed Matter, 378:920–924, 2006.
  * [101] J J Joyce, T Durakiewicz, K S Graham, E D Bauer, D P Moore, J N Mitchell, J A Kennison, R L Martin, L E Roy, and G E Scuseria.  Pu electronic structure and photoelectron spectroscopy.  Journal of Physics: Conference Series, 273(1):012023, 2011.
  * [102] R Eloirdi, L Havela, T Gouder, A Shick, J Rebizant, F Huber, and R Caciuffo.  Photoelectron spectroscopy study of PuCoGa5 thin films.  Journal of nuclear materials, 385(1):8–10, 2009.
  * [103] T Gouder, R Eloirdi, and R Caciuffo.  Direct observation of pure pentavalent uranium in U2O5 thin films by high resolution photoemission spectroscopy.  Scientific reports, 8(1):1–7, 2018.
  * [104] S Fujimori, T Ohkochi, T Okane, Y Saitoh, A Fujimori, H Yamagami, Y Haga, E Yamamoto, and Y Ōnuki.  Itinerant nature of U 5$f$ states in uranium mononitride revealed by angle-resolved photoelectron spectroscopy.  Phys. Rev. B, 86(23):235108, 2012.
  * [105] T Durakiewicz, P S Riseborough, C G Olson, J J Joyce, P M Oppeneer, S Elgazzar, E D Bauer, J L Sarrao, E Guziewicz, D P Moore, and et al. Observation of a kink in the dispersion of $f$-electrons.  Europhys. Lett., 84(3):37003, 2008.
  * [106] T Das, T Durakiewicz, J-X Zhu, J J Joyce, J L Sarrao, and M J Graf.  Imaging the formation of high-energy dispersion anomalies in the actinide UCoGa5.  Phys. Rev. X, 2(4):041012, 2012.
  * [107] I Kawasaki, S Fujimori, Y Takeda, T Okane, A Yasui, Y Saitoh, H Yamagami, Y Haga, E Yamamoto, and Y Ōnuki.  Band structure and fermi surface of UPd3 studied by soft x-ray angle-resolved photoemission spectroscopy.  Phys. Rev. B, 87(7):075142, 2013.
  * [108] M F Beaux II, T Durakiewicz, L Moreschini, M Grioni, F Offi, G Monaco, G Panaccione, J J Joyce, E D Bauer, J L Sarrao, and et al. Electronic structure of single crystal UPd3, UGe2, and USb2 from hard X-ray and angle-resolved photoelectron spectroscopy.  Journal of Electron Spectroscopy and Related Phenomena, 184(8-10):517–524, 2011.
  * [109] J D Denlinger, G-H Gweon, J W Allen, C G Olson, M B Maple, J L Sarrao, P E Armstrong, Z Fisk, and H Yamagami.  Comparative study of the electronic structure of XRu2Si2: probing the anderson lattice.  Journal of Electron Spectroscopy and Related Phenomena, 117:347–369, 2001.
  * [110] T Ito, H Kumigashira, T Takahashi, Y Haga, E Yamamoto, T Honma, H Ohkuni, and Y. Ōnuki.  Band structure and fermi surface of URu2Si2 studied by high-resolution angle-resolved photoemission spectroscopy.  Phys. Rev. B, 60(19):13390, 1999.
  * [111] A F Santander-Syro, M Klein, F L Boariu, A Nuber, P Lejay, and F Reinert.  Fermi-surface instability at the ’hidden-order’ transition of URu2Si2.  Nat. Phys., 5(9):637–641, 2009.
  * [112] R Yoshida, Y Nakamura, M Fukui, Y Haga, E Yamamoto, Y Ōnuki, M Okawa, S Shin, M Hirai, Y Muraoka, and et al. Signature of hidden order and evidence for periodicity modification in URu2Si2.  Phys. Rev. B, 82(20):205108, 2010.
  * [113] I Kawasaki, S Fujimori, Y Takeda, T Okane, A Yasui, Y Saitoh, H Yamagami, Y Haga, E Yamamoto, and Y Ōnuki.  Electronic structure of URu2Si2 in paramagnetic phase studied by soft x-ray photoemission spectroscopy.  Journal of Physics: Conference Series, 273(1):012039, 2011.
  * [114] G L Dakovski, Y Li, S M Gilbertson, G Rodriguez, A V Balatsky, J-X Zhu, K Gofryk, E D Bauer, P H Tobash, A Taylor, and et al. Anomalous femtosecond quasiparticle dynamics of hidden order state in URu2Si2.  Physical Review B, 84(16):161103, 2011.
  * [115] R Yoshida, M Fukui, Y Haga, E Yamamoto, Y Onuki, M Okawa, W Malaeb, S Shin, Y Muraoka, and T Yokoya.  Observation of two fine structures related to the hidden order in the spectral functions of URu2Si2.  Physical Review B, 85(24):241102, 2012.
  * [116] I Kawasaki, S Fujimori, Y Takeda, T Okane, A Yasui, Y Saitoh, H Yamagami, Y Haga, E Yamamoto, and Y Ōnuki.  Band structure and fermi surface of URu2Si2 studied by soft x-ray angle-resolved photoemission spectroscopy.  Physical Review B, 83(23):235121, 2011.
  * [117] F L Boariu, C Bareille, H Schwab, A Nuber, P Lejay, T Durakiewicz, F Reinert, and A F Santander-Syro.  Momentum-resolved evolution of the kondo lattice into ‘’hidden order” in URu2Si2.  Physical Review Letters, 110(15):156404, 2013.
  * [118] S Chatterjee, J Trinckauf, T Hänke, D E Shai, J W Harter, T J Williams, G M Luke, K M Shen, and J Geck.  Formation of the coherent heavy fermion liquid at the hidden order transition in URu2Si2.  Physical Review Letters, 110(18):186401, 2013.
  * [119] R Yoshida, K Tsubota, T Ishiga, M Sunagawa, J Sonoyama, D Aoki, J Flouquet, T Wakita, Y Muraoka, and T Yokoya.  Translational symmetry breaking and gapping of heavy-quasiparticle pocket in URu2Si2.  Scientific reports, 3(1):1–6, 2013.
  * [120] J-Q Meng, P M Oppeneer, J A Mydosh, P S Riseborough, K Gofryk, J J Joyce, E D Bauer, Y Li, and T Durakiewicz.  Imaging the three-dimensional fermi-surface pairing near the hidden-order transition in URu2Si2 using angle-resolved photoemission spectroscopy.  Physical Review Letters, 111(12):127002, 2013.
  * [121] C Bareille, F L Boariu, H Schwab, P Lejay, F Reinert, and A F Santander-Syro.  Momentum-resolved hidden-order gap reveals symmetry breaking and origin of entropy loss in URu2Si2.  Nature Communications, 5(1):1–11, 2014.
  * [122] T Durakiewicz.  Photoemission investigations of URu2Si2.  Philosophical Magazine, 94(32-33):3723–3736, 2014.
  * [123] P Ruello, K D Becker, K Ullrich, L Desgranges, C Petot, and G Petot-Ervas.  Thermal variation of the optical absorption of UO2: determination of the small polaron self-energy.  Journal of nuclear materials, 328(1):46–54, 2004.
  * [124] J G Tobin and S-W Yu.  Orbital specificity in the unoccupied states of UO2 from resonant inverse photoelectron spectroscopy.  Physical Review Letters, 107(16):167406, 2011.
  * [125] S M Gilbertson, T Durakiewicz, G L Dakovski, Y Li, J-X Zhu, S D Conradson, S A Trugman, and G Rodriguez.  Ultrafast photoemission spectroscopy of the uranium dioxide UO2 mott insulator: Evidence for a robust energy gap structure.  Physical Review Letters, 112(8):087402, 2014.
  * [126] L E Roy, T Durakiewicz, R L Martin, J E Peralta, G E Scuseria, C G Olson, J J Joyce, and E Guziewicz.  Dispersion in the mott insulator UO2: A comparison of photoemission spectroscopy and screened hybrid density functional theory.  Journal of computational chemistry, 29(13):2288–2294, 2008.
  * [127] J J Joyce, A J Arko, L E Cox, and S Czuchlewski.  Resonance photoelectron spectroscopy using a tunable laser plasma light source.  Surface and Interface Analysis: An International Journal devoted to the development and application of techniques for the analysis of surfaces, interfaces and thin films, 26(2):121–123, 1998.
  * [128] S L Molodtsov, J Boysen, M Richter, P Segovia, C Laubschat, S A Gorovikov, A M Ionov, G V Prudnikova, and V K Adamchuk.  Dispersion of 5f electron states: Angle-resolved photoemission on ordered films of U metal.  Physical Review B, 57(20):13241, 1998.
  * [129] L Berbil-Bautista, T Hänke, M Getzlaff, R Wiesendanger, I Opahle, K Koepernik, and M Richter.  Observation of 5 f states in U/W (110) films by means of scanning tunneling spectroscopy.  Physical Review B, 70(11):113401, 2004.
  * [130] M Zarshenas and S J Asadabadi.  Theoretical study of $\alpha$-U/W (110) thin films from density functional theory calculations: structural, magnetic and electronic properties.  Thin Solid Films, 520(7):2901–2908, 2012.
  * [131] R Springell, B Detlefs, G H Lander, R C C Ward, R A Cowley, N Ling, W Goetze, R Ahuja, W Luo, and B Johansson.  Elemental engineering: Epitaxial uranium thin films.  Physical Review B, 78(19):193403, 2008.
  * [132] C P Opeil, R K Schulze, H M Volz, J C Lashley, M E Manley, W L Hults, R J Hanrahan Jr, J L Smith, B Mihaila, K B Blagoev, et al.  Angle-resolved photoemission and first-principles electronic structure of single-crystalline $\alpha$-u (001).  Physical Review B, 75(4):045120, 2007.
  * [133] C P Opeil, R K Schulze, M E Manley, J C Lashley, W L Hults, R J Hanrahan Jr, J L Smith, B Mihaila, K B Blagoev, R C Albers, et al.  Valence-band UPS, 6p core-level XPS, and LEED of a uranium (001) single crystal.  Physical Review B, 73(16):165109, 2006.
  * [134] Q Chen, S Tan, W Feng, L Luo, X Zhu, and X Lai.  Direct observation of the f–c hybridization in the ordered uranium films on W (110).  Chinese Physics B, 28(7):077404, 2019.
  * [135] J J Joyce, T Durakiewicz, K S Graham, E D Bauer, D P Moore, J N Mitchell, J A Kennison, T M McCleskey, Q Jia, A K Burrell, and et al. 5f electronic structure and fermiology of Pu materials.  MRS Online Proceedings Library, 1264(1):1–6, 2010.
  * [136] R E Jilek, E Bauer, A K Burrell, T M McCleskey, Q Jia, B L Scott, M F Beaux, T Durakiewicz, J J Joyce, K D Rector, and et al. Preparation of epitaxial uranium dicarbide thin films by polymer-assisted deposition.  Chemistry of Materials, 25(21):4373–4377, 2013.
  * [137] S Middlemas, Z Hua, V Chauhan, W T Yorgason, R Schley, A Khanolkar, M Khafizov, and D Hurley.  Determining local thermal transport in a composite uranium-nitride/silicide nuclear fuel using square-pulse transient thermoreflectance technique.  Journal of Nuclear Materials, 528:151842, 2020.
  * [138] M Sanati, R C Albers, T Lookman, and A Saxena.  Elastic constants, phonon density of states, and thermal properties of UO2.  Physical Review B, 84(1):014116, 2011.
  * [139] P Santini, S Carretta, G Amoretti, R Caciuffo, N Magnani, and G H Lander.  Multipolar interactions in f-electron systems: The paradigm of actinide dioxides.  Reviews of Modern Physics, 81(2):807, 2009.
  * [140] N Magnani, P Santini, G Amoretti, and R Caciuffo.  Perturbative approach to j mixing in f-electron systems: Application to actinide dioxides.  Physical Review B, 71(5):054405, 2005.
  * [141] M Mann, D Thompson, K Serivalsatit, T M Tritt, J Ballato, and J Kolis.  Hydrothermal growth and thermal property characterization of ThO2 single crystals.  Crystal growth & design, 10(5):2146–2151, 2010.
  * [142] K Gofryk, S Du, C R Stanek, J C Lashley, X-Y Liu, R K Schulze, J L Smith, D J Safarik, D D Byler, K J McClellan, and et al. Anisotropic thermal conductivity in uranium dioxide.  Nature communications, 5(1):1–7, 2014.
  * [143] T Nishi, A Itoh, M Takano, M Numata, M Akabori, Y Arai, and K Minato.  Thermal conductivity of neptunium dioxide.  Journal of nuclear materials, 376(1):78–82, 2008.
  * [144] J F Lagedrost, D F Askey, V W Storhok, and J E Gates.  Thermal conductivity of PuO2 as determined from thermal diffusivity measurements.  Nuclear Applications, 4(1):54–61, 1968.
  * [145] S S Parker, J T White, P Hosemann, and A T Nelson.  Thermophysical properties of thorium mononitride from 298 to 1700 K.  Journal of Nuclear Materials, 526:151760, 2019.
  * [146] M Khafizov, V Chauhan, Y Wang, F Riyad, N Hang, and D H Hurley.  Investigation of thermal transport in composites and ion beam irradiated materials for nuclear energy applications.  Journal of Materials Research, 32(1):204–216, 2017.
  * [147] M N Luckyanova, J A Johnson, A A Maznev, J Garg, A Jandi, M T Bulsara, E A Fitzgerald, K A Nelson, and G Chen.  Anisotropy of the thermal conductivity in GaAs/AlAs superlatttices.  Nano Lett., 13:3973–3977, 2013.
  * [148] A Giri, J L Braun, and P E Hopkins.  Effect of crystalline/amorphous interfaces on thermal transport across confined thin films and superlattices.  Journal of Applied Physics, 119(23):235305, 2016.
  * [149] P Santini, R Lemanski, and P Erdős.  Magnetism of actinide compounds.  Advances in Physics, 48(5):537–653, 1999.
  * [150] G J-P Deblonde, M Zavarin, and A B Kersting.  The coordination properties and ionic radius of actinium: A 120-year-old enigma.  Coordination Chemistry Reviews, 446:214130, 2021.
  * [151] L Severin, M S S Brooks, and B Johansson.  Relationship between the Coulomb integral U and the stoner parameter I.  Physical Review Letters, 71(19):3214, 1993.
  * [152] J C Lashley, A Lawson, R J McQueeney, and G H Lander.  Absence of magnetic moments in plutonium.  Physical Review B, 72(5):054416, 2005.
  * [153] M Janoschek, P Das, B Chakrabarti, D L Abernathy, M D Lumsden, J M Lawrence, J D Thompson, G H Lander, J N Mitchell, S Richmond, and et al. The valence-fluctuating ground state of plutonium.  Science Advances, 1(6):e1500188, 2015.
  * [154] G H Lander, J-C Griveau, R Eloirdi, N Magnani, E Colineau, F Wilhelm, SD Brown, D Wermeille, A Rogalev, RG Haire, and et a. Measurements related to the magnetism of curium metal.  Physical Review B, 99(22):224419, 2019.
  * [155] H R Ott, Z Fisk, A J Freeman, and G H Lander.  Handbook on the physics and chemistry of the actinides.  AJ Freeman, GH Lander, Noth-Holland, Amsterdam, pages 85–225, 1987\.
  * [156] M Marutzky, U Barkow, J Schoenes, and R Troć.  Optical and magneto-optical properties of single crystalline uranium nitride.  Journal of magnetism and magnetic materials, 299(1):225–230, 2006\.
  * [157] H Sakai, H Kato, Y Tokunaga, S Kambe, R E Walstedt, A Nakamura, N Tateiwa, and T C Kobayashi.  Magnetism of uranium dioxide UO2 under high pressure.  Journal of Magnetism and Magnetic Materials, 272:E413–E414, 2004\.
  * [158] V Ichas, S Zwirner, D Braithwaite, J C Spirlet, J Rebizant, and W Potzel.  Electrical resistance and magnetic properties of the neptunium monopnictides NpAs, NpSb, and NpBi at high pressures.  Physical Review B, 56(22):14481, 1997.
  * [159] S E Nave, R G Haire, and P G Huray.  Magnetic properties of actinide elements having the 5f6 and 5f7 electronic configurations.  Physical Review B, 28(5):2317, 1983.
  * [160] M Jin, Y Guo, B Li, X Niu, and Y Zhang.  Magnetic and electronic properties of $\alpha$-U2N3 and its role in preventing uranium from oxidation: First-principles studies.  Journal of Nuclear Materials, 512:72–78, 2018.
  * [161] E L Bright, R Springell, D G Porter, S P Collins, and G H Lander.  Synchrotron x-ray scattering of magnetic and electronic structure of UN and U2N3 epitaxial films.  Physical Review B, 100(13):134426, 2019.
  * [162] K Shrestha, D Antonio, M Jaime, N Harrison, D S Mast, D Safarik, T Durakiewicz, J-C Griveau, and K Gofryk.  Tricritical point from high-field magnetoelastic and metamagnetic effects in UN.  Scientific Reports, 7(1):1–8, 2017.
  * [163] R Troć, M Samsel-Czekała, A Pikul, A V Andreev, D I Gorbunov, Y Skourski, and J Sznajd.  Electronic structure of UN based on specific heat and field-induced transitions up to 65 T.  Physical Review B, 94(22):224415, 2016.
  * [164] S Hamann, T Förster, D I Gorbunov, M König, M Uhlarz, J Wosnitza, and T Helm.  Fermi-surface reconstruction at the metamagnetic high-field transition in uranium mononitride.  Physical Review B, 104(15):155123, 2021.
  * [165] N M Wolcott and R A Hein.  Superconductivity of thorium below l∘ K.  Philosophical Magazine, 3(30):591–596, 1958.
  * [166] R D Fowler, B T Matthias, L B Asprey, H H Hill, J D G Lindsay, C E Olsen, and R W White.  Superconductivity of protactinium.  Physical Review Letters, 15(22):860, 1965.
  * [167] J L Smith, J C Spirlet, and W Müller.  Superconducting properties of protactinium.  Science, 205(4402):188–190, 1979.
  * [168] V G Aschermann and E Justi.  Elektrische leitfähigkeit, magnetische widerstandsvermehrung.  Hall-effekt und Supraleitung von Rhenium, Phys, 43:207, 1942.
  * [169] B B Goodman and D Shoenberg.  Superconductivity of uranium.  Nature, 165(4194):441–442, 1950.
  * [170] J E Gordon, H Montgomery, R J Noer, G R Pickett, and R Tobón.  Superconductivity of thorium and uranium.  Physical Review, 152(1):432, 1966.
  * [171] Kevin T Moore and Gerrit van der Laan.  Nature of the 5 f states in actinide metals.  Reviews of Modern Physics, 81(1):235, 2009.
  * [172] J L Smith and R G Haire.  Superconductivity of americium.  Science, 200(4341):535–537, 1978.
  * [173] J-C Griveau and E Colineau.  Superconductivity in transuranium elements and compounds.  Comptes Rendus Physique, 15(7):599–615, 2014.
  * [174] M B Maple, M C de Andrade, J Herrmann, R P Dickey, N R Dilley, and S Han.  Superconductivity in rare earth and actinide compounds.  Journal of alloys and compounds, 250(1-2):585–595, 1997.
  * [175] D Aoki, Y Haga, T D Matsuda, N Tateiwa, S Ikeda, Y Homma, H Sakai, Y Shiokawa, E Yamamoto, A Nakamura, and et al. Unconventional heavy-fermion superconductivity of a new transuranium compound NpPd5Al2.  Journal of the Physical Society of Japan, 76(6):063701, 2007.
  * [176] J-C Griveau, K Gofryk, and J Rebizant.  Transport and magnetic properties of the superconductor NpPd5Al2.  Physical Review B, 77(21):212502, 2008.
  * [177] E D Bauer, M M Altarawneh, P H Tobash, K Gofryk, O E Ayala-Valenzuela, J N Mitchell, R D McDonald, C H Mielke, F Ronning, J C Griveau, and et al. Localized 5f electrons in superconducting PuCoIn5: consequences for superconductivity in PuCoGa5.  Journal of Physics: Condensed Matter, 24(5):052206, 2011.
  * [178] F Wastin, P Boulet, J Rebizant, E Colineau, and GH Lander.  Advances in the preparation and characterization of transuranium systems.  Journal of Physics: Condensed Matter, 15(28):S2279, 2003.
  * [179] E D Bauer and J D Thompson.  Plutonium-based heavy-fermion systems.  Annu. Rev. Condens. Matter Phys., 6(1):137–153, 2015.
  * [180] H R Ott, H Rudigier, Z Fisk, and J L Smith.  UBe13: An unconventional actinide superconductor.  Physical review letters, 50(20):1595, 1983.
  * [181] B J Ramshaw, A Shekhter, R D McDonald, J B Betts, J N Mitchell, P H Tobash, C H Mielke, E D Bauer, and A Migliori.  Avoided valence transition in a plutonium superconductor.  Proceedings of the National Academy of Sciences, 112(11):3285–3289, 2015.
  * [182] K Gofryk, J-C Griveau, P S Riseborough, and T Durakiewicz.  Thermoelectric power as a probe of density of states in correlated actinide materials: The case of PuCoGa5 superconductor.  Physical Review B, 94(19):195117, 2016.
  * [183] S Ran, C Eckberg, Q-P Ding, Y Furukawa, T Metz, S R Saha, I-L Liu, M Zic, H Kim, J Paglione, and et al. Nearly ferromagnetic spin-triplet superconductivity.  Science, 365(6454):684–687, 2019.
  * [184] G Nakamine, S Kitagawa, K Ishida, Y Tokunaga, H Sakai, S Kambe, A Nakamura, Y Shimizu, Y Homma, D Li, and et al. Superconducting properties of heavy fermion UTe2 revealed by 125te-nuclear magnetic resonance.  journal of the physical society of japan, 88(11):113703, 2019.
  * [185] D Braithwaite, M Vališka, G Knebel, G Lapertot, J-P Brison, A Pourret, ME Zhitomirsky, J Flouquet, F Honda, and D Aoki.  Multiple superconducting phases in a nearly ferromagnetic system.  Communications Physics, 2(1):1–6, 2019.
  * [186] I M Hayes, D S Wei, T Metz, J Zhang, Y S Eo, S Ran, S R Saha, J Collini, N P Butch, D F Agterberg, and et al. Weyl superconductivity in UTe2.  arXiv preprint arXiv:2002.02539, 2020.
  * [187] S Bae, H Kim, Y S Eo, S Ran, I Liu, W T Fuhrman, J Paglione, N P Butch, S M Anlage, and et al. Anomalous normal fluid response in a chiral superconductor UTe2.  Nature communications, 12(1):1–5, 2021.
  * [188] C Brun, T Cren, and D Roditchev.  Review of 2D superconductivity: the ultimate case of epitaxial monolayers.  Superconductor Science and Technology, 30(1):013003, 2016.
  * [189] V Bhardwaj, A Bhattacharya, L K Varga, A K Ganguli, and R Chatterjee.  Thickness-dependent magneto-transport properties of topologically nontrivial DyPdBi thin films.  Nanotechnology, 31(38):384001, 2020.
  * [190] A Cossard, J K Desmarais, S Casassa, C Gatti, and A Erba.  Charge density analysis of actinide compounds from the quantum theory of atoms in molecules and crystals.  The journal of physical chemistry letters, 12(7):1862–1868, 2021\.
  * [191] P Richmond and G L Sewell.  Electron correlations, magnetic ordering, and mott insulation in solids.  Journal of Mathematical Physics, 9(3):349–356, 1968.
  * [192] J Krumrain, G Mussler, S Borisova, T Stoica, L Plucinski, CM Schneider, and D Grützmacher.  Mbe growth optimization of topological insulator Bi2Te3 films.  Journal of Crystal Growth, 324(1):115–118, 2011.
  * [193] S Tang, C Zhang, D Wong, Z Pedramrazi, H-Z Tsai, C Jia, B Moritz, M Claassen, H Ryu, S Kahn, and et al. Quantum spin hall state in monolayer 1T’-WTe2.  Nature Physics, 13(7):683–687, 2017.
  * [194] T Hesjedal.  Rare earth doping of topological insulators: A brief review of thin film and heterostructure systems.  physica status solidi (a), 216(8):1800726, 2019.
  * [195] V Ivanov, X Wan, and S Y Savrasov.  Topological insulator-to-weyl semimetal transition in strongly correlated actinide system UNiSn.  Physical Review X, 9(4):041055, 2019.
  * [196] X Zhang, H Zhang, J Wang, C Felser, and S-C Zhang.  Actinide topological insulator materials with strong interaction.  Science, 335(6075):1464–1466, 2012.
  * [197] H Choi, W Zhu, S K Cary, L E Winter, Z Huang, R D McDonald, V Mocko, B L Scott, P H Tobash, J D Thompson, and et al. Experimental and theoretical study of topology and electronic correlations in PuB4.  Physical Review B, 97(20):201114, 2018.
  * [198] A P Dioguardi, H Yasuoka, S M Thomas, H Sakai, S K Cary, S A Kozimor, T E Albrecht-Schmitt, H C Choi, J-X Zhu, J D Thompson, and et al. 239Pu nuclear magnetic resonance in the candidate topological insulator PuB4.  Physical Review B, 99(3):035104, 2019.
  * [199] X Deng, K Haule, and G Kotliar.  Plutonium hexaboride is a correlated topological insulator.  Physical review letters, 111(17):176404, 2013.
  * [200] M Dzero, K Sun, V Galitski, and P Coleman.  Topological Kondo insulators.  Physical review letters, 104(10):106408, 2010.
  * [201] W Kohn and L J Sham.  Self-consistent equations including exchange and correlation effects.  Physical Review, 140:1133, 1965.
  * [202] P Hohenberg and W Kohn.  Inhomogeneous electron gas.  Phys. Rev., 136:864, 1964.
  * [203] K Burke.  Perspective on density functional theory.  Journal Of Chemical Physics, 136:150901, 2012.
  * [204] R O Jones.  Density functional theory: its origins, rise to prominence, and future.  Rev. Mod. Phys., 87:897, 2015.
  * [205] C J Cramer and D G Truhlar.  Density functional theory for transition metals and transition metal chemistry.  Physical Chemistry Chemical Physics, 11:10757, 2009.
  * [206] P J Hasnip, K Refson, M Probert, J R Yates, S J Clark, and C J Pickard.  Density functional theory in the solid state.  Philosophical Transactions Of The Royal Society A-mathematical Physical And Engineering Sciences, 372:20130270, 2014.
  * [207] A Jain, Y Shin, and K A Persson.  Computational predictions of energy materials using density functional theory.  Nature Reviews Materials, 1:15004, 2016.
  * [208] R J Maurer, C Freysoldt, A M Reilly, J G Brandenburg, O T Hofmann, T Bjorkman, S Lebegue, and A Tkatchenko.  Advances in density-functional calculations for materials modeling.  Annual Review Of Materials Research, Vol 49 Se Annual Review Of Materials Research, 49:1, 2019.
  * [209] J P Perdew and K Schmidt.  Jacob’s ladder of density functional approximations for the exchange-correlation energy.  Density Functional Theory And Its Application To Materials, edited by V. Van Doren, C. Van Alsenoy, and P. Geerlings, 577:1, 2001.
  * [210] G Kotliar, S Y Savrasov, K Haule, V S Oudovenko, O Parcollet, and C A Marianetti.  Electronic structure calculations with dynamical mean-field theory.  Rev. Mod. Phys., 78:865, 2006.
  * [211] A J Cohen, P Mori-Sanchez, and W T Yang.  Insights into current limitations of density functional theory.  Science, 321:792, 2008.
  * [212] M Imada, A Fujimori, and Y Tokura.  Metal-insulator transitions.  Rev. Mod. Phys., 70:1039, 1998.
  * [213] J P Perdew, W T Yang, K Burke, Z H Yang, E Gross, M Scheffler, G E Scuseria, T M Henderson, I Y Zhang, A Ruzsinszky, and et al. Understanding band gaps of solids in generalized Kohn-Sham theory.  Proceedings Of The National Academy Of Sciences Of The United States Of America, 114:2801, 2017.
  * [214] A Georges, G Kotliar, W Krauth, and M J Rozenberg.  Dynamical mean-field theory of strongly correlated fermion systems and the limit of infinite dimensions.  Rev. Mod. Phys., 68:13, 1996.
  * [215] P Werner, A Comanac, L D Medici, M Troyer, and A J Millis.  Continuous-time solver for quantum impurity models.  Phys. Rev. Lett., 97:076405, 2006.
  * [216] K Haule.  Quantum monte carlo impurity solver for cluster dynamical mean-field theory and electronic structure calculations with adjustable cluster base.  Phys. Rev. B, 75:155113, 2007.
  * [217] E Gull, A J. Millis, A I. Lichtenstein, A N. Rubtsov, M Troyer, and P Werner.  Continuous-time monte-carlo methods for quantum impurity models.  Rev. Mod. Phys., 83:349, May 2011.
  * [218] C A Marianetti, K Haule, G Kotliar, and M J Fluss.  Electronic coherence in delta-Pu: a dynamical mean-field theory study.  Phys. Rev. Lett., 101:056403, 2008.
  * [219] H Park, A J Millis, and C A Marianetti.  Total energy calculations using DFT plus DMFT: computing the pressure phase diagram of the rare earth nickelates.  Phys. Rev. B, 89:245133, 2014.
  * [220] E B Isaacs and C A Marianetti.  Compositional phase stability of correlated electron materials within DFT+DMFT.  Phys. Rev. B, 102:045146, Jul 2020.
  * [221] R Adler, C J Kang, C H Yee, and G Kotliar.  Correlated materials design: prospects and challenges.  Reports On Progress In Physics, 82:012504, 2019.
  * [222] B Chakrabarti, T Birol, and K Haule.  Role of entropy and structural parameters in the spin-state transition of LaCoO3.  Physical Review Materials, 1:064403, 2017.
  * [223] F Lechermann.  Multiorbital processes rule the Nd1-xSrxNiO2 normal state.  Physical Review X, 10:041002, 2020.
  * [224] V I Anisimov, F Aryasetiawan, and A I Lichtenstein.  First-principles calculations of the electronic structure and spectra of strongly correlated systems: the LDA+U method.  Journal Of Physics-condensed Matter, 9:767, 1997.
  * [225] B D Sahoo, K D Joshi, and T C Kaushik.  Structural, elastic, vibrational, thermophysical properties and pressure-induced phase transitions of ThN2, Th2N3, and Th3N4: An ab initio investigation.  Journal of Applied Physics, 128(3):035902, 2020.
  * [226] J Adachi, K Kurosaki, M Uno, and S Yamanaka.  A molecular dynamics study of thorium nitride.  Journal of alloys and compounds, 394(1-2):312–316, 2005.
  * [227] R Atta-Fynn and Asok K Ray.  Density functional study of the actinide nitrides.  Physical Review B, 76(11):115101, 2007.
  * [228] N Reyren, S Thiel, A D Caviglia, L F Kourkoutis, G Hammerl, C Richter, C W Schneider, T Kopp, A S Ruetschi, D Jaccard, M Gabay, D A Muller, J M Triscone, and J Mannhart.  Superconducting interfaces between insulating oxides.  Science, 317:1196, 2007.
  * [229] D D Castro, C Cantoni, F Ridolfi, C Aruta, A Tebano, N Yang, and G Balestrino.  High-Tc superconductivity at the interface between the CaCuO2 and SrTiO3 insulating oxides.  Phys. Rev. Lett., 115:147001, 2015.
  * [230] U Luders, W C Sheets, A David, W Prellier, and R Fresard.  Room-temperature magnetism in LaVO3/SrVO3 superlattices by geometrically confined doping.  Phys. Rev. B, 80:241102, 2009.
  * [231] A V Boris, Y Matiks, E Benckiser, A Frano, P Popovich, V Hinkov, P Wochner, M Castro-colin, E Detemple, V K Malik, C Bernhard, T Prokscha, A Suter, Z Salman, E Morenzoni, G Cristiani, H U Habermeier, and B Keimer.  Dimensionality control of electronic phase transitions in nickel-oxide superlattices.  Science, 332:937, 2011.
  * [232] J Chakhalian, J W Freeland, H U Habermeier, G Cristiani, G Khaliullin, M VanVeenendaal, and B Keimer.  Orbital reconstruction and covalent bonding at an oxide interface.  Science, 318:1114, 2007.
  * [233] M Bibes, J E Villegas, and A Barthelemy.  Ultrathin oxide films and interfaces for electronics and spintronics.  Advances In Physics, 60:5, 2011.
  * [234] H H Chen, A J Millis, and C A Marianetti.  Engineering correlation effects via artificially designed oxide superlattices.  Phys. Rev. Lett., 111:116403, 2013.
  * [235] Z C Zhong, M Wallerberger, J M Tomczak, C Taranto, N Parragh, A Toschi, G Sangiovanni, and K Held.  Electronics with correlated oxides: SrVO3/SrTiO3 as a Mott transistor.  Phys. Rev. Lett., 114:246401, 2015.
  * [236] A James, M Aichhorn, and J Laverock.  Quantum confinement induced metal-insulator transition in strongly correlated quantum wells of SrVO3 superlattices.  Physical Review Research, 3:023149, 2021.
  * [237] D Gao and A K Ray.  On the convergence of the electronic structure properties of the fcc americium (0 0 1) surface.  Surface science, 600(22):4941–4952, 2006.
  * [238] D Gao and A K Ray.  Relativistic density functional study of fcc americium and the (111) surface.  Physical Review B, 77(3):035123, 2008.
  * [239] E L Sikorski, B J Jaques, and L Li.  First-principles magnetic treatment of the uranium nitride (100) surface and effect on corrosion initiation.  Journal of Applied Physics, 130(9):095301, 2021.
  * [240] H Kroemer.  Quasi-electric fields and band offsets: Teaching electrons new tricks (nobel lecture).  ChemPhysChem, 2(8-9):490–499, 2001.
  * [241] C A Dennett, N Poudel, P J Simmonds, A Tiwari, D H Hurley, and K Gofryk.  Towards actinide heterostructure synthesis and science.  Nat. Commun., 2022.  Accepted.
  * [242] M Block, M Laatiaoui, and S Raeder.  Recent progress in laser spectroscopy of the actinides.  Progress in Particle and Nuclear Physics, 116:103834, 2021.
  * [243] P Grover, L S Ferch, and G Schreckenbach.  Adsorption of actinide (U–Pu) complexes on the silicene and germanene surface: A theoretical study.  The Journal of Physical Chemistry A, 124(8):1522–1534, 2020.
  * [244] A Lopez-Bezanilla.  f-orbital based Dirac states in a two-dimensional uranium compound.  Journal of Physics: Materials, 3(2):024002, 2020.
  * [245] S-J Park and M-K Seo.  Interface science and composites, volume 18.  Academic Press, 2011.
  * [246] S Murali, C Zhang, M Goryll, R R King, and C B Honsberg.  Study of pit formation in MBE grown GaP on misoriented Si.  Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena, 38(3):032201, 2020.
  * [247] J Simon, S Tomasulo, P J Simmonds, M Romero, and M L Lee.  Metamorphic gaasp buffers for growth of wide-bandgap ingap solar cells.  Journal of Applied Physics, 109:013708, 2011.
  * [248] D Benyahia, Ł. Kubiszyn, K Michalczewski, A Kȩbłowski, P Martyniuk, J Piotrowski, A Rogalski, and et al.  Optimization of the interfacial misfit array growth mode of GaSb epilayers on GaAs substrate.  Journal of Crystal Growth, 483:26–30, 2018.
  * [249] K E Sautter, K D Vallejo, and P J Simmonds.  Strain-driven quantum dot self-assembly by molecular beam epitaxy.  Journal of Applied Physics, 128(3):031101, 2020.
  * [250] T Makimoto, K Kumakura, M Maeda, H Yamamoto, and Y Horikoshi.  A new AlON buffer layer for RF-MBE growth of AlN on a sapphire substrate.  Journal of Crystal Growth, 425:138–140, 2015.
  * [251] K D Vallejo, T A Garrett, C I Cabrera, B Liang, K A Grossklaus, and P J Simmonds.  Tensile-strained self-assembly of InGaAs on InAs (111) a.  Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena, 39(6):062809, 2021.
  * [252] X He, Y Feng, X Yang, S Wu, Z Cai, J Wei, J Shen, H Huang, D Liu, Z Chen, and et al. Step-graded AlGaN vs superlattice: role of strain relief layer in dynamic on-resistance degradation.  Applied Physics Express, 15(1):011001, 2021.
  * [253] U Nandi, M Mohammadi, H Lu, J Norman, A C Gossard, L Alff, and S Preu.  Material properties and performance of ErAs: In(Al)GaAs photoconductors for 1550 nm laser operation.  Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 39(2):023407, 2021.
  * [254] P Jin, X L Ye, and Z G Wang.  Growth of low-density InAs/GaAs quantum dots on a substrate with an intentional temperature gradient by molecular beam epitaxy.  Nanotechnology, 16(12):2775, 2005.
  * [255] J W Matthews and A E Blakeslee.  Defects in epitaxial multilayers: I. misfit dislocations.  Journal of Crystal growth, 27:118–125, 1974.
  * [256] F A Rough and A A Bauer.  Constitution of uranium and thorium alloys.  Technical report, Battelle Memorial Inst., Columbus, Ohio, 1958.