title:
Strength of the Spin-Fluctuation-Mediated Pairing Interaction in a
  High-Temperature Superconductor

abstract:
  Theories based on the coupling between spin fluctuations and fermionic
quasiparticles are among the leading contenders to explain the origin of
high-temperature superconductivity, but estimates of the strength of this
interaction differ widely. Here we analyze the charge- and spin-excitation
spectra determined by angle-resolved photoemission and inelastic neutron
scattering, respectively, on the same crystals of the high-temperature
superconductor YBa2Cu3O6.6. We show that a self-consistent description of both
spectra can be obtained by adjusting a single parameter, the spin-fermion
coupling constant. In particular, we find a quantitative link between two
spectral features that have been established as universal for the cuprates,
namely high-energy spin excitations and "kinks" in the fermionic band
dispersions along the nodal direction. The superconducting transition
temperature computed with this coupling constant exceeds 150 K, demonstrating
that spin fluctuations have sufficient strength to mediate high-temperature
superconductivity.


title: The number of orientable small covers over cubes

abstract:
  We count orientable small covers over cubes. We also get estimates for
$O_n/R_n$, where $O_n$ is the number of orientable small covers and $R_n$ is
the number of all small covers over an $n$-cube up to the Davis-Januszkiewicz
equivalence.


title: Invariant solutions of the supersymmetric sine-Gordon equation

abstract:
  A comprehensive symmetry analysis of the N=1 supersymmetric sine-Gordon
equation is performed. Two different forms of the supersymmetric system are
considered. We begin by studying a system of partial differential equations
corresponding to the coefficients of the various powers of the anticommuting
independent variables. Next, we consider the super-sine-Gordon equation
expressed in terms of a bosonic superfield involving anticommuting independent
variables.
  In each case, a Lie (super)algebra of symmetries is determined and a
classification of all subgroups having generic orbits of codimension 1 in the
space of independent variables is performed. The method of symmetry reduction
is systematically applied in order to derive invariant solutions of the
supersymmetric model. Several types of algebraic, hyperbolic and doubly
periodic solutions are obtained in explicit form.


title: Birational geometry of Fano double spaces of index two

abstract:
  We study birational geometry of Fano varieties, realized as double covers
$\sigma\colon V\to {\mathbb P}^M$, $M\geq 5$, branched over generic
hypersurfaces $W=W_{2(M-1)}$ of degree $2(M-1)$. We prove that the only
structures of a rationally connected fiber space on $V$ are the
pencils-subsystems of the free linear system $|-\frac12 K_V|$. The groups of
birational and biregular self-maps of the variety $V$ coincide.


title: Constructing de Sitter vacua in no-scale string models without uplifting

abstract:
  We develop a method for constructing metastable de Sitter vacua in N=1
supergravity models describing the no-scale volume moduli sector of Calabi-Yau
string compactifications. We consider both heterotic and orientifold models.
Our main guideline is the necessary condition for the existence of metastable
vacua coming from the Goldstino multiplet, which constrains the allowed scalar
geometries and supersymmetry-breaking directions. In the simplest non-trivial
case where the volume is controlled by two moduli, this condition simplifies
and turns out to be fully characterised by the intersection numbers of the
Calabi-Yau manifold. We analyse this case in detail and show that once the
metastability condition is satisfied it is possible to reconstruct in a
systematic way the local form of the superpotential that is needed to stabilise
all the fields. We apply then this procedure to construct some examples of
models where the superpotential takes a realistic form allowed by flux
backgrounds and gaugino condensation effects, for which a viable vacuum arises
without the need of invoking corrections to the Kahler potential breaking the
no-scale property or uplifting terms. We finally discuss the prospects of
constructing potentially realistic models along these lines.


title: Accelerated expansion and matter creation

abstract:
  A set of cosmological models that takes into account the variation of the
particle number is presented. In this context both dark matter and dark energy
can be explained using a single component, without assuming any exotic equation
of state, solving directly the cosmic coincidence problem.


title:
Quantum calculations of H2-H2 collisions: from ultracold to thermal
  energies

abstract:
  We present quantum dynamics of collisions between two para-H2 molecules from
low (1 mK) to high collision energies (1 eV). The calculations are carried out
using a quantum scattering code that solves the time-independent Schrodinger
equation in its full dimensionality without any decoupling approximations. The
six-dimensional potential energy surface for the H4 system developed by
Boothroyd et al. [J. Chem. Phys. 116, 666 (2002)] is used in the calculations.
Elastic, inelastic and state-to-state cross sections as well as rate
coefficients from T = 1 K to 400 K obtained from our calculations are compared
with available experimental and theoretical results. Overall, good agreement is
obtained with previous studies.


title:
Statistical physics of a model binary genetic switch with linear
  feedback

abstract:
  We study the statistical properties of a simple genetic regulatory network
that provides heterogeneity within a population of cells. This network consists
of a binary genetic switch in which stochastic flipping between the two switch
states is mediated by a "flipping" enzyme. Feedback between the switch state
and the flipping rate is provided by a linear feedback mechanism: the flipping
enzyme is only produced in the on switch state and the switching rate depends
linearly on the copy number of the enzyme. This work generalises the model of
[Phys. Rev. Lett., 101, 118104] to a broader class of linear feedback systems.
We present a complete analytical solution for the steady-state statistics of
the number of enzyme molecules in the on and off states, for the general case
where the enzyme can mediate flipping in either direction. For this general
case we also solve for the flip time distribution, making a connection to first
passage and persistence problems in statistical physics. We show that the
statistics of the model are non-Poissonian, leading to a peak in the flip time
distribution. The occurrence of such a peak is analysed as a function of the
parameter space. We present a new relation between the flip time distributions
measured for two relevant choices of initial condition. We also introduce a new
correlation measure to show that this model can exhibit long-lived temporal
correlations, thus providing a primitive form of cellular memory. Motivated by
DNA replication as well as by evolutionary mechanisms involving gene
duplication, we study the case of two switches in the same cell. This results
in correlations between the two switches; these can either positive or negative
depending on the parameter regime.


title: Magnetic structure of actinide metals

abstract:
  In comparison to 3d or 4f metals, magnetism in actinides remains poorly
understood due to experimental complications and the exotic behavior of the 5f
states. In particular, plutonium metal is most especially vexing. Over the last
five decades theories proposed the presence of either ordered or disordered
local moments at low temperatures. However, experiments such as magnetic
susceptibility, electrical resistivity, nuclear magnetic resonance, specific
heat, and elastic and inelastic neutron scattering show no evidence for ordered
or disordered magnetic moments in any of the six phases of plutonium. Beyond
plutonium, the magnetic structure of other actinides is an active area of
research given that temperature, pressure, and chemistry can quickly alter the
magnetic structure of the 5f states. For instance, curium metal has an
exceedingly large spin polarization that results in a total moment of about 8
Bohr magneton/atom, which influences the phase stability of the metal. Insight
in the actinide ground state can be obtained from core-level x-ray absorption
spectroscopy (XAS) and electron energy-loss spectroscopy (EELS). A sum rule
relates the branching ratio of the core-level spectra measured by XAS or EELS
to the expectation value of the angular part of the spin-orbit interaction.


title: Multiple Time Dimensions

abstract:
  The possibility of physics in multiple time dimensions is investigated.
Drawing on recent work by Walter Craig and myself, I show that, contrary to
conventional wisdom, there is a well-posed initial value
problem--deterministic, stable evolution--for theories in multiple time
dimensions. Though similar in many ways to ordinary, single-time theories,
multi-time theories have some rather intriguing properties which suggest new
directions for the understanding of fundamental physics.


title: Depth Zero Representations of Nonlinear Covers of $p$-adic Groups

abstract:
  We generalize the methods of Moy-Prasad, in order to define and study the
genuine depth zero representations of some nonlinear covers of reductive groups
over $p$-adic local fields. In particular, we construct all depth zero
supercuspidal representations of the metaplectic group $Mp_{2n}$ over a
$p$-adic field of odd residue characteristic.


title: Decting Errors in Reversible Circuits With Invariant Relationships

abstract:
  Reversible logic is experience renewed interest as we are approach the limits
of CMOS technologies. While physical implementations of reversible gates have
yet to materialize, it is safe to assume that they will rely on faulty
individual components. In this work we present a present a method to provide
fault tolerance to a reversible circuit based on invariant relationships.


title: Unveiling the birth and evolution of the HII region Sh2-173

abstract:
  Based on a multiwavelength study, the ISM around the HII region Sh2-173 has
been analyzed. The ionized region is clearly detected in the optical and in the
radio continuum images. The analysis of the HI data shows a region of low
emissivity that has an excellent morphological correlation with the radio
continuum emission. The HII region is partially bordered by a photodissociation
region, which, in turn, is encircled by a molecular structure. Taking into
account the presence of noncircular motions in the Perseus spiral arm, together
with previous distance estimates for the region, we adopt a distance of 2.5 +-
0.5 kpc for Sh2-173. Seven hot stars were identified in the field of Sh2-173,
being only one an O-type star. The amount of energetic photons emitted by this
star is enough to keep the region ionized and heat the dust. Given that an
expanding HII region may trigger star formation, a search for YSO candidates
was made using different infrared point source catalogues. A population of 46
YSO candidates was identified projected onto the molecular clouds. On the other
hand, Sh2-173 is located in a dense edge of a large HI shell. The possibility
for Sh2-173 of being part of a hierarchical system of three generations is
suggested. The ages of both, the HII region and the large shell, were estimated
and compared. We concluded that Sh2-173 is a young HII region of about 0.6 -
1.0 Myr old. As for the large shell we obtained a dynamical age of 5 +- 1 Myr.
These age estimates, together with the relative location of the different
structures, support the hypothesis that Sh2-173 is part of a hierarchical
system.


title: The K-Receiver Broadcast Channel with Confidential Messages

abstract:
  The secrecy capacity region for the K-receiver degraded broadcast channel
(BC) is given for confidential messages sent to the receivers and to be kept
secret from an external wiretapper. Superposition coding and Wyner's random
code partitioning are used to show the achievable rate tuples. Error
probability analysis and equivocation calculation are also provided. In the
converse proof, a new definition for the auxiliary random variables is used,
which is different from either the case of the 2-receiver BC without common
message or the K-receiver BC with common message, both with an external
wiretapper; or the K-receiver BC without a wiretapper.