title: Superconductivity and Spin Fluctuations

abstract:
  The organizers of the Memorial Session for Herman Rietschel asked that I
review some of the history of the interplay of superconductivity and spin
fluctuations. Initially, Berk and Schrieffer showed how paramagnon spin
fluctuations could suppress superconductivity in nearly-ferromagnetic
materials. Following this, Rietschel and various co-workers wrote a number of
papers in which they investigated the role of spin fluctuations in reducing the
T_c of various electron-phonon superconductors. Paramagnon spin fluctuations
are also believed to provide the p-wave pairing mechanism responsible for the
superfluid phases of $^3He$. More recently, antiferromagnetic spin fluctuations
have been proposed as the mechanism for d-wave pairing in the heavy-fermion
superconductors and in some organic materials as well as possibly the high T_c
cuprates. Here I will review some of this early history and discuss some of the
things we have learned more recently from numerical simulations.


title:
Phase diagram for morphological transitions of wetting films on
  chemically structured substrates

abstract:
  Using an interface displacement model we calculate the shapes of thin
liquidlike films adsorbed on flat substrates containing a chemical stripe. We
determine the entire phase diagram of morphological phase transitions in these
films as function of temperature, undersaturation, and stripe width


title: Mixed-spin cluster expansion for a quasi-one-dimensional Haldane system

abstract:
  We present a novel mixed-spin cluster expansion method for a
quasi-one-dimensional Haldane system with bond alternation. By mapping the s=1
antiferromagnetic spin model on square and cubic lattices to the equivalent
mixed-spin model, we study the competition among the Haldane, the dimer and the
magnetically ordered phases. The mixed-spin cluster expansion proposed here
allows us to directly deal with the Haldane phase, which may not be reached by
standard series expansion methods. The phase diagram is determined rather
precisely by making use of an additional symmetry property in the effective
mixed-spin model introduced.


title: Dynamical correlation functions of the spin Calogero-Sutherland model

abstract:
  The thermodynamic limit of the dynamical density and spin-density two-point
correlation functions for the spin Calogero-Sutherland model are derived from
Uglov's finite-size results. The resultant formula for the density two-point
correlation function is consistent with the previous conjecture on the basis of
the minimal number of elementary excitations.

categories: cond-mat.str-el hep-th math-ph math.MP nlin.SI solv-int


title: Ising cubes with enhanced surface couplings

abstract:
  Using Monte Carlo techniques, Ising cubes with ferromagnetic nearest-neighbor
interactions and enhanced couplings between surface spins are studied. In
particular, at the surface transition, the corner magnetization shows
non-universal, coupling-dependent critical behavior in the thermodynamic limit.
Results on the critical exponent of the corner magnetization are compared to
previous findings on two-dimensional Ising models with three intersecting
defect lines.


title:
The ac-Driven Motion of Dislocations in a Weakly Damped
  Frenkel-Kontorova Lattice

abstract:
  By means of numerical simulations, we demonstrate that ac field can support
stably moving collective nonlinear excitations in the form of dislocations
(topological solitons, or kinks) in the Frenkel-Kontorova (FK) lattice with
weak friction, which was qualitatively predicted by Bonilla and Malomed [Phys.
Rev. B{\bf 43}, 11539 (1991)]. Direct generation of the moving dislocations
turns out to be virtually impossible; however, they can be generated initially
in the lattice subject to an auxiliary spatial modulation of the on-site
potential strength. Gradually relaxing the modulation, we are able to get the
stable moving dislocations in the uniform FK lattice with the periodic boundary
conditions, provided that the driving frequency is close to the gap frequency
of the linear excitations in the uniform lattice. The excitations have a large
and noninteger index of commensurability with the lattice (suggesting that its
actual value is irrational). The simulations reveal two different types of the
moving dislocations: broad ones, that extend, roughly, to half the full length
of the periodic lattice (in that sense, they cannot be called solitons), and
localized soliton-like dislocations, that can be found in an excited state,
demonstrating strong persistent internal vibrations. The minimum (threshold)
amplitude of the driving force necessary to support the traveling excitation is
found as a function of the friction coefficient. Its extrapolation suggests
that the threshold does not vanish at the zero friction, which may be explained
by radiation losses. The moving dislocation can be observed experimentally in
an array of coupled small Josephson junctions in the form of an {\it inverse
Josephson effect}, i.e., a dc-voltage response to the uniformly applied ac bias
current.


title: ESR experiments in the Kondo insulator CeNiSn

abstract:
  Below a characteristic temperature, due to hybridisation effects Kondo
insulators exhibit a gap in the electronic density of states and behave like
semiconductors. By using Gd3+ electron spin resonance (ESR), the compound
CeNiSn was investigated as a representative of this class. In addition, the
metal-to-insulator transition was studied as a function of doping for
CeNi(1-x)Co(x)Sn and CeNi(1-y)Pt(y)Sn. The linewidth of the Gd resonance yields
direct information about the density of states at the Fermi energy. So the size
of the gap can clearly be estimated for the pure compound, and the closing of
the gap by substitution of Ni by Co or Pt can be followed in detail. These
results are compared to measurements of NMR, specific heat and susceptibility.


title: Relativistic photoemission theory for general nonlocal potentials

abstract:
  An improved formulation of the one-step model of photoemission from crystal
surfaces is proposed which overcomes different limitations of the original
theory. Considering the results of an electronic-structure calculation, the
electronic (one-particle) potential and the (many-body) self-energy, as given
quantities, we derive explicit expressions for the dipole transition-matrix
elements. The theory is formulated within a spin-polarized, relativistic
framework for general nonspherical and space-filling one-particle potentials
and general nonlocal, complex and energy-dependent self-energies. It applies to
semi-infinite lattices with perfect lateral translational invariance and
arbitrary number of atoms per unit cell.


title:
Mode coupling theory for molecular liquids: What can we learn from a
  system of hard ellipsoids?

abstract:
  Molecular fluids show rich and complicated dynamics close to the glass
transition. Some of these observations are related to the fact that
translational and orientational degrees of freedom couple in nontrivial ways. A
model system which can serve as a paradigm to understand these couplings is a
system of hard ellipsoids of revolution. To test this we compare at the ideal
glass transition the static molecular correlators of a linear A-B Lennard-Jones
molecule obtained from a molecular dynamics simulation with a selected fluid of
hard ellipsoids for which the static correlators have been obtained using
Percus-Yevick theory. We also demonstrate that the critical non-ergodicity
parameters obtained from molecular mode coupling theory for both systems show a
remarkable similarity at the glass transition, provided the aspect ratio is
chosen properly. Therefore we conclude that a system of hard ellipsoids can
indeed be used to understand part of the essential behaviour of such a simple
molecular system like the A-B Lennard-Jones molecules in the vicinity of the
ideal glass transition.


title:
Correlation functions for a two-dimensional electron system with bosonic
  interactions and a square Fermi surface

abstract:
  We calculate zero-temperature correlation functions for a model of 2D
interacting electrons with short-range interactions and a square Fermi surface.
The model was arrived at by mapping electronic states near a square Fermi
surface with Hubbard-like interactions onto one-dimensional quantum chains,
retaining terms which can be written in terms of bosonic density operators.
Interactions between orthogonal chains, corresponding to orthogonal faces of
the square Fermi surface, are neglected. The correlation functions become sums
of Luttinger-type correlation functions due to the bosonic model. However, the
correlation function exponents differ in form from those of the Luttinger
model. As a consequence, the simple scaling relations found to exist between
the Luttinger model exponents, do not carry over to the leading exponents of
our model. We find that for repulsive effective interactions, charge-density
wave/spin-density wave instabilities are dominant. We do not consider d-wave
instabilities here.


title: How large can be SDW and CDW apmlitudes in underdoped cuprates

abstract:
  Self-consistent calculation of spin (charge) density wave order (SDW/CDW)
parameters have been performed for bilayered cuprates on the basis of a singlet
correlated band model. Evolution of the Fermi surface in the strongly
underdoped regime is described by using a two-band approach. The smooth
development of the pseudogap formation temperature is explained from underdoped
to overdoped states and the Fourier amplitudes <s_q> (spin) and <e_q> (charge)
modulations have been calculated. We have found a maximum of the
incommensurability for doping 0.09-0.11 holes per copper site.


title: Johnson-Nyquist noise in narrow wires

abstract:
  The Johnson-Nyquist noise in narrow semiconducting wires having a transverse
size smaller than the screening length is shown to be white up to frequency
$D/L^2$ and to decay at higher frequencies as $\omega^{-{1/2}}$. This result is
contrasted with the noise spectra in neutral and charged liquids.


title:
Nonequilibrium Phase Transition in Non-Local and Nonlinear Diffusion
  Model

abstract:
  We present the results of analytical and numerical studies of a
one-dimensional nonlocal and nonlinear diffusion equation describing
non-equilibrium processes ranging from aggregation phenomena to cooperation of
individuals. We study a dynamical phase transition that is obtained on tuning
the initial conditions and demonstrate universality and characterize the
critical behavior. The critical state is shown to be reached in a
self-organized manner on dynamically evolving the diffusion equation subjected
to a mirror symmmetry transformation.


title:
Density-functional-theory calculations of molecular nitrogen on
  ruthenium cluster

abstract:
  Density-functional-theory calculations for the adsorption of molecular
nitrogen on ruthenium surface are reported. It is found that nitrogen molecule
is adsorbed while standing perpendicular to the triangular surface of
ruthenium. The doping by K atoms favours the adsorption of molecular nitrogen.
The calculation of the adsorption energy has also been performed on iron and
osmium clusters which shows that ruthenium adsorbs better than iron or osmium.
It is found that potential for atomic nitrogen is deeper and more stable than
that of molecular nitrogen and hence atomic nitrogen is important for
catalysis.


title: Configurational Entropy and Diffusivity of Supercooled Water

abstract:
  We calculate the configurational entropy S_conf for the SPC/E model of water
for state points covering a large region of the (T,rho) plane. We find that (i)
the (T,rho) dependence of S_conf correlates with the diffusion constant and
(ii) that the line of maxima in S_conf tracks the line of density maxima. Our
simulation data indicate that the dynamics are strongly influenced by S_conf
even above the mode-coupling temperature T_MCT(rho).


title:
Frequency Evolution of Neutron Peaks Below Tc: Commensurate and
  Incommensurate Structure in LaSrCuO and YBaCuO

abstract:
  We study the evolution of the neutron cross-section with variable frequency
$\omega$ and fixed $T$ below $T_c$ in two different cuprate families. Our
calculations, which predominantly probe the role of d-wave pairing, lead to
generic features, independent of Fermi surface shapes. Among our findings,
reasonably consistent with experiment, are (i) for $\omega $ near the gap
energy $\Delta$, both optimal {LaSrCuO} and slightly underdoped YBCO exhibit
(comparably) incommensurate peaks (ii) peak sharpening below $T_c$ is seen in
{LaSrCuO}, (iii) quite generically, a frequency evolution from incommensurate
to commensurate and then back to incommensurate structure is found with
increasing $\omega$. Due to their narrow $\omega$ regime of stability,
commensurate peaks in {LaSrCuO} should be extremely difficult to observe.


title:
Super-reflection of light from a random amplifying medium with disorder
  in the complex refractive index : Statistics of fluctuations

abstract:
  The probability distribution of the reflection coefficient for light
reflected from a one-dimensional random amplifying medium with {\it
cross-correlated} spatial disorder in the real and the imaginary parts of the
refractive index is derived using the method of invariant imbedding. The
statistics of fluctuations have been obtained for both the correlated telegraph
noise and the Gaussian white-noise models for the disorder. In both cases, an
enhanced backscattering (super-reflection with reflection coefficient greater
than unity) results because of coherent feedback due to Anderson localization
and coherent amplification in the medium. The results show that the effect of
randomness in the imaginary part of the refractive index on localization and
super-reflection is qualitatively different.


title:
Quasiparticle Interactions in Fractional Quantum Hall Systems:
  Justification of Different Hierarchy Schemes

abstract:
  The pseudopotentials describing the interactions of quasiparticles in
fractional quantum Hall (FQH) states are studied. Rules for the identification
of incompressible quantum fluid ground states are found, based upon the form of
the pseudopotentials. States belonging to the Jain sequence nu=n/(1+2pn), where
n and p are integers, appear to be the only incompressible states in the
thermodynamic limit, although other FQH hierarchy states occur for finite size
systems. This explains the success of the composite Fermion picture.


title:
Optical spectral weights and the ferromagnetic transition temperature of
  CMR manganites: relevance of double-exchange to real materials

abstract:
  We present a thorough and quantitative comparison of double-exchange models
to experimental data on the colossal magnetoresistance manganese perovskites.
Our results settle a controversy by showing that physics beyond double-exchange
is important even in La$_{0.7}$Sr$_{0.3}$MnO$_3$, which has been regarded as a
conventional double-exchange system. We show that the crucial quantity for
comparisons of different calculations to each other and to data is the
conduction band kinetic energy $K$, which is insensitive to the details of the
band structure and can be experimentally determined from optical conductivity
measurements. The seemingly complicated dependence of $T_c$ on the Hund's
coupling $J$ and carrier concentration $n$ is shown to reflect the variation of
$K$ with $J$, $n$ and temperature. We present results for the optical
conductivity which allow interpretation of experiments and show that a feature
previously interpreted in terms of the Hund's coupling was misidentified. We
also correct minor errors in the phase diagram presented in previous work.


title:
Field-induced vacancy localization in a driven lattice gas: Scaling of
  steady states

abstract:
  With the help of Monte Carlo simulations and a mean-field theory, we
investigate the ordered steady-state structures resulting from the motion of a
single vacancy on a periodic lattice which is filled with two species of
oppositely ``charged'' particles. An external field biases particle-vacancy
exchanges according to the particle's charge, subject to an excluded volume
constraint. The steady state exhibits charge segregation, and the vacancy is
localized at one of the two characteristic interfaces. Charge and hole density
profiles, an appropriate order parameter and the interfacial regions themselves
exhibit characteristic scaling properties with system size and field strength.
The lattice spacing is found to play a significant role within the mean-field
theory.


title: The General Structure of Bose Gases with Arbitrary Spin

abstract:
  Motivated by the recent discoveries of spin-1 and spin-1/2 Bose gas, we have
studied the general structure of the Bose gases with arbitrary spin. A general
method is developed to uncover the elementary building blocks of the angular
momentum eigenstates, as well as the relations (or interactions) between them.
Applications of this method to Bose gases with integer spins (f=1,2,3) and half
integer spins (f=1/2,3/2) reveal many surprising structures.


title:
The ARPES Pseudogap and Anomalous Background of Underdoped BiSCO 2212 as
  Evidence for a Fermi Level Band Crossing

abstract:
  We show that the unusual observations of a pseudogap in the normal state of
underdoped Bi_2Sr_2CaCu_2O_8+delta (BiSCO) using angle resolved photoemission
spectroscopy (ARPES) is consistent with a new band structure for the cuprate
superconductors in which the x2-y2 and z2 bands are seen to cross at the Fermi
level. Limitations in the experimental method prevent the narrow 3D z2 band
from being fully resolved, leading instead to a broad background with
"stepfunction" character. As a consequence, the Fermi surface is mis-assigned
and a pseudogap of approximately d-wave symmetry develops.


title: Simplified dynamics for glass model

abstract:
  In spin glass models one can remove minimization of free energy by some order
parameter. One can consider hierarchy of order parameters. It is possible to
divide energy among these parts. We can consider relaxation process in glass
system phenomonologically, as exchange of energy between 2 parts. It is
possible to identify trap points in phase space. We suggest some
phenomonological approximation-truncated Langevine.
  The mean field statics is used to introduce a phenomenologic dynamics as its
natural extension.
  Purely kinetical phase transitions are investigated..


title: Thermodynamic Phase Diagram of the Quantum Hall Skyrmion System

abstract:
  We numerically study the interacting quantum Hall skyrmion system based on
the Chern-Simons action. By noticing that the action is invariant under global
spin rotations in the spin space with respect to the magnetic field direction,
we obtain the low-energy effective action for a many skyrmion system.
Performing extensive molecular dynamics simulations, we establish the
thermodynamic phase diagram for a many skyrmion system.


title:
Field-Theoretical Analysis of Critical and Coexistence Singularities at
  Critical End Points

abstract:
  Continuum models with critical end points are considered whose Hamiltonian
${\mathcal{H}}[\phi,\psi]$ depends on two densities $\phi$ and $\psi$.
Field-theoretic methods are used to show the equivalence of the critical
behavior on the critical line and at the critical end point and to give a
systematic derivation of critical-end-point singularities like the thermal
singularity $\sim|{t}|^{2-\alpha}$ of the spectator-phase boundary and the
coexistence singularities $\sim |{t}|^{1-\alpha}$ or $\sim|{t}|^{\beta}$ of the
secondary density $<\psi>$. The appearance of a discontinuity eigenexponent
associated with the critical end point is confirmed, and the mechanism by which
it arises in field theory is clarified.


title: Random Energy Model as a paradigm of complex systems

abstract:
  A quadratic extension of REM has been treated. Discussed here is the origin
of relation of REM to strings and other complex physical phenomena. Two basic
features of the REM class of complex phenomena were identified: the double
thermodynamic reflection (a hierarchy of free energies) including the strong
reflection at the upper level (the free energy on the order of a logarithm of
the degrees of freedom) and the loss (complete or partial) of the local
symmetry property. Two main classes of complex phenomena related to REM are
seen: the spin glass phase of REM and the boundary the spin glass-ferromagnetic
phases. Some examples of physics interest are analyzed from this viewpoint.


title: Phase Diagram of Adsorbate-Induced Row-Type-Alignments

abstract:
  The phase diagram of adsorbate-induced row-type-alignments, such as
missing-row reconstructions induced by adsorbate-atoms on the FCC(110) surface,
is calculated by the Blume-Emmery-Griffiths (BEG) model. In the model, we
introduce adatom-adatom and dipole-dipole interactions between nearest-neighbor
(NN) and next-nearest-neighbor (NNN) rows. The calculation of the temperature
versus adatom chemical potential phase diagram is performed using mean-field
approximation. It is indicated that when NN and NNN interactions are
competitive, there appear either dipole or coverage modulated (incommensurate)
phases at high temperatures for wide regime of the interactions.


title: Conformally Invariant Fractals and Potential Theory

abstract:
  The multifractal (MF) distribution of the electrostatic potential near any
conformally invariant fractal boundary, like a critical O(N) loop or a $Q$
-state Potts cluster, is solved in two dimensions. The dimension $\hat
f(\theta)$ of the boundary set with local wedge angle $\theta$ is $\hat
f(\theta)=\frac{\pi}{\theta} -\frac{25-c}{12}
\frac{(\pi-\theta)^2}{\theta(2\pi-\theta)}$, with $c$ the central charge of the
model. As a corollary, the dimensions $D_{\rm EP} =sup_{\theta}\hat f(\theta)$
of the external perimeter and $D_{\rm H}$ of the hull of a Potts cluster obey
the duality equation $(D_{\rm EP}-1)(D_{\rm H}-1)={1/4}$. A related covariant
MF spectrum is obtained for self-avoiding walks anchored at cluster boundaries.

categories: cond-mat.stat-mech math-ph math.MP math.PR


title:
Supersymmetry on a lattice and Dirac fermions in a random vector
  potential

abstract:
  We study two-dimensional Dirac fermions in a random non-Abelian vector
potential by using lattice regularization. We consider U(N) random vector
potential for large $N$. The ensemble average with respect to random vector
potential is taken by using lattice supersymmetry which we introduced before in
order to investigate phase structure of supersymmetric gauge theory. We show
that a phase transition occurs at a certain critical disorder strength. The
ground state and low-energy excitations are studied in detail in the
strong-disorder phase. Correlation function of the fermion local density of
states decays algebraically at the band center because of a quasi-long-range
order of chiral symmetry and the chiral anomaly cancellation in the lattice
regularization (the species doubling). In the present study, we use the lattice
regularization and also the Haar measure of U(N) for the average over the
random vector potential. Therefore topologically nontrivial configurations of
the vector potential are all included in the average. Implication of the
present results for the system of Dirac fermions in a random vector potential
with noncompact Gaussian distribution is discussed.


title: Sandpile Models of Self-Organized Criticality

abstract:
  Self-Organized Criticality is the emergence of long-ranged spatio-temporal
correlations in non-equilibrium steady states of slowly driven systems without
fine tuning of any control parameter. Sandpiles were proposed as prototypical
examples of self-organized criticality. However, only some of the laboratory
experiments looking for the evidence of criticality in sandpiles have reported
a positive outcome. On the other hand a large number of theoretical models have
been constructed that do show the existence of such a critical state. We
discuss here some of the theoretical models as well as some experiments.


title:
Many-body vortex effect on the transverse force acting on a moving
  vortex

abstract:
  The nondissipative transverse force acting on one moving vortex under the
influence of another vortex is discussed in fermionic superfluid systems, where
the relative velocity between the vortices is finite.
  On the basis of detailed numerical solutions of the Bogoliubov-de Gennes
equation, the Berry phase for an adiabatic motion of the vortex line is
examined for a two-vortex system.
  It is found that the detailed electronic structure of a vortex core can
affect the transverse force, without abandoning the previous discussions of the
robust Magnus force on a single vortex.


title: Tails of Localized Density of States of Two-dimensional Dirac Fermions

abstract:
  The density of states of Dirac fermions with a random mass on a
two-dimensional lattice is considered. We give the explicit asymptotic form of
the single-electron density of states as a function of both energy and
(average) Dirac mass, in the regime where all states are localized. We make use
of a weak-disorder expansion in the parameter g/m^2, where g is the strength of
disorder and m the average Dirac mass for the case in which the evaluation of
the (supersymmetric) integrals corresponds to non-uniform solutions of the
saddle point equation. The resulting density of states has tails which deviate
from the typical pure Gaussian form by an analytic prefactor.

categories: cond-mat.mes-hall cond-mat.stat-mech cond-mat.str-el cond-mat.supr-con


title: 4f-spin dynamics in La(2-x-y)Sr(x)Nd(y)CuO(4)

abstract:
  We have performed inelastic magnetic neutron scattering experiments on
La(2-x-y)Sr(x)Nd(y)CuO(4) in order to study the Nd 4f-spin dynamics at low
energies. In all samples we find at high temperatures a quasielastic line
(Lorentzian) with a line width which decreases on lowering the temperature. The
temperature dependence of the quasielastic line width Gamma/2(T) can be
explained with an Orbach-process, i.e. a relaxation via the coupling between
crystal field excitations and phonons. At low temperatures the Nd-4f magnetic
response S(Q,omega) correlates with the electronic properties of the
CuO(2)-layers. In the insulator La(2-y)Nd(y)CuO(4) the quasielastic line
vanishes below 80 K and an inelastic excitation occurs. This directly indicates
the splitting of the Nd3+ ground state Kramers doublet due to the static
antiferromagnetic order of the Cu moments. In La(1.7-x)Sr(x)Nd(0.3)CuO(4) with
x = 0.12, 0.15 and La(1.4-x)Sr(x)Nd(0.6)CuO(4) with x = 0.1, 0.12, 0.15, 0.18
superconductivity is strongly suppressed. In these compounds we observe a
temperature independent broad quasielastic line of Gaussian shape below T about
30 K. This suggests a distribution of various internal fields on different Nd
sites and is interpreted in the frame of the stripe model. In
La(1.8-y)Sr(0.2)Nd(y)CuO(4) (y = 0.3, 0.6) such a quasielastic broadening is
not observed even at lowest temperature.


title:
Anomalous Low Temperature Behavior of Superconducting
  Nd(1.85)Ce(0.15)CuO(4-y)

abstract:
  We have measured the temperature dependence of the in-plane London
penetration depth lambda(T) and the maximum Josephson current Ic(T) using
bicrystal grain boundary Josephson junctions of the electron-doped cuprate
superconductor Nd(1.85)Ce(0.15)CuO(4-y). Both quantities reveal an anomalous
temperature dependence below about 4 K. In contrast to the usual monotonous
decrease (increase) of lambda(T) (Ic(T)) with decreasing temperature, lambda(T)
and Ic(T) are found to increase and decrease, respectively, with decreasing
temperature below 4 K resulting in a non-monotonous overall temperature
dependence. This anomalous behavior was found to be absent in analogous
measurements performed on Pr(1.85)Ce(0.15)CuO(4-y). From this we conclude that
the anomalous behavior of Nd(1.85)Ce(0.15)CuO(4-y) is caused by the presence of
the Nd3+ paramagnetic moments. Correcting the measured lambda(T) dependence of
Nd(1.85)Ce(0.15)CuO(4-y) for the temperature dependent susceptibility due to
the Nd moments, an exponential dependence is obtained indicating isotropic
s-wave pairing. This result is fully consistent with the lambda(T) dependence
measured for Pr(1.85)Ce(0.15)CuO(4-y).


title: Novel surface state in a class of incommensurate system

abstract:
  We study the Landau model of the class of incommensurate systems with a
scalar order parameter where the modulated phase is driven by a
gradient-squared term with negative coefficient. For example, theoretical
studies of cholesteric liquid crystals in a field (electric or magnetic)
suggest that such an modulated phase should exist at high chirality. The bulk
phase diagram in the presence of a bulk external field which couples linearly
to the order parameter exhibits a modulated phase inside a loop in the
temperature-field plane, and a homogeneous phase outside. On analyzing the same
model for a semi-infinite system, we find a surprising result; the system
exhibits surface states in a region where the bulk phase is homogeneous (but
close to the modulated region). These states are very different from the
well-known surface states induced either by a surface field or by enhanced
interactions at the surface, for they exist and are energetically favored even
when the sole effect of the surface is to terminate the bulk, as expressed by
free boundary conditions taken at the surface. Near the surface, the
surface-state order parameter is very different from the bulk value (in fact,
it has the opposite sign). When the temperature or the bulk field are varied to
move away from the modulated state, we find a surface phase transition at which
the surface states become energetically unfavorable, though they continue to
exist as metastable states. We then study how a surface field changes the
surface phase diagram.


title: Where is 99% of the condensation energy of Tl_2Ba_2CuO_y coming from?

abstract:
  Anderson's interlayer tunneling model can account for up to 1% of the
condensation energy of Tl_2Ba_2CuO_y. Here we account for the remaining 99%. We
predict an in-plane kinetic energy gain of 1 to 3 meV per planar oxygen in the
optimally doped material when it goes superconducting. It is suggested that the
effect may be most easily detected in underdoped dirty samples.


title:
T=0 Partition Functions for Potts Antiferromagnets on Moebius Strips and
  Effects of Graph Topology

abstract:
  We present exact calculations of the zero-temperature partition function of
the $q$-state Potts antiferromagnet (equivalently the chromatic polynomial) for
Moebius strips, with width $L_y=2$ or 3, of regular lattices and homeomorphic
expansions thereof. These are compared with the corresponding partition
functions for strip graphs with (untwisted) periodic longitudinal boundary
conditions.


title:
Voltage-biased I-V characteristics in the multi-Josephson junction model
  of high T$_c$ superconductor

abstract:
  By use of the multi-Josephson junction model, we investigate voltage-biased
I-V characteristics. Differently from the case of the single junction, I-V
characteristics show a complicated behavior due to inter-layer couplings among
superconducting phase differences mediated by the charging effect. We show that
there exist three characteristic regions, which are identified by jumps and
cusps in the I-V curve. In the low voltage region, the total current is
periodic with trigonometric functional increases and rapid drops. Then a kind
of chaotic region is followed. Above certain voltage, the total current behaves
with a simple harmonic oscillation and the I-V characteristics form a
multi-branch structure as in the current-biased case. The above behavior is the
result of the inter-layer coupling, and may be used to confirm the inter-layer
coupling mechanism of the formation of hysteresis branches.


title:
Distribution of the reflection eigenvalues of a weakly absorbing chaotic
  cavity

abstract:
  The scattering-matrix product SS+ of a weakly absorbing medium is related by
a unitary transformation to the time-delay matrix without absorption. It
follows from this relationship that the eigenvalues of SS+ for a weakly
absorbing chaotic cavity are distributed according to a generalized Laguerre
ensemble.


title: Gaudin Hypothesis for the XYZ Spin Chain

abstract:
  The XYZ spin chain is considered in the framework of the generalized
algebraic Bethe ansatz developed by Takhtajan and Faddeev. The sum of norms of
the Bethe vectors is computed and expressed in the form of a Jacobian. This
result corresponds to the Gaudin hypothesis for the XYZ spin chain.

categories: cond-mat hep-th math-ph math.MP math.QA nlin.SI solv-int


title:
Partially Asymmetric Simple Exclusion Model in the Presence of an
  Impurity on a Ring

abstract:
  We study a generalized two-species model on a ring. The original model [1]
describes ordinary particles hopping exclusively in one direction in the
presence of an impurity. The impurity hops with a rate different from that of
ordinary particles and can be overtaken by them. Here we let the ordinary
particles hop also backward with the rate q. Using Matrix Product Ansatz (MPA),
we obtain the relevant quadratic algebra. A finite dimensional representation
of this algebra enables us to compute the stationary bulk density of the
ordinary particles, as well as the speed of impurity on a set of special
surfaces of the parameter space. We will obtain the phase structure of this
model in the accessible region and show how the phase structure of the original
model is modified. In the infinite-volume limit this model presents a shock in
one of its phases.


title: Reversible electrowetting and trapping of charge: model and experiments

abstract:
  We derive a model for voltage-induced wetting, so-called electrowetting, from
the principle of virtual displacement. Our model includes the possibility that
charge is trapped in or on the wetted surface. Experimentally, we show
reversible electrowetting for an aqueous droplet on an insulating layer of 10
micrometer thickness. The insulator is coated with a highly fluorinated layer
impregnated with oil, providing a contact-angle hysteresis lower than 2
degrees. Analyzing the data with our model, we find that until a threshold
voltage of 240 V, the induced charge remains in the liquid and is not trapped.
For potentials beyond the threshold, the wetting force and the contact angle
saturate, in line with the occurrence of trapping of charge in or on the
insulating layer. The data are independent of the polarity of the applied
electric field, and of the ion type and molarity. We suggest possible
microscopic origins for charge trapping.


title: Material failure time and the fiber bundle model with thermal noise

abstract:
  The statistical properties of failure are studied in a fiber bundle model
with thermal noise. We find that in agreement with recent experiments the
macroscopic failure is produced by a thermal activation of microcracks. Most
importantly the effective temperature of the system is amplified by the spatial
disorder (heterogeneity) of the fiber bundle.


title: Nonequilibrium Growth problems

abstract:
  We discuss the features of nonequilibrium growth problems, their scaling
description and their differences from equilibrium problems. The emphasis is on
the Kardar-Parisi-Zhang equation and the renormalization group point of view.
Some of the recent developments along these lines are mentioned.


title: A model for the fragile-to-strong transition in water

abstract:
  A model based on the existence of two different competing local structures in
water is described. It is shown that it can explain the transition between
fragile and strong behavior that supercooled water has around 220 K. The high
temperature behavior is similar to that observed in standard fragile
glass-formers. The strong behavior at low temperatures is associated with the
existence of a remanent configurational entropy coming from the possibility of
locally choosing between the two possible environments.


title: Nonlinear Current Response of a d-wave Superfluid

abstract:
  Despite several efforts the nonlinear Meissner effect in d-wave
superconductors, as has been discussed by Yip and Sauls in 1992, has not been
verified experimentally in high-Tc superconductors at present. Here, we
reinvestigate the nonlinear response expected in a d-wave superconductor. While
the linear H field dependence of the penetration depth, predicted by Yip and
Sauls, is restricted by the lower critical field and can be masked by nonlocal
effects, we argue that the upturn of the nonlinear coefficient of the quadratic
field dependence is more stable and remains observable over a broader range of
parameters. We investigate this by studying the influence of nonmagnetic
impurities on the nonlinear response. We discuss the difficulties of observing
this intrinsic d-wave signature in present day high-Tc films and single
crystals.


title: Spin Dynamics of Rare Earth Ions in Phosphate Laser Glasses

abstract:
  The spin dynamics of rare earth ions (Er, Nd, Sm and Gd) in heavily doped
phosphate glasses have been investigated using 31P NMR. Correlation times in
the range 10^-2 to 10^-12 s have been obtained for temperatures between 4 K and
100 K. No evidence of spin-spin coupling between the ions has been found and
spin relaxation occurs via conventional phonon processes, including the Orbach
process.
  A model involving inhomogeneous broadening of the NMR resonance lines, with
distinct sample regions corresponding to the presence or absence of nuclear
spin diffusion, has been used in extracting the electron spin correlation times
from the NMR measurements.


title: From Molecular Dynamics to Dissipative Particle Dynamics

abstract:
  A procedure is introduced for deriving a coarse-grained dissipative particle
dynamics from molecular dynamics. The rules of the dissipative particle
dynamics are derived from the underlying molecular interactions, and a Langevin
equation is obtained that describes the forces experienced by the dissipative
particles and specifies the associated canonical Gibbs distribution for the
system.


title:
Topological transitions and freezing in XY models and Coulomb gases with
  quenched disorder: renormalization via traveling waves

abstract:
  We study the two dimensional XY model with quenched random phases and its
Coulomb gas formulation. A novel renormalization group (RG) method is developed
which allows to study perturbatively the glassy low temperature XY phase and
the transition at which frozen topological defects (vortices) proliferate. This
RG approach is constructed both from the replicated Coulomb gas and,
equivalently without the use of replicas, using the probability distribution of
the local disorder (random defect core energy). By taking into account the
fusion of environments (i.e charge fusion in the replicated Coulomb gas) this
distribution is shown to obey a Kolmogorov's type (KPP) non linear RG equation
which admits travelling wave solutions and exhibits a freezing phenomenon
analogous to glassy freezing in Derrida's random energy models. The resulting
physical picture is that the distribution of local disorder becomes broad below
a freezing temperature and that the transition is controlled by rare favorable
regions for the defects, the density of which can be used as the new
perturbative parameter. The determination of marginal directions at the
disorder induced transition is shown to be related to the well studied front
velocity selection problem in the KPP equation and the universality of the
novel critical behaviour obtained here to the known universality of the
corrections to the front velocity. Applications to other two dimensional
problems are mentionned at the end.


title:
Dynamics and statistics of simple models with infinite-range attractive
  interaction

abstract:
  In this paper we review a series of results obtained for 1D and 2D simple
N-body dynamical models with infinite-range attractive interactions and without
short distance singularities. The free energy of both models can be exactly
obtained in the canonical ensemble, while microcanonical results can be derived
from numerical simulations. Both models show a phase transition from a low
energy clustered phase to a high energy gaseous state, in analogy with the
models introduced in the early 70's by Thirring and Hertel. The phase
transition is second order for the 1D model, first order for the 2D model.
Negative specific heat appears in both models near the phase transition point.
For both models, in the presence of a negative specific heat, a cluster of
collapsed particles coexists with a halo of higher energy particles which
perform long correlated flights, which lead to anomalous diffusion. The
dynamical origin of the "superdiffusion" is different in the two models, being
related to particle trapping and untrapping in the cluster in 1D, while in 2D
the channelling of particles in an egg-crate effective potential is responsible
of the effect. Both models are Lyapunov unstable and the maximal Lyapunov
exponent $\lambda$ has a peak just in the region preceeding the phase
transition. Moreover, in the low energy limit $\lambda$ increases
proportionally to the square root of the internal energy, while in the high
energy region it vanishes as $N^{-1/3}$.

categories: cond-mat.stat-mech astro-ph chao-dyn cond-mat.dis-nn hep-th nlin.CD


title: Magnetic impurities in gapless Fermi systems: perturbation theory

abstract:
  We consider a symmetric Anderson impurity model, with a soft-gap
hybridization vanishing at the Fermi level with a power law r > 0. Three facets
of the problem are examined. First the non-interacting limit, which despite its
simplicity contains much physics relevant to the U > 0 case: it exhibits both
strong coupling (SC) states (for r < 1) and local moment (LM) states (for r >
1), with characteristic signatures in both spectral properties and
thermodynamic functions. Second, we establish general conditions upon the
interaction self-energy for the occurence of a SC state for U > 0. This leads
to a pinning theorem, whereby the modified spectral function is pinned at the
Fermi level for any U where a SC state exists; it generalizes to arbitrary r
the familiar pinning condition for the normal r = 0 Anderson model. Finally, we
consider explicitly spectral functions at the simplest level: second order
perturbation theory in U, which we conclude is applicable for r < 1/2 and r > 1
but not for 1/2 < r < 1. Characteristic spectral features observed in numerical
renormalization group calculations are thereby recovered, for both SC and LM
phases; and for the SC state the modified spectral functions are found to
contain a generalized Abrikosov-Suhl resonance exhibiting a characteristic
low-energy Kondo scale with increasing interaction strength.


title: Stokes' drift: a rocking ratchet

abstract:
  We derive the explicit analytic expression for the Stokes' drift in one
dimension in the presence of a dichotomic Markov forcing. For small amplitudes
of the forcing, the drift is enhanced, but the enhancement is reduced with
increasing frequency of the forcing. On the other hand, a reduction of the
drift or even a flux reversal can be induced at larger amplitudes, while the
flux is now found to be an increasing function of the perturbation frequency.


title: Tunable Charge Density Wave Transport in a Current-Effect Transistor

abstract:
  The collective charge density wave (CDW) conduction is modulated by a
transverse single-particle current in a transistor-like device. Nonequilibrium
conditions in this geometry lead to an exponential reduction of the depinning
threshold, allowing the CDWs to slide for much lower bias fields. The results
are in excellent agreement with a recently proposed dynamical model in which
''wrinkles'' in the CDW wavefronts are ''ironed'' by the transverse current.
The experiment might have important implications for other driven periodic
media, such as moving vortex lattices or ''striped phases'' in high-Tc
superconductors.


title: Surface Excitations in a Bose-Einstein Condensate

abstract:
  Surface modes in a Bose-Einstein condensate of sodium atoms have been
studied. We observed excitations of standing and rotating quadrupolar and
octopolar modes. The modes were excited with high spatial and temporal
resolution using the optical dipole force of a rapidly scanning laser beam.
This novel technique is very flexible and should be useful for the study of
rotating Bose-Einstein condensates and vortices.


title: Metastability and paramagnetism in superconducting mesoscopic disks

abstract:
  A projected order parameter is used to calculate, not only local minima of
the Ginzburg-Landau energy functional, but also saddle points or energy
barriers responsible for the metastabilities observed in superconducting
mesoscopic disks (Geim et al. Nature {\bf 396}, 144 (1998)). We calculate the
local minima magnetization and find the energetic instability points between
vortex configurations with different vorticity. We also find that, for any
vorticity, the supercurrent can reverse its flow direction on decreasing the
magnetic field before one vortex can escape.


title: A ratchet driven by quasimonochromatic noise

abstract:
  The currents generated by noise-induced activation processes in a periodic
potential are investigated analytically, by digital simulation and by
performing analogue experiments.


title:
Theoretical Continuous Equation Derived from the Microscopic Dynamics
  for Growing Interfaces in Quenched Media

abstract:
  We present an analytical continuous equation for the Tang and Leschhorn model
[Phys. Rev A {\bf 45}, R8309 (1992)] derived from his microscopic rules using a
regularization procedure. As well in this approach the nonlinear term $(\nabla
h)^2$ arises naturally from the microscopic dynamics even if the continuous
equation is not the Kardar-Parisi-Zhang equation [Phys. Rev. Lett. {\bf 56},
889 (1986)] with quenched noise (QKPZ). Our equation looks like a QKPZ but with
multiplicative quenched and thermal noise. The numerical integration of our
equation reproduce the scaling exponents of the roughness of this directed
percolation depinning model.


title:
Spin-orbit coupling in the actinide elements: a critical evaluation of
  theoretical equilibrium volumes

abstract:
  In a recent paper by Jones et al., it is argued, based on FP-LAPW
band-structure calculations, that previous calculations of ground-state
properties for actinides, using the FP-LMTO method implemented by J. M. Wills,
are in error. We demonstrate in this paper that the conclusions of Jones et.
al. are unfounded. Calculations using the FP-LMTO method are compared with
calculations performed with the FP-LAPW method and, in contradiction to
statements by Jones et. al., they agree very well. The limitation of the
present implementation of the spin-orbit coupling is discussed, where
especially the 6p states pose a difficulty. The observed discrepancy (0-10%)
between the FP-LMTO calculations and the recent FP-LAPW calculation of Jones
et. al. is shown to be due to the choice of muffin-tin radius in the
calculations. We argue that the choice of a constant muffin-tin radius, common
for all calculated volumes, is less satisfactory compared to the choice of a
muffin-tin radius that scales with the volume, especially when a large volume
interval and open packed structures are considered. The conclusion of Jones et
al. that alpha-Pu has partially delocalized 5f electrons is argued to be
erroneous.


title: A Local Moment Approach to magnetic impurities in gapless Fermi systems

abstract:
  A local moment approach is developed for the single-particle excitations of a
symmetric Anderson impurity model (AIM), with a soft-gap hybridization
vanishing at the Fermi level with a power law r > 0. Local moments are
introduced explicitly from the outset, and a two-self-energy description is
employed in which the single-particle excitations are coupled dynamically to
low-energy transverse spin fluctuations. The resultant theory is applicable on
all energy scales, and captures both the spin-fluctuation regime of strong
coupling (large-U), as well as the weak coupling regime. While the primary
emphasis is on single particle dynamics, the quantum phase transition between
strong coupling (SC) and (LM) phases can also be addressed directly; for the
spin-fluctuation regime in particular a number of asymptotically exact results
are thereby obtained. Results for both single-particle spectra and SC/LM phase
boundaries are found to agree well with recent numerical renormalization group
(NRG) studies. A number of further testable predictions are made; in
particular, for r < 1/2, spectra characteristic of the SC state are predicted
to exhibit an r-dependent universal scaling form as the SC/LM phase boundary is
approached and the Kondo scale vanishes. Results for the `normal' r = 0 AIM are
moreover recovered smoothly from the limit r -> 0, where the resultant
description of single-particle dynamics includes recovery of Doniach-Sunjic
tails in the Kondo resonance, as well as characteristic low-energy Fermi liquid
behaviour.


title:
Reply on `Comment on ``Comment on `Macroscopic Equation for the
  Roughness of Growing Interfaces in Quenched Disorder'' '

abstract: Reply to ``Comment on [Phys. Rev. Lett. 81, 630 (1998)]''


title: Electron Spin Resonance of defects in the Haldane System Y(2)BaNiO(5)

abstract:
  We calculate the electron paramagnetic resonance (EPR) spectra of the
antiferromagnetic spin-1 chain compound Y(2)BaNi(1-x)Mg(x)O(5) for different
values of x and temperature T much lower than the Haldane gap (~100K). The
low-energy spectrum of an anisotropic Heisenberg Hamiltonian, with all
parameters determined from experiment, has been solved using DMRG. The observed
EPR spectra are quantitatively reproduced by this model. The presence of
end-chain S=1/2 states is clearly observed as the main peak in the spectrum and
the remaining structure is completely understood.


title: Low temperature magnetic hysteresis in Mn$_{12}$ acetate single crystals

abstract:
  Precise magnetic hysteresis measurements of small single crystals of
Mn$_{12}$ acetate of spin 10 have been conducted down to 0.4 K using a high
sensitivity Hall magnetometer. At higher temperature (>1.6K) step-like changes
in magnetization are observed at regularly spaced magnetic field intervals, as
previously reported. However, on lowering the temperature the steps in
magnetization shift to higher magnetic fields, initially gradually. These
results are consistent with the presence of a second order uniaxial magnetic
anisotropy, first observed by EPR spectroscopy, and thermally assisted
tunnelling with tunnelling relaxation occurring from levels of progressively
lower energy as the temperature is reduced. At lower temperature an abrupt
shift in step positions is found. We suggest that this shift may be the first
evidence of an abrupt, or first-order, transition between thermally assisted
and pure quantum tunnelling, suggested by recent theory.


title: Bandgap renormalization and excitonic binding in T-shaped quantum wires

abstract:
  We calculate the electronic structure for a modulation doped and gated
T-shaped quantum wire using density functional theory. We calculate the bandgap
renormalization as a function of the density of conduction band electrons,
induced by the donor layer and/or the gate, for the translationally invariant
wire, incorporating all growth and geometric properties of the structure
completely. We show that most of the bandgap renormalization arises from
exchange-correlation effects, but that a small shift also results from the
difference of wave function evolution between electrons and holes. We calculate
the binding energy of excitons in the wire, which breaks translational
invariance, using a simpler, cylindrical model of the wire. For a single hole
and a one dimensional electron gas of density n_e, screening of the exciton
binding energy is shown to approximately compensate for bandgap
renormalization, suggesting that the recombination energy remains approximately
constant with n_e, in agreement with experiment. We find that the nature of
screening, as treated within our non-linear model, is significantly different
from that of the various linear screening treatments, and the orthogonality of
free carrier states with the bound electron states has a profound effect on the
screening charge. We find that the electron and hole remain bound for all
densities up to about 3 x 10^6 cm^{-1} and that, as n_e increases from zero,
trion and even ``quadron'' formation becomes allowed.


title: Understanding depletion forces beyond entropy

abstract:
  The effective interaction energy of a colloidal sphere in a suspension
containing small amounts of non-ionic polymers and a flat glass surface has
been measured and calculated using total internal reflection microscopy (TIRM)
and a novel approach within density functional theory (DFT), respectively.
Quantitative agreement between experiment and theory demonstrates that the
resulting repulsive part of the depletion forces cannot be interpreted entirely
in terms of entropic arguments but that particularly at small distances
($\lesssim$ 100 nm) attractive dispersion forces have to be taken into account.


title:
Nonequilibrium phase transitions induced by multiplicative noise:
  effects of self-correlation

abstract:
  A recently introduced lattice model, describing an extended system which
exhibits a reentrant (symmetry-breaking, second-order) noise-induced
nonequilibrium phase transition, is studied under the assumption that the
multiplicative noise leading to the transition is colored. Within an effective
Markovian approximation and a mean-field scheme it is found that when the
self-correlation time of the noise is different from zero, the transition is
also reentrant with respect to the spatial coupling D. In other words, at
variance with what one expects for equilibrium phase transitions, a large
enough value of D favors disorder. Moreover, except for a small region in the
parameter subspace determined by the noise intensity and D, an increase in the
self-correlation time usually preventsthe formation of an ordered state. These
effects are supported by numerical simulations.


title:
A non perturbative approach of the principal chiral model between two
  and four dimensions

abstract:
  We investigate the principal chiral model between two and four dimensions by
means of a non perturbative Wilson-like renormalization group equation. We are
thus able to follow the evolution of the effective coupling constants within
this whole range of dimensions without having recourse to any kind of small
parameter expansion. This allows us to identify its three dimensional critical
physics and to solve the long-standing discrepancy between the different
perturbative approaches that characterizes the class of models to which the
principal chiral model belongs.


title: Disorder Driven Lock-In Transitions of CDWs and Related Structures

abstract:
  Thermal fluctuations are known to play an important role in low-dimensional
systems which may undergo incommensurate-commensurate or (for an accidentally
commensurate wavevector) lock-in transitions. In particular, an intermediate
floating phase with algebraically decaying correlations exists only in D=2
dimensions, whereas in higher dimensions most features of the phase diagram are
mean-field like.
  Here we will show, that the introduction of frozen-in disorder leads to
strong fluctuation effects even in D<4 dimensions. For commensurate wavevectors
the lattice pinning potential dominates always over weak impurity pinning if p
\le p_c=6/\pi (D=3), where p denotes the degeneracy of the commensurate phase.
For larger p a disorder driven continuous transition between a long-range
ordered locked-in phase and quasi-long-range ordered phase, dominated by
impurity pinning, occurs. Critical exponents of this transition, which is
characterized by a zero temperature fixed point, are calculated within an
expansion in 4-D. The generalization to incommensurate wavevectors will be
discussed. If the modulation in the quasi-long-range ordered phase has
hexagonal symmetry, as e.g.for flux-line lattices, the algebraic decay is
non-universal and depends on the Poisson ratio of the elastic constants. Weakly
driven transport is dominated by thermally activated creep in both phases, but
with different creep exponents.


title: Transport measurements in BSCCO-2212 in high magnetic fields

abstract:
  We present results for the in-plane resistivity as a function of temperature
for clean single crystal samples of BSCCO-2212 in applied magnetic fields above
0.5 T. Several fixed angles between the magnetic field and the crystal c-axis
were studied. The data were analysed in terms of the vortex-glass (VG) model,
as suggested by previous experimental studies. Several features of the data
were found to be inconsistent with the VG scenario. Recent theoretical studies
have challenged the VG interpretation, and these are discussed in terms of our
data.


title:
Anomalous Self-Energy Effects of the B_1g Phonon in
  Y_{1-x}(Pr,Ca)_xBa_2Cu_3O_7 Films

abstract:
  In Raman spectra of cuprate superconductors the gap shows up both directly,
via a redistribution of the electronic background, the so-called "2Delta
peaks", and indirectly, e.g. via the renormalization of phononic excitations.
We use a model that allows us to study the redistribution and the related
phonon self-energy effects simultaneously. We apply this model to the B_1g
phonon of Y_{1-x}(Pr,Ca)_xBa_2Cu_3O_7 films, where Pr or Ca substitution
enables us to investigate under- and overdoped samples. While various
self-energy effects can be explained by the strength and energy of the 2\Delta
peaks, anomalies remain. We discuss possible origins of these anomalies.


title: Electron-Phonon Coupling Deduced from Phonon Line Shapes

abstract:
  We investigate the Fano-type line shape of the Ba mode of
Y_{1-x}Ca_xBa_2Cu_3O_{6+y} films observed in Raman spectra with A_1g symmetry.
The line shape is described with an extended Fano formula that allows us to
obtain the bare phonon parameters and the self-energy effects based on a
phenomenological description of the real and imaginary part of the low-energy
electronic response. It turns out that the phonon intensity originates almost
entirely from a coupling to this electronic response with negligible
contributions from interband (high-energy) electronic excitations. In the
normal state we obtain a measure of the electron-phonon coupling via the
mass-enhancement factor lambda. We find lambda=6.8 +/- 0.5 % around optimum
doping and only weak changes of the self-energy in the superconducting state.
With increased disorder at the Ba site we find a decreased intensity of the Ba
mode which we can relate to a decreased electron-phonon coupling.


title:
Experimental evidence of multiple magnetic relaxation processes in Mn12
  acetate and Mn12 2-Cl benzoate

abstract:
  The Mn cluster complexes Mn12(RCOO)16(H2O)4O12 with R=CH3 and 2ClPh
(hereafter referred to as Mn12 acetate and Mn12 2-Cl benzoate, respectively)
exhibit macroscopic magnetic quantum tunneling. Dynamical magnetic measurements
indicate for both samples that the anisotropy energy barrier is nearly the same
and that the relaxation times have sharp minima at the same resonant fields.
However, we show that for both compounds a minority portion of the clusters
does not undergo this main relaxation process. From ac-susceptibility
experiments, we report evidence of a continuous distribution of faster
relaxation times, giving rise to a second relaxation regime. The zero-field
effective relaxation times of such a distribution follow an Arrhenius law with
energy barrier of approximately 23 and 30 K for the acetate and the benzoate,
respectively. In the case of Mn12 2-Cl benzoate, the field dependence of the
effective relaxation time points towards the existence of spin tunneling with
Hz=0 being a resonant field. The existence of a broad distribution of energy
barriers is corroborated in magnetization experiments by the observation of
logarithmic relaxation at short times.


title: Statistical Physics of Irregular Low-Density Parity-Check Codes

abstract:
  Low-density parity-check codes with irregular constructions have been
recently shown to outperform the most advanced error-correcting codes to date.
In this paper we apply methods of statistical physics to study the typical
properties of simple irregular codes.
  We use the replica method to find a phase transition which coincides with
Shannon's coding bound when appropriate parameters are chosen.
  The decoding by belief propagation is also studied using statistical physics
arguments; the theoretical solutions obtained are in good agreement with
simulations. We compare the performance of irregular with that of regular codes
and discuss the factors that contribute to the improvement in performance.


title: Dilatancy and Friction in sheared granular media

abstract:
  We introduce a simple model to describe the frictional properties of granular
media under shear. We model the friction force in term of the horizontal
velocity v and the vertical position z of the slider, interpreting z as a
constitutive variable characterizing the contact. Dilatancy is shown to play an
essential role in the dynamic, inducing a stick-slip instability at low
velocity. We compute the phase diagram, analyze numerically the model for a
wide range of parameters and compare our results with experiments on dry and
wet granular media, obtaining a good agreement. In particular, we reproduce the
hysteretic velocity dependence of the frictional force.


title:
Electronic Raman Scattering and Phonon Self-Energy Effects in the R-123
  System: Signatures of Gap and Pseudogap

abstract:
  Using Raman spectroscopy and transport measurements we investigate thin
epitaxial films of Y{1-x}(Pr,Ca)xBa2Cu3O{6+y}. We explore the electronic Raman
responses obtained after subtraction of phononic excitations, and the 2Delta
peaks that form out of the electronic background below Tc. We find that the
energy of the B1g 2Delta peak increases monotonically with decreasing doping
until the peaks become unresolvable. In contrast, the peaks in A1g symmetry
follow Tc being resolvable in the Pr-doped films. The B2g responses are weak
and a 2Delta peak is only detected at the highest doping level. As a
consequence of strong electron-phonon coupling, the B1g phonon at 340 cm^-1
exhibits a pronounced Fano-type line shape. We use a phenomenological model to
describe the line shape that takes into account real and imaginary part of the
electronic response. It allows us to obtain the self-energy corrections and the
mass- enhancement factor lambda as a measure of the coupling. In the normal
state we find lambda=0.015 around optimal doping and decreasing values with
decreasing doping. The electron-phonon coupling increases strongly below T_c in
overdoped samples in which the B1g 2Delta peaks appear in the vicinity of the
phonon. Self-energy effects observed in the superconducting state can only
partly be assigned to the redistributing electronic response. Anomalies with
respect to frequency, linewidth, and intensity remain. They appear at
increasing tempera- tures with decreasing doping and we provide evidence that
they are connected to the presence of the pseudogap. We supplement our study by
a comparison with single crystal data and investigate the influence of
site-substitutional disorder on the electronic response and the electron-phonon
interaction.


title: Hall effect of epitaxial double-perovskite Sr_2FeMoO_6 thin films

abstract:
  We prepared high epitaxial thin films of the compound Sr_2FeMoO_6 with narrow
rocking curves by pulsed laser deposition. The diagonal and nondiagonal
elements of the resistivity tensor were investigated at temperatures from 4 K
up to room temperature in magnetic fields up to 8 T. An electronlike ordinary
Hall effect and a holelike anomalous Hall contribution are observed. Both
coefficients have reversed sign compared to the colossal magnetoresistive
manganites. We found at 300 K an ordinary Hall coefficent of -1.87x10^{-10}
m^3/As, corresponding to a nominal charge carrier density of four electrons per
formula unit. At low temperature only a small negative magnetoresistance is
observed which vanishes at higher temperatures. The temperature coefficient of
the resistivity is negative over the whole temperature range. A Kondo like
behavior is observed below 30 K while above 100 K variable range hopping like
transport occurs.


title:
Unusual thermodynamical and transport signatures of short coherence
  length superconductors: A BCS Bose-Einstein crossover approach

abstract:
  In this paper we present predictions for thermodynamic and transport
properties of a BCS Bose-Einstein crossover theory, below Tc, which theory
satisfies the reasonable constraints that it yield (i) the Leggett ground state
and (ii) BCS theory at weak coupling and all temperatures. The nature of the
strong coupling limit is inferred, along with the behavior of the Knight shift,
superfluid density, and specific heat. Comparisons with existing data on short
coherence length superconductors, such as organic and high Tc systems, are
presented, which provide some support for the present picture.


title: Theory of PbTiO3, BaTiO3, and SrTiO3 Surfaces

abstract:
  First-principles total-energy calculations are carried out for (001) surfaces
of the cubic perovskite ATiO3 compounds PbTiO3, BaTiO3, and SrTiO3. Both
AO-terminated and TiO2-terminated surfaces are considered, and fully-relaxed
atomic configurations are determined. In general, BaTiO3 and SrTiO3 are found
to have a rather similar behavior, while PbTiO3 is different in many respects
because of the partially covalent character of the Pb-O bonds. PbTiO3 and
BaTiO3 are ferroelectrics, and the influence of the surface upon the
ferroelectric distortions is studied for the case of a tetragonal ferroelectric
distortion parallel to the surface. The surface relaxation energies are found
to be substantial, i.e., many times larger than the bulk ferroelectric well
depth. Nevertheless, the influence of the surface upon the ferroelectric order
parameter is modest, and is qualitatively as well as quantitatively different
for the two materials. Surface energies and electronic properties are also
computed. It is found that for BaTiO3 and SrTiO3 surfaces, both AO-terminated
and TiO2-terminated surfaces can be thermodynamically stable, whereas for
PbTiO3 only the PbO surface termination is stable.


title:
The virtual crystal approximation revisited: Application to dielectric
  and piezoelectric properties of perovskites

abstract:
  We present an approach to the implementation of the virtual crystal
approximation (VCA) for the study of properties of solid solutions in the
context of density-functional methods. Our approach can easily be applied to
any type of pseudopotential, and also has the advantage that it can be used to
obtain estimates of the atomic forces that would arise if the real atoms were
present, thus giving insight into the expected displacements in the real alloy.
We have applied this VCA technique within the Vanderbilt
ultrasoft-pseudopotential scheme to predict dielectric and piezoelectric
properties of the Pb(Zr[0.5],Ti[0.5])O[3] solid solution in its paraelectric
and ferroelectric phases, respectively. Comparison with calculations performed
on ordered alloy supercells and with data on parents compounds demonstrates the
adequacy of using the VCA for perovskite solid solutions. In particular, the
VCA approach reproduces the anomalous Born effective charges and the large
value of the piezoelectric coefficients.


title:
Possible charge inhomogeneities in the CuO2 planes of YBa2Cu3O6+x
  (x=0.25, 0.45, 0.65, 0.94) from pulsed neutron diffraction

abstract:
  The atomic pair distribution functions (PDF) of four powder samples of
YBa2Cu3O6+x (x=0.25, 0.45, 0.65, 0.94) at 15 K have been measured by means of
pulsed neutron diffraction. The PDF is modelled using a full-profile fitting
approach to yield structural parameters. In contrast to earlier XAFS work we
find no evidence of a split apical oxygen site. However, a slightly improved
fit over the average crystallographic model results when the planar Cu(2) site
is split along the z-direction. This is interpreted in terms of charge
inhomogeneities in the CuO2 planes.


title:
Quantum fluctuations and the c-axis optical conductivity of High-T_c
  Superconductors

abstract:
  A theory of the frequency dependence of the interplane conductivity of a
strongly anisotropic superconductor is presented. The form of the conductivity
is shown to be a sensitive probe of the strength of quantum and thermal
fluctuations of the phase of the superconducting order parameter. The
temperature dependence of the superfluid stiffness and of the form of the
absorbtion at frequencies of the order of twice the superconducting gap is
shown to depend on the interplay between superconducting pairing, phase
coherence and the mechanism by which electrons are scattered. Measurements of
the c-axis conductivity of high-T_c superconductors are interpreted in terms of
the theory.


title: Gibbs entropy and irreversible thermodynamics

abstract:
  Recently a number of approaches has been developed to connect the microscopic
dynamics of particle systems to the macroscopic properties of systems in
nonequilibrium stationary states, via the theory of dynamical systems. This way
a direct connection between dynamics and Irreversible Thermodynamics has been
claimed to have been found. However, the main quantity used in these studies is
a (coarse-grained) Gibbs entropy, which to us does not seem suitable, in its
present form, to characterize nonequilibrium states. Various simplified models
have also been devised to give explicit examples of how the coarse-grained
approach may succeed in giving a full description of the Irreversible
Thermodynamics. We analyze some of these models pointing out a number of
difficulties which, in our opinion, need to be overcome in order to establish a
physically relevant connection between these models and Irreversible
Thermodynamics.


title: Low Temperature AC Conductivity in BSCCO (2212)

abstract:
  We report measurements of anamolously large dissipative conductivities in
BiSrCaCuO(2212) at low temperatures. We have measured the complex conductivity
of BSCCO thin films at 100-600 GHz as a function of doping from the underdoped
to the overdoped state. At low temperatures there exists a residual dissipative
conductivity which scales with the T=0 superfluid density as the doping is
varied. This residual dissipative conductivity is larger than the possible
contribution from a thermal population of quasiparticles at the d-wave gap
nodes.


title: Reentrant Spin-Peierls Transition in Mg-Doped CuGeO_3

abstract:
  We report a synchrotron x-ray scattering study of the diluted spin-Peierls
(SP) material Cu_{1-x}Mg_xGeO_3. In a recent paper we have shown that the SP
dimerization attains long-range order only for x < x_c = 0.022(0.001). Here we
report that the SP transition is reentrant in the vicinity of the critical
concentration x_c. This is manifested by broadening of the SP dimerization
superlattice peaks below the reentrance temperature, T_r, which may mean either
the complete loss of the long-range SP order or the development of a
short-range ordered component within the long-range ordered SP state. Marked
hysteresis and very large relaxation times are found in the samples with Mg
concentrations in the vicinity of x_c. The reentrant transition is likely
related to the competing Neel transition which occurs at a temperature similar
to T_r. We argue that impurity-induced competing interchain interactions play
an essential role in these phenomena.


title:
A New Method to Calculate the Spin-Glass Order Parameter of the
  Two-Dimensional +/-J Ising Model

abstract:
  A new method to numerically calculate the $n$th moment of the spin overlap of
the two-dimensional $\pm J$ Ising model is developed using the identity derived
by one of the authors (HK) several years ago. By using the method, the $n$th
moment of the spin overlap can be calculated as a simple average of the $n$th
moment of the total spins with a modified bond probability distribution. The
values of the Binder parameter etc have been extensively calculated with the
linear size, $L$, up to L=23. The accuracy of the calculations in the present
method is similar to that in the conventional transfer matrix method with about
$10^{5}$ bond samples. The simple scaling plots of the Binder parameter and the
spin-glass susceptibility indicate the existence of a finite-temperature
spin-glass phase transition. We find, however, that the estimation of $T_{\rm
c}$ is strongly affected by the corrections to scaling within the present data
($L\leq 23$). Thus, there still remains the possibility that $T_{\rm c}=0$,
contrary to the recent results which suggest the existence of a
finite-temperature spin-glass phase transition.


title:
Percolative phase separation induced by nonuniformly distributed excess
  oxygens

abstract:
  The zero-field $^{139}$La and $^{55}$Mn nuclear magnetic resonances were
studied in $\rm La_{0.8}Ca_{0.2}MnO_{3+\delta}$ with different oxygen
stoichiometry $\delta$. The signal intensity, peak frequency and line
broadening of the $^{139}$La NMR spectrum show that excess oxygens have a
tendency to concentrate and establish local ferromagnetic ordering around
themselves. These connect the previously existed ferromagnetic clusters
embedded in the antiferromagnetic host, resulting in percolative conduction
paths. This phase separation is not a charge segregation type, but a
electroneutral type. The magnetoresistance peak at the temperature where
percolative paths start to form provides a direct evidence that phase
separation is one source of colossal magnetoresistance effect.


title: Enhancements to the GW space-time method

abstract:
  We describe the following new features which significantly enhance the power
of the recently developed real-space imaginary-time GW scheme (Rieger et al.,
Comp. Phys. Commun. 117, 211 (1999)) for the calculation of self-energies and
related quantities of solids: (i) to fit the smoothly decaying time/energy
tails of the dynamically screened Coulomb interaction and other quantities to
model functions, treating only the remaining time/energy region close to zero
numerically and performing the Fourier transformation from time to energy and
vice versa by a combination of analytic integration of the tails and
Gauss-Legendre quadrature of the remaining part and (ii) to accelerate the
convergence of the band sum in the calculation of the Green's function by
replacing higher unoccupied eigenstates by free electron states (plane waves).
These improvements make the calculation of larger systems (surfaces, clusters,
defects etc.) accessible.


title: Compressible Anisotropic States around the Half-Filled Landau Levels

abstract:
  Using the von Neumann lattice formalism, we study compressible anisotropic
states around the half-filled Landau levels in the quantum Hall system. In
these states the unidirectional charge density wave (UCDW) state seems to be
the most plausible state. The charge density profile and Hartree-Fock energy of
the UCDW are calculated self-consistently. The wave length dependence of the
energy for the UCDW is also obtained numerically. We show that the UCDW is
regarded as a collection of the one-dimensional lattice Fermi-gas systems which
extend to the uniform direction. The kinetic energy of the gas system is
generated dynamically from the Coulomb interaction.


title:
Influence of the cooperative Jahn-Teller effect on the transport- and
  magnetic properties of La_{7/8}Sr_{1/8}MnO_3 single crystals

abstract:
  The low-doped magnetic perovskite La_{7/8}Sr_{1/8}MnO_3 undergoes within the
paramagnetic-semiconducting phase a first-order structural transition due to
antiferrodistorsive ordering of Jahn-Teller deformed MnO_6 octahedra. This
allows to study not only the influence of the spin configuration on the
magneto-transport properties (CMR effect) but also the role of orbital order
and disorder. The orbital ordering transition (at 269 K in zero magnetic field)
causes a doubling of the resistivity (regardless of the CMR effect in applied
magnetic fields) and a drop of the paramagnetic susceptibility. The latter
might be interpreted in terms of a shrinking of spin polarons. External
magnetic fields shift the ordering transition to lower temperatures according
to the field-induced decrease of the carrier localization. The magnetic field -
temperature phase boundary line was investigated by means of magnetoresistance
(up to 12 T) and pulsed-fields magnetization measurements up to 50 T. The
pronounced magnetization anomalies, associated with the phase transition,
vanish for fields exceeding 20 T. This behaviour has been attributed to a
field-induced crossover from antiferrodistorsive order to a
nondistorsive/ferromagnetic orbital configuration.


title: Optically Detected Magnetophonon Resonances in Polar Semiconductors

abstract:
  Magnetophonon resonances are found for $\omega_{c}=\omega_{LO}/N$ with
N=1,2,3 .... in the polaron cyclotron resonance (CR) linewidth and effective
mass of bulk polar semiconductors. The CR mass and the linewidth are obtained
from the full polaron magneto-optical absorption spectrum which are calculated
using the memory function technique. The amplitude of the resonant peak in the
linewidth can be described by an exponential law at low temperature.


title:
Correlation length-exponent relation for the two-dimensional random
  Ising model

abstract:
  We consider the two-dimensional (2d) random Ising model on a diagonal strip
of the square lattice, where the bonds take two values, $J_1>J_2$, with equal
probability. Using an iterative method, based on a successive application of
the star-triangle transformation, we have determined at the bulk critical
temperature the correlation length along the strip, $\xi_L$, for different
widths of the strip, $L \le 21$. The ratio of the two lengths, $\xi_L/L=A$, is
found to approach the universal value, $A=2/\pi$ for large $L$, independent of
the dilution parameter, $J_1/J_2$. With our method we have demonstrated with
high numerical precision, that the surface correlation function of the 2d
dilute Ising model is self-averaging, in the critical point conformally
coovariant and the corresponding decay exponent is $\eta_{\parallel}=1$.


title: Parallel flow in Hele-Shaw cells with ferrofluids

abstract:
  Parallel flow in a Hele-Shaw cell occurs when two immiscible liquids flow
with relative velocity parallel to the interface between them. The interface is
unstable due to a Kelvin-Helmholtz type of instability in which fluid flow
couples with inertial effects to cause an initial small perturbation to grow.
Large amplitude disturbances form stable solitons. We consider the effects of
applied magnetic fields when one of the two fluids is a ferrofluid. The
dispersion relation governing mode growth is modified so that the magnetic
field can destabilize the interface even in the absence of inertial effects.
However, the magnetic field does not affect the speed of wave propagation for a
given wavenumber. We note that the magnetic field creates an effective
interaction between the solitons.


title: Supercurrent Stability in a Quasi-1D Weakly Interacting Bose Gas

abstract:
  We discuss a possibility of observing superfluid phenomena in a quasi-1D
weakly interacting Bose gas at finite temperatures. The weakness of interaction
in combination with generic properties of 1D liquids can result in a situation
when relaxational time of supercurrent is essentially larger than the time of
experimental observation, and the behavior of the system is indistinguishable
from that of a genuine superfluid.


title: The 6-vertex model of hydrogen-bonded crystals with bond defects

abstract:
  It is shown that the percolation model of hydrogen-bonded crystals, which is
a 6-vertex model with bond defects, is completely equivalent with an 8-vertex
model in an external electric field. Using this equivalence we solve exactly a
particular 6-vertex model with bond defects. The general solution for the
Bethe-like lattice is also analyzed.


title: Search for a quantum phase transition in U(Pt_(1-x)Pd_x)_3

abstract:
  Pd in U(Pt_{1-x}Pd_x)_3 suppresses the superconducting T_c to 0 K at critical
concentration x_c of 0.007 and induces a conventional AFM state for x > x_c.
The resistivity below 1 K shows a deviation from Fermi liquid behavior
described by a power law where the exponent ranges from 2 at x=0 to 1.6 for x =
x_c. This suggests that a quantum phase transition (QPT) may exist near x_c
associated with either the magnetic or superconducting transition temperature =
0 K. Transport for a sample with x = 0.004 < x_c has constant exponent of 1.77
as increasing pressure suppresses T_c to 0 K, suggesting that if a QPT exists
it may be associated with the magnetic transition.


title: Bosonization, vicinal surfaces, and hydrodynamic fluctuation theory

abstract:
  Through a Euclidean path integral we establish that the density fluctuations
of a Fermi fluid in one dimension are related to vicinal surfaces and to the
stochastic dynamics of particles interacting through long range forces with
inverse distance decay. In the surface picture one easily obtains the Haldane
relation and identifies the scaling exponents governing the low energy,
Luttinger liquid behavior. For the stochastic particle model we develop a
hydrodynamic fluctuation theory, through which in some cases the large distance
Gaussian fluctuations are proved nonperturbatively.


title:
Liquid 4He near the superfluid transition in the presence of a heat
  current and gravity

abstract:
  The effects of a heat current and gravity in liquid 4He near the superfluid
transition are investigated for temperatures above and below T_lambda. We
present a renormalization-group calculation based on model F for the Green's
function in a self-consistent approximation which in quantum many-particle
theory is known as the Hartree approximation. The approach can handle a zero
average order parameter above and below T_lambda and includes effects of
vortices. We calculate the thermal conductivity and the specific heat for all
temperatures T and heat currents Q in the critical regime. Furthermore, we
calculate the temperature profile. Below T_lambda we find a second correlation
length which describes the dephasing of the order parameter field due to
vortices. We find dissipation and mutual friction of the superfluid-normal
fluid counterflow and calculate the Gorter-Mellink coefficient A. We compare
our theoretical results with recent experiments.


title:
Universal relationship between the penetration depth and the
  normal-state conductivity in YBaCuO

abstract:
  The absolute values of the conductivity in the normal state sigma_n and of
the low temperature penetration depths lambda(0) were measured for a number of
different samples of the YBaCuO family. We found a striking correlation between
sigma_n and 1/lambda^2, regardless of doping, oxygen reduction or defects, thus
providing a simple method to predict the superconducting penetration depth and
to have an estimate of the sample quality by measuring the normal-state
conductivity.


title:
Non-equilibrium steady state of the superfluid mixtures of helium
  isotopes at classical temperatures

abstract:
  The size effect in the gas of impuritons of the superfluid mixtures of helium
isotopes is investigated by taking into consideration the contribution of
thermal excitations. The solution is obtained for the set of kinetic equations
describing a non-equilibrium state of the phonon-impuriton system of a
superfluid mixture situated in the volume filled with the macroparticles. It
allows to find the condition describing a steady, thermodynamically
non-equilibrium state of $^3{He}-^4{He}$ mixture in confined geometry. The
Knudsen effect in the gas of impuritons of a superfluid mixture is investigated
by taking into account the contribution of phonons. A model for the collision
operator has been proposed to analyze the exact results in the context of the
concrete physical situations. An experiment for the investigation of the
Knudsen effect in a superfluid mixture of helium isotopes is proposed.


title: Spin-1/2 periodic nonuniform XX chains and the spin-Peierls instability

abstract:
  Using continued fractions we obtained the exact result for the density of
magnon states of the regularly alternating spin-1/2 XX chain with
Dzyaloshinskii-Moriya interaction. We examined the stability of the magnetic
chain with respect to the spin-Peierls dimerization.


title: Magnons in real materials from density-functional theory

abstract:
  We present an implementation of the adiabatic spin-wave dynamics of Niu and
Kleinman. This technique allows to decouple the spin and charge excitations of
a many-electron system using a generalization of the adiabatic approximation.
The only input for the spin-wave equations of motion are the energies and Berry
curvatures of many-electron states describing frozen spin spirals. The latter
are computed using a newly developed technique based on constrained
density-functional theory, within the local spin density approximation and the
pseudo-potential plane-wave method. Calculations for iron show an excellent
agreement with experiments.


title:
Chromatic Polynomials and their Zeros and Asymptotic Limits for Families
  of Graphs

abstract:
  Let $P(G,q)$ be the chromatic polynomial for coloring the $n$-vertex graph
$G$ with $q$ colors, and define $W=\lim_{n \to \infty}P(G,q)^{1/n}$. Besides
their mathematical interest, these functions are important in statistical
physics. We give a comparative discussion of exact calculations of $P$ and $W$
for a variety of recursive families of graphs, including strips of regular
lattices with various boundary conditions and homeomorphic expansions thereof.
Generalizing to $q \in {\mathbb C}$, we determine the accumulation sets of the
chromatic zeros constituting the continuous loci of points on which $W$ is
nonanalytic. Various families of graphs with the property that the chromatic
zeros and/or their accumulation sets (i) include support for $Re(q) < 0$; (ii)
bound regions and pass through $q=0$; and (iii) are noncompact are discussed,
and the role of boundary conditions is analyzed. Some corresponding results are
presented for Potts model partition functions for nonzero temperature,
equivalent to the full Tutte polynomials for various families of graphs.


title: Role of confined phonons in thin film superconductivity

abstract:
  We calculate the critical temperature $T_c$ and the superconducting energy
gaps $\Delta_n$ of a thin film superconductor system, where $\Delta_n$ is the
superconducting energy gap of the $n$-th subband. Since the quantization of
both the electron energy and phonon spectrum arises due to dimensional
confinement in one direction, the effective electron-electron interaction
mediated by the quantized confined phonons is different from that mediated by
the bulk phonon, leading to the modification of $T_c$ in the thin film system.
We investigate the dependence of $T_c$ and $\Delta_n$ on the film thickness $d$
with this modified interaction.


title: Magnetic Domains and Stripes in the Spin-Fermion Model for Cuprates

abstract:
  Monte Carlo simulations applied to the Spin-Fermion model for cuprates show
the existence of antiferromagnetic spin domains and charge stripes upon doping.
The stripes are partially filled, with a filling of approximately 1/2 hole per
site, and they separate spin domains with a $\pi$ phase shift among them. The
stripes observed run either along the x or y axes and they are separated by a
large energy barrier. No special boundary conditions or external fields are
needed to stabilize these structures at low temperatures. When magnetic
incommensurate peaks are observed at momentum $\pi(1,1-\delta)$ and symmetrical
points, charge incommensurate peaks appear at $(0,2 \delta)$ and symmetrical
points, as experimentally observed. The strong charge fluctuations responsible
for the formation of the stripes also induce a pseudogap in the density of
states.


title:
Destruction of the Mott Insulating Ground State of Ca_2RuO_4 by a
  Structural Transition

abstract:
  We report a first-order phase transition at T_M=357 K in single crystal
Ca_2RuO_4, an isomorph to the superconductor Sr_2RuO_4. The discontinuous
decrease in electrical resistivity signals the near destruction of the Mott
insulating phase and is triggered by a structural transition from the low
temperature orthorhombic to a high temperature tetragonal phase. The magnetic
susceptibility, which is temperature dependent but not Curie-like decreases
abruptly at TM and becomes less temperature dependent. Unlike most insulator to
metal transitions, the system is not magnetically ordered in either phase,
though the Mott insulator phase is antiferromagnetic below T_N=110 K.


title: Antiresonances in Molecular Wires

abstract:
  We present analytic and numerical studies based on Landauer theory of
conductance antiresonances of molecular wires. Our analytic treatment is a
solution of the Lippmann-Schwinger equation for the wire that includes the
effects of the non-orthogonality of the atomic orbitals on different atoms
exactly. The problem of non-orthogonality is treated by solving the transport
problem in a new Hilbert space which is spanned by an orthogonal basis. An
expression is derived for the energies at which antiresonances should occur for
a molecular wire connected to a pair of single-channel 1D leads. From this
expression we identify two distinct mechanisms that give rise to antiresonances
under different circumstances. The exact treatment of non-orthogonality in the
theory is found to be necessary to obtain reliable results. Our numerical
simulations extend this work to multichannel leads and to molecular wires
connected to 3D metallic nanocontacts. They demonstrate that our analytic
results also provide a good description of these more complicated systems
provided that certain well-defined conditions are met. These calculations
suggest that antiresonances should be experimentally observable in the
differential conductance of molecular wires of certain types.


title: Electrical Conductance of Molecular Wires

abstract:
  Molecular wires (MW) are the fundamental building blocks for molecular
electronic devices. They consist of a molecular unit connected to two continuum
reservoirs of electrons (usually metallic leads). We rely on Landauer theory as
the basis for studying the conductance properties of MW systems. This relates
the lead to lead current to the transmission probability for an electron to
scatter through the molecule. Two different methods have been developed for the
study of this scattering. One is based on a solution of the Lippmann-Schwinger
equation and the other solves for the {\bf t} matrix using Schroedinger's
equation. We use our methodology to study two problems of current interest. The
first MW system consists of 1,4 benzene-dithiolate (BDT) bonded to two gold
nanocontacts. Our calculations show that the conductance is sensitive to the
chemical bonding between the molecule and the leads. The second system we study
highlights the interesting phenomenon of antiresonances in MW. We derive an
analytic formula predicting at what energies antiresonances should occur in the
transmission spectra of MW. A numerical calculation for a MW consisting of
filter molecules attached to an active molecule shows the existence of an
antiresonance at the energy predicted by our formula.


title: Chiral hedgehog textures in 2D XY-like ordered domains

abstract:
  The textures associated with a point defect centered in a circular domain of
a thin film with XY-like ordering have been analyzed. The family of equilibrium
textures, both stable and metastable, can be classified by a new radial
topological number in addition to the winding number of the defect. Chiral
textures are supported in an achiral system as a result of spontaneously broken
chiral symmetry. Among these chiral textures, our theoretical analysis
accurately describes two categories of recently discovered ``reversing spiral''
textures, ones that are energetically stable and metastable.


title:
Continuous topological phase transitions between clean quantum Hall
  states

abstract:
  Continuous transitions between states with the {\em same} symmetry but
different topological orders are studied. Clean quantum Hall (QH) liquids with
neutral quasiparticles are shown to have such transitions. For clean bilayer
(nnm) states, a continous transition to other QH states (including non-Abelian
states) can be driven by increasing interlayer repulsion/tunneling. The
effective theories describing the critical points at some transitions are
derived.


title: Charging Effects in a Quantum Wire with Leads

abstract:
  We investigate the distribution of the electron density and the potential in
a quantum wire coupled to reservoirs, treating this structure as a unified
quantum system and taking into account the Coulomb interaction of electrons.
The chemical potential difference that exists between a decoupled, isolated
quantum wire and the reservoirs gives rise to charge transfer in the coupled
system. We show that the quantum wire can be charged positively or negatively
or remain neutral as a whole, depending on such factors as the wire radius and
the background charge density in the wire. The magnitude of the charge and its
sign are to a large extent determined by the exchange interaction of the
electrons in the wire. Using a Hartree-Fock approach, we develop a model of a
quantum wire which includes the reservoirs. This model allows us to find the
self-consistent distribution of the electron density and the potential in the
wire both at equilibrium and in the presence of transport. The linear
conductance is investigated as a function of the chemical potential. The
nonadiabatic transition from the reservoirs to the wire leads to conductance
oscillations caused by multiple scattering of electron waves. The period of the
oscillations depends on the charge acquired by the wire and the exchange
energy. We find that the exchange interaction strongly enhances the Friedel
oscillations near the contacts. However, they do not noticeably suppress the
conductance because the wire has a finite length and is charged. Under far from
equilibrium conditions, which appear when the applied voltage exceeds the Fermi
energy in the wire, the system becomes unstable with respect to fluctuations of
the electric potential and the electron density. The instability results in the
appearance of multistable electron states.


title:
New mechanism of X-ray radiation from a relativistic charged particle in
  a dielectric random medium

abstract:
  We have considered X-ray radiation of a relativistic charged particle moving
in a system consisting of microspheres distributed randomly in a dielectric
material. A new mechanism based on the diffusional scattering of pseudophotons
is suggested. The result leads to a stronger dependence of radiation intensity
on the particle energy, than that predicted by the traditional transition
radiation theory,and explains a recent experiment on such a system of randomly
distributed superconducting granules.


title: Spontaneous emission of phonons by coupled quantum dots

abstract:
  We find an interference effect for electron-phonon interactions in coupled
semiconductor quantum dots that can dominate the nonlinear transport properties
even for temperatures close to zero. The intradot electron tunnel process leads
to a `shake up' of the phonon system and is dominated by a double-slit-like
interference effect of spontaneously emitted phonons. The effect is closely
related to subradiance of photons (Dicke effect) in a laser-trapped two-ion
system and explains the oscillations in the nonlinear current-voltage
characteristics of coupled dots observed recently.


title: Doping-dependent magnetization plateaux in p-merized Hubbard chains

abstract:
  We study zero-temperature Hubbard chains with periodically modulated hopping
at arbitrary filling n and magnetization m. We show that the magnetization
curves have plateaux at certain values of m which depend on the periodicity p
and the filling. At commensurate filling n a charge gap opens and then
magnetization plateaux correspond to fully gapped situations. However, plateaux
also arise in the magnetization curves at fixed n between the commensurate
values and then the plateau-value of of m depends continuously on n and can
thus also become irrational. In particular for the case of dimerized hopping
(p=2) and fixed doping we find that a plateau appears at m=1-n. In this case,
there is still a gapless mode on the plateau leading to thermodynamic behavior
which is different from a completely gapped situation.


title:
Coulomb effects on the transmittance of open quantum dots in a
  tight-binding model

abstract:
  A quantum-mechanical calculation of conductance in an open quantum dot is
performed in the Landauer-B\"{u}ttiker formalism using a tight binding
Hamiltonian with direct Coulomb interaction. The charge distribution in the dot
is calculated self-consistently as function of a gate potential, for various
dot-leads couplings. The interaction is active only inside the dot, but not in
the leads, its strength being an input parameter. Our calculations are
complementary to the master-equation approach, go beyond the "orthodox theory",
and account for the size, tunneling, and interaction effects in quantum dots.