title: Hall constant of strongly correlated electrons on a ladder

abstract:
  The Hall constant R_H in a tight-binding model of correlated electrons on a
ladder at T=0 is expressed in terms of derivatives of the ground state energy
with respect to external magnetic and electric fields. This novel method is
used for the analysis of the t-J model on finite size ladders. It is found that
for a single hole R_H is hole-like and close to the semiclassical value, while
for two holes it can vary with ladder geometry. In odd-leg ladders, R_H behaves
quite regularly changing sign as a function of doping, the variation being
quantitatively close to experimental results in cuprates.


title:
A Nonequilibrium Statistical Ensemble Formalism. Maxent-Nesom: Concepts,
  Construction, Application, Open Questions and Criticisms

abstract:
  We describe a particular approach for the construction of a nonequilibrium
statistical ensemble formalism for the treatment of dissipative many-body
systems. This is the so-called Nonequilibrium Statistical Operator Method,
based on the seminal and fundamental ideas set forward by Boltzmann and Gibbs.
The existing approaches can be unified under a unique variational principle,
namely, MaxEnt, which we consider here. The main six basic steps that are at
the foundations of the formalism are presented and the fundamental concepts are
discussed. The associated nonlinear quantum kinetic theory and the accompanying
Statistial Thermodynamics (the Informational Statistical Thermodynamics) are
very briefly described. The corresponding response function theory for systems
away from equilibrium allows to connected the theory with experiments, and some
examples are summarized; there follows a good agreement between theory and
experimental data in the cases in which the latter are presently available. We
also present an overview of some conceptual questions and associated
criticisms.


title: On Equivalence of Thermostatistical Formalisms

abstract:
  We make some simple observations on basic issues pertaining to
thermostatistical formalisms.


title: Finite-size corrections to the free energy of crystalline solids

abstract:
  We analyse the finite-size corrections to the free energy of crystals with a
fixed center of mass. When we explicitly correct for the leading ($\ln N/N$)
corrections, the remaining free energy is found to depend linearly on 1/N.
Extrapolating to the thermodynamic limit ($N\to\infty$), we estimate the free
energy of a defect free crystal of particles interacting through an
$r^{-12}$-potential. We also estimate the free energy of perfect hard-sphere
crystal near coexistence: at $\rho \sigma^{3}$=1.0409, the excess free energy
of a defect-free hard-sphere crystal is $5.91889(4)kT$ per particle. This,
however, is not the free energy of an equilibrium hard-sphere crystal. The
presence of an finite concentration of vacancies results in a reduction of the
free energy that is some two orders of magnitude larger than the present error
estimate.


title: Preparing Topological States of a Bose-Einstein Condensate

abstract:
  The burgeoning field of Bose-Einstein condensation in dilute alkali and
hydrogen gases has stimulated a great deal of research into the statistical
physics of weakly interacting quantum degenerate systems. The recent
experiments offer the possibility for exploring fundamental properties of low
temperature physics in a very controllable and accessible way. One current goal
of experimenters in this field is to observe superfluid-like behavior in these
trapped Bose gases, analogous to persistent currents in superfluid liquid
helium, which flow without observable viscosity, and electric currents in
superconductors, which flow without observable resistance. These ``super''
properties of Bose-condensed systems occur because the macroscopic occupation
of a quantized mode provides a stabilizing mechanism that inhibits decay due to
thermal relaxation. Here we solve the time-dependent Gross-Pitaevskii equation
of motion of the condensate involving two hyperfine atomic states and show how
to generate, with extremely high fidelity, topological modes such as vortices
that open the door to the study of superfluidity in these new systems. Our
approach is inspired by recent experiments investigating a trapped condensate
with two strongly coupled internal states. We show how the interplay between
the internal and motional dynamics can be utilized to prepare the condensate in
a variety of interesting configurations.


title: Ideal Gases in Time-Dependent Traps

abstract:
  We investigate theoretically the properties of an ideal trapped gas in a
time-dependent harmonic potential. Using a scaling formalism, we are able to
present simple analytical results for two important classes of experiments:
free expansion of the gas upon release of the trap; and the response of the gas
to a harmonic modulation of the trapping potential is investigated. We present
specific results relevant to current experiments on trapped Fermions.


title: Mean-field solution of the small-world network model

abstract:
  The small-world network model is a simple model of the structure of social
networks, which simultaneously possesses characteristics of both regular
lattices and random graphs. The model consists of a one-dimensional lattice
with a low density of shortcuts added between randomly selected pairs of
points. These shortcuts greatly reduce the typical path length between any two
points on the lattice. We present a mean-field solution for the average path
length and for the distribution of path lengths in the model. This solution is
exact in the limit of large system size and either large or small number of
shortcuts.


title: Tunnelling of topological line defects in strongly coupled superfluids

abstract:
  The geometric theory of vortex tunnelling in superfluid liquids is developed.
Geometry rules the tunnelling process in the approximation of an incompressible
superfluid, which yields the identity of phase and configuration space in the
vortex collective co-ordinate. To exemplify the implications of this approach
to tunnelling, we solve explicitly for the two-dimensional motion of a point
vortex in the presence of an ellipse, showing that the hydrodynamic collective
co-ordinate description limits the constant energy paths allowed for the vortex
in configuration space. We outline the experimental procedure used in helium II
to observe tunnelling events, and compare the conclusions we draw to the
experimental results obtained so far. Tunnelling in Fermi superfluids is
discussed, where it is assumed that the low energy quasiparticle excitations
localised in the vortex core govern the vortex dynamical equations. The
tunnelling process can be dominated by Hall or dissipative terms, respectively
be under the influence of both, with a possible realization of this last
intermediate case in unconventional, high-temperature superconductors.


title:
Clusters of interstitial carbon atoms near the graphite surface as a
  possible origin of dome-like features observed by STM

abstract:
  Formation of clusters of interstitial carbon atoms between the surface and
second atomic layers of graphite is demonstrated by means of molecular dynamics
simulations. It is shown that interstitial clusters result in the dome-like
surface features that may be associated with some of the hillocks observed by
STM on the irradiated graphite surface.


title: Combinatorial and topological approach to the 3D Ising model

abstract:
  We extend the planar Pfaffian formalism for the evaluation of the Ising
partition function to lattices of high topological genus g. The 3D Ising model
on a cubic lattice, where g is proportional to the number of sites, is
discussed in detail. The expansion of the partition function is given in terms
of 2^{2 g} Pfaffians classified by the oriented homology cycles of the lattice,
i.e. by its spin-structures. Correct counting is guaranteed by a signature term
which depends on the topological intersection of the oriented cycles through a
simple bilinear formula. The role of a gauge symmetry arising in the above
expansion is discussed. The same formalism can be applied to the counting
problem of perfect matchings over general lattices and provides a determinant
expansion of the permanent of 0-1 matrices.


title: Path integral Monte Carlo simulation of helium at negative pressures

abstract:
  Path integral Monte Carlo (PIMC) simulations of liquid helium at negative
pressure have been carried out for a temperature range from the critical
temperature to below the superfluid transition. We have calculated the
temperature dependence of the spinodal line as well as the pressure dependence
of the isothermal sound velocity in the region of the spinodal. We discuss the
slope of the superfluid transition line and the shape of the dispersion curve
at negative pressures.


title:
Nonequilibrium Superconductor-Normal Metal Tunnel Contact and the Phonon
  Deficit Effect

abstract:
  We consider tunnel microrefrigerators at low temperature. There is a number
of experimental studies performed on microrefrigeration in tunneling
superconductor--normal metal (SN) structures. Related to these experiments,
only the electron subsystem has been considered theoretically. Independently,
the phonon deficit effect has been studied a while ago in
superconductor-superconductor tunnel junctions. It can be regarded as a
possible prototype scheme for superconducting microrefrigerators. We try to
provide the missing link between experiments on the SN tunnel junction
refrigerators and the theory which includes microscopically phonons in
combination with the mechanism of the phonon deficit effect.


title:
Effects of Coulomb interaction and tunneling on electron transport in
  coupled one-dimensional systems: from ballistic to diffusive regime

abstract:
  A linear theory of electron transport is developed for a system of two ideal
quantum wires, of length L, coupled by tunneling and Coulomb interaction. The
interaction of electrons with acoustical phonons is included and the results
are valid in both the ballistic and diffusive regime. In the {\it ballistic}
regime, both tunneling and Coulomb drag lead to a {\it negative}
transresistance R_{TR}, while in the {\it diffusive} regime the tunneling
opposes the drag and leads to a {\it positive} R_{TR}. If L is smaller than the
phase-breaking length, the tunneling leads to interference oscillations of the
resistance that are damped exponentially with L.


title: Underlying Pairing States of High $T_c$ Superconductivity

abstract:
  I present a microscopic theory I proposed recently to describe high-T_c
superconductivity in cuprates. I show that coherent pairing states consisting
of extended singlet Cooper pairs and triplet $\pi$ pairs can manifest both the
Mott insulating antiferromagnetic order and the d-wave superconducting order.
  From this configuration of coherent pairing states, I can describe both the
single electron properties and the low energy collective excitations in high
T_c superconductivity in the same framework. The quasiparticle can be derived
directly with respect to the coherent pairing states. While, the low-lying
quantum fluctuations associated with spin-wave and charge excitation can be
investigated from the path integral of the coherent pairing states.


title: Intermittent Granular Flow and Clogging with Internal Avalanches

abstract:
  The dynamics of intermittent granular flow through an orifice in a granular
bin and the associated clogging due to formation of arches blocking the outlet,
is studied numerically in two-dimensions. Our numerical results indicate that
for small hole sizes, the system self-organizes to a steady state and the
distribution of the grain displacements decays as power laws. On the other
hand, for large holes, the outflow distributions are also observed to follow
power law distributions.


title:
Equilibrial Charge of Grains and Low-temperature Conductivity of
  Granular Metals

abstract:
  The low-temperature equilibrial state of a system of small metal grains,
embedded into insulator, is studied. We find, that the grains may be charged
due to the fluctuations of the surface energy of electron gas in grains, rather
than quantization of electron states. The higherst-occupied level in a grain
fluctuates within the range of order of charging energy below the overall
chemical potential. The system, called a gapless Hubbard insulator, has no
overall energy gap, while the transfer of an electron on finite distances costs
finite energy. The ionization energy is determined mostly by the intragrain
Coulomb repulsion, rather than a weak intergrain interaction, responsible for
the Coulomb gap.
  The hopping transport in the system is studied. The hopping energy is
determined by the charging energy. At low temperature the transport has gapless
character.


title: Superconductivity in Ferromagnetic RuSr2GdCu2O8

abstract:
  Relying on the inhomogeneous (layered) crystal, electronic, and magnetic
structure, we show how superconductivity can coexist with the ferromagnetic
phase of RuSr2GdCu2O8 as observed by Tallon and coworkers. Since the Cu
d_{x^2-y^2} orbitals couple only to apical O p_x, p_y orbitals (and only
weakly), which also couple only weakly to the magnetic Ru t_{2g} orbitals,
there is sufficiently weak exchange splitting, especially of the symmetric CuO2
bilayer Fermi surface, to allow singlet pairing. The exchange splitting is
calculated to be large enough that the superconducting order parameter may be
of the Fulde-Ferrell-Larkin-Ovchinnikov type. We also note that \pi-phase
formation is preferred by the magnetic characteristics of RuSr2GdCu2O8.


title: Multiscaling in Infinite Dimensional Collision Processes

abstract:
  We study relaxation properties of two-body collisions in infinite spatial
dimension. We show that this process exhibits multiscaling asymptotic behavior
as the underlying distribution is characterized by an infinite set of
nontrivial exponents. These nonequilibrium relaxation characteristics are found
to be closely related to the steady state properties of the system.


title: Flat histogram Monte Carlo method

abstract:
  We discuss a sampling algorithm which generates flat histogram in energy. In
combination with transition matrix Monte Carlo, the density of states and
derived quantities such as entropy and free energy as a function of temperature
can be computed in a single simulation.


title: On non-Riemannian geometry of twisted nematic liquid crystals

abstract:
  A teleparallel geometrical description of the nematic phases of liquid
crystals is proposed.In the case of the twisted geometry of nematics Cartan
torsion is given by a spatial helical form which depends on the twist angle.
Geodesics of a test particle around the teleparallel liquid crystal are given.


title: The partition function for "composite particles"

abstract:
  We calculate the partition function for "composite particles". For any finite
number of states d, and in the following two cases: 1)all states have the same
energy, 2)the energy is linearly distributed over the states, we transform the
partition function into a finite sum of terms which exhibit trivially their $%
d $dependance. The infinite d thermodynamic limit is obtained as well as the
finite d-size corrections, etc. In the second case, we obtain the finite d-size
corrections to "the universal chiral partition function for exclusion
statistics".


title:
A self-consistent Ornstein-Zernike approximation for the
  Edwards-Anderson spin glass model

abstract:
  We propose a self-consistent Ornstein-Zernike approximation for studying the
Edwards-Anderson spin glass model. By performing two Legendre transforms in
replica space, we introduce a Gibbs free energy depending on both the
magnetizations and the overlap order parameters. The correlation functions and
the thermodynamics are then obtained from the solution of a set of coupled
partial differential equations. The approximation becomes exact in the limit of
infinite dimension and it provides a potential route for studying the stability
of the high-temperature phase against replica-symmetry breaking fluctuations in
finite dimensions. As a first step, we present the numerical predictions for
the freezing temperature and the zero-field thermodynamic properties above
freezing as a function of dimensionality.


title:
A look at the crossover region between BCS superconductivity and Bose
  Einstein Condensation

abstract:
  Pair fluctuation theory has been used to study the crossover from the weak
coupling BCS theory to the strong coupling Bose Einstein Condensation. The
effect of fluctuations has been studied over the whole crossover regime. It has
been shown that the pair fluctuations are enhanced considerably in both two and
three dimensions and hence mean field theory is inadequate to study the
physical properties in this regime. A self consistent scheme for calculating
the pair susceptibility is given.


title:
Atomic structure and vibrational properties of icosahedral B$_4$C boron
  carbide

abstract:
  The atomic structure of icosahedral B$_4$C boron carbide is determined by
comparing existing infra-red absorption and Raman diffusion measurements with
the predictions of accurate {\it ab initio} lattice-dynamical calculations
performed for different structural models. This allows us to unambiguously
determine the location of the carbon atom within the boron icosahedron, a task
presently beyond X-ray and neutron diffraction ability. By examining the inter-
and intra-icosahedral contributions to the stiffness we show that, contrary to
recent conjectures, intra-icosahedral bonds are harder.


title: A Fault-Tolerant Superconducting Associative Memory

abstract:
  The demand for high-density data storage with ultrafast accessibility
motivates the search for new memory implementations. Ideally such storage
devices should be robust to input error and to unreliability of individual
elements; furthermore information should be addressed by its content rather
than by its location. Here we present a concept for an associative memory whose
key component is a superconducting array with natural multiconnectivity. Its
intrinsic redundancy is crucial for the content-addressability of the resulting
storage device and also leads to parallel image retrieval. Because patterns are
stored nonlocally both physically and logically in the proposed device,
information access and retrieval are fault-tolerant. This superconducting
memory should exhibit picosecond single-bit acquisition times with negligible
energy dissipation during switching and multiple non-destructive read-outs of
the stored data.


title: Classification of phase transitions in small systems

abstract:
  We present a classification scheme for phase transitions in finite systems
like atomic and molecular clusters based on the Lee-Yang zeros in the complex
temperature plane. In the limit of infinite particle numbers the scheme reduces
to the Ehrenfest definition of phase transitions and gives the right critical
indices. We apply this classification scheme to Bose-Einstein condensates in a
harmonic trap as an example of a higher order phase transitions in a finite
system and to small Ar clusters.


title: Inferring degree of nonextensivity for generalized entropies

abstract:
  The purpose of this note is to argue that degree of nonextensivity as given
by Tsallis distribution obtained from maximum entropy principle has a different
origin than nonextensivity inferred from pseudo-additive property of Tsallis
entropy.


title: Impurity correlations in dilute Kondo alloys

abstract:
  The single impurity Kondo model is often used to describe metals with dilute
concentrations (n_i) of magnetic impurities. Here we examine how dilute the
impurities must be for this to be valid by developing a virial expansion in
impurity density. The O(n_i^2) term is determined from results on the
2-impurity Kondo problem by averaging over the RKKY coupling. The non-trivial
fixed point of the 2-impurity problem could produce novel singularities in the
heat capacity of dilute alloys at O(n_i^2).


title: Screening, nonadiabaticity, and quantized acoustoelectric current

abstract:
  Quantized single-electron transport driven by surface acoustic waves (SAW)
through a pinched-off narrow constriction is studied theoretically. Long-range
Coulomb interaction causes the tunneling coupling between the two-dimensional
electron gas (2DEG) and the moving minimum of the SAW-induced potential to
decay rapidly with time. The energy scale, associated with the characteristic
time of this decay, controls both the width of the transition regions between
the plateaus and the slope of the plateaus. This sets a limit for the accuracy
of the quantization of acoustoelectric current at low temperature.


title:
Analytic Approximations for the Velocity of Field-Driven Ising
  Interfaces

abstract:
  We present analytic approximations for the field, temperature, and
orientation dependences of the interface velocity in a two-dimensional kinetic
Ising model in a nonzero field. The model, which has nonconserved order
parameter, is useful for ferromagnets, ferroelectrics, and other systems
undergoing order-disorder phase transformations driven by a bulk free-energy
difference. The Solid-on-Solid (SOS) approximation for the microscopic surface
structure is used to estimate mean spin-class populations, from which the mean
interface velocity can be obtained for any specific single-spin-flip dynamic.
This linear-response approximation remains accurate for higher temperatures
than the single-step and polynuclear growth models, while it reduces to these
in the appropriate low-temperature limits. The equilibrium SOS approximation is
generalized by mean-field arguments to obtain field-dependent class populations
for moving interfaces, and thereby a nonlinear-response approximation for the
velocity. The analytic results are compared with Monte Carlo simulations.
Excellent agreement is found in a wide range of field, temperature, and
interface orientation.


title:
Disorder Effects in the Bipolaron System Ti$_{4}$O$_{7}$ Studied by
  Photoemission Spectroscopy

abstract:
  We have performed a photoemission study of Ti$_{4}$O$_{7}$ around its two
transition temperatures so as to cover the metallic, high-temperature
insulating (bipolaron-liquid), and low-temperature insulating
(bipolaron-crystal) phases. While the spectra of the low-temperature insulating
phase show a finite gap at the Fermi level, the spectra of the high-temperature
insulating phase are gapless, which is interpreted as a soft Coulomb gap due to
dynamical disorder. We suggest that the spectra of the high-temperature
disordered phase of Fe$_{3}$O$_{4}$, which exhibits a charge order-disorder
transition (Verwey transition), can be interpreted in terms of a Coulomb gap.


title:
Dislocations and the critical endpoint of the melting line of vortex
  line lattices

abstract:
  We develop a theory for dislocation-mediated structural transitions in the
vortex lattice which allows for a unified description of phase transitions
between the three phases, the elastic vortex glass, the amorphous vortex glass,
and the vortex liquid, in terms of a free energy functional for the dislocation
density. The origin of a critical endpoint of the melting line at high magnetic
fields, which has been recently observed experimentally, is explained.


title:
Fluctuation of inverse compressibility for electronic systems with
  random capacitive matrices

abstract:
  This article is concerned with statistics of addition spectra for systems of
identical charged particles. A classical model is suggested in order to study
fluctuations of Coulomb blockade peak spacings in large two-dimensional
semiconductor quantum dots. It is based on the electrostatics of several
electron islands among which there are random inductive and capacitive
couplings. Each island can accommodate electrons on quantum orbitals whose
energy depend also on an external magnetic field. | In contrast with a single
island quantum dot where the spacing distribution between conductance peaks is
close to Gaussian, here the distribution has a peak at small spacing value. The
fluctuations are mainly due to charging effects. The model can explain the
occasional occurrence of couples or even triples of closely spaced Coulomb
blockade peaks, as well as the qualitative behavior of peak positions with the
applied magnetic field.


title:
Paired Hall States versus Unidirectional CDW in Tilted Field for
  $\nu={5/2}$

abstract:
  We formulate the composite fermions in the presence of an in-plane magnetic
field. As the in-plane field increases, if we assume the state at $\nu=5/2$
turns into the mixed state between the unidirectional charge density wave
domains and paired Hall state, we can phenomenologically fit the theoretically
defined gap to the experimental measured results. We explain the destruction of
the paired Hall states and then a phase transition from the paired Hall state
to the unidirectional charge density wave from a symmetry point of view.


title: Thermodynamics of Mesoscopic Vortex Systems in 1+1 Dimensions

abstract:
  The thermodynamics of a disordered planar vortex array is studied numerically
using a new polynomial algorithm which circumvents slow glassy dynamics. Close
to the glass transition, the anomalous vortex displacement is found to agree
well with the prediction of the renormalization-group theory. Interesting
behaviors such as the universal statistics of magnetic susceptibility
variations are observed in both the dense and dilute regimes of this mesoscopic
vortex system.


title: Anomalous magneto-oscillations in two-dimensional systems

abstract:
  The frequencies of Shubnikov-de Haas oscillations have long been used to
measure the unequal population of spin-split two-dimensional subbands in
inversion asymmetric systems. We report self-consistent numerical calculations
and experimental results which indicate that these oscillations are not simply
related to the zero-magnetic-field spin-subband densities.


title:
Oscillation of the tunnel splitting in nanospin systems within the
  particle mapping formalism

abstract:
  The oscillation of tunnel splitting in the biaxial spin system within
magnetic field along the anisotropy axis is analyzed within the particle
mapping approach, rather than in the (\theta-\phi) spin coherent-state
representation. In our mapping procedure, the spin system is transformed into a
particle moving in the restricted $S^1$ geometry whose wave function subjects
to the boundary condition involving additional phase shift. We obtain the new
topological phase that plays the same role as the Wess-Zumino action in spin
coherent-state representation. Considering the interference of two possible
trajectories, instanton and anti-instanton, we get the identical condition for
the field at which tunneling is quenched, with the previous result within spin
coherent-state representation.


title:
Anomalous temperature behavior of resistivity in lightly doped
  manganites around a metal-insulator phase transition

abstract:
  An unusual temperature and concentration behavior of resistivity in
$La_{0.7}Ca_{0.3}Mn_{1-x}Cu_xO_3$ has been observed at slight $Cu$ doping
($0\leq x \leq 0.05$). Namely, introduction of copper results in a splitting of
the resistivity maximum around a metal-insulator transition temperature
$T_0(x)$ into two differently evolving peaks. Unlike the original $Cu$-free
maximum which steadily increases with doping, the second (satellite) peak
remains virtually unchanged for $x<x_c$, increases for $x\ge x_c$ and finally
disappears at $x_m\simeq 2x_c$ with $x_c\simeq 0.03$. The observed phenomenon
is thought to arise from competition between substitution induced strengthening
of potential barriers (which hamper the charge hopping between neighboring $Mn$
sites) and weakening of carrier's kinetic energy. The data are well fitted
assuming a nonthermal tunneling conductivity theory with randomly distributed
hopping sites.


title: Muon localization site in U(Pt,Pd)3

abstract:
  The angular and temperature (10-250 K) variation of the Knight shift of
single-crystalline U(Pt0.95Pd0.05)3 has been measured in transverse field
(B=0.6 T) mSR experiments. By analysing the temperature variation of the Knight
shift with a modified Curie-Weiss expression the muon localization site in this
hexagonal material is determined at (0,0,0).


title: Phase separation in supersolids

abstract:
  We study quantum phase transitions in the ground state of the two dimensional
hard-core boson Hubbard Hamiltonian. Recent work on this and related models has
suggested ``supersolid'' phases with simultaneous diagonal and off-diagonal
long range order. We show numerically that, contrary to the generally held
belief, the most commonly discussed ``checkerboard'' supersolid is
thermodynamically unstable. Furthermore, this supersolid cannot be stabilized
by next near neighbour interaction. We obtain the correct phase diagram using
the Maxwell construction. We demonstrate the ``striped'' supersolid is
thermodynamically stable and is separated from the superfluid phase by a
continuous phase transition.


title: Short-range particle correlations in dilute Bose gas

abstract:
  The thermodynamics of a homogeneous dilute Bose gas with an arbitrary strong
repulsion between particles is investigated on the basis of the exact relation
connecting the pair correlation function with the in-medium pair wave functions
and occupation numbers. It is shown that the effective-interaction scheme which
is reduced to the Bogoliubov model with the effective pairwise potential, is
not acceptable for investigating the short-range particle correlations in a
dilute strongly interacting Bose gas. In contrast to this scheme, our model is
thermodynamically consistent and free of the ultraviolet divergences due to
accurate treatment of the short-range boson correlations. The equation for the
in-medium scattering amplitude is derived that makes it possible to find the
in-medium renormalization for the pair wave functions at short boson
separations. Low-density expansions for the main thermodynamic quantities are
reinvestigated on the basis of this equation. Besides, the expansions are found
for the interaction and kinetic energies per particle. It is demonstrated that
for the many-boson system of the hard spheres the interaction energy is equal
to zero for any boson density. The exact relationship between the chemical
potential and in-medium pair wave functions is also established.


title: Finite-Size Scaling in Two-dimensional Continuum Percolation Models

abstract:
  We test the universal finite-size scaling of the cluster mass order parameter
in two-dimensional (2D) isotropic and directed continuum percolation models
below the percolation threshold by computer simulations. We found that the
simulation data in the 2D continuum models obey the same scaling expression of
mass M to sample size L as generally accepted for isotropic lattice problems,
but with a positive sign of the slope in the ln-ln plot of M versus L. Another
interesting aspect of the finite-size 2D models is also suggested by plotting
the normalized mass in 2D continuum and lattice bond percolation models, versus
an effective percolation parameter, independently of the system structure (i.e.
lattice or continuum) and of the possible directions allowed for percolation
(i.e. isotropic or directed) in regions close to the percolation thresholds.
Our study is the first attempt to map the scaling behaviour of the mass for
both lattice and continuum model systems into one curve.


title:
Wavelength dependent negative and positive persistent photoconductivity
  in Sn delta-doped GaAs structures

abstract:
  The photoconductivity of GaAs structures delta-doped by Sn has been
investigated for wavelengths lambda= 650-1200 nm in the temperature interval T=
4.2-300 K. The electron densities and mobilities, before and after
illumination, have been determined by magnetoresistance, Shubnikov-de Haas
effect and Hall effect measurements, in high magnetic fields. For the heavily
doped structures (n_H> 2x10^13 cm^-2) we observe under illumination by light
with wavelengths larger than the band-gap wavelength of the host material
(lambda= 815 nm at T= 4.2 K) first positive (PPPC) and then negative (NPPC)
persistent photoconductivity. The NPPC is attributed to the ionisation of DX
centres and PPPC is explained by the excitation of electrons from Cr impurity
states in the substrate. For lambda< 815 nm in addition the excitation of
electron over the band gap of GaAs contributes to the PPPC. For the lightly
doped structures (n_H<= 2x10^13 cm^-2) the photoconductivity effect is always
positive.


title: Barrier crossing induced by very slow external noise

abstract:
  We consider the motion of a particle in a force field subjected to adiabatic,
fluctuations of external origin. We do not put the restriction on the type of
stochastic process that the noise is Gaussian. Based on a method developed
earlier by us [ J. Phys. A {\bf 31} (1998) 3937, 7301] we have derived the
equation of motion for probability distribution function for the particle on a
coarse-grained timescale $\Delta t$ assuming that it satisfies the separation
of timescales; $|\mu |^{-1} \ll \Delta t \ll \tau_c$, where $\tau_c$ is the
correlation time of fluctuations. $|\mu |^{-1}$ refers to the inverse of the
damping rate (or, the largest of the eigenvalues of the unperturbed system) and
sets the shortest timescale in the dynamics in contrast to the conventional
theory of fast fluctuations. The equation includes a third order noise term. We
solve the equation for a Kramers' type potential and show that although the
system is thermodynamically open, appropriate boundary conditions allow the
distinct steady states. Based on the exact solution of the third order equation
for the linearized potential and the condition for attainment of the steady
states we calculate the adiabatic noise-induced rate of escape of a particle
confined in a well. A typical variation of the escape rate as a function of
dissipation which is reminiscent of Kramers' turn-over problem, has been
demonstrated.


title: A geometrical picture for finite dimensional spin glasses

abstract:
  A controversial issue in spin glass theory is whether mean field correctly
describes 3-dimensional spin glasses. If it does, how can replica symmetry
breaking arise in terms of spin clusters in Euclidean space? Here we argue that
there exist system-size low energy excitations that are sponge-like, generating
multiple valleys separated by diverging energy barriers. The droplet model
should be valid for length scales smaller than the size of the system (theta >
0), but nevertheless there can be system-size excitations of constant energy
without destroying the spin glass phase. The picture we propose then combines
droplet-like behavior at finite length scales with a potentially mean field
behavior at the system-size scale.


title:
General technique of calculating drift velocity and diffusion
  coefficient in arbitrary periodic systems

abstract:
  We develop a practical method of computing the stationary drift velocity V
and the diffusion coefficient D of a particle (or a few particles) in a
periodic system with arbitrary transition rates. We solve this problem both in
a physically relevant continuous-time approach as well as for models with
discrete-time kinetics, which are often used in computer simulations. We show
that both approaches yield the same value of the drift, but the difference
between the diffusion coefficients obtained in each of them equals V*V/2.
Generalization to spaces of arbitrary dimension and several applications of the
method are also presented.


title: Granular Flow, Collisional Cooling and Charged Grains

abstract:
  The non-Newtonian character of granular flow in a vertical pipe is analyzed.
The time evolution of the flow velocity and the velocity fluctuations (or
granular temperature) is derived. The steady state velocity has a power law
dependence on the pipe width with an exponent between 3/4 and 3/2. The flow
becomes faster the more efficient the collisional cooling is, provided the
density remains low enough. The dependence of collisional cooling on the solid
fraction, the restitution coefficient and a possible electric charge of all
grains is discussed in detail.


title: Width of Rough Interfaces on Asymmetric Lattices

abstract:
  I present a calculation of the interfacial width within the capillary wave
(Gaussian) approximation. The calculation is done on rectangular lattices of
size L_1 times L_2, with periodic boundary conditions.


title:
Lro in Lattice Systems of Linear Classical and Quantum Oscillators.
  Strong N-N Pair Quadratic Interaction

abstract:
  For systems of one-component interacting oscillators on the d-dimensional
lattice, d>1, whose potential energy besides a large nearest-neighbour (n-n)
ferromagnetic translation-invariant quadratic term contains small
non-nearest-neighbour translation invariant term, an existence of a
ferromagnetic long-range order for two valued lattice spins, equal to a sign of
oscillator variables, is established for sufficiently large magnitude g of the
n-n interaction with the help of the Peierls type contour bound. The Ruelle
superstability bound is used for a derivation of the contour bound.


title: Charge Fluctuations in the Two-Dimensional One-Component Plasma

abstract:
  We study, via computer simulations, the fluctuations in the net electric
charge, in a two dimensional one component plasma (OCP) with uniform background
charge density $-e \rho$, in a region $\Lambda$ inside a much larger overall
neutral system. Setting $e=1$ this is the same as the fluctuations in
$N_\Lambda$, the number of mobile particles of charge $e$. As expected the
distribution of $ N_\Lambda$ has, for large $\Lambda$, a Gaussian form with a
variance which grows only as $\hat \kappa |\partial \Lambda|$, where $|\partial
\Lambda|$ is the length of the perimeter of $\Lambda$. The properties of this
system depend only on the coupling parameter $\Gamma = kT$ which is the same as
the reciprocal temperature in our units. Our simulations show that when the
coupling parameter $\Gamma$ increases, $\hat \kappa(\Gamma)$ decreases to an
asymptotic value $\hat \kappa(\infty) \sim \hat \kappa(2)/2$ which is equal (or
very close) to that obtained for the corresponding variance of particles on a
rigid triangular lattice. Thus, for large $\Gamma$, the characteristic length
$\xi_L = 2\hat \kappa/\rho$ associated with charge fluctuations behaves very
differently from that of the Debye length, $\xi_D \sim 1/\sqrt \Gamma$, which
it approaches as $\Gamma \to 0$. The pair correlation function of the OCP is
also studied.


title: Chern numbers and localization by non-Hermitean operators

abstract:
  In Hermitean quantum mechanics, extended current-carrying states are
distinguished from localized ones by their non-zero Chern number. We generalize
the notion of Chern number to non-Hermitean localization problems such as
tiltedflux lines in type-II superconductors with line defects and passive
scalar transport in random vorticity fields. We find that the usualpoint
eigenvalue degeneracies that occur in the Hermitean case are now loops bounding
a branch cut sheet with the same quantized total Berry flux, and hence Chern
number, as the original point. If a loop cuts the integration surface
non-quantized contributions to the total flux result.


title:
Energy-level statistics at the metal-insulator transition in anisotropic
  systems

abstract:
  We study the three-dimensional Anderson model of localization with
anisotropic hopping, i.e. weakly coupled chains and weakly coupled planes. In
our extensive numerical study we identify and characterize the metal-insulator
transition using energy-level statistics. The values of the critical disorder
$W_c$ are consistent with results of previous studies, including the
transfer-matrix method and multifractal analysis of the wave functions. $W_c$
decreases from its isotropic value with a power law as a function of
anisotropy. Using high accuracy data for large system sizes we estimate the
critical exponent $\nu=1.45\pm0.2$. This is in agreement with its value in the
isotropic case and in other models of the orthogonal universality class. The
critical level statistics which is independent of the system size at the
transition changes from its isotropic form towards the Poisson statistics with
increasing anisotropy.


title: Self-Organized Criticality in a Fibre-Bundle type model

abstract:
  The dynamics of a fibre-bundle type model with equal load sharing rule is
numerically studied. The system, formed by N elements, is driven by a slow
increase of the load upon it which is removed in a novel way through internal
transfers to the elements broken during avalanches. When an avalanche ends,
failed elements are regenerated with strengths taken from a probability
distribution. For a large enough N and certain restrictions on the distribution
of individual strengths, the system reaches a self-organized critical state
where the spectrum of avalanche sizes is a power law with an exponent
$\tau\simeq 1.5$.


title: Andreev Bound States and Josephson Coupling in YBCO/Pb tunnel junction

abstract:
  We have made in-situ [103] and [110]YBCO/Pb junctions, which display an
hysteretic behaviour with both a well-defined Quasi-Particule branch and a
Josephson current. A zero energy conductance peak is observed in the tunneling
conductance which is a signature of Andreev Bound States (ABS) at a surface of
a d-wave superconductor. The observed splitting of this peak at low temperature
may confirm the existence of a surface state which breaks time-reversal
symmetry. However, up to now, we have not seen any evidence of a coupling
between Josephson current and the zero-energy ABS, predicted by Y. Tanaka and
S. Kashiwaya (PRB 56 p.892).


title: Spin Wave Theory of Double Exchange Magnets

abstract:
  An isotropic half-metallic double exchange magnet with a direct superexchange
coupling between the localized spins is studied within the spin-wave (1/S)
expansion. The momentum dependence of the spin wave spectrum (including quantum
corrections) in the ferromagnetic phase at T=0 is investigated. Based on the
calculated spin wave spectrum of the canted state, it is shown that as long as
the external magnetic field is not too strong, the double exchange --
superexchange competition does not result in a stabilization of a
two-sublattice canted spin ordering.


title: Selection Rule in Josephson Coupling between In and Sr2RuO4

abstract:
  We have studied the Josephson coupling between a conventional s-wave
superconductor (In) and Sr2RuO4 and found that the coupling is allowed in the
in-plane direction, but not along the c axis. This selection rule indicates
that the symmetry of superconducting order parameter of Sr2RuO4 is either p- or
alternatively, purely d-wave. If Sr2RuO4 is a p-wave superconductor, as
strongly favored by other experimental evidence, our result suggests that the
pairing state of Sr2RuO4 is Gamma5 with d(k) = z(kx +/- iky), a nodeless state
in which the spins of the superconducting electrons lie in the RuO2 planes.


title: Disorder-Driven Non-Fermi-Liquid Behavior in CeRhRuSi_2

abstract:
  We report measurements of the bulk magnetic susceptibility and ^{29}Si
nuclear magnetic resonance (NMR) linewidth in the heavy-fermion alloy
CeRhRuSi_2. The linewidth increases rapidly with decreasing temperature and
reaches large values at low temperatures, which strongly suggests the wide
distributions of local susceptibilities \chi_j obtained in disorder-driven
theories of non-Fermi-liquid (NFL) behavior. The NMR linewidths agree well with
distribution functions P(\chi) which fit bulk susceptibility and specific heat
data. The apparent return to Fermi-liquid behavior observed below 1 K is
manifested in the vanishing of P(\chi) as \chi \to \infty, suggesting the
absence of strong magnetic response at low energies. Our results indicate the
need for an extension of some current theories of disorder-driven NFL behavior
in order to incorporate this low-temperature crossover.


title:
Coupled phonon-ripplon modes in a single wire of electrons on the
  liquid-helium surface

abstract:
  The coupled phonon-ripplon modes of the quasi-one-dimensional electron chain
on the liquid helium sutface are studied. It is shown that the electron-ripplon
coupling leads to the splitting of the collective modes of the wire with the
appearance of low-frequency modes and high-frequency optical modes starting
from threshold frequencies. The effective masses of an electron plus the
associated dimple for low frequency modes are estimated and the values of the
threshold frequencies are calculated. The results obtained can be used in
experimental attempts to observe the phase transition of the electron wire into
a quasi-ordered phase.


title:
Quantum-classical phase transition of escape rate in biaxial spin system
  with an arbitrarily directed magnetic field

abstract:
  We investigate the escape rate of a biaxial spin particle with an arbitrarily
dierected magnetic field in the easy plane, described by Hamiltonian ${\cal H}
= -AS_z^2 - BS_x^2 -H_x S_x -H_z S_z, (A>B>0)$. We derive an effective particle
potential by using the method of particle mapping. With the help of the
criterion for the presence of a first-order quantum-classical transition of the
escape rate we obtained various phase boundary curves depending on the
anisotropy parameter $b \equiv B/A$ and the field parameters $\alpha_{x,z}
\equiv H_{x,z}/AS$ : $\alpha_{zc}(b_c)'s, \alpha_{xc}(b_c)'s$, and $\alpha_{zc}
= \alpha_{zc}(\alpha_{xc})$. It is found from $\alpha_{zc}(b_c)'s$ and
$\alpha_{xc}(b_c)'s$ that the-first-order region decreases as $b$ and $\alpha_x
$ (or $\alpha_z$) increase. The phase boundary line $\alpha_{zc} =
\alpha_{zc}(\alpha_{xc}) shows that compared with the uniaxial system, both the
first- and second-oredr regions are diminished due to the transverse
anisotropy. Moreover, it is observed that, in the limit $\alpha_{xc} \to 0$,
$\alpha_{zc}$ does not coinsides with the coercive field line, which yields
more reduction in the first-order region. We have also computed the crossover
temperatures at the phase boundary :$T_c(b_c), T_c(\alpha_{xc}, \alpha_{zc})$.


title: Nonextensive thermodynamic formalism for chaotic dynamical systems

abstract:
  A nonextensive thermostatic approach to chaotic dynamical systems is
developed by expressing generalized Tsallis distribution as escort
distribution. We explicitly show the thermodynamic limit and also derive the
Legendre Transform structure. As an application, bit variance is calculated for
ergodic logistic map. Consistency of the formalism demands a relation between
box size ($\epsilon$) and degree of nonextensivity, given as $(1-q)\sim -1/{\rm
ln} \epsilon$. This relation is numerically verified for the case of bit
variance as well as using basic definition of Tsallis entropy.


title:
Effect of a Normal-State Pseudogap on Optical Conductivity in Underdoped
  Cuprate Superconductors

abstract:
  We calculate the c-axis infrared conductivity $\sigma_c(\omega)$ in
underdoped cuprate superconductors for spinfluctuation exchange scattering
within the CuO$_2$-planes including a phenomenological d-wave pseudogap of
amplitude $E_g$. For temperatures decreasing below a temperature $T^* \sim
E_g/2$, a gap for $\omega < 2E_g$ develops in $\sigma_c(\omega)$ in the
incoherent (diffuse) transmission limit. The resistivity shows 'semiconducting'
behavior, i.e. it increases for low temperatures above the constant behavior
for $E_g=0$. We find that the pseudogap structure in the in-plane optical
conductivity is about twice as big as in the interplane conductivity
$\sigma_c(\omega)$, in qualitative agreement with experiment. This is a
consequence of the fact that the spinfluctuation exchange interaction is
suppressed at low frequencies as a result of the opening of the pseudogap.
While the c-axis conductivity in the underdoped regime is described best by
incoherent transmission, in the overdoped regime coherent conductance gives a
better description.


title:
Disorder and interaction induced pairing in the addition spectra of
  quantum dots

abstract:
  We have investigated numerically the electron addition spectra in quantum
dots containing a small number (N < 11) of interacting electrons, in presence
of strong disorder. For a short-range Coulomb repulsion, we find regimes in
which two successive electrons enter the dot at very close values of the
chemical potential. In the strongly correlated regime these close additions, or
pairing, are associated with electrons tunneling into distinct electron puddles
within the dot. We discuss the tunneling rates at pairing, and we argue that
our results are related to a phenomenon known as "bunching", recently observed
experimentally.


title: Lexicon of Topological Defects in He-3 Superfluids

abstract:
  Defects in the multi-dimensional macroscopic quantum field of the He-3
superfluids are localized objects with a topological charge and are
topologically stable. They include point-like objects, vortex lines, planar
domain-wall-like structures, and 3-dimensional textures, which may or may not
include singular points or lines. An inventory of illustrations is presented
which concisely lists the different experimentally confirmed defect structures
in the A and B phases of superfluid He-3.


title:
The role of long-range forces in the phase behavior of colloids and
  proteins

abstract:
  The phase behavior of colloid-polymer mixtures, and of solutions of globular
proteins, is often interpreted in terms of a simple model of hard spheres with
short-ranged attraction. While such a model yields a qualitative understanding
of the generic phase diagrams of both colloids and proteins, it fails to
capture one important difference: the model predicts fluid-fluid phase
separation in the metastable regime below the freezing curve. Such demixing has
been observed for globular proteins, but for colloids it appears to be
pre-empted by the appearance of a gel. In this paper, we study the effect of
additional long-range attractions on the phase behavior of spheres with
short-ranged attraction. We find that such attractions can shift the
(metastable) fluid-fluid critical point out of the gel region. As this
metastable critical point may be important for crystal nucleation, our results
suggest that long-ranged attractive forces may play an important role in the
crystallization of globular proteins. However, in colloids, where refractive
index matching is often used to switch off long-ranged dispersion forces,
gelation is likely to inhibit phase separation.


title: Temperature Effects on Edge Magnetoplasmons in the Quantum Hall Regime

abstract:
  A microscopic treatment of edge magnetoplasmons (EMPs) is presented for the
case of not-too-low temperatures in which the inequality $k_{B}T\gg \hbar
v_{g}/\ell_{0}$, where $v_{g}$ is the group velocity of the edge states and
$\ell_{0}$ is the magnetic length, is fulfilled, and for filling factors $\nu
=1(2)$. We have obtained independent EMP modes spatially symmetric and
antisymmetric with respect to the edge. We describe in detail the spatial
structure and dispersion relations of the new edge waves (edge helicons,
dipole, quadrupole and octupole EMPs), which have the characteristic length
$\ell_{T}=\ell_{0}^{2}k_{B}T/\hbar v_{g}$. We have found that, in contrast to
well-known results for a spatially homogeneous dissipation within the channel,
the damping of the fundamental EMP at not-too-low temperatures is not quantized
and has a $T^{-1}$ dependence.


title: The magnetism of the t-t' Hubbard model

abstract:
  The magnetic properties of the t-t' Hubbard Model in the two dimensional
square lattice are studied within an unrestricted Hartree-Fock approximation in
real space. The interplay between antiferromagnetism, ferromagnetism, phase
separation and inhomogeneous magnetic textures is studied. It is shown that, at
sufficiently large values of t'/t, a rich fenomenology is to be expected
between the antiferromagnetic phase at half filling and the ferromagnetic phase
a lower fillings.


title: Two-finger selection theory in the Saffman-Taylor problem

abstract:
  We find that solvability theory selects a set of stationary solutions of the
Saffman-Taylor problem with coexistence of two \it unequal \rm fingers
advancing with the same velocity but with different relative widths $\lambda_1$
and $\lambda_2$ and different tip positions. For vanishingly small
dimensionless surface tension $d_0$, an infinite discrete set of values of the
total filling fraction $\lambda = \lambda_1 + \lambda_2$ and of the relative
individual finger width $p=\lambda_1/\lambda_2$ are selected out of a
two-parameter continuous degeneracy. They scale as $\lambda-1/2 \sim d_0^{2/3}$
and $|p-1/2| \sim d_0^{1/3}$. The selected values of $\lambda$ differ from
those of the single finger case. Explicit approximate expressions for both
spectra are given.

categories: cond-mat.stat-mech chao-dyn cond-mat.mtrl-sci nlin.CD nlin.PS patt-sol


title: Solution of classical stochastic one dimensional many-body systems

abstract:
  We propose a simple method that allows, in one dimension, to solve exactly a
wide class of classical stochastic many-body systems far from equilibrium. For
the sake of illustration and without loss of generality, we focus on a model
that describes the asymmetric diffusion of hard core particles in the presence
of an external source and instantaneous annihilation. Starting from a Master
equation formulation of the problem we show that the density and multi-point
correlation functions obey a closed set of integro-differential equations which
in turn can be solved numerically and/or analytically


title: Extended and localized vibrations models in disordered systems

abstract:
  We discuss vibrational localization problems in glasses and disordered media
in this paper. It is claimed that the essence of the localization problem is
already observed in disordered lattice models. These kinds of vibrations belong
to a different universality class than bonded electrons. Specifically, The
eigenvectors are extended even in two dimensions. Moreover, the correlation
exponent does not diverge in the transition from localized to delocalized
states. Furthermore, the volume of the extended states is scaled according to
the distance from the transition as $V\sim |\omega-\omega_c| L^d$.
Interestingly, boson peaks can be observed in the density of states in both two
and three dimensions. We studied the eigenstates of this problem and analyzed
the scattering effects in these lattices. Importantly, we found that in two
dimensions the boson peak, the localization edge, and the beginning of
anomalous scattering are at the same frequency. In three dimensions, however,
there are three separate regions: (1) localized, (2) weakly scattered, and (3)
anomalously scattered. Finally, we discuss the relevance of this study to
actual experiments and glasses.


title:
Numerical Study on Aging Dynamics in the 3D Ising Spin-Glass Model ---
  Quasi-Equilibrium Behaviour of Spin Auto-Correlation Functions

abstract:
  Using Monte Carlo simulations, we have studied aging phenomena in
three-dimensional Gaussian Ising spin-glass model focusing on quasi-equilibrium
behavior of the spin auto-correlation functions. Weak violation of the time
translational invariance in the quasi-equilibrium regime is analyzed in terms
of effective stiffness for droplet excitations in the presence of domain walls.
The simulated results in not only isothermal but also $T$-shift aging processes
exhibit the expected scaling behavior with respect to the characteristic length
scales associated with droplet excitations and domain walls in spite of the
fact that the growth law for these length scales still shows a pre-asymptotic
behavior compared with the asymptotic form proposed by the droplet theory.
Implications of our simulational results are also discussed in relation to
experimental observations.


title: Spin dependent scattering of a domain-wall of controlled size

abstract:
  Magnetoresistance measurements in the CPP geometry have been performed on
single electrodeposited Co nanowires exchange biased on one side by a sputtered
amorphous GdCo layer. This geometry allows the stabilization of a single domain
wall in the Co wire, the thickness of which can be controlled by an external
magnetic field. Comparing magnetization, resistivity, and magnetoresistance
studies of single Co nanowires, of GdCo layers, and of the coupled system,
gives evidence for an additional contribution to the magnetoresistance when the
domain wall is compressed by a magnetic field. This contribution is interpreted
as the spin dependent scattering within the domain wall when the wall thickness
becomes smaller than the spin diffusion length.


title:
The two-dimensional bond-diluted quantum Heisenberg model at the
  classical percolation threshold

abstract:
  The two-dimensional antiferromagnetic S=1/2 Heisenberg model with random bond
dilution is studied using quantum Monte Carlo simulation at the percolation
threshold (50% of the bonds removed). Finite-size scaling of the staggered
structure factor averaged over the largest connected clusters of sites on L*L
lattices shows that long-range order exists within the percolating fractal
clusters in the thermodynamic limit. This implies that the order-disorder
transition driven by bond-dilution occurs exactly at the percolation threshold
and that the exponents are classical. This result should apply also to the
site-diluted system.


title: Enhancement of Coulomb drag away from half filled Landau levels

abstract:
  Coulomb drag between two parallel two dimensional electron gases has been
measured at high magnetic fields in samples with two different layer spacings.
As the layer filling factor deviates from nu=1/2, we find that the magnitude of
the drag is enhanced quadratically with Delta nu = nu - 1/2, and the curvature
of the enhancement is insensitive to both the sign of Delta nu and the spacing
between the layers. Our results suggest that the enhancement is not due to
non-perturbative interlayer correlations.


title:
Exciton Beats in GaAs Quantum Wells: Bosonic Representation and
  Collective Effects

abstract:
  We discuss light-heavy hole beats observed in transient optical experiments
in GaAs quantum wells in terms of a free-boson coherent state model. This
approach is compared with descriptions based on few-level representations.
Results lead to an interpretation of the beats as due to classical
electromagnetic interference. The boson picture correctly describes photon
excitation of extended states and accounts for experiments involving coherent
control of the exciton density and Rayleigh scattering beating.


title:
Internal Diffusion-Limited Aggregation: Parallel Algorithms and
  Complexity

abstract:
  The computational complexity of internal diffusion-limited aggregation (DLA)
is examined from both a theoretical and a practical point of view. We show that
for two or more dimensions, the problem of predicting the cluster from a given
set of paths is complete for the complexity class CC, the subset of P
characterized by circuits composed of comparator gates. CC-completeness is
believed to imply that, in the worst case, growing a cluster of size n requires
polynomial time in n even on a parallel computer.
  A parallel relaxation algorithm is presented that uses the fact that clusters
are nearly spherical to guess the cluster from a given set of paths, and then
corrects defects in the guessed cluster through a non-local annihilation
process. The parallel running time of the relaxation algorithm for
two-dimensional internal DLA is studied by simulating it on a serial computer.
The numerical results are compatible with a running time that is either
polylogarithmic in n or a small power of n. Thus the computational resources
needed to grow large clusters are significantly less on average than the
worst-case analysis would suggest.
  For a parallel machine with k processors, we show that random clusters in d
dimensions can be generated in O((n/k + log k) n^{2/d}) steps. This is a
significant speedup over explicit sequential simulation, which takes
O(n^{1+2/d}) time on average.
  Finally, we show that in one dimension internal DLA can be predicted in O(log
n) parallel time, and so is in the complexity class NC.


title: Eigenvalue Distribution In The Self-Dual Non-Hermitian Ensemble

abstract:
  We consider an ensemble of self-dual matrices with arbitrary complex entries.
This ensemble is closely related to a previously defined ensemble of
anti-symmetric matrices with arbitrary complex entries. We study the two-level
correlation functions numerically. Although no evidence of non-monotonicity is
found in the real space correlation function, a definite shoulder is found. On
the analytical side, we discuss the relationship between this ensemble and the
$\beta=4$ two-dimensional one-component plasma, and also argue that this
ensemble, combined with other ensembles, exhausts the possible universality
classes in non-hermitian random matrix theory. This argument is based on
combining the method of hermitization of Feinberg and Zee with Zirnbauer's
classification of ensembles in terms of symmetric spaces.


title:
A theory of void formation in charge-stabilised colloidal suspensions at
  low ionic strength

abstract:
  Using a carefully justified development of Debye-Huckel theory for highly
asymmetric electrolytes, one finds that a region of expanded phase instability,
or miscibility gap, can appear for charge-stabilised colloidal suspensions at
high charges and low ionic strengths. It is argued that this is offers a
straightforward explanation for the observations of void structures and other
anomalies in such suspensions in this region. The nature of the interface
between coexisting phases, and general arguments that many-body attractions
form a key part of the underlying physical picture, are also examined. The
present analysis may also generate new insights into old problems such as
coacervation in oppositely charged colloid or protein / polyelectrolyte
mixtures, and suggests interesting new possibilities such as the appearance of
charge density wave phases in colloidal systems in the vicinity of the critical
solution points.


title: Introduction to Multicanonical Monte Carlo Simulations

abstract:
  Monte Carlo simulation with {\it a-priori} unknown weights have attracted
recent attention and progress has been made in understanding (i) the technical
feasibility of such simulations and (ii) classes of systems for which such
simulations lead to major improvements over conventional Monte Carlo
simulations. After briefly sketching the history of multicanonical calculations
and their range of application, a general introduction in the context of the
statistical physics of the d-dimensional generalized Potts models is given.
Multicanonical simulations yield canonical expectation values for a range of
temperatures or any other parameter(s) for which appropriate weights can be
constructed. We shall address in some details the question how the
multicanonical weights are actually obtained. Subsequently miscellaneous topics
related to the considered algorithms are reviewed. Then multicanonical studies
of first order phase transitions are discussed and finally applications to
complex systems such as spin glasses and proteins.


title: Microwave study of quantum n-disk scattering

abstract:
  We describe a wave-mechanical implementation of classically chaotic n-disk
scattering based on thin 2-D microwave cavities. Two, three, and four-disk
scattering are investigated in detail. The experiments, which are able to probe
the stationary Green's function of the system, yield both frequencies and
widths of the low-lying quantum resonances. The observed spectra are found to
be in good agreement with calculations based on semiclassical periodic orbit
theory. Wave-vector autocorrelation functions are analyzed for various
scattering geometries, the small wave-vector behavior allowing one to extract
the escape rate from the quantum repeller. Quantitative agreement is found with
the value predicted from classical scattering theory. For intermediate
energies, non-universal oscillations are detected in the autocorrelation
function, reflecting the presence of periodic orbits.


title: New Physics in High Landau Levels

abstract:
  Recent magneto-transport experiments on ultra-high mobility 2D electron
systems in GaAs/AlGaAs heterostructures have revealed the existence of whole
new classes of correlated many-electron states in highly excited Landau levels.
These new states, which appear only at extremely low temperatures, are
distinctly different from the familiar fractional quantum Hall liquids of the
lowest Landau level. Prominent among the recent findings are the discoveries of
giant anisotropies in the resistivity near half filling of the third and higher
Landau levels and the observation of re- entrant integer quantum Hall states in
the flanks of these same levels. This contribution will survey the present
status of this emerging field.


title: Mean-Field Theory of a Quantum Heisenberg Spin Glass

abstract:
  A full mean field solution of a quantum Heisenberg spin glass model is
presented in a large-N limit. A spin glass transition is found for all values
of the spin S. The quantum critical regime associated with the quantum
transition at S=0, and the various regimes in the spin glass phase at high spin
are analyzed. The specific heat is shown to vanish linearly with temperature.
In the spin-glass phase, intriguing connections between the equilibrium
properties of the quantum problem and the out-of-equilibrium dynamics of
classical models are pointed out.


title: Monte Carlo Hamiltonian - From Statistical Physics to Quantum Theory

abstract:
  Monte Carlo techniques have been widely employed in statistical physics as
well as in quantum theory in the Lagrangian formulation. However, in some areas
of application to quantum theories computational progress has been slow. Here
we present a recently developed approach: the Monte Carlo Hamiltonian method,
designed to overcome the difficulties of the conventional approach.

categories: cond-mat.stat-mech hep-lat hep-ph hep-th


title:
Spin Splitting in de Haas-van Alp-hen Oscillation in Two-Dimensional
  Two-Band Systems

abstract:
  We study the effects of the Zeeman term on the de Haas van Alphen (dHvA)
oscillation in two-dimensional two-band systems. We found that the Fourier
transform amplitudes of the oscillations are not described by the spin
reduction factor in the Lifshitz-Kosevich formalism in two-dimensional systems.
The anomalous dependence on the effective $g$-factor can be observed by
tilting-angle dependence of the dHvA oscillation in quasi-two-dimensional
organic conductors and Sr$_2$RuO$_4$.


title:
Scale-free energy dissipation and dynamic phase transition in stochastic
  sandpiles

abstract:
  We study numerically scaling properties of the distribution of cumulative
energy dissipated in an avalanche and the dynamic phase transition in a
stochastic directed cellular automaton [B. Tadi\'c and D. Dhar, Phys. Rev.
Lett. {\bf 79}, 1519 (1997)] in d=1+1 dimensions. In the critical steady state
occurring for the probability of toppling $p\ge p^\star$= 0.70548, the
dissipated energy distribution exhibits scaling behavior with new scaling
exponents $\tau_E $ and D_E for slope and cut-off energy, respectively,
indicating that the sandpile surface is a fractal. In contrast to avalanche
exponents, the energy exponents appear to be p- dependent in the region
$p^\star \le p <1$, however the product $(\tau_E-1)D_E$ remains universal. We
estimate the roughness exponent of the transverse section of the pile as $\chi
=0.44\pm 0.04$. Critical exponents characterizing the dynamic phase transition
at $p^\star $ are obtained by direct simulation and scaling analysis of the
survival probability distribution and the average outflow current. The
transition belongs to a new universality class with the critical exponents
$\nu_\| =\gamma =1.22 \pm 0.02$, $\beta =0.56\pm 0.02$ and $\nu_\bot = 0.761
\pm 0.029$, with apparent violation of hyperscaling. Generalized hyperscaling
relation leads to $\beta + \beta ^\prime = (d-1)\nu_\bot $, where $\beta
^\prime = 0.195 \pm 0.012$ is the exponent governed by the ultimate survival
probability.


title:
Magnetic properties of the three-dimensional Hubbard model at half
  filling

abstract:
  We study the magnetic properties of the 3d Hubbard model at half-filling in
the TPSC formalism, previously developed for the 2d model. We focus on the
N\'eel transition approached from the disordered side and on the paramagnetic
phase. We find a very good quantitative agreement with Dynamical Mean-Field
results for the isotropic 3d model. Calculations on finite size lattices also
provide satisfactory comparisons with Monte Carlo results up to the
intermediate coupling regime. We point out a qualitative difference between the
isotropic 3d case, and the 2d or anisotropic 3d cases for the double occupation
factor. Even for this local correlation function, 2d or anisotropic 3d cases
are out of reach of DMF: this comes from the inability of DMF to account for
antiferromagnetic fluctuations, which are crucial.


title:
Operation of Quantum Cellular Automaton cells with more than two
  electrons

abstract:
  We present evidence that operation of QCA (Quantum Cellular Automaton) cells
with four dots is possible with an occupancy of 4N+2 electrons per cell (N
being an integer). We show that interaction between cells can be described in
terms of a revised formula for cell polarization, which is based only on the
difference between diagonal occupancies. We validate our conjectures with full
quantum simulations of QCA cells for a number of electrons varying from 2 to 6,
using the Configuration-Interaction method.


title: Worse fluctuation method for fast Value-at-Risk estimates

abstract:
  We show how one can actually take advantage of the strongly non-Gaussian
nature of the fluctuations of financial assets to simplify the calculation of
the Value-at-Risk of complex non linear portfolios. The resulting equations are
not hard to solve numerically, and should allow fast VaR and $\Delta$VaR
estimates of large portfolios, where {\it by construction} the influence of
rare events is taken into account reliably. Our method can be seen as a
correctly probabilized `scenario' calculation (or `stress-testing').


title:
Investigation of the Mesoscopic Aharonov-Bohm effect in Low Magnetic
  Fields

abstract:
  We have investigated the Aharonov-Bohm effect in mesoscopic semiconductor
GaAs/GaAlAs rings in low magnetic fields. The oscillatory magnetoconductance of
these systems is systematically studied as a function of electron density. We
observe phase-shifts of $\pi$ in the magnetoconductance oscillations, and
halving of the fundamental $h/e$ period, as the density is varied.
Theoretically we find agreement with the experiment, by introducing an
asymmetry between the two arms of the ring.


title: Evolution of a sandpile in a thick flow regime

abstract:
  We solve a one-dimensional sandpile problem analytically in a thick flow
regime when the pile evolution may be described by a set of linear equations.
We demonstrate that, if an income flow is constant, a space periodicity takes
place while the sandpile evolves even for a pile of only one type of particles.
Hence, grains are piling layer by layer. The thickness of the layers is
proportional to the input flow of particles $r_0$ and coincides with the
thickness of stratified layers in a two-component sandpile problem which were
observed recently. We find that the surface angle $\theta$ of the pile reaches
its final critical value ($\theta_f$) only at long times after a complicated
relaxation process. The deviation ($\theta_f - \theta $) behaves asymptotically
as $(t/r_{0})^{-1/2}$. It appears that the pile evolution depends on initial
conditions. We consider two cases: (i) grains are absent at the initial moment,
and (ii) there is already a pile with a critical slope initially. Although at
long times the behavior appears to be similar in both cases, some differences
are observed for the different initial conditions are observed. We show that
the periodicity disappears if the input flow increases with time.


title: Storage capacity of a constructive learning algorithm

abstract:
  Upper and lower bounds for the typical storage capacity of a constructive
algorithm, the Tilinglike Learning Algorithm for the Parity Machine [M. Biehl
and M. Opper, Phys. Rev. A {\bf 44} 6888 (1991)], are determined in the
asymptotic limit of large training set sizes. The properties of a perceptron
with threshold, learning a training set of patterns having a biased
distribution of targets, needed as an intermediate step in the capacity
calculation, are determined analytically. The lower bound for the capacity,
determined with a cavity method, is proportional to the number of hidden units.
The upper bound, obtained with the hypothesis of replica symmetry, is close to
the one predicted by Mitchinson and Durbin [Biol. Cyber. {\bf 60} 345 (1989)].


title: Slow $^{4}He$ Quenches Produce Fuzzy, Transient Vortices

abstract:
  We examine the Zurek scenario for the production of vortices in quenches of
liquid $^{4}He$ in the light of recent experiments. Extending our previous
results to later times, we argue that short wavelength thermal fluctuations
make vortices poorly defined until after the transition has occurred. Further,
if and when vortices appear, it is plausible that that they will decay faster
than anticipated from turbulence experiments, irrespective of quench rates.


title: The six-vertex model on random lattices

abstract:
  In this letter, the 6-vertex model on dynamical random lattices is defined
via a matrix model and rewritten (following I. Kostov) as a deformation of the
O(2) model. In the large N planar limit, an exact solution is found at
criticality. The critical exponents of the model are determined; they vary
continously along the critical line. The vicinity of the latter is explored,
which confirms that we have a line of c=1 conformal field theories coupled to
gravity.

categories: cond-mat.stat-mech hep-th math-ph math.MP


title:
The spin-1/2 transverse XX chain with regularly alternating bonds and
  fields

abstract:
  We use continued fractions for a study of the thermodynamic properties of the
periodic nonuniform spin-1/2 isotropic XY chain in a non-random/random
(Lorentzian) transverse field. The obtained results permit to examine the
influence of a magnetic field and randomness on the spin-Peierls dimerization.


title:
Effect of Excluded Volume and Anisotropy on Granular Statistics: 'Fermi
  Statistics' and Condensation

abstract:
  We explore the consequences of the excluded volume interaction of hard
spheres at high densities and present a theory for excited granular materials.
We first demonstrate that, in the presence of gravity, the granular density
crosses over from Boltzmann to Fermi statistics, when temperature is decreased
in the weak excitation limit. Comparisons of numerical simulations with our
predictions concerning the scaling behavior of temperature with agitation
frequency, gravity and particle-diameter show satisfying agreement. Next,
within the framework of the Enskog theory of hard spheres, we interpret this
crossover as a 'condensation' of hard spheres from the dilute gas-state to a
high density solid-like state. In the high density, low temperature limit
Enskog theory fails because it predicts densities larger than the closed packed
density below a certain temperature. We show how to extend the range of
applicability of the Enskoq theory to arbitrarily low temperatures by
constructing a physical solution: all particles that are situated in regions
with densities larger than a certain maximum density are assumed to be
'condensed.'


title:
High-field transport properties of bulk Si: A test for the Fokker-Planck
  approach

abstract:
  High electric-field transport parameters are calculated using an analytical
Fokker-Planck approach (FPA), where transport is modeled as a drift-diffusion
process in energy space. We have applied the theory to the case of Si, taking
into account the six intervalley phonons, aiming to test the FPA. The obtained
results show a quite reasonable agreement with experimental data and Monte
Carlo simulations confirming in this case that the FPA works very well for high
enough electric fields.


title:
Ferromagnetic, A-type, and Charge-Ordered CE-type States in Doped
  Manganites using Jahn-Teller Phonons

abstract:
  The two-orbital model for manganites with both non-cooperative and
cooperative Jahn-Teller phonons is studied at hole density x=0.5 using Monte
Carlo techniques. The phase diagram is obtained varying the electron-phonon
coupling and the $\rm t_{2g}$-spins exchange. The insulating CE-type charge-
and orbital-ordered state with the $z$-axis charge $stacking$ observed in
narrow-bandwidth manganites is stabilized in the simulations. Its charge gap
$\Delta_{\rm CO}$ is much larger than the critical temperature $\rm k_B
T_{CO}$. Metallic-like A-type and ferromagnetic states are also obtained in the
same framework, and the phase boundaries among them have first-order
characteristics.


title:
The Influence of Electrostatic Truncation on Simulations of Polarizable
  Systems

abstract:
  Different schemes for the treatment of long-ranged electrostatic interactions
will be examined for water simulations using the polarizable fluctuating charge
potential. Several different methods are compared, including Ewald sums,
potential shifting, spherical truncation and reaction field corrections. For
liquid water, properties such as the energy, pressure, dynamics and structure
are more sensitive to the treatment of the long-ranged interactions with
polarizable than with non-polarizable potentials.


title: Commensurate dynamic magnetic correlations in La2(Cu,Li)O4

abstract:
  When sufficient numbers of holes are introduced into the two-dimensional CuO2
square lattice, dynamic magnetic correlations become incommensurate with
underlying lattice in all previously investigated La_{2-x}A_xCu_{1-z}B_zO_{4+y}
(A=Sr or Nd, B=Zn) including high T_C superconductors and insulators, and in
bilayered superconducting YBa_2Cu_3O_{6.6} and Bi_2Sr_2CaCu_2O_8. Magnetic
correlations also become incommensurate in structurally related La_2NiO_4 when
doped with Sr or O. We report an exception to this so-far well established
experimental "rule" in La_2Cu_{1-z}Li_{z}O_4 in which magnetic correlations
remain commensurate.


title:
Calculation of Neel temperature for S=1/2 quasi-one-dimensional
  Heisenberg antiferromagnets

abstract:
  Isotropic S=1/2 quasi-one-dimensional antiferromagnets are considered within
the bosonization method. The 1/zp-corrections to the interchain mean-field
theory (zp is the number of nearest neighbors in transverse to chain
directions) are obtained for the ground-state sublattice magnetization and Neel
temperature T_N. The corrections to T_N make up about 25% of mean-feld value,
while those to ground-state sublattice magnetization are small enough
(especially in the three-dimensional case). The fluctuation corrections
obtained improve considerably the agreement with the experimental data for
magnetic-chain compounds KCuF3, Sr2CuO3 and Ca2CuO3.


title:
Integrable open-boundary conditions for the q-deformed extended Hubbard
  model

abstract:
  Integrable open-boundary condition for the q-deformed
Essler-Korepin-Schoutens extended Hubbard model of strongly correlated
electrons, are studied in the framework of the boundary quantum inverse
scattering method. Diagonal boundary K-matrices are found, and nine classes of
integrable boundary terms are determined.


title:
Integrable one-dimensional supersymmetric q-deformed extended Hubbard
  model with boundary Kondo impurity

abstract:
  This paper has been withdrawn by the author, due to an error in relation
(11).


title: Gel transitions in colloidal suspensions

abstract:
  The idealized mode coupling theory (MCT) is applied to colloidal systems
interacting via short-range attractive interactions of Yukawa form. At low
temperatures MCT predicts a slowing down of the local dynamics and ergodicity
breaking transitions. The nonergodicity transitions share many features with
the colloidal gel transition, and are proposed to be the source of gelation in
colloidal systems. Previous calculations of the phase diagram are complemented
with additional data for shorter ranges of the attractive interaction, showing
that the path of the nonergodicity transition line is then unimpeded by the
gas-liquid critical curve at low temperatures. Particular attention is given to
the critical nonergodicity parameters, motivated by recent experimental
measurements. An asymptotic model is developed, valid for dilute systems of
spheres interacting via strong short-range attractions, and is shown to capture
all aspects of the low temperature MCT nonergodicity transitions.


title:
Another Derivation of a Sum Rule for the Two-Dimensional Two-Component
  Plasma

abstract:
  In a two-dimensional two-component plasma, the second moment of the number
density correlation function has the simple value $\{12 \pi
[1-(\Gamma/4)]^2\}^{-1}$, where $\Gamma$ is the dimensionless coupling
constant. This result is derived directly by using diagrammatic methods.


title:
Shape Effects of Finite-Size Scaling Functions for Anisotropic
  Three-Dimensional Ising Models

abstract:
  The finite-size scaling functions for anisotropic three-dimensional Ising
models of size $L_1 \times L_1 \times aL_1$ ($a$: anisotropy parameter) are
studied by Monte Carlo simulations. We study the $a$ dependence of finite-size
scaling functions of the Binder parameter $g$ and the magnetization
distribution function $p(m)$. We have shown that the finite-size scaling
functions for $p(m)$ at the critical temperature change from a two-peak
structure to a single-peak one by increasing or decreasing $a$ from 1. We also
study the finite-size scaling near the critical temperature of the layered
square-lattice Ising model, when the systems have a large two-dimensional
anisotropy. We have found the three-dimensional and two-dimensional finite-size
scaling behavior depending on the parameter which is fixed; a unified view of
3D and 2D finite-size scaling behavior has been obtained for the anisotropic 3D
Ising models.


title: Magneto-Transport Properties of the Rutheno-Cuprate RuSr_2GdCu_2O_8

abstract:
  RuSr_2GdCu_2O_8 (Ru-1212) is a triple perovskite containing both CuO_2 and
RuO_2 layers. It has attracted a great deal of interest recently because it
displays electronic ferromagnetism and superconductivity coexisting on a
microscopic scale, with T_Curie of 135K and T_c up to 50K.
  We report magnetisation and magnetoresistance (MR) data that exhibit effects
due to the interaction between the ferromagnetic ruthenium moments and the
conduction electrons. The MR is negative at temperatures above T_Curie, and is
proportional to the square of the applied field well above this temperature.
Below T_Curie the MR displays a positive peak at fields of around 15 kOe, but
at high fields it becomes negative again, and approximately linear. We analyse
the high temperature data in terms of spin-disorder scattering theory and
extract a value for the exchange interaction between the carriers in the CuO_2
planes and the Ru spins.


title:
Some Peculiarities of Proton Transport in Quasi-One-Dimensional
  Hydrogen-Bonded Chains

abstract:
  The protonic conductivity in the hydrogen bonded chains is investigated
theoretically in the framework of the two-stage transport model. The strong
interactions with optical phonon stretching mode are considered. We obtain the
transition from the insulator to the metal-type state from the temperature
dependencies of the hopping conductivity and analyze the influence of the
specific Grotthuss mechanism on the transition character. We investigate also
the main peculiarities in the frequency dependencies of the band and hopping
conductivity parts which appear due to above-mentioned two-stage process of the
proton migration along the chain.


title: Modeling Market Mechanism with Minority Game

abstract:
  Using the Minority Game model we study a broad spectrum of problems of market
mechanism. We study the role of different types of agents: producers,
speculators as well as noise traders. The central issue here is the information
flow : producers feed in the information whereas speculators make it away. How
well each agent fares in the common game depends on the market conditions, as
well as their sophistication. Sometimes there is much to gain with little
effort, sometimes great effort virtually brings no more incremental gain.
Market impact is shown to play also an important role, a strategy should be
judged when it is actually used in play for its quality. Though the Minority
Game is an extremely simplified market model, it allows to ask, analyze and
answer many questions which arise in real markets.


title:
Onsager reaction-field theory for magnetic models on diamond and hcp
  lattices

abstract:
  The Onsager reaction-field (ORF) theory is extended so that it applies to any
three-dimensional Bravais lattice with a basis. The ORF calculation is used to
predict the critical temperature for classical Ising, XY, and Heisenberg
magnetic models, in particular, on diamond and hexagonal close packed lattices.
Results are compared with series extrapolations and other theoretical
approaches where available. For the hcp lattice the ORF calculation is seen to
be exactly equivalent to a Green's function approach.


title:
Electronic Raman scattering on under-doped Hg-1223 high-Tc
  superconductors:investigations on the symmetry of the order parameter

abstract:
  In order to obtain high quality, reliable electronic Raman spectra of a
high-Tc superconductor compound, we have studied strongly under-doped
HgBa2Ca2Cu3O8+d. This choice was made such as to i)minimize oxygen disorder in
the Hg-plane generated by oxygen doping ii) avoid the need of phonon background
subtraction from the raw data iii)eliminate traces of parasitic phases
identified and monitored on the crystal surface. Under these experimental
conditions we are able to present the pure electronic Raman response function
in the B2g, B1g, A1g+B2g and A1g+B1g channels. The B2g spectrum exhibits a
linear frequency dependence at low energy whereas the B1g one shows a
cubic-like dependence. The B2g and B1g spectra display two well defined maxima
at 5.6kBTc and 9kBTc respectively. In mixed A1g channels an intense electronic
peak centered around 6.4 kBTc is observed. The low energy parts of the spectra
correspond to the electronic response expected for a pure dx2-y2 gap symmetry
and can be fitted up to the gap energy for the B1g channel. However, in the
upper parts, the relative position of the B1g and B2g peaks needs expanding the
B2g Raman vertex to second order Fermi surface harmonics to fit the data with
the dx2-y2 model. The sharper and more intense A1g peak appears to challenge
the Coulomb screening efficiency present for this channel. As compared to
previous data on more optimally doped, less stoichiometric
  Hg-1223 compounds, this work reconciles the electronic Raman spectra of
under- doped Hg-1223 crystals with the dx2-y2 model, provided that the oxygen
doping is not too strong. This apparent extreme sensitivity of the electronic
Raman spectra to the low lying excitations induced by oxygen doping in the
superconducting state is emphasized here and remains an open question.


title: Finite size Spin Wave theory of the triangular Heisenberg model

abstract:
  We present a finite size spin wave calculation on the Heisenberg
antiferromagnet on the triangular lattice focusing in particular on the
low-energy part of the excitation spectrum. For s=1/2 the good agreement with
the exact diagonalization and quantum Monte Carlo results supports the
reliability of the spin wave expansion to describe the low-energy spin
excitations of the Heisenberg model even in presence of frustration. This
indicates that the spin susceptibility of the triangular antiferromagnet is
very close to the linear spin wave result.


title: Statistical mechanics of a nonlinear discrete system

abstract:
  Statistical mechanics of the discrete nonlinear Schr\"odinger equation is
studied by means of analytical and numerical techniques. The lower bound of the
Hamiltonian permits the construction of standard Gibbsian equilibrium measures
for positive temperatures. Beyond the line of $T=\infty$, we identify a phase
transition, through a discontinuity in the partition function. The phase
transition is demonstrated to manifest itself in the creation of breather-like
localized excitations. Interrelation between the statistical mechanics and the
nonlinear dynamics of the system is explored numerically in both regimes.


title: Stripe Conductivity in La(1.775)Sr(0.225)NiO(4)

abstract:
  We report Raman light-scattering and optical conductivity measurements on a
single crystal of La(1.775)Sr(0.225)NiO(4) which exhibits incommensurate
charge-stripe order. The extra phonon peaks induced by stripe order can be
understood in terms of the energies of phonons that occur at the charge-order
wave vector, Q_c. A strong Fano antiresonance for a Ni-O bond-stretching mode
provides clear evidence for finite dynamical conductivity within the charge
stripes.


title: Thermoelastic Damping in Micro- and Nano-Mechanical Systems

abstract:
  The importance of thermoelastic damping as a fundamental dissipation
mechanism for small-scale mechanical resonators is evaluated in light of recent
efforts to design high-Q micrometer- and nanometer-scale electro-mechanical
systems (MEMS and NEMS). The equations of linear thermoelasticity are used to
give a simple derivation for thermoelastic damping of small flexural vibrations
in thin beams. It is shown that Zener's well-known approximation by a
Lorentzian with a single thermal relaxation time slightly deviates from the
exact expression.


title:
Magnetic quasicrystals: What can we expect to see in their neutron
  diffraction data?

abstract:
  The theory of magnetic symmetry in quasicrystals is used to characterize the
nature of magnetic peaks, expected in elastic neutron diffraction experiments.
It is established that there is no symmetry-based argument which forbids the
existence of quasiperiodic long-range magnetic order. Suggestions are offered
as to where one should look for the simplest kinds of antiferromagnetic
quasicrystals.


title:
Universal Scaling of Wave Propagation Failure in Arrays of Coupled
  Nonlinear Cells

abstract:
  We study the onset of the propagation failure of wave fronts in systems of
coupled cells. We introduce a new method to analyze the scaling of the critical
external field at which fronts cease to propagate, as a function of
intercellular coupling. We find the universal scaling of the field throughout
the range of couplings, and show that the field becomes exponentially small for
large couplings. Our method is generic and applicable to a wide class of
cellular dynamics in chemical, biological, and engineering systems. We confirm
our results by direct numerical simulations.


title: Band structure of local pairs. Mathematical tools

abstract:
  A system of strongly interacting fermions in a solid state is discussed. A
structure of singlet and triplet coupled 2-particle states and their excitation
spectra are investigated. It is shown that an account of intersite fermion
interaction leads to new boson modes. A problem of interaction of pairs with
external fields are discussed. The same analysis can be performed for another
sorts of pairings like d-wave.


title:
Some linear differential expressions for an electron scattering problem
  in a field of the one-dimentional arbitrary potential

abstract:
  The linear system of differential equations for determination of transmission
and reflection amplytudes of scattered electron in the field of one dimensional
arbitrary potential is obtained. It is shown that in general the scattering
problem can be reduced to the Caushy problem for the stationary
  Shrodinger equation. An explicit functional for a dependance of transmission
amplitude from scattered field is found.


title:
Bi-stability of mixed states in neural network storing hierarchical
  patterns

abstract:
  We discuss the properties of equilibrium states in an autoassociative memory
model storing hierarchically correlated patterns (hereafter, hierarchical
patterns). We will show that symmetric mixed states (hereafter, mixed states)
are bi-stable on the associative memory model storing the hierarchical patterns
in a region of the ferromagnetic phase. This means that the first-order
transition occurs in this ferromagnetic phase. We treat these contents with a
statistical mechanical method (SCSNA) and by computer simulation. Finally, we
discuss a physiological implication of this model. Sugase et al. analyzed the
time-course of the information carried by the firing of face-responsive neurons
in the inferior temporal cortex. We also discuss the relation between the
theoretical results and the physiological experiments of Sugase et al.


title: Tricritical Behavior in the Extended Hubbard Chains

abstract:
  Phase diagrams of the one-dimensional extended Hubbard model (including
nearest-neighbor interaction $V$) at half- and quarter-filling are studied by
observing level crossings of excitation spectra using the exact
diagonalization. This method is based on the Tomonaga-Luttinger liquid theory
including logarithmic corrections which stem from the renormalization of the
Umklapp- and the backward-scattering effects.
  Using this approach, the phase boundaries are determined with high accuracy,
and then the structure of the phase diagram is clarified. At half-filling, the
phase diagram consists of two
  Berezinskii-Kosterlitz-Thouless (BKT) transition lines and one Gaussian
transition line in the charge sector, and one spin-gap transition line.
  This structure reflects the U(1) $\otimes$ SU(2) symmetry of the electron
system. Near the $U=2V$ line, the Gaussian and the spin-gap transitions take
place independently from the weak- to the intermediate-coupling region, but
these two transition lines are coupled in the strong-coupling region. This
result demonstrates existence of a tricritical point and a
bond-charge-density-wave (BCDW) phase between charge- and spin-density-wave
(CDW, SDW) phases. To clarify this mechanism of the transition, we also
investigate effect of a correlated hopping term which plays a role to enlarge
BCDW and bond-spin-density-wave (BSDW) phases. At quarter-filling, a similar
crossover phenomenon also takes place in the large-$V$ region involving
spin-gap and BKT-type metal-insulator transitions.


title: Many-Spin Effects and Tunneling Properties of Magnetic Molecules

abstract:
  Spin tunneling in molecular magnets has attracted much attention, however
theoretical considerations of this phenomenon up to now have not taken into
account the many-spin nature of molecular magnets. We present, to our
knowledge, the first successful attempt of a realistic calculation of tunneling
splittings for Mn$_{12}$ molecules, thus achieving a quantitatively accurate
many-spin description of a real molecular magnet in the energy interval ranging
from about 100 K down to 10$^{-12}$ K. Comparison with the results of the
standard single-spin model shows that many-spin effects affect the tunneling
splittings considerably. The values of ground state splitting given by
single-spin and many-spin models differ from each other by a factor of five.


title:
A Nonequilibrium Quantum Field Theory Description of the Bose-Einstein
  Condensate

abstract:
  We study the detailed out of equilibrium time evolution of a homogeneous
Bose-Einstein condensate.We consider a nonrelativistic quantum theory for a
self-interacting complex scalar field, immersed in a thermal bath, as an
effective microscopic model for the description of the Bose-Einstein
condensate. This approach yields the following main results:(i) the interaction
between fluctuations proves to be crucial in the mechanism of instability
generation; (ii) there are essentially two regimes in the $k$-space, with a
crossover for $k^2/2m \sim 2\lambda |\phi_0|^2$, where, in our notation,
$\lambda$ is the coupling constant and $|\phi_0|^2$ is the condensate density;
(iii) a set of coupled equations that determines completely the nonequilibrium
dynamics of the condensate density as a function of the temperature and of the
total density of the gas.

categories: cond-mat.soft hep-ph hep-th nucl-th quant-ph


title: Studies of the t-J two-leg ladder via series expansions

abstract:
  Series expansions at T=0 are used to study properties of the half-filled
$t-J$ ladder doped with one or two holes, and at quarter filling. Dispersion
curves are obtained for one-hole symmetric and antisymmetric (bonding and
antibonding) excitations and for the two-hole bound state. The line in the
phase diagram which separates bound and unbound states is determined. For
quarter filling we compute the ground state energy and estimate the location of
the phase separation line. Comparisons with other numerical and analytical
results are presented.


title: Electron States in a Staircase Infinitely Deep Well in a Magnetic Field

abstract:
  The electron states in a size-quantaized coated semiconductor wire in a
uniform magnetic field applied parallel to the wire axis is considered. The
wave functions are found and the equation for determination of energy
eigenvalues depending both on magnetic field and parameters of the wire and the
coating is obtained. The binding energy of a hydrogen-like impurity located on
the wire axis is computed by the use of the variational method, and it is shown
that this energy increases with the increase of the magnetic field.