title: A three-dimensional lattice gas model for amphiphilic fluid dynamics

abstract:
  We describe a three-dimensional hydrodynamic lattice-gas model of amphiphilic
fluids. This model of the non-equilibrium properties of oil-water-surfactant
systems, which is a non-trivial extension of an earlier two-dimensional
realisation due to Boghosian, Coveney and Emerton [Boghosian, Coveney, and
Emerton 1996, Proc. Roy. Soc. A 452, 1221-1250], can be studied effectively
only when it is implemented using high-performance computing and visualisation
techniques. We describe essential aspects of the model's theoretical basis and
computer implementation, and report on the phenomenological properties of the
model which confirm that it correctly captures binary oil-water and
surfactant-water behaviour, as well as the complex phase behaviour of ternary
amphiphilic fluids.


title: Drift of a polymer chain in disordered media

abstract:
  We consider the drift of a polymer chain in a disordered medium, which is
caused by a constant force applied to the one end of the polymer, under
neglecting the thermal fluctuations. In the lowest order of the perturbation
theory we have computed the transversal fluctuations of the centre of mass of
the polymer, the transversal and the longitudinal size of the polymer, and the
average velocity of the polymer. The corrections to the quantities under
consideration, which are due to the interplay between the motion and the
quenched forces, are controlled by the driving force and the degree of
polymerization. The transversal fluctuations of the Brownian particle and of
the centre of mass of the polymer are obtained to be diffusive. The transversal
fluctuations studied in the present Letter may also be of relevance for the
related problem of the drift of a directed polymer in disordered media and its
applications.


title: Local Time-Reversal Symmetry Breaking in d_(x2-y2) Superconductors

abstract:
  We show that an isolated impurity in a spin singlet d_{x2-y2} superconductor
generates a d_{xy} order parameter with locally broken time-reversal symmetry.
The origin of this effect is a coupling between the d_{x2-y2} and the d_{xy}
order parameter induced by spin-orbit scattering off the impurity. The
signature of locally broken time-reversal symmetry is an induced orbital charge
current near the impurity, which generates a localized magnetic field in the
vicinity of the impurity. We present a microscopic theory for the impurity
induced d_{xy} component, discuss its spatial structure as well as the pattern
of induced current and local magnetic field near the localized impurity spin.


title: Pairing Correlations on t-U-J Ladders

abstract:
  Pairing correlations on generalized t-U-J two-leg ladders are reported. We
find that the pairing correlations on the usual t-U Hubbard ladder are
significantly enhanced by the addition of a nearest-neighbor exchange
interaction J. Likewise, these correlations are also enhanced for the t-J model
when the onsite Coulomb interaction is reduced from infinity. Moreover, the
pairing correlations are larger on a t-U-J ladder than on a t-Jeff ladder in
which Jeff has been adjusted so that the two models have the same spin gap at
half-filling. This enhancement of the pairing correlations is associated with
an increase in the pair-binding energy and the pair mobility in the t-U-J model
and point to the importance of the charge transfer nature of the cuprate
systems.


title:
Fermionic Mapping For Eigenvalue Correlation Functions Of (Weakly)
  Non-Hermitian Symplectic Ensemble

abstract:
  The eigenvalues of an arbitrary quaternionic matrix have a joint probability
distribution function first derived by Ginibre. We show that there exists a
mapping of this system onto a fermionic field theory and then use this mapping
to integrate over the positions of the eigenvalues and obtain eigenvalue
density as well as all higher correlation functions for both the strongly and
weakly non-Hermitian cases.


title: The fate of spinons in spontaneously dimerised spin-1/2 ladders

abstract:
  We study a weakly coupled, frustrated two-leg spin-1/2 Heisenberg ladder. For
vanishing coupling between the chains, elementary excitations are deconfined,
gapless spin-1/2 objects called spinons. We investigate the fate of spinons for
the case of a weak interchain interaction. We show that despite a drastic
change in ground state, which becomes spontaneously dimerised, spinons survive
as elementary excitations but acquire a spectral gap. We furthermore determine
the exact dynamical structure factor for several values of momentum transfer.


title: Charge melting and polaron collapse in $La_{1.2}Sr_{1.8}Mn_{2}O_{7}$

abstract:
  X-ray and neutron scattering measurements directly demonstrate the existence
of polarons in the paramagnetic phase of optimally-doped colossal
magnetoresistive oxides. The polarons exhibit short-range correlations that
grow with decreasing temperature, but disappear abruptly at the ferromagnetic
transition because of the sudden charge delocalization. The "melting" of the
charge ordering as we cool through $T_C$ occurs with the collapse of the
quasi-static polaron scattering, and provides important new insights into the
relation of polarons to colossal magnetoresistance.


title:
Exact critical exponent for the shortest-path scaling function in
  percolation

abstract:
  It is shown that the critical exponent $g_1$ related to pair-connectiveness
and shortest-path (or chemical distance) scaling, recently studied by Porto et
al., Dokholyan et al., and Grassberger, can be found exactly in 2d by using a
crossing-probability result of Cardy, with the outcome $g_1 = 25/24$. This
prediction is consistent with existing simulation results.


title: Conductivity of Layered Crystals

abstract:
  We show that the resistive anisotropy of an anisotropic medium is determined
by the ratio of the phase coherence lengths. In layered crystals in which the
interlayer transport is incoherent, the out-of-plane phase coherence length is
fixed and temperature independent. This leads to a temperature dependent
resistive anisotropy and to the coexistence of metallic in-plane and
non-metallic out-of-plane conductivities. Our approach provides a description
of the c-axis conductivity in the highly nonclassical regime, characteristic of
layered cuprates.


title: Universality classes in directed sandpile models

abstract:
  We perform large scale numerical simulations of a directed version of the
two-state stochastic sandpile model. Numerical results show that this
stochastic model defines a new universality class with respect to the Abelian
directed sandpile. The physical origin of the different critical behavior has
to be ascribed to the presence of multiple topplings in the stochastic model.
These results provide new insights onto the long debated question of
universality in abelian and stochastic sandpiles.


title: Bose-Einstein condensation with magnetic dipole-dipole forces

abstract:
  Ground-state solutions in a dilute gas interacting via contact and magnetic
dipole-dipole forces are investigated. To the best of our knowledge, it is the
first example of studies of the Bose-Einstein condensation in a system with
realistic long-range interactions. We find that for the magnetic moment of e.g.
chromium and a typical value of the scattering length all solutions are stable
and only differ in size from condensates without long-range interactions. By
lowering the value of the scattering length we find a region of unstable
solutions. In the neighborhood of this region the ground state wavefunctions
show internal structures not seen before in condensates. Finally, we find an
analytic estimate for the characteristic length appearing in these solutions.


title: Quantum Monte Carlo treatment of elastic exciton-exciton scattering

abstract:
  We calculate cross sections for low energy elastic exciton-exciton scattering
within the effective mass approximation. Unlike previous theoretical
approaches, we give a complete, non-perturbative treatment of the four-particle
scattering problem. Diffusion Monte Carlo is used to calculate the essentially
exact energies of scattering states, from which phase shifts are determined.
For the case of equal-mass electrons and holes, which is equivalent to
positronium-positronium scattering, we find a_s = 2.1 a_x for scattering of
singlet-excitons and a_s= 1.5 a_x for triplet-excitons, where a_x is the
excitonic radius. The spin dependence of the cross sections arises from the
spatial exchange symmetry of the scattering wavefunctions. A significant
triplet-triplet to singlet-singlet scattering process is found, which is
similar to reported effects in recent experiments and theory for excitons in
quantum wells. We also show that the scattering length can change sign and
diverge for some values of the mass ratio m_h/m_e, an effect not seen in
previous perturbative treatments.


title:
Instrumentation for Millimeter-wave Magnetoelectrodynamic Investigations
  of Low-Dimensional Conductors and Superconductors

abstract:
  We describe instrumentation for conducting high sensitivity millimeter-wave
cavity perturbation measurements over a broad frequency range (40-200 GHz) and
in the presence of strong magnetic fields (up to 33 tesla). A Millimeter-wave
Vector Network Analyzer (MVNA) acts as a continuously tunable microwave source
and phase sensitive detector (8-350 GHz), enabling simultaneous measurements of
the complex cavity parameters (resonance frequency and Q-value) at a rapid
repetition rate (approx. 10 kHz). We discuss the principal of operation of the
MVNA and the construction of a probe for coupling the MVNA to various
cylindrical resonator configurations which can easily be inserted into a high
field magnet cryostat. We also present several experimental results which
demonstrate the potential of the instrument for studies of low-dimensional
conducting systems.


title: Posture Sway and the Transition Rate for a Fall

abstract:
  Postural body sway displacements for quiet standing subjects (measured with a
new ultrasonic device) are reported. Two of the well known strategies for
balancing, namely ankle and hip movements were probed. The data is modeled
using a Fokker-Plank-Langevin stochastic theory. Both analytic and computer
simulation techniques are employed. The Kramers transition rate for a fall is
expressed as a function of experimental parameters. The root mean square
velocity is especially important in determining the fall probability.


title:
Distribution functions in non-equilibrium theory of superconductivity
  and Andreev spectroscopy in unconventional superconductors

abstract:
  We present a new theoretical formulation of non-equilibrium superconducting
phenomena, including singlet and triplet pairing. We start from the general
Keldysh-Nambu-Gor'kov Green's functions in the quasiclassical approximation and
represent them in terms of 2x2 spin-matrix coherence functions and distribution
functions for particle-type and hole-type excitations. The resulting transport
equations for the distribution functions may be interpreted as a generalization
to the superconducting state of Landau's transport equation for the normal
Fermi liquid of conduction electrons. The equations are well suited for
numerical simulations of dynamical phenomena. Using our formulation we solve an
open problem in quasiclassical theory of superconductivity, the derivation of
an explicit representation of Zaitsev's nonlinear boundary conditions [A.V.
Zaitsev, JETP 59, 1015 (1984)] at surfaces and interfaces. These boundary
conditions include non-equilibrium phenomena and spin singlet and triplet
unconventional pairing. We eliminate spurious solutions as well as numerical
stability problems present in the original formulation. Finally, we formulate
the Andreev scattering problem at interfaces in terms of the introduced
distribution functions and present a theoretical analysis for the study of time
reversal symmetry breaking states in unconventional superconductors via Andreev
spectroscopy experiments at N-S interfaces with finite transmission. We include
impurity scattering self consistently.


title:
Dynamic Entropy as a Measure of Caging and Persistent Particle Motion in
  Supercooled Liquids

abstract:
  The length-scale dependence of the dynamic entropy is studied in a molecular
dynamics simulation of a binary Lennard-Jones liquid above the mode-coupling
critical temperature $T_c$. A number of methods exist for estimating the
entropy of dynamical systems and we utilize an approximation based on
calculating the mean first-passage time (MFPT) for particle displacement
because of its tractability and its accessibility in real and simulation
measurements. The MFPT dynamic entropy $S(\epsilon)$ is defined to equal the
inverse of the average first-passage time for a particle to exit a sphere of
radius $\epsilon$. This measure of the degree of chaotic motion allows us to
identify characteristic time and space scales and to quantify the increasingly
correlated particle motion and intermittency occurring in supercooled liquids.
In particular, we identify a ``cage'' size defining the scale at which the
particles are transiently localized, and we observe persistent particle motion
at intermediate length scales beyond the scale where caging occurs.
Furthermore, we find that the dynamic entropy at the scale of one interparticle
spacing extrapolates to zero as the mode-coupling temperature $T_c$ is
approached.


title:
Moment Formalisms applied to a solvable Model with a Quantum Phase
  Transition. I. Exponential Moment Methods

abstract:
  We examine the Ising chain in a transverse field at zero temperature from the
point of view of a family of moment formalisms based upon the cumulant
generating function, where we find exact solutions for the generating functions
and cumulants at arbitrary couplings and hence for both the ordered and
disordered phases of the model. In a t-expansion analysis, the exact
Horn-Weinstein function E(t) has cuts along an infinite set of curves in the
complex Jt-plane which are confined to the left-hand half-plane Im Jt < -1/4
for the phase containing the trial state (disordered), but are not so for the
other phase (ordered). For finite couplings the expansion has a finite radius
of convergence. Asymptotic forms for this function exhibit a crossover at the
critical point, giving the excited state gap in the ground state sector for the
disordered phase, and the first excited state gap in the ordered phase.
Convergence of the t-expansion with respect to truncation order is found in the
disordered phase right up to the critical point, for both the ground state
energy and the excited state gap. However convergence is found in only one of
the Connected Moments Expansions (CMX), the CMX-LT, and the ground state energy
shows convergence right to the critical point again, although to a limited
accuracy.


title: Integrable open supersymmetric U model with boundary impurity

abstract:
  An integrable version of the supersymmetric U model with open boundary
conditions and an impurity situated at one end of the chain is introduced. The
model is solved through the algebraic Bethe ansatz method and the Bethe ansatz
equations are obtained.


title: Intrinsic Josephson Effect in the Layered Two-dimensional t-J Model

abstract:
  The intrinsic Josephson effect in the high-Tc superconductors is studied
using the layered two-dimensional t-J model. The d.c.Josephson current which
flows perpendicular to the t-J planes is obtained within the mean-field
approximation and the Gutzwiller approximation. We find that the Josephson
current has its maximum near the optimum doping region as a function of the
doping rate.


title: Andreev Spectroscopy for Superconductive Phase Qubit

abstract:
  New method to probe quantum dynamics of the order parameter phase of a small
superconductive island is proposed. The idea is to measure Andreev subgap
conductance between the island and a dirty normal-metal wire connected with the
island by very high-resistance tunnel barrier. The phase on the island is
supposed to stay primarly in one of the two nearly degenerate classical minima
i.e. the island is an active element of the Superconductive Phase Qubit.
Voltage-dependence of the differential subgap conductance measured at low
temperatures are predicted to demonstrate crucially different behavior
depending on the type of the internal dynamics of phase (coherent tunnelling of
phase, non-coherent tunnelling, or no tunneling). Thus the measurements of
nonliner Andreev conductance is a tool to probe the type of quantum behavior of
the artificial two-level system.


title:
Metallic phase between the Fermi glass and the Wigner crystal in two
  dimensions

abstract:
  For intermediate Coulomb energy to Fermi energy ratios $r_s$, spinless
fermions in a two-dimensional random potential form a new quantum phase,
different from the Fermi glass (weakly interacting Anderson localized states)
and the Wigner crystal (regular array of charges pinned by the disorder). The
intermediate phase is characterized by an ordered flow of persistent currents
with a typical value decreasing with excitation energy. Extending finite size
scaling analysis to the many body ground state, we find that electron-electron
interactions can drive the Fermi glass towards an intermediate metallic phase
(Coulomb metal).


title:
Numerical study on Anderson transitions in three-dimensional disordered
  systems in random magnetic fields

abstract:
  The Anderson transitions in a random magnetic field in three dimensions are
investigated numerically. The critical behavior near the transition point is
analyzed in detail by means of the transfer matrix method with high accuracy
for systems both with and without an additional random scalar potential. We
find the critical exponent $\nu$ for the localization length to be $1.45 \pm
0.09$ with a strong random scalar potential. Without it, the exponent is
smaller but increases with the system sizes and extrapolates to the above value
within the error bars. These results support the conventional classification of
universality classes due to symmetry. Fractal dimensionality of the wave
function at the critical point is also estimated by the equation-of-motion
method.


title:
Power-law distributions and Levy-stable intermittent fluctuations in
  stochastic systems of many autocatalytic elements

abstract:
  A generic model of stochastic autocatalytic dynamics with many degrees of
freedom $w_i$ $i=1,...,N$ is studied using computer simulations. The time
evolution of the $w_i$'s combines a random multiplicative dynamics $w_i(t+1) =
\lambda w_i(t)$ at the individual level with a global coupling through a
constraint which does not allow the $w_i$'s to fall below a lower cutoff given
by $c \cdot \bar w$, where $\bar w$ is their momentary average and $0<c<1$ is a
constant. The dynamic variables $w_i$ are found to exhibit a power-law
distribution of the form $p(w) \sim w^{-1-\alpha}$. The exponent $\alpha (c,N)$
is quite insensitive to the distribution $\Pi(\lambda)$ of the random factor
$\lambda$, but it is non-universal, and increases monotonically as a function
of $c$. The "thermodynamic" limit, N goes to infty and the limit of decoupled
free multiplicative random walks c goes to 0, do not commute: $\alpha(0,N) = 0$
for any finite $N$ while $ \alpha(c,\infty) \ge 1$ (which is the common range
in empirical systems) for any positive $c$. The time evolution of ${\bar w (t)}
$ exhibits intermittent fluctuations parametrized by a (truncated)
L\'evy-stable distribution $L_{\alpha}(r)$ with the same index $\alpha$. This
non-trivial relation between the distribution of the $w_i$'s at a given time
and the temporal fluctuations of their average is examined and its relevance to
empirical systems is discussed.


title:
Wave-vector dependence of spin and density multipole excitations in
  quantum dots

abstract:
  We have employed time-dependent local-spin density functional theory to
analyze the multipole spin and charge density excitations in GaAs-AlGaAs
quantum dots. The on-plane transferred momentum degree of freedom has been
taken into account, and the wave-vector dependence of the excitations is
discussed. In agreement with previous experiments, we have found that the
energies of these modes do not depend on the transferred wave-vector, although
their intensities do. Comparison with a recent resonant Raman scattering
experiment [C. Sch\"uller et al, Phys. Rev. Lett {\bf 80}, 2673 (1998)] is
made. This allows to identify the angular momentum of several of the observed
modes as well as to reproduce their energies.


title: Theory for atomic diffusion on fixed and deformable crystal lattices

abstract:
  We develop a theoretical framework for the diffusion of a single
unconstrained species of atoms on a crystal lattice that provides a
generalization of the classical theories of atomic diffusion and
diffusion-induced phase separation to account for constitutive nonlinearities,
external forces, and the deformation of the lattice. In this framework, we
regard atomic diffusion as a microscopic process described by two independent
kinematic variables: (i) the atomic flux, which reckons the local motion of
atoms relative to the motion of the underlying lattice, and (ii) the time-rate
of the atomic density, which encompasses nonlocal interactions between
migrating atoms and characterizes the kinematics of phase separation. We
introduce generalized forces power-conjugate to each of these rates and require
that these forces satisfy ancillary microbalances distinct from the
conventional balance involving the forces that expend power over the rate at
which the lattice deforms. A mechanical version of the second law, which takes
the form of an energy imbalance accounting for all power expenditures
(including those due to the atomic diffusion and phase separation), is used to
derive restrictions on the constitutive equations. With these restrictions, the
microbalance involving the forces conjugate to the atomic flux provides a
generalization of the usual constitutive relation between the atomic flux and
the gradient of the diffusion potential, a relation that in conjunction with
the atomic balance yields a generalized Cahn-Hilliard equation.


title: Multi-Phase Equilibrium of Crystalline Solids

abstract:
  A continuum model of crystalline solid equilibrium is presented in which the
underlying periodic lattice structure is taken explicitly into account. This
model also allows for both point and line defects in the bulk of the lattice
and at interfaces, and the kinematics of such defects is discussed in some
detail. A Gibbsian variational argument is used to derive the necessary bulk
and interfacial conditions for multi-phase equilibrium (crystal-crystal and
crystal-melt) where the allowed lattice variations involve the creation and
transport of defects in the bulk and at the phase interface. An interfacial
energy, assumed to depend on the interfacial dislocation density and the
orientation of the interface with respect to the lattices of both phases, is
also included in the analysis. Previous equilibrium results based on nonlinear
elastic models for incoherent and coherent interfaces are recovered as special
cases for when the lattice distortion is constrained to coincide with the
macroscopic deformation gradient, thereby excluding bulk dislocations. The
formulation is purely spatial and needs no recourse to a fixed reference
configuration or an elastic-plastic decomposition of the strain. Such a
decomposition can be introduced however through an incremental elastic
deformation superposed onto an already dislocated state, but leads to
additional equilibrium conditions. The presentation emphasizes the role of
{configurational forces} as they provide a natural framework for the
description and interpretation of singularities and phase transitions.


title:
Nonmonotonic External Field Dependence of the Magnetization in a Finite
  Ising Model: Theory and MC Simulation

abstract:
  Using $\phi^4$ field theory and Monte Carlo (MC) simulation we investigate
the finite-size effects of the magnetization $M$ for the three-dimensional
Ising model in a finite cubic geometry with periodic boundary conditions. The
field theory with infinite cutoff gives a scaling form of the equation of state
$h/M^\delta = f(hL^{\beta\delta/\nu}, t/h^{1/\beta\delta})$ where
$t=(T-T_c)/T_c$ is the reduced temperature, $h$ is the external field and $L$
is the size of system. Below $T_c$ and at $T_c$ the theory predicts a
nonmonotonic dependence of $f(x,y)$ with respect to $x \equiv
hL^{\beta\delta/\nu}$ at fixed $y \equiv t/h^{1/\beta \delta}$ and a crossover
from nonmonotonic to monotonic behaviour when $y$ is further increased. These
results are confirmed by MC simulation. The scaling function $f(x,y)$ obtained
from the field theory is in good quantitative agreement with the finite-size MC
data. Good agreement is also found for the bulk value $f(\infty,0)$ at $T_c$.


title: Vortex line and ring dynamics in trapped Bose-Einstein condensates

abstract:
  Vortex dynamics in inhomogeneous Bose-Einstein condensates are studied
numerically in two and three dimensions. We simulate the precession of a single
vortex around the center of a trapped condensate, and use the Magnus force to
estimate the precession frequency. Vortex ring dynamics in a spherical trap are
also simulated, and we discover that a ring undergoes oscillatory motion around
a circle of maximum energy. The position of this locus is calculated as a
function of the number of condensed atoms. In the presence of dissipation, the
amplitude of the oscillation will increase, eventually resulting in
self-annihilation of the ring.


title: Charge Order Driven spin-Peierls Transition in NaV2O5

abstract:
  We conclude from 23Na and 51V NMR measurements in NaxV2O5(x=0.996) a charge
ordering transition starting at T=37 K and preceding the lattice distortion and
the formation of a spin gap Delta=106 K at Tc=34.7 K. Above Tc, only a single
Na site is observed in agreement with the Pmmn space group of this first
1/4-filled ladder system. Below Tc=34.7 K, this line evolves into eight
distinct 23Na quadrupolar split lines, which evidences a lattice distortion
with, at least, a doubling of the unit cell in the (a,b) plane. A model for
this unique transition implying both charge density wave and spin-Peierls order
is discussed.


title: Bogoliubov's Theory: A Paradigm of Quantum Phase Transitions

abstract:
  This short essay discusses the application of Bogoliubov's theory of
superfluidity in the context of quantum phase transitions.


title: Optical and Magneto-optical Response of a Doped Mott Insulator

abstract:
  We study the optical, Raman, and ac Hall response of the doped Mott insulator
within the dynamical mean-field theory ($d=\infty$) for strongly correlated
electron systems. The occurence of the {\it isosbectic} point in the optical
conductivity is shown to be associated with the frequency dependence of the
generalized charge susceptibility. We compute the Raman response, which probes
the fluctuations of the "stress tensor", and show that the scattering is
characterized by appreciable incoherent contributions. The calculated ac Hall
constant and Hall angle also exhibit the isosbectic points. These results are
also compared with those obtained for a {\it non-FL} metal in $d=\infty$. The
role of low-energy coherence (FL) or incoherence (non-FL) in determining the
finite frequency response of strongly correlated metals in $d=\infty$ is
discussed in detail.


title:
Evidence for charge localization in the ferromagnetic phase of
  La_(1-x)Ca_(x)MnO_3 from High real-space-resolution x-ray diffraction

abstract:
  High real-space-resolution atomic pair distribution functions of
La_(1-x)Ca_(x)MnO_3 (x=0.12, 0.25 and 0.33) have been measured using
high-energy x-ray powder diffraction to study the size and shape of the MnO_6
octahedron as a function of temperature and doping. In the paramagnetic
insulating phase we find evidence for three distinct bond-lengths (~ 1.88, 1.95
and 2.15A) which we ascribe to Mn^{4+}-O, Mn^{3+}-O short and Mn^{3+}-O long
bonds respectively. In the ferromagnetic metallic (FM) phase, for x=0.33 and
T=20K, we find a single Mn-O bond-length; however, as the metal-insulator
transition is approached either by increasing T or decreasing x, intensity
progressively appears around r=2.15 and in the region 1.8 - 1.9A suggesting the
appearance of Mn^{3+}-O long bonds and short Mn^{4+}-O bonds. This is strong
evidence that charge localized and delocalized phases coexist close to the
metal-insulator transition in the FM phase.


title: Phase Transitions in a Two-Component Site-Bond Percolation Model

abstract:
  A method to treat a N-component percolation model as effective one component
model is presented by introducing a scaled control variable $p_{+}$. In Monte
Carlo simulations on $16^{3}$, $32^{3}$, $64^{3}$ and $128^{3}$ simple cubic
lattices the percolation threshold in terms of $p_{+}$ is determined for N=2.
Phase transitions are reported in two limits for the bond existence
probabilities $p_{=}$ and $p_{\neq}$. In the same limits, empirical formulas
for the percolation threshold $p_{+}^{c}$ as function of one
component-concentration, $f_{b}$, are proposed. In the limit $p_{=} = 0$ a new
site percolation threshold, $f_{b}^{c} \simeq 0.145$, is reported.


title: Exotic ground states and impurities in multiband superconductors

abstract:
  We consider the effect of isotropic impurity scattering on the exotic
superconducting states that arise from the usual BCS mechanism in substances of
cubic and hexagonal symmetry where the Fermi surface contains inequivalent but
degenerate pockets (e.g. around several points of high symmetry). As examples
we look at CeCo$_2$, CeRu$_2$, and LaB$_6$; all of which have such Fermi
surface topologies and the former exhibits unconventional superconducting
behavior. We find that while these non s-wave states are suppressed by
non-magnetic impurities, the suppression is much weaker than would be expected
for unconventional superconductors with isotropic non-magnetic impurity
scattering.


title:
Electron Correlation and the c-axis Dispersion of Cu d_z^2: a New Band
  Structure for High Temperature Superconductors

abstract:
  Previously we showed the major effect of electron correlation in the cuprate
superconductors is to lower the energy of the Cu d_x^2-y^2/O p_sigma (x^2-y^2)
band with respect to the Cu d_z^2/O' p_z (z^2) band. In our 2D Hubbard model
for La_1.85Sr_0.15CuO_4 (LaSCO), the z^2 band is narrow and crosses the
standard x^2-y^2 band just below the Fermi level. In this work, we introduce
c-axis dispersion to the model and find the z^2 band to have considerable
anisotropic 3D character. An additional hole-like surface opens up in the z^2
band at (0,0,2pi/c) which expands with doping. At sufficient doping levels, a
symmetry allowed x^2-y^2/z^2 band crossing along the (0,0)-(pi,pi) direction of
the Brillouin zone appears at the Fermi level. At this point, Cooper pairs
between the two bands (e.g. (k uparrow x^2-y^2/k downarrow z^2)) can form,
providing the basis for the Interband Pairing Theory of superconductivity in
these materials.


title: Vortex dynamics in a three-state model under cyclic dominance

abstract:
  The evolution of domain structure is investigated in a two-dimensional voter
model with three states under cyclic dominance. The study focus on the dynamics
of vortices, defined by the points where three states (domains) meet. We can
distinguish vortices and antivortices which walk randomly and annihilate each
other. The domain wall motion can create vortex-antivortex pairs at a rate
which is increased by the spiral formation due to the cyclic dominance. This
mechanism is contrasted with a branching annihilating random walk (BARW) in a
particle antiparticle system with density dependent pair creation rate.
Numerical estimates for the critical indices of the vortex density
($\beta=0.29(4)$) and of its fluctuation ($\gamma=0.34(6)$) improve an earlier
Monte Carlo study [Tainaka and Itoh, Europhys. Lett. 15, 399 (1991)] of the
three-state cyclic voter model in two dimensions.


title: Vicinal Surfaces, Fractional Statistics and Universality

abstract:
  We propose that the phases of all vicinal surfaces can be characterized by
four fixed lines, in the renormalization group sense, in a three-dimensional
space of coupling constants. The observed configurations of several Si surfaces
are consistent with this picture. One of these fixed lines also describes
one-dimensional quantum particles with fractional exclusion statistics. The
featureless steps of a vicinal surface can therefore be thought of as a
realization of fractional-statistics particles, possibly with additional
short-range interactions.


title: Loop Model with Generalized Fugacity in Three Dimensions

abstract:
  A statistical model of loops on the three-dimensional lattice is proposed and
is investigated. It is O(n)-type but has loop fugacity that depends on global
three-dimensional shapes of loops in a particular fashion. It is shown that,
despite this non-locality and the dimensionality, a layer-to-layer transfer
matrix can be constructed as a product of local vertex weights for infinitely
many points in the parameter space. Using this transfer matrix, the site
entropy is estimated numerically in the fully packed limit.

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


title:
Using Corbino disk sample geometry to separate quasiparticle and vortex
  motion contributions to the mixed state dissipation of YBCO

abstract:
  A method, using a Corbino disk sample geometry, is described and applied to
study of resistive tails of sintered $YBCO$. When the transport current passes
radially from the rim of the disk sample to its center, the two component
potential drop signal is detected below $T_c$, which is due to (i)
quasiparticles $W_q$ and (ii) vortex-core-motion related contribution $W_{\phi
}$. When the contact pairs for $W$ are placed radially, $W_q(r)$ and $W_{\phi
}(r)$ are found to follow the markedly distinctive functional dependences,
providing a unique possibility to deconvolute the relative strengths of both
contributions. The results obtained suggest that the mixed state dissipation of
high-$T_c$ superconductors is strongly influenced by the quasiparticle
excitations.


title: Nucleation in dilute 3He-4He liquid mixtures at low temperatures

abstract:
  We present a study of phase separation from supersaturated 3He-4He liquid
mixtures at low temperatures addressing both the degree of critical
supersaturation Dx and the thermal-to-quantum crossover temperature T* for the
nucleation process. Two different nucleation seeds are investigated, namely 3He
droplets and 4He vortex lines with cores filled with 3He. We have found that
the experimental T* is reproduced when we consider that nucleation proceeds
from 3He droplets, whereas Dx is reproduced when we consider 4He vortex lines
filled with 3He. However, neither nucleation configuration is able to
simultaneously reproduce the current experimental information on Dx and T*.


title:
Coupling strength of charge carriers to spin fluctuations in
  high-temperature superconductors

abstract:
  In conventional superconductors, the most direct evidence of the mechanism
responsible for superconductivity comes from tunneling experiments in which a
clear image of the electron-phonon interaction is revealed. The observed
structure in the current voltage characteristics at the phonon energies can be
used to measure, through inversion of the Eliashberg equations, the electron
phonon spectral density \alpha^2F(\omega). The coherence length in conventional
materials is long and the tunneling process probes several atomic layers into
the bulk of the superconductor. On the contrary, in the high-T_c oxides,
particularly for c-axis tunneling, the coherence length can be quite short and
in an optical experiment or in neutron scattering experiments the bulk of the
sample is probed. Therefore, these spectroscopies become the methods of choice
for the investigation of mechanisms of high-T_c superconductivity. Accurate
reflectance measurements in the infrared range and precise polarized neutron
scattering data are available for a variety of oxides. In this paper we show
that conducting carriers studied by means of infrared spectroscopy reveal
strong coupling to a resonance structure in the spectrum of spin fluctuations
examined with neutron scattering. The coupling strength inferred from
experiment is sufficient to account for high values of T_c which signals the
prominent role of spin excitations in the superconductivity of oxides.


title: Transaction costs: a new point of view

abstract:
  We consider a new approach to portfolio selection in presence of transaction
costs which allows to map the problem into one without costs. The proposed
approach connects all the quantities of interest to exit times and
probabilities to reach barriers. This leads to analytic results in the Wiener
case and to directly measurable quantities on a historical dataset in real
markets.


title: Noisy regression and classification with continuous multilayer networks

abstract:
  We investigate zero temperature Gibbs learning for two classes of
unrealizable rules which play an important role in practical applications of
multilayer neural networks with differentiable activation functions:
classification problems and noisy regression problems. Considering one step of
replica symmetry breaking, we surprisingly find that for sufficiently large
training sets the stable state is replica symmetric even though the target rule
is unrealizable. Further, the classification problem is shown to be formally
equivalent to the noisy regression problem.


title:
Dimensional crossover in a mesoscopic superconducting loop of finite
  width

abstract:
  Superconducting structures with the size of the order of the superconducting
coherence length xi(T) have a critical temperature T_c, oscillating as a
function of the applied perpendicular magnetic field H (or flux Phi). For a
thin-wire superconducting loop, the oscillations in T_c are perfectly periodic
with H (this is the well-known Little-Parks effect), while for a singly
connected superconducting disk the oscillations are pseudoperiodic, i.e. the
magnetic period decreases as H grows. In the present paper, we study the
intermediate case: a loop made of thick wires. By increasing the size of the
opening in the middle, the disk-like behaviour of T_c(H) with a quasi-linear
background (characteristic for '3-dimensional' (3D) behaviour) is shown to
evolve into a parabolic T_c(H) background ('2D'), superimposed with perfectly
periodic oscillations. The calculations are performed using the linearized
Ginzburg-Landau theory, with the proper normal/vacuum boundary conditions at
both the internal and the external interface. Above a certain crossover
magnetic flux Phi, T_c(Phi) of the loops becomes quasi-linear, and the flux
period matches with the case of the filled disk. This dimensional transition is
similar to the 2D-3D transition for thin films in a parallel magnetic field,
where vortices enter the material as soon as the film thickness t > 1.8 xi(T).
For the loops studied here, the crossover point appears for w = 1.8 xi(T) as
well, with w the width of the wires forming the loop. In the 3D regime, a
"giant vortex state" establishes, where superconductivity is concentrated near
the sample's outer interface. The vortex is then localized inside the loop's
opening.


title: Strong Phase Separation in a Model of Sedimenting Lattices

abstract:
  We study the steady state resulting from instabilities in crystals driven
through a dissipative medium, for instance, a colloidal crystal which is
steadily sedimenting through a viscous fluid. The problem involves two coupled
fields, the density and the tilt; the latter describes the orientation of the
mass tensor with respect to the driving field. We map the problem to a 1-d
lattice model with two coupled species of spins evolving through conserved
dynamics. In the steady state of this model each of the two species shows
macroscopic phase separation. This phase separation is robust and survives at
all temperatures or noise levels--- hence the term Strong Phase Separation.
This sort of phase separation can be understood in terms of barriers to
remixing which grow with system size and result in a logarithmically slow
approach to the steady state. In a particular symmetric limit, it is shown that
the condition of detailed balance holds with a Hamiltonian which has
infinite-ranged interactions, even though the initial model has only local
dynamics. The long-ranged character of the interactions is responsible for
phase separation, and for the fact that it persists at all temperatures.
Possible experimental tests of the phenomenon are discussed.


title:
A variational description of the ground state structure in random
  satisfiability problems

abstract:
  A variational approach to finite connectivity spin-glass-like models is
developed and applied to describe the structure of optimal solutions in random
satisfiability problems. Our variational scheme accurately reproduces the known
replica symmetric results and also allows for the inclusion of replica symmetry
breaking effects. For the 3-SAT problem, we find two transitions as the ratio
$\alpha$ of logical clauses per Boolean variables increases. At the first one
$\alpha_s \simeq 3.96$, a non-trivial organization of the solution space in
geometrically separated clusters emerges. The multiplicity of these clusters as
well as the typical distances between different solutions are calculated. At
the second threshold $\alpha_c \simeq 4.48$, satisfying assignments disappear
and a finite fraction $B_0 \simeq 0.13$ of variables are overconstrained and
take the same values in all optimal (though unsatisfying) assignments. These
values have to be compared to $\alpha_c \simeq 4.27, B_0 \simeq 0.4$ obtained
from numerical experiments on small instances. Within the present variational
approach, the SAT-UNSAT transition naturally appears as a mixture of a first
and a second order transition. For the mixed $2+p$-SAT with $p<2/5$, the
behavior is as expected much simpler: a unique smooth transition from SAT to
UNSAT takes place at $\alpha_c=1/(1-p)$.


title: Integrable model of interacting XX and Fateev-Zamolodchikov chains

abstract:
  We consider the exact solution of a model of correlated particles, which is
presented as a system of interacting XX and Fateev-Zamolodchikov chains. This
model can also be considered as a generalization of the multiband anisotropic
$t-J$ model in the case we restrict the site occupations to at most two
electrons. The exact solution is obtained for the eigenvalues and eigenvectors
using the Bethe-ansatz method.


title:
Theory for underdoped high-T_c superconductors: effects of phase
  fluctuations

abstract:
  In underdoped cuprates, T_c is thought to be determined by Cooper pair phase
fluctuations because of the small superfluid density n_s. Experimentally, T_c
is found to scale with n_s. The fluctuation-exchange approximation (FLEX) in
its standard form fails to predict this behavior of T_c since it does not
include phase fluctuations. We therefore extend the FLEX to include them
selfconsistently. We present results for T_c[n_s,x], where x is the doping.


title:
Charge carrier density collapse in La_0.67Ca_0.33MnO_3 and
  La_0.67Sr_0.33MnO_3 epitaxial thin films

abstract:
  We measured the temperature dependence of the linear high field Hall
resistivity of La_0.67Ca_0.33MnO_3 (T_C=232K) and La_0.67Sr_0.33MnO_3
(T_C=345K) thin films in the temperature range from 4K up to 360K in magnetic
fields up to 20T. At low temperatures we find a charge carrier density of 1.3
and 1.4 holes per unit cell for the Ca- and Sr-doped compound, respectively. In
this temperature range electron-magnon scattering contributes to the
longitudinal resistivity. At the ferromagnetic transition temperature T_C a
dramatic drop in the number of current carriers $n$ down to 0.6 holes per unit
cell, accompanied by an increase in unit cell volume, is observed. Corrections
of the Hall data due to a non saturated magnetic state will lead a more
pronounced charge carrier density collapse.


title:
The irreversibility line of overdoped Bi_{2+x}Sr_{2-(x+y)}Cu_{1+y}O_{6
  +- delta} at ultra-low temperatures and high magnetic fields

abstract:
  The irreversible magnetization of the layered high-T_{c} superconductor
Bi_{2+x}Sr_{2-(x+y)}Cu_{1+y}O_{6 +- delta} (Bi-2201) has been measured by means
of a capacitive torquemeter up to B=28 T and down to T=60 mK. No magnetization
jumps, peak effects or crossovers between different pinning mechanisms appear
to be present. The deduced irreversibility field B_{irr} can not be described
by the law B_{irr}(T)=B_{irr}(0)(1-T/T_{c})^n based on flux creep, but an
excellent agreement is found with the analytical form of the melting line of
the flux lattice as calculated from the Lindemann criterion. The behavior of
B_{irr}(T) obtained here is very similar to the resistive critical field of a
Bi-2201 thin film, suggesting that magnetoresistive experiments are likely to
be strongly influenced by flux lattice melting.


title: Intermittency and Nonextensivity in Turbulence and Financial Markets

abstract:
  We present a new framework for modeling the statistical behavior of both
fully developed turbulence and short-term dynamics of financial markets based
on the nonextensive thermostatistics proposed by Tsallis. We also show that
intermittency -- strong bursts in the energy dissipation or clusters of high
price volatility -- and nonextensivity -- anomalous scaling of usually
extensive properties like entropy -- are naturally linked by a single parameter
q, from the nonextensive thermostatistics.


title:
Interpretation of Nuclear Quadrupole Resonance Spectra in Doped
  La$_2$CuO$_4$

abstract:
  The nuclear quadrupole resonance (NQR) spectrum of strontium doped
La$_2$CuO$_4$ surprisingly resembles the NQR spectrum of La$_2$CuO$_4$ doped
with excess oxygen, both spectra being dominated by a main peak and one
principal satellite peak at similar frequencies. Using first-principles cluster
calculations this is investigated here by calculating the electric field
gradient (EFG) at the central copper site of the cluster after replacing a
lanthanum atom in the cluster with a strontium atom or adding an interstitial
oxygen to the cluster. In each case the EFG was increased by approximately 10 %
leading unexpectedly to the explanation that the NQR spectra are only
accidentally similar and the origins are quite different. Additionally the
widths of the peaks in the NQR spectra are explained by the different EFG of
copper centres remote from the impurity. A model, based on holes moving rapidly
across the planar oxygen atoms, is proposed to explain the observed increase in
frequency of both the main and satellite peaks in the NQR spectrum as the
doping concentration is increased.


title:
Two Landmarks in Polymer Physics: The Edwards Model and de Gennes
  Observation

abstract:
  The impact of two landmark papers one by Edwards and the other by de Gennes
on the field of polymer physics is highlighted.


title:
Screening of a macroion by multivalent ions: Correlation induced
  inversion of charge

abstract:
  Screening of a strongly charged macroion by multivalent counterions is
considered. It is shown that counterions form a strongly correlated liquid at
the surface of the macroion. Cohesive energy of this liquid leads to additional
attraction of counterions to the surface which is absent in conventional
solutions of Poisson-Boltzmann equation. Away from the surface this attraction
can be taken into account by a new boundary condition for the concentration of
counterions near the surface. Poisson-Boltzmann equation is solved with this
boundary condition for a charged flat surface, a cylinder and a sphere. In all
three cases, screening is much stronger than in the conventional approach. At
some critical exponentially small concentration of multivalent counterions in
the solution they totally neutralize the surface charge at small distances from
the surface. At larger concentrations they invert the sign the net macroion
charge. Absolute value of the inverted charge density can be as large as 20% of
that of the bare one. In particular, for a cylindrical macroion, it is shown
that for screening by multivalent counterions predictions of the
Onsager-Manning theory are quantitatively incorrect. The net charge density of
the cylinder is smaller than their theory predicts and inverts sign with
growing concentration of counterions. Moreover the condensation looses its
universality and the net charge linear density depends on the bare one.


title: A Critical Reexamination of the Peierls-Nabarro Model

abstract:
  We reexamine two essential issues within the Peierls-Nabarro model which are
critical in obtaining accurate values for the Peierls stress. The first issue
is related to the sampling scheme of the misfit energy across the glide plane
and the second one is the effect of relaxation on the Peierls stress. It is
shown that most of the current applications of the Peierls-Nabarro model do not
treat properly the two issues and therefore are not able to predict reliable
values for the Peierls stress. We argue that the double counting scheme for the
misfit energy at both sides of the glide plane is physically more reasonable,
and it can predict more accurate values for the Peierls stress, especially for
dislocations with equal spacing between alternating atomic rows. We also show
the importance for allowing atomic relaxation when a dislocation traverses the
Peierls barrier, which in turn lowers the Peierls stress for narrow
dislocations by an order of magnitude.


title: Mobility and stochastic resonance in spatially inhomogeneous system

abstract:
  The mobility of an overdamped particle, in a periodic potential tilted by a
constant external field and moving in a medium with periodic friction
coefficient is examined. When the potential and the friction coefficient have
the same periodicity but have a phase difference, the mobility shows many
interesting features as a function of the applied force, the temperature, etc.
The mobility shows stochastic resonance even for constant applied force, an
issue of much recent interest. The mobility also exhibits a resonance like
phenomenon as a function of the field strength and noise induced slowing down
of the particle in an appropriate parameter regime.


title: A Unified Theory of Dynamical Mean Field and Spin Fluctuations

abstract:
  A new approach is proposed which encompasses the dynamical mean field theory
(DMFT) for strongly correlated electron systems and the self-consistent
renormalization (SCR) theory of spin fluctuations. The latter is incorporated
into DMFT as a $O(1/d)$ correction, where $d$ is the spatial dimensionality,
but in a phenomenological way. The DMFT is completely recovered when the effect
of spin fluctuation is omitted, whereas the properties at the quantum critical
point (QCP) is the same as the SCR theory. An example of calculation is
presented for the single band Hubbard model and the non-Fermi liquid behavior
is demonstrated at QCP.


title: Simulation Studies on the Stability of the Vortex-Glass Order

abstract:
  The stability of the three-dimensional vortex-glass order in random type-II
superconductors with point disorder is investigated by equilibrium Monte Carlo
simulations based on a lattice XY model with a uniform field threading the
system. It is found that the vortex-glass order, which stably exists in the
absence of screening, is destroyed by the screenng effect, corroborating the
previous finding based on the spatially isotropic gauge-glass model. Estimated
critical exponents, however, deviate considerably from the values reported for
the gauge-glass model.


title:
Exact Quantization of Even-Denominator Fractional Quantum Hall State at
  $\nu$=5/2 Landau Level Filling Factor

abstract:
  We report ultra-low temperature experiments on the obscure fractional quantum
Hall effect (FQHE) at Landau level filling factor $\nu$=5/2 in a very high
mobility specimen of $\mu=1.7 \times 10^7$ cm$^2$/Vs. We achieve an electron
temperature as low as $\sim$ 4~mK, where we observe vanishing $R_{xx}$ and, for
the first time, a quantized Hall resistance, $R_{xy}=h/(5/2e^2)$ to within 2
ppm. $R_{xy}$ at the neighboring odd-denominator states $\nu$=7/3 and 8/3 is
also quantized. The temperature dependences of the $R_{xx}$-minima at these
fractional fillings yield activation energy gaps $\Delta_{5/2}$=0.11K,
$\Delta_{7/3}$=0.10K, and $\Delta_{8/3}$=0.055K.


title: Coupling of carbon nanotubes to metallic contacts

abstract:
  The modeling of carbon nanotube-metal contacts is important from both basic
and applied view points. For many applications, it is important to design
contacts such that the transmission is dictated by intrinsic properties of the
nanotube rather than by details of the contact. In this paper, we calculate the
electron transmission probability from a nanotube to a free electron metal,
which is side-contacted. If the metal-nanotube interface is sufficiently
ordered, we find that k-vector conservation plays an important role in
determining the coupling, with the physics depending on the area of contact,
tube diameter and chirality. The main results of this paper are: (i)
conductance scales with contact length, a phenomena that has been observed in
experiments and (ii) in the case of uniform coupling between metal and
nanotube, the threshold value of the metal Fermi wave vector (below which
coupling is insignificant) depends on chirality. Disorder and small phase
coherence length relax the need for k-vector conservation, thereby making the
coupling stronger.


title: Keldysh action for disordered superconductors

abstract:
  Keldysh representation of the functional integral for the interacting
electron system with disorder is used to derive microscopically an effective
action for dirty superconductors. In the most general case this action is a
functional of the 8 x 8 matrix Q(t,t') which depends on two time variables, and
on the fluctuating order parameter field and electric potential. We show that
this approach reproduces, without the use of the replica trick, the well-known
result for the Coulomb-induced renormalization of the electron-electron
coupling constant in the Cooper channel. Turning to the new results, we
calculate the effects of the Coulomb interaction upon: i) the subgap Andreev
conductance between superconductor and 2D dirty normal metal, and ii) the
Josephson proximity coupling between superconductive islands via such a metal.
These quantities are shown to be strongly suppressed by the Coulomb interaction
at sufficiently low temperatures due to both zero-bias anomaly in the density
of states and disorder-enhanced repulsion in the Cooper channel.


title:
A scaling approximation for structure factors in the integral equation
  theory of polydisperse nonionic colloidal fluids

abstract:
  Integral equation of pure liquids, combined with a new "scaling
approximation" based on a corresponding states treatment of pair correlation
functions, is used to evaluate approximate structure factors for colloidal
fluids constituted of uncharged particles with polydispersity in size and
energy parameters. Both hard spheres and Lennard-Jones interactions are
considered. For polydisperse hard spheres, the scaling approximation is
compared to theories utilized by small angle scattering experimentalists
(decoupling approximation, local monodisperse approximation)and to the van der
Waals one-fluid theory. The results are tested against predictions from
analytical expressions, exact within the Percus-Yevick approximation. For
polydisperse Lennard-Jones particles, the scaling approximation combined with a
"modified hypernetted chain" integral equation, is tested against molecular
dynamics data generated for the present work. Despite ist simplicity, the
scaling approximation exhibits a satisfactory performance for both potentials
and represents a considerable improvement over the above mentioned theories.
Shortcomings of the proposed theory, its applicability to the analysis of
experimental scattering data, and its possible extensions to different
potentials are finally discussed.


title: Theory and applications of the stress density

abstract:
  Drawing on the theory of quantum mechanical stress, we introduce the stress
density in density functional theory. In analogy with the Chetty-Martin energy
density, the stress density provides a spatial resolution of the contributions
to the integrated macroscopic stress tensor. We give specific prescriptions for
a practical and efficient implementation in the plane wave ultrasoft
pseudopotential method within the local-density approximation. We demonstrate
the abilities of the stress density studying a set of representative test cases
from surface and interface physics. In perspective, the stress density emerges
as vastly more powerful and predictive than the integrated macroscopic stress.


title:
Structural phase transitions and sound velocity anomalies in
  La(1-x)Sr(x)MnO3

abstract:
  The sound velocities dependencies on temperature V(T) were measured by means
of resonance method have on the thin plates of quasimonocrystalline manganites
La(1-x)Sr(x)MnO3. Results are presented for x=0.1, 0.125, 0.15, 0.175 and 0.2
in the temperature range from 70 to 350K. The V(T) behavior correlates with
those of susceptibilities and magnetic moment M(T). For x=0.175 and 0.2 the
transition to ferromagnetic state is accompanied by appearance of large but
smooth rise in V(T) whereas structural phase transition between the rhomboedral
(R) and orthorhombic (O) structures is accompanied by sharp increase up to
15-20% in V(T). For x=0.1, 0.125 and 0.15 the structural phase transition
between Jahn-Teller distorted orthorhombic structure (O') and pseudocubic one
(O'') is accompanied by deep minimum (~20-30%) in V(T).


title:
Magnon Broadening Effects in Double Layered Manganite La_1.2 Sr_1.8 Mn_2
  O_7

abstract:
  Magnon linewidth of La_1.2 Sr_1.8 Mn_2 O_7 near the Brillouin zone boundary
is investigated from both theoretical and experimental points of view. Abrupt
magnon broadening is ascribed to a strong magnon-phonon coupling. Magnon
broadening observed in cubic perovskite manganites is also discussed.


title: Unconventional one-magnon scattering resistivity in half metals

abstract:
  Low-temperature resistivity of half-metals is investigated. To date it has
been discussed that the one-magnon scattering process in half-metals is
irrelevant for low-temperature resistivity, due to the fully spin-polarized
electronic structure at the ground state. If one takes into account the
non-rigid-band behavior of the minority band due to spin fluctuations at finite
temperatures, however, the unconventional one-magnon scattering process is
shown to be most relevant and gives T^3 dependence in resistivity. This
behavior may be used as a crucial test in the search for half-metallic
materials which are potentially important for applications. Comparison with
resistivity data of
  La_1-x Sr_x MnO_3 as candidates for half-metals shows good agreement.


title: Effect of impact energy on the shape of granular heaps

abstract:
  We study experimentally the shape of a granular heap formed pouring a
granular material into a vertical Hele-Shaw cell and analyze the effect of the
grain impact energy. We propose a continuous model for the steady profile of
the heap that explicitly considers energy dissipation of flowing grains through
inelastic collisions. We solve the model analytically and analyze the resulting
height profile as a function of several parameters, such as the restitution
coefficient of the grains and their impact energies. We find good agreement
between theory and experiments.


title: A Sum Rule for the Two-Dimensional Two-Component Plasma

abstract:
  In a two-dimensional two-component plasma, the second moment of the 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 by using
analogies with critical systems.


title: Influence of nanoscale regions on Raman spectra of complex perovskites

abstract:
  New approach on interpretation and processing of Raman spectra of complex
perovskites is suggested. Raman spectra of $PbMg_{1/3}Nb_{2/3}O_{3}$ and $%
PbSc_{1/2}Ta_{1/2}O_{3}$ are successfully described on the basis of the
phonon-confinement model and validity of the method is demonstrated on good
agreement of experimentally obtained and computer simulated spectra of another
complex perovskite, $BaMg_{1/3}Ta_{2/3}O_{3}$. The study showed that the size
of nanoscale regions is a kind of an inborn characteristic of the sample and is
not temperature dependent. For $PbSc_{1/2}Ta_{1/2}O_{3}$, two special
temperature points, approximately 400 K and 600 K are found from the analysis
of the temperature behaviour of obtained mode parameters.


title: Spin relaxation in semiconductor quantum dots

abstract:
  We have studied the physical processes responsible for the spin -flip in GaAs
quantum dots. We have calculated the rates for different mechanisms which are
related to spin-orbit coupling and cause a spin-flip during the inelastic
relaxation of the electron in the dot both with and without a magnetic field.
We have shown that the zero-dimensional character of the problem when electron
wave functions are localized in all directions leads to freezing out of the
most effective spin-flip mechanisms related to the absence of the inversion
centers in the elementary crystal cell and at the heterointerface and, as a
result, to unusually low spin-flip rates.


title: Minimum Thermal Conductivity of Superlattices

abstract:
  The phonon thermal conductivity of a multilayer is calculated for transport
perpendicular to the layers. There is a cross over between particle transport
for thick layers to wave transport for thin layers. The calculations shows that
the conductivity has a minimum value for a layer thickness somewhat smaller
then the mean free path of the phonons.


title:
First-Principles Study Of The Structural Instabilities In Hexagonal
  Barium Titanate: Coupling Between The Soft Optical And The Acoustic Modes

abstract:
  Hexagonal BaTiO_3 undergoes a structural phase transition to an orthorhombic
C222_1 phase at T_0 = 222 K. The transition is driven by a soft optical mode
with E_2u symmetry whose couplings force the appearance of a spontaneous E_2g
strain (improper ferroelastic character). Staying within the same E_2u
subspace, the system could in principle settle into a second (not observed)
orthorhombic phase (Cmc2_1). We have carried out a first-principles
investigation of these questions, studying the structure of the existing C222_1
and the "virtual" Cmc2_1 phases, and describing the spontaneous E_2g strain in
accord with the experimental observations. In addition, we show that the
occurrence of C222_1 instead of Cmc2_1 cannot be explained by the E_2u soft
modes themselves and, therefore, must be related to their couplings with
secondary order parameters. A more detailed analysis proves that the E_2g
strains do not account for the experimental preference.


title: Collective excitations of a trapped degenerate Fermi gas

abstract:
  We evaluate the small-amplitude excitations of a spin-polarized vapour of
Fermi atoms confined inside a harmonic trap. The dispersion law
$\omega=\omega_{f}[l+4n(n+l+2)/3]^{1/2}$ is obtained for the vapour in the
collisional regime inside a spherical trap of frequency $\omega_{f}$, with $n$
the number of radial nodes and $l$ the orbital angular momentum. The low-energy
excitations are also treated in the case of an axially symmetric harmonic
confinement. The collisionless regime is discussed with main reference to a
Landau-Boltzmann equation for the Wigner distribution function: this equation
is solved within a variational approach allowing an account for
non-linearities. A comparative discussion of the eigenmodes of oscillation for
confined Fermi and Bose vapours is presented in an Appendix.


title:
Download relaxation dynamics on the WWW following newspaper publication
  of URL

abstract:
  A few key properties of the World-Wide-Web (WWW) has been established
indicating the lack of any characteristic scales for the WWW, both in its
topology and in its dynamics. Here, we report an experiment which quantifies
another power law describing the dynamical response of the WWW to a Dirac-like
perturbation, specifically how the popularity of a web site evolves and relaxes
as a function of time, in response to the publication of a notice/advertisement
in a newspaper. Following the publication of an interview of the authors by a
journalist which contained our URL, we monitored the rate of downloads of our
papers and found it to obey a $1/t^b$ power law with exponent $b=0.58\pm 0.03$.
This small exponent implies long-term memory and can be rationalized using the
concept of persistence, which specifies how long a relaxing dynamical system
remains in a neighborhood of its initial configuration.


title:
Biological evolution through mutation, selection, and drift: An
  introductory review

abstract:
  Motivated by present activities in (statistical) physics directed towards
biological evolution, we review the interplay of three evolutionary forces:
mutation, selection, and genetic drift. The review addresses itself to
physicists and intends to bridge the gap between the biological and the
physical literature. We first clarify the terminology and recapitulate the
basic models of population genetics, which describe the evolution of the
composition of a population under the joint action of the various evolutionary
forces. Building on these foundations, we specify the ingredients explicitly,
namely, the various mutation models and fitness landscapes. We then review
recent developments concerning models of mutational degradation. These predict
upper limits for the mutation rate above which mutation can no longer be
controlled by selection, the most important phenomena being error thresholds,
Muller's ratchet, and mutational meltdowns. Error thresholds are deterministic
phenomena, whereas Muller's ratchet requires the stochastic component brought
about by finite population size. Mutational meltdowns additionally rely on an
explicit model of population dynamics, and describe the extinction of
populations. Special emphasis is put on the mutual relationship between these
phenomena. Finally, a few connections with the process of molecular evolution
are established.


title:
Experimental observations of non-equilibrium distributions and
  transitions in a 2D granular gas

abstract:
  A large number (~10,000) of uniform stainless steel balls comprising less
than one layer coverage on a vertically shaken plate provides a rich system for
the study of excited granular media. Viewed from above, the horizontal motion
in the layer shows interesting collective behavior as a result of inelastic
particle-particle collisions. Clusters appear as localized fluctuations from
purely random density distributions, as demonstrated by increased particle
correlations. The clusters grow as the medium is "cooled" by reducing the rate
of energy input. Further reduction of the energy input leads to the nucleation
of a collapse: a close-packed crystal of particles at rest. High speed
photography allows for measurement of particle velocities between collisions.
The velocity distributions deviate strongly from a Maxwell distribution at low
accelerations, and show approximately exponential tails, possibly due to an
observed cross-correlation between density and velocity fluctuations. When the
layer is confined with a lid, the velocity distributions at higher
accelerations are non-Maxwellian and independent of the granular temperature.


title:
New insulating phases of two-dimensional electrons in high Landau
  levels: observation of sharp thresholds to conduction

abstract:
  The intriguing re-entrant integer quantized Hall states recently discovered
in high Landau levels of high-mobility 2D electron systems are found to exhibit
extremely non-linear transport. At small currents these states reflect
insulating behavior of the electrons in the uppermost Landau level. At larger
currents, however, a discontinuous and hysteretic transition to a conducting
state is observed. These phenomena, found only in very narrow magnetic field
ranges, are suggestive of the depinning of a charge density wave state, but
other explanations can also be constructed.


title:
Phase separation and stripe formation in the 2D t-J model: a comparison
  of numerical results

abstract:
  We make a critical analysis of numerical results for and against phase
separation and stripe formation in the t-J model. We argue that the frustrated
phase separation mechanism for stripe formation requires phase separation at
too high a doping for it to be consistent with existing numerical studies of
the t-J model. We compare variational energies for various methods, and
conclude that the most accurate calculations for large systems appear to be
from the density matrix renormalization group. These calculations imply that
the ground state of the doped t-J model is striped, not phase separated.


title:
Non-linear electrical conduction and broadband noise in charge-ordered
  rare earth manganate Nd_0.5Ca_0.5MnO_3

abstract:
  Measurements of the dc transport properties and the low-frequency
conductivity noise in films of charge ordered Nd_0.5Ca_0.5MnO_3 grown on Si
subtrate reveal the existence of a threshold field in the charge ordered regime
beyond which strong non linear conduction sets in along with a large broad band
conductivity noise. Threshold-dependent conduction disappears as T -> T_{CO},
the charge ordering temperature. This observation suggests that the charge
ordered state gets depinned at the onset of the non-linear conduction.


title: Comment on "Level statistics of quantum dots coupled to reservoirs"

abstract:
  This is a comment to J. Konig, Y. Gefen and G. Schon, Phys. Rev. Lett. 81,
4468 (1998) who consider the coupling of two discrete levels of a dot to the
continuum of states in reservoirs. We have generalized this work to the case of
several discrete levels.


title:
Critical Behaviour of 3D Systems with Long-Range Correlated Quenched
  Defects

abstract:
  A field-theoretic description of the critical behaviour of systems with
quenched defects obeying a power law correlations $\sim |{\bf x}|^{-a}$ for
large separations ${\bf x}$ is given. Directly for three-dimensional systems
and different values of correlation parameter $2\leq a \leq 3$ a
renormalization analysis of scaling function in the two-loop approximation is
carried out, and the fixed points corresponding to stability of the various
types of critical behaviour are identified. The obtained results essentially
differ from results evaluated by double $\epsilon, \delta$ - expansion. The
critical exponents in the two-loop approximation are calculated with the use of
the Pade-Borel summation technique.


title: Spin-dependent transport in a clean one-dimensional channel

abstract:
  A shoulder-like feature close to $(0.7\times 2e^{2}/h)$, "the 0.7 structure"
at zero magnetic field was observed in clean one-dimensional (1D) channels
[K.J. Thomas et al., Phys. Rev. Lett. 77, 135 (1996)]. To provide further
understanding of this structure, we have performed low-temperature measurements
of a novel design of 1D channel with overlaying finger gates to study the 0.7
structure as a function of lateral confinement strength and potential profile.
We found that the structure persists when the lateral confinement strength is
changed by a factor of 2. We have also shown that the 0.7 structure present in
two 1D channels in series behaves like a single 1D channel which shows the 0.7
structure, demonstrating that the 0.7 structure is not a transmission effect
through a ballistic channel at zero in-plane magnetic field.


title:
Atomic layering at the liquid silicon surface: a first- principles
  simulation

abstract:
  We simulate the liquid silicon surface with first-principles molecular
dynamics in a slab geometry. We find that the atom-density profile presents a
pronounced layering, similar to those observed in low-temperature liquid metals
like Ga and Hg. The depth-dependent pair correlation function shows that the
effect originates from directional bonding of Si atoms at the surface, and
propagates into the bulk. The layering has no major effects in the electronic
and dynamical properties of the system, that are very similar to those of bulk
liquid Si. To our knowledge, this is the first study of a liquid surface by
first-principles molecular dynamics.


title: Waves on Icicles

abstract:
  Icicles with wave patterns on their surfaces can sometimes be seen hanging
from roofs of buildings. Surprisingly, most of these wave patterns are at
intervals of about 1 cm. The reason for this uniformity of interval has not
been clarified. Here we show a formula to explain this remarkable phenomenon by
introducing a new instability theory. This theory is given by thermal diffusion
in thin water layer streams flowing along the icicles. The streams change the
temperature distribution and control waves of short wavelengths. The specific
wavelength (about 1 cm) can be determined by Laplace instability of the heat
field in the atmosphere and by the thermal diffusion effect in thin-layer
streams.


title: Superconductivity and Quantum Spin Disorder in Cuprates

abstract:
  A fundamental connection between superconductivity and quantum spin
fluctuations in underdoped cuprates, is revealed. A variational calculation
shows that {\em Cooper pair hopping} strongly reduces the local magnetization
$m_0$. This effect pertains to recent neutron scattering and muon spin rotation
measurements in which $m_0$ varies weakly with hole doping in the poorly
conducting regime, but drops precipitously above the onset of
superconductivity.


title: The disordered Fermi-liquid fixed point and its instabilities

abstract:
  The concept of a disordered Fermi-liquid fixed point is introduced and used
to understand various properties of disordered metals within a unifying
framework. Corrections to scaling near this fixed point give what are commonly
called weak-localization effects. Various instabilities of the disordered
Fermi-liquid phase are discussed. These include two distinct types of
superconducting-to-normal-metal quantum phase transitions. First, the quantum
phase transition from a disordered metal to a conventional superconductor in
bulk materials is considered. Second, a quantum phase transition in two
dimensions from metallic-like behavior to a novel type of disorder induced,
spin-triplet, even parity, superconductivity is treated. The paper is concluded
with a discussion of the nature of the ground state of two-dimensional,
disordered electron systems.


title: Rare regions and annealed disorder in quantum phase transitions

abstract:
  The Griffiths region that is due to rare regions and the resulting local
moments in disordered itinerant quantum magnets, and its influence on the
critical behavior, is considered within the framework of an effective field
theory. It is shown that the local moments can be described in terms of static,
annealed disorder, and the physical consequences of this description are
discussed.


title:
Topological phase-fluctuations, amplitude fluctuations, and criticality
  in extreme type-II superconductors

abstract:
  We study the effect of critical fluctuations on the $(B,T)$ phase diagram in
extreme type-II superconductors in zero and finite magnetic field using
large-scale Monte Carlo simulations on the Ginzburg-Landau model in a frozen
gauge approximation. We show that a vortex-loop unbinding gives a correct
picture of the zero field superconducting-normal transition even in the
presence of amplitude fluctuations, which are far from being critical at $T_c$.
We extract critical exponents of the dual model by studying the topological
excitations of the original model. From the vortex-loop distribution function
we extract the anomalous dimension of the dual field $\eta \simeq -0.18$, and
conclude that the charged Ginzburg-Landau model and the neutral 3DXY model
belong to different universality classes. We find are two distinct scaling
regimes for the vortex-line lattice melting line: a high-field scaling regime
and a distinct low-field 3DXY critical scaling regime. We also find indications
of an abrupt change in the connectivity of the vortex-tangle in the vortex
liquid along a line $T_L \geq T_M$. This is the finite field counter-part of
the zero-field vortex-loop blowout. Which at low enough fields appears to
coincide with $T_M$. Here, a description of the vortex system only in terms of
field induced vortex lines is inadequate at and above the VLL melting
temperature.


title:
Anomalous scaling dimensions and critical points in type-II
  superconductors

abstract:
  The existence of a {\it stable critical point}, separate from the Gaussian
and XY critical points, of the Ginzburg-Landau theory for superconductors, is
demonstrated by direct extraction via Monte-Carlo simulations, of a negative
anomalous dimension $\eta_{\phi}$ of a complex scalar field $\phi$ forming a
dual description of a neutral superfluid. The dual of the neutral superfluid is
isomorphic to a charged superfluid coupled to a massless gauge-field. The
anomalous scaling dimension of the superfluid order-field is positive, while we
find that the anomalous dimension of the dual field is negative. The dual
gauge-field does not decouple from the dual complex matter-field at the
critical point. {\it These two critical theories represent separate fixed
points.} The physical meaning of a negative $\eta_{\phi}$ is that the
vortex-loop tangle of the superfluid at the critical point fills space {\it
more} efficiently than random walkers, {\it without collapsing}. This is due to
the presence of the massless dual gauge-field, and the resulting long-ranged
{\it vectorial} Biot-Savart interaction between vortex-loop segments, which is
a relevant perturbation to the steric $|\psi|^4$ repulsion term. Hence, the
critical dual theory is not in the universality class of the $|\psi|^4$-theory.


title:
A quantitative comparison between spike-train responses of the
  Hodgkin-Huxley and integrate-and-fire neurons

abstract:
  Spike-train responses of single Hodgkin-Huxley (HH) and integrate-and-fire
(IF) neurons with and without the refractory period, are calculated and
compared. The HH and IF neurons are assumed to receive spike-train inputs with
the constant interspike intervals (ISIs) and stochastic ISIs given by the Gamma
distribution, through excitatory and inhibitory synaptic couplings: for both
the couplings the HH neuron can fire while the IF neuron can only for the
excitatory one. It is shown that the response to the constant-ISI inputs of the
IF neuron strongly depends on the refractory period and the synaptic strength
and that its response is rather different from that of the HH neuron. The
variability of HH and IF neurons depends not only on the jitter of the
stochastic inputs but also on their mean and the synaptic strength. Even for
the excitatory inputs, the type-I IF neuron may be a good substitute of the
type-II HH neuron only in the limited parameter range


title:
Ordered phase and scaling in $Z_n$ models and the three-state
  antiferromagnetic Potts model in three dimensions

abstract:
  Based on a Renormalization-Group picture of $Z_n$ symmetric models in three
dimensions, we derive a scaling law for the $Z_n$ order parameter in the
ordered phase. An existing Monte Carlo calculation on the three-state
antiferromagnetic Potts model, which has the effective $Z_6$ symmetry, is shown
to be consistent with the proposed scaling law. It strongly supports the
Renormalization-Group picture that there is a single massive ordered phase,
although an apparently rotationally symmetric region in the intermediate
temperature was observed numerically.


title: A hidden Goldstone mechanism in the Kagom\'e lattice antiferromagnet

abstract:
  In this paper, we study the phases of the Heisenberg model on the \kagome
lattice with antiferromagnetic nearest neighbour coupling $J_1$ and
ferromagnetic next neighbour coupling $J_2$. Analysing the long wavelength, low
energy effective action that describes this model, we arrive at the phase
diagram as a function of $\chi = \frac{J_2}{J_1} $. The interesting part of
this phase diagram is that for small $\chi$, which includes $\chi =0$, there is
a phase with no long range spin order and with gapless and spin zero low lying
excitations. We discuss our results in the context of earlier, numerical and
experimental work.


title:
Q-Ising neural network dynamics: a comparative review of various
  architectures

abstract:
  This contribution reviews the parallel dynamics of Q-Ising neural networks
for various architectures: extremely diluted asymmetric, layered feedforward,
extremely diluted symmetric, and fully connected. Using a probabilistic
signal-to-noise ratio analysis, taking into account all feedback correlations,
which are strongly dependent upon these architectures the evolution of the
distribution of the local field is found. This leads to a recursive scheme
determining the complete time evolution of the order parameters of the network.
Arbitrary Q and mainly zero temperature are considered. For the asymmetrically
diluted and the layered feedforward network a closed-form solution is obtained
while for the symmetrically diluted and fully connected architecture the
feedback correlations prevent such a closed-form solution. For these symmetric
networks equilibrium fixed-point equations can be derived under certain
conditions on the noise in the system. They are the same as those obtained in a
thermodynamic replica-symmetric mean-field theory approach.


title:
Polarization Instability in Polydomain Ferroelectric Epitaxial Thin
  Films and the Formation of Heterophase Structures

abstract:
  A thermodynamic theory is developed for dense laminar domain structures in
epitaxial ferrolectric films. It is found that, at some critical misfit strain
between the film and substrate, the 90 degrees c/a/c/a domain structure becomes
unstable with respect to the appearance of the polarization component parallel
to domain walls, which results in the formation of a heterophase structure. For
PbTiO_3 and BaTiO_3 films, the stability ranges of polydomain and heterophase
states are determined using "misfit strain - temperature" diagrams. Dielectric
anomalies accompanying misfit-strain-driven structural transformations are
described.


title: Correlated patterns in non-monotonic graded-response perceptrons

abstract:
  The optimal capacity of graded-response perceptrons storing biased and
spatially correlated patterns with non-monotonic input-output relations is
studied. It is shown that only the structure of the output patterns is
important for the overall performance of the perceptrons.


title: Magnetic susceptibility of the double exchange model

abstract:
  Previously a many-body coherent potential approximation (CPA) was used to
study the double exchange (DE) model with quantum local spins S, both for S=1/2
and for general S in the paramagnetic state. This approximation, exact in the
atomic limit, was considered to be a many-body extension of Kubo's one-electron
dynamical CPA for the DE model. We now extend our CPA treatment to the case of
general S and spin polarization. We show that Kubo's one-electron CPA is always
recovered in the empty-band limit and that our CPA is equivalent to dynamical
mean field theory in the classical spin limit. We then solve our CPA equations
self-consistently to obtain the static magnetic susceptibility chi in the
strong-coupling limit. As in the case of the CPA for the Hubbard model we find
unphysical behaviour in chi at half-filling and no magnetic transition for any
finite S. We identify the reason for this failure of our approximation and
propose a modification which gives the correct Curie-law behaviour of chi at
half-filling and a transition to ferromagnetism for all S.


title: Liquid Conservation and Non-local Interface Dynamics in Imbibition

abstract:
  The propagation and roughening of a liquid-gas interface moving through a
disordered medium under the influence of capillary forces is considered. The
system is described by a phase-field model with conserved dynamics and spatial
disorder is introduced through a quenched random field. Liquid conservation
leads to slowing down of the average interface position H and imposes an
intrinsic correlation length \xi_\times \sim H^{1/2} on the spatial
fluctuations of the interface. The interface is statistically self affine in
space, with global roughness exponent \chi \simeq 1.25 and exhibits anomalous
scaling.


title:
Sharp Transition to a Spin-Ordered Phase in the 2D Electron System in
  Silicon MOSFET's

abstract:
  This paper has been withdrawn by the authors. Measurements at many different
angles spanning a broad range of electron densities indicate that the observed
doubling of the period of Shubnikov-de Haas oscillations, which we had
attributed to a transition to a new spin-ordered phase, is also consistent with
an alternative explanation. A full interpretation of our data will be posted at
a later date.


title: Missing 2k_F Response for Composite Fermions in Phonon Drag

abstract:
  The response of composite Fermions to large wavevector scattering has been
studied through phonon drag measurements. While the response retains
qualitative features of the electron system at zero magnetic field, notable
discrepancies develop as the system is varied from a half-filled Landau level
by changing density or field. These deviations, which appear to be inconsistent
with the current picture of composite Fermions, are absent if half-filling is
maintained while changing density. There remains, however, a clear deviation
from the temperature dependence anticipated for 2k_F scattering.


title: Glassy Dynamics of Protein Folding

abstract:
  A coarse grained model of a random polypeptide chain, with only discrete
torsional degrees of freedom and Hookean springs connecting pairs of
hydrophobic residues is shown to display stretched exponential relaxation under
Metropolis dynamics at low temperatures with the exponent $\beta\simeq 1/4$, in
agreement with the best experimental results. The time dependent correlation
functions for fluctuations about the native state, computed in the Gaussian
approximation for real proteins, have also been found to have the same
functional form. Our results indicate that the energy landscape exhibits
universal features over a very large range of energies and is relatively
independent of the specific dynamics.


title: Supersymmetry Theory of Disordered Heteropolymers

abstract:
  The effective motion equation that describes the different monomer
alternation along the heteropolymer chain is proposed. On its basis the
supersymmetry field scheme that allows to obtain the equations for the
structure factor and Green function is built up. The memory and ergodicity
breaking effects are investigated depending on the temperature and quenched
disorder of the monomer alternation. The phase diagram that determines the
existence of the non-ergodic and freezing states is provided.


title: Statistical theory of hierarchical avalanche ensemble

abstract:
  The statistical ensemble of avalanche intensities is considered to
investigate diffusion in ultrametric space of hierarchically subordinated
avalanches. The stationary intensity distribution and the steady-state current
are obtained. The critical avalanche intensity needed to initiate the global
avalanche formation is calculated depending on noise intensity. The large time
asymptotic for the probability of the global avalanche appearance is derived.


title: Spontaneous spin stripe dimerization in the doped t-J model

abstract:
  To investigate spin dimerization in the t-J model we consider the extended
t-J-\delta model with explicit spin dimerization introduced via parameter
\delta. At zero doping the dimerized spin liquid is unstable at \delta <
\delta_c = 0.3. We demonstrate that the doping stabilizes the dimerized stripe
phase: the \delta_c(x) decreases when doping x increases. At doping larger than
critical, x > x_c, the dimerized phase is stable even without explicit
dimerization, i.e. at \delta=0.


title:
Time-dependent Effects in the Metallic Phase in Si-MOS: Evidence for
  Non-Diffusive Transport

abstract:
  We have found that the conduction in Si-MOS structures has a substantial
imaginary component in the metallic phase for the density range 6 \times n_c >
n > n_c, where n_c is the critical density of the metal-insulator transition.
For high mobility samples, the corresponding delay (or advance) time equals
approximately to 0.1 - 10ms and increases exponentially as density and
temperature decrease. In very low mobility samples, at temperature of 0.3K, the
time-lag in establishing the equilibrium resistance reaches hundreds of
seconds. The delay (advance) times are approximately 10^2-10^8 times larger
than the overall RC-time of the gated structure. These results give evidence
for a non-Boltzmann character of the transport in the low-density metallic
phase. We relate the time-dependent effects to tunneling of carries between the
2D bulk and localized states.


title:
Fixed-N Superconductivity: The Exact Crossover from the Bulk to the
  Few-Electron Limit

abstract:
  We use two truly canonical approaches to describe superconductivity in
ultrasmall metallic grains: (a) a variational fixed-N projected BCS-like theory
and (b) an exact solution of the model Hamiltonian developed by Richardson in
context with Nuclear Physics. Thereby we obtain a description of the entire
crossover from the bulk BCS regime (mean level spacing $d\ll$ bulk gap
$\tilde\Delta$) to the `fluctuation-dominated' few-electron regime
($d\gg\tilde\Delta$). A wave-function analysis shows in detail how the BCS
limit is recovered and how for $d\gg\tilde\Delta$ pairing correlations become
delocalized in energy space.


title:
Exact Partition Functions for Potts Antiferromagnets on Cyclic Lattice
  Strips

abstract:
  We present exact calculations of the zero-temperature partition function of
the $q$-state Potts antiferromagnet on arbitrarily long strips of the square,
triangular, and kagom\'e lattices with width $L_y=2$ or 3 vertices and with
periodic longitudinal boundary conditions. From these, in the limit of infinite
length, we obtain the exact ground-state entropy $S_0=k_B \ln W$. These results
are of interest since this model exhibits nonzero ground state entropy $S_0 >
0$ for sufficiently large $q$ and hence is an exception to the third law of
thermodynamics. We also include results for homeomorphic expansions of the
square lattice strip. The analytic properties of $W(q)$ are determined and
related to zeros of the chromatic polynomial for long finite strips.


title: Quantum critical behavior of itinerant ferromagnets

abstract:
  We investigate the quantum phase transition of itinerant ferromagnets. It is
shown that correlation effects in the underlying itinerant electron system lead
to singularities in the order parameter field theory that result in an
effective long-range interaction between the spin fluctuations. This
interaction turns out to be generically {\em antiferromagnetic} for clean
systems. In disordered systems analogous correlation effects lead to even
stronger singularities. The resulting long-range interaction is, however,
generically ferromagnetic.
  We discuss two possibilities for the ferromagnetic quantum phase transition.
In clean systems, the transition is generically of first order, as is
experimentally observed in MnSi. However, under certain conditions the
transition may be continuous with non-mean field critical behavior. In
disordered systems, one finds a very rich phase diagram showing first order and
continuous phase transitions and several multicritical points.


abstract:
  The structural and electronic properties of germanium clathrates Ge$_{46}$
and K$_8$Ge$_{46}$ are studied by first principles calculations within the
local density approximation. The equilibrium structures are obtained by {\em ab
initio} pseudopotential calculation combined with dynamic minimizations. The
clathrate structure is found as a low energy phase for germanium. The
electronic band structures for Ge$_{46}$ clathrates are calculated and the band
gap is found to be considerably larger than that of the diamond phase. Due to
the effect of pentagonal rings, strong similarity in electronic properties
between clathrate and Ge$_{24}$ fullerene structure are found. The effect of
doping clathrate cages with metal atoms are examined. The K$_8$Ge$_{46}$
clathrate is found to be metallic with the conduction bands only slightly
modified by K dopants.


title: Anomalous electronic conductance in quasicrystals

abstract:
  Generic quantum interference effects occuring in 1D-quasicrystals are
reviewed with emphasis on the joint effect of phason disorder on electronic
localization and propagation modes. In close conjunction with properties of
real materials, the contributions of quantum interferences in several regimes
close to the metal insulator transition are outlined.


title: Persistence in Nonequilibrium Systems

abstract:
  This is a brief review of recent theoretical efforts to understand
persistence in nonequilibrium systems. Some of the recent experimental results
are also briefly mentioned. I also discuss recent generalizations of
persistence in various directions and conclude with a summary of open
questions.


title: Stability criterion for attractive Bose-Einstein condensates

abstract:
  A general stability criterion is derived for the D-dimensional ground states
of the Gross-Pitaevskii equation, which describes attractive Bose-Einstein
condensates confined in a magnetic trap. These ground states are shown to avoid
the collapse in finite time and are proven to be stable in two and three
spatial dimensions.


title:
Modified Perturbation Theory Applied to Kondo-Type Transport through a
  Quantum Dot under a Magnetic Field

abstract:
  Linear conductance through a quantum dot is calculated under a finite
magnetic field using the modified perturbation theory. The method is based on
the second-order perturbation theory with respect to the Coulomb repulsion, but
the self-energy is modified to reproduce the correct atomic limit and to
fulfill the Friedel sum rule exactly. Although this method is applicable only
to zero temperature in a strict sense, it is approximately extended to finite
temperatures. It is found that the conductance near electron-hole symmetry is
suppressed by the application of the magnetic field at low temperatures.
Positive magnetoconductance is observed in the case of large electron-hole
asymmetry.


title: Shapiro steps in a superconducting film with an antidot lattice

abstract:
  Shapiro voltage steps at voltages V_n=nV_0 (n integer) have been observed in
the voltage-current characteristics of a superconducting film with a square
lattice of perforating microholes (antidots)in the presence of radiofrequent
radiation. These equidistant steps appear at the second matching field H_2 when
the flow of the interstitial vortex lattice in the periodic potential created
by the antidots and the vortices trapped by them, is in phase with the applied
rf frequency. Therefore, the observation of Shapiro steps clearly reveals the
presence of mobile intersitial vortices in superconducting films with regular
pinning arrays. The interstitial vortices, moved by the driving current,
coexist with immobile vortices strongly pinned at the antidots.


title:
`Composite particles' and the eigenstates of Calogero-Sutherland and
  Ruijsenaars-Schneider

abstract:
  We establish a one-to-one correspondance between the ''composite particles''
with $N$ particles and the Young tableaux with at most $N$ rows. We apply this
correspondance to the models of Calogero-Sutherland and Ruijsenaars-Schneider
and we obtain a momentum space representation of the ''composite particles'' in
terms of creation operators attached to the Young tableaux. Using the technique
of bosonisation, we obtain a position space representation of the ''composite
particles'' in terms of products of vertex operators. In the special case where
the ''composite particles'' are bosons and if we add one extra quasiparticle or
quasihole, we construct the ground state wave functions corresponding to the
Jain series $\nu =p/(2np\pm 1) $ of the fractional quantum Hall effect.


title: The rate of entropy increase at the edge of chaos

abstract:
  Under certain conditions, the rate of increase of the statistical entropy of
a simple, fully chaotic, conservative system is known to be given by a single
number, characteristic of this system, the Kolmogorov-Sinai entropy rate. This
connection is here generalized to a simple dissipative system, the logistic
map, and especially to the chaos threshold of the latter, the edge of chaos. It
is found that, in the edge-of-chaos case, the usual Boltzmann-Gibbs-Shannon
entropy is not appropriate. Instead, the non-extensive entropy $S_q\equiv
\frac{1-\sum_{i=1}^W p_i^q}{q-1}$, must be used. The latter contains a
parameter q, the entropic index which must be given a special value $q^*\ne 1$
(for q=1 one recovers the usual entropy) characteristic of the edge-of-chaos
under consideration. The same q^* enters also in the description of the
sensitivity to initial conditions, as well as in that of the multifractal
spectrum of the attractor.

categories: cond-mat.stat-mech astro-ph chao-dyn comp-gas hep-th nlin.CD nlin.CG nucl-th


title:
Metal-insulator transition in three dimensional Anderson model:
  universal scaling of higher Lyapunov exponents

abstract:
  Numerical studies of the Anderson transition are based on the finite-size
scaling analysis of the smallest positive Lyapunov exponent. We prove
numerically that the same scaling holds also for higher Lyapunov exponents.
This scaling supports the hypothesis of the one-parameter scaling of the
conductance distribution. From the collected numerical data for quasi one
dimensional systems up to the system size 24 x 24 x infinity we found the
critical disorder 16.50 < Wc < 16.53 and the critical exponent 1.50 < \nu <
1.54. Finite-size effects and the role of irrelevant scaling parameters are
discussed.