title:
Low-Temperature Dephasing in Disordered Conductors: the Effect of
  ``1/f'' Fluctuations

abstract:
  Electronic quantum effects in disordered conductors are controlled by the
dephasing rate of conduction electrons. This rate is expected to vanish with
the temperature. We consider the very intriguing recently reported apparent
saturation of this dephasing rate in several systems at very low temperatures.
We show that the ``standard model'' of a conductor with static defects can {\em
not} have such an effect. However, allowing some dynamics of the defects may
produce it.


title:
Phase Ordering and Onset of Collective Behavior in Chaotic Coupled Map
  Lattices

abstract:
  The phase ordering properties of lattices of band-chaotic maps coupled
diffusively with some coupling strength $g$ are studied in order to determine
the limit value $g_e$ beyond which multistability disappears and non-trivial
collective behavior is observed. The persistence of equivalent discrete spin
variables and the characteristic length of the patterns observed scale
algebraically with time during phase ordering. The associated exponents vary
continuously with $g$ but remain proportional to each other, with a ratio close
to that of the time-dependent Ginzburg-Landau equation. The corresponding
individual values seem to be recovered in the space-continuous limit.

categories: cond-mat.dis-nn chao-dyn cond-mat.stat-mech nlin.CD


title: Scaling properties in off equilibrium dynamical processes

abstract:
  In the present paper, we analyze the consequences of scaling hypotheses on
dynamic functions, as two times correlations $C(t,t')$. We show, under general
conditions, that $C(t,t')$ must obey the following scaling behavior $C(t,t') =
\phi_1(t)^{f(\beta)}{\cal{S}}(\beta)$, where the scaling variable is
$\beta=\beta(\phi_1(t')/\phi_1(t))$ and $\phi_1(t')$, $\phi_1(t)$ two
undetermined functions. The presence of a non constant exponent $f(\beta)$
signals the appearance of multiscaling properties in the dynamics.


title: Diffusion of particles moving with constant speed

abstract:
  The propagation of light in a scattering medium is described as the motion of
a special kind of a Brownian particle on which the fluctuating forces act only
perpendicular to its velocity. This enforces strictly and dynamically the
constraint of constant speed of the photon in the medium. A Fokker-Planck
equation is derived for the probability distribution in the phase space
assuming the transverse fluctuating force to be a white noise. Analytic
expressions for the moments of the displacement $<x^{n}>$ along with an
approximate expression for the marginal probability distribution function
$P(x,t)$ are obtained. Exact numerical solutions for the phase space
probability distribution for various geometries are presented. The results show
that the velocity distribution randomizes in a time of about eight times the
mean free time ($8t^*$) only after which the diffusion approximation becomes
valid. This factor of eight is a well known experimental fact. A persistence
exponent of $0.435 \pm 0.005$ is calculated for this process in two dimensions
by studying the survival probability of the particle in a semi-infinite medium.
The case of a stochastic amplifying medium is also discussed.


title: Floppy Membranes

abstract:
  Floppy membranes are tensionless surfaces without extrinsic stiffness, whose
fluctuations are governed by fourth-order bending elasticity. This suppresses
spiky superstructures and ensures that floppy membranes remain smooth over any
distance, with Hausdorff dimension D=2, in contrast to surfaces with stiffness,
which are rough on the scale of some finite persistence length.


title:
On the Coulomb interaction in chiral-invariant one-dimensional electron
  systems

abstract:
  We consider a one-dimensional electron system, suitable for the description
of the electronic correlations in a metallic carbon nanotube. Renormalization
group methods are used to study the low-energy behavior of the unscreened
Coulomb interaction between currents of well-defined chirality. In the limit of
a very large number n of subbands we find a strong renormalization of the Fermi
velocity, reminiscent of a similar phenomenon in the graphite sheet. For small
n or sufficiently low energy, the Luttinger liquid behavior takes over, with a
strong wavefunction renormalization leading to a vanishing quasiparticle
weight. Our approach is appropriate to study the crossover from two-dimensional
to one-dimensional behavior in carbon nanotubes of large radius.


title:
Anisotropic resistivity of the antiferromagnetic insulator
  Bi_2Sr_2ErCu_2O_8

abstract:
  Anisotropic resistivities of Bi_2Sr_2Ca_{1-x}Er_xCu_2O_8 single crystals were
measured and analyzed from 4.2 to 500 K with special interest in the parent
antiferromagnetic insulator of x=1.0. Although the resistivity is
semiconducting along both the in- and out-of-plane directions, the temperature
dependence is found to be significantly different. As a result, the resistivity
ratio for x=1.0 takes a broad maximum near room temperature. The electric
conduction in parent antiferromagnetic insulators is different from other
semiconductors, and is as unconventional as that in high-temperature
superconductors.


title: The shape of a moving fluxon in stacked Josephson junctions

abstract:
  We study numerically and analytically the shape of a single fluxon moving in
a double stacked Josephson junctions (SJJ's) for various junction parameters.
We show that the fluxon in a double SJJ's consists of two components, which are
characterized by different Swihart velocities and Josephson penetration depths.
The weight coefficients of the two components depend on the parameters of the
junctions and the velocity of the fluxon. It is shown that the fluxon in SJJ's
may have an unusual shape with an inverted magnetic field in the second
junction when the velocity of the fluxon is approaching the lower Swihart
velocity. Finally, we study the influence of fluxon shape on flux-flow
current-voltage characteristics and analyze the spectrum of Cherenkov radiation
for fluxon velocity above the lower Swihart velocity. Analytic expression for
the wavelength of Cherenkov radiation is derived.


title:
Non-perturbative results for level correlations from the replica
  nonlinear sigma model

abstract:
  We show that for all the three standard symmetry classes (unitary, orthogonal
and symplectic), the conventional replica nonlinear sigma model gives the
correct non-perturbative result for the two-level correlation functions
R_2(\omega) of electrons in disordered metals in the limit of large \omega. In
this limit, non-perturbative oscillatory contributions arise from a degenerate
saddle-point manifold within this sigma model which corresponds to the
replica-symmetry breaking. Moreover, we demonstrate that in the unitary case
the very same results can be extracted from the well known exact integral
representation for R_2(\omega).


title: The random magnetic flux problem in a quantum wire

abstract:
  The random magnetic flux problem on a lattice and in a quasi one-dimensional
(wire) geometry is studied both analytically and numerically. The first two
moments of the conductance are obtained analytically. Numerical simulations for
the average and variance of the conductance agree with the theory. We find that
the center of the band $\epsilon=0$ plays a special role. Away from
$\epsilon=0$, transport properties are those of a disordered quantum wire in
the standard unitary symmetry class. At the band center $\epsilon=0$, the
dependence on the wire length of the conductance departs from the standard
unitary symmetry class and is governed by a new universality class, the chiral
unitary symmetry class. The most remarkable property of this new universality
class is the existence of an even-odd effect in the localized regime:
Exponential decay of the average conductance for an even number of channels is
replaced by algebraic decay for an odd number of channels.


title:
Defect-Induced Resonant Tunneling of Electromagnetic Waves Through a
  Polariton Gap

abstract:
  We consider tunneling of electromagnetic waves through a polariton band gap
of a 1-D chain of atoms. We analytically demonstrate that a defect embedded in
the structure gives rise to the resonance transmission at the frequency of a
local polariton state associated with the defect.


title:
Superconducting pairing, and the collective magnetic excitation in the
  extended 2-dimensional t-J model

abstract:
  To investigate the non-Fermi liquid behavior we consider the extended
two-dimensional t-J model which includes additional hopping t''. In the regime
t,J << t'' we were able to solve the model analytically. It has a very rich
phase diagram including antiferromagnetic (AF) insulator and AF strange metal
with different kinds of pseudospin-singlet superconducting pairings
(p,d,g-waves). We also demonstrate a collective triplet excitation with energy
below the superconducting gap


title: Dynamics of trapped Bose gases at finite temperatures

abstract:
  Starting from an approximate microscopic model of a trapped Bose-condensed
gas at finite temperatures, we derive an equation of motion for the condensate
wavefunction and a quantum kinetic equation for the distribution function for
the excited atoms. The kinetic equation includes collisions between the
condensate and non-condensate atoms ($C_{12}$), in addition to collisions
between the excited atoms as described by the Uehling-Uhlenbeck ($C_{22}$)
collision integral. Assuming that the $C_{22}$ collision rate is sufficiently
rapid to produce a local equilibrium Bose distribution, the kinetic equation
can be used to derive hydrodynamic equations for the non-condensate. These
equations include a description of the equilibration of the local chemical
potentials of the condensate and non-condensate components which gives rise to
a new relaxational mode associated with the exchange of atoms between the two
components. We show how the Landau two-fluid equations emerge in the frequency
domain $\omega \tau_\mu \ll 1$, where $\tau_\mu$ is a characteristic relaxation
time of the equilibration process. This process provides an additional source
of damping of the collective modes (first and second sound in the case of a
uniform system). Our equations are consistent with the generalized Kohn
theorem. Finally, a variational solution of the equations is developed which is
used to determine some of the monopole, dipole and quadrupole normal modes of a
trapped Bose gas in an isotropic trap.


title:
d_{x^2-y^2}-Wave Pairing Fluctuations and Pseudo Spin Gap in
  Two-Dimensional Electron Systems

abstract:
  Pseudogap phenomena of high-T_c cuprates are examined. In terms of AFM
(antiferromagnetic) and dSC (d_{x^2-y^2}-wave superconducting) auxiliary fields
introduced to integrate out the fermions, the effective action for 2D electron
systems with AFM and dSC fluctuations is considered. By the self-consistent
renormalization (SCR), the NMR relaxation rate T_1^{-1}, the spin correlation
length \xi_\sigma and the pairing correlation length \xi_d are calculated. From
this calculation, a mechanism of the pseudogap formation emerges as the region
of dominant d-wave short-range order (SRO) over AFM-SRO. When damping for the
AFM fluctuation strongly depends on the dSC correlation length through the
formation of precursor singlets around (\pi,0) and (0,\pi) points in the
momentum space, the pseudogap appears in a region of the normal state
characterized by decreasing 1/T_1T and increasing AFM correlation length with
decrease in temperature. This reproduces a characteristic feature of the
pseudogap phenomena in many underdoped cuprates. When the damping becomes
insensitive to the dSC correlation length, the pseudogap region shrinks as in
the overdoped cuprates.


title:
Vortex phase transformations probed by the local ac response of
  Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta} single crystals with various doping

abstract:
  The linear ac response of the vortex system is measured locally in Bi-2212
single crystals at various doping, using a miniature two-coil mutual-inductance
technique. It was found that a step-like change in the local ac response takes
place exactly at the first-order transition (FOT) temperature T_{FOT}(H)
determined by a global dc magnetization measurement. The T_{FOT}(H) line in the
H-T phase diagram becomes steeper with increasing doping. In the higher-field
region where the FOT is not observed, the local ac response still shows a
broadened but distinct feature, which can be interpreted to mark the growth of
a short-range order in the vortex system.


title: AB Initio Studies of Electronic Excitations in Real Solids

abstract:
  The first part of this article centers on the fact that key features of the
dynamical response of weakly-correlated materials (the alkalis, Al), have been
found experimentally to differ qualitatively from simple-model behavior. In the
absence of ab initio theory, the surprises embodied in the experimental data
were imputed to effects of dynamical correlations. We summarize results of ab
initio investigations of linear response, performed within time-dependent
density-functional theory (TDDFT), in which the unexpected features of the
observed spectra are shown to be due to band-structure effects. Contrary to
conventional wisdom, the response cannot be understood "universally," in terms
of a simple scaling with the density, on going from metal to metal (e.g.,
through the alkali series) --even the shape of the dispersion curve for the
plasmon energy is system-specific. The second part of this article starts out
with the observation that a similar ab initio study of systems with more
complex electronic structures would require the availability of a realistic
approximation for the dynamical many-body kernel entering the density-response
function in TDDFT. Thus, we outline a diagrammatic alternative, framed within
the conserving-approximation method of Baym and Kadanoff. Using as a benchmark
the band gap of Si obtained in the GW approximation, together with a
diagrammatic (and conserving) solution of the ensuing Bethe-Salpeter equation,
we discuss issues involving conservation laws, self-consistency, and sum rules.
These conceptual issues are particularly important for the development of ab
initio methods for the study of dynamical response and quasiparticle band
structure of strongly-correlated materials. We argue that inclusion of
short-range correlations absent in the GW approximation is a must, even in Si.


title:
Molecular dynamics calculation of the thermal conductivity of vitreous
  silica

abstract:
  We use extensive classical molecular dynamics simulations to calculate the
thermal conductivity of a model silica glass. Apart from the potential
parameters, this is done with no other adjustable quantity and the standard
equations of heat transport are used directly in the simulation box. The
calculations have been done between 10 and 1000 Kelvin and the results are in
good agreement with the experimental data at temperatures above 20K. The
plateau observed around 10K can be accounted for by correcting our results
taking into account finite size effects in a phenomenological way.


title:
Theory of the Josephson effect in superconductor / ferromagnet /
  superconductor junctions

abstract:
  To reveal the influence of the exchange interaction on the Josephson effect,
the d.c. Josephson current in unconventional-superconductor / ferromagnet /
unconventional-superconductor junctions is studied. When the two
superconductors have d-wave symmetry, the Josephson current is drastically
suppressed with the increase of the exchange interaction. On the other hand,
when the two superconductors have different parities, e.g. s-wave and p-wave,
an enhancement of the Josephson current is obtained with the increment of the
exchange interaction due to the break-down of SU(2) symmetry in the spin space.


title: The Haldane gap for the S=2 antiferromagnetic Heisenberg chain revisited

abstract:
  Using the density matrix renormalization group (DMRG) technique, we carry out
a large scale numerical calculation for the S=2 antiferromagnetic Heisenberg
chain. Performing systematic scaling analysis for both the chain length $L$ and
the number of optimal states kept in the iterations $m$, the Haldane gap
$\Delta (2)$ is estimated accurately as $(0.0876\pm0.0013)J$. Our systematic
analysis for the S=2 chains not only ends the controversies arising from
various DMRG calculations and Monte Carlo simulations, but also sheds light on
how to obtain reliable results from the DMRG calculations for other complicated
systems.


title: Dynamic States of a Continuum Traffic Equation with On-Ramp

abstract:
  We study the phase diagram of the continuum traffic flow model of a highway
with an on-ramp. Using an open boundary condition, traffic states and
metastabilities are investigated numerically for several representative values
of the upstream boundary flux $f_{up}$ and for the whole range of the on-ramp
flux $f_{rmp}$. An inhomogeneous but time-independent traffic state (standing
localized cluster state) is found and related to a recently measured traffic
state. Due to the density gradient near the on-ramp, a novel traffic jam can
occur even when the downstream density is below the critical density of the
usual traffic jam formation in homogeneous highways, and its structure varies
qualitatively with $f_{rmp}$. The free flow, the recurring hump (RH) state, and
the traffic jam can all coexist in a certain metastable region where the free
flow can undergo phase transitions either to the RH state or to the traffic jam
state. We also find two nontrivial analytic solutions. These solutions
correspond to the standing localized cluster state and the homogeneous
congested traffic state (one form of the novel traffic jam),which are observed
in numerical simulations.


title: Dynamics of Vortices in Two-Dimensional Magnets

abstract:
  Theories, simulations and experiments on vortex dynamics in
quasi-two-dimensional magnetic materials are reviewed. These materials can be
modelled by the classical two-dimensional anisotropic Heisenberg model with XY
(easy-plane) symmetry. There are two types of vortices, characterized by their
polarization (a second topological charge in addition to the vorticity): Planar
vortices have Newtonian dynamics (even-order equations of motion) and exhibit
strong discreteness effects, while non-planar vortices have non-Newtonian
dynamics (odd-order equations of motion) and smooth trajectories. The influence
of thermal fluctuations on single vortices is investigated. Different types of
noise and damping are discussed and implemented into the microscopic equations
which yields stochastic equations of motion for the vortices. The solutions of
the these equations are compared with Langevin dynamics simulations. Moreover,
noise-induced transitions between opposite polarizations of a vortex are
investigated. For temperatures above the Kosterlitz-Thouless vortex-antivortex
unbinding transition, a phenomenological theory, namely the vortex gas
approach, yields central peaks in the dynamic form factors for the spin
correlations. Such peaks are observed both in combined Monte Carlo- and Spin
Dynamics-Simulations and in inelastic neutron scattering experiments. However,
the assumption of ballistic vortex motion appears questionable.


title:
Quantum Group based Modelling for the description of high temperature
  superconductivity and antiferromagnetism

abstract:
  Following our recent conjecture to model the phenomenona of
antiferromagnetism and superconductivity by quantum symmetry groups, we propose
in the present note three toy models, namely, one based on $SO_{q}(3)$ the
other two constructed with the $SO_{q}(4)$ and $SO_{q}(5)$ quantum groups.
  Possible motivations and rationale for these choices are outlined. One of the
prime motivations underlying our proposal is the experimental observation of
stripe structure [phase] in high T_c superconductivity [HTSC] materials. A
number of experimental techniques have recently observed that the CuO_2 are
rather inhomogeneous, providing evidence for phase separation into a two
component system. i.e. carrier-rich and carrier-poor regions.
  In particular, extended x-ray absorption fine structure [EXAFS] demonstrated
that these domains forms stripes of undistorted and distorted local structures
alternating with mesoscopic length scale comparable with coherence length in
HTSC.


title:
Crystal Field, Magnetic Anisotropy and Excitations in Rare-Earth
  Hexaborides

abstract:
  We clarify the role of crystalline electric field (CEF) induced magnetic
anisotropy in the ground state and spin-wave spectrum of cubic rare-earth
materials with dominating isotropic magnetic exchange interactions. In
particular we study the hexaboride NdB_6 which is shown to exhibit strong
spin-quadrupolar coupling. The CEF scheme is analyzed and a non-collinear
magnetization response is found. The spin orientation in the
antiferromagnetically ordered ground-state is identified. Moreover, the spin
excitations are evaluated and in agreement with inelastic neutron scattering a
suppression of one of the two magnetic modes in the strong-coupling regime is
predicted.


title:
Influence of magnetic fields on the spin reorientation transition in
  ultra-thin films

abstract:
  The dependence of the spin reorientation transition in ultra-thin
ferromagnetic films on external magnetic fields is studied. For different
orientations of the applied field with respect to the film, phase diagrams are
calculated within a mean field theory for the classical Heisenberg model. In
particular we find that the spin reorientation transition present in this model
is not suppressed completely by an applied field, as the magnetization
component perpendicular to the field may show spontaneous order in a certain
temperature interval.


title: Conserved Ordering Dynamics of Heisenberg Spins with Torque

abstract:
  We show that a torque induced by the local molecular field drives the
zero-temperature ordering dynamics of a conserved Heisenberg magnet to a new
fixed point, characterised by exponents z=2 and $\lambda \approx 5.15$.
Numerical solutions of the Langevin equation indicate that theories using a
Gaussian closure are inconsistent even when the torque is absent. The torque is
relevant even for quenches to T_c, with exponents $z=4-\epsilon/2$ and $\lambda
= d$ (where $\epsilon = 6-d$). Indeed $\lambda$ is always equal to d for
quenches to T_c whenever the order parameter is conserved.


title:
Delocalization in an open one-dimensional chain in an imaginary vector
  potential

abstract:
  We present first results for the transmittance, T, through a 1D disordered
system with an imaginary vector potential, ih, which provide a new analytical
criterion for a delocalization transition in the model. It turns out that the
position of the critical curve on the complex energy plane (i.e. the curve
where an exponential decay of <T> is changed by a power-law one) is different
from that obtained previously from the complex energy spectra. Corresponding
curves for <T^n> or <ln T> are also different. This happens because of
different scales of the exponential decay of one-particle Green's functions
(GF) defining the spectra and many-particle GF governing transport
characteristics, and reflects higher-order correlations in localized
eigenstates of the non-Hermitian model.


title:
Systematic Analytical Approach to Correlation Functions of Resonances in
  Quantum Chaotic Scattering

abstract:
  We solve the problem of resonance statistics in systems with broken
time-reversal invariance by deriving the joint probability density of all
resonances in the framework of a random matrix approach and calculating
explicitly all n-point correlation functions in the complex plane. As a
by-product, we establish the Ginibre-like statistics of resonances for many
open channels. Our method is a combination of Itzykson-Zuber integration and a
variant of nonlinear $\sigma-$model and can be applied when the use of
orthogonal polynomials is problematic.

categories: cond-mat chao-dyn nlin.CD nlin.SI solv-int


title:
Spin-gap physics, ground state degeneracy, and bound states on the
  depleted kagome lattice

abstract:
  We analyse the antiferromagnetic spin-1/2 Heisenberg model on a depleted
kagome lattice, where some bonds have been reduced to exchange integral J_2 <<
J_1. The fully depleted system consists of 1D chains, each with a doubly
degenerate singlet-pair ground state and a spectral gap (like the
Majumdar-Ghosh model). There are localised and itinerant low-energy
excitations. The modes from the lowest branch of excitations are incapable of
lifting the 2D system's ground state degeneracy at finite J_2/J_1 << 1.
Low-energy excitations of the 2D system are dominated by coherently propagating
bound states of the 1D excitations.


title: Non-abelian Exclusion Statistics

abstract:
  We introduce the notion of `order-$k$ non-abelian exclusion statistics'. We
derive the associated thermodynamic equations by employing the Thermodynamic
Bethe Ansatz for specific non-diagonal scattering matrices. We make contact
with results obtained by different methods and we point out connections with
`fermionic sum formulas' for characters in a Conformal Field Theory. As an
application, we derive thermodynamic distribution functions for quasi-holes
over a class of non-abelian quantum Hall states recently proposed by Read and
Rezayi.


title:
Spin susceptibility of interacting electrons in one dimension: Luttinger
  liquid and lattice effects

abstract:
  The temperature-dependent uniform magnetic susceptibility of interacting
electrons in one dimension is calculated using several methods. At low
temperature, the renormalization group reaveals that the Luttinger liquid spin
susceptibility $\chi (T) $ approaches zero temperature with an infinite slope
in striking contrast with the Fermi liquid result and with the behavior of the
compressibility in the absence of umklapp scattering. This effect comes from
the leading marginally irrelevant operator, in analogy with the Heisenberg spin
1/2 antiferromagnetic chain. Comparisons with Monte Carlo simulations at higher
temperature reveal that non-logarithmic terms are important in that regime.
These contributions are evaluated from an effective interaction that includes
the same set of diagrams as those that give the leading logarithmic terms in
the renormalization group approach. Comments on the third law of thermodynamics
as well as reasons for the failure of approaches that work in higher dimensions
are given.


title: Theory Of The One Dimensional SU(4) Spin-Orbital Model

abstract:
  The one-dimensional spin-orbital model is studied by means of Abelian
bosonization. We derive the low-energy effective theory which enables us to
study small deviations from the SU(4) symmetric point. We show that there
exists a massless region with algebraically decaying correlation functions,
$\sim \cos(\pi/{2a_0} x) x^{-3/2}$. When entering the massive phase, the system
displays an approximate SO(6) enlarged symmetry with a dimerization type of
ordering consisting in alternating spin and orbital singlets.


title:
Stationary Coverage of a Stochastic Adsorption-Desorption Process with
  Diffusional Relaxation

abstract:
  We show that it is possible to derive the stationary coverage of an
adsorption-desorption process of dimers with diffusional relaxation with a very
simple ansatz for the stationary distribution of the process supplemented by a
hypothesis of global balance. Our approach is contrasted to the exact result
and we seek to understand its validity within an instance of the model.


title:
Triangular anisotropies in Driven Diffusive Systems: reconciliation of
  Up and Down

abstract:
  Deterministic coarse-grained descriptions of driven diffusive systems (DDS)
have been hampered by apparent inconsistencies with kinetic Ising models of
DDS. In the evolution towards the driven steady-state, ``triangular''
anisotropies in the two systems point in opposite directions with respect to
the drive field. We show that this is non-universal behavior in the sense that
the triangular anisotropy ``flips'' with local modifications of the Ising
interactions. The sign and magnitude of the triangular anisotropy also vary
with temperature. We have also flipped the anisotropy of coarse-grained models,
though not yet at the latest stages of evolution. Our results illustrate the
comparison of deterministic coarse-grained and stochastic Ising DDS studies to
identify universal phenomena in driven systems. Coarse-grained systems are
particularly attractive in terms of analysis and computational efficiency.


title: Onset of Self-Assembly in Polymer-Surfactant Systems

abstract:
  The onset of self-assembly in a dilute aqueous solution containing a flexible
polymer and surfactant is theoretically studied. Focusing on the effect of the
surfactant on polymer conformation and using a conjecture of partial collapse
of the polymer at the onset of self-assembly, we obtain results which agree
with known experimental observations: (i) polymer-surfactant self-assembly
always starts at a lower concentration (cac) than the one required for
surfactant-surfactant self-assembly (cmc); (ii) in charged systems the cac
increases with salt concentration and is almost independent of polymer charge;
(iii) in weakly interacting systems the cac remains roughly proportional to the
cmc over a wide range of cmc values. The special case of amphiphilic side-chain
polymers strongly supports our basic conjecture. A similarity is found between
the partial collapse induced by the surfactant and general results concerning
the effect of impurities on critical phenomena.


title:
Towards an effective potential for the monomer, dimer, hexamer, solid
  and liquid forms of hydrogen fluoride

abstract:
  We present an attempt to build up a new two-body effective potential for
hydrogen fluoride, fitted to theoretical and experimental data relevant not
only to the gas and liquid phases, but also to the crystal. The model is simple
enough to be used in Molecular Dynamics and Monte Carlo simulations. The
potential consists of: a) an intra-molecular contribution, allowing for
variations of the molecular length, plus b) an inter-molecular part, with three
charged sites on each monomer and a Buckingham "exp-6" interaction between
fluorines. The model is able to reproduce a significant number of observables
on the monomer, dimer, hexamer, solid and liquid forms of HF. The shortcomings
of the model are pointed out and possible improvements are finally discussed.


title: Dynamics of Ordering of Isotropic Magnets

abstract:
  We study the dynamics of ordering of the nonconserved and conserved
Heisenberg magnet. The dynamics consists of two parts - an irreversible
dissipation into a heat bath and a reversible precession induced by a torque
due to the local molecular field. For quenches both to T=0 and T=T_c, we show
that the torque is irrelevant when the dynamics is nonconserved but relevant
when the dynamics is conserved and is governed by a new nontrivial fixed point.


title: Renormalized Field Theory of Infinitely Driven Lattice Gases

abstract:
  We use field theoretic renormalization group methods to study the critical
behavior of a recently proposed Langevin equation for driven lattice gases
under infinitely fast drive.
  We perform an expansion around the upper critical dimension, d_c=4, and
obtain the critical exponents to one loop order. The main features of the two
loop calculation are then outlined. The renormalized theory is shown to exhibit
a behavior different from the standard field theory for the DLG, i.e. it is not
mean field like.


title: Theory of magnetoresistance in films of dilute magnetic alloys

abstract:
  Earlier a magnetic anisotropy for magnetic impurities nearby the surface of
non-magnetic host was proposed in order to explain the size dependence of the
Kondo effect in dilute magnetic alloys. Recently Giordano has measured the
magnetoresistance of dilute Au(Fe) films for different thicknesses well above
the Kondo temperature $T_K$. In this way he verified the existence of that
anisotropy even for such a case where the Kondo effect is not dominating. For
detailed comparison of that suggestion with experiments, the magnetic field
dependence of the magnetoresistance is calculated in the lowest approximation,
thus in the second order of the exchange coupling. The strength of the
anisotropy is very close to earlier estimates deduced from the size dependence
of the Kondo resistivity amplitude.


title:
Diffusion of a Test Chain in a Quenched Background of Semidilute
  Polymers

abstract:
  Based on a recently established formalism (U. Ebert, J. Stat. Phys. 82, 183
(1996)) we analyze the diffusive motion of a long polymer in a quenched random
medium. The medium is modeled by a frozen semidilute polymer system. In the
framework of standard renormalization group (RG) theory we present a systematic
perturbative approach to handle such a many chain system. In contrast to the
work of Ebert we here deal with long range correlated disorder and find an
attractive RG fixed point. Unlike in polymer statics the semidilute limit here
yields new nontrivial power laws for dynamic quantities. The exponents are
intermediate between the Rouse and reptation results. An explicit one loop
calculation for the center of mass motion is given.


title:
Spin-Orbit-Induced Kondo Size Effect in Thin Films with 5/2-spin
  Impurities

abstract:
  Recently, for spin $S=5/2$ impurities quite different size dependence of the
Kondo contribution to the resistivity was found experimentally than for S=2.
Therefore previous calculation about the effect of the spin-orbit-induced
magnetic anisotropy on the Kondo amplitude of the resistivity is extended to
the case of $S=5/2$ impurity spin which differs from the integer spin case as
the ground state is degenerated. In this case the Kondo contribution remains
finite when the sample size goes to zero and the thickness dependence in the
Kondo resistivity is much weaker for Cu(Mn). The behavior of the Kondo
coefficient as a function of the thickness depends on the Kondo temperature,
that is somewhat stronger for larger $T_K$. Comparing our results with a recent
experiment in thin Cu(Mn) films, we find a good agreement.


title: Metal-insulator transition in the one-dimensional SU(N) Hubbard model

abstract:
  We investigate the metal-insulator transition of the one-dimensional SU(N)
Hubbard model for repulsive interaction. Using the bosonization approach a Mott
transition in the charge sector at half-filling (k_F=\pi/Na_0) is conjectured
for N > 2. Expressions for the charge and spin velocities as well as for the
Luttinger liquid parameters and some correlation functions are given. The
theoretical predictions are compared with numerical results obtained with an
improved zero-temperature quantum Monte Carlo approach. The method used is a
generalized Green's function Monte Carlo scheme in which the stochastic time
evolution is partially integrated out. Very accurate results for the gaps,
velocities, and Luttinger liquid parameters as a function of the Coulomb
interaction U are given for the cases N=3 and N=4. Our results strongly support
the existence of a Mott-Hubbard transition at a {\it non-zero} value of the
Coulomb interaction. We find $U_c \sim 2.2$ for N=3 and $U_c \sim 2.8$ for N=4.


title: Mass enhancement of two-dimensional electrons in thin-oxide Si-MOSFET's

abstract:
  We wish to report in this paper a study of the effective mass (m^*) in
thin-oxide Si-metal-oxide-semiconductor field-effect-transistors, using the
temperature dependence of the Shubnikov-de Haas (SdH) effect and following the
methodology developed by J.L. Smith and P.J. Stiles, Phys. Rev. Lett. {\bf 29},
102 (1972). We find that in the thin oxide limit, when the oxide thickness
$d_{ox}$ is smaller than the average two-dimensional electron-electron
separation r, m^* is still enhanced and the enhancement can be described by
$m^*/m_B = 0.815 + 0.23(r/d_{ox})$, where $m_B = 0.195 m_e$ is the bulk
electron mass, $m_e$ the free electron mass. At $n_s = 6 \times 10^{11}/cm^2$,
for example, $m^* \simeq 0.25 m_e$, an enhancement doubles that previously
reported by Smith and Stiles. Our result shows that the interaction between
electrons in the semiconductor and the neutralizing positive charges on the
metallic gate electrode is important for mass enhancement. We also studied the
magnetic-field orientation dependence of the SdH effect and deduced a value of
$3.0 \pm 0.5$ for the effective $g$ factor in our thin oxide samples.


title: The two-leg t-J ladder: A mean-field description

abstract:
  Two-leg t-J ladders are investigated in the framework of a combination of the
phase string formulation and bond-operator representation. We develope a
mean-field theory in the strong rung interaction regime, i.e. $J_{\perp}\gg J,
t$, which provides a unified description of the undoped insulating phase and
the low doping phase --- the so-called C1S0 phase. Both of them are
characterized by the resonating-valence-bond (RVB) order parameter, with gap
opened up in all spin excitations. The ground state of the doped phase is
intrinsically a superconductor with a d-wave symmetry, which is driven by the
RVB correlations. The ground-state energy is in good agreement with numerical
results. Phase separation is shown to occur beyond some critical value of J/t
for given doping concentration. We also show that the spin gap in the doped
phase is determined by quasi-particle-like excitations. The local structure of
hole pairs as well as the spectra of various spin and charge modes are analyzed
in comparison with other approaches.


title: Non linear flux flow in TiN superconducting thin film

abstract:
  We have studied the superconducting behavior of 100 nm Titanium Nitride (TiN)
thin film in a perpendicular magnetic field. We found a zero field transition
temperature of 4.6 K and a slope in the H-T plane of -0.745 T/K. At 4.2 K, we
have performed careful transport measurements by measuring both the
differential resistivity and voltage as a function of a DC current. Our results
are analyzed in the framework of linear and non linear flux flow behavior. In
particular, we have observed an electronic instability at high vortex
velocities and from its dependence with respect to the applied magnetic field,
we can exctract the inelastic scattering time and diffusion length of the
quasiparticles.


title: Suppression and restoration of superconductivity in PrBa2Cu3O7

abstract:
  I review the principal experimental findings about superconductivity,
suppression thereof, and related properties of RE_{1-x}Pr_xBa_2Cu3O7 in a
decision-tree manner, eliminating the models substantially incompatible with
established experimental facts, eventually focussing on the pf\sigma
hybridization models and listing on a verbal level the interpretations of the
existing experiments as they emerge from such models.


title: Detection by NMR of a "local spin-gap" in quenched CsC60

abstract:
  We present a 13C and 133Cs NMR investigation of the CsC60 cubic quenched
phase. Previous ESR measurements suggest that this phase is metallic, but NMR
reveals contrasting electronic behavior on the local scale. The 13C
spin-lattice relaxation time (T1) exhibits a typical metallic behavior down to
50 K, but indicates that a partial spin-gap opens for T<50 K. Unexpectedly,
133Cs NMR shows that there are two inequivalent Cs sites. For one of these
sites, the NMR shift and (T1T)^{-1} follow an activated law, confirming the
existence of a spin-gap. We ascribe this spin-gap to the occurrence of
localized spin-singlets on a small fraction of the C60 molecules.


title: X-ray phase determination by the principle of minimum charge

abstract:
  When the charge density in a crystal or a quasicrystal is reconstructed from
its Fourier modes, the global minimum value of the density is sensitively
dependent on the relative phases of the modes. The set of phases that maximizes
the value of the global minimum, corresponds, by positivity of the density, to
the charge density having the minimum total charge that is consistent with the
measured Fourier amplitudes. Phases which minimize the total charge (i.e. the
average charge density) also have the property that the lowest minima of the
charge density become exactly degenerate and proliferate within the unit cell.
The large number of degenerate minima have the effect that density maxima are
forced to occupy ever smaller regions of the unit cell. Thus by minimizing
charge, the atomicity of the charge density is enhanced as well. Charge
minimization applied to simulated crystalline and quasicrystalline diffraction
data successfully reproduces the correct phases starting from random initial
values.


title: Hamiltonian Description of Composite Fermions: Aftermath

abstract:
  The Lowest Landau Level (LLL), long distance theory of Composite Fermions
(CF) developed by Murthy and myself is minimally extended to all distances,
guided by very general principles. The resulting theory is mathematically
consistent, and physically appealing: we clearly see the electron and the
vortices binding to form the CF. The meaning of the constraints, their role in
ensuring compressibility of dipolar objects at $\nu =1/2$, and the
observability of dipoles are clarified.


title: c-axis Tunneling in Nb/Au/YBaCuO Structures

abstract:
  We present the experimental results for Nb/Au/YBaCuO structures, in which the
current flows along (001) direction of YBaCuO film. The theoretical evaluations
show, that at the experimental values of the Au/YBaCuO interface transparency,
determined from the interface resistance D~10^-6, the critical current of the
structure is of the fluctuation order of magnitude due to the sharp decrease of
the amplitude potential of the superconducting carriers on this interface.
Obtained I-V-curves could be interpreted in terms of contact between d-type
pairing superconductor or gapless isotropic superconductor with normal metal.
No critical current was observed for investigated structures with
characteristic interface resistance RnS~10^-6 Ohm cm^2, 2 orders of magnitude
lower, than for known experimental data. PACS: 74.50.+r, 74.72.Bk


title: Traveling time and traveling length for flow in porous media

abstract:
  We study traveling time and traveling length for tracer dispersion in porous
media. We model porous media by two-dimensional bond percolation, and we model
flow by tracer particles driven by a pressure difference between two points
separated by Euclidean distance $r$. We find that the minimal traveling time
$t_{min}$ scales as $t_{min} \sim r^{1.40}$, which is different from the
scaling of the most probable traveling time, ${\tilde t} \sim r^{1.64}$. We
also calculate the length of the path corresponding to the minimal traveling
time and find $\ell_{min} \sim r^{1.13}$ and that the most probable traveling
length scales as ${\tilde \ell} \sim r^{1.21}$. We present the relevant
distribution functions and scaling relations.


title: Hysteresis at low Reynolds number: the onset of 2D vortex shedding

abstract:
  Hysteresis has been observed in a study of the transition between laminar
flow and vortex shedding in a quasi-two dimensional system. The system is a
vertical, rapidly flowing soap film which is penetrated by a rod oriented
perpendicular to the film plane. Our experiments show that the transition from
laminar flow to a periodic K\'arm\'an vortex street can be hysteretic, i.e.
vortices can survive at velocities lower than the velocity needed to generate
them.


title: Monte Carlo Study of the Critical Behavior of Random Bond Potts Models

abstract:
  We present results of Monte Carlo simulations of random bond Potts models in
two dimensions, for different numbers of Potts states, q. We introduce a simple
scheme which yields continuous self-dual distributions of the interactions. As
expected, we find multifractal behavior of the correlation functions at the
critical point and obtain estimates of the exponent eta_n for several moments,
n, of the correlation functions, including typical (n -> 0), average (n=1) and
others. In addition, for q=8, we find that there is only a single correlation
length exponent describing the correlation length away from criticality. This
is numerically very close to the pure Ising value of unity.


title: Nonequilibrium Josephson-like effects in wide mesoscopic S-N-S junctions

abstract:
  Mesoscopic superconducting-normal-metal-superconducting (S-N-S) junctions
with a large separation between the superconducting electrodes (i.e. wide
junctions) exhibit nonequilibrium supercurrents, even at temperatures for which
the equilibrium Josephson effect is exponentially small. The second harmonic of
the Josephson frequency dominates these currents, as observed in recent
experiments. A simple description of these effects, in the spirit of the
Resistively-Shunted-Junction model, is suggested here. It is used to calculate
dc I-V characteristics, and to examine the effects of various types of noise
and of external microwave radiation (Shapiro steps). It is found that the
nonequilibrium supercurrents are excited when the junction is driven by a dc
bias or an ac bias, or even by external noise. In the case of junctions which
are also long in the direction perpendicular to the current flow, thermodynamic
phase fluctuations (thermal noise) alone can drive the quasiparticles out of
local equilibrium. Magnetic flux is then predicted to be trapped in units of
Phi_0 /2 = hc/4e.


title: Optimization of Gutzwiller Wavefunctions in Quantum Monte Carlo

abstract:
  Gutzwiller functions are popular variational wavefunctions for correlated
electrons in Hubbard models. Following the variational principle, we are
interested in the Gutzwiller parameters that minimize e.g. the expectation
value of the energy. Rewriting the expectation value as a rational function in
the Gutzwiller parameters, we find a very efficient way for performing that
minimization. The method can be used to optimize general Gutzwiller-type
wavefunctions both, in variational and in fixed-node diffusion Monte Carlo.


title: Parameter-free expression for superconducting Tc in cuprates

abstract:
  A parameter-free expression for the superconducting critical temperature of
layered cuprates is derived which allows us to express Tc in terms of
experimentally measured parameters. It yields Tc values observed in about 30
lanthanum, yttrium and mercury-based samples for different levels of doping.
This remarkable agreement with the experiment as well as the unusual critical
behaviour and the normal-state gap indicate that many cuprates are close to the
Bose-Einstein condensation regime.


title:
Transport in Materials with Disclination Dipoles: Applications to
  Polycrystals and Amorphous Dielectrics

abstract:
  The problem of both electron and phonon scattering by wedge disclination
dipoles (WDD) is studied in the framework of the deformation potential
approach. The exact analytical results for the mean free path are obtained
within the Born approximation. The WDD-induced contribution to the residual
resistivity in nanocrystalline metals is estimated. Using the WDD-based model
of a grain boundary, the thermal conductivity, kappa, of polycrystals and
amorphous dielectrics is studied. It is shown that the low-temperature
crossover of kappa experimentally observed in LiF, NaCl, and sapphire can be
explained by the grain-boundary phonon scattering. A combination of two
scattering processes, the phonon scattering due to biaxial WDD and the
Rayleigh-type scattering, is suggested to be of importance in amorphous
dielectrics. Our results are in a good agreement with the experimentally
observed kappa in a-SiO_2, a-GeO_2 a-Se, and polystyrene over a wide
temperature range.


title: The Dilute Bose Gas Revised

abstract:
  The well-known results concerning a dilute Bose gas with the short-range
repulsive interaction should be reconsidered due to a thermodynamic
inconsistency of the method being basic to much of the present understanding of
this subject. The aim of our paper is to propose a new way of treating the
dilute Bose gas with an arbitrary strong interaction. Using the reduced density
matrix of the second order and a variational procedure, this way allows us to
escape the inconsistency mentioned and operate with singular potentials of the
Lennard-Jones type. The derived expansion of the condensate depletion in powers
of the boson density n=N/V reproduces the familiar result, while the expansion
for the mean energy per particle is of the new form: E=2\pi \hbar^2 a n/m
[1+128/(15 \sqrt{\pi})\sqrt{na^3}(1-5b/8a)+ ...], where a is the scattering
length and b>0 stands for one more characteristic length depending on the shape
of the interaction potential (in particular, for the hard spheres a=b). All the
consideration concerns the zero temperature.


title: Modelling Quasicrystal Growth

abstract:
  Understanding the growth of quasicrystals poses a challenging problem, not
the least because the quasiperiodic order present in idealized mathematical
models of quasicrystals prohibit simple local growth algorithms. This can only
be circumvented by allowing for some degree of disorder, which of course is
always present in real quasicrystalline samples. In this review, we give an
overview of the present state of theoretical research, addressing the problems,
the different approaches and the results obtained so far.


title:
Suppression of Superconductivity in Single Crystals of UPt_3 by Pd
  Substitution

abstract:
  The suppression of superconductivity by substitution effects has been
measured in high quality single crystals of U(Pt_{1-x} Pd_x)_3 with 0 <= x <=
0.002. While the superconducting transition temperature T_c varies linearly
with residual resistivity r_0, consistent with pair-breaking by impurity
potential scattering, the rate of suppression of T_c with r_0 is much larger
for Pd substitution than for other impurity substitutions or by increased
defect density. This effect is correlated with an increase in the inelastic
scattering coefficient, and may be related to Pd-induced changes in the
magnetic fluctuation spectrum.


title: Genetic algorithm dynamics on a rugged landscape

abstract:
  The genetic algorithm is an optimization procedure motivated by biological
evolution and is successfully applied to optimization problems in different
areas. A statistical mechanics model for its dynamics is proposed based on the
parent-child fitness correlation of the genetic operators, making it applicable
to general fitness landscapes. It is compared to a recent model based on a
maximum entropy ansatz. Finally it is applied to modeling the dynamics of a
genetic algorithm on the rugged fitness landscape of the NK model.

categories: cond-mat.stat-mech adap-org nlin.AO q-bio.PE


title: Annealing schedule from population dynamics

abstract:
  We introduce a dynamical annealing schedule for population-based optimization
algorithms with mutation. On the basis of a statistical mechanics formulation
of the population dynamics, the mutation rate adapts to a value maximizing
expected rewards at each time step. Thereby, the mutation rate is eliminated as
a free parameter from the algorithm.


title:
Comparing different approaches to model the atomic structure of a
  ternary decagonal quasicrystal

abstract:
  It is shown that the covering approach with a single decagonal prototile can
be transformed into a hexagon, boat and star tiling. Particularly, the atomic
decoration recently proposed by Cockayne and Widom (Phys. Rev. Lett. 81, 598
(1998)) as a structure model for the decagonal phase is investigated. There,
conflicts with the prototile approach arise which are due to specific
peculiarities of the decoration. The above model is compared with recent
experimental images which give strong support to its main features, but
contradict competing structure proposals. The implications for the
stabilization mechanism are discussed.


title: A prognosis oriented microscopic stock market model

abstract:
  We present a new microscopic stochastic model for an ensemble of interacting
investors that buy and sell stocks in discrete time steps via limit orders
based on individual forecasts about the price of the stock. These orders
determine the supply and demand fixing after each round (time step) the new
price of the stock according to which the limited buy and sell orders are then
executed and new forecasts are made. We show via numerical simulation of this
model that the distribution of price differences obeys an exponentially
truncated Levy-distribution with a self similarity exponent mu~5.


title:
Two lifetimes and the pseudogap in the orbital magnetoresistance of
  Zn-substituted La{1.85}Sr{0.15}CuO{4}

abstract:
  The effect of zinc doping on the anomalous temperature dependence of the
magnetoresistance and the Hall effect in the normal state was studied in a
series of La{1.85}Sr{0.15}Cu{1-y}Zn{y}O{4} films, with values of y between zero
and 0.12. The orbital magnetoresistance at high temperatures is found to be
proportional to the square of the tangent of the Hall angle, as predicted by
the model of two relaxation rates, for all Zn-doped specimens, including
nonsuperconducting films. The proportionality constant is equal to 13.7+/-0.5
independent of doping. This is very different from the behavior observed in
underdoped La{2-x}Sr{x}CuO{4} films where a decrease of x destroys the
proportionality. In addition, the behavior of the orbital magnetoresistance at
low temperatures is found to be different depending on whether x is changed or
y. We suggest that these differences reflect a different evolution of the
pseudogap in the two cases.


title:
A consistent description of kinetics and hydrodynamics of systems of
  interacting particles by means of the nonequilibrium statistical operator
  method

abstract:
  A statistical approach to a self-consistent description of kinetic and
hydrodynamic processes in systems of interacting particles is formulated on the
basis of the nonequilibrium statistical operator method by D.N.Zubarev. It is
shown how to obtain the kinetic equation of the revised Enskog theory for a
hard sphere model, the kinetic equations for multistep potentials of
interaction and the Enskog-Landau kinetic equation for a system of charged hard
spheres. The BBGKY hierarchy is analyzed on the basis of modified group
expansions. Generalized transport equations are obtained in view of a
self-consistent description of kinetics and hydrodynamics. Time correlation
functions, spectra of collective excitations and generalized transport
coefficients are investigated in the case of weakly nonequilibrium systems of
interacting particles.


title: Reduced O diffusion through Be doped Pt electrodes

abstract:
  Using first principles electronic structure calculations we screen nine
elements for their potential to retard oxygen diffusion through
poly-crystalline Pt (p-Pt) films. We determine that O diffuses preferentially
as interstitial along Pt grain boundaries (GBs). The calculated barriers are
compatible with experimental estimates. We find that Be controls O diffusion
through p-Pt. Beryllium segregates to Pt GBs at interstitial (i) and
substitutional (s) sites. i-Be is slightly less mobile than O and it repels O,
thus stuffing the GB. s-Be has a high diffusion barrier and it forms strong
bonds to O, trapping O in the GB. Experiments confirm our theoretical
predictions.


title: Spatial Correlations in Dynamical Mean Field Theory

abstract:
  We further develop an extended dynamical mean field approach introduced
earlier. It goes beyond the standard $D=\infty$ dynamical mean field theory by
incorporating quantum fluctuations associated with intersite (RKKY-like)
interactions. This is achieved by scaling the intersite interactions to the
same power in 1/D as that for the kinetic terms. In this approach, a correlated
lattice problem is reduced to a single-impurity Anderson model with additional
self-consistent bosonic baths. Here, we formulate the approach in terms of
perturbation expansions. We show that the two-particle vertex functions are
momentum-dependent, while the single-particle self-energy remains local. In
spite of this, the approach is conserving. Finally, we also determine the form
of a momentum-dependent dynamical susceptibility; the resulting expression
relates it to the corresponding Weiss field, local correlation function and
(momentum-dependent) intersite coupling.


title: Measurement of the local Jahn-Teller distortion in LaMnO_3.006

abstract:
  The atomic pair distribution function (PDF) of stoichiometric LaMnO_3 has
been measured. This has been fit with a structural model to extract the local
Jahn-Teller distortion for an ideal Mn(3+)O_6 octahedron. These results are
compared to Rietveld refinements of the same data which give the average
structure. Since the local structure is being measured in the PDF there is no
assumption of long-range orbital order and the real, local, Jahn-Teller
distortion is measured directly. We find good agreement both with published
crystallographic results and our own Rietveld refinements suggesting that in an
accurately stoichiometric material there is long range orbital order as
expected. The local Jahn-Teller distortion has 2 short, 2 medium and 2 long
bonds.


title: Critical and Near-Critical Branching Processes

abstract:
  Scale-free dynamics in physical and biological systems can arise from a
variety of causes. Here, we explore a branching process which leads to such
dynamics. We find conditions for the appearance of power laws and study
quantitatively what happens to these power laws when such conditions are
violated. From a branching process model, we predict the behavior of two
systems which seem to exhibit near scale-free behavior--rank-frequency
distributions of number of subtaxa in biology, and abundance distributions of
genotypes in an artificial life system. In the light of these, we discuss
distributions of avalanche sizes in the Bak-Tang-Wiesenfeld sandpile model.

categories: cond-mat.dis-nn adap-org cond-mat.stat-mech nlin.AO q-bio.PE


title:
Comment on ``Evidence for Anisotropic State of Two-Dimensional Electrons
  in High Landau Levels''

abstract:
  In a recent letter M. Lilly et al [PRL 82, 394 (1999)] have shown that a
highly anisotropic state can arise in certain two dimensional electron systems.
In the large square samples studied, resistances measured in the two
perpendicular directions are found to have a ratio that may be 60 or larger at
low temperature and at certain magnetic fields. In Hall bar measurements, the
anisotropy ratio is found to be much smaller (roughly 5). In this comment we
resolve this discrepancy by noting that the anisotropy of the underlying sheet
resistivities is correctly represented by Hall bar resistance measurements but
shows up exponentially enhanced in resistance measurements on square samples
due to simple geometric effects. We note, however, that the origin of this
underlying resistivity anisotropy remains unknown, and is not addressed here.


title: Possible cooling by resonant Fowler-Nordheim emission

abstract:
  A new method of electronic refrigeration based on resonant Fowler-Nordheim
emission is proposed and analyzed. In this method, a bulk emitter is covered
with a-few-nm-thick film of a widegap semiconductor, creating an intermediate
step between electron energies in the emitter and in vacuum. An external
electric field tilts this potential profile, forming a quantum well, and hence
2D electron subbands at the semiconductor-vacuum boundary. Alignment of the
lowest subband with the energy levels of the hottest electrons of the emitter
(a few $k_{B}T$ above its Fermi level) leads to a resonant, selective emission
of these electrons, providing emitter cooling. Calculations show that cooling
power as high as 10^{4} W/cm^{2} (at 300 K), and temperatures down to 10 K may
be achieved using this effect.


title:
The Importance of Static Correlation in the Band Structure of High
  Temperature Superconductors

abstract:
  Recently we presented a new band structure for La(2-x)Sr(x)CuO(4) and other
high temperature superconductors in which a second narrow band was seen to
cross the primary band at the Fermi level. The existence of this second Fermi
level band is in complete disagreement with the commonly accepted LDA band
structure. Yet it provided a crucial piece of physics which led to an
explanation for superconductivity and other unusual phenomena in these
materials. In this work we present details as to the nature of the failure of
conventional methods in deriving the band structure of the cuprates. In
particular, we use a number of chemical analogues to describe the problem of
static correlation in the band structure calculations and show how this can be
corrected with the predictable outcome of a Fermi level band crossing.


title:
RNA secondary structure formation: a solvable model of heteropolymer
  folding

abstract:
  The statistical mechanics of heteropolymer structure formation is studied in
the context of RNA secondary structures. A designed RNA sequence biased
energetically towards a particular native structure (a hairpin) is used to
study the transition between the native and molten phase of the RNA as a
function of temperature. The transition is driven by a competition between the
energy gained from the polymer's overlap with the native structure and the
entropic gain of forming random contacts. A simplified Go-like model is
proposed and solved exactly. The predicted critical behavior is verified via
exact numerical enumeration of a large ensemble of similarly designed
sequences.

categories: cond-mat.stat-mech cond-mat.soft physics.bio-ph q-bio.BM


title:
Barkhausen avalanches in anisotropic ferromagnets with $180^\circ$
  domain walls

abstract:
  We show that Barkhausen noise in two-dimensional disordered ferromagnets with
extended domain walls is characterized by the avalanche size exponent $\tau_s
=1.54$ at low disorder. With increasing disorder the characteristic domain size
is reduced relative to the system size due to nucleation of new domains and a
dynamic phase transition occurs to the scaling behavior with $\tau_s=1.30$. The
exponents decrease at finite driving rate. The results agree with recently
observed behavior in amorphous Metglas and Fe-Co-B ribbons when the applied
anisotropic stress is varied.


title: Response Function of Coarsening Systems

abstract:
  The response function of domain growth processes, and in particular the
violation of the fluctuation-dissipation theorem, are studied both analytically
and numerically. In the asymptotic limit of large times, the
fluctuation-dissipation ratio $X$, which quantifies this violation, tends to
one if $C>m^2$ and to zero if $C<m^2$, corresponding to the fast (`bulk') and
slow (`domain-wall') responses, respectively. In this paper, we focus on the
pre-asymptotic behavior of the domain-wall response. This response is shown to
scale with the typical domain length $L(t)$ as $1/L(t)$ for dimension $d>2$,
and as $\ln (L(t))/L(t)$ for $d=2$. Numerical results confirming this analysis
are presented.


title: Direct current generation due to wave mixing in semiconductors

abstract:
  We describe a novel effect of the generation of direct current which may
arise in semiconductors or semiconductor microstructures due to a mixing of
coherent electromagnetic radiations of commensurate frequencies. The effect is,
in essence, due to a nonparabolicity of the electron energy bands and is
stronger in systems where this nonparabolicity is greater. We have made exact
calculations in the framework of the Kane model, applicable to narrow gap
semiconductors and the tight-binding model which we employ for a description of
a semiconductor superlattice.


title: The Kondo Lattice Model

abstract:
  In this lecture, we review the experimental situation of heavy Fermions with
emphasis on the existence of a quantum phase transition (QPT) and related
non-Fermi liquid (NFL) effects. We overview the Kondo lattice model (KLM) which
is believed to describe the physics of those systems. After recalling the
existing theories based on large-N expansion and various N=2 schemes, we
present two alternative approaches: (i) a spin fluctuation-Kondo functional
integral approach treating the spin-fluctuation and Kondo effects on an equal
footing, and (ii) a supersymmetric theory enlarging the usual fermionic
representation of the spin into a mixed fermionic-bosonic representation in
order to describe the spin degrees of freedom as well as the Fermi-liquid type
excitations. This kind of approaches may open up new prospects for the
description of the critical phenomena associated to the quantum phase
transition in Heavy-Fermion systems.


title: Microscopic equation for growing interfaces in quenched disordered media

abstract:
  We present the microscopic equation of growing interface with quenched noise
for the Tang and Leschhorn model [L. H. Tang and H. Leschhorn, Phys. Rev. A
{\bf 45}, R8309 (1992)]. The evolution equation for the height, the mean
height, and the roughness are reached in a simple way. An equation for the
interface activity density (or free sites density) as function of time is
obtained. The microscopic equation allows us to express these equations into
two contributions: the diffusion and the substratum contributions. All these
equations shows the strong interplay between the diffusion and the substratum
contribution in the dynamics.


title:
Universality in the off-equilibrium critical dynamics of the $3d$
  diluted Ising model

abstract:
  We study the off-equilibrium critical dynamics of the three dimensional
diluted Ising model. We compute the dynamical critical exponent $z$ and we show
that it is independent of the dilution only when we take into account the
scaling-corrections to the dynamics. Finally we will compare our results with
the experimental data.


title: Growing interfaces in quenched disordered media

abstract:
  We present the microscopic equation of growing interface with quenched noise
for the Tang and Leschhorn model [{\em Phys. Rev.} {\bf A 45}, R8309 (1992)].
The evolution equations for the mean heigth and the roughness are reached in a
simple way. Also, an equation for the interface activity density (i.e.
interface density of free sites) as function of time is obtained. The
microscopic equation allows us to express these equations in two contributions:
the diffusion and the substratum one. All the equation shows the strong
interplay between both contributions in the dynamics. A macroscopic evolution
equation for the roughness is presented for this model for the critical
pressure $p=0.461$. The dynamical exponent $\beta=0.629$ is analitically
obtained in a simple way. Theoretical results are in excellent agreement with
the Monte Carlo simulation.


title:
SC state in the underdoped high-T_c cuprates as a quantum spin liquid. A
  microscopic theory

abstract:
  We have discovered a new example of quantum spin liquid which is a
superconducting (SC) phase in 2D electron system close to electronic
topological transition. As a quantum spin liquid in low dimensional localized
spin systems it is characterized by a resonance spin mode and a well separated
two-particle continuum. Application of the theory to the high-T_c cuprates
allows to shed light on many observed in the underdoped regime features such as
the so called 40 meV resonance peak, incommensurability at lower \omega, spin
gap etc.


title:
Sedimentation of strongly and weakly charged colloidal particles:
  Prediction of fractional density dependence

abstract:
  We report on calculations of the reduced sedimentation velocity $U/U_{0}$ in
homogenous suspensions of strongly and weakly charged colloidal spheres as a
function of particle volume fraction $\phi$. For dilute suspensions of strongly
charged spheres at low salinity, $U/U_{0}$ is well represented by the
parametric form $1-p\phi^\alpha$ with a fractional exponent $\alpha=1/3$ and a
parameter $p\simeq 1.8$, which is essentially independent from the macroion
charge $Z$. This non-linear volume fraction dependence can be quantitatively
understood in terms of a model of effective hard spheres with $\phi$-dependent
diameter. For weakly charged spheres in a deionized solvent, we show that the
exponent $\alpha$ can be equal to 1/2, if an expression for $U/U_0$ given by
Petsev and Denkov [J. Colloid Interface Sci. 149, 329 (1992)] is employed. We
further show that the range of validity of this expression is limited to very
small values of $\phi$ and $Z$, which are probably not accessible in
sedimentation experiments. The presented results might also hold for other
systems like spherical proteins or ionic micelles.


title: Ab initio calculations of the dynamical response of copper

abstract:
  The role of localized $d$-bands in the dynamical response of Cu is
investigated, on the basis of {\em ab initio} pseudopotential calculations. The
density-response function is evaluated in both the random-phase approximation
(RPA) and a time-dependent local-density functional approximation (TDLDA). Our
results indicate that in addition to providing a polarizable background which
lowers the free-electron plasma frequency, d-electrons are responsible, at
higher energies and small momenta, for a double-peak structure in the dynamical
structure factor. These results are in agreement with the experimentally
determined optical response of copper. We also analyze the dependence of
dynamical scattering cross sections on the momentum transfer.


title:
The Density Matrix Renormalization Group applied to single-particle
  Quantum Mechanics

abstract:
  A simplified version of White's Density Matrix Renormalization Group (DMRG)
algorithm has been used to find the ground state of the free particle on a
tight-binding lattice. We generalize this algorithm to treat the tight-binding
particle in an arbitrary potential and to find excited states. We thereby solve
a discretized version of the single-particle Schr\"odinger equation, which we
can then take to the continuum limit. This allows us to obtain very accurate
results for the lowest energy levels of the quantum harmonic oscillator,
anharmonic oscillator and double-well potential. We compare the DMRG results
thus obtained with those achieved by other methods.


title: The normal phase of quasi-one-dimensional organic superconductors

abstract:
  We review the properties of quasi-one-dimensional organic superconductors:
the Bechgaard salts and their sulfur analogs in their normal phase precursor
tolong-range order. We go through the main observations made in the normal
state of these systems at low magnetic field and tackle the issue of their
description under the angles of the Fermi and Luttinger liquid pictures.


title: Cutoff and lattice effects in the $\varphy^4$ theory of confined systems

abstract:
  We study cutoff and lattice effects in the O(n) symmetric $\phi^4$ theory for
a $d$-dimensional cubic geometry of size $L$ with periodic boundary conditions.
In the large-N limit above $T_c$, we show that $\phi^4$ field theory at finite
cutoff $\Lambda$ predicts the nonuniversal deviation $\sim (\Lambda L)^{-2}$
from asymptotic bulk critical behavior that violates finite-size scaling and
disagrees with the deviation $\sim e^{-cL}$ that we find in the $\phi^4$
lattice model. The exponential size dependence requires a non-perturbative
treatment of the $\phi^4$ model. Our arguments indicate that these results
should be valid for general $n$ and $d > 2$.


title: Violation of Finite-Size Scaling in Three Dimensions

abstract:
  We reexamine the range of validity of finite-size scaling in the $\phi^4$
lattice model and the $\phi^4$ field theory below four dimensions. We show that
general renormalization-group arguments based on the renormalizability of the
$\phi^4$ theory do not rule out the possibility of a violation of finite-size
scaling due to a finite lattice constant and a finite cutoff. For a confined
geometry of linear size $L$ with periodic boundary conditions we analyze the
approach towards bulk critical behavior as $L \to \infty$ at fixed $\xi$ for $T
> T_c$ where $\xi$ is the bulk correlation length. We show that for this
analysis ordinary renormalized perturbation theory is sufficient. On the basis
of one-loop results and of exact results in the spherical limit we find that
finite-size scaling is violated for both the $\phi^4$ lattice model and the
$\phi^4$ field theory in the region $L \gg \xi$. The non-scaling effects in the
field theory and in the lattice model differ significantly from each other.


title: Thermodynamics of the dissipative two-state system: a Bethe Ansatz study

abstract:
  The thermodynamics of the dissipative two-state system is calculated exactly
for all temperatures and level asymmetries for the case of Ohmic dissipation.
We exploit the equivalence of the two-state system to the anisotropic Kondo
model and extract the thermodynamics of the former by solving the thermodynamic
Bethe Ansatz equations of the latter. The universal scaling functions for the
specific heat $C_{\alpha}(T)$ and static dielectric susceptibility
$\chi_{\alpha}(T)$ are extracted for all dissipation strengths $0<\alpha<1$ for
both symmetric and asymmetric two-state systems. The logarithmic corrections to
these quantities at high temperatures are found in the Kondo limit $\alpha\to
1^{-}$, whereas for $\alpha< 1$ we find the expected power law temperature
dependences with the powers being functions of the dissipative coupling
$\alpha$. The low temperature behaviour is always that of a Fermi liquid.


title:
The NMR of High Temperature Superconductors without Anti-Ferromagnetic
  Spin Fluctuations

abstract:
  A microscopic theory for the NMR anomalies of the planar Cu and O sites in
superconducting La_1.85Sr_0.15CuO_4 is presented that quantitatively explains
the observations without the need to invoke anit-ferromagnetic spin
fluctuations on the planar Cu sites and its significant discrepancy with the
observed incommensurate neutron spin fluctuations. The theory is derived from
the recently published ab-initio band structure calculations that correct LDA
computations tendency to overestimate the self-coulomb repulsion for the
half-filled Cu d_x2-y2 orbital for these ionic systems. The new band structure
leads to two bands at the Fermi level with holes in the Cu d_z2 and apical O
p_z orbitals in addition to the standard Cu d_x2-y2 and planar O p_sigma
orbitals. This band structure is part of a new theory for the cuprates that
explains a broad range of experiments and is based upon the formation of Cooper
pairs comprised of a k up spin electron from one band and a -k down spin
electron from another band (Interband Pairing Model).


title:
Generalization of the Lie-Trotter Product Formula for q-Exponential
  Operators

abstract:
  The Lie-Trotter formula $e^{\hat{A}+\hat{B}} = \lim_{N\to \infty}
(e^{\hat{A}/N} e^{\hat{B}/N})^N$ is of great utility in a variety of quantum
problems ranging from the theory of path integrals and Monte Carlo methods in
theoretical chemistry, to many-body and thermostatistical calculations. We
generalize it for the q-exponential function $e_q (x) = [1+ (1-q)
x]^{(1/(1-q))}$ (with $e_1(x)=e^x$), and prove $e_q(\hat{A}+\hat{B}+(1-q)
[\hat{A}\hat{B}+\hat{B}\hat{A}] /2) = \lim_{N\to \infty}
{[e_{1-(1-q)N}(\hat{A}/N)] [e_{1-(1-q)N}(\hat{B}/N)]}^N$. This extended formula
is expected to be similarly useful in the nonextensive situations


title: Diffusional growth of wetting droplets

abstract:
  The diffusional growth of wetting droplets on the boundary wall of a
semi-infinite system is considered in different regions of a first-order
wetting phase diagram. In a quasistationary approximation of the concentration
field, a general growth equation is established on the basis of a generalized
Gibbs-Thomson relation which includes the van der Waals interaction between the
droplet and the wall. Asymptotic scaling solutions of these equations are found
in the partial-, complete- and pre-wetting regimes.


title: Small-world networks: Evidence for a crossover picture

abstract:
  Watts and Strogatz [Nature 393, 440 (1998)] have recently introduced a model
for disordered networks and reported that, even for very small values of the
disorder $p$ in the links, the network behaves as a small-world. Here, we test
the hypothesis that the appearance of small-world behavior is not a
phase-transition but a crossover phenomenon which depends both on the network
size $n$ and on the degree of disorder $p$. We propose that the average
distance $\ell$ between any two vertices of the network is a scaling function
of $n / n^*$. The crossover size $n^*$ above which the network behaves as a
small-world is shown to scale as $n^*(p \ll 1) \sim p^{-\tau}$ with $\tau
\approx 2/3$.

categories: cond-mat.stat-mech adap-org cond-mat.dis-nn nlin.AO


title: Experiments in Dilute Atomic Bose-Einstein Condensation

abstract:
  Both theoretical and experimental activity in BEC has expanded dramatically
in the three years since the first observation of BEC in a dilute atomic gas.
One is tempted to ask, why? Why is there so much interest in the field? Is this
burst of activity something that could have been foreseen, in, say, 1990?


title:
Collapse of the charge ordering gap of Nd_{0.5}Sr_{0.5}MnO_{3} in an
  applied magnetic field

abstract:
  We report results of tunneling studies on the charge ordering compound
Nd_{0.5}Sr_{0.5}MnO_{3} in a magnetic field up to 6T and for temperature down
to 25K.We show that a gap (2\Delta_{CO} \approx 0.5eV opens up in the density
of state (DOS) at the Fermilevel (E_F) on charge ordering (T_{CO}=150K) which
collapses in an applied magnetic field when the charge ordered state melts.
There is a clear correspondence between the behavior of the resistivity and the
gap formation and its collapse in an applied magnetic field. We conclude that a
gap in the DOS at E_F is necessary for the stability of the charge ordered
state.


title: Crossover from Fermi liquid to Wigner molecule behavior in quantum dots

abstract:
  The crossover from weak to strong correlations in parabolic quantum dots at
zero magnetic field is studied by numerically exact path-integral Monte Carlo
simulations for up to eight electrons. By the use of a multilevel blocking
algorithm, the simulations are carried out free of the fermion sign problem. We
obtain a universal crossover only governed by the density parameter $r_s$. For
$r_s>r_c$, the data are consistent with a Wigner molecule description, while
for $r_s<r_c$, Fermi liquid behavior is recovered. The crossover value $r_c
\approx 4$ is surprisingly small.


title: Excited state TBA and functional relations in spinless Fermion model

abstract:
  The excited state thermodynamic Bethe ansatz (TBA) equations for the spinless
Fermion model are presented by the quantum transfer matrix (QTM) approach. We
introduce a more general family called T-functions and explore functional
relations among them (T-system) and their certain combinations (Y-system).
{}From their analytical property, we derive a closed set of non-linear integral
equations which characterize the correlation length of $<c_j^{\dagger}c_i>$ at
any finite temperatures. Solving these equations numerically, we explicitly
determine the correlation length, which coincides with earlier results with
high accuracy.

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


title:
Effective Field Theory for Layered Quantum Antiferromagnets with
  Non-Magnetic Impurities

abstract:
  We propose an effective two-dimensional quantum non-linear sigma model
combined with classical percolation theory to study the magnetic properties of
site diluted layered quantum antiferromagnets like
La$_{2}$Cu$_{1-x}$M$_x$O$_{4}$ (M$=$Zn, Mg). We calculate the staggered
magnetization at zero temperature, $M_s(x)$, the magnetic correlation length,
$\xi(x,T)$, the NMR relaxation rate, $1/T_1(x,T)$, and the N\'eel temperature,
$T_N(x)$, in the renormalized classical regime. Due to quantum fluctuations we
find a quantum critical point (QCP) at $x_c \approx 0.305$ at lower doping than
the two-dimensional percolation threshold $x_p \approx 0.41$. We compare our
results with the available experimental data.


title:
Dynamical properties of a randomly diluted neural network with variable
  activity

abstract:
  The subject of study is a neural network with binary neurons, randomly
diluted synapses and variable pattern activity. We look at the system with
parallel updating using a probabilistic approach to solve the one step dynamics
with one condensed pattern. We derive restrictions on the storage capacity and
the mutual information content occuring during the retrieval process. Special
focus is on the constraints on the threshold for optimal performance. We also
look at the effect of noisy updating, giving a dynamical version of the
critical temperature, the corresponding threshold and an approximation for the
time evolution for small temperatures. The description is applicable to the
whole retrieval process in the limit of strong dilution. The analysis is
carried out as exactly as possible and over the full parameter ranges,
generalizing some former results.


title:
Non-Universal Quasi-Long Range Order in the Glassy Phase of Impure
  Superconductors

abstract:
  The structural correlation functions of a weakly disordered Abrikosov lattice
are calculated for the first time in a systematic RG-expansion in d=4-\epsilon
dimensions. It is shown, that in the asymptotic limit the Abrikosov lattice
exhibits still quasi long range translational order described by a
non-universal exponent \bar\eta_{\bf G} which depends on the ratio of the
renormalized elastic constants \kappa =\tilde c_{66}/\tilde c_{11} of the flux
line (FL) lattice. Our calculations show clearly three distinct scaling regimes
corresponding to the Larkin, the manifold and the asymptotic Bragg glass
regime. On a wide range of intermediate length scales the FL displacement
correlation function increases as a power law with twice of the manifold
roughness exponent \zeta_{rm}(\kappa), which is also non-universal. Our
results, in particular the \kappa-dependence of the exponents, are in variance
with those of the variational treatment with replica symmetry breaking which
allows in principle an experimental discrimination between the two approaches.


title:
Observation of Type I/II Transition in GaAs/InGaP Heterostructure by C-V
  Profiling

abstract:
  It is known in the literature that InGaP alloy synthesized within a certain
range of growth temperature (520 - 720 degree Centrigrade) shows Cu-Pt crystal
ordering. This ordering reduces the band gap energy . It was predicted that the
ordering induced modification of energy levels of InGaP lowers its conduction
band edge, which could change a type I band alignment at a GaAs/InGaP
heterojunction to type II . Here we report the first observation of this
ordering related type I/II transition for a GaAs/InGaP heterojunction by C-V
profiling experiments done at room temperature .


title:
Effects of carrier concentration on the superfluid density of high-T_c
  cuprates

abstract:
  The absolute values and temperature, T, dependence of the in-plane magnetic
penetration depth, $\lambda_{ab}$, of La_{2-x}Sr_xCuO_4 and
HgBa_2CuO_{4+\delta} have been measured as a function of carrier concentration.
We find that the superfluid density, $\rho_s$, changes substantially and
systematically with doping. The values of $\rho_{s}(0)$ are closely linked to
the available low energy spectral weight as determined by the electronic
entropy just above T_c and the initial slope of $\rho_{s}(T)/\rho_{s}(0)$
increases rapidly with carrier concentration. The results are discussed in the
context of a possible relationship between $\rho_s$ and the normal-state (or
pseudo) energy gap.


title:
Correlation between Spin Polarization and Magnetic Moment in
  Ferromagnetic Alloys

abstract:
  The correlation between the magnetic moment in ferromagnetic alloys and the
tunneling spin polarization in ferromagnet-insulator-superconductor tunneling
experiments has been a mystery. The measured spin polarization for Fe, Co, Ni,
and various Ni alloys is positive and roughly proportional to their magnetic
moments, which can not be explained by considering the net density of states.
Using a tight-binding coherent potential approximation (CPA) model, we show
that while the polarization of the net density of states is not correlated with
the magnetic moment, the polarization of the density of states of {\it s}
electrons is correlated with the magnetic moment in the same manner as observed
by the tunneling experiments.
  We also discuss the spin polarization measurements by Andreev reflection
experiments, some of which obtained different results from the tunneling
experiments and our calculations.


title:
Reaction Diffusion and Ballistic Annihilation Near an Impenetrable
  Boundary

abstract:
  The behavior of the single-species reaction process $A+A\to O$ is examined
near an impenetrable boundary, representing the flask containing the reactants.
Two types of dynamics are considered for the reactants: diffusive and ballistic
propagation. It is shown that the effect of the boundary is quite different in
both cases: diffusion-reaction leads to a density excess, whereas ballistic
annihilation exhibits a density deficit, and in both cases the effect is not
localized at the boundary but penetrates into the system. The field-theoretic
renormalization group is used to obtain the universal properties of the density
excess in two dimensions and below for the reaction-diffusion system. In one
dimension the excess decays with the same exponent as the bulk and is found by
an exact solution. In two dimensions the excess is marginally less relevant
than the bulk decay and the density profile is again found exactly for late
times from the RG-improved field theory. The results obtained for the diffusive
case are relevant for Mg$^{2+}$ or Cd$^{2+}$ doping in the TMMC crystal's
exciton coalescence process and also imply a surprising result for the dynamic
magnetization in the critical one-dimensional Ising model with a fixed spin.
For the case of ballistic reactants, a model is introduced and solved exactly
in one dimension. The density-deficit profile is obtained, as is the density of
left and right moving reactants near the impenetrable boundary.


title:
Energy Landscape and Overlap Distribution of Binary Lennard-Jones
  Glasses

abstract:
  We study the distribution of overlaps of glassy minima, taking proper care of
residual symmetries of the system. Ensembles of locally stable, low lying
glassy states are efficiently generated by rapid cooling from the liquid phase
which has been equilibrated at a temperature $T_{run}$. Varying $T_{run}$, we
observe a transition from a regime where a broad range of states are sampled to
a regime where the system is almost always trapped in a metastable glassy
state. We do not observe any structure in the distribution of overlaps of
glassy minima, but find only very weak correlations, comparable in size to
those of two liquid configurations.


title: Boundary effects on one-particle spectra of Luttinger liquids

abstract:
  We calculate one-particle spectra for a variety of models of Luttinger
liquids with open boundary conditions. For the repulsive Hubbard model the
spectral weight close to the boundary is enhanced in a large energy range
around the chemical potential. A power law suppression, previously predicted by
bosonization, only occurs after a crossover at energies very close to the
chemical potential. Our comparison with exact spectra shows that the effects of
boundaries can partly be understood within the Hartree-Fock approximation.


title:
The Ground State of the ``Frozen'' Electron Phase in Two-Dimensional
  Narrow-Band Conductors with a Long-Range Interelectron Repulsion. Stripe
  Formation and Effective Lowering of Dimension

abstract:
  In narrow-band conductors a weakly screened Coulomb interelectron repulsion
can supress narrow-band electrons' hopping, resulting in formation of a
``frozen'' electron phase which differs principally from any known macroscopic
self-localized electron state including the Wigner crystal. In a zero-bandwidth
limit the ``frozen'' electron phase is a classical lattice system with a
long-range interparticle repulsion. The ground state of such systems has been
considered in the case of two dimensions for an isotropic pair potential of the
mutual particle repulsion. It has been shown that particle ordering into
stripes and effective lowering of dimension universally resides in the ground
state for any physically reasonable pair potential and for any geometry of the
conductor lattice. On the basis of this fact a rigorous general procedure to
fully describe the ground state has been formulated. Arguments have been
adduced that charge ordering in High-T_c superconductors testifies to presence
of a ``frozen'' electron phase in these systems.


title:
Effect of the pseudogap on the Hall conductivity in underdoped
  YBa_2Cu_3O_6+x

abstract:
  In underdoped YBa_2Cu_3O_x (YBCO) with x = 6.63, the opening of the pseudogap
at T^* ~ 160 K has a strong effect on the Hall angle tan(theta). While the Hall
response is significantly reduced, the diagonal current is relatively
unaffected. The Hall conductivity suppression continues deep into the flux-flow
state (from T_c to 40 K), as an anomalous suppression of the vortex Hall
current.


title: Instantaneous Spin Correlations in La2CuO4

abstract:
  We have carried out a neutron scattering study of the instantaneous spin-spin
correlations in La2CuO4 (T_N = 325 K) over the temperature range 337 K to 824
K. Incident neutron energies varying from 14.7 meV to 115 meV have been
employed in order to guarantee that the energy integration is carried out
properly. The results so-obtained for the spin correlation length as a function
of temperature when expressed in reduced units agree quantitatively both with
previous results for the two dimensional (2D) tetragonal material Sr2CuO2Cl2
and with quantum Monte Carlo results for the nearest neighbor square lattice
S=1/2 Heisenberg model. All of the experimental and numerical results for the
correlation length are well described without any adjustable parameters by the
behavior predicted for the quantum non-linear sigma model in the low
temperature renormalized classical regime. The amplitude, on the other hand,
deviates subtly from the predicted low temperature behavior. These results are
discussed in the context of recent theory for the 2D quantum Heisenberg model.


title: Effective Mass of a Vortex in a Clean Superconductor

abstract:
  We calculate the effective mass of a single quantized vortex in the BCS
superconductor at finite temperature. Using the effective action for a vortex,
we arrive at the mass formula as the integral of the spectral function
$J(\omega)/\omega^3$ over frequency. The spectral function is given in terms of
the transition elements of the gradient of the Hamiltonian between
Bogoliubov-deGennes eigenstates. We show that core-core, and core-extended
transitions yield the vortex mass, near T=0, of order of electron mass
displaced by the normal core. The extended-extended states contributions are
linearly divergent with the low frequency cutoff $\omega_c$, in accordance with
the Ohmic character of the spectral function at low energies. We argue that the
mass and friction are closely related in this system and arise from the same
mechanism - interaction with the surrounding fermionic degrees of freedom.


title: Transport through a finite Hubbard chain connected to reservoirs

abstract:
  The dc conductance through a finite Hubbard chain of size N coupled to two
noninteracting leads is studied at T = 0 in an electron-hole symmetric case.
Assuming that the perturbation expansion in U is valid for small N (=1,2,3,...)
owing to the presence of the noninteracting leads, we obtain the self-energy at
\omega = 0 analytically in the real space within the second order in U. Then,
we calculate the inter-site Green's function which connects the two boundaries
of the chain, G_{N1}, solving the Dyson equation. The conductance can be
obtained through G_{N1}, and the result shows an oscillatory behavior as a
function of N. For odd N, a perfect transmission occurs independent of U. This
is due to the inversion and electron-hole symmetries, and is attributed to a
Kondo resonance appearing at the Fermi level. On the other hand, for even N,
the conductance is a decreasing function of N and U.


title: Spontaneous emission of polaritons from a Bose-Einstein condensate

abstract:
  We study the spontaneous emission of a partially excited Bose-Einstein
condensate composed of two-level atoms. The formation of polaritons induced by
the ground-state part of the condensate leads to an avoided crossing in the
photon spectrum. This avoided crossing acts similarly to a photonic band gap
and modifies the spontaneous emission rate.


title: Non-degenerate Two Photon Absorption Spectra of Si Nanocrystallites

abstract:
  We propose an efficient linear-scaling time-dependent method for calculating
nonlinear response function, and study the size effects in non-degenerate two
photon absorption spectra of Si nanocrystallites by using semi-empirical
pseudopotentials.


title: Thermodynamics of binary mixture glasses

abstract:
  We compute the thermodynamic properties of the glass phase in a binary
mixture of soft spheres. Our approach is a generalization to mixtures of the
replica strategy, recently proposed by Mezard and Parisi, providing a first
principle statistical mechanics computation of the thermodynamics of glasses.
The method starts from the inter-atomic potentials, and translates the problem
into the study of a molecular liquid. We compare uor analytical predictions to
numerical simulations, focusing onto the values of the thermodynamic transition
and the configurational entropy.


title: Ultrametricity in 3D Edwards-Anderson spin glasses

abstract:
  We perform an accurate test of Ultrametricity in the aging dynamics of the
three dimensional Edwards-Anderson spin glass. Our method consists in
considering the evolution in parallel of two identical systems constrained to
have fixed overlap. This turns out to be a particularly efficient way to study
the geometrical relations between configurations at distant large times. Our
findings strongly hint towards dynamical ultrametricity in spin glasses, while
this is absent in simpler aging systems with domain growth dynamics. A recently
developed theory of linear response in glassy systems allows to infer that
dynamical ultrametricity implies the same property at the level of equilibrium
states.


title:
Comment on "Macrospopic Equation for the Roughness of Growing Interfaces
  in Quenched Disorder"

abstract:
  We comment on a recent Letter by Braunstein and Buceta [PRL vol.81, 630
(1998)], in which a novel equation has been proposed to describe the dynamics
of interfaces in the presence of quenched disorder. We argue that the ansatz
Braunstein and Buceta presented is just an effective description of the very
short times regime and has no significance for the macroscopic (large scale)
behaviour of the interface. We calculate an expression for the short time
behaviour of the interface that is valid for any q (and not only at q_c).


title: On the universality of compact polymers

abstract:
  Fully packed loop models on the square and the honeycomb lattice constitute
new classes of critical behaviour, distinct from those of the low-temperature
O(n) model. A simple symmetry argument suggests that such compact phases are
only possible when the underlying lattice is bipartite. Motivated by the hope
of identifying further compact universality classes we therefore study the
fully packed loop model on the square-octagon lattice. Surprisingly, this model
is only critical for loop weights n < 1.88, and its scaling limit coincides
with the dense phase of the O(n) model. For n=2 it is exactly equivalent to the
selfdual 9-state Potts model. These analytical predictions are confirmed by
numerical transfer matrix results. Our conclusions extend to a large class of
bipartite decorated lattices.


title:
Magnetic Susceptibility Due to Disorder-Induced Neutral Solitons in
  Interacting Polymer Chains

abstract:
  We study the magnetic response due to neutral solitons induced by disorder in
polymer materials. We account for interchain interactions, which, if
sufficiently strong, result in a bond-ordered phase, in which the neutral
solitons are bound into pairs. We analytically calculate the corresponding pair
size distribution. As the spins of the solitons have a distance dependent
antiferromagnetic coupling, this allows us to calculate the magnetic
susceptibility in the ordered phase. At low temperatures, the result deviates
from the usual Curie behavior in a way that depends on the relative strength of
the disorder and the interchain interactions. We compare our results to the
observed magnetic susceptibility of trans-polyacetylene and we suggest new
experiments extending towards lower temperatures.


title:
Experimental Evidence for Coulomb Charging Effects in the Submicron
  Bi-2212 Stacks

abstract:
  We developed the focused ion beam (FIB) and ion milling techniques for a
fabrication of the Bi_2Sr_2CaCu_2O_{8+\delta} (Bi-2212) stacked junctions with
in-plane size L_{ab} from several microns down to the submicron scale without
degradation of T_c. We found that behaviour of submicron junctions (L_{ab} < 1
{\mu}m) is quite different from the bigger ones. The critical current density
is considerably suppressed, the hysteresis and multibranched structure of the
IV characteristics are eliminated, the periodic structure of current peaks
reproducibly appears on the IV curves at low temperatures. A period of the
structure, {\Delta}V, is consistent with the Coulomb charging energy of a
single pair, {\Delta}V = e/C with C the effective capacitance of the stack. We
consider this behaviour to originate from the Coulomb blockade of the intrinsic
Josephson tunneling in submicron Bi-2212 stacks.


title:
Confinement-deconfinement transition in two-coupled chains with umklapp
  scattering

abstract:
  A role of umklapp scattering has been examined for two-coupled chains with
both forward and backward scatterings by applying renormalization group method
to bosonized Hamiltonian. It has been found that a state with relevant
interchain hopping changes into a state with irrelevant (confined) one when the
magnitude of umklapp scattering becomes larger than that of interchain hopping.
Critical value of umklapp scattering for such a confinement-deconfinement
transition is calculated as the function of interchain hopping and intrachain
interactions. A crossover from one-dimensional regime into that of coupled
chains is also shown with decreasing temperature.