title:
Nonlinear response scaling of the two-dimensional XY spin glass in the
  Coulomb-gas representation

abstract:
  Vortex critical dynamics of the two dimensional XY spin glass is studied by
Monte Carlo methods in the Coulomb-gas representation. A scaling analysis of
the nonlinear response is used to calculate the correlation length exponent
$\nu $ of the zero-temperature glass transition. The estimate, $\nu =1.3(2)$,
is in agreement witth a recent estimate in the phase representation using the
same analysis and indicates that the relevant length scale for vortex motion is
set by the spin-glass correlation length and that spin and chiralities may
order with different correlation length exponents.


title:
Microwave spectroscopy on a double quantum dot with an on-chip Josephson
  oscillator

abstract:
  We present measurements on microwave spectroscopy on a double quantum dot
with an on-chip microwave source. The quantum dots are realized in the
two-dimensional electron gas of an AlGaAs/GaAs heterostructure and are weakly
coupled in series by a tunnelling barrier forming an 'ionic' molecular state.
We employ a Josephson oscillator formed by a long Nb/Al-AlO$_x$/Nb junction as
a microwave source. We find photon-assisted tunnelling sidebands induced by the
Josephson oscillator, and compare the results with those obtained using an
externally operated microwave source.


title: Entropy for relaxation dynamics in granular media

abstract:
  We investigate the role of entropic concepts for the relaxation dynamics in
granular systems. In these systems the existence of a geometrical frustration
induces a drastic modification of the allowed phase space, which in its turn
induces a dynamic behavior characterized by hierarchical relaxation phenomena
with several time scales associated. In particular we show how, in the
framework of a mean-field model introduced for the compaction phenomenon, there
exists a free-energy-like functional which decreases along the trajectories of
the dynamics and which allows to account for the asymptotic behavior: e.g.
density profile, segregation phenomena. Also we are able to perform the
continuous limit of the above mentioned model which turns out to be a diffusive
limit. In this framework one can single out two separate physical ingredients:
the free-energy-like functional that defines the phase-space and the asymptotic
states and a diffusion coefficient $D(\rho)$ accounting for the velocity of
approach to the asymptotic stationary states.


title:
Criticality of the "critical state" of granular media: Dilatancy angle
  in the tetris model

abstract:
  The dilatancy angle describes the propensity of a granular medium to dilate
under an applied shear. Using a simple spin model (the ``tetris'' model) which
accounts for geometrical ``frustration'' effects, we study such a dilatancy
angle as a function of density. An exact mapping can be drawn with a directed
percolation process which proves that there exists a critical density $\rho_c$
above which the system expands and below which it contracts under shear. When
applied to packings constructed by a random deposition under gravity, the
dilatancy angle is shown to be strongly anisotropic, and it constitutes an
efficient tool to characterize the texture of the medium.


title: Bosons in a Lattice: Exciton-Phonon Condensate in Cu2O

abstract:
  We explore a nonlinear field model to describe the interplay between the
ability of excitons to be Bose-condensed and their interaction with other modes
of a crystal. We apply our consideration to the long-living para-excitons in
Cu2O. Taking into account the exciton-phonon interaction and introducing a
coherent phonon part of the moving condensate, we derive the dynamic equations
for the exciton-phonon condensate. These equations can support localized
solutions, and we discuss the conditions for the moving inhomogeneous
condensate to appear in the crystal. We calculate the condensate wave function
and energy, and a collective excitation spectrum in the semiclassical
approximation; the inside-excitations were found to follow the asymptotic
behavior of the macroscopic wave function exactly. The stability conditions of
the moving condensate are analyzed by use of Landau arguments, and Landau
critical parameters appear in the theory. Finally, we apply our model to
describe the recently observed interference and strong nonlinear interaction
between two coherent exciton-phonon packets in Cu2O.


title:
2+p-SAT: Relation of Typical-Case Complexity to the Nature of the Phase
  Transition

abstract:
  Heuristic methods for solution of problems in the NP-Complete class of
decision problems often reach exact solutions, but fail badly at "phase
boundaries", across which the decision to be reached changes from almost always
having one value to almost having a different value. We report an analytic
solution and experimental investigations of the phase transition that occurs in
the limit of very large problems in K-SAT. The nature of its "random
first-order" phase transition, seen at values of K large enough to make the
computational cost of solving typical instances increase exponenitally with
problem size, suggest a mechanism for the cost increase. There has been
evidence for features like the "backbone" of frozen inputs which characterizes
the UNSAT phase in K-SAT in the study of models of disordered materials, but
this feature and this transition are uniquely accessible to analysis in K-SAT.
The random first order transition combines properties of the 1st order
(discontinuous onset of order) and 2nd order (with power law scaling, e.g. of
the width of the the critical region in a finite system) transitions known in
the physics of pure solids. Such transitions should occur in other combinatoric
problems in the large N limit. Finally, improved search heuristics may be
developed when a "backbone" is known to exist.


title: Collisional relaxation in diffuse clouds of trapped bosons

abstract:
  The damping of collective modes and the relaxation of temperature
anisotropies in a trapped Bose gas is determined at temperatures above the
Bose-Einstein condensation temperature in the collisionless regime. We
demonstrate for both cases how the effects of collisions may be treated
perturbatively and calculate relaxation rates based on a variational principle.
Our results are compared with experiment and with previous theoretical
calculations.


title: Density Matrix Renormalization

abstract:
  The Density Matrix Renormalization Group (DMRG) has become a powerful
numerical method that can be applied to low-dimensional strongly correlated
fermionic and bosonic systems. It allows for a very precise calculation of
static, dynamic and thermodynamic properties. Its field of applicability has
now extended beyond Condensed Matter, and is successfully used in Statistical
Mechanics and High Energy Physics as well. In this article, we briefly review
the main aspects of the method. We also comment on some of the most relevant
applications so as to give an overview on the scope and possibilities of DMRG
and mention the most important extensions of the method such as the calculation
of dynamical properties, the application to classical systems, inclusion of
temperature, phonons and disorder and a recent modification for the {\it ab
initio} calculation of electronic states in molecules.


title: Surface Properties of a Selective Dissagregation Model

abstract:
  There are three fundamental physical processes that gives rise to the
morphology of a surface: deposition, surface diffusion and desorption. The
characteristics of the interfaces generated by the combination of deposition
and surface diffusion has been well studied during the past decades . In
particular for growth models, particles are added to the surface and then are
allowed to relax by different mechanisms. Many of this models have been shown
to lead to the formation of self-affine surfaces, characterized by scaling
exponents. From a theoretical point of view, the studies dedicated to the
self-affine interfaces generated by growth models can be considered to follow
two main branches. The studies about the properties of discrete models and the
studies about continuous models. The first ones where dedicated mainly to the
study of the properties of computational models in which the growth proceeds on
an initially empty lattice representing a d-dimensional substrate. At each time
step, the height of the lattice sites is increased by units (usually one unit)
representing the incoming particles. Different models only differs on the
relaxation mechanisms proposed to capture specific experimental
characteristics. Then, the models are classified according to the values of the
scaling exponents in several universality classes.


title: A Simple Method to Mix Granular Materials

abstract:
  We show that a mixture of two species of granular particles with equal sizes
but differing densities can be either segregated or mixed by adjusting the
granular temperature gradient and the magnitude of the gravitational force. In
the absence of gravity, the dense, heavy particles move to the colder regions.
If the temperature gradient is put into a gravitational field with the colder
regions above the hotter, a uniform mixture of light and heavy particles can be
attained. This situation can be realized in a container of finite height with a
vibrating bottom, placed in a gravitational field. We present a relation
between the height of the container, the particle properties, and the strength
of gravity required to minimize segregation.


title:
Segregation of polydisperse granular media in the presence of a
  temperature gradient

abstract:
  Granular media are examined with the focus on polydisperse mixtures in the
presence of two localized heat-baths. If the two driving energies are similar,
the large particles prefer to stay in the `cold' regions of the system -- as
far away from the energy source as possible. If one of the temperatures is
larger than the other, the cold region is shifted towards the colder reservoir;
if the temperature of one source is much higher, a strong, almost constant
temperature gradient builds up between the two reservoirs and the large
particles are found close to the cold reservoir. Furthermore, clustering is
observed between the heat reservoirs, if dissipation is strong enough.


title: Anomalous diffusion with absorption: Exact time-dependent solutions

abstract:
  Recently, analytical solutions of a nonlinear Fokker-Planck equation
describing anomalous diffusion with an external linear force were found using a
non extensive thermostatistical Ansatz. We have extended these solutions to the
case when an homogeneous absorption process is also present. Some peculiar
aspects of the interrelation between the deterministic force, the nonlinear
diffusion and the absorption process are discussed.


title:
A study of the superconducting gap in RNi$_2$B$_2$C (R = Y, Lu) single
  crystals by inelastic light scattering

abstract:
  Superconductivity-induced changes in the electronic Raman scattering response
were observed for the RNi$_2$B$_2$C (R = Y, Lu) system in different scattering
geometries. In the superconducting state, 2$\Delta$-like peaks were observed in
A$_{1g}$, B$_{1g}$, and B$_{2g}$ spectra from single crystals. The peaks in
A$_{1g}$ and B$_{2g}$ symmetries are significantly sharper and stronger than
the peak in B$_{1g}$ symmetry. The temperature dependence of the frequencies of
the 2$\Delta$-like peaks shows typical BCS-type behavior, but the apparent
values of the $2\Delta$ gap are strongly anisotropic for both systems. In
addition, for both YNi$_2$B$_2$C and LuNi$_2$B$_2$C systems, there exists
reproducible scattering strength below the $2\Delta$ gap which is roughly
linear to the frequency in B$_{1g}$ and B$_{2g}$ symmetries. This discovery of
scattering below the gap in non-magnetic borocarbide superconductors, which are
thought to be conventional BCS-type superconductors, is a challenge for current
understanding of superconductivity in this system.


title:
Reentrant AC magnetic susceptibility in Josephson-junction arrays: An
  alternative explanation for the paramagnetic Meissner effect

abstract:
  The paramagnetic Meissner effect (PME) measured in high $T_{C}$ granular
superconductors has been attributed to the presence of $\pi$-junctions between
the grains. Here we present measurements of complex AC magnetic susceptibility
from two-dimensional arrays of conventional (non $\pi$) Nb/Al/AlOx/Nb Josephson
junctions. We measured the susceptibility as a function of the temperature $T$,
the AC amplitude of the excitation field, $h_{AC}$, and the external magnetic
field, $H_{DC}$. The experiments show a strong paramagnetic contribution from
the multi-junction loops, which manifests itself as a reentrant screening at
low temperature, for values of $h_{AC}$ higher than 50 mOe. A highly simplified
model, based on a single loop containing four junction, accounts for this
paramagnetic contribution and the range of parameters in which it appears. This
model offers an alternative explanation of PME which does not involve
$\pi$-junctions.


title: Cluster-Growth in Freely Cooling Granular Media

abstract:
  When dissipative particles are left alone, their fluctuation energy decays
due to collisional interactions, clusters build up and grow with time until the
system size is reached. When the effective dissipation is strong enough, this
may lead to the `inelastic collapse', i.e. the divergence of the collision
frequency of some particles. The cluster growth is an interesting physical
phenomenon, whereas the inelastic collapse is an intrinsic effect of the
inelastic hard sphere (IHS) model used to study the cluster growth - involving
only a negligible number of particles in the system. Here, we extend the IHS
model by introducing an elastic contact energy and the related contact duration
t_c. This avoids the inelastic collapse and allows to examine the long-time
behavior of the system. For a quantitative description of the cluster growth,
we propose a burning - like algorithm in continuous space, that readily
identifies all particles that belong to the same cluster. The criterion for
this is here chosen to be only the particle distance.
  With this method we identify three regimes of behavior. First, for short
times a homogeneous cooling state (HCS) exists, where a mean-field theory works
nicely, and the clusters are tiny and grow very slowly. Second, at a certain
time which depends on the system's properties, cluster growth starts and the
clusters increase in size and mass until, in the third regime, the system size
is reached and most of the particles are collected in one huge cluster.


title:
On the relative positions of the $2\Delta$ peaks in Raman and tunneling
  spectra of d-wave superconductors

abstract:
  We study $B_{1g}$ Raman intensity $R(\Omega)$ and the density of states
$N(\omega)$ in isotropic 2D d-wave superconductors. For an ideal gas,
$R(\Omega)$ and $N(\omega)$ have sharp peaks at $\Omega =2\Delta$ and $\omega
=\Delta$, respectively, where $\Delta$ is the maximum value of the gap.
  We study how the peak positions are affected by the fermionic damping due to
impurity scattering.
  We show that while the damping generally shifts the peak positions to larger
frequencies, the peak in $R(\Omega)$ still occurs at almost twice the peak
position in $N(\omega)$ and therefore cannot account for the experimentally
observed downturn shift of the peak frequency in $R(\Omega)$ in underdoped
cuprates compared to twice that in $N(\omega)$. We also discuss how the
fermionic damping affects the dynamical spin susceptibility.


title:
Linear Relaxation Processes Governed by Fractional Symmetric Kinetic
  Equations

abstract:
  We get fractional symmetric Fokker - Planck and Einstein - Smoluchowski
kinetic equations, which describe evolution of the systems influenced by
stochastic forces distributed with stable probability laws. These equations
generalize known kinetic equations of the Brownian motion theory and contain
symmetric fractional derivatives over velocity and space, respectively. With
the help of these equations we study analytically the processes of linear
relaxation in a force - free case and for linear oscillator. For a weakly
damped oscillator we also get kinetic equation for the distribution in slow
variables. Linear relaxation processes are also studied numerically by solving
corresponding Langevin equations with the source which is a discrete - time
approximation to a white Levy noise. Numerical and analytical results agree
quantitatively.


title:
Transition from quantum to classical Heisenberg trimers: Thermodynamics
  and time correlation functions

abstract:
  We focus on the transition from quantum to classical behavior in
thermodynamic functions and time correlation functions of a system consisting
of three identical quantum spins s that interact via isotropic Heisenberg
exchange. The partition function and the zero-field magnetic susceptibility are
readily shown to adopt their classical forms with increasing s. The behavior of
the spin autocorrelation function (ACF) is more subtle. Unlike the classical
Heisenberg trimer where the ACF approaches a unique non-zero limit for long
times, for the quantum trimer the ACF is periodic in time. We present exact
values of the time average over one period of the quantum trimer for s less or
equal 7 and for infinite temperature. These averages differ from the long-time
limit, (9/40)\ln3+(7/30), of the corresponding classical trimer by terms of
order 1/(s*s). However, upon applying the Levin u-sequence acceleration method
to our quantum results we can reproduce the classical value to six significant
figures.


title: Local mode behaviour in quasi-1D CDW systems

abstract:
  We analyze numerically the ground-state and spectral properties of the
three-quarter filled Peierls-Hubbard Hamiltonian. Various charge- and
spin-ordered states are identified. In the strong-coupling regime, we find
clear signatures of local lattice distortions accompanied by intrinsic
multi-phonon localization. The results are discussed in relation to recent
experiments on MX chain [-PtCl-] complexes. In particular we are able to
reproduce the observed red shift of the overtone resonance Raman spectrum.


title:
Spin correlation functions and N\'{e}el order in the 2D Heisenberg
  model: Effects of spatial anisotropy

abstract:
  The ground-state properties of the square-lattice spin-1/2 Heisenberg
antiferromagnet with spatially anisotropic couplings are investigated by
Green's-function projection approaches. The staggered magnetization and the
two-spin correlators are calculated; the competition between magnetic long- and
short-range order is discussed in comparison with experiments on $\rm
Sr_2[Ca_2]CuO_3$.


title: Oscillatory behaviour in a lattice prey-predator system

abstract:
  Using Monte Carlo simulations we study a lattice model of a prey-predator
system. We show that in the three-dimensional model populations of preys and
predators exhibit coherent periodic oscillations but such a behaviour is absent
in lower-dimensional models. Finite-size analysis indicate that amplitude of
these oscillations is finite even in the thermodynamic limit. In our opinion,
this is the first example of a microscopic model with stochastic dynamics which
exhibits oscillatory behaviour without any external driving force. We suggest
that oscillations in our model are induced by some kind of stochastic
resonance.


title:
Interaction of surface acoustic waves with a two-dimensional electron
  gas in the presence of spin splitting of the Landau bands

abstract:
  The absorption and variation of the velocity of a surface acoustic wave of
frequency $f$= 30 MHz interacting with two-dimensional electrons are
investigated in GaAs/AlGaAs heterostructures with an electron density $n=(1.3 -
2.8) \times 10^{11} cm^{-2}$ at $T$=1.5 - 4.2 K in magnetic fields up to 7 T.
Characteristic features associated with spin splitting of the Landau level are
observed. The effective g factor and the width of the spin-split Landau bands
are determined: $g^* \simeq 5$ and $A$=0.6 meV. The greater width of the
orbital-split Landau bands (2 meV) relative to the spin-split bands is
attributed to different shielding of the random fluctuation potential of
charged impurities by 2D electrons. The mechanisms of the nonlinearities
manifested in the dependence of the absorption and the velocity increment of
the SAW on the SAW power in the presence of spin splitting of the Landau levels
are investigated.


title: Correlations in the cotunneling regime of a quantum dot

abstract:
  Off-resonance conductance through weakly coupled quantum dots ("valley
conductance") is governed by cotunneling processes in which a large number of
dot states participate. Virtually the same states participate in the transport
at consecutive valleys, which leads to significant valley-valley conductance
correlations. These correlations are calculated within the constant interaction
model. Comparison with experiment shows that these correlations are less robust
in reality. Among the possible reasons for this is the breakdown of the
constant interaction model, accompanied by "scrambling" of the dot as the
particle number is varied.


title:
Effects of doping on thermally excited quasiparticles in the high-$T_c$
  superconducting state

abstract:
  The physical properties of low energy superconducting quasiparticles in high-
$T_c$ superconductors are examined using magnetic penetration depth and
specific heat experimental data. We find that the low energy density of states
of quasiparticles of La$_{2-x}$Sr$_x$CuO$_4$ scales with $(x-x_c)/T_c$ to the
leading order approximation, where $x_c $ is the critical doping concentration
below which $T_c=0$. The linear temperature term of the superfluid density is
renormalized by quasiparticle interactions and the renormalization factor times
the Fermi velocity is found to be doping independent.


title: High-Acceleration Patterns in Thin Vibrated Granular Layers

abstract:
  Theoretical and experimental study of high-acceleration patterns in vibrated
granular layers is presented. The order parameter model based on parametric
Ginzburg-Landau equation is used to describe strongly nonlinear excitations
including hexagons, interface between flat anti-phase domains and new localized
objects, super-oscillons. The experiments confirmed the existence of
super-oscillons and bound states of super-oscillons and interfaces. On the
basis of order parameter model we predict analytically and confirm
experimentally that the additional subharmonic driving results in controlled
motion of the interfaces.


title: Negative Domain Wall Resistance in Ferromagnets

abstract:
  The electrical resistance of a diffusive ferromagnet with magnetic domain
walls is studied theoretically, taking into account the spatial dependence of
the magnetization. The semiclassical domain wall resistance is found to be
either negative or positive depending on the difference between the
spin-dependent scattering life-times. The predictions can be tested
experimentally by transport studies in doped ferromagnets.


title:
Bloch-like oscillations induced by charge discreteness in quantum
  mesoscopic rings

abstract:
  We study the effect of charge discreteness in a quantum mesoscopic ring wih
inductance L. The ring is pierced by a time depending external magnetic field.
When the external magnetic flux varies uniformly, the current induced in the
ring oscillates with a frequency proportional to the charge discreteness and
the flux variation. This phenomenon is very similar to the well known Bloch's
oscillation in crystals. The similitude is related to the charge discreteness
in the charge-current representation, which plays the same role as the constant
lattice in crystals.


title:
Evolution of a polaron band through the phase diagram of
  Nd[2-x]Ce[x]CuO[4-y]

abstract:
  The evolution with doping and temperature of the polaronic absorption in a
family of high-T$_c$ superconductors is first followed here across the whole
phase diagram. The polaron band softens with increasing doping, is still
present in the superconducting phase, and vanishes in the overdoped metallic
phase. These results point toward the existence of polaron aggregates at low
temperature and may provide an experimental ground for the increasing number of
theoretical descriptions of many-polaron systems.


title: Phase diagram of ferrimagnetic ladders with bond-alternation

abstract:
  We study the phase diagram of a 2-leg bond-alternation spin-(1/2, 1) ladder
for two different configurations using a quantum renormalization group
approach. Although d-dimensional ferrimagnets show gapless behavior, we will
explicitly show that the effect of the spin mixing and the bond-alternation can
open the possibility for observing an energy gap. We show that the gapless
phases of such systems can be equivalent to the 1-dimensional half-integer
antiferroamgnets, besides the gapless ferrimagnetic phases. We therefore
propose a phase transition between these two gapless phases that can be seen in
the parameter space.


title: Phase diagram of an exactly solvable t-J ladder model

abstract:
  We study a system of one-dimensional t-J models coupled to a ladder system. A
special choice of the interaction between neighbouring rungs leads to an
integrable model with supersymmetry, which is broken by the presence of rung
interactions. We analyze the spectrum of low-lying excitations and ground state
phase diagram at zero temperature.


title:
Spin interactions and switching in vertically tunnel-coupled quantum
  dots

abstract:
  We determine the spin exchange coupling J between two electrons located in
two vertically tunnel-coupled quantum dots, and its variation when magnetic (B)
and electric (E) fields (both in-plane and perpendicular) are applied. We
predict a strong decrease of J as the in-plane B field is increased, mainly due
to orbital compression. Combined with the Zeeman splitting, this leads to a
singlet-triplet crossing, which can be observed as a pronounced jump in the
magnetization at in-plane fields of a few Tesla, and perpendicular fields of
the order of 10 Tesla for typical self-assembled dots. We use harmonic
potentials to model the confining of electrons, and calculate the exchange J
using the Heitler-London and Hund-Mulliken technique, including the long-range
Coulomb interaction. With our results we provide experimental criteria for the
distinction of singlet and triplet states and therefore for microscopic spin
measurements. In the case where dots of different sizes are coupled, we present
a simple method to switch on and off the spin coupling with exponential
sensitivity using an in-plane electric field. Switching the spin coupling is
essential for quantum computation using electronic spins as qubits.


title: Theory of anomalous magnon softening in ferromagnetic manganites

abstract:
  In metallic manganites with low Curie temperatures, a peculiar softening of
the magnon spectrum close to the magnetic zone boundary has experimentally been
observed. Here we present a theory of the renormalization of the magnetic
excitation spectrum in colossal magnetoresistance compounds. The theory is
based on the modulation of magnetic exchange bonds by the orbital degree of
freedom of double-degenerate e_g electrons. The model considered is an
orbitally degenerate double-exchange system coupled to Jahn-Teller active
phonons which we treat in the limit of strong onsite repulsions. Charge and
coupled orbital-lattice fluctuations are identified as the main origin of the
unusual softening of the magnetic spectrum.


title: Numerical Study of the Pairing Correlation of the t-J Type Models

abstract:
  We reported that the pair-pair correlation function of the two-dimensional
t-J model does not have long-range d-wave superconducting correlations in the
interesting parameter range of $J/t \leq 0.5$. The power-Lanczos method is used
under the assumption of monotonic behavior. This assumption has been well
checked in the two-dimensional t-J and attractive Hubbard model. Here we
re-examine this criterion of monotonic behavior of the pairing correlation
function for the one-dimensional and two-leg t-J ladder where other accurate
numerical results are available. The method seems to be working well.


title:
Study of the connection between hysteresis and thermal relaxation in
  magnetic materials

abstract:
  The connection between hysteresis and thermal relaxation in magnetic
materials is studied from both the experimental and the theoretical viewpoint.
Hysteresis and viscosity effects are measured in Finemet-type nanocrystalline
materials above the Curie temperature of the amorphous phase, where the system
consists of ferromagnetic nanograins imbedded in a paramagnetic matrix. The
hysteresis loop dependence on field rate, the magnetization time decay at
different constant fields and the magnetization curve shape after field
reversal are all consistent with a single value of the fluctuation field Hf =
8A/m (at 430 C). In addition, it is shown that all data collapse onto a single
curve M(Hath), when magnetization is plotted as a function of a properly
defined field Hath, dependent on time and field rate. Experimental data are
interpreted by assuming that the system consists of an assembly of elementary
bistable units, distributed in energy levels and energy barriers. The
approximations under which one predicts data collapse onto a single curve
M(Hath) are discussed.


title: Phase diagram of a 1 dimensional spin-orbital model

abstract:
  We study a 1 dimensional spin-orbital model using both analytical and
numerical methods. Renormalization group calculations are performed in the
vicinity of a special integrable point in the phase diagram with SU(4)
symmetry. These indicate the existence of a gapless phase in an extended region
of the phase diagram, missed in previous studies. This phase is SU(4) invariant
at low energies apart from the presence of different velocities for spin and
orbital degrees of freedom. The phase transition into a gapped dimerized phase
is in a generalized Kosterlitz-Thouless universality class. The phase diagram
of this model is sketched using the density matrix renormalization group
technique.


title: Transitions between different superconducting states in mesoscopic disks

abstract:
  Using a linear analysis, we study the stability of giant-vortex states in
very thin disks. The vortex expulsion and penetration fields are obtained for
finite thickness disks from a numerical solution of the non-linear
Ginzburg-Landau (GL) equations. Using an extension of the London approximation,
in which the phase distribution of the order parameter is prescribed and the
superconducting electron density is found numerically, we consider the free
energy behavior for transitions between different superconducting states.


title:
Computer simulations of the mechanism of thickness selection in polymer
  crystals

abstract:
  In this paper I describe the computer simulations that I have performed to
critically examine the Lauritzen-Hoffman and the Sadler-Gilmer theories of
polymer crystallization. In particular, I have computed the free energy profile
for nucleation of a new crystalline layer on the growth face to compare with
that assumed by the Lauritzen-Hoffman theory, I have analysed the mechanism of
thickness selection in a multi-pathway model in which some of the constraints
in the Lauritzen-Hoffman theory are relaxed, and I have re-examined the model
used by Sadler-Gilmer. These investigations have lead to a mechanism of
thickness selection of lamellar polymer crystals that differs from the two
theories that I set out to examine.


title:
Metal-insulator transition of isotopically enriched
  neutron-transmutation-doped ^{70}Ge:Ga in magnetic fields

abstract:
  We have investigated the temperature dependence of the electrical
conductivity sigma(N,B,T) of nominally uncompensated,
neutron-transmutation-doped ^{70}Ge:Ga samples in magnetic fields up to B=8 T
at low temperatures (T=0.05-0.5 K). In our earlier studies at B=0, the critical
exponent mu=0.5 defined by sigma(N,0,0) \propto (N-N_c)^{mu} has been
determined for the same series of ^{70}Ge:Ga samples with the doping
concentration N ranging from 1.861 \times 10^{17} cm^{-3} to 2.434 \times
10^{17} cm^{-3}. In magnetic fields, the motion of carriers loses time-reversal
symmetry, the universality class may change and with it the value of mu. In
this work, we show that magnetic fields indeed affect the value of mu (mu
changes from 0.5 at B=0 to 1.1 at B \geq 4 T). The same exponent mu'=1.1 is
also found in the magnetic-field-induced MIT for three different ^{70}Ge:Ga
samples, i.e., sigma(N,B,0) \propto [B_c(N)-B]^{mu'} where B_c(N) is the
concentration-dependent critical magnetic induction. We show that sigma(N,B,0)
obeys a simple scaling rule on the (N,B) plane. Based on this finding, we
derive from a simple mathematical argument that mu=mu' as has been observed in
our experiment.


title: Scaling Behavior of Cyclical Surface Growth

abstract:
  The scaling behavior of cyclical surface growth (e.g. deposition/desorption),
with the number of cycles n, is investigated. The roughness of surfaces grown
by two linear primary processes follows a scaling behavior with asymptotic
exponents inherited from the dominant process while the effective amplitudes
are determined by both. Relevant non-linear effects in the primary processes
may remain so or be rendered irrelevant. Numerical simulations for several
pairs of generic primary processes confirm these conclusions. Experimental
results for the surface roughness during cyclical electrodeposition/dissolution
of silver show a power-law dependence on n, consistent with the scaling
description.


title:
Crossover to Fermi-liquid behavior for weakly-coupled Luttinger liquids
  in the anisotropic large-dimension limit

abstract:
  We study the problem of the crossover from one- to higher-dimensional metals
by considering an array of Luttinger liquids (one-dimensional chains) coupled
by a weak interchain hopping {\tp.} We evaluate the exact asymptotic low-energy
behavior of the self-energy in the anisotropic infinite-dimension limit. This
limit extends the dinamical mean field concept to the case of a chain embedded
in a self-consistent medium. The system flows to a Fermi-liquid fixed point for
energies below the dimensional crossover temperature, and the anomalous
exponent $\al$ renormalizes to zero, in the case of equal spin and charge
velocities. In particular, the single-particle spectral function shows sharp
quasiparticle peaks with nonvanishing weight along the whole Fermi surface, in
contrast to the lowest-order result. Our result is obtained by carring out a
resummation of all diagrams of the expansion in \tp contributing to the
anisotropic $D\to\infty$ limit. This is done by solving, in an almost
completely analytic way, an asymptotically exact recursive equation for the
renormalized vertices, within a skeleton expansion. Our outcome shows that
perturbation expansions in \tp restricted to lowest orders are unreliable below
the crossover temperature. The extension to finite dimensions is discussed.
This work extends our recent Letter [Phys. Rev. Lett. {\bf 83}, 128 (1999)],
and includes all mathematical details.


title:
In-plane fluxon in layered superconductors with arbitrary number of
  layers

abstract:
  I derive an approximate analytic solution for the in-plane vortex (fluxon) in
layered superconductors and stacked Josephson junctions (SJJ's) with arbitrary
number of layers. The validity of the solution is verified by numerical
simulation. It is shown that in SJJ's with large number of thin layers,
phase/current and magnetic field of the fluxon are decoupled from each other.
The variation of phase/current is confined within the Josephson penetration
depth, $\lambda_J$, along the layers, while magnetic field decays at the
effective London penetration depth, $\lambda_c \gg \lambda_J$. For comparison
with real high-$T_c$ superconducting samples, large scale numerical simulations
with up to 600 SJJ's and with in-plane length up to 4000 $\lambda_J$%, are
presented. It is shown, that the most striking feature of the fluxon is a
Josephson core, manifesting itself as a sharp peak in magnetic induction at the
fluxon center.


title:
Unexpected Behavior of the Local Compressibility Near the B=0
  Metal-Insulator Transition

abstract:
  We have measured the local electronic compressibility of a two-dimensional
hole gas as it crosses the B=0 Metal-Insulator Transition. In the metallic
phase, the compressibility follows the mean-field Hartree-Fock (HF) theory and
is found to be spatially homogeneous. In the insulating phase it deviates by
more than an order of magnitude from the HF predictions and is spatially
inhomogeneous. The crossover density between the two types of behavior, agrees
quantitatively with the transport critical density, suggesting that the system
undergoes a thermodynamic change at the transition.


title:
Butterfly hysteresis loop and dissipative spin reversal in the S=1/2,
  V15 molecular complex

abstract:
  Time resolved magnetization measurements have been performed on a spin 1/2
molecular complex, so called V$_{15}$. Despite the absence of a barrier,
magnetic hysteresis is observed over a timescale of several seconds. A detailed
analysis in terms of a dissipative two level model is given, in which
fluctuations and splittings are of same energy. Spin-phonon coupling leads to
long relaxation times and to a particular "butterfly" hysteresis loop.


title: Magnetic remanence of Josephson junction arrays

abstract:
  In this work we study the magnetic remanence exhibited by Josephson junction
arrays in response to an excitation with an AC magnetic field. The effect,
predicted by numerical simulations to occur in a range of temperatures, is
clearly seen in our tridimensional disordered arrays. We also discuss the
influence of the critical current distribution on the temperature interval
within which the array develops a magnetic remanence. This effect can be used
to determine the critical current distribution of an array.


title: Selforganized 3-band structure of the doped fermionic Ising spin glass

abstract:
  The fermionic Ising spin glass is analyzed for arbitrary filling and for all
temperatures. A selforganized 3-band structure of the model is obtained in the
magnetically ordered phase. Deviation from half filling generates a central
nonmagnetic band, which becomes sharply separated at T=0 by (pseudo)gaps from
upper and lower magnetic bands. Replica symmetry breaking effects are derived
for several observables and correlations. They determine the shape of the
3-band DoS, and, for given chemical potential, influence the fermion filling
strongly in the low temperature regime.


title:
Observation of the Josephson plasma mode for a superfluid $^3$He weak
  link

abstract:
  The quantum phase difference across a superfluid Josephson-like weak link may
exhibit periodic motion describable by a nonlinear equation identical to that
of a rigid pendulum. We have directly observed this mode and find that the
phase oscillation frequency decreases toward zero as the phase amplitude
increases toward $\pi$. We also find that the low amplitude frequency is a
direct measure of the weak link's critical current in quantitative agreement
with theory.


title: Orthogonality Catastrophe Revisited

abstract:
  We prove a simple theorem on the overlap of the wavefunctions of a manybody
system with and without a single impurity and show how, and under which
conditions, this leads to the ``Orthogonality Catastrophe'' (OC) described by
Anderson. A compact new derivation of the known result for the OC exponent in a
free electron gas, $\alpha=2 \sum_l(2l+1) (\delta_l(\epsilon_f)/\pi)^2 $
%$\alpha = 2\sum_l [2l+1] (\delta_l(\epsilon_f)/\pi)^2$, is given. We also
introduce a simple way of calculating core level photoemission (XPS) lineshapes
from the finite size scaling of groundstate energies, reproducing and extending
results previously derived using boundary conformal field theory. Different
conditions under which the OC fails are discussed, and simple physical
predictions for XPS lineshapes made in each case.


title: Deconstruction of the Trap Model for the New Conducting State in 2D

abstract:
  A key prediction of the trap model for the new conducting state in 2D is that
the resistivity turns upwards below some characteristic temperature, $T_{\rm
min}$. Altshuler, Maslov, and Pudalov have argued that the reason why no upturn
has been observed for the low density conducting samples is that the
temperature was not low enough in the experiments. We show here that $T_{\rm
min}$ within the Altshuler, Maslov, and Pudalov trap model actually increases
with decreasing density, contrary to their claim. Consequently, the trap model
is not consistent with the experimental trends.


title:
Anisotropic effect of field on the orthorhombic-to-tetragonal transition
  in the striped cuprate (La,Nd)_{2-x}Sr_xCuO_4

abstract:
  The Nd-doped cuprate La_{2-y-x}Nd_ySr_xCuO_4 displays a first-order phase
transition at T_d (= 74 K for x=0.10, y = 0.60) to a low-temperature tetragonal
(LTT) phase. A magnetic field H applied || the a-axis leads to an increase in
T_d, whereas T_d is decreased when H || c. These effects show that magnetic
ordering involving both Nd and Cu spins plays a key role in driving the LTO-LTT
transition. Related anisotropic effects are observed in the uniform
susceptibility and the in-plane magnetoresistance.


title:
On the universality of distribution of ranked cluster masses at critical
  percolation

abstract:
  The distribution of masses of clusters smaller than the infinite cluster is
evaluated at the percolation threshold. The clusters are ranked according to
their masses and the distribution $P(M/L^D,r)$ of the scaled masses M for any
rank r shows a universal behaviour for different lattice sizes L (D is the
fractal dimension). For different ranks however, there is a universal
distribution function only in the large rank limit, i.e., $P(M/L^D,r)r^{-y\zeta
} \sim g(Mr^y/L^D)$ (y and $\zeta$ are defined in the text), where the
universal scaling function g is found to be Gaussian in nature.


title:
Injection statistics simulator for dynamic analysis of noise in
  mesoscopic devices

abstract:
  We present a model for electron injection from thermal reservoirs which is
applied to particle simulations of one-dimensional mesoscopic conductors. The
statistics of injected carriers is correctly described from nondegenerate to
completely degenerate conditions. The model is validated by comparing Monte
Carlo simulations with existing analytical results for the case of ballistic
conductors. An excellent agreement is found for average and noise
characteristics, in particular, the fundamental unities of electrical and
thermal conductances are exactly reproduced.


title:
One and two dimensional tunnel junction arrays in weak Coulomb blockade
  regime-absolute accuracy in thermometry

abstract:
  We have investigated one and two dimensional (1D and 2D) arrays of tunnel
junctions in partial Coulomb blockade regime. The absolute accuracy of the
Coulomb blockade thermometer is influenced by the external impedance of the
array, which is not the same in the different topologies of 1D and 2D arrays.
We demonstrate, both by experiment and by theoretical calculations in simple
geometries, that the 1D structures are better in this respect. Yet in both 1D
and 2D, the influence of the environment can be made arbitrarily small by
making the array sufficiently large.


title:
Interface dynamics in Hele-Shaw flows with centrifugal forces.
  Preventing cusp singularities with rotation

abstract:
  A class of exact solutions of Hele-Shaw flows without surface tension in a
rotating cell is reported. We show that the interplay between injection and
rotation modifies drastically the scenario of formation of finite-time cusp
singularities. For a subclass of solutions, we show that, for any given initial
condition, there exists a critical rotation rate above which cusp formation is
prevented. We also find an exact sufficient condition to avoid cusps
simultaneously for all initial conditions. This condition admits a simple
interpretation related to the linear stability problem.

categories: cond-mat chao-dyn nlin.CD nlin.PS patt-sol


title: Glass Transition in a 2D Lattice Model

abstract:
  The dynamics of compaction of hard cross-shaped pentamers on the 2D square
lattice is investigated. The addition of new particles is controlled by
diffusive relaxation. It is shown that the filling process terminates at a
glassy phase with a limiting coverage density \rho_{rcp}=0.171626(3), lower
than the density of closest packing \rho_{cp}=0.2, and the long time filling
rate vanishes like (\rho_{rcp}-\rho(t))^2. For the entire density regime the
particles form an amorphous phase, devoid of any crystalline order. Therefore,
the model supports a stable random packing state, as opposed to the hard disks
system. Our results may be relevant to recent experiments studying the
clustering of proteins on bilayer lipid membranes.


title: Helium mixtures in nanotube bundles

abstract:
  An analogue to Raoult's law is determined for the case of a 3He-4He mixture
adsorbed in the interstitial channels of a bundle of carbon nanotubes. Unlike
the case of He mixtures in other environments, the ratio of the partial
pressures of the coexisting vapor is found to be a simple function of the ratio
of concentrations within the nanotube bundle.


title:
Commensurability oscillations due to pinned and drifting orbits in a
  two-dimensional lateral surface superlattice

abstract:
  We have simulated conduction in a two-dimensional electron gas subject to a
weak two-dimensional periodic potential, $V_x \cos(2\pi x/a) + V_y \cos(2\pi
y/a)$. The usual commensurability oscillations in $\rho_{xx}(B)$ are seen with
$V_x$ alone. An increase of $V_y$ suppresses these oscillations, rather than
introducing the additional oscillations in $\rho_{yy}(B)$ expected from
previous perturbation theories. We show that this behavior arises from drift of
the guiding center of cyclotron motion along contours of an effective
potential. Periodic modulation in the magnetic field can be treated in the same
way.


title:
Carrier induced ferromagnetic interactions in p-doped Zn(1-x)MnxTe
  epilayers

abstract:
  p-type doping of molecular-beam-epitaxy grown layers of the diluted magnetic
semiconductor Zn(1-x)MnxTe is achieved by using an active nitrogen cell. The
strong interaction between the localized Mn spins and the holes deeply modifies
the transport properties (metal-insulator transition, spin-dependent Hall
effect). In spite of the weak localization of the carriers at low temperature,
the holes clearly induce a ferromagnetic interaction between the localized
spins, which is discussed as a function of Mn content and hole concentration.


title: Conductivity tensor of striped quantum Hall phases

abstract:
  We study the transport properties of pinned striped quantum Hall phases. We
show that under quite general assumptions, the macroscopic conductivity tensor
satisfies a semicircle law. In particular, this result is valid for both
smectic and nematic stripe phases, independent of the presence of topological
defects such as dislocations and grain boundaries. As a special case, our
results explain the experimental validity of a product rule for the dissipative
part of the resistivity tensor, which was previously derived by MacDonald and
Fisher for a perfect stripe structure.


title:
Electronic States in Half-Filled Correlated System with Alternating
  Potential

abstract:
  The effect of an alternating potential on a one-dimensional half-filled
Hubbard model with repulsive interaction has been examined by applying the
renormalization group method to the bosonized Hamiltonian. The electronic
state, which is determined by the competition between alternating potential and
umklapp scattering, is calculated where the relevance and the irrelevance of
the alternating potential leads to the band insulator and the Mott insulator
respectively. The excitation gaps for charge and spin fluctuations are
calculated for both states.


title:
Apertureless Near-Field Second Harmonic Microscopy with Bare Tapered
  Optical Fiber Tips

abstract:
  We present a near-field optical technique for second harmonic imaging using
bare tapered optical fiber tip illuminated with femtosecond laser pulses.
Enhancement of electric field at the tip of the fiber results in enhanced
second harmonic generation (SHG) from the sample region near the tip. This SH
emission is collected by the same tapered fiber. Spatial distribution of SHG
from thin ferroelectric Pb(Zr_xTi_1-x)O_3 films and model metal-ferroelectric
devices has been studied. Spatial resolution on the order of 80 nm has been
achieved. This is the first time diffraction limit is surpassed in SH
microscopic measurements. Electric field induced changes in SHG from individual
grains and/or domains of ferroelectric thin films have been observed.


title:
Random Walks in Logarithmic and Power-Law Potentials, Nonuniversal
  Persistence, and Vortex Dynamics in the Two-Dimensional XY Model

abstract:
  The Langevin equation for a particle (`random walker') moving in
d-dimensional space under an attractive central force, and driven by a Gaussian
white noise, is considered for the case of a power-law force, F(r) = -
Ar^{-sigma}. The `persistence probability', P_0(t), that the particle has not
visited the origin up to time t, is calculated. For sigma > 1, the force is
asymptotically irrelevant (with respect to the noise), and the asymptotics of
P_0(t) are those of a free random walker. For sigma < 1, the noise is
(dangerously) irrelevant and the asymptotics of P_0(t) can be extracted from a
weak noise limit within a path-integral formalism. For the case sigma=1,
corresponding to a logarithmic potential, the noise is exactly marginal. In
this case, P_0(t) decays as a power-law, P_0(t) \sim t^{-theta}, with an
exponent theta that depends continuously on the ratio of the strength of the
potential to the strength of the noise. This case, with d=2, is relevant to the
annihilation dynamics of a vortex-antivortex pair in the two-dimensional XY
model. Although the noise is multiplicative in the latter case, the relevant
Langevin equation can be transformed to the standard form discussed in the
first part of the paper. The mean annihilation time for a pair initially
separated by r is given by t(r) \sim r^2 ln(r/a) where a is a microscopic
cut-off (the vortex core size). Implications for the nonequilibrium critical
dynamics of the system are discussed and compared to numerical simulation
results.


title:
Invaded cluster algorithm for critical properties of periodic and
  aperiodic planar Ising models

abstract:
  We demonstrate that the invaded cluster algorithm, recently introduced by
Machta et al, is a fast and reliable tool for determining the critical
temperature and the magnetic critical exponent of periodic and aperiodic
ferromagnetic Ising models in two dimensions. The algorithm is shown to
reproduce the known values of the critical temperature on various periodic and
quasiperiodic graphs with an accuracy of more than three significant digits. On
two quasiperiodic graphs which were not investigated in this respect before,
the twelvefold symmetric square-triangle tiling and the tenfold symmetric
T\"ubingen triangle tiling, we determine the critical temperature. Furthermore,
a generalization of the algorithm to non-identical coupling strengths is
presented and applied to a class of Ising models on the Labyrinth tiling. For
generic cases in which the heuristic Harris-Luck criterion predicts deviations
from the Onsager universality class, we find a magnetic critical exponent
different from the Onsager value. But also notable exceptions to the criterion
are found which consist not only of the exactly solvable cases, in agreement
with a recent exact result, but also of the self-dual ones and maybe more.


title: Schottky barriers in carbon nanotube heterojunctions

abstract:
  We investigate electronic properties of heterojunctions between metallic and
semiconducting single-wall carbon nanotubes. Ineffective screening of the long
range Coulomb interaction in one-dimensional nanotube systems drastically
modifies the charge transfer phenomena compared to conventional semiconductor
heterostructures. The length of depletion region varies over a wide range (from
the nanotube radius to the nanotube length) sensitively depending on the doping
strength. The Schottky barrier gives rise to the asymmetry of the I-V
characteristics of heterojunctions, in agreement with recent experimental
results by Yao {\it et al.} and Fuhrer {\it et al.} Dynamic charge build-up
near the junction results in a step-like growth of the current at reverse bias.


title: Scaling for vibrational modes of fractals tethered at the boundaries

abstract:
  Using a recently introduced mapping between a scalar elastic network tethered
at its boundaries and a diffusion problem with permanent traps, we study
various vibrational properties of progressively tethered disordered fractals.
Different scaling forms are proposed for different types of boundary tethering
and numerical investigation of the scaling is performed by the approximate
diagonalization of the corresponding large, sparse transition probability
matrices. Rather different localization behaviors are found for the leading
modes depending on the types of tethering.


title:
On the Thomas-Fermi approximation in the bulk of trapped Bose-Einstein
  condensed gases

abstract:
  Corrections to the Thomas-Fermi-type solution of the Gross-Pitaevskii
equation are inevitable in order to get correctly the frequencies of the low
lying modes out of the Bogolyubov equations. These corrections are important in
the bulk, too, thus the failure of the Thomas-Fermi approximation is not
confined to the surface. We discuss this effect quantitatively and consider
similar phenomena of spin fluctuations in Bose-Einstein condensed gases in an
optical trap.


title:
Exact expression for the diffusion propagator in a family of
  time-dependent anharmonic potentials

abstract:
  We have obtained the exact expression of the diffusion propagator in the
time-dependent anharmonic potential $V(x,t)={1/2}a(t)x^2+b\ln x$. The
underlying Euclidean metric of the problem allows us to obtain analytical
solutions for a whole family of the elastic parameter a(t), exploiting the
relation between the path integral representation of the short time propagator
and the modified Bessel functions. We have also analyzed the conditions for the
appearance of a non-zero flow of particles through the infinite barrier located
at the origin (b<0).


title: On Rational Bubbles and Fat Tails

abstract:
  This paper addresses the statistical properties of time series driven by
rational bubbles a la Blanchard and Watson (1982), corresponding to
multiplicative maps, whose study has recently be revived recently in physics as
a mechanism of intermittent dynamics generating power law distributions. Using
insights on the behavior of multiplicative stochastic processes, we demonstrate
that the tails of the unconditional distribution emerging from such bubble
processes follow power-laws (exhibit hyperbolic decline). More precisely, we
find that rational bubbles predict a 'fat' power tail for both the bubble
component and price differences with an exponent smaller than 1, implying
absence of convergence of the mean. The distribution of returns is dominated by
the same power-law over an extended range of large returns. Although power-law
tails are a pervasive feature of empirical data, these numerical predictions
are in disagreement with the usual empirical estimates of an exponent between 2
and 4. It, therefore, appears that exogenous rational bubbles are hardly
reconcilable with some of the stylized facts of financial data at a very
elementary level.


title: Reply to Comment on "Stripes and the t-J Model"

abstract:
  This is a Reply to the Comment of S.R. White and D.J. Scalapino
[cond-mat/9907243] on our recent paper ``Stripes and the t-J Model'' [Physical
Review Letters 83, 132 (1999) and cond-mat/9812022].


title: First principles study of Li intercalated carbon nanotube ropes

abstract:
  We studied Li intercalated carbon nanotube ropes by using first principles
methods. Our results show charge transfer between Li and C and small structural
deformation of nanotube due to intercalation. Both inside of nanotube and the
interstitial space are susceptible for intercalation. The Li intercalation
potential of SWNT rope is comparable to that of graphite and almost independent
of Li density up to around LiC$_2$, as observed in recent experiments. This
density is significantly higher than that of Li intercalated graphite, making
nanorope to be a promising candidate for anode material in battery
applications.


title:
On the relationship between the noise-induced persistent current and
  dephasing rate

abstract:
  AC noise in disordered conductors causes both dephasing of the electron wave
functions and a DC current around a mesoscopic ring. We demonstrate that the
dephasing rate tau_{\phi}^{-1} in long wires and the DC current, induced by the
same noise and averaged over an ensemble of small rings are connected. The
ensemble-averaged h/2e flux harmonic <I> of the current and the dephasing rate
caused by the same uniform in space high frequency AC field are related in a
remarkably simple way: <I> tau_{\phi}=C e. Here e is an electron charge, and
the constant C depends on the Dyson symmetry class. For a pure potential
disorder the current <I> is diamagnetic C = -(4/\pi) and in the presence of
strong spin-orbit scattering it is paramagnetic C =(2/\pi). The relationship
seems to agree reasonably with experiments. This suggests that the two puzzles:
anomalously large persistent current [L.P.Levy et al., Phys.Rev.Lett., v.64,
2074 (1990)] and the low-temperature saturation of the dephasing [P.Mohanty et
al., Phys.Rev.Lett., v.78, 3366 (1997)] may have a common solution.


title:
Three-dimensional Ising model in the fixed-magnetization ensemble: a
  Monte Carlo study

abstract:
  We study the three-dimensional Ising model at the critical point in the
fixed-magnetization ensemble, by means of the recently developed geometric
cluster Monte Carlo algorithm. We define a magnetic-field-like quantity in
terms of microscopic spin-up and spin-down probabilities in a given
configuration of neighbors. In the thermodynamic limit, the relation between
this field and the magnetization reduces to the canonical relation M(h).
However, for finite systems, the relation is different. We establish a close
connection between this relation and the probability distribution of the
magnetization of a finite-size system in the canonical ensemble.


title:
Thin superconducting disk with field-dependent critical current:
  Magnetization and ac susceptibilities

abstract:
  Magnetization hysteresis loops and the ac susceptibility of a superconducting
thin disk are calculated in the critical-state model assuming a field-dependent
critical current density, Jc(B). The results are obtained by solving
numerically the set of coupled integral equations for the flux and current
distributions [PRB 60(17) p.13112 (1999)] for a disk placed in a perpendicular
applied field Ba. From the magnetization curves the range of fields where the
vertical width of the loop relates directly to Jc(Ba) is determined. Comparing
our results on susceptibility with experimental data for parametric plot
Im(chi) vs. Re(chi) shows that by taking the B-dependence of Jc into account
the agreement improves dramatically. We show that the asymptotic behavior for
large ac-field amplitudes Bam changes from Re(chi) ~ 1/Bam^{3/2} and Im(chi) ~
1/Bam for the Bean model, to Re(chi) ~ 1/Bam^3 and Im(chi) ~ 1/Bam^2 for Jc
decreasing with |B| as 1/|B| or faster. For small Bam the behavior can always
be described by an effective Bean model with a renormalized Jc. We also find
that in the Im(chi) vs. Re(chi) plot the peak of Im(chi) increases in magnitude
and shifts to lower |Re(chi)|. This allows an easy experimental discrimination
between a Bean model behavior, one with Jc(B), and one where flux creep is an
ingredient.


title: Shocks in non-loaded bead chains with impurities

abstract:
  We numerically investigate the problem of the propagation of a shock in an
horizontal non-loaded granular chain with a bead interaction force exponent
varying from unity to large values. When $\alpha$ is close to unity we observed
a cross-over between a nonlinearity-dominated regime and a solitonic one, the
latest being the final steady state of the propagating wave. In the case of
large values of $\alpha$ the deformation field given by the numerical
simulations is completely different from the one obtained by analytical
calculation. In the following we studied the interaction of these shock waves
with a mass impurity placed in the bead chain. Two different physical pictures
emerge whether we consider a light or a heavy impurity mass. The scatter of the
shock wave with a light impurity yields damped oscillations of the impurity
which then behave as a solitary wave source. Differently an heavy impurity is
just shifted by the shock and the transmitted wave loses its solitonic
character being fragmented into waves of decreasing amplitudes.


title:
Local-field study of phase conjugation in metallic quantum wells with
  probe fields of both propagating and evanescent character

abstract:
  The phase conjugated response from nonmagnetic multi-level metallic quantum
wells is analyzed and an essentially complete analytical solution is presented
and discussed. The description is based on a semi-classical local-field theory
for degenerate four-wave mixing in mesoscopic interaction volumes of condensed
media developed by the present authors [T. Andersen and O. Keller, Phys.
Scripta 58, 132 (1998)]. The analytical solution is supplemented by a numerical
analysis of the phase conjugated response from a two-level quantum well in the
case where one level is below the Fermi level and the other level is above.
This is the simplest configuration of a quantum well phase conjugator in which
the light-matter interaction can be tuned to resonance. The phase conjugated
response is examined in the case where all the scattering takes place in one
plane, and linearly polarized light is used in the mixing. In the numerical
work we study a two-monolayer thick copper quantum well using the infinite
barrier model potential. Our results show that the phase conjugated response
from such a quantum-well system is highly dependent on the spatial dispersion
of the matter response. The resonances showing up in the numerical results are
analytically identified from the expressions for the linear and nonlinear
response tensors. In addition to the general discussion of the phase conjugated
response with varying frequency and parallel component of the wavevector, we
present the phase conjugated response in the special case where the light is in
resonance with the interband transition.


title:
'Theory for the enhanced induced magnetization in coupled magnetic
  trilayers in the presence of spin fluctuations'

abstract:
  Motivated by recent experiments, the effect of the interlayer exchange
interaction $J_{inter}$ on the magnetic properties of coupled Co/Cu/Ni
trilayers is studied theoretically. Here the Ni film has a lower Curie
temperature $T_{C,\rm Ni}$ than the Co film in case of decoupled layers. We
show that by taking into account magnetic fluctuations the interlayer coupling
induces a strong magnetization for $T\gtsim T_{C,\rm Ni}$ in the Ni film. For
an increasing $J_{inter}$ the resonance-like peak of the longitudinal Ni
susceptibility is shifted to larger temperatures, whereas its maximum value
decreases strongly. A decreasing Ni film thickness enhances the induced Ni
magnetization for $T\gtsim T_{C,\rm Ni}$. The measurements cannot be explained
properly by a mean field estimate, which yields a ten times smaller effect.
Thus, the observed magnetic properties indicate the strong effect of 2D
magnetic fluctuations in these layered magnetic systems. The calculations are
performed with the help of a Heisenberg Hamiltonian and a Green's function
approach.


title:
Stochastic resonance in bistable systems: The effect of simultaneous
  additive and multiplicative correlated noises

abstract:
  We analyze the effect of the simultaneous presence of correlated additive and
multiplicative noises on the stochastic resonance response of a modulated
bistable system. We find that when the correlation parameter is also modulated,
the system's response, measured through the output signal-to-noise ratio,
becomes largely independent of the additive noise intensity.


title:
Non-Invariant Ground States, Thermal Average, and generalized Fermionic
  Statistics

abstract:
  We present an approach to generalised fermionic statistics which relates the
existence of a generalised statistical behaviour to non-invariant ground
states. Considering the thermal average of an operatorial generalization of the
Heisenberg algebra, we get an occupation number which depends on the degree of
mixing between symmetric and antisymmetric sectors of the ground state. A
natural prescription is given for the construction of a supersymmetric
statistics. We also show that the structure of the vacuum, and therefore the
statistical behaviour of the system, can be accounted for in terms of a second
order phase transition.


title:
Suppression of ferromagnetic ordering in doped manganites: Effects of
  the superexchange interaction

abstract:
  From a Monte Carlo study of the ferromagnetic Kondo lattice model for doped
manganites, including the antiferromagnetic superexchange interaction
($J_{AF}$), we found that the ferromagnetic ordering was suppressed as $J_{AF}$
increased. The ferromagnetic transition temperature $T_c$, as obtained from a
mean field fit to the calculated susceptibilities, was found to decrease
monotonically with increasing $J_{AF}$. Further, the suppression in $T_c$
scales with the bandwidth narrowing induced by the antiferromagnetic
frustration originating from $J_{AF}$. From these results, we propose that the
change in the superexchange interaction strength between the $t_{2g}$ electrons
of the Mn ions is one of the mechanisms responsible for the suppression in
$T_c$ observed in manganites of the type
(La$_{0.7-y}$Pr$_{y}$)Ca$_{0.3}$MnO$_3$.


title:
Anomalous spin susceptibility and magnetic polaron formation in the
  double exchange systems

abstract:
  The magnetic susceptibility and spin-spin correlation of the double-exchange
model for doped manganites are investigated through the Monte Carlo
calculations on the three-dimensional lattice model. Deviations of the
susceptibility from the Curie-Weiss behavior above the ferromagnetic ordering
temperature $T_c$ seem to indicate a formation of local ferromagnetic clusters
in the vicinity of $T_c$, which is consistent with recent electron paramagnetic
resonance experiments for La$_{2/3}$Ca$_{1/3}$MnO$_3$. A further analysis of
the spin-spin correlations show the ferromagnetic cluster size to be
three-to-four lattice spacings, suggesting that the charge carriers may form
magnetic polarons.


title: Vertex Corrections in the Spin-fluctuation-induced Superconductivity

abstract:
  We evaluate vertex corrections to $T_c$ on the basis of the antiferromagnetic
spin-fluctuation model of the high-$T_c$ superconductivity. It is found that
the corrections are attractive in the $d_{x^2-y^2}$ channel, and they become
appreciable as we go through an intermediate-coupling regime of $T_c\simeq
100$K, the maximum $T_c$ attainable in the one-loop \'Eliashberg calculation.


title:
Fluid-fluid transitions of hard spheres with a very short-range
  attraction

abstract:
  Hard spheres with an attraction of range a tenth to a hundredth of the sphere
diameter are constrained to remain fluid even at densities when monodisperse
particles at equilibrium would have crystallised, in order to compare with
experimental systems which remain fluid. They are found to have a fluid-fluid
transition at high density. As the range of the attraction tends to zero, the
density at the critical point tends towards the random-close-packing density of
hard spheres.


title: Order Parameter as an Additional State Variable of Unstable Traffic Flow

abstract:
  We discuss a phenomenological approach to the description of unstable vehicle
motion on multilane highways that could explain in a simple way such observed
self-organizing phenomena as the sequence of the phase transitions "free flow
-> synchronized motion -> jam" and the hysteresis in them.
  We introduce a new variable called order parameter that accounts for possible
correlations in the vehicle motion at different lanes. So, it is principally
due to "many-body" effects in the car interaction in contrast to such variables
as the mean car density and velocity being actually the zeroth and first
moments of the "one-particle" distribution function. Therefore, we regard the
order parameter as an additional independent state variable of traffic flow and
formulate the corresponding evolution equation governing the lane changing
rate.
  In this context we analyze the instability of homogeneous traffic flow
manifesting itself in both of the mentioned above phase transitions where and
endowing them with the hysteresis. Besides, the jam state is characterized by
the vehicle flows at different lanes being independent of one another.


title:
Collective oscillations driven by correlation in the nonlinear optical
  regime

abstract:
  We present an analytical and numerical study of the coherent exciton
polarization including exciton-exciton correlation. The time evolution after
excitation with ultrashort optical pulses can be divided into a slowly varying
polarization component and novel ultrafast collective modes. The frequency and
damping of the collective modes are determined by the high-frequency properties
of the retarded two-exciton correlation function, which includes Coulomb
effects beyond the mean-field approximation. The overall time evolution depends
on the low-frequency spectral behavior. The collective mode, well separated
from the slower coherent density evolution, manifests itself in the coherent
emission of a resonantly excited excitonic system, as demonstrated numerically.


title: Shot Noise in Mesoscopic Conductors

abstract:
  Theoretical and experimental work concerned with dynamic fluctuations has
developed into a very active and fascinating subfield of mesoscopic physics. We
present a review of this development focusing on shot noise in small electric
conductors. Shot noise is a consequence of the quantization of charge. It can
be used to obtain information on a system which is not available through
conductance measurements. In particular, shot noise experiments can determine
the charge and statistics of the quasiparticles relevant for transport, and
reveal information on the potential profile and internal energy scales of
mesoscopic systems. Shot noise is generally more sensitive to the effects of
electron-electron interactions than the average conductance. We present a
discussion based on the conceptually transparent scattering approach and on the
classical Langevin and Boltzmann-Langevin methods; in addition a discussion of
results which cannot be obtained by these methods is provided. We conclude the
review by pointing out a number of unsolved problems and an outlook on the
likely future development of the field.

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


title:
Ground State Phase Diagram of Frustrated S=1 XXZ chains : Chiral Ordered
  Phases

abstract:
  The ground-state phase diagram of frustrated S=1 XXZ spin chains with the
competing nearest- and next-nearest-neighbor antiferromagnetic couplings is
studied using the infinite-system density-matrix renormalization-group method.
We find six different phases, namely, the Haldane, gapped chiral, gapless
chiral, double Haldane, N\'{e}el, and double N\'{e}el (uudd) phases. The gapped
and gapless chiral phases are characterized by the spontaneous breaking of
parity, in which the long-range order parameter is a chirality, \kappa_l =
S_l^xS_{l+1}^y-S_l^yS_{l+1}^x, whereas the spin correlation decays either
exponentially or algebraically. These chiral ordered phases appear in a broad
region in the phase diagram for \Delta < 0.95, where \Delta is an
exchange-anisotropy parameter. The critical properties of phase transitions are
also studied.


title: Stress Relaxation of Entangled Polymer Networks

abstract:
  The non-linear stress-strain relation for crosslinked polymer networks is
studied using molecular dynamics simulations. Previously we demonstrated the
importance of trapped entanglements in determining the elastic and relaxational
properties of networks. Here we present new results for the stress versus
strain for both dry and swollen networks. Models which limit the fluctuations
of the network strands like the tube model are shown to describe the stress for
both elongation and compression. For swollen networks, the total modulus is
found to decrease like (V_0/V)^{2/3} and goes to the phantom model result only
for short strand networks.


title:
Competition between Superconductivity and Charge Stripe Order in High Tc
  Cuprates

abstract:
  Members of the high Tc cuprate family (with defect free $CuO_2$ planes) are
suggested to have an `{\em Intrinsic single layer superconducting} Tc', larger
than the experimentally observed Tc, at a given hole concentration. This
difference occurs to varying degrees among the different members, due to an
anomalous response of the d-wave superconducting state to self generated charge
perturbations. In particular, any quasi elastic charge stripe order, arising
from electron-electron and electron- lattice coupling, can suppress the large
intrinsic superconducting Tc. Existing experimental results on the wide Tc
variation in the single layer cuprate families, such as the low Tc ($\approx 38
K, 33 K$) of LSCO and Bi-2201 compared to high Tc ($\approx 95 K, 98 K$) of
Tl-2201 and Hg-1201, the strain induced increase in Tc (25 to 50 K) observed in
LSCO are qualitatively explained by our mechanism. We predict that under proper
epitaxial strain the Tc of LSCO and Bi-2201 should increase from 30's all the
way upto the intrinsic Tc $\sim $ 90 K, at optimal doping.


title: Binding of molecules to DNA and other semiflexible polymers

abstract:
  A theory is presented for the binding of small molecules such as surfactants
to semiflexible polymers. The persistence length is assumed to be large
compared to the monomer size but much smaller than the total chain length. Such
polymers (e.g. DNA) represent an intermediate case between flexible polymers
and stiff, rod-like ones, whose association with small molecules was previously
studied. The chains are not flexible enough to actively participate in the
self-assembly, yet their fluctuations induce long-range attractive interactions
between bound molecules. In cases where the binding significantly affects the
local chain stiffness, those interactions lead to a very sharp, cooperative
association. This scenario is of relevance to the association of DNA with
surfactants and compact proteins such as RecA. External tension exerted on the
chain is found to significantly modify the binding by suppressing the
fluctuation-induced interaction.

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


title: Optimal Fluctuations and Tail States of non-Hermitian Operators

abstract:
  We develop a general variational approach to study the statistical properties
of the tail states of a wide class of non-Hermitian operators. The utility of
the method, which is a refinement of the instanton approach introduced by
Zittartz and Langer, is illustrated in detail by reference to the problem of a
quantum particle propagating in an imaginary scalar potential.


title:
Novel coexisting density wave ground state in strongly correlated,
  two-dimensional electronic materials

abstract:
  Two-dimensional (2D) strongly correlated electron systems underlie many of
the most important phenomena in contemporary condensed matter physics,
including the Quantum Hall Effect (QHE), ``high T_c'' superconductivity, and
possible exotic conducting states in silicon MOSFETs. We demonstrate the
existence of yet another exotic ground state in strongly correlated, 2D
electronic materials: a novel, insulating bond-order/charge density wave state
(BCDW) in the commensurate 1/4-filled band that persists for all anisotropies
within the 2D lattice, in contradiction to the non-interacting electron
prediction of the vanishing of density waves in 2D for non-1/2-filled bands.
The persistence of the BCDW in the 2D lattice is a consequence of strong
electron-electron (e-e) interaction and the resultant ``confinement,'' a
concept recently widely debated. Our results have implications for experiments
in the organic charge transfer solids (CTS), where they explain the observation
of a ``mysterious'' coexistence of density waves, clarify the optical
conductivity of the ``metallic'', and suggest an approach to the observed
organic superconductivity.


title:
Exact Demonstration of Magnetization Pleateaus and First Order
  Dimer-N\'eel Phase Transitions in a Modified Shastry-Sutherland Model for
  SrCu$_2$(BO$_3$)$_2$

abstract:
  We study a generalized Shastry-Sutherland model for the material
SrCu$_2$(BO$_3$)$_2$. Along a line in the parameter space, we can show
rigorously that the model has a first order phase transition between Dimerized
and N\'eel-ordered ground states. Furthermore, when a magnetic field is applied
in the Dimerized phase, magnetization plateaus develop at commensurate values
of the magnetization. We also discuss various aspects of the phase diagram and
properties of this model away from this exactly soluble line, which include
gap-closing continuous transitions between Dimerized and magnetically ordered
phases.


title: Linear theory of unstable growth on rough surfaces

abstract:
  Unstable homoepitaxy on rough substrates is treated within a linear continuum
theory. The time dependence of the surface width W(t) is governed by three
length scales: The characteristic scale $l_0$ of the substrate roughness, the
terrace size $l_D$ and the Ehrlich-Schwoebel length $l_{ES}$. If $l_{ES} \ll
l_D$ (weak step edge barriers) and $l_0 \ll l_m \sim l_D \sqrt{l_D/l_{ES}}$,
then W(t) displays a minimum at a coverage $\theta_{\rm min} \sim
(l_D/l_{ES})^2$, where the initial surface width is reduced by a factor
$l_0/l_m$. The r\^{o}le of deposition and diffusion noise is analyzed. The
results are applied to recent experiments on the growth of InAs buffer layers
[M.F. Gyure {\em et al.}, Phys. Rev. Lett. {\bf 81}, 4931 (1998)]. The overall
features of the observed roughness evolution are captured by the linear theory,
but the detailed time dependence shows distinct deviations which suggest a
significant influence of nonlinearities.


title:
Scaling Approach to Calculate Critical Exponents in Anomalous Surface
  Roughening

abstract:
  We study surface growth models exhibiting anomalous scaling of the local
surface fluctuations. An analytical approach to determine the local scaling
exponents of continuum growth models is proposed. The method allows to predict
when a particular growth model will have anomalous properties ($\alpha \neq
\alpha_{loc}$) and to calculate the local exponents. Several continuum growth
equations are examined as examples.


title: The Free Energy Formula

abstract:
  The Onsager formula for the free energy of the two dimensional Ising model
with periodic boundary conditions is proved.


title:
Hysteresis in One-Dimensional Anti-Ferromagnetic Random-Field Ising
  Model at Zero-Temperature

abstract:
  We analyse hysteresis in a one-dimensional anti-ferromagnetic random field
Ising model at zero-temperature. The random field is taken to have a
rectangular distribution of width $2 \Delta$ centered about the origin. A
uniform applied field is varied slowly from $-\infty$ to $+\infty$ and back.
Analytic expression for the hysteresis loop is obtained in the case $\Delta \le
|J|$, where $|J|$ is the strength of the nearest neighbor interaction.


title: Transport in a class of exactly solvable models of interacting fermions

abstract:
  We study transport in a class of exactly solvable models of interacting
fermions in one dimension. We contrast these models with models of
non-interacting fermions in an Aharanov-Bohm ring to which they are
superficially similar. We introduce magnetic and non-magnetic impurities at a
site, through either a weak $\delta$-function potential or through a weak link.
Using a renormalisation group analysis, we show that the strength of the
nonmagnetic impurity is not affected by the interaction, whereas the magnetic
impurity cuts the wire at the impurity site.


title: Quasiparticle-quasiparticle Scattering in High Tc Superconductors

abstract:
  The quasiparticle lifetime and the related transport relaxation times are the
fundamental quantities which must be known in order to obtain a description of
the transport properties of the high T_c superconductors. Studies of these
quantities have been undertaken previously for the d-wave, high T_c
superconductors for the case of temperature-independent elastic impurity
scattering. However, much less is known about the temperature-dependent
inelastic scattering. Here we give a detailed description of the
characteristics of the temperature-dependent quasiparticle-quasiparticle
scattering in d-wave superconductors, and find that this process gives a
natural explanation of the rapid variation with temperature of the electrical
transport relaxation rate.


title: Vortex Matter in Mesoscopic Superconducting Disks and Rings

abstract:
  Phase transitions between different (i.e. giant and multi-vortex)
superconducting states and between the superconducting-normal state of
mesoscopic disks and rings are studied in the presence of an external magnetic
field by solving the two non-linear Ginzburg-Landau equations
self-consistently. The flux through a circular disk with a hole in the middle
is not quantized.


title:
Vehicular Traffic: A System of Interacting Particles Driven Far From
  Equilibrium

abstract:
  In recent years statistical physicists have developed {\it discrete}
"particle-hopping" models of vehicular traffic, usually formulated in terms of
{\it cellular automata}, which are similar to the microscopic models of
interacting charged particles in the presence of an external electric field.
Concepts and techniques of non-equilibrium statistical mechanics are being used
to understand the nature of the steady states and fluctuations in these
so-called "microscopic" models. In this brief review we explain, primarily to
the nonexperts, these models and the physical implications of the results.


title:
Dynamics of Flux Creep in Underdoped Single Crystals of
  Y_1-xPr_xBa_2Cu_3O_7-d

abstract:
  Transport as well as magnetic relaxation properties of the mixed state were
studied on strongly underdoped Y_1-xPr_xBa_2Cu_3O_7-d crystals. We observed two
correlated phenomena - a coupling transition and a transition to quantum creep.
The distribution of transport current below the coupling transition is highly
nonuniform, which facilitates quantum creep. We speculate that in the mixed
state below the coupling transition, where dissipation is nonohmic, the current
distribution may be unstable with respect to self-channeling resulting in the
formation of very thin current-carrying layers.


title: Surface Instability in Windblown Sand

abstract:
  We investigate the formation of ripples on the surface of windblown sand
based on the one-dimensional model of Nishimori and Ouchi [Phys. Rev. Lett. 71,
197 (1993)], which contains the processes of saltation and grain relaxation. We
carry out a nonlinear analysis to determine the propagation speed of the
restabilized ripple patterns, and the amplitudes and phases of their first,
second, and third harmonics. The agreement between the theory and our numerical
simulations is excellent near the onset of instability. We also determine the
Eckhaus boundary, outside which the steady ripple patterns are unstable.


title: Adiabatic connection from accurate wavefunction calculations

abstract:
  An extremely easy method for accurately calculating the adiabatic connection
of density functional theory is presented, and its accuracy tested on both
Hooke's atom and the He atom. The method is easy because calculations are
needed only for different values of parameters in the external potential, which
can be achieved with almost any electronic structure code.


title: Freezing of Simple Liquid Metals

abstract:
  Freezing of simple liquid metals and the relative stabilities of competing
crystalline solids are investigated using thermodynamic perturbation theory,
the interactions between ions being modeled by effective pair potentials
derived from pseudopotential theory. The ionic free energy of the solid phase
is calculated, to first order in the perturbation potential, using classical
density-functional theory and an accurate approximation to the hard-sphere
radial distribution function. Free energy calculations for Na, Mg, and Al yield
well-defined freezing transitions and structural free energy differences for
bcc, fcc, and hcp crystals in qualitative agreement with experiment.


title: Lattice Independent Approach to Thermal Phase Mixing

abstract:
  We show how to achieve lattice-spacing independent results in numerical
simulations of finite-temperature stochastic scalar field theories. We
generalize the previous approach of hep-lat/9607026 by obtaining results which
are independent of the renormalization scale. As an application of our method,
we examine thermal phase mixing in the context of Ginzburg-Landau models with
short-range interactions. In particular, we obtain the lattice-spacing and
renormalization-scale independent critical value of the control parameter which
determines the free-energy barrier between the two low-temperature phases. We
also propose a simple procedure to extract the critical value of control
parameters for different choices of lattice spacing.


title:
Superconducting single-charge transistor in a tunable dissipative
  environment

abstract:
  We study a superconducting single-charge transistor, where the coherence of
Cooper pair tunneling is destroyed by the coupling to a tunable dissipative
environment. Sequential tunneling and cotunneling processes are analyzed to
construct the shape of the conductance peaks and their dependence on the
dissipation and temperature. Unexpected features are found due to a cross-over
between two distinct regimes, one `environment-assisted' the other
`environment-dominated'. Several of the predictions have been confirmed by
recent experiments. The model and results apply also to the dynamics of
Josephson junction quantum bits on a conducting ground plane, thus explaining
the influence of dissipation on the coherence.


title:
Superconductivity and Topological Numbers in the Hubbard Chain with
  Bond-Charge Interaction

abstract:
  We determine the quantum phase diagram of the Hubbard chain with
electron-hole symmetric correlated hopping at 1/2- and 1/4-filling using
geometric concepts and continuum limit field theory. The long distance behavior
of various correlation functions show a very rich phase diagram with several
insulating, metallic, and superconducting phases, which might be relevant to
(TMTSF)$_2$X compounds. The closing of charge and spin gaps are accurately
resolved as topological transitions (jumps in $\pi$ of Berry phases). The
metallic or insulating character of each thermodynamic phase is obtained from
the ground-state expectation value of a displacement operator in reciprocal
space.


title: Random Defect Lines in Conformal Minimal Models

abstract:
  We analyze the effect of adding quenched disorder along a defect line in the
2D conformal minimal models using replicas. The disorder is realized by a
random applied magnetic field in the Ising model, by fluctuations in the
ferromagnetic bond coupling in the Tricritical Ising model and Tricritical
Three-state Potts model (the $\phi_{12}$ operator), etc.. We find that for the
Ising model, the defect renormalizes to two decoupled half-planes without
disorder, but that for all other models, the defect renormalizes to a
disorder-dominated fixed point. Its critical properties are studied with an
expansion in $\eps \propto 1/m$ for the mth Virasoro minimal model. The decay
exponents $X_N=\frac{N}{2}(1-\frac{9(3N-4)}{4(m+1)^2}+
\mathcal{O}(\frac{3}{m+1})^3)$ of the Nth moment of the two-point function of
$\phi_{12}$ along the defect are obtained to 2-loop order, exhibiting
multifractal behavior.This leads to a typical decay exponent $X_{\rm typ}={1/2}
(1+\frac{9}{(m+1)^2}+\mathcal{O}(\frac{3}{m+1})^3)$. One-point functions are
seen to have a non-self-averaging amplitude. The boundary entropy is larger
than that of the pure system by order 1/m^3.
  As a byproduct of our calculations, we also obtain to 2-loop order the
exponent $\tilde{X}_N=N(1-\frac{2}{9\pi^2}(3N-4)(q-2)^2+\mathcal{O}(q-2)^3)$ of
the Nth moment of the energy operator in the q-state Potts model with bulk bond
disorder.


title: Effective Mass of the Four Flux Composite Fermion at $\nu = 1/4$

abstract:
  We have measured the effective mass ($m^*$) of the four flux composite
fermion at Landau level filling factor $\nu = 1/4$ ($^4$CF), using the
activation energy gaps at the fractional quantum Hall effect (FQHE) states
$\nu$ = 2/7, 3/11, and 4/15 and the temperature dependence of the Shubnikov-de
Haas (SdH) oscillations around $\nu = 1/4$. We find that the energy gaps show a
linear dependence on the effective magnetic field $B_{eff}$ ($\equiv
B-B_{\nu=1/4}$), and from this linear dependence we obtain $m^* = 1.0 m_e$ and
a disorder broadening $\Gamma \sim$ 1 K for a sample of density $n = 0.87
\times 10^{11}$ /cm$^2$. The $m^*$ deduced from the temperature dependence of
the SdH effect shows large differences for $\nu > 1/4$ and $\nu < 1/4$. For
$\nu > 1/4$, $m^* \sim 1.0 m_e$. It scales as $\sqrt{B_{\nu}}$ with the mass
derived from the data around $\nu =1/2$ and shows an increase in $m^*$ as $\nu
\to 1/4$, resembling the findings around $\nu =1/2$. For $\nu < 1/4$, $m^*$
increases rapidly with increasing $B_{eff}$ and can be described by $m^*/m_e =
-3.3 + 5.7 \times B_{eff}$. This anomalous dependence on $B_{eff}$ is
precursory to the formation of the insulating phase at still lower filling.


title: Kinetic Theory and Mesoscopic Noise

abstract:
  In the theory of noise processes for mesoscopic conductors, the relationship
between shot noise and hot-electron noise is absolutely fundamental to
understanding the underlying microscopic fluctuations. From the vantage point
of orthodox microscopics and kinetics, their relation is a long way from being
settled. Its resolution calls for the tools of many-body theory. We motivate
the many-body approach to noise, review the analysis of conductance and current
noise within linear diffusive theories, and discuss the smooth crossover
between thermal noise and shot noise. We outline a kinetic theory of
nonequilibrium fluctuations, and show how this completely orthodox approach
directly negates drift-diffusive explanations of the crossover.


title:
Structure of Flux Line Lattices with Weak Disorder at Large Length
  Scales

abstract:
  Dislocation-free decoration images containing up to 80,000 vortices have been
obtained on high quality Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ superconducting
single crystals. The observed flux line lattices are in the random manifold
regime with a roughening exponent of 0.44 for length scales up to 80-100
lattice constants. At larger length scales, the data exhibit nonequilibrium
features that persist for different cooling rates and field histories.


title: Coupled-mode theory for Bose-Einstein condensates

abstract:
  We apply the concepts of nonlinear guided-wave optics to a Bose-Einstein
condensate (BEC) trapped in an external potential. As an example, we consider a
parabolic double-well potential and derive coupled-mode equations for the
complex amplitudes of the BEC macroscopic collective modes. Our equations
describe different regimes of the condensate dynamics, including the nonlinear
Josephson effect for any separation between the wells. We demonstrate
macroscopic self-trapping for both repulsive and attractive interactions, and
confirm our results by numerical simulations.


title:
Summability of the perturbative expansion for a zero-dimensional
  disordered spin model

abstract:
  We show analytically that the perturbative expansion for the free energy of
the zero dimensional (quenched) disordered Ising model is Borel-summable in a
certain range of parameters, provided that the summation is carried out in two
steps: first, in the strength of the original coupling of the Ising model and
subsequently in the variance of the quenched disorder. This result is
illustrated by some high-precision calculations of the free energy obtained by
a straightforward numerical implementation of our sequential summation method.


title: Phases of the two-band model of spinless fermions in one dimension

abstract:
  We study the two-band model of spinless fermions in one dimension for weak
repulsive interactions. In this case, the model is equivalent to the weakly
interacting spinless two-leg ladder. We obtain analytic expressions for the
superconducting pairing correlation function and the charge density correlation
function, which show, that a finite interchain hopping t_p results in dominant
superconductivity for repulsive interactions (for vanishing t_p, we recover
previous results). We furthermore find that the transition from the
superconducting phase to the usual one-dimensional (Luttinger) metal at large
doping occurs via a mixed phase, where superconducting pairs are formed in the
bonding band only. We give the phase diagram as a function of temperature and
doping.


title: A Simple Model for the DNA Denaturation Transition

abstract:
  We study pairs of interacting self-avoiding walks on the 3d simple cubic
lattice. They have a common origin and are allowed to overlap only at the same
monomer position along the chain. The latter overlaps are indeed favored by an
energetic gain.
  This is inspired by a model introduced long ago by Poland and Sheraga [J.
Chem. Phys. {\bf 45}, 1464 (1966)] for the denaturation transition in DNA
where, however, self avoidance was not fully taken into account. For both
models, there exists a temperature T_m above which the entropic advantage to
open up overcomes the energy gained by forming tightly bound two-stranded
structures.
  Numerical simulations of our model indicate that the transition is of first
order (the energy density is discontinuous), but the analog of the surface
tension vanishes and the scaling laws near the transition point are exactly
those of a second order transition with crossover exponent \phi=1. Numerical
and exact analytic results show that the transition is second order in modified
models where the self-avoidance is partially or completely neglected.


title:
Breakdown of Luttinger liquid state in one-dimensional frustrated
  spinless fermion model

abstract:
  Haldane hypothesis about the universality of Luttinger liquid (LL) behavior
in conducting one-dimensional (1D) fermion systems is checked numerically for
spinless fermion model with next-nearest-neighbor interactions. It is shown
that for large enough interactions the ground state can be gapless (metallic)
due to frustrations but not be LL. The exponents of correlation functions for
this unusual conducting state are found numerically by finite-size method.


title:
Interpolative method for transport properties of quantum dots in the
  Kondo regime

abstract:
  We present an interpolative method for describing coherent transport through
an interacting quantum dot. The idea of the method is to construct an
approximate electron self-energy which becomes exact both in the limits of weak
and strong coupling to the leads. The validity of the approximation is first
checked for the case of a single (spin-degenerate) dot level. A generalization
to the multilevel case is then discussed. We present results both for the
density of states and the temperature dependent linear conductance showing the
transition from the Kondo to the Coulomb blockade regime.


title:
Gauge field theory of transport and magnetic relaxation in underdoped
  cuprates

abstract:
  Based on recently proposed U(1)xSU(2) Chern-Simons gauge field theory, an
interpretation of the transport and magnetic relaxation properties of
underdoped cuprates is proposed, taking into account the short range
antiferromagnetic order. The interplay of the doping-dependent spin gap
(explicitly derived by us) effect and dissipation due to gauge fluctuations
gives rise to a crossover from metallic to insulating behavior of conductivity
as temperature decreases, in semi-quantitative agreement with experimental
data. For the same reason the magnetic relaxation rate shows a maximum nearby.
Various crossover temperatures related to spin gap effects are shown to be
different manifestations of the same energy scale.


title:
On modulational instability and energy localization in anharmonic
  lattices at finite energy density

abstract:
  The localization of vibrational energy, induced by the modulational
instability of the Brillouin-zone-boundary mode in a chain of classical
anharmonic oscillators with finite initial energy density, is studied within a
continuum theory. We describe the initial localization stage as a gas of
envelope solitons and explain their merging, eventually leading to a single
localized object containing a macroscopic fraction of the total energy of the
lattice. The initial-energy-density dependences of all characteristic time
scales of the soliton formation and merging are described analytically. Spatial
power spectra are computed and used for the quantitative explanation of the
numerical results.