title: Phase Transition in the Passive Scalar Advection

abstract:
  The paper studies the behavior of the trajectories of fluid particles in a
compressible generalization of the Kraichnan ensemble of turbulent velocities.
We show that, depending on the degree of compressibility, the trajectories
either explosively separate or implosively collapse. The two behaviors are
shown to result in drastically different statistical properties of scalar
quantities passively advected by the flow. At weak compressibility, the
explosive separation of trajectories induces a familiar direct cascade of the
energy of a scalar tracer with a short-distance intermittency and dissipative
anomaly. At strong compressibility, the implosive collapse of trajectories
leads to an inverse cascade of the tracer energy with suppressed intermittency
and with the energy evacuated by large scale friction. A scalar density whose
advection preserves mass exhibits in the two regimes opposite cascades of the
total mass squared. We expect that the explosive separation and collapse of
Lagrangian trajectories occur also in more realistic high Reynolds number
velocity ensembles and that the two phenomena play a crucial role in fully
developed turbulence.


title: Large-N treatment of the Abrikosov transition at low temperatures

abstract:
  We investigate the influence of order parameter fluctuations on the
transition between normal and mixed superconducting states at low temperatures.
We show that in case of clean quasi-two-dimensional superconductors the
transition can be described by the functional of the Ginzburg-Landau type. We
consider the large-N generalization of this functional and using the lowest
Landau level approximation we get the large-N equations which describe the
phase transition. In case of physical dimensionality we found that the
transition is of the first order. The fluctuations significantly affect the
temperature dependence of the upper critical field.


title: Fermions in an anisotropic random magnetic field

abstract:
  We study the localization of fermions in an anisotropic random magnetic field
in two dimensions. It is assumed that the randomness in a particular direction
is stronger than those in the other directions. We consider a network model of
zero field contours, where there are two types of randomness - the random
tunneling matrix element at the saddle points and unidirectional random
variation of the number of fermionic states following zero field contours.
After averaging over the random complex tunneling amplitude, the problem is
mapped to an SU(2N) random exchange quantum spin chain in the $N \to 0$ limit.
We suggest that the fermionic state becomes critical in an anisotropic fashion.


title:
Negative Differential Resistance in the Scanning Tunneling Spectroscopy
  of Organic molecules

abstract:
  The conductance-voltage spectrum of molecular nanostructures measured by
scanning tunneling spectroscopy (STS) is generally assumed to reflect the local
density of states of the molecule. This excludes the possibility of observing
negative differential resistance (NDR). We report here the observation of NDR
in the scanning tunneling microscope (STM) current-voltage (I-V)
characteristics of self-assembled monolayer (SAM) of 4-p-Terphenylthiol
molecules on gold substrate measured using a platinum probe. We argue that the
NDR arises from narrow structures in the local density of states at the tip
apex atom and show that depending on the electrostatic potential profile across
the system, NDR could be observed in one or both bias directions.


title:
Response of an Excitatory-Inhibitory Neural Network to External
  Stimulation: An Application to Image Segmentation

abstract:
  Neural network models comprising elements which have exclusively excitatory
or inhibitory synapses are capable of a wide range of dynamic behavior,
including chaos. In this paper, a simple excitatory-inhibitory neural pair,
which forms the building block of larger networks, is subjected to external
stimulation. The response shows transition between various types of dynamics,
depending upon the magnitude of the stimulus. Coupling such pairs over a local
neighborhood in a two-dimensional plane, the resultant network can achieve a
satisfactory segmentation of an image into ``object'' and ``background''.
Results for synthetic and and ``real-life'' images are given.

categories: cond-mat.dis-nn chao-dyn nlin.CD q-bio.NC


title: Optimal packing of polydisperse hard-sphere fluids

abstract:
  We consider the effect of intermolecular interactions on the optimal
size-distribution of $N$ hard spheres that occupy a fixed total volume. When we
minimize the free-energy of this system, within the Percus-Yevick
approximation, we find that no solution exists beyond a quite low threshold
($\eta \thickapprox 0.260$). Monte Carlo simulations reveal that beyond this
density, the size-distribution becomes bi-modal. Such distributions cannot be
reproduced within the Percus-Yevick approximation. We present a theoretical
argument that supports the occurrence of a non-monotonic size-distribution and
emphasizing the importance of finite size effects.


title: On the 'Strong-Coupling' Generalization of the Bogoliubov Model

abstract:
  A generalized Bogoliubov model of the Bose gas in the ground state is
proposed which properly takes into account both the long-range and short-range
spatial boson correlations. It concerns equilibrium characteristics and
operates with in-medium Schrodinger equations for the pair wave functions of
bosons being the eigenfunctions of the second-order reduced density matrix. The
approach developed provides reasonable results for a dilute Bose gas with
arbitrary strong interaction between particles (the 'strong-coupling' case) and
comes to the canonical Bogoliubov model in the weak-coupling regime.


title: Statistical Mechanics and error-correction Codes

abstract:
  I will show that there is a deep relation between error-correction codes and
certain mathematical models of spin glasses. In particular minimum error
probability decoding is equivalent to finding the ground state of the
corresponding spin system. The most probable value of a symbol is related to
the magnetization at a different temperature. Convolutional codes correspond to
one-dimensional spin systems and Viterbi's decoding algorithm to the transfer
matrix algorithm of Statistical Mechanics. A particular spin-glass model, which
is exactly soluble, corresponds to an ideal code, i.e. a code which allows
error-free communication if the rate is below channel capacity.


title:
Direct Measurement of the Destruction of Charge Quantization in a Single
  Electron Box

abstract:
  We report here direct measurements of the destruction of charge quantization
in a single electron box, the first over the full range of box-to-lead
conductance values from G ~ 0 to the conductance quantum G_q = 2e^2/h, using a
sensitive single-electron transistor (SET) electrometer. The sensitivity of the
electrometer is measured to be dq ~ 6 x 10^-5 e/sqrt{Hz} and its superiority to
conductance measurements of charge fluctuations is clearly demonstrated. As the
rate of quantum mechanical tunneling from the box to its lead is increased, the
quantization of charge is destroyed, disappearing entirely at G = G_q in
agreement with theory.


title: On the stability of standing matter waves in a trap

abstract:
  We discuss excited Bose-condensed states and find the criterion of dynamical
stability of a kink-wise state, i.e., a standing matter wave with one nodal
plane perpendicular to the axis of a cylindrical trap. The dynamical stability
requires a strong radial confinement corresponding to the radial frequency
larger than the mean-field interparticle interaction. We address the question
of thermodynamic instability related to the presence of excitations with
negative energy.


title: Stripe phase ordering as quantum interference phenomenon

abstract:
  A quantum interference mechanism of the stripe phase instability in quasi
one-dimensional (1D) repulsive electron system is proposed. The leading
spin-charge coupling term in Landau functional is derived microscopically. It
is shown that away from half filling, periodic lattice potential causes
cooperative condensation of the spin and charge superlattices, which constitute
a 1D analogue of the experimentally observed quasi 2D structures in lanthanum
cuprates and nickelates. Renormalization group analysis qualitatively supports
the Hartree-Fock picture.


title:
Ground State Entropy of Potts Antiferromagnets: Homeomorphic Classes
  with Noncompact W Boundaries

abstract:
  We present exact calculations of the zero-temperature partition function
$Z(G,q,T=0)$ and ground-state degeneracy $W(\{G\},q)$ for the $q$-state Potts
antiferromagnet on a number of families of graphs $G$ for which (generalizing
$q$ from ${\mathbb Z}_+$ to ${\mathbb C}$) the boundary ${\cal B}$ of regions
of analyticity of $W$ in the complex $q$ plane is noncompact, passing through
$z=1/q=0$. For these types of graphs, since the reduced function
$W_{red.}=q^{-1}W$ is nonanalytic at $z=0$, there is no large--$q$ Taylor
series expansion of $W_{red.}$. The study of these graphs thus gives insight
into the conditions for the validity of the large--$q$ expansions. It is shown
how such (families of) graphs can be generated from known families by
homeomorphic expansion.


title:
Specific Heat (1.2-108 K) and Thermal Expansion (4.4-297 K) Measurements
  of the 3d Heavy Fermion Compound LiV_{2}O_{4}

abstract:
  Specific heat C_{p}(T) measurements of the heavy fermion normal-spinel
structure compound LiV_{2}O_{4} were carried out using a heat-pulse calorimeter
over the temperature T range from 1.2 to 108 K. The electronic specific heat
C_{e}(T) of LiV_{2}O_{4} is extracted from the C_{p}(T) data using the lattice
contribution obtained for LiTi_{2}O_{4}, a superconductor with T_{c} = 11.8 K.
The electronic specific heat coefficient \gamma(T) = C_{e}(T)/T of LiV_{2}O_{4}
is found to be 0.42 and 0.43 J/mol K^2 at T = 1 K for two different high
magnetic purity samples, respectively. \gamma(T) decreases rapidly with
increasing temperature from 4 to 30 K and then decreases much more slowly from
0.13 J/mol K^2 at 30 K to 0.08 J/mol K^2 at 108 K. The C_{e}(T) is compared
with theoretical predictions for the spin S = 1/2 Kondo model, a generic Fermi
liquid model, and an antiferromagnetically coupled quantum-disordered metal.
Each of these theories can adequately describe C_{e}(T) in the Fermi liquid
regime at low (~ 1 to 10 K) temperatures, consistently yielding a large
extrapolated \gamma(0) = 428(3) mJ/mol K^2. However, none of these theories
describes C_{e}(T) from ~ 10 K to 108 K. Our C_{e}(T) data are also in severe
disagreement with the magnetic specific heat of the spin S = 1/2 Heisenberg
model. Thermal expansion measurements of LiV_{2}O_{4} were carried out from 4.4
to 297 K using a differential capacitance dilatometer. Strong increases in the
thermal expansion coefficient and Gr\"uneisen parameter \Gamma are found below
\~ 20 K, confirming the results of Chmaissem et al. [Phys. Rev. Lett. 79, 4866
(1997)] obtained using neutron diffraction. We estimate that \Gamma(0) is about
11.4.


title: Compression of finite size polymer brushes

abstract:
  We consider edge effects in grafted polymer layers under compression. For a
semi-infinite brush, the penetration depth of edge effects $\xi\propto
h_0(h_0/h)^{1/2}$ is larger than the natural height $h_0$ and the actual height
$h$. For a brush of finite lateral size $S$ (width of a stripe or radius of a
disk), the lateral extension $u_S$ of the border chains follows the scaling law
$u_S = \xi \phi (S/\xi)$. The scaling function $\phi (x)$ is estimated within
the framework of a local Flory theory for stripe-shaped grafting surfaces. For
small $x$, $\phi (x)$ decays as a power law in agreement with simple arguments.
The effective line tension and the variation with compression height of the
force applied on the brush are also calculated.


title:
The Expansion in Width for Domain Walls in Nematic Liquid Crystals in
  External Magnetic Field

abstract:
  The improved expansion in width is applied to curved domain walls in uniaxial
nematic liquid crystals in external magnetic field. In the present paper we
concentrate on the case of equal elastic constants. We obtain approximate form
of the director field up to second order in magnetic coherence length.


title: Universality classes in the random-storage sandpile model

abstract:
  The avalanche statistics in a stochastic sandpile model where toppling takes
place with a probability p is investigated. The limiting case p=1 corresponds
to the Bak-Tang-Wiesenfeld (BTW) model with deterministic toppling rule. Based
on the moment analysis of the distribution of avalanche sizes we conclude that
for 0<p<p_c the model belongs to the DP universality class while for p_c<p<1 it
belongs to the BTW universality class, where p_c is identified with the
critical probability for directed percolation in the corresponding lattice.


title:
Realization of Haldane's Exclusion Statistics in a Model of
  Electron-Phonon Interactions

abstract:
  We discuss an integrable model describing one-dimensional electrons
interacting with two-dimensional anharmonic phonons. In the low temperature
limit it is possible to decouple phonons and consider one-dimensional
excitations separately. They have a trivial two-body scattering matrix and obey
fractional statistics. As far as we know the original model presents the first
example of a model with local bare interactions generating purely statistical
interactions between renormalized particles. As a by-product we obtain
non-trivial thermodynamic equations for the interacting system of
two-dimensional phonons.


title:
Site percolation thresholds and universal formulas for the Archimedean
  lattices

abstract:
  The site percolation thresholds p_c are determined to high precision for
eight Archimedean lattices, by the hull-walk gradient-percolation simulation
technique, with the results p_c = 0.697043, honeycomb or (6^3), 0.807904
(3,12^{2}), 0.747806 (4,6,12), 0.729724 (4,8^{2}), 0.579498 (3^{4},6), 0.621819
(3,4,6,4), 0.550213 (3^{3},4^{2}), and 0.550806 (3^{2},4,3,4), and errors of
about +/- 2\times10^{-6}. (The remaining Archimedean lattices are the square
(4^4), triangular (3^6) and Kagom\'e (3,6,3,6), for which p_c is already known
exactly or to a high degree of accuracy.) The numerical result for the
(3,12^{2}) lattice is consistent with the exact value [1-2 sin(\pi/18)]^{1/2},
which we also derive.
  The values of p_c for all Archimedean lattices are found to be linearly
related to the density of sites within an error of about 1%, which predicts p_c
more accurately than correlations based just upon the coordination number can
give. Comparison with anisotropic lattices is also made.


title:
Self-organized criticality in linear interface depinning and sandpile
  models

abstract:
  The dynamics of an elastic interface profile h(x,t) under a driving force
increasing at rate c, a restored force -epsilon h, and disorder is
investigated. Using perturbation theory and functional renormalization group
the phase diagram and the scaling exponents, up to the first order in 4-d, are
obtained. The model is found to be critical in the double limit epsilon->0$ and
c/epsilon->0$ and belongs to a different universality class as that of constant
force models. It is shown that undirected sandpile models with stochastic rules
and linear interface models with extremal dynamics belong to this new
universality class.


title: Landau theory of bi-criticality in a random quantum rotor system

abstract:
  We consider here a generalization of the random quantum rotor model in which
each rotor is characterized by an M-component vector spin. We focus entirely on
the case not considered previously, namely when the distribution of exchange
interactions has non-zero mean. Inclusion of non-zero mean permits
ferromagnetic and superconducting phases for M=1 and M=2, respectively. We find
that quite generally, the Landau theory for this system can be recast as a
zero-mean problem in the presence of a magnetic field. Naturally then, we find
that a Gabay-Toulouse line exists for $M>1$ when the distribution of exchange
interactions has non-zero mean. The solution to the saddle point equations is
presented in the vicinity of the bi-critical point characterized by the
intersection of the ferromagnetic (M=1) or superconducting (M=2) phase with the
paramagnetic and spin glass phases. All transitions are observed to be second
order. At zero temperature, we find that the ferromagnetic order parameter is
non-analytic in the parameter that controls the paramagnet/ferromagnet
transition in the absence of disorder. Also for M=1, we find that replica
symmetry breaking is present but vanishes at low temperatures. In addition, at
finite temperature, we find that the qualitative features of the phase diagram,
for M=1, are {\it identical} to what is observed experimentally in the random
magnetic alloy $LiHo_xY_{1-x}F_4$.


title: Ising Spin Glasses in a Magnetic Field

abstract:
  Ground states of the three dimensional Edwards-Anderson spin glass are
computed in the presence of an external magnetic field. Our algorithm is
sufficiently powerful for us to treat systems with up to 600 spins. We perform
a statistical analysis of how the ground state changes as the field is
increased, and reach the conclusion that the spin glass phase at zero
temperature does not survive in the presence of any finite field. This is in
agreement with the droplet model or scaling predictions, but in sharp
disagreement with the mean field picture. For comparison, we also investigate a
dilute mean field spin glass model where an Almeida-Thouless line is present.


title:
Low-Field Anomaly in the Specific Heat of s-wave Superconductors due to
  the Expansion of the Vortex Cores

abstract:
  The magnetic field dependence of the electronic specific heat C(H) in the
s-wave superconductor NbSe_2 shows curvature at low fields, resembling the near
H^1/2 term in C(H) which has been reported in high-T_c superconductors and
attributed to a d-wave pairing state. In NbSe_2 we find that the low field
behavior in C(H) is described quantitatively by the expansion of vortex cores
and the field dependence of the magnetic induction B above H_c1. The associated
change in the density of quasiparticle states localized in the vortex cores
provides a simple explanation for the "low-field anomaly" in C(H) observed in
s-wave superconductors.


title: Statistical Mechanics of Support Vector Networks

abstract:
  Using methods of Statistical Physics, we investigate the generalization
performance of support vector machines (SVMs), which have been recently
introduced as a general alternative to neural networks. For nonlinear
classification rules, the generalization error saturates on a plateau, when the
number of examples is too small to properly estimate the coefficients of the
nonlinear part. When trained on simple rules, we find that SVMs overfit only
weakly. The performance of SVMs is strongly enhanced, when the distribution of
the inputs has a gap in feature space.


title: Survival-Time Distribution for Inelastic Collapse

abstract:
  In a recent publication [PRL {\bf 81}, 1142 (1998)] it was argued that a
randomly forced particle which collides inelastically with a boundary can
undergo inelastic collapse and come to rest in a finite time. Here we discuss
the survival probability for the inelastic collapse transition. It is found
that the collapse-time distribution behaves asymptotically as a power-law in
time, and that the exponent governing this decay is non-universal. An
approximate calculation of the collapse-time exponent confirms this behaviour
and shows how inelastic collapse can be viewed as a generalised persistence
phenomenon.


title: Energy barriers of spin glasses from multi-overlap simulations

abstract:
  We report large-scale simulations of the three-dimensional Edwards-Anderson
Ising spin glass system using the recently introduced multi-overlap Monte Carlo
algorithm. In this approach the temperature is fixed and two replica are
coupled through a weight factor such that a broad distribution of the Parisi
overlap parameter $q$ is achieved. Canonical expectation values for the entire
$q$-range (multi-overlap) follow by reweighting. We present an analysis of the
performance of the algorithm and in particular discuss results on spin glass
free-energy barriers which are hard to obtain with conventional algorithms. In
addition we discuss the non-trivial scaling behavior of the canonical
$q$-distributions in the broken phase.


title: Inversion of Optical Reflectance in the Fullerenes

abstract:
  Since the discovery of superconductivity in the alkali-doped fullerenes
\cite{hebard}, the electron-phonon interaction has been the primary suspect for
superconductivity in this class of compounds. In this paper we first provide a
pedagogical review of how the question of mechanism has traditionally been
settled, and then some well-known properties of the optical reflectance (and
the derived conductance) are summarized. Finally we demonstrate how a recently
derived inversion procedure can use the optical properties of a metal to infer
the magnitude of the electron-phonon interaction. We conclude that in the case
of the alkali-doped fullerenes, the electron-phonon interaction is sufficiently
strong to explain the transition temperatues found in these the materials.


title:
Jahn-Teller Impurity States in LaSrCuO: XAFS Evidence and Implications
  for High Tc Superconductivity

abstract:
  Polarized XAFS measurements on powder of LaSrCuO with aligned c-axes find
that apical oxygens neighboring only the Sr dopants have a double-site
distribution. This result requires that the doped holes reside in impurity
states peaked on the CuO6 octahedron neighboring the dopant (denoted as
Sr-octahedrons). A model of the double site is presented of two co-existing
spin differentiated (singlet and triplet) Jahn-Teller (JT) distortions of the
Sr-octahedrons induced by an extrinsic doped hole pairing with the intrinsic
hole. It is speculated that Bose-condensation of the singlet pairs, bound by
more than 0.1 eV, produces superconductivity.


title: Compact polymers on decorated square lattices

abstract:
  A Hamiltonian cycle of a graph is a closed path that visits every vertex once
and only once. It serves as a model of a compact polymer on a lattice. I study
the number of Hamiltonian cycles, or equivalently the entropy of a compact
polymer, on various lattices that are not homogeneous but with a sublattice
structure. Estimates for the number are obtained by two methods. One is the
saddle point approximation for a field theoretic representation. The other is
the numerical diagonalization of the transfer matrix of a fully packed loop
model in the zero fugacity limit. In the latter method, several scaling
exponents are also obtained.

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


title:
Impurity corrections to the thermodynamics in spin chains using a
  transfer-matrix DMRG method

abstract:
  We use the density matrix renormalization group (DMRG) for transfer matrices
to numerically calculate impurity corrections to thermodynamic properties. The
method is applied to two impurity models in the spin-1/2 chain, namely a weak
link in the chain and an external impurity spin. The numerical analysis
confirms the field theory calculations and gives new results for the crossover
behavior.


title:
Flux-line entanglement as the mechanism of melting transition in
  high-temperature superconductors in a magnetic field

abstract:
  The mechanism of the flux-line-lattice (FLL) melting in anisotropic high-T_c
superconductors in ${\bf B}\parallel {\bf \hat{c}}$ is clarified by Monte Carlo
simulations of the 3D frustrated XY model. The percentage of entangled flux
lines abruptly changes at the melting temperature T_m, while no sharp change
can be found in the number and size distribution of vortex loops around T_m.
Therefore, the origin of this melting transition is the entanglement of flux
lines. Scaling behaviors of physical quantities are consistent with the above
mechanism of the FLL melting. The Lindemann number is also evaluated without
any phenomenological arguments.


title: Transport Mean Free Path for Magneto-Transverse Light Diffusion

abstract:
  We derive an expression for the transport mean free path $\ell^*_\perp$
associated with magneto-transverse light diffusion for a random collection of
Faraday-active
  Mie scatterers. This expression relates the magneto-transverse diffusion in
multiple scattering directly to the magneto-transverse scattering of a single
scatterer.


title:
A New Type of Electron Nuclear-Spin Interaction from Resistively
  Detected NMR in the Fractional Quantum Hall Effect Regime

abstract:
  Two dimensional electron gases in narrow GaAs quantum wells show huge
longitudinal resistance (HLR) values at certain fractional filling factors.
Applying an RF field with frequencies corresponding to the nuclear spin
splittings of {69}Ga, {71}Ga and {75}As leads to a substantial decreases of the
HLR establishing a novel type of resistively detected NMR. These resonances are
split into four sub lines each. Neither the number of sub lines nor the size of
the splitting can be explained by established interaction mechanisms.


title:
Branching annihilating random walks with parity conservation on a square
  lattice

abstract:
  Using Monte Carlo simulations we have studied the transition from an "active"
steady state to an absorbing "inactive" state for two versions of the branching
annihilating random walks with parity conservation on a square lattice. In the
first model the randomly walking particles annihilate when they meet and the
branching process creates two additional particles; in the second case we
distinguish particles and antiparticles created and annihilated in pairs. Quite
distinct critical behavior is found in the two cases, raising the question of
what determines universality in this kind of systems.


title: Effect of band filling in the Kondo lattice: A mean-field approach

abstract:
  The usual Kondo-lattice, including an antiferromagnetic exchange interaction
between nearest-neighboring localized spins, is treated here in a mean-field
scheme that introduces two mean-field parameters: one associated with the local
Kondo effect, and the other related to the magnetic correlations between
localized spins. Phases with short-range magnetic correlations or coexistence
between those and the Kondo effect are obtained. By varying the number of
electrons in the conduction band, we notice that the Kondo effect tends to be
suppressed away from half filling, while magnetic correlations can survive if
the Heisenberg coupling is strong enough. An enhanced linear coefficient of the
specific heat is obtained at low temperatures in the metallic state.


title: Orbital Magnetism in Small Quantum Dots with Closed Shells

abstract:
  It is found that various kind of shell structure which occurs at specific
values of the magnetic field leads to the disappearance of the orbital
magnetization for particular magic numbers of small quantum dots with an
electron number $A < 30$.


title:
Random Magnetic Interactions and Spin Glass Order Competing with
  Superconductivity: Interference of the Quantum Parisi Phase

abstract:
  We analyse the competition between spin glass (SG) order and local pairing
superconductivity (SC) in the fermionic Ising spin glass with frustrated
fermionic spin interaction and nonrandom attractive interaction. The phase
diagram is presented for all temperatures T and chemical potentials \mu. SC-SG
transitions are derived for the relevant ratios between attractive and
frustrated-magnetic interaction. Characteristic features of pairbreaking caused
by random magnetic interaction and/or by spin glass proximity are found. The
existence of low-energy excitations, arising from replica permutation symmetry
breaking (RPSB) in the Quantum Parisi Phase, is shown to be relevant for the
SC-SG phase boundary. Complete 1-step RPSB-calculations for the SG-phase are
presented together with a few results for infinity-step breaking. Suppression
of reentrant SG - SC - SG transitions due to RPSB is found and discussed in
context of ferromagnet - SG boundaries. The relative positioning of the SC and
SG phases presents a theoretical landmark for comparison with experiments in
heavy fermion systems and high T_c superconductors. We find a crossover line
traversing the SG-phase with (\mu=0,T=0) as its quantum critical (end)point in
complete RPSB, and scaling is proposed for its vicinity. We argue that this
line indicates a random field instability and suggest Dotsenko-Mezard vector
replica symmetry breaking to occur at low temperatures beyond.


title: Determination of the correlation length from short-time dynamics

abstract:
  We investigate the short-time dynamic relaxation of the two-dimensional XY
model in the high temperature phase. Starting from the ordered state, we
measure the autocorrelation function and determine the autocorrelation time. It
is shown that apart from a constant the equilibrium spatial correlation length
can be calculated in this way.


title:
Simulation of Magnetization Switching in Biaxial Single-Domain
  Ferromagnetic Particles

abstract:
  The magnetization switching dynamics of biaxial single-domain homogeneous
ferromagnetic particles, in which the two easy axes are perpendicular to each
other, is simulated using a 4-state clock model. A zero-field mapping of the
statics between the symmetric 4-state clock model and two decoupled Ising
models is extended to non-zero field statics and to the dynamics. This
significantly simplifies the analysis of the simulation results. We measure the
magnetization switching time of the model and analyze the results using droplet
theory. The switching dynamics in the asymmetric model is more complicated. If
the easy axis is perpendicular to the stable magnetization direction, the
system can switch its magnetization via two different channels, one very fast
and the other very slow. A maximum value for the switching field as a function
of system size is obtained. The asymmetry affects the switching fields
differently, depending on whether the switching involves one single droplet or
many droplets of spins in the stable magnetization configuration. The angular
dependence of the switching field in symmetric and asymmetric models is also
studied.


title:
A tale of two condensates: the odd "Bose - Einstein" condensation of
  atomic Hydrogen

abstract:
  The recent report of the observation of Bose-Einstein condensation in atomic
Hydrogen, characterized by an "anomalous" density spectrum, is shown to be in
agreement with the prediction of the existence of two condensates for
temperatures lower than a well defined temperature (which for Hydrogen is $
105~ \mu K $), based on the QED coherent interaction in a gas of ultracold
atoms at a density $n > n_0 (n_0=(1/\lambda)^3, \lambda$ being the wave-length
of the e.m. modes resonantly coupled to the Hydrogen atoms)


title:
Temperature dependent spatial oscillations in the correlations of the
  XXZ spin chain

abstract:
  We study the correlation $<\sigma^z_0\sigma^z_n>$ for the XXZ chain in the
massless attractive (ferromagnetic) region at positive temperatures by means of
a numerical study of the quantum transfer matrix. We find that there is a range
of temperature where the behavior of the correlation for large separations is
oscillatory with an incommensurate period which depends on temperature.


title: Magnetic field independence of the spin gap in YBa_2Cu_3O_{7-delta}

abstract:
  We report, for magnetic fields of 0, 8.8, and 14.8 Tesla, measurements of the
temperature dependent ^{63}Cu NMR spin lattice relaxation rate for near
optimally doped YBa_2Cu_3O_{7-delta}, near and above T_c. In sharp contrast
with previous work we find no magnetic field dependence. We discuss
experimental issues arising in measurements of this required precision, and
implications of the experiment regarding issues including the spin or pseudo
gap.


title:
Physical origin of the buckling in CuO$_2$: Electron-phonon coupling and
  Raman spectra

abstract:
  It is shown theoretically that the buckling of the CuO$_{2}$ planes in
certain cuprate systems can be explained in terms of an electric field across
the planes which originates from different valences of atoms above and below
the plane. This field results also in a strong coupling of the Raman-active
out-of-phase vibration of the oxygen atoms ($B_{1g}$ mode) to the electronic
charge transfer between the two oxygens in the CuO$_{2}$ plane. Consequently,
the electric field can be deduced from the Fano-type line shape of the $B_{1g}$
phonon. Using the electric field estimated from the electron-phonon coupling
the amplitude of the buckling is calculated and found to be in good agreement
with the structural data. Direct experimental support for the idea proposed is
obtained in studies of YBa$_{2}$Cu$_{3}$O$_{6+x}$ and
Bi$_{2}$Sr$_{2}$(Ca$_{1-x}$Y$_{x}$)Cu$_{2}$O$_{8}$ with different oxygen and
yttrium doping, respectively, including antiferromagnetic samples. In the
latter compound, symmetry breaking by replacing Ca partially by Y leads to an
enhancement of the electron-phonon coupling by an order of magnitude.


title:
Angular Dependence of C-Axis Magnetoresistance in Bi-2212 Single
  Crystals With Columnar Defects

abstract:
  We measured the angular dependence of the c-axis magnetoresistance rho_c(B)
of Bi-2212 irradiated with heavy ions. At temperatures near 68 K the scaling of
rho_c(B) with the c-axis magnetic field component B_perp is broken and the
in-plane field, B_parallel, affects rho_c. At this temperature, at a specific
field B_cr \approx B_Phi/2, magnetoresistance becomes independent of field
orientation. This crossing point allows us to estimate the correlation length L
of pancake positions along the c axis. We find L/s is about 100 at B=B_cr,
where s is the interlayer spacing. This provides evidence of strong enhancement
of pancake alignment in the vortex liquid in crystals with columnar defects.


title:
Testing quantum correlations in a confined atomic cloud by scattering
  fast atoms

abstract:
  We suggest measuring one-particle density matrix of a trapped ultracold
atomic cloud by scattering fast atoms in a pure momentum state off the cloud.
The lowest-order probability of the inelastic process, resulting in a pair of
outcoming fast atoms for each incoming one, turns out to be given by a Fourier
transform of the density matrix. Accordingly, important information about
quantum correlations can be deduced directly from the differential scattering
cross-section. A possible design of the atomic detector is also discussed.


title: Monte Carlo Study of the Phase Structure of Compact Polymer Chains

abstract:
  We study the phase behavior of single homopolymers in a simple
hydrophobic/hydrophilic off-lattice model with sequence independent local
interactions. The specific heat is, not unexpectedly, found to exhibit a
pronounced peak well below the collapse temperature, signalling a possible
low-temperature phase transition. The system size dependence at this maximum is
investigated both with and without the local interactions, using chains with up
to 50 monomers. The size dependence is found to be weak. The specific heat
itself seems not to diverge. The homopolymer results are compared with those
for two non-uniform sequences. Our calculations are performed using the methods
of simulated and parallel tempering. The performances of these algorithms are
discussed, based on careful tests for a small system.


title:
Bose-Einstein vs. electrodynamic condensates: the question of order and
  coherence

abstract:
  The remarkable recent experiments on ensembles of magnetically trapped
ultracold alkali atoms have demonstrated the transition to a highly ordered
phase, that has been attributed to the process of quantum-mechanical
condensation, predicted long ago (1924) by Bose and Einstein. After having
presented our argument against the above attribution, we show that the known
phenomenology, including a discrepancy of about one order of magnitude with the
predictions of Bose- Einstein condensation, is in good agreement with the
electromagnetic coherence induced on the alkali atoms by the long range
electrodynamic interactions. We also predict that for temperatures lower than a
well defined $T_{BEC}$, the state predicted by Bose and Einstein coexists with
the new coherent electrodynamic state.


title:
Low field vortex dynamics over seven time decades in a
  Bi_2Sr_2CaCu_2O_{8+\delta} single crystal for temperatures 13 K < T < 83 K

abstract:
  Using a custom made dc-SQUID magnetometer, we have measured the time
relaxation of the remanent magnetization M_rem of a Bi_2Sr_2CaCu_2O_{8+\delta}
single crystal from the fully critical state for temperatures 13 K < T < 83 K.
The measurements cover a time window of seven decades 10^{-2} s < t < 10^5 s,
so that the current density j can be studied from values very close to j_c down
to values considerably smaller than j_c. From the data we have obtained: (i)
the flux creep activation barriers U as a function of current density j, (ii)
the current-voltage characteristics E(j) in a typical range of 10^{-7} V/cm to
10^{-15} V/cm, and (iii) the critical current density j_c(0) at T = 0. Three
different regimes of vortex dynamics are observed: For temperatures T < 20 K
the activation barrier U(j) is logarithmic, no unique functional dependence
U(j) could be found for the intermediate temperature interval 20 K < T < 40 K,
and finally for T > 40 K the activation barrier U(j) follows a power-law
behavior with an exponent mu = 0.6. From the analysis of the data within the
weak collective pinning theory for strongly layered superconductors, it is
argued that for temperatures T < 20 K pancake-vortices are pinned individually,
while for temperatures T > 40 K pinning involves large collectively pinned
vortex bundles. A description of the vortex dynamics in the intermediate
temperature interval 20 K < T < 40 K is given on the basis of a qualitative low
field phase diagram of the vortex state in Bi_2Sr_2CaCu_2O_{8+\delta}. Within
this description a second peak in the magnetization loop should occur for
temperatures between 20 K and 40 K, as it has been observed in several
magnetization measurements in the literature.


title:
Synthesis, Characterization and Magnetic Susceptibility of the Heavy
  Fermion Transition Metal Oxide LiV_{2}O_{4}

abstract:
  The preparative method, characterization and magnetic susceptibility \chi
measurements versus temperature T of the heavy fermion transition metal oxide
LiV_{2}O_{4} are reported in detail. The intrinsic \chi(T) shows a nearly
T-independent behavior below ~ 30 K with a shallow broad maximum at about 16 K,
whereas Curie-Weiss-like behavior is observed above 50-100 K. Field-cooled and
zero-field-cooled magnetization M measurements in applied magnetic fields H =
10 to 100 G from 1.8 to 50 K showed no evidence for spin-glass ordering.
Crystalline electric field theory for an assumed cubic V point group symmetry
is found insufficient to describe the observed temperature variation of the
effective magnetic moment. The Kondo and Coqblin-Schrieffer models do not
describe the magnitude and T dependence of \chi with realistic parameters. In
the high T range, fits of \chi(T) by the predictions of high temperature series
expansion calculations provide estimates of the V-V antiferromagnetic exchange
coupling constant J/k_{B} ~ 20 K, g-factor g ~ 2 and the T-independent
susceptibility. Other possible models to describe the \chi(T) are discussed.
The paramagnetic impurities in the samples were characterized using isothermal
M(H) measurements with 0 < H <= 5.5 Tesla at 2 to 6 K. These impurities are
inferred to have spin S_{imp} ~ 3/2 to 4, g_{imp} ~ 2 and molar concentrations
of 0.01 to 0.8 %, depending on the sample.


title: Polarons in Wurtzite Nitride Semiconductors

abstract:
  Polaron binding energy and effective mass are calculated for semiconductors
with wurtzite crystalline structure from the first order electron-phonon
corrections to the self-energy. A recently introduced Frohlich-like
electron-phonon interaction Hamiltonian which accounts for the LO and TO
polarizations mixing due to the anisotropy is used in the calculation. The
polaronic damping rates are evaluated for finite temperature. Numerical results
are reported for GaN. It is shown that the electron-phonon coupling is strong
enough to justify the necessity of the inclusion of second-order corrections.


title: Stripes and the t-J model

abstract:
  We investigate the two-dimensional t-J model at a hole doping of x = 1/8 and
J/t = 0.35 with exact diagonalization. The low-energy states are uniform (not
striped). We find numerous excited states with charge density wave structures,
which may be interpreted as striped phases. Some of these are consistent with
neutron scattering data on the cuprates and nickelates.


title: Anomalous thermal conductivity in glasses at low temperatures

abstract: This paper has been withdrawn for extensive revision.


title: How to compute the thermodynamics of a glass using a cloned liquid

abstract:
  The recently proposed strategy for studying the equilibrium thermodynamics of
the glass phase using a molecular liquid is reviewed and tested in details on
the solvable case of the $p$-spin model. We derive the general phase diagram,
and confirm the validity of this procedure. We point out the efficacy of a
system of two weakly coupled copies in order to identify the glass transition,
and the necessity to study a system with $m<1$ copies ('clones') of the
original problem in order to derive the thermodynamic properties of the glass
phase.


title: Strongly Anisotropic Transport in Higher Two-Dimensional Landau Levels

abstract:
  Low-temperature, electronic transport in Landau levels N>1 of a
two-dimensional electron system is strongly anisotropic. At half-filling of
either spin level of each such Landau level the magnetoresistance either
collapses to form a deep minimum or is peaked in a sharp maximum, depending on
the in-plane current direction. Such anisotropies are absent in the N=0 and N=1
Landau level, which are dominated by the states of the fractional quantum Hall
effect. The transport anisotropies may be indicative of a new many particle
state, which forms exclusively in higher Landau levels.


title:
Superconductivity and spin correlation in organic conductors: a quantum
  Monte Carlo study

abstract:
  The d-wave pairing correlations along with spin correlation are calculated
with quantum Monte Carlo method for the two-dimensional Hubbard model on
lattice structures representing organic superconductors
$\kappa$-(BEDT-TTF)$_2$X and (TMTSF)$_2$X. In both cases the pairing
correlations for superconducting order parameters with nodes are found to be
enhanced. The symmetry and the enhancement of the pairing is systematically
correlated with the spin structure factor, suggesting a spin-fluctuation
mediated pairing. We have further found that, as we deform the Fermi surface to
make the system approach the half-filled square lattice, the coherence of the
pairing saturates while the local pairing amplitude continues to increase.


title:
Observation of magnetic order in La_{1.9}Sr_{0.1}CuO_{4} from two-magnon
  Raman scattering

abstract:
  We reported two-magnon Raman scattering from La_{1.9}Sr_{0.1}CuO_4, which has
a suppressed Tc=12 K, as the temperature is lowered below 37 K and an ordered
spin phase is formed. The two-magnon Raman intensity increases with decreasing
temperature. The magnetic scattering in La_{1.9}Sr_{0.1}CuO_4 is totally
different from that reported in the parent compound La_2CuO_4. We analyze the
line shape of the two-magnon scattering within the traditional Loudon-Fleury
theory and find the superexchange constant J=1052 cm^{-1}. The calculation of
the frequency moment suggests that the quantum fluctuations are very weak in
the system. The room temperature Raman scattering from La_2CuO_4 is also
measured. Strong features appear in the one-phonon spectrum at the frequencies
of the longitudinal optical (LO) infrared modes which we suggest become Raman
active through a Fr\"{o}hlich-interaction.


title:
Theory of Orbital Excitation and Resonant Inelastic X-ray Scattering in
  Manganites

abstract:
  We study theoretically the collective orbital excitation named orbital wave
in the orbital ordered manganites.The dispersion relation of the orbital wave
is affected by the static spin structure through the coupling between spin and
orbital degrees of freedom. As a probe to detect the dispersion relation, we
propose two possible methods by utilizing resonant inelastic x-ray scattering.
The transition probability of the orbital wave scattering is formulated, and
the momentum and polarization dependences of the structure factor are
calculated in several types of the orbital and spin structures. The elastic
x-ray scattering in the L-edge case to observe the orbital ordering is also
discussed.


title: Quantum kinetics and thermalization in a particle bath model

abstract:
  We study the dynamics of relaxation and thermalization in an exactly solvable
model of a particle interacting with a harmonic oscillator bath. Our goal is to
understand the effects of non-Markovian processes on the relaxational dynamics
and to compare the exact evolution of the distribution function with
approximate Markovian and Non-Markovian quantum kinetics. There are two
different cases that are studied in detail: i) a quasiparticle (resonance) when
the renormalized frequency of the particle is above the frequency threshold of
the bath and ii) a stable renormalized `particle' state below this threshold.
The time evolution of the occupation number for the particle is evaluated
exactly using different approaches that yield to complementary insights. The
exact solution allows us to investigate the concept of the formation time of a
quasiparticle and to study the difference between the relaxation of the
distribution of bare particles and that of quasiparticles. We derive a
non-Markovian quantum kinetic equation which resums the perturbative series and
includes off-shell effects. A Markovian approximation that includes off-shell
contributions and the usual Boltzmann equation (energy conserving) are obtained
from the quantum kinetic equation in the limit of wide separation of time
scales upon different coarse-graining assumptions. The relaxational dynamics
predicted by the non-Markovian, Markovian and Boltzmann approximations are
compared to the exact result. The Boltzmann approach is seen to fail in the
case of wide resonances and when threshold and renormalization effects are
important.


title:
Phase Locking, Devil's Staircases, Farey Trees, and Arnold Tongues in
  Driven Vortex Lattices with Periodic Pinning

abstract:
  Using numerical simulations, we observe phase locking, Arnold tongues, and
Devil's staircases for vortex lattices driven at varying angles with respect to
an underlying superconducting periodic pinning array. This rich structure
should be observalble in transport measurments. The transverse $V(I)$ curves
have a Devil's staircase structure, with plateaus occurring near the driving
angles along symmetry directions of the pinning array. Each of the plateaus
corresponds to a different dyanmical phase with a distinctive vortex structure
and flow pattern.


title:
Phase Diagram of a Superconducting and Antiferromagnetic System with
  SO(5) Symmetry

abstract:
  Temperature vs. chemical-potential phase diagrams of an SO(5) model for
high-(T_c) cuprates are calculated by Monte Carlo simulation. There is a
bicritical point where the second-order antiferromagnetism (AF) and
superconductivity transition lines merge tangentially into a first-order line,
and the SO(5) symmetry is achieved. In an external magnetic field, the AF
ordering is first order in the region where the first-order melting line of
flux lattice joins in. There is a tricritical point on the AF transition line
from which the AF ordering becomes second order.


title: On the ground state of solids with strong electron correlations

abstract:
  We formulate the calculation of the ground-state wavefunction and energy of a
system of strongly correlated electrons in terms of scattering matrices. A
hierarchy of approximations is introduced which results in an incremental
expansion of the energy. The present approach generalizes previous work
designed for weakly correlated electronic systems.


title:
Effects of Charge Density Modulation on Incommensurate
  Antiferromagnetism: Ginzburg-Landau Study

abstract:
  In the vicinity of hole density, 1/8, La-based high-$T_{c}$ superconductors
exhibit long-range order of incommensurate antiferromagnetism (IC-AF).
Motivated by this observation, we explore the possible stabilization of IC-AF
ordering due to the existence of static charge density modulation (CDM). We use
Ginzburg-Landau free energy based on the mean field theory of the t-J model. It
is shown numerically that three kinds of CDM including a stripe pattern can
stabilize static IC-AF ordering. We also argue that such CDMs can cause the
saturation of the degree of incommensurability ($\eta$) as a function of hole
density for IC-AF fluctuation. In our framework a principal effect of
low-temperature tetragonal (LTT) structure is just to increase $\eta$ of IC-AF
slightly larger only for the stripe pattern.


title:
Scaling and infrared divergences in the replica field theory of the
  Ising spin glass

abstract:
  Replica field theory for the Ising spin glass in zero magnetic field is
studied around the upper critical dimension d=6. A scaling theory of the spin
glass phase, based on Parisi's ultrametrically organised order parameter, is
proposed. We argue that this infinite step replica symmetry broken (RSB) phase
is nonperturbative in the sense that amplitudes of scaling forms cannot be
expanded in term of the coupling constant w^2. Infrared divergent integrals
inevitably appear when we try to compute amplitudes perturbatively,
nevertheless the \epsilon-expansion of critical exponents seems to be
well-behaved. The origin of these problems can be traced back to the unusual
behaviour of the free propagator having two mass scales, the smaller one being
proportional to the perturbation parameter w^2 and providing a natural infrared
cutoff. Keeping the free propagator unexpanded makes it possible to avoid
producing infrared divergent integrals. The role of Ward-identities and the
problem of the lower critical dimension are also discussed.


title:
Probing the plateau-insulator quantum phase transition in the quantum
  Hall regime

abstract:
  We report quantum Hall experiments on the plateau-insulator transition in a
low mobility In_{.53} Ga_{.47} As/InP heterostructure. The data for the
longitudinal resistance \rho_{xx} follow an exponential law and we extract a
critical exponent \kappa= .55 \pm .05 which is slightly different from the
established value \kappa = .42 \pm .04 for the plateau transitions. Upon
correction for inhomogeneity effects, which cause the critical conductance
\sigma_{xx}^* to depend marginally on temperature, our data indicate that the
plateau-plateau and plateau- insulator transitions are in the same universality
class.


title: Dissipative properties of vibrated granular materials

abstract:
  We investigate collective dissipative properties of vibrated granular
materials by means of molecular dynamics simulations. Rates of energy losses
indicate three different regimes or "phases"in the amplitude-frequency plane of
the external forcing, namely, solid, convective, and gas-like regimes. The
behavior of effective damping decrement in the solid regime is glassy.
Practical applications are dicussed.


title:
Band structures of periodic carbon nanotube junctions and their
  symmetries analyzed by the effective mass approximation

abstract:
  The band structures of the periodic nanotube junctions are investigated by
the effective mass theory and the tight binding model.
  The periodic junctions are constructed by introducing pairs of a pentagonal
defect and a heptagonal defect periodically in the carbon nanotube.
  We treat the periodic junctions whose unit cell is composed by two kinds of
metallic nanotubes with almost same radii, the ratio of which is between 0.7
and 1 .
  The discussed energy region is near the undoped Fermi level where the channel
number is kept to two, so there are two bands.
  The energy bands are expressed with closed analytical forms by the effective
mass theory with some assumptions, and they coincide well with the numerical
results by the tight binding model. Differences between the two methods are
also discussed. Origin of correspondence between the band structures and the
phason pattern discussed in Phys. Rev. B {\bf 53}, 2114, is clarified. The
width of the gap and the band are in inverse proportion to the length of the
unit cell, which is the sum of the lengths measured along the tube axis in each
tube part and along 'radial' direction in the junction part. The degeneracy and
repulsion between the two bands are determined only from symmetries.


title:
The reconstruction of Ni and Rh (001) surfaces upon Carbon, Nitrogen, or
  Oxygen adsorption

abstract:
  Nickel and Rhodium (001) surfaces display a similar - as from STM images -
clock reconstruction when half a monolayer of C/Ni, N/Ni or O/Rh is adsorbed;
no reconstruction is observed instead for O/Ni. Adsorbate atoms sit at the
center of the black squares of a chess-board, $c(2\times 2)$, pattern and two
different reconstructions are actually compatible with the observed STM images
- showing a $(2\times 2)p4g$ pattern - according to whether a rotation of the
black or white squares occurs. We report on a first - principles study of the
structure of X/Ni(001) and X/Rh(001) surfaces (X=C,N,O) at half a monolayer
coverage, performed using density-functional theory. Our findings are in
agreement with all available experimental information and shed new light on the
mechanisms responsible for the reconstructions. We show that the same substrate
may display different reconstructions - or no reconstruction - upon adsorption
of different atomic species, depending on the relative importance of the
chemical and steric factors which determine the reconstruction.


title:
Absence of a Finite-Temperature Melting Transition in the Classical
  Two-Dimensional One-Component Plasma

abstract:
  Vortices in thin-film superconductors are often modelled as a system of
particles interacting via a repulsive logarithmic potential. Arguments are
presented to show that the hypothetical (Abrikosov) crystalline state for such
particles is unstable at any finite temperature against proliferation of
screened disclinations. The correlation length of crystalline order is
predicted to grow as $\sqrt{1/T}$ as the temperature $T$ is reduced to zero, in
excellent agreement with our simulations of this two-dimensional system.


title:
Vortex lattice melting and the damping of the dHvA oscillations in the
  mixed state

abstract:
  Phase fluctuations in the superconducting order parameter, which are
responsible for the melting of the Abrikosov vortex lattice below the mean
field $H_{c2}$, are shown to dramatically enhance the scattering of
quasi-particles by the fluctuating pair potential, thus leading to enhanced
damping of the dHvA oscillations in the liquid mixed state. This effect is
shown to quantitatively account for the detailed field dependence of the dHvA
amplitude observed recently in the mixed state of a Quasi 2D organic SC.


title: The quantum Heisenberg antiferromagnet on the square lattice

abstract:
  The pure-quantum self-consistent harmonic approximation, a semiclassical
method based on the path-integral formulation of quantum statistical mechanics,
is applied to the study of the thermodynamic behaviour of the quantum
Heisenberg antiferromagnet on the square lattice (QHAF). Results for various
properties are obtained for different values of the spin and successfully
compared with experimental data.


title: Pauli-Limited Superconductivity in Small Grains

abstract:
  We report on an exploration of the mean-field phase diagram for Pauli-limited
superconductivity in small metallic grains. Emphasis is placed on the crossover
from the ultra-small grain limit where superconductivity disappears to the bulk
thin-film limit as the single-particle level spacing in the grain decreases. We
find that the maximum Zeeman coupling strength compatible with
superconductivity increases with decreasing grain size, in spite of a
monotonically decreasing condensation energy per unit volume.


title: Effective Hamiltonian with holomorphic variables

abstract:
  The pure-quantum self-consistent harmonic approximation (PQSCHA) permits to
study a quantum system by means of an effective classical Hamiltonian. In this
work the PQSCHA is reformulated in terms of the holomorphic variables connected
to a set of bosonic operators. The holomorphic formulation, based on the
olomorphic path integral for the Weyl symbol of the density matrix, makes it
possible to directly approach general Hamiltonians given in terms of bosonic
creation and annihilation operators.


title:
Strongly Charged, Flexible Polyelectrolytes in Poor Solvents --
  Molecular Dynamics Simulations

abstract:
  We present a set of molecular dynamics (MD) simulations of strongly charged,
flexible polyelectrolyte chains under poor solvent conditions in a salt free
solution. Structural properties of the chains and of the solutions are
reported. By varying the polymer density and the electrostatic interaction
strength we study the crossover from a dominating electrostatic interaction to
the regime of strong screening, where the hydrophobic interactions dominate.
During the crossover a multitude of structures is observed. In the limit of low
polymer density strongly stretched, necklace like conformations are found. In
the opposite limit of high polymer density which is equivalent to strongly
screened electrostatic interactions, we find that the chains are extremely
collapsed, however we observe no agglomeration or phase separation. The
investigations show that the density of free charges is one of the relevant
parameters which rules the behavior of the system and hence should be used as a
parameter to explain experimental results.


title:
Perturbation of infra-red fixed points and duality in quantum impurity
  problems

abstract:
  We explain in this paper how a meaningful irrelevant perturbation theory
around the infra-red (strong coupling) fixed point can be carried out for
integrable quantum impurity problems. This is illustrated in details for the
spin 1/2 Kondo model, where our approach gives rise to the complete low
temperature expansion of the resistivity, beyond the well known $T^2$ Fermi
liquid behaviour. We also consider the edge states tunneling problem, and
demonstrate by Keldysh techniques that the DC current satisfies an exact
duality between the UV and IR regimes. This corresponds physically to a duality
between the tunneling of Laughlin quasi particles and electrons, and, more
formally, to the existence of an exact instantons expansion. The duality is
deeply connected with integrability, and could not have been expected a priori.


title: Unbounded fluctuations in transport through an integrable cavity

abstract:
  We derive a semiclassical scheme for the conductance through a rectangular
cavity. The transmission amplitudes are expressed as a sum over families of
trajectories rather than a sum over isolated trajectories. The contributing
families are obtained from the evaluation of a finite number of continued
fractions. We find that, contrary to the chaotic case, the conductance
fluctuations increase with the incoming energy and the correlation function
exhibits a singularity at the origin.


title:
Effect of Nonmagnetic Impurities on the Magnetic Resonance Peak in
  YBa2Cu3O7

abstract:
  The magnetic excitation spectrum of a YBa_2 Cu_3 O_7 crystal containing 0.5%
of nonmagnetic (Zn) impurities has been determined by inelastic neutron
scattering. Whereas in the pure system a sharp resonance peak at E ~ 40 meV is
observed exclusively below the superconducting transition temperature T_c, the
magnetic response in the Zn-substituted system is broadened significantly and
vanishes at a temperature much higher than T_c. The energy-integrated spectral
weight observed near q = (pi,pi) increases with Zn substitution, and only about
half of the spectral weight is removed at T_c.


title:
The level shifting induced negative magnetoresistance in the
  nearest-neighbor hopping conduction

abstract:
  We propose a new mechanism of negative magnetoresistance in non-magnetic
granular materials in which electron transport is dominated by hopping between
two nearest-neighbor clusters. We study the dependence of magnetoresistance on
temperature and separation between neighboring clusters. At a small separation
we find a negative magnetoresistance at low temperatures and it changes over to
a positive value as temperature increases. For a fixed temperature,
magnetoresistance changes from negative to positive when the cluster separation
increases. The change of magnetoresistance $\Delta R/R$ can be more than 80% at
low temperatures.


title: Superconducting gap within a modified interlayer tunneling model

abstract:
  A modified version of the interlayer tunneling model, including interlayer
single particle hopping (ISPH), is considered as a phenomenological model to
describe cuprate superconductors. The effective ISPH (t_\perp^{eff}) is taken
along with a probability factor P, that involves the normal state pseudogap
(E_g). This makes t_\perp^{eff} to mimic experimental observations that, ISPH
is small in the underdoped regime and increases towards overdoping. Within the
modified model, we establish the absence of bilayer splitting as observed in
case of layered cuprates. Transition temperature (T_c) and the superconducting
gap are calculated. A match, to the T-dependent superconducting gap data from
experiment, is obtained and high values of the ratio of the superconducting gap
to T_c are recovered. Depending on the values of E_g, T_c as a function of
interlayer coupling shows mixed behaviour. This is a prediction and can be
checked further.


abstract:
  The pure-quantum self-consistent harmonic approximation (PQSCHA) permits to
study a quantum system by means of an effective classical Hamiltonian -
depending on quantum coupling and temperature - and classical-like expressions
for the averages of observables. In this work the PQSCHA is derived in terms of
the holomorphic variables connected to a set of bosonic operators. The
holomorphic formulation, based on the path integral for the Weyl symbol of the
density matrix, makes it possible to approach directly general Hamiltonians
given in terms of bosonic creation and annihilation operators.


title: Jahn-Teller Effect in Diamond-like Carbon Film

abstract:
  The Jahn-Teller effect used semi-theoretically to analyse UV-visible spectra
of the diamond like-carbon films on Si substrates. By deconvolution of
UV-visible absorption spectra of the typical films, different absorption lines
found. For each sample, six lines which were related to $\V^\circ$ by
Jahn-Teller effect are distinguished. Our theoretical approach based on
distortion of the $\V^\circ$ as the consequence of the stress in the films, is
in agreement with experimental results. The shift of the Jahn-Teller lines
toward the zero phonon line of $\V^\circ$ were found in accordance to stress
decrease in the films. The difference between room temperature and low
temperature spectra is discussed. We found that the splitting of the excited
state of $\V^\circ$ in the films under stress is twice as large as that of the
ground state.


title:
Residual resistivity ratio and its relation to the positive
  magnetoresistance behavior in natural multilayer LaMn2Ge2; relevance to
  artificial multilayer physics

abstract:
  Results of low temperature magnetoresistance ($\Delta\rho/\rho$) and
isothermal magnetization (M) measurements on polycrystalline ferromagnetic (T_C
close to 300 K) natural multilayers, LaMn_{2+x}Ge_{2-y}Si_y, are reported. It
is found that the samples with large residual resistivity ratio,
$\rho(300K)/\rho(4.2K)$, exhibit large positive magnetoresistance at high
magnetic fields. The Kohler's rule is not obeyed in these alloys. In addition,
at 4.5 K, there is a tendency towards linear variation of $\Delta\rho/\rho$
with magnetic field with increasing $\rho(300K)/\rho(4.2K$); however, the field
dependence of $\Delta\rho/\rho$ does not track that of M, thereby suggesting
that the magnetoresistance originates from non-magnetic layers. It is
interesting that these experimental findings on bulk polycrystals are
qualitatively similar to what is seen in artificially grown multilayer systems
recently.


title: The Haldane-Rezayi Quantum Hall State and Magnetic Flux

abstract:
  We consider the general abelian background configurations for the
Haldane-Rezayi quantum Hall state. We determine the stable configurations to be
the ones with the spontaneous flux of $(\Z+1/2) \phi_0$ with $\phi_0 = hc/e$.
This gives the physical mechanism by which the edge theory of the state becomes
identical to the one for the 331 state. It also provides a new experimental
consequence which can be tested in the enigmatic $\nu=5/2$ plateau in a single
layer system.


title: Riemannian geometry of the Pauli paramagnetic gas

abstract:
  We investigate the thermodynamic curvature resulting from a Riemannian
geometry approach to thermodynamics for the Pauli paramagnetic gas which is a
system of identical fermions each with spin 1/2. We observe that the absolute
value of thermodynamic curvature can be interpreted as a measure of the
stability of the considered system only in the classical and semiclassical
regime. But in quantum regime some exceptions are observed.


title: Non-Cayley-tree model for quasiparticle decay in a quantum dot

abstract:
  The decay of a quasiparticle in a confined geometry, resulting from
electron-electron interactions, has been mapped onto the single-electron
problem of diffusion on a Cayley tree by Altshuler et al. [Phys.Rev.Lett. 78,
2803 (1997)]. We study an alternative model, that captures the strong
correlations between the self-energies of different excitations with the same
number of quasiparticles. The model has a recursion relation for the
single-particle density of states that is markedly different from the Cayley
tree. It remains tractable enough that sufficiently large systems can be
studied to observe the localization transition in Fock space predicted by
Altshuler et al.


title:
Critical properties and phase diagram of the quantum anisotropic XY spin
  chain in a random magnetic field: A density-matrix renormalization-group
  analysis

abstract:
  The spin-1/2 quantum anisotropic XY spin chain in a transverse random
magnetic field parallel to the z axis is numerically studied by means of the
density-matrix renormalization group. The dependence of the spontaneous
magnetization and the energy gap on both the strength of the random magnetic
field and the factor of anisotropy is determined. The critical line for the
order-disorder phase transition is obtained and the resulting phase diagram is
drawn. Our results are compatible with the fact that models with different
factors of anisotropy fall within the same universality class as the quantum
Ising model in a transverse random field.


title:
Conductance oscillations in tunnel-coupled quantum dots in the quantum
  Hall regime

abstract:
  We present measurements of transport through two tunnel-coupled quantum dots
of different sizes connected in series in a strong, variable, perpendicular
magnetic field. Double dot conductance was measured both as a function of
magnetic field, which was varied across the filling factor nu = 4 quantum Hall
plateau, and as a function of charge induced evenly on the two dots. The
conductance peaks undergo position shifts and height modulations as the
magnetic field is varied. These shifts and modulations form a pattern that
repeats over large ranges of magnetic field and with the addition of double dot
charge. The robust pattern repetition is consistent with a frequency locking
effect.


title: Nonclassical effects and off-diagonal couplings in a model for FeBr2

abstract:
  Using Monte Carlo techniques, we show that the recently experimentally
observed transitionlike phenomena in the transverse spin-ordering close to the
anomalies in the antiferromagnetic phase of FeBr2 (Petracic et al. [Phys. Rev.
B 57, R11051 (1998)]) may result from the quantum nature of the S=1 spins and
an off-diagonal exchange between axial and planar spin components.


title: Overlap properties and adsorption transition of two Hamiltonian paths

abstract:
  We consider a model of two (fully) compact polymer chains, coupled through an
attractive interaction. These compact chains are represented by Hamiltonian
paths (HP), and the coupling favors the existence of common bonds between the
chains. Using a ($n=0$ component) spin representation for these paths, we show
the existence of a phase transition for strong coupling (i.e. at low
temperature) towards a ``frozen'' phase where one chain is completely adsorbed
onto the other. By performing a Legendre transform, we obtain the probability
distribution of overlaps. The fraction of common bonds between two HP, i.e.
their overlap $q$, has both lower ($q_m$) and upper ($q_M$) bounds. This means
in particuliar that two HP with overlap greater than $q_M$ coincide. These
results may be of interest in (bio)polymers and in optimization problems.


title:
A Conjecture for possible theory for the description of high temperature
  superconductivity and antiferromagnetism

abstract:
  Keeping in mind some salient features of quantum symmetry groups and in lieu
of the interesting critique by Baskaran and Anderson of the hypothesis of the
SO(5) model of Zhang and the related works of others such as Sachdev to
describe superconductivity and antiferromagnetism we propose a conjecture to
model these complex phenomenona by quantum groups, at least as a starting
point.


title:
Strongly Correlated Electrons on a Silicon Surface: Theory of a Mott
  Insulator

abstract:
  We demonstrate theoretically that the electronic ground state of the
potassium-covered Si(111)-B surface is a Mott insulator, explicitly
contradicting band theory but in good agreement with recent experiments. We
determine the physical structure by standard density-functional methods, and
obtain the electronic ground state by exact diagonalization of a many-body
Hamiltonian. The many-body conductivity reveals a Brinkman-Rice metal-insulator
transition at a critical interaction strength; the calculated interaction
strength is well above this critical value.


title: A field-oriented chain of dipolar particles in elongational flow

abstract:
  We study the behavior of an isolated field-oriented chain of dipolar
particles in elongational fluid flow. Our main goal is to emphasize the effect
of dipolar interactions on the chain's contribution to the pressure tensor and
to the viscosities of a dilute suspension of these linear aggregates. In our
model, despite the overall rigid appearance of the chain at rest, the
constituent beads may move slightly relative to one another, conferring a
certain degree of flexibility to the chain. This flexibility is quantified in
terms of a dimensionless parameter, $\lambda^{-1}$, comparing thermal and
dipolar energies. We perform an expansion in $\lambda^{-1}$, and obtain the
first correction to the rigid chain contribution to the Kramers' pressure
tensor for different flow geometries. The interplay of the elongational flow
field and the field-induced chain orientation gives rise to a rich variety of
scenarios. We compute the elongational, shear, and rotational viscosities in
some representative situations.


title:
Anomalous Magnetothermal Resistance of High-Tc Superconductors:
  Anomalous Cyclotron Orbits at a Dirac Point

abstract:
  I derive equations of motion for quasiparticles near the nodes in the d-wave
gap of high Tc superconductors. Previous versions have not properly taken into
account the spatial dependence of the gap parameter phase. The results are
compatible with magnetothermal conductivity measurements in the superconducting
phase by Ong and Krishana.


title:
Interlayer Coherence in the $\nu=1$ and $\nu=2$ Bilayer Quantum Hall
  States

abstract:
  We have measured the Hall-plateau width and the activation energy of the
bilayer quantum Hall (BLQH) states at the Landau-level filling factor $\nu=1$
and 2 by tilting the sample and simultaneously changing the electron density in
each quantum well. The phase transition between the commensurate and
incommensurate states are confirmed at $\nu =1$ and discovered at $\nu =2$. In
particular, three different $\nu =2$ BLQH states are identified; the compound
state, the coherent commensurate state, and the coherent incommensurate state.


title: Corrections to scaling at the Anderson transition

abstract:
  We report a numerical analysis of corrections to finite size scaling at the
Anderson transition due to irrelevant scaling variables and non-linearities of
the scaling variables. By taking proper account of these corrections, the
universality of the critical exponent for the orthogonal universality class for
three different distributions of the random potential is convincingly
demonstrated.


title: Thermodynamics of the Double Exchange Systems

abstract:
  This article gives a comprehensive review on the recent studies of the double
exchange systems using non-perturbative approaches; the dynamical mean-field
theory and the Monte Carlo method. Investigations beyond mean-field type
treatments are described. Taking into account strong spin fluctuations which
create large changes in conduction electron structure, finite temperature
properties as well as dynamics of the system are calculated. Comparisons with
experimental data for colossal magnetoresistance manganites are made. We show
that high Curie temperature (Tc) compounds, e.g. (La,Sr)MnO3, are canonical
double-exchange systems. Properties of other compounds with lower Tc are
discussed in relation to inhomogeneities of the system including the issue of
phase separation.


title: The 2-d Coulomb Gas on a 1-d lattice

abstract:
  The statistical mechanics of a two dimensional Coulomb gas confined to one
dimension is studied, wherein hard core particles move on a ring. Exact
self-duality is shown for a version of the sine-Gordon model arising in this
context, thereby locating the transition temperature exactly. We present
asymptotically exact results for the correlations in the model and characterize
the low and high temperature phases. Numerical simulations provide support to
these renormalization group calculations. Connections with other interesting
problems, the quantum Brownian motion of a particle in a periodic potential and
impurity problems, are pointed out.


title: Incipient failure in sandpile models

abstract:
  Elastoplastic and constitutive equation theories are two approaches based on
very different assumptions for creating a continuum theory for the stress
distributions in a static sandpile. Both models produce the same surprising
prediction that in a two dimensional granular pile constructed at its angle of
repose, the outside wedge will be on the verge of failure. We show how these
predictions can be tested experimentally.


title:
The magnetoresistance in GdNi: Magnetic-polaronic-like effect near Curie
  temperature and low temperature sign reversal

abstract:
  The results of magnetoresistance ($\Delta \rho /\rho$)measurements in GdNi,
in the temperature range 4.2 to 300 K are reported. The sign of $\Delta \rho
/\rho $ above the Curie temperature (Tc= 70 K) is negative and its magnitude in
a magnetic field of 80kOe grows with decreasing temperature below 150 K with a
peak value of about -20% at Tc. These features, qualitatively resembling those
in giant magnetoresistance systems (manganates), are attributed to the
formation of some kind of magnetic polarons induced by Gd. The
magnetoresistance changes sign below 12 K which is attributed to subtle band
structure effects.


title:
A possibility of emission of high frequency gravitational radiation from
  junctions between d-wave and s-wave superconductors

abstract:
  Recent measurements on a class of high-Tc superconductors (HTSCs) have shown
that Cooper-pairs wavefunction is a d-wave, while in another class, d-wave and
s-wave may coexist. Conventional low-Tc superconductors are s-wave
superconductors. When d-wave Cooper-pairs are injected in a superconductor that
can sustain s-wave pairing, d-wave pairs are subject to transition to s-wave
pairs and energy is irradiated by means of gravitons. We show that in s-wave to
d-wave type superconductor (SDS) junctions in an equilibrium condition no net
gravitational wave energy is emitted, on the other hand under non equilibrium
conditions a net gravitational wave energy is emitted by the junction.
Experiments that show a gravitational interaction between inhomogeneous high-Tc
superconductors, under non-equilibrium conditions, and test objects may be
understood by accepting a possibility of emission of gravitational radiation
from SDS junctions.


title: Light scattering from mesoscopic objects in diffusive media

abstract:
  The diffuse intensity propagating in turbid media is sensitive to the
presence of any kind of object embedded in the medium, e.g. obstacles or
defects. The long-ranged effects of isolated objects can be described by a
stationary diffusion equation, the effect of any single object being
parametrized in terms of a multipole expansion. An absorbing object is chiefly
characterized by a negative charge, while the leading effect of a non-absorbing
object is due to its dipole moment. The associated intrinsic characteristics of
the object (capacitance $Q$ or effective radius $R_{\rm eff}$, polarizability
$P$) can be evaluated within the diffusion approximation for large enough
objects. The situation of mesoscopic objects, with a size comparable to the
mean free path, requires a more careful treatment, for which the appropriate
framework is radiative transfer theory. This formalism is worked out in detail
for spheres and cylinders of the following kinds: totally absorbing (black),
transparent, and totally reflecting.


title:
Electronic Structure, Correlation Effects and Physical Properties of d-
  and f-Metals and Their Compounds

abstract:
  The book includes all main physical properties of d- and f-transition-metal
systems and corresponding theoretical concepts. Especial attention is paid to
the theory of magnetism and transport phenomena. Some examples of
non-traditional questions which are treated in detail in the book: the
influence of density of states singularities on electron properties;
many-electron description of strong itinerant magnetism; mechanisms of magnetic
anisotropy; microscopic theory of anomalous transport phenomena in
ferromagnets. Besides considering classical problems of solid state physics as
applied to transition metals, modern developments in the theory of correlation
effects in d- and f-compounds are considered within many-electron models. The
book contains, where possible, a simple physical discussion. More difficult
questions are considered in Appendices.


title: KPZ Equation and Surface Growth Model

abstract:
  We consider the ultra-discrete Burgers equation. All variables of the
equation are discrete. We classify the equation into five regions in the
parameter space. We discuss behavior of solutions. Using this equation we
construct the deterministic surface growth models respectively. Furthermore we
introduce noise into the ultra-discrete Burgers equation. We present the
automata models of the KPZ equation. One model corresponds to the discrete
version of the ASEP and the other to the Kim-Kosterlitz model.


title: Self-Structuring of Granular Media under Internal Avalanches

abstract:
  We study the phenomenon of internal avalanching within the context of
recently proposed ``Tetris'' lattice models for granular media. We define a
recycling dynamics under which the system reaches a steady state which is
self-structured, i.e. it shows a complex interplay between textured internal
structures and critical avalanche behavior. Furthermore we develop a general
mean-field theory for this class of systems and discuss possible scenarios for
the breakdown of universality.


title: Internal avalanches in models of granular media

abstract:
  We study the phenomenon of internal avalanching within the context of
recently introduced lattice models of granular media. The avalanche is produced
by pulling out a grain at the base of the packing and studying how many grains
have to rearrange before the packing is once more stable. We find that the
avalanches are long-ranged, decaying as a power-law. We study the distriution
of avalanches as a function of the density of the packing and find that the
avalanche distribution is a very sensitive structural probe of the system.


title:
Effect of disorder on the temperature dependence of radiative lifetimes
  in V-groove quantum wires

abstract:
  We have studied the effect of disorder on the radiative properties of
semiconductor quantum wires by time-resolved photoluminescence spectroscopy.
The dependence of the radiative lifetimes is measured over a temperature range
extending from 8 to 150 K. At low temperatures we find that the measured
dependence does not conform to the theoretical prediction for recombination of
free excitons in a quasi one-dimensional system due to the major influence of
disorder. An extension of the theory is developped in order to take into
account exciton ionization and the contribution of free carriers. A mean
localization length for excitons is also estimated from the radiative lifetime
at 8 K.


title: Finite size scaling in three-dimensional bootstrap percolation

abstract:
  We consider the problem of bootstrap percolation on a three dimensional
lattice and we study its finite size scaling behavior. Bootstrap percolation is
an example of Cellular Automata defined on the $d$-dimensional lattice
$\{1,2,...,L\}^d$ in which each site can be empty or occupied by a single
particle; in the starting configuration each site is occupied with probability
$p$, occupied sites remain occupied for ever, while empty sites are occupied by
a particle if at least $\ell$ among their $2d$ nearest neighbor sites are
occupied. When $d$ is fixed, the most interesting case is the one $\ell=d$:
this is a sort of threshold, in the sense that the critical probability $p_c$
for the dynamics on the infinite lattice ${\Bbb Z}^d$ switches from zero to one
when this limit is crossed. Finite size effects in the three-dimensional case
are already known in the cases $\ell\le 2$: in this paper we discuss the case
$\ell=3$ and we show that the finite size scaling function for this problem is
of the form $f(L)={\mathrm{const}}/\ln\ln L$. We prove a conjecture proposed by
A.C.D. van Enter.


title: Computer simulations of defects in perovskite KNbO3 crystals

abstract:
  An ab initio LMTO approach and semi-empirical quantum chemical INDO method
have been used for supercell calculations of basic point defects - F-type
centers and hole polarons bound to cation vacancy - in partly covalent
perovskite KNbO3. We predict the existence of both one-site and two-site
(molecular) polarons with close absorption energies (~ 1 eV). The relevant
experimental data are discussed and interpreted.


title:
Evidence for crossover from a Bose-Einstein condensate to a BCS-like
  superconductor with doping in YBa2Cu3O(7-x) from quasiparticle relaxation
  dynamics experiments

abstract:
  Time resolved measurements of quasiparticle (QP) relaxation dynamics on the
femtosecond timescale are reported as a function of temperature and doping in
YBa2Cu3O(7-x) for (0.48 < x < 0.1). In the underdoped state (x > 0.15), there
is no evidence for any changes in the low-energy gap structure at Tc from
either photoinduced QP absorption or QP relaxation time data. In combination
with the sum rule, this implies the existence of pre-formed pairs up to a much
higher temperature T*. Near x = 0.1 a rapid cross-over is observed, to a state
where the QP recombination time diverges and the photoinduced QP density falls
to zero at Tc, indicating the existence of a temperature-dependent
superconducting gap which closes at Tc.


title: Pressure drag in linear and nonlinear quantum fluids

abstract:
  We study the flow of a weakly-interacting Bose-Einstein condensate around an
obstacle by numerical solution of the Gross-Pitaevskii equation. We observe
vortex emission and the formation of bow waves leading to pressure drag. We
compare the drag law with that of an ideal Bose gas, and show that interactions
reduce the drag force. This reduction can be explained in terms of a
`collisional screening' of the obstacle.


title:
Fundamental constraints for the mechanism of superconductivity in
  cuprates

abstract:
  Considerable progress has been made over the last decade in understanding the
phenomenological properties of the cuprate high-T$_{c}$ superconductors and in
producing well characterized high quality materials. Nevertheless, the pairing
mechanism itself remains controversial. We establish a criterion to test
theories for layered superconductors relying on a substantial interlayer
contribution. The criterion is based on the ratio of the interlayer
contribution to the total superfluid density, which is traced back to the
inverse squared effective mass anisotropy. The anisotropy can be measured
rather accurately by various experimental techniques. It turns out that models
relying on interlayer pairing cannot be considered as serious candidates for
the mechanism of superconductivity in cuprate superconductors.


title:
Magnetic Field induced Dimensional Crossover Phenomena in Cuprate
  Superconductors and their Implications

abstract:
  We discuss the occurrence of crossing points in the magnetization -
temperature $(m,T$) plane within the framework of critical phenomena. It is
shown that in a two-dimensional superconducting slab of thickness $d_{s}$
$m_{z}(\delta)$ versus temperature $T$ curves measured in different fields
$\mathbf{H} = H(0,\sin (\delta) ,\cos (\delta))$ will cross at the critical
temperature T_c of the slab. In contrast, in a 3D anisotropic bulk
superconductor the crossing point occurs in the plot $m_{z}(\delta)
/H_{z}^{1/2}$ versus $T$. The experimental facts that 2D crossing point
features have been observed in ceramics and in single crystals for $\mathbf{H}$
close to $\mathbf{H} = H(0,0,1)$, but not for $\mathbf{H} = H(0,1,0)$, is
explained in terms of an angle-dependent crossover field separating the regions
where 2D or 3D thermal fluctuations dominate. The measured 2D-crossing point
data are used to estimate one of the fundamental parameters of cuprate
superconductors, the minimum thickness of the slab $(d_{s})$, which remains
superconducting. Our estimates, based on experimental 2D-crossing point data
for single crystals, reveal that this length adopts material dependent values.
Therefore, experimental data for T_c and $\lambda_{\Vert}^{2}(T=0)$, plotted in
terms of T_c versus $1/\lambda_{\Vert}^{2}(T=0)$ will not tend to a straight
line with universal slope as the underdoped limit is approached. Implications
for magnetic torque measurements are also worked out.


title:
Interaction-Induced Enhancement of Spin-Orbit Coupling in
  Two-Dimensional Electronic System

abstract:
  We study theoretically the renormalization of the spin-orbit coupling
constant of two-dimensional electrons by electron-electron interactions. We
demonstrate that, similarly to the $g$ factor, the renormalization corresponds
to the enhancement, although the magnitude of the enhancement is weaker than
that for the $g$ factor. For high electron concentrations (small interaction
parameter $r_s$) the enhancement factor is evaluated analytically within the
static random phase approximation. For large $r_s\sim 10$ we use an approximate
expression for effective electron-electron interaction, which takes into
account the local field factor, and calculate the enhancement numerically. We
also study the interplay between the interaction-enhanced Zeeman splitting and
interaction-enhanced spin-orbit coupling.


title: Thermoelectric Properties of Anisotropic Systems

abstract:
  The effective transport coefficients and figure of merit ZT for anisotropic
systems are derived from a macroscopic formalism. The full tensorial structure
of the transport coefficients and the effect of the sample boundaries are
included. Induced transverse fields develop which can be larger than the
applied fields and which reduce the effective transport coefficients. A
microscopic model relevant for multi-valleyed materials is introduced which
utilizes the effective-mass and relaxation-time approximations. The thermopower
and Lorentz number are independent of the tensorial structure of the transport
coefficients in this case and are therefore isotropic. ZT is also isotropic for
vanishing lattice thermal conductivity \kappa_\ell. For non-vanishing but
sufficiently isotropic \kappa_\ell, ZT is maximal along the direction of
highest electrical conductivity \sigma. Numerical calculations suggest that
maximal ZT generally occurs along the principal direction with the largest
\sigma/\kappa_\ell. An explicit bound on ZT is derived. Several results for
specific systems are obtained: (1) Bulk n-type Bi_2Te_3 exhibits easily
observable induced transverse fields and anisotropic ZTs. (2) Increased
anisotropy in HgTe/ Hg_{1-x}Cd_xTe superlattices (SLs) is associated with
larger induced fields. (3) The valley degeneracy is split and the bulk masses
modified in isolated Bi_2Te_3 quantum wells, resulting in optimal ZTs for wells
grown along the trigonal direction. (4) Non-parabolic dispersion in SLs has
little effect on the thermopower at the carrier concentrations which maximize
ZT.


title: Kinky Behavior in Josephson Junctions

abstract:
  We analyze nonperturbatively the behavior of a Josephson junction in which
two BCS superconductors are coupled through an Anderson impurity. We recover
earlier perturbative results which found that a $\delta=\pi$ phase difference
is preferred when the impurity is singly occupied and the on-site Coulomb
interaction is large. We find a novel intermediate phase in which one of
$\delta=0$ and $\delta=\pi$ is stable while the other is metastable, with the
energy $E(\delta)$ having a kink somewhere in between. As a consequence of the
kink, the $I-V$ characteristics of the junction are modified at low voltages.


title: Hidden Integrability of a Kondo Impurity in an Unconventional Host

abstract:
  We study a spin-1/2 Kondo impurity coupled to an unconventional host in which
the density of band states vanishes either precisely at (``gapless'' systems)
or on some interval around the Fermi level (``gapped''systems). Despite an
essentially nonlinear band dispersion, the system is proven to exhibit hidden
integrability and is diagonalized exactly by the Bethe ansatz.


title:
Fluctuations in confined nematic liquid crystals in a regime of critical
  wetting

abstract:
  Within the macroscopic Landau-de Gennes approach, we examine the Gaussian
normal mode fluctuations of semi-infinite nematic liquid crystals in a regime
of critical wetting. It is argued that surface free-energy potentials that
strongly suppress the long-range nematic order favor the appearance of bound
biaxial nematic-director fluctuation modes, located in the domain occupied by
the thermodynamic phase wetting the wall. Instead, substrates enhancing the
orientational order promote the existence of uniaxial nematic-director local
excitations. Close to the phase coexistence temperature both types of local
exciations are strongly softened as compared to their bulk counterparts and
acquire characteristic cusplike low-energy spectra. These spectrum
peculiarities are directly connected to the critical behavior of the mean-field
interface position and can provide a valuable insight on the nature of surface
interactions and critical wetting phenomena in nematic liquid crystals.
Possible changes in the local director mode properties resulting from the
critical interface position fluctuations and order electricity effects are also
discussed.


title:
The mechanism of thickness selection in the Sadler-Gilmer model of
  polymer crystallization

abstract:
  Recent work on the mechanism of polymer crystallization has led to a proposal
for the mechanism of thickness selection which differs from those proposed by
the surface nucleation theory of Lauritzen and Hoffman and the entropic barrier
model of Sadler and Gilmer. This has motivated us to reexamine the model used
by Sadler and Gilmer. We again find a fixed-point attractor which describes the
dynamical convergence of the crystal thickness to a value just larger than the
minimum stable thickness, l_min. This convergence arises from the combined
effect of two constraints on the length of stems in a layer: it is unfavourable
for a stem to be shorter than l_min and for a stem to overhang the edge of the
previous layer. The relationship between this new mechanism and the explanation
given by Sadler and Gilmer in terms of an entropic barrier is discussed. We
also examine the behaviour of the Sadler-Gilmer model when an energetic
contribution from chain folds is included.


title: Propagating front in an excited granular layer

abstract:
  A partial monolayer of ~ 20000 uniform spherical steel beads, vibrated
vertically on a flat plate, shows remarkable ordering transitions and
cooperative behavior just below 1g maximum acceleration. We study the stability
of a quiescent disordered or ``amorphous'' state formed when the acceleration
is switched off in the excited ``gaseous'' state. The transition from the
amorphous state back to the gaseous state upon increasing the plate's
acceleration is generally subcritical: An external perturbation applied to one
bead initiates a propagating front that produces a rapid transition. We measure
the front velocity as a function of the applied acceleration. This phenomenon
is explained by a model based on a single vibrated particle with multiple
attractors that is perturbed by collisions. A simulation shows that a
sufficiently high rate of interparticle collisions can prevent trapping in the
attractor corresponding to the nonmoving ground state.


title: The dynamo effect - A dynamic renormalisation group approach

abstract:
  The Dynamo effect is used to describe the generation of magnetic fields in
astrophysical objects. However, no rigorous derivation of the dynamo equation
is available. We justify the form of the equation using an Operator Product
Expansion (OPE) of the relevant fields. We also calculate the coefficients of
the OPE series using a dynamic renormalisation group approach and discuss the
time evolution of the initial conditions on the initial seed magnetic field.


title: Canonical phase space approach to the noisy Burgers equation

abstract:
  Presenting a general phase approach to stochastic processes we analyze in
particular the Fokker-Planck equation for the noisy Burgers equation and
discuss the time dependent and stationary probability distributions. In one
dimension we derive the long-time skew distribution approaching the symmetric
stationary Gaussian distribution. In the short time regime we discuss
heuristically the nonlinear soliton contributions and derive an expression for
the distribution in accordance with the directed polymer-replica model and
asymmetric exclusion model results.


title:
Controlling Josephson transport by manipulation of Andreev levels in
  ballistic mesoscopic junctions

abstract:
  We discuss how to control dc Josephson current by influencing the structure
and nonequilibrium population of Andreev levels via external electrostatic
gates, external current injection and electromagnetic radiation. In particular
we will consider the "giant" Josephson current in "long" SIS tunnel junctions
and the regular and anomalous nonequilibrium Josephson currents in three
terminal SNS junctions. We will briefly discuss applications to the Josephson
field effect transistor (JOFET) and to the newly invented Josephson
interference transistor (JOINT).


title:
Phonon Mediated Transresistivity in a Double Layer Composite Fermion
  System

abstract:
  We consider the fractional drag in a double layer system of two-dimensional
electrons in the half-filled lowest Landau level. At sufficiently large
inter-layer separations the drag is dominated by exchange of acoustic phonons
and exhibits novel temperature and inter-layer distance dependences. At low
temperatures the phonon mediated drag is strongly enhanced with respect to the
case of zero magnetic field.


title:
Perturbative and non-perturbative parts of eigenstates and local
  spectral density of states: the Wigner band random matrix model

abstract:
  A generalization of Brillouin-Wigner perturbation theory is applied
numerically to the Wigner Band Random Matrix model. The perturbation theory
tells that a perturbed energy eigenstate can be divided into a perturbative
part and a non-perturbative part with the perturbative part expressed as a
perturbation expansion. Numerically it is found that such a division is
important in understanding many properties of both eigenstates and the
so-called local spectral density of states (LDOS). For the average shape of
eigenstates, its central part is found to be composed of its non-perturbative
part and a region of its perturbative part, which is close to the
non-perturbative part. A relationship between the average shape of eigenstates
and that of LDOS can be explained. Numerical results also show that the
transition for the average shape of LDOS from the Breit-Wigner form to the
semicircle form is related to a qualitative change in some properties of the
perturbation expansion of the perturbative parts of eigenstates. The transition
for the half-width of the LDOS from quadratic dependence to linear dependence
on the perturbation strength is accompanied by a transition of a similar form
for the average size of the non-perturbative parts of eigenstates. For both
transitions, the same critical perturbation strength $\lambda_b$ has been found
to play important roles. When perturbation strength is larger than $\lambda_b$,
the average shape of LDOS obeys an approximate scaling law.