title:
Finite-temperature phase transitions in $\nu=2$ bilayer quantum Hall
  systems

abstract:
  In this paper, the influence of an in-plane magnetic field B_\parallel on the
finite-temperature phase transitions in nu=2 bilayer quantum Hall systems are
examined. It is found that there can exist two types of finite-temperature
phase transitions. The first is the Kosterlitz-Thouless (KT) transitions, which
can have an unusual non-monotonic dependence on B_\parallel; the second type
originates from the crossing of energy levels and always increases with
B_\parallel. Based on these results, we point out that the threshold
temperature observed in the inelastic light scattering experiments cannot be
the KT transition temperature, because the latter shows a totally different
B_\parallel-dependence as compared with the experimental observation. Instead,
it should be the level-crossing temperature, which we found agrees with the
B_\parallel-dependence observed. Moreover, combining the knowledge of these two
transition temperatures, a complete finite-temperature phase diagram is
presented.


title:
Phase transitions in two dimensions - the case of Sn adsorbed on Ge(111)
  surfaces

abstract:
  Accurate atomic coordinates of the room-temperature (root3xroot3)R30degree
and low-temperature (3x3) phases of 1/3 ML Sn on Ge(111) have been established
by grazing-incidence x-ray diffraction with synchrotron radiation. The Sn atoms
are located solely at T4-sites in the (root3xroot3)R30degree structure. In the
low temperature phase one of the three Sn atoms per (3x3) unit cell is
displaced outwards by 0.26 +/- 0.04 A relative to the other two. This
displacement is accompanied by an increase in the first to second double-layer
spacing in the Ge substrate.


title:
Far infrared study of the two dimensional dimer spin system
  SrCu_2(BO_3)_2

abstract:
  Using far-infrared spectroscopy in magnetic fields up to 12T we have studied
a two-dimensional dimer spin gap system SrCu_2(BO_3)_2. We found several
infrared active modes in the dimerized state (below 10K) in the frequency range
from 3 to 100cm^-1. The measured splitting from the ground state to the excited
triplet M_S=0 sublevel is Delta_1=24.2cm^-1 and the other two triplet state
sublevels in zero magnetic field are 1.4cm^-1 below and above the M_S=0
sublevel. Another multiplet is at Delta_2=37.6cm^-1 from the ground state. A
strong electric dipole active transition polarized in the (ab)-plane is
activated in the dimer spin system below 15K at 52cm^-1.


title: Critical statistics in a power-law random banded matrix ensemble

abstract:
  We investigate the statistical properties of the eigenvalues and eigenvectors
in a random matrix ensemble with $H_{ij}\sim |i-j|^{-\mu}$. It is known that
this model shows a localization-delocalization transition (LDT) as a function
of the parameter $\mu$. The model is critical at $\mu=1$ and the eigenstates
are multifractals. Based on numerical simulations we demonstrate that the
spectral statistics at criticality differs from semi-Poisson statistics which
is expected to be a general feature of systems exhibiting a LDT or `weak
chaos'.

categories: cond-mat.dis-nn chao-dyn cond-mat.mes-hall nlin.CD


title:
Structure determination of the indium-induced Si(111)-(4x1)
  reconstruction by surface x-ray diffraction

abstract:
  A detailed structural model for the indium-induced Si(111)-(4x1) surface
reconstruction has been determined by analyzing an extensive set of
x-ray-diffraction data recorded with monochromatic ($\hbar\omega$ = 9.1 keV)
synchrotron radiation. The reconstruction is quasi-one-dimensional. The main
features in the structure are chains of silicon atoms alternating with zigzag
chains of indium atoms on top of an essentially unperturbed silicon lattice.
The indium coverage corresponds to one monolayer. The structural model
consistently explains all previously published experimental data.


title: Temporally disordered granular flow: A model of landslides

abstract:
  We propose and study numerically a stochastic cellular automaton model for
the dynamics of granular materials with temporal disorder representing random
variation of the diffusion probability $1-\mu (t)$ around threshold value
$1-\mu_0$ during the course of an avalanche. Combined with the slope threshold
dynamics, the temporal disorder yields a series of secondary instabilities,
resembling those in realistic granular slides. When the parameter $\mu_0$ is
lower than the critical value $\mu_{0}^\star \approx 0.4$, the dynamics is
dominated by occasional huge sandslides. For the range of values $\mu_{0}^\star
\le \mu_0 < 1$ the critical steady states occur, which are characterized by
multifractal scaling properties of the slide distributions and continuously
varying critical exponents $\tau_X(\mu_0)$. The mass distribution exponent for
$\mu_0\approx 0.45$ is in agreement with the reported value that characterizes
Himalayan sandslides. At $\mu_{0}= \mu_{0}^\star$ the exponents governing
distributions of large relaxation events reach numerical values which are close
to those of parity-conserving universality class, whereas for small avalanches
they are close to the mean-field exponents.


title: Generalized bit cumulants for chaotic systems: Numerical results

abstract:
  We propose generalized bit cumulants for chaotic systems, within nonextensive
thermodynamic approach. In this work, we apply the first and second generalized
cumulants to one dimensional logistic and logistic-like family of maps.


title:
Thermodynamic chaos and infinitely many critical exponents in the
  Baxter-Wu model

abstract:
  The mechanisms leading to thermodynamic chaos in the Baxter-Wu model is
considered. We compare the Baxter-Wu model with triangular antiferromagnets and
discuss the difficulties related to the modeling of thermodynamic chaos by
disordered models. We also discuss how to overcome the problem of infinitely
many order parameters. Then we consider the Baxter-Wu model in a complex
magnetic field and show the existence of infinitely many critical exponents in
this model.


title: Phase diagram of S=1/2 XXZ chain with NNN interaction

abstract:
  We study the ground state properties of one-dimensional XXZ model with
next-nearest neighbor coupling alpha and anisotropy Delta. We find the direct
transition between the ferromagnetic phase and the spontaneously dimerized
phase. This is surprising, because the ferromagnetic phase is classical,
whereas the dimer phase is a purely quantum and nonmagnetic phase. We also
discuss the effect of bond alternation which arises in realistic systems due to
lattice distortion. Our results mean that the direct transition between the
ferromagnetic and spin-Peierls phase occur.


title: Stars and statistical physics: a teaching experience

abstract:
  The physics of stars, their workings and their evolution, is a goldmine of
problems in statistical mechanics and thermodynamics. We discuss many examples
that illustrate the possibility of deepening student's knowledge of statistical
mechanics by an introductory study of stars. The matter constituting the
various stellar objects provides examples of equations of state for classical
or quantal and relativistic or non-relativistic gases. Maximum entropy can be
used to characterize thermodynamic and gravitational equilibrium which
determines the structure of stars and predicts their instability above a
certain mass. Contraction accompanying radiation induces either heating or
cooling, which explains the formation of stars above a minimum mass. The
characteristics of the emitted light are understood from black-body radiation
and more precisely from the Boltzmann-Lorentz kinetic equation for photons. The
luminosity is governed by the transport of heat by photons from the center to
the surface. Heat production by thermonuclear fusion is determined by
microscopic balance equations. The stability of the steady state of stars is
controlled by the interplay of thermodynamics and gravitation.


title: Quantum Phase Transitions in the p-SiGe system

abstract:
  The rich variety of phase transitions observed in the strained p-SiGe system
are considered and compared. It is shown that the integer quantum Hall effect
transitions, the Hall insulating transition and the re-entrant transition into
an insulating phase near filling factor 3/2 are all very similar and good
examples of quantum critical phase transitions. The B=0 metal insulator
transition also shows many similarities to these transitions but requires the
inclusion of an extra impurity scattering term to fully explain the data.


title: Why does a metal-superconductor junction have a resistance?

abstract:
  This is a tutorial article based on a lecture delivered in June 1999 at the
NATO Advanced Study Institute in Ankara. The phenomenon of Andreev reflection
is introduced as the electronic analogue of optical phase-conjugation. In the
optical problem, a disordered medium backed by a phase-conjugating mirror can
become completely transparent. Yet, a disordered metal connected to a
superconductor has the same resistance as in the normal state. The resolution
of this paradox teaches us a fundamental difference between phase conjugation
of light and electrons.


title:
The energy spectrum of complex periodic potentials of the Kronig-Penney
  type

abstract:
  We consider a complex periodic PT-symmetric potential of the Kronig-Penney
type, in order to elucidate the peculiar properties found by Bender et al. for
potentials of the form $V=i(\sin x)^{2N+1}$, and in particular the absence of
anti-periodic solutions. In this model we show explicitly why these solutions
disappear as soon as $V^*(x)\neq V(x)$, and spell out the consequences for the
form of the dispersion relation.


title: Fractional excitations in the Luttinger liquid

abstract:
  We reconsider the spectrum of the Luttinger liquid (LL) usually understood in
terms of phonons (density fluctuations), and within the context of bosonization
we give an alternative representation in terms of fractional states. This
allows to make contact with Bethe Ansatz which predicts similar fractional
states. As an example we study the spinon operator in the absence of spin
rotational invariance and derive it from first principles: we find that it is
not a semion in general; a trial Jastrow wavefunction is also given for that
spinon state. Our construction of the new spectroscopy based on fractional
states leads to several new physical insights: in the low-energy limit, we find
that the $S_{z}=0$ continuum of gapless spin chains is due to pairs of
fractional quasiparticle-quasihole states which are the 1D counterpart of the
Laughlin FQHE quasiparticles. The holon operator for the Luttinger liquid with
spin is also derived. In the presence of a magnetic field, spin-charge
separation is not realized any longer in a LL: the holon and the spinon are
then replaced by new fractional states which we are able to describe.


title:
One-dimensional electron gas interacting with a Heisenberg spin-1/2
  chain

abstract:
  We analyse a model of a one-dimensional electron gas interacting with an
antiferromagnetic Heisenberg spin-1/2 chain via the spin exchange interactions.
Using a solution at a special limit, we characterize the gapless modes of the
spin gap fixed point at weak coupling $ J_{K}\ll J_{H},E_{F} $. we show that
the only gapless pairing mode with divergent susceptibility is a composite
odd-parity odd-frequency singlet pairing order parameter, while the ordinary
BCS even-parity singlet pairing mode is incoherent. For 2-leg ladder systems,
we note that it is possible to have a range of doping where the chemical
potential cuts only the anti-bonding band while the bonding band remains
half-filled. We propose that, in such a state, the 2-leg ladder is effectively
realizing the one-dimensional Kondo-Heisenberg model.


title:
Peak Effect in Ca$_3$Rh$_4$Sn$_{13}$ : Vortex Phase Diagram and
  Evidences for Stepwise Amorphization of Flux Line Lattice

abstract:
  The Peak Effect (PE) regime in a single crystal of Ca$_3$Rh$_4$Sn$_{13}$ has
been investigated in detail via ac susceptibility as well as dc magnetization
measurements. The PE region comprises two discontinuous first order like
transitions, located near its onset and peak positions and reflects a stepwise
fracturing of the flux line lattice (FLL). This scenario can be construed as an
alternative to a scheme involving the appearance of Generalized
Fulde-Ferrel-Larkin-Ovchinnikov (GFFLO) state as considered in the case of PE
phenomenon in isostructural Yb$_3$Rh$_4$Sn$_{13}$ system. The procedure of
thermal cycling across the onset position of PE produces an open hysteresis
loop, which is consistent with the notion of the fracturing of the FLL. A study
of thermomagnetic history dependence of different metastable vortex states
shows the path dependence in the critical current density J_c(H,T) over a large
part of the (H,T) parameter space. The path dependence in J_c ceases above the
peak position of the PE, suggesting a complete amorphization and the loss of
long range correlations of the FLL at (T_p, H_p) line. A plausible vortex phase
diagram has been illustrated for Ca$_3$Rh$_4$Sn$_{13}$n and phases like an
elastic solid, a plastic solid, a pinned amorphous and an unpinned amorphous
states have been identified.


title: Finite-temperature properties of the two-orbital Anderson model

abstract:
  The metallic phase of the two-orbital Anderson lattice is study in the limit
of infinite spatial dimensions, where a second order perturbation treatment is
used to solve the single-site problem. Using this approximation, in the Kondo
regime, we find that the finite temperature properties of the conduction
electrons exhibit the same behaviour as observed in the metallic phase of the
two-channel Kondo lattice. Possible connections between these two models are
discussed.


title: Disorder Averaging and Finite Size Scaling

abstract:
  We propose a new picture of the renormalization group (RG) approach in the
presence of disorder, which considers the RG trajectories of each random sample
(realization) separately instead of the usual renormalization of the averaged
free energy. The main consequence of the theory is that the average over
randomness has to be taken after finding the critical point of each
realization. To demonstrate these concepts, we study the finite-size scaling
properties of the two-dimensional random-bond Ising model. We find that most of
the previously observed finite-size corrections are due to the sample-to-sample
fluctuation of the critical temperature and scaling is more adequate in terms
of the new scaling variables.


title:
Peierls transition with acoustic phonons and twist deformation in carbon
  nanotubes

abstract:
  We consider the Peierls instability due to the interaction of electrons with
both acoustic and optical phonons. We suggest that such a transition takes
place in carbon nanotubes with small radius. The topological excitations and
the temperature dependence of the conductivity resulting from the
electron-lattice interactions are considered.


title: Time distribution and loss of scaling in granular flow

abstract:
  Two cellular automata models with directed mass flow and internal time scales
are studied by numerical simulations. Relaxation rules are a combination of
probabilistic critical height (probability of toppling $p$) and deterministic
critical slope processes with internal correlation time $t_c$ equal to the
avalanche lifetime, in Model A, and $t_c\equiv 1$, in Model B. In both cases
nonuniversal scaling properties of avalanche distributions are found for $p\ge
p^\star $, where $p^\star$ is related to directed percolation threshold in
$d=3$. Distributions of avalanche durations for $p\ge p^\star $ are studied in
detail, exhibiting multifractal scaling behavior in model A, and finite size
scaling behavior in model B, and scaling exponents are determined as a function
of $p$. At $p=p^\star$ a phase transition to noncritical steady state occurs.
Due to difference in the relaxation mechanisms, avalanche statistics at
$p^\star$ approaches the parity conserving universality class in Model A, and
the mean-field universality class in Model B. We also estimate roughness
exponent at the transition.


title: Statistical Properties of Statistical Ensembles of Stock Returns

abstract:
  We select n stocks traded in the New York Stock Exchange and we form a
statistical ensemble of daily stock returns for each of the k trading days of
our database from the stock price time series. We analyze each ensemble of
stock returns by extracting its first four central moments. We observe that
these moments are fluctuating in time and are stochastic processes themselves.
We characterize the statistical properties of central moments by investigating
their probability density function and temporal correlation properties.


title: Pauli and orbital effects of magnetic field on charge density waves

abstract:
  Taking into account both Pauli and orbital effects of external magnetic field
we compute the mean field phase diagram for charge density waves in
quasi-one-dimensional electronic systems. The magnetic field can cause
transitions to CDW states with two types of the shifts of wave vector from its
zero-field value. It can also stabilize the field-induced charge density wave.
Furthermore, the critical temperature shows peaks at a new kind of magic
angles.


title: Electron locking in semiconductor superlattices

abstract:
  We describe a novel state of electrons and phonons arising in semiconductor
superlattices (SSL) due to strong electron-phonon interactions. These states
are characterized by a localization of phonons and a self-trapping or locking
of electrons in one or several quantum wells due to an additional,
deformational potential arising around these locking wells in SSL. The effect
is enhanced in a longitudinal magnetic field.
  Using the tight-binding and adiabatic approximations the whole energy
spectrum of the self-trapped states is found and accurate, analytic expressions
are included for strong electron-phonon coupling. Finally, we discuss possible
experiments which may detect these predicted self-trapped states.


title: A new variational approach for the Holstein Molecular Crystal Model

abstract:
  A new variational technique is developed to investigate the polaronic
features of the Holstein Molecular Crystal Model. It is based on a linear
superposition of Bloch states that describe large and small polaron wave
functions. It is shown that this method provides a very good description of the
regime characterized by intermediate values of the electron-phonon coupling
constant (the so-called intermediate polaron) for any value of the adiabatic
parameter $\omega_0/t$. The polaron ground state energy in one and two
dimensions is calculated and successfully compared with the best estimates
available providing a clear physical interpretation of the intermediate
polaron. The band structure, the spectral weight of the ground state and the
lattice displacement associated to the polaron are also calculated and
discussed. The new method has the advantage to require a very little
computational effort.


title:
Emergence of macroscopic temperatures in systems that are not
  thermodynamical microscopically: towards a thermodynamical description of
  slow granular rheology

abstract:
  A scenario for systems with slow dynamics is characterised by stating that
there are several temperatures coexisting in the sample, with a single
temperature shared by all observables at each (widely separate) time-scale.
  In preparation for the study of granular rheology, we show within this
framework that glassy systems with driving and friction that are generic and do
not correspond to a thermal bath --- and whose microscopic `fast' motion is
hence not thermal --- have a well-defined macroscopic temperature associated to
the slow degrees of freedom.
  This temperature is what a thermometer coupled to the system will measure if
tuned to respond to low frequencies, and since it can be related to the number
of stationary configurations, it is the formalisation of Edwards'
`compactivity' ideas.


title: Melting in two dimensions with particle conservation

abstract: This paper has been withdrawn.


title: Soliton Solutions of Integrable Hierarchies and Coulomb Plasmas

abstract:
  Some direct relations between soliton solutions of integrable hierarchies and
thermodynamical quantities of the Coulomb plasmas on the plane are revealed. We
find that certain soliton solutions of the Kadomtsev-Petviashvili (KP) and
B-type KP (BKP) hierarchies describe two-dimensional one or two component
plasmas at special boundary conditions and fixed temperatures. It is shown that
different reductions of integrable hierarchies describe one (two) component
plasmas or dipole gases on one-dimensional submanifolds embedded in the
two-dimensional space. We demonstrate application of the methods of soliton
theory to statistical mechanics of such systems.

categories: cond-mat.stat-mech cond-mat.str-el hep-th nlin.SI solv-int


title:
The Extension of Rod-Coil Multiblock Copolymers and the Effect of the
  Helix-Coil Transition

abstract:
  The extension elasticity of rod-coil mutliblock copolymers is analyzed for
two experimentally accessible situations. In the quenched case, when the
architecture is fixed by the synthesis, the force law is distinguished by a
sharp change in the slope. In the annealed case, where interconversion between
rod and coil states is possible, the resulting force law is sigmoid with a
pronounced plateau. This last case is realized, for example, when
homopolypeptides capable of undergoing a helix-coil transition are extended
from a coil state. Both scenarios are relevant to the analysis and design of
experiments involving single molecule mechanical measurements of biopolymers
and synthetic macromolecules.


title: Comments on Extended t-J Models, Nodal Liquids and Supersymmetry

abstract:
  In the context of extended t-J models, with intersite Coulomb interactions,
nodal liquids are discussed. We use the spin-charge separation ansatz as
applied to the nodes of a d-wave superconducting gap. Such a situation may be
of relevance to the physics of high-temperature superconductivity. We point out
the possibility that at certain points of the parameter space supersymmetric
points may occur, characterized by dynamical supersymmetries between the spinon
and holon degrees of freedom, which are quite different from the symmetries in
conventional supersymmetric t-J models. Such symmetries pertain to the
continuum effective field theory of the nodal liquid, and one's hope is that
the ancestor lattice model may differ from the continuum theory only by
renormalization-group irrelevant operators in the infrared. We give plausible
arguments that nodal liquids at such supersymmetric points are characterized by
superconductivity of Kosterlitz-Thouless type.


title: Electronic Correlations in Manganites

abstract:
  The influence of local electronic correlations on the properties of colossal
magnetoresistance manganites is investigated. To this end, a ferromagnetic
two-band Kondo lattice model is supplemented with the local Coulomb repulsion
missing in this model, and is analyzed within dynamical mean-field theory.
Results for the spectral function, optical conductivity, and the
paramagnetic-to-ferromagnetic phase transition show that electronic
correlations have drastic effects and may explain some experimental
observations.


title:
Wall bounded turbulent shear flow: Analytic result for an universal
  amplitude

abstract:
  In the turbulent boundary layer above a flat plate, the velocity profile is
known to have the form v=v_0[(1/\kappa) ln z + constant]. The distance from the
wall in dimensionless units is z and v_0 is an uniquely defined velocity scale.
The number \kappa is universal and measurements over several decades have shown
that it is nearly 0.42. We use a randomly stirred model of turbulence to derive
the above law and find \kappa=\sqrt{\frac{108} {125 \pi}} \simeq 0.52.


title:
Iordanskii Force and the Gravitational Aharonov-Bohm effect for a Moving
  Vortex

abstract:
  I discuss the scattering of phonons by a vortex moving with respect to a
superfluid condensate. This allows us to test the compatibility of the
scattering-theory derivation of the Iordanskii force with the galilean
invariance of the underlying fluid dynamics. In order to obtain the correct
result we must retain $O(v_s^2)$ terms in the sound-wave equation, and this
reinforces the interpretation, due to Volovik, of the Iordanskii force as an
analogue of the gravitational Bohm-Aharonov effect.


title: Thermodynamic Signature of a Two-Dimensional Metal-Insulator Transition

abstract:
  We present a study of the compressibility, K, of a two-dimensional hole
system which exhibits a metal-insulator phase transition at zero magnetic
field. It has been observed that dK/dp changes sign at the critical density for
the metal-insulator transition. Measurements also indicate that the insulating
phase is incompressible for all values of B. Finally, we show how the phase
transition evolves as the magnetic field is varied and construct a phase
diagram in the density-magnetic field plane for this system.


title: Acoustic Energy and Momentum in a Moving Medium

abstract:
  By exploiting the mathematical analogy between the propagation of sound in a
non-homogeneous potential flow and the propagation of a scalar field in a
background gravitational field, various wave ``energy'' and wave ``momentum''
conservation laws are established in a systematic manner. In particular the
acoustic energy conservation law due to Blokhintsev appears as the result of
the conservation of a mixed co- and contravariant energy-momentum tensor, while
the exchange of relative energy between the wave and the mean flow mediated by
the radiation stress tensor, first noted by Longuet-Higgins and Stewart in the
context of ocean waves, appears as the covariant conservation of the doubly
contravariant form of the same energy-momentum tensor.


title: Spin triplet superconductivity with line nodes in Sr2RuO4

abstract:
  Several possible odd-parity states are listed up group-theoretically and
examined in light of recent experiments on Sr$_2$RuO$_4$. Those include some of
the $f$-wave pairing states, ${\mib d}({\mib k})\propto{\hat{\mib z}}
k_xk_y(k_x + {\rm i}k_y)$ and ${\hat{\mib z}} (k_x^2-k_y^2)(k_x + {\rm i}k_y)$
and other ${\hat{\mib z}} (k_x + {\rm i}k_y)\cos ck_z$ ($c$ is the $c$-axis
lattice constant) as most plausible candidates. These are time-reversal
symmetry broken states and have line nodes running either vertically (the
former two) or horizontally (the latter), consistent with experiments.
Characterizations of these states and other possibilities are given.


title:
Construction of size-consistent effective Hamiltonians for systems with
  arbitrary Hilbert space

abstract:
  Effective Hamiltonians are usually constructed by using canonical
transformations or projection techniques. In contrast to this, we present a
method for systems with arbitrary Hilbert space based on the introduction of
cumulants. Cumulants guarantee size consistency, a property that is not always
evident in other treatments. As a nontrivial example of use the derived method
is applied to the strong-coupling limit of the half-filled Hubbard model on a
general lattice in arbitrary spatial dimension for which the fourth-order
expansion in t/U of the effective Hamiltonian is derived.


title:
Theory for the coupling between longitudinal phonons and intrinsic
  Josephson oscillations in layered superconductors

abstract:
  In this publication a microscopic theory for the coupling of intrinsic
Josephson oscillations in layered superconductors with longitudinal
c-axis-phonons is developed. It is shown that the influence of lattice
vibrations on the c-axis transport can be fully described by introducing an
effective longitudinal dielectric function. Resonances in the
I-V-characteristic appear at van Hove singularities of both acoustical and
optical longitudinal phonon branches. This provides a natural explanation of
the recently discovered subgap structures in the I-V-characteristic of highly
anisotropic cuprate superconductors. The effect of the phonon dispersion on the
damping of these resonances and the coupling of Josephson oscillations in
different resistive junctions due to phonons are discussed in detail.


title:
Comment on "Photonic Band Gaps: Noncommuting Limits and the 'Acoustic
  Band'"

abstract:
  This Comment concerns the effective dielectric constant for 2D photonic
crystal. We show that even in the limit of very high dielectric constant of
photonic crystal's "atoms", static and quasistatic approaches give the same
results, unlike it was claimed by Nicorovichi et al, PRL, 75, 1507 (1995).


title:
Quantum dynamics of two-dimensional vortex pairs with arbitrary total
  vorticity

abstract:
  Quantum dynamics of a vortex pair is investigated by considering the pair
Hamiltonian within various, unequivalent algebraic frameworks. First the vortex
pair spectrum is constructed in the standard contest of the e(2)-like dynamical
symmetry and its degeneracy is thoroughly examined. Then the Berry phase
phenomenon is studied through an su(1,1) realization of the pair Hamiltonian
when its parameters are assumed to be time-dependent, whereas the Feynman-
Onsager quantization conditions are recovered by means of symmetry arguments
within a third approach based on a magneticlike description of the vortex pair.
Finally, it is shown how recasting the dynamical algebra in terms of
two-particle realization of both su(2) and su(1,1) provides the correct
approach for the quantization of the model Hamiltonian accounting for the pair
scattering from a disklike obstacle.


title: Phase Transition in Perovskite Manganites with Orbital Degree of Freedom

abstract:
  Roles of orbital degree of freedom of Mn ions in phase transition as a
function of temperature and hole concentration in perovskite manganites are
studied. It is shown that the orbital order-disorder transition is of the first
order in the wide region of hole concentration and the N$\rm \acute{e}$el
temperature for the anisotropic spin ordering, such as the layer-type
antiferromagnetic one, is lower than the orbital ordering temperature due to
the anisotropy in the orbital space. The calculated results of the temperature
dependence of the spin and orbital order parameters explain a variety of the
experiments observed in manganites.


title: Deterministic Equations of Motion and Dynamic Critical Phenomena

abstract:
  Taking the two-dimensional $\phi^4$ theory as an example, we numerically
solve the deterministic equations of motion with random initial states.
Short-time behavior of the solutions is systematically investigated. Assuming
that the solutions generate a microcanonical ensemble of the system, we
demonstrate that the second order phase transition point can be determined
already from the short-time dynamic behavior. Initial increase of the
magnetization and critical slowing down are observed. The dynamic critical
exponent z, the new exponent $\theta$ and the static exponents $\beta$ and
$\nu$ are estimated. Interestingly, the deterministic dynamics with random
initial states is in a same dynamic universality class of Monte Carlo dynamics.


title: Microscopic Deterministic Dynamics and Persistence Exponent

abstract:
  Numerically we solve the microscopic deterministic equations of motion with
random initial states for the two-dimensional $\phi^4$ theory. Scaling behavior
of the persistence probability at criticality is systematically investigated
and the persistence exponent is estimated.


title: Deterministic Equations of Motion and Phase Ordering Dynamics

abstract:
  We numerically solve microscopic deterministic equations of motion for the 2D
$\phi^4$ theory with random initial states. Phase ordering dynamics is
investigated. Dynamic scaling is found and it is dominated by a fixed point
corresponding to the minimum energy of random initial states.


title: The Critical Exponent $\theta'$ in Spin Glasses

abstract:
  Short-time dynamic scaling behavior of the 3D $\pm J$ Ising spin glass is
studied by Monte Carlo methods. Starting the replicas with independent initial
configurations with a small pseudo magnetization, the dynamic evolution of the
overlap q(t) between two replicas is measured. The initial increase of the
overlap q(t) is observed and the corresponding exponent $\theta'$ is obtained.
  From the scaling relation $\lambda =d/z-\theta'$, the dynamic exponent z is
estimated.


title:
Dielectric Response Near the Density-Driven Mott Transition in infinite
  dimensions

abstract:
  We study the dielectric response of correlated systems which undergo a Mott
transition as a function of band filling within the dynamical mean field
framework. We compute the dielectric figure of merit (DFOM), which is a measure
of dielectric efficiency and an important number for potential device
applications. It is suggested how the DFOM can be optimized in real transition
metal oxides. The structures seen in the computed Faraday rotation are
explained on the basis of the underlying local spectral density of the
$d=\infty$ Hubbard model.


title: Quantum Stochastic Absorption

abstract:
  We report a detailed and systematic study of wave propagation through a
stochastic absorbing random medium. Stochastic absorption is modeled by
introducing an attenuation constant per unit length $\alpha$ in the free
propagation region of the one-dimensional disordered chain of delta function
scatterers. The average value of the logarithm of transmission coefficient
decreases linearly with the length of the sample. The localization length is
given by $\xi ~ = ~ \xi_w \xi_\alpha / (\xi_w + \xi_\alpha)$, where $\xi_w$ and
$\xi_\alpha$ are the localization lengths in the presence of only disorder and
of only absorption respectively. Absorption does not introduce any additional
reflection in the limit of large $\alpha$, i.e., reflection shows a monotonic
decrease with $\alpha$ and tends to zero in the limit of $\alpha\to\infty$, in
contrast to the behavior observed in case of coherent absorption. The
stationary distribution of reflection coefficient agrees well with the
analytical results obtained within random phase approximation (RPA) in a larger
parameter space. We also emphasize the major differences between the results of
stochastic and coherent absorption.


title: Strain in heteroepitaxial growth

abstract:
  We use atomistic simulations with an empirical potential (EAM) to study the
elastic effects of heteroepitaxial islands on adatom diffusion. We measure the
diffusion barrier on pure stressed substrate and near a misfit island, as well
as the detachment barrier from islands of different size.


title:
Absence of a Zero Temperature Vortex Solid Phase in Strongly Disordered
  Superconducting Bi Films

abstract:
  We present low temperature measurements of the resistance in magnetic field
of superconducting ultrathin amorphous Bi films with normal state sheet
resistances, $R_N$, near the resistance quantum, $R_Q={\hbar\over {e^2}}$. For
$R_N<R_Q$, the tails of the resistive transitions show the thermally activated
flux flow signature characteristic of defect motion in a vortex solid with a
finite correlation length. When $R_N$ exceeds $R_Q$, the tails become
non-activated. We conclude that in films where $R_N>R_Q$ there is no vortex
solid and, hence, no zero resistance state in magnetic field. We describe how
disorder induced quantum and/or mesoscopic fluctuations can eliminate the
vortex solid and also discuss implications for the magnetic-field-tuned
superconductor-insulator transition.


title: Mechanisms for slow strengthening in granular materials

abstract:
  Several mechanisms cause a granular material to strengthen over time at low
applied stress. The strength is determined from the maximum frictional force
F_max experienced by a shearing plate in contact with wet or dry granular
material after the layer has been at rest for a waiting time \tau. The layer
strength increases roughly logarithmically with \tau -only- if a shear stress
is applied during the waiting time. The mechanisms of strengthening are
investigated by sensitive displacement measurements and by imaging of particle
motion in the shear zone. Granular matter can strengthen due to a slow shift in
the particle arrangement under shear stress. Humidity also leads to
strengthening, but is found not to be its sole cause. In addition to these time
dependent effects, the static friction coefficient can also be increased by
compaction of the granular material under some circumstances, and by cycling of
the applied shear stress.


title:
Evidence for exhaustion in the conductivity of the infinite-dimensional
  periodic Anderson model

abstract:
  Monte Carlo-maximum entropy calculations of the conductivity of the
infinite-dimensional periodic Anderson model are presented. We show that the
optical conductivity displays anomalies associated with the exhaustion of
conduction-band states near the Fermi energy including a Drude weight which
rises with temperature, with weight transferred from a temperature and
doping-dependent mid-IR peak and a low-frequency incoherent contribution. Both
the Drude and mid-IR peaks persist up to very high temperatures. The
resistivity displays a non-universal peak and two other regions associated with
impurity-like physics at high temperatures and Fermi Liquid formation at low T.


title: Magnetization-controlled spin transport in DyAs/GaAs layers

abstract:
  Electrical transport properties of DyAs epitaxial layers grown on GaAs have
been investigated at various temperatures and magnetic fields up to 12T. The
measured longitudinal resistances show two distinct peaks at fields around 0.2
and 2.5T which are believed to be related to the strong spin-disorder
scattering occurring at the phase transition boundaries induced by external
magnetic field. An empirical magnetic phase diagram is deduced from the
temperature dependent experiment, and the anisotropic transport properties are
also presented for various magnetic field directions with respect to the
current flow.


title:
Interplay between Structure and Electronic Properties in Organic
  Conductors

abstract:
  It is known that the ground states of organic conductors have a diversity
reflecting the spatial arrangement of the constituent molecules within the unit
cell. A systematic theoretical search for the unifying view behind such
possible ground states has been made based on the Hartree-Fock mean field
approximation not only to the on-site but also to intersite Coulomb
interactions with special emphasis on the families of typical organic
conductors (TMTCF)$_2X$ and (ET)$_2X$.


title: Hydrodynamic approximation for the nonlinear response of a metal surface

abstract:
  We present semi-classical and quantized hydrodynamic models to obtain the
quadratic electronic response of a plane-bounded electron gas. Explicit
expressions for the dynamic image potential experienced by charged particles
moving near a jellium surface are derived, up to third order in the projectile
charge. These expressions are employed to compute the image potential at all
distances outside the surface. Though nonlinear corrections are found to be
more important far inside the solid than outside, our results indicate that the
nonlinear image potential is enhanced with respect to the linear image
potential by a factor that is for Al as large as $\sim 1.15$ near the surface
in the case of a stationary particle ($v\to 0$) with positive unit charge $e$.


title:
The watching operators method in the theory of Frenkel exciton. Novel
  criterion of localization and its exact calculation for the non diagonal
  disordered 1D chain's zero-state

abstract:
  A method is proposed for manipulating with diagrammatic expansion of Green's
function of Frenkel's exciton random walks on the perfect lattice. The method
allows one to select diagrams, to supply diagrams with factors containing
information about the number of sites the diagram has passed through, etc.
Simple problems related to the defect lattices are considered using the
proposed method. The new criterion of localization of Frenkel exciton - the
number of sites covered by the wave function - is established. The number of
sites covered by the zero state of 1D non-diagonally disordered chain is
studied. It is shown that this problem can be solved by calculating the random
walks Green's function with modified diagrammatic expansion. By means of the
developed method an exact analitical expression for the number of sites covered
by the zero-state is obtained and zero-state is shown to be localized.


title: Modified Renormalization Strategy for Sandpile Models

abstract:
  Following the Renormalization Group scheme recently developed by Pietronero
{\it et al}, we introduce a simplifying strategy for the renormalization of the
relaxation dynamics of sandpile models. In our scheme, five sub-cells at a
generic scale $b$ form the renormalized cell at the next larger scale. Now the
fixed point has a unique nonzero dynamical component that allows for a great
simplification in the computation of the critical exponent $z$. The values
obtained are in good agreement with both numerical and theoretical results
previously reported.


title:
Shifting a Quantum Wire through a Disordered Crystal: Observation of
  Conductance Fluctuations in Real Space

abstract:
  A quantum wire is spatially displaced by suitable electric fields with
respect to the scatterers inside a semiconductor crystal. As a function of the
wire position, the low-temperature resistance shows reproducible fluctuations.
Their characteristic temperature scale is a few hundred millikelvin, indicating
a phase-coherent effect. Each fluctuation corresponds to a single scatterer
entering or leaving the wire. This way, scattering centers can be counted one
by one.


title:
Charge inversion in DNA-amphiphile complexes: Possible application to
  gene therapy

abstract:
  We study a complex formation between the DNA and cationic amphiphilic
molecules. As the amphiphile is added to the solution containing DNA, a
cooperative binding of surfactants to the DNA molecules is found. This binding
transition occurs at specific density of amphiphile, which is strongly
dependent on the concentration of the salt and on the hydrophobicity of the
surfactant molecules. We find that for amphiphiles which are sufficiently
hydrophobic, a charge neutralization, or even charge inversion of the complex
is possible. This is of particular importance in applications to gene therapy,
for which the functional delivery of specific base sequence into living cells
remains an outstanding problem. The charge inversion could, in principle, allow
the DNA-surfactant complexes to approach negatively charged cell membranes
permitting the transfection to take place.


title: Local size segregation in polydisperse hard sphere fluids

abstract:
  The structure of polydisperse hard sphere fluids, in the presence of a wall,
is studied by the Rosenfeld density functional theory. Within this approach,
the local excess free energy depends on only four combinations of the full set
of density fields. The case of continuous polydispersity thereby becomes
tractable. We predict, generically, an oscillatory size segregation close to
the wall, and connect this, by a perturbation theory for narrow distributions,
with the reversible work for changing the size of one particle in a
monodisperse reference fluid.


title:
In-plane gate single-electron transistor in Ga[Al]As fabricated by
  scanning probe lithography

abstract:
  A single-electron transistor has been realized in a Ga[Al]As heterostructure
by oxidizing lines in the GaAs cap layer with an atomic force microscope. The
oxide lines define the boundaries of the quantum dot, the in-plane gate
electrodes, and the contacts of the dot to source and drain. Both the number of
electrons in the dot as well as its coupling to the leads can be tuned with an
additional, homogeneous top gate electrode. Pronounced Coulomb blockade
oscillations are observed as a function of voltages applied to different gates.
We find that, for positive top-gate voltages, the lithographic pattern is
transferred with high accuracy to the electron gas. Furthermore, the dot shape
does not change significantly when in-plane voltages are tuned.


title:
Level spacing statistics of disordered finite superlattices spectra and
  motional narrowing as a random matrix theory effect

abstract:
  In the present work the problem of coupled disordered quantum wells is
addressed in a random matrix theory framework. The quantum wells are short
repulsive binary alloys embeded by ordered barriers and show well defined
quantized levels as a consequence of spatial confinement. Finite disordered
superlattices may show both diffusive-like and localized minibands. Three
different level repulsion suppression mechanisms are discussed by analysing the
evolution of nearest-level-spacing distribution function within each
superlattice miniband. The present numerical results show a motional narrowing
effect, which is in fact a consequence of the random matrix theory.


title: The Pseudogap in La(2-x)Sr(x)CuO(4): A Raman Viewpoint

abstract:
  We report the results of Raman scattering experiments on single crystals of
La(2-x)Sr(x)CuO(4) [La214] as a function of temperature and doping. In
underdoped compounds low-energy B1g spectral weight is depleted in association
with the opening of a pseudogap on regions of the Fermi surface located near
(pi, 0) and (0, pi). The magnitude of the depletion increases with decreasing
doping, and in the most underdoped samples, with decreasing temperature. The
spectral weight that is lost at low-energies (omega < 800 cm-1) is transferred
to the higher energy region normally occupied by multi-magnon scattering. From
the normal state B2g spectra we have determined the scattering rate
Gamma(omega, T) of qausiparticles located near the diagonal directions in
k-space, (pi/2, pi/2) regions. In underdoped compounds, Gamma(omega, T) is
suppressed at low temperatures for energies less than Eg(x) ~ 800 cm-1. The
observed doping dependence of the two-magnon scattering and the scattering rate
suppression thus suggest that the pseudogap is characterized by an energy scale
Eg ~ J, where J is the antiferromagnetic super-exchange energy. Comparison with
the results from other techniques provides a consistent picture of the
pseudogap in La214.


title:
Probability distribution of the order parameter for the 3D Ising model
  universality class: a high precision Monte Carlo study

abstract:
  We study the probability distribution P(M) of the order parameter (average
magnetization) M, for the finite-size systems at the critical point. The
systems under consideration are the 3-dimensional Ising model on a simple cubic
lattice, and its 3-state generalization known to have remarkably small
corrections to scaling. Both models are studied in a cubic box with periodic
boundary conditions. The model with reduced corrections to scaling makes it
possible to determine P(M) with unprecedented precision. We also obtain a
simple, but remarkably accurate approximate formula describing the universal
shape of P(M).


title: Bose-Einstein condensation in quasi2D trapped gases

abstract:
  We discuss BEC in (quasi)2D trapped gases and find that well below the
transition temperature $T_c$ the equilibrium state is a true condensate,
whereas at intermediate temperatures $T<T_c$ one has a quasicondensate
(condensate with fluctuating phase). The mean-field interaction in a quasi2D
gas is sensitive to the frequency $\omega_0$ of the (tight) confinement in the
"frozen" direction, and one can switch the sign of the interaction by changing
$\omega_0$. Variation of $\omega_0$ can also reduce the rates of inelastic
processes, which opens prospects for tunable BEC in trapped quasi2D gases.


title:
Liquid-Gas phase transition in Bose-Einstein Condensates with time
  evolution

abstract:
  We study the effects of a repulsive three-body interaction on a system of
trapped ultra-cold atoms in Bose-Einstein condensed state. The stationary
solutions of the corresponding $s-$wave non-linear Schr\"{o}dinger equation
suggest a scenario of first-order liquid-gas phase transition in the condensed
state up to a critical strength of the effective three-body force. The time
evolution of the condensate with feeding process and three-body recombination
losses has a new characteristic pattern. Also, the decay time of the dense
(liquid) phase is longer than expected due to strong oscillations of the
mean-square-radius.


title: Resonant inelastic x-ray scattering in one-dimensional copper oxides

abstract:
  The Cu K-edge resonant inelastic x-ray scattering (RIXS) spectrum in
one-dimensional insulating cuprates is theoretically examined by using the
exact diagonalization technique for the extended one-dimensional
  Hubbard model with nearest neighbor Coulomb interaction. We find the
following characteristic features that can be detectable by RIXS experiments:
(i) The spectrum with large momentum transfer indicates the formation of
excitons, i.e., bound states of holon and doublon. (ii) The spectrum with small
momentum transfer depends on the incident photon energy. We propose that the
RIXS provides a unique opportunity to study the upper
  Hubbard band in one-dimensional cuprates.


title:
Nonuniversality and critical point shift in systems with infinitely many
  absorbing configurations

abstract:
  A detailed study of critical spreading in the one-dimensional pair contact
process is performed using a recently devised reweighting method. The results
confirm the validity of a generalized hyperscaling relation among the
(nonuniversal) spreading exponents, and support the assertion that the critical
point location does not depend on the particle density \phi. It appears that
the exponents z and delta+eta, once thought to be invariant, exhibit a
systematic dependence on \phi.


title: Transport and Noise of Entangled Electrons

abstract:
  We consider a scattering set-up with an entangler and beam splitter where the
current noise exhibits bunching behavior for electronic singlet states and
antibunching behavior for triplet states. We show that the entanglement of two
electrons in the double-dot can be detected in mesoscopic transport
measurements. In the cotunneling regime the singlet and triplet states lead to
phase-coherent current contributions of opposite signs and to Aharonov-Bohm and
Berry phase oscillations in response to magnetic fields. We analyze the Fermi
liquid effects in the transport of entangled electrons.


title: Phase separation, percolation and giant isotope effect in manganites

abstract:
  Phase separation and a tendency to form inhomogeneous structures seems to be
a generic property of systems with strongly correlated electrons. After shortly
summarising the existing theoretical results in this direction, I concentrate
on the phenomena in doped manganites. I discuss general theoretical results on
the phase separation at small doping and close to the doping x=0.5. The
"global" phase diagram in this region is constructed. These general results are
illustrated on the example of the particular system with rich and complicated
properties - (LaPr)_(1-x)Ca_xMnO_3 in which there exist a ferromagnetic
metallic (FM) phase and a charge ordered (CO) insulating one. The experimental
situation in this system is discussed and the interpretation is given in the
framework of the model with competition of FM and CO, and the indications of
phase separation and percolative nature of this system are given. Giant isotope
effect observed in this situation is shortly discussed.


title:
Dissipative Dynamics and the Statistics of Energy States of a Hookean
  Model for Protein Folding

abstract:
  A generic model of a random polypeptide chain, with discrete torsional
degrees of freedom and Hookean springs connecting pairs of hydrophobic
residues, reproduces the energy probability distribution of real proteins over
a very large range of energies. We show that this system with harmonic
interactions, under dissipative dynamics driven by random noise, leads to a
distribution of energy states obeying a modified one-dimensional
Ornstein-Uhlenbeck process and giving rise to the so called Wigner
distribution. A tunably fine- or coarse-grained sampling of the energy
landscape yields a family of distributions for the energies and energy
spacings.


title:
NQR Study of Spin-Freezing in Superconducting La2-xSrxCuO4: The Example
  of x=0.06

abstract:
  NQR 139La and 63Cu spin-lattice relaxation rate (1/T_1) measurements in a
La1.94Sr0.06CuO4 single crystal are described. Slowing-down of Cu2+ spin
fluctuations is evidenced through a dramatic increase of 139La 1/T_1 on
cooling. While the onset of diamagnetism occurs at Tc=8 K, 1/^{139}T_1 has a
peak at Tg~5 K, when the characteristic frequency of magnetic fluctuations
reaches the NQR frequency nu_Q~19 MHz. In agreement with a number of previous
studies, these results show that the so-called "cluster spin-glass" phase
persists in the superconducting regime. Issues concerning the coexistence of
the two phases are discussed.


title: Strong coupling theory for the Hubbard model

abstract:
  We reanalyze the Hubbard-I approximation by showing that it is equivalent to
an effective Hamiltonian describing Fermionic charge fluctuations, which can be
solved by Bogoliubov transformation. As the most important correction in the
limit of large U and weak spin correlations we augment this Hamiltonian by
further effective particles, which describe composite objects of a Fermionic
charge fluctuation and a spin-, density- or eta- excitation. The scheme is
valid for positive and negative U. We present results for the single particle
Green's function for the two-dimensional Hubbard model with and without t' and
t'' terms, and compare to Quantum Monte-Carlo (QMC) results for the
paramagnetic phase. The overall agreement is significantly improved over the
conventional Hubbard-I or two-pole approximation.


title:
Metal-insulator transition in 2D: Anderson localization by
  temperature-dependent disorder?

abstract:
  A generalization of the single-parameter scaling theory of localization is
proposed for the case when the random potential depends on temperature. The
scaling equation describing the behavior of the resistance is derived. It is
shown that the competition between the metallic-like temperature dependence of
the Drude resistivity and localization leads to a maximum (minimum) at higher
(lower) temperatures. An illustration of a metal-insulator transition in the
model of charged traps, whose concentration depends on temperature, is
presented.


title: Shock-Like Dynamics of Inelastic Gases

abstract:
  We provide a simple physical picture which suggests that the asymptotic
dynamics of inelastic gases in one dimension is independent of the degree of
inelasticity. Statistical characteristics, including velocity fluctuations and
the velocity distribution are identical to those of a perfectly inelastic
sticky gas, which in turn is described by the inviscid Burgers equation.
Asymptotic predictions of this continuum theory, including the t^{-2/3}
temperature decay and the development of discontinuities in the velocity
profile, are verified numerically for inelastic gases.

categories: cond-mat.stat-mech cond-mat.soft nlin.PS patt-sol


title:
Remnant Fermi surface in a pseudogap regime of the two-dimensional
  Hubbard model at finite temperature

abstract:
  A precursor effect on the Fermi surface in the two-dimensional Hubbard model
at finite temperatures near the antiferromagnetic instability is studied using
three different itinerant approaches: the second order perturbation theory, the
paramagnon theory (PT), and the two-particle self-consistent (TPSC) approach.
In general, at finite temperature, the Fermi surface of the interacting
electron systems is not sharply defined due to the broadening effects of the
self-energy. In order to take account of those effects we consider the
single-particle spectral function $A({\bf k},0)$ at the Fermi level, to
describe the counterpart of the Fermi surface at T=0. We find that the Fermi
surface is destroyed close to the pseudogap regime due to the spin-fluctuation
effects in both PT and TPSC approaches. Moreover, the top of the effective
valence band is located around ${\bf k}=(\pi/2,\pi/2)$ in agreement with
earlier investigations on the single-hole motion in the antiferromagnetic
background. A crossover behavior from the Fermi-liquid regime to the pseudogap
regime is observed in the electron concentration dependence of the spectral
function and the self-energy.


title:
Critical behavior of YBa_{2}Cu_{3}O_{7-\delta} and
  Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta} from the dual Ginzburg-Landau model

abstract:
  An unifying scenario for the critical behavior of
YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ (YBCO) and
Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ (BSCCO) is proposed. It is shown that
both critical crossovers observed in these materials follow by considering two
different scalings in the dual Ginzburg-Landau model. The first scaling leads
to the critical exponents $\nu\approx 2/3$, $\nu'\approx 1/3$ and
$\alpha\approx 0$, with $\nu$ and $\alpha$ being respectively the correlation
lenght and specific heat exponents while $\nu'$ is the magnetic field
penetration depth exponent. These values for the critical exponents agree with
the ones obtained experimentally for YBCO single crystals. For the second
scaling we obtain $\nu=1$ and $\alpha=-1$ which must be compared with the
measured values $\nu\approx 1$ and $\alpha\approx-0.7$ for BSCCO. For the
penetration depth exponent it is predicted the value $\nu'=1$.


title:
Biquadratic interactions and spin-Peierls transition in the spin 1 chain
  LiVGe2O6

abstract:
  The magnetic susceptibility of a new one-dimensional, S=1 system, the
vanadium oxide LiVGe2O6, has been measured. Contrary to previous S=1 chains, it
exhibits an abrupt drop at 22 K typical of a spin-Peierls transition, and it is
consistent with a gapless spectrum above this temperature. We propose that this
behaviour is due to the presence of a significant biquadratic exchange
interaction, a suggestion supported by quantum chemistry calculations that take
into account the quasi-degeneracy of the t2g levels.


title: Weak Localization Effect in Superconductors by Radiation Damage

abstract:
  Large reductions of the superconducting transition temperature $T_{c}$ and
the accompanying loss of the thermal electrical resistivity (electron-phonon
interaction) due to radiation damage have been observed for several A15
compounds, Chevrel phase and Ternary superconductors, and $\rm{NbSe_{2}}$ in
the high fluence regime. We examine these behaviors based on the recent theory
of weak localization effect in superconductors. We find a good fitting to the
experimental data. In particular, weak localization correction to the
phonon-mediated interaction is derived from the density correlation function.
It is shown that weak localization has a strong influence on both the
phonon-mediated interaction and the electron-phonon interaction, which leads to
the universal correlation of $T_{c}$ and resistance ratio.


title: LDA+DMFT study for $La_{1-x}Sr_{x}TiO_{3}$

abstract:
  The dynamical mean-field theory together with the non-crossing approximation
is used to set up a novel scheme to study the electronic structure of strongly
correlated electron systems. The non-interacting band structure is obtained
from a density functional calculation within the local density approximation.
With this method the doped Mott insulator $\rm La_{1-x}Sr_x Ti O_3$ is studied.
Starting from first-principle calculations for a cubic and an orthorhombic
system we determined the one-particle spectrum. Both one-particle spectra show
a lower Hubbard band (seen as $d^1 \to d^o$ transitions in photo emission
experiments), a Kondo resonance near the Fermi energy and the upper Hubbard
band ($d^1\to d^2$ transitions in an inverse photoemission experiment). The
upper Hubbard band develops a multipeak structure, a consequence of the
consideration of all local two-particle correlations, which leads to the full
multiplet structure in the atomic limit. The calculation for the orthorhombic
system shows qualitative good agreement when compared with experimental
photoemission spectra.


title:
Effects of Long Range Electronic Interactions on a One-dimensional
  Electron System

abstract:
  The effects of a long range electronic potential on a one dimensional chain
of spinless fermions are investigated by numerical techniques (Exact
Diagonalisation of rings with up to 30 sites complemented by finite size
analysis) and analytic calculations. Due to a competition between the $2 k_F$
oscillations in the density and the (very slow) log divergence of the long
wavelength part of the Coulomb potential, the metallic character of the system
is enhanced at intermediate (and up to quite large) lengthscales. Despite some
similarities, we found that this quasi-metallic regime is not of the Luttinger
Liquid type, as evidenced by strong deviations from predictions of conformal
field theory and in agreement with the picture of a (very weakly pinned) Wigner
crystal. When the strength of the Coulomb interaction is substancially
increased, we observe a smooth cross-over to a strongly localized charge
density wave.


title:
Cluster variation method and disorder varieties of two-dimensional
  Ising-like models

abstract:
  I show that the cluster variation method, long used as a powerful hierarchy
of approximations for discrete (Ising-like) two-dimensional lattice models,
yields exact results on the disorder varieties which appear when competitive
interactions are put into these models. I consider, as an example, the
plaquette approximation of the cluster variation method for the square lattice
Ising model with nearest-neighbor, next-nearest-neighbor and plaquette
interactions, and, after rederiving known results, report simple closed-form
expressions for the pair and plaquette correlation functions.


title: Mixed Phases for the t-J Model

abstract:
  We study the competition between non-magnetic (dimer or flux) states and
short-range antiferromagnetically ordered RVB states in the t-J model and
present a finite temperature phase diagram. We show that, for a wide range of
temperatures and dopings, the stable phase results to be a state in which both
the flux and the RVB parameters are nonzero.


title:
Quasiparticle Resonant States Induced by a Unitary Impurity in a d-Wave
  Superconductor

abstract:
  The quasiparticle resonant states around a single nonmagnetic impurity with
unitary scattering in a d-wave superconductor is studied by solving the
Bogoliubov-de Gennes equations based on a t-J model. Both the spatial variation
of the order parameter and the local density of states (LDOS) around the
impurity have been investigated. We find: (i) A particle-hole symmetric system
has a single symmetric zero-energy peak in the LDOS regardless of the size of
the superconducting coherence length \xi_0; (ii) For the particle-hole
asymmetric case, an asymmetric splitting of the zero-energy peak is intrinsic
to a system with a small value of k_{F}\xi_0.


title:
Simulations of metastable decay in two- and three-dimensional models
  with microscopic dynamics

abstract:
  We present a brief analysis of the crossover phase diagram for the decay of a
metastable phase in a simple dynamic lattice-gas model of a two-phase system.
We illustrate the nucleation-theoretical analysis with dynamic Monte Carlo
simulations of a kinetic Ising lattice gas on square and cubic lattices. We
predict several regimes in which the metastable lifetime has different
functional forms, and provide estimates for the crossovers between the
different regimes. In the multidroplet regime, the
Kolmogorov-Johnson-Mehl-Avrami theory for the time dependence of the
order-parameter decay and the two-point density correlation function allows
extraction of both the order parameter in the metastable phase and the
interfacial velocity from the simulation data.


title:
Imaging the Effects of Individual Zinc Impurity Atoms on
  Superconductivity in Bi2Sr2CaCu2O8+delta

abstract:
  Although their crystal structures are complex, all high temperature
superconductors contain some crystal planes consisting of only Cu and O atoms
in a square lattice. Superconductivity is believed to originate from strongly
interacting electrons in these CuO2 planes. Substitution of a single impurity
atom at a Cu site creates a simple but powerful perturbation to these
interactions. Detailed knowledge of the effects of such an impurity atom on the
superconducting order parameter and on the quasi-particle local density of
states (LDOS) could allow competing theories of high temperature
superconductivity (HTSC) to be tested at the atomic scale. The fundamental
implications of results from numerous bulk measurements on samples doped with
impurity atoms could also be clarified with such data. Here we describe
scanning tunneling microscopy studies of the effects of individual Zn impurity
atoms located at the Cu site in the high-Tc superconductor Bi2Sr2CaCu2O8+delta.
Tunneling spectroscopy shows intense quasi-particle scattering resonances at
the Zn sites, coincident with strong suppression of superconductivity within
about 1.5 nm. Imaging of the quasi-particle LDOS at these sites reveals the
long sought four-fold symmetric "quasi-particle cloud" aligned with the d-wave
gap nodes. Several unexpected phenomena, which can shed new light on the
atomic-scale response of HTSC to a probe impurity atom, are also observed.


title: The rheology of field-responsive suspensions

abstract:
  We present some aspects of the rheology of field-responsive suspensions, a
class of field-responsive liquid matter systems possessing the ability to
undergo significant changes in their strength upon application of an external
field. Both the single-particle and the many-particle domains are discussed. In
the former, consideration of the full nonlinear dynamics of the particles leads
to an anomalous behavior of the viscosity whereas in the latter the most
salient feature is the formation of chains and fractal structures. We indicate
how to deal with the rheology at moderately concentrations leaving open the
problem at higher concentrations for which the complexity of the emergent
structures strongly limits the knowledge of their dynamics.


title: Conserved Linking in Single- and Double-Stranded Polymers

abstract:
  We demonstrate a variant of the Bond Fluctuation lattice Monte Carlo model in
which moves through cis conformations are forbidden. Ring polymers in this
model have a conserved quantity that amounts to a topological linking number.
Increased linking number reduces the radius of gyration mildly. A linking
number of order 0.2 per bond leads to an eight-percent reduction of the radius
for 128-bond chains. This percentage appears to rise with increasing chain
length, contrary to expectation. For ring chains evolving without the
conservation of linking number, we demonstrate a substantial anti-correlation
between the twist and writhe variables whose sum yields the linking number. We
raise the possibility that our observed anti-correlations may have counterparts
in the most important practical polymer that conserves linking number, DNA.


title: Scattering of Hot Excitons in Quantum Wells

abstract:
  The scattering probabilities of hot excitons in narrow quantum wells (QWs)are
obtained. The exciton-phonon matrix element is considered by using an envelope
function Hamiltonian approach in the strong quantization limit where the QW
width is smaller than the exciton bulk Bohr radius. The Fr\"{o}hlich-like
interaction is taken into account and the contribution of the confined and
interface modes to the scattering probability are calculated as a function of
quantum well width, electron and hole effective masses, and in-plane
center-of-mass kinetic energy. Inter- and intra-subband excitonic transitions
are discussed in term of the phonon scalar potential selectionrules. It is
shown that even parity electrostatic potential states for confined and
interface modes give the main contribution to the excitonic scattering rate.
The consequences of exciton relaxation and scattering probability present for
the multiphonon resonance Raman scattering in narrow QWs are briefly discussed.


title: Electrical transport properties of ultrathin disordered films

abstract:
  We report an experimental study of quench condensed ($2K\le T \le 15K$)
disordered ultrathin films of {\rm Bi} where localisation effects and
superconductivity compete. Experiments are done with different substrates
and/or different underlayers. Quasi-free standing films of {\rm Bi}, prepared
by quenching {\rm Bi} vapours onto solid {\rm Xe}, are also studied. The
results show a dependence of the transport properties both on the dielectric
constant of the substrate/underlayer as well as the temperature of quench
condensation. RHEED studies indicate that quantum size effects are important in
these systems. In this paper, we try to correlate the structure of the films to
the transport properties obtained.


title: Onset of superconductivity in 2D granular films composed of Bi clusters

abstract:
  Two dimensional films comprising of Bi clusters ranging in size from 25 to
100 $\mathrm{\AA}$ show an influence of the underlying matrix on normal state
resistivity and the superconducting transition temperature $T_\mathrm{c}$. This
is in contrast to crystalline Bi. Quantum size effects are observed by changing
the deposition temperature, which determines cluster size. These observations
can be attributed to superconductivity at the surface. Implications for the
requirement of a critical number of superconducting clusters before the
establishment of global phase coherence are briefly discussed.


title:
Macroscopic Quantum Coherence in Small Antiferromagnetic Particle and
  the Quantum Interference Effects

abstract:
  Starting from the Hamiltonian operator of the noncompensated two-sublattice
model of a small antiferromagnetic particle, we derive the effective Lagrangian
of a biaxial antiferromagnetic particle in an external magnetic field with the
help of spin-coherent-state path integrals. Two unequal level-shifts induced by
tunneling through two types of barriers are obtained using the instanton
method. The energy spectrum is found from Bloch theory regarding the periodic
potential as a superlattice. The external magnetic field indeed removes
Kramers' degeneracy, however a new quenching of the energy splitting depending
on the applied magnetic field is observed for both integer and half-integer
spins due to the quantum interference between transitions through two types of
barriers.


title:
Interaction of photons with plasmas and liquid metals: photoabsorption
  and scattering

abstract:
  Formulas to describe the photoabsorption and the photon scattering by a
plasma or a liquid metal are derived in a unified manner with each other. It is
shown how the nuclear motion, the free-electron motion and the core-electron
behaviour in each ion in the system determine the structure of photoabsorption
and scattering in an electron-ion mixture. The absorption cross section in the
dipole approximation consists of three terms which represent the absorption
caused by the nuclear motion, the absorption owing to the free-electron motion
producing optical conductivity or inverse Bremsstrahlung, and the absorption
ascribed to the core-electron behaviour in each ion with the Doppler
correction. Also, the photon scattering formula provides an analysis method for
experiments observing the ion-ion dynamical structure factor (DSF), the
electron-electron DSF giving plasma oscillations, and the core-electron DSF
yielding the X-ray Raman (Compton) scattering with a clear definition of the
background scattering for each experiment, in a unified manner. A formula for
anomalous X-ray scattering is also derived for a liquid metal. At the same
time, Thomson scattering in plasma physics is discussed from this general point
of view.


title:
Spectral Line Shape of High-Frequency Local Vibrations in Adsorbed
  Molecular Lattices

abstract:
  We consider high-frequency local vibrations anharmonically coupled with
low-frequency modes in a planar lattice of adsorbed molecules. The effect of
lateral intermolecular interactions on the spectral line shape for local
vibrations is analyzed in the limit of the high density of adsorbed molecules.
It is shown that the spectral line positions and widths depend on behaviour of
low-frequency distribution function for a system of adsorbed molecules. The
results obtained allows the spectral line characteristics of the local
vibrations for isotopically diluted {$^{13}$}C{$^{16}$}O{$_{2}$} molecules in
the {$^{12}$}C{$^{16}$}O{$_{2}$} monolayer on the NaCl(100) surface to be
described in agreement with the experimentally measured values.


title: Minimal model to describe the magnetism of $CuGeO_{3}$

abstract:
  We show that to describe properly the low energy excitations of $CuGeO_{3}$
one must include the effects of a transverse antiferromagnetic coupling, which
is estimated to be $J_{\perp}=0.15 J$. Owing to this coupling the frustration
in the chains is significantly lower than recent 1D estimates based on purely
one-dimensional arguments, we find $J_{2}=0.2 J$. Furthermore we have found a
strong modulation of the nearest neighbour coupling due to the static
distortion $\delta = 0.065$, which is 5 times higher than that previously
deduced from a 1D chain approach. Our set of parameters gives, i) a value of
the distorsion which agreeswell with some recent estimate for a lower bound, we
are able to perfectly reproduce ii)the dispersions, iii) the experimental
susceptibility at both high and low temperatures. By performing DMRG
calculations for 2 coupled chains we have analysed the effect of the transverse
coupling on the ratio of singlet to triplet gaps. The ratio is very sensitive
to the parameters and the universality reported in the strict one dimensional
case is lost. As an additional point, we provide a simple picture to explain
the interesting new feature observed in recent inelastic neutron scattering
experiments: the existence of a second branch of excitations.


title: Flowing sand - a possible physical realization of Directed Percolation

abstract:
  A simple model for flowing sand on an inclined plane is introduced. The model
is related to recent experiments by Douady and Daerr [Nature 399, 241 (1999)]
and reproduces some of the experimentally observed features. Avalanches of
intermediate size appear to be compact, placing the critical behavior of the
model into the universality class of compact directed percolation. On very
large scales, however, the avalanches break up into several branches leading to
a crossover from compact to ordinary directed percolation. Thus, systems of
flowing granular matter on an inclined plane could serve as a first physical
realization of directed percolation.


title: Anomalous anharmonicity in doped cuprate superconductors

abstract:
  Strong nonlinear elastic effects arise due to the Lifshitz transition
associated with a change in the Fermi surface topology. It is shown that an
electronic contribution to the elastic characteristics of cuprates becomes
important if the chemical potential is near but not too near the Van Hove
critical energy $\epsilon_c$. In the case of a saddle point singularity in the
energy spectrum an enhancement factor arises for the "electronic" anharmonic
constants of the 3rd and 4th order, that is proportional to
$\epsilon_F/(\epsilon_F-\epsilon_c)$ to the 1st and 2nd power, respectively.
The behaviour of the Grueneisen coefficient in the vicinity of the Lifshitz
transition is considered.


title: Evaporative cooling of an atomic beam

abstract:
  We present a theoretical analysis of the evaporative cooling of an atomic
beam propagating in a magnetic guide. Cooling is provided by transverse
evaporation. The atomic dynamics inside the guide is analyzed by solving the
Boltzmann equation with two different approaches: an approximate analytical
ansatz and a Monte-Carlo simulation. Within their domain of validity, these two
methods are found to be in very good agreement with each other. They allow us
to determine how the phase-space density and the flux of the beam vary along
its direction of propagation. We find a significant increase for the
phase-space density along the guide for realistic experimental parameters. By
extrapolation, we estimate the length of the beam needed to reach quantum
degeneracy.


title:
Fast Response and Temporal Coding on Coherent Oscillations in
  Small-World Networks

abstract:
  We have investigated the role that different connectivity regimes play on the
dynamics of a network of Hodgkin-Huxley neurons by computer simulations. The
different connectivity topologies exhibit the following features: random
connectivity topologies give rise to fast system response yet are unable to
produce coherent oscillations in the average activity of the network; on the
other hand, regular connectivity topologies give rise to coherent oscillations
and temporal coding, but in a temporal scale that is not in accordance with
fast signal processing. Finally, small-world (SW) connectivity topologies,
which fall between random and regular ones, take advantage of the best features
of both, giving rise to fast system response with coherent oscillations along
with reproducible temporal coding on clusters of neurons. Our work is the
first, to the best of our knowledge, to show the need for a small-world
topology in order to obtain all these features in synergy within a biologically
plausible time scale.


title: Model for a Josephson junction array coupled to a resonant cavity

abstract:
  We describe a simple Hamiltonian for an underdamped Josephson array coupled
to a single photon mode in a resonant cavity. Using a Hartree-like mean-field
theory, we show that, for any given strength of coupling between the photon
field and the Josephson junctions, there is a transition from incoherence to
coherence as a function of N, the number of Josephson junctions in the array.
Above that value of N, the energy in the photon field is proportional to N^2,
indicative of coherent emission. Both of these features agree with recent
experiments by Barbara et al. The experimentally observed voltage steps may
require an explicit solution of the dynamical equations.


title: Distribution of Attraction Basins in a Family of Simple Glasses

abstract:
  We study the distribution of attraction basins as a function of energy in
simple glasses. We find that it is always broad. Furthermore we identify two
types of glasses, both with an exponentially large number of metastable states.
In one type the largest attraction basin is exponentially small, whereas in the
other it is polynomially small in the system size N. If there exists a tuning
parameter that connects one regime with another, then these two phases are
separated by a critical point. We discuss implications for optimization
problems.


title: Vortex Physics in Confined Geometries

abstract:
  Patterned irradiation of cuprate superconductors with columnar defects allows
a new generation of experiments which can probe the properties of vortex
liquids by forcing them to flow in confined geometries. Such experiments can be
used to distinguish experimentally between continuous disorder-driven glass
transitions of vortex matter, such as the vortex glass or the Bose glass
transition, and nonequilibrium polymer-like glass transitions driven by
interaction and entanglement. For continuous glass transitions, an analysis of
such experiments that combines an inhomogeneous scaling theory with the
hydrodynamic description of viscous flow of vortex liquids can be used to infer
the critical behavior. After generalizing vortex hydrodynamics to incorporate
currents and field gradients both longitudinal and transverse to the applied
field, the critical exponents for all six vortex liquid viscosities are
obtained. In particular, the shear viscosity is predicted to diverge as
$|T-T_{BG}|^{-\nu z}$ at the Bose glass transition, with $\nu\simeq 1$ and
$z\simeq 4.6$ the dynamical critical exponent. The scaling behavior of the ac
resistivity is also derived. As concrete examples of flux flow in confined
geometries, flow in a channel and in the Corbino disk geometry are discussed in
detail. Finally, the implications of scaling for the hydrodynamic description
of transport in the dc flux transformer geometry are discussed.


title:
Dipole-quadrupole interactions and the nature of phase III of compressed
  hydrogen

abstract:
  A new class of strongly infrared active structures is identified for phase
III of compressed molecular H2 by constant-pressure ab initio molecular
dynamics and density-functional perturbation calculations. These are planar
quadrupolar structures obtained as a distortion of low-pressure quadrupolar
phases, after they become unstable at about 150 GPa due to a zone-boundary soft
phonon. The nature of the II-III transition and the origin of the IR activity
are rationalized by means of simple electrostatics, as the onset of a
stabilizing dipole-quadrupole interaction.


title:
Finite-size effects and the stabilized spin-polarized jellium model for
  metal clusters

abstract:
  In the framework of spherical geometry for jellium and local spin density
approximation, we have obtained the equilibrium $r_s$ values,
$\bar{r}_s(N,\zeta)$, of neutral and singly ionized "generic" $N$-electron
clusters for their various spin polarizations, $\zeta$. Our results reveal that
$\bar{r}_s(N,\zeta)$ as a function of $\zeta$ behaves differently depending on
whether $N$ corresponds to a closed-shell or an open-shell cluster. That is,
for a closed-shell one, $\bar{r}_s(N,\zeta)$ is an increasing function of
$\zeta$ over the whole range $0\le\zeta\le 1$, and for an open-shell one, it
has a decreasing part corresponding to the range $0<\zeta\le\zeta_0$, where
$\zeta_0$ is a polarization that the cluster assumes in a configuration
consistent with Hund's first rule.
  In the context of the stabilized spin-polarized jellium model, our
calculations based on these equilibrium $r_s$ values, $\bar{r}_s(N,\zeta)$,
show that instead of the maximum spin compensation (MSC) rule, Hund's first
rule governs the minimum-energy configuration. We therefore conclude that the
increasing behavior of the equilibrium $r_s$ values over the whole range of
$\zeta$ is a necessary condition for obtaining the MSC rule for the
minimum-energy configuration; and the only way to end up with an increasing
behavior over the whole range of $\zeta$ is to break the spherical geometry of
the jellium background. This is the reason why the results based on simple
jellium with spheroidal or ellipsoidal geometries show up MSC rule.


title: Condensation energy in the spin-fermion model for cuprates

abstract:
  We use the Scalapino-White relation between the condensation energy and the
difference between the dynamical structure factor in the normal and the
superconducting states to compute the condensation energy in the spin-fermion
model. We show that at strong coupling, the extra low-frequency spectral weight
associated with the resonance peak in the dynamical structure factor in a
superconductor is compensated only at energies $\sim J$ which are much larger
than the superconducting gap $\Delta$. We argue that in this situation, the
condensation energy is large and well accounts for the data for cuprates.


title: Superconductivity in Mesoscopic Metal Particles: The Role of Degeneracy

abstract:
  Recently, it has been possible to construct single-electron transistors to
study electronic properties, including superconductivity, in metallic grains of
nanometer size. Among several theoretical results are suppression of
superconductivity with decreasing grain size and parity effect, i.e. dependence
on the parity of the number of electrons on the grain. We study how these
results are affected by degeneracy of energy levels. In addition to the
time-reversal symmetry, for certain energy spectra and more generally for
lattice symmetries, energy levels are strongly degenerate near the Fermi
energy. For a parabolic dispersion, degeneracy d is of the order of k_FL while
the typical distance between the levels is of the order of epsilon_F/(k_FL)^2
where k_F and epsilon_F are the Fermi wave vector and energy, respectively, and
L is the particle size. First, using an exact solution method for BCS
Hamiltonian with finite number of energy levels, for the well studied
non-degenerate case we find a new feature. In that case, parity effect exhibits
a minimum instead of a monotonic behavior. For d-fold degenerate states we find
that the ratio of two successive parity effect parameters Delta_p is nearly
1+1/d. Our numerical solutions for the exact ground state energy of negative U
Hubbard model on a cubic cluster also give very similar results. Hence we
conclude that parity effect is a general property of small Fermi systems with
attractive interaction, and it is closely related to degeneracy of energy
levels.


title:
Supersymmetry approach in the field theory of ergodicity breaking
  transitions

abstract:
  The supersymmetry self-consistent approximation for the model of
non-equilibrium thermodynamic system with quenched disorder is derived from the
dynamical action calculated by means of generalized second Legendre
transformation. The equations for adiabatic and isothermal susceptibilities,
memory and field induced parameters are obtained on the basis of asymptotic
analysis of dynamical Dyson equations. It is shown that the marginal stability
condition that defines the critical point is governed by fluctuations violating
FDT. The temperature of ergodicity breaking transition is calculated as a
function of quenched disorder intensities. Transformation of superfields
related to the mapping between an instanton process and the corresponding
causal solution is discussed.


title:
Finite size spectrum, magnon interactions and magnetization of S=1
  Heisenberg spin chains

abstract:
  We report our density matrix renormalization-group and analytical work on S=1
antiferromagnetic Heisenberg spin chains. We study the finite size behavior
within the framework of the non-linear sigma model. We study the effect of
magnon-magnon interactions on the finite size spectrum and on the magnetization
curve close to the critical magnetic field, determine the magnon scattering
length and compare it to the prediction from the non-linear $\sigma$ model.


title: An interpretation of Tsallis statistics based on polydispersity

abstract:
  It is argued that polydispersed systems like colloids provide a direct
example where Tsallis' statistical distribution is useful for describing the
heirarchical nature of the system based on particle size.


title:
Microcanonical Transfer Matrix and Yang-Lee Zeros of the Q-State Potts
  Model

abstract:
  The microcanonical transfer matrix and its extensions offer a new way of
obtaining exact partition functions on finite two dimensional lattices. We show
the density of the partition function zeros in the complex x- plane for the
Ising model at $y=y_c$ and $y=0.5y_c$, and the density of the partition
function zeros in the complex y-plane for the three-state Potts model.


title: Partition function zeros of the Q-state Potts model for non-integer Q

abstract:
  The distribution of the zeros of the partition function in the complex
temperature plane (Fisher zeros) of the two-dimensional Q-state Potts model is
studied for non-integer Q. On $L\times L$ self-dual lattices studied ($L\le8$),
no Fisher zero lies on the unit circle $p_0=e^{i\theta}$ in the complex
$p=(e^{\beta J}-1)/\sqrt{Q}$ plane for Q<1, while some of the Fisher zeros lie
on the unit circle for Q>1 and the number of such zeros increases with
increasing Q. The ferromagnetic and antiferromagnetic properties of the Potts
model are investigated using the distribution of the Fisher zeros. For the
Potts ferromagnet we verify the den Nijs formula for the thermal exponent
$y_t$. For the Potts antiferromagnet we also verify the Baxter conjecture for
the critical temperature and present new results for the thermal exponents in
the range 0<Q<3.


title:
Exact results for the zeros of the partition function of the Potts model
  on finite lattices

abstract:
  The Yang-Lee zeros of the Q-state Potts model are investigated in 1, 2 and 3
dimensions. Analytical results derived from the transfer matrix for the
one-dimensional model reveal a systematic behavior of the locus of zeros as a
function of Q. For 1<Q<2 the zeros in the complex $x=\exp(\beta H_q)$ plane lie
inside the unit circle, while for Q>2 they lie outside the unit circle for
finite temperature. In the special case Q=2 the zeros lie exactly on the unit
circle as proved by Lee and Yang. In two and three dimensions the zeros are
calculated numerically and behave in the same way. Results are also presented
for the critical line of the Potts model in an external field as determined
from the zeros of the partition function in the complex temperature plane.


title:
Magnetotransport properties of (Ga,Mn)As investigated at low temperature
  and high magnetic field

abstract:
  Magnetotransport properties of ferromagnetic semiconductor (Ga,Mn)As have
been investigated. Measurements at low temperature (50 mK) and high magnetic
field (<= 27 T) have been employed in order to determine the hole concentration
p = 3.5x10^20 cm ^-3 of a metallic (Ga0.947Mn0.053)As layer. The analysis of
the temperature and magnetic field dependencies of the resistivity in the
paramagnetic region was performed with the use of the above value of p, which
gave the magnitude of p-d exchange energy |N0beta | ~ 1.5 eV.


title: Effects of a magnetic field on the one-dimensional spin-orbital model

abstract:
  We study the effects of a uniform magnetic field on the one-dimensional
spin-orbital model in terms of effective field theories. Two regions are
examined: one around the SU(4) point (J=K/4) and the other with K<<J. We found
that when $J\leq K/4$, the spin and orbital correlation functions exhibit
power-law decay with nonuniversal exponents. In the region with J>K/4, the
excitation spectrum has a gap. When the magnetic field is beyond some critical
value, a quantum phase transition occurs. However, the correlation functions
around the SU(4) point and the region with K<<J exhibit distinct behavior. This
results from different structures of excitation spectra in both regime.


title: Adsorption-like Collapse of Diblock Copolymers

abstract:
  A linear copolymer made of two reciprocally attracting N-monomer blocks
collapses to a compact phase through a novel transition, whose exponents are
determined with extensive MC simulations in two and three dimensions. In the
former case, an identification with the statistical geometry of suitable
percolation paths allows to predict that the number of contacts between the
blocks grows like $N^{9/16}$. In the compact phase the blocks are mixed and, in
two dimensions, also zipped, in such a way to form a spiral, double chain
structure.


title: Explaining the Forward Interest Rate Term Structure

abstract:
  We present compelling empirical evidence for a new interpretation of the
Forward Rate Curve (FRC) term structure. We find that the average FRC follows a
square-root law, with a prefactor related to the spot volatility, suggesting a
Value-at-Risk like pricing. We find a striking correlation between the
instantaneous FRC and the past spot trend over a certain time horizon. This
confirms the idea of an anticipated trend mechanism proposed earlier and
provides a natural explanation for the observed shape of the FRC volatility. We
find that the one-factor Gaussian Heath-Jarrow-Morton model calibrated to the
empirical volatility function fails to adequately describe these features.


title:
Enhancement of Stochastic Resonance in distributed systems due to a
  selective coupling

abstract:
  Recent massive numerical simulations have shown that the response of a
"stochastic resonator" is enhanced as a consequence of spatial coupling.
Similar results have been analytically obtained in a reaction-diffusion model,
using "nonequilibrium potential" techniques. We now consider a field-dependent
diffusivity and show that the "selectivity" of the coupling is more efficient
for achieving stochastic-resonance enhancement than its overall value in the
constant-diffusivity case.


title:
Dopant-induced crossover from 1D to 3D charge transport in conjugated
  polymers

abstract:
  The interplay between inter- and intra-chain charge transport in bulk
polythiophene in the hopping regime has been clarified by studying the
conductivity as a function of frequency (up to 3 THz), temperature and doping
level. We present a model which quantitatively explains the observed crossover
from quasi-one-dimensional transport to three-dimensional hopping conduction
with increasing doping level. At high frequencies the conductivity is dominated
by charge transport on one-dimensional conducting chains.


title:
Zeeman Coupling and Abnormal Thermal Conductivities in BSCCO
  Superconductors

abstract:
  Using the path-integral formulation we derive microscopically a
Ginzburg-Landau free energy with a Zeeman coupling between the magnetic field
and the orbital angular momentum of the Cooper pairs in a superconductor with
singlet pairing in the $d_{x^{2}-y^{2}}$- and the sub-dominant $d_{xy}$-
channels. The Zeeman coupling induces a time-reversal-symmetry-breaking pairing
state. Based on careful examinations of the energy gain due to the Zeeman
coupling, the energy lost due to the kinetic energy of the excess superfluid,
and the Doppler energy shift for quasi-particle excitations, we present a
coherent interpretation for the puzzling and conflicting thermal conductivites
observed at above 5K (K. Krishana, {\it et al}, Science {\bf 277}, 83(1997))
and at sub-Kelvins (H. Aubin, {\it et al}, Phys. Rev. Lett. {\bf 82}, 624
(1999)).


title: An introduction to real-time renormalization group

abstract:
  This article presents a tutorial introduction to a recently developed
real-time renormalization group method. It describes nonequilibrium properties
of discrete quantum systems coupled linearly to an environment. We illustrate
the technique by a simple and exactly solvable model: A quantum dot consisting
of a single non-degenerate level coupled to two reservoirs. The article is
intended for advanced students. Besides elementary quantum mechanics and
statistical mechanics, it requires only knowledge of second quantization and
Wick's theorem.


title: Transmission Phase Shift of a Quantum Dot with Kondo Correlations

abstract:
  We study the effects of Kondo correlations on the transmission phase shift of
a quantum dot in an Aharonov-Bohm ring. We predict in detail how the
development of a Kondo resonance should affect the dependence of the phase
shift on transport voltage, gate voltage and temperature. This system should
allow the first direct observation of the well-known scattering phase shift of
pi/2 expected (but not directly measurable in bulk systems) at zero temperature
for an electron scattering off a spin-1/2 impurity that is screened into a
singlet.


title: Dynamics of Learning with Restricted Training Sets I: General Theory

abstract:
  We study the dynamics of supervised learning in layered neural networks, in
the regime where the size $p$ of the training set is proportional to the number
$N$ of inputs. Here the local fields are no longer described by Gaussian
probability distributions and the learning dynamics is of a spin-glass nature,
with the composition of the training set playing the role of quenched disorder.
We show how dynamical replica theory can be used to predict the evolution of
macroscopic observables, including the two relevant performance measures
(training error and generalization error), incorporating the old formalism
developed for complete training sets in the limit $\alpha=p/N\to\infty$ as a
special case. For simplicity we restrict ourselves in this paper to
single-layer networks and realizable tasks.


title: Features of level broadening in a ring-stub system

abstract:
  When a one dimensional (1D) ring-stub system is coupled to an electron
reservoir, the states acquire a width (or broadening characterized by poles in
the complex energy plane) due to finite life time effects. We show that this
broadening is limited by anti-resonances due to the stub. The differences in
level broadening in presence and absence of anti-resonance is exemplified by
comparison to a 1D ring coupled to an infinite reservoir. We also show that the
anti-resonances due to the stub has an anchoring effect on the poles when a
magnetic flux through the ring is varied. This will have implication on change
in distribution of the poles in disordered multichannel situation as magnetic
flux is varied.


title:
Kinks in the discrete sine-Gordon model with Kac-Baker long-range
  interactions

abstract:
  We study effects of Kac-Baker long-range dispersive interaction (LRI) between
particles on kink properties in the discrete sine-Gordon model. We show that
the kink width increases indefinitely as the range of LRI grows only in the
case of strong interparticle coupling. On the contrary, the kink becomes
intrinsically localized if the coupling is under some critical value.
Correspondingly, the Peierls-Nabarro barrier vanishes as the range of LRI
increases for supercritical values of the coupling but remains finite for
subcritical values. We demonstrate that LRI essentially transforms the internal
dynamics of the kinks, specifically creating their internal localized and
quasilocalized modes. We also show that moving kinks radiate plane waves due to
break of the Lorentz invariance by LRI.


title: Gold nanowires and their chemical modifications

abstract:
  Equilibrium structure, local densities of states, and electronic transport in
a gold nanowire made of a four-atom chain supported by two gold electrodes,
which has been imaged recently by high-resolution electron microscopy, and
chemical modification of the wire via the adsorption of a methylthiol molecule,
are investigated with ab-initio local density functional simulations. In the
bare wire at the imaged geometry the middle two atoms dimerize, and the
structure is strongly modified by the adsorption of the molecule with an
accompanying increase of the ballistic conductance through the wire.