title: Active Width at a Slanted Active Boundary in Directed Percolation

abstract:
  The width W of the active region around an active moving wall in a directed
percolation process diverges at the percolation threshold p_c as W \simeq A
\epsilon^{-\nu_\parallel} \ln(\epsilon_0/\epsilon), with \epsilon=p_c-p,
\epsilon_0 a constant, and \nu_\parallel=1.734 the critical exponent of the
characteristic time needed to reach the stationary state \xi_\parallel \sim
\epsilon^{-\nu_\parallel}. The logarithmic factor arises from screening of
statistically independent needle shaped sub clusters in the active region.
Numerical data confirm this scaling behaviour.


title:
Detecting Josephson effect in the excitonic condensate by coherent
  emission of light

abstract:
  Coherent emission of light by a split excitonic Bose-Einstein condensate --
excitonic Josephson junction -- can be extremely sensitive to the Josephson
phase established across the junction. As a result of this, the emission can be
redistributed between different directions and even cancelled. The cases of the
dipole- and of the quadrupole-active excitons are considered.


title: Scaling and percolation in the small-world network model

abstract:
  In this paper we study the small-world network model of Watts and Strogatz,
which mimics some aspects of the structure of networks of social interactions.
We argue that there is one non-trivial length-scale in the model, analogous to
the correlation length in other systems, which is well-defined in the limit of
infinite system size and which diverges continuously as the randomness in the
network tends to zero, giving a normal critical point in this limit. This
length-scale governs the cross-over from large- to small-world behavior in the
model, as well as the number of vertices in a neighborhood of given radius on
the network. We derive the value of the single critical exponent controlling
behavior in the critical region and the finite size scaling form for the
average vertex-vertex distance on the network, and, using series expansion and
Pade approximants, find an approximate analytic form for the scaling function.
We calculate the effective dimension of small-world graphs and show that this
dimension varies as a function of the length-scale on which it is measured, in
a manner reminiscent of multifractals. We also study the problem of site
percolation on small-world networks as a simple model of disease propagation,
and derive an approximate expression for the percolation probability at which a
giant component of connected vertices first forms (in epidemiological terms,
the point at which an epidemic occurs). The typical cluster radius satisfies
the expected finite size scaling form with a cluster size exponent close to
that for a random graph. All our analytic results are confirmed by extensive
numerical simulations of the model.

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


title:
An effective Hamiltonian for an extended Kondo lattice model and a
  possible origin of charge ordering in half-doped manganites

abstract:
  An effective Hamiltonian is derived in the case of the strong Hund coupling
and on-site Coulomb interaction by means of a projective perturbation approach.
A physical mechanism for charge ordering in half-doped manganites
(R_{0.5}X_{0.5}MnO_3) is proposed. The virtual process of electron hopping
results in antiferromagnetic superexchange and a repulsive interaction, which
may drive electrons to form a Wigner lattice. The phase diagram of the ground
state of the model is presented at half doping. In the case of formation of
Wigner lattice, we prove that spins of electrons are aligned ferromagnetically
as well as that the localized spin background is antiferromagnetic. The
influence of the on-site Coulomb interaction is also discussed.


title: Simple models of proteins with repulsive non-native contacts

abstract:
  The Go model is extended to the case when the non-native contact energies may
be either attractive or repulsive. The folding temperature is found to increase
with the energy of non-native contacts. The repulsive non-native contact
energies may lead to folding at T=0 for some two-dimensional sequences and to
reduction in complexity of disconnectivity graphs for local energy minima.


title:
Phase-ordering dynamics of binary mixtures with field-dependent mobility
  in shear flow

abstract:
  The effect of shear flow on the phase-ordering dynamics of a binary mixture
with field-dependent mobility is investigated. The problem is addressed in the
context of the time-dependent Ginzburg-Landau equation with an external
velocity term, studied in self-consistent approximation. Assuming a scaling
ansatz for the structure factor, the asymptotic behavior of the observables in
the scaling regime can be analytically calculated. All the observables show
log-time periodic oscillations which we interpret as due to a cyclical
mechanism of stretching and break-up of domains. These oscillations are dumped
as consequence of the vanishing of the mobility in the bulk phase.


title: Two-scale competition in phase separation with shear

abstract:
  The behavior of a phase separating binary mixture in uniform shear flow is
investigated by numerical simulations and in a renormalization group (RG)
approach. Results show the simultaneous existence of domains of two
characteristic scales. Stretching and cooperative ruptures of the network
produce a rich interplay where the recurrent prevalence of thick and thin
domains determines log-time periodic oscillations. A power law growth $ R(t)
\sim t^{\alpha}$ of the average domain size, with $\alpha =4/3$ and $\alpha =
1/3$ in the flow and shear direction respectively, is shown to be obeyed.


title: Plateau transitions in fractional quantum Hall liquids

abstract:
  Effects of backward scattering between fractional quantum Hall (FQH) edge
modes are studied. Based on the edge-state picture for hierarchical FQH
liquids, we discuss the possibility of the transitions between different
plateaux of the tunneling conductance $G$. We find a selection rule for the
sequence which begins with a conductance $G=m/(mp\pm 1)$ ($m$: integer, $p$:
even integer) in units of $e^2/h$. The shot-noise spectrum as well as the
scaling behavior of the tunneling current is calculated explicitly.


title:
The effect of geometrical confinement on the interaction between charged
  colloidal suspensions

abstract:
  The effective interaction between charged colloidal particles confined
between two planar like-charged walls is investigated using computer
simulations of the primitive model describing asymmetric electrolytes. In
detail, we calculate the effective force acting onto a single macroion and onto
a macroion pair in the presence of slit-like confinement. For moderate Coulomb
coupling, we find that this force is repulsive. Under strong coupling
conditions, however, the sign of the force depends on the distance to the
plates and on the interparticle distance. In particular, the particle-plate
interaction becomes strongly attractive for small distances which may explain
the occurrence of colloidal crystalline layers near the plates observed in
recent experiments.


title: Phase behaviour of a simple model of globular proteins

abstract:
  A simple model of globular proteins which incorporates anisotropic
attractions is proposed. It is closely related to models used to model simple
hydrogen-bonding molecules such as water. Theories for both the fluid and solid
phases are presented, and phase diagrams calculated. The model protein exhibits
a fluid-fluid transition which is metastable with respect to the fluid-solid
transition for most values of the model parameters. This is behaviour often
observed for globular proteins. The model offers an explanation of the
difficulty observed in crystallising some globular proteins and suggests that
some proteins may not have a solid phase at all under all but extreme
conditions.


title: Spin and orbital ordering in double-layered manganites

abstract:
  We study theoretically the phase diagram of the double-layered perovskite
manganites taking into account the orbital degeneracy, the strong Coulombic
repulsion, and the coupling with the lattice deformation. Observed spin
structural changes as the increased doping are explained in terms of the
orbital ordering and the bond-length dependence of the hopping integral along
$c$-axis. Temperature dependence of the neutron diffraction peak corresponding
to the canting structure is also explained. Comparison with the 3D cubic system
is made.


title:
First- principle calculations of magnetic interactions in correlated
  systems

abstract:
  We present a novel approach to calculate the effective exchange interaction
parameters based on the realistic electronic structure of correlated magnetic
crystals in local approach with the frequency dependent self energy. The analog
of ``local force theorem'' in the density functional theory is proven for
highly correlated systems. The expressions for effective exchange parameters,
Dzialoshinskii- Moriya interaction, and magnetic anisotropy are derived. The
first-principle calculations of magnetic excitation spectrum for ferromagnetic
iron, with the local correlation effects from the numerically exact QMC-scheme
is presented.


title: Prewetting transitions of Ar and Ne on alkali metal surfaces

abstract:
  We have studied by means of Density-Functional calculations the wetting
properties of Ar and Ne adsorbed on a plane whose adsorption properties
simulate the Li and Na surfaces. We use reliable ab-initio potentials to model
the gas-substrate interactions. Evidence for prewetting transitions is found
for all the systems investigated and their wetting phase diagrams are
calculated.


title: Intermittency in Dynamics of Two-Dimensional Vortex-like Defects

abstract:
  We examine high-order dynamical correlations of defects (vortices,
disclinations etc) in thin films starting from the Langevin equation for the
defect motion. We demonstrate that dynamical correlation functions $F_{2n}$ of
vorticity and disclinicity behave as $F_{2n}\sim y^2/r^{4n}$ where $r$ is the
characteristic scale and $y$ is the fugacity. As a consequence, below the
Berezinskii-Kosterlitz-Thouless transition temperature $F_{2n}$ are
characterized by anomalous scaling exponents. The behavior strongly differs
from the normal law $F_{2n}\sim F_2^n$ occurring for simultaneous correlation
functions, the non-simultaneous correlation functions appear to be much larger.
The phenomenon resembles intermittency in turbulence.

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


title:
First-Principles Elastic Constants for the hcp Transition Metals Fe, Co,
  and Re at High Pressure

abstract:
  The elastic constant tensors for the hcp phases of three transition metals
(Co, Re, and Fe) are computed as functions of pressure using the Linearized
Augmented Plane Wave method with both the local density and generalized
gradient approximations. Spin-polarized states are found to be stable for Co
(ferromagnetic) and Fe (antiferromagnetic at low pressure). The elastic
constants of Co and Re are compared to experimental measurements near ambient
conditions and excellent agreement is found. Recent measurements of the lattice
strain in high pressure experiments when interpreted in terms of elastic
constants for Re and Fe are inconsistent with the calculated moduli.


title:
Spin glass behavior upon diluting frustrated magnets and spin liquids: a
  Bethe-Peierls treatment

abstract:
  A Bethe-Peierls treatment to dilution in frustrated magnets and spin liquids
is given. A spin glass phase is present at low temperatures and close to the
percolation point as soon as frustration takes a finite value in the dilute
magnet model; the spin glass phase is reentrant inside the ferromagnetic phase.
An extension of the model is given, in which the spin glass / ferromagnet phase
boundary is shown not to reenter inside the ferromagnetic phase asymptotically
close to the tricritical point whereas it has a turning point at lower
temperatures. We conjecture similar phase diagrams to exist in finite
dimensional models not constraint by a Nishimori's line. We increase
frustration to study the effect of dilution in a spin liquid state. This
provides a ``minimal'' ordering by disorder from an Ising paramagnet to an
Ising spin glass.


title: Theory of Dephasing by External Perturbation in Open Quantum Dots

abstract:
  We propose a random matrix theory describing the influence of a time
dependent external field on the average magnetoresistance of open quantum dots.
The effect is taken into account in all orders of perturbation theory, and the
result is applicable to both weak and strong external fields.


title: Scale Invariant Dynamics of Surface Growth

abstract:
  We describe in detail and extend a recently introduced nonperturbative
renormalization group (RG) method for surface growth. The scale invariant
dynamics which is the key ingredient of the calculation is obtained as the
fixed point of a RG transformation relating the representation of the
microscopic process at two different coarse-grained scales. We review the RG
calculation for systems in the Kardar-Parisi-Zhang universality class and
compute the roughness exponent for the strong coupling phase in dimensions from
1 to 9. Discussions of the approximations involved and possible improvements
are also presented. Moreover, very strong evidence of the absence of a finite
upper critical dimension for KPZ growth is presented. Finally, we apply the
method to the linear Edwards-Wilkinson dynamics where we reproduce the known
exact results, proving the ability of the method to capture qualitatively
different behaviors.


title: Reservoir Layers In High Tc Mercury Cuprates

abstract:
  We put forward the hypothesis that cations with 6s electrons (Hg,Tl,Pb,Bi) in
the charge reservoir layers of high Tc cuprate superconductors actively
participate in the pairing interaction as negative-U centers. We further argue
that the Hg-cuprates are outstanding superconductors (Tc > 160 K) because they
can exist as two-site negative-U centers, Hg2+2. Their electrons are less
localized than in single-site centers (negative-U or bipolaron) and can have a
strong pairing interaction with a smaller increase in effective mass. The Hg2+2
centers are oriented in the x and y directions and can have phase differences
compatible with the d-wave symmetry of the CuO2 planes.


title: Role of symmetry and dimension on pseudogap phenomena

abstract:
  The attractive Hubbard model in d=2 is studied through Monte Carlo
simulations at intermediate coupling. There is a crossover temperature $T_X$
where a pseudogap appears with concomitant precursors of Bogoliubov
quasiparticles that are not local pairs. The pseudogap in $A(k,\omega)$ occurs
in the renormalized classical regime when the correlation length is larger than
the direction-dependent thermal de Broglie wave length, $\xi_{th}=\hbar
v_{F}(k)/k_{B}T.$ The ratio $T_{X}/T_{c}$ for the pseudogap may be made
arbitrarily large when the system is close to a point where the order parameter
has SO(n) symmetry with n>2. This is relevant in the context of SO(5) theories
of high $T_c$ but has more general applicability.


title: Consilience of High-Tc Theories

abstract:
  Improvements both in the quality and in the variety of experiments on
high-temperature superconductors have yielded new insights into the microscopic
origins of pairing. A number of competing theories have already been ruled out.
Some of the more promising descriptions -- gauge theories, coupled-chains,
nesting instabilities, nodal liquids, and stripes -- share features in common.
A unified picture of the cuprates is beginning to emerge.


title: The Origin of Extra Modes in (VO)_2P_2O_7

abstract:
  Recent inelastic neutron scattering experiments on (VO)_2P_2O_7 (VOPO) have
seen previously unanticipated sharp peaks in the dynamic structure factor in
addition to the pair of triplet modes observed earlier. We argue that the
additional features are in essence `shadows' of the previously studied features
arising due to umklapp scattering, and suggest experimental tests of this
proposal. The basic point is illustrated by some elementary calculations within
a strong-coupling expansion for the alternating chain with the right geometry
(on which the matrix element for umklapp scattering depends sensitively) taken
fully into account.


title: First principles theory of fluctuations in vortex liquids and solids

abstract:
  Consistent perturbation theory for thermodynamical quantities in type II
superconductors in magnetic field at low temperatures is developed. It is
complementary to the existing expansion valid at high temperatures.
Magnetization and specific heat are calculated to two loop order and compare
well to existing Monte Carlo simulations and experiments.


title:
Origin of the Charge-Orbital Stripe Structure in La_(1-x)Ca_(x)MnO_(3)
  (x=1/2, 2/3)

abstract:
  We propose the origin of the charge-ordered stripe structure with the orbital
ordering observed experimentally in La_(1-x)Ca_(x)MnO_(3) (x=1/2, 2/3), in
which the long-range Coulomb interaction plays an essential role. We study a
Hubbard model with doubly-degenerate e_g orbitals, and treat the on-site
Coulomb interaction (U) and the nearest-neighbor one (V) with the Hartree-Fock
approximation. Both the charge and orbital ordering structures observed in
experiments are reproduced in a wide region of the U-V phase diagram determined
by the present study. The stability of the orbital ordering is also confirmed
by the perturbation theory.


title:
Structure and Rheology of the Defect-gel States of Pure and
  Particle-dispersed Lyotropic Lamellar Phases

abstract:
  We present important new results from light-microscopy and rheometry on a
moderately concentrated lyotropic smectic, with and without particulate
additives. Shear-treatment aligns the phase rapidly, except for a striking
network of oily-streak defects, which anneals out much more slowly. If
spherical particles several microns in diameter are dispersed in the lamellar
medium, part of the defect network persists under shear-treatment, its nodes
anchored on the particles. The sample as prepared has substantial storage and
loss moduli, both of which decrease steadily under shear-treatment. Adding
particles enhances the moduli and retards their decay under shear. The data for
the frequency-dependent storage modulus after various durations of
shear-treatment can be scaled to collapse onto a single curve. The elasticity
and dissipation in these samples thus arises mainly from the defect network,
not directly from the smectic elasticity and hydrodynamics.


title:
Radiation From a Sine-Gordon Soliton Propagating in an External
  Potential

abstract:
  The interaction of a fast moving sine-Gordon soliton with an external
periodic potential is examined. The resulting equation of motion for the
collective coordinate representing the position of the soliton is given in
relativistic form. We examine the radiation emitted due to the interaction of
the soliton with the potential and we calculate the potential dependent part of
the time evolution equation for the creation and annihilation operators for
fluctuations.


title: Magnetic excitations in charge- ordered NaV2O5

abstract:
  An investigation of the spin excitation spectrum of charge ordered (CO)
NaV2O5 is presented. We discuss several different exchange models which may be
relevant for this compound, namely in- line and zig-zag chain models with weak
as well as strong inter- chain coupling and also a ladder model and a CO/MV
(mixed valent) model. We put special emphasis on the importance of large
additional exchange across the diagonals of V- ladders and the presence of
exchange anisotropies on the excitation spectrum. It is shown that the observed
splitting of transverse dispersion branches may both be interpreted as
anisotropy effect as well as acoustic- optic mode splitting in the weakly
coupled chain models. In addition we calculate the field dependence of
excitation modes in these models. Furthermore we show that for strong inter-
chain coupling, as suggested by recent LDA+U results, an additional high energy
optical excitation appears and the spin gap is determined by anisotropies. The
most promising CO/MV model predicts a spin wave dispersion perpendicular to the
chains which agrees very well with recent results obtained by inelastic neutron
scattering.


title: Effective interaction between star polymers

abstract:
  The distance-resolved effective interaction between two star polymers in a
good solvent is calculated by Molecular Dynamics computer simulations. The
results are compared with a pair potential proposed recently by Likos et al.
[Phys. Rev. Lett. 1998, 80, 4450] which is exponentially decaying for large
distances and crosses over, at the corona diameter of the star, to an ultrasoft
logarithmic repulsion for small distances. Excellent agreement is found in a
broad range of star arm numbers.


title:
The MCS Model of the Superconductivity in Hole-Doped Cuprates: an
  Experimental Evidence

abstract:
  We present experimental data obtained by different techniques. Some of them
are not easy to explain by any theoretical model presented in the literature.
However, a MCS model of the superconductivity in hole-doped cuprates, proposed
in our previous work, is capable to explain them. Thus, we present an evidence
in favor of the MCS model.


title:
Non-universality of compact support probability distributions in random
  matrix theory

abstract:
  The two-point resolvent is calculated in the large-n limit for the
generalized fixed and bounded trace ensembles. It is shown to disagree with the
one of the canonical Gaussian ensemble by a non-universal part which is given
explicitly for all monomial potentials $V(M)=M^{2p}$. Moreover, we prove that
for the generalized fixed and bounded trace ensemble all k-point resolvents
agree in the large-n limit, despite their non-universality.


title: Complexation of DNA with Cationic Surfactant

abstract:
  Transfection of an anionic polynucleotide through a negatively charged
membrane is an important problem in genetic engineering. The direct association
of cationic surfactant to DNA decreases the effective negative charge of the
nucleic acid, allowing the DNA-surfactant complex to approach a negatively
charged membrane. The paper develops a theory for solutions composed of
polyelectrolyte, salt, and ionic surfactant. The theoretical predictions are
compared with the experimental measurements.


title: Semi-classical Theory of Conductance and Noise in Open Chaotic Cavities

abstract:
  Conductance and shot noise of an open cavity with diffusive boundary
scattering are calculated within the Boltzmann-Langevin approach. In
particular, conductance contains a non-universal geometric contribution,
originating from the presence of open contacts. Subsequently, universal
expressions for multi-terminal conductance and noise valid for all chaotic
cavities are obtained classically basing on the fact that the distribution
function in the cavity depends only on energy and using the principle of
minimal correlations.


title:
Many-body Effects in Angle-resolved Photoemission: Quasiparticle Energy
  and Lifetime of a Mo(110) Surface State

abstract:
  In a high-resolution photoemission study of a Mo(110) surface state various
contributions to the measured width and energy of the quasiparticle peak are
investigated. Electron-phonon coupling, electron-electron interactions and
scattering from defects are all identified mechanisms responsible for the
finite lifetime of a valence photo-hole. The electron-phonon induced mass
enhancement and rapid change of the photo-hole lifetime near the Fermi level
are observed for the first time.


title: Nuclear spin driven resonant tunnelling of magnetisation in Mn12 acetate

abstract:
  Current theories still fail to give a satisfactory explanation of the
observed quantum phenomena in the relaxation of the magnetisation of the
molecular cluster Mn12 acetate. In the very low temperature regime, Prokof'ev
and Stamp recently proposed that slowly changing dipolar fields and rapidly
fluctuating hyperfine fields play a major role in the tunnelling process. By
means of a faster relaxing minor species of Mn12ac and a new experimental 'hole
digging' method, we measured the intrinsic line width broadening due to local
fluctuating fields, and found strong evidence for the influence of nuclear
spins on resonance tunnelling at very low temperatures (0.04 - 0.3K). At higher
temperature (1.5 - 4K), we observed a homogeneous line width broadening of the
resonance transitions being in agreement with a recent calculation of
Leuenberger and Loss.


title:
Does spin-orbit coupling play a role in metal-nonmetal transition in
  two-dimensional systems?

abstract:
  We propose an experiment, which would allow to pinpoint the role of
spin-orbit coupling in the metal-nonmetal transition observed in a number of
two-dimensional systems at low densities. Namely, we demonstrate that in a
parallel magnetic field the interplay between the spin-orbit coupling and the
Zeeman splitting leads to a characteristic anisotropy of resistivity with
respect to the direction of the in-plane magnetic field. Though our analytic
calculation is done in the deeply insulating regime, the anisotropy is expected
to persist far beyond that regime.


title: Entropy-driven pumping across zeolites and biological channels

abstract:
  When two binary solutions are separated by a permeable barrier, the
individual species typically diffuse and mix, dissipating their chemical
potential gradients. However, we use model lattice simulations to show that
single-file molecular-sized channels (such biomembrane channels and zeolites)
can exhibit diffusional pumping, where one type of particle uses its entropy of
mixing to drive another up its chemical potential gradient. Quantitative
analyses of rates and efficiencies of transport are plotted as functions of
transmembrane potential, pore length, and particle-pore interactions. Our
results qualitatively explain recent measurements of ``negative'' osmosis and
suggest new, more systematic experiments, particularly in zeolite transport
systems.


title: On the Field-Induced Gap in Cu Benzoate and Other S=1/2 Antiferromagnets

abstract:
  Recent experiments on the S=1/2 antiferromagnetic chain compound, Cu
benzoate, discovered an unexpected gap scaling as approximately the 2/3 power
of an applied magnetic field. A theory of this gap, based on an effective
staggered field, orthogonal to the applied uniform field, resulting from a
staggered gyromagnetic tensor and a Dzyaloshinskii-Moriya interaction, leading
to a sine-Gordon quantum field theory, has been developed. Here we discuss many
aspects of this subject in considerable detail, including a review of the S=1/2
chain in a uniform field, a spin-wave theory analysis of the uniform plus
staggered field problem, exact amplitudes for the scaling of gap, staggered
susceptibility and staggered magnetization with field or temperature,
intensities of soliton and breather peaks in the structure function and field
and temperature dependence of the total susceptibility.


title:
One-Dimensional Arrays of Oscillators: Energy Localization in Thermal
  Equilibrium

abstract:
  All systems in thermal equilibrium exhibit a spatially variable energy
landscape due to thermal fluctuations. Thus at any instant there is naturally a
thermodynamically driven localization of energy in parts of the system relative
to other parts of the system. The specific characteristics of the spatial
landscape such as, for example, the energy variance, depend on the
thermodynamic properties of the system and vary from one system to another. The
temporal persistence of a given energy landscape, that is, the way in which
energy fluctuations (high or low) decay toward the thermal mean, depends on the
dynamical features of the system. We discuss the spatial and temporal
characteristics of spontaneous energy localization in 1D anharmonic chains in
thermal equilibrium.


title:
Phase transitions of electron-hole and unbalanced electron systems in
  coupled quantum wells in high magnetic field

abstract:
  Superfluidity of spatially separated electrons and holes and unbalanced
two-layer electron system in high magnetic field is considered.
  The temperature of the Kosterlitz-Thouless transition to a superfluid state
is obtained as a function of magnetic field and interlayer separation. The
equation of state for magnetoexciton system in quasi-classical regime is
analyzed. The transition from excitonic phase to electron-hole phase is
considered.
  Possible experimental manifestations of the predicted effects are briefly
discussed.


title: Thermodynamics of Kondo Model with Electronic Interactions

abstract:
  On the basis of Bethe ansatz solution of one dimensional Kondo model with
electronic interaction, the thermodynamics equilibrium of the system in finite
temperature is studied in terms of the strategy of Yang and Yang. The string
hypothesis in the spin rapidity is discussed extensively. The thermodynamics
quantities, such as specific heat and magnetic susceptibility, are obtained.


title: Quantum Critical Behavior in Kondo Systems

abstract:
  This article briefly reviews three topics related to the quantum critical
behavior of certain heavy-fermion systems. First, we summarize an extended
dynamical mean-field theory for the Kondo lattice, which treats on an equal
footing the quantum fluctuations associated with the Kondo and RKKY couplings.
The dynamical mean-field equations describe an effective Kondo impurity model
with an additional coupling to vector bosons. Two types of quantum phase
transition appear to be possible within this approach---the first a
conventional spin-density-wave transition, the second driven by local physics.
For the second type of transition to be realized, the effective impurity model
must have a quantum critical point exhibiting an anomalous local spin
susceptibility. In the second part of the paper, such a critical point is shown
to occur in two variants of the Kondo impurity problem. Finally, we propose an
operational test for the existence of quantum critical behavior driven by local
physics. Neutron scattering results suggest that CeCu$_{6-x}$Au$_x$ passes this
test.


title:
Berry Phase Theory of Anomalous Hall Effect: Application to Colossal
  Magnetoresistance Manganites

abstract:
  We show that the Anomalous Hall Effect (AHE) observed in Colossal
Magnetoresistance Manganites is a manifestation of Berry phase effects caused
by carrier hopping in a non-trivial spin background.
  We determine the magnitude and temperature dependence of the Berry phase
contribution to the AHE, finding that it increases rapidly in magnitude as the
temperature is raised from zero through the magnetic transition temperature
T_c, peaks at a temperature $T_{max} > T_c$ and decays as a power of T, in
agreement with experimental data. We suggest that our theory may be relevant to
the anomalous hall effect in conventional ferromagnets as well.


title:
Finite Temperature Time-Dependent Effective Theory For The Goldstone
  Field In A BCS-Type Superfluid

abstract:
  We extend to finite temperature the time-dependent effective theory for the
Goldstone field (the phase of the pair field) $ \theta $ which is appropriate
for a superfluid containing one species of fermions with s-wave interactions,
described by the BCS Lagrangian. We show that, when Landau damping is
neglected, the effective theory can be written as a local time-dependent
non-linear Schr\"{o}dinger Lagrangian (TDNLSL) which preserves the Galilean
invariance of the zero temperature effective theory and is identified with the
superfluid component. We then calculate the relevant Landau terms which are
non-local and which destroy the Galilean invariance. We show that the retarded
$\theta$-propagator (in momentum space) can be well represented by two poles in
the lower-half frequency plane, describing damping with a predicted
temperature, frequency and momentum dependence. It is argued that the real
parts of the Landau terms can be approximately interpreted as contributing to
the normal fluid component.


title:
Disorder-induced phase transition in a one-dimensional model of rice
  pile

abstract:
  We propose a one-dimensional rice-pile model which connects the 1D BTW
sandpile model (Phys. Rev. A 38, 364 (1988)) and the Oslo rice-pile model
(Phys. Rev. lett. 77, 107 (1997)) in a continuous manner. We found that for a
sufficiently large system, there is a sharp transition between the trivial
critical behaviour of the 1D BTW model and the self-organized critical (SOC)
behaviour. When there is SOC, the model belongs to a known universality class
with the avalanche exponent $\tau=1.53$.


title: The new integral relaxation time for thermal activation of spins

abstract:
  The integral relaxation time for the difference of populations of the two
potential wells for double-well classical spin systems is introduced. For the
uniaxial symmetry, it is given by a quadrature. Unlike the previously
introduced integral relaxation time for the magnetization, the former at low
temperatures describes the rate of crossing the barrier under all conditions,
including the strongly biased case. In the high-barrier case, the new integral
relaxation time approaches the inverse of the lowest eigenvalue of the
Fokker-Planck equation. It can be more conveniently found by numerical methods
than the latter.


title:
Similarities between the Hubbard and Periodic Anderson Models at Finite
  Temperatures

abstract:
  The single band Hubbard and the two band Periodic Anderson Hamiltonians have
traditionally been applied to rather different physical problems - the Mott
transition and itinerant magnetism, and Kondo singlet formation and scattering
off localized magnetic states, respectively. In this paper, we compare the
magnetic and charge correlations, and spectral functions, of the two systems.
We show quantitatively that they exhibit remarkably similar behavior, including
a nearly identical topology of the finite temperature phase diagrams at
half-filling. We address potential implications of this for theories of the
rare earth ``volume collapse'' transition.


title:
Jarzynski equality for the transitions between nonequilibrium steady
  states

abstract:
  Jarzynski equality [Phys. Rev. E {\bf 56}, 5018 (1997)] is found to be valid
with slight modefication for the transitions between nonequilibrium stationary
states, as well as the one between equilibrium states. Also numerical results
confirm its validity. Its relevance for nonequilibrium thermodynamics of the
operational formalism is discussed.


title: Low Energy Effective Hamiltonian for the Fractional Quantum Hall Effect

abstract:
  A low energy effective Hamiltonian for the fractional quantum Hall effect is
obtained by using irreducible representations of the symmetry group. It is
found that the model described by the effective Hamiltonian is similar to the
Heisenberg spin chain. Symmetries of the effective Hamiltonian are studied in
order to decompose the relevant Hilbert space. This decomposition will be
useful for further numerical analysis on the gap of the quantum Hall system.


title:
Phase Diagram Of The Biham-Middleton-Levine Traffic Model In Three
  Dimensions

abstract:
  We study numerically the behavior of the Biham-Middleton-Levine traffic model
in three dimensions. Our extensive numerical simulations show that the phase
diagram for this model in three dimensions is markedly different from that in
one and two dimensions. In addition to the full speed moving as well as the
completely jamming phases, whose respective average asymptotic car speeds $<v>$
equal one and zero, we observe an extensive region of car densities $\rho$ with
a low but non-zero average asymptotic car speed. The transition from this
extensive low average asymptotic car speed region to the completely jamming
region is at least second order. We argue that this low speed region is a
result of the formation of a spatially-limited-extended percolating cluster.
Thus, this low speed phase is present in $n > 3$ dimensional
Biham-Middleton-Levine model as well.


title: Universality in quantum parametric correlations

abstract:
  We investigate the universality of correlation functions of chaotic and
disordered quantum systems as an external parameter is varied. A new, general
scaling procedure is introduced which makes the theory invariant under
reparametrizations. Under certain general conditions we show that this
procedure is unique. The approach is illustrated with the particular case of
the distribution of eigenvalue curvatures. We also derive a semiclassical
formula for the non-universal scaling factor, and give an explicit expression
valid for arbitrary deformations of a billiard system.


title: Influence of the confinement geometry on surface superconductivity

abstract:
  The nucleation field for surface superconductivity, $H_{c3}$, depends on the
geometrical shape of the mesoscopic superconducting sample and is substantially
enhanced with decreasing sample size. As an example we studied circular,
square, triangular and wedge shaped disks. For the wedge the nucleation field
diverges as $H_{c3}/H_{c2}=\sqrt{3}/\alpha$ with decreasing angle ($\alpha$) of
the wedge, where $H_{c2}$ is the bulk upper critical field.


title:
Intermediate Regime between the Fermi Glass and the Mott Insulator in
  one Dimension

abstract:
  We consider the ground state reorganization driven by an increasing nearest
neighbor repulsion U for spinless fermions in a strongly disordered ring. When
U -> 0, the electrons form a glass with Anderson localized states. At half
filling, a regular array of charges (Mott insulator) is pinned by the random
substrate when U -> \infty. Between those two insulating limits, we show that
there is an intermediate regime where the electron glass becomes more liquid
before crystallizing. The liquid-like behavior of the density-density
correlation function is accompanied by an enhancement of the persistent
current.


title:
Crossover from BCS superconductivity to BEC of pairs: The role of the
  lifetime of the pairs

abstract:
  The understanding of an electron gas with short coherence length pairs formed
by an attractive interaction is believed to be one of the major keys to our
theoretical knowledge of the high-T_c-superconductors. Mainly the deviations of
the cuprates from usual metallic Fermi liquid behaviour already in the normal
state like e.g. a linear resistivity or the observation of a pseudo gap can
result from electron-electron correlations.
  We therefore investigate the negative U Hubbard model in two dimensions at
low densities using the T-matrix approximation. In the non selfconsistent
formulation of the theory the system always shows an instability towards Bose
condensation of pairs into an infinite lifetime two-particle bound state. If
the calculations are performed selfconsistently pair-pair scattering is
included which causes the pairs to have finite lifetime. The physics of these
finite lifetime pairs is discussed.


title:
Self-consistent treatment of dynamical correlation functions using a
  spectral representation technique

abstract:
  A system of equations resulting from an approximation of the equation of
motion of Green functions for correlated electron systems is usually solved
using Matsubara technique. In this work we propose an alternative method which
works entirely along the real frequency axis. Using the example of the
attractive Hubbard model studied in the T-matrix approximation both
self-consistently and non-self-consistently we demonstrate how powerful such a
treatment is especially when dynamic quantities are calculated.


title: Wavefunctions for the Luttinger liquid

abstract:
  Standard bosonization techniques lead to phonon-like excitations in a
Luttinger liquid (LL), reflecting the absence of Landau quasiparticles in these
systems. Yet in addition to the above excitations some LL are known to possess
solitonic states carrying fractional quantum numbers (e.g. the spin 1/2
Heisenberg chain). We have reconsidered the zero modes in the low-energy
spectrum of the gaussian boson LL hamiltonian both for fermionic and bosonic
LL: in the spinless case we find that two elementary excitations carrying
fractional quantum numbers allow to generate all the charge and current excited
states of the LL. We explicitly compute the wavefunctions of these two objects
and show that one of them can be identified with the 1D version of the Laughlin
quasiparticle introduced in the context of the Fractional Quantum Hall effect.
For bosons, the other quasiparticle corresponds to a spinon excitation. The
eigenfunctions of Wen's chiral LL hamiltonian are also derived: they are quite
simply the one dimensional restrictions of the 2D bulk Laughlin wavefunctions.


title: Optical properties of an interacting large polaron gas

abstract:
  The normal state conductivity of a system of interacting large polarons is
calculated within the Random Phase approximation and some numerical results are
presented. The behaviour of the optical absorption as a function of the charge
carrier density and of the temperature is analyzed for different values of the
electron-phonon coupling constant. It is shown that the conductivity exihibits
features similar to thos observed in the infrared spectra of the cuprates.


title: Theory of interlayer exchange coupling

abstract:
  This paper contains the notes of lectures on the theory of interlayer
exchange coupling presented at the 30-th Ferienschule of the Institut fuer
Festkoerperforschung, Forschungszentrum Juelich, March 1999.


title:
Coulomb blockade conductance peak fluctuations in quantum dots and the
  independent particle model

abstract:
  We study the combined effect of finite temperature, underlying classical
dynamics, and deformations on the statistical properties of Coulomb blockade
conductance peaks in quantum dots. These effects are considered in the context
of the single-particle plus constant-interaction theory of the Coulomb
blockade. We present numerical studies of two chaotic models, representative of
different mean-field potentials: a parametric random Hamiltonian and the smooth
stadium. In addition, we study conductance fluctuations for different
integrable confining potentials. For temperatures smaller than the mean level
spacing, our results indicate that the peak height distribution is nearly
always in good agreement with the available experimental data, irrespective of
the confining potential (integrable or chaotic). We find that the peak bunching
effect seen in the experiments is reproduced in the theoretical models under
certain special conditions. Although the independent particle model fails, in
general, to explain quantitatively the short-range part of the peak height
correlations observed experimentally, we argue that it allows for an
understanding of the long-range part.


title: Photon shot noise

abstract:
  A recent theory is reviewed for the shot noise of coherent radiation
propagating through a random medium. The Fano factor P/I (the ratio of the
noise power and the mean transmitted current) is related to the scattering
matrix of the medium. This is the optical analogue of Buttiker's formula for
electronic shot noise. Scattering by itself has no effect on the Fano factor,
which remains equal to 1 (as for a Poisson process). Absorption and
amplification both increase the Fano factor above the Poisson value. For strong
absorption P/I has the universal limit 1+3f/2 with f the Bose-Einstein function
at the frequency of the incident radiation. This is the optical analogue of the
one-third reduction factor of electronic shot noise in diffusive conductors. In
the amplifying case the Fano factor diverges at the laser threshold, while the
signal-to-noise ratio I^2/P reaches a finite, universal limit.


title: Vortex Plasma in a Superconducting Film with Magnetic Dots

abstract:
  We consider a superconducting film, placed upon a magnetic dot array.
Magnetic moments of the dots are normal to the film and randomly oriented. We
determine how the concentration of the vortices in the film depends on the
magnetic moment of a dot at low temperatures. The concentration of the
vortices, bound to the dots, is proportional to the density of the dots and
depends on the magnetization of a dot in a step-like way. The concentration of
the unbound vortices oscillates about a value, proportional to the magnetic
moment of the dots. The period of the oscillations is equal to the width of a
step in the concentration of the bound vortices.


title:
The superconducting gap \Delta_{c}, the pseudogap \Delta_{p} and pair
  fluctuations above Tc in overdoped YCaBaCuO from femtosecond time-domain
  spectroscopy

abstract:
  The low-energy electronic excitation spectrum and gap structure in optimally
doped and overdoped Y_{1-x}Ca_{x}Ba_{2}Cu_{3}O_{7-\delta} single crystals are
investigated by real-time measurements of the quasiparticle relaxation dynamics
with femtosecond optical spectroscopy. From the amplitude of the photoinduced
reflectivity as a function of time, temperature and doping x we find clear
evidence for the coexistence of two distinct gaps in the entire overdoped
phase. One is a temperature-independent pseudogap $\Delta_{p}$ and the other is
a T-dependent collective gap $\Delta_{c}(T)$ which has a BCS-like T-dependence
closing at Tc. From quasiparticle relaxation time measurements above Tc we
ascertain that fluctuations associated with the collective gap $\Delta_{c}(T)$
are limited to a few K, consistent with time-dependent Ginzburg-Landau theory
and are distinct from the pseudogap whose presence is apparent well above Tc
for all x.


title:
Kinetic approach to the Gaussian thermostat in a dilute sheared gas in
  the thermodynamic limit

abstract:
  A dilute gas of particles with short range interactions is considered in a
shearing stationary state. A Gaussian thermostat keeps the total kinetic energy
constant. For infinitely many particles it is shown that the thermostat becomes
a friction force with constant friction coefficient. For finite number of
particles N, the fluctuations around this constant are of order one over the
square root of N, and distributed approximately Gaussian with deviations for
large fluctuations. These deviations prohibit a derivation of
fluctuation-dissipation relations far from equilibrium, based on the
Fluctuation Theorem.


title: A new quantum phase in two dimensions

abstract:
  For intermediate Coulomb energy to Fermi energy ratios $r_s$, spinless
fermions in a random potential form a new quantum phase which is different from
the Fermi glass and the Wigner crystal. From a numerical study of small
clusters, we show that this phase is characterized by an ordered flow of
enhanced persistent currents and occurs for $r_s$ values where a metallic phase
has been observed in two dimensions.


title: Drag and Hall drag in a bi-layer system with pinholes

abstract:
  The transresistance and the Hall transresistance of dirty two-dimensional
bi-layer systems in the presence of tunneling bridges (pinholes) are studied
theoretically. We find, at weak magnetic field, a non-zero Hall
transresistance. In a geometry where the pinholes are concentrated in the
middle of the sample, a quantum process gives the dominant contribution to both
the ordinary transresistance and the Hall transresistance. Arising from the
interplay between Coulomb repulsion, disorder and tunneling, the quantum
contribution increases in a singular way as the temperature decreases.


title:
Quantum spin ladder with dynamic phonons. Bound states of magnons and
  phonons

abstract:
  We consider the two-chain Heisenberg ladder with antiferromagnetic
interactions. Our approach is based on the description of spin excitations as
triplets above a strong-coupling singlet ground state. It is shown that
interaction with dynamic phonons drastically changes the dispersion of the
elementary triplet excitation (magnon) and the spin structure factor. It is
also shown that a new triplet excitation appears in the spectrum: the bound
state of a magnon and a phonon.


title: The profile of a decaying crystalline cone

abstract:
  The decay of a crystalline cone below the roughening transition is studied.
We consider local mass transport through surface diffusion, focusing on the two
cases of diffusion limited and attachment-detachment limited step kinetics. In
both cases, we describe the decay kinetics in terms of step flow models.
Numerical simulations of the models indicate that in the attachment-detachment
limited case the system undergoes a step bunching instability if the repulsive
interactions between steps are weak. Such an instability does not occur in the
diffusion limited case. In stable cases the height profile, h(r,t), is flat at
radii r<R(t)\sim t^{1/4}. Outside this flat region the height profile obeys the
scaling scenario \partial h/\partial r = {\cal F}(r t^{-1/4}). A scaling ansatz
for the time-dependent profile of the cone yields analytical values for the
scaling exponents and a differential equation for the scaling function. In the
long time limit this equation provides an exact description of the discrete
step dynamics. It admits a family of solutions and the mechanism responsible
for the selection of a unique scaling function is discussed in detail. Finally
we generalize the model and consider permeable steps by allowing direct adatom
hops between neighboring terraces. We argue that step permeability does not
change the scaling behavior of the system, and its only effect is a
renormalization of some of the parameters.


title:
Determination of critical current density in melt-processed HTS bulks
  from levitation force measurements

abstract:
  A simple approach to describe the levitation force measurements on
melt-processed HTS bulks was developed. A couple of methods to determine the
critical current density $J_c$ were introduced. The averaged $ab$-plane $J_c$
values for the field parallel to this plane were determined. The first and
second levitation force hysteresis loops calculated with these $J_c$ values
coincide remarkably well with the experimental data.


title: Observation of Quantum Asymmetry in an Aharonov-Bohm Ring

abstract:
  We have investigated the Aharonov-Bohm effect in a one-dimensional
GaAs/GaAlAs ring at low magnetic fields. The oscillatory magnetoconductance of
these systems are for the first time systematically studied as a function of
density. We observe phase-shifts of $\pi$ in the magnetoconductance
oscillations, and halving of the fundamental $h/e$ period, as the density is
varied. Theoretically we find agreement with the experiment, by introducing an
asymmetry between the two arms of the ring.


title: Quasiperiodicity, bistability and chaos in the Landau-Lifshitz equation

abstract:
  The dynamics of an individual magnetic moment is studied through the
Landau-Lifshitz equation with a periodic driving in the direction perpendicular
to the applied field. For fields lower than the anisotropy field and small
values of the perturbation amplitude we have observed the magnetic moment
bistability. At intermediate values we have found quasiperiodic bands
alternating with periodic motion. At even larger values a chaotic regime is
found. When the applied field is larger than the anisotropy one, the behavior
is periodic with quasiperiodic regions. Those appear periodically in the
amplitude of the oscillating field. Also, even for low values of the driving
force, the moment is not parallel to the applied field.


title:
Zero-Point Fluctuations and the Quenching of the Persistent Current in
  Normal Metal Rings

abstract:
  The ground state of a phase-coherent mesoscopic system is sensitive to its
environment. We investigate the persistent current of a ring with a quantum dot
which is capacitively coupled to an external circuit with a dissipative
impedance. At zero temperature, zero-point quantum fluctuations lead to a
strong suppression of the persistent current with decreasing external
impedance. We emphasize the role of displacement currents in the dynamical
fluctuations of the persistent current and show that with decreasing external
impedance the fluctuations exceed the average persistent current.


title: Infrared Studies of the Onset of Conductivity in Ultra-Thin Pb Films

abstract:
  In this paper we report the first experimental measurement of the infrared
conductivity of ultra-thin quenched-condensed Pb films. For dc sheet
resistances such that $\omega \tau \ll 1$ the ac conductance increases with
frequency but is in disagreement with the predictions of weak localization. We
attribute this behavior to the effects of an inhomogeneous granular structure
of these films, which is manifested at the very small probing scale of infrared
measurements. Our data are consistent with predictions of two-dimensional
percolation theory.


title: Study of the one-dimensional off-lattice hot-monomer reaction model

abstract:
  Hot monomers are particles having a transient mobility (a ballistic flight)
prior to being definitely absorbed on a surface. After arriving at a surface,
the excess energy coming from the kinetic energy in the gas phase is dissipated
through degrees of freedom parallel to the surface plane. In this paper we
study the hot monomer-monomer adsorption-reaction process on a continuum
(off-lattice) one-dimensional space by means of Monte Carlo simulations. The
system exhibits second-order irreversible phase transition between a reactive
and saturated (absorbing) phases which belong to the directed percolation (DP)
universality class. This result is interpreted by means of a coarse-grained
Langevin description which allows as to extend the DP conjecture to transitions
occurring in continuous media.


title:
Many-Body Renormalization of Semiconductor Quantum Wire Excitons:
  Absorption, Gain, Binding, Unbinding, and Mott Transition

abstract:
  We consider theoretically the formation and stability of quasi-one
dimensional many-body excitons in GaAs quantum wire structures under external
photoexcitation conditions by solving the dynamically screened Bethe-Salpeter
equation for realistic Coulomb interaction. In agreement with several recent
experimental findings the calculated excitonic peak shows very weak carrier
density dependence upto (and even above) the Mott transition density,
$n_c\sim3\times10^5$ cm$^{-1}$. Above $n_c$ we find considerable optical gain
demonstrating compellingly the possibility of one-dimensional quantum wire
laser operation.


title: Evolutionary freezing in a competitive population

abstract:
  We show that evolution in a population of adaptive agents, repeatedly
competing for a limited resource, can come to an abrupt halt. This transition
from evolutionary to non-evolutionary behavior arises as the global resource
level is changed, and is reminiscent of a phase transition to a frozen state.
Its origin lies in the inductive decision-making of the agents, the limited
global information that they possess and the dynamical feedback inherent in the
system.

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


title:
Critical Current Enhancement due to an Electric Field in a Granular
  d-Wave Superconductor

abstract:
  We study the effects of an electric-field in the transport properties of bulk
granular superconductors with different kinds of disorder. We find that for a
d-wave granular superconductor with random $\pi$-junctions the critical current
always increases after applying a strong electric field, regardless of the
polarity of the field. This result plus a change in the voltage as a function
of the electric field are in good agreement with experimental results in
ceramic high T_c superconductors.


title: Heavy-Fermions in a Transition-Metal Compound: $LiV_2O_4$

abstract:
  The recent discovery of heavy-Fermion properties in Lithium Vanadate and the
enormous difference in its properties from the properties of Lithium Titanate
as well as of the manganite compounds raise some puzzling questions about
strongly correlated Fermions. These are disscussed as well as a solution to the
puzzles provided.


title:
Physics on the edge: contour dynamics, waves and solitons in the quantum
  Hall effect

abstract:
  We present a theoretical study of the excitations on the edge of a
two-dimensional electron system in a perpendicular magnetic field in terms of a
contour dynamics formalism. In particular, we focus on edge excitations in the
quantum Hall effect. Beyond the usual linear approximation, a non-linear
analysis of the shape deformations of an incompressible droplet yields soliton
solutions which correspond to shapes that propagate without distortion. A
perturbative analysis is used and the results are compared to analogous
systems, like vortex patches in ideal hydrodynamics. Under a local induction
approximation we find that the contour dynamics is described by a non-linear
partial differential equation for the curvature: the modified Korteweg-de Vries
equation.
  PACS number(s): 73.40.Hm, 02.40.Ma, 03.40.Gc, 11.10.Lm


title: Spectroscopy, Interactions and Level Splittings in Au Nanoparticles

abstract:
  We have measured the electronic energy spectra of nm-scale Au particles using
a new tunneling spectroscopy configuration. The particle diameters ranged from
5nm to 9nm, and at low energies the spectrum is discrete, as expected by the
electron-in-a-box model. The density of tunneling resonances increases rapidly
with energy, and at higher energies the resonances overlap forming broad
resonances. Near the Thouless energy, the broad resonances merge into a
continuum. The tunneling resonances display Zeeman splitting in a magnetic
field. Surprisingly, the g-factors (~0.3) of energy levels in Au nano-particles
are much smaller than the g-factor (2.1) in bulk gold.


title:
Wilson's ratio and the spin splitting of magnetic oscillations in
  quasi-two-dimensional metals

abstract:
  A simple consistency check is proposed for the Fermi liquid description of
the low-temperature properties of quasi-two-dimensional metals. In a
quasi-two-dimensional Fermi liquid the Zeeman splitting of magnetic
oscillations can be used to determine g^*, the gyro-magnetic ratio which is
renormalised by many-body effects. It is shown that g^*/g is equal to Wilson's
ratio R, the dimensionless ratio of the low-temperature spin susceptibility to
the specific heat coefficient. Measured values of g^*/g and R are compared for
the layered perovskite Sr2RuO4 and for a range of organic metals based on the
BEDT-TTF molecule. It is also shown that the Pauli paramagnetic limiting
magnetic field, at which singlet superconductivity should be destroyed by the
Zeeman splitting of the electron spins, is changed from the values given by BCS
theory by a factor of g^*/g.


title:
External-field-induced tricritical point in a fluctuation-driven
  nematic-smectic-A transition

abstract:
  We study theoretically the effect of an external field on the
nematic-smectic-A (NA) transition close to the tricritical point, where
fluctuation effects govern the qualitative behavior of the transition. An
external field suppresses nematic director fluctuations, by making them
massive. For a fluctuation-driven first-order transition, we show that an
external field can drive the transition second-order. In an appropriate liquid
crystal system, we predict the required magnetic field to be of order 10 T. The
equivalent electric field is of order $1 V/\mu m$.


abstract:
  Time-dependent Ginzburg-Landau model with noise for neutral s-wave
superconductor is derived from real-time finite-temperature quantum field
theory by integrating over fermions in the density matrix propagator. Quantum
decoherence due to environment of pair breaking fluctuations is identified.
Dynamics is described by Langevin equation in the near critical regime and by a
nonlinear Schrodinger equation at zero temperature.

categories: cond-mat.supr-con cond-mat.stat-mech hep-th quant-ph


title:
On the critical behavior of disordered quantum magnets: The relevance of
  rare regions

abstract:
  The effects of quenched disorder on the critical properties of itinerant
quantum antiferromagnets and ferromagnets are considered. Particular attention
is paid to locally ordered spatial regions that are formed in the presence of
quenched disorder even when the bulk system is still in the paramagnetic phase.
These rare regions or local moments are reflected in the existence of spatially
inhomogeneous saddle points of the Landau-Ginzburg-Wilson functional. We derive
an effective theory that takes into account small fluctuations around all of
these saddle points. The resulting free energy functional contains a new term
in addition to those obtained within the conventional perturbative approach,
and it comprises what would be considered non-perturbative effects within the
latter. A renormalization group analysis shows that in the case of
antiferromagnets, the previously found critical fixed point is unstable with
respect to this new term, and that no stable critical fixed point exists at
one-loop order. This is contrasted with the case of itinerant ferromagnets,
where we find that the previously found critical behavior is unaffected by the
rare regions due to an effective long-ranged interaction between the order
parameter fluctuations.


title: Trapped Bose-Einstein Condensates: Role of Dimensionality

abstract:
  We analyse systematically, from the viewpoint of the nonlinear physics of
solitary waves, the effect of the spatial dimension (D = 1,2,3) on the
structure and stability of the Bose-Einstein condensates (BECs) trapped in an
external anisotropic parabolic potential. While for the positive scattering
length the stationary ground-state solutions of the Gross-Pitaevskii equation
are shown to be always stable independently of the spatial dimension, for the
negative scattering length the ground-state condensate is stable only in the 1D
and 2D cases, whereas in the 3D case it becomes unstable. A direct link between
nonlinear modes of BECs and (bright and dark) solitary waves of the nonlinear
Schrodinger equation is demonstrated for all the dimensions.


title: Transport in Luttinger Liquids

abstract:
  We give a brief introduction to Luttinger liquids and to the phenomena of
electronic transport or conductance in quantum wires. We explain why the
subject of transport in Luttinger liquids is relevant and fascinating and
review some important results on tunneling through barriers in a
one-dimensional quantum wire and the phenomena of persistent currents in
mesoscopic rings. We give a brief description of our own work on transport
through doubly-crossed Luttinger liquids and transport in the Schulz-Shastry
exactly solvable Luttinger-like model.


title: On the possibility of optimal investment

abstract:
  We analyze the theory of optimal investment in risky assets, developed
recently by Marsili, Maslov and Zhang [Physica A 253 (1998) 403]. When the real
data are used instead of abstract stochastic process, it appears that a
non-trivial investment strategy is rarely possible. We show that non-zero
transaction costs make the applicability of the method even more difficult. We
generalize the method in order to take into account possible correlations in
the asset price.


title: Vortex dynamics in bulk HTS with levitation techniques

abstract:
  We have developed a set of experimental techniques with levitation to
investigate the macroscopic magnetic properties of bulk HTS: (i) resonance
oscillation technique; (ii) high speed magnetic rotor technique; (iii) the
method of viscous motion of a permanent magnet through a HTS aperture under a
steady force. Early, we used these techniques to investigate the magnetic flux
dynamics in granular HTS. Now, we have extended and modified these techniques
to study the large grain melt-textured HTS. With resonance oscillations
technique we obtain the energy losses vs ac magnetic field amplitude at HTS
surface and can derive the values of critical current density with high
accuracy. We extract information about distribution of critical current in a
thin undersurface layer and, in this way, check the HTS degradation processes.
The treatment of the HTS surface that has influence on the surface pinning was
investigated with this technique.


title: How sandpiles spill: Sandpile problem in a thick flow regime

abstract:
  We obtain an analytical solution of a one-dimensional sandpile problem in a
thick flow regime, when it can be formulated in terms of linear equations. It
is shown that a space periodicity takes place during the sandpile evolution
even for a sandpile of only one type of particles. Similar periodicity was
observed previo usly for many component sandpiles. Space periods are
proportional to an input flow of particles $r_0$. We find that the surface
angle $\theta$ of the pile reaches it's final critical value ($\theta_f$) from
lower values only at long times. The deviation ($\theta_f - \theta$) behaves as
$(t/r_{0})^{-1/2}$.


title: The Yrast Spectra of Weakly Interacting Bose-Einstein Condensates

abstract:
  The low energy quantal spectrum is considered as a function of the total
angular momentum for a system of weakly interacting bosonic atoms held together
by an external isotropic harmonic potential. It is found that besides the usual
condensation into the lowest state of the oscillator, the system exhibits two
additional kinds of condensate and associated thermodynamic phase transitions.
These new phenomena are derived from the degrees of freedom of "partition
space" which describes the multitude of different ways in which the angular
momentum can be distributed among the atoms while remaining all the time in the
lowest state of the oscillator.


title:
Effects of Spin Fluctuations in Quasi-One-Dimensional Organic
  Superconductors

abstract:
  We study the electronic states of quasi-one-dimensional organic conductors
using the single band Hubbard model at half-filling. We treat the effects of
the on-site Coulomb interaction by the fluctuation-exchange (FLEX) method, and
calculate the phase diagram and physical properties. The calculated pressure
dependence of the Neel temperature coincides well with the experimental one. We
also show that a pseudogap is formed in the density of states near the chemical
potential and that d-wave superconductivity appears next to the
antiferromagnetic state. Moreover the NMR relaxation rate increases on cooling
in the low-temperature region.


title:
Fluids of hard ellipsoids: Phase diagram including a nematic instability
  from Percus-Yevick theory

abstract:
  An important aspect of molecular fluids is the relation between orientation
and translation parts of the two-particle correlations. Especially the detailed
knowledge of the influence of orientation correlations is needed to explain and
calculate in detail the occurrence of a nematic phase.
  The simplest model system which shows both orientation and translation
correlations is a system of hard ellipsoids. We investigate an isotropic fluid
formed of hard ellipsoids with Percus-Yevick theory.
  Solving the Percus-Yevick equations self-consistently in the high density
regime gives a clear criterion for a nematic instability. We calculate in
detail the equilibrium phase diagram for a fluid of hard ellipsoids of
revolution. Our results compare well with Monte Carlo Simulations and density
functional theory.


title: Exact statistical properties of the Burgers equation

abstract:
  The one dimensional Burgers equation in the inviscid limit with white noise
initial condition is revisited. The one- and two-point distributions of the
Burgers field as well as the related distributions of shocks are obtained in
closed analytical forms. In particular, the large distance behavior of spatial
correlations of the field is determined. Since higher order distributions
factorize in terms of the one and two points functions, our analysis provides
an explicit and complete statistical description of this problem.


title: Weak localisation in AlGaAs/GaAs p-type quantum wells

abstract:
  We have for the first time experimentally investigated the weak localisation
magnetoresistance in a AlGaAs/GaAs p-type quantum well. The peculiarity of such
systems is that spin-orbit interaction is strong. On the theoretical side it is
not possible to treat the spin-orbit interaction as a perturbation. This is in
contrast to all prior investigations of weak localisation. In this letter we
compare the experimental results with a newly developed diffusion theory, which
explicitly describes the weak localisation regime when the spin-orbit coupling
is strong. The spin relaxation rates calculated from the fitting parameters was
found to agree with theoretical expectations. Furthermore the fitting
parameters indicate an enhanced phase breaking rate compared to theoretical
predictions.


title:
Simulation of the Zero Temperature Behavior of a 3-Dimensional Elastic
  Medium

abstract:
  We have performed numerical simulation of a 3-dimensional elastic medium,
with scalar displacements, subject to quenched disorder. We applied an
efficient combinatorial optimization algorithm to generate exact ground states
for an interface representation. Our results indicate that this Bragg glass is
characterized by power law divergences in the structure factor $S(k)\sim A
k^{-3}$. We have found numerically consistent values of the coefficient $A$ for
two lattice discretizations of the medium, supporting universality for $A$ in
the isotropic systems considered here. We also examine the response of the
ground state to the change in boundary conditions that corresponds to
introducing a single dislocation loop encircling the system. Our results
indicate that the domain walls formed by this change are highly convoluted,
with a fractal dimension $d_f=2.60(5)$. We also discuss the implications of the
domain wall energetics for the stability of the Bragg glass phase. As in other
disordered systems, perturbations of relative strength $\delta$ introduce a new
length scale $L^* \sim \delta^{-1/\zeta}$ beyond which the perturbed ground
state becomes uncorrelated with the reference (unperturbed) ground state. We
have performed scaling analysis of the response of the ground state to the
perturbations and obtain $\zeta = 0.385(40)$. This value is consistent with the
scaling relation $\zeta=d_f/2- \theta$, where $\theta$ characterizes the
scaling of the energy fluctuations of low energy excitations.


title:
Macroscopic Quantum Fluctuations in the Josephson Dynamics of Two Weakly
  Linked Bose-Einstein Condensates

abstract:
  We study the quantum corrections to the Gross-Pitaevskii equation for two
weakly linked Bose-Einstein condensates. The goals are: 1) to investigate
dynamical regimes at the borderline between the classical and quantum behaviour
of the bosonic field; 2) to search for new macroscopic quantum coherence
phenomena not observable with other superfluid/superconducting systems. Quantum
fluctuations renormalize the classical Josephson oscillation frequencies. Large
amplitude phase oscillations are modulated, exhibiting collapses and revivals.
We describe a new inter-well oscillation mode, with a vanishing (ensemble
averaged) mean value of the observables, but with oscillating mean square
fluctuations. Increasing the number of condensate atoms, we recover the
classical Gross-Pitaevskii (Josephson) dynamics, without invoking the
symmetry-breaking of the Gauge invariance.

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


title:
Weak Localization in the Conductance Peaks of Coulomb Blockade Quantum
  Dots

abstract:
  We derive closed expressions for the universal weak localization peak of the
average conductance peak heights in Coulomb blockade quantum dots in the
crossover from orthogonal to unitary symmetry. The scale for the crossover is
independent of the number of channels in each lead, in contrast with the case
of open dots. The functional form of the weak localization peak is independent
of temperature. We also derive analytically the variance of the conductance
peak heights as a function of the crossover parameter.


title: Quasiclassical Hamiltonians for large-spin systems

abstract:
  We propose a method for obtaining effective classical Hamiltonians \cal H for
many-body quantum spin systems with large spins. This method uses the
coherent-state representation of the partition function Z and the cumulant
expansion in powers of 1/S. For the quantum Hamiltonian \hat H of a Heisenberg
form, the 1/S corrections in \cal H have a non-Heisenberg many-spin form. The
effective Hamiltonian \cal H can be treated by methods familiar for classical
systems. The non-Heisenberg terms in \cal H may be responsible for such effects
as spin-Peierls transition and uplifting of the classical degeneracy by quantum
fluctuations.


title: Dissipative dynamics of a kink state in a Bose-condensed gas

abstract:
  We develop a theory of dissipative dynamics of a kink state in a
finite-temperature Bose-condensed gas. We find that due to the interaction with
the thermal cloud the kink state accelerates towards the velocity of sound and
continuously transforms to the ground-state condensate. We calculate the
life-time of a kink state in a trapped gas and discuss possible experimental
implications.


title:
Modeling nonlinearity and hysteresis due to critical-state flux
  penetration in hard superconductors

abstract:
  The nonlinear surface impedance of a thin superconducting strip carrying a
microwave current has been calculated numerically from first principles based
upon flux penetration due to a current-induced critical state. The calculations
approach analytic results in the appropriate limit. The technique is useful for
design of passive superconducting microwave circuits.


title: Statistical-thermodynamical foundations of anomalous diffusion

abstract:
  It is shown that Tsallis' generalized statistics provides a natural frame for
the statistical-thermodynamical description of anomalous diffusion. Within this
generalized theory, a maximum-entropy formalism makes it possible to derive a
mathematical formulation for the mechanisms that underly Levy-like
superdiffusion, and for solving the nonlinear Fokker-Planck equation.


title:
Observation of Coherent Charge-State Mixing in Asymmetric Bloch
  Transistors

abstract:
  The reduced switching current of an excited energy-band in a Bloch transistor
can be used to detect microwave-induced interband transitions. Spectroscopic
measurements of the band-gap are performed by mapping the frequency dependence
of the excitation threshold where the microwave photon energy and band-gap are
equal. This excitation threshold also provides a probe of charge-state mixing
on the island of the transistor. Any asymmetry in the junctions of the
transistor makes the coherent mixing of charge-states appear as a finite
gap-splitting at the electrostatic degeneracy point between states differing by
one excess Cooper-pair on the island. The measured gap-splitting in an
asymmetric transistor clearly demonstrates the effect.


title: Evolutionary dynamics in the Bak-Sneppen model on small-world networks

abstract:
  We study the dynamics of the Bak-Sneppen model on small-world networks. For
each site in the network, we define a ``connectance,'' which measures the
distance to all other sites. We find radically different patterns of activity
for different sites, depending on their connectance and also on the topology of
the network. For a given network, the site with the minimal connectance shows
long periods of stasis interrupted by much smaller periods of activity. In
contrast, the activity pattern for the maximally connected site appears uniform
on the same time scale. We discuss the significance of these results for
speciation events.


title: Quasiparticles of d-wave superconductors in finite magnetic fields

abstract:
  We study quasiparticles of d-wave superconductors in the vortex lattice by
self-consistently solving the Bogoliubov-de Gennes equations. It is found for a
pure $d_{x^2-y^2}$ state that: (i) low-energy quasiparticle bands in the
magnetic Brillouin zone have rather large dispersion even in low magnetic
fields, indicating absense of bound states for an isolated vortex; (ii) in
finite fields with $k_F \xi_0$ small, the calculated tunneling conductance at
the vortex core shows a double-peak structure near zero bias, as qualitatively
consistent with the STM experiment by Maggio-Aprile et al. [Phys. Rev. Lett.
{\bf 75} (1995) 2754]. We also find that mixing of a $d_{xy}$- or an s-wave
component, if any, develops gradually without transitions as the field is
increased, having little effect on the tunneling spectra.


title: Vortex states of the E_{u} model for Sr_{2}RuO_{4}

abstract:
  Based on the Ginzburg-Landau functional of E_{u} symmetry presented by
Agterberg, vortex states of Sr_{2}RuO_{4} are studied in detail over
$H_{c1}\lesssim H \leq H_{c2}$ by using the Landau-level expansion method. For
the field in the basal plane, it is found that (i) the second superconducting
transition should be present irrespective of the field direction; (ii) below
this transition, a characteristic double-peak structure may develop in the
magnetic-field distribution; (iii) a third transition may occur between two
different vortex states. It is also found that, when the field is along the c
axis, the square vortex lattice may deform through a second-order transition
into a rectangular one as the field is lowered from H_{c2}. These predictions
will be helpful in establishing the E_{u} model for Sr_{2}RuO_{4}.


title: Microstructure analysis of reconstructed porous media

abstract:
  We compare the quantitative microstructural properties of Berea Sandstone
with stochastic reconstructions of the same sandstone. The comparison is based
on local porosity theory. The reconstructions employ Fourier space filtering of
Gaussian random fields and match the average porosity and two-point correlation
function of the experimental model. Connectivity properties of the stochastic
models differ significantly from the experimental model. Reconstruction models
with different levels of coarse graining also show different average local
connectivity.


title:
Coarse-grained surface energies and temperature-induced anchoring
  transitions in nematic liquid crystals

abstract:
  We introduce a coarse-grained description of the surface energy of a nematic
liquid crystal. The thermal fluctuations of the nematic director close to the
surface renormalize at macroscopic scales the bare surface potential in a
temperature-dependent way. The angular dependence of the renormalized potential
is dramatically smoothed, thus explaining the success of the Rapini-Papoular
form. Close to the isotropic phase, the anchoring energy is strongly suppressed
and the change of its shape allows for anchoring transitions. Our theory
describes quantitatively the temperature dependence of the anchoring energy and
the temperature-induced anchoring transitions reported in the literature.


title:
Magnetoresistance Anomalies in Antiferromagnetic YBa_2Cu_3O_{6+x}:
  Fingerprints of Charged Stripes

abstract:
  We report novel features in the in-plane magnetoresistance (MR) of heavily
underdoped YBa_2Cu_3O_{6+x}, which unveil a developed ``charged stripe''
structure in this system. One of the striking features is an anisotropy of the
MR with a "d-wave" symmetry upon rotating the magnetic field H within the ab
plane, which is caused by the rotation of the stripes with the external field.
With decreasing temperature, a hysteresis shows up below ~20 K in the MR curve
as a function of H and finally below 10 K the magnetic-field application
produces a persistent change in the resistivity. This "memory effect" is caused
by the freezing of the directionally-ordered stripes.


title: Role of Orbitals in Manganese Oxides - Ordering and Fluctuation

abstract:
  We study the manganese oxides from the viewpoint of the strongly correlated
doped Mott insulator. The magnetic ordering and the charge transport are
governed by the orbital degrees of freedom, and their dimensionality is
controlled by the anisotropic transfer integrals between the $e_g$ orbitals. As
x increases the magnetic structure is predicted to change as $A \to F \to A \to
C \to G$ (F: ferromagnet, A: layered antiferromagnet, C: rod-type
antiferromagnet, G: usual antiferromagnet), in agreement with experiments.
Especially the orbital is aligned as $d_{x^2-y^2}$ in the metallic A state,
which explains the quasi 2D transport and no canting of the spin observed
experimentally. Next we discuss the ferromagnetic state without the orbital
ordering due to the quantum fluctuation. Here the interplay between the
electron repulsion U and the Jahn-Teller electron-phonon interation $E_{LR}$ is
studied with a large d model. In addition to this strong correlation, we
propose that the dynamical phase separation could explain the specific heat as
well as the various anomalous physical properties, e.g., resistivity,
photo-emission, etc.


title: Directed rigidity and bootstrap percolation in (1+1) dimensions

abstract:
  We study directed rigidity percolation (equivalent to directed bootstrap
percolation) on three different lattices: square, triangular, and augmented
triangular. The first two of these display a first-order transition at p=1,
while the augmented triangular lattice shows a continuous transition at a
non-trivial p_c. On the augmented triangular lattice we find, by extensive
numerical simulation, that the directed rigidity percolation transition belongs
to the same universality class as directed percolation. The same conclusion is
reached by studying its surface critical behavior, i.e. the spreading of
rigidity from finite clusters close to a non-rigid wall. Near the discontinuous
transition at p=1 on the triangular lattice, we are able to calculate the
finite-size behavior of the density of rigid sites analytically. Our results
are confirmed by numerical simulation.


title: Statistics of DNA sequences: a low frequency analysis

abstract:
  We study statistical properties of DNA chains of thirteen microbial complete
genomes. We find that the power spectrum of several of the sequences studied
flattens off in the low frequency limit. This implies that the correlation
length in those sequences is much smaller than the entire DNA chain.
Consequently, in contradiction with previous studies, we show that the fractal
behavior of DNA chains not always prevail through the entire DNA molecule.


title:
``Linearized'' Dynamical Mean-Field Theory for the Mott-Hubbard
  transition

abstract:
  The Mott-Hubbard metal-insulator transition is studied within a simplified
version of the Dynamical Mean-Field Theory (DMFT) in which the coupling between
the impurity level and the conduction band is approximated by a single pole at
the Fermi energy. In this approach, the DMFT equations are linearized, and the
value for the critical Coulomb repulsion U_{\rm c} can be calculated
analytically. For the symmetric single-band Hubbard model at zero temperature,
the critical value is found to be given by 6 times the square root of the
second moment of the free (U=0) density of states. This result is in good
agreement with the numerical value obtained from the Projective Selfconsistent
Method and recent Numerical Renormalization Group calculations for the Bethe
and the hypercubic lattice in infinite dimensions. The generalization to more
complicated lattices is discussed. The ``linearized DMFT'' yields plausible
results for the complete geometry dependence of the critical interaction.


title: Entropy and typical properties of Nash equilibria in two-player games

abstract:
  We use techniques from the statistical mechanics of disordered systems to
analyse the properties of Nash equilibria of bimatrix games with large random
payoff matrices. By means of an annealed bound, we calculate their number and
analyse the properties of typical Nash equilibria, which are exponentially
dominant in number. We find that a randomly chosen equilibrium realizes almost
always equal payoffs to either player. This value and the fraction of
strategies played at an equilibrium point are calculated as a function of the
correlation between the two payoff matrices. The picture is complemented by the
calculation of the properties of Nash equilibria in pure strategies.

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


title: Magic Islands and Barriers to Attachment: A Si/Si(111)7x7 Growth Model

abstract:
  Surface reconstructions can drastically modify growth kinetics during initial
stages of epitaxial growth as well as during the process of surface
equilibration after termination of growth. We investigate the effect of
activation barriers hindering attachment of material to existing islands on the
density and size distribution of islands in a model of homoepitaxial growth on
Si(111)7x7 reconstructed surface. An unusual distribution of island sizes
peaked around "magic" sizes and a steep dependence of the island density on the
growth rate are observed. "Magic" islands (of a different shape as compared to
those obtained during growth) are observed also during surface equilibration.


title:
Improved high-temperature expansion and critical equation of state of
  three-dimensional Ising-like systems

abstract:
  High-temperature series are computed for a generalized $3d$ Ising model with
arbitrary potential. Two specific ``improved'' potentials (suppressing leading
scaling corrections) are selected by Monte Carlo computation. Critical
exponents are extracted from high-temperature series specialized to improved
potentials, achieving high accuracy; our best estimates are:
$\gamma=1.2371(4)$, $\nu=0.63002(23)$, $\alpha=0.1099(7)$, $\eta=0.0364(4)$,
$\beta=0.32648(18)$. By the same technique, the coefficients of the small-field
expansion for the effective potential (Helmholtz free energy) are computed.
These results are applied to the construction of parametric representations of
the critical equation of state. A systematic approximation scheme, based on a
global stationarity condition, is introduced (the lowest-order approximation
reproduces the linear parametric model). This scheme is used for an accurate
determination of universal ratios of amplitudes. A comparison with other
theoretical and experimental determinations of universal quantities is
presented.


title: A measure of conductivity for lattice fermions at finite density

abstract:
  We study the linear response to an external electric field of a system of
fermions in a lattice at zero temperature. This allows to measure numerically
the Euclidean conductivity which turns out to be compatible with an analytical
calculation for free fermions. The numerical method is generalizable to systems
with dynamical interactions where no analytical approach is possible.


title: Correlated Fermions in a one-dimensional quasiperiodic potential

abstract:
  We study analytically one-dimensional interacting spinless fermions in a
Fibonacci potential. We show that the effects of the quasiperiodic modulation
are intermediate between those of a commensurate potential and a disordered
one. The system exhibits a metal-insulator transition whose position depends
both on the strength of the correlations and on the position of the Fermi
level. Consequently, the conductivity displays a power law like size and
frequency behaviour characterized by a non trivial exponent.


title: Analysis of the approximations used in the van Hove-BCS theory

abstract:
  An analysis of the different approaches used within the van Hove BCS model
for the high temperature superconductors has been done. How far the employment
of an asymptotic expression for the density of states underestimates the
thermodynamic parameters, as for example, the critical temperature, the gap at
zero temperature, and the known universal ratios of the BCS model, is
discussed. Analytical expressions obtained for some thermodynamic functions are
compared with the numerical results for which the exact 2D density of states
has been considered. Approximate analytical expressions for the temperature
dependence of the reduced gap, the superconducting electronic specific heat,
and the critical magnetic field have been obtained and compared with the
corresponding numerical results. The validity of the different approximations
and of the equations obtained for the thermodynamic functions in the framework
of the van Hove model are also analyzed.


title: Self-organized Networks of Competing Boolean Agents

abstract:
  A model of Boolean agents competing in a market is presented where each agent
bases his action on information obtained from a small group of other agents.
The agents play a competitive game that rewards those in the minority. After a
long time interval, the poorest player's strategy is changed randomly, and the
process is repeated. Eventually the network evolves to a stationary but
intermittent state where random mutation of the worst strategy can change the
behavior of the entire network, often causing a switch in the dynamics between
attractors of vastly different lengths.


title:
Mutual synchronization and clustering in randomly coupled chaotic
  dynamical networks

abstract:
  We introduce and study systems of randomly coupled maps (RCM) where the
relevant parameter is the degree of connectivity in the system. Global
(almost-) synchronized states are found (equivalent to the synchronization
observed in globally coupled maps) until a certain critical threshold for the
connectivity is reached. We further show that not only the average
connectivity, but also the architecture of the couplings is responsible for the
cluster structure observed. We analyse the different phases of the system and
use various correlation measures in order to detect ordered non-synchronized
states. Finally, it is shown that the system displays a dynamical hierarchical
clustering which allows the definition of emerging graphs.


title:
Measuring the interaction force between a high temperature
  superconductor and a permanent magnet

abstract:
  Repulsive and attractive forces are both possible between a superconducting
sample and a permanent magnet, and they can give place to magnetic levitation
or free-suspension phenomena, respectively. We show experiments to quantify
this magnetic interaction which represents a promising field regarding to
short-term technological applications of high temperature superconductors. The
measuring technique employs an electronic balance and a rare-earth magnet that
induces a magnetic moment in a melt-textured YBa2Cu3O7 superconductor immersed
in liquid nitrogen. The simple design of the experiments allows a fast and easy
implementation in the advanced physics laboratory with a minimum cost. Actual
levitation and suspension demonstrations can be done simultaneously as a help
to interpret magnetic force measurements.


title: Renormalized Bosonic Interaction of Excitons

abstract:
  An effective bosonic Hamiltonian of $1s$ excitons with ``spin'' degrees of
freedom in two dimension is obtained through a projection procedure, starting
from a conventional electron-hole Hamiltonian ${\cal H}_{eh}$. We first
demonstrate that a straightforward transformation of ${\cal H}_{eh}$ into a
Hamiltonian of bosonic excitons does not give the two-body interaction between
an ``up-spin'' exciton and a ``down-spin'' exciton, which are created by the
left- and right-circularly polarized light beams, respectively. We then show
that this interaction is generated through a projection procedure onto the
subspace spanned by $1s$ excitons, as a renormalization effect coming from
higher exciton states. The projection also renormalizes the interaction between
$1s$ excitons with the same spins by a large amount. These renormalization
effects are crucial for the polarization dependence of the optical responses
from semiconductors. The present theory gives the microscopic foundation of the
phenomenology that was successfully applied to the analysis of four-wave mixing
experiments in GaAs quantum wells strongly coupled to the radiation field in a
high-Q micro cavity.


title: Mesoscopic Noise Theory: Microscopics, or Phenomenology?

abstract:
  We argue, physically and formally, that existing diffusive models of noise
yield inaccurate microscopic descriptions of nonequilibrium current
fluctuations. The theoretical shortfall becomes pronounced in quantum-confined
metallic systems, such as the two-dimensional electron gas. In such systems we
propose a simple experimental test of mesoscopic validity for diffusive
theory's central claim: the smooth crossover between Johnson-Nyquist and shot
noise.


title:
Quantum Kinetic Theory V: Quantum kinetic master equation for mutual
  interaction of condensate and noncondensate

abstract:
  A detailed quantum kinetic master equation is developed which couples the
kinetics of a trapped condensate to the vapor of non-condensed particles. This
generalizes previous work which treated the vapor as being undepleted.


title: Adiabatic noise-induced escape rate for nonequilibrium open systems

abstract:
  We consider the motion of an overdamped particle in a force field in presence
of an external, adiabatic noise, without the restriction that the noise process
is Gaussian or the stochastic process is Markovian. We examine the condition
for attainment of steady state for this nonequilibrium open system and
calculate the adiabatic noise-induced rate of escape of the particle over a
barrier.


title:
The asymmetric single-impurity Anderson model - the modified
  perturbation theory

abstract:
  We investigate the single-impurity Anderson model by means of the recently
introduced modified perturbation theory. This approximation scheme yields
reasonable results away from the symmetric case. The agreement with exactly
known results for the symmetric case is checked, and results for the
non-symmetric case are presented. With decreasing conduction band occupation,
the breakdown of the screening of the local moment is observed. In the
crossover regime between Kondo limit and mixed-valence regime, an enhanced
zero-temperature susceptibility is found.


title: Aging as dynamics in configuration space

abstract:
  The relaxation dynamics of many disordered systems, such as structural
glasses, proteins, granular materials or spin glasses, is not completely frozen
even at very low temperatures. This residual motion leads to a change of the
properties of the material, a process commonly called aging. Despite recent
advances in the theoretical description of such aging processes, the
microscopic mechanisms leading to the aging dynamics are still a matter of
dispute. In this Letter we investigate the aging dynamics of a simple glass
former by means of molecular dynamics computer simulation. Using the concept of
the inherent structure we give evidence that aging dynamics can be understood
as a decrease of the effective configurational temperature T of the system.
  From our results we conclude that the equilibration process is faster when
the system is quenched to T_c, the critical T of mode-coupling theory, and that
thermodynamic concepts are useful to describe the out-of-equilibrium aging
process.


title:
Molecular mode-coupling theory for supercooled liquids: Application to
  water

abstract:
  We present mode-coupling equations for the description of the slow dynamics
observed in supercooled molecular liquids close to the glass transition. The
mode-coupling theory (MCT) originally formulated to study the slow relaxation
in simple atomic liquids, and then extended to the analysis of liquids composed
by linear molecules, is here generalized to systems of arbitrarily shaped,
rigid molecules. We compare the predictions of the theory for the $q$-vector
dependence of the molecular nonergodicity parameters, calculated by solving
numerically the molecular MCT equations in two different approximation schemes,
with ``exact'' results calculated from a molecular dynamics simulation of
supercooled water. The agreement between theory and simulation data supports
the view that MCT succeeds in describing the dynamics of supercooled molecular
liquids, even for network forming ones.


title:
Statistical Mechanics of Stress Transmission in Disordered Granular
  Arrays

abstract:
  We give a statistical-mechanical theory of stress transmission in disordered
arrays of rigid grains with perfect friction. Starting from the equations of
microscopic force and torque balance we derive the fundamental equations of
stress equilibrium. We illustrate the validity of our approach by solving the
stress distribution of a homogeneous and isotropic array.


title: Giant vortex state in perforated aluminum microsquares

abstract:
  We investigate the nucleation of superconductivity in a uniform perpendicular
magnetic field H in aluminum microsquares containing a few (2 and 4) submicron
holes (antidots). The normal/superconducting phase boundary T_c(H) of these
structures shows a quite different behavior in low and high fields. In the low
magnetic field regime fluxoid quantization around each antidot leads to
oscillations in T_c(H), expected from the specific sample geometry, and
reminiscent of the network behavior. In high magnetic fields, the T_c(H)
boundaries of the perforated and a reference non-perforated microsquare reveal
cusps at the same values of Phi/Phi_0 (where Phi is the applied flux threading
the total square area and Phi_0 is the superconducting flux quantum), while the
background on T_c(H) becomes quasi-linear, indicating that a giant vortex state
is established. The influence of the actual geometries on T_c(H) is analyzed in
the framework of the linearized Ginzburg-Landau theory.


title:
Transitions in the Horizontal Transport of Vertically Vibrated Granular
  Layers

abstract:
  Motivated by recent advances in the investigation of fluctuation-driven
ratchets and flows in excited granular media, we have carried out experimental
and simulational studies to explore the horizontal transport of granular
particles in a vertically vibrated system whose base has a sawtooth-shaped
profile. The resulting material flow exhibits novel collective behavior, both
as a function of the number of layers of particles and the driving frequency;
in particular, under certain conditions, increasing the layer thickness leads
to a reversal of the current, while the onset of transport as a function of
frequency occurs gradually in a manner reminiscent of a phase transition. Our
experimental findings are interpreted here with the help of extensive, event
driven Molecular Dynamics simulations. In addition to reproducing the
experimental results, the simulations revealed that the current may be reversed
as a function of the driving frequency as well. We also give details about the
simulations so that similar numerical studies can be carried out in a more
straightforward manner in the future.