title:
Mechanisms of Carrier-Induced Ferromagnetism in Diluted Magnetic
  Semiconductors

abstract:
  Two different approaches to the problem of carrier-induced ferromagnetism in
the system of the disordered magnetic ions, one bases on self-consistent
procedure for the exchange mean fields, other one bases on the RKKY
interaction, used in present literature as the alternative approximations is
analyzed. Our calculations in the framework of exactly solvable model show that
two different contributions to the magnetic characteristics of the system
represent these approaches. One stems from the diagonal part of carrier-ion
exchange interaction that corresponds to mean field approximation. Other one
stems from the off-diagonal part that describes the interaction between ion
spins via free carriers. These two contributions can be responsible for the
different magnetic properties, so aforementioned approaches are complementary,
not alternative. A general approach is proposed and compared with different
approximations to the problem under consideration.


title:
Elasticity and melting of vortex crystals in anisotropic
  superconductors: Beyond the continuum regime

abstract:
  The elastic moduli of vortex crystals in anisotropic superconductors are
frequently involved in the investigation of their phase diagram and transport
properties. We provide a detailed analysis of the harmonic eigenvalues (normal
modes) of the vortex lattice for general values of the magnetic field strength,
going beyond the elastic continuum regime. The detailed behavior of these
wavevector-dependent eigenvalues within the Brillouin zone (BZ), is compared
with several frequently used approximations that we also recalculate.
Throughout the BZ, transverse modes are less costly than their longitudinal
counterparts, and there is an angular dependence which becomes more marked
close to the zone boundary. Based on these results, we propose an analytic
correction to the nonlocal continuum formulas which fits quite well the
numerical behavior of the eigenvalues in the London regime. We use this
approximate expression to calculate thermal fluctuations and the full melting
line (according to Lindeman's criterion) for various values of the anisotropy
parameter.


title: Looking at friction through "shearons"

abstract:
  We study the response to shear of a one-dimensional monolayer embedded
between two rigid plates, where the upper one is externally driven. The shear
is shown to excite ``shearons'', which are collective modes of the embedded
system with well defined spatial and temporal pattern, and which dominate the
frictional properties of the driven system. We demonstrate that static
friction, stick-slip events, and memory effects are well described in terms of
the creation and/or annihilation of a shearon. This raises the possibility to
control friction by modifying the excited shearon, which we examplify by
introducing a defect at the bottom plate.


title:
Kinetic Theory of Collective Modes in Atomic Clouds above the
  Bose-Einstein Transition Temperature

abstract:
  We calculate frequencies and damping rates of the lowest collective modes of
a dilute Bose gas confined in an anisotropic trapping potential above the
Bose-Einstein transition temperature. From the Boltzmann equation with a
simplified collision integral we derive a general dispersion relation that
interpolates between the collisionless and hydrodynamic regimes. In the case of
axially symmetric traps we obtain explicit expressions for the frequencies and
damping rates of the lowest modes in terms of a phenomenological collision
time. Our results are compared with microscopic calculations and experiments.


title:
Spin Chains in an External Magnetic Field. Closure of the Haldane Gap
  and Effective Field Theories

abstract:
  We investigate both numerically and analytically the behaviour of a spin-1
antiferromagnetic (AFM) isotropic Heisenberg chain in an external magnetic
field. Extensive DMRG studies of chains up to N=80 sites extend previous
analyses and exhibit the well known phenomenon of the closure of the Haldane
gap at a lower critical field H_c1. We obtain an estimate of the gap below
H_c1. Above the lower critical field, when the correlation functions exhibit
algebraic decay, we obtain the critical exponent as a function of the net
magnetization as well as the magnetization curve up to the saturation (upper
critical) field H_c2. We argue that, despite the fact that the SO(3) symmetry
of the model is explicitly broken by the field, the Haldane phase of the model
is still well described by an SO(3) nonlinear sigma-model. A mean-field theory
is developed for the latter and its predictions are compared with those of the
numerical analysis and with the existing literature.


title:
The spectral weight of the Hubbard model through cluster perturbation
  theory

abstract:
  We calculate the spectral weight of the one- and two-dimensional Hubbard
models, by performing exact diagonalizations of finite clusters and treating
inter-cluster hopping with perturbation theory. Even with relatively modest
clusters (e.g. 12 sites), the spectra thus obtained give an accurate
description of the exact results. Thus, spin-charge separation (i.e. an
extended spectral weight bounded by singularities) is clearly recognized in the
one-dimensional Hubbard model, and so is extended spectral weight in the
two-dimensional Hubbard model.


title: Electron spin resonance study of Na_{1-x}Li_xV_2O_5

abstract:
  We measured X-band electron-spin resonance of single crystalline sodium
vanadate doped with lithium, Na_{1-x}Li_xV_2O_5 for 0 < x < 1.3% . The phase
transition into a dimerized phase that is observed at 34 K in the undoped
compound, was found to be strongly suppressed upon doping with lithium. The
spin susceptibility was analyzed to determine the transition temperature and
the energy gap with respect to the lithium content. The transition temperature
Tsp is suppressed following a square dependence of the lithium concentration
while the energy gap is found to decrease linearly. At high temperatures
(T>Tsp) the susceptibility remains nearly independent of doping.


title:
The anomaly of the oxygen bond-bending mode at 320 cm$^{-1}$ and the
  additional absorption peak in the c-axis infrared conductivity of underdoped
  YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ single crystals revisited by
  ellipsometricmeasurements

abstract:
  We have performed ellipsometric measurements of the far-infrared c-axis
dielectric response of underdoped YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ single
crystals. Here we report a detailed analysis of the temperature-dependent
renormalization of the oxygen bending phonon mode at 320 cm$^{-1}$ and the
formation of the additional absorption peak around 400-500 cm$^{-1}$. For a
strongly underdoped YBa$_{2}$Cu$_{3}$O$_{6.5}$ crystal with T$_{c}$=52 K we
find that, in agreement with previous reports based on conventional reflection
measurements, the gradual onset of both features occurs well above T$_{c}$ at
T*$\sim $150 K. Contrary to some of these reports, however, our data establish
that the phonon anomaly and the formation of the additional peak exhibit very
pronounced and steep changes right at T$_{c}$. For a less underdoped
YBa$_{2}$Cu$_{3}$O$_{6.75}$ crystal with T$_{c}$=80 K, the onset temperature of
the phonon anomaly almost coincides with T$_{c}$. Also in contrast to some
previous reports, we find for both crystals that a sizeable fraction of the
spectral weight of the additional absorption peak cannot be accounted for by
the spectral-weight loss of the phonon modes but instead arises from a
redistribution of the electronic continuum. Our ellipsometric data are
consistent with a model where the bilayer cuprate compounds are treated as a
superlattice of intra- and inter-bilayer Josephson-junctions.


title:
Flow equation solution for the weak to strong-coupling crossover in the
  sine-Gordon model

abstract:
  A continuous sequence of infinitesimal unitary transformations, combined with
an operator product expansion for vertex operators, is used to diagonalize the
quantum sine-Gordon model for 2 pi < beta^2 < infinity. The leading order of
this approximation already gives very accurate results for the single-particle
gap in the strong-coupling phase. This approach can be understood as an
extension of perturbative scaling theory since it links weak to strong-coupling
behavior in a systematic expansion. The approach should also be useful for
other strong-coupling problems that can be formulated in terms of vertex
operators.


title: Oscillatory Exchange Coupling across Cr$_{(1-x)}$V$_x$ Alloy spacers

abstract:
  We have identified the pieces of the Fermi surface responsible for the long
period oscillations of magnetic coupling across Cr and Cr$_{(1-x)}$V$_x$ alloy
spacers in metallic multilayers. Analysing experiments and results of KKR-CPA
calculations we find that the periods are determined by the extremal
wave~vectors of the hole pockets centered on the N-point in the Brillouin zone.


title:
Time dependent mean field theory of the superfluid-insulator phase
  transition

abstract:
  We develop a time-dependent mean field approach, within the time-dependent
variational principle, to describe the Superfluid-Insulator quantum phase
transition. We construct the zero temperature phase diagram both of the
Bose-Hubbard model (BHM), and of a spin-S Heisenberg model (SHM) with the XXZ
anisotropy. The phase diagram of the BHM indicates a phase transition from a
Mott insulator to a compressibile superfluid phase, and shows the expected
lobe-like structure. The SHM phase diagram displays a quantum phase transition
between a paramagnetic and a canted phases showing as well a lobe-like
structure. We show how the BHM and Quantum Phase model (QPM) can be rigorously
derived from the SHM. Based on such results, the phase boundaries of the SHM
are mapped to the BHM ones, while the phase diagram of the QPM is related to
that of the SHM. The QPM's phase diagram obtained through the application of
our approach to the SHM, describes the known onset of the macroscopic phase
coherence from the Coulomb blockade regime for increasing Josephson coupling
constant. The BHM and the QPM phase diagrams are in good agreement with Quantum
Monte Carlo results, and with the third order strong coupling perturbative
expansion.


title: Diffusion in disordered lattices and related Heisenberg ferromagnets

abstract:
  We study the diffusion of classical hard-core particles in disordered
lattices within the formalism of a quantum spin representation. This analogy
enables an exact treatment of non-instantaneous correlation functions at finite
particle densities in terms of single spin excitations in disordered
ferromagnetic backgrounds. Applications to diluted chains and percolation
clusters are discussed. It is found that density fluctuations in the former
exhibit a stretched exponential decay while an anomalous power law asymptotic
decay is conjectured for the latter.


title:
The Order Parameters for Pairing and Phase Coherence in Cuprates; the
  Magnetic Origin of the Coherent Gap. The MCS Model of High-Tc
  Superconductivity

abstract:
  The phase diagram of order parameters for pairing and phase coherence in
hole-doped cuprates is discussed. By examining carefully some recent inelastic
neutron scattering data obtained on hole-doped cuprates and heavy fermion
compound UPd2Al3 in which the superconductivity is mediated by spin
fluctuations, we conclude that the coherent gap in hole-doped cuprates has most
likely the magnetic origin and scales with Tc, on average, as 2Dc/kBTc = 5.4.
We discuss a model of the superconductivity in hole-doped cuprates and the
symmetries of two order parameters.


title: Many-body Theory vs Simulations for the pseudogap in the Hubbard model

abstract:
  The opening of a critical-fluctuation induced pseudogap (or precursor
pseudogap) in the one-particle spectral weight of the half-filled
two-dimensional Hubbard model is discussed. This pseudogap, appearing in our
Monte Carlo simulations, may be obtained from many-body techniques that use
Green functions and vertex corrections that are at the same level of
approximation. Self-consistent theories of the Eliashberg type (such as the
Fluctuation Exchange Approximation) use renormalized Green functions and bare
vertices in a context where there is no Migdal theorem. They do not find the
pseudogap, in quantitative and qualitative disagreement with simulations,
suggesting these methods are inadequate for this problem. Differences between
precursor pseudogaps and strong-coupling pseudogaps are also discussed.


title:
Algebraic Bethe ansatz for the one-dimensional Hubbard model with open
  boundaries

abstract:
  The one-dimensional Hubbard model with open boundary conditions is exactly
solved by means of algebraic Bethe ansatz. The eigenvalue of the transfer
matrix, the energy spectrum as well as the Bethe ansatz equations are obtained.


title: Semiclassical mechanics of a non-integrable spin cluster

abstract:
  We study detailed classical-quantum correspondence for a cluster system of
three spins with single-axis anisotropic exchange coupling. With autoregressive
spectral estimation, we find oscillating terms in the quantum density of states
caused by classical periodic orbits: in the slowly varying part of the density
of states we see signs of nontrivial topology changes happening to the energy
surface as the energy is varied. Also, we can explain the hierarchy of quantum
energy levels near the ferromagnetic and antiferromagnetic states with EKB
quantization to explain large structures and tunneling to explain small
structures.


title:
Fermi Surface of Bi2212: a Systematic Revisit and Identification of
  Almost Perfectly Nested Fermi Surface Segments

abstract:
  The Fermi surface of the Bi2212 system is systematically studied using a
variety of photon energies. In addition to reconciling the conflicting reports
on the Fermi surface of this important material, we identify almost perfectly
nested Fermi surface segments parallel to the $(1,0)$ or $(0,1)$ direction.
This general yet long overlooked feature of the Fermi surface is intriguingly
similar to the Fermi surface of the $La_{1.48}Nd_{0.4}Sr_{0.12}CuO_4$ system.


title: Level Fluctuations and Many-Body Effects in Disorder-Free Quantum Dots

abstract:
  We have investigated whether many-body effects can induce significant level
fluctuations in a disorder-free quantum dot. The closed energy shell structures
and relaxation of the Hartree-Fock potentials are found to play a significant
role. The level degeneracy consistent with the rotational symmetry of the
confining potential determines the structure of the energy shells. A closed
shell state of a dot can give rise to large fluctuations. When a strong
magnetic field is present the shell structure is absent and fluctuations are
significantly reduced.


title: Control of Superconducting Correlations in High-Tc Cuprates

abstract:
  A strategy to enhance d-wave superconducting correlations is proposed based
on our numerical study for correlated electron models for high-Tc cuprates. We
observe that the pairing is enhanced when the single-electron level around
(pi,0) is close to the Fermi level E_F, while the d-wave pairing interaction
itself contains elements to disfavor the pairing due to shift of the
(pi,0)-level. Angle-resolved photoemission results in the cuprates are
consistently explained in the presence of the d-wave pairing interaction. Our
proposal is the tuning of the (pi,0)-level under the many-body effects to E_F
by optimal design of band structure.


title:
Exact eigenstates and transmission for two interacting electrons on
  quantum dots

abstract:
  The eigenstates and the scattering transmission for two interacting electrons
are found exactly for I quantum dots, including the hybridization with the
states on the leads. The results imply limitations on the validity of the
Coulomb blockade picture. The ground states for I=1, and 2 on a
one--dimensional chain (modeling single and double quantum dots) exhibit
quantum delocalization and magnetic transitions. The effective transmission T
of two interacting electrons through one impurity (I=1) is enhanced by a
renormalization of the repulsive interaction, when one of the electrons is
captured in a strongly localized state.


title: Polarization fields in nitride nanostructures: ten points to think about

abstract:
  Macroscopic polarization, both of intrinsic and piezoelectric nature, is
unusually strong in III-V nitrides, and the built in electric fields in the
layers of nitride-based nanostructures, stemming from polarization changes at
heterointerfaces, have a major impact on the properties of single and multiple
quantum wells, high mobility transistors, and thin films. The concepts involved
in the theory and applications of polarization in nitrides have encountered
some resistance in the field. Here we discuss critically ten ``propositions''
aimed at clarifying the main controversial issues.


title:
Large Magnetoresistance Ratio in Ferromagnetic Single-Electron
  Transistors in the Strong Tunneling Regime

abstract:
  We study transport through a ferromagnetic single-electron transistor. The
resistance is represented as a path integral, so that systems where the tunnel
resistances are smaller than the quantum resistance can be investigated. Beyond
the low order sequential tunneling and co-tunneling regimes, a large
magnetoresistance ratio at sufficiently low temperatures is found. In the
opposite limit, when the thermal energy is larger than the charging energy, the
magnetoresistance ratio is only slightly enhanced.


title:
The Recurrent Variational Approach applied to the Electronic Structure
  of Conjugated Polymers

abstract:
  In this note, first the Recurrent Variational Approach (RVA) is introduced by
using as example a non-trivial spin-model, the spin-1/2 antiferromagnetic
two-leg-ladder. Then, a first application of this scheme to the electronic
structure of conjugated polymers is proposed. An analogy between the two-leg
ladder and the dimerized chain described by the Extended Peierls-Hubbard
Hamiltonian is underlined and comparisons are made with DMRG calculations. The
proposed ansatz is useful to get some analytical insight and it is a good
starting point for further improvements using the RVA.


title: Universal level-spacing statistics in quasiperiodic tight-binding models

abstract:
  We study statistical properties of the energy spectra of two-dimensional
quasiperiodic tight-binding models. The multifractal nature of the eigenstates
of these models is corroborated by the scaling of the participation numbers
with the systems size. Hence one might have expected `critical' or
`intermediate' statistics for the level-spacing distributions as observed at
the metal-insulator transition in the three-dimensional Anderson model of
disorder. However, our numerical results are in perfect agreement with the
universal level-spacing distributions of the Gaussian orthogonal random matrix
ensemble, including the distribution of spacings between second, third, and
forth neighbour energy levels.


title: Correlation functions in the two-dimensional random-field Ising model

abstract:
  Transfer-matrix methods are used to study the probability distributions of
spin-spin correlation functions $G$ in the two-dimensional random-field Ising
model, on long strips of width $L = 3 - 15$ sites, for binary field
distributions at generic distance $R$, temperature $T$ and field intensity
$h_0$. For moderately high $T$, and $h_0$ of the order of magnitude used in
most experiments, the distributions are singly-peaked, though rather
asymmetric. For low temperatures the single-peaked shape deteriorates, crossing
over towards a double-$\delta$ ground-state structure. A connection is obtained
between the probability distribution for correlation functions and the
underlying distribution of accumulated field fluctuations. Analytical
expressions are in good agreement with numerical results for $R/L \gtrsim 1$,
low $T$, $h_0$ not too small, and near G=1. From a finite-size {\it ansatz} at
$T=T_c (h_0=0)$, $h_0 \to 0$, averaged correlation functions are predicted to
scale with $L^y h_0$, $y =7/8$. From numerical data we estimate y=0.875 \pm
0.025$, in excellent agreement with theory. In the same region, the RMS
relative width $W$ of the probability distributions varies for fixed $R/L=1$ as
$W \sim h_0^{\kappa} f(L h_0^u)$ with $\kappa \simeq 0.45$, $u \simeq 0.8$ ;
$f(x)$ appears to saturate when $x \to \infty$, thus implying $W \sim
h_0^{\kappa}$ in $d=2$.


title:
Exact Numerical Calculation of the Density of States of the Fluctuating
  Gap Model

abstract:
  We develop a powerful numerical algorithm for calculating the density of
states rho(omega) of the fluctuating gap model, which describes the low-energy
physics of disordered Peierls and spin-Peierls chains. We obtain rho(omega)
with unprecedented accuracy from the solution of a simple initial value problem
for a single Riccati equation. Generating Gaussian disorder with large
correlation length xi by means of a simple Markov process, we present a
quantitative study of the behavior of rho (omega) in the pseudogap regime. In
particular, we show that in the commensurate case and in the absence of forward
scattering the pseudogap is overshadowed by a Dyson singularity below a certain
energy scale omega^{ast}, which we explicitly calculate as a function of xi.


title: Matrix Product Approach to Conjugated Polymers

abstract:
  The Matrix Product method (MPM) has been used in the past to generate
variational ansatzs of the ground state (GS) of spin chains and ladders. In
this paper we apply the MPM to study the GS of conjugated polymers in the
valence bond basis, exploiting the charge and spin conservation as well as the
electron-hole and spin-parity symmetries. We employ the $U-V-\delta$
Hamiltonian which is a simplified version of the PPP Hamiltonian. For several
coupling constants $U$ and $V$ and dimerizations $\delta$ we compute the GS
energy per monomer which agrees within a $2%-4%$ accuracy with the DMRG
results. We also show the evolution of the MP-variational parameters in the
weak and strong dimerization regimes.


title: Zero-temperature criticality in a simple glass model

abstract:
  We introduce the strongly-interacting trap model, a version of Bouchaud's
trap model for glasses [Bouchaud J-P 1992 {\em J. Phys. I France {\bf 2}}
1705]. At finite temperatures the model exhibits glassy relaxation over
intermediate timeframes but reaches a steady state at finite times. In limit of
zero temperature and with a suitably renormalised timescale the model maps onto
the Bak-Sneppen model, widely studied in the context of self-organised
criticality [Bak P and Sneppen K 1993 {\em Phys. Rev. Lett. {\bf 71}} 4083].
Hence zero temperature is a critical point in all dimensions. These claims are
supported by mean field analysis of the stationary solution and numerical
simulations of a finite-dimensional lattice model.


title:
Duality symmetry, strong coupling expansion and universal critical
  amplitudes in two-dimensional \Phi^{4} field models

abstract:
  We show that the exact beta-function \beta(g) in the continuous 2D g\Phi^{4}
model possesses the Kramers-Wannier duality symmetry. The duality symmetry
transformation \tilde{g}=d(g) such that \beta(d(g))=d'(g)\beta(g) is
constructed and the approximate values of g^{*} computed from the duality
equation d(g^{*})=g^{*} are shown to agree with the available numerical
results. The calculation of the beta-function \beta(g) for the 2D scalar
g\Phi^{4} field theory based on the strong coupling expansion is developed and
the expansion of \beta(g) in powers of g^{-1} is obtained up to order g^{-8}.
The numerical values calculated for the renormalized coupling constant
g_{+}^{*} are in reasonable good agreement with the best modern estimates
recently obtained from the high-temperature series expansion and with those
known from the perturbative four-loop renormalization-group calculations. The
application of Cardy's theorem for calculating the renormalized isothermal
coupling constant g_{c} of the 2D Ising model and the related universal
critical amplitudes is also discussed.


title:
Transmission of Information and Herd Behavior: an Application to
  Financial Markets

abstract:
  We propose a model for stochastic formation of opinion clusters, modelled by
an evolving network, and herd behaviour to account for the observed fat-tail
distribution in returns of financial-price data. The only parameter of the
model is h, the rate of information dispersion per trade, which is a measure of
herding behavior. For h below a critical h* the system displays a power-law
distribution of the returns with exponential cut-off. However for h>h* an
increase in the probability of large returns is found, and may be associated to
the occurrence of large crashes.


title: Ferroelectric Phase Transitions in Films with Depletion Charge

abstract:
  We consider ferroelectric phase transitions in both short-circuited and
biased ferroelectric-semiconductor films with a space (depletion) charge which
leads to some unusual behavior. It is shown that in the presence of the charge
the polarization separates into `switchable' and `non-switchable' parts. The
electric field, appearing due to the space charge, does not wash out the phase
transition, which remains second order but takes place at somewhat reduced
temperature. At the same time, it leads to a suppression of the
ferroelectricity in a near-electrode layer. This conclusion is valid for
materials with both second and first order phase transitions in pure bulk
samples. Influence of the depletion charge on thermodynamic coercive field
reduces mainly to the lowering of the phase transition temperature, and its
effect is negligible. The depletion charge can, however, facilitate an
appearance of the domain structure which would be detrimental for device
performance (fatigue). We discuss some issues of conceptual character, which
are generally known but were overlooked in previous works. The present results
have general implications for small systems with depletion charge.


title: Modified Extrapolation Length Renormalization Group Equation

abstract:
  A modified renormalization group equation for the inverse extrapolation
length $c$ is derived by considering the phase shifts of order parameter
fluctuations. The resulting non-linear equation is also derived using standard
methods and some additional assumptions. The associated renormalized flow
$c(l)$ exhibits the correct behavior near both the special and ordinary fixed
points and in particular yields a canonical scaling of $c$ with cross-over
exponent $\phi_{\rm ord} = -\nu$ near the ordinary transition.


title: Nonlinear Dynamics of Nuclear-Electronic Spin Processes in Ferromagnets

abstract:
  Spin dynamics is considered in ferromagnets consisting of electron and
nuclear subsystems interacting with each other through hyperfine forces. In
addition, the ferromagnetic sample is coupled with a resonance electric
circuit. Under these conditions, spin relaxation from a strongly nonequilibrium
initial state displays several peculiarities absent for the standard set-up in
studying spin relaxation. The main feature of the nonlinear spin dynamics
considered in this communication is the appearance of ultrafast coherent
relaxation, with characteristic relaxation times several orders shorter than
the transverse relaxation time $T_2$. This type of coherent spin relaxation can
be used for extracting additional information on the intrinsic properties of
ferromagnetic materials and also can be employed for different technical
applications.


title:
Fermi-level alignment at metal-carbon nanotube interfaces: application
  to scanning tunneling spectroscopy

abstract:
  At any metal-carbon nanotube interface there is charge transfer and the
induced interfacial field determines the position of the carbon nanotube band
structure relative to the metal Fermi-level. In the case of a single-wall
carbon nanotube (SWNT) supported on a gold substrate, we show that the charge
transfers induce a local electrostatic potential perturbation which gives rise
to the observed Fermi-level shift in scanning tunneling spectroscopy (STS)
measurements. We also discuss the relevance of this study to recent experiments
on carbon nanotube transistors and argue that the Fermi-level alignment will be
different for carbon nanotube transistors with low resistance and high
resistance contacts.


title:
Impact of magnetic frustration on the Mott transition within a
  slave-boson mean-field theory

abstract:
  We investigate the paramagnetic-metal-to-antiferromagnetic-metal and
antiferromagnetic-metal-to- antiferromagnetic-insulator transitions using a
slave-boson mean-field theory. To this effect, we discuss the ground state of
the half-filled Hubbard model as a function of t'/t and correlation strength U,
where t and t' are the hopping amplitudes between nearest and next-nearest
neighbors, respectively. The metal-insulator transition at a critical U_{MIT}
is of second order for small levels of magnetic frustration, t'/t<0.06, and of
first order for large ones, t'/t>0.06. The insulator is always
antiferromagnetically ordered, while the metal exhibits a second-order
transition from a paramagnetic to an antiferromagnetic state up to t'/t=0.14,
as U is increased. We also contrast these findings with what we obtain in
Hartree-Fock approximation.


title: Proximity Effect in Presence of Quantum Fluctuations

abstract:
  The effect of Coulomb interaction upon superconductive proximity effect in
disordered metals is studied, employing newly developed Keldysh functional
approach (cond-mat/9907358).
  We have calculated subgap Andreev conductance between superconductor and 2D
dirty film, as well as Josephson coupling via such a film. Both two
qualitatively different Coulomb effects - suppression of the tunneling density
of states and disorder-enchanced repulsion in the Cooper channel - are shown to
be important at sufficiently low temperatures.


title: Coulomb Blockade in low mobility nanometer size Si:MOSFETs

abstract:
  We investigate coherent transport in Si:MOSFETs with nominal gate lengths 50
to 100nm and various widths at very low temperature. Independent of the
geometry, localized states appear when G=e^{2}/h and transport is dominated by
resonant tunnelling through a single quantum dot formed by an impurity
potential. We find that the typical size of the relevant impurity quantum dot
is comparable to the channel length and that the periodicity of the observed
Coulomb blockade oscillations is roughly inversely proportional to the channel
length. The spectrum of resonances and the nonlinear I-V curves allow to
measure the charging energy and the mean level energy spacing for electrons in
the localized state. Furthermore, we find that in the dielectric regime, the
variance var(lng) of the logarithmic conductance lng is proportional to its
average value <lng> consistent with one-electron scaling models.


title: Energy barriers for diffusion on stepped Rh(111) surfaces

abstract:
  Energy barriers for different moves of a single Rh adatom in the vicinity of
steps on Rh(111) surface are studied with molecular statics. Interatomic
interactions are modeled by the semi-empirical many-body
Rosato-Guillope-Legrand potential. We calculate systematically barriers for the
descent at straight steps, steps with the kink and small islands as well as
barriers for diffusion along the step edges. The descent is more probable on
steps with a {111} microfacet and near kinks. Diffusion along a step with a
{100} microfacet is faster than along a step with a {111} microfacet. We also
calculate barriers for diffusion on several surfaces vicinal to Rh(111).


title:
Numerical Study on Aging Dynamics in the 3D Ising Spin-Glass Model. II.
  Quasi-Equilibrium Regime of Spin Auto-Correlation Function

abstract:
  Using Monte Carlo simulations, we have studied isothermal aging of
three-dimensional Ising spin-glass model focusing on quasi-equilibrium behavior
of the spin auto-correlation function. Weak violation of the time translational
invariance in the quasi-equilibrium regime is analyzed in terms of {\it
effective stiffness} for droplet excitations in the presence of domain walls.
Within the range of computational time window, we have confirmed that the
effective stiffness follows the expected scaling behavior with respect to the
characteristic length scales associated with droplet excitations and domain
walls, whose growth law has been extracted from our simulated data. Implication
of the results are discussed in relation to experimental works on ac
susceptibilities.


title:
A History-Dependent Stochastic Predator-Prey Model : Chaos and its
  Elimination

abstract:
  A non-Markovian stochastic predator-prey model is introduced in which the
prey are immobile plants and predators are diffusing herbivors. The model is
studied by both mean-field approximation (MFA)and computer simulations. The MFA
results a series of bifurcations in the phase space of mean predator and prey
densities, leading to a chaotic phase. Because of emerging correlations between
the two species distributions, the interaction rate alters and if it is set the
value which is obtained from the simulation, then the chaotic phase disappears.


title:
Numerical Study of the Vortex Phase Diagram Using the Bose Model in STLS
  approximation

abstract:
  We study the phase diagram of the flux lines using the mapping to 2D bosons
in the self-consistent-field approximation of Singwi, Tosi, Land, and Sjolander
(STLS). The pair correlation function, static structure factor, interaction
energy, and spectrum of the excited energies are calculated over a wide range
of the parameters in this approximation. These quantities are used for studying
the melting transition from the Abrikosov lattice into the entangled vortex
liquid. The resulting $B-T$ phase diagram is in good agreement with the known
estimates for the vortex lattice melting and the Monte Carlo simulations. We
also discuss the effect of van der Waals interaction, induced by thermal
fluctuations, together with the repulsion potential on the phase diagram.


title:
Phase transition from a $d_{x^2-y^2}$ to $d_{x^2-y^2}+id_{xy}$
  superconductor

abstract:
  The temperature dependencies of specific heat and spin susceptibility of a
coupled $d_{x^2-y^2} +id_{xy}$ superconductor in the presence of a weak
$d_{xy}$ component are investigated in the tight-binding model (1) on square
lattice and (2) on a lattice with orthorhombic distortion. As the temperature
is lowered past the critical temperature $T_c$, first a less ordered
$d_{x^2-y^2}$ superconductor is created, which changes to a more ordered
$d_{x^2-y^2} +id_{xy}$ superconductor at $T_{c1} (<T_c)$. This manifests in two
second order phase transitions identified by two jumps in specific heat at
$T_c$ and $T_{c1}$. The temperature dependencies of the superconducting
observables exhibit a change from power-law to exponential behavior as
temperature is lowered below $T_{c1}$ and confirm the new phase transition.


title: Water-like anomalies for core-softened models of fluids: One dimension

abstract:
  We use a one-dimensional (1d) core-softened potential to develop a physical
picture for some of the anomalies present in liquid water. The core-softened
potential mimics the effect of hydrogen bonding. The interest in the 1d system
stems from the facts that closed-form results are possible and that the
qualitative behavior in 1d is reproduced in the liquid phase for higher
dimensions. We discuss the relation between the shape of the potential and the
density anomaly, and we study the entropy anomaly resulting from the density
anomaly. We find that certain forms of the two-step square well potential lead
to the existence at T=0 of a low-density phase favored at low pressures and of
a high-density phase favored at high pressures, and to the appearance of a
point $C'$ at a positive pressure, which is the analog of the T=0 ``critical
point'' in the $1d$ Ising model. The existence of point $C'$ leads to anomalous
behavior of the isothermal compressibility $K_T$ and the isobaric specific heat
$C_P$.


title: Correlated Persistent Tunneling Currents in Glasses

abstract:
  Low temperature properties of glasses are derived within a generalized
tunneling model, considering the motion of charged particles on a closed path
in a double-well potential. The presence of a magnetic induction field B
violates the time reversal invariance due to the Aharonov-Bohm phase, and leads
to flux periodic energy levels. At low temperature, this effect is shown to be
strongly enhanced by dipole-dipole and elastic interactions between tunneling
systems and becomes measurable. Thus, the recently observed strong sensitivity
of the electric permittivity to weak magnetic fields can be explained. In
addition, superimposed oscillations as a function of the magnetic field are
predicted.


title: Collapse of Flexible Polyelectrolytes in Multivalent Salt Solutions

abstract:
  The collapse of flexible polyelectrolytes in a solution of multivalent
counterions is studied by means of a two state model. The states correspond to
rod-like and spherically collapsed conformations respectively. We focus on the
very dilute monomer concentration regime where the collapse transition is found
to occur when the charge of the multivalent salt is comparable (but smaller) to
that of the monomers. The main contribution to the free energy of the collapsed
conformation is linear in the number of monomers $N$, since the internal state
of the collapsed polymer approaches that of an amorphous ionic solid. The free
energy of the rod-like state grows as $N\ln N$, due to the electrostatic energy
associated with that shape. We show that practically all multivalent
counterions added to the system are condensed into the polymer chain, even
before the collapse.


title: Tunneling in the moving domain wall

abstract:
  The mechanism of the parametrical stimulated tunneling in the spectrum of the
moving domain wall considered. It is shown that such a mechanism can to cause
the initial phase of the parametrical evolution of domain wall's surface waves.
In turn, this evolution leads to an appearance of essential features in domain
walls dynamics.


title:
Ideal Bose gas in fractal dimensions and superfluid $^4$He in porous
  media

abstract:
  Physical properties of ideal Bose gas with the fractal dimensionality between
D=2 and D=3 are theoretically investigated. Calculation shows that the
characteristic features of the specific heat and the superfluid density of
ideal Bose gas in fractal dimensions are strikingly similar to those of
superfluid Helium-4 in porous media. This result indicates that the geometrical
factor is dominant over mutual interactions in determining physical properties
of Helium-4 in porous media.


title:
Spontaneous vs. piezoelectric polarization in III-V nitrides: conceptual
  aspects and practical consequences

abstract:
  Macroscopic polarization plays a major role in determining the optical and
electrical properties of nitride nanostructures via polarization-induced
built-in electrostatic fields. While currently fashionable, this field of
endeavour is still by far in its early infancy. Here we contribute some
clarifications on the conceptual issues involved in determining built-in fields
in III-V nitride nanostructures, sorting out in particular the roles of
spontaneous and piezoelectric polarization.


title: Planar quasiperiodic Ising models

abstract:
  We investigate zero-field Ising models on periodic approximants of planar
quasiperiodic tilings by means of partition function zeros and high-temperature
expansions. These are obtained by employing a determinant expression for the
partition function. The partition function zeros in the complex temperature
plane yield precise estimates of the critical temperature of the quasiperiodic
model. Concerning the critical behaviour, our results are compatible with
Onsager universality, in agreement with the Harris-Luck criterion based on
scaling arguments.


title:
Inversion Symmetry and Critical Exponents of Dissipating Waves in the
  Sandpile Model

abstract:
  Statistics of waves of topplings in the Sandpile model is analysed both
analytically and numerically. It is shown that the probability distribution of
dissipating waves of topplings that touch the boundary of the system obeys
power-law with critical exponent 5/8. This exponent is not indeendent and is
related to the well-known exponent of the probability distribution of last
waves of topplings by exact inversion symmetry s -> 1/s.


title:
Quantum transport through ballistic cavities: soft vs. hard quantum
  chaos

abstract:
  We study transport through a two-dimensional billiard attached to two
infinite leads by numerically calculating the Landauer conductance and the
Wigner time delay. In the generic case of a mixed phase space we find a power
law distribution of resonance widths and a power law dependence of conductance
increments apparently reflecting the classical dwell time exponent, in striking
difference to the case of a fully chaotic phase space. Surprisingly, these
power laws appear on energy scales {\em below} the mean level spacing, in
contrast to semiclassical expectations.


title: Spin Dynamics of a Fermi Liquid in an Electric Field

abstract:
  The influence of an external electric field on the spin dynamics of an
electrically neutral Fermi liquid is considered, the mechanism of such an
influence being the relativistic spin-orbital interaction. As a result,
Leggett's equations for the spin dynamics of weakly polarized Fermi liquids are
generalized to the case of non-zero external electric field. In addition, we
obtained the transverse spin dynamics equation for strongly spin-polarized
liquids in an electric field at zero temperature. In both situations covariant
derivatives depending on the electric field are shown to be substituted for
spatial gradients in line with the SU(2) gauge invariance of the microscopic
Hamiltonian. The new equations are applied to the study of spin flow along a
channel, where an electric field is found to bring about an additional phase
shift of the order of magnitude of the phase shift in superfluid $^3$He-$B$ but
growing with time.


title: Spontaneous Currents in Josephson Devices

abstract:
  The unconventional Josephson coupling in a ferromagnetic weak link between
d-wave superconductors is studied theoretically.
  For strong ferromagnetic barrier influence, the unconventional coupling, with
ground state phase difference across the link $0<\phi_{\rm gs}\leq \pi$, is
obtained at small crystal misorientation of the superconducting electrodes, in
contrast to the case of normal metal barrier, where it appears at large
misorientations. In both cases, with decreasing temperature there is an
increasing range of misorientations, where $\phi_{\rm gs}$ varies continuously
between 0 and $\pi$.
  When the weak link is a part of a superconducting ring, this is accompanied
by the flow of spontaneous supercurrent, of intensity which depends (for a
given misorientation) on the reduced inductance $l=2\pi LI_c(T)/\Phi_0$, and is
non-zero only for $l$ greater than a critical value. For $l\gg 1$, another
consequence of the unconventional coupling is the anomalous quantization of the
magnetic flux.


title: Evolution of the System with Singular Multiplicative Noise

abstract:
  The governed equations for the order parameter, one-time and two-time
correlators are obtained on the basis of the Langevin equation with the white
multiplicative noise which amplitude $x^{a}$ is determined by an exponent
$0<a<1$ ($x$ being a stochastic variable). It turns out that equation for
autocorrelator includes an anomalous average of the power-law function with the
fractional exponent $2a$. Determination of this average for the stochastic
system with a self-similar phase space is performed. It is shown that at
$a>1/2$, when the system is disordered, the correlator behaves
non-monotonically in the course of time, whereas the autocorrelator is
increased monotonically. At $a<1/2$ the phase portrait of the system evolution
divides into two domains: at small initial values of the order parameter, the
system evolves to a disordered state, as above; within the ordered domain it is
attracted to the point having the finite values of the autocorrelator and order
parameter. The long-time asymptotes are defined to show that, within the
disordered domain, the autocorrelator decays hyperbolically and the order
parameter behaves as the power-law function with fractional exponent $-2(1-a)$.
Correspondingly, within the ordered domain, the behavior of both dependencies
is exponential with an index proportional to $-t\ln t$.


title:
The influence of correlated hopping on valence and metal-insulator
  transitions in the Falicov-Kimball model

abstract:
  The extrapolation of small-cluster exact-diagonalization calculations is used
to examine the influence of correlated hopping on valence and metal-insulator
transitions in the one-dimensional Falicov-Kimball model. It is shown that the
ground-state phase diagram as well as the picture of valence and
metal-insulator transitions found for the conventional Falicov-Kimball model
(without correlated hopping) are strongly changed when the correlated hopping
term is added. The effect of correlated hopping is so strong that it can induce
the insulator-metal transition, even in the half-filled band case.


title:
Interference of a Bose-Einstein condensate in a hard-wall trap:
  Formation of vorticity

abstract:
  We theoretically study the coherent expansion of a Bose-Einstein condensate
in the presence of a confining impenetrable hard-wall potential. The nonlinear
dynamics of the macroscopically coherent matter field results in rich and
complex spatio-temporal interference patterns demonstrating the formation of
vorticity and solitonlike structures, and the fragmentation of the condensate
into coherently coupled pieces.


title:
Non-adiabaticity and single-electron transport driven by surface
  acoustic waves

abstract:
  Single-electron transport driven by surface acoustic waves (SAW) through a
narrow constriction, formed in two-dimensional electron gas, is studied
theoretically. Due to long-range Coulomb interaction, the tunneling coupling
between the electron gas and the moving minimum of the SAW-induced potential
rapidly decays with time. As a result, nonadiabaticiy sets a limit for the
accuracy of the quantization of acoustoelectric current.


title: Electron-Assisted Hopping in Two Dimensions

abstract:
  We have studied the non-ohmic effects in the conductivity of a
two-dimensional system which undergoes the crossover from weak to strong
localization with decreasing electron concentration. When the electrons are
removed from equilibrium with phonons, the hopping conductivity depends only on
the electron temperature. This indicates that the hopping transport in a system
with a large localization length is assisted by electron-electron interactions
rather than by the phonons.


title: NMR Determination of 2D Electron Spin Polarization at $\nu=1/2$

abstract:
  Using a `standard' NMR spin-echo technique we determined the spin
polarization of two-dimensional electrons, confined to GaAs quantum wells, from
the hyperfine shift of Ga nuclei in the wells. Concentrating on the temperature
and magnetic field dependencies of spin polarization at Landau level filling
factor $\nu =1/2$, we find that the results are described well by a simple
model of non-interacting composite fermions, although some inconsistencies
remain when the two-dimensional electron system is tilted in the magnetic
field.


title: Low-temperature dephasing in disordered conductors: experimental aspects

abstract:
  What is the lowest temperature to which one can trace the growth of the
dephasing time in low-dimensional conductors? I consider the fundamental
limitation, the crossover from weak to strong localization, as well as several
experimental reasons for frequently observed saturation of the dephasing time
(hot-electron effects, dephasing by external noise). Recent progress in our
understanding of the electron-phonon interaction in disordered conductors is
also briefly discussed.


title:
Phase transition from a $d_{x^2-y^2}$ to $d_{x^2-y^2}+d_{xy}$
  superconductor

abstract:
  We study the phase transition from a $d_{x^2-y^2}$ to $d_{x^2-y^2}+d_{xy}$
superconductor using the tight-binding model of two-dimensional cuprates. As
the temperature is lowered past the critical temperature $T_c$, first a $
d_{x^2-y^2}$ superconducting phase is created. With further reduction of
temperature, the $ d_{x^2-y^2}+d_{xy}$ phase is created at temperature
$T=T_{c1}$. We study the temperature dependencies of the order parameter,
specific heat and spin susceptibility in these mixed-angular-momentum states on
square lattice and on a lattice with orthorhombic distortion. The
above-mentioned phase transitions are identified by two jumps in specific heat
at $T_c$ and $T_{c1}$.


title: Two phase transitions in ${(d_{x^2-y^2}+is)}$-wave superconductors

abstract:
  We study numerically the temperature dependencies of specific heat,
susceptibility, penetration depth, and thermal conductivity of a coupled
$(d_{x^2-y^2}+is)$-wave Bardeen-Cooper-Schreiffer superconductor in the
presence of a weak s-wave component (1) on square lattice and (2) on a lattice
with orthorhombic distorsion. As the temperature is lowered past the critical
temperature $T_c$, a less ordered superconducting phase is created in
$d_{x^2-y^2}$ wave, which changes to a more ordered phase in $(d_{x^2-y^2}+is)$
wave at $T_{c1}$. This manifests in two second-order phase transitions. The two
phase transitions are identified by two jumps in specific heat at $T_c$ and
$T_{c1}$. The temperature dependencies of the superconducting observables
exhibit a change from power-law to exponential behavior as temperature is
lowered below $T_{c1}$ and confirm the new phase transition.


title: Sub-Doppler Laser Cooling of Fermionic K-40 Atoms

abstract:
  We report laser cooling of fermionic K-40 atoms, with temperatures down to
(15 +/- 5) microK, for an enriched sample trapped in a MOT and additionaly
cooled in optical molasses. This temperature is a factor of 10 below the
Doppler-cooling limit and corresponds to an rms velocity within a factor of two
of the lowest realizable rms velocity (~3.5v rec) in 3D optical molasses.
Realization of such low atom temperatures, up to now only accessible with
evaporative cooling techniques, is an important precursor to producing a
degenerate Fermi gas of K-40 atoms.


title: Flowing sand - a physical realization of Directed Percolation

abstract:
  We introduce and investigate a simple model to describe recent experiments by
Douady and Daerr on flowing sand. The model reproduces experimentally observed
compact avalanches, whose opening angle decreases linearly as a threshold is
approached. On large scales the model exhibits a crossover from compact
directed percolation to directed percolation; we predict similar behavior for
the experimental system. We estimate the regime where "true" directed
percolation morphology and exponents will be observed, providing the first
experimental realization for this class of models.


title: Typical Performance of Gallager-type Error-Correcting Codes

abstract:
  The performance of Gallager's error-correcting code is investigated via
methods of statistical physics. In this approach, the transmitted codeword
comprises products of the original message bits selected by two
randomly-constructed sparse matrices; the number of non-zero row/column
elements in these matrices constitutes a family of codes. We show that
Shannon's channel capacity is saturated for many of the codes while slightly
lower performance is obtained for others which may be of higher practical
relevance. Decoding aspects are considered by employing the TAP approach which
is identical to the commonly used belief-propagation-based decoding.


title: Coherent Ultrafast Optical Dynamics of the Fermi Edge Singularity

abstract:
  We develop a non-equilibrium many-body theory of the coherent femtosecond
nonlinear optical response of the Fermi edge singularity. We study the role of
the dynamical Fermi sea response in the time-evolution of the pump-probe
spectra. The electron-hole correlations are treated nonperturbatively with the
time-dependent coupled cluster cxpansion combined with the effective
Hamiltonian approach. For short pulse durations, we find a non-exponential
decay of the differential transmission during negative time delays, which is
governed by the interactions. This is in contrast to the results obtained
within the Hartree-Fock approximation, which predicts an exponential decay
governed by the dephasing time. We discuss the role of the optically-induced
dephasing effects in the coherent regime.


title: A Thermodynamic Model for Receptor Clustering

abstract:
  Intracellular signaling often arises from ligand-induced oligomerization of
cell surface receptors. This oligomerization or clustering process is
fundamentally a cooperative behavior between near-neighbor receptor molecules;
the properties of this cooperative process clearly affects the signal
transduction. Recent investigations have revealed the molecular basis of
receptor-receptor interactions, but a simple theoretical framework for using
this data to predict cluster formation has been lacking. Here, we propose a
simple, coarse-grained, phenomenological model for ligand-modulated receptor
interactions and discuss its equilibrium properties via mean-field theory. The
existence of a first-order transition for this model has immediate implications
regarding the robustness of the cellular signaling response.


title:
The Angular Intensity Correlation Functions $C^{(1)}$ and $C^{(10)}$ for
  the Scattering of S-Polarized Light from a One-Dimensional Randomly Rough
  Dielectric Surface

abstract:
  We calculate the short-range contributions $C^{(1)}$ and $C^{(10)}$ to the
angular intensity correlation function for the scattering of s-polarized light
from a one-dimensional random interface between two dielectric media. The
calculations are carried out on the basis of a new approach that separates out
explicitly the contributions $C^{(1)}$ a nd $C^{(10)}$ to the angular intensity
correlation function. The contribution $C^{(1)}$ displays peaks associated with
the memory effect and the reciprocal memory effect. In the case of a
dielectric-dielectric interface, which does not support surface electromagnetic
surface waves, these peaks arise from the co herent interference of
multiply-scattered lateral waves supported by the in terface. The contribution
$C^{(10)}$ is a structureless function of its arguments.


title: Two State Behavior in a Solvable Model of $\beta$-hairpin folding

abstract:
  Understanding the mechanism of protein secondary structure formation is an
essential part of protein-folding puzzle. Here we describe a simple model for
the formation of the $\beta$-hairpin, motivated by the fact that folding of a
$\beta$-hairpin captures much of the basic physics of protein folding. We argue
that the coupling of ``primary'' backbone stiffness and ``secondary'' contact
formation (similar to the coupling between the ``secondary'' and ``tertiary''
structure in globular proteins), caused for example by side-chain packing
regularities, is responsible for producing an all-or-none 2-state
$\beta$-hairpin formation. We also develop a recursive relation to compute the
phase diagram and single exponential folding/unfolding rate arising via a
dominant transition state.


title: Subband population in a single-wall carbon nanotube diode

abstract:
  We observe current rectification in a molecular diode consisting of a
semiconducting single-wall carbon nanotube and an impurity. One half of the
nanotube has no impurity, and it has a current-voltage (I-V) charcteristic of a
typical semiconducting nanotube. The other half of the nanotube has the
impurity on it, and its I-V characteristic is that of a diode. Current in the
nanotube diode is carried by holes transported through the molecule's
one-dimensional subbands. At 77 Kelvin we observe a step-wise increase in the
current through the diode as a function of gate voltage, showing that we can
control the number of occupied one-dimensional subbands through electrostatic
doping.


title:
Ab Initio Study of Screw Dislocations in Mo and Ta: A new picture of
  plasticity in bcc transition metals

abstract:
  We report the first ab initio density-functional study of <111> screw
dislocations cores in the bcc transition metals Mo and Ta. Our results suggest
a new picture of bcc plasticity with symmetric and compact dislocation cores,
contrary to the presently accepted picture based on continuum and interatomic
potentials. Core energy scales in this new picture are in much better agreement
with the Peierls energy barriers to dislocation motion suggested by
experiments.


title: Marginal Fermi Liquid with a Two-Dimensional Patched Fermi Surface

abstract:
  We consider a model composed of Landau quasiparticle states with patched
Fermi surfaces (FS) sandwiched by states with flat FS to simulate the ``cold''
spot regions in cuprates. We calculate the one particle irreducible function
and the self-energy up to two-loop order. Using renormalization group arguments
we show that in the forward scattering channel the renormalized coupling
constant is never infrared stable due to the flat FS sectors. Furthemore we
show that the self-energy scales with energy as ${\rm Re} \Sigma \sim \omega
\ln \omega $ as $\omega \to 0$, and thus the Fermi liquid state within each FS
patch is turned into a marginal Fermi liquid.


title: ESR study in lightly doped La_(1-x)Sr_(x)MnO_(3)

abstract:
  This is the first systematic ESR investigation of La_(1-x)Sr_(x)MnO_(3)
single crystals for Sr concentrations x < 0.2. The rich phase diagram of this
compound, which is dominated by the Jahn-Teller effect of Mn^(3+), is a matter
of actual interest in connection with giant magnetoresistance phenomena. The
ESR signal, which is due to both Mn^(3+) and Mn^(4+), directely probes the
suppression of the Jahn-Teller distortion with increasing Sr concentration and
reveals all characteristic structural and magnetic phase transitions.


title: Vortex with Fractional Quantum Numbers in Chiral p-Wave Superconductor

abstract:
  We show that a vortex in a chiral p-wave superconductor, which has the p_{x}+
i p_{y}-wave pairing state and breaks U(1), parity and time reversal symmetry
simultaneously, has fractional charge -{n e}/{4} and fractional angular
momentum -n^{2}/{16} (n; vorticity). This suggests that the vortex could be
anyon and could obey fractional statistics. Electromagnetic property of the
vortex is also discussed and we find that an electric field is induced near the
vortex core.


title: The Hubbard chain: Lieb-Wu equations and norm of the eigenfunctions

abstract:
  We argue that the square of the norm of the Hubbard wave function is
proportional to the determinant of a matrix, which is obtained by linearization
of the Lieb-Wu equations around a solution. This means that in the vicinity of
a solution the Lieb-Wu equations are non-degenerate, if the corresponding wave
function is non-zero. We further derive an action that generates the Lieb-Wu
equations and express our determinant formula for the square of the norm in
terms of the Hessian determinant of this action.

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


title:
Direct Observation of a One Dimensional Static Spin Modulation in
  Insulating La1.95Sr0.05CuO4

abstract:
  We report the results of an extensive elastic neutron scattering study of the
incommensurate (IC) static spin correlations in La1.95Sr0.05CuO4 which is an
insulating spin glass at low temperatures. The present neutron scattering
experiments on the same x=0.05 crystal employ a narrower instrumental
Q-resolution and thereby have revealed that the crystal has only two
orthorhombic twins at low temperatures with relative populations of 2:1. We
find that, in a single twin, only two satellites are observed at (1, +/-0.064,
L)(ortho) and (0, 1+/-0.064, L)(ortho), that is, the modulation vector is only
along the orthorhombic b*-axis. This demonstrates unambiguously that
La1.95Sr0.05CuO4 has a one-dimensional static diagonal spin modulation at low
temperatures, consistent with certain stripe models. We have also reexamined
the x=0.04 crystal that previously was reported to show a single commensurate
peak. By mounting the sample in the (H, K, 0) zone, we have discovered that the
x=0.04 sample in fact has the same IC structure as the $x=0.05$ sample. The
incommensurability parameter d for x=0.04 and 0.05, where d is the distance
from (1/2, 1/2) in tetragonal reciprocal lattice units, follows the linear
relation d=x. These results demonstrate that the insulator to superconductor
transition in the under doped regime (0.05 </= x </= 0.06) in La2-xSrxCuO4 is
coincident with a transition from diagonal to collinear static stripes at low
temperatures thereby evincing the intimate coupling between the one dimensional
spin density modulation and the superconductivity.


title:
Shot noise induced charge and potential fluctuations of edge states in
  proximity of a gate

abstract:
  We evaluate the RC-time of edge states capacitively coupled to a gate located
away from a QPC which allows for partial transmission of an edge channel. At
long times or low frequencies the RC-time governs the relaxation of charge and
current and governs the fluctuations of the equilibrium electrostatic
potential. The RC-time in mesoscopic structures is determined by an
electrochemical capacitance which depends on the density of states of the edge
states and a charge relaxation resistance. In the non-equilibrium case, in the
presence of transport, the shot noise leads to charge fluctuations in proximity
of the gate which are again determined by the equilibrium electrochemical
capacitance but with a novel resistance. The case of multiple edge states is
discussed and the effect of a dephasing voltage probe on these resistances is
investigated. The potential fluctuations characterized by these capacitances
and resistances are of interest since they determine the dephasing rate in
Coulomb coupled mesoscopic conductors.


title: B(H) Constitutive Relations Near H_c1 in Disordered Superconductors

abstract:
  We provide a self-contained account of the B vs. H constitutive relation near
H_c1 in Type II superconductors with various types of quenched random disorder.
The traditional Abrikosov result B ~ [ln (H - H_c1)]^{-2}, valid in the absence
of disorder and thermal fluctuations, changes significantly in the presence of
disorder. Moreover, the constitutive relations will depend strongly on the type
of disorder. In the presence of point disorder, B ~ (H - H_c1)^{3/2} in
three-dimensional (thick) superconductors, as shown by Nattermann and Lipowsky.
In two-dimensional (thin film) superconductors with point disorder, B ~ (H -
H_c1). In the presence of parallel columnar disorder, we find that B ~ exp[-C /
(H - H_c1)] in three dimensions, while B ~ exp[-K / (H - H_c1)^{1/2}] in two
dimensions. In the presence of nearly isotropically splayed disorder, we find
that B ~ (H - H_c1)^{3/2} in both two and three dimensions.


title:
Realization of Bose-Einstein condensation of dilute magnons in
  TlCuCl$_3$

abstract:
  The recent observation [Oosawa et al. J. Phys. : Condens. Matter {\bf 11},
265 (1999)] of the field-induced N\'{e}el ordering in a spin-gap magnetic
compound TlCuCl$_3$ is interpreted as a Bose-Einstein Condensation of magnons.
A mean-field calculation based on this picture is shown to describe well the
temperature dependence of the magnetization. The present system opens a new
area for studying Bose-Einstein condensation of thermodynamically large number
of particles in a grand-canonical ensemble.


title: Temperature and Pressure Effects on Lattice Properties of Pure C60 Solid

abstract:
  A simple model based on atom-atom potential has been used to calculate
various bulk, structural and thermodynamic properties of pure C_{60} solid by
including implicit anharmonicity. The pressure and temperature dependent
properties such as bulk modulus, lattice and orientational structure, thermal
expansion, phonon frequency shift, Gruneisen parameters, heat capacity and
entropy have been calculated and compared with available data. Though results
for some of these properties have been calculated earlier, using improved
potential models to suit C60 solid, this paper ascertains the extent of
validity of a potential used successfully in the past explaining the observed
properties of aromatic hydrocarbons. The same potential used without
modification does reproduce, barring orientational absolute minimum
configuration, broadly, all the other calculated properties to similar degree
of accuracy. It thus provides a platform to include additional terms for
interpreting properties of doped C60 solids and polymerized C60 systems.


title: Magnetic subband structure of an electron on a triangular lattice

abstract:
  The effects of anisotropic nearest-neighbor (NN) and isotropic
next-nearest-neighbor (NNN) hopping integrals on the magnetic subband structure
of an electron on a triangular lattice are studied within the tight-binding
approximation. A generalized Harper equation that includes both the NN and the
NNN hopping terms is first derived. Then, the effects of anisotropic NN hopping
integrals are examined and the results are compared in detail with those of an
existing literature [Phys. Rev. B {\bf 56}, 3787 (1997)]. It is found that the
hopping anisotropy generically leads to the occurrence of band broadening and
gap closing. The effects of isotropic NNN hopping integrals are next examined,
and it is found that introducing the NNN hopping integral changes considerably
the magnetic subband structure; band broadening, gap closing, gap reopening,
and band crossing occur depending on the strength of the isotropic NNN hopping
integral. Symmetries of the magnetic subband structures with and without both
the hopping anisotropy and the isotropic NNN hopping integrals are also
discussed.


title: Peierls substitution in the energy dispersion of a hexagonal lattice

abstract:
  The method of the Peierls substitution in studying the magnetic subband
structure of a hexagonal lattice is re-examined. Several errors in the
formalism of a couple of recent papers are pointed out and rectified so as to
describe the effect of the magnetic field pertinently.


title:
Localization of electrons in two-dimensional spatially-correlated random
  magnetic fields

abstract:
  The localization properties of electrons moving in a plane perpendicular to a
spatially-correlated static magnetic field of random amplitude and vanishing
mean are investigated. We apply the method of level statistics to the
eigenvalues and perform a multifractal analysis for the eigenstates. From the
size and disorder dependence of the variance of the nearest neighbor energy
spacing distribution, $P_{W,L}(s)$, a single branch scaling curve is obtained.
Contrary to a recent claim, we find no metal-insulator-transition in the
presence of diagonal disorder. Instead, as in the uncorrelated random magnetic
field case, conventional unitary behavior (all states are localized) is
observed. The eigenstates at the band center, which in the absence of diagonal
disorder are believed to belong to the chiral unitary symmetry class, are shown
to exhibit a $f(\alpha)$-distribution for not too weak random fields. The
corresponding generalized multifractal dimensions are calculated and found to
be different from the results known for a QHE-system.


title: Frequency dependent conductivity in the integer quantum Hall effect

abstract:
  Frequency dependent electronic transport is investigated for a
two-dimensional disordered system in the presence of a strong perpendicular
static magnetic field. The ac-conductivity is calculated numerically from
Kubo's linear response theory using a recursive Green's function technique. In
the tail of the lowest Landau band, we find a linear frequency dependence for
the imaginary part of $\sigma_{xx}(\omega)$ which agrees well with earlier
analytical calculations. On the other hand, the frequency dependence of the
real part can not be expressed by a simple power law. The broadening of the
$\sigma_{xx}$-peak with frequency in the lowest Landau band is found to exhibit
a scaling relation from which the critical exponent can be extracted.


title:
Unusual Charge Localization in Zn-doped and Heavily Underdoped
  YBa2Cu3O7-d at Low Temperatures

abstract:
  The in-plane normal-state resistivity of Zn-doped YBa2Cu3O7-d and heavily
underdoped pure YBCO single crystals is measured down to low temperatures under
magnetic fields up to 18 T. We found that the temperature dependence of the
normal-state rho_ab does not obey log(1/T) and tends not to diverge in the low
temperature limit. The result suggests that the ``ground state'' of the normal
state of YBCO is metallic.


title:
Critical Properties of Spin-1 Antiferromagnetic Heisenberg Chains with
  Bond Alternation and Uniaxial Single-Ion-Type Anisotropy

abstract:
  One dimensional S=1 antiferromagnetic Heisenberg chains with bond alternation
and uniaxial single-ion-type anisotropy are studied by numerical exact
diagonalization of finite size systems. We calculate the ground state phase
diagram using the twisted boundary condition method for $D\geq 0$. The ground
states of this model contain the Haldane state, large-D state and the dimer
state. Using conformal field theory and the level spectroscopy method, we
calculate the D and $\delta$-dependence of the Luttinger liquid paraments K and
the critical exponent $\nu$ of the energy gap.


title: Theory of inelastic lifetimes of low-energy electrons in metals

abstract:
  Electron dynamics in the bulk and at the surface of solid materials are well
known to play a key role in a variety of physical and chemical phenomena. In
this article we describe the main aspects of the interaction of low-energy
electrons with solids, and report extensive calculations of inelastic lifetimes
of both low-energy electrons in bulk materials and image-potential states at
metal surfaces. New calculations of inelastic lifetimes in a homogeneous
electron gas are presented, by using various well-known representations of the
electronic response of the medium. Band-structure calculations, which have been
recently carried out by the authors and collaborators, are reviewed, and future
work is addressed.


title: Synchronization of non-chaotic dynamical systems

abstract:
  A synchronization mechanism driven by annealed noise is studied for two
replicas of a coupled-map lattice which exhibits stable chaos (SC), i.e.
irregular behavior despite a negative Lyapunov spectrum. We show that the
observed synchronization transition, on changing the strength of the stochastic
coupling between replicas, belongs to the directed percolation universality
class. This result is consistent with the behavior of chaotic deterministic
cellular automata (DCA), supporting the equivalence Ansatz between SC models
and DCA. The coupling threshold above which the two system replicas synchronize
is strictly related to the propagation velocity of perturbations in the system.


title:
Magnetic anisotropy, first-order-like metamagnetic transitions and large
  negative magnetoresistance in the single crystal of Gd$_{2}$PdSi$_3$

abstract:
  Electrical resistivity ($\rho$), magnetoresistance (MR), magnetization,
thermopower and Hall effect measurements on the single crystal
Gd$_{2}$PdSi$_3$, crystallizing in an AlB$_2$-derived hexagonal structure are
reported. The well-defined minimum in $\rho$ at a temperature above N\'eel
temperature (T$_N$= 21 K) and large negative MR below $\sim$ 3T$_N$, reported
earlier for the polycrystals, are reproducible even in single crystals. Such
features are generally uncharacteristic of Gd alloys. In addition, we also
found interesting features in other data, e.g., two-step first-order-like
metamagnetic transitions for the magnetic field along [0001] direction. The
alloy exhibits anisotropy in all these properties, though Gd is a S-state ion.


title:
Can pulling cause right- to left-handed structural transitions in
  negatively supercoiled DNA double-helix?

abstract:
  The folding angle distribution of stretched and negatively supercoiled DNA
double-helix is investigated based on a theoretical model we proposed earlier
[H. Zhou et al., Phys. Rev. Lett. 82, 4560 (1999)]. It is shown that pulling
can transit a negatively supercoiled DNA double-helix from the right-handed
B-form to a left-handed configuration which resembles DNA Z-form in some
important respects. The energetics of this possible transition is calculated
and the comparison with recent experimental observations are qualitatively
discussed.

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


title: Random matrices and the replica method

abstract:
  Recent developments [Kamenev and Mezard, cond-mat/9901110, cond-mat/9903001;
Yurkevich and Lerner, cond-mat/9903025; Zirnbauer, cond-mat/9903338] have
revived a discussion about applicability of the replica approach to description
of spectral fluctuations in the context of random matrix theory and beyond. The
present paper, concentrating on invariant non-Gaussian random matrix ensembles
with orthogonal, unitary and symplectic symmetries, aims to demonstrate that
both the bosonic and the fermionic replicas are capable of reproducing
nonperturbative fluctuation formulas for spectral correlation functions in
entire energy scale, including the self-correlation of energy levels, provided
no sigma-model mapping is used.

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


title: One-Component Plasma: Going Beyond Debye-Hueckel

abstract:
  Using field-theoretic methods, we calculate the internal energy for the
One-Component Plasma (OCP). We go beyond the recent calculation by Brilliantov
[N. Brilliantov, Contrib. Plasma Phys. 38, pg. 489 (1998) / cond-mat/9805358]
by including non-Gaussian terms. We show that, for the whole range of the
plasma parameter Gamma, the effect of the higher-order terms is small and that
the final result is not improved relative to the Gaussian theory when compared
to simulations.


title:
Comment on `Glassy Transition in a Disordered Model for the RNA
  Secondary Structure'

abstract:
  In cond-mat/9907125 the low-temperature behavior of a model for RNA secondary
structure was studied. It is claimed that the model exhibits a breaking of the
replica symmetry, since the width of the distribution P(q) of overlaps may
converge to a finite value at T=0. The authors used an exact enumeration method
to obtain all ground states for a given RNA sequence. Because of the
exponential growing degeneracy, only sequences up to length L=256 could be
studied.
  Here it is shown that, in contrast to the previous results, by going to much
larger sizes as L=2000 the variance coverges towards zero, i.e. P(q) is a
delta-function in the thermodynamic limit.


title: Finite Temperature Collapse of a Bose Gas with Attractive Interactions

abstract:
  We study the mechanical stability of the weakly interacting Bose gas with
attractive interactions, and construct a unified picture of the collapse valid
from the low temperature condensed region to the high temperature classical
regime. As we show, the non-condensed particles play a crucial role in
determining the region of stability. We extend our results to describe domain
formation in spinor condensates.


title:
Low-energy excitations of spin-Peierls chains with modified
  bond-impurities

abstract:
  The introduction of modified bond-defects in spin-Peierls systems is
investigated in a model of antiferromagnetic Heisenberg spin chains coupled to
adiabatic phonons. Generically, new low-energy magnetic or non-magnetic
excitations appear below the bulk spin gap energy. When two adjacent bonds are
modified, these excitations can be interpreted in terms of boundstates of a
soliton with the localized spin-1/2 located on the impurity site. It is shown
that the confining potential occurs even in the case of {\it isolated} chains.


title:
Non-Kondo zero bias anomaly in electronic transport through an
  ultra-small Si quantum dot

abstract:
  We have studied low-temperature single electron transport through ultra-small
Si quantum dots. We find that at low temperatures Coulomb blockade is partially
lifted at certain gate voltages. Furthermore, we observed an enhancement of
differential conductance at zero bias. The magnetic field dependence of this
zero bias anomaly is very different from the one reported in GaAs quantum dots,
inconsistent with predictions for the Kondo effect.


title: Diffusion of hydrogen in crystalline silicon

abstract:
  The coefficient of diffusion of hydrogen in crystalline silicon is calculated
using tight-binding molecular dynamics. Our results are in good quantitative
agreement with an earlier study by Panzarini and Colombo [Phys. Rev. Lett. 73,
1636 (1994)]. However, while our calculations indicate that long jumps dominate
over single hops at high temperatures, no abrupt change in the diffusion
coefficient can be observed with decreasing temperature. The (classical)
Arrhenius diffusion parameters, as a consequence, should extrapolate to low
temperatures.


title:
Prospecting for the superfluid transition in electron-hole coupled
  quantum wells using Coulomb drag

abstract:
  I investigate the possibility of using Coulomb drag to detect a precursor of
the predicted (but as yet not definitively observed) superfluid transition in
electron-hole coupled quantum wells. The drag transresisitivity $\rho_{21}$ is
shown to be significantly enhanced above the transition temperature $T_c$ and
to diverge logarithmically as $T\to T_c^+$, due to electron-hole pairing
fluctuations which are somewhat analogous to the Maki-Thompson contribution to
conductivity in metals above the superconducting $T_c$. The enhancement in
$\rho_{21}$ is estimated to be detectable at temperatures much higher than
$T_c$.


title: Numerical studies of Anderson transition

abstract:
  We present numerical results for the statistics of $z$'s ($z$'s are defined
as logarithm of eigenvalues of the transfermatrix $T^\dag T$) at the critical
points of Anderson transition in 3D and 4D. The change of the density of $z$
due to the crossover from the metallic to the localized regime is described.
Linear behavior $\rho(z)= z$ at the critical point in 3D is proven and
discussed. In the insulating regime, the universal form of $\rho$ has been
found.


title:
Conductance fluctuations as a tool for investigating the quantum modes
  in atomic size metallic contacts

abstract:
  Recently it has been observed that the conductance fluctuations of atomic
size gold contacts are suppressed when the conductance is equal to an integer
multiple of the conductance quantum. The fact that these contacts tend to
consist exclusively of fully open or closed modes has been argued to be the
origin for this suppression. Here, the experiments have been extended to a wide
range of metallic elements with different chemical valence and they provide new
information about the relation between the mode composition and statistically
preferred conductance values observed in conductance histograms.


title:
Topological Aspects Of Spin Pairing in High $T_c$ Superconductivity and
  Berry Phase

abstract:
  From topological viewpoint we have analysed the role of Berry phase in spin
pairing mechanism of high $T_c$ superconducting state.


title: Thermally Activated Dynamics of the Capillary Condensation

abstract:
  This paper is devoted to the thermally activated dynamics of the capillary
condensation. We present a simple model which enables us to identify the
critical nucleus involved in the transition mechanism. This simple model is
then applied to calculate the nucleation barrier from which we can obtain
informations on the nucleation time. We present a simple estimation of the
nucleation barrier in slab geometry both in the two dimensional case and in the
three dimensional case. We extend the model in the case of rough surfaces which
is closer to the experimental case and allows comparison with experimental
datas.


title: Humidity Effects and Aging Behavior in Granular Media

abstract:
  We present a study of humidity effects on the maximum stability angle in
granular media. We show that a granular medium of small glass beads exhibits
aging properties : the first avalanche angle increases logarithmically with the
resting time of the pile. This aging behavior is found to depend on the
relative humidity of the surrounding atmosphere. A short interpretation of this
effect, based on a model of activated capillary condensation, is proposed.


title: Universalities of weak localization

abstract:
  We present here a novel approach to evaluation of the weak localization
correction (WLC) to transport properties of a mesoscopic conductor. It is based
on an extension of Keldysh technique and allows one to evaluate the full
counting statistics of the current in the conductor. In our opinion, it
provides a fresh look on the theory of weak localization.


title:
Existence of Saturated Ferromagnetic and Spiral States in 1D
  Lieb-Ferrimagnetic Models away from Half-Filling

abstract:
  In order to study conditions for the appearance of ferromagnetism in a wide
filling region, we investigate numerically three types of one-dimensional
Lieb-ferrimagnetic Hubbard models: a periodic diamond (PD) chain, a periodic
alternately-attached leg (PAAL) chain and an open diamond (OD) chain. All of
these models have a flat band (or equivalently, degenerate single-electron
eigenvalues). The PD and OD chains commonly have a local-loop structure.
Nagaoka's theorem holds only in the PD chain. At half-filling, it have been
rigorously proven that all of these models are ferrimagnet. Away from
half-filling, however, quite different magnetic properties are found. In the
fillings 1/3< rho_e <1/2, the ground state of the PD chain for a
infinitely-large U is the extended ferromagnetic state, that is, the saturated
ferromagnetic state or the spiral state for odd or even number of electrons,
respectively. In the PAAL chain, on the other hand, there is no magnetic order.
Thus, the flat band is found to be not a sufficient condition of the extended
ferromagnetic state. We find, moreover, that the saturated ferromagnetism
appears in the OD chain, although the Nagaoka theorem does not hold on this
chain. This indicates that the local-loop structure plays an important role on
the appearance of the extended ferromagnetic state.


title: On the scaling of the (H-T)-phase diagram of CuGeO$_3$

abstract:
  The H-T phase diagram of CuGeO$_3$ has been determined, for different values
of the hydrostatic pressure, utilizing optical absorption spectroscopy on the
Cu$^{2+}$ d-d transitions. It is shown that the intensity of the related zero
phonon line transition is very sensitive to the local environment of Cu$^{2+}$,
allowing for precise determination of all phase transitions. It is found that
the phase diagrams at various pressures do not scale according to the
Cross-Fischer theory. An alternative scaling is proposed.


title:
Bose Condensate in a Double-Well Trap: Ground State and Elementary
  Excitations

abstract:
  We study the Bose-Einstein condensate in the MIT double-well trap. We
calculate the ground state density profile of $^{23}$Na atoms and the
Bogoliubov spectrum of the elementary excitations as function of the strength
of the double-well barrier. In particular, we analyze the behavior of
quantum-mechanical collective excitations. Finally, we discuss the
observability criteria for macroscopic quantum tunneling and macroscopic
quantum self-trapping.


title: Time-Dependent Variational Approach to Bose-Einstein Condensation

abstract:
  We discuss the mean-field approximation for a trapped weakly-interacting
Bose-Einstein condensate (BEC) and its connection with the exact many-body
problem by deriving the Gross-Pitaevskii action of the condensate. The
mechanics of the BEC in a harmonic potential is studied by using a variational
approach with time-dependent Gaussian trial wave-functions. In particular, we
analyze the static configurations, the stability and the collective
oscillations for both ground-state and vortices.


title: Exact Solution of an Evolutionary Model without Ageing

abstract:
  We introduce an age-structured asexual population model containing all the
relevant features of evolutionary ageing theories. Beneficial as well as
deleterious mutations, heredity and arbitrary fecundity are present and managed
by natural selection. An exact solution without ageing is found. We show that
fertility is associated with generalized forms of the Fibonacci sequence, while
mutations and natural selection are merged into an integral equation which is
solved by Fourier series. Average survival probabilities and Malthusian growth
exponents are calculated indicating that the system may exhibit mutational
meltdown. The relevance of the model in the context of fissile reproduction
groups as many protozoa and coelenterates is discussed.


title: The Optimized Model of Multiple Invasion Percolation

abstract:
  We study the optimized version of the multiple invasion percolation model.
Some topological aspects as the behavior of the acceptance profile,
coordination number and vertex type abundance were investigated and compared to
those of the ordinary invasion. Our results indicate that the clusters show a
very high degree of connectivity, spoiling the usual nodes-links-blobs
geometrical picture.


title: Magnetic relaxation in a classical spin chain as model for nanowires

abstract:
  With decreasing particle size, different mechanisms dominate the thermally
activated magnetization reversal in ferromagnetic particles. We investigate
some of these mechanisms for the case of elongated, single-domain nanoparticles
which we describe by a classical Heisenberg spin chain driven by an external
magnetic field. For sufficiently small system size the magnetic moments rotate
coherently. With increasing size a crossover to a reversal due to
soliton-antisoliton nucleation sets in. For even larger systems many of these
soliton-antisoliton pairs nucleate at the same time. These effects give rise to
a complex size dependence of the energy barriers and characteristic time scales
of the relaxation. We study these quantities using Monte Carlo simulations as
well as a direct integration of the Landau-Lifshitz-Gilbert equation of motion
with Langevin dynamics and we compare our results with asymptotic solutions for
the escape rate following from the Fokker-Planck equation. Also, we investigate
the crossover from coherent rotation to soliton-antisoliton nucleation and
multi-droplet nucleation, especially its dependence on the system size, the
external field and the anisotropy of the system.


title:
The influence of the Wolff's cluster structure on the determination of
  the critical dynamic exponent

abstract:
  Recently we reported some interesting features of the Wolff's algorithm
behavior when applied to the site-bond-correlated Ising model.Our main results
were that a stronger correlation diminishes the autocorrelation time but it
does not change or affectthe critical dynamic exponent.In this paper, we
analyse the Wolff's cluster structure and how it varies with the spatial
correlation. The fractal dimensions are determined for several values of the
magnetic atoms concentration and spacial correlation, giving an estimative of
the critical dynamic exponent.


title: Crystal symmetry, step-edge diffusion and unstable growth

abstract:
  We study the effect of crystal symmetry and step-edge diffusion on the
surface current governing the evolution of a growing crystal surface. We find
there are two possible contributions to anisotropic currents, which both lead
to the destabilization of the flat surface: terrace current (j_t), which is
parallel to the surface slope, and step current (j_s), which has components
parallel (j_pa) and perpendicular (j_pe) to the slope. On a high-symmetry
surface, terrace and step currents are generically singular at zero slope, and
this does not allow to perform the standard linear stability analysis. As far
as a one-dimensional profile is considered, (j_pe) is irrelevant and (j_pa)
suggests that mound sides align along [110] and [1-10] axes. On a vicinal
surface, (j_s) destabilizes against step bunching; its effect against step
meandering depends on the step orientation, in agreement with the recent
findings by O.Pierre-Louis et al. [Phys. Rev. Lett. 82, 3661 (1999)].


title:
Strong enhancement of spin fluctuations in the
  low-temperature-tetragonal phase of antiferromagnetically ordered
  La_{2-x-y}Eu_ySr_xCuO_4

abstract:
  Measurements of the static magnetization, susceptibility and ESR of Gd spin
probes have been performed to study the properties of antiferromagnetically
ordered La_{2-x-y}Eu_ySr_xCuO_4 (x less or equal 0.02) with the low temperature
tetragonal structure. According to the static magnetic measurements the CuO_2
planes are magnetically decoupled in this structural phase. The ESR study
reveals strong magnetic fluctuations at the ESR frequency which are not present
in the orthorhombic phase. It is argued that this drastic enhancement of the
spin fluctuations is due to a considerable weakening of the interlayer exchange
and a pronounced influence of hole motion on the antiferromagnetic properties
of lightly hole doped La_2CuO_4. No evidence for the stripe phase formation at
small hole doping is obtained in the present study.


title: Fractional quantum conductance in carbon nanotubes

abstract:
  Using a scattering technique based on a parametrized linear combination of
atomic orbitals Hamiltonian, we calculate the ballistic quantum conductance of
multi-wall carbon nanotubes. We find that inter-wall interactions not only
block some of the quantum conductance channels, but also redistribute the
current non-uniformly over the individual tubes across the structure. Our
results provide a natural explanation for the unexpected integer and
non-integer conductance values reported for multi-wall nanotubes in Ref.[S.
Frank, P. Poncharal, Z.L. Wang, and W. A. de Heer, Science 280, 1744 (1998)]


title: Critical adsorption near edges

abstract:
  Symmetry breaking surface fields give rise to nontrivial and long-ranged
order parameter profiles for critical systems such as fluids, alloys or magnets
confined to wedges. We discuss the properties of the corresponding universal
scaling functions of the order parameter profile and the two-point correlation
function and determine the critical exponents eta_parallel and
eta_perpendicular for the so-called normal transition.


title:
Pressure Dependence of the Irreversibility Line in
  Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$:Role of Anisotropy in Flux-Line
  Formation

abstract:
  One of the important problems of high-temperature superconductivity is to
understand and ultimately to control fluxoid motion. We present the results of
a new technique for measuring the pressure dependence of the transition to
superconductivity in a diamond anvil cell. By measuring the third harmonic of
the {\it ac} susceptibility, we determine the onset of irreversible flux
motion. This enables us to study the effects of pressure on flux motion. The
application of pressure changes interplanar spacing, and hence the interplanar
coupling, without significantly disturbing the intraplanar superconductivity.
Thus we are able to separate the effects of coupling from other properties that
might affect the flux motion. Our results directly show the relationship
between lattice spacing, effective- mass anisotropy, and the irreversibility
line in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$. Our results also demonstrate
that an application of 2.5 GPa pressure causes a dramatic increase in
interplanar coupling.


title:
The influence of Anderson localization on the mode decay of excited
  nonlinear systems

abstract:
  A one-dimensional system of masses with nearest-neighbor interactions and
periodic boundary conditions is used to study mode decay and ergodicity in
nonlinear, disordered systems. The system is given an initial periodic
displacement, and the total system energy within a specific frequency channel
is measured as a function of time. Results indicate that the rate of mode decay
at early times increases when impurities are added. However, for long times the
rate of mode decay decreases with increasing impurity mass and impurity
concentration. This behavior at long times can be explained by Anderson
localization effects and the nonergodic response of the system.


title:
A pseudopotential study of electron-hole excitations in colloidal,
  free-standing InAs quantum dots

abstract:
  Excitonic spectra are calculated for free-standing, surface passivated InAs
quantum dots using atomic pseudopotentials for the single-particle states and
screened Coulomb interactions for the two-body terms. We present an analysis of
the single particle states involved in each excitation in terms of their
angular momenta and Bloch-wave parentage. We find that (i) in agreement with
other pseudopotential studies of CdSe and InP quantum dots, but in contrast to
k.p calculations, dot states wavefunction exhibit strong odd-even angular
momentum envelope function mixing (e.g. $s$ with $p$) and large
valence-conduction coupling. (ii) While the pseudopotential approach produced
very good agreement with experiment for free-standing, colloidal CdSe and InP
dots, and for self-assembled (GaAs-embedded) InAs dots, here the predicted
spectrum does {\em not} agree well with the measured (ensemble average over dot
sizes) spectra. (1) Our calculated excitonic gap is larger than the PL measure
one, and (2) while the spacing between the lowest excitons is reproduced, the
spacings between higher excitons is not fit well. Discrepancy (1) could result
from surface states emission. As for (2), agreement is improved when account is
taken of the finite size distribution in the experimental data. (iii) We find
that the single particle gap scales as $R^{-1.01}$ (not $R^{-2}$), that the
screened (unscreened) electron-hole Coulomb interaction scales as $R^{-1.79}$
($R^{-0.7}$), and that the eccitonic gap sclaes as $R^{-0.9}$. These scaling
laws are different from those expected from simple models.


title:
Microwave Conductivity due to Impurity Scattering in a d-wave
  Superconductor

abstract:
  The self-consistent t-matrix approximation for impurity scattering in
unconventional superconductors is used to interpret recent measurements of the
temperature and frequency dependence of the microwave conductivity of YBCO
crystals below 20K. In this theory, the conductivity is expressed in terms of a
fequency dependent single particle self-energy, determined by the impurity
scattering phase shift which is small for weak (Born) scattering and approaches
$\pi / 2$ for unitary scattering. Inverting this process, microwave
conductivity data are used to extract an effective single-particle self-energy
and obtain insight into the nature of the operative scattering processes. It is
found that the effective self-energy is well approximated by a constant plus a
linear term in frequency with a small positive slope for thermal quasiparticle
energies below 20K. Possible physical origins of this form of self-energy are
discussed.


title: Mach-Zehnder Bragg interferometer for a Bose-Einstein Condensate

abstract:
  We construct a Mach-Zehnder interferometer using Bose-Einstein condensed
rubidium atoms and optical Bragg diffraction. In contrast to interferometers
based on normal diffraction, where only a small percentage of the atoms
contribute to the signal, our Bragg diffraction interferometer uses all the
condensate atoms. The condensate coherence properties and high phase-space
density result in an interference pattern of nearly 100% contrast. In
principle, the enclosed area of the interferometer may be arbitrarily large,
making it an ideal tool that could be used in the detection of vortices, or
possibly even gravitational waves.


title: Supersymmetric Mean-Field Theory of t-J Model

abstract:
  The supersymmetric formulation of t-J model is studied in this paper at the
mean-field level where $\delta$-T phase diagram is computed. We find that
slave-fermion-like spiral phase is stable at low doping concentration, and the
slave-boson-like d-wave fermionic spin pairing state becomes energetically
favourable when $\delta\geq$ 0.23. An improvement in free energy using
Gutzwiller's method lowers the transition doping concentration to 0.06. We also
point out the existence of new branches of excitations in the supersymmetric
theory.


title: A model of marginal Fermi liquid superconductor at two dimensions

abstract:
  In this paper we study a model of s-wave marginal Fermi liquid superconductor
at two dimensions. Besides the usual Bardeen-Cooper-Schrieffer(BCS) point
attractive interaction, the fermions in our system also interact {\em via} a
long range repulsive interaction $v(q)=g_{D}/q^2$. We find that although
superconductivity is not destroyed by the long-ranged repulsion interaction
(demonstrated by the existence of Meissner effect), nevertheless the BCS
pairing function $\Delta_{BCS}$ is strongly suppressed resulting in power-law
correlation $<\Delta_{BCS}(\vec{x})\Delta_{BCS}(0)>\sim|\vec{x}|^{-2\gamma}$.
This observation is explained within a general picture of marginal Fermi
liquid.


title: Bosonization theory of fermions interacting via a confining potential

abstract:
  We study in this paper the properties of a gas of fermions interacting {\em
via} a scalar potential $v(q)=4\pi{e}^2/q^2$ for $q<\Lambda<<k_F$ at dimensions
larger than one, where $\Lambda$ is a high momentum cutoff and $k_F$ is the
fermi wave vector. In particular, we shall consider the $e^2\to\infty$ limit
where the potential becomes confining. Within a bosonization approximation,
effective Hamiltonians describing the low energy physics of the system are
constructed, where we show that the system can be described as a fermi liquid
formed by chargeless quasi-particles which has vanishing wavefunction overlap
with the bare fermions in the system.