title: Single-file Diffusion with Random Diffusion Constants

abstract:
  The single-file problem of N particles in one spatial dimension is analyzed,
when each particle has a randomly distributed diffusion constant D sampled in a
density $\rho(D)$. The averaged one-particle distributions of the edge
particles and the asymptotic ($N\gg 1$) behaviours of their transport
coefficients (anomalous velocity and diffusion constant) are strongly dependent
on the D-distribution law, broad or narrow. When $\rho$ is exponential, it is
shown that the average one-particle front for the edge particles does not
shrink when N becomes very large, as contrasted to the pure (non-disordered)
case. In addition, when $\rho$ is a broad law, the same occurs for the averaged
front, which can even have infinite mean and variance. On the other hand, it is
shown that the central particle, dynamically trapped by all others as it is,
follows a narrow distribution, which is a Gaussian (with a diffusion constant
scaling as $N^{-1}$) when the fractional moment $<D^{-1/2}>$ exists and is
finite; otherwise ($\rho(D)\propto D^{\alpha-1}, \alpha\le\case12$), this
density is, far from the origin, a stretched exponential with an exponent in
the range $]0, 2]$; then the effective diffusion constant scales as
$N^{-\beta}$, with $\beta = 1/(2\alpha)$.


title:
Stochastic vortex dynamics in two-dimensional easy-plane ferromagnets:
  Multiplicative versus additive noise

abstract:
  We study how thermal fluctuations affect the dynamics of vortices in the
two-dimensional classical, ferromagnetic, anisotropic Heisenberg model
depending on their additive or multiplicative character. Using a collective
coordinate theory, we analytically show that multiplicative noise, arising from
fluctuations in the local field term of the Landau-Lifshitz equations, and
Langevin-like additive noise both have the same effect on vortex dynamics
(within a very plausible assumption consistent with the collective coordinate
approach). This is a non-trivial result, as multiplicative and additive noises
usually modify the dynamics quite differently. We also carry out numerical
simulations of both versions of the model finding that they indeed give rise to
very similar vortex dynamics.


title:
Noise-induced switching between vortex states with different
  polarization in classical two-dimensional easy-plane magnets

abstract:
  In the 2-dimensional anisotropic Heisenberg model with XY-symmetry there are
non-planar vortices which exhibit a localized structure of the z-components of
the spins around the vortex center. We study how thermal noise induces a
transition of this structure from one polarization to the opposite one. We
describe the vortex core by a discrete Hamiltonian and consider a stationary
solution of the Fokker-Planck equation. We find a bimodal distribution function
and calculate the transition rate using Langer's instanton theory (1969). The
result is compared with Langevin dynamics simulations for the full many-spin
model.


title: Dielectric susceptibility of the Coulomb-glass

abstract:
  We derive a microscopic expression for the dielectric susceptibility $\chi$
of a Coulomb glass, which corresponds to the definition used in classical
electrodynamics, the derivative of the polarization with respect to the
electric field. The fluctuation-dissipation theorem tells us that $\chi$ is a
function of the thermal fluctuations of the dipole moment of the system. We
calculate $\chi$ numerically for three-dimensional Coulomb glasses as a
function of temperature and frequency.


title: Universal amplitude ratios in the 2D four state Potts model

abstract:
  We present a Monte Carlo study of various universal amplitude ratios of the
two dimensional q=4 Potts model. We simulated the model close to criticality in
both phases taking care to keep the systematic errors, due to finite size
effects and logarithmic corrections in the scaling functions, under control.
Our results are compatible with those recently obtained using the form-factors
approach and with the existing low temperature series for the model.


title: Phase diagram of magnetic polymers

abstract:
  We consider polymers made of magnetic monomers (Ising or Heisenberg-like) in
a good solvent. These polymers are modeled as self-avoiding walks on a cubic
lattice, and the ferromagnetic interaction between the spins carried by the
monomers is short-ranged in space. At low temperature, these polymers undergo a
magnetic induced first order collapse transition, that we study at the mean
field level. Contrasting with an ordinary $\Theta$ point, there is a strong
jump in the polymer density, as well as in its magnetization. In the presence
of a magnetic field, the collapse temperature increases, while the
discontinuities decrease. Beyond a multicritical point, the transition becomes
second order and $\Theta$-like. Monte Carlo simulations for the Ising case are
in qualitative agreement with these results.


title:
Flux-noise spectra around the Kosterlitz-Thouless transition for
  two-dimensional superconductors

abstract:
  The flux-noise spectra around the Kosterlitz-Thouless transition are obtained
from simulations of the two-dimensional resistively shunted junction model. In
particular the dependence on the distance $d$ between the pick-up coil and the
sample is investigated. The typical experimental situation corresponds to the
large-$d$ limit and a simple relation valid in this limit between the complex
impedance and the noise spectra is clarified. Features, which distinguish
between the large- and small-$d$ limit, are identified and the possibility of
observing these features in experiments is discussed.


title:
Low Temperature Static and Dynamic Behavior of the Two-Dimensional
  Easy-Axis Heisenberg Model

abstract:
  We apply the self-consistent harmonic approximation (SCHA) to study static
and dynamic properties of the two-dimensional classical Heisenberg model with
easy-axis anisotropy. The static properties obtained are magnetization and spin
wave energy as functions of temperature, and the critical temperature as a
function of the easy-axis anisotropy. We also calculate the dynamic correlation
functions using the SCHA renormalized spin wave energy. Our analytical results,
for both static properties and dynamic correlation functions, are compared to
numerical simulation data combining cluster-Monte Carlo algorithms and Spin
Dynamics. The comparison allows us to conclude that far below the transition
temperature, where the SCHA is valid, spin waves are responsible for all
relevant features observed in the numerical simulation data; topological
excitations do not seem to contribute appreciably. For temperatures closer to
the transition temperature, there are differences between the dynamic
correlation functions from SCHA theory and Spin Dynamics; these may be due to
the presence of domain walls and solitons.


title: Kondo screening in gapless magnetic alloys

abstract:
  The low-energy physics of a spin-1/2 Kondo impurity in a gapless host, where
a density of band states $\rho_0(\epsilon)=|\epsilon|^r/(|\epsilon|^r+\beta^r)$
vanishes at the Fermi level $\epsilon=0$, is studied by the Bethe ansatz. The
growth of the parameter $\Gamma_r=\beta{\rm g}^{-1/r}$ (where ${\rm g}$ is an
exchange constant) is shown to drive the system ground state from the Kondo
regime with the screened impurity spin to the Anderson regime, where the
impurity spin is unscreened, however, in a weak magnetic field $H$, it exceeds
its free value, $S_i(H)>{1/2}$, due to a strong coupling to a band. It is shown
also that a sufficiently strong potential scattering at the impurity site
destroys the Anderson regime.


title:
Hierarchy of equations of motion for nonlinear coherent excitations
  applied to magnetic vortices

abstract:
  Starting from a travelling wave ansatz we show successively that the shape of
a nonlinear excitation generally depends also on the 1st, 2nd, ... time
derivative of the position X of the excitation. From the Hamilton equations we
derive a hierarchy of equations of motion for X. The type of the excitation
determines on which levels the hierarchy can be truncated consistently:
"Gyrotropic" excitations are governed by odd-order equations, non-gyrotropic
ones by even-order equations. Examples for the latter case are kinks in
1-dimensional models and planar vortices of the 2D anisotropic (easy-plane)
Heisenberg model. The non-planar vortices of this model are the simplest
gyrotropic example. For this case we solve the Hamilton equations for a finite
system with one vortex and free boundary conditions and calculate the
parameters of the 3rd-order equation of motion. This equation yields
trajectories which are a superposition of two cycloids with different
frequencies, which is in full agreement with computer simulations of the full
many-spin model. Finally we demonstrate that the additional effects from the
5th-order equation are negligible.


title:
Interaction of nonuniform elastic waves with two-dimensional electrons
  in AlGaAs-GaAs-AlGaAs heterostructures

abstract:
  An interaction of a double layer electron system realized in an
AlGaAs-GaAs-AlGaAs heterostructure with nonuniform elastic modes localized in
the GaAs layer is considered. The dependence of the coupling constant on the
ratio between the thickness of the GaAs layer and the wavelength is calculated
for the wave vector directed along the [110] axis, the displacement vector
lying in the (1-10) plane and the interface boundaries parallel to the (001)
plane. It is shown, that the coupling constant reaches the maximal value at the
wavelength which is of order of the thickness of the GaAs layer. The
renormalization of the velocity of the elastic modes is found for the case,
when the electron system is in the fractional quantum Hall regime. It is shown,
that for certain modes the dependence of the velocity shift on the wave vector
is modified qualitatively under a transition of the electron system to the
state, which corresponds to the Halperin wave function.


title: Theory of strong outcoupling from Bose-Einstein condensates

abstract:
  We study the dynamics of a magnetically trapped Bose-Einstein condensate in
the presence of an external electromagnetic field coupling trapped, untrapped
and antitrapped Zeeman sublevels. For large condensates an approximate
analytical solution of the coupled Gross-Pitaevskii equations is given in the
regime of strong outcoupling. The theory is developed for the cases of
rf-outcoupling within a hyperfine manifold of states, micowave-outcoupling
connecting states in two different hyperfine manifolds, and Raman outcoupling.


title:
Quantum Hall fluctuations and evidence for charging in the quantum Hall
  effect

abstract:
  We find that mesoscopic conductance fluctuations in the quantum Hall regime
in silicon MOSFETs display simple and striking patterns. The fluctuations fall
into distinct groups which move along lines parallel to loci of integer filling
factor in the gate voltage-magnetic field plane. Also, a relationship appears
between the fluctuations on quantum Hall transitions and those found at low
densities in zero magnetic field. These phenomena are most naturally attributed
to charging effects. We argue that they are the first unambiguous manifestation
of interactions in dc transport in the integer quantum Hall effect.


title:
Theory of Magneto--Acoustic Transport in Modulated Quantum Hall Systems
  Near $\nu=1/2$

abstract:
  Motivated by the experimental results of Willett et al [Phys.Rev. Lett., {\bf
78}, 4478 (1997)] we develop a magneto-transport theory for the response of a
two dimensional electron gas (2DEG) in the Fractional Quantum Hall Regime near
Landau level filling factor $\nu = 1/2$ to the surface acoustic wave (SAW) in
the presence of an added periodic density modulation. We assume there exists a
Composite Fermion Fermi Surface (CF-FS) at $\nu = 1/2$, and we show that the
deformation of the (CF-FS) due to the density modulation can be at the origin
of the observed transport anomalies for the experimental conditions. Our
analysis is carried out particularly for the non-local case which corresponds
to the SAW experiments. We introduce a new model of a deformed CF-FS. The model
permits us to explain anomalous features of the response of the modulated 2DEG
to the SAW near $\nu = 1/2:$ namely the nonlinear wave vector dependence of the
electron conductivity, the appearance of peaks in the SAW velocity shift and
attenuation and the anisotropy of the effect, all of which originate from
contributions to the conductivity tensor due to the regions of the CF-FS which
are flattened by the applied modulation.


title:
Self-organized criticality in the hysteresis of the Sherrington -
  Kirkpatrick model

abstract:
  We study hysteretic phenomena in random ferromagnets. We argue that the angle
dependent magnetostatic (dipolar) terms introduce frustration and long range
interactions in these systems. This makes it plausible that the Sherrington -
Kirkpatrick model may be able to capture some of the relevant physics of these
systems. We use scaling arguments, replica calculations and large scale
numerical simulations to characterize the hysteresis of the zero temperature SK
model. By constructing the distribution functions of the avalanche sizes,
magnetization jumps and local fields, we conclude that the system exhibits
self-organized criticality everywhere on the hysteresis loop.


title:
Neutron Scattering Study of Spin Density Wave Order in the
  Superconducting State of Excess-Oxygen-Doped La2CuO4+y

abstract:
  We report neutron scattering measurements of spin density wave order within
the superconducting state of a single crystal of predominately stage-4
La2CuO4+y with a Tc(onset) of 42 K. The low temperature elastic magnetic
scattering is incommensurate with the lattice and is characterized by
long-range order in the copper-oxide plane with the spin direction identical to
that in the insulator. Between neighboring planes, the spins exhibit
short-range correlations with a stacking arrangement reminiscent of that in the
undoped antiferromagnetic insulator. The elastic magnetic peak intensity
appears at the same temperature within the errors as the superconductivity,
suggesting that the two phenomena are strongly correlated. These observations
directly reveal the persistent influence of the antiferromagnetic order as the
doping level increases from the insulator to the superconductor. In addition,
our results confirm that spin density wave order for incommensurabilities near
1/8 is a robust feature of the La2CuO4-based superconductors.


title:
A two-dimensional interacting system obeying Fractional Exclusion
  Statistics

abstract:
  We consider N fermions in a two-dimensional harmonic oscillator potential
interacting with a very short-range repulsive pair-wise potential. The
ground-state energy of this system is obtained by performing a Thomas-Fermi as
well as a self-consistent Hartree-Fock calculation. The two results are shown
to agree even for a small number of particles. We next use the finite
temperature Thomas-Fermi method to demonstrate that in the local density
approximation, these interacting fermions are equivalent to a system of
noninteracting particles obeying the Haldane-Wu fractional exclusion
statistics. It is also shown that mapping onto to a system of N noninteracting
quasi-particles enables us to predict the energies of the excited states of the
N-body system.


title:
``X-Ray Edge'' Singularities in Nanotubes and Quantum Wires with
  Multiple Subbands

abstract:
  Band theory predicts an inverse square root van Hove singularity in the
tunneling density of states at the minimum energy of an unoccupied subband in a
one-dimensional quantum wire. With interactions, an orthogonality catastrophe
analogous to the x-ray edge effect for core levels in a metal strongly reduces
this singularity by a power B of the energy above threshold, with B
approximately 0.3 for typical carbon nanotubes. Despite the anomalous tunneling
characteristic, good quasiparticles corresponding to the unoccupied subband
states do exist.


title:
Generic transmission zeros and in-phase resonances in time-reversal
  symmetric single channel transport

abstract:
  We study phase coherent transport in a single channel system using the
scattering matrix approach. It is shown that identical vanishing of the
transmission amplitude occurs generically in quasi-1D systems if the
time-reversal is a good symmetry. The transmission zeros naturally lead to
abrupt phase changes (without any intrinsic energy scale) and in-phase
resonances, providing insights to recent experiments on phase coherent
transport through a quantum dot.


title: L\'{e}vy flights in quantum transport in quasi-ballistic wires

abstract:
  Conductance fluctuations, localization and statistics of Lyapunov exponents
are studied numerically in pure metallic wires with rough boundaries
(quasi-ballistic wires). We find that the correlation energy of conductance
fluctuations scales anomalously with the sample dimensions, indicating the role
of L\'{e}vy flights. Application of a magnetic field deflects the L\'{e}vy
flights which reduces the localization length. This deflection also breaks the
geometrical flux cancellation and restores the usual Aharonov-Bohm type
magneto-conductance fluctuations.


title:
A Simple Analytical Model of Vortex Lattice Melting in 2D
  Superconductors

abstract:
  The melting of the Abrikosov vortex lattice in a 2D type-II superconductor at
high magnetic fields is studied analytically within the framework of the
phenomenological Ginzburg-Landau theory. It is shown that local phase
fluctuations in the superconducting order parameter, associated with low
energies sliding motions of Bragg chains along the principal crystallographic
axes of the vortex lattice, lead to a weak first order 'melting' transition at
a certain temperature $T_{m}$, well below the mean field $T_{c\text{}}$, where
the shear modulus drops abruptly to a nonzero value. The residual shear modulus
above $T_{m}$ decreases asymptotically to zero with increasing temperature.
Despite the large phase fluctuations, the average positions of Bragg chains at
fimite temperature correspond to a regular vortex lattice, slightly distorted
with respect to the triangular Abrikosov lattice. It is also shown that a
genuine long range phase coherence exists only at zero temperature; however,
below the melting point the vortex state is very close to the triangular
Abrikosov lattice. A study of the size dependence of the structure factor at
finite temperature indicates the existence of quasi-long range order with
$S(\overrightarrow{G}) \sim N^{\sigma}$, and $1/2<\sigma <1$, where
superconducting crystallites of correlated Bragg chains grow only along pinning
chains. This finding may suggest a very efficient way of generating pinning
defects in quasi 2D superconductors. Our results for the melting temperature
and for the entropy jump agree with the state of the art Monte Carlo
simulations.


title:
A Comparative Study of the Magnetization Process of Two-Dimensional
  Antiferromagnets

abstract:
  Plateaux in the magnetization curves of the square, triangular and hexagonal
lattice spin-1/2 XXZ antiferromagnet are investigated. One finds a zero
magnetization plateau (corresponding to a spin-gap) on the square and hexagonal
lattice with Ising-like anisotropies, and a plateau with one third of the
saturation magnetization on the triangular lattice which survives a small
amount of easy-plane anisotropy. Here we start with transfer matrix
computations for the Ising limit and continue with series in the XXZ-anisotropy
for plateau-boundaries using the groundstates of the Ising limit. The main
focus is then a numerical computation of the magnetization curves with
anisotropies in the vicinity of the isotropic situation. Finally, we discuss
the universality class associated to the asymptotic behaviour of the
magnetization curve close to saturation, as observed numerically in two and
higher dimensions.


title: Stick-slip statistics for two fractal surfaces: A model for earthquakes

abstract:
  Following the observations of the self-similarity in various length scales in
the roughness of the fractured solid surfaces, we propose here a new model for
the earthquake. We demonstrate rigorously that the contact area distribution
between two fractal surfaces follows an unique power law. This is then utilised
to show that the elastic energy releases for slips between two rough fractal
surfaces indeed follow a Guttenberg-Richter like power law.


title: Atomic-scale simulations of nanocrystalline metals

abstract:
  Nanocrystalline metals, i.e. metals in which the grain size is in the
nanometer range, have a range of technologically interesting properties
including increased hardness and yield strength. We present atomic-scale
simulations of the plastic behavior of nanocrystalline copper. The simulations
show that the main deformation mode is sliding in the grain boundaries through
a large number of uncorrelated events, where a few atoms (or a few tens of
atoms) slide with respect to each other. Little dislocation activity is seen in
the grain interiors. The localization of the deformation to the grain
boundaries leads to a hardening as the grain size is increased (reverse
Hall-Petch effect), implying a maximum in hardness for a grain size above the
ones studied here. We investigate the effects of varying temperature, strain
rate and porosity, and discuss the relation to recent experiments. At
increasing temperatures the material becomes softer in both the plastic and
elastic regime. Porosity in the samples result in a softening of the material,
this may be a significant effect in many experiments.


title: The Surface of a Bose-Einstein Condensed Atomic Cloud

abstract:
  We investigate the structure and collective modes of a planar surface of a
trapped Bose-Einstein condensed gas at zero temperature. In the long-wavelength
limit we find a mode similar to the gravity wave on the surface of a fluid with
the frequency $\omega$ and the wavenumber $q$ related by $\omega^2=Fq/m$. Here
$F$ is the force due to the confining potential at the surface and $m$ is the
particle mass. At shorter wavelengths we use a variational approach and find
corrections to $\omega^2$ of order $q^4 \ln{q}$. We demonstrate the usefulness
of the concept of an effective surface tension for describing both static and
dynamic properties of condensed atomic clouds.


title:
Renormalization group approach of itinerant electron systems near the
  Lifshitz point

abstract:
  Using the renormalization approach proposed by Millis for the itinerant
electron systems we calculated the specific heat coefficient $\gamma(T)$ for
the magnetic fluctuations with susceptibility $\chi^{-1}\sim
|\delta+\omega|^\alpha+f(q)$ near the Lifshitz point. The constant value
obtained for $\alpha=4/5$ and the logarithmic temperature dependence, specific
for the non-Fermi behavior, have been obtained in agreement with the
experimental dat.


title:
Chaos and energy spreading for time-Dependent Hamiltonians, and the
  various Regimes in the theory of Quantum Dissipation

abstract:
  We make the first steps towards a generic theory for energy spreading and
quantum dissipation. The Wall formula for the calculation of friction in
nuclear physics and the Drude formula for the calculation of conductivity in
mesoscopic physics can be regarded as two special results of the general
formulation. We assume a time-dependent Hamiltonian $H(Q,P;x(t))$ with
$x(t)=Vt$, where $V$ is slow in a classical sense. The rate-of-change $V$ is
not necessarily slow in the quantum-mechanical sense. Dissipation means an
irreversible systematic growth of the (average) energy. It is associated with
the stochastic spreading of energy across levels. The latter can be
characterized by a transition probability kernel $P_t(n|m)$ where $n$ and $m$
are level indices. This kernel is the main object of the present study. In the
classical limit, due to the (assumed) chaotic nature of the dynamics, the
second moment of $P_t(n|m)$ exhibits a crossover from ballistic to diffusive
behavior. We define the $V$ regimes where either perturbation theory or
semiclassical considerations are applicable in order to establish this
crossover in the quantal case. In the limit $\hbar\to 0$ perturbation theory
does not apply but semiclassical considerations can be used in order to argue
that there is detailed correspondence, during the crossover time. In the
perturbative regime there is a lack of such correspondence. Namely, $P_t(n|m)$
is characterized by a perturbative core-tail structure that persists during the
crossover time. In spite of this lack of (detailed) correspondence there may be
still a restricted correspondence as far as the second-moment is concerned.
Such restricted correspondence is essential in order to establish the universal
fluctuation-dissipation relation.


title:
Coulomb Gap in a Doped Semiconductor near the Metal-Insulator
  Transition: Tunneling Experiment and Scaling Ansatz

abstract:
  Electron tunneling experiments are used to probe Coulomb correlation effects
in the single-particle density-of-states (DOS) of boron-doped silicon crystals
near the critical density of the metal-insulator transition (MIT). At low
energies, a DOS measurement distinguishes between insulating and metallic
samples with densities 10 to 15 % on either side of the MIT. However, at higher
energies the DOS of both insulators and metals show a common behavior,
increasing roughly as the square-root of energy. The observed characteristics
of the DOS can be understood using a classical treatment of Coulomb
interactions combined with a phenomenological scaling ansatz to describe the
length-scale dependence of the dielectric constant as the MIT is approached
from the insulating side.


title: The Nagel-Schreckenberg model revisited

abstract:
  The Nagel-Schreckenberg model is a simple cellular automaton for a realistic
description of single-lane traffic on highways. For the case $v_{max}=1$ the
properties of the stationary state can be obtained exactly. For the more
relevant case $v_{max}>1$, however, one has to rely on Monte Carlo simulations
or approximative methods. Here we study several analytical approximations and
compare with the results of computer simulations. The role of the braking
parameter $p$ is emphasized. It is shown how the local structure of the
stationary state depends on the value of $p$. This is done by combining the
results of computer simulations with those of the approximative methods.


title: 3He and Universe parallelism

abstract:
  We discuss topological properties of the ground state of spatially
homogeneous ensemble of fermions. There are several classes of topologically
different fermionic vacua; in each case the momentum space topology of the
vacuum determines the low-energy (infrared) properties of the fermionic energy
spectrum. Among them there is a class of the gapless systems which is
characterized by the Fermi-hypersurface, which is the topologically stable
singularity in momentum space. This class contains the conventional Landau
Fermi-liquid and also the non-Landau Luttinger Fermi-liquid. Another important
class of gapless systems is characterized by the topologically stable point
nodes (Fermi points). Superfluid 3He-A and electroweak vacuum belong to this
universality class. The fermionic quasiparticles (particles) in this class are
chiral: close to the Fermi points they are left-handed or right-handed massless
relativistic particles. Since the spectrum becomes relativistic at low energy,
the symmetry of the system is enhanced in the low-energy edge. The low-energy
dynamics acquires local invariance, Lorentz invariance and general covariance,
which become better and better when the energy decreases. Interaction of the
fermions near the Fermi point leads to collective bosonic modes, which look
like effective gauge and gravitational fields. Since the vacuum of superfluid
3He-A and electroweak vacuum are topologically similar, we can use 3He-A for
simulation of many phenomena in high energy physics, including axial anomaly.
3He-A textures induce a nontrivial effective metrics of the space, where the
free quasiparticles move along geodesics. Possible simulation in 3He-A of event
horizon, Hawking radiation, rotating vacuum and conical space are discussed.


title: Optical linewidth of a low density Fermi-Dirac gas

abstract:
  We study propagation of light in a Fermi-Dirac gas at zero temperature. We
analytically obtain the leading density correction to the optical linewidth.
This correction is a direct consequence of the quantum statistical correlations
of atomic positions that modify the optical interactions between the atoms at
small interatomic separations. The gas exhibits a dramatic line narrowing
already at very low densities.


title: Even-Odd and Super-Even Effects in the Attractive Hubbard Model

abstract:
  The canonical BCS wave function is tested for the attractive Hubbard model.
Results are presented for one dimension, and are compared with the exact
solutions by the Bethe ansatz and the results from the conventional grand
canonical BCS approximation, for various chain lengths, electron densities, and
coupling strengths. While the exact ground state energies are reproduced very
well both by the canonical and grand canonical BCS approximations, the
canonical method significantly improves the energy gaps for small systems and
weak coupling. The ``parity'' effect due to the number of electrons being even
or odd naturally emerges in our canonical results. Furthermore, we find a
``super-even'' effect: the energy gap oscillates as a function of even electron
number, depending on whether the number of electrons is $4 m$ or $4 m + 2$ (m
integer). Such oscillations as a function of electron number should be
observable with tunneling measurements in ultrasmall metallic grains.


title: Impurities and orbital dependent superconductivity in Sr_2RuO_4

abstract:
  There now exists a wealth of experimental evidence that Sr_2RuO_4 is an
odd-parity superconductor. Experiments further indicate that among the bands
stemming from the Ru {xy,xz,yz} orbitals, the portion of the Fermi surface
arising from the xy orbitals exhibits a much larger gap than the portions of
the Fermi surface arising from the {xz,yz} orbitals. In this paper the role of
impurities on such an orbital dependent superconducting state is examined
within the Born approximation. In contrast to expected results for a nodeless
p-wave superconductor the unique nature of the superconducting state in
Sr_2RuO_4 implies that a low concentration of impurities strongly influences
the low temperature behavior.


title: Quantum Glass Transition in a Periodic Long-Range Josephson Array

abstract:
  We show that the ground state of the periodic long range Josephson array
frustrated by magnetic field is a glass for a sufficiently large Josephson
energies despite the absence of a quenched disorder. Like superconductors, this
glass state has non-zero phase stiffness and Meissner response; for smaller
Josephson energies the glass "melts" and the ground state loses the phase
stiffness and becomes insulating. We find the critical scaling behavior near
this quantum phase transition: the excitation gap vanishes as (J-J_c)^2, the
frequency-dependent magnetic susceptibility behaves as \chi(\omega) ~
\sqrt{\omega}\ln{\omega}.


title: Dephasing in Metals by Two-Level Systems in the 2-Channel-Kondo Regime

abstract:
  We point out a novel, non-universal contribution to the dephasing rate
1/\tau_\phi \equiv \gamma_\phi of conduction electrons in metallic systems:
scattering off non-magnetic two-level systems (TLSs) having almost degenerate
Kondo ground states. In the regime \Delta_{ren} < T < T_K (\Delta_{ren} =
renormalized level splitting, T_K = Kondo temperature), such TLSs exhibit
non-Fermi-liquid physics that can cause \gamma_\phi, which generally decreases
with decreasing T, to seemingly saturate in a limited temperature range before
vanishing for T \to 0. This could explain the saturation of dephasing recently
observed in gold wires [Mohanty et al. Phys. Rev. Lett. 78, 3366 (1997)].


title:
Origin of Anomalous Diffusion and Non-Gaussian Effects for Hard Spheres:
  Analysis of Three-Time Correlations

abstract:
  We present new simulation results on a hard sphere system at high densities.
Using three-time correlations, we can account for the anomalous diffusion,
which results from a homogeneous back-dragging effect. Furthermore, we
calculate the non-gaussian parameter and connect it to the existence of dynamic
heterogeneities.


title: Small Energy Scale for Mixed-Valent Uranium Materials

abstract:
  We investigate a two-channel Anderson impurity model with a $5f^1$ magnetic
and a $5f^2$ quadrupolar ground doublet, and a $5f^2$ excited triplet. Using
the numerical renormalization group method, we find a crossover to a non-Fermi
liquid state below a temperature $T^*$ varying as the $5f^2$ triplet-doublet
splitting to the 7/2 power. To within numerical accuracy, the non-linear
magnetic susceptibility and the $5f^1$ contribution to the linear
susceptibility are given by universal one-parameter scaling functions. These
results may explain UBe$_{13}$ as mixed valent with a small crossover scale
$T^*$.


title: Local Electronic Structure and High Temperature Superconductivity

abstract:
  It is argued that a new mechanism and many-body theory of superconductivity
are required for doped correlated insulators. Here we review the essential
features of and the experimental support for such a theory, in which the
physics is driven by the kinetic energy.


title:
Comment on "Isotope Dependence of the Spin Gap in YBa2Cu4O8 as
  determined by Cu NQR Relaxation"

abstract: Comment is made on the isotope dependence of the spin gap in YBa2Cu4O8.


title:
Aperiodicity-induced effects on the transmission resonances in
  multibarrier systems

abstract:
  We study the resonant tunneling properties of an electron through a few types
of binary periodic and aperiodic multibarrier systems. Within the framework of
the effective-mass approximation, we calculate the transmission coefficients to
investigate the dependence of the transmission resonances on the system
parameters such as the kind of aperiodicity, the generation number, and the
widths of the wells and barriers. Similarities and differences of the
resonances between the binary periodic and aperiodic systems are discussed in
detail. Transmission resonances in aperiodic systems are found to be
characterized by complex resonance splitting and a variety of peak-to-valley
ratios which are not exhibited in the periodic system. For some energy ranges,
transmission resonances in aperiodic systems are also found to resemble those
in the periodic system, despite the existence of aperiodicity.


title: Interfacial adsorption in two-dimensional Potts models

abstract:
  The interfacial adsorption W at the first-order transition in two-dimensional
q-state Potts models is studied. In particular, findings of Monte Carlo
simulations and of density-matrix renormalization group calculations, at q= 16,
are consistent with the analytic result, obtained in the Hamiltonian limit at
large values of q, that W \propto t^{-1/3} on approach to the bulk critical
temperature T_c, t=|T_c-T|/T_c. In addition, the numerical findings allow to
estimate corrections to scaling. Our study supports and specifies a previous
conclusion by Bricmont and Lebowitz based on low-temperature expansions.


title: Mean Field Theory of Josephson Junction Arrays with Charge Frustration

abstract:
  Using the path integral approach, we provide an explicit derivation of the
equation for the phase boundary for quantum Josephson junction arrays with
offset charges and non-diagonal capacitance matrix. For the model with nearest
neighbor capacitance matrix and uniform offset charge $q/2e=1/2$, we determine,
in the low critical temperature expansion, the most relevant contributions to
the equation for the phase boundary. We explicitly construct the charge
distributions on the lattice corresponding to the lowest energies. We find a
reentrant behavior even with a short ranged interaction. A merit of the path
integral approach is that it allows to provide an elegant derivation of the
Ginzburg-Landau free energy for a general model with charge frustration and
non-diagonal capacitance matrix. The partition function factorizes as a product
of a topological term, depending only on a set of integers, and a
non-topological one, which is explicitly evaluated.


title:
A condition for first order phase transitions in quantum mechanical
  tunneling models

abstract:
  A criterion is derived for the determination of parameter domains of first
order phase transitions in quantum mechanical tunneling models. The criterion
is tested by application to various models, in particular to some which have
been used recently to explore spin tunneling in macroscopic particles. In each
case agreement is found with previously heuristically determined domains.


title:
Ground state of a homogeneous Bose gas: a diffusion Monte Carlo
  calculation

abstract:
  We use a diffusion Monte Carlo method to calculate the lowest energy state of
a uniform gas of bosons interacting through different model potentials, both
strictly repulsive and with an attractive well. We explicitly verify that at
low density the energy per particle follows a universal behavior fixed by the
gas parameter na^3. In the regime of densities typical for experiments in
trapped Bose-condensed gases, the corrections to the mean-field energies
greatly exceed the differences due to the details of the potential.


title:
Analytic Solution for the Critical State in Superconducting Elliptic
  Films

abstract:
  A thin superconductor platelet with elliptic shape in a perpendicular
magnetic field is considered. Using a method originally applied to circular
disks, we obtain an approximate analytic solution for the two-dimensional
critical state of this ellipse. In the limits of the circular disk and the long
strip this solution is exact, i.e. the current density is constant in the
region penetrated by flux. For ellipses with arbitrary axis ratio the obtained
current density is constant to typically 0.001, and the magnetic moment
deviates by less than 0.001 from the exact value. This analytic solution is
thus very accurate. In increasing applied magnetic field, the penetrating flux
fronts are approximately concentric ellipses whose axis ratio b/a < 1 decreases
and shrinks to zero when the flux front reaches the center, the long axis
staying finite in the fully penetrated state. Analytic expressions for these
axes, the sheet current, the magnetic moment, and the perpendicular magnetic
field are presented and discussed. This solution applies also to
superconductors with anisotropic critical current if the anisotropy has a
particular, rather realistic form.


title:
AC Response of Thin Film Superconductors at Various Temperatures and
  Magnetic Fields

abstract:
  The basic theory of the linear and nonlinear ac susceptibilities of type-II
superconductors is outlined for various geometries, e.g. circular and elliptic
disks, rings, and strips of thin films or of thicker platelets in a
perpendicular magnetic field. It is shown how the linear susceptibility depends
on the linear complex resistivity or on the complex penetration depth, and the
nonlinear susceptibility on the nonlinear dependence of the electric field on
the current density and magnetic induction. The dependence of these
constitutive laws on the temperature and on various material properties like
pinning forces, pinning energies, structural defects, and granularity, leads to
an implicit dependence of the ac susceptibility on these parameters.


title: Trapped Bose-Einstein Condensed Gas with Two and Three-Atom Interactions

abstract:
  The stability of a Bose-Einstein condensed state of trapped ultra-cold atoms
is investigated under the assumption of an attractive two-body and a repulsive
three-body interaction. The Ginzburg-Pitaevskii-Gross (GPG) nonlinear
Schr\"odinger equation is extended to include an effective potential dependent
on the square of the density and solved numerically for the $s-$wave. The
lowest frequency of the collective mode is determined through the Fourier
transform of the time dependent solution and its dependences on the number of
atoms and the strength of the three-body force are studied. We show that the
addition of three-body dynamics can allow the number of condensed atoms to
increase considerably, even when the strength of the three-body force is very
small compared with the strength of the two-body force.


title: Depletion potential in hard sphere fluids

abstract:
  A versatile new approach for calculating the depletion potential in a hard
sphere mixture is presented. This is valid for any number of components and for
arbitrary densities. We describe two different routes to the depletion
potential for the limit in which the density of one component goes to zero.
Both routes can be implemented within density functional theory and simulation.
We illustrate the approach by calculating the depletion potential for a big
hard sphere in a fluid of small spheres near a planar hard wall. The density
functional results are in excellent agreement with simulations.


title:
Local densities, distribution functions, and wave function correlations
  for spatially resolved shot noise at nanocontacts

abstract:
  We consider a current-carrying, phase-coherent multi-probe conductor to which
a small tunneling contact is attached. We treat the conductor and the tunneling
contact as a phase-coherent entity and use a Green's function formulation of
the scattering approach. We show that the average current and the current
fluctuations at the tunneling contact are determined by an effective local
non-equilibrium distribution function. This function characterizes the
distribution of charge-carriers (or quasi-particles) inside the conductor. It
is an exact quantum-mechanical expression and contains the phase-coherence of
the particles via local partial densities of states, called injectivities. The
distribution function is analyzed for different systems in the zero-temperature
limit as well as at finite temperature. Furthermore, we investigate in detail
the correlations of the currents measured at two different contacts of a
four-probe sample, where two of the probes are only weakly coupled contacts. In
particular, we show that the correlations of the currents are at
zero-temperature given by spatially non-diagonal injectivities and
emissivities. These non-diagonal densities are sensitive to correlations of
wave functions and the phase of the wave functions. We consider ballistic
conductors and metallic diffusive conductors. We also analyze the Aharonov-Bohm
oscillations in the shot noise correlations of a conductor which in the absence
of the nano-contacts exhibits no flux-sensitivity in the conductance.


title: Phonon line shape in disordered A3C60 (A= K, Rb)

abstract:
  We present a calculation of the Hg phonon self-energy for a model of A3C60
(A= K, Rb). The orientational disorder of the C60 molecules is included, and
the lowest order self-energy diagram is considered. The calculations illustrate
that due to the violation of momentum conservation by the orientational
disorder, Raman scattering can measure the decay of a phonon in an
electron-hole pair, allowing the estimate of the electron-phonon coupling from
such experiments. Comparison with experimental line shapes further provides
support for a local correlation of the molecular orientations, with neighboring
molecules preferentially having orientations differing by a 90 degree rotation.


title:
Optical Conductivity in a Simple Model of Pseudogap State in
  Two-Dimensional System

abstract:
  We present calculation of optical conductivity in a simple model of
electronic spectrum of two-dimensional system with "hot patches" on the Fermi
surface, leading to non Fermi-liquid renormalization of the spectral density
(pseudogap) on these patches. It is shown that this model qualitatively
reproduces basic anomalies of optical experiments in the pseudogap state of
copper oxides.


title:
Competitions in layered ruthenates: ferro- vs. antiferromagnetism and
  triplet vs. singlet pairing

abstract:
  Ru based perovskites demonstrate an amazing richness in their magnetic
properties, including 3D and quasi-2D ferromagnetism, antiferromagnetism, and
unconventional superconductivity. Tendency to ferromagnetism, stemming from the
unusually large involvement of O in magnetism in ruthenates, leads to
ferromagnetic spin fluctuations in Sr2RuO4 and eventually to p-wave
superconductivity. A related compound Ca2RuO4 was measured to be
antiferromagnetic, suggesting a possibility of antiferromagnetic fluctuations
in Sr2RuO4 as well. Here we report first principles calculations that
demonstrate that in both compounds the ferro- and antiferromagnetic
fluctuations coexist, leading to an actual instability in Ca2RuO4 and to a
close competition between p-wave and d-wave superconducting symmetries in
Sr2RuO4. The antiferromagnetism in this system appears to be mostly related
with the nesting, which is the strongest at Q=(2pi/3,2pi/3,0). Surprisingly,
for the Fermiology of Sr2RuO4 the p-wave state wins over the d-wave one
everywhere except in close vicinity of the antiferromagnetic instability. The
most stable state within the d-wave channel has vanishing order parameter at
one out of three Fermi surfaces in Sr2RuO4, while in the p channel its
amplitude is comparable at all three of them.


title:
The Non-local Kardar-Parisi-Zhang Equation With Spatially Correlated
  Noise

abstract:
  The effects of spatially correlated noise on a phenomenological equation
equivalent to a non-local version of the Kardar-Parisi-Zhang equation are
studied via the dynamic renormalization group (DRG) techniques. The correlated
noise coupled with the long ranged nature of interactions prove the existence
of different phases in different regimes, giving rise to a range of roughness
exponents defined by their corresponding critical dimensions. Finally
self-consistent mode analysis is employed to compare the non-KPZ exponents
obtained as a result of the long range -long range interactions with the DRG
results.


title: SOC in a population model with global control

abstract:
  We study a plant population model introduced recently by J. Wallinga [OIKOS
{\bf 74}, 377 (1995)]. It is similar to the contact process (`simple epidemic',
`directed percolation'), but instead of using an infection or recovery rate as
control parameter, the population size is controlled directly and globally by
removing excess plants. We show that the model is very closely related to
directed percolation (DP). Anomalous scaling laws appear in the limit of large
populations, small densities, and long times. These laws, associated critical
exponents, and even some non-universal parameters, can be related to those of
DP. As in invasion percolation and in other models where the r\^oles of control
and order parameters are interchanged, the critical value $p_c$ of the wetting
probability $p$ is obtained in the scaling limit as singular point in the
distribution of infection rates. We show that a mean field type approximation
leads to a model studied by Y.C. Zhang et al. [J. Stat. Phys. {\bf 58}, 849
(1990)]. Finally, we verify the claim of Wallinga that family extinction in a
marginally surviving population is governed by DP scaling laws, and speculate
on applications to human mitochondrial DNA.


title: Comment on ``Diffusion of Ionic Particles in Charged Disordered Media''

abstract:
  This comment points out that simulation results of Mehrabi and Sahimi [Phys.
Rev. Lett. 82, 735 (1999)] are iconsistent with exact bounds, renormalization
group calculations, and previous numerical simulations.


title:
Spin-isospin textured excitations in a double layer at filling factor
  $\nu =2$

abstract:
  We study the charged excitations of a double layer at filling factor 2 in the
ferromagnetic regime. In a wide range of Zeeman and tunneling splittings we
find that the low energy charged excitations are spin-isospin textures with the
charge mostly located in one of the layers. As tunneling increases, the parent
spin texture in one layer becomes larger and it induces, in the other layer, a
shadow spin texture antiferromagnetically coupled to the parent texture. These
new quasiparticles should be observable by measuring the strong dependence of
its spin on tunneling and Zeeman couplings.


title: Directed paths on hierarchical lattices with random sign weights

abstract:
  We study sums of directed paths on a hierarchical lattice where each bond has
either a positive or negative sign with a probability $p$. Such path sums $J$
have been used to model interference effects by hopping electrons in the
strongly localized regime. The advantage of hierarchical lattices is that they
include path crossings, ignored by mean field approaches, while still
permitting analytical treatment. Here, we perform a scaling analysis of the
controversial ``sign transition'' using Monte Carlo sampling, and conclude that
the transition exists and is second order. Furthermore, we make use of exact
moment recursion relations to find that the moments $<J^n>$ always determine,
uniquely, the probability distribution $P(J)$. We also derive, exactly, the
moment behavior as a function of $p$ in the thermodynamic limit. Extrapolations
($n\to 0$) to obtain $<\ln J>$ for odd and even moments yield a new signal for
the transition that coincides with Monte Carlo simulations. Analysis of high
moments yield interesting ``solitonic'' structures that propagate as a function
of $p$. Finally, we derive the exact probability distribution for path sums $J$
up to length L=64 for all sign probabilities.


title: Equation of motion approach to the solution of Anderson model

abstract:
  Based on an equation of motion approach the single impurity Anderson
model(SIAM) is reexamined. Using the cluster expansions the equations of motion
of Green functions are transformed into the corresponding equations of motion
of connected Green functions, which provides a natural and uniform truncation
scheme. A factor of two missing in the Lacroix's approximation for the Kondo
temperature is gained in the next higher order truncation beyond Lacroix's. A
quantitative improvement in the density of states at the Fermi level is also
obtained.


title:
Vortex Dynamics and Entropic Coulomb Forces in Ising and Potts
  Antiferromagnets and Ice Models

abstract:
  We study the dynamics of topological defects in the triangular Ising
antiferromagnet, a related model on the square lattice equivalent to the
six-vertex ice model, and the three-state antiferromagnetic Potts model on the
square lattice. Since each of these models has a height representation in which
defects are screw dislocations, we expect them to be attracted or repelled with
an entropy-driven Coulomb force. In each case we show explicitly how this force
is felt through local fields. We measure the force numerically, both by
quenching the system to zero temperature and by measuring the motion of vortex
pairs. For the three-state Potts model, we calculate both the force and the
defect mobility, and find reasonable agreement with theory.


title:
Observation of New Incommensurate Magnetic Correlations at the Lower
  Critical Concentration for Superconductivity (x=0.05) in La(2-x)Sr(x)CuO4

abstract:
  Neutron-scattering experiments have been performed on lightly-doped
La(2-x)Sr(x)CuO4 single crystals in both the insulating (x=0.03,0.04,0.05) and
superconducting (x=0.06) regions. Elastic magnetic peaks are observed at low
temperatures in all samples with the maximum peak linewidth occuring at the
critical concentration x_c=0.05. New incommensurate peaks are observed only at
x=0.05, the positions of which are rotated by 45 degrees in reciprocal space
about (pi,pi) from those observed for x>=0.06 in the superconducting phase.


title: Non-equilibrium current noise in mesoscopic disordered SNS junctions

abstract:
  Current noise in superconductor-normal metal-superconductor (SNS) junctions
is calculated within the scattering theory of multiple Andreev reflections
(MAR). It is shown that the noise exhibits subharmonic gap singularities at
$eV=2\Delta/n$, $n=1,2,... $ both in single-mode junctions with arbitrary
transparency $D$ and in multi-mode disordered junctions. The subharmonic
structure is superimposed with monotonic increase of the effective transferred
charge $q^*=S_I(0)/2I$ with decreasing bias voltage. Other features of the
noise include a step-like increase of $q^*$ in junctions with small $D$, and a
divergence $S_I(0) \propto V^{-1/2}$ at small voltages and excess noise $S_{ex}
= 2eI_{ex}$, where $I_{ex}$ is the excess current, at large voltages, in
junctions with diffusive transport.


title:
Computational Complexity of Determining the Barriers to Interface Motion
  in Random Systems

abstract:
  The low-temperature driven or thermally activated motion of several condensed
matter systems is often modeled by the dynamics of interfaces (co-dimension-1
elastic manifolds) subject to a random potential. Two characteristic
quantitative features of the energy landscape of such a many-degree-of-freedom
system are the ground-state energy and the magnitude of the energy barriers
between given configurations. While the numerical determination of the former
can be accomplished in time polynomial in the system size, it is shown here
that the problem of determining the latter quantity is NP-complete. Exact
computation of barriers is therefore (almost certainly) much more difficult
than determining the exact ground states of interfaces.


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

abstract:
  We discuss dissipative dynamics of a vortex state in a trapped Bose-condensed
gas at finite temperature and draw a scenario of decay of this state in a
static trap. The interaction of the vortex with the thermal cloud transfers
energy from the vortex to the cloud and induces the motion of the vortex core
to the border of the condensate. Once the vortex reaches the border, it
immediately decays through the creation of excitations. We calculate the
characteristic life-time of a vortex state and address the question of how the
dissipative dynamics of vortices can be studied experimentally.


title:
Coexistence of double alternating antiferromagnetic chains in
  (VO)_2P_2O_7 : NMR study

abstract:
  Nuclear magnetic resonance (NMR) of 31P and 51V nuclei has been measured in a
spin-1/2 alternating-chain compound (VO)_2P_2O_7. By analyzing the temperature
variation of the 31P NMR spectra, we have found that (VO)_2P_2O_7 has two
independent spin components with different spin-gap energies. The spin gaps are
determined from the temperature dependence of the shifts at 31P and 51V sites
to be 35 K and 68 K, which are in excellent agreement with those observed in
the recent inelastic neutron scattering experiments [A.W. Garrett et al., Phys.
Rev. Lett. 79, 745 (1997)]. This suggests that (VO)_2P_2O_7 is composed of two
magnetic subsystems showing distinct magnetic excitations, which are associated
with the two crystallographically-inequivalent V chains running along the b
axis. The difference of the spin-gap energies between the chains is attributed
to the small differences in the V-V distances, which may result in the
different exchange alternation in each magnetic chain. The exchange
interactions in each alternating chain are estimated and are discussed based on
the empirical relation between the exchange interaction and the interatomic
distance.


title:
Charge sensitivity of radio frequency single-electron transistor
  (RF-SET)

abstract:
  A theoretical analysis of the charge sensitivity of the RF-SET is presented.
We use the ``orthodox'' approach and consider the case when the carrier
frequency is much less than $I/e$ where $I$ is the typical current through
RF-SET. The optimized noise-limited sensitivity is determined by the
temperature $T$, and at low $T$ it is only 1.4 times less than the sensitivity
of conventional single-electron transistor.


title: Periodic Vortex Structures in Superfluid 3He-A

abstract:
  We discuss the general properties of periodic vortex arrangements in rotating
superfluids. The different possible structures are classified according to the
symmetry space-groups and the circulation number. We calculate numerically
several types of vortex structures in superfluid 3He-A. The calculations are
done in the Ginzburg-Landau region, but the method is applicable at all
temperatures. A phase diagram of vortices is constructed in the plane formed by
the magnetic field and the rotation velocity. The characteristics of the six
equilibrium vortex solutions are discussed. One of these, the locked vortex 3,
has not been considered in the literature before. The vortex sheet forms the
equilibrium state of rotating 3He-A at rotation velocities exceeding 2.6 rad/s.
The results are in qualitative agreement with experiments.


title:
Length and time scale divergences at the magnetization-reversal
  transition in the Ising model

abstract:
  The divergences of both the length and time scales, at the magnetization-
reversal transition in Ising model under a pulsed field, have been studied in
the linearized limit of the mean field theory. Both length and time scales are
shown to diverge at the transition point and it has been checked that the
nature of the time scale divergence agrees well with the result obtained from
the numerical solution of the mean field equation of motion. Similar growths in
length and time scales are also observed, as one approaches the transition
point, using Monte Carlo simulations. However, these are not of the same nature
as the mean field case. Nucleation theory provides a qualitative argument which
explains the nature of the time scale growth. To study the nature of growth of
the characteristic length scale, we have looked at the cluster size
distribution of the reversed spin domains and defined a pseudo-correlation
length which has been observed to grow at the phase boundary of the transition.


title:
Fractional Brownian Motion Approximation Based on Fractional Integration
  of a White Noise

abstract:
  We study simple approximations to fractional Gaussian noise and fractional
Brownian motion. The approximations are based on spectral properties of the
noise. They allow one to consider the noise as the result of fractional
integration/differentiation of a white Gaussian noise. We study correlation
properties of the approximation to fractional Gaussian noise and point to the
peculiarities of persistent and anti-persistent behaviors. We also investigate
self-similar properties of the approximation to fractional Brownian motion,
namely, ``tH laws`` for the structure function and the range. We conclude that
the models proposed serve as a convenient tool for the natural processes
modelling and testing and improvement of the methods aimed at analysis and
interpretation of experimental data.


title: Ballistic and diffuse transport through a ferromagnetic domain wall

abstract:
  We study transport through ballistic and diffuse ferromagnetic domain walls
in a two-band Stoner model with a rotating magnetization direction. For a
ballistic domain wall, the change in the conductance due to the domain wall
scattering is obtained from an adiabatic approximation valid when the length of
the domain wall is much longer than the Fermi wavelength. In diffuse systems,
the change in the resistivity is calculated using a diagrammatic technique to
the lowest order in the domain wall scattering and taking into account
spin-dependent scattering lifetimes and screening of the domain wall potential.


title:
Green Function Monte Carlo with Stochastic Reconfiguration: an effective
  remedy for the sign problem disease

abstract:
  A recent technique, proposed to alleviate the ``sign problem disease'', is
discussed in details. As well known the ground state of a given Hamiltonian $H$
can be obtained by applying the imaginary time propagator $e^{-H \tau}$ to a
given trial state $\psi_T$ for large imaginary time $\tau$ and sampling
statistically the propagated state $ \psi_{\tau} = e^{-H \tau} \psi_T$. However
the so called ``sign problem'' may appear in the simulation and such
statistical propagation would be practically impossible without employing some
approximation such as the well known ``fixed node'' approximation (FN). This
method allows to improve the FN dynamic with a systematic correction scheme.
This is possible by the simple requirement that, after a short imaginary time
propagation via the FN dynamic, a number $p$ of correlation functions can be
further constrained to be {\em exact} by small perturbation of the FN
propagated state, which is free of the sign problem. By iterating this scheme
the Monte Carlo average sign, which is almost zero when there is sign problem,
remains stable and finite even for large $\tau$. The proposed algorithm is
tested against the exact diagonalization results available on finite lattice.
It is also shown in few test cases that the dependence of the results upon the
few parameters entering the stochastic technique can be very easily controlled,
unless for exceptional cases.


title:
Magnetic critical behavior of two-dimensional random-bond Potts
  ferromagnets in confined geometries

abstract:
  We present a numerical study of 2D random-bond Potts ferromagnets. The model
is studied both below and above the critical value $Q_c=4$ which discriminates
between second and first-order transitions in the pure system. Two geometries
are considered, namely cylinders and square-shaped systems, and the critical
behavior is investigated through conformal invariance techniques which were
recently shown to be valid, even in the randomness-induced second-order phase
transition regime Q>4. In the cylinder geometry, connectivity transfer matrix
calculations provide a simple test to find the range of disorder amplitudes
which is characteristic of the disordered fixed point. The scaling dimensions
then follow from the exponential decay of correlations along the strip. Monte
Carlo simulations of spin systems on the other hand are generally performed on
systems of rectangular shape on the square lattice, but the data are then
perturbed by strong surface effects. The conformal mapping of a semi-infinite
system inside a square enables us to take into account boundary effects
explicitly and leads to an accurate determination of the scaling dimensions.
The techniques are applied to different values of Q in the range 3-64.


title: Absence of simulation evidence for critical depletion in slit-pores

abstract:
  Recent Monte Carlo simulation studies of a Lennard-Jones fluid confined to a
mesoscopic slit-pore have reported evidence for ``critical depletion'' in the
pore local number density near the liquid-vapour critical point. In this note
we demonstrate that the observed depletion effect is in fact a simulation
artifact arising from small systematic errors associated with the use of long
range corrections for the potential truncation. Owing to the large
near-critical compressibility, these errors lead to significant changes in the
pore local number density. We suggest ways of avoiding similar problems in
future studies of confined fluids.


title: Phenomenology of the superconducting state in Sr2RuO4

abstract:
  The symmetry of the superconducting phase of Sr2RuO4 is identified as the
odd-parity pairing state d(k)=\hat{z}(k_x \pm i k_y) based on recent
experiments. The experimental evidence for the so-called orbital dependent
superconductivity leads to a single-band description of superconductivity based
on spin fluctuation mechanism. It is shown that the state \hat{z}(k_x \pm i
k_y) can be stabilized by the spin fluctuation feedback mechanism analogous to
the A-phase in 3He and by spin-orbit coupling effects.


title:
Enhancement of de Haas-van Alphen Oscillation due to Spin in the
  Magnetic Breakdown System

abstract:
  The effects of the Zeeman term on the de Haas-van Alphen oscillation is
studied in the magnetic breakdown system. We find that the amplitude of the
oscillation with the frequencies of $f_{\beta} + f_{\alpha}$ and $f_{\beta} +
2f_{\alpha}$ are enhanced by the Zeeman term, while they are expected to be
reduced in the semiclassical theory. A possible interpretation of the
experiments in organic conductors is discussed.


title: Stochastic Resonance in Washboard Potentials

abstract:
  We study the mobility of an overdamped particle in a periodic potential
tilted by a constant force. The mobility exhibits a stochastic resonance in
inhomogeneous systems with space dependent friction coefficient. The result
indicates that the presence of oscillating external field is not essential for
the observability of stochastic resonance, at least in the inhomogenous medium.


title: Analytic model for the ballistic adsorption of polydisperse mixtures

abstract:
  We study the ballistic adsorption of a polydisperse mixture of spheres onto a
line. Within a mean-field approximation, the problem can be analytically solved
by means of a kinetic equation for the gap distribution. In the mean-field
approach, the adsorbed substrate as approximated as composed by {\em effective}
particles with the {\em same} size, equal to the average diameter of the
spheres in the original mixture. The analytic solution in the case of binary
mixtures agrees quantitatively with direct Monte Carlo simulations of the
model, and qualitatively with previous simulations of a related model in $d=2$.


title:
Non-Kramers Freezing and Unfreezing of Tunneling in the Biaxial Spin
  Model

abstract:
  The ground state tunnel splitting for the biaxial spin model in the magnetic
field, H = -D S_{x}^2 + E S_{z}^2 - g \mu_B S_z H_z, has been investigated
using an instanton approach. We find a new type of spin instanton and a new
quantum interference phenomenon associated with it: at a certain field, H_2 =
2SE^{1/2}(D+E)^{1/2}/(g \mu_B), the dependence of the tunneling splitting on
the field switches from oscillations to a monotonic growth. The predictions of
the theory can be tested in Fe_8 molecular nanomagnets.


title: Sign Rules for Anisotropic Quantum Spin Systems

abstract:
  We present new and exact ``sign rules'' for various spin-s anisotropic
spin-lattice models. It is shown that, after a simple transformation which
utilizes these sign rules, the ground-state wave function of the transformed
Hamiltonian is positive-definite. Using these results exact statements for
various expectation values of off-diagonal operators are presented, and
transitions in the behavior of these expectation values are observed at
particular values of the anisotropy. Furthermore, the effects of sign rules in
variational calculations and quantum Monte Carlo calculations are considered.
They are illustrated by a simple variational treatment of a one-dimensional
anisotropic spin model.


title: One and two-component hard core plasmas

abstract:
  A field theory is presented for particles which interact via Coulomb and
hard-core potentials. We apply the method to the one-component plasma (OCP)
with hard cores, consisting of identical particles of fixed charge and diameter
in a neutralizing background, and the symmetric two-component plasma (TCP) with
hard cores, consisting of equal numbers of positively and negatively charged
particles of identical size. We obtain exactly the first few coefficients of a
systematic low-density expansion of the free energy for both models. The OCP
coefficients go over to the classical Abe result in the high-charge (or small
hard-core diameter) limit. The TCP, on the other hand, exhibits diverging
coefficients in the high-charge limit, which is due to the formation of
strongly bound ion-pairs.


title: Three-body correlations in the Nagaoka state on the square lattice

abstract:
  A three-body scattering theory previously proposed by one of the present
authors is developed to be applied to the saturated ferromagnetic state in the
two-dimensional Hubbard model. The single-particle Green's function is
calculated by taking account of the multiple scattering between two electrons
and one hole. Several limiting cases are discussed and the relation to the
variational principle is examined. The importance of the three-body correlation
is demonstrated in comparison with the results of the ladder approximation. A
possible phase boundary for the Nagaoka ground state is presented for the
square lattice, which improves the previous variational results.


title:
Dependence of Magnetic Anisotropy and Magnetoresistance of
  Ni81Fe19-Films on Annealing

abstract:
  Permalloy (Py:Ni81Fe19) exhibits an anisotropic magnetoresistance (AMR) which
is very often used to read magnetic signals from storage devices. Py-films of
thickness 20nm were prepared by dc-magnetron sputtering in a magnetic field
onto thermally oxidized Si-wafers and annealed ex situ at temperatures up to
1000K in order to investigate the dependence of the magnetic anisotropy and the
AMR on heat treatments. The films exhibit an uniaxial anisotropy after
preparation which changes during annealing above 520K. The AMR along the former
magnetically easy axis as well as the corresponding field sensitivity are
increased by a heat treatment around 700K reaching maxima of about 8% and a
maximum sensitivity of 1.5%/Oe, respectively. We discuss possible sources for
the change in anisotropy, i.e. strain effects, inhomogeneities, and changes of
the local atomic order.


title:
Analytical results for random walks in the presence of disorder and
  traps

abstract:
  In this paper, we study the dynamics of a random walker diffusing on a
disordered one-dimensional lattice with random trappings. The distribution of
escape probabilities is computed exactly for any strength of the disorder.
These probabilities do not display any multifractal properties contrary to
previous numerical claims. The explanation for this apparent multifractal
behavior is given, and our conclusion are supported by numerical calculations.
These exact results are exploited to compute the large time asymptotics of the
survival probability (or the density) which is found to decay as $\exp
[-Ct^{1/3}\log^{2/3}(t)]$. An exact lower bound for the density is found to
decay in a similar way.


title:
Miscibility behavior and single chain properties in polymer blends: a
  bond fluctuation model study

abstract:
  Computer simulation studies on the miscibility behavior and single chain
properties in binary polymer blends are reviewed. We consider blends of various
architectures in order to identify important architectural parameters on a
coarse grained level and study their qualitative consequences for the
miscibility behavior. The phase diagram, the relation between the exchange
chemical potential and the composition, and the intermolecular paircorrelation
functions for symmetric blends of linear chains, blends of cyclic polymers,
blends with an asymmetry in cohesive energies, blends with different chain
lengths, blends with distinct monomer shapes, and blends with a stiffness
disparity between the components are discussed. We investiagte the temperature
and composition dependence of the single chain conformations in symmetric and
asymmetric blends and compare our findings to scaling arguments and detailed
SCF calculations.
  Two aspects of the single chain dynamics in blends are discussed: the
dynamics of short non--entangled chains in a binary blend and irreversible
reactions of a small fraction of reactive polymers at a strongly segregated
interface. Pertinent off-lattice simulations and analytical theories are
briefly discussed.


title: Magnetic Ordering in Layered High Temperature Superconductors

abstract:
  We discuss the scenario of two-step magnetic ordering in layered high
temperature superconductors after charge ordering. As the temperature
decreases, the transition from 3D Heisenberg spin behavior to 2D XY coupling of
the Cu spins occurs at Berezinskii-Kosterlitz-Thouless temperature in
dielectric stripes. Further temperature decreasing leads to the 3D spin glass
transition.


title: Stochastic series expansion method with operator-loop update

abstract:
  A cluster update (the ``operator-loop'') is developed within the framework of
a numerically exact quantum Monte Carlo method based on the power series
expansion of exp(-BH) (stochastic series expansion). The method is generally
applicable to a wide class of lattice Hamiltonians for which the expansion is
positive definite. For some important models the operator-loop algorithm is
more efficient than loop updates previously developed for ``worldline''
simulations. The method is here tested on a two-dimensional anisotropic
Heisenberg antiferromagnet in a magnetic field.


title: Protein Design is a Key Factor for Subunit-subunit Association

abstract:
  Fundamental questions about the role of the quaternary structures are
addressed using a statistical mechanics off-lattice model of a dimer protein.
The model, in spite of its simplicity, captures key features of the
monomer-monomer interactions revealed by atomic force experiments. Force curves
during association and dissociation are characterized by sudden jumps followed
by smooth behavior and form hysteresis loops. Furthermore, the process is
reversible in a finite range of temperature stabilizing the dimer. It is shown
that in the interface between the two monomeric subunits the design procedure
naturally favors those amino acids whose mutual interaction is stronger.
Furthermore it is shown that the width of the hysteresis loop increases as the
design procedure improves, i.e. stabilizes more the dimer.


title: Folding Lennard-Jones proteins by a contact potential

abstract:
  We studied the possibility to approximate a Lennard Jones interaction by a
pairwise contact potential. First we used a Lennard-Jones potential to design
off-lattice, protein-like heteropolymer sequences, whose lowest energy (native)
conformations were then identified by Molecular Dynamics. Then we turned to
investigate whether one can find a pairwise contact potential, whose ground
states are the contact maps associated with these native conformations. We show
that such a requirement cannot be satisfied exactly - i.e. no such contact
parameters exist. Nevertheless, we found that one can find contact energy
parameters for which an energy minimization procedure, acting in the space of
contact maps, yields maps whose corresponding structures are close to the
native ones. Finally we show that when these structures are used as the initial
point of a Molecular Dynamics energy minimization process, the correct native
folds are recovered with high probability.


title:
Periodicity-dependent stiffness of periodic hydrophilic-hydrophobic
  hetero-polymers

abstract:
  From extensive Monte Carlo simulations of a Larson model of perfectly
periodic heteropolymers (PHP) in water a striking stiffening is observed as the
period of the alternating hydrophobic and hydrophilic blocks is shortened. At
short period and low temperature needle-like conformations are the stable
conformation. As temperature is increased thermal fluctuations induce kinks and
bends. At large periods compact oligomeric globules are observed. From the
generalized Larson prescription, originally developed for modelling surfactant
molecules in aqueous solutions, we find that the shorter is the period the more
stretched is the PHP. This novel effect is expected to stimulate polymer
synthesis and trigger research on the rheology of aqueous periodic
heteropolymer solutions.


title:
Spin-Peierls transition in the Heisenberg chain with finite-frequency
  phonons

abstract:
  We study the spin-Peierls transition in a Heisenberg spin chain coupled to
optical bond-phonons. Quantum Monte Carlo results for systems with up to N=256
spins show unambiguously that the transition occurs only when the spin-phonon
coupling alpha exceeds a critical value alpha_c. Using sum rules, we show that
the phonon spectral function has divergent (for infinite N) weight extending to
zero frequency for alpha < alpha_c. The equal-time phonon-phonon correlations
decay with distance r as 1/r. This behavior is characteristic for all 0 < alpha
< alpha_c and the q=pi phonon does not soften (to zero frequency) at the
transition.


title: Universal scaling, beta function, and metal-insulator transitions

abstract:
  We demonstrate a universal scaling form of longitudinal resistance in the
quantum critical region of metal-insulator transitions, based on numerical
results of three-dimensional Anderson transitions (with and without magnetic
field), two-dimensional quantum Hall plateau to insulator transition, as well
as experimental data of the recently discovered two-dimensional metal-insulator
transition. The associated reflection symmetry and a peculiar logarithmic form
of the beta function exist over a wide range in which the resistance can change
by more than one order of magnitude. Interesting implications for the
two-dimensional metal-insulator transition are discussed.


title: Forward Electron-Phonon Scattering and HTS

abstract:
  Tunneling and point contact spectroscopy show clear phonon features and
together with optic measurements give strong support that the electron-phonon
interaction (EPI) is large in HTS oxides. Strong correlations in HTS oxides
renormalize the EPI (and interaction with impurities) so that the forward
scattering peak (FSP) develops for small hole doping \delta<<1. The FSP
mechanism explains important properties of the normal and superconducting
state.


title: Delocalization transitions of semi-flexible manifolds

abstract:
  Semi-flexible manifolds such as fluid membranes or semi-flexible polymers
undergo delocalization transitions if they are subject to attractive
interactions. We study manifolds with short-ranged interactions by
field-theoretic methods based on the operator product expansion of local
interaction fields. We apply this approach to manifolds in a random potential.
Randomness is always relevant for fluid membranes, while for semi-flexible
polymers there is a first order transition to the strong coupling regime at a
finite temperature.


title:
Comment on ``Mechanisms of synchronization and pattern formation in a
  lattice of pulse-coupled oscillators'' by Diaz-Guilera et al., Phys. Rev. E
  57, 3820 (1998)

abstract:
  In a recent paper, Diaz-Guilera et al. [Phys. Rev. E 57, 3820 (1998)] analyze
the mechanisms of synchronization and pattern formation in a lattice of
pulse-coupled oscillators. In essence, their analysis consists in the study of
the stability of the fixed points of several linear return maps which are
obtained from the original system by means of matrix manipulations. We show
that although the model they consider is very specific and actually unable to
account even for a linear phase response curve, their method does not give
correct information on the original system since many of the assumptions
involved are in general not correct. To clarify these aspects, several issues
concerning the real dynamics are also discussed.


title: Optical Conductivity of the Two-Dimensional Hubbard Model

abstract:
  Charge dynamics of the two-dimensional Hubbard model is investigated.
Lancz$\ddot{\rm o}$s-diagonalization results for the optical conductivity and
the Drude weight of this model are presented. Near the Mott transition, large
incoherence below the upper-Hubbard band is obtained together with a remarkably
suppressed Drude weight in two dimensions while the clearly coherent character
is shown in one dimension. The two-dimensional results are consistent with
previous results from quantum Monte Carlo calculations indicating that the Mott
transition in this two-dimensional model belongs to the universality class
characterized by the dynamical exponent of $z=4$.


title:
Trapping and Mobilization of Residual Fluid During Capillary
  Desaturation in Porous Media

abstract:
  We discuss the problem of trapping and mobilization of nonwetting fluids
during immiscible two phase displacement processes in porous media. Capillary
desaturation curves give residual saturations as a function of capillary
number. Interpreting capillary numbers as the ratio of viscous to capillary
forces the breakpoint in experimental curves contradicts the theoretically
predicted force balance. We show that replotting the data against a novel
macroscopic capillary number resolves the problem for discontinuous mode
displacement.


title:
Spin-ordering in S = 1 anisotropic Heisenberg models with nondiagonal
  spin exchange

abstract:
  The properties of S = 1 anisotropic Heisenberg models with nondiagonal
exchange between axial and planar spin components are investigated using Monte
Carlo techniques. The quantum nature is taken into account in a semi-classical
approximation. The ordering of the spins when applying an external field with
axial and planar components is discussed. It is argued that the quantum nature
of the spins and the nondiagonal exchange may explain the peculiar shape of the
magnetic specific heat of FeBr2 as well as the weakly first-order phase
transition observed in the same compound when a tilted field is applied.


title: Exact Ground States in Spin Systems with Orbital Degeneracy

abstract:
  We present exact ground states in spin models with orbital generacy in one
and higher dimensions. A method to obtain the exact ground states of the models
when the Hamiltonians are composed of the products of two commutable operators
is proposed. For the case of the spin-1/2 model with two-fold degeneracy some
exact ground states are given, such as the Valence-Bond (VB), the magnetically
ordered, and the orbitally ordered states under particular parameter regimes.
We also find the models with the higher spin and degeneracy which have the new
types of VB ground states in the spin and the orbital sectors.


title:
Renormalization group study of one-dimensional systems with roughening
  transitions

abstract:
  A recently introduced real space renormalization group technique, developed
for the analysis of processes in the Kardar-Parisi-Zhang universality class, is
generalized and tested by applying it to a different family of surface growth
processes.
  In particular, we consider a growth model exhibiting a rich phenomenology
even in one dimension. It has four different phases and a directed percolation
related roughening transition. The renormalization method reproduces extremely
well all the phase diagram, the roughness exponents in all the phases and the
separatrix among them. This proves the versatility of the method and elucidates
interesting physical mechanisms.


title:
Interplay between tunneling and exchange effects in the two electron
  double quantum dot molecule

abstract:
  The electronic structure of two vertically coupled quantum dots containing
two electrons is investigated in the presence of interdot tunneling. Our
approach also includes the interdot exchange and we find that the tunneling is
affected by the Coulomb interaction. There is an interplay between direct
tunneling, the interdot exchange and the Coulomb interaction influence on the
tunneling, which can lead to a zero angular momentum spin-singlet -
spin-triplet transition of the ground state, even in the absence of a magnetic
field.


title:
Screening, Coulomb pseudopotential, and superconductivity in
  alkali-doped Fullerenes

abstract:
  We study the static screening in a Hubbard-like model using quantum Monte
Carlo. We find that the random phase approximation is surprisingly accurate
almost up to the Mott transition. We argue that in alkali-doped Fullerenes the
Coulomb pseudopotential $\mu^\ast$ is not very much reduced by retardation
effects. Therefore efficient screening is important in reducing $\mu^{\ast}$
sufficiently to allow for an electron-phonon driven superconductivity. In this
way the Fullerides differ from the conventional picture, where retardation
effects play a major role in reducing the electron-electron repulsion.


title: Non-perturbative approach to nearly antiferromagnetic Fermi liquids

abstract:
  We present a non-perturbative approach to the problem of quasiparticles
coupled to spin-fluctuations. If the fully dressed spin-fluctuation propagator
is used in the Feynman graph expansion of the single-particle Green's function,
the problem of summing all spin-fluctuation exchange graphs (i.e without
virtual fermion loops) can be cast as a functional integral over gaussian
distributed random vector fields. A Monte Carlo sampling of this functional
integral does not suffer from the 'fermion sign problem' and offers an
attractive alternative to perturbative calculations. We compare the results of
our computer simulations with perturbation theory and self-consistent one-loop
calculations.


title:
Time-Resolved Optical Studies of Quasiparticle Dynamics in
  High-Temperature Superconductors: Experiments and theory

abstract:
  Ultrafast time-resolved optical spectroscopy in high-temperature
superconductors enables the direct real-time measurement of non-equilibrium
quasiparticle recombination dynamics. In addition, it also gives detailed
information about the symmetry of the superconducting gap and the "pseudogap",
their doping dependence and their temperature dependence. Experimental data,
together with theoretical models on the photoinduced transmission amplitude and
relaxation time as a function of temperature and doping in YBa2Cu3O7-d gives a
consistent picture of the evolution of low-energy structure, where a
temperature-independent gap is shown to exist in the underdoped state which
evolves with doping into a two-component state near optimum doping, where the
dominant response is from a T-dependent BCS-like superconducting gap.


title: Defect-Unbinding Transition in Layered Superconductors

abstract:
  We establish a new interstitial-vacancy unbinding transition of the
Berezinskii-Kosterlitz-Thouless type, transforming the three dimensional
pancake vortex lattice of a decoupled layered superconductor into a defected
solid. This transition is the natural finite-field extension of the
vortex-anti-vortex unbinding transition establishing the zero-field superfluid
stiffness. At finite Josephson coupling, the defect unbinding transition turns
into a topological decoupling transition.


title: Full Symmetry of Single- and Multi-wall Nanotubes

abstract:
  The full symmetry groups for all single- and multi-wall carbon nanotubes are
found. As for the single-wall tubes, the symmetries form nonabelian
nonsymorphic line groups, enlarging the groups reported in literature. In the
multi-wall case, any type of the line and the axial point groups can be
obtained, depending on single-wall constituents and their relative position.
Several other consequences are discussed: quantum numbers and related selection
rules, electronic and phonon bands, and their degeneracy, application to tensor
properties.


title: 3D Anderson transition for two electrons in 2D

abstract:
  It is shown that the Coulomb interaction can lead to delocalization of two
electron states in two-dimensional (2D) disordered potential in a way similar
to the Anderson transition in three dimensions (3D). At fixed disorder strength
the localized phase corresponds to low electron density and large value of
parameter r_s.


title:
Punctuated vortex coalescence and discrete scale invariance in
  two-dimensional turbulence

abstract:
  We present experimental evidence and theoretical arguments showing that the
time-evolution of freely decaying 2-d turbulence is governed by a {\it
discrete} time scale invariance rather than a continuous time scale invariance.
Physically, this reflects that the time-evolution of the merging of vortices is
not smooth but punctuated, leading to a prefered scale factor and as a
consequence to log-periodic oscillations. From a thorough analysis of freely
decaying 2-d turbulence experiments, we show that the number of vortices, their
radius and separation display log-periodic oscillations as a function of time
with an average log-frequency of ~ 4-5 corresponding to a prefered scaling
ratio of ~ 1.2-1.3


title: Quantum Monte Carlo Study of Weakly Coupled Spin Ladders

abstract:
  We report a quantum Monte Carlo study of the thermodynamic properties of
arrays of spin ladders with various widths ($n$), coupled via a weak
inter-ladder exchange coupling $\alpha J$, where $J$ is the intra-ladder
coupling both along and between the chains. This coupled ladder system serves
as a simplified model for the magnetism of presumed ordered spin and charge
stripes in the two-dimensional CuO$_2$ planes of hole-doped copper oxides. Our
results for $n=3$ with weak inter-ladder coupling $\alpha=0.05$, estimated from
the $t-t'-t''-J$ model, show good agreement with the ordering temperature of
the recently observed spin density wave condensation in La$_2$CuO$_{4+y}$. We
show that there exists a quantum critical point at $\alpha_c \simeq 0.07$ for
$n=4$, and determine the phase diagram. Our data at this quantum critical point
agree quantitatively with the universal scaling predicted by the quantum
nonlinear $\sigma$ model. We also report results on random mixtures of $n=2$
and $n=3$ ladders, which correspond to the doping region near but above 1/8.
Our study on the magnetic static structure factor reveals a saturation of the
incommensurability of the spin correlations around 1/8, while the
incommensurability of the charge stripes grows linearly with hole
concentration. The implications of this result for the interpretation of
neutron scattering experiments on the dynamic spin fluctuations in
La$_{2-x}$Sr$_x$CuO$_4$ are discussed.


title:
One and Two Dimensional Spin Systems in the Regime Close to
  Deconfinement of Spinons

abstract:
  Based on the Majumdar-Ghosh chain we construct several spin models which
allow us to investigate spinon dynamics in the regime close to deconfinement of
spinons. We consider the J_1 - J_2 - \delta model, two coupled J_1 - J_2 chains
(ladder), and a 2D array of coupled J_1 - J_2 chains. Using the picture of two
spinons interacting with a string confining potential we calculate the singlet-
triplet splitting, magnetic structure factor, tunneling amplitude of two
spinons and the excitation spectra for the ladder and the array.


title:
Hopf invariant for long-wavelength skyrmions in quantum Hall systems for
  integer and fractional fillings

abstract:
  We show that a Hopf term exists in the effective action of long-wavelength
skyrmions in quantum Hall systems for both odd integer and fractional filling
factors $\nu = 1/(2s +1)$, where $s$ is an integer. We evaluate the prefactor
of the Hopf term using Green function method in the limit of strong external
magnetic field using model of local interaction. The prefactor ($N$) of the
Hopf term is found to be equal to $\nu$. The spin and charge densities and
hence the total spin and charge of the skyrmion are computed from the effective
action. The total spin is found to have a dominant contribution from the Berry
term in the effective action and to increase with the size of the skyrmion. The
charge and the statistics of the skyrmion, on the other hand, are completely
determined by the prefactor of the Hopf term. Consequently, the skyrmions have
charge $\nu e$ and are Fermions (anyons) for odd integer (fractional) fillings.
We also obtain the effective action of the skyrmions at finite temperature. It
is shown that at finite temperature, the value of the prefactor of the Hopf
term depends on the order in which the zero-momentum and zero-frequency limits
are taken.


title:
BaCu_2Si_2O_7: a new quasi 1-dimensional S = 1/2 antiferromagnetic chain
  system

abstract:
  The new antiferromagnetic (AF) compound BaCu_2Si_2O_7 is studied by magnetic
susceptibility and neutron scattering techniques. The observed behavior is
dominated by the presence of loosely coupled S=1/2 chains with the intrachain
AF exchange constant J = 24.1 meV. Long-range Neel ordering is observed below
Tn = 9.2 K. The results are discussed within the framework of the Mean
Field-RPA model for weakly interacting quantum spin chains.


title:
Monte Carlo Study of the Separation of Energy Scales in Quantum Spin 1/2
  Chains with Bond Disorder

abstract:
  One-dimensional Heisenberg spin 1/2 chains with random ferro- and
antiferromagnetic bonds are realized in systems such as $Sr_3 CuPt_{1-x} Ir_x
O_6$. We have investigated numerically the thermodynamic properties of a
generic random bond model and of a realistic model of $Sr_3 CuPt_{1-x} Ir_x
O_6$ by the quantum Monte Carlo loop algorithm. For the first time we
demonstrate the separation into three different temperature regimes for the
original Hamiltonian based on an exact treatment, especially we show that the
intermediate temperature regime is well-defined and observable in both the
specific heat and the magnetic susceptibility. The crossover between the
regimes is indicated by peaks in the specific heat. The uniform magnetic
susceptibility shows Curie-like behavior in the high-, intermediate- and
low-temperature regime, with different values of the Curie constant in each
regime. We show that these regimes are overlapping in the realistic model and
give numerical data for the analysis of experimental tests.


title: Spin Dynamical Properties of the Layered Perovskite La1.2Sr1.8Mn2O7

abstract:
  Inelastic neutron-scattering measurements were performed on a single crystal
of the layered colossal magnetoresistance (CMR) material La1.2Sr1.8Mn2O7 (Tc ~
120K). We found that the spin wave dispersion is almost perfectly
two-dimensional with the in-plane spin stiffness constant D ~ 151meVA. The
value is similar to that of similarly doped La1-xSrxMnO3 though its Tc is three
times higher, indicating a large renormalization due to low dimensionality.
There exist two branches due to a coupling between layers within a
double-layer. The out-of-plane coupling is about 30% of the in-plane coupling
though the Mn-O bond lengths are similar.


title: Observation of shell structure in sodium nanowires

abstract:
  The quantum states of a system of particles in a finite spatial domain in
general consist of a set of discrete energy eigenvalues; these are usually
grouped into bunches of degenerate or close-lying levels, called shells. In
fermionic systems, this gives rise to a local minimum in the total energy when
all the states of a given shell are occupied. In particular, the closed-shell
electronic configuration of the noble gases produces their exceptional
stability. Shell effects have previously been observed for protons and neutrons
in nuclei and for clusters of metal atoms. Here we report the observation of
shell effects in an open system - a sodium metal nanowire connecting two bulk
sodium metal electrodes, which are progressively pulled apart. We measure
oscillations in the statistical distribution of conductance values, for contact
cross-sections containing up to a hundred atoms or more. The period follows the
law expected from shell-closure effects, similar to the abundance peaks at
`magic numbers' of atoms in metal clusters.


title: On Local Symmetric Order Parameters of Vortex Lattice States

abstract:
  This paper gives a new refined definition of local symmetric order parameters
(OPs)(s-wave, d-wave and p-wave order parameters) of vortex lattice states for
singlet superconductivity. s-wave, d-wave and p-wave OPs at a site (m,n) are
defined as A, B and E representations of the four fold rotation C_4 at the site
(m,n) of nearest neighbor OPs <a_{(m,n)\downarrow}a_{(m+1,n)\uparrow}> etc. The
new OPs have a well defined nature such that an OP(e.g. d-wave) at the site
obtained under translation by a lattice vector (of the vortex lattice) from a
site (m,n) is expressed by the corresponding OP (e.g. d-wave) at the site (m,n)
times a phase factor. The winding numbers of s-wave and d-wave OPs are given.


title:
Dephasing Effects by Ferromagnetic Boundary on Resistivity in Disordered
  Metallic Layer

abstract:
  The resistivity of disordered metallic layer sandwiched by two ferromagnetic
layers at low-temperature is investigated theoretically. It is shown that the
magnetic field acting at the interface does not affect the classical Boltzmann
resistivity but causes a dephasing among electrons in the presence of the
spin-orbit interaction, suppressing the anti-localization due to the spin-orbit
interaction. The dephasing turns out to be stronger in the case where the
magnetization of the two layers is parallel, contributing to a positive
magnetoresistance close to a switching field at low temperature.


title: Coherent Polariton Radiation and Light Localization

abstract:
  A system of resonance atoms is placed in a medium with developed polariton
effect. In the spectrum of polariton states there can exist a band gap. If an
atom with a resonance frequency inside the polariton gap is incorporated into
the medium, the atomic spontaneous emission can be strongly suppressed. This is
what is called the localization of light. Nevertheless, an ensemble of
resonance atoms inside the polariton gap can radiate if their coherent
interaction is sufficiently strong. Conditions when coherent polariton
radiation can appear and the properties of this coherent polariton emission are
studied.


title: The Addition Spectrum and Koopmans' Theorem for Disordered Quantum Dots

abstract:
  We investigate the addition spectrum of disordered quantum dots containing
spinless interacting fermions using the self-consistent Hartree-Fock
approximation. We concentrate on the regime r_s >~1, with finite dimensionless
conductance g. We find that in this approximation the peak spacing fluctuations
do not scale with the mean single particle level spacing for either Coulomb or
nearest neighbour interactions when r_s >~1. We also show that Koopmans'
approximation to the addition spectrum can lead to errors that are of order the
mean level spacing or larger, both in the mean addition spectrum peak spacings,
and in the peak spacing fluctuations.