title:
Charged impurity scattering limited low temperature resistivity of low
  density silicon inversion layers

abstract:
  We calculate within the Boltzmann equation approach the charged impurity
scattering limited low temperature electronic resistivity of low density
$n$-type inversion layers in Si MOSFET structures. We find a rather sharp
quantum to classical crossover in the transport behavior in the $0 - 5$K
temperature range, with the low density, low temperature mobility showing a
strikingly strong non-monotonic temperature dependence, which may qualitatively
explain the recently observed anomalously strong temperature dependent
resistivity in low-density, high-mobility MOSFETs.


title: Optical excitations in a non-ideal Bose gas

abstract:
  Optical excitations in a Bose gas are demonstrated to be very sensitive to
many-body effects. At low temperature the momentum relaxation is provided by
momentum exchange collisions, rather than by elastic collisions. A collective
excitation mode forms, which in a Boltzmann gas is manifest in a collision
shift and dramatic narrowing of spectral lines.
  In the BEC state, each spectral line splits into two components. The doubling
of the optical excitations results from the physics analogous to that of the
second sound. We present a theory of the line doubling, and calculate the
oscillator strengths and linewidth.


title:
Influence of wetting properties on the hydrodynamic boundary condition
  at a fluid-solid interface

abstract:
  It is well known that, at a macroscopic level, the boundary condition for a
viscous fluid at a solid wall is one of "no-slip". The liquid velocity field
vanishes at a fixed solid boundary. In this paper, we consider the special case
of a liquid that partially wets the solid, i.e. a drop of liquid in equilibrium
with its vapor on the solid substrate has a finite contact angle. Using
extensive Non-Equilibrium Molecular Dynamics (NEMD) simulations, we show that
when the contact angle is large enough, the boundary condition can drastically
differ (at a microscopic level) from a "no-slip" condition. Slipping lengths
exceeding 30 molecular diameters are obtained for a contact angle of 140
degrees, characteristic of Mercury on glass. On the basis of a Kubo expression
for $\delta$, we derive an expression for the slipping length in terms of
equilibrium quantities of the system. The predicted behaviour is in very good
agreement with the numerical results for the slipping length obtained in the
NEMD simulations. The existence of large slipping lentgh may have important
implications for the transport properties in nanoporous media under such
"nonwetting" conditions.


title:
Estimation of the charge carrier localization length from Gaussian
  fluctuations in the magneto-thermopower of La_{0.6}Y_{0.1}Ca_{0.3}MnO_3

abstract:
  The magneto-thermoelectric power (TEP) $\Delta S(T,H)$ of perovskite type
manganise oxide $La_{0.6}Y_{0.1}Ca_{0.3}MnO_3$ is found to exhibit a sharp peak
at some temperature $T^{*}=170K$. By approximating the true shape of the
measured magneto-TEP in the vicinity of $T^{*}$ by a linear triangle of the
form $\Delta S(T,H)\simeq S_p(H)\pm B^{\pm}(H)(T^{*}-T)$, we observe that $B
^{-}(H)\simeq 2B ^{+}(H)$. We adopt the electron localization scenario and
introduce a Ginzburg-Landau (GL) type theory which incorporates the two
concurrent phase transitions, viz., the paramagnetic-ferromagnetic transition
at the Curie point $T_C$ and the "metal-insulator" (M-I) transition at
$T_{MI}$. The latter is characterized by the divergence of the field-dependent
charge carrier localization length $\xi (T,H)$ at some characteristic field
$H_0$. Calculating the average and fluctuation contributions to the total
magnetization and the transport entropy related magneto-TEP $\Delta S(T,H)$
within the GL theory, we obtain a simple relationship between $T^{*}$ and the
above two critical temperatures ($T_{C}$ and $T_{MI}$). The observed slope
ratio $B ^{-}(H)/B ^{+}(H)$ is found to be governed by the competition between
the electron-spin exchange $JS$ and the induced magnetic energy $M_sH_0$. The
comparison of our data with the model predictions produce $T_{C}=195K$,
$JS=40meV$, $M_0=0.4M_s$, $\xi_0=10\AA$, and $n_e/n_i=2/3$ for the estimates of
the Curie temperature, the exchange coupling constant, the critical
magnetization, the localization length, and the free-to-localized carrier
number density ratio, respectively.


title: A Lattice Boltzmann Model for Wave and Fracture phenomena

abstract:
  We show that the lattice Boltzmann formalism can be used to describe wave
propagation in a heterogeneous media, as well as solid-body-like systems and
fracture propagation. Several fundamental properties of real fractures (such as
propagation speed and transition between rough and smooth crack surfaces) are
well captured by our approach.

categories: cond-mat.stat-mech comp-gas cond-mat.mtrl-sci nlin.CG


title:
Synchronization in a ring of pulsating oscillators with bidirectional
  couplings

abstract:
  We study the dynamical behavior of an ensemble of oscillators interacting
through short range bidirectional pulses. The geometry is 1D with periodic
boundary conditions. Our interest is twofold. To explore the conditions
required to reach fully synchronization and to invewstigate the time needed to
get such state. We present both theoretical and numerical results.


title:
Central peak position in magnetization loops of high-$T_c$
  superconductors

abstract:
  Exact analytical results are obtained for the magnetization of a
superconducting thin strip with a general behavior J_c(B) of the critical
current density. We show that within the critical-state model the magnetization
as function of applied field, B_a, has an extremum located exactly at B_a=0.
This result is in excellent agreement with presented experimental data for a
YBCO thin film. After introducing granularity by patterning the film, the
central peak becomes shifted to positive fields on the descending field branch
of the loop. Our results show that a positive peak position is a definite
signature of granularity in superconductors.


title: Exact Stationary State for an ASEP with Fully Parallel Dynamics

abstract:
  The exact stationary state of an asymmetric exclusion process with fully
parallel dynamics is obtained using the matrix product Ansatz. We give a simple
derivation for the deterministic case by a physical interpretation of the
dimension of the matrices. We prove the stationarity via a cancellation
mechanism and by making use of an explicit representation of the matrix algebra
we easily find closed expressions for the correlation functions in the general
probabalistic case. Asymptotic expressions, obtained by making use of earlier
results, allow us to derive the exact phase diagram.


title:
Non-equilibrium spin accumulation in ferromagnetic single-electron
  transistors

abstract:
  We study transport in ferromagnetic single-electron transistors. The non-
equilibrium spin accumulation on the island caused by a finite current through
the system is described by a generalized theory of the Coulomb blockade. It
enhances the tunnel magnetoresistance and has a drastic effect on the time-
dependent transport properties. A transient decay of the spin accumulation may
reverse the electric current on time scales of the order of the spin-flip
relaxation time. This can be used as an experimental signature of the non-
equilibrium spin accumulation.


title:
Interference Phenomena in Electronic Transport Through Chaotic Cavities:
  An Information-Theoretic Approach

abstract:
  We develop a statistical theory describing quantum-mechanical scattering of a
particle by a cavity when the geometry is such that the classical dynamics is
chaotic. This picture is relevant to a variety of systems, ranging from atomic
nuclei to microwave cavities; the main application here is to electronic
transport through ballistic microstructures. The theory describes the regime in
which there are two distinct time scales, associated with a prompt and an
equilibrated response, and is cast in terms of the matrix of scattering
amplitudes S. The prompt response is related to the energy average of S which,
through ergodicity, is expressed as the average over an ensemble of systems. We
use an information-theoretic approach: the ensemble of S-matrices is determined
by (1) general physical features-- symmetry, causality, and ergodicity, (2) the
specific energy average of S, and (3) the notion of minimum information in the
ensemble. This ensemble, known as Poisson's kernel, is meant to describe those
situations in which any other information is irrelevant. Thus, one constructs
the one-energy statistical distribution of S using only information expressible
in terms of S itself without ever invoking the underlying Hamiltonian. This
formulation has a remarkable predictive power: from the distribution of S we
derive properties of the quantum conductance of cavities, including its
average, its fluctuations, and its full distribution in certain cases, both in
the absence and presence prompt response. We obtain good agreement with the
results of the numerical solution of the Schrodinger equation for cavities in
which either prompt response is absent or there are two widely separated time
scales. Good agreement with experimental data is obtained once temperature
smearing and dephasing effects are taken into account.


title: Pseudogaps in Underdoped Cuprates

abstract:
  It has become clear in the past several years that the cuprates show many
unusual properties, both in the normal and superconducting states, especially
in the underdoped region. In particular, gap-like behavior is observed in
magnetic properties, c-axis conductivity and photoemission, whereas in-plane
transport properties are only slightly affected by the pseudogap. I shall argue
that these experimental evidences must be viewed in the context of the physics
of a doped Mott insulator and that they support the notion of spin charge
separation. I shall review recent theoretical developments, concentrating on
studies based on the t-J model. I shall describe a model based on quasiparticle
excitations, which predicts the doping dependence of T_c and anomalous
energy-gap-to-T_c ratios. Finally, I shall outline how the model may be derived
from a microscopic formulation of the t-J model. After a brief review of the
U(1) formulation, I shall explain some of the difficulties encountered there,
and how a new SU(2) formulation can resolve some of the difficulties.


title: Coupled Potts models: Self-duality and fixed point structure

abstract:
  We consider q-state Potts models coupled by their energy operators.
Restricting our study to self-dual couplings, numerical simulations demonstrate
the existence of non-trivial fixed points for 2 <= q <= 4. These fixed points
were first predicted by perturbative renormalisation group calculations.
Accurate values for the central charge and the multiscaling exponents of the
spin and energy operators are calculated using a series of novel transfer
matrix algorithms employing clusters and loops. These results compare well with
those of the perturbative expansion, in the range of parameter values where the
latter is valid. The criticality of the fixed-point models is independently
verified by examining higher eigenvalues in the even sector, and by
demonstrating the existence of scaling laws from Monte Carlo simulations. This
might be a first step towards the identification of the conformal field
theories describing the critical behaviour of this class of models.


title: Ab initio optical properties of Si(100)

abstract:
  We compute the linear optical properties of different reconstructions of the
clean and hydrogenated Si(100) surface within DFT-LDA, using norm-conserving
pseudopotentials. The equilibrium atomic geometries of the surfaces, determined
from self-consistent total energy calculations within the Car-Parrinello
scheme, strongly influence Reflectance Anisotropy Spectra (RAS), showing
differences between the p(2x2) and c(4x2)reconstructions. The Differential
Reflectivity spectrum for the c(4x2) reconstruction shows a positive peak at
energies < 1 eV, in agreement with experimental results.


title: Mean-field theory of temperature cycling experiments in spin-glasses

abstract:
  We study analytically the effect of temperature cyclings in mean-field
spin-glasses. In accordance with real experiments, we obtain a strong
reinitialization of the dynamics on decreasing the temperature combined with
memory effects when the original high temperature is restored. The same
calculation applied to mean-field models of structural glasses shows no such
reinitialization, again in accordance with experiments. In this context, we
derive some relations between experimentally accessible quantities and propose
new experimental protocols. Finally, we briefly discuss the effect of field
cyclings during isothermal aging.


title:
Continuous phase transition in a spin-glass model without time-reversal
  symmetry

abstract:
  We investigate the phase transition in a strongly disordered short-range
three-spin interaction model characterized by the absence of time reversal
symmetry in the Hamiltonian. In the mean-field limit the model is well
described by the Adam-Gibbs-DiMarzio scenario for the glass transition; however
in the short-range case this picture turns out to be modified. The model
presents a finite temperature continuous phase transition characterized by a
divergent spin-glass susceptibility and a negative specific heat exponent. We
expect the nature of the transition in this 3-spin model to be the same as the
transition in the Edwards-Anderson model in a magnetic field, with the
advantage that the strong crossover effects present in the latter case are
absent.


title:
Quantifying Spatially Heterogeneous Dynamics in Computer Simulations of
  Glassforming Liquids

abstract:
  We examine the phenomenon of dynamical heterogeneity in computer simulations
of an equilibrium, glass-forming liquid. We describe several approaches to
quantify the spatial correlation of single-particle motion, and show that
spatial correlations of particle displacements become increasingly long-range
as the temperature decreases toward the mode coupling critical temperature.


title: Coupling of Length Scales and Atomistic Simulation of MEMS Resonators

abstract:
  We present simulations of the dynamic and temperature dependent behavior of
Micro-Electro-Mechanical Systems (MEMS) by utilizing recently developed
parallel codes which enable a coupling of length scales. The novel techniques
used in this simulation accurately model the behavior of the mechanical
components of MEMS down to the atomic scale. We study the vibrational behavior
of one class of MEMS devices: micron-scale resonators made of silicon and
quartz. The algorithmic and computational avenue applied here represents a
significant departure from the usual finite element approach based on continuum
elastic theory. The approach is to use an atomistic simulation in regions of
significantly anharmonic forces and large surface area to volume ratios or
where internal friction due to defects is anticipated. Peripheral regions of
MEMS which are well-described by continuum elastic theory are simulated using
finite elements for efficiency. Thus, in central regions of the device, the
motion of millions of individual atoms is simulated, while the relatively large
peripheral regions are modeled with finite elements. The two techniques run
concurrently and mesh seamlessly, passing information back and forth. This
coupling of length scales gives a natural domain decomposition, so that the
code runs on multiprocessor workstations and supercomputers. We present novel
simulations of the vibrational behavior of micron-scale silicon and quartz
oscillators. Our results are contrasted with the predictions of continuum
elastic theory as a function of size, and the failure of the continuum
techniques is clear in the limit of small sizes. We also extract the Q value
for the resonators and study the corresponding dissipative processes.


title: Luttinger Liquid Behavior in Carbon Nanotubes

abstract:
  An interacting one-dimensional (1D) electron system is predicted to behave
very differently than its higher-dimensional counterparts. Coulomb interactions
strongly modify the properties away from those of a Fermi liquid, resulting in
a Luttinger liquid (LL) characterized by a power-law vanishing of the density
of states at the Fermi level. Experiments on one-dimensional semiconductor
wires and fractional quantum Hall conductors have been interpreted using this
picture, but questions remain about the connection between theory and
experiment. Recently, single-walled carbon nanotubes (SWNTs) have emerged as a
new type of 1D conductor that may exhibit LL behavior. Here we present
measurements of the conductance of individual ropes of such SWNTs as a function
of temperature and voltage. Power law behavior as a function of temperature or
bias voltage is observed: G~ T^a and dI/dV ~ V^a. Both the power-law functional
forms and the inferred exponents are in good agreement with theoretical
predictions for tunneling into a LL.


title:
Angular Dependence of the Nonlinear Transverse Magnetic Moment of YBCO
  in the Meissner state

abstract:
  The angular dependence of the nonlinear transverse magnetic moment of
untwinned high-quality single crystals of optimally doped YBCO have been
studied at a temperature of 2.5K using a low frequency AC technique. The
absence of any signature at angular period 2\pi/4is analyzed in light of the
numerical predictions of such a signal for a pure d-wave order parameter with
line nodes. Implications of this null result for the existence of a non-zero
gap at all angles on the Fermi surface are discussed.


title:
Topological defects and the short-distance behavior of the structure
  factor in nematic liquid crystals

abstract:
  The scattering of light at large wave-vector magnitudes k in nematic systems
containing topological defects is investigated theoretically. At large k the
structure factor S(k) is dominated by power-law contributions originating from
singular order-parameter variations associated with topological defects and
from transverse thermal fluctuations of the nematic director. These defects
(nematic disclinations and hedgehogs) lead to contributions of the form rho A
k^{-xi} (``the Porod tail''), where rho is the number density of a given type
of defect, A is a dimensionless Porod amplitude, and xi is an integer-valued
Porod exponent. The Porod amplitudes and exponents are calculated for all types
of topologically stable defects occurring in uniaxial and biaxial nematics in
two or three spatial dimensions. The range of wave-vectors in which the
contributions to the scattering intensity due to defects dominate the
contribution due to thermal fluctuations is estimated, and it is concluded that
for experimentally accessible defect densities the range of observability of
the Porod tail extends over one to three decades in scattering wave-vector
magnitude k. Available experimental results on phase ordering in uniaxial
nematics are analyzed, and applications of our results are suggested for
light-scattering studies of other nematic systems containing numerous defects.


title:
Collective charge density excitations in two-component one-dimensional
  quantum plasmas: Phase fluctuation mode dispersion and spectral weight in
  semiconductor quantum wire nanostructures

abstract:
  Collective charge density excitation spectra of both a spatially separated
two-component quasi-one dimensional (1D) quantum plasma, as existing in a
semiconductor double quantum wire structure, and a 1D homogeneous electron-hole
plasma, as appropriate for a photoexcited semiconductor quantum wire system,
are calculated within the two-component random-phase-approximation. We also
investigate the effect of impurity scattering on the collective mode dispersion
and damping, and calculate the collective mode spectral weight by obtaining the
dynamical structure factor. We find that both OP and AP modes are overdamped by
impurity scattering below some critical wave vector. We find that in the long
wavelength limit the spectral weight is carried mostly by the OP, but the
spectral weight of the undamped AP mode at finite (but not too large) wave
vectors is comparable with that of the OP mode, making it viable to observe the
AP mode in semiconductor quantum wire systems. The effect of the interwire
electron tunneling in a biwire system on the collective charge density
excitation spectra is also studied. We discuss the mode dispersion and damping
from an effective Luttinger liquid perspective as well and in some cases
include local field corrections in our calculations.


title:
Transport in the XX chain at zero temperature: Emergence of flat
  magnetization profiles

abstract:
  We study the connection between magnetization transport and magnetization
profiles in zero-temperature XX chains. The time evolution of the transverse
magnetization, m(x,t), is calculated using an inhomogeneous initial state that
is the ground state at fixed magnetization but with m reversed from -m_0 for
x<0 to m_0 for x>0. In the long-time limit, the magnetization evolves into a
scaling form m(x,t)=P(x/t) and the profile develops a flat part (m=P=0) in the
|x/t|< c(m_0) region. The flat region shrinks to zero if m_0->1/2 while it
expands with the maximum velocity, c_0=1, for m_0->0. The states emerging in
the scaling limit are compared to those of a homogeneous system where the same
magnetization current is driven by a bulk field, and we find that the
expectation values of various quantities (energy, occupation number in the
fermionic representation) agree in the two systems.


title:
Mean-Field HP Model, Designability and Alpha-Helices in Protein
  Structures

abstract:
  Analysis of the geometric properties of a mean-field HP model on a square
lattice for protein structure shows that structures with large number of switch
backs between surface and core sites are chosen favorably by peptides as unique
ground states. Global comparison of model (binary) peptide sequences with
concatenated (binary) protein sequences listed in the Protein Data Bank and the
Dali Domain Dictionary indicates that the highest correlation occurs between
model peptides choosing the favored structures and those portions of protein
sequences containing alpha-helices.


title: Eigenvalue Correlations For Banded Matrices

abstract:
  We study the evolution of the distribution of eigenvalues of $N\times N$
matrix ensembles subject to a change of variances of its matrix elements. Our
results indicate that the evolution of the probability density is governed by a
Fokker- Planck equation similar to the one governing the time-evolution of the
particle- distribution in Wigner-Dyson gas, with relative variances now playing
the role of time. This is also similar to the Fokker-Planck equation for the
distribution of eigenvalues of a $N\times N$ matrix subject to a random
perturbation taken from the standard Gaussian ensembles with
perturbation-strength as the "time" variable. This equivalence alonwith the
already known correlations of standard Gaussian ensembles can therefore help us
to obtain the same for various physically-significant cases modeled by random
banded Gaussian ensembles.


title: Potential scattering and the Kondo effect

abstract:
  We study a generalized Kondo model in which a spin-1/2 impurity is coupled to
a conduction band by both s-d exchange and potential interactions. A strong
potential scattering is shown to screen an exchange scattering, and the Kondo
temperature of the system is decreased.


title: Theory and Data Analysis for the High Momentum End of $^4$He spectrum

abstract:
  The hybridization of the single-excitation branch with the two-excitation
continuum is reconsidered from the theoretical point of view by including the
effect of the interference term between one and two excitations. The
phenomenological theory presented is used to reproduce the experimental data
over a wide region of momentum (2.3-3.6 A^{-1}) and energy (0-12 meV). It is
thus possible to extract the final part of $^4$He spectrum with higher
accuracy. It is found that data agree with a dispersion relation always below
twice the roton energy. This is consistent with the negative value of the
roton-roton interaction found.


title:
Magnetic and Charge Correlations of the 2-dimensional $t-t'-U$ Hubbard
  model

abstract:
  Using a spin-rotation invariant six-slave boson representation, we study the
square lattice Hubbard model with nearest-neighbor hopping $t$ and next-nearest
neighbor hopping $t'$. We discuss the influence of $t'$ on the charge and
magnetic properties. In the hole-doped domain, we find that a negative $t'$
strongly favors itinerant ferromagnetism over any incommensurate phase,
especially in the strong coupling regime. For positive $t'$ magnetic
fluctuations are suppressed. A tight connection between frustrated charge
dynamics and large magnetic fluctuations is pointed out. A clear tendency
towards striped charge ordering is found in the regime of large positive $t'$.


title: Percolation-like phase transition in a non-equilibrium steady state

abstract:
  We study the Gierer-Meinhardt model of reaction-diffusion on a
site-disordered square lattice. Let $p$ be the site occupation probability of
the square lattice. For $p$ greater than a critical value $p_c$, the steady
state consists of stripe-like patterns with long-range connectivity. For $p <
p_c$, the connectivity is lost. The value of $p_c$ is found to be much greater
than that of the site percolation threshold for the square lattice. In the
vicinity of $p_c$, the cluster-related quantities exhibit power-law scaling
behaviour. The method of finite-size scaling is used to determine the values of
the fractal dimension $d_f$, the ratio, $\frac{\gamma}{\nu}$, of the average
cluster size exponent $\gamma$ and the correlation length exponent $\nu$ and
also $\nu$ itself. The values appear to indicate that the disordered GM model
belongs to the universality class of ordinary percolation.


title:
The peculiarities of phase transitions in layered
  ferroelectrics-semiconductors $ TlIn_{1-x}Fe_xS_2 $

abstract:
  The results of differential-thermal analysis of various samples as well as
dielectric measurements of solid solutions $TlIn_{1-x}Fe_x^{56,57}S_2$ are
presented. It has been established that phase transitions become blurred and
shift to low temperatures with increasing the concentration of defects in the
layered structure. at some threshold value the structure loses layerity, the
crystal becomes isotropig and the distribution of defects does homogeneous.


title:
Spinon Superconductivity and Superconductivities Mediated by Spin-Waves
  and Phonons in Cuprates

abstract:
  The disclosure of spinon superconductivity and superconductivity mediated by
spin-waves in hole-doped Bi2212 cuprate raises the question about the origin of
the superconductivity in other cuprates and specially in an electron-doped NCCO
cuprate.


title:
Self-consistent calculation of the autolocalization barrier for
  quasiparticles in anisotropic crystal

abstract:
  The energy of the electron wave packet interacting with lattice distortion,
is considered in anisotropic crystal. Anisotropy of the electron and phonon
spectra as well as of the electron-phonon interaction are taken into account.
The height of the barrier between free and self-trapped states is calculated in
dependence on the anisotropy parameters. The calculation is done numerically,
using continual aproximation. The analytical solution is obtained for some
cases of quasi-two and quasi-one-dimensional spectra. Key words: anisotropy;
polarons; barrier.


title: Electronic transport in disordered interacting systems

abstract:
  We numerically investigate the transport properties of disordered interacting
electrons in three dimensions in the metallic as well as in the insulating
phases. The disordered many-particle problem is modeled by the quantum Coulomb
glass which contains a random potential, long-range unscreened Coulomb
interactions and quantum hopping between different sites. We have recently
developed the Hartree-Fock based diagonalization (HFD) method which amounts to
diagonalizing the Hamiltonian in a suitably chosen energetically truncated
basis. This method allows us to investigate comparatively large systems. Here
we calculate the combined effect of disorder and interactions on the
dissipative conductance. We find that the qualitative influence of the
interactions on the conductance depends on the relative disorder strength. For
strong disorder interactions can significantly enhance the transport while they
suppress the conductance for weak disorder.


title: The spin gap of CaV4O9 revisited

abstract:
  The large-plaquette scenario of the spin gap in CaV4O9 is investigated on the
basis of extensive exact diagonalizations. We confirm the existence of a
large-plaquette phase in a wide range of parameters, and we show that the most
recent neutron scattering data actually require an intra-plaquette second
neighbor exchange integral much larger than the inter-plaquette one, thus
justifying the perturbative calculation used in the interpretation of the
neutron scattering experiments.


title: Statistics of photodissociation spectra: nonuniversal properties

abstract:
  We consider the two-point correlation function of the photodissociation cross
section in molecules where the fragmentation process is indirect, passing
through resonances above the dissociation threshold. In the limit of
overlapping resonances, a formula is derived, relating this correlation
function to the behavior of the corresponding classical system. It is shown
that nonuniversal features of the two-point correlation function may have
significant experimental manifestations.

categories: cond-mat chao-dyn nlin.CD physics.chem-ph


title:
Residual entropy and spin gap in a one-dimensional analog of the
  pyrochlore antiferromagnet

abstract:
  We show that the low-energy sector of the S=1/2, antiferromagnetic Heisenberg
model on a one-dimensional lattice of coupled tetrahedra consists of 2^N
replica of the spectrum of the dimerized Heisenberg chain, where N is the
number of tetrahedra.
  This provides a proof of the following properties: i) there is a residual
ground-state entropy per spin equal to 2^{1/4}; ii) there is a singlet-triplet
gap as long as the coupling between the tetrahedra is smaller than the internal
one. These properties are compared to available results on the pyrochlore
lattice.


title:
U(1)XSU(2) Gauge theory of underdoped high Tc cuprates via Chern-Simons
  bosonization

abstract:
  We outline the basic ideas involved in a recently proposed derivation of a
gauge theory for underdoped cuprates in the "spin-gap phase", performed
essentially step by step starting from the t-J model, considered as a model
Hamiltonian for the CuO_2 layers. The basic tool is the U(1)XSU(2) Chern-Simons
bosonization, to which it is dedicated a somewhat detailed discussion. The
basic output is a "spin-gap" not vanishing in any direction and an
antiferromagnetic correlation length proportional to the inverse square root of
doping concentration, in agreement with data deduced from the neutron
experiments. The model also exhibits a small half-pocket Fermi surface around
(\pi/2, \pi/2) and a linear in temperature dependence of in-plane resistivity
in certain temperature range.


title: The Holstein Polaron

abstract:
  We describe a variational method to solve the Holstein model for an electron
coupled to dynamical, quantum phonons on an infinite lattice. The variational
space can be systematically expanded to achieve high accuracy with modest
computational resources (12-digit accuracy for the 1d polaron energy at
intermediate coupling). We compute ground and low-lying excited state
properties of the model at continuous values of the wavevector $k$ in
essentially all parameter regimes. Our results for the polaron energy band,
effective mass and correlation functions compare favorably with those of other
numerical techniques including DMRG, Global Local and exact diagonalization. We
find a phase transition for the first excited state between a bound and unbound
system of a polaron and an additional phonon excitation. The phase transition
is also treated in strong coupling perturbation theory.


title:
Strong-coupling perturbation theory for the two-dimensional Bose-Hubbard
  model in a magnetic field

abstract:
  The Bose-Hubbard model in an external magnetic field is investigated with
strong-coupling perturbation theory. The lowest-order secular equation leads to
the problem of a charged particle moving on a lattice in the presence of a
magnetic field, which was first treated by Hofstadter. We present phase
diagrams for the two-dimensional square and triangular lattices, showing a
change in shape of the phase lobes away from the well-known power-law behavior
in zero magnetic field. Some qualitative agreement with experimental work on
Josephson-junction arrays is found for the insulating phase behavior at small
fields.


title:
Bulk and edge correlations in the compressible half-filled quantum Hall
  state

abstract:
  We study bulk and edge correlations in the compressible half-filled state,
using a modified version of the plasma analogy. The corresponding plasma has
anomalously weak screening properties, and as a consequence we find that the
correlations along the edge do not decay algebraically as in the Laughlin
(incompressible) case, while the bulk correlations decay in the same way. The
results suggest that due to the strong coupling between charged modes on the
edge and the neutral Fermions in the bulk, reflected by the weak screening in
the plasma analogue, the (attractive) correlation hole is not well defined on
the edge. Hence, the system there can be modeled as a free Fermi gas of {\em
electrons} (with an appropriate boundary condition). We finally comment on a
possible scenario, in which the Laughlin-like dynamical edge correlations may
nevertheless be realized.


title: Dynamics of SiO$_2$ glasses

abstract:
  Results on the dynamics of silica are presented: vibrations and relaxations.
Using molecular dynamics, glass structures are generated by rapidly quenching
melts below the glass transition. For the local minima of the structures the
vibrational density of states is determined, the structures of the eigenmodes
are analyzed, and the influence of the single components is discussed.
Relaxations are studied in both amorphous SiO$_2$ and silica melts. Our main
focus is on the type of relaxation, i.e., whether the main contributions are
caused by small atomic displacements, or by bond-breaking processes, e.g.
creation and annihilation of dangling bonds. The eigenvector of the relaxations
is found to be similar to the low-frequency one.


title:
Fermi Surface and gap parameter in high-Tc superconductors: the Stripe
  Quantum Critical Point scenario

abstract:
  We study the single-particle spectral properties of electrons coupled to
quasicritical charge and spin fluctuations close to a stripe-phase, which is
governed by a Quantum Critical Point near optimum doping. We find that spectral
weight is transferred from the quasiparticle peak to incoherent dispersive
features. As a consequence the distribution of low-laying spectral weight is
modified with respect to the quasiparticle Fermi surface. The interplay of
charge and spin fluctuations reproduces features of the observed Fermi surface,
such as the asymmetric suppression of spectral weight near the M points of the
Brillouin zone.
  Within the model, we also analyze the interplay between repulsive spin and
attractive charge fluctuations in determining the symmetry and the peculiar
momentum dependence of the superconducting gap parameter. When both spin and
charge fluctuations are coupled to the electrons, we find $d_{x^2-y^2}$-wave
gap symmetry in a wide range of parameter. A crossover $d$- vs $s$-wave
symmetry of the gap may occur when the strength of charge fluctuations
increases with respect to spin fluctuations.


title:
Broadband dielectric response of glycerol and propylene carbonate: a
  comparison

abstract:
  Dielectric data on glycerol and propylene carbonate covering 18 decades of
frequency are presented and compared to each other. Both materials exhibit
qualitatively similar behavior except for marked differences in the
high-frequency region just below the boson peak. The results on both materials
are consistent with the mode coupling theory of the glass transition.


title: An Atom Laser with a cw Output Coupler

abstract:
  We demonstrate a continuous output coupler for magnetically trapped atoms.
Over a period of up to 100 ms a collimated and monoenergetic beam of atoms is
continuously extracted from a Bose- Einstein condensate. The intensity and
kinetic energy of the output beam of this atom laser are controlled by a weak
rf-field that induces spin flips between trapped and untrapped states.
Furthermore, the output coupler is used to perform a spectroscopic measurement
of the condensate, which reveals the spatial distribution of the magnetically
trapped condensate and allows manipulation of the condensate on a micrometer
scale.


title: The Renormalization Group and its Finite Lattice Approximations

abstract:
  We investigate finite lattice approximations to the Wilson Renormalization
Group in models of unconstrained spins. We discuss first the properties of the
Renormalization Group Transformation (RGT) that control the accuracy of this
type of approximations and explain different methods and techniques to
practically identify them. We also discuss how to determine the anomalous
dimension of the field. We apply our considerations to a linear sigma model in
two dimensions in the domain of attraction of the Ising Fixed Point using a
Bell-Wilson RGT. We are able to identify optimal RGTs which allow accurate
computations of quantities such as critical exponents, fixed point couplings
and eigenvectors with modest statistics. We finally discuss the advantages and
limitations of this type of approach.


title: Phase coherence and ``fragmented'' Bose condensates

abstract:
  We show that a ``fragmented Bose condensate'' in which two or more distinct
single-particle states are macroscopically occupied by the same species of
boson is inherently unstable to the formation of a conventional Bose condensate
whose macroscopically occupied state is a linear combination of the
``fragments'' with definite relative phases. A related analysis shows that a
reproducible relative phase develops when two initially decoupled condensates
are placed in contact.


title: The Chalker-Coddington Network Model is Quantum Critical

abstract:
  We show that the localization transition in the integer quantum Hall effect
as described by the Chalker-Coddington network model is quantum critical. We
first map the anisotropic network model to the problem of diagonalizing a
one-dimensional non-Hermitian non-compact supersymmetric lattice Hamiltonian of
interacting bosons and fermions. Its behavior is investigated numerically using
the density matrix renormalization group method, and critical behavior is found
at the plateau transition. This result is confirmed by an exact, analytic,
generalization of the Lieb-Schultz-Mattis theorem.


title:
Phase diagram for a class of spin-half Heisenberg models interpolating
  between the square-lattice, the triangular-lattice and the linear chain
  limits

abstract:
  We study the spin-half Heisenberg models on an anisotropic two-dimensional
lattice which interpolates between the square-lattice at one end, a set of
decoupled spin-chains on the other end, and the triangular-lattice Heisenberg
model in between. By series expansions around two different dimer ground states
and around various commensurate and incommensurate magnetically ordered states,
we establish the phase diagram for this model of a frustrated antiferromagnet.
We find a particularly rich phase diagram due to the interplay of magnetic
frustration, quantum fluctuations and varying dimensionality. There is a large
region of the usual 2-sublattice Ne\'el phase, a 3-sublattice phase for the
triangular-lattice model, a region of incommensurate magnetic order around the
triangular-lattice model, and regions in parameter space where there is no
magnetic order. We find that the incommensurate ordering wavevector is in
general altered from its classical value by quantum fluctuations. The regime of
weakly coupled chains is particularly interesting and appears to be nearly
critical.


title: On Superconductors and Torsion Vortices

abstract:
  The Meissner effect for superconductors in spacetimes with torsion is
revisited.Two new physical interpretaions are presented.The first considers the
Landau-Ginzburg theory yields a new symmetry-breaking vacuum depending on
torsion.In the second interpretation a gravitational Meissner torsional effect
where when the Higgs field vanishes, torsion and electromagnetic fields need
not vanish and outside the Abrikosov tubes a torsion vector analogous to the
Maxwell potential is obtained.The analogy is even stronger if we think that in
this case the torsion vector has to be derivable from a torsion
potential.Another solution of Landau-Ginzburg equation is shown to lead
naturally to the geometrization of the electromagnetism in terms of the torsion
field.


title: Disorder Induced Ferromagnetism in Restricted Geometries

abstract:
  We study the influence of on-site disorder on the magnetic properties of the
ground state of the infinite $U$ Hubbard model. We find that for one
dimensional systems disorder has no influence, while for two dimensional
systems disorder enhances the spin polarization of the system. The tendency of
disorder to enhance magnetism in the ground state may be relevant to recent
experimental observations of spin polarized ground states in quantum dots and
small metallic grains.


title:
Temperature- and magnetic-field-dependent thermal conductivity of pure
  and Zn-doped Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta} single crystals

abstract:
  The thermal conductivity \kappa of Bi-2212 is measured in pure and Zn-doped
crystals as a function of temperature and magnetic field. The in-plane
resistivity is also measured on the identical samples. Using these data, we
make a crude estimate of the impurity-scattering rate \Gamma of the pure and
the Zn-doped crystals. Our measurement show that the "plateau" in the \kappa(H)
profile is not observed in the majority of our Bi-2212 crystals, including one
of the cleanest crystals available to date. The estimated values of \Gamma for
the pure and Zn-doped samples allow us to compare the \kappa(H) data with the
existing theories of the quasiparticle heat transport in d-wave superconductors
under magnetic field. Our analysis indicates that a proper inclusion of the
quasiparticle-vortex scattering, which is expected to play the key role in the
peculiar behavior of the \kappa(H), is important for a quantitative
understanding of the QP heat transport in the presence of the vortices.


title: Quantitative study of laterally inhomogeneous wetting films

abstract:
  Based on a microscopic density functional theory we calculate the internal
structure of the three-phase contact line between liquid, vapor, and a
confining wall as well as the morphology of liquid wetting films on a substrate
exhibiting a chemical step. We present a refined numerical analysis of the
nonlocal density functional which describes the interface morphologies and the
corresponding line tensions. These results are compared with those predicted by
a more simple phenomenological interface displacement model. Except for the
case that the interface exhibits large curvatures, we find that the interface
displacement model provides a quantitatively reliable description of the
interfacial structures.


title:
Determination of the exponent gamma for SAWs on the two-dimensional
  Manhattan lattice

abstract:
  We present a high-statistics Monte Carlo determination of the exponent gamma
for self-avoiding walks on a Manhattan lattice in two dimensions. A
conservative estimate is $\gamma \gtapprox 1.3425(3)$, in agreement with the
universal value 43/32 on regular lattices, but in conflict with predictions
from conformal field theory and with a recent estimate from exact enumerations.
We find strong corrections to scaling that seem to indicate the presence of a
non-analytic exponent Delta < 1. If we assume Delta = 11/16 we find gamma =
1.3436(3), where the error is purely statistical.


title: Superfluid Flow Past an Array of Scatterers

abstract:
  We consider a model of nonlinear superfluid flow past a periodic array of
point-like scatterers in one dimension. An application of this model is the
determination of the critical current of a Josephson array in a regime
appropriate to a Ginzburg-Landau formulation. Here, the array consists of short
normal-metal regions, in the presence of a Hartree electron-electron
interaction, and embedded within a one-dimensional superconducting wire near
its critical temperature, $Tc$. We predict the critical current to depend
linearly as $A (Tc-T)$, while the coefficient $A$ depends sensitively on the
sizes of the superconducting and normal-metal regions and the strength and sign
of the Hartree interaction. In the case of an attractive interaction, we find a
further feature: the critical current vanishes linearly at some temperature
$T*$ less than $Tc$, as well as at $Tc$ itself. We rule out a simple
explanation for the zero value of the critical current, at this temperature
$T*$, in terms of order parameter fluctuations at low frequencies.


title: Phase Diagram of a Loop on the Square Lattice

abstract:
  The phase diagram of the O(n) model, in particular the special case $n=0$, is
studied by means of transfer-matrix calculations on the loop representation of
the O(n) model. The model is defined on the square lattice; the loops are
allowed to collide at the lattice vertices, but not to intersect. The loop
model contains three variable parameters that determine the loop density or
temperature, the energy of a bend in a loop, and the interaction energy of
colliding loop segments. A finite-size analysis of the transfer-matrix results
yields the phase diagram in a special plane of the parameter space. These
results confirm the existence of a multicritical point and an Ising-like
critical line in the low-temperature O(n) phase.


title: First and second order transitions in dilute O(n) models

abstract:
  We explore the phase diagram of an O(n) model on the honeycomb lattice with
vacancies, using finite-size scaling and transfer-matrix methods. We make use
of the loop representation of the O(n) model, so that $n$ is not restricted to
positive integers. For low activities of the vacancies, we observe critical
points of the known universality class. At high activities the transition
becomes first order. For n=0 the model includes an exactly known theta point,
used to describe a collapsing polymer in two dimensions. When we vary $n$ from
0 to 1, we observe a tricritical point which interpolates between the
universality classes of the theta point and the Ising tricritical point.


title:
Backflow and dissipation during the quantum decay of a metastable Fermi
  liquid

abstract:
  The particle current in a metastable Fermi liquid against a first-order phase
transition is calculated at zero temperature. During fluctuations of a droplet
of the stable phase, in accordance with the conservation law, not only does an
unperturbed current arise from the continuity at the boundary, but a backflow
is induced by the density response. Quasiparticles carrying these currents are
scattered by the boundary, yielding a dissipative backflow around the droplet.
An energy of the hydrodynamic mass flow of the liquid and a friction force
exerted on the droplet by the quasiparticles have been obtained in terms of a
potential of their interaction with the droplet.


title: Single fluxon in double stacked Josephson junctions: Analytic solution

abstract:
  We derive an approximate analytic solution for a single fluxon in a double
stacked Josephson junctions (SJJ's) for arbitrary junction parameters and
coupling strengths. It is shown that the fluxon in a double SJJ's can be
characterized by two components, with different Swihart velocities and
Josephson penetration depths. Using the perturbation theory we find the second
order correction to the solution and analyze its accuracy. Comparison with
direct numerical simulations shows a quantitative agreement between exact and
approximate analytic solutions. It is shown that due to the presence of two
components, the fluxon in SJJ's may have an unusual shape with an inverted
magnetic field in the second junction when the velocity of the fluxon is
approaching the lower Swihart velocity.


title: Multiphonon anharmonic decay of a quantum mode

abstract:
  A nonperturbative theory of multiphonon anharmonic transitions between energy
levels of a local mode is presented. It is shown that the rate of transitions
rearranges near the critical level number $n_{cr}$: at smaller $n$ the process
slows down, while at larger $n$ it accelerates in time, causing a jump-like
loss of energy followed by the generation of phonon bursts. Depending on
parameters, phonons are emitted in pairs, triplets etc.


title: Statistical Green's Functions

abstract:
  The basic mathematical properties of Green's functions used in statistical
mechanics as well as the equations defining these functions and the techniques
of solving these equations are reviewed. An approach is presented called the
Correlated Iteration Theory, which has been developed by the author. This
approach differs from all other known variants of perturbation theory for
Green's functions by the combination of two factors: the systematic formulation
of an algorithm for obtaining subsequent approximations and the consistent
consideration of interparticle correlations at each step of the procedure.


title: Vortices in Ginzburg-Landau billiards

abstract:
  We present an analysis of the Ginzburg-Landau equations for the description
of a two-dimensional superconductor in a bounded domain. Using the properties
of a special integrability point of these equations which allows vortex
solutions, we obtain a closed expression for the energy of the superconductor.
  The role of the boundary of the system is to provide a selection mechanism
for the number of vortices.
  A geometrical interpretation of these results is presented and they are
applied to the analysis of the magnetization recently measured on small
superconducting disks. Problems related to the interaction and nucleation of
vortices are discussed.


title: The phase diagramm of La{1-x}Sr{x}MnO3 revisited

abstract:
  We report on detailed sucseptibility and magnetization studies in
La{1-x}Sr{x}MnO3 for x<=0.2. We arrive at a phase diagram which shows a
ferromagnetic ground state for 0.1<=x<=0.15 followed by a canted
antiferromagnetic phase at higher temperatures, not in agreement with present
canonical conjectures. The phase diagram indicates the importance of structural
aspects and superexchange interactions for the magnetic phases at low Sr
concentrations.


title:
Microscopic theory of weakly coupled superconducting multilayers in an
  external magnetic field

abstract:
  We present the first fully microscopic, self-consistent, and self-contained
theory of superconducting weakly coupled periodic multilayers with tunnel
barriers in the presence of externally applied parallel magnetic fields, in the
local Ginzburg-Landau regime. We solve a nontrivial mathematical problem of a
microscopic derivation and exact minimization of the free-energy functional. In
the thin-layer limit that corresponds to the domain of validity of the
phenomenological Lawrence-Doniach model, our physical results strikingly
contrast with those of our predecessors. In particular, we completely revise
previous calculations of the lower critical field and refute the concept of a
triangular Josephson vortex lattice. We show that Josephson vortices penetrate
into all the barriers simultaneously and form peculiar structures that we term
''vortex planes''. We calculate the superheating field of the Meissner state
and predict hysteresis in the magnetization. In the vortex state, the
magnetization exhibits distinctive oscillatory behavior and jumps due to
successive penetration of the vortex planes. We prove that the vortex-plane
penetration and pinning by the edges of the sample cause the Fraunhofer pattern
of the critical Josephson current. We calculate the critical temperature and
the upper critical field of infinite (along the layers) multilayers. For finite
multilayers, we predict a series of first-order phase transitions to the normal
state and oscillations of the critical temperature versus the applied field.
Finally, we discuss some theoretical and experimental implications of the
obtained results.
  PACS numbers: 74.50.+r, 74.80.Dm


title:
Superconducting and Magnetic Properties of Nb/Pd_1-xFe_x/Nb Triple
  Layers

abstract:
  The superconducting and magnetic properties of Nb/Pd_1-xFe_x/Nb triple layers
with constant Nb layer thickness d_Nb=200 A and different interlayer
thicknesses are investigated. The thickness dependence of the magnetization and
of the superconducting transition temperature shows that for small iron
concentration x the Pd_1-xFe_x layer is likely to be in the paramagnetic state
for very thin films whereas ferromagnetic order is established for x>=0.13. The
parallel critical field B_c2II(T) exhibits a crossover from two-dimensional
(2D) behavior where the Nb films are coupled across the interlayer, towards a
2D behavior of coupled Nb films with increasing d_PdFe or x. This 2D-2D
crossover allows a determination of the penetration depth xi_F of Cooper pairs
into the Pd_1-xFe_x layer as a function of x. For samples with a ferromagnetic
interlayer xi_ is found to be independent of x.


title:
The Stripe-Phase Quantum-Critical-Point Scenario for High-Tc
  Superconductors

abstract:
  A summary is given of the main outcomes of the quantum-critical-point
scenario for high-$T_c$ superconductors, developed in the last few years by the
Rome group. Phase separation, which commonly occurs in strongly correlated
electronic systems, turns into a stripe instability when Coulomb interaction is
taken into account. The stripe phase continuously connects the high doping
regime, dominated by charge degrees of freedom to the low doping regime where
spin degrees of freedom are most relevant. Dynamical stripe fluctuations
enslave antiferromagnetic fluctuations at high doping. Critical fluctuations
near the stripe instability mediate a singular interaction between
quasiparticles, which is responsible for the non-Fermi liquid behavior in the
metallic phase and for the Cooper pairing with d-wave symmetry in the
superconducting phase.


title: Quasi-long range order in the random anisotropy Heisenberg model

abstract:
  The large distance behaviors of the random field and random anisotropy
Heisenberg models are studied with the functional renormalization group in
$4-\epsilon$ dimensions. The random anisotropy model is found to have a phase
with the infinite correlation radius at low temperatures and weak disorder. The
correlation function of the magnetization obeys a power law $<{\bf m}({\bf
r}_1) {\bf m}({\bf r}_2)>\sim| {\bf r}_1-{\bf r}_2|^{-0.62\epsilon}$. The
magnetic susceptibility diverges at low fields as $\chi\sim
H^{-1+0.15\epsilon}$. In the random field model the correlation radius is found
to be finite at the arbitrarily weak disorder.


title: Coupled Hartree-Fock-Bogoliubov kinetic equations for a trapped Bose gas

abstract:
  Using the Kadanoff-Baym non-equilibrium Green's function formalism, we derive
the self-consistent Hartree-Fock-Bogoliubov (HFB) collisionless kinetic
equations and the associated equation of motion for the condensate wavefunction
for a trapped Bose-condensed gas. Our work generalizes earlier work by Kane and
Kadanoff (KK) for a uniform Bose gas. We include the off-diagonal (anomalous)
pair correlations, and thus we have to introduce an off-diagonal distribution
function in addition to the normal (diagonal) distribution function. This
results in two coupled kinetic equations. If the off-diagonal distribution
function can be neglected as a higher-order contribution, we obtain the
semi-classical kinetic equation recently used by Zaremba, Griffin and Nikuni
(based on the simpler Popov approximation). We discuss the static local
equilibrium solution of our coupled HFB kinetic equations within the
semi-classical approximation. We also verify that a solution is the rigid
in-phase oscillation of the equilibrium condensate and non-condensate density
profiles, oscillating with the trap frequency.


title: Stripe Formation within SO(5) Theory

abstract:
  We study the formation of stripe order within the SO(5) theory of high T_c
superconductivity. We show that spin and charge modulations arise as a result
of the competition between a local tendency to phase separate and the
long-range Coulomb interaction. This frustrated phase separation leads to
hole-rich and hole-poor regions which are respectively superconducting and
antiferromagnetic. A rich variety of microstructures ranging from droplet and
striped to inverted-droplet phases are stabilized, depending on the charge
carrier concentration. We show that the SO(5) energy functional favors
non-topological stripes.


title:
Vortices with antiferromagnetic cores in the SO(5) theory of high
  temperature superconductivity

abstract:
  We consider the problem of superconducting Ginzburg-Landau (G-L) vortices
with antiferromagnetic cores which arise in Zhang's SO(5) model of
antiferromagnetism (AF) and high temperature superconductivity (SC). This
problem was previously considered by Arovas et al. who constructed approximate
"variational" solutions, in the large kappa limit, to estimate the domain of
stability of such vortices in the temperature-chemical potential phase diagram.
By solving the G-L equations numerically for general kappa, we show that the
amplitude of the antiferromagnetic component at the vortex core decreases to
zero continuously at a critical value of the AF-SC anisotropy (g~0.25) which is
essentially independent of kappa for large kappa. The magnetic field profile,
the vortex line energy and the value of the B-field at the center of the vortex
core, as functions of anisotropy are also presented.


title: Identifying the protein folding nucleus using molecular dynamics

abstract:
  Molecular dynamics simulations of folding in an off-lattice protein model
reveal a nucleation scenario, in which a few well-defined contacts are formed
with high probability in the transition state ensemble of conformations. Their
appearance determines folding cooperativity and drives the model protein into
its folded conformation.


title: Absence of non-linear Meissner effect in YBa2Cu3O6.95

abstract:
  We present measurements the field and temperature dependence of the
penetration depth (lambda) in high purity, untwinned single crystals of
YBa2Cu3O6.95 in all three crystallographic directions. The temperature
dependence of lambda is linear down to low temperatures, showing that our
crystals are extremely clean. Both the magnitude and temperature dependence of
the field dependent correction to lambda however, are considerably different
from that predicted from the theory of the non-linear Meissner effect for a
d-wave superconductor (Yip-Sauls theory). Our results suggest that the
Yip-Sauls effect is either absent or is unobservably small in the Meissner
state of YBa2Cu3O6.95.


title: Supersymmetric Field Theory of Non-Equilibrium Thermodynamic System

abstract:
  On the basis of Langevin equation the optimal SUSY field scheme is formulated
to discribe a non-equilibrium thermodynamic system with quenched disorder and
non-ergodicity effects. Thermodynamic and isothermal susceptibilities, memory
parameter and irreversible response are determined at different temperatures
and quenched disorder intensities.


title:
Neutron Scattering Studies of Spin Fluctuations in High Temperature
  Superconductors

abstract:
  Neutron scattering can provide detailed information about the energy and
momentum dependence of the magnetic dynamics of materials provided sufficiently
large single crystals are available. This requirement has limited the number of
rare earth high temperature superconducting materials that have been studied in
any detail. However, improvements in crystal growth in recent years has
resulted in considerable progress in our understanding of the behaviour of the
magnetism of the CuO planes in both the superconducting and normal state. This
review will focus primarily on the spin fluctuations in La_{2-x}Sr_{x}CuO_{4}
and YBa_{2}Cu_{3}O_{7-x} since these are the two systems for which the most
detailed results are available. Although gaps in our understanding remain,
there is now a consistent picture of on the spin fluctuation spectra in both
systems as well as the changes induced by the superconducting transition. For
both La_{2-x}Sr_{x}CuO_{4} and underdoped YBa_{2}Cu_{3}O_{7-x} the normal state
response is characterised by incommensurate magnetic fluctuations. The low
energy excitations are suppressed by the superconducting transition with a
corresponding enhancement in the response at higher energies. For
YBa_{2}Cu_{3}O_{7-x} the superconducting state is accompanied by the rapid
development of a commensurate resonant response whose energy varies with T_{c}.
In underdoped samples this resonance persists above T_{c}.


title:
Parafermion Statistics and Quasihole Excitations for the Generalizations
  of the Paired Quantum Hall States

abstract:
  We continue the program started in cond-mat/9809384 and explain the
statistics of the excitations for the generalizations of the paired states in
the quantum Hall effect in terms of the parafermion statistics. We show that
these excitations behave as combinations of bosons and parafermions. That
generalizes the prior treatment of the paired (Pfaffian) state where the
excitations behave as combinations of bosons and fermions. We explain what it
means, from a quantum mechanical point of view, for a particle to be a
`parafermion' rather than a boson or a fermion and work through several
explicit examples. The resulting multiplets coincide exactly with the angular
momentum multiplets found numerically for the $k+1$ particle interaction
Hamiltonian on a sphere. We also present a proof that the wave functions found
in cond-mat/9809384 are indeed the correlation functions of the parafermion
conformal field theory.


title:
Elementary excitations of magnetically ordered systems with orbital
  degeneracy

abstract:
  The Holstein-Primakoff transformation is generalized to develop a quantum
flavor wave theory for spin systems with orbital degeneracy. Elementary
excitations consist of spin, orbital, and spin-orbital waves. Spin and
spin-orbital waves couple each other due to orbital anisotropy and Hund's rule,
resulting in new modes observable by inelastic neutron scattering. In the SU(4)
limit, flavor waves are dispersionless along one or more directions, and give
rise to quantum fluctuations of reduced dimensionality.


title: Comment on "Is the nonlinear Meissner effect unobservable?"

abstract:
  In a recent Letter (Phys. Rev. Lett. 81, p.5640 (1998), cond-mat/9808249 v3),
it was suggested that nonlocal effects may prevent observation of the nonlinear
Meissner effect in YBCO. We argue that this claim is incorrect with regards to
measurements of the nonlinear transverse magnetic moment, and that the most
likely reason for a null result lies elsewhere.


title:
Discrete molecular dynamics studies of the folding of a protein-like
  model

abstract:
  Background: Many attempts have been made to resolve in time the folding of
model proteins in computer simulations. Different computational approaches have
emerged. Some of these approaches suffer from the insensitivity to the
geometrical properties of the proteins (lattice models), while others are
computationally heavy (traditional MD).
  Results: We use a recently-proposed approach of Zhou and Karplus to study the
folding of the protein model based on the discrete time molecular dynamics
algorithm. We show that this algorithm resolves with respect to time the
folding --- unfolding transition. In addition, we demonstrate the ability to
study the coreof the model protein.
  Conclusion: The algorithm along with the model of inter-residue interactions
can serve as a tool to study the thermodynamics and kinetics of protein models.


title:
Anomalous tunneling conductances of a spin singlet \nu=2/3 edge states:
  Interplay of Zeeman splitting and Long Range Coulomb Interaction

abstract:
  The point contact tunneling conductance between edges of the spin singlet
$\nu=2/3,\hat{K}=(3/3/0)$ quantum Hall states is studied both in the
quasiparticle tunneling picture and in the electron tunneling picture. Due to
the interplay of Zeeman splitting and the long range Coulomb interaction
between edges of opposite chirality novel spin excitations emerge, and their
effect is characterized by anomalous exponents of the charge and spin tunneling
conductances in various temperature ranges. Depending on the kinds of
scatterings at the point contact and the tunneling mechanism the anomalous
interaction in spin sector may enhance or suppress the tunneling conductances.
The effects of novel spin excitation are also relevant to the recent NMR
experiments on quantum Hall edges.


title:
A planar force-constant model for phonons in wurtzite GaN and AlN:
  Application to hexagonal GaN/AlN superlattices

abstract:
  A planar force-constant model is developed for longitudinal phonons of
wurtzite GaN and AlN propagating along the [0001] direction. The proposed model
is then applied to the study of the phonon modes in hexagonal GaN/AlN
superlattices in the longitudinal polarization. The confinement of the
superlattice phonon mode is discussed.


title: Strongly Correlated Electrons and Neutron Scattering

abstract:
  Various aspects of close and interesting relationship between
antiferromagnetism and singlet ground states are introduced for which neutron
scattering have been playing vital roles. The special emphasis is on the
disorder-induced antiferromagnetism in spin-Peierls systems, which can be
viewed as a nucleation process of classical magnetic order in the background of
singlet state whose origin is purely quantum-mechanical. It is then pointed out
that similar features will be found in Ce_xCu_2Si_2 and high T_c cuprates.
Finally the possible charge ordering process in NaV_2O_5 is discussed which
leads to the quenching of localized spins.


title:
Cell theory of the electroosmosis in the concentrated diaphragms
  consisting of spheric particles of colloid dispersity with account of the
  insoluble boundary layer influence

abstract:
  The electroosmotic transfer (ratio of velocity of liquid to electric current
density) and conductivity of disperse system were calculated as functions of
volume fraction of disperse particles. The considered model of electric double
layer (EDL) was generalized by taking into account the reducing dissolving
ability of liquid near the surface (non-dissolving boundary layer). The problem
was solved for limiting case of large degree of EDL overlapping in
interparticle space. The obtained results explain the main features of
experimental data.


title:
Incommensurability in the magnetic excitations of the
  bilinear-biquadratic spin-1 chain

abstract:
  We study the magnetic excitation spectrum of the S=1 quantum Heisenberg spin
chain with Hamiltonian : H = sum_i cos(theta) S_i S_i+1 + sin(theta) (S_i
S_i+1)^2. We focus on the range -pi/4 < theta < +pi/4 where the spin chain is
in the gapped Haldane phase. The excitation spectrum and static structure
factor is studied using direct Lanczos diagonalization of small systems and
density-matrix renormalization group techniques combined with the single-mode
approximation. The magnon dispersion has a minimum at q=pi until a critical
value theta_c = 0.38 is reached at which the curvature (velocity) vanishes.
Beyond this point, which is distinct from the VBS point and the Lifshitz point,
the minimum lies at an incommensurate value that goes smoothly to 2pi/3 when
theta approaches pi/4, the Lai-Sutherland point. The mode remains isolated from
the other states: there is no evidence of spinon deconfinement before the point
theta =+pi/4. These findings explain recent observation of the magnetization
curve M approx (H -H_c)^1/4 for theta =theta_c.


title: Quasiclassical Green's function approach to mesoscopic superconductivity

abstract:
  Recent experiments on mesoscopic normal-metal--superconductor
heterostructures resolve properties on length scales and at low temperatures
such that the temperature is below the Thouless energy $k_B T \le E_{Th}$. We
describe the properties of these systems within the framework of quasiclassical
many-body techniques. Diffusive and ballistic systems are covered, both in
equilibrium and nonequilibrium situations. Thereby we demonstrate the common
physical basis of various subtopics.


title: Spin Maser under Stationary Pumping

abstract:
  Spin dynamics of a polarized spin system is studied when the latter is
coupled with a resonant electric circuit and is under the action of an external
pumping supporting a stationary nonequilibrium magnetization. A complete
classification of possible regimes of spin motion is given. In addition to
seven regimes considered earlier, two other transient regimes are found and
thoroughly described: One is an oscillatory regime, when spins always move
coherently but the degree of coherence fluctuates with time. Another is a
pulsing regime, when spins reveal coherent motion during short pulses separated
from each other by intervals of incoherent motion. These regimes are, in
principle, transient, although may be extremely long lasting; their duration
may be several orders longer than the transverse relaxation time and twice
longer than the longitudinal relaxation time. Both transient regimes end with a
coherent quasistationary regime.


title: Jams, Waves, and Clusters

abstract:
  Have you been suffering from traffic jams lately and asking yourself why
freeways are no free ways anymore? After several great advances in traffic
theory, Korean physicists have now offered an interpretation of a recently
discovered state of congested traffic, called ``synchronized'' traffic. Their
fluid-dynamic simulations can be a useful tool for an optimization of traffic
flow on motorways.

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


title: Cellular Automata Simulating Experimental Properties of Traffic Flows

abstract:
  A model for 1D traffic flow is developed, which is discrete in space and
time. Like the cellular automaton model by Nagel and Schreckenberg [J. Phys. I
France 2, 2221 (1992)], it is simple, fast, and can describe stop-and-go
traffic. Due to its relation to the optimal velocity model by Bando et al.
[Phys. Rev. E 51, 1035 (1995)], its instability mechanism is of deterministic
nature. The model can be easily calibrated to empirical data and displays the
experimental features of traffic data recently reported by Kerner and Rehborn
[Phys. Rev. E 53, R1297 (1996)].


title: Magnetic Field Induced Phase Transitions in YBa2Cu4O8

abstract:
  The $c$-axis resistivity measurements in YBa_2Cu_4O_8 from Hussey et al. for
magnetic field orientations along the c-axis as well as within the ab-plane are
analyzed and interpreted using the scaling theory for static and dynamic
classical critical phenomena. We identify a superconductor to normal conductor
transition for both field orientations as well as a normal conductor to
insulator transition at a critical field H_c||a with dynamical critical
exponent z=1, leading to a multicritical point where superconducting, normal
conducting and insulating phases coexist.


title: Andreev Tunneling in Strongly Interacting Quantum Dots

abstract:
  We review recent work on resonant Andreev tunneling through a strongly
interacting quantum dot connected to a normal and to a superconducting lead. We
derive a general expression for the current flowing in the structure and
discuss the linear and non-linear transport in the nonperturbative regime. New
effects associated to the Kondo resonance combined with the two-particle
tunneling arise. The Kondo anomaly in the $I-V$ characteristics depends on the
relative size of the gap energy and the Kondo temperature.


title:
Weakly correlated electrons on a square lattice: a renormalization group
  theory

abstract:
  We formulate the exact Wilsonian renormalization group for a system of
interacting fermions on a lattice. The flow equations for all vertices of the
Wilson effective action are expressed in form of the Polchinski equation. We
apply this method to the Hubbard model on a square lattice using both zero- and
finite- temperature methods. Truncating the effective action at the sixth term
in fermionic variables we obtain the one-loop functional renormalization
equations for the effective interaction. We find the temperature of the
instability Tc^{RG} as function of doping. We calculate furthermore the
renormalization of the angle-resolved correlation functions for the
superconductivity (SC) and for the antiferromagnetism (AF). The dominant
component of the SC correlations is of the type d while the AF fluctuations are
of the type s Following the strength of both SC and AF fluctuation along the
instability line we obtain the phase diagram. The temperature Tc^{RG} can be
identified with the crossover temperature T{co} found in the underdoped regime
of the high-temperature superconductors, while in the overdoped regime Tc^{RG}
corresponds to the superconducting critical temperature.


title: Tilt Grain-Boundary Effects in S- and D-Wave Superconductors

abstract:
  We calculate the s- and d-wave superconductor order parameter in the vicinity
of a tilt grain boundary. We do this self-consistently within the Bogoliubov de
Gennes equations, using a realistic microscopic model of the grain boundary. We
present the first self-consistent calculations of supercurrent flows in such
boundaries, obtaining the current-phase characteristics of grain boundaries in
both s-wave and d-wave superconductors.


title: Positive magnetoresistance and orbital ordering in La(1-x)Sr(x)MnO(3)

abstract:
  We report on detailed transprort measurements of single crystalline
La(1-x)Sr(x)MnO(3) (x<=0.2). We have found a giantpositive magnetoresistance in
the compositions range between 0.1<=x<=0.125 and give an explaination in terms
of orbital ordering due to the interplay between superexchange interactions and
Jahn-Teller distortions.


title:
Generalized Fokker-Planck Equation For Multichannel Disordered Quantum
  Conductors

abstract:
  The Dorokhov-Mello-Pereyra-Kumar (DMPK) equation, which describes the
distribution of transmission eigenvalues of multichannel disordered conductors,
has been enormously successful in describing a variety of detailed transport
properties of mesoscopic wires. However, it is limited to the regime of quasi
one dimension only. We derive a one parameter generalization of the DMPK
equation, which should broaden the scope of the equation beyond the limit of
quasi one dimension.


title:
Quantum Dot Lattice Embedded in An Organic Medium: Hybrid Exciton State
  and Optical Response

abstract:
  We propose a new model to implement organic exciton-semiconductor exciton
hybridization by embedding a semiconductor quantum dot array into an organic
medium. A Wannier-Mott transfer exciton is formed when the exciton in each
semiconductor dot interacts via the multipole-multipole coupling with other
excitons in the different dtos of the array. A new hybrid exciton appears in
the system owing to strong dipole-dipole interaction of the Frenkel exciton of
the organic molecules with the Wannier Mott transfer exciton of the quantum dot
array. This hybrid exciton has both a large oscillator strength (Frenkel like)
and a large Bohr radius (Wannier like). At resonance between these two types of
excitons, the optical non-linearity is very large and can be controlled by
changing parameters of the system such as dot radius and dot spacing.


title:
Fermi surface of the colossal magnetoresistance perovskite
  La_{0.7}Sr_{0.3}MnO_{3}

abstract:
  Materials that exhibit colossal magnetoresistance (CMR) are currently the
focus of an intense research effort, driven by the technological applications
that their sensitivity lends them to. Using the angular correlation of photons
from electron-positron annihilation, we present a first glimpse of the Fermi
surface of a material that exhibits CMR, supported by ``virtual crystal''
electronic structure calculations. The Fermi surface is shown to be
sufficiently cubic in nature that it is likely to support nesting.


title: Variation of elastic scattering across a quantum well

abstract:
  The Drude scattering times of electrons in two subbands of a parabolic
quantum well have been studied at constant electron sheet density and different
positions of the electron distribution along the growth direction. The
scattering times obtained by magnetotransport measurements decrease as the
electrons are displaced towards the well edges, although the lowest-subband
density increases. By comparing the measurements with calculations of the
scattering times of a two-subband system, new information on the location of
the relevant scatterers and the anisotropy of intersubband scattering is
obtained. It is found that the scattering time of electrons in the lower
subband depends sensitively on the position of the scatterers, which also
explains the measured dependence of the scattering on the carrier density. The
measurements indicate segregation of scatterers from the substrate side towards
the quantum well during growth.


title:
The Mott-Hubbard transition and the paramagnetic insulating state in the
  two-dimensional Hubbard model

abstract:
  The Mott-Hubbard transition is studied in the context of the two-dimensional
Hubbard model. Analytical calculations show the existence of a critical value
Uc of the potential strength which separates a paramagnetic metallic phase from
a paramagnetic insulating phase. Calculations of the density of states and
double occupancy show that the ground state in the insulating phase contains
always a small fraction of empty and doubly occupied sites. The structure of
the ground state is studied by considering the probability amplitude of
intersite hopping. The results indicate that the ground state of the Mott
insulator is characterized by a local antiferromagnetic order; the electrons
keep some mobility, but this mobility must be compatible with the local
ordering. The vanishing of some intersite probability amplitudes at U=Uc puts a
constrain on the electron mobility. It is suggested that such quantities might
be taken as the quantities which control the order in the insulating phase.


title:
Spin wave analysis to the spatially-anisotropic Heisenberg
  antiferromagnet on triangular lattice

abstract:
  We study the phase diagram at T=0 of the antiferromagnetic Heisenberg model
on the triangular lattice with spatially-anisotropic interactions. For values
of the anisotropy very close to J_alpha/J_beta=0.50, conventional spin wave
theory predicts that quantum fluctuations melt the classical structures, for
S=1/2. For the regime J_beta<J_alpha, it is shown that the incommensurate
spiral phases survive until J_beta/J_alpha=0.27, leaving a wide region where
the ground state is disordered. The existence of such nonmagnetic states
suggests the possibility of spin liquid behavior for intermediate values of the
anisotropy.


title:
Edge helicons and repulsion of fundamental edge magnetoplasmons in the
  quantum Hall regime

abstract:
  A quasi-microscopic treatment of edge magnetoplasmons (EMP) is presented for
very low temperatures and confining potentials smooth on the scale of the
magnetic length $\ell_{0}$ but sufficiently steep at the edges such that Landau
level (LL) flattening can be discarded. The profile of the unperturbed electron
density is sharp and the dissipation taken into account comes only from
electron intra-edge and intra-LL transitions due to scattering by acoustic
phonons. For wide channels and filling factors $\nu =1$ and 2, there exist
independent EMP modes spatially symmetric and antisymmetric with respect to the
edge. Some of these modes, named edge helicons, can propagate nearly undamped
even when the dissipation is strong. Their density profile changes
qualitatively during propagation and is given by a rotation of a complex vector
function. For $\nu >2,$ the Coulomb coupling between the LLs leads to a
repulsion of the uncoupled fundamental LL modes: the new modes have very
different group velocities and are nearly undamped. The theory accounts well
for the experimentally observed plateau structure of the delay times as well as
for the EMP's period and decay rates.


title: Resonant Processes in a Frozen Gas

abstract:
  We present a theory of resonant processes in a frozen gas of atoms
interacting via dipole-dipole potentials that vary as $r^{-3}$, where $r$ is
the interatomic separation. We supply an exact result for a single atom in a
given state interacting resonantly with a random gas of atoms in a different
state. The time development of the transition process is calculated both on-
and off-resonance, and the linewidth with respect to detuning is obtained as a
function of time $t$. We introduce a random spin Hamiltonian to model a dense
system of resonators and show how it reduces to the previous model in the limit
of a sparse system. We derive approximate equations for the average effective
spin, and we use them to model the behavior seen in the experiments of Anderson
et al. and Lowell et al. The approach to equilibrium is found to be
proportional to $\exp (-\sqrt{\gamma_{eq}t}$), where the constant $\gamma
_{eq}$ is explicitly related to the system's parameters.


title: Glass Model, Hubbard Model and High-Temperature Superconductivity

abstract:
  In this paper we revisit the glass model describing the macroscopic behavior
of the High-Temperature superconductors. We link the glass model at the
microscopic level to the striped phase phenomenon, recently discussed widely.
The size of the striped phase domains is consistent with earlier predictions of
the glass model when it was introduced for High-Temperature Superconductivity
in 1987. In an additional step we use the Hubbard model to describe the
microscopic mechanism for d-wave pairing within these finite size stripes. We
discuss the implications for superconducting correlations of Hubbard model,
which are much higher for stripes than for squares, for finite size scaling,
and for the new view of the glass model picture.


title:
Nonequilibrium effects in the tunneling conductance spectra of small
  metallic particles

abstract:
  The tunneling spectra of small metallic grains shows an unusual structure of
the differential conductance peaks. Namely, resonance peaks appear in clusters,
or develop substructure as the the gate voltage is changed. These features are
manifestations of a nonequilibrium behavior which appears when the applied
source-drain voltage is sufficiently large. Electron-electron Coulomb
interaction as well as an attractive pairing interaction play an important role
in determining the magnitude and energy scales of the phenomena. For normal
grains each cluster of resonances is identified with one excited
single-electron state of the metal particle, shifted as a result of the
different nonequilibrium occupancy configurations of the other single-electron
states. For superconducting grains in the odd charging state, the substructure
of the resonance is explained by nonequilibrium ``gapless'' excitations
associated with different energy levels occupied by the unpaired electron. The
excitations are generated by inelastic cotunneling.


title: The Origin of the Designability of Protein Structures

abstract:
  We examined what determines the designability of 2-letter codes (H and P)
lattice proteins from three points of view. First, whether the native structure
is searched within all possible structures or within maximally compact
structures. Second, whether the structure of the used lattice is bipartite or
not. Third, the effect of the length of the chain, namely, the number of
monomers on the chain. We found that the bipartiteness of the lattice structure
is not a main factor which determines the designability. Our results suggest
that highly designable structures will be found when the length of the chain is
sufficiently long to make the hydrophobic core consisting of enough number of
monomers.


title: Bethe Ansatz solution of a new class of Hubbard-type models

abstract:
  We define one-dimensional particles with generalized exchange statistics. The
exact solution of a Hubbard-type Hamiltonian constructed with such particles is
achieved using the Coordinate Bethe Ansatz. The chosen deformation of the
statistics is equivalent to the presence of a magnetic field produced by the
particles themselves, which is present also in a ``free gas'' of these
particles.


title: Using path integrals to price interest rate derivatives

abstract:
  We present a new approach for the pricing of interest rate derivatives which
allows a direct computation of option premiums without deriving a
(Black-Scholes type) partial differential equation and without explicitly
solving the stochastic process for the underlying variable. The approach is
tested by rederiving the prices of a zero bond and a zero bond option for a
short rate environment which is governed by Vasicek dynamics. Furthermore, a
generalization of the method to general short rate models is outlined. In the
case, where analytical solutions are not accessible, numerical implementations
of the path integral method in terms of lattice calculations as well as path
integral Monte Carlo simulations are possible.


title: Possibility of long-range order in clean mesoscopic cylinders

abstract:
  A microscopic Hamiltonian of the magnetostatic interaction is discussed. This
long-range interaction can play an important role in mesoscopic systems leading
to an ordered ground state.
  The self-consistent mean field approximation of the magnetostatic interaction
is performed to give an effective Hamiltonian from which the spontaneous,
self-sustaining currents can be obtained.
  To go beyond the mean field approximation the mean square fluctuation of the
total momentum is calculated and its influence on self-sustaining currents in
mesoscopic cylinders with quasi-1D and quasi-2D conduction is considered. Then,
by the use of the microscopic Hamiltonian of the magnetostatic interaction for
a set of stacked rings, the problem of long-range order is discussed. The
temperature $T^{*}$ below which the system is in an ordered state is
determined.


title:
Two-fluid dynamics for a Bose-Einstein condensate out of local
  equilibrium with the non-condensate

abstract:
  We extend our recent work on the two-fluid hydrodynamics of a Bose-condensed
gas by including collisions involving both condensate and non-condensate atoms.
These collisions are essential for establishing a state of local thermodynamic
equilibrium between the condensate and non-condensate. Our theory is more
general than the usual Landau two-fluid theory, to which it reduces in the
appropriate limit, in that it allows one to describe situations in which a
state of complete local equilibrium between the two components has not been
reached. The exchange of atoms between the condensate and non-condensate is
associated with a new relaxational mode of the gas.


title:
A Variational Approach to the Localization Transition of Heteropolymers
  at Interfaces

abstract:
  A chain with random hydrophobic-hydrophilic charges is studied in the
presence of an interface separating a polar from a non polar solvent. Within a
Gaussian variational approach in replica space, a transition is found, from a
high temperature region, where the chain is delocalized, to a low temperature
region, where the chain is localized at the interface. The transition
temperature diverges as the neutrality of the chain is approached. Our results
are in agreement with an Imry-Ma type argument by Garel et al. The problem of
replica symmetry breaking is also addressed, and the replica-symmetric solution
is found to be unstable.


title:
Topological Features of the Magnetic Response in Inhomogeneous Magnetic
  Fields

abstract:
  We present topological features of the magnetic response (orbital and spin)
of a two-dimensional degenerate and non interacting electron gas due to
inhomogeneous applied magnetic fields. These issues are analysed from the point
of view of the Index theory with a special emphasis on the non perturbative
aspects of this response. The limiting case of a Aharonov-Bohm magnetic flux
line is studied in details and the results are extended to more general
situations.


title: Operator method in solving non-linear equations of the Hartree-Fock type

abstract:
  The operator method is used to construct the solutions of the problem of the
polaron in the strong coupling limit and of the helium atom on the basis of the
Hartree-Fock equation. $E_0=-0.1085128052\alpha^2$ is obtained for the polaron
ground-state energy. Energies for 2s- and 3s-states are also calculated. The
other excited states are briefly discussed.


title: Comment on "Evidence for the Droplet/Scaling Picture of Spin Glasses"

abstract:
  In a recent letter Moore et al. claim to exhibit evidence for a
non-mean-field behavior of the $3d$ Ising spin glass. We show that their claim
is insubstantial, and by analyzing in detail the behavior of the
Migdal-Kadanoff approximation (MKA) as compared to the behavior of the
Edwards-Anderson (EA) spin glass we find further evidence of a mean-field like
behavior of the $3d$ spin glass.


title: Angle resolved Cu and O photoemission intensities in CuO_2 planes

abstract:
  Using a mapping from the three-band extended Hubbard model for cuprate
superconductors into a generalized t-J model, and exact diagonalization of the
latter in a 4x4 cluster, we determine the quasiparticle weight for destruction
of a Cu or an O electrons with definite wave vector k. We also derive an
approximate but accurate analytical expression which relates the O intensity
with the quasiparticle weight in the generalized t-J model, and use it to
obtain the O intensity from results of the self-consistent Born approximation.
The k-dependence of Cu and O intensities is markedly different. In particular
the O intensity vanishes for k=(0,0). Our results are relevant for the
interpretation of angle-resolved photoemission experiments.


title:
Spinon signatures in the critical phase of the (1,1/2) ferrimagnet in a
  magnetic field

abstract:
  We propose an effective theory for the critical phase of a quantum
ferrimagnetic chain with alternating spins 1 and 1/2 in an external magnetic
field. With the help of the matrix product variational approach, the system is
mapped to a spin-1/2 XXZ chain in an (effective) magnetic field; as a
byproduct, we obtain an excellent description of the optical magnon branch in
the gapped phase. Recent finite-temperature DMRG results for the
low-temperature part of the specific heat are well described by the present
approach, and the ``pop-up'' peaks, developing near the critical field values
and in the middle of the critical phase, are identified with the contributions
from two different spinon bands of the effective spin-1/2 chain. The effect
should be as well observable in other spin-gap systems in an external field,
particularly in spin ladders.


title: How to calculate the degree of spin polarization in ferromagnets?

abstract:
  Different ways to define and to calculate the degree of spin polarization in
a ferromagnet are discussed, particularly with respect to spin-polarized
tunneling and Andreev reflection at the boundary between a superconductor and
ferromagnet. As an example, the degrees of spin polarization for different
experiments in Fe and Ni are calculated in the framework of the local spin
density approximation (LSDA) and used to illustrate the differences between
various definitions of spin polarization.


title: Comment on ``Critical temperature of trapped hard-sphere Bose gases''

abstract:
  In this comment, I discuss a recent path-integral Monte Carlo calculation by
Pearson, Pang, and Chen (Phys. Rev. A 58, 4796 (1998)). For bosons with a small
hard-core interaction in a harmonic trap, the authors find a critical
temperature which does not change with respect to the non-interacting gas. The
calculation suffers from a serious discretization error of the many-particle
density matrix.


title:
Spin-Liquid phase of the Multiple-Spin Exchange Hamiltonian on the
  Triangular Lattice

abstract:
  We performed an exact diagonalization study of the spin liquid phase of the
Multiple-Spin Exchange model on the triangular lattice. It is characterized by
no Neel Long Range Order (NLRO), short-ranged magnetic correlations and a spin
gap. We found no LRO in any local order parameter we investigated (chiral,
dimer,...). The probable asymptotic ground state degeneracy is discussed. We
argue that it could be of topological origin and that the system is probably
not a chiral spin liquid. A possible relation to the
Affleck-Kennedy-Lieb-Tasaki (AKLT) phase is discussed.


title:
Excitation spectrum and ground state properties of the S=1/2 Heisenberg
  ladder with staggered dimerization

abstract:
  We have studied the excitation spectrum of the $S=1/2$ quantum spin ladder
with staggered dimerization by dimer series expansions, diagrammatic analysis
of an effective interacting Bose gas of local triplets, and exact
diagonalization of small clusters. We find that the model has two massive
phases, with predominant inter-chain (rung) or intra-chain correlations. The
transition from the rung dimer into the intra-chain dimer phase is
characterized by softening of the triplet spectrum at $k=\pi$. The excitation
spectrum as well as the spin correlations away from and close to the critical
line are calculated. The location of the phase boundary is also determined.


title:
Comment on "Charged impurity scattering limited low temperature
  resistivity of low density silicon inversion layers" (Das Sarma and Hwang,
  cond-mat/9812216)

abstract:
  In a recent preprint cond-mat/9812216, Das Sarma and Hwang propose an
explanation of the sharp decrease in resistivity at low temperatures which has
been attributed to a transition to an unexpected conducting phase in dilute
high-mobility two-dimensional systems at B=0. In this Comment, we examine
whether their model is supported by the available experimental data.


title:
Numerical diagonalization analysis of the ground-state
  superfluid-localization transition in two dimensions

abstract:
  Ground state of the two-dimensional hard-core-boson system in the presence of
the quenched random chemical potential is investigated by means of the
exact-diagonalization method for the system sizes up to L=5. The criticality
and the DC conductivity at the superfluid-localization transition have been
controversial so far. We estimate, with the finite-size scaling analysis, the
correlation-length and the dynamical critical exponents as nu=2.3(0.6) and z=2,
respectively. The AC conductivity is computed with the Gagliano-Balseiro
formula, with which the resolvent (dynamical response function) is expressed in
terms of the continued-fraction form consisted of Lanczos tri-diagonal
elements. Thereby, we estimate the universal DC conductivity as
sigma_c(omega=0)=0.135(0.01) ((2e)^2/h).


title:
Effects of the CuO chains on the anisotropic penertration depth of
  YBa2Cu4O8

abstract:
  The temperatuer dependence of the magnetic penetration depth of grain aligned
YBa2Cu4O8 has been measured along the ab plane and c-axis. Both $\lambda_ab$
and $\lambda_c$ vary as \sqrt{T} up to 0.4Tc implying a square root density of
states at low energy. The results are discussed in terms of a proximity model
of alternating stacked superconducting and normal layers


title:
Non-Universal Power Law of the "Hall Scattering Rate" in a Single-Layer
  Cuprate Bi_{2}Sr_{2-x}La_{x}CuO_{6}

abstract:
  In-plane resistivity \rho_{ab}, Hall coefficient, and magnetoresistance (MR)
are measured in a series of high-quality Bi_{2}Sr_{2-x}La_{x}CuO_{6} crystals
with various carrier concentrations, from underdope to overdope. Our crystals
show the highest T_c (33 K) and the smallest residual resistivity ever reported
for Bi-2201 at optimum doping. It is found that the temperature dependence of
the Hall angle obeys a power law T^n with n systematically decreasing with
increasing doping, which questions the universality of the Fermi-liquid-like
T^2 dependence of the "Hall scattering rate". In particular, the Hall angle of
the optimally-doped sample changes as T^{1.7}, not as T^2, while \rho_{ab}
shows a good T-linear behavior. The systematics of the MR indicates an
increasing role of spin scattering in underdoped samples.


title:
Normal-state magnetotransport in La_{1.905}Ba_{0.095}CuO_{4} single
  crystals

abstract:
  The normal-state magnetotransport properties of La_{2-x}Ba_{x}CuO_{4} single
crystals with x=0.095 are measured; at this composition, a structural
transition to a low-temperature-tetragonal (LTT) phase occurs without
suppression of superconductivity. None of the measured properties (in-plane and
out-of-plane resistivity, magnetoresistance, and Hall coefficient) shows any
sudden change at the LTT phase transition, indicating that the occurrence of
the LTT phase does not necessarily cause an immediate change in the electronic
state such as the charge-stripe stabilization.


title:
Non-local conservation in the coupling field: effect on critical
  dynamics

abstract:
  We consider the critical dynamics of a system with an $n$-component
non-conserved order parameter coupled to a conserved field with long range
diffusion. An exponent $\sigma$ characterizes the long range transport,
$\sigma=2$ being the known locally conserved case. With renormalisation group
calculations done upto one loop order, several regions are found with different
values of the dynamic exponent $z$ in the $\sigma -n$ plane. For $n<4$, there
are three regimes, I: nonuniversal, $\sigma$ dependent $z$, II: universal with
$z$ depending on $n$ and III': conservation law irrelevant, $z$ being equal to
that in the nonconserved case. The known locally conserved case belongs to
regions I and II.