title: Kinetic induced phase transition

abstract:
  An Ising model with local Glauber dynamics is studied under the influence of
additional kinetic restrictions for the spin-flip rates depending on the
orientation of neighboring spins. Even when the static interaction between the
spins is completely eliminated and only an external field is taken into account
the system offers a phase transition at a finite value of the applied field.
The transition is realized due to a competition between the activation
processes driven by the field and the dynamical rules for the spin-flips. The
result is based on a master equation approach in a quantum formulation.


title: Triplet superconductivity in a one-dimensional ferromagnetic t-J model

abstract:
  In this paper we study the ground state phase diagram of a one-dimensional
$t-U-J$ model, at half-filling. In the large-bandwidth limit and for
ferromagnetic exchange with easy-plane anisotropy, a phase with gapless charge
and massive spin excitations, characterized by the coexistence of triplet
superconducting ($TS$) and spin density wave ($SDW^{z}$) instabilities is
realized in the ground state. With reduction of the bandwidth, a transition
into an insulating phase showing properties of the spin-1/2 XY model takes
place. In the case of weakly anisotropic antiferromagnetic exchange the system
shows a long range dimerized (Peierls) ordering in the ground state. The
complete weak-coupling phase diagram of the model, including effects of the
on-site Hubbard interaction, is obtained.


title: Dynamic correlations in doped 1D Kondo insulator: Finite-T DMRG study

abstract:
  The finite-T DMRG method is applied to the one-dimensional Kondo lattice
model to calculate dynamic correlation functions. Dynamic spin and charge
correlations, S_f(omega), S_c(omega), and N_c(omega), and quasiparticle density
of states rho(omega) are calculated in the paramagnetic metallic phase for
various temperatures and hole densities. Near half filling, it is shown that a
pseudogap grows in these dynamic correlation functions below the crossover
temperature characterized by the spin gap at half filling. A sharp peak at
omega=0 evolves at low temperatures in S_f(omega) and N_c(omega). This may be
an evidence of the formation of the collective excitations, and this confirms
that the metallic phase is a Tomonaga-Luttinger liquid in the low temperature
limit.


title: Exact Exponents of Supersonic Turbulence in Arbitrary Dimensions

abstract: This paper has been withdrawn

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


title: Cascading Parity-Check Error-Correcting Codes

abstract:
  A method for improving the performance of sparse-matrix based parity check
codes is proposed, based on insight gained from methods of statistical physics.
The advantages of the new approach are demonstrated on an existing
encoding/decoding paradigm suggested by Sourlas. We also discuss the
application of the same method to more advanced codes of a similar type.


title:
Full potential LAPW calculation of electron momentum density and related
  properties of Li

abstract:
  Electron momentum density and Compton profiles in Lithium along $<100 >$,
$<110>$, and $<111>$ directions are calculated using Full-Potential Linear
Augmented Plane Wave basis within generalized gradient approximation. The
profiles have been corrected for correlations with Lam-Platzman formulation
using self-consistent charge density. The first and second derivatives of
Compton profiles are studied to investigate the Fermi surface breaks. Decent
agreement is observed between recent experimental and our calculated values.
Our values for the derivatives are found to be in better agreement with
experiments than earlier theoretical results. Two-photon momentum density and
one- and two-dimensional angular correlation of positron annihilation radiation
are also calculated within the same formalism and including the
electron-positron enhancement factor.


title: Charge and spin dynamics of the Hubbard chains

abstract:
  We calculate the local correlation functions of charge and spin for the
one-chain and two-chain Hubbard model using the density matrix renormalization
group method and the recursion technique. Keeping only finite number of states
we get good accuracy for the low energy excitations. We study the charge and
spin gaps, bandwidths and weights of the spectra for various values of the
on-site Coulomb interaction U and the electron filling. In the low energy part,
the local correlation functions are different for the charge and spin. The
bandwidths are proportional to t for the charge and J for the spin,
respectively.


title: Minimal Variance Hedging of Options with Student-t Underlying

abstract:
  I explicitly work out closed form solutions for the optimal hedging
strategies (in the sense of Bouchaud and Sornette) in the case of European call
options, where the underlying is modeled by (unbiased) iid additive returns
with Student-t distributions. The results may serve as illustrative examples
for option pricing in the presence of fat tails.


title: Weighted Fixed Points in Self-Similar Analysis of Time Series

abstract:
  The self-similar analysis of time series is generalized by introducing the
notion of scenario probabilities. This makes it possible to give a complete
statistical description for the forecast spectrum by defining the average
forecast as a weighted fixed point and by calculating the corresponding a
priori standard deviation and variance coefficient. Several examples of
stock-market time series illustrate the method.


title: Entropy and Time

abstract:
  The emergence of a direction of time in statistical mechanics from an
underlying time-reversal-invariant dynamics is explained by examining a simple
model. The manner in which time-reversal symmetry is preserved and the role of
initial conditions are emphasized. An extension of the model to finite
temperatures is also discussed.


title: Finite size effects and error-free communication in Gaussian channels

abstract:
  The efficacy of a specially constructed Gallager-type error-correcting code
to communication in a Gaussian channel is being examined. The construction is
based on the introduction of complex matrices, used in both encoding and
decoding, which comprise sub-matrices of cascading connection values. The
finite size effects are estimated for comparing the results to the bounds set
by Shannon. The critical noise level achieved for certain code-rates and
infinitely large systems nearly saturates the bounds set by Shannon even when
the connectivity used is low.


title:
Comment on ``Quantum Suppression of Shot Noise in Atom-Size Metallic
  Contacts''

abstract:
  In a recent letter (Phys.Rev.Lett. 82, 1526 (1999)), van den Brom and van
Ruitenbeek found a pronounced suppression of the shot noise in atom-size gold
contacts with conductances near integer multiples of $G_0=2e^2/h$, revealing
unambiguously the quantized nature of the electronic transport. However, the ad
hoc model they introduced to describe the contribution of partially-open
conductance channels to the shot noise is unable to fit either the maxima or
minima of their shot noise data. Here we point out that a model of
quantum-confined electrons with disorder quantitatively reproduces their
measurements.


title: Theory of interlayer exchange interactions in magnetic multilayers

abstract:
  This paper presents a review of the phenomenon of interlayer exchange
coupling in magnetic multilayers. The emphasis is put on a pedagogical
presentation of the mechanism of the phenomenon, which has been successfully
explained in terms of a spin-dependent quantum confinement effect. The
theoretical predictions are discussed in connection with corresponding
experimental investigations.


title:
Observation of the Lambda Point in the 4He-Vycor System: A Test of
  Hyperuniversality

abstract:
  We have performed a high resolution specific heat measurement on 4He
completely filling the pores of Vycor glass. Within 10mK of the superfluid
transition we have found a peak in the heat capacity which is only 0.02% the
size of the background. The peak can be fit with a rounded version of the
``logarithmic singularity'' observed in bulk 4He. Along with the previously
observed ``2/3'' power law dependence of the superfluid density on temperature,
this strongly suggests that the disorder imposed by the Vycor is irrelevant to
the 3DXY superfluid phase transition. The critical amplitude of the peak, while
in agreement with that found in other experiments in dilute superfluid 4He, is
considerably larger than that predicted by the theory of hyperuniversality.


title: The nature of attraction between like charged rods

abstract:
  Comment on the paper of Ha and Liu (Phys. Rev. Lett. {\bf 79}, 1289 (1997))
regarding the nature of attraction between like charged rods. We demostrate
that their results do not produce the correct low temperature limit.


title:
Identity of the universal repulsive-core singularity with Yang-Lee edge
  criticality

abstract:
  Lattice and continuum fluid models with repulsive-core interactions typically
display a dominant, critical-type singularity on the real, negative activity
axis. Lai and Fisher recently suggested, mainly on numerical grounds, that this
repulsive-core singularity is universal and in the same class as the Yang-Lee
edge singularities, which arise above criticality at complex activities with
positive real part. A general analytic demonstration of this identification is
presented here using a field-theory approach with separate representation of
the repulsive and attractive parts of the pair interactions.


title:
Cooperative Jahn-Teller Effect on the Magnetic Structure of Manganese
  Oxides

abstract:
  The magnetic structure of LaMnO$_3$ is investigated on three-dimensional
clusters of MnO$_6$ octahedra by using a combination of relaxation and Monte
Carlo techniques. It is found that the cooperative Jahn-Teller phonons lead to
the stabilization of A-type antiferromagnetic and C-type orbital structures in
the physically relevant region of parameter space for LaMnO$_3$ with small
corrections due to tilting effects. The results suggest that strong Coulomb
interactions are not necessary for a qualitative description of undoped
manganites. In fact, it is shown that the present result is not essentially
changed even if the Coulomb interaction is explicitly included.


title:
The Three-Magnon Contribution to the Spin Correlation Function in
  Integer-Spin Antiferromagnetic Chains

abstract:
  The exact form factor for the O(3) non-linear sigma model is used to predict
the three-magnon contribution to the spin correlation function, S(q,w), near
wavevector q=pi in an integer spin, one-dimensional antiferromagnet. The
three-magnon contribution is extrememly broad and extremely weak; the
integrated intensity is <2% of the single-magnon contribution.


title: Origin of Native Driving Force in Protein Folding

abstract:
  We derive an expression with four adjustable parameters that reproduces well
the 20x20 Miyazawa-Jernigan potential matrix extracted from known protein
structures. The numerical values of the parameters can be approximately
computed from the surface tension of water, water-screened dipole interactions
between residues and water and among residues, and average exposures of
residues in folded proteins.


title:
First-order scaling near a second-order phase transition: Tricritical
  polymer collapse

abstract:
  The coil-globule transition of an isolated polymer has been well established
to be a second-order phase transition described by a standard tricritical O(0)
field theory. We provide compelling evidence from Monte Carlo simulations in
four dimensions, where mean-field theory should apply, that the approach to
this (tri)critical point is dominated by the build-up of first-order-like
singularities masking the second-order nature of the coil-globule transition:
the distribution of the internal energy having two clear peaks that become more
distinct and sharp as the tricritical point is approached. However, the
distance between the peaks slowly decays to zero. The evidence shows that the
position of this (pseudo) first-order transition is shifted by an amount from
the tricritical point that is asymptotically much larger than the width of the
transition region. We suggest an explanation for the apparently contradictory
scaling predictions in the literature.


title:
Four-dimensional polymer collapse II: Pseudo-First-Order Transition in
  Interacting Self-avoiding Walks

abstract:
  In earlier work we provided the first evidence that the collapse, or
coil-globule, transition of an isolated polymer in solution can be seen in a
four-dimensional model. Here we investigate, via Monte Carlo simulations, the
canonical lattice model of polymer collapse, namely interacting self-avoiding
walks, to show that it not only has a distinct collapse transition at finite
temperature but that for any finite polymer length this collapse has many
characteristics of a rounded first-order phase transition. However, we also
show that there exists a `$\theta$-point' where the polymer behaves in a simple
Gaussian manner (which is a critical state), to which these finite-size
transition temperatures approach as the polymer length is increased. The
resolution of these seemingly incompatible conclusions involves the argument
that the first-order-like rounded transition is scaled away in the
thermodynamic limit to leave a mean-field second-order transition. Essentially
this happens because the finite-size \emph{shift} of the transition is
asymptotically much larger than the \emph{width} of the pseudo-transition and
the latent heat decays to zero (algebraically) with polymer length. This
scenario can be inferred from the application of the theory of Lifshitz,
Grosberg and Khokhlov (based upon the framework of Lifshitz) to four
dimensions: the conclusions of which were written down some time ago by
Khokhlov. In fact it is precisely above the upper critical dimension, which is
3 for this problem, that the theory of Lifshitz may be quantitatively
applicable to polymer collapse.


title: Freezing of anisotropic spin clusters in La1.98Sr0.02CuO4

abstract:
  A spin-glass compound, La$_{1.98}$Sr$_{0.02}$CuO$_4$, shows
quasi-three-dimensional magnetic ordering below $\sim$40 K. A remarkable
feature is that the magnetic correlation length along the orthorhombic
$a\rm_{ortho}$ axis is much longer than that along the $b\rm_{ortho}$ axis,
suggesting that the spin structure is closely related to the diagonal stripe
structure. The spin-glass state can be expressed as a random freezing of
quasi-three-dimensional spin clusters with anisotropic spin correlations
($\xi'_a\sim160$ \AA, $\xi'_b\sim25$ \AA, and $\xi'_c\sim4.7$ \AA at 1.6 K).
The new magnetic state is important as an intermediate phase between the
three-dimensional antiferromagnetic ordered phase in La$_2$CuO$_4$ and the
incommensurate phase in La$_{1.95}$Sr$_{0.05}$CuO$_4$ in which the positions of
the incommensurate peaks are rotated by 45$^\circ$ in reciprocal space about
($\pi$,$\pi$) from those observed in the superconducting La$_2$CuO$_4$
compounds.


title: Intrinsic localized modes in the charge-transfer solid PtCl

abstract:
  We report a theoretical analysis of intrinsic localized modes in a
quasi-one-dimensional charge-transfer-solid $[Pt(en)_2][Pt(en)_2
Cl_2](ClO_4)_4$(PtCl). We discuss strongly nonlinear features of resonant Raman
overtone scattering measurements on PtCl, arising from quantum intrinsic
localized (multiphonon) modes (ILMs) and ILM-plus-phonon states. We show, that
Raman scattering data displays clear signs of a non-thermalization of lattice
degrees-of-freedom, manifested in a nonequilibrium density of intrinsic
localized modes.


title:
Comparison of the $\pi$ and $\Delta$ operator approximations for the
  two-dimensional t-J model

abstract:
  We compare the spectra of the new $\pi$ operator of the SO(5) theory and the
conventional $\Delta$ operator for the two-dimensional t-J model. We also
calculate the weight transferred to the two-hole ground state from half-filling
by these operators. We find that the spectra of these operators are quite
similar and the weight for the $\pi$ operator is smaller than the weight for
the $\Delta$ operator. We argue that the two-dimensional t-J model does not
have a good approximate SO(5) symmetry claimed in Ref. 1.


title: Quasi-equilibrium interpretation of aging dynamics

abstract:
  We develop an interpretation of the off-equilibrium dynamical solution of
mean-field glassy models in terms of quasi-equilibrium concepts. We show that
the relaxation of the "thermoremanent magnetization" follows a generalized
version of the Onsager regression postulate of induced fluctuations. We then
find the rationale for the equality between the fluctuation-dissipation ratio
and the rate of growth of the configurational entropy close to the asymptotic
state, found empirically in mean-field solutions.


title:
Quantum diffusion at high interstitial concentrations: Application to
  surface diffusion

abstract:
  In this work we first study the quantum diffusion in a volume of a
crystalline solid at high interstitial concentrations when the effects of the
short-range interactions between the diffusing particles are to be factors.
Within the scope of the small-polaron formalism the transition rate depending
on both the temperature and the interstitial concentration has been calculated.
Then, using the obtained result, we consider the model of surface diffusion
that reproduces qualitatively the diffusion behaviour of the hydrogen isotopes
on W(110) surface, observed in the lowtemperature experiments [S.C. Wang and R.
Gomer, J.Chem.Phys. v.83, 4193 (1985)]. The coverage dependence of the
diffusion coefficient is determined by Monte Carlo simulation. The model allows
to suppose that substantially the different diffusion of hydrogen (tritium) and
deuterium can be the outcome of their different quantum statistic and the
direct interactions between the adparticles.


title:
Ground state of an $S=1/2$ distorted diamond chain - model of $\rm Cu_3
  Cl_6 (H_2 O)_2 \cdot 2H_8 C_4 SO_2$

abstract:
  We study the ground state of the model Hamiltonian of the trimerized $S=1/2$
quantum Heisenberg chain $\rm Cu_3 Cl_6 (H_2 O)_2 \cdot 2H_8 C_4 SO_2$ in which
the non-magnetic ground state is observed recently. This model consists of
stacked trimers and has three kinds of coupling constants between spins; the
intra-trimer coupling constant $J_1$ and the inter-trimer coupling constants
$J_2$ and $J_3$. All of these constants are assumed to be antiferromagnetic. By
use of the analytical method and physical considerations, we show that there
are three phases on the $\tilde J_2 - \tilde J_3$ plane ($\tilde J_2 \equiv
J_2/J_1$, $\tilde J_3 \equiv J_3/J_1$), the dimer phase, the spin fluid phase
and the ferrimagnetic phase. The dimer phase is caused by the frustration
effect. In the dimer phase, there exists the excitation gap between the
two-fold degenerate ground state and the first excited state, which explains
the non-magnetic ground state observed in $\rm Cu_3 Cl_6 (H_2 O)_2 \cdot 2H_8
C_4 SO_2$. We also obtain the phase diagram on the $ \tilde J_2 - \tilde J_3$
plane from the numerical diagonalization data for finite systems by use of the
Lanczos algorithm.


title: Geometrical description of vortices in Ginzburg-Landau billiards

abstract:
  In these notes we discuss the topological nature of some problems in
condensed matter physics.
  We adopt the language of differential geometry to present this subject and
our aim is to develop some intuition towards concepts like curvature, fiber
bundles, connexions, characteristic classes and topological invariants. The
lectures end with a theoretical analysis of recent experimental results
obtained on mesoscopic superconducting films.


title:
Tight-binding Hamiltonians for Sr filled ruthenates: application to the
  gap anisotropy and Hall coefficient in Sr2RuO4

abstract:
  Accurate orthogonal tight-binding Hamiltonians are constructed for
ferromagnetic SrRuO$_3$ and the layered perovskite superconductor,
Sr$_2$RuO$_4$ by fitting to all-electron full-potential local density band
structures obtained by the linearized augmented planewave method. These
Hamiltonians allow the band structure to be computed on very fine meshes in the
Brillouin zone at low cost, and additionally have analytic band velocities,
while retaining the accuracy of the full-potential electronic structure
calculations. This greatly facilitates calculation of transport and
superconducting parameters related to the fermiology. These features are
exploited to calculate the Hall coefficient and vortex lattice geometry for
Sr$_2$RuO$_4$ with fine integration meshes. We find the lower limit for the
interband order parameter anisotropy to be compatible with the observed square
geometry. We also find that the sign reversal of the Hall coefficient can be
explained in a conventional way if the bands are shifted by a few mRy so as to
match the experimental de Haas-van Alphen areas exactly, {\it and} the
temperature dependence of the relaxation time is strongly dependent on the band
character.


title: A Ball in a Groove

abstract:
  We study the static equilibrium of an elastic sphere held in a rigid groove
by gravity and frictional contacts, as determined by contact mechanics. As a
function of the opening angle of the groove and the tilt of the groove with
respect to the vertical, we identify two regimes of static equilibrium for the
ball. In the first of these, at large opening angle or low tilt, the ball rolls
at both contacts as it is loaded. This is an analog of the "elastic" regime in
the mechanics of granular media. At smaller opening angles or larger tilts, the
ball rolls at one contact and slides at the other as it is loaded, analogously
with the "plastic" regime in the mechanics of granular media. In the elastic
regime, the stress indeterminacy is resolved by the underlying kinetics of the
ball response to loading.


title: Quantum ergodicity for electrons in two dimensions

abstract:
  We study the effect of electron-electron interaction on a two dimensional
(2D) disordered lattice. For the case of two electrons the analytical estimates
are presented showing a transition from localized to delocalized states in a
way similar to the Anderson transition in 3D. The localized phase corresponds
to large values of the parameter r_s, which is determined by the ratio of the
Coulomb and Fermi energies. The numerical investigations of the spectral
statistics P(s) in a system with up to N_p=30 spin polarized electrons show the
transition from the Poisson to the Wigner-Dyson distribution at a total
electron energy independent of N_p. The relation to experiments on the
metal-insulator transition in 2D is also discussed.


title: Exactly solvable analogy of small-world networks

abstract:
  We present an exact description of a crossover between two different regimes
of simple analogies of small-world networks. Each of the sites chosen with a
probability $p$ from $n$ sites of an ordered system defined on a circle is
connected to all other sites selected in such a way. Every link is of a unit
length. Thus, while $p$ changes from 0 to 1, an averaged shortest distance
between a pair of sites changes from $\bar{\ell} \sim n$ to $\bar{\ell} = 1$.
We find the distribution of the shortest distances $P(\ell)$ and obtain a
scaling form of $\bar{\ell}(p,n)$. In spite of the simplicity of the models
under consideration, the results appear to be surprisingly close to those
obtained numerically for usual small-world networks.


title:
Surface Critical Behavior in Systems with Non-Equilibrium Phase
  Transitions

abstract:
  We study the surface critical behavior of branching-annihilating random walks
with an even number of offspring (BARW) and directed percolation (DP) using a
variety of theoretical techniques. Above the upper critical dimensions d_c,
with d_c=4 (DP) and d_c=2 (BARW), we use mean field theory to analyze the
surface phase diagrams using the standard classification into ordinary,
special, surface, and extraordinary transitions. For the case of BARW, at or
below the upper critical dimension, we use field theoretic methods to study the
effects of fluctuations. As in the bulk, the field theory suffers from
technical difficulties associated with the presence of a second critical
dimension. However, we are still able to analyze the phase diagrams for BARW in
d=1,2, which turn out to be very different from their mean field analog.
Furthermore, for the case of BARW only (and not for DP), we find two
independent surface beta_1 exponents in d=1, arising from two distinct
definitions of the order parameter. Using an exact duality transformation on a
lattice BARW model in d=1, we uncover a relationship between these two surface
beta_1 exponents at the ordinary and special transitions. Many of our
predictions are supported using Monte-Carlo simulations of two different models
belonging to the BARW universality class.


title: Stripe formation in electron-doped cuprates

abstract:
  We investigate the formation of charge domain walls in an electron-doped
extended Hubbard model for the superconducting cuprates. Within an unrestricted
Hartree-Fock approach, extended by the introduction of slave-bosons to obtain a
more proper treatment of strong correlations, we demonstrate the occurrence of
stripes in the (1,1) and (1,-1) directions having one doped electron per stripe
site. The different filling, direction and width of these electron-doped
stripes with respect to those obtained in the hole-doped systems have
interesting observable consequences, which are discussed.


title: s-wave superconductivity in the presence of local Coulomb correlations

abstract:
  We derive the superconductiong mean-field equations for an attractive
interaction, V, in the s-wave channel when local Coulomb interactions are taken
into account for any value of U. Our results show that the Coulomb repulsion is
detrimental to the critical temperature, $T_c$, and the order parameter,
$\Delta(T)$, for values of $U \geq |V|$. Furthermore, our results depend on
band filling in a sensible way. In the presence of local correlations,
$2\Delta(0)/T_c$ differs from the BCS ratio, since Coulomb interactions affect
much more the superconducting critical temperature, $T_c$, than the
superconducting order parameter, $\Delta(T)$. We conclude that the presence of
Coulomb interactions play an additional role in the analysis of experimental
data, specially in narrow band systems.


title:
Partitioning of a Polymer Chain between Two Confining Cavities: The
  Roles of Excluded Volume and Finite Size Conduits

abstract:
  Lattice-field calculations are performed on a Gaussian polymer chain confined
to move within the region defined by two fused spheres. The results of the
calculations are in accord with recent experimental measurements and computer
simulations, and suggest that current theoretical understanding of polymer
partitioning phenomena is not adequate when excluded volume interactions
between the monomers are present. It is also shown that the notion of ground
state dominance can fail even in the large monomer limit.


title:
Ordering in the dilute weakly-anisotropic antiferromagnet
  Mn(0.35)Zn(0.65)F2

abstract:
  The highly diluted antiferromagnet Mn(0.35)Zn(0.65)F2 has been investigated
by neutron scattering in zero field. The Bragg peaks observed below the Neel
temperature TN (approximately 10.9 K) indicate stable antiferromagnetic
long-range ordering at low temperature. The critical behavior is governed by
random-exchange Ising model critical exponents (nu approximately 0.69 and gamma
approximately 1.31), as reported for Mn(x)Zn(1-x)F2 with higher x and for the
isostructural compound Fe(x)Zn(1-x)F2. However, in addition to the Bragg peaks,
unusual scattering behavior appears for |q|>0 below a glassy temperature Tg
approximately 7.0 K. The glassy region T<Tg corresponds to that of noticeable
frequency dependence in earlier zero-field ac susceptibility measurements on
this sample. These results indicate that long-range order coexists with
short-range nonequilibrium clusters in this highly diluted magnet.


title: Spin Injection in a Ballistic Two-Dimensional Electron Gas

abstract:
  We explore electrically injected, spin polarized transport in a ballistic
two-dimensional electron gas. We augment the Buettiker-Landauer picture with a
simple, but realistic model for spin-selective contacts to describe multimode
reservoir-to-reservoir transport of ballistic spin 1/2 particles. Clear and
unambiguous signatures of spin transport are established in this regime, for
the simplest measurement configuration that demonstrates them directly. These
new effects originate from spin precession of ballistic carriers; they exhibit
strong dependence upon device geometry and vanish in the diffusive limit. Our
results have important implications for prospective ``spin transistor''
devices.


title: Optical Generation of Vortices in trapped Bose-Einstein Condensates

abstract:
  We demonstrate numerically the efficient generation of vortices in
Bose-Einstein condensates (BEC) by using a ``phase imprinting'' method. The
method consist of passing a far off resonant laser pulse through an absorption
plate with azimuthally dependent absorption coefficient, imaging the laser beam
onto a BEC, and thus creating the corresponding non-dissipative Stark shift
potential and condensate phase shift. In our calculations we take into account
experimental imperfections. We also propose an interference method to detect
vortices by coherently pushing part of the condensate using optically induced
Bragg scattering.


title: Conductance distribution of disordered quasi one-dimensional wires

abstract:
  We determine analytically the distribution of conductances of quasi
one-dimensional disordered electron systems, neglecting electron-electron
interaction, for all strengths of disorder. We find that in the crossover
region between the metallic and insulating regimes, P(g) is highly asymmetric,
given by ``one-sided'' log-normal distribution. For larger disorder, the tail
of the log-normal distribution is cut-off for g > 1 by a Gaussian.


title:
Charge Frustration Effects in Capacitively Coupled Two-Dimensional
  Josephson-Junction Arrays

abstract:
  We investigate the quantum phase transitions in two capacitively coupled
two-dimensional Josephson-junction arrays with charge frustration. The system
is mapped onto the S=1 and $S=1/2$ anisotropic Heisenberg antiferromagnets near
the particle-hole symmetry line and near the maximal-frustration line,
respectively, which are in turn argued to be effectively described by a single
quantum phase model. Based on the resulting model, it is suggested that near
the maximal frustration line the system may undergo a quantum phase transition
from the charge-density wave to the super-solid phase, which displays both
diagonal and off- diagonal long-range order.


title: Computer investigation of the energy landscape of amorphous silica

abstract:
  The multidimensional topography of the collective potential energy function
of a so-called strong glass former (silica) is analyzed by means of classical
molecular dynamics calculations. Features qualitatively similar to those of
fragile glasses are recovered at high temperatures : in particular an intrinsic
characteristic temperature $T_c\simeq 3500$K is evidenced above which the
system starts to investigate non-harmonic potential energy basins. It is shown
that the anharmonicities are essentially characterized by a roughness appearing
in the potential energy valleys explored by the system for temperatures above
$T_c$.


title: Explosive crystallization mechanism of ultradisperse amorphous films

abstract:
  The explosive crystallization of germanium ultradisperse amorphous films is
studied experimentally. We show that crystallization may be initiated by local
heating at the small film thickness but it realizes spontaneously at the large
ones. The fractal pattern of the crystallized phase is discovered that is
inherent in the phenomena of diffusion limited aggregation. It is shown that in
contrast to the ordinary crystallization mode the explosive one is connected
with the instability which is caused by the self-heating. A transition from the
first mechanism to the second one is modelled by Lorenz system. The process of
explosive crystallization is represented on the basis of the self-organized
criticality conception. The front movement is described as the effective
diffusion in the ultrametric space of hierarchically subordinated avalanches,
corresponding to the explosive crystallization of elementary volumes of
ultradisperse powder. The expressions for the stationary crystallization heat
distribution and the steady-state heat current are obtained. The heat needed
for initiation of the explosive crystallization is obtained as a function of
the thermometric conductivity. The time dependence of the spontaneous
crystallization probability in a thin films is examined.


title: Motion of Quantized Vortices as Elementary Objects

abstract:
  The general local, nondissipative equations of motion for a quantized vortex
moving in an uncharged laboratory superfluid are derived from a relativistic,
co-ordinate invariant framework, having vortices as its elementary objects in
the form of stable topological excitations. This derivation is carried out for
a pure superfluid with isotropic gap at the absolute zero of temperature, on
the level of a hydrodynamic, collective co-ordinate description. In the
formalism, we use as fundamental ingredients that particle number as well as
vorticity are conserved, and that the fluid is perfect. No assumptions are
involved as regards the dynamical behaviour of the order parameter. The
interaction of the vortex with the background fluid, representing the Magnus
force, and with itself via phonons, giving rise to the hydrodynamic vortex
mass, are separated. For a description of the motion of the vortex in a dense
laboratory superfluid like helium II, two limits have to be considered: The
nonrelativistic limit for the superfluid background is taken, and the motion of
the vortex is restricted to velocities much less than the speed of sound. The
canonical structure of vortex motion in terms of the collective co-ordinate is
used for the quantization of this motion.


title: Non-extensive thermodynamics and stationary processes of localization

abstract:
  We focus our attention on dynamical processes characterized by an entropic
index Q<1. According to the probabilistic arguments of Tsallis and Bukman
[C.Tsallis, D.J. Bukman, Phys. Rev. E 54,R2197 (1996)] these processes are
subdiffusional in nature. The non-extensive generalization of the
Kolmogorov-Sinai entropy yielding the same entropic index implies the
stationary condition. We note, on the other hand, that enforcing the stationary
property on subdiffusion has the effect of producing a localization process
occurring within a finite time scale. We thus conclude that the stationary
dynamic processes with Q<1 must undergo a localization process occurring at
finite time. We check the validity of this conclusion by means of a numerical
treatment of the dynamics of the logistic map at the critical point.


title: Magnetotunneling as a Probe of Luttinger-Liquid Behavior

abstract:
  A novel method for detecting Luttinger-liquid behavior is proposed. The idea
is to measure the tunneling conductance between a quantum wire and a parallel
two-dimensional electron system as a function of both the potential difference
between them, $V$, and an in-plane magnetic field, $B$. We show that the
two-parameter dependence on $B$ and $V$ allows for a determination of the
characteristic dependence on wave vector $q$ and frequency $\omega$ of the {\it
spectral function}, $A_{\rm LL}(q,\omega)$, of the quantum wire. In particular,
the separation of spin and charge in the Luttinger liquid should manifest
itself as singularities in the $I$-$V$-characteristic. The experimental
feasibility of the proposal is discussed.


title: Supercurrent Quantization in Narrow Channel SNS Junctions

abstract:
  We determine the quasi-particle excitation spectrum in the normal region of a
narrow ballistic superconductor--normal-metal--superconductor (SNS) Josephson
contact. Increasing the effective chemical potential in the contact converts
the electronic levels into Andreev-levels carrying supercurrent. The opening of
these superchannels leads to a supercurrent quantization which exhibits a
non-universal behavior in general and we discuss its dependence on the junction
parameters.


title:
Fluctuation broadening of plasma resonance line in the vortex liquid
  state of layered superconductors

abstract:
  The Josephson plasma resonance (JPR) provides a sensitive probe of vortex
states in layered superconductors. We demonstrate that in the case of weak
damping in the liquid phase, broadening of the JPR line is caused mainly by
random Josephson coupling arising from the density fluctuations of pancake
vortices. In this case the JPR line has the universal shape, which is
determined only by parameters of the superconductors and temperature. This
mechanism gives a natural explanation for the experimentally observed
asymmetric lineshape. The tail at high frequencies arises due to mixing of the
propagating plasma modes by random Josephson coupling, while the tail at small
frequencies is caused by the localized plasma modes originating from a rare
fluctuation suppression of the Josephson coupling in large areas.


title:
Plasma resonance at low magnetic fields as a probe of vortex line
  meandering in layered superconductors

abstract:
  We consider the magnetic field dependence of the plasma resonance frequency
in pristine and in irradiated Bi$_2$Sr$_2$CaCu$_2$O$_8$ crystals near $T_c$. At
low magnetic fields we relate linear in field corrections to the plasma
frequency to the average distance between the pancake vortices in the
neighboring layers (wandering length). We calculate the wandering length in the
case of thermal wiggling of vortex lines, taking into account both Josephson
and magnetic interlayer coupling of pancakes. Analyzing experimental data, we
found that (i) the wandering length becomes comparable with the London
penetration depth near T$_{c}$ and (ii) at small melting fields ($< 20$ G) the
wandering length does not change much at the melting transition. This shows
existence of the line liquid phase in this field range. We also found that
pinning by columnar defects affects weakly the field dependence of the plasma
resonance frequency near $T_c$.


title: Towards the Thermodynamics of Localization Processes

abstract:
  We study the entropy time evolution of a quantum mechanical model, which is
frequently used as a prototype for Anderson's localization. Recently Latora and
Baranger [V. Latora, M. Baranger, Phys. Rev.Lett. 82, 520(1999)] found that
there exist three entropy regimes, a transient regime of passage from dynamics
to thermodynamics, a linear in time regime of entropy increase, namely a
thermodynamic regime of Kolmogorov kind, and a saturation regime. We use the
non-extensive entropic indicator recently advocated by Tsallis [ C. Tsallis, J.
Stat. Phys. 52, 479 (1988)] with a mobile entropic index q, and we find that
with the adoption of the ``magic'' value q = Q = 1/2 the Kolmogorov regime
becomes more extended and more distinct than with the traditional entropic
index q = 1. We adopt a two-site model to explain these properties by means of
an analytical treatment and we argue that Q =1/2 might be a typical signature
of the occurrence of Anderson's localization.


title: The Levy diffusion as an effect of sporadic randomness

abstract:
  The Levy diffusion processes are a form of non ordinary statistical mechanics
resting, however, on the conventional
  Markov property. As a consequence of this, their dynamic derivation is
possible provided that (i) a source of randomness is present in the
corresponding microscopic dynamics and (ii) that the consequent process of
memory erasure is properly taken into account by the theoretical treatment.


title: Complexity and line of critical points in a short-range spin-glass model

abstract:
  We investigate the critical behavior of a three-dimensional short-range spin
glass model in the presence of an external field $\eps$ conjugated to the
Edwards-Anderson order parameter. In the mean-field approximation this model is
described by the Adam-Gibbs-DiMarzio approach for the glass transition. By
Monte Carlo numerical simulations we find indications for the existence of a
line of critical points in the plane $(\eps,T)$ which separates two
paramagnetic phases and terminates in a critical endpoint. This line of
critical points appears due to the large degeneracy of metastable states
present in the system (configurational entropy) and is reminiscent of the
first-order phase transition present in the mean-field limit. We propose a
scenario for the spin-glass transition at $\eps=0$, driven by a spinodal point
present above $T_c$, which induces strong metastability through Griffiths
singularities effects and induces the absence of a two-step shape relaxation
curve characteristic of glasses.


title:
Detection of Coulomb Charging around an Antidot in the Quantum Hall
  Regime

abstract:
  We have detected oscillations of the charge around a potential hill (antidot)
in a two-dimensional electron gas as a function of a large magnetic field B.
The field confines electrons around the antidot in closed orbits, the areas of
which are quantised through the Aharonov-Bohm effect. Increasing B reduces each
state's area, pushing electrons closer to the centre, until enough charge
builds up for an electron to tunnel out. This is a new form of the Coulomb
blockade seen in electrostatically confined dots. Addition and excitation
spectra in DC bias confirm the Coulomb blockade of tunnelling.


title: Detection of Coulomb Charging around an Antidot

abstract:
  We have detected oscillations of the charge around a potential hill (antidot)
in a two-dimensional electron gas as a function of a perpendicular magnetic
field B. The field confines electrons around the antidot in closed orbits, the
areas of which are quantised through the Aharonov-Bohm effect. Increasing B
reduces each state's area, pushing electrons closer to the centre, until enough
charge builds up for an electron to tunnel out. This is a new form of the
Coulomb blockade seen in electrostatically confined dots. We have also studied
h/2e oscillations and found evidence for coupling of opposite spin states of
the lowest Landau level.


title:
Calculation Method for the Three-Dimensional Ising Ferromagnet
  Thermodynamics within the Frames of $\rho^6$ Model

abstract:
  Calculation of thermodynamic functions of the three-dimensional Ising
ferromagnet above and below critical temperature is performed in the
approximation of sixfold basis distribution ($\rho^6$ model). Comparison with
the results for the $\rho^4$ model indicates that dependence of the
thermodynamic functions on the renormalization group parameter $s$ becomes
weaker. The optimal interval of the renormalization group parameter values is
determined.


title: Hall tunneling of vortices in superclean superconductors

abstract:
  We discuss the main features of Hall tunneling of pancake vortices in
superclean high-$T_{c}$ superconductors. The general formalism for the
calculation of the lifetime of a vortex pinned in a metastable configuration is
described. The results are applied to the problem of quantum tunneling of a
pancake vortex from a columnar defect in the limit of a small driving current.


title: Marginal Pinning of Quenched Random Polymers

abstract:
  An elastic string embedded in 3D space and subject to a short-range
correlated random potential exhibits marginal pinning at high temperatures,
with the pinning length $L_c(T)$ becoming exponentially sensitive to
temperature. Using a functional renormalization group (FRG) approach we find
$L_c(T) \propto \exp[(32/\pi)(T/T_{\rm dp})^3]$, with $T_{\rm dp}$ the
depinning temperature. A slow decay of disorder correlations as it appears in
the problem of flux line pinning in superconductors modifies this result, $\ln
L_c(T)\propto T^{3/2}$.


title: Steady-State Cracks in Viscoelastic Lattice Models II

abstract:
  We present the analytic solution of the Mode III steady-state crack in a
square lattice with piecewise linear springs and Kelvin viscosity. We show how
the results simplify in the limit of large width. We relate our results to a
model where the continuum limit is taken only along the crack direction. We
present results for small velocity, and for large viscosity, and discuss the
structure of the critical bifurcation for small velocity. We compute the size
of the process zone wherein standard continuum elasticity theory breaks down.


title: Holons on a meandering stripe

abstract:
  To study a possible effect of transverse fluctuations of a stripe in a 2D
antiferromagnet on its charge dynamics, we identify elementary excitations of a
weakly doped domain wall in the Hubbard model. Hartree-Fock numerics and
analysis of fermion zero modes suggest that for U>=3t charged excitations are
mobile holons, Q=1, S=0. Each holon resides on a kink in the position of the
domain wall. We construct a simple model in which transverse stripe dynamics is
induced solely by motion of the holons. In the absence of spin excitations
(spinons, Q=0, S=1/2), stripe fluctuations DO NOT suppress a tendency to form a
global charge-density order.


title: A mesoscopic Tera-hertz pulse detector

abstract:
  We show that, under the passage of an electromagnetic terahertz pulse, an
asymmetric double barrier device may act as an on/off current switch, depending
on the bias. The time-dependent response of the device is discussed.


title:
Fermionic R-operator approach for the small-polaron model with open
  boundary condition

abstract:
  Exact integrability and algebraic Bethe ansatz of the small-polaron model
with the open boundary condition are discussed in the framework of the quantum
inverse scattering method (QISM). We employ a new approach where the fermionic
R-operator which consists of fermion operators is a key object. It satisfies
the Yang-Baxter equation and the reflection equation with its corresponding
K-operator. Two kinds of 'super-transposition' for the fermion operators are
defined and the dual reflection equation is obtained. These equations prove the
integrability and the Bethe ansatz equation which agrees with the one obtained
from the graded Yang-Baxter equation and the graded reflection equations.


title: Nonlinear Modes of a Macroscopic Quantum Oscillator

abstract:
  We consider the Bose-Einstein condensate in a parabolic trap as a macroscopic
quantum oscillator and describe, analytically and numerically, its collective
modes - a nonlinear generalisation of the (symmetric and antisymmetric)
Hermite-Gauss eigenmodes of a harmonic quantum oscillator.


title: Quantum interferences in quasicrystals

abstract:
  Contributions of quantum interference effects occuring in quasicrystals are
emphasized. First conversely to metallic systems, quasiperiodic ones are shown
to enclose original alterations of their conductive properties while
downgrading long range order. Besides, origins of localization mechanisms are
outlined within the context of the original metal-insulator transition (MIT)
found in these materials.


title: Synchronization and resonance in a driven system of coupled oscillators

abstract:
  We study the noise effects in a driven system of globally coupled
oscillators, with particular attention to the interplay between driving and
noise. The self-consistency equation for the order parameter, which measures
the collective synchronization of the system, is derived; it is found that the
total order parameter decreases monotonically with noise, indicating overall
suppression of synchronization. Still, for large coupling strengths, there
exists an optimal noise level at which the periodic (ac) component of the order
parameter reaches its maximum. The response of the phase velocity is also
examined and found to display resonance behavior.


title: Scaling and universality of AC conduction in disordered solids

abstract:
  Recent scaling results for the AC conductivity of ionic glasses by Roling et
al. [Phys. Rev. Lett. vol 78, 2160 (1997)] and Sidebottom [Phys. Rev. Lett. vol
82, 3653 (1999)] are discussed. It is shown that Sidebottom's version of
scaling is completely general. A new analytical approximation to the universal
AC conductivity of hopping in the extreme disorder limit, the ``diffusion
cluster approximation,'' is presented and compared to simulations and
experiments.


title:
Electronic structure and exchange interactions of the ladder vanadates
  CaV2O5 and MgV2O5

abstract:
  We have performed ab-initio calculations of the electronic structure and
exchange couplings in the layered vanadates CaV2O5 and MgV2O5. Based on our
results we provide a possible explanation of the unusual magnetic properties of
these materials, in particular the large difference in the spin gap between
CaV2O5 and MgV2O5.


title: Ground state energy and mass gap of a generalised quantum spin ladder

abstract:
  We show that a 2-leg ladder hamiltonian introduced recently by Albeverio and
Fei (cond-mat/9807341) can be made to satisfy the Hecke algebra. As a result we
have found an equivalent representation of the eigenspectrum in terms of the
spin-1/2 antiferromagnetic XXZ chain at $\Delta = - 5/3$. The values of
thermodynamic quantities such as the ground state energy and mass gap follow
from the known XXZ results.


title:
Quantum Phase Transition of S=1/2 Trimerized XXZ Spin Chain in Magnetic
  Field

abstract:
  We study the magnetization plateau at a third of the saturation magnetization
of the S=1/2 trimerized XXZ spin chain at T=0. The appearance of the plateau
depends on the values of the XXZ anisotropy and the magnitude of the
trimerization. This plateauful-plateauless transition is a quantum phase
transition of the Berezinskii-Kosterlitz-Thouless type, which is difficult to
precisely detect from the numerical data. To determine the phase boundary line
of this transition precisely, we use the level crossing of low-lying
excitations obtained from the numerical diagonalization. We also discuss the
S=1/2 ferromagnetic-ferromagnetic-antiferromagnetic chain.


title: Crossover between displacive and order-disorder phase transition

abstract:
  The phase transition in a 3D array of classical anharmonic oscillators with
harmonic nearest-neighbour coupling (discrete $\phi^4$ model) is studied by
Monte Carlo (MC) simulations and by analytical methods. The model allows to
choose a single dimensionless parameter a determining completely the behaviour
of the system. Changing a from 0 to $+\infty$ allows to go continuously from
the displacive to the order-disorder limit. We calculate the transition
temperature $T_c$ and the temperature dependence of the order parameter down to
T=0 for a wide range of the parameter a. The $T_c$ from MC calculations shows
an excellent agreement with the known asymptotic values for small and large a.
The obtained MC results are further compared with predictions of the mean-field
and independent-mode approximations as well as with predictions of our own
approximation scheme.
  In this approximation, we introduce an auxiliary system, which yields
approximately the same temperature behaviour of the order parameter, but allows
the decoupling of the phonon modes.
  Our approximation gives the value of $T_c$ within an error of 5% and
satisfactorily describes the temperature dependence of the order parameter for
all values of a.


title: Spectral Features of the Proximity Effect

abstract:
  We calculate the local density of states (LDOS) of a superconductor-normal
metal sandwich at arbitrary impurity concentration. The presence of the
superconductor induces a gap in the normal metal spectrum that is proportional
to the inverse of the elastic mean free path $l$ for rather clean systems. For
a mean free path much shorter than the thickness of the normal metal, we find a
gap size proportional to $l$ that approaches the behavior predicted by the
Usadel equation (diffusive limit).


title: Dipoles at $\nu =1$

abstract:
  We consider the problem of Bosonic particles interacting repulsively in a
strong magnetic field at the filling factor $\nu =1.$ We project the system in
the Lowest Landau Level and map the dynamics into an interacting Fermion
system. We study the resulting Hamiltonian in the Hartree--Fock approximation
in the case of a $\delta $ repulsive potential. The physical picture which
emerges is in agreement with the proposal of N. Read that the composite
Fermions behave as a gas of dipoles. We argue that the consequence of this is
that the composite Fermions interact with screened short range interactions. We
develop a Landau theory which we also expect to describe the physical $\nu
=1/2$ Fermionic state. The Form factor, the effective mass and the conductivity
are analised in this model.


title: A theory of phonon spectroscopy in the fractional quantum Hall regime

abstract:
  We describe a theoretical framework for the interpretation of time-resolved
phonon absorption experiments carried out in the fractional quantum Hall regime
of a magnetically quantized two-dimensional electron system (2des). The only
phonons which can be absorbed at low temperature are those whose energies
exceed the magnetoroton gap predicted by Girvin, MacDonald and Platzman. The
rate of energy transfer from the phonons to the electron liquid is entirely
controlled by the creation of these collective excitations. Using simple
isotropic approximations for the phonon propagation and electron-phonon
coupling we obtain analytic results for the regime in which the electron
temperature and the characteristic temperature of the phonons are much less
than the gap and identify the way in which the dispersion curve of the
magnetorotons could be extracted from time and angle resolved experiments.


title:
Bolometric detection of mechanical bending waves in suspended carbon
  nanotubes

abstract:
  We show that it is possible to detect mechanical bending modes on 1 micron
long ropes of single walled-carbon nanotubes suspended between 2 metallic
contacts. This is done by measuring either their dc resistance in a region of
strong temperature dependence (in the vicinity of superconducting or
metal-insulator transition), or their critical current. The vibrations are
excited by a radio-frequency electric field produced by an antenna located in
the vicinity of the sample. We analyze the mechanism of detection of the
mechanical resonances in terms of heating and phase breaking effects.


title: Online Learning with Ensembles

abstract:
  Supervised online learning with an ensemble of students randomized by the
choice of initial conditions is analyzed. For the case of the perceptron
learning rule, asymptotically the same improvement in the generalization error
of the ensemble compared to the performance of a single student is found as in
Gibbs learning. For more optimized learning rules, however, using an ensemble
yields no improvement. This is explained by showing that for any learning rule
$f$ a transform $\tilde{f}$ exists, such that a single student using
$\tilde{f}$ has the same generalization behaviour as an ensemble of
$f$-students.


title:
Linear and non-linear dielectric spectroscopy on ammonium doped Rochelle
  salt

abstract:
  In this work we present a thorough investigation of the mixed system of
Rochelle salt doped with ammonium, with 0<=x<=1 for [Na(K{1-x}(NH4)x)C4H4O6 *
4H2O]. This interesting system is known to exhibit a rich phase diagram with a
variety of polar and non-polar phases. However, most of the earlier
investigations date back many decades and the experimental advances made since
then should enable new insight into the physical nature of its phase diagram.
We studied single crystals of 18 doping concentrations using linear and
non-linear frequency-dependent dielectric response spectroscopy. In the low
field measurements a variety of relaxation features are detected and analyzed
in detail. The results for two concentrations of x=0 and x=0.15 are described
qualitatively with a simple model. Possibly the most important outcome of the
present work is the detection of a new electrically ordered phase for
concentrations 0.18<x<0.89.


title:
Observation of supercurrent enhancement in SNS junctions by
  non-equilibrium injection into supercurrent carrying bound Andreev states

abstract:
  We report for the first time enhancement of the supercurrent by means of
injection in a mesoscopic three terminal planar SNSNS device made of Al on
GaAs. When a current is injected from one of the superconducting Al electrodes
at an injection bias $V=\Delta(T)/e$, the DC Josephson current between the
other two superconducting electrodes has a maximum, giving evidence for an
enhancement due to a non-equilibrium injection into bound Andreev states of the
underlying semiconductor. The effect persists to temperatures where the
equilibrium supercurrent has vanished.


title: Inelastic lifetimes of hot electrons in real metals

abstract:
  We report a first-principles description of inelastic lifetimes of excited
electrons in real Cu and Al, which we compute, within the GW approximation of
many-body theory, from the knowledge of the self-energy of the excited
quasiparticle. Our full band-structure calculations indicate that actual
lifetimes are the result of a delicate balance between localization, density of
states, screening, and Fermi-surface topology. A major contribution from
$d$-electrons participating in the screening of electron-electron interactions
yields lifetimes of excited electrons in copper that are larger than those of
electrons in a free-electron gas with the electron density equal to that of
valence ($4s^1$) electrons. In aluminum, a simple metal with no $d$-bands,
splitting of the band structure over the Fermi level results in electron
lifetimes that are smaller than those of electrons in a free-electron gas.


title: Triplet superconductivity in quasi one-dimensional systems

abstract:
  We study a Hubbard hamiltonian, including a quite general nearest-neighbor
interaction, parametrized by repulsion V, exchange interactions Jz, Jperp,
bond-charge interaction X and hopping of pairs W. The case of correlated
hopping, in which the hopping between nearest neighbors depends upon the
occupation of the two sites involved, is also described by the model for
sufficiently weak interactions. We study the model in one dimension with usual
continuum-limit field theory techniques, and determine the phase diagram. For
arbitrary filling, we find a very simple necessary condition for the existence
of dominant triplet superconducting correlations at large distance in the spin
SU(2) symmetric case: 4V+J<0. In the correlated hopping model, the three-body
interaction should be negative for positive V. We also compare the predictions
of this weak-coupling treatment with numerical exact results for the
correlated-hopping model obtained by diagonalizing small chains, and using
novel techniques to determine the opening of the spin gap.


title:
Phase transition into superconducting mixed state and de Haas - van
  Alphen effect

abstract:
  The Landau expansion for the free energy of the superconducting mixed state
near the upper critical field in powers of the square modulus of the order
parameter averaged over Abrikosov lattice is derived.
  The analytical calculations has been carried out in frame of Gor'kov
formalism for 3-dimensional isotropic BCS model beyond the limits of
quasiclassical approximation, another words with Landau quantization of the
quasiparticle energy levels taken into consideration. The derivation is
performed at low temperature and high enough but finite crystal's purity.
  The effect of Pauli paramagnetic terms is taken into account. The quantum
oscillations of the critical temperature, the order parameter's amplitude and
the magnetization (de Haas-van Alphen effect) in the mixed state are found. The
limitation of validity of a mean field approach due to critical fluctuations
(Ginzburg criterion) for the phase transition under consideration is
established.


title: Dipoles and fractional quantum Hall masses

abstract:
  We develop a microscopic formalism to study the fractional quantum Hall
plateaus at filling factors $\nu $ away from $1/2\beta$ $\beta$ an integer. The
theory is in terms of quasiparticles which carry a charge $e^{\ast}$ equal to
$1-2\beta\nu $ times the charge of the electron. The wave functions obtained
following our approach are shown to coincide precisely with the form predicted
by Jain and this holds independently of the interaction potential.\
Microscopically this rigidity originates from the fact that two different
charges interacting attractively in their lowest Landau levels form a bound
state with a universal wave function. From the expressions of the gaps we
compute an effective mass which agrees well with the experiments carried at
$\nu=1/2$ and 1/4.


title: Interactions and Weak Localization: Perturbation Theory and Beyond

abstract:
  We establish an explicit correspondence between perturbative and
nonperturbative results in the problem of quantum decoherence in disordered
conductors. We demonstrate that the dephasing time $\tau_{\phi}$ cannot be
unambiguously extracted from a perturbative calculation. We show that the
effect of the electron-electron interaction on the magnetoconductance is
described by the function $A_d(t)\exp (-f_d(t))$. The dephasing time is
determined by $f_d(t)$, i.e. in order to evaluate $\tau_{\phi}$ it is
sufficient to perform a nonperturbative analysis with an exponential accuracy.
The effect of interaction on the pre-exponent $A_d(t)$ is important if one
calculates the interaction-dependent part of the weak localization correction
for strong magnetic fields. The zero temperature dephasing time drops out of
this correction in the first order due to the exact cancellation of the linear
in time $T$-independent contributions from the exponent and the pre-exponent.
Nonlinear in time $T$-independent contributions do not cancel out already in
the first order of the perturbation theory and yield an additional contribution
to dephasing at all temperatures including T=0.


title:
A Modified Approach to Single-Spin Detection Using Magnetic Resonance
  Force Microscopy

abstract:
  The magnetic moment of a single spin interacting with a cantilever in
magnetic resonance force microscopy (MRFM) experiences quantum jumps in
orientation rather than smooth oscillations. These jumps cannot be detected by
a conventional MRFM based on observation of driven resonant oscillations of a
cantilever. In this paper, we propose a method which will allow detection of
the magnetic signal from a single spin using a modification of a conventional
MRFM. We estimate the opportunity to detect the magnetic signal from a single
proton.


title:
The problem of phase breaking in the electronic conduction in mesoscopic
  systems: a linear-response theory approach

abstract:
  We study the problem of electronic conduction in mesoscopic systems when the
electrons are allowed to interact not only with static impurities, but also
with a scatterer (a phase breaker(PB)) that possesses internal degrees of
freedom. We first analyze the role of the PB in reducing the coherent
interference effects in a one-electron quantum-mechanical system. In the
many-electron system we can make a number of quite general statements within
the framework of linear-response theory and the random-phase approximation. We
cannot calculate the conductivity tensor in full generality: we thus resort to
a model, in which that tensor can be expressed entirely in a single-electron
picture. The resulting zero-temperature conductance can be written in terms of
the total transmission coefficient at the Fermi energy, containing an
additional trace over the states of the PB.


title: Quantum Decoherence and the "P(E)-Theory"

abstract:
  We point out a close physical and formal similarity between the problems of
electron tunneling in the effective environment and the weak localization
effects in the presence of interactions. In both cases the results are
expressed in terms of the ``energy probability distribution function'' $P(E)$
which has a finite width even at T=0 due to interactions.


title: A Composite Fermion Description of Rotating Bose-Einstein Condensates

abstract:
  We study the properties of rotating Bose-Einstein condensates in parabolic
traps, with coherence length large compared to the system size. In this limit,
it has been shown that unusual groundstates form which cannot be understood
within a conventional many-vortex picture. Using comparisons with exact
numerical results, we show that these groundstates can be well-described by a
model of non-interacting ``composite fermions''. Our work emphasises the
similarities between the novel states that appear in rotating Bose-Einstein
condensates and incompressible fractional quantum Hall states.


title: Potential energy landscape of finite-size mean-field models for glasses

abstract:
  connected spin-glass models with a discontinuous transition. In the
thermodynamic limit the equilibrium properties in the high temperature phase
are described by the schematic Mode Coupling Theory of super-cooled liquids. We
show that {\it finite-size} fully connected spin-glass models do exhibit
properties typical of Lennard-Jones systems when both are near the critical
glass transition, where thermodynamics is ruled by energy minima distribution.
Our study opens the way to consider activated processes in real glasses through
finite-size corrections (i.e. calculations beyond the saddle point
approximation) in mean-field spin-glass models.


title: The physical limits of communication

abstract:
  It has been well-known since the pioneering work of Claude Shannon in the
1940s that a message transmitted with optimal efficiency over a channel of
limited bandwidth is indistinguishable from random noise to a receiver who is
unfamiliar with the language in which the message is written. In this letter we
demonstrate an equivalent result about electromagnetic transmissions. We show
that when electromagnetic radiation is used as the transmission medium, the
most information-efficient format for a given message is indistinguishable from
black-body radiation to a receiver who is unfamiliar with that format. The
characteristic temperature of the radiation is set by the amount of energy used
to make the transmission. If information is not encoded in the direction of the
radiation, but only its timing, energy or polarization, then the most efficient
format has the form of a one-dimensional black-body spectrum which is easily
distinguished from the three-dimensional case.


title: Thermal vortex dynamics in a two-dimensional condensate

abstract:
  We carry out an analytical and numerical study of the motion of an isolated
vortex in thermal equilibrium, the vortex being defined as the point
singularity of a complex scalar field $\psi(\r,t)$ obeying a nonlinear
stochastic Schr\"odinger equation. Because hydrodynamic fluctuations are
included in this description, the dynamical picture of the vortex emerges as
that of both a massive particle in contact with a heat bath, and as a passive
scalar advected to a background random flow. We show that the vortex does not
execute a simple random walk and that the probability distribution of vortex
flights has non-Gaussian (exponential) tails.


title: Disorder Induced Phase Transition for Random 2D Dirac Fermions

abstract:
  We present evidence that two dimensional Dirac fermions in the presence of
random Abelian gauge potential exhibit a phase transition when the disorder
strength exceeds a certain critical value. We argue that this phase transition
has novel properties unique to disordered systems. It resembles in many ways
the transition from dilute to dense polymers phase in two dimension. In
particular, we argue that the central charge of the disordered Dirac fermions,
being $c=0$ before the transition, changes to $c=-2$. We discuss possible
implications to quantum Hall transitions, in view of recently proposed model
for quantum Hall transitions with $c=-2$.


title: Particle Dynamics in a Mass-Conserving Coalescence Process

abstract:
  We consider a fully asymmetric one-dimensional model with mass-conserving
coalescence. Particles of unit mass enter at one edge of the chain and
coalescence while performing a biased random walk towards the other edge where
they exit. The conserved particle mass acts as a passive scalar in the reaction
process $A+A\to A$, and allows an exact mapping to a restricted ballistic
surface deposition model for which exact results exist. In particular, the
mass- mass correlation function is exactly known. These results complement
earlier exact results for the $A+A\to A$ process without mass. We introduce a
comprehensive scaling theory for this process. The exact anaytical and
numerical results confirm its validity.


title:
Transitions from the Quantum Hall State to the Anderson Insulator: Fa te
  of Delocalized States

abstract:
  Transitions between the quantum Hall state and the Anderson insulator are
studied in a two dimensional tight binding model with a uniform magnetic field
and a random potential. By the string (anyon) gauge, the weak magnetic field
regime is explored numerically. The regime is closely related to the continuum
model. The change of the Hall conductance and the trajectoy of the delocalized
states are investigated by the topological arguments and the Thouless number
study.


title:
Wave-packet dynamics in slowly perturbed crystals: Gradient corrections
  and Berry-phase effects

abstract:
  We present a unified theory for wave-packet dynamics of electrons in crystals
subject to perturbations varying slowly in space and time. We derive the
wave-packet energy up to the first order gradient correction and obtain all
kinds of Berry-phase terms for the semiclassical dynamics and the quantization
rule. For electromagnetic perturbations, we recover the orbital magnetization
energy and the anomalous velocity purely within a single-band picture without
invoking inter-band couplings. For deformations in crystals, besides a
deformation potential, we obtain a Berry-phase term in the Lagrangian due to
lattice tracking, which gives rise to new terms in the expressions for the
wave-packet velocity and the semiclassical force. For multiple-valued
displacement fields surrounding dislocations, this term manifests as a Berry
phase, which we show to be proportional to the Burgers vector around each
dislocation.


title:
Quantum dots with even number of electrons: Kondo effect in a finite
  magnetic field

abstract:
  We study a small spin-degenerate quantum dot with even number of electrons,
weakly connected by point contacts to the metallic electrodes, and subject to
an external magnetic field. If the Zeeman energy B is equal to the
single-particle level spacing $\Delta $ in the dot, the ground state of the dot
becomes doubly degenerate, and the system exhibits Kondo effect, despite the
fact that B exceeds by far the Kondo temperature $T_{K}$. A possible
realization of this in tunneling experiments is discussed.


title: Extreme Value Statistics and Error Correcting Codes

abstract:
  Derrida's model can be used for construction and transmission of error
correcting codes, in an optimal way. we use the extreme statistics method to
derive the optimal signal to noise ratio.


title:
Characterisation, Raman, Magnetic and Resistivity measurements in
  polycrystalline samples of LaMnO3 doped with Cd

abstract:
  We report a study of polycrystalline samples of the family La1-xCdxMnO3+dwith
different percentage of Mn4+ ions. X-rays diffraction, Iodometric titration,
Raman, Magnetic and Electrical Resistivity measurements provide a general
characterisation of the physical properties. Results are qualitatively similar
to the ones found in Ca doped manganese perovskites.
  Key words: Cd, manganese perovskites, magnetoresistance, Raman, magnetic,
resistivity


title: Inertia, coarsening and fluid motion in binary mixtures

abstract:
  Symmetric binary fluids, quenched into a regime of immiscibility, undergo
phase separation by spinodal decomposition. In the late stages, the fluids are
separated by sharply defined, but curved, interfaces: the resulting Laplace
pressure drives fluid flow. Scaling ideas (of Siggia and of Furukawa) predict
that, ultimately, this flow should become turbulent as inertial effects
dominate over viscous ones. The physics here is complex: mesoscale simulation
methods (such as Lattice Boltzmann and Dissipative Particle Dynamics) can play
an essential role in its elucidation, as we describe. Likewise, it is a matter
of experience that immiscible fluids will mix, on some lengthscale at least, if
stirred vigorously enough. A scaling theory (of Doi and Ohta) predicts the
dependence of a steady state domain size on shear rate, but assumes low
Reynolds number (inertia is neglected). Our preliminary simulation results
(three-dimensional, so far only on small systems) show little sign of the kind
of steady state envisaged by Doi and Ohta; they raise instead the possibility
of an oriented domain texture which can continue to coarsen until either
inertial effects, or (in our simulations) finite size effects, come into play.


title: Quantum phase transitions in antiferromagnets and superfluids

abstract:
  We present a general introduction to the non-zero temperature dynamic and
transport properties of low-dimensional systems near a quantum phase
transition. Basic results are reviewed in the context of experiments on the
spin-ladder compounds, insulating two-dimensional antiferromagnets, and
double-layer quantum Hall systems. Recent large N computations on an extended
t-J model (cond-mat/9906104) motivate a global scenario of the quantum phases
and transitions in the high temperature superconductors, and connections are
made to numerous experiments.


title: Dynamics and Scaling of One Dimensional Surface Structures

abstract:
  We study several one dimensional step flow models. Numerical simulations show
that the slope of the profile exhibits scaling in all cases. We apply a scaling
ansatz to the various step flow models and investigate their long time
evolution. This evolution is described in terms of a continuous step density
function, which scales in time according to D(x,t)=F(xt^{-1/\gamma}). The value
of the scaling exponent \gamma depends on the mass transport mechanism. When
steps exchange atoms with a global reservoir the value of \gamma is 2. On the
other hand, when the steps can only exchange atoms with neighboring terraces,
\gamma=4. We compute the step density scaling function for three different
profiles for both global and local exchange mechanisms. The computed density
functions coincide with simulations of the discrete systems. These results are
compared to those given by the continuum approach of Mullins.


title:
A Possible Mechanism of the Pseudogap in Organic Superconductor Based on
  the Superconducting Fluctuation

abstract:
  The explanation of the anomalous behavior in $\kappa$-type (BEDT-TTF)$_2$X
which was revealed by the nuclear magnetic resonance experiments is presented.
We calculate the electronic properties by using the one-loop approximation for
the superconducting fluctuation. The decrease of the density of states due to
the large damping effect is obtained and the one-particle spectrum has a
pseudogap structure around the Fermi level. It is found that these behaviors
are peculiar to the quasi two-dimensional system and are enhanced in the
incoherent metal. The similarity between the cuprates and this compounds is
discussed and an experiment to check this proposal is suggested.


title:
Perturbation Theory on the Transition Temperature and Electronic
  Properties of Organic Superconductor

abstract:
  We study the superconducting transition temperature and the electronic
properties of the metallic phase of $\kappa$-type (BEDT-TTF)$_2$X which shows
unconventional properties in experiments, on the basis of the third order
perturbation theory for a simple effective Hubbard model of a nearly triangular
lattice. Appropriate transition temperatures and $d_{x^2-y^2}$ symmetry of the
gap function are obtained in good agreement with experimental results. We also
calculate the transition temperature by the fluctuation-exchange
approximation(FLEX) in order to compare the two approaches; FLEX gives higher
transition temperatures rather than the perturbation approach. However, it is
also found that the vertex corrections, which are ignored in FLEX, have a
crucial effect on $T_{\rm c}$ for strongly frustrated systems. The density of
states and the normal self-energy calculated in this perturbation scheme show
the nature of the conventional Fermi liquid near the Mott-insulator. Thus, our
perturbation approach is applicable to the conventional metallic phase of this
compound, while it cannot explain the (pseudo-)spin gap phenomenon which
signals the non-Fermi liquid.


title:
Comment on "quantum theory for mesosocopic electric circuits".
  Cond-mat/9907171 and cond-mat/9606206

abstract:
  In references cond-mat/9907171 and cond-mat/9606206 (Phys.Rev.B.53, 4927
(1996)) by You-Quan Li and Bin Chen, was considered a mesoscopic LC circuit
with charge discreteness. So, it was proposed a finite difference Schroedinger
equation for the charge time behavior. In this comment, we generalize the
corresponding mesoscopic Hamiltonian in order to taken into account the
dissipative effects (resistance R). Namely, a quantum term RI, proportional to
the current, is added to the mesoscopic LC circuit equation. This is
carried-out in analogy with the theory of Caldirola-Kanai for quantum one
particle damping.


title:
Mound formation in nonequilibrium surface growth morphology does not
  necessarily imply a Schwoebel instability

abstract:
  We demonstrate, using well-established nonequilibrium growth models, that
mound formation in the dynamical surface growth morphology does not necessarily
imply a surface edge diffusion bias (the Schwoebel barrier) as has been almost
universally accepted in the literature. We find mounded morphologies in several
nonequilibrium growth models which incorporate no Schwoebel barrier. Our work
should lead to a critical re-evaluation of recent experimental observations of
mounded morphologies which have been theoretically interpreted in terms of
Schwoebel barrier effects.


title:
Charge ordering and chemical potential shift in La$_{2-x}$Sr$_x$NiO$_4$
  studied by photoemission spectroscopy

abstract:
  We have studied the chemical potential shift in La$_{2-x}$Sr$_x$NiO$_4$ and
the charge ordering transition in La$_{1.67}$Sr$_{0.33}$NiO$_4$ by
photoemission spectroscopy. The result shows a large ($\sim$ 1 eV/hole)
downward shift of the chemical potential with hole doping in the high-doping
regime ($\delta \gtrsim$ 0.33) while the shift is suppressed in the low-doping
regime ($\delta \lesssim$ 0.33). This suppression is attributed to a
segregation of doped holes on a microscopic scale when the hole concentration
is lower than $\delta \simeq 1/3$. In the $\delta = 1/3$ sample, the
photoemission intensity at the chemical potential vanishes below the charge
ordering transition temperature $T_{\rm CO}=$ 240 K.


title:
Angle-resolved photoemission study of untwinned PrBa$_2$Cu$_3$O$_7$:
  undoped CuO$_2$ plane and doped CuO$_3$ chain

abstract:
  We have performed an angle-resolved photoemission study on untwinned
PrBa$_2$Cu$_3$O$_7$, which has low resistivity but does not show
superconductivity. We have observed a dispersive feature with a band maximum
around ($\pi$/2,$\pi$/2), indicating that this band is derived from the undoped
CuO$_2$ plane. We have observed another dispersive band exhibiting
one-dimensional character, which we attribute to signals from the doped CuO$_3$
chain. The overall band dispersion of the one-dimensional band agrees with the
prediction of $t-J$ model calculation with parameters relevant to cuprates
except that the intensity near the Fermi level is considerably suppressed in
the experiment.


title: Spin and charge ordering in self-doped Mott insulators

abstract:
  We have investigated possible spin and charge ordered states in 3d
transition-metal oxides with small or negative charge-transfer energy, which
can be regarded as self-doped Mott insulators, using Hartree-Fock calculations
on d-p-type lattice models. It was found that an antiferromagnetic state with
charge ordering in oxygen 2p orbitals is favored for relatively large
charge-transfer energy and may be relevant for PrNiO$_3$ and NdNiO$_3$. On the
other hand, an antiferromagnetic state with charge ordering in transition-metal
3$d$ orbitals tends to be stable for highly negative charge-transfer energy and
can be stabilized by the breathing-type lattice distortion; this is probably
realized in YNiO$_3$.


title: Magnetization Process in the One-Dimensional Doped Kondo Lattice Model

abstract:
  The magnetization process in the one-dimensional Kondo lattice model for the
doped (n_{c}<1) case is studied by the density matrix renormalization group
(DMRG) method. A rapid increase of the magnetization is caused by the collapse
of the intersite incommensurate correlation of f spins. On the contrary, the
intrasite f-c singlet correlation survives in the larger magnetic field. The
crossover from large to small Fermi surfaces for majority and minority spins is
observed, whereas the Fermi surfaces are always contributed by f spins. A
magnetization plateau appears with the magnitude of 1-n_{c}. Both ends of the
plateau are related to the coherence temperature and the Kondo temperature
which are characteristic energies essential in heavy electron systems.


title: The Phase Diagrams of F=2 Spinor Bose Condensates

abstract:
  We show that there are three possible phases for a spin-2 spinor Bose
condensate, one more compared to the spin-1 case. The order parameters of these
phases are the spontaneous magnetization and the singlet pair amplitude.
Current estimates of scattering lengths show that all three phases have
realizations in optically trapped alkali atoms. There is also a one-to-one
correspondence between the structure of a spin-2 spinor Bose condensate and
that of a d-wave BCS superfluid.


title: Channel Flow of Smectic Films

abstract:
  The hydrodynamics of smectic films at an air-water interface is discussed,
with particular focus on the viscous response of the film under flow normal to
the layers. The corrections to the response functions of the smectic phase,
arising from the coupling between the flow and the smectic order parameter, are
calculated. The results for the effective viscosity are illustrated by
analysing smectic film flow in a channel geometry. Two limiting cases of the
flow, namely, motion dominated by dislocation-induced shear-softening and
dislocation-free motion dominated by the permeation mode of mass transfer, are
studied. The effect of drag from a finite depth liquid subphase is considered.
The results are compared to those for hexatic and liquid films.


title:
Quasi-one-dimensional superconductors: from weak to strong magnetic
  field

abstract:
  We discuss the possible existence of a superconducting phase at high magnetic
field in organic quasi-one-dimensional conductors. We consider in particular
(i) the formation of a Larkin-Ovchinnikov-Fulde-Ferrell state, (ii) the role of
a temperature-induced dimensional crossover occuring when the transverse
coherence length $\xi_z(T)$ becomes of the order of the lattice spacing, and
(iii) the effect of a magnetic-field-induced dimensional crossover resulting
from the localization of the wave functions at high magnetic field. In the case
of singlet spin pairing, only the combination of (i) and (iii) yields a picture
consistent with recent experiments in the Bechgaard salts showing the existence
of a high-field superconducting phase. We point out that the vortex lattice is
expected to exhibit unusual characteristics at high magnetic field.


title: Economics-Based Optimization of Unstable Flows

abstract:
  As an example for the optimization of unstable flows, we present an
economics-based method for deciding the optimal rates at which vehicles are
allowed to enter a highway. It exploits the naturally occuring fluctuations of
traffic flow and is flexible enough to adapt in real time to the transient flow
characteristics of road traffic. Simulations based on realistic parameter
values show that this strategy is feasible for naturally occurring traffic, and
that even far from optimality, injection policies can improve traffic flow.
Moreover, the same method can be applied to the optimization of flows of gases
and granular media.


title: Threshold effect during dissolution of 3He inclusions in solid 4He

abstract:
  A pressure jump has been found at the onset of the dissolution of bcc
inclusions in separated solid 3He-4He mixture if the crystal is overheated
above a certain critical value. This effect can be explained in the framework
of a multistage dissolution process model.


title: Non-exponential kinetic behavior of confined water

abstract:
  We present the results of molecular dynamics simulations of SPC/E water
confined in a realistic model of a silica pore. The single-particle dynamics
have been studied at ambient temperature for different hydration levels. The
confinement near the hydrophilic surface makes the dynamic behaviour of the
liquid strongly dependent on the hydration level. Upon decrease of the number
of water molecules in the pore we observe the onset of a slow dynamics due to
the ``cage effect''. The conventional picture of a stochastic single-particle
diffusion process thus looses its validity.


title: Magnon-Paramagnon Effective Theory of Itinerant Ferromagnets

abstract:
  The present work is devoted to the derivation of an effective
magnon-paramagnon theory starting from a microscopic lattice model of
ferromagnetic metals. For some values of the microscopic parameters it
reproduces the Heisenberg theory of localized spins. For small magnetization
the effective model describes the physics of weak ferromagnets in accordance
with the experimental results. It is written in a way which keeps O(3) symmetry
manifest,and describes both the order and disordered phases of the system.
  Analytical expression for the Curie temperature,which takes the magnon
fluctuations into account exactly, is obtained. For weak ferromagnets $T_c$ is
well below the Stoner's critical temperature and the critical temperature
obtained within Moriya's theory.


title: Critical-point finite-size scaling in the microcanonical ensemble

abstract:
  We develop a scaling theory for the finite-size critical behavior of the
microcanonical entropy (density of states) of a system with a
critically-divergent heat capacity. The link between the microcanonical entropy
and the canonical energy distribution is exploited to establish the former, and
corroborate its predicted scaling form, in the case of the 3d Ising
universality class. We show that the scaling behavior emerges clearly when one
accounts for the effects of the negative background constant contribution to
the canonical critical specific heat. We show that this same constant plays a
significant role in determining the observed differences between the canonical
and microcanonical specific heats of systems of finite size, in the critical
region.


title: Submonolayer growth with decorated island edges

abstract:
  We study the dynamics of island nucleation in the presence of adsorbates
using kinetic Monte Carlo simulations of a two-species growth model. Adatoms
(A-atoms) and impurities (B-atoms) are codeposited, diffuse and aggregate
subject to attractive AA- and AB-interactions. Activated exchange of adatoms
with impurities is identified as the key process to maintain decoration of
island edges by impurities during growth. While the presence of impurities
strongly increases the island density, a change in the scaling of island
density with flux, predicted by a rate equation theory for attachment-limited
growth [D. Kandel, Phys. Rev. Lett. 78, 499 (1997)], is not observed. We argue
that, within the present model, even completely covered island edges do not
provide efficient barriers to attachment.


title:
Breakdown of the magnetization plateau of the S=1/2
  ferromagnetic-ferromagnetic-antiferromagnetic trimerized spin chain with
  anisotropy

abstract:
  We study the breakdown of the magnetization plateau at the magnetization
M=M_{S}/3 (M_{S} is the saturation magnetization) of the S=1/2 anisotropic spin
chain with ferromagnetic-ferromagnetic-antiferromagnetic interactions. We
consider the model with the isotropic ferromagnetic (trimer) coupling J_{F},
and anisotropic antiferromagnetic coupling (J_{x}=J_{y}=J_{AF} and J_{z}=\Delta
J_{AF}). For the limit of large \gamma\equiv J_{F}/J_{AF}, the model is
equivalent to the S=3/2 XXZ chain with the exchange anisotropy \Delta. There is
a phase transition between the plateau (small-\gamma) and the no-plateau
(large-\gamma) regions. This phase transition is of the
Berezinskii-Kosterlitz-Thouless type, and we determine the phase boundary from
the numerical diagonalization data. For \Delta=1, in particular, the phase
transition between the plateau and the no-plateau regions occurs at the point
\gamma_{c}=15.4.


title:
The Fermion-ladder Models: Extensions of the Hubbard Model with
  $\eta$-pairing

abstract:
  We propose the two-leg fermion-ladder models for SU(2|2) and SU(4) cases. The
former is exactly the extended Hubbard model proposed by Essler, Korepin and
Schoutens. The later is a new model also with $\eta$-pairing symmetry which is
important for superconductivity. This new extension of the Hubbard model can be
solved exactly.


title: Skyrmion Dynamics and NMR Line Shapes in QHE Ferromagnets

abstract:
  The low energy charged excitations in quantum Hall ferromagnets are
topological defects in the spin orientation known as skyrmions. Recent
experimental studies on nuclear magnetic resonance spectral line shapes in
quantum well heterostructures show a transition from a motionally narrowed to a
broader `frozen' line shape as the temperature is lowered at fixed filling
factor. We present a skyrmion diffusion model that describes the experimental
observations qualitatively and shows a time scale of $\sim 50 \mu{\rm sec}$ for
the transport relaxation time of the skyrmions. The transition is characterized
by an intermediate time regime that we demonstrate is weakly sensitive to the
dynamics of the charged spin texture excitations and the sub-band electronic
wave functions within our model. We also show that the spectral line shape is
very sensitive to the nuclear polarization profile along the z-axis obtained
through the optical pumping technique.


title:
Aging phenomena in spin glass and ferromagnetic phases: domain growth
  and wall dynamics

abstract:
  We compare aging in a disordered ferromagnet and in a spin glass, by studying
the different phases of a reentrant system. We have measured the relaxation of
the low-frequency ac susceptibility, in both the ferromagnetic and spin-glass
phases of a CdCr_{1.9}In_{0.1}S_4 sample. A restart of aging processes when the
temperature is lowered (`chaos-like' effect) is observed in both phases. The
memory of previous aging at a higher temperature can be retrieved upon
re-heating, but in the ferromagnetic phase it can rapidly be erased by the
growth of ferromagnetic domains. We interpret the behaviour observed in the
ferromagnetic phase in terms of a combination of domain growth and pinned wall
reconformations, and suggest that aging in spin glasses is dominated by such
wall reconformation processes.


title:
Tuning of dynamic localization in coupled minibands: signatures of a
  field induced insulator-metal transition

abstract:
  We follow the evolution of dressed coupled mini-bands as a function of an AC
field intensity, non perturbatively, for a wide field frequency range. High and
low frequency limits are characterized by two different dynamic localization
regimes, clearly separated by a breakdown region in a quasi-energy map.
Signatures of a insulator-metal like transition by means of a field induced
suppression of a Peierls-like instability are identified.


title:
Isotropic dispersion, line shape, and remnant Fermi surface in one hole
  problem

abstract:
  It is shown that the isotropic dispersion relation, line shape of the
spectral function, and remnant Fermi surface found in recent photoemission
experiment in insulating Ca_2CuO_2Cl_2 by Ronning et al.(Science, 282, 2067
(1998)) can be consistently explained by a direct holon hopping process which
fundamentally undermines the conventional self-consistent Born approximation
approach to the t-J model.


title:
Infrared Hall effect in high Tc superconductors: Evidence for non-Fermi
  liquid Hall scattering

abstract:
  Infrared (20-120 cm-1 and 900-1100 cm-1) Faraday rotation and circular
dichroism are measured in high Tc superconductors using sensitive polarization
modulation techniques. Optimally doped YBCO thin films are studied at
temperatures down to 15 K and magnetic fields up to 8 T. At 1000 cm-1 the Hall
conductivity varies strongly with temperature in contrast to the longitudinal
conductivity which is nearly independent of temperature. The Hall scattering
rate has a T^2 temperature dependence but, unlike a Fermi liquid, depends only
weakly on frequency. The experiment puts severe constraints on theories of
transport in the normal state of high Tc superconductors.


title: Similarity of percolation thresholds on the hcp and fcc lattices

abstract:
  Extensive Monte-Carlo simulations were performed in order to determine the
precise values of the critical thresholds for site ($p^{hcp}_{c,S} = 0.199 255
5 \pm 0.000 001 0$) and bond ($p^{hcp}_{c,B} = 0.120 164 0 \pm 0.000 001 0$)
percolation on the hcp lattice to compare with previous precise measuremens on
the fcc lattice. Also, exact enumeration of the hcp and fcc lattices was
performed and yielded generating functions and series for the zeroth, first,
and second moments of both lattices. When these series and the values of $p_c$
are compared to those for the fcc lattice, it is apparent that the site
percolation thresholds are different; however, the bond percolation thresholds
are equal within error bars, and the series only differ slightly in the higher
order terms, suggesting the actual values are very close to each other, if not
identical.


title:
The Fermi surfaces of Metallic Alloys and the Oscillatory Magnetic
  Coupling between Magnetic Layers separated by such Alloy Spacers

abstract:
  We review the theory of oscillatory magnetic coupling in Metallic Multilayers
across alloy spacers. We illustrate the relationship between the frequencies of
the oscillations and the extremal caliper vectors of the Fermi surface of the
spacer by explicit calculations for Cu$_{(1-x)}$Ni$_x$, Cr$_{(1-x)}$V$_x$ and
Cr$_{(1-x)}$Mo$_x$ alloys. We argue the measurement of the frequencies of such
oscillations can be an extremely useful and cheap probe of the Fermi surface of
random alloys.


title: Integrable Kondo impurities in one-dimensional extended Hubbard models

abstract:
  Three kinds of integrable Kondo problems in one-dimensional extended Hubbard
models are studied by means of the boundary graded quantum inverse scattering
method. The boundary K matrices depending on the local moments of the
impurities are presented as a nontrivial realization of the graded reflection
equation algebras acting in a $(2 s_\alpha+1)$-dimensional impurity Hilbert
space. Further, these models are solved using the algebraic Bethe ansatz method
and the Bethe ansatz equations are obtained.


title:
Exact determination of the phase structure of the p-species asymmetric
  exclusion process

abstract:
  We consider a multi-species generalization of the Asymmetric Simple Exclusion
Process on an open chain, in which particles hop with their characteristic
hopping rates and fast particles can overtake slow ones. The number of species
is arbitrary and the hopping rates can be selected from a discrete or
continuous distribution. We determine exactly the phase structure of this model
and show how the phase diagram of the 1-species ASEP is modified. Depending on
the distribution of hopping rates, the system can exist in a three-phase regime
or a two phase regime. In the three- phase regime the phase structure is almost
the same as in the one species case, that is, there are the low density, the
high density and the maximal current phases, while in the two-phase regime
there is no high density phase.


title:
Extrinsic levels, diffusion, and unusual incorporation mechanism of
  lithium in GaN

abstract:
  Results of a first-principles study of the Li impurity in GaN are presented.
We find Li is a channel interstitial, with an onset for diffusion at T$\sim$
600 K. Above this temperature, Li can transform to a Ga-substitutional acceptor
by exothermic recombination with Ga vacancies. This process implies capture of
at least one electron; therefore Li acts as an electron sink. Li$_{\rm Ga}$ is
stable again interstitialcy, and has a shallow first ionization levels of 0.16
eV, and second ionization at 0.63 eV. Lattice locations and their temperature
dependence are in close agreement with recent experiments.


title: Transport Properties in (Na,Ca)Co_2O_4 Ceramics

abstract:
  The resistivity and thermopower of polycrystalline (Na,Ca)Co_2O_4 were
measured and analyzed. Both the quantities increase with x, suggesting that the
carrier density is decreased by the substitutions of Ca^{2+} for Na^{+}.
Considering that the temperature dependence of the resistivity show a
characteristic change with x, the conduction mechanism is unlikely to come from
a simple electron-phonon scattering. As a reference for NaCo_2O_4, single
crystals of a two-dimensional Co oxide (Bi,Pb)_2M_3Co_2O_9 (M=Sr and Ba) were
studied. The Pb substitution decreases the resistivity, leaving the thermopower
nearly intact.