title:
Charge Localization from Local Destruction of Antiferromagnetic
  Correlation in Zn-doped YBa2Cu3O7-d

abstract:
  The in-plane normal-state resistivity of Zn-doped YBa2Cu3O7-d single crystals
is measured down to low temperatures by suppressing superconductivity with
magnetic fields up to 18 T. Substitution of Cu with Zn in the CuO2 planes is
found to induce carrier localization at low temperatures in "clean" samples
with kF l > 5, where the mean free path l is larger than the electron wave
length and thus localization is not normally expected. The destruction of the
local antiferromagnetic correlation among Cu spins by Zn is discussed to be the
possible origin of this unusual charge localization.


title:
Comment on ``Effective Mass and g-Factor of Four Flux Quanta Composite
  Fermions"

abstract:
  In a recent Letter, Yeh et al.[Phys. Rev. Lett. 82, 592 (1999)] have shown
beautiful experimental results which indicate that the composite fermions with
four flux quanta ($^4$CF) behave as fermions with mass and spin just like those
with two flux quanta. They observed the collapse of the fractional quantum Hall
gaps when the following condition is satisfied with some integer $j$,
$g^*\mu_{\rm B}B_{\rm tot} = j \hbar \omega_{\rm c}^*$, where $g^*$ and
$\omega_{\rm c}^*$ are the g-factor and the cyclotron frequency of the $^4$CF,
respectively. However, in their picture the gap at the Fermi energy remains
always finite even if the above condition is satisfied, thus the reason of the
collapse was left as a mystery. In this comment it is shown that part of the
mystery is resolved by considering the electron-hole symmetry properly.


title: Phase diagram of the two-chain Hubbard model

abstract:
  We have calculated the charge gap and spin gap for the two-chain Hubbard
model as a function of the on-site Coulomb interaction and the interchain
hopping amplitude. We used the density matrix renormalization group method and
developed a method to calculate separately the gaps numerically for the
symmetric and antisymmetric modes with respect to the exchange of the chain
indices. We have found very different behaviors for the weak and strong
interaction cases. Our calculated phase diagram is compared to the one obtained
by Balents and Fisher using the weak coupling renormalization group technique.


title:
Plasmon Lifetime in K: A Case Study of Correlated Electrons in Solids
  Amenable to Ab Initio Theory

abstract:
  On the basis of a new ab initio, all-electron response scheme, formulated
within time-dependent density-functional theory, we solve the puzzle posed by
the anomalous dispersion of the plasmon linewidth in K. The key damping
mechanism is shown to be decay into particle-hole pairs involving empty states
of d-symmetry. While the effect of many-particle correlations is small, the
correlations built into the "final-state" -d-bands play an important, and
novel, role ---which is related to the phase-space complexity associated with
these flat bands. Our case study of plasmon lifetime in K illustrates the
importance of ab initio paradigms for the study of excitations in
correlated-electron systems.


title:
Dynamical Response of Correlated Electrons in Solids Probed by Inelastic
  Scattering Experiments: An Ab Initio Theoretical Perspective

abstract:
  We present results of ab initio theoretical investigations of the excitation
spectra of correlated electrons in metals (Al, K, and Li) and their interplay
with inelastic scattering experiments. We resolve various anomalies contained
in the data, which were originally viewed as signatures of strong dynamical
electronic correlations; we show that, instead, the anomalies are due to
band-structure effects. The underlying theoretical framework in our
density-response calculations is time-dependent density-functional theory; with
this scheme we discuss the lifetime of the K plasmon, and the dynamical
structure factor of Al. From a self-consistent solution of the Dyson equation
in the GW approximation for the electron self-energy, we discuss the electron
momentum density in Li. Our results and methods point to a new way of thinking
about electronic excitations in real materials. The main challenge ahead is the
proper treatment of dynamical correlations for realistic representations of the
band structure.


title: Vesicles in solutions of hard rods

abstract:
  The surface free energy of ideal hard rods near curved hard surfaces is
determined to second order in curvature for surfaces of general shape. In
accordance with previous results for spherical and cylindrical surfaces it is
found that this quantity is non-analytical when one of the principal curvatures
changes signs. This prohibits writing it in the common Helfrich form. It is
shown that the non-analytical terms are the same for any aspect ratio of the
rods. These results are used to find the equilibrium shape of vesicles immersed
in solutions of rod-like (colloidal) particles. The presence of the particles
induces a change in the equilibrium shape and to a shift of the prolate-oblate
transition in the vesicle phase diagram, which are calculated within the
framework of the spontaneous curvature model. As a consequence of the special
form of the energy contribution due to the rods these changes cannot be
accounted for by a simple rescaling of the elastic constants of the vesicle as
for solutions of spherical colloids or polymers.


title:
The Magnetic Origin of d-Wave High-Tc Superconductivity from Tunneling
  Spectroscopy Measurements on Bi2212 Single Crystals

abstract:
  The complete scenario of high-Tc superconductivity based on experimental data
from electron-tunneling spectroscopy on underdoped, overdoped and Ni-doped
Bi2212 single crystals using a break-junction technique is presented. There are
two different types of superconductivity in cuprates: superconductivity due to
pairing of spinons on charged stripes and magnetic superconductivity mediated
by spin-waves. The coherent state of the spinon superconductivity is
established via the magnetic superconductivity. In NCCO, there is only the
spinon superconductivity and the coherent state is established due to the
Josephson coupling between charged stripes. Below Tc, we observe four different
gaps, namely, (i) a SDW gap due to antiferromagnetic correlations; (ii) a
superconducting gap due to spinon pairing; (iii) a d-wave magnetic
superconducting gap, and (iv) a small superconducting gap most likely having
g-wave symmetry. We show that the d-wave superconductivity is mediated by
spin-waves and the magnitude of the superconducting gap due to pairing of
spinons is larger than the magnitude of the d-wave gap, however, the d-wave gap
is more intense. The superconducting gap due to pairing of spinons most likely
has a s-wave symmetry. The maximum magnitudes of the SDW, the spinon and d-wave
gaps are located at (p/2, p/2), (p/2, p/2) and (p, 0) on the Fermi surface,
respectively. The d-g-wave superconductivity mediated by spin-waves can be
considered as a pairing of magnetic polarons. We formulate a theorem for
cuprates by analogy with the Anderson's theorem for classical superconductors.
The presented model of high-Tc superconductivity naturally explains other
experimental data.


title:
Temperature-Induced Metal-Insulator Transition in a Narrow-Band Model
  with Non-Equivalent Hubbard Subbands at Half-Filling

abstract:
  In the previous papers (Journ. of Phys. Stud. 2, 362 (1998);
cond-mat/9811213) we have studied metal-insulator transition in a generalized
Hubbard model with correlated hopping at half-filling and zero temperature. The
present paper is devoted to a further study of metal-insulator transition in
this model,in particular, an investigation of temperature-induced
metal-to-insulator transition. The dependence of energy gap on concentration of
doubly occupancy leads to increasing energy gap width with increase of
temperature. Thus narrow-band system can undergo transition from a metallic
state to an insulating state with the increase of temperature. For some values
of intra-atomic Coulomb repulsion $U$ and $w_0$ ($w_0$ is half-bandwidth) we
find the values of temperature when narrow-band material undergoes transition
from a metallic state to an insulating state. We show that at given $U/w_0$
metal-insulator transition in model with non-equivalent Hubbard sub-bands can
occur at smaller temperature than in the Hubbard model. It testifies on the
fact that taking into account of correlated hopping is important for a
consideration of metal-insulator transition problem.


title: Magnetism and Electronic States of Systems with Strong Hund Coupling

abstract:
  This paper is a brief review of our recent studies concerning on magnetism
and electronic states of lattice systems with Hund coupling. First we examined
the effectiveness of the Hund coupling in realizing ferromagnetism in the
doubly degenerate Hubbard model. One- and infinite-dimensional systems were
studied and thereby dimensional dependence was discussed. In quarter-filled
systems the insulating ferromagnetic state accompanied by alternating orbital
order was found stable. In more-than-quarter filling cases metallic
ferromagnetism is stabilized by the ``double exchange mechanism''. These
results are common to one and infinite dimensions. In less-than-quarter filling
cases the ferromagnetic ground state is stable in one dimension but not in
infinite dimensions. Secondly we examined the electronic states and the
resistivity in the double exchange model by using the one-particle Green
function. The splitting and narrowing of the one-particle spectrum due to the
Hund coupling were clarified in the framework of a single-site approximation.
The resistivity due to the scattering by random localized spins was shown to be
too small to explain the experimental results of doped manganites.


title: Avalanche and spreading exponents in systems with absorbing states

abstract:
  We present generic scaling laws relating spreading critical exponents and
avalanche exponents (in the sense of self-organized criticality) in general
systems with absorbing states. Using these scaling laws we present a collection
of the state-of-the-art exponents for directed percolation, dynamical
percolation and other universality classes. This collection of results should
help to elucidate the connections of self-organized criticality and systems
with absorbing states. In particular, some non-universality in avalanche
exponents is predicted for systems with many absorbing states.


title: Fun with Quantum Dots

abstract:
  We consider quantum dots with a parabolic confining potential. The
qualitative features of such mesoscopic systems as functions of the total
number of electrons N and their total angular momentum J, e.g. magic numbers,
overall symmetries etc., are derived solely from combinatoric principles. The
key is one simple hypothesis about such quantum dots yielding a basis of states
(different from the usual single electron states one starts with) which is
extremely easy to handle. Within this basis all qualitative features are
already present without the need of any perturbation theory.


title: Hamiltonian Cycles on Random Eulerian Triangulations

abstract:
  A random Eulerian triangulation is a random triangulation where an even
number of triangles meet at any given vertex. We argue that the central charge
increases by one if the fully packed O(n) model is defined on a random Eulerian
triangulation instead of an ordinary random triangulation. Considering the case
n -> 0, this implies that the system of random Eulerian triangulations equipped
with Hamiltonian cycles describes a c=-1 matter field coupled to 2D quantum
gravity as opposed to the system of usual random triangulations equipped with
Hamiltonian cycles which has c=-2. Hence, in this case one should see a change
in the entropy exponent from the value gamma=-1 to the irrational value
gamma=(-1-\sqrt{13})/6=-0.76759... when going from a usual random triangulation
to an Eulerian one. A direct enumeration of configurations confirms this change
in gamma.


title:
General transport properties of superconducting quantum point contacts:
  a Green functions approach

abstract:
  We discuss the general transport properties of superconducting quantum point
contacts. We show how these properties can be obtained from a microscopic model
using nonequilibrium Green function techniques. For the case of a one-channel
contact we analyze the response under different biasing conditions: constant
applied voltage, current bias and microwave-induced transport. Current
fluctuations are also analyzed with particular emphasis on thermal and
shot-noise. Finally, the case of superconducting transport through a resonant
level is discussed. The calculated properties show a remarkable agreement with
the available experimental data from atomic-size contacts measurements. We
suggest the possibility of extending this comparison to several other
predictions of the theory.


title:
Nonlocal Effects on the Surface Resistance of High Temperature
  Superconductors with (100) and (110) Surfaces

abstract:
  The low temperature surface resistance R_s of d-wave superconductors is
calculated as function of frequency assuming normal state quasiparticle mean
free paths l in excess of the penetration depth. Results depend strongly on the
geometric configuration. In the clean limit, two contributions to R_s with
different temperature dependencies are identified: photon absorption by
quasiparticles and pair breaking. The size of nonlocal corrections, which can
be positive or negative depending on frequency decreases for given l as the
scattering phase shift \delta_N is increased. However, except in the unitarity
limit \delta_N = 0.5 \pi, nonlocal effects should be observable.


title: Minimizing volatility increases large risks

abstract:
  We introduce a faithful representation of the heavy tail multivariate
distribution of asset returns, as parsimonous as the Gaussian framework. Using
calculation techniques of functional integration and Feynman diagrams borrowed
from particle physics, we characterize precisely, through its cumulants of high
order, the distribution of wealth variations of a portfolio composed of an
arbitrary mixture of assets. The portfolio which minimizes the variance, i.e.
the relatively small risks, often increases larger risks as measured by higher
normalized cumulants and by the Value-at-risk.


title: The Singlet-Triplet Pseudo-Jahn-Teller Centers in Copper Oxides

abstract:
  One of the most exciting features of the hole centers CuO_{4}^{5-} in doped
cuprates is an unusually complicated ground state which is the result of the
electronic quasi-degeneracy. An additional hole, doped to the basic
CuO_{4}^{6-} cluster with the b_{1g} hole can occupy both the same hybrid
Cu3d-O2p orbital state resulting in a Zhang-Rice singlet ^1A_{1g} and the
purely oxygen e_u molecular orbital resulting in a singlet or triplet ^{1,3}E_u
term with the close energies. We present detailed analysis of the
(pseudo)-Jahn-Teller effect driven by the near-degeneracy within the
(^1A_{1g},^{1,3}E_u)-manifold.


title:
Metastable states and information propagation in a 1D array of
  locally-coupled bistable cells

abstract:
  We study the effect of metastable states on the relaxation process (and hence
information propagation) in locally coupled and boundary-driven structures. We
first give a general argument to show that metastable states are inevitable
even in the simplest of structures, a wire. At finite temperatures, the
relaxation mechanism is a thermally assisted random walk. The time required to
reach the ground state and its life time are determined by the coupling
parameters. These time scales are studied in a model based on an array of
quantum dots.


title:
Charging effects in the ac conductance of a double barrier resonant
  tunneling structure

abstract:
  There have been many studies of the linear response ac conductance of a
double barrier resonant tunneling structure (DBRTS). While these studies are
important, they fail to self-consistently include the effect of time dependent
charge density in the well. In this paper, we calculate the ac conductance by
including the effect of time dependent charge density in the well in a
self-consistent manner. The charge density in the well contributes to both the
flow of displacement currents and the time dependent potential in the well. We
find that including these effects can make a significant difference to the ac
conductance and the total ac current is not equal to the average of
non-selfconsitently calculated conduction currents in the two contacts, an
often made assumption. This is illustrated by comparing the results obtained
with and without the effect of the time dependent charge density included
properly.


title: Electron transport through a circular constriction

abstract:
  We calculate the conductance of a circular constriction of radius $a$ in an
insulating diaphragm which separates two conducting half-spaces characterized
by the mean free path $\ell$. Our exact result interpolates between the Maxwell
conductance in diffusive ($\ell \ll a$) and the Sharvin conductance in
ballistic ($\ell \gg a$) transport regime. Following the earlier approach of
Wexler we find the explicit form of the Green's function for the linearized
Boltzmann operator. The formula for the conductance deviates by less than 11%
from the naive interpolation formula obtained by adding resistances in the
diffusive and the ballistic regime.


title:
Scaling in the time-dependent failure of a fiber bundle with local load
  sharing

abstract:
  We study the scaling behaviors of a time-dependent fiber-bundle model with
local load sharing. Upon approaching the complete failure of the bundle, the
breaking rate of fibers diverges according to $r(t)\propto (T_f-t)^{-\xi}$,
where $T_f$ is the lifetime of the bundle, and $\xi \approx 1.0$ is a quite
universal scaling exponent. The average lifetime of the bundle $<T_f>$ scales
with the system size as $N^{-\delta}$, where $\delta$ depends on the
distribution of individual fiber as well as the breakdown rule.


title:
A Configuration Interaction approach to hole pairing in the
  Two-Dimensional Hubbard Model

abstract:
  The interactions between holes in the Hubbard model, in the low density,
intermediate to strong coupling limit, are investigated by systematically
improving mean field calculations. The Configuration Interaction basis set is
constructed by applying to local Unrestricted Hartree-Fock configurations all
lattice translations and rotations. It is shown that this technique reproduces,
correctly, the properties of the Heisenberg model, in the limit of large U.
Upon doping, dressed spin polarons in neighboring sites have an increased
kinetic energy and an enhanced hopping rate. Both effects are of the order of
the hopping integral and lead to an effective attraction at intermediate
couplings.


title: Falicov-Kimball Models: A Partial Review of the Ground States Problem

abstract:
  In this review we present a biased review of the ground state properties of
the Falicov-Kimball models in 1,2 and infinite dimensions, considering either
fermions or hard-core bosons. In particular we want to show the very rich
structure that these models exhibit and to point out the analogies and
differences associated with the statistic of the quantum particles and the
nature of the lattice (bipartite or not). The flux phase problem is also
discussed.


title:
Random Walkers in 1-D Random Environments: Exact Renormalization Group
  Analysis

abstract:
  Sinai's model of diffusion in one-dimension with random local bias is studied
by a real space renormalization group which yields exact results at long times.
The effects of an additional small uniform bias force are also studied. We
obtain analytically the scaling form of the distribution of the position $x(t)$
of a particle, the probability of it not returning to the origin and the
distributions of first passage times, in an infinite sample as well as in the
presence of a boundary and in a finite size sample. We compute the distribution
of meeting time of two particles. We also obtain a detailed analytic
description of thermally averaged trajectories: we compute the distributions of
the number of returns and of the number of jumps forward. They obey
multifractal scaling, characterized by generalized persistence exponents
$\theta(g)$ which we compute. With a small bias, the number of returns is
finite, characterized by a universal scaling function. The statistics of the
successive times of return of thermally averaged trajectories is obtained. The
two time distribution of the positions of a particle, $x(t)$ and $x(t')$
($t>t'$) is computed exactly. It exhibits ``aging'' with several regimes:
without a bias, for $t-t' \sim t'^\alpha, \alpha > 1$, it exhibits a $(\ln
t)/(\ln t')$ scaling, with a novel singularity at rescaled positions
$x(t)=x(t')$. For closer times $\alpha<1$ there is a quasi-equilibrium regime
with $\ln(t-t')/\ln t'$ scaling. The crossover to a $t/t'$ aging form under a
small bias is obtained analytically. Rare events, e.g. splitting of the thermal
packet between wells, are also studied. Connections with the Green's function
of a 1D Schr\"odinger problem and quantum spin chains are discussed.


title: Snaking of a fluid in two dimensions

abstract:
  The nature of physical processes can depend substantially on the
dimensionality of a system. One such example are buckling instabilities, which
arise from the competition between axial compression and bending in elastic
filaments. Thus, coiling of a jet of viscous fluid falling on a substrate
(honey poured on a toast) \cite{maha1,griffiths} behaves quite differently from
the coiling of viscous sheets (volcanous lava sliding on a crust)
\cite{johnson,ramberg}. Here we consider a novel effect of coiling of a fluid
jet confined to a two dimensional film.


title:
Resonant transport through midgap states in voltage-biased Josephson
  junctions of d-wave superconductors

abstract:
  We study theoretically the ac Josephson effect in voltage biased planar
junctions of d-wave superconductors. For some orientations of the
superconductors a current peak is found at finite voltage in the
current-voltage characteristics. We pick out the relevant physical processes
and write down an analytical formula for the current which clearly shows how
the midgap state acts as a resonance and produces the peak. We present a
possible explanation for the zero-bias conductance peak, recently found in
experiments on grain boundary junctions of high-temperature superconductors, in
terms of resonant transmission through midgap state of quasiparticles
undergoing multiple Andreev reflections. We note that within our framework the
zero-bias conductance peak appears in rather transparent Josephson junctions of
d-wave superconductors.


title: Order Parameter Symmetries in High Temperature Superconductors

abstract:
  The symmetry operations of the crystal groups relevant for the high
temperature superconductors HgBa2CuO4+x (Hg1201), YBa2Cu3O7-x (YBCO), and
  Bi2Sr2CaCu2O8+x (BSCCO) are elucidated. The allowable combinations of the
superconducting order parameter (OP) components are presented for both the
angular momentum and lattice representations. For tetragonal Hg1201, the spin
singlet OP components are composed from four sets of compatible basis
functions, which combine to give the generalized s-, dx2-y2-, dxy-, and
gxy(x2-y2)- wave OPs. In YBCO, elements of s- and dx2-y2- wave sets are
compatible, but in BSCCO, elements of s- and dxy- wave sets are compatible. The
Josephson critical current density JcJ across c-axis twist junctions in the
vicinity of Tc is then evaluated as a function of the twist angle phi0, for
each allowable OP combination, for both coherent and incoherent tunneling.
Recent experiments of Li et al. demonstrated the independence of JcJ(phi0)/JcS
upon phi0 at and below Tc, where JcS is the critical current density of a
constituent single crystal. These experiments are shown to be consistent with
an OP containing an s-wave component, but inconsistent with an OP containing
the purported dx2-y2-wave component. In addition, they demonstrate that the
interlayer tunneling across untwisted layers in single crystal BSCCO is
entirely incoherent. We propose a new type of tricrystal experiment using
single crystal c-axis twist junctions, that does not employ substrate grain
boundaries.


title:
Reply to "Comment on `General Method to Determine Replica Symmetry
  Breaking Transitions'"

abstract:
  In a comment cond-mat/9811080 to our paper (PRL, vol 81, 1698 (1998)) H.
Bokil, A. Bray, B. Drossel and M. Moore claimed that we have reached wrong
conclusions. We show here why their claims are not correct, especially when
compared to the analysis of reference cond-mat/9808140.


title: Chaos and Synchronized Chaos in an Earthquake Model

abstract:
  We show that chaos is present in the symmetric two-block Burridge-Knopoff
model for earthquakes. This is in contrast with previous numerical studies, but
in agreement with experimental results. In this system, we have found a rich
dynamical behavior with an unusual route to chaos. In the three-block system,
we see the appearance of synchronized chaos, showing that this concept can have
potential applications in the field of seismology.


title:
Effects of disorder on the optical properties of the (Zn,Mg)(S,Se)
  quaternary alloy

abstract:
  The electronic and optical properties of (Zn,Mg)(S,Se) wide-gap solid
solutions are studied using ab initio techniques and starting from the
previously determined atomistic structure of the alloy. Compositional disorder
is shown to close substantially the gap with respect to the predictions of the
virtual-crystal approximation. The bowing of the fundamental gap vs.
composition predicted by our calculations is in very good agreement with
experiments available for the Zn(S,Se) pseudo-binary alloy. At temperatures
typical for MBE growth, the quaternary alloy displays a rather large amount of
short-range order whose effect is to slightly but unmistakably open the gap.
Our results agree well with recent experimental data for the quaternary alloy.


title:
Theory of Ferromagnetic Metal to Paramagnetic Insulator Transition i in
  R_{1-x}A_{x}MnO_{3}

abstract:
  The double-exchange model for the Mn oxides with orbital degeneracy is
studied with including on-site Coulomb repulsion, Jahn-Teller (J-T) coupling
and doping-induced disorder. In the strong interaction limit, it is mapped onto
a single-band Anderson model, in which all scattering mechanisms can be treated
on an equal footing. A sharp rise in the mean square fluctuation of lattice
distortions is found near the Curie temperature $T_c$, in agreement with
experiments. We show that the spin and J-T disorders lead to a metal-insulator
transition (MIT) only at low carrier density. The MIT observed in samples with
$0.2\leq x<0.5$ can be explained by further including the disorder effect of
cation size mismatch.


title: Pressure of Membrane between Walls

abstract:
  For a single membrane of stiffness kappa fluctuating between two planar walls
of distance d, we calculate analytically the proportionality constant in the
pressure law p proportional to T^2/kappa^2 d^3, in very good agreement with
results from Monte Carlo simulations.


title: Sine-Gordon low-energy effective theory for Copper Benzoate

abstract:
  Specific heat data for the quasi one dimensional quantum magnet Copper
Benzoate (Cu(C_6D_5COO)_2 \cdot 3D_2O) is analyzed in the framework of an
effective low-energy description in terms of a Sine-Gordon theory.


title: One-dimensional diffusion in a semi-infinite Poisson random force

abstract:
  We consider the one-dimensional diffusion of a particle on a semi-infinite
line and in a piecewise linear random potential. We first present a new
formalism which yields an analytical expression for the Green function of the
Fokker-Planck equation, valid for any deterministic construction of the
potential profile. The force is then taken to be an asymmetric dichotomic
process. Solving the corresponding energy-dependent stochastic Riccati equation
in the space-asymptotic regime, we give an exact probabilistic description of
returns to the origin. This method allows for a time-asymptotic
characterization of the underlying dynamical phases. When the two values taken
by the dichotomic force are of different signs, there occur trapping potential
wells with a broad distribution of trapping times and dynamical phases may
appear, depending on the mean force. If both values are negative, the
time-asymptotic mean value of the probability density at the origin is
proportional to the absolute value of the mean force. If they are both
positive, traps no more exist and the dynamics is always normal. Problems with
a shot-noise force and with a Gaussian white-noise force are solved as
appropriate limiting cases.


title: An X-Ray Induced Structural Transition in La_0.875Sr_0.125MnO_3

abstract:
  We report a synchrotron x-ray scattering study of the magnetoresistive
manganite La_0.875Sr_0.125MnO_3. At low temperatures, this material undergoes
an x-ray induced structural transition at which charge ordering of Mn^3+ and
Mn^4+ ions characteristic to the low-temperature state of this compound is
destroyed. The transition is persistent but the charge-ordered state can be
restored by heating above the charge-ordering transition temperature and
subsequently cooling. The charge-ordering diffraction peaks, which are
broadened at all temperatures, broaden more upon x-ray irradiation, indicating
the finite correlation length of the charge-ordered state. Together with the
recent reports on x-ray induced transitions in Pr_(1-x)Ca_xMnO_3, our results
demonstrate that the photoinduced structural change is a common property of the
charge-ordered perovskite manganites.


title: Bound States for a Magnetic Impurity in a Superconductor

abstract:
  We discuss a solvable model describing an Anderson like impurity in a BCS
superconductor. The model can be mapped onto an Ising field theory in a
boundary magnetic field, with the Ising fermions being the quasi-particles of
the Bogoliubov transformation in BCS theory. The reflection S-matrix exhibits
Andreev scattering, and the existence of bound states of the quasi-particles
with the impurity lying inside the superconducting gap.


title: Electronic correlations in transport through coupled quantum dots

abstract:
  The conductance through two quantum dots in series is studied using general
qualitative arguments and quantitative slave-boson mean-field theory. It is
demonstrated that measurements of the conductance can explore the phase diagram
of the two-impurity Anderson model. Competition between the Kondo effect and
the inter-dot magnetic exchange leads to a two-plateau structure in the
conductance as a function of gate voltage and a two or three peak structure in
the conductance vs. inter-dot tunneling.


title: Berry's Phase Induced Bose-Einstein condensation into a Vortex State

abstract:
  The existence of a geometric phase in magnetic traps can be used to Bose
condense a magnetically trapped atomic gas into a vortex state. We propose an
experimental setup where a magnetic trap together with a blue detuned laser
beam form a multiply connected trap geometry. The local variation of the
magnetic quantization axis induces a geometric or Berry's phase that allows the
atoms to acquire an effective gauge charge interacting with the analog of a
magnetic solenoid. It is shown that the ground state of such a system may be
given by a vortex state. We also discuss the influence of atomic interactions
on the proposed vortex production scheme in the context of present
Bose-Einstein condensation experiments with dilute gases.


title:
Anisotropic thermodynamics and $T/\sqrt H$ scaling of d-wave
  superconductors in the vortex state

abstract:
  The density of states of a 2D d-wave superconductor in the vortex state with
applied magnetic field $\bf H$ in the plane is shown to exhibit fourfold
oscillations as a function of the angle of the field with respect to the
crystal axes. We find further that the frequency dependence of the density of
states and the temperature dependence of transport coefficients obey different
power laws, thus leading to different magnetic scaling functions, for field in
the nodal and anti-nodal direction. We discuss the consequences of this
anisotropy for measurements of the specific heat.


title: Spin-Wave Excitations of Half-Filled Kondo Lattice Model

abstract:
  The spin excitations in the antiferromagnetic phase of half-filled Kondo
lattice model are studied by means of the decoupling approximation for spin
Green's function. The spin-wave spectrum is calculated as a function of Kondo
coupling, and this is used to calculate the thermodynamic quantities at low
temperatures. The N\'{e}el temperature of the form
  $T_{\rm N}=0.087{J}^2\ln{(1/J)}$ is obtained for the 3-dimensional case in
the weak-coupling limit. It is pointed out that the ratio of the spin-wave
velocity to the N\'{e}el temperature $v_s/T_{\rm N}$ is enhanced as the Kondo
coupling becomes small, reflecting the long-range nature of effective
interactions between localized spins.


title:
Imaging Electron Wave Functions of Quantized Energy Levels in Carbon
  Nanotubes

abstract:
  Carbon nanotubes provide a unique system to study one-dimensional
quantization phenomena. Scanning tunneling microscopy is used to observe the
electronic wave functions that correspond to quantized energy levels in short
metallic carbon nanotubes. Discrete electron waves are apparent from periodic
oscillations in the differential conductance as a function of the position
along the tube axis, with a period that differs from that of the atomic
lattice. Wave functions can be observed for several electron states at adjacent
discrete energies. The measured wavelengths are in good agreement with the
calculated Fermi wavelength for armchair nanotubes.


title: Novel Nonreciprocal Acoustic Effects in Antiferromagnets

abstract:
  The possible occurrence of nonreciprocal acoustic effects in antiferromagnets
in the absence of an external magnetic field is investigated using both (i) a
microscopic formulation of the magnetoelastic interaction between spins and
phonons and (ii) symmetry arguments. We predict for certain antiferromagnets
the existence of two new nonreciprocal (non-time invariant) effects:
  A boundary-condition induced nonreciprocal effect and the occurrence of
transversal phonon modes propagating in opposite directions having different
velocities. Estimates are given and possible materials for these effects to be
observed are suggested.


title: Shadow bands, gap and pseudogaps in high-T_c superconductors

abstract:
  Within the framework of the Charge Density Wave Quantum Critical Point
(CDW-QCP) scenario for high-T_c superconductors (HTCS), we introduce a model
for tight-binding electrons coupled to quasi-critical fluctuations. In the
normal state our model reproduces features of the Fermi Surface (FS) observed
in ARPES measurements on optimally doped Bi2212, such as the anisotropic
suppression of spectral weight around the M points of the Brillouin zone. The
spectral density is characterized by a transfer of spectral weight from the
main quasi-particle peak to dispersing shadow peaks which originate branches of
a shadow FS.
  In the superconducting state our model reproduces the d-wave symmetry of the
gap parameter, which results from a balance between small-q attraction and
large-q repulsion. The gap parameter is enhanced due to cooperative effects of
charge and spin fluctuations.


title: Polymer Adsorption on Fractal Walls

abstract:
  Polymer adsorption on fractally rough walls of varying dimensionality is
studied by renormalization group methods on hierarchical lattices. Exact
results are obtained for deterministic walls. The adsorption transition is
found continuous for low dimension $d_w$ of the adsorbing wall and the
corresponding crossover exponent $\phi$ monotonically increases with $d_w$,
eventually overcoming previously conjectured bounds. For $d_w$ exceeding a
threshold value $d_w^*$, $\phi$ becomes 1 and the transition turns
first--order. $d_w^*>d_{saw}$, the fractal dimension of the polymer in the
bulk. An accurate numerical approach to the same problem with random walls
gives evidence of the same scenario.


title:
Atomic structure of silver chloride formed on Ag(111) surface upon low
  temperature chlorination

abstract:
  The structure of AgCl formed in the course of chlorine adsorption on Ag(111)
surface was studied by STM. For the first time atomic resolution STM images of
silver chloride were obtained. The silver chloride was detected as small (30-60
A in diameter) islands located on the atomic terraces. The upper plane of the
islands was found to be AgCl(111) with interatomic distance close to the bulk
value. The chloride surface lattice was not rotated with respect to the
substrate. A possible mechanism of AgCl growth on Ag(111) is discussed.


title:
Spectral correlations in systems undergoing a transition from
  periodicity to disorder

abstract:
  We study the spectral statistics for extended yet finite quasi 1-d systems
which undergo a transition from periodicity to disorder. In particular we
compute the spectral two-point form factor, and the resulting expression
depends on the degree of disorder. It interpolates smoothly between the two
extreme limits -- the approach to Poissonian statistics in the (weakly)
disordered case, and the universal expressions derived for the periodic case.
The theoretical results agree very well with the spectral statistics obtained
numerically for chains of chaotic billiards and graphs.


title: Spectrum and diffusion for a class of tight-binding models on hypercubes

abstract:
  We propose a class of exactly solvable anisotropic tight-binding models on an
infinite-dimensional hypercube. The energy spectrum is analytically computed
and is shown to be fractal and/or absolutely continuous according to the value
hopping parameters. In both cases, the spectral and diffusion exponents are
derived. The main result is that, even if the spectrum is absolutely
continuous, the diffusion exponent for the wave packet may be anything between
0 and 1 depending upon the class of models.


title:
Neutron Scattering Study of Temperature-Concentration Phase Diagram of
  (Cu1-xMgx)GeO3

abstract:
  In doped CuGeO3 systems, such as (Cu1-xZnx)GeO3 and Cu(Ge1-xSix)O3, the
spin-Peierls (SP) ordering (T<Tsp) coexists with the antiferromagnetic (AF)
phase (T<TN<Tsp). Tsp decreases while TN increases with increasing x in low
doping region. For higher x, however, the SP state disappears and only the AF
state remains. These features are common for all the doped CuGeO3 systems so
far studied, indicating the existence of universal T-x phase diagram. Recently,
Masuda et al. carried out comprehensive magnetic susceptibility (chi)
measurements of (Cu1-xMgx)GeO3, in which doping concentration can be controlled
significantly better than the Zn doped systems. They found that TN suddenly
jumps from 3.43 to 3.98K at the critical concentration xc sim 0.023 and that a
drop in chi corresponding to the SP ordering also disappears at x>xc. They thus
concluded that there is a compositional phase boundary between two distinct
magnetic phases. To clarify the nature of two phases, we performed
neutron-scattering measurements on (Cu1-xMgx)GeO3 single crystals with various
x. Analysis of the data at fixed temperature points as a function of doping
concentration has revealed sudden changes of order parameters at the critical
concentration xc=0.027 +- 0.001. At finite temperatures below TN, the drastic
increase of the AF moment takes place at xc. The spin-Peierls order parameter
delta associated with lattice dimerization shows a precipitous decrease at all
temperature below Tsp. However, it goes to zero above xc only at the low
temperature limit.


title: Conductances in normal and normal-superconductor structures

abstract:
  We study theoretically electronic transport through a normal metal--
superconductor (NS) interface and show that more than one conductance may be
defined, depending on the pair of chemical potentials whose difference one
chooses to relate linearly to the current. We argue that the situation is
analogous to that found for purely normal transport, where different
conductance formulae can be invoked. We revisit the problem of the "right"
conductance formula in a simple language, and analyze its extension to the case
of mesoscopic superconductivity. The well-known result that the standard
conductance of a NS interface becomes 2 (in units of $2e^2/h$) in the
transmissive limit, is viewed here in a different light. We show that it is not
directly related to the presence of Andreev reflection, but rather to a
particular choice of chemical potentials. This value of 2 is measurable because
only one single-contact resistance is involved in a typical experimental setup,
in contrast with the purely normal case, where two of them intervene. We
introduce an alternative NS conductance that diverges in the transmissive limit
due to the inability of Andreev reflection to generate a voltage drop. We
illustrate numerically how different choices of chemical potential can yield
widely differing I--V curves for a given NS interface.


title: Non Magnetic Impurities in the Spin-Gap Phase of the Cuprate

abstract:
  It is now well established that Zn doping of high-$T_C$ cuprates reduces
their $T_C$ and triggers the appearence of a spin glass phase. In this context,
we have solved exactly the problem of N non magnetic impurities in the
staggered flux phase of the Heisenberg model which we assume to be a good
mean-field approximation for the spin-gap phase of the cuprates. In this model,
the quasiparticule spectrum has four nodes on the Fermi surface, and
linearization of the spectrum in the neighbourhood of these nodes leads to a
system of 2D Dirac fermions. In the presence of a macroscopic number of (non
magnetic) impurities, the problem has a characteristic logarithmic structure
that renders ineffective the usual perturbative expansions. We have used this
logarithmic structure to calculate an exact solution. For a concentration ni of
impurities in the unitary scattering limit, the additional density of states is
found to be proportional to $ni/(w \ln^2 (|w|/D))$ (where D is the infrared
cut-off of the linearized spectrum) in analogy with the 1D case of doped
spin-Peierls and two-leg ladders compounds. We argue that the system exhibits a
quasi long-range order at T=0 with instantaneous spin-spin correlations
decreasing as $ni/ \ln^4 (ni/R_{ij})$ for large distances $R_{ij}$ between
impurity sites. We predict enhanced low energy fluctuations and compare these
results to NMR and inelastic neutron scattering experiments in the high-$T_C$
cuprates.


title: Giant enhancement of anisotropy by electron-phonon interaction

abstract:
  Anisotropic electron-phonon interaction is shown to lead to the anisotropic
polaron effect. The resulting anisotropy of the polaron band is an exponential
function of the electron-phonon coupling and might be as big as $10^3$. This
also makes anisotropy very sensitive to small changes of coupling and implies
wide variations of anisotropy among compounds of similar structure. The isotope
effect on mass anisotropy is predicted. Polaron masses are obtained by an exact
Quantum Monte Carlo method. Implications for high-temperature superconductors
are briefly discussed.


title:
Exact Density of States in a System of Disordered Two Dimensional Dirac
  Fermions in the Unitarity Limit: The d-wave Superconductor

abstract:
  In this paper we present a method to compute the exact density of states
induced by $N$ non magnetic impurities in a system of two dimensional Dirac
fermions in the unitarity limit. We review the case of the $\pi$-flux phase of
the Heisenberg model and also treat the disordered d-wave superconductor. In
both case we find additional states in the gap with $\delta \rho (\omega)
\simeq n_i/ |\omega(\ln^2 |\omega/\Delta_0| + (\pi/2)^2)|$.


title: Optical study of the metal-nonmetal transition in NiS

abstract:
  Optical reflectivity study has been made on the hexagonal (NiAs-type)
Ni_{1-d}S in order to probe its electronic properties, in particular those
associated with the metal-nonmetal transition in this compound. Samples with
d=0.005 and 0.02 are studied, which have transition temperatures Tt=246 K and
161 K, respectively. A pronounced dip appears in the reflectivity spectra upon
the transition, and the optical conductivity spectra show that the electronic
structure below Tt is similar to that of a carrier-doped semiconductor with an
energy gap of ~ 0.15 eV. The optical spectra indicate that the gap becomes
larger with decreasing temperature, and it becomes smaller as d increases. It
is also found that the overall spectrum including the violet region can be
described based on a charge-transfer-type semiconductor, consistent with recent
photoemission results.


title:
One-dimensional fermions with incommensurate hopping close to
  dimerization

abstract:
  We study the spectrum of fermions hopping on a chain with a weak
incommensuration close to dimerization; both q, the deviation of the wave
number from pi, and delta, the strength of the incommensuration, are small. For
free fermions, we use a continuum Dirac theory to show that there are an
infinite number of bands which meet at zero energy as q approaches zero. In the
limit that the ratio q/delta ---> 0, the number of states lying inside the q =
0 gap is nonzero and equal to 2 delta / pi^2. Thus the limit q ---> 0 differs
from q = 0; this can be seen clearly in the behavior of the specific heat at
low temperature. For interacting fermions or the XXZ spin-1/2 chain, we use
bosonization to argue that similar results hold.


title: Metastability in stochastic dynamics of disordered mean-field models

abstract:
  We study a class of Markov chains that describe reversible stochastic
dynamics of a large class of disordered mean field models at low temperatures.
Our main purpose is to give a precise relation between the metastable time
scales in the problem to the properties of the rate functions of the
corresponding Gibbs measures. We derive the analog of the Wentzell-Freidlin
theory in this case, showing that any transition can be decomposed, with
probability exponentially close to one, into a deterministic sequence of
``admissible transitions''. For these admissible transitions we give upper and
lower bounds on the expected transition times that differ only by a constant.
  The distribution rescaled transition times are shown to converge to the
exponential distribution. We exemplify our results in the context of the random
field Curie-Weiss model.


title:
Critical properties of the three-dimensional equivalent-neighbor model
  and crossover scaling in finite systems

abstract:
  Accurate numerical results are presented for the three-dimensional
equivalent-neighbor model on a cubic lattice, for twelve different interaction
ranges (coordination number between 18 and 250). These results allow the
determination of the range dependences of the critical temperature and various
critical amplitudes, which are compared to renormalization-group predictions.
In addition, the analysis yields an estimate for the interaction range at which
the leading corrections to scaling vanish for the spin-1/2 model and confirms
earlier conclusions that the leading Wegner correction must be negative for the
three-dimensional (nearest-neighbor) Ising model. By complementing these
results with Monte Carlo data for systems with coordination numbers as large as
52514, the full finite-size crossover curves between classical and Ising-like
behavior are obtained as a function of a generalized Ginzburg parameter. Also
the crossover function for the effective magnetic exponent is determined.


title:
Comment on "Electronic Properties of Armchair Carbon Nanotubes:
  Bosonization Approach"

abstract:
  We comment on the paper by H. Yoshioka and A. Odintsov, to appear in PRL, see
cond-mat/9805106.


title: Local Density of States in Superconductor-Ferromagnetic Hybrid Systems

abstract:
  In this Letter we discuss various properties of the local density of states
(DOS) for a superconductor-ferromagnet hybrid system. The DOS is modified at
small energies on both sides of the interface. Due to the interplay of
superconductivity and ferromagnetism, the local DOS depends on the spin
direction. The spin polarization effects extend over a long distance from the
interface both in the superconductor and in the ferromagnet. If the ferromagnet
is of finite lenght, the DOS shows a (spin dependent) gap.


title:
Ac transport studies in polymers by a resistor network and transfer
  matrix approaches: application to polyaniline

abstract:
  A statistical model of resistor network is proposed to describe a polymer
structure and to simulate the real and imaginary components of its ac
resistivity. It takes into account the polydispersiveness of the material as
well as intrachain and interchain charge transport processes. By the
application of a transfer matrix technique, it reproduces ac resistivity
measurements carried out with polyaniline films in different doping degrees and
at different temperatures. Our results indicate that interchain processes
govern the resistivity behavior in the low frequency region while, for higher
frequencies, intrachain mechanisms are dominant.


title: Alternating steady state in one-dimensional flocking

abstract:
  We study flocking in one dimension, introducing a lattice model in which
particles can move either left or right. We find that the model exhibits a
continuous nonequilibrium phase transition from a condensed phase, in which a
single `flock' contains a finite fraction of the particles, to a homogeneous
phase; we study the transition using numerical finite-size scaling.
Surprisingly, in the condensed phase the steady state is alternating, with the
mean direction of motion of particles reversing stochastically on a timescale
proportional to the logarithm of the system size. We present a simple argument
to explain this logarithmic dependence. We argue that the reversals are
essential to the survival of the condensate. Thus, the discrete directional
symmetry is not spontaneously broken.


title: Surface electronic Structure and magnetic properties of doped manganites

abstract:
  The electronic structure and magnetic properties of La_{1-x}A_{x}MnO_3 are
investigated. It is assumed that, at the outermost layer, the environment of
the Mn ions does not have cubic symmetry. The e_g orbitals are split and the
double exchange mechanism is weakened. The charge state of the Mn ions is
modified, and the magnetic ordering of the spins tends to be antiferromagnetic.
The surface magnetization and the dependence of the transport properties
through the resulting surface barrier on applied magnetic field and temperature
is analyzed.


title: Andreev reflection in Si-engineered Al/InGaAs hybrid junctions

abstract:
  Andreev-reflection dominated transport is demonstrated in Al/n-In0.38Ga0.62As
superconductor-semiconductor junctions grown by molecular beam epitaxy on
GaAs(001). High junction transparency was achieved in low-doped devices by
exploiting Si interface bilayers to suppress the native Schottky barrier. It is
argued that this technique is ideally suited for the fabrication of ballistic
transport hybrid microstructures.


title: Three-body decay of a rubidium Bose-Einstein condensate

abstract:
  We have measured the three-body decay of a Bose-Einstein condensate of
rubidium ($^{87}$Rb) atoms prepared in the doubly polarized ground state
$F=m_F=2$. Our data are taken for a peak atomic density in the condensate
varying between $2\times 10^{14}$ cm$^{-3}$ at initial time and $7\times
10^{13}$ cm$^{-3}$, 16 seconds later. Taking into account the influence of the
uncondensed atoms onto the decay of the condensate, we deduce a rate constant
for condensed atoms $L=1.8 (\pm 0.5) \times 10^{-29}$ cm$^{6} $s$^{-1}$. For
these densities we did not find a significant contribution of two-body
processes such as spin dipole relaxation.


title: Spin monopoles with Bose-Einstein condenstates

abstract:
  We study the feasibility of preparing a Bose-Einstein condensed sample of
atoms in a 2D spin monopole. In this state, the atomic internal spins lie in
the x-y plane, and point in the radial direction.


title:
Electronic spectrum and hopping conductivity in highly doped lattice
  systems

abstract:
  A model of strongly disordered lattice system with long-range Coulomb
interactions between localized charge carriers has been considered. The total
electronic energy is characterized by the presence of multiple metastable
minima (including the true ground state), and different types of excitation
spectra over these minima. A numeric procedure, accounting for all many-body
correlations in finite size samples, confirms the existence of Coulomb gap in
the single-particle spectrum and also provides corrections to the known
mean-field theory results, as asymmetry of the gap at non-half-filling,
vanishing density of low energy pair excitations, modified temperature exponent
for hopping conductivity.


title: Abelian-Higgs-Navier-Stokes Hydrodynamics for Nematic Films with Defects

abstract:
  A new theory of hydrodynamics of uniaxial nematic liquid crystal films in the
presence of defects is developed. A gauge field incorporating screening is
introduced, resulting in the static elastic free energy having the form of a
two-dimensional Abelian-Higgs model. Hydrodynamic equations are derived via the
standard methods of de~Groot and Mazur. By working in the vicinity of the
Bogomol'nyi equations consequences for defect centre motion are outlined.


title:
Casimir Force between Vortex Matter in Anisotropic and Layered
  Superconductors

abstract:
  We present a new approach to calculate the attractive long range
vortex-vortex interaction of the van der Waals type present in anisotropic and
layered superconductors. The mapping of the statistical mechanics of vortex
lines onto the imaginary time quantum mechanics of two dimensional charged
bosons allows us to define a 2D Casimir problem: Two half-spaces of (dilute)
vortex matter separated by a gap of width R are mapped to two dielectric
half-planes of charged bosons interacting via a massive gauge field. We
determine the attractive Casimir force between the two half-planes and show,
that it agrees with the pairwise summation of the van der Waals force between
vortices previously found by Blatter and Geshkenbein [Phys. Rev. Lett. 77, 4958
(1996)]


title:
A numerical approach to the ground and excited states of a Bose-Einstein
  condensed gas confined in a completely anisotropic trap

abstract:
  The ground and excited states of a weakly interacting and dilute
Bose-Einstein condensed gas, confined in a completely anisotropic harmonic
oscillator potential, are determined at zero temperature within the Bogoliubov
approximation. The numerical calculations employ a computationally efficient
procedure based on a discrete variable representation (DVR) of the Hamiltonian.
The DVR is efficient for problems where the interaction potential may be
expressed as a local function of interparticle coordinates. In order to address
condensates that are both very large (millions of atoms) and fully anisotropic,
the ground state is found using a self-consistent field approach. Experience
has demonstrated, however, that standard iterative techniques applied to the
solution of the non-linear partial differential equation for the condensate are
non-convergent. This limitation is overcome using the method of direct
inversion in the iterated subspace (DIIS). In addition, the sparse structure of
the DVR enables the efficient application of iterative techniques such as the
Davidson and/or Lanczos methods, to extract the eigenvalues of physical
interest. The results are compared with recent experimental data obtained for
Bose-Einstein condensed alkali metal vapors confined in magnetic traps.


title:
Antiferromagnetic Potts Models on the Square Lattice: A High-Precision
  Monte Carlo Study

abstract:
  We study the antiferromagnetic q-state Potts model on the square lattice for
q=3 and q=4, using the Wang-Swendsen-Kotecky (WSK) Monte Carlo algorithm and a
powerful finite-size-scaling extrapolation method. For q=3 we obtain good
control up to correlation length $\xi \sim 5000$; the data are consistent with
$\xi(\beta) = A e^{2\beta} \beta^p (1 + a_1 e^{-\beta} + ...)$ as
$\beta\to\infty$, with $p \approx 1$. The staggered susceptibility behaves as
$\chi_{stagg} \sim \xi^{5/3}$. For q=4 the model is disordered ($\xi \ltapprox
2$) even at zero temperature. In appendices we prove a correlation inequality
for Potts antiferromagnets on a bipartite lattice, and we prove ergodicity of
the WSK algorithm at zero temperature for Potts antiferromagnets on a bipartite
lattice.


title: Metallic ferromagnetism without exchange splitting

abstract:
  In the band theory of ferromagnetism there is a relative shift in the
position of majority and minority spin bands due to the self-consistent field
due to opposite spin electrons. In the simplest realization, the Stoner model,
the majority and minority spin bands are rigidly shifted with respect to each
other. Here we consider models at the opposite extreme, where there is no
overall shift of the energy bands. Instead, upon spin polarization one of the
bands broadens relative to the other. Ferromagnetism is driven by the resulting
gain in kinetic energy. A signature of this class of mechanisms is that a
transfer of spectral weight in optical absorption from high to low frequencies
occurs upon spin polarization. We show that such models arise from generalized
tight binding models that include off-diagonal matrix elements of the Coulomb
interaction. For certain parameter ranges it is also found that reentrant
ferromagnetism occurs. We examine properties of these models at zero and finite
temperatures, and discuss their possible relevance to real materials.


title:
Microwave surface impedance of proximity-coupled superconducting (Nb) /
  spin-glass (CuMn) bilayers

abstract:
  The surface impedance of Nb/CuMn (superconducting/spin-glass) bilayers has
been measured at 10 GHz with the parallel plate resonator technique to obtain
information about the exotic behavior of the order parameter in
superconducting/magnetic proximity systems. The data strongly differs from the
superconducting/normal-metal case, showing the magnetic nature of the CuMn
layer, which acts as a weak ferromagnet. The results are described in the
framework of two models for the electrodynamics of
superconducting/ferromagnetic (S/M) bilayers characterized by a
proximity-coupling length scale which is independent of temperature.


title: Stability of a Vortex in a Trapped Bose-Einstein Condensate

abstract:
  Based on the method of matched asymptotic expansion and on a time-dependent
variational analysis, we study the dynamics of a vortex in the large-condensate
(Thomas-Fermi) limit. Both methods as well as an analytical solution of the
Bogoliubov equations show that a vortex in a trapped Bose-Einstein condensate
has formally unstable normal mode(s) with positive normalization and negative
frequency, corresponding to a precession of the vortex line around the center
of the trap. In a rotating trap, the solution becomes stable above an angular
velocity $\Omega_m$ characterizing the onset of metastability with respect to
small transverse displacements of the vortex from the central axis.


title:
Temperature-induced spectral weight transfer in
  Bi_2Sr_2CaCu_2O_{8+delta}: a conventional view

abstract:
  In contrast with the recent photoemission observations by Shen et al. of
anomalous temperature-induced momentum-dependent spectral weight transfer in
Bi_2Sr_2CaCu_2O_{8+delta}, we find that in the same material, in spite of the
unusual spectral lineshape change between superconducting and normal states,
the integrated spectral weight displays minimum variation as a function of
temperature, which is in agreement with the sum rule of angle-resolved
photoemission: the integrated spectral weight is proportional to the momentum
density, n(k).


title:
Multifractal scaling of electronic transmission resonances in perfect
  and imperfect Fibonacci $\delta$-function potentials

abstract:
  We present here a detailed multifractal scaling study for the electronic
transmission resonances with the system size for an infinitely large one
dimensional perfect and imperfect quasiperiodic system represented by a
sequence of $\delta$-function potentials. The electronic transmission
resonances in the energy minibands manifest more and more fragmented nature of
the transmittance with the change of system sizes. We claim that when a small
perturbation is randomly present at a few number of sites, the nature of
electronic states will change and this can be understood by studying the
electronic transmittance with the change of system size. We report the
different critical states manifested in the size variation of the transmittance
corresponding to the resonant energies for both perfect and imperfect cases
through multifractal scaling study for few of these resonances.


title:
Transport and magnetic anomalies due to A-site ionic size mismatch in
  La$_{0.5}$Ca$_{0.5-x}$Ba_{x}MnO$_3$

abstract:
  We present results of electrical resistivity, magnetoresistance and ac and dc
magnetic susceptibility on polycrystalline samples of the type
La(0.5)Ca(0.5-x)Ba(x)MnO(3) synthesized under identical heat treatment
conditions. The substitution of larger Ba ions for Ca results in a non-
monotonic variation of the curie temperature as the system evolves from a
charge ordered insulating state for x=0 to a ferromagnetic metallic state for
x=0.5. An intermediate compositino, x=0.1, interestingly exhibits
ferromagnetic. insulating behaviour with thermal hysteresis in ac chi around
the curie tem- perature (120K). The x=0.2 and 0.3 compounds exhibit
semiconducting like behavior as the temperature is lowered below 300K, with a
broad peak in rho around 80-100K: These compositions exhibit a weak increase in
rho as the temperature lowered below 30K, indicative of electron localization
effects. These compositions also undergo ferromagnetic transitions below about
200 and 235K respectively, though these are non-hysteretic; above all, for
these compositions, MR is large and conveniently measurable over the entire
tempera- ture range of measurement below Tc. This experimental finding may be
of interest from the application point of view. We infer that the A-site
ionic-size mismatch plays a crucial role in the deciding these properties.


title: New Class of Non-Abelian Spin-Singlet Quantum Hall States

abstract:
  We present a new class of non-abelian spin-singlet quantum Hall states,
generalizing Halperin's abelian spin-singlet states and the Read-Rezayi
non-abelian quantum Hall states for spin-polarized electrons. We label the
states by (k,M) with M odd (even) for fermionic (bosonic) states, and find a
filling fraction $\nu=2k/(2kM+3)$. The states with M=0 are bosonic spin-singlet
states characterized by an SU(3)_k symmetry. We explain how an effective
Landau-Ginzburg theory for the SU(3)_2 state can be constructed. In general,
the quasi-particles over these new quantum Hall states carry spin, fractional
charge and non-abelian quantum statistics.


title:
Bifurcation at the c=3/2 Takhtajan-Babujian point to the c=1 critical
  lines

abstract:
  We study the S=1 quantum spin chains with bilinear, biquadratic, plus bond
alternation in the vicinity of the S=1 Takhtajan-Babujian model. Transition
line between the Haldane and the dimer phases are determined numerically. To
see the crossover behavior from c=3/2 (k=2 SU(2) WZW model) at the
Takhtajan-Babujian point to c=1 (k=1 SU(2) WZW model), we calculate the
conformal anomaly c and scaling dimensions of the primary fields on the
transition line.


title:
Coherent Transport through a Quantum Dot Embedded in an Aharonov-Bohm
  Ring

abstract:
  We study the coherent transport in multi-terminal mesoscopic Aharonov-Bohm
ring with a quantum dot embedded in an arm. Employing the Friedel sum rule for
the effective single-particle levels in the quantum dot, we explain some
anomalous features which have been observed in the experiment. We attribute
these anomalies to the result of nontrivial quantum interference of the quantum
dot with the attached ring. Further, we propose a new feature of conductance
oscillations which can be a test for the validity of our model.


title:
Protein Foldability and Designability: General Physics and Pretty
  Chemistry

abstract:
  Making use of a simplified model for protein folding, it can be shown that
conformations which are particularly stable when their energy is minimized with
respect to amino acid sequence (in the sense that they display a large energy
gap to the lowest structrally dissimilar conformation), aside from leading to
fast folding, are highly designable (in the sense that many sequences target
onto it in the folding process). These results are quite general, do not depend
on the particular simmetries displayed by the compact conformation chosen as
native, and can be obtained making use of a large class of contact energies. On
the other hand, the design of sequences which fold onto native conformations
displaying secondary structures and eventually tertiary simmetries, is a
difficult task requiring a delicate tuning of the contact energies.


title:
Evolution of One-Particle and Double-Occupied Green Functions for the
  Hubbard Model at Half-Filling With Lifetime Effects Within The Moment
  Approach

abstract:
  We evaluate the one-particle and double-occupied Green functions for the
Hubbard model at half-filling using the moment approach of Nolting. Our
starting point is a self-energy, $\Sigma(\vec{k},\omega)$, which has a single
pole, $\Omega(\vec{k})$, with {\it spectral} weight, $\alpha(\vec{k})$, and
quasi-particle lifetime, $\gamma(\vec{k})$. In our approach,
$\Sigma(\vec{k},\omega)$ becomes the central feature of the many-body problem
and due to three unkown $\vec{k}$-parameters we have to satisfy only the first
three sum rules instead of four as in the canonical formulation of Nolting.
This self-energy choice forces our system to be a non-Fermi liquid for any
value of the interaction, since it does not vanish at zero frequency. The
one-particle Green function, $G(\vec{k},\omega)$, shows the finger-print of a
strongly correlated system, i.e., a double peak structure in the one-particle
spectral density, $A(\vec{k},\omega)$, vs $\omega$ for intermediate values of
the interaction. Close to the Mott Insulator-Transition, $A(\vec{k},\omega)$,
becomes a wide single peak, signaling the absence of quasi-particles.


title: Effective potential for dissipative quantum systems

abstract:
  We consider quantum nonlinear systems with dissipation described within the
Caldeira-Leggett model, i.e., by a nonlocal action in the path integral for the
density matrix. Approximate classical-like formulas are derived in order to
evaluate thermal averages of observables: the cases of linear and nonlinear
dissipation are considered, and the framework is extended to the case of many
degrees of freedom.


title: Population Inversion Induced by Resonant States in Semiconductors

abstract:
  We present a theoretical prediction of a new mechanism for carrier population
inversion in semiconductors under an applied electric field. The mechanism is
originated from a coherent capture-emission type inelastic scattering of
resonant states. We support our theory with concrete calculations for shallow
acceptor resonant states in strained p-Ge where a lasing in THz frequency
region has been recently observed.


title:
Energy Levels of Quasiperiodic Hamiltonians, Spectral Unfolding, and
  Random Matrix Theory

abstract:
  We consider a tight-binding Hamiltonian defined on the quasiperiodic
Ammann-Beenker tiling. Although the density of states (DOS) is rather spiky,
the integrated DOS is quite smooth and can be used to perform spectral
unfolding. The effect of unfolding on the integrated level-spacing distribution
is investigated for various parts of the spectrum which show different
behaviour of the DOS. For energy intervals with approximately constant DOS, we
find good agreement with the distribution of the Gaussian orthogonal random
matrix ensemble (GOE) even without unfolding. For energy ranges with
fluctuating DOS, we observe deviations from the GOE result. After unfolding, we
always recover the GOE distribution.


title: Electronic states in topologically disordered systems

abstract:
  Networks generated from Voronoi tessellations of space are prototypes for
topologically disordered systems. In order to assess the effects of random
coordination on the statistical properties of energy spectra, we analyze
tight-binding models for planar topologically disordered systems. To this end,
the networks are generated by a simple topological model covering a wide range
of naturally observed random structures. We find that the energy-level-spacing
distributions exhibit level repulsion, similar to the spacings in the
energetically disordered Anderson model of localization in the metallic regime.


title: Theory of the NO+CO surface reaction model

abstract:
  We derive a pair approximation (PA) for the NO+CO model with instantaneous
reactions. For both the triangular and square lattices, the PA, derived here
using a simpler approach, yields a phase diagram with an active state for
CO-fractions y in the interval y_1 < y < y_2, with a continuous (discontinuous)
phase transition to a poisoned state at y_1 (y_2). This is in qualitative
agreement with simulation for the triangular lattice, where our theory gives a
rather accurate prediction for y_2. To obtain the correct phase diagram for the
square lattice, i.e., no active state, we reformulate the PA using sublattices.
The (formerly) active regime is then replaced by a poisoned state with broken
symmetry (unequal sub- lattice coverages), as observed recently by Kortluke et
al. [Chem. Phys. Lett. 275, 85 (1997)]. In contrast with their approach, in
which the active state persists, although reduced in extent, we report here the
first qualitatively correct theory of the NO+CO model on the square lattice.
Surface diffusion of nitrogen can lead to an active state in this case. In one
dimension, the PA predicts that diffusion is required for the existence of an
active state.


title: One Dimensional Phonon Coupled Electron Tunneling: A Realistic Model

abstract:
  The transition probability for a one dimensional tunneling electron coupled
to acoustical phonons is calculated, with the Feynman path-integral method for
zero temperature. We considered a realistic electron phonon interaction
(deformation potential, piezoelectric), making use of slowness of the phonon
system compared to electron tunneling. We show that the problem of the complex
non-linear coupling of a tunneling electron to the zero point fluctuations of a
phonon field is equivalent to that of an electron tunneling through a slow
fluctuating spatially uniform barrier, thus resulting in an increase of the
tunneling probability due to electron coupling with zero-point phonon
oscillations. We calculated also the the energy change of the tunneling
electron due to phonon emission.


title:
Ab-initio structural, elastic, and vibrational properties of carbon
  nanotubes

abstract:
  A study based on ab initio calculations is presented on the estructural,
elastic, and vibrational properties of single-wall carbon nanotubes with
different radii and chiralities. We use SIESTA, an implementation of
pseudopotential-density-functional theory which allows calculations on systems
with a large number of atoms per cell. Different quantities like bond
distances, Young moduli, Poisson ratio and the frequencies of different phonon
branches are monitored versus tube radius. The validity of expectations based
on graphite is explored down to small radii, where some deviations appear
related to the curvature effects. For the phonon spectra, the results are
compared with the predictions of the simple zone-folding approximation. Except
for the known defficiencies of this approximation in the low-frequency
vibrational regions, it offers quite accurate results, even for relatively
small radii.


title: Quantum Oscillations and Overcritical Torque Interaction in Sr2RuO4

abstract:
  Sr2RuO4 is the only known layered perovskite oxide superconductor without
copper; there is strong evidence for an unconventional ("p-wave") pairing
mechanism, and it has recently been shown to possess a cylindrical,
"quasi-two-dimensional" Fermi surface. Using Sr2RuO4 as a test case for the
detection of quantum oscillations with piezoresistive microcantilevers, the
piezolever torque magnetometry technique was successfully implemented on a
dilution refrigerator. It was possible to reproduce the quantum-oscillatory
magnetization data on all three Fermi surface sheets, in a crystal of microgram
mass. Moreover, an absolute estimate of the amplitude of the oscillation is
provided. We also investigated the phenomenon of torque interaction which
distorts the magnetization signal and introduces harmonics and sidebands to the
dHvA spectrum. As the torque interaction effect grows in strength,
overcriticality is shown to lead to discrete magnetization jumps and to a
near-asymptotically damped "sproing" effect.


title: Critical exponents in stochastic sandpile models

abstract:
  We present large scale simulations of a stochastic sandpile model in two
dimensions. We use moments analysis to evaluate critical exponents and finite
size scaling method to consistently test the obtained results. The general
picture resulting from our analysis allows us to characterize the large scale
behavior of the present model with great accuracy.


title: Theory of a dilute low-temperature trapped Bose condensate

abstract:
  This set of four lectures reviews various aspects of the theory of a dilute
low-temperature trapped Bose gas, starting with (I) a review of the Bogoliubov
description of the elementary excitations in a uniform system. The treatment is
then generalized (II) to include the new physical effects of a confining
harmonic trap potential on the condensate and its normal modes. An equivalent
hydrodynamic description (III) focuses directly on the density and velocity
fluctuations. The physics of vortices (IV) in an incompressible fluid is
summarized and extended to the case of a trapped Bose condensate.


title: Vortex Stabilization in Dilute Bose-Einstein Condensate Under Rotation

abstract:
  The stability of a quantized vortex state in Bose-Einstein condensation is
examined within Bogoliubov theory for alkali atom gases confined in a harmonic
potential under forced rotation. By solving the non-linear Bogoliubov equations
coupled with the Gross-Pitavskii equation, the elementary excitations and the
total energy of the systems are calculated as a function of rotation velocity.
There are two distinct criteria of vortex stability; The position of the
excitation energy levels relative to the condensate energy level yields the
local stability criterion, and the total energy relative to that of the
non-vortex state yields the global stability criterion. The vortex stability
phase diagram in the rotation velocity vs the particle density of the system is
obtained, allowing one to locate the appropriate region to observe the singly
quantized vortex.


title:
Blocking of inter-subspace tunneling by intra subspace inelastic
  scattering

abstract:
  In recent years the notion of intrinsic decoherence and dephasing of a
particle interacting with its environment is being investigated intensively.
This has an important bearing on a plausible causal connection between
incoherent c-axis resistivity and high-temperature superconductivity. In our
work we study the tunnel supression and incoherent motion of a particle
tunneling between two sites. The bosonic excitations of the environment are
coupled to only one site inducing on-site spin flips. We show that this on-site
spin flip scattering makes the tunnel motion incoherent. In the
high-temperature limit incoherent rate or hopping rate has been calculated. We
also briefly discuss the renormalization of the effective tunneling by
environmental coupling at zero temperature following Wegner's renormalization
group procedure.


title:
Griffiths-McCoy Singularities in the Random Transverse-Field Ising Spin
  Chain

abstract:
  We consider the paramagnetic phase of the random transverse-field Ising spin
chain and study the dynamical properties by numerical methods and scaling
considerations. We extend our previous work [Phys. Rev. B 57, 11404 (1998)] to
new quantities, such as the non-linear susceptibility, higher excitations and
the energy-density autocorrelation function. We show that in the Griffiths
phase all the above quantities exhibit power-law singularities and the
corresponding critical exponents, which vary with the distance from the
critical point, can be related to the dynamical exponent z, the latter being
the positive root of [(J/h)^{1/z}]_av=1. Particularly, whereas the average spin
autocorrelation function in imaginary time decays as [G]_av(t)~t^{-1/z}, the
average energy-density autocorrelations decay with another exponent as
[G^e]_av(t)~t^{-2-1/z}.


title:
The structure of the ripple phase in chiral and racemic DMPC
  multibilayers

abstract:
  We present electron density maps of the ripple phase of chiral and racemic
dimyristoylphosphatidylcholine. The structures of the two systems are found to
be identical within experimental errors, thus unambiguously showing that the
chirality of the lipid molecules does not influence the structure of this
phase.


title:
On the semiclassical treatment of anharmonic quantum oscillators via
  coherent states - The Toda chain revisited

abstract:
  We use coherent states as a time-dependent variational ansatz for a
semiclassical treatment of the dynamics of anharmonic quantum oscillators. In
this approach the square variance of the Hamiltonian within coherent states is
of particular interest. This quantity turns out to have natural interpretation
with respect to time-dependent solutions of the semiclassical equations of
motion. Moreover, our approach allows for an estimate of the decoherence time
of a classical object due to quantum fluctuations. We illustrate our findings
at the example of the Toda chain.


title: Understanding multifractality: reconstructing images from edges

abstract:
  It has been recently proven that natural images exhibit scaling properties
analogue to those of turbulent flows. These properties allow regarding each
image as a multifractal object, for which its most singular manifold conveys
the most of the non-redundant structure. In the present work, we go further in
this analysis, proposing a simple propagator that reconstructs the whole image
from this set. This fact could have deep implications for biology, technology
and statistical mechanics.


title: Competition between glassiness and order in a multi-spin glass

abstract:
  A mean-field multi-spin interaction spin glass model is analyzed in the
presence of a ferromagnetic coupling. The static and dynamical phase diagrams
contain four phases (paramagnet, spin glass, ordinary ferromagnet and glassy
ferromagnet) and exhibit reentrant behavior. The glassy ferromagnet phase has
anomalous dynamical properties. The results are consistent with a
nonequilibrium thermodynamics that has been proposed for glasses.


title:
Generalized off-equilibrium fluctuation-dissipation relations in random
  Ising systems

abstract:
  We show that the numerical method based on the off-equilibrium
fluctuation-dissipation relation does work and is very useful and powerful in
the study of disordered systems which show a very slow dynamics. We have
verified that it gives the right information in the known cases (diluted
ferromagnets and random field Ising model far from the critical point) and we
used it to obtain more convincing results on the frozen phase of
finite-dimensional spin glasses. Moreover we used it to study the Griffiths
phase of the diluted and the random field Ising models.


title:
Does the Falicov-Kimball model allow for a ferroelectric ground state
  with a spontaneous polarization?

abstract:
  The extrapolation of small-cluster exact-diagonalization calculations is used
to examine the possibility of electronic ferroelectricity in the one
dimensional spinless and spin-one-half Falicov-Kimball model (FKM). It is found
that neither spinless nor spin-one-half version of the FKM does not allow for a
ferroelectric ground state with a spontaneous polarization, i.e. there is no
nonvanishing $<d^{+}f>$-expectation value for vanishing hybridization $V$.


title:
Coexistence of long-range order for two observables at finite
  temperatures

abstract:
  We give a criterion for the simultaneous existence or non existence of two
long-range orders for two observables, at finite temperatures, for quantum
lattice many body systems. Our analysis extends previous results of G.-S. Tian
limited to the ground state of similar models. The proof involves an inequality
of Dyson-Lieb-Simon which connects the Duhamel two-point function to the usual
correlation function. An application to the special case of the Holstein model
is discussed.


title:
Kinetics of electric field induced oxygen ion migration in epitaxial
  metallic oxide films

abstract:
  In this paper we report the observation of curent induced change of
resistance of thin metallic oxide films. The resistance changes at a very low
current (current density $J \geq 10^{3}$ A/cm$^{2}$). We find that the time
dependence associated with the processes (increase of resistance) show a
streched exponential type dependence at lower temperature, which crosses over
to a creep type behavior at $T \geq$ 350 K. The time scale associated shows a
drastic drop in the magnitude at $T \approx$ 350 K, where a long range
diffusion sets in increasing the conductivity noise. The phenomena is like a
"glass-transition" in the random lattice of oxygen ions.


title: Stability of a hard-sphere binary quasicrystal

abstract:
  The stability of a quasicrystalline structure, recently obtained in a
molecular-dynamics simulation of rapid cooling of a binary melt, is analyzed
for binary hard-sphere mixtures within a density-functional approach. It is
found that this quasicrystal is metastable relative to crystalline and fluid
phases for diameter ratios above 0.83. Such trend is partially reversed for
lower diameter ratios, since the quasicrystal becomes stable with respect to
the crystal but does not reach a coexistence with the fluid.


title: Spin Dynamics near the Quantum Critical Point of Heavy-Fermions

abstract:
  The dynamical spin susceptibility is studied in the magnetically-disordered
phase of heavy-Fermion systems near the antiferromagnetic quantum phase
transition. In the framework of the $S=1/2$ Kondo lattice model, we introduce a
perturbative expansion treating the spin and Kondo-like degrees of freedom on
an equal footing. The general expression of the dynamical spin susceptibility
that we derive presents a two-component behaviour: a quasielastic peak as in a
Fermi liquid theory, and a strongly q-dependent inelastic peak typical of a
non-Fermi liquid behaviour. Very strikingly, the position of the inelastic peak
is found to be pushed to zero at the antiferromagnetic transition with a
vanishing relaxation rate. The comparison has been quantitatively made with
Inelastic Neutron Scattering (INS) experiments performed in $CeCu_{6}$ and
$Ce_{1-x}La_{x}Ru_{2}Si_{2}$. The excellent agreement that we have found gives
strong support to a two-band model with new prospects for the study of the
quantum critical phenomena in the vicinity of the magnetic phase transition.


title: Supersymmetric Approach to Heavy-Fermion Systems

abstract:
  We present a new supersymmetric approach to the Kondo lattice model in order
to describe simultaneously the quasiparticle excitations and the low-energy
magnetic fluctuations in heavy-Fermion systems. This approach mixes the
fermionic and the bosonic representation of the spin following the standard
rules of superalgebra. Our results show the formation of a bosonic band within
the hybridization gap reflecting the spin collective modes. The density of
states at the Fermi level is strongly renormalized while the Fermi surface sum
rule includes $n_{c}+1$ states. The dynamical susceptibility is made of a Fermi
liquid superimposed on a localized magnetism contribution.


title: Longitudinal and transverse forces on a vortex in superfluid $^4$He

abstract:
  We examine the transverse and longitudinal components of the drag force upon
a straight vortex line due to the scattering of liquid $^4$He excitations. For
this purpose, we consider a recently proposed Hamiltonian that describes the
dissipative motion of a vortex, giving an explicit expression for the
vortex-quasiparticle interaction. The involved dissipative coefficients are
obtained in terms of the reservoir correlation function. Most of our explicit
calculations are concerned to the range of temperatures below 0.4 K, at which
the reservoir is composed by phonon quasiparticle excitations. We also discuss
some important implications in the determination of possible scattering
processes leading to dissipation, according to the values of vortex mass found
in the literature.


title:
Observation of a temperature dependent electrical resistance minimum
  above the magnetic ordering temperature in Gd$_2$PdSi$_3$

abstract:
  Results on electrical resistivity, magnetoresistance, magnetic Results on
electrical resistivity, magnetoresistance, magnetic susceptibility, heat
capacity and Gd Mossbauer measurements on a Gd-based intermetallic compound,
Gd$_{2}$PdSi$_{3}$ are reported. A finding of interest is that the resistivity
unexpectedly shows a well-defined minimum at about 45 K, well above the long
range magnetic ordering temperature (21 K), a feature which gets suppressed by
the application of a magnetic field. This observation in a Gd alloy presents an
interesting scenario. On the basis of our results, we propose electron
localization induced by s-f (or d-f) exchange interaction prior to long range
magnetic order as a mechanism for the electrical resistance minimum.


title:
Entropy flux and Lagrange multipliers: information theory and
  thermodynamics

abstract:
  We analyze the relation between the nonequilibrium Lagrange multipliers used
in information theory and those used in Liu's technique to exploit the entropy
inequality. We observe that there is not a complete equivalence between Liu's
technique and information theory.


title: Hole Dynamics in Two-Dimensional Antiferromagnetic Mott Insulators

abstract:
  The dispersion relation of a doped hole in the half-filled 2D Hubbard model
is shown to follow a k^4 law around the (0,pi) and (pi,0) points in the
Brillouin zone. Upon addition of pair-hopping processes this dispersion
relation is unstable towards a k^2 law. The above follows from T=0 Quantum
Monte calculations of the single particle spectral function A(k, omega) on 16 X
16 lattices. We discuss finite dopings and argue that the added term restores
coherence to charge dynamics and drives the system towards a d_x^2 - y^2
superconductor.


title:
Analysis of the valence band photoemission spectrum of
  Sr$_2$CuO$_2$Cl$_2$ along the high-symmetry directions

abstract:
  Band structure calculations have been used to identify the different bands
contributing to the polarisation-dependent photoemission spectra of the undoped
model cuprate Sr$_2$CuO$_2$Cl$_2$ at the high-symmetry points of the CuO$_2$
plane $\Gamma$, $(\pi/a,0)$ and $(\pi/a,\pi/a)$ and along the high-symmetry
directions $\Gamma - (\pi/a,\pi/a)$ and $\Gamma - (\pi/a,0)$. Results from
calculations within the local density approximation (LDA) have been compared
with calculations taking into account the strong electron correlations by
LDA+U, with the result that the experimental order of energy levels at the
high-symmetry points is better described by the LDA+U calculation than by the
simple LDA. All the main peaks in the photoemission spectra at the high
symmetry points could be assigned to different Cu 3$d$ and O 2$p$ orbitals
which we have classified according to their point symmetries. The dispersions
along the high-symmetry directions were compared with an 11-band tight-binding
model which was fitted both to the LDA+U band structure calculation and the
angle-resolved photoemission data. The mean field treatment successfully
describes the oxygen derived bands but shows discrepancies for the copper ones.


title: Aperiodic Extended Surface Perturbations in the Ising Model

abstract:
  We study the influence of an aperiodic extended surface perturbation on the
surface critical behaviour of the two-dimensional Ising model in the extreme
anisotropic limit. The perturbation decays as a power of the distance from the
free surface with an oscillating amplitude following some aperiodic sequence.
The asymptotic density is 1/2 so that the mean ampltitude vanishes. The
relevance of the perturbation is discussed by combining scaling arguments of
Cordery and Burkhardt for the Hilhorst-van Leeuwen model and Luck for aperiodic
perturbations. The relevance-irrelevance criterion involves the decay exponent
of the perturbation, the wandering exponent which governs the fluctuation of
the sequence and the bulk correlation length exponent. Analytical results are
obtained for the surface magnetization which displays a rich variety of
critical behaviours. The results are checked through a numerical
finite-size-scaling study. They show that second-order effects must be taken
into account in the discussion of the relevance-irrelevance criterion. The
scaling behaviours of the first gap and the surface energy are also discussed.


title:
Large Low Temperature Magnetoresistance and Magnetic Anomalies in
  Tb$_2$PdSi$_3$ and Dy$_2$PdSi$_3$

abstract:
  The results of heat-capacity, magnetic susceptibility, electrical resistivity
and magnetoresistance $(\Delta \rho/\rho)$ measurements on the compounds
Tb$_2$PdSi$_3$ and Dy$_2$PdSi$_3$, are reported. The results establish that
these compounds undergo long-range magnetic ordering (presumably with a complex
magnetic structure) below (Tc=) 23 and 8 K respectively. The $\Delta \rho/\rho
$ is negative in the vicinity of Tc and the magnitude grows as Tc is approached
from higher temperature as in the case of well-known giant magnetoresistance
systems (La manganite based perovskites); this is attributed to the formation
of some kind of magnetic polarons. The magnitude of magnetoresistance at low
temperatures is quite large, for instance, about 30% in the presence of 60 kOe
field at 5 K in the Dy sample.


title: Shear viscosity of a crosslinked polymer melt

abstract:
  We investigate the static shear viscosity on the sol side of the
vulcanization transition within a minimal mesoscopic model for the
Rouse-dynamics of a randomly crosslinked melt of phantom polymers. We derive an
exact relation between the viscosity and the resistances measured in a
corresponding random resistor network. This enables us to calculate the
viscosity exactly for an ensemble of crosslinks without correlations. The
viscosity diverges logarithmically as the critical point is approached. For a
more realistic ensemble of crosslinks amenable to the scaling description of
percolation, we prove the scaling relation $k=\phi-\beta$ between the critical
exponent $k$ of the viscosity, the thermal exponent $\beta$ associated with the
gel fraction and the crossover exponent $\phi$ of a random resistor network.


title:
The importance of self-consistency in determining interface properties
  of S-I-N and D-I-N structures

abstract:
  We develop a method to solve the Bogoliubov de Gennes equation for
superconductors self-consistently, using the recursion method. The method
allows the pairing interaction to be either local or non-local corresponding to
$s$ and $d$--wave superconductivity, respectively. Using this method we examine
the properties of various $S-I-N$ and $D-I-N$ interfaces. In particular we
self-consistently calculate the spatially varying density of states and the
superconducting order parameter. We see that changing the strength of the
insulating barrier, at the interface, does not, in the case of an $s$--wave
superconductor, dramatically, change the low energy local density of states, in
the superconducting region near the interface. This is in stark contrast to
what we see in the case of a $D-I-N$ interface where the local particle density
of states is changed dramatically. Hence we deduce that in calculating such
properties as the conductance of $S-I-N$ and $D-I-N$ structures it is far more
important to carry out a self-consistent calculations in the $d$--wave case.


title: Thermodynamics of the glassy state

abstract:
  A picture for thermodynamics of the glassy state is introduced. It assumes
that one extra parameter, the effective temperature, is needed to describe the
glassy state. This explains the classical paradoxes concerning the Ehrenfest
relations and the Prigogine-Defay ratio. As a second part, the approach
connects the response of macroscopic observables to a field change with their
temporal fluctuations, and with the fluctuation-dissipation relation, in a
generalized non-equilibrium way.


title: Conductance and Shot Noise for Particles with Exclusion Statistics

abstract:
  The first quantized Landauer approach to conductance and noise is generalized
to particles obeying exclusion statistics. We derive an explicit formula for
the crossover between the shot and thermal noise limits and argue that such a
crossover can be used to determine experimentally whether charge carriers in
FQHE devices obey exclusion statistics.


title:
Renormalization group analysis of the quantum non-linear sigma model
  with a damping term

abstract:
  We investigate the behavior of the zero-temperature quantum non-linear sigma
model in d dimensions in the presence of a damping term of the form f(w)~
|w|^alpha, with 1 \le alpha <2. We find two fixed points: a spin-wave fixed
point FP1 showing a dynamic scaling exponent z=1 and a dissipative fixed point
FP2 with z>1. In the framework of the \epsilon-expansion it is seen that there
is a range of values alpha_*(d) \le alpha \le 2 where the point FP1 is stable
with respect to FP2, so that the system realizes a z=1 quantum critical
behavior even in the presence of a dissipative term. However, reasonable
arguments suggest that in d=2 this range is very narrow. In the broken symmetry
phase we discuss a phenomenological scaling approach, treating damping as a
perturbation of the ordered ground state. The relation of these results with
the pseudogap effect observed in underdoped layered cuprates is discussed.


title: Quantum Phase Transitions in 2d Quantum Liquids

abstract:
  These lectures provide a pedagogical introduction to the theory of continuous
quantum phase transitions. Various two-dimensional condensed matter systems,
such as a superconducting film, a quantum Hall liquid, and an array of
Josephson junctions, undergo such a phase transition at the absolute zero of
temperature. The universality classes defined by repulsively interacting bosons
with and without disorder are shown to play a central role in understanding
these systems.


title:
The effect of the total density of states at Fermi level on spin
  polarised tunnelling in granular La0.55Ho0.15Sr0.3MnO3

abstract:
  We study the spin polarised tunnelling mechanism through magnetisation and
magnetoresistance in the granular polycrystalline colossal magnetoresistive
manganite, La0.55Ho0.15Sr0.3MnO3. This system has a ferromagnetic transition
temperature (Tc) of 255 K associated with a metal-insulator transition around
the same temperature. We have investigated dependence of the magnetoresistance
due to spin polarised tunnelling on temperature and reduced magnetisation of
the sample. We discuss the significance of our results within the realm of a
model recently proposed by us to explain the spin polarised tunnelling in
granular La0.7Sr0.3MnO3, in the light of the recent finding by A. Biswas et al.
(cond-mat/9806084) regarding the evolution of the total density of states at
Fermi level as a function of temperature in colossal magnetoresistive
materials.


title:
Localized states due to the coupling of exciton with the coupled lattice
  oscillators

abstract:
  Discrete nonlinear Schr\"oginger equation (DNLS) of the form, $i \frac{dC_n}
{dt}$ = $C_{n+1}$ + $C_{n-1}$ - $ \chi_n [|C_{n+1}|^2 + |C_{n-1}|^2 - 2
|C_n|^2] C_n$ is used to study the formation of stationary localized states in
one dimensional system due to a single as well as a dimeric nonlinear impurity.
The fully nonlinear chain is also considered. The stability of the states and
its connection with the nonlinear strength is presented. Results are compared
with those obtained from other DNLS. It is found that the DNLS used in this
paper has more impact in the formation of stationary localized states.


title: Heat conduction in the diatomic Toda lattice revisited

abstract:
  The problem of the diverging thermal conductivity in one-dimensional (1-D)
lattices is considered. By numerical simulations, it is confirmed that the
thermal conductivity of the diatomic Toda lattice diverges, which is opposite
to what one has believed before. Also the diverging exponent is found to be
almost the same as the FPU chain. It is reconfirmed that the diverging thermal
conductivity is universal in 1-D systems where the total momentum preserves.


title: Virtual lattice dynamics method in quantum mechanics

abstract:
  General molecular dynamic approach, making possible direct calculation of
eigen values and eigen functions for a quantum-mechanical system of an
arbitrary symmetry is proposed. The method is based on analogy between discrete
representation of the Schr\"{o}dinger equation and the system of Newton
equations describing dynamics of specially constructed virtual lattice. Few
examples demonstrating the method capabilities are considered.


title:
Low energy excitations in crystalline perovskite oxides: Evidence from
  noise experiments

abstract:
  In this paper we report measurements of 1/f noise in a crystalline metallic
oxide with perovskite structure down to 4.2K. The results show existence of
localized excitations with average activation energy $\approx$ 70-80 meV which
produce peak in the noise at T $\approx$ 35-40K. In addition, it shows clear
evidence of tunnelling type two-level-systems (as in glasses) which show up in
noise measurements below 30K.