title: New integrable version of the degenerate supersymmetric t-J model

abstract:
  A new integrable version of the degenerate supersymmetric t-J model is
proposed. In this formulation instead of restricting single occupancy of
electrons at each lattice site we may have up to two electrons at each site. As
a requirement of exact integrability the hopping interaction turns out to be
correlated with the density of electrons in the neighboring sites. The exact
solution of the model is obtained through the coordinate Bethe anzatz.


title: An Exactly Solvable Constrained XXZ Chain

abstract:
  A new family of exactly solvable models is introduced. These models are
generalizations of the XXZ chain where the distance among spins up
($\sigma^z$-basis) cannot be smaller or equal to t (t=0,1,2,...). The case t=0
recovers the standard XXZ chain. The coordinate Bethe ansatz is applied and the
phase diagram is calculated. Exploring the finite-size consequences of
conformal invariance the critical exponents are evaluated exactly at the
critical regions of the phase diagram


title: Parametric level statistics in random matrix theory: Exact solution

abstract:
  An exact solution to the problem of parametric level statistics in
non-Gaussian ensembles of N by N Hermitian random matrices with either soft or
strong level confinement is formulated within the framework of the orthogonal
polynomial technique. Being applied to random matrices with strong level
confinement, the solution obtained leads to emergence of a new connection
relation that makes a link between the parametric level statistics and the
scalar two-point kernel in the thermodynamic limit.

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


title: Diffusion in disordered systems under iterative measurement

abstract:
  We consider a sequence of idealized measurements of time-separation $\Delta
t$ onto a discrete one-dimensional disordered system. A connection with Markov
chains is found. For a rapid sequence of measurements, a diffusive regime
occurs and the diffusion coefficient $D$ is analytically calculated. In a
general point of view, this result suggests the possibility to break the
Anderson localization due to decoherence effects. Quantum Zeno effect emerges
because the diffusion coefficient $D$ vanishes at the limit $\Delta t \to 0$.


title: Lattice Displacements Above Tc in the Layered Manganite La1.2Sr1.8Mn2O7

abstract:
  Neutron diffraction data presented in this paper demonstrates the relevance
of lattice displacement above TC, in our understanding of the evolution of the
crystal structure with temperature in the layered CMR manganite
La1.2Sr1.8Mn2O7. The anomalous temperature behavior of thermal diffuse
scattering (TDS) in La1.2Sr1.8Mn2O7 strongly suggests that it arises from
lattice displacements and correlates directly with anomalies in the
displacement parameters of the O- and Mn-atoms and Mn-O bond lengths. From our
measurements, the insulator - metal transition can be described as a transition
from a high temperature state with disordered Mn-O bond lengths to a low
temperature state with a more uniform distribution on Mn-O bonds. These
observations are in agreement with polaronic charge transport above TC in the
perovskite manganites; as electron hopping is responsible for bond disorder
above TC, below the transition where eg carries are delocalized, any lattice
displacements are uniformly averaged.


title: Theory of Transmission through disordered superlattices

abstract:
  We derive a theory for transmission through disordered finite superlattices
in which the interface roughness scattering is treated by disorder averaging.
This procedure permits efficient calculation of the transmission thr ough
samples with large cross-sections. These calculations can be performed
utilizing either the Keldysh or the Landauer-B\"uttiker transmission
formalisms, both of which yield identical equations. For energies close to the
lowest miniband, we demonstrate the accuracy of the computationally efficient
Wannier-function approximation. Our calculations indicate that the transmission
is strongly affected by interface roughness and that information about scale
and size of the imperfections can be obtained from transmission data.


title:
Direct Imaging of the First Order Spin Flop Transition in the Layered
  Manganite La1.4Sr1.6Mn2O7

abstract:
  Magnetic field induced transitions in the antiferromagnetic layered manganite
La1.4Sr1.6Mn2O7 were studied using magnetization measurements and a
high-resolution magneto-optical imaging technique. We report the first direct
observation of the formation of ferromagnetic domains appearing at the first
order spin-flop transition. The magnetization process proceeds through
nucleation of polarized domains at crystal defect sites and not by domain wall
motion. A small magnetic hysteresis is caused by the nucleation and
annihilation of domains in the mixed state. These results establish a direct
link betweeen the magnetic structure on the atomic scale as seen in neutron
scattering and the macroscopic properties of the sample as seen in
magnetization and conductivity measurements.


title: Fermi Surfaces, Fermi Patches, and Fermi Arcs in High Tc Superconductors

abstract:
  A defining property of metals is the existence of a Fermi surface: for two
dimensions, a continuous contour in momentum space which separates occupied
from unoccupied states. In this paper, I discuss angle resolved photoemission
data on the cuprate superconductor BSCCO and argue that it is not best thought
of in this conventional picture. Rather, the data are consistent with
``patches'' of finite area connected by more conventional ``arcs''. Novel
physics is associated with the patches, in that the states contained in a patch
are dispersionless and thus interaction dominated. In the pseudogap phase, the
patches are gapped out, leaving the Fermi arcs disconnected. This unusual
situation may be the key to understanding the microscopic physics of the high
temperature superconductors, in that the pairing correlations are strongest in
the patches, yet the superfluid density lives only on the arcs.


title: withdrawn

abstract: This paper has been withdrawn by the author.


title:
A re-examination of the electronic structure of Bi_2Sr_2CaCu_2O_{8+d}
  and Bi_2Sr_2Cu_1O_{6+d} - An electron-like Fermi Surface and the absence of
  flat bands at $E_F$

abstract:
  We present a re-examination of the electronic structure and Fermi Surface
(FS) of Bi-Sr-Ca-Cu-O (BSCCO) as obtained from angle-resolved photoemission
experiments. By applying a stricter set of FS crossing criteria as well as by
varying the incident photon energy outside the usual range, we have found very
different behavior from that previously observed. In particular we have found
an electron-like FS centered around the Gamma point, and the flat bands at E_F
near the M point of the zone are absent. These results are robust over a large
range of dopings and from single to double layer samples.


title: Excess Spin and the Dynamics of Antiferromagnetic Ferritin

abstract:
  Temperature-dependent magnetization measurements on a series of synthetic
ferritin proteins containing from 100 to 3000 Fe(III) ions are used to
determine the uncompensated moment of these antiferromagnetic particles. The
results are compared with recent theories of macroscopic quantum coherence
which explicitly include the effect of this excess moment. The scaling of the
excess moment with protein size is consistent with a simple model of finite
size effects and sublattice noncompensation.


title:
Orbital Magnetism and Current Distribution of Two-Dimensional Electrons
  under Confining Potential

abstract:
  The spatial distribution of electric current under magnetic field and the
resultant orbital magnetism have been studied for two-dimensional electrons
under a harmonic confining potential $V(\vecvar{r})=m \omega_0^2 r^2/2$ in
various regimes of temperature and magnetic field, and the microscopic
conditions for the validity of Landau diamagnetism are clarified. Under a weak
magnetic field $(\omega_c\lsim\omega_0, \omega_c$ being a cyclotron frequency)
and at low temperature $(T\lsim\hbar\omega_0)$, where the orbital magnetic
moment fluctuates as a function of the field, the currents are irregularly
distributed paramagnetically or diamagnetically inside the bulk region. As the
temperature is raised under such a weak field, however, the currents in the
bulk region are immediately reduced and finally there only remains the
diamagnetic current flowing along the edge. At the same time, the usual Landau
diamagnetism results for the total magnetic moment. The origin of this dramatic
temperature dependence is seen to be in the multiple reflection of electron
waves by the boundary confining potential, which becomes important once the
coherence length of electrons gets longer than the system length. Under a
stronger field $(\omega_c\gsim\omega_0)$, on the other hand, the currents in
the bulk region cause de Haas-van Alphen effect at low temperature as
$T\lsim\hbar\omega_c$. As the temperature gets higher $(T\gsim\hbar\omega_c)$
under such a strong field, the bulk currents are reduced and the Landau
diamagnetism by the edge current is recovered.


title:
High-Frequency Hopping conductivity of Disordered 2D-system in the IQHE
  Regime

abstract:
  High frequency (hf) conductivity in the form $\sigma^{hf} = \sigma_1^{hf} -
i\sigma_2^{hf}$ was obtained from the measurement of Surface Acoustic Waves
(SAW) attenuation and velocity (f=30 MHz) in GaAs/AlGaAs heterostructures
($n=1.3-7\cdot 10^{11}cm^{-2}$). It has been shown that in the Integer Quantum
Hall Effect (IQHE) regime for all the samples at magnetic fields corresponding
to the middle of the Hall plateaus and T=1.5 K, $\sigma_1 / \sigma_2 =0.14 \pm
0.03$. The ratio $\sigma_1 / \sigma_2=0.15$ points the case when the
high-frequency hopping conductivity mechanism (electronic transition between
the localized states formed by "tight" pairs) is valid \cite{1}. Dependencies
of $\sigma_1$ and $\sigma_2$ on temperature and magnetic field is analyzed
width of the Landau band broadened by the impurity random potential is
determined.


title: The Upper Critical Dimension of the Abelian Sandpile Model

abstract:
  The existing estimation of the upper critical dimension of the Abelian
Sandpile Model is based on a qualitative consideration of avalanches as
self-avoiding branching processes. We find an exact representation of an
avalanche as a sequence of spanning sub-trees of two-component spanning trees.
Using equivalence between chemical paths on the spanning tree and loop-erased
random walks, we reduce the problem to determination of the fractal dimension
of spanning sub-trees. Then, the upper critical dimension $d_u=4$ follows from
Lawler's theorems for intersection probabilities of random walks and
loop-erased random walks.


title: Multiple Scales in Small-World Graphs

abstract:
  Small-world architectures may be implicated in a range of phenomena from
disease propagation to networks of neurons in the cerebral cortex. While most
of the recent attention on small-world networks has focussed on the effect of
introducing disorder/randomness into a regular network, we show that that the
fundamental mechanism behind the small-world phenomenon is not
disorder/randomness, but the presence of connections of many different length
scales. Consequently, in order to explain the small-world phenomenon, we
introduce the concept of multiple scale graphs and then state the multiple
length scale hypothesis. Multiple scale graphs form a unifying conceptual
framework for the study of evolving graphs. Moreover, small-world behavior in
randomly rewired graphs is a consequence of features common to all multiple
scale graphs. To support the multiple length scale hypothesis, novel graph
architectures are introduced that need not be a result of random rewiring of a
regular graph. In each case it is shown that whenever the graph exhibits
small-world behavior, it also has connections of diverse length scales. We also
show that the distribution of the length scales of the new connections is
significantly more important than whether the new connections are long range,
medium range or short range connections.


title:
Hubbard model with bond-charge interaction on a triangular lattice: a
  renormalization group study

abstract:
  We have studied the Hubbard model with bond-charge interaction on a
triangular lattice for a half-filled band. At the point of particle-hole
symmetry the model could be analyzed in detail in two opposite regimes of the
parameter space. Using a real space renormalization group we calculate the
ground state energy and the local moment over the whole parameter space. The RG
results obey the exact results in the respective limits. In the intermediate
region of the parameter space the RG results clearly show the effects of the
non-bipartite geometry of the lattice as well as the absence of symmetry in the
reversal of the sign of the hopping matrix element.


title:
Spin structure of impurity band of semiconductors in two and
  three-dimensional cases

abstract:
  The exchange interaction between electrons located at different randomly
distributed impurities is studied for small density of impurities. The
singlet-triplet splitting 2J(R) is calculated for two Coulomb centers at a
distance R. Interpolated formulas are found which work for all distances R from
zero to infinity. The data from atomic physics are used for the interpolation
in three-dimensional case. For two-dimensional case the original calculations
are performed to find asymptotic behavior of the splitting at large R, the
splitting for the ``two-dimensional helium atom'' (R=0) and the splitting at
$R=a_B$, where $a_B$ is the effective Bohr radius. The spin structure of
impurity band is described by the Heisenberg Hamiltonian. The ground state of a
system consists of localized singlets. The new results are obtained for the
distribution of the singlet pairs in the ground state. These results are exact
at low density. The problem is reduced to a non-trivial geometric problem which
is solved in the mean field approximation and by computer modeling. The density
of free electrons is found as a function of temperature and the distribution
function of the singlet-triplet transitions energies is calculated. Both
functions are given in an analytical form.


title: Spin Degree of Freedom in a Two-Dimensional Electron Liquid

abstract:
  We have investigated correlation between spin polarization and
magnetotransport in a high mobility silicon inversion layer which shows the
metal-insulator transition. Increase in the resistivity in a parallel magnetic
field reaches saturation at the critical field for the full polarization
evaluated from an analysis of low-field Shubnikov-de Haas oscillations. By
rotating the sample at various total strength of the magnetic field, we found
that the normal component of the magnetic field at minima in the diagonal
resistivity increases linearly with the concentration of ``spin-up'' electrons.


title: Decay of the metastable phase in d=1 and d=2 Ising models

abstract:
  We calculate perturbatively the tunneling decay rate $\Gamma$ of the
metastable phase in the quantum d=1 Ising model in a skew magnetic field near
the coexistence line $0<h_{x}<1, h_{z}\to -0$ at T=0. It is shown that $\Gamma$
oscillates in the magnetic field $h_{z}$ due to discreteness of the excitation
energy spectrum. After mapping of the obtained results onto the extreme
anisotropic d=2 Ising model at $T<T_c$, we verify in the latter model the
droplet theory predictions for the free energy analytically continued to the
metastable phase. We find also evidence for the discrete-lattice corrections in
this metastable phase free energy.


title: Energy landscapes, supergraphs, and "folding funnels" in spin systems

abstract:
  Dynamical connectivity graphs, which describe dynamical transition rates
between local energy minima of a system, can be displayed against the
background of a disconnectivity graph which represents the energy landscape of
the system. The resulting supergraph describes both dynamics and statics of the
system in a unified coarse-grained sense. We give examples of the supergraphs
for several two dimensional spin and protein-related systems. We demonstrate
that disordered ferromagnets have supergraphs akin to those of model proteins
whereas spin glasses behave like random sequences of aminoacids which fold
badly.


title: Nondissipative Drag Conductance as a Topological Quantum Number

abstract:
  We show in this paper that the boundary condition averaged nondissipative
drag conductance of two coupled mesoscopic rings with no tunneling, evaluated
in a particular many-particle eigenstate, is a topological invariant
characterized by a Chern integer. Physical implications of this observation are
discussed.


title: Scaling for the Percolation Backbone

abstract:
  We study the backbone connecting two given sites of a two-dimensional lattice
separated by an arbitrary distance $r$ in a system of size $L$. We find a
scaling form for the average backbone mass: $<M_B>\sim L^{d_B}G(r/L)$, where
$G$ can be well approximated by a power law for $0\le x\le 1$: $G(x)\sim
x^{\psi}$ with $\psi=0.37\pm 0.02$. This result implies that $<M_B> \sim
L^{d_B-\psi}r^{\psi}$ for the entire range $0<r<L$. We also propose a scaling
form for the probability distribution $P(M_B)$ of backbone mass for a given
$r$. For $r\approx L, P(M_B)$ is peaked around $L^{d_B}$, whereas for $r\ll L,
P(M_B)$ decreases as a power law, $M_B^{-\tau_B}$, with $\tau_B\simeq 1.20\pm
0.03$. The exponents $\psi$ and $\tau_B$ satisfy the relation
$\psi=d_B(\tau_B-1)$, and $\psi$ is the codimension of the backbone,
$\psi=d-d_B$.


title:
A link between the spin fluctuation and Fermi surface in high T_C
  cuprates --- A consistent description within the single-band Hubbard model

abstract:
  A link between the spin fluctuation and the "fermiology" is explored for the
single-band Hubbard model within the fluctuation exchange (FLEX) approximation.
We show that the experimentally observed peak position of the spin structure in
the high T_C cuprates can be understood from the model that reproduces the
experimentally observed Fermi surface. In particular, both the variation of the
incommensurability of the peak in the spin structure and the evolution of the
Fermi surface with hole doping in La_{2-x}Sr_xCuO_4 may be understood with a
second nearest neighbor hopping decreasing with hole doping.


title: Dimer Order with Striped Correlations in the J_1-J_2 Heisenberg Model

abstract:
  Ground state energies for plaquette and dimer order in the $J_1-J_2$
square-lattice spin-half Heisenberg model are compared using series expansion
methods. We find that these energies are remarkably close to each other at
intermediate values of $J_2/J_1$, where the model is believed to have a quantum
disordered ground state. They join smoothly with those obtained from the Ising
expansions for the 2-sublattice N\'eel-state at $J_2/J_1 \approx 0.4$,
suggesting a second order transition from a N\'eel state to a quantum
disordered state, whereas they cross the energy for the 4-sublattice ordered
state at $J_2/J_1 \approx 0.6$ at a large angle, implying a first order
transition to the 4-sublattice magnetic state. The strongest evidence that the
plaquette phase is not realized in this model comes from the analysis of the
series for the singlet and triplet excitation spectra, which suggest an
instability in the plaquette phase. Thus, our study supports the recent work of
Kotov et al, which presents a strong picture for columnar dimer order in this
model. We also discuss the striped nature of spin correlations in this phase,
with substantial resonance all along columns of dimers.


title:
Morphological phase transitions of thin fluid films on chemically
  structured substrates

abstract:
  Using an interface displacement model derived from a microscopic density
functional theory we investigate thin liquidlike wetting layers adsorbed on
flat substrates with an embedded chemical heterogeneity forming a stripe. For a
wide range of effective interface potentials we find first-order phase
transitions as well as continuous changes between lateral interfacial
configurations bound to and repelled from the stripe area. We determine phase
diagrams and discuss the conditions under which these morphological changes
arise.


title: The Structure and Dynamics of Sodium Disilicate

abstract:
  We investigate the structure and dynamics of sodium disilicate by means of
molecular dynamics computer simulation. We show that the structure is described
by a partially destroyed tetrahedral SiO_4 network and a spherical super
structure formed by the silicon and sodium atoms. The static structure factor
of our simulation is in very good agreement with one from a neutron scattering
experiment. For 1008 particles we find strong finite size effects in the
dynamics which are due to the missing of modes contributing to the boson peak.


title:
Quantum dots in magnetic fields: Phase diagram and broken symmetry of
  the Chamon-Wen edge

abstract:
  Quantum dots in magnetic fields are studied within the current spin density
functional formalism avoiding any spatial symmetry restrictions of the
solutions. We find that the maximum density droplet reconstructs into states
with broken internal symmetry: The Chamon-Wen edge co-exists with a modulation
of the charge density along the edge. The phase boundaries between the
polarization transition, the maximum density droplet and its reconstruction are
in agreement with recent experimental results.


title:
Asymptotic limit of high spatial dimensions and thermodynamic
  consistence

abstract:
  The question of thermodynamic consistence and $\Phi$-derivability of the
asymptotic limit of high spatial dimensions for quantum itinerant models is
addressed. It is shown that although the irreducible $n$-particle Green
functions are local, reducible vertex functions retain different momentum
dependence. As a consequence, the vertex corrections to conductivity do not
generally vanish in the mean-field limit. The mean-field theory is a
$\Phi$-derivable approximation only if regular nonlocal or anomalous local
external sources are admitted.


title:
Stability of self-consistent solutions for the Hubbard model at
  intermediate and strong coupling

abstract:
  We present a general framework how to investigate stability of solutions
within a single self-consistent renormalization scheme being a parquet-type
extension of the Baym-Kadanoff construction of conserving approximations. To
obtain a consistent description of one- and two-particle quantities, needed for
the stability analysis, we impose equations of motion on the one- as well on
the two-particle Green functions simultaneously and introduce approximations in
their input, the completely irreducible two-particle vertex. Thereby we do not
loose singularities caused by multiple two-particle scatterings. We find a
complete set of stability criteria and show that each instability, singularity
in a two-particle function, is connected with a symmetry-breaking order
parameter, either of density type or anomalous. We explicitly study the Hubbard
model at intermediate coupling and demonstrate that approximations with static
vertices get unstable before a long-range order or a metal-insulator transition
can be reached. We use the parquet approximation and turn it to a workable
scheme with dynamical vertex corrections. We derive a qualitatively new theory
with two-particle self-consistence, the complexity of which is comparable with
FLEX-type approximations. We show that it is the simplest consistent and stable
theory being able to describe qualitatively correctly quantum critical points
and the transition from weak to strong coupling in correlated electron systems.


title: Dual description of nonextensive ensembles

abstract:
  In the case of a system with an unbounded hamiltonian the entropic index q of
non-extensive thermodynamics has an upperbound q_c>1 beyond which the formalism
becomes meaningless. The expression 1/(q_c-1) is the dimension of the state
space (i.e. the manifold of density matrices) in the context of non-commutative
geometry. For q=q_c an ultraviolet cutoff E< E_ub is needed to guarantee the
existence of the equilibrium density matrix. Duality between q>1 and
q<1-statistics via a q --> 1/q transformation is established. It leads to an
overall picture in which the meaning of both q>1-statistics and q<1-statistics
is clarified.


title: Phase Transition and Symmetry Breaking in the Minority Game

abstract:
  We show that the Minority Game, a model of interacting heterogeneous agents,
can be described as a spin systems and it displays a phase transition between a
symmetric phase and a symmetry broken phase where the games outcome is
predicable. As a result a ``spontaneous magnetization'' arises in the spin
formalism.

categories: cond-mat.stat-mech adap-org cond-mat.dis-nn nlin.AO


title:
Non-Fermi-liquid behavior in the Kondo lattices induced by peculiarities
  of magnetic ordering and spin dynamics

abstract:
  A scaling consideration of the Kondo lattices is performed with account of
singularities in the spin excitation spectral function. It is shown that a
non-Fermi-liquid (NFL) behavior between two critical values of the bare $s-f$
coupling constant occurs naturally for complicated magnetic structures with
several magnon branches. This may explain the fact that a NFL behavior takes
place often in the heavy-fermion systems with peculiar spin dynamics. Another
kind of a NFL-like state (with different critical exponents) can occur for
simple antiferromagnets with account of magnon damping, and for paramagnets,
especially with two-dimensional character of spin fluctuations. The mechanisms
proposed lead to some predictions about behavior of specific heat, resistivity,
magnetic susceptibility, and anisotropy parameter, which can be verified
experimentally.


title:
Phase transitions in finite systems = topological peculiarities of the
  microcanonical entropy surface

abstract:
  It is discussed how phase transitions of first order (with phase separation
and surface tension), continuous transitions and (multi)-critical points can be
defined and classified for finite systems from the topology of the energy
surface $e^{S(E,N)}$ of the mechanical N-body phase space or more precisely of
the curvature determinant $D(E,N)=\partial^2S/\partial E^2*\partial^2S/\partial
N^2-(\partial^2S/\partial E\partial N)^2$ without taking the thermodynamic
limit. The first calculation of the entire entropy surface $S(E,N)$ for a
q=3-states Potts lattice gas on a 50*50 square lattice is shown. There are two
lines, where $S(E,N)$ has a maximum curvature $\sim 0$. One is the border
between the regions in \{$E,N$\} with $D(E,N)>0$ and with $D(E,N)<0$, the other
line is critical starting as a valley in $D(E,N)$ running from the continuous
transition in the ordinary q=3-Potts model, converting at $P_m$ into a flat
ridge/plateau (maximum) deep inside the convex intruder of $S(E,N)$ which
characterizes the first order liquid-gas transition. The multi-critical point
$P_m$ is their crossing.


title:
On the estimate of the spin-gap in quasi-1D Heisenberg antiferromagnets
  from nuclear spin-lattice relaxation

abstract:
  We present a careful analysis of the temperature dependence of the nuclear
spin-lattice relaxation rate $1/T_1$ in gapped quasi-1D Heisenberg
antiferromagnets. It is found that in order to estimate the value of the gap
correctly from $1/T_1$ the peculiar features of the dispersion curve for the
triplet excitations must be taken into account. The temperature dependence of
$1/T_1$ due to two-magnon processes, is reported for different values of the
ratio $r=J_{\perp}/J_{\parallel}$ between the superexchange constants in a
2-leg-ladder. As an illustrative example we compare our results to the
experimental findings for $^{63}Cu 1/T_1$ in the dimerized chains and
2-leg-ladders contained in Sr$_{14}Cu_{24}O_{41}$


title: On the scaling behavior of the abelian sandpile model

abstract:
  The abelian sandpile model in two dimensions does not show the type of
critical behavior familar from equilibrium systems. Rather, the properties of
the stationary state follow from the condition that an avalanche started at a
distance r from the system boundary has a probability proportional to 1/sqrt(r)
to reach the boundary. As a consequence, the scaling behavior of the model can
be obtained from evaluating dissipative avalanches alone, allowing not only to
determine the values of all exponents, but showing also the breakdown of
finite-size scaling.


title: Tunnelling Spectroscopy of Localized States near the Quantum Hall Edge

abstract:
  In the paper we dscuss experimental results of M. Grayson et al. on tunneling
$I$-$V$ characteristics of the quantum Hall edge. We suggest a two step
tunneling mechanism involving localized electron states near the edge, which
might account for discrepancy between the experimental data and the predictions
of the chiral Luttinger liquid theory of the quantum Hall edge.


title:
Quasiparticle thermal conductivity in the vortex state of high-T$_c$
  cuprates

abstract:
  We present the results of a microscopic calculation of the longitudinal
thermal conductivity, $\kappa$, of a d-wave superconductor in the mixed state.
Our results show an increase in the thermal conductivity with the applied field
at low temperatures, and a decrease followed by a nearly field independent
$\kappa(H)$ at higher temperatures, in qualitative agreement with the
experimental results. We discuss the relationship between the slope of the
superconducting gap and the plateau in $\kappa(H)$.


title:
Extending the n=2 Ruddlesden-Popper Solid Solution La2-2xSr1+2xMn2O7
  Beyond x=0.5: Synthesis of Mn4+-rich Compounds

abstract:
  We report the synthesis and room temperature crystal structures of the
heretofore unknown, metastable manganites La2-2xSr1+2xMn2O7+delta (0.5 < x <
0.9) via high-temperature (T=1650 C) quenching followed by low-temperature
(T=400 C) annealing to fill oxygen vacancies; this approach enables access to
the electronic, magnetic, and structural properties of previously unexplored
compositions in this important CMR system.


title:
Thermodynamics of boson and fermion systems with fractal distribution
  functions

abstract:
  Starting with the fractal inspired distribution functions for
Maxwell-Boltzmann, Bose-Einstein and Fermi systems, as reported by F.
B\"{u}y\"{u}kkili\c{c} and D. Demirhan, we obtain the corresponding probability
distributions and study their thermodynamic behavior. We compare our results
with those corresponding to ideal gases (q=1), and Bose-Einstein and Fermi
systems with quantum group symmetry. In particular, we show that the
hamiltonian that gives the Bose-Einstein generalized distribution function can
be interpreted as a q-deformation of the ideal gas hamiltonian.


title: Harmonic generation from YBCO microwave resonators

abstract:
  Measurement of harmonic power on a suspended stripline microwave resonator
patterned out of thin film YBa_2Cu_3O_7 (YBCO) has been carried out as a
function of temperature and microwave power. The third harmonic power P_3 shows
a subtle nonlinear dependence on the fundamental power P_1 on a log-log scale.
Fit to a straight line yields slopes of P_3 vs. P_1 between 1.5 to 3, contrary
to elementary calculations that predict a slope of exactly 3. It is shown that
third harmonic power generated from higher order terms in the nonlinear
impedance could account for the discrepancy quantitatively without resorting to
any additional ad-hoc assumptions. Results of measurement of fifth harmonic
power is presented as a confirmation. A small second harmonic power is also
observed that does not show anomalous power dependence within the experimental
accuracy.


title: Pyroelectric charge release in rhombohedral PZT

abstract:
  A new experimental set-up controlled by computer has been made to measure the
pyroelectric charge of ferroelectric materials with a relatively high
conductivity at slow rates of temperature variation. It allowed us to obtain
the polarisation vs. temperature behaviour of PZT with various compositions in
the whole range of temperatures with a high accuracy. The ferroelectric Zr-rich
region of the PZT phase diagram, where the isostructural phase transition
FRL-FRH takes place, has been studied.


title: Blume-Emery-Griffiths Model in a Random Crystal Field

abstract:
  We study the Blume-Emery-Griffiths model in a random crystal field in two and
three dimensions, through a real-space renormalization-group approach and a
mean-field approximation, respectively. According to the two-dimensional
renormalization-group calculation, non-symmetry-breaking first-order phase
transitions are eliminated and symmetry-breaking discontinuous transitions are
replaced by continuous ones, when disorder is introduced. On the other hand,
the mean-field calculation predicts that first-order transitions are not
eliminated by disorder, although some changes are introduced in the phase
diagrams. We make some comments on the consequences of a degeneracy parameter,
which may be relevant in martensitic transitions.


title: Conductance of Distorted Carbon Nanotubes

abstract:
  We have calculated the effects of structural distortions of armchair carbon
nanotubes on their electrical transport properties. We found that the bending
of the nanotubes decreases their transmission function in certain energy ranges
and leads to an increased electrical resistance. Electronic structure
calculations show that these energy ranges contain localized states with
significant $\sigma$-$\pi$ hybridization resulting from the increased curvature
produced by bending. Our calculations of the contact resistance show that the
large contact resistances observed for SWNTs are likely due to the weak
coupling of the NT to the metal in side bonded NT-metal configurations.


title: Off equilibrium dynamics of the Frustrated Ising Lattice Gas

abstract:
  We study by means of Monte Carlo simulations the off equilibrium properties
of a model glass, the Frustrated Ising Lattice Gas (FILG) in three dimensions.
We have computed typical two times quantities, like density-density
autocorrelations and the autocorrelation of internal degrees of freedom. We
find an aging scenario particularly interesting in the case of the density
autocorrelations in real space which is very reminiscent of spin glass
phenomenology. While this model captures the essential features of structural
glass dynamics, its analogy with spin glasses may bring the possibility of its
complete description using the tools developed in spin glass theory.


title: Synchronous clusters in a noisy inhibitory neural network

abstract:
  We study the stability and information encoding capacity of synchronized
states in a neuronal network model that represents part of thalamic circuitry.
Our model neurons have a Hodgkin-Huxley-type low threshold Calcium channel,
display post inhibitory rebound, and are connected via GABAergic inhibitory
synapses. We find that there is a threshold in synaptic strength, $\tau_c$,
below which there are no stable spiking network states. Above threshold the
stable spiking state is a cluster state, where different groups of neurons fire
consecutively, and each neuron fires with the same cluster each time. Weak
noise destabilizes this state, but stronger noise drives the system into a
different, self-organized, stochastically synchronized state. Neuronal firing
is still organized in clusters, but individual neurons can hop from cluster to
cluster. Noise can actually induce and sustain such a state below the threshold
of synaptic strength. We do find a qualitative difference in the firing
patterns between small ($\sim 10$ neurons) and large ($\sim 1000$ neurons)
networks. We determine the information content of the spike trains in terms of
two separate contributions: the spike time jitter around cluster firing times,
and the hopping from cluster to cluster. We quantify the information loss due
to temporally correlated interspike intervals. Recent experiments on the locust
olfactory system and striatal neurons suggest that the nervous system may
actually use these two channels to encode separate and unique information.


title:
Perpendicular transport and magnetization processes in magnetic
  multilayers with strongly and weakly coupled magnetic layers

abstract:
  Within the framework of a two-band tight-binding model, we have performed
calculations of giant magnetoresistance, exchange coupling and thermoelectric
power (TEP) for a system consisting of three magnetic layers separated by two
non-magnetic spacers with the first two magnetic layers strongly
antiferromagnetically exchange-coupled. We have shown how does the GMR relate
with the corresponding regions of magnetic structure phase diagrams and
computed some relevant hysteresis loops, too. The GMR may take negative values
for specific layers thicknesses, and the TEP reveals quite pronounced
oscillations around a negative bias.


title:
Two-Dimensional Gross-Pitaevskii Equation: Theory of Bose-Einstein
  Condensation and the Vortex State

abstract:
  We derive the Gross-Pitaevskii equation in two-dimension from the first
principles of two-dimensional scattering theory. Numerical calculation of the
condensate wave function shows that atoms in a two-dimensional harmonic trap
can be condensed into its ground state. Moreover, the ground state energy and
the wave function of two-dimensional vortex state are also obtained.
Quantitative comparisons between 2D results and 3D ones are made in detail.


title: Two-dimensional photonic crystal polarizer

abstract:
  A novel polarizer made from two-dimensional photonic bandgap materials was
demonstrated theoretically. This polarizer is fundamentally different from the
conventinal ones. It can function in a wide frequency range with high
performance and the size can be made very compact, which renders it useful as a
micropolarizer in microoptics.


title: Andreev Reflection Enhanced Shot Noise in Mesoscopic SNS Junctions

abstract:
  Current noise is measured with a SQUID in low impedance and transparent
Nb-Al-Nb j unctions of length comparable to the phase breaking length and much
longer than the thermal length. The shot noise amplitude is compared with
theoretical predictions of doubled shot noise in diffusive
normal/superconductor (NS) junctions due to the Andreev reflections. We discuss
the heat dissipation away from the normal part through the NS interfaces. A
weak applied magnetic field reduces the amplitude of the 1/f noise by a factor
of two, showing that even far from equilibrium the sample is in the mesoscopic
regime.


title: Pressure screening and fluctuations at the bottom of a granular column

abstract:
  We report sets of precise and reproducible measurements on the static
pressure at the bottom of a granular column. We make a quantitative analysis of
the pressure saturation when the column height is increased. We evidence a
great sensitivity of the measurements with the global packing fraction and the
eventual presence of shear bands at the boundaries. We also show the limit of
the classical Janssen model and discuss these experimental results under the
scope of recently proposed theoretical frameworks.


title:
Numerical tests of conjectures of conformal field theory for
  three-dimensional systems

abstract:
  The concept of conformal field theory provides a general classification of
statistical systems on two-dimensional geometries at the point of a continuous
phase transition. Considering the finite-size scaling of certain special
observables, one thus obtains not only the critical exponents but even the
corresponding amplitudes of the divergences analytically. A first numerical
analysis brought up the question whether analogous results can be obtained for
those systems on three-dimensional manifolds.
  Using Monte Carlo simulations based on the Wolff single-cluster update
algorithm we investigate the scaling properties of O(n) symmetric classical
spin models on a three-dimensional, hyper-cylindrical geometry with a toroidal
cross-section considering both periodic and antiperiodic boundary conditions.
Studying the correlation lengths of the Ising, the XY, and the Heisenberg
model, we find strong evidence for a scaling relation analogous to the
two-dimensional case, but in contrast here for the systems with antiperiodic
boundary conditions.


title: Time-Dependent Ginzburg-Landau Theory and Duality

abstract:
  In the first part of this review paper, the time-dependent Ginzburg-Landau
theory is derived starting from the microscopic BCS model with the help of a
derivative expansion. Special attention is paid to two space dimensions, where
the entire crossover from the weak-coupling BCS limit to the strong-coupling
BEC limit of tightly bound fermion pairs is accessible analytically. The second
part deals with the dual approach to the time-independent Ginzburg-Landau
theory in three space dimensions. In this approach, the magnetic vortices of a
superconductor play the central role, and the superconductor-to-normal phase
transition is understood as a proliferation of these vortices.


title: Scattering of magnetic solitons in two dimensions

abstract:
  Solitons which have the form of a vortex-antivortex pair have recently been
found in the Landau-Lifshitz equation which is the standard model for the
ferromagnet. We simulate numerically head-on collisions of two
vortex-antivortex pairs and observe a right angle scattering pattern. We offer
a resolution of this highly nontrivial dynamical behavior by examining the
Hamiltonian structure of the model, specifically the linear momentum of the two
solitons. We further investigate the dynamics of vortices in a modified
nonlinear sigma-model which arises in the description of antiferromagnets. We
confirm numerically that a robust feature of the dynamics is the right angle
scattering of two vortices which collide head-on. A generalization of our
theory is given for this model which offers arguments towards an understanding
of the observed dynamical behavior.


title:
Stresses in silos: Comparison between theoretical models and new
  experiments

abstract:
  We present precise and reproducible mean pressure measurements at the bottom
of a cylindrical granular column. If a constant overload is added, the pressure
is linear in overload and nonmonotonic in the column height. The results are
{\em quantitatively} consistent with a local, linear relation between stress
components, as was recently proposed by some of us. They contradict the
simplest classical (Janssen) approximation, and may pose a rather severe test
of competing models.


title:
Momentum Distribution Spectroscopy Using Deep Inelastic Neutron
  Scattering

abstract:
  We show that deep inelastic neutron scattering from hydrogen(or other light
nuclei) can be used to measure a spectrum of anharmonic contributions to the
target atom momentum distribution with high and known accuracy . The method is
applied here to determine the momentum distribution of the hydrogen in the
hydrogen bonded system KHC2O4(potassium binoxalate), where 13 anharmonic
coefficients are obtained at the 2-sigma to 3-sigma level. The momentum
distribution is obtained to an accuracy of better than few percent at all
significant values of momentum.


title:
A new quantum phase between the Fermi glass and the Wigner crystal in
  two dimensions

abstract:
  For intermediate Coulomb energy to Fermi energy ratios $r_s$, spinless
fermions in a random potential form a new quantum phase which is nor a Fermi
glass, neither a Wigner crystal. Studying small clusters, we show that this
phase gives rise to an ordered flow of enhanced persistent currents, for
disorder strength and ratios $r_s$ where a metallic phase has been recently
observed in two dimensions.


title: Exact and approximate results of non-extensive quantum statistics

abstract:
  We develop an analytical technique to derive explicit forms of
thermodynamical quantities within the asymptotic approach to non-extensive
quantum distribution functions. Using it, we find an expression for the number
of particles in a boson system which we compare with other approximate scheme
(i.e. factorization approach), and with the recently obtained exact result. To
do this, we investigate the predictions on Bose-Einstein condensation and the
blackbody radiation. We find that both approximation techniques give results
similar to (up to ${\cal O}(q-1)$) the exact ones, making them a useful tool
for computations. Because of the simplicity of the factorization approach
formulae, it appears that this is the easiest way to handle with physical
systems which might exhibit slight deviations from extensivity.


title:
Generalized quantal distribution functions within factorization
  approach: Some general results for bosons and fermions

abstract:
  The generalized quantal distribution functions are investigated concerning
systems of non-interacting bosons and fermions. The formulae for the number of
particles and energy are presented and applications to the Chandrasekhar limit
of white dwarfs stars and to the Bose-Einstein condensation are commented.


title:
Quantal distribution functions in non-extensive statistics and an early
  universe test revisited

abstract:
  Within the context of non-extensive thermostatistics, we use the
factorization approximation to study a recently proposed early universe test. A
very restrictive bound upon the non-extensive parameter is presented: $|q-1| <
4.01 \times 10^{-3}$.


title: Quantum Statistical Mechanics for Nonextensive Systems II

abstract:
  In this paper, the Green function theory of quantum many-particle systems
recently presented is reworked within the framework of nonextensive statistical
mechanics with a new normalized $q$-expectation values. This reformulation
introduces a renormalization of temperature of the earlier theory and a
self-consistency condition. The linear response theory is also presented, along
with its two-particle Green function version. Finally, a Boltzmann
transport-like equation is also developed here.


title: Statistics of Shear-induced Rearrangements in a Model Foam

abstract:
  Under steady shear, a foam relaxes stress through intermittent rearrangements
of bubbles accompanied by sudden drops in the stored elastic energy. We use a
simple model of foam that incorporates both elasticity and dissipation to study
the statistics of bubble rearrangements in terms of energy drops, the number of
nearest neighbor changes, and the rate of neighbor-switching (T1) events. We do
this for a two-dimensional system as a function of system size, shear rate,
dissipation mechanism, and gas area fraction. We find that for dry foams, there
is a well-defined quasistatic limit at low shear rates where localized
rearrangements occur at a constant rate per unit strain, independent of both
system size and dissipation mechanism. These results are in good qualitative
agreement with experiments on two-dimensional and three-dimensional foams. In
contrast, we find for progessively wetter foams that the event size
distribution broadens into a power law that is cut off only by system size.
This is consistent with criticality at the melting transition.


title:
Statistical Mechanics of Vacancy and Interstitial Strings in Hexagonal
  Columnar Crystals

abstract:
  Columnar crystals contain defects in the form of vacancy/interstitial loops
or strings of vacancies and interstitials bounded by column ``heads'' and
``tails''. These defect strings are oriented by the columnar lattice and can
change size and shape by movement of the ends and forming kinks along the
length. Hence an analysis in terms of directed living polymers is appropriate
to study their size and shape distribution, volume fraction, etc. If the
entropy of transverse fluctuations overcomes the string line tension in the
crystalline phase, a string proliferation transition occurs, leading to a
supersolid phase. We estimate the wandering entropy and examine the behaviour
in the transition regime. We also calculate numerically the line tension of
various species of vacancies and interstitials in a triangular lattice for
power-law potentials as well as for a modified Bessel function interaction
between columns as occurs in the case of flux lines in type-II superconductors
or long polyelectrolytes in an ionic solution. We find that the centered
interstitial is the lowest energy defect for a very wide range of interactions;
the symmetric vacancy is preferred only for extremely short interaction ranges.


title: Image restoration using the chiral Potts spin-glass

abstract:
  We report on the image reconstruction (IR) problem by making use of the
random chiral q-state Potts model, whose Hamiltonian possesses the same gauge
invariance as the usual Ising spin glass model. We show that the pixel
representation by means of the Potts variables is suitable for the gray-scale
level image which can not be represented by the Ising model. We find that the
IR quality is highly improved by the presence of a glassy term, besides the
usual ferromagnetic term under random external fields, as very recently pointed
out by Nishimori and Wong. We give the exact solution of the infinite range
model with q=3, the three gray-scale level case. In order to check our
analytical result and the efficiency of our model, 2D Monte Carlo simulations
have been carried out on real-world pictures with three and eight gray-scale
levels.


title: Correlated scattering of photons from a trapped Bose-Einstein condensate

abstract:
  The second-order coherence of photons scattered from a trapped Bose-Einstein
condensate is found to be enhanced for the scattering angles that are either
the same or symmetrical with respect to the direction of laser propagation. The
enhancement occurs periodically with respect to the time interval between
photons, and becomes less and less pronounced with increasing the strength of
the interaction between atoms.


title: Travelling waves in a drifting flux lattice

abstract:
  Starting from the time-dependent Ginzburg-Landau (TDGL) equations for a type
II superconductor, we derive the equations of motion for the displacement field
of a moving vortex lattice without inertia or pinning. We show that it is
linearly stable and, surprisingly, that it supports wavelike long-wavelength
excitations arising not from inertia or elasticity but from the
strain-dependent mobility of the moving lattice. It should be possible to image
these waves, whose speeds are a few \mu m/s, using fast scanning tunnelling
microscopy.


title: Directed Polymer -- Directed Percolation Transition

abstract:
  We study the relation between the directed polymer and the directed
percolation models, for the case of a disordered energy landscape where the
energies are taken from bimodal distribution. We find that at the critical
concentration of the directed percolation, the directed polymer undergoes a
transition from the directed polymer universality class to the directed
percolation universality class. We also find that directed percolation clusters
affect the characterisrics of the directed polymer below the critical
concentration.


title: Molecular dynamic simulation of directional crystal growth

abstract:
  We use molecular dynamic to simulate the directional growth of binary
mixtures. our results compare very well with analitical and experimental
results. This opens up the possibility to probe growth situations which are
difficult to reach experimentally, being an important tool for further
experimental and theoretical developments in the area of crystal growth.


title: Heat Capacity of Mesoscopic Superconducting Disks

abstract:
  We study the heat capacity of isolated giant vortex states, which are good
angular momentum ($L$) states, in a mesoscopic superconducting disk using the
Ginzburg-Landau (GL) theory. At small magnetic fields the $L$=0 state
qualitatively behaves like the bulk sample characterized by a discontinuity in
heat capacity at $T_c$. As the field is increased the discontinuity slowly
turns into a continuous change which is a finite size effect. The higher $L$
states show a continuous change in heat capacity at $T_c$ at all fields. We
also show that for these higher $L$ states, the behavior of the peak position
with change in field is related to the paramagnetic Meissner effect
(irreversible) and can lead to an unambiguous observation of positive
magnetization in mesoscopic superconductors.


title:
Magnetic and quadrupolar order in a one-dimensional ferromagnet with
  cubic crystal-field anisotropy

abstract:
  The zero temperature phase diagram of a one-dimensional S=2 Heisenberg
ferromagnet with single-ion cubic anisotropy is studied numerically using the
density-matrix renormalization group method. Evidence is found that although
the model does not involve quadrupolar couplings, there is a purely quadrupolar
phase for large values of the anisotropy. The phase transition between the
magnetic and quadrupolar phases is continuous and it seems to be characterized
by Ising critical exponents.


title: Spread option and exchange option with stochastic interest rates

abstract:
  In this work, we consider the issue of pricing exchange options and spread
options with stochastic interest rates. We provide the closed form solution for
the exchange option price when interest rate is stochastic. Our result holds
when interest rate is modeled with a stochastic term structure of general form,
which includes Vasicek model, CIR term structure, and other well-known term
structure models as special cases. In particular, we have discussed the
possibility of using our closed form solution as a control variate in pricing
spread options with stochastic interest rate.


title:
Timesaving Double-Grid Method for Real-Space Electronic-Structure
  Calculations

abstract:
  We present a simple and efficient technique in ab initio electronic-structure
calculation utilizing real-space double-grid with a high density of grid points
in the vicinity of nuclei. This technique promises to greatly reduce the
overhead for performing the integrals that involves non-local parts of
pseudopotentials, with keeping a high degree of accuracy. Our procedure gives
rise to no Pulay forces, unlike other real-space methods using adaptive
coordinates. Moreover, we demonstrate the potential power of the method by
calculating several properties of atoms and molecules.


title: Persistent currents on graphs

abstract:
  We develop a method to calculate the persistent currents and their spatial
distribution (and transport properties) on graphs made of quasi-1D diffusive
wires. They are directly related to the field derivatives of the determinant of
a matrix which describes the topology of the graph. In certain limits, they are
obtained by simple counting of the nodes and their connectivity. We relate the
average current of a disordered graph with interactions and the non-interacting
current of the same graph with clean 1D wires. A similar relation exists for
orbital magnetism in general.


title:
Reduction formula for fermion loops and density correlations of the 1D
  Fermi gas

abstract:
  Fermion N-loops with an arbitrary number of density vertices N > d+1 in d
spatial dimensions can be expressed as a linear combination of (d+1)-loops with
coefficients that are rational functions of external momentum and energy
variables. A theorem on symmetrized products then implies that divergencies of
single loops for low energy and small momenta cancel each other when loops with
permuted external variables are summed. We apply these results to the
one-dimensional Fermi gas, where an explicit formula for arbitrary N-loops can
be derived. The symmetrized N-loop, which describes the dynamical N-point
density correlations of the 1D Fermi gas, does not diverge for low energies and
small momenta. We derive the precise scaling behavior of the symmetrized N-loop
in various important infrared limits.


title: Density-functional theory for attraction between like-charged plates

abstract:
  We study the interactions between two negatively charged macroscopic surfaces
confining positive counterions. A density-functional approach is introduced
which, besides the usual mean-field interactions, takes into account the
correlations in the positions of counterions. The excess free energy is derived
in the framework of the Debye-H{\"u}ckel theory of the one-component plasma,
with the homogeneous density replaced by a weighted density. The minimization
of the total free energy yields the density profile of the microions. The
pressure is calculated and compared with the simulations and the results
derived from integral equations theories. We find that the interaction between
the two plates becomes attractive when their separation distance is
sufficiently small and the surface charge density is larger than a threshold
value.


title:
Convergent Approximation for the 2-Body Correlation Function in an
  Interface

abstract:
  A convergent approximation is proposed for a mean field density-density
correlation function in a system with a two-phase interface. It is based on a
fourth-order expansion of the Hamiltonian in terms of fluctuations around the
equilibrium profile. The approach is illustrated by one and three dimensional
calculations for systems characterized by the Ginzburg-Landau functional.


title:
1/8 Doping Anomalies and Oxygen Vacancies in Underdoped Superconducting
  Cuprates

abstract:
  Measurements of thermal conductivity ($\kappa$) versus temperature and doping
for several cuprate superconductors are discussed. Suppressed values of the
normal-state $\kappa$ and the slope change in $\kappa$ at $T_c$, observed near
1/8 doping in both YBa$_2$Cu$_3$O$_{6+x}$ and Hg cuprates, are attributed to
local lattice distortions and the suppression of the superconducting
condensate, respectively. Both phenomena are proposed to arise from small
domains of localized planar holes, presumably a manifestation of phase
separation. It is suggested that the phase behavior of $\kappa$ reflects stripe
dynamics and the 1/8 doping anomalies stripe pinning by oxygen-vacancy
clusters.


title: A Model for the Optical Absorption in Porous Silicon Quantum Wires

abstract:
  In this paper, we analyse the optical absorption in porous silicon . This is
the first attempt to explicitly demonstrate that it is not possible to extract
the band gap of such low dimensional nanostructures like porous silicon from a
Tauc plot of Square Root(alpha h nu) vs (h nu) . These objections are also
valid for other reduced dimensional systems like quantum wires and quantum dots
etc.
  So we model the absorption process assuming that porous silicon is a pseudo
1D material system having a distribution of band gaps. We show that in order to
explain the absorption we specifically need to invoke - (a) k is not conserved
in optical transitions,(b) the oscillator strength of these transitions depends
on the size of the nanostructure in which the absorption takes place and (c)
the distribution of band gaps significantly influences the optical absorption.
A natural explanation of the temperature dependence of absorption in porous
silicon also follows from our model . We have also shown that porosity can be
inferred non-destructively from transmission measurements in the region of low
absorption.
  One can easily generalize our equations and use them to analyse the
absorption process of other low dimensional materials too .


title: Cooper Instability in the Occupation Dependent Hopping Hamiltonians

abstract:
  A generic Hamiltonian, which incorporates the effect of the orbital
contraction on the hopping amplitude between the nearest sites, is studied both
analytically at the weak coupling limit and numerically at the intermediate and
strong coupling regimes for finite atomic cluster. The effect of the orbital
contraction due to hole localization at atomic sites is specified with two
coupling parameters V and W (multiplicative and additive contraction terms).
The singularity of the vertex part of the two-particle Green's function
determines the critical temperature Tc and the relaxation rate Gamma(T) of the
order parameter at temperature above Tc. Unlike in conventional BCS
superconductors, Gamma has a non-zero imaginary part which may influence the
fluctuation conductivity of superconductor above Tc. We compute the ground
state energy as a function of the particle number and magnetic flux through the
cluster, and show the existence of the parity gap Delta appearing at the range
of system parameters consistent with the appearance of Cooper instability.
Numeric calculation of the Hubbard model (with U>0) at arbitrary occupation
does not show any sign of superconductivity in small cluster.


title:
Properties of the ferrimagnetic double-perovskite A_{2}FeReO_{6} (A=Ba
  and Ca)

abstract:
  Ceramics of A_{2}FeReO_{6} double-perovskite have been prepared and studied
for A=Ba and Ca. Ba_{2}FeReO_{6} has a cubic structure (Fm3m) with $a\approx
$8.0854(1) \AA whereas Ca_{2}FeReO_{6} has a distorted monoclinic symmetry with
$a\approx 5.396(1) \AA, b\approx 5.522(1) \AA, c\approx 7.688(2) \AA$ and
$\beta =90.4^{\circ} (P21/n)$. The barium compound is metallic from 5 K to 385
K, i.e. no metal-insulator transition has been seen up to 385 K, and the
calcium compound is semiconducting from 5 K to 385 K. Magnetization
measurements show a ferrimagnetic behavior for both materials, with T_{c}=315 K
for Ba_{2}FeReO_{6} and above 385 K for Ca_{2}FeReO_{6}. A specific heat
measurement on the barium compound gave an electron density of states at the
Fermi level, N(E_{F}) equal to 6.1$\times 10^{24} eV^{-1}mole^{-1}$. At 5 K, we
observed a negative magnetoresistance of 10 % in a magnetic field of 5 T, but
only for Ba_{2}FeReO_{6}. Electrical, thermal and magnetic properties are
discussed and compared to the analogous compounds Sr_{2}Fe(Mo,Re)O_{6}.


title: Pseudogap phase in the U(1) gauge theory with incoherent spinon pairs

abstract:
  The pseudogap effect of underdoped high-$T_c$ superconductors is studied in
the U(1) gauge theory of the t-J model including the spinon pairing
fluctuation. The gauge fluctuation breaks the long range correlation between
the spinon pairs. The pairing fluctuation, however, suppresses significantly
the low-lying gauge fluctuations and leads to a stable but phase incoherent
spin gap phase which is responsible for the pseudogap effects. This quantum
disordered spin gap phase emerges below a characteristic temperature $T^{*}$
which is determined by the effective potential for the spinon pairing gap
amplitude. The resistivity is suppressed by the phase fluctuation below $%
T^{*}$, consistent with experiments.


title: An exactly solvable many-body problem in one dimension

abstract:
  For N impenetrable particles in one dimension where only the nearest and
next-to-nearest neighbours interact, we obtain the complete spectrum both on a
line and on a circle. Further, we establish a mapping between these N-body
problems and the short-range Dyson model introduced recently to model
intermediate spectral statistics in some systems using which we compute the
two-point correlation function and prove the absence of long-range order in the
corresponding many-body theory. Further, we also show the absence of
off-diagonal long-range order in these systems.


title: Coulomb systems at low density

abstract:
  Results on the correlations of low density classical and quantum Coulomb
systems at equilibrium in three dimensions are reviewed. The exponential decay
of particle correlations in the classical Coulomb system -- Debye-H\"uckel
screening -- is compared and contrasted with the quantum case where strong
arguments are presented for the absence of exponential screening. Results and
techniques for detailed calculations that determine the asymptotic decay of
correlations for quantum systems are discussed. Theorems on the existence of
molecules in the Saha regime are reviewed. Finally, new combinatoric formulas
for the coefficients of Mayer expansions are presented and their role in proofs
of results on Debye-H\"uckel screening is discussed.


title: Pulsed neutron diffraction study of Zr-rich PZT

abstract:
  Pulsed neutron diffraction investigations have been performed in the
ferroelectric PZT system, Pb(Zr1-xTix)O3, doped with 1%wt. of Nb2O5, as a
function of both temperature and composition. The study has been made in a wide
range of temperatures encompassing the three known phases in Zr-rich PZT:
ferroelectric low temperature (R3c), ferroelectric high temperature (R3m) and
paraelectric (Pm3m). The combination of the temperature and the composition
dependence of the structural parameters allowed the determination of the
special relationship, recently pointed out by Corker et al., between the
octahedral strain (z) and the tilt angle (w) in the ferroelectric low
temperature phase (FL) of PZT. The strain-tilt coupling coefficient has been
found to decrease linearly with Ti content and the composition at which z and w
de-couple to be of x=0.30.


title:
Lennard-Jones binary mixture: a thermodynamical approach to glass
  transition

abstract:
  We study the liquid-glass transition of the Lennard-Jones binary mixture
introduced by Kob and Andersen from a thermodynamic point of view. By means of
the replica approach, translating the problem in the study of a molecular
liquid, we study the phase transition due to the entropy crisis and we find
that the Kauzmann's temperature $\tk$ is $\sim 0.32$. At the end we compare
analytical predictions with numerical results.


title:
Potential Energy Landscape and Long Time Dynamics in a Simple Model
  Glass

abstract:
  We analyze the properties of a Lennard-Jones system at the level of the
potential energy landscape. After an exhaustive investigation of the
topological features of the landscape of the systems, obtained studying small
size sample, we describe the dynamics of the systems in the multi-dimensional
configurational space by a simple model. This consider the configurational
space as a connected network of minima where the dynamics proceeds by jumps
described by an appropriate master equation. Using this model we are able to
reproduce the long time dynamics and the low temperature regime. We investigate
both the equilibrium regime and the off-equilibrium one, finding those typical
glassy behavior usually observed in the experiments such as: {\it i)} stretched
exponential relaxation, {\it ii)} temperature-dependent stretching parameter,
{\it iii)} breakdown of the Stokes-Einstein relation, and {\it iv)} appearance
of a critical temperature below which one observes deviation from the
fluctuation-dissipation relation as consequence of the lack of equilibrium in
the system.


title:
Phase Fluctuations and Pseudogap Properties: Influence of Nonmagnetic
  Impurities

abstract:
  The presence of nonmagnetic impurities in a 2D ``bad metal'' depresses the
superconducting Berezinskii-Kosterlitz-Thouless transition temperature, while
leaving the pairing energy scale unchanged. Thus the region of the pseudogap
non-superconducting phase, where the modulus of the order parameter is non-zero
but its phase is random, and which opens at the pairing temperature is
substantially bigger than for the clean system. This supports the premise that
fluctuations in the phase of the order parameter can in principle describe the
pseudogap phenomena in high-$T_c$ materials over a rather wide range of
temperatures and carrier densities. The temperature dependence of the bare
superfluid density is also discussed.


title: Transport properties of the hierarchical model for stretched polymers

abstract:
  We prove that the hierarchical fractal model recently proposed for describing
the stretched polymers [A. N. Samukhin et al, Phys. Rev. Lett.{\bf 78},
326(1997)] is equivalent to a one-dimensional chain with hierarchical aperiodic
structure. By use of the transfer matrix technique we calculate the electronic
transmission and the dc conductance. We find that there exist sharp-edged
transmission subbands and gaps, but the transmission subbands are rich in
substructures which show self-similarity. The temperature dependence of
conductance $g (T)$ is sensitive to the variation of the Fermi level and to the
structure parameters $m$ and $n$ of the original fractal structure. The
relationship of the obtained results to the experimental data and other
theoretical works is discussed.


title:
Maximal Stability Regions for Superconducting Ground States of
  Generalized Hubbard Models

abstract:
  For a class of generalized Hubbard models, we determine the maximal stability
region for the superconducting eta-pairing ground state. We exploit the
Optimized Ground State (OGS) approach and the Lanczos diagonalization procedure
to derive a sequence of improved bounds. We show that some pieces of the
stability boundary are asymptotic, namely independent on the OGS cluster size.
In this way, necessary and sufficient conditions are obtained to realize
superconductivity in terms of an eta-pairing ground state. The phenomenon is
explained by studying the properties of certain exact eigenstates of the OGS
hamiltonians.


title:
New Integrable Models of Strongly Correlated Particles with Correlated
  Hopping

abstract:
  The exact solution is obtained for the eigenvalues and eigenvectors for two
models of strongly correlated particles with single-particle correlated and
uncorrelated pair hoppings. The asymptotic behavior of correlation functions
are analysed in different regions, where the models exhibit different physical
behavior.


title:
Exact Solution of the Asymmetric Exclusion Model with Particles of
  Arbitrary Size

abstract:
  A generalization of the simple exclusion asymmetric model is introduced. In
this model an arbitrary mixture of molecules with distinct sizes $s =
0,1,2,...$, in units of lattice space, diffuses asymmetrically on the lattice.
A related surface growth model is also presented. Variations of the
distribution of molecules's sizes may change the excluded volume almost
continuously. We solve the model exactly through the Bethe ansatz and the
dynamical critical exponent $z$ is calculated from the finite-size corrections
of the mass gap of the related quantum chain. Our results show that for an
arbitrary distribution of molecules the dynamical critical behavior is on the
Kardar-Parizi-Zhang (KPZ) universality.


title:
Specific Heat of Zn-Doped YBa_{2}Cu_3O_{6.95}: Possible Evidence for
  Kondo Screening in the Superconducting State

abstract:
  The magnetic field dependence of the specific heat of Zn-doped single
crystals of YBa_{2}Cu_3O_{6.95} was measured between 2 and 10 K and up to 8
Tesla. Doping levels of 0, 0.15%, 0.31%, and 1% were studied and compared. In
particular we searched for the Schottky anomaly associated with the Zn-induced
magnetic moments.


title:
Transport Coefficients for Granular Media from Molecular Dynamics
  Simulations

abstract:
  Under many conditions, macroscopic grains flow like a fluid; kinetic theory
pred icts continuum equations of motion for this granular fluid. In order to
test the theory, we perform event driven molecular simulations of a
two-dimensional gas of inelastic hard disks, driven by contact with a heat
bath. Even for strong dissipation, high densities, and small numbers of
particles, we find that continuum theory describes the system well. With a bath
that heats the gas homogeneously, strong velocity correlations produce a
slightly smaller energy loss due to inelastic collisions than that predicted by
kinetic theory. With an inhomogeneous heat bath, thermal or velocity gradients
are induced. Determination of the resulting fluxes allows calculation of the
thermal conductivity and shear viscosity, which are compared to the predictions
of granular kinetic theory, and which can be used in continuum modeling of
granular flows. The shear viscosity is close to the prediction of kinetic
theory, while the thermal conductivity can be overestimated by a factor of 2;
in each case, transport is lowered with increasing inelasticity.


title:
Exact Results for Thermodynamics of the Classical Field Theories: Sine-
  and Sinh-Gordon Models

abstract:
  Using the recently obtained exact results for the expectation values of
operators in the sine- and sinh-Gordon models [A. B. Zamolodchikov and S.
Lukyanov, Nucl. Phys. B{\bf 493}, 571 (1997), V. Fateev, S. Lukyanov, A. B.
Zamolodchikov and Al. B. Zamolodchikov, Phys. Lett. B{\bf 406}, 83 (1997)] we
calculate the specific heat of the corresponding two dimensional Euclidean
(classical) models.
  We show that the temperature dependence of the specific heat of the
sine-Gordon model, in the commensurate phase, has a maximum well below the
Kosterlitz-Thouless transition and that the sinh-Gordon model is
thermodynamically unstable in the strong coupling regime. We give also the
temperature dependence of the specific heat in the incommensurate phase of the
sine-Gordon model.


title:
What happened to the gas-liquid transition in the system of dipolar hard
  spheres?

abstract:
  We explore the equilibrium properties of a system composed of dipolar hard
spheres. A new theory based on the ideas derived from the work of Debye and
H\"uckel, Bjerrum, and Onsager is proposed to explain the absence of the
anticipated critical point in this system.


title: Velocity Correlations in Driven Two-Dimensional Granular Media

abstract:
  Simulations of volumetrically forced granular media in two dimensions produce
s tates with nearly homogeneous density. In these states, long-range velocity
correlations with a characteristic vortex structure develop; given sufficient
time, the correlations fill the entire simulated area. These velocity
correlations reduce the rate and violence of collisions, so that pressure is
smaller for driven inelastic particles than for undriven elastic particles in
the same thermodynamic state. As the simulation box size increases, the effects
of veloc ity correlations on the pressure are enhanced rather than reduced.


title:
Glassy timescale divergence and anomalous coarsening in a kinetically
  constrained spin chain

abstract:
  We analyse the out of equilibrium behavior of an Ising spin chain with an
asymmetric kinetic constraint after a quench to a low temperature T. In the
limit T\to 0, we provide an exact solution of the resulting coarsening process.
The equilibration time exhibits a `glassy' divergence \teq=\exp(const/T^2)
(popular as an alternative to the Vogel-Fulcher law), while the average domain
length grows with a temperature dependent exponent, \dbar ~ t^{T\ln 2}. We show
that the equilibration time \teq also sets the timescale for the linear
response of the system at low temperatures.


title:
Unusual features in the nonlinear microwave surface impedance of
  Y-Ba-Cu-O thin films

abstract:
  Striking features have been found in the nonlinear microwave (8 GHz) surface
impedance $Z_s=R_s + jX_s$ of high-quality YBaCuO thin films with comparable
low power characteristics [$R_{res}\sim 35--60 \mu\Omega$ and $\lambda_L(15
K)\sim 130--260 nm$]. The surface resistance $R_s$ is found to increase,
decrease, or remain independent of the microwave field $H_{rf}$ (up to 60 mT)
at different temperatures and for different samples. However, the surface
reactance $X_s$ always follows the same functional form. Mechanisms which may
be responsible for the observed variations in $R_s$ and $X_s$ are briefly
discussed.


title: Quasiparticle thermal Hall angle and magnetoconductance in YBa_2Cu_3O_x

abstract:
  We present a way to extract the quasiparticle (qp) thermal conductivity
Kappa_e and mean-free-path in YBa_2Cu_3O_x, using the thermal Hall effect and
the magnetoconductance of Kappa_e. The results are very consistent with heat
capacity experiments. Moreover, we find a simple relation between the thermal
Hall angle Theta_Q and the H-dependence of Kappa_e, as well as numerical
equality between Theta_Q and the electrical Hall angle. The findings also
reveal an anomalously anisotropic scattering process in the normal state.


title: Geometrical model for a particle on a rough inclined surface

abstract:
  A simple geometrical model is presented for the gravity-driven motion of a
single particle on a rough inclined surface. Adopting a simple restitution law
for the collisions between the particle and the surface, we arrive at a model
in which the dynamics is described by a one-dimensional map. This map is
studied in detail and it is shown to exhibit several dynamical regimes (steady
state, chaotic behavior, and accelerated motion) as the model parameters vary.
A phase diagram showing the corresponding domain of existence for these regimes
is presented. The model is also found to be in good qualitative agreement with
recent experiments on a ball moving on a rough inclined line.


title: Phonon-induced spin relaxation of conduction electrons in aluminum

abstract:
  Spin-flip Eliashberg function $\alpha_S^2F$ and temperature-dependent spin
relaxation time $T_1(T)$ are calculated for aluminum using realistic
pseudopotentials. The spin-flip electron-phonon coupling constant $\lambda_S$
is found to be $2.5\times 10^{-5}$. The calculations agree with experiments
validating the Elliott-Yafet theory and the spin-hot-spot picture of spin
relaxation for polyvalent metals.


title: Flow Induced Organization and Memory of a Vortex Lattice

abstract:
  We report on experiments probing the evolution of a vortex state in response
to a driving current in 2H-NbSe$_2$ crystals. By following the vortex motion
with fast transport measurements we find that the current enables the system to
reorganize and access new configurations. During this process the system
exhibits a long-term memory: if the current is turned off the vortices freeze
in place remembering their prior motion. When the current is restored the
motion resumes where it stopped. The experiments provide evidence for a
dynamically driven structural change of the vortex lattice and a corresponding
dynamic phase diagram that contains a previously unknown regime where the
critical current can be either $increased$ or $decreased$ by applying an
appropriate driving current.


title: Projected SO(5) Models

abstract:
  We construct a class of projected SO(5) models where the Gutzwiller
constraint of no-double-occupancy is implemented exactly. We introduce the
concept of projected SO(5) symmetry where all static correlation functions are
exactly SO(5) symmetric and discuss the signature of the projected SO(5)
symmetry in dynamical correlation functions. We show that this class of
projected SO(5) models can give a realistic description of the global phase
diagram of the high T_c superconductors and account for many of their physical
properties.


title:
Numerical Study on Aging Dynamics in the 3D Ising Spin-Glass Model. I.
  Energy Relaxation and Domain Coarsening

abstract:
  Using Monte Carlo simulations, we have studied the relaxation of energy of
the three-dimensional Ising spin-glass model in aging process. Our
finite-size-scaling analysis on the isothermal energy decay after the quench
suggests strongly that the energy relaxes by coarsening of domain walls in
agreement with the droplet theory. We have also evaluated relaxation times
required for spin configurations in small systems, which are regarded as
isolated droplets, to flip globally. The results support the fundamental
assumption of the droplet theory that coarsening of domain walls, in case of
spin glasses, is driven by successive nucleation of thermally activated
droplets.


title:
Discrete breathers in nonlinear lattices: Experimental detection in a
  Josephson array

abstract:
  We present an experimental study of discrete breathers in an underdamped
Josephson-junction array. Breathers exist under a range of dc current biases
and temperatures, and are detected by measuring dc voltages. We find the
maximum allowable bias current for the breather is proportional to the array
depinning current while the minimum current seems to be related to a junction
retrapping mechanism. We have observed that this latter instability leads to
the formation of multi-site breather states in the array. We have also studied
the domain of existence of the breather at different values of the array
parameters by varying the temperature.


title: Classical versus Quantum Transport near Quantum Hall Transitions

abstract:
  In attempt to settle the apparent disagreements between different
experimental results, transport data near quantum Hall transitions are
interpreted by identifying two distinct conduction regimes. The ``classical''
regime, dominated by nearest neighbor hopping between localized conducting
puddles, manifests an activated-like resistivity formula, and the quantized
Hall insulator behavior. At very low temperatures $T$, or farther from the
critical point, a crossover occurs to a "quantum" transport regime dominated by
variable range hopping. The latter is characterized by a different
T-dependence, yet the dependence on filling fraction is, coincidentally, hard
to distinguish.


title:
Bounds on the Complex Zeros of (Di)Chromatic Polynomials and Potts-Model
  Partition Functions

abstract:
  I show that there exist universal constants $C(r) < \infty$ such that, for
all loopless graphs $G$ of maximum degree $\le r$, the zeros (real or complex)
of the chromatic polynomial $P_G(q)$ lie in the disc $|q| < C(r)$. Furthermore,
$C(r) \le 7.963906... r$. This result is a corollary of a more general result
on the zeros of the Potts-model partition function $Z_G(q, {v_e})$ in the
complex antiferromagnetic regime $|1 + v_e| \le 1$. The proof is based on a
transformation of the Whitney-Tutte-Fortuin-Kasteleyn representation of $Z_G(q,
{v_e})$ to a polymer gas, followed by verification of the
Dobrushin-Koteck\'y-Preiss condition for nonvanishing of a polymer-model
partition function. I also show that, for all loopless graphs $G$ of
second-largest degree $\le r$, the zeros of $P_G(q)$ lie in the disc $|q| <
C(r) + 1$. Along the way, I give a simple proof of a generalized (multivariate)
Brown-Colbourn conjecture on the zeros of the reliability polynomial for the
special case of series-parallel graphs.

categories: cond-mat.stat-mech hep-lat math-ph math.CO math.MP


title: Josephson tunneling between weakly interacting Bose-Einstein condensates

abstract:
  Based on a tunneling Hamiltonian description, we calculate the Josephson,
normal and interference currents between two Bose-Einstein condensates
described by the Bogoliubov theory. The dominant Josephson term is of first
order in the tunneling with a critical current density proportional to the
ground state pressure. In contrast to superconductors, the normal current
remains finite at zero temperature. We discuss the dynamics of the relative
phase in a semiclassical approximation derived from an exact functional
integral approach, which includes the interaction effects at fixed total
particle number. It is shown that the normal current leads to a damping of the
Josephson oscillations and, at long times, eliminates the macroscopic quantum
self trapping predicted by Smerzi et.al. Finally we give estimates for an
experimental realization of Josephson tunneling in cold atomic gases, which
indicate that coherent transfer of atoms might be realized with a $^{23}$Na
condensate.


title: Sliding Objects with Random Friction

abstract:
  We study the motion of elastic networks driven over a random substrate. Our
model which includes local friction forces leads to complex dynamical behavior.
We find a transition to a sliding state which belongs to a new universality
class. The phase diagram comprises of a pinned state, a stick-slip motion
phase, and a free motion phase.


title: Accuracy of a mechanical single electron shuttle

abstract:
  Motivated by recent experiments, we calculate both the average current and
the current fluctuations for a metallic island which oscillates between two
symmetric electrodes. Electrons can only tunnel on or off the island when it is
close to one of the electrodes. Using a Master equation we investigate the
accuracy of such an electron shuttle both analytically and numerically. It is
shown that optimum operation is reached when the contact time is much larger
than the RC-time.


title: Single hole dynamics in the one dimensional t-J model

abstract:
  We present a new finite-temperature quantum Monte Carlo algorithm to compute
imaginary-time Green functions for a single hole in the t-J model on
non-frustrated lattices. Spectral functions are then obtained with the Maximum
Entropy method. Simulations of the one-dimensional case show that a simple
charge-spin separation Ansatz is able to describe the overall features of the
spectral function over the whole energy range for values of J/t from 1/3 to 4.
This includes the bandwidth W \sim 4t + J and the compact support of the
spectral function. The quasiparticle weight Z_k is computed on lattices up to
L=96 sites, and scales as Z_k\propto L^{-1/2}.


title: Pauli susceptibility of nonadiabatic Fermi liquids

abstract:
  The nonadiabatic regime of the electron-phonon interaction leads to behaviors
of some physical measurable quantities qualitatively different from those
expected from the Migdal-Eliashberg theory. Here we identify in the Pauli
paramagnetic susceptibility $\chi$ one of such quantities and show that the
nonadiabatic corrections reduce $\chi$ with respect to its adiabatic limit. We
show also that the nonadiabatic regime induces an isotope dependence of $\chi$,
which in principle could be measured.


title: Distribution of Dangling Ends on the Incipient Percolation Cluster

abstract:
  We study numerically and by scaling arguments the probability P(M)dM that a
given dangling end of the incipient percolation cluster has a mass between M
and M + dM. We find by scaling arguments that P(M) decays with a power law,
P(M)~M^(-(1+k)), with an exponent k=dBf/df, where df and dBf are the fractal
dimensions of the cluster and its backbone, respectively. Our numerical results
yield k=0.83 in d=2 and k=0.74 in d=3 in very good agreement with theory.


title:
Energy Transduction of Isothermal Ratchets: Generic Aspects and Specific
  Examples Close to and Far from Equilibrium

abstract:
  We study the energetics of isothermal ratchets which are driven by a chemical
reaction between two states and operate in contact with a single heat bath of
constant temperature. We discuss generic aspects of energy transduction such as
Onsager relations in the linear response regime as well as the efficiency and
dissipation close to and far from equilibrium. In the linear response regime
where the system operates reversibly the efficiency is in general nonzero.
Studying the properties for specific examples of energy landscapes and
transitions, we observe in the linear response regime that the efficiency can
have a maximum as a function of temperature. Far from equilibrium in the fully
irreversible regime, we find a maximum of the efficiency with values larger
than in the linear regime for an optimal choice of the chemical driving force.
We show that corresponding efficiencies can be of the order of 50%. A simple
analytic argument allows us to estimate the efficiency in this irreversible
regime for small external forces.


title:
Observation of Critical Amplitude Fluctuations near the Two Dimensional
  Superconductor-Insulator Transition

abstract:
  We report results of transport measurements in the quantum critical regime of
the disorder tuned, 2D superconductor-insulator transition (SIT) in
homogeneously disordered films. We show that, as the superconducting transition
temperature decreases, the transition width grows, appearing to diverge at the
SIT. In addition, structure develops in the DC current-voltage characteristics
of films closest to the SIT indicating that the 2D superconductivity is driven
into a regime of extreme inhomogeneity. The data suggest a picture of the phase
transition in which large amplitude fluctuations occur as the amplitude is
suppressed to near zero by disorder.


title:
Middle-Field Cusp Singularities in the Magnetization Process of
  One-Dimensional Quantum Antiferromagnets

abstract:
  We study the zero-temperature magnetization process (M-H curve) of
one-dimensional quantum antiferromagnets using a variant of the density-matrix
renormalization group method. For both the S=1/2 zig-zag spin ladder and the
S=1 bilinear-biquadratic chain, we find clear cusp-type singularities in the
middle-field region of the M-H curve. These singularities are successfully
explained in terms of the double-minimum shape of the energy dispersion of the
low-lying excitations. For the S=1/2 zig-zag spin ladder, we find that the cusp
formation accompanies the Fermi-liquid to non-Fermi-liquid transition.


title: Ratchet potential for fluxons in Josephson-Junction arrays

abstract:
  We propose a simple configuration of a one-dimensional parallel array of
Josephson junctions in which the pinning potential for trapped fluxons lacks
inversion symmetry (ratchet potential). This sytem can be modelised by a set of
non-linear pendula with alternating lengths and harmonic couplings. We show, by
molecular dynamics simulations, that fluxons behave as single particles in
which the predictions for overdamped thermal ratchet can be easily verified.


title:
Doped Heisenberg chains: spin-S generalizations of the supersymmetric
  t-J model

abstract:
  A family of exactly solvable models describing a spin-S Heisenberg chain
doped with mobile spin-(S-1/2) carriers is constructed from gl(2|1)-invariant
solutions of the Yang-Baxter equation. The models are generalizations of the
supersymmetric t-J model which is obtained for S=1/2. We solve the model by
means of the algebraic Bethe Ansatz and present results for the zero
temperature and thermodynamic properties. At low temperatures the models show
spin charge separation, i.e. contain contributions of a free bosonic theory in
the charge sector and an SU(2)-invariant theory describing the magnetic
excitations. For small carrier concentration the latter can be decomposed
further into an SU(2) level-2S Wess-Zumino-Novikov-Witten model and the minimal
unitary model M_p with p=2S+1.


title:
Effects of Spin-Orbit Interactions on Tunneling via Discrete Energy
  Levels in Metal Nanoparticles

abstract:
  The presence of spin-orbit scattering within an aluminum nanoparticle affects
measurements of the discrete energy levels within the particle by (1) reducing
the effective g-factor below the free-electron value of 2, (2) causing avoided
crossings as a function of magnetic field between predominantly-spin-up and
predominantly-spin-down levels, and (3) introducing magnetic-field-dependent
changes in the amount of current transported by the tunneling resonances. All
three effects can be understood in a unified fashion by considering a simple
Hamiltonian. Spin-orbit scattering from 4% gold impurities in superconducting
aluminum nanoparticles produces no dramatic effect on the superconducting gap
at zero magnetic field, but we argue that it does modify the nature of the
superconducting transition in a magnetic field.


title: Linear-scaling ab-initio calculations for large and complex systems

abstract:
  A brief review of the SIESTA project is presented in the context of
linear-scaling density-functional methods for electronic-structure calculations
and molecular-dynamics simulations of systems with a large number of atoms.
Applications of the method to different systems are reviewed, including carbon
nanotubes, gold nanostructures, adsorbates on silicon surfaces, and nucleic
acids. Also, progress in atomic-orbital bases adapted to linear-scaling
methodology is presented.


title: Thermal Conductivity as a Probe of Quasi-Particles in the Cuprates

abstract:
  In underdoped YBa_2Cu_3O_x (x=6.63), the low-T thermal conductivity Kappa_xx
varies steeply with field B at small B, and saturates to a nearly
field-independent value at high fields. The simple expression [1+p(T)|B|]^(-1)
provides an excellent fit to Kappa_xx(B) over a wide range of fields. From the
fit, we extract the zero-field mean-free-path, and the low temperature behavior
of the QP current. The procedure also allows the QP Hall angle Theta_QP to be
obtained. We find that Theta_QP falls on the 1/T^2 curve extrapolated from the
electrical Hall angle above Tc. Moreover, it shares the same T dependence as
the field scale p(T) extracted from Kappa_xx. We discuss implications of these
results.


title: The Pair Contact Process in Two Dimensions

abstract:
  We study the stationary properties of the two-dimensional pair contact
process, a nonequilibrium lattice model exhibiting a phase transition to an
absorbing state with an infinite number of configurations. The critical
probability and static critical exponents are determined via Monte Carlo
simulations, as well as order-parameter moment ratios and the scaling of the
initial density decay. The static critical properties are consistent with the
directed percolation universality class.


title: Collisional relaxation in a fermionic gas

abstract:
  We propose a method to study the degeneracy of a trapped atomic gas of
fermions through the relaxation of the motion of a test particle. In the
degenerate regime, and for an energy of the test particle well below the Fermi
energy, we show that the Fermi-Dirac statistics is responsible for a strong
decrease of the relaxation rate. This method can be used to directly measure
the temperature of the fermionic gas.


title: Time Dependent Development of the Coulomb Gap

abstract:
  We show that the time development of the Coulomb gap in a Coulomb glass can
involve very long relaxation times due to electron rearrangement and hopping.
We find that an applied magnetic field reduces the rate of electron hopping
and, hence, Coulomb gap formation. These results are consistent with recent
conductance experiments on thin semiconducting and metallic films.


title: Effects of Defects on Friction for a Xe Film Sliding on Ag(111)

abstract:
  The effects of a step defect and a random array of point defects (such as
vacancies or substitutional impurities) on the force of friction acting on a
xenon monolayer film as it slides on a silver (111) substrate are studied by
molecular dynamic simulations and compared with the results of lowest order
perturbation theory in the substrate corrugation potential. For the case of a
step, the magnitude and velocity dependence of the friction force are strongly
dependent on the direction of sliding respect to the step and the corrugation
strength. When the applied force F is perpendicular to the step, the film is
pinned forF less than a critical force Fc. Motion of the film along the step,
however, is not pinned. Fluctuations in the sliding velocity in time provide
evidence of both stick-slip motion and thermally activated creep. Simulations
done with a substrate containing a 5 percent concentration of random point
defects for various directions of the applied force show that the film is
pinned for the force below a critical value. The critical force, however, is
still much lower than the effective inertial force exerted on the film by the
oscillations of the substrate in experiments done with a quartz crystal
microbalance (QCM). Lowest order perturbation theory in the substrate potential
is shown to give results consistent with the simulations, and it is used to
give a physical picture of what could be expected for real surfaces which
contain many defects.


title: Discrete breathers in dc biased Josephson-junction arrays

abstract:
  We propose a method to excite and detect a rotor localized mode
(rotobreather) in a Josephson-junction array biased by dc currents. In our
numerical studies of the dynamics we have used experimentally realizable
parameters and included self-inductances. We have uncovered two families of
rotobreathers. Both types are stable under thermal fluctuations and exist for a
broad range of array parameters and sizes including arrays as small as a single
plaquette. We suggest a single Josephson-junction plaquette as an ideal system
to experimentally investigate these solutions.


title: Anomalous quantum Hall conductivity and resonances in coupled layers

abstract:
  A two-layer system coupled via tunneling and with different carrier masses in
each layer is investigated in the integer quantum Hall regime. Striking
deviations of the one-layer Hall conductivity from the usual quantization are
found, if resonance between Landau levels of different layers is achieved. The
appearance of negative jumps in the Hall conductivity is also predicted under
suitable conditions. The results obtained are robust against disorder in the
system.


title:
A frictional Cosserat model for the flow of granular materials through a
  vertical channel

abstract:
  A rigid-plastic Cosserat model has been used to study dense, fully developed
flow of granular materials through a vertical channel. Frictional models based
on the classical continuum do not predict the occurrence of shear layers, at
variance with experimental observations. This feature has been attributed to
the absence of a material length scale in their constitutive equations. The
present model incorporates such a material length scale by treating the
granular material as a Cosserat continuum. Thus localised couple stresses exist
and the stress tensor is asymmetric. The velocity profiles predicted by the
model are in close agreement with available experimental data. The predicted
dependence of the shear layer thickness on the width of the channel is in
reasonable agreement with data. In the limit of the ratio of the particle
diameter to the half-width of the channel being small, the model predicts that
the shear layer thickness scaled by the particle diameter grows.


title: The Superconductor-Insulator Transition in 2D

abstract:
  The superconductor-insulator transition of ultrathin films of bismuth, grown
on liquid helium cooled substrates, has been studied. The transition was tuned
by changing both film thickness and perpendicular magnetic field. Assuming that
the transition is controlled by a T=0 critical point, a finite size scaling
analysis was carried out to determine the correlation length exponent v and the
dynamical critical exponent z. The phase diagram and the critical resistance
have been studied as a function of film thickness and magnetic field. The
results are discussed in terms of bosonic models of the
superconductor-insulator transition, as well as the percolation models which
predict finite dissipation at T=0.