title: Anomalous charge transport of the parent antiferromagnet Bi2Sr2RCu2O8

abstract:
  The resistivity and the thermopower for Bi2Sr2Ca{1-x}RxCu2O8 single crystals
are measured and analyzed with a special interest in the parent antiferromagnet
insulators. Above room temperature, the parent insulators show an electric
conduction confined in the CuO2 plane and a decreasing thermopower with
temperature, which are very similar to those for high-$T_c$ cuprates. These
data strongly suggest that essential anomalies of high-Tc cuprates inheres in
the parent insulators.


title:
An electron correlation originated negative magnetoresistance in a
  system having a partly flat band

abstract:
  Inspired from an experimentally examined organic conductor, a novel mechanism
for negative magnetoresistance is proposed for repulsively interacting
electrons on a lattice whose band dispersion contains a flat portion (a flat
bottom below a dispersive part here). When the Fermi level lies in the flat
part, the electron correlation should cause ferromagnetic spin fluctuations to
develop with an enhanced susceptibility. A relatively small magnetic field will
then shift the majority-spin Fermi level to the dispersive part, resulting in a
negative magnetoresistance. We have actually confirmed the idea by calculating
the conductivity in magnetic fields, with the fluctuation exchange
approximation, for the repulsive Hubbard model on a square lattice having a
large second nearest-neighbor hopping.


title:
Effects of substituting rare-earth ion R by non-magnetic impurities in
  $R_2BaNiO_5$ - theory and numerical DMRG results

abstract:
  In this paper we study the effect of substituting R (rare-earth ion) by
non-magnetic ions in the spin-1 chain material $R_2BaNiO_5$. Using a
strong-coupling expansion and numerical density matrix renormalization group
calculations, we show that spin-wave bound states are formed at the impurity
site. Experimental consequences of the bound states are pointed out.


title:
Anisotropic dielectric constant of the parent antiferromagnet
  Bi2Sr2MCu2O8 (M=Dy, Y and Er) single crystals

abstract:
  The anisotropic dielectric constants of the parent antiferromagnet
Bi2Sr2MCu2O8 (M=Dy, Y and Er) single crystals were measured from 80 to 300 K.
The in-plane dielectric constant is found to be very huge (10^4-10^5). This
suggests a remnant of the Fremi surface of the parent antiferromagnet. The
out-of-plane dielectric constant is 50-200, which is three orders of magnitude
smaller than the in-plane one. A significant anomaly is that a similar
out-of-plane dielectric constant is observed in superconducting samples.


title:
Critical doping in overdoped high-Tc superconductors - a Quantum
  Critical Point?

abstract:
  Evidence is presented from the scaling of the Knight shift, entropy and
transport properties together with the sharp peaking of condensation energy,
critical currents, superfluid density and a variety of other physical
properties for the occurrence of a common critical doping point in lightly
overdoped high-Tc superconductors (HTS). This critical doping lies at the point
where the doping-dependent normal-state pseudogap energy, Eg, falls to zero and
bears a strong, though incomplete, resemblance to a quantum critical point
(QCP). A QCP scenario could lead directly to an explanation of the non-Fermi
liquid behaviour of the normal-state metallic phase and the overall generic
behaviour of the HTSC.


title:
Experimental observation of the mobility edge in a waveguide with
  correlated disorder

abstract:
  The tight-binding model with correlated disorder introduced by Izrailev and
Krokhin [PRL 82, 4062 (1999)] has been extended to the Kronig-Penney model. The
results of the calculations have been compared with microwave transmission
spectra through a single-mode waveguide with inserted correlated scatterers.
All predicted bands and mobility edges have been found in the experiment, thus
demonstrating that any wanted combination of transparent and non-transparent
frequency intervals can be realized experimentally by introducing appropriate
correlations between scatterers.


title:
Random solids and random solidification: What can be learned by
  exploring systems obeying permanent random constraints?

abstract:
  In many interesting physical settings, such as the vulcanization of rubber,
the introduction of permanent random constraints between the constituents of a
homogeneous fluid can cause a phase transition to a random solid state. In this
random solid state, particles are permanently but randomly localized in space,
and a rigidity to shear deformations emerges. Owing to the permanence of the
random constraints, this phase transition is an equilibrium transition, which
confers on it a simplicity (at least relative to the conventional glass
transition) in the sense that it is amenable to established techniques of
equilibrium statistical mechanics. In this Paper I shall review recent
developments in the theory of random solidification for systems obeying
permanent random constraints, with the aim of bringing to the fore the
similarities and differences between such systems and those exhibiting the
conventional glass transition. I shall also report new results, obtained in
collaboration with Weiqun Peng, on equilibrium correlations and
susceptibilities that signal the approach of the random solidification
transition, discussing the physical interpretation and values of these
quantities both at the Gaussian level of approximation and, via a
renormalization-group approach, beyond.


title: Excitonic Strings in one dimensional organic compounds

abstract:
  Important questions concern the existence of excitonic strings in organic
compounds and their signatures in the photophysics of these systems. A model in
terms of Hard Core Bosons is proposed to study this problem in one dimension.
Mainly the cases with two and three particles are studied for finite and
infinite lattices, where analytical results are accessible. It is shown that if
bi-excitonic states exist, three-excitonic and even, n-excitonic strings, at
least in a certain range of parameters, will exist. Moreover, the behaviour of
the transitions from one exciton to the biexciton is fully clarified. The
results are in agreement with exact finite cluster diagonalizations of several
model Hamiltonians.


title: Spontaneous plaquette dimerization in the $J_1-J_2$ Heisenberg model

abstract:
  We investigate the non magnetic phase of the spin-half frustrated Heisenberg
antiferromagnet on the square lattice using exact diagonalization (up to 36
sites) and quantum Monte Carlo techniques (up to 144 sites). The spin gap and
the susceptibilities for the most important crystal symmetry breaking operators
are computed. A genuine and somehow unexpected `plaquette RVB', with
spontaneously broken translation symmetry and no broken rotation symmetry,
comes out from our numerical simulations as the most plausible ground state for
$J_2/J_1 \simeq 0.5 $.


title:
Exact properties of the chemical potential-density relation at finite
  temperature in the Hubbard model

abstract:
  We draw some rigorous conclusions about the functional properties of the
$\mu-\rho$ relation in the Hubbard model based on symmetry considerations and
unitary transformations. It is shown that the charge susceptibility reaches its
local extreme at half-filling. Exact expressions are obtained in two limiting
cases.


title: Two Theorems on Pseudo-spin in the Hubbard Model

abstract:
  An inequality of the eigenvalues of the reduced density matrix $\rho_2$ at
finite temperature in the Hubbard model is obtained by means of the Bogolyubov
inequality. The quasi-average of $\tilde{S}^{+}$ in a simple symmetry-breaking
perturbation of the Hamiltonian for a bipartite lattice is shown to be zero.


title:
Field-induced segregation of ferromagnetic nano-domains in
  Pr$_{0.5}$Sr$_{0.5}$MnO$_3$, detected by $^{55}$Mn NMR

abstract:
  The antiferromagnetic manganite Pr$_{0.5}$Sr$_{0.5}$MnO$_3$ was investigated
at low temperature by means of magnetometry and $^{55}$Mn NMR. A field-induced
transition to a ferromagnetic state is detected by magnetization measurements
at a threshold field of a few tesla. NMR shows that the ferromagnetic phase
develops from zero field by the nucleation of microscopic ferromagnetic
domains, consisting of an inhomogeneous mixture of tilted and fully aligned
parts. At the threshold the NMR spectrum changes discontinuously into that of a
homogeneous, fully aligned, ferromagnetic state, suggesting a percolative
origin for the ferromagnetic transition.


title: Coherent Backscattering of light in a magnetic field

abstract:
  This paper describes how coherent backscattering is altered by an external
magnetic field. In the theory presented, magneto-optical effects occur inside
Mie scatterers embedded in a non-magnetic medium. Unlike previous theories
based on point-like scatterers, the decrease of coherent backscattering is
obtained in leading order of the magnetic field using rigorous Mie theory. This
decrease is strongly enhanced in the proximity of resonances, which cause the
path length of the wave inside a scatterer to be increased. Also presented is a
novel analysis of the shape of the backscattering cone in a magnetic field.


title: Bethe ansatz for the SU(4) extension of the Hubbard Model

abstract:
  We apply the nested algebraic Bethe ansatz method to solve the eigenvalue
problem for the SU(4) extension of the Hubbard model. The Hamiltonian is
equivalent to the SU(4) graded permutation operator. The graded Yang-Baxter
equation and the graded Quantum Inverse Scattering Method are used to obtain
the eigenvalue of the SU(4) extension of the Hubbard model.


title:
Critical temperature and phase diagram for a 2D superfluid Fermi-gas
  with repulsion

abstract:
  In the framework of a 2D Fermi-gas with short-range repulsive interaction we
find the critical temperature of the superfluid phase transition based on
Kohn-Luttinger effect, and analyze its dependence on magnetic field. We
calculate strong-coupling corrections to the Ginzburg-Landau free energy
functional and analyze the staibility of 2D superfluid phases. Possible
applications of the results to real systems are discussed.


title:
Dynamics of competition between collectivity and noise in the stock
  market

abstract:
  Detailed study of the financial empirical correlation matrix of the 30
companies comprised by DAX within the period of the last 11 years, using the
time-window of 30 trading days, is presented. This allows to clearly identify a
nontrivial time-dependence of the resulting correlations. In addition, as a
rule, the draw downs are always accompanied by a sizable separation of one
strong collective eigenstate of the correlation matrix which, at the same time,
reduces the variance of the noise states. The opposite applies to draw ups. In
this case the dynamics spreads more uniformly over the eigenstates which
results in an increase of the total information entropy.

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


title: Superfluid critical temperature in 3D Fermi gas with repulsion

abstract:
  The critical temperature of a superfluid phase transition in a Fermi gas with
repulsive interaction is found. The influence of a magnetic field on the
transition is analyzed. The estimates for the critical temperature for a
trapped gas of $^6$Li atoms and $^3$He--$^4$He mixtures are presented.


title:
Comment on ``Density Matrix Renormalization Group Study of the Haldane
  Phase in Random One-Dimensional Antiferromagnets"

abstract:
  In a recent Letter (PRL 83, 3297 (1999)), Hida presented numerical results
indicating that the Haldane phase of the Heisenberg antiferromagnetic spin-1
chain is stable against bond randomness, for box distributions of the bond
strength, even when the box distribution stretches to zero bond strength. The
author thus concluded that the Haldane phase is stable against bond randomness
for any distribution of the bond strength, no matter how broad. In this
Comment, we (i) point out that the randomness distributions studied in this
Letter do not represent the broadest possible distributions, and therefore
these numerical results do not lead to the conclusion that the Haldane phase is
stable against any randomness; and (ii) provide a semiquantitative estimate of
the critical randomness beyond which the Haldane phase yields to the Random
Singlet phase, in a specific class of random distribution functions for the
bond strength.


title: Stripes Disorder and Correlation lengths in doped antiferromagnets

abstract:
  For stripes in doped antiferromagnets, we find that the ratio of spin and
charge correlation lenghts, $\xi_{s}/\xi_{c}$, provide a sharp criterion for
determining the dominant form of disorder in the system. If stripes disorder is
controlled by topological defects then $\xi_{s}/\xi_{c}\lesssim 1$. In contast,
if stripes correlations are disordered primarily by non-topological elastic
deformations (i.e., a Bragg-Glass type of disorder) then $1<\xi
_{s}/\xi_{c}\lesssim 4$ is expected. Therefore, the observation of $\xi
_{s}/\xi_{c}\approx 4$ in $(LaNd)_{2-x}Sr_{x}CuO_{4}$ and $\xi_{s}/\xi
_{c}\approx 3$ in $La_{2/3}Sr_{1/3}NiO_{4}$ invariably implies that the stripes
are in a Bragg glass type state, and topological defects are much less relevant
than commonly assumed. Expected spectral properties are discussed. Thus, we
establish the basis for any theoretical analysis of the experimentally
obsereved glassy state in these material.


title: Maximum velocity of a fluxon in a stack of coupled Josephson junctions

abstract:
  Dynamics of a fluxon in a stack of inductively coupled long Josephson
junctions is studied analytically and numerically. We demonstrate that the
fluxon has a maximum velocity, which does not necessarily coincide with any of
the characteristic Josephson plasma wave velocities. The maximum fluxon
velocity is found by means of numerical simulations of the quasi-infinite
system. Using the variational approximation, we propose a simple analytical
formula for the dependence of the fluxon's maximum velocity on the coupling
constant and on the distribution of critical currents in different layers. This
analysis yields rather precise results in the limit of small dissipation. The
simulations also show that nonzero dissipation additionally stabilizes the
fluxon.


title:
Conductance Suppression due to Correlated Electron Transport in Coupled
  Double-dots

abstract:
  The electrostatic interaction between two capacitively-coupled metal
double-dots is studied at low temperatures. Experiments show that when the
Coulomb blockade is lifted by applying appropriate gate biases to both
double-dots, the conductance through each double-dot becomes significantly
lower than when only one double-dot is conducting. A master equation is derived
for the system and the results obtained agree well with the experimental data.
The model suggests that the conductance lowering in each double-dot is caused
by a single-electron tunneling in the other double-dot. Here, each double-dot
responds to the instantaneous, rather than average, potentials on the other
double-dot. This leads to correlated electron motion within the system, where
the position of a single electron in one double-dot controls the tunneling rate
through the other double-dot.


title: The mean-field theory for attraction between like-charged macromolecules

abstract:
  A mean-field theory based on Gibbs-Bogoliubov inequality is constructed to
study the interactions between two like-charged polyions. It is shown that
contrary to the previously established paradigm, a properly constructed
mean-field theory can quantitatively account for the attractive interactions
between two like-charged rods.


title: Ferromagnetic correlations in quasi-one-dimensional conducting channels

abstract:
  We propose a model which explains the experimental observation of spontaneous
spin polarization of conducting electrons in quasi- one-dimensional GaAs/AlGaAs
channels. We show that a ferromagnetic order is a generic property of a
quasi-one-dimensional conducting channel embedded in a Wigner crystal. We also
discuss gate voltage, magnetic field and temperature dependences of the
channel's conductance.


title: Bound states of interacting helium atoms

abstract:
  We study the possibility of existence of bound states for two interacting
He-4 atoms. It is shown that for some potentials, there exist not only discrete
levels but also bands akin to those in the Kronig--Penney model.


title:
Stationary solutions of the one-dimensional nonlinear Schroedinger
  equation: I. Case of repulsive nonlinearity

abstract:
  All stationary solutions to the one-dimensional nonlinear Schroedinger
equation under box and periodic boundary conditions are presented in analytic
form. We consider the case of repulsive nonlinearity; in a companion paper we
treat the attractive case. Our solutions take the form of stationary trains of
dark or grey density-notch solitons. Real stationary states are in one-to-one
correspondence with those of the linear Schr\"odinger equation. Complex
stationary states are uniquely nonlinear, nodeless, and symmetry-breaking. Our
solutions apply to many physical contexts, including the Bose-Einstein
condensate and optical pulses in fibers.


title:
Stationary solutions of the one-dimensional nonlinear Schroedinger
  equation: II. Case of attractive nonlinearity

abstract:
  All stationary solutions to the one-dimensional nonlinear Schroedinger
equation under box or periodic boundary conditions are presented in analytic
form for the case of attractive nonlinearity. A companion paper has treated the
repulsive case. Our solutions take the form of bounded, quantized, stationary
trains of bright solitons. Among them are two uniquely nonlinear classes of
nodeless solutions, whose properties and physical meaning are discussed in
detail. The full set of symmetry-breaking stationary states are described by
the $C_{n}$ character tables from the theory of point groups. We make
experimental predictions for the Bose-Einstein condensate and show that, though
these are the analog of some of the simplest problems in linear quantum
mechanics, nonlinearity introduces new and surprising phenomena.


title:
Two-dimensional limit of exchange-correlation energy functional
  approximations in density functional theory

abstract:
  We investigate the behavior of three-dimensional (3D) exchange-correlation
energy functional approximations of density functional theory in anisotropic
systems with two-dimensional (2D) character. Using two simple models, quasi-2D
electron gas and two-electron quantum dot, we show a {\it fundamental
limitation} of the local density approximation (LDA), and its semi-local
extensions, generalized gradient approximation (GGA) and meta-GGA (MGGA), the
most widely used forms of which are worse than the LDA in the strong 2D limit.
The origin of these shortcomings is in the inability of the local (LDA) and
semi-local (GGA/MGGA) approximations to describe systems with 2D character in
which the nature of the exchange-correlation hole is very nonlocal. Nonlocal
functionals provide an alternative approach, and explicitly the average density
approximation (ADA) is shown to be remarkably accurate for the quasi-2D
electron gas system. Our study is not only relevant for understanding of the
functionals but also practical applications to semiconductor quantum structures
and materials such as graphite and metal surfaces. We also comment on the
implication of our findings to the practical device simulations based on the
(semi-)local density functional method.


title: Non-adiabatic Landau-Zener transitions in low spin molecular magnet V15

abstract:
  The V_{15} polyoxovanadate molecule is made of 15 spins 1/2 with
antiferromagnetic couplings. It belongs to the class of molecules with very
large Hilbert space dimension (2^{15} in V_{15}, 10^8 in Mn_{12}-ac). It is a
low-spin/big-molecule with spin S=1/2. Contrary large-spins/big-molecules of
the Mn_{12}-ac type, V_{15} has no energy barrier against spin rotation.
Magnetization measurements have been performed and despite the absence of a
barrier, magnetic hysteresis is observed over a timescale of several seconds.
This new phenomenon characterized by a "butterfly" hysteresis loop is due to
the effect of the environment on the quantum rotation of the entangled 15 spins
of the molecule, in which the phonon density of states is not at its
equilibrium (phonon bottleneck).


title: Thermodynamic Irreversibility from high-dimensional Hamiltonian Chaos

abstract:
  This paper discusses the thermodynamic irreversibility realized in
high-dimensional Hamiltonian systems with a time-dependent parameter. A new
quantity, the irreversible information loss, is defined from the Lyapunov
analysis so as to characterize the thermodynamic irreversibility. It is proved
that this new quantity satisfies an inequality associated with the second law
of thermodynamics. Based on the assumption that these systems possess the
mixing property and certain large deviation properties in the thermodynamic
limit, it is argued reasonably that the most probable value of the irreversible
information loss is equal to the change of the Boltzmann entropy in statistical
mechanics, and that it is always a non-negative value. The consistency of our
argument is confirmed by numerical experiments with the aid of the definition
of a quantity we refer to as the excess information loss.


title:
Superconducting fluctuations and anomalous diamagnetism in underdoped
  YBa2Cu3O6+x from magnetization and 63Cu NMR-NQR relaxation measurements

abstract:
  Magnetization and 63Cu NMR-NQR relaxation measurements are used to study the
superconducting fluctuations in YBa2Cu3O6+x (YBCO) oriented powders. In
optimally doped YBCO the fluctuating negative magnetization M_{fl}(H,T) is
rather well described by an anisotropic Ginzburg-Landau (GL) functional and the
curves M_{fl}/sqrt{H} cross at Tc. In underdoped YBCO, instead, over a wide
temperature range an anomalous diamagnetism is observed, stronger than in the
optimally doped compound by about an order of magnitude. The field and
temperature dependences of M_{fl} cannot be described either by an anisotropic
GL functional or on the basis of scaling arguments. The anomalous diamagnetism
is more pronounced in samples with a defined order in the Cu(1)O chains. The
63Cu(2) relaxation rate shows little, if any, field dependence in the vicinity
of the transition temperature Tc(H=0). It is argued how the results in the
underdoped compounds can be accounted for by the presence of charge
inhomogeneities, favoured by chains ordering.


title: Spectral determinant on quantum graphs

abstract:
  We study the spectral determinant of the Laplacian on finite graphs
characterized by their number of vertices V and of bonds B. We present a path
integral derivation which leads to two equivalent expressions of the spectral
determinant of the Laplacian either in terms of a V x V vertex matrix or a 2B x
2B link matrix that couples the arcs (oriented bonds) together. This latter
expression allows us to rewrite the spectral determinant as an infinite product
of contributions of periodic orbits on the graph. We also present a
diagrammatic method that permits us to write the spectral determinant in terms
of a finite number of periodic orbit contributions. These results are
generalized to the case of graphs in a magnetic field. Several examples
illustrating this formalism are presented and its application to the
thermodynamic and transport properties of weakly disordered and coherent
mesoscopic networks is discussed.

categories: cond-mat.mes-hall chao-dyn hep-th nlin.CD


title: On the Helix-Coil transition in grafted chains

abstract:
  The helix-coil transition is modified by grafting to a surface. This
modification is studied for short peptides capable of forming $\alpha$-helices.
Three factors are involved: (i) the grafting can induced change of the boundary
free energy of the helical domain (ii) the van der Waals attraction between the
helices and (iii) the crowding induced stretching of the coils. As a result the
helix-coil transition acquires ``all or nothing'' characteristics. In addition
the transition temperature is elevated and the transition itself sharpens as
the grafting density increases.


title: Superconductivity from Flat Dispersion Designed in Doped Mott Insulators

abstract:
  Routes to enhance superconducting instability are explored for doped Mott
insulators. With the help of insights for criticalities of metal-insulator
transitions, geometrical design of lattice structure is proposed to control the
instability. A guideline is to explicitly make flat band dispersions near the
Fermi level without suppressing two-particle channels. In a one-dimensional
model, numerical studies show that our prescription with finite-ranged hoppings
realizes large enhancement of spin-gap and pairing dominant regions. We also
propose several multi-band systems, where the pairing is driven by intersite
Coulomb repulsion.


title: High-Field Electrical Transport in Single-Wall Carbon Nanotubes

abstract:
  Using low-resistance electrical contacts, we have measured the intrinsic
high-field transport properties of metallic single-wall carbon nanotubes.
Individual nanotubes appear to be able to carry currents with a density
exceeding 10^9 A/cm^2. As the bias voltage is increased, the conductance drops
dramatically due to scattering of electrons. We show that the current-voltage
characteristics can be explained by considering optical or zone-boundary phonon
emission as the dominant scattering mechanism at high field.


title: Mixed state on a sparsely encoded associative memory model

abstract:
  In the present paper, we analyze symmetric mixed states corresponding to the
so-called concept formation on a sparsely encoded associative memory model with
0-1 neurons. Three types of mixed states, OR, AND and a majority decision mixed
state are described as typical examples. Each element of the OR mixed state is
composed of corresponding memory pattern elements by means of the OR-operation.
The other two types are similarly defined. By analyzing their stabilities
through the SCSNA and the computer simulation, we found that the storage
capacity of the OR mixed state diverges in the sparse limit, but that the other
states do not diverge. In addition, we found that the optimal threshold values,
which maximize the storage capacity, for the memory pattern and the OR mixed
state coincide with each other in the spare limit. Thus, we conclude that the
OR mixed state is a reasonable representative of the mixed state in the spare
limit. Finally, the paper examines the relationship between our results and
recently reported physiological findings regarding face-responsive neurons in
the inferior temporal cortex.


title: Charge fluctuations and dephasing in coulomb coupled conductors

abstract:
  It is shown that the dephasing rate in Coulomb coupled mesoscopic structures
is determined by charge relaxation resistances. The charge relaxation
resistance together with the capacitance determines the RC-time of the
mesoscopic structure and at small frequencies determines the voltage
fluctuation spectrum. Self-consistent expressions are presented which give the
charge relaxation resistance and consequently the dephasing rate in terms of
the diagonal and off-diagonal elements of a generalized Wigner-Smith delay-time
matrix. Dephasing rates are discussed both for the equilibrium state and in the
transport state in which charge fluctuations are generated by shot noise. A
number of different geometries are discussed. This article is to appear in {\it
Quantum Mesoscopic Phenomena and Mesoscopic Devices}, edited by I. O. Kulik and
R. Ellialtioglu, (Kluwer, unpublished).


title: Temperature-Dependent Pseudogaps in Colossal Magnetoresistive Oxides

abstract:
  Direct electronic structure measurements of a variety of the colossal
magnetoresistive oxides show the presence of a pseudogap at the Fermi energy
E_F which drastically suppresses the electron spectral function at E_F. The
pseudogap is a strong function of the layer number of the samples (sample
dimensionality) and is strongly temperature dependent, with the changes
beginning at the ferromagnetic transition temperature T_c. These trends are
consistent with the major transport trends of the CMR oxides, implying a direct
relationship between the pseudogap and transport, including the "colossal"
conductivity changes which occur across T_c. The k-dependence of the
temperature-dependent effects indicate that the pseudogap observed in these
compounds is not due to the extrinsic effects proposed by Joynt.


title: Some aspects of functional Bethe ansatz

abstract:
  The validity of Bethe ansatz wave function without the backward scattering
for the problem of impurity in correlated hosts with periodic boundary
condition is illustrated by a simple example of quantum mechanics. An long
being overlooked point in solving Hubbard model by means of Bethe ansatz is
indicated. A possible repairing on the Hamiltonian is suggested so that the
well known solution is still valid.


title:
AC Loss and Contact Resistance In Copper-Stabilized Nb3Al Rutherford
  Cables with and without a Stainless Steel Core

abstract:
  Calorimetric measurements of AC loss and hence interstrand contact resistance
(ICR), were measured on three samples of Rutherford cable wound with
Cu-stabilized jelly-roll type unplated Nb3Al strand. One of the cable types was
furnished with a thin core of AISI 316L stainless steel and the other two were
both uncored but insulated in different ways. The cables were subjected to a
room-temperature-applied uniaxial pressure of 12 MPa that was maintained during
the reaction heat treatment (RHT), then vacuum impregnated with CTD 101 epoxy,
and repressurized to 100 MPa during AC-loss measurement. The measurements were
performed at 4.2 K in a sinusoidal field of amplitude 400 mT at frequencies of
1 to 90 mHz (no DC-bias field) that was applied both perpendicular and parallel
to the face of the cable (the face-on, FO, and edge-on, EO, directions,
respectively). For the cored cable the FO-measured effective ICR (FO-ICR), was
5.27 . Those for the uncored cables were less than 0.08
. As shown previously for NbTi- and Nb3Sn-based Rutherford
cables, the FO-ICR can be significantly increased by the insertion of a core,
although in this case it is still below the range recommended for
accelerator-magnet use. Post-measurement dissection of one of the cables showed
that the impregnating resin had permeated between the strands and coated the
core with a thin, insulating layer excepting for some sintered points of
contact. In the uncored cables the strands were coated with resin except for
the points of interstrand contact. It is suggested that in the latter case this
tendency for partial coating leads to a processing-sensitive FO-ICR.


title: Theory of electrons with orbital degeneracy

abstract:
  The Hubbard model for electrons with orbital degeneracy is shown to have an
underlying SU_d(4) symmetry of spin-orbital double. A hidden charge SU_c(4)
symmetry is exposed and an extended Lieb-Mattis transformation which maps these
two symmetries into each other is given. On the basis of elementary degenerate
perturbative theory, it is shown that the system with strong repulsive coupling
is equivalent to a SO(6) Heisenberg magnet at half-filling and a SU(4) one at
quarter-filling. The band is half-filled at all temperature for $\mu=3U/2$. The
features of ground state and low-lying excitations in one dimension are
indicated according to exact solutions. Various possibilities of symmetry
breaking are also given.


title:
The short-time critical behaviour of the Ginzburg-Landau model with
  long-range interaction

abstract:
  The renormalisation group approach is applied to the study of the short-time
critical behaviour of the $d$-dimensional Ginzburg-Landau model with long-range
interaction of the form $p^{\sigma} s_{p}s_{-p}$ in momentum space. Firstly the
system is quenched from a high temperature to the critical temperature and then
relaxes to equilibrium within the model A dynamics. The asymptotic scaling laws
and the initial slip exponents $\theta^{\prime}$ and $\theta$ of the order
parameter and the response function respectively, are calculated to the second
order in $\epsilon=2\sigma-d$.


title:
Cu NMR evidence for enhanced antiferromagnetic correlations around Zn
  impurities in YBa2Cu3O6.7

abstract:
  Doping the high-Tc superconductor YBa2Cu3O6.7 with 1.5 % of non-magnetic Zn
impurities in CuO2 planes is shown to produce a considerable broadening of 63Cu
NMR spectra, as well as an increase of low-energy magnetic fluctuations
detected in 63Cu spin-lattice relaxation measurements. A model-independent
analysis demonstrates that these effects are due to the development of
staggered magnetic moments on many Cu sites around each Zn and that the
Zn-induced moment in the bulk susceptibility might be explained by this
staggered magnetization. Several implications of these enhanced
antiferromagnetic correlations are discussed.


title:
Observation of the scissors mode and superfluidity of a trapped
  Bose-Einstein condensed gas

abstract:
  We report the observation of the scissors mode of a Bose-Einstein condensed
gas of 87^Rb atoms in a magnetic trap, which gives direct evidence of
superfluidity in this system. The scissors mode of oscillation is excited by a
sudden rotation of the anisotropic trapping potential. For a gas above T_c
(normal fluid) we detect the occurrence of oscillations at two frequencies,
with the lower frequency corresponding to the rigid body value of the moment of
inertia. Well below T_c the condensate oscillates at a single frequency,
without damping, as expected for a superfluid.


title:
Coupling Current Control in Stabrite-Coated Nbti Rutherford Cables by
  Varying the Width of a Stainless Steel Core

abstract:
  AC loss measurements were made on a series of Rutherford cables wound from
stabrite-coated NbTi/Cu multifilamentary strand, from the results of which
interstrand contact resistances were calculated. The 28-strand, 15 mm wide,
LHC-inner type cables contained stainless steel cores of various widths.
Calorimetric loss measurements were made on all samples using a sinusoidal
field with an amplitude of 400 mT. It is concluded that by adjusting the width
of a stainless steel core the effective ICR of a stabrite-coated Rutherford
cable can be varied over a wide range, and in particular that a published
target of 15 micro-ohm could be attained.


title: Energy and entropy of metastable states in glassy systems

abstract:
  We investigate the multi-valley energy landscape of a 3-D on-lattice network
model for covalent glasses, numerically determining the shape of the valleys,
the local density of states, the density of minima and the local connectivity.
We present some of these quantities in a graphical birds-eye view of the
landscape, and discuss their implications for the relaxation dynamics and
cooling behavior of glasses. The strong similarities between the landscape of
this model and those of other complex systems point to the possibility of a
common low-temperature dynamical description.


title:
The short-time behavior of kinetic spherical model with long-ranged
  interactions

abstract:
  The kinetic spherical model with long-ranged interactions and an arbitrary
initial order m_{0} quenched from a very high temperature to T < T_{c} is
solved. In the short-time regime, the bulk order increases with a power law in
both the critical and phase-ordering dynamics. To the latter dynamics, a power
law for the relative order m_{r} ~ -t^{-k} is found in the intermediate
time-regime. The short-time scaling relation of small m_{0} are generalized to
an arbitrary m_{0} and all the time larger than t_{mic}. The characteristic
functions $\phi (b,m_{0})$ for the scaling of m_{0} and $\epsilon (b,T')$ for
T'=T/T_{c} are obtained. The crossover between scaling regimes is discussed in
detail.


title: 4f spin density in the reentrant ferromagnet SmMn2Ge2

abstract:
  The spin contribution to the magnetic moment in SmMn2Ge2 has been measured by
magnetic Compton scattering in both the low and high temperature ferromagnetic
phases. At low temperature, the Sm site is shown to possess a large 4f spin
moment of 3.4 +/- 0.1 Bohr magnetons, aligned antiparallel to the total
magnetic moment. At high temperature, the data show conclusively that ordered
magnetic moments are present on the samarium site.


title: Quantum Poincare Recurrences for Hydrogen Atom in a Microwave Field

abstract:
  We study the time dependence of the ionization probability of Rydberg atoms
driven by a microwave field, both in classical and in quantum mechanics. The
quantum survival probability follows the classical one up to the Heisenberg
time and then decays algebraically as P(t) ~ 1/t. This decay law derives from
the exponentially long times required to escape from some region of the phase
space, due to tunneling and localization effects. We also provide parameter
values which should allow to observe such decay in laboratory experiments.


title: Bose-Einstein condensation in a rotating anisotropic TOP trap

abstract:
  We describe the construction and operation of a time-orbiting potential trap
(TOP trap) that has different oscillation frequencies along its three principal
axes. These axes can be rotated and we have observed Bose-Einstein condensates
of 87^Rb with a rotating ellipsoidal shape. Under these conditions it has been
predicted that quantized vortices form and are stable.


title: Ferromagnetic Liquid Thin Films Under Applied Field

abstract:
  Theoretical calculations, computer simulations and experiments indicate the
possible existence of a ferromagnetic liquid state, although definitive
experimental evidence is lacking. Should such a state exist, demagnetization
effects would force a nontrivial magnetization texture. Since liquid droplets
are deformable, the droplet shape is coupled with the magnetization texture. In
a thin-film geometry in zero applied field, the droplet has a circular shape
and a rotating magnetization texture with a point vortex at the center. We
calculate the elongation and magnetization texture of such ferromagnetic thin
film liquid droplet confined between two parallel plates under a weak applied
magnetic field. The vortex stretches into a domain wall and exchange forces
break the reflection symmetry. This behavior contrasts qualitatively and
quantitatively with the elongation of paramagnetic thin films.


title: AC susceptibility in deoxygenated YBCO single crystals

abstract:
  We measured the first and third harmonic of the complex AC susceptibility in
YBCO single crystals with different oxygen content. The amplitude of the AC
field was varied in presence of an external DC field both applied parallel to
the c axis of the crystals. We show evidence that deoxygenation leads to a
reduction of bulk pinning strength and consequently to a stronger contribution
of geometrical barriers.


title:
Synchronization in a System of Globally Coupled Oscillators with Time
  Delay

abstract:
  We study the synchronization phenomena in a system of globally coupled
oscillators with time delay in the coupling. The self-consistency equations for
the order parameter are derived, which depend explicitly on the amount of
delay. Analysis of these equations reveals that the system in general exhibits
discontinuous transitions in addition to the usual continuous transition,
between the incoherent state and a multitude of coherent states with different
synchronization frequencies. In particular, the phase diagram is obtained on
the plane of the coupling strength and the delay time, and ubiquity of
multistability as well as suppression of the synchronization frequency is
manifested. Numerical simulations are also performed to give consistent
results.


title: Coulomb blockade in quantum dots with overlapping resonances

abstract:
  Coulomb blockade (CB) in a quantum dot (QD) with one anomalously broad level
is considered. In this case many consecutive pronounced CB peaks correspond to
occupation of one and the same broad level. Between the peaks the electron
jumps from this level to one of the narrow levels and the transmission through
the dot at the next resonance essentially repeats that at the previous one.
This offers a natural explanation to the recently observed behavior of the
transmission phase in an interferometer with a QD. Single particle resonances
of very different width are natural if the dot is not fully chaotic. This idea
is illustrated by the numerical simulations for a non-integrable QD whose
classical dynamics is intermediate between integrable and chaotic. Possible
manifestations for the Kondo experiments in the QD are discussed.


title:
LAPW frozen-phonon calculation, shell model lattice dynamics and
  specific-heat measurement of SnO

abstract:
  An ab-initio Linear Augmented Plane-Wave (LAPW) calculation of the
zone-centered phonon frequencies of SnO has been performed. E$_g$ symmetry has
been ascribed to the mode observed at 113 cm$^{-1}$ in Raman measurements,
discarding a previous B$_{1g}$ assignement. The other phonon modes measured by
Raman spectroscopy are also well reproduced. A new shell-model has also been
developed, that gives good agreement of the zone-centered frequencies compared
to the measured data and the LAPW results. Specific heat measurements have been
performed between 5 K and 110 K. Computation of the specific heat and the
M\"{o}ssbauer recoilless fraction with the improved shell-model shows a good
agreement with the experimental data as a function of temperature.


title:
Noise in disordered systems: The power spectrum and dynamic exponents in
  avalanche models

abstract:
  For a long time, it has been known that the power spectrum of Barkhausen
noise had a power-law decay at high frequencies. Up to now, the theoretical
predictions for this decay have been incorrect, or have only applied to a small
set of models. In this paper, we describe a careful derivation of the power
spectrum exponent in avalanche models, and in particular, in variations of the
zero-temperature random-field Ising model. We find that the naive exponent,
(3-tau)/(sigma nu z), which has been derived in several other papers, is in
general incorrect for small tau, when large avalanches are common. (tau is the
exponent describing the distribution of avalanche sizes, and (sigma nu z) is
the exponent describing the relationship between avalanche size and avalanche
duration.) We find that for a large class of avalanche models, including
several models of Barkhausen noise, the correct exponent for tau<2 is 1/(sigma
nu z). We explicitly derive the mean-field exponent of 2. In the process, we
calculate the average avalanche shape for avalanches of fixed duration and
scaling forms for a number of physical properties.


title:
Predicting phase transition pressure in solids:a semiclasical
  possibility

abstract:
  This is a short review of the physical ideas,algorithm for the calculation of
the phase transition pressure and some results of a semi-classical theory of
the behaviour of materials under high pressure proposed by P.Savic and
R.Kasanin.It is base on the Coulomb interaction,supplemented by a microscopic
selection rule and a set of experimentally founded postulates,and it has found
applications from DAC experiments to planetary structure modelling.


title:
Gap anisotropy in the angle-resolved photoemission spectroscopy of
  Bi_2Sr_2CaCu_2O_{8+\delta}

abstract:
  The gap anisotropy in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ is revisited in the
framework of a d-wave scenario in view of the recent angle-resolved
photoemission experiment. Based on a tight-binding fit to the normal state
dispersion, a detail analysis on the effects of the inclusion of the next
harmonic in the d-wave has been presented. Significant effect has been observed
in the superconducting T$_c$. The density of states is linear at the nodes with
enhanced weight, caused by a marked increase in the low energy excitaions which
affect the thermodynamics considerably. The slope of the $\rho_s - T$ curve in
the low temperature regime increases and the specific heat reflects the
enhanced entropy at low temperatures. The leading edge of the ARPES energy
distribution curves have been calculated and found to shift towards higher
energy. The effect of scattering by non-magnetic impurities in this context is
also outlined.


title:
Condensation and interaction range in harmonic boson traps: a
  variational approach

abstract:
  For a gas of N bosons interacting through a two-body Morse potential a
variational bound of the free energy of a confined system is obtained. The
calculation method is based on the Feynman-Kac functional projected on the
symmetric representation. Within the harmonic approximation a variational
estimate of the effect of the interaction range on the existence of
many-particle bound states, and on the N-T phase diagram is obtained.


title:
Phenomenological description of the microwave surface impedance and
  complex conductivity of high-$T_c$ single crystals

abstract:
  Measurements of the microwave surface impedance $Z_s(T)=R_s(T)+iX_s(T)$ and
of the complex conductivity $\sigma_s(T)$ of high-quality, high-$T_c$ single
crystals of YBCO, BSCCO, TBCCO, and TBCO are analyzed. Experimental data of
$Z_s(T)$ and $\sigma_s(T)$ are compared with calculations based on a modified
two-fluid model which includes temperature-dependent quasiparticle scattering
and a unique temperature variation of the density of superconducting carriers.
We elucidate agreement as well as disagreement of our analysis with the salient
features of the experimental data. Existing microscopic models are reviewed
which are based on unconventional symmetry types of the order parameter and on
novel mechanisms of quasiparticle relaxation.


title: Velocity distribution in granular gases of viscoelastic particles

abstract:
  The velocity distribution in a homogeneously cooling granular gas has been
studied in the viscoelastic regime when the restitution coefficient of
colliding particles depends on the impact velocity. We show that for
viscoelastic particles the simple scaling hypothesis is violated, i.e., that
the time dependence of the velocity distribution does not scale with the mean
square velocity as in the case of particles interacting via a constant
restitution coefficient. The deviation from the Maxwellian distribution does
not depend on time monotonously. For the case of small dissipation we detected
two regimes of evolution of the velocity distribution function: Starting from
the initial Maxwellian distribution, the deviation first increases with time on
a collision time-scale saturating at some maximal value; then it decays to zero
on much larger time-scale which corresponds to the temperature relaxation. For
larger values of the dissipation parameter there appears an additional
intermediate relaxation regime. Analytical calculations for small dissipation
agrees well with the results of a numerical analysis.


title:
Review of recent progress on numerical studies of the Anderson
  transition

abstract:
  A review of recent progress in numerical studies of the Anderson transition
in three dimensional systems is presented. From high precision calculations the
critical exponent $\nu$ for the divergence of the localization length is
estimated to be $\nu=1.57\pm 0.02$ for the orthogonal universality class, which
is clearly distinguished from $\nu=1.43\pm 0.03$ for the unitary universality
class. The boundary condition dependences of some quantities at the Anderson
transition are also discussed.


title: 3D quantum percolation studied by level statistics

abstract:
  We study the metal-insulator transition on a three dimensional quantum
percolation model by analyzing energy level statistics. The quantum percolation
threshold $\pq$, which is larger than the classical percolation threshold
$\pc$, becomes smaller when the time reversal symmetry (TRS) is broken, i.e.
$\pq({\rm with TRS})>\pq({\rm without TRS})>\pc$. It is shown that critical
exponents are consistent with the result of the Anderson transition, suggesting
that the quantum percolation problem can be classified into the same
universality classes of the Anderson transition. The shape of level statistics
at the critical point is also reported.


title:
Selection Rules for Resonant Inelastic X-Ray Scattering from Tetragonal
  Copper-Oxides

abstract:
  We demonstrate the utility of point group representation theory for symmetry
analysis in resonant inelastic x-ray scattering. From its
polarization-dependence, we show that a 5 eV inelastic feature in Sr2CuO2Cl2
has pure B1g symmetry and assign it to a transition in the cell-perturbation
calculations of Simon, et. al. [Phys. Rev. B., 54, R3780 (1996)]. We discuss
how Raman selection rules are broken at nonzero momentum transfer and how this
can also act as a probe of wave function symmetry.


title: Flux Jumps Driven by a Pulsed Magnetic Field

abstract:
  The understanding of flux jumps in the high temperature superconductors is of
importance since the occurrence of these jumps may limit the perspectives of
the practical use of these materials. In this work we present the experimental
study of the role of heavy ion irradiation in stabilizing the HTSC against flux
jumps by comparing un-irradiated and 7.5 10^10 Kr-ion/cm2 irradiated
(YxTm1-x)Ba2Cu3O7 single crystals. Using pulsed field magnetization
measurements, we have applied a broad range of field sweep rates from 0.1T/s up
to 1800 T/s to investigate the behavior of the flux jumps. The observed flux
jumps, which may be attributed to thermal instabilities, are incomplete and
have different amplitudes. The flux jumps strongly depend on the magnetic
field, on the magneto-thermal history of the sample, on the magnetic field
sweep rate, on the critical current density jc, on the temperature and on the
thermal contact with the bath in which the sample is immersed.


title:
Comparison of the Transport Mechanism in Underdoped High Temperature
  Superconductors and in Spin Ladders

abstract:
  Recently, the normal state resistivity of high temperature superconductors
(in particular in La2-xSrxCuO4 single crystals) has been studied extensively in
the region below Tc by suppressing the superconducting state in high magnetic
fields. In the present work we report on the normal state resistance of
underdoped La2-xSrxCuO4 thin films under epitaxial strain, measured far below
Tc by applying pulsed fields up to 60 T. We will compare the transport
measurements on these high temperature superconductors with transport data
reported for the Sr2.5Ca11.5Cu24O41 spin ladder compound. This comparison leads
to an interpretation of the data in terms of the recently proposed 1D quantum
transport model and the charge-stripe models.


title:
Lattice polaron formation: Effects of non-screened electron-phonon
  interaction

abstract:
  We explore the quasiparticle properties of lattice polarons on the basis of a
quite general electron-phonon Hamiltonian with a long-range displacement-type
of interaction. To treat the dynamical quantum phonons without significant loss
of accuracy we adapt an exact Lanczos diagonalization method and compute
various static and dynamical quantities, such as the electron-lattice
correlation function, the polaron band dispersion, the effective polaron mass,
the kinetic energy, the single-particle spectral function, and the optical
conductivity, on finite one-dimensional lattices for a wide range of model
parameters. We compare the results with those obtained for the standard
Holstein model with short-range electron-phonon interaction only.


title: Electron-phonon interaction in CdTe/CdMnTe/CdMgTe quantum wells

abstract:
  The optic vibrational (confined and interface) modes and the electron-phonon
and hole-phonon interactions are obtained, in the dielectric continuum model,
for two diluted magnetic semiconductor structures: a single well of
Cd1-xMnxTe/Cd1-yMgyTe with the magnetic ions in the well, and a double well of
CdTe/Cd1-xMnxTe, in which a thin layer of the magnetic material is grown in the
middle of the structure. The scattering rates for the intra- and the
inter-subband transitions are obtained for electrons and holes.


title: Chern-Simons Correlations on (2+1)D Lattice

abstract:
  We have computed the contribution of zero modes to the value of the number of
particles in the model of discrete (2+1)-dimensional nonlinear Schr\"odinger
equation. It is shown for the first time that in the region of small values of
the Chern-Simons coefficient k there exists a universal attraction between
field configurations. For k=2 this phenomenon may be a dynamic origin of the
semion pairing in high temperature superconducting state of planar systems.


title:
Universality and multifractal behaviour of spin-spin correlation
  functions in disordered Potts models

abstract:
  We report a transfer matrix study of the random bond $q-$state Potts model in
the vicinity of the Ising model $q=2$. We draw attention to a precise
determination of magnetic scaling dimensions in order to compare with
perturbative results. Universality is checked by the computation of the
spin-spin correlation function decay exponent obtained with different types of
probability distributions of the coupling strengths. Our numerical data,
compared to perturbative results for the second moment of the correlation
function, obtained with both replica symmetry and replica symmetry breaking
schemes, are conclusively in favour of the replica symmetric calculations. The
multifractal behaviour of higher moments as well as that of typical correlation
functions are also investigated and a comparison is made with the perturbative
expansions. Finally, the shape of the correlation function probability
distribution is analyzed.


title: Random matrices, random polynomials and Coulomb systems

abstract:
  It is well known that the joint probability density of the eigenvalues of
Gaussian ensembles of random matrices may be interpreted as a Coulomb gas. We
review these classical results for hermitian and complex random matrices, with
special attention devoted to electrostatic analogies. We also discuss the joint
probability density of the zeros of polynomials whose coefficients are complex
Gaussian variables. This leads to a new two-dimensional solvable gas of
interacting particles, with non-trivial interactions between particles.

categories: cond-mat.stat-mech chao-dyn cond-mat.str-el nlin.CD


title: Effective action approach to strongly correlated fermion systems

abstract:
  We construct a new functional for the single particle Green's function, which
is a variant of the standard Baym Kadanoff functional.
  The stability of the stationary solutions to the new functional is directly
related to aspects of the irreducible particle hole interaction through the
Bethe Salpeter equation.
  A startling aspect of this functional is that it allows a simple and rigorous
derivation of both the standard and extended dynamical mean field (DMFT)
equations as stationary conditions. Though the DMFT equations were formerly
obtained only in the limit of infinite lattice coordination, the new functional
described in the work, presents a way of directly extending DMFT to finite
dimensional systems, both on a lattice and in a continuum. Instabilities of the
stationary solution at the bifurcation point of the functional, signal the
appearance of a zero mode at the Mott transition which then couples t o
physical quantities resulting in divergences at the transition.


title:
Sequential magnetotunneling in a vertical Quantum Dot tuned at the
  crossing to higher spin states

abstract:
  We have calculated the linear magnetoconductance across a vertical parabolic
Quantum Dot with a magnetic field in the direction of the current. Gate voltage
and magnetic field are tuned at the degeneracy point between the occupancies
N=2 and N=3, close to the Singlet-Triplet transition for N=2. We find that the
conductance is enhanced prior to the transition by nearby crossings of the
levels of the 3 particle dot. Immediately after it is depressed by roughly 1/3,
as long as the total spin S of the 3 electron ground state doesn't change from
S=1/2 to S=3/2, due to spin selection rule.
  At low temperature this dip is very sharp, but the peak is recovered by
increasing the temperature.


title:
Nagel scaling, relaxation and universality in the kinetic Ising model in
  an alternating isotopic chain

abstract:
  The dynamic critical exponent and the frequency and wave-vector dependent
susceptibility of the kinetic Ising model with Glauber dynamics on an
alternating isotopic chain are examined. The analysis provides to our knowledge
the first connection between a microscopic model and the Nagel scaling curve
originally proposed to describe dielectric susceptibility measurements of
several glass-forming liquids. While support is given to the hypothesis
relating the Nagel scaling to multiple relaxation processes, it is also found
that the scaling function may also exhibit plateau regions and does not hold
for low temperatures.


title:
Activated processes and Inherent Structure dynamics of finite-size
  mean-field models for glasses

abstract:
  We investigate the inherent structure (IS) dynamics of mean-field {\it
finite-size} spin-glass models whose high-temperature dynamics is described in
the thermodynamic limit by the schematic Mode Coupling Theory for super-cooled
liquids. Near the threshold energy the dynamics is ruled by activated processes
which induce a logarithmic slow relaxation. We show the presence of aging in
both the IS correlation and integrated response functions and check the
validity of the one-step replica symmetry breaking scenario in the presence of
activated processes. Our work shows: 1) The violation of the
fluctuation-dissipation theorem is given by the configurational entropy, 2) The
intermediate time regime ($\log(t)\sim N$) in mean-field theory automatically
includes activated processes opening the way to analytically investigate
activated processes by computing corrections beyond mean-field.


title: Bose-Einstein Condensation in a Constant Magnetic Field

abstract:
  We discuss the occurrence of Bose-Einstein condensation in systems of
noninteracting charged particles in three in one dimensions and in presence of
an external magnetic field. In the one dimensional, as well as in the magnetic
field cases, although not a critical temperature, a characteristic temperature
can be found, corresponding to the case in which the ground state density
becomes a macroscopic fraction of the total density. The case of relativistic
charged scalar and vector particles is studied. The results obtainedgive
support to the existence of superconductivity in extremely strong magnetic
fields, and leads to the prediction of superconductive-ferromagnetic behavior
in the vector field case, which might be of interest in condensed matter as
well as in cosmology. Some features of the magnetization in the early universe
are conjectured.


title: Dynamics of Kinks: Nucleation, Diffusion and Annihilation

abstract:
  We investigate the nucleation, annihilation, and dynamics of kinks in a
classical (1+1)-dimensional Phi^4 field theory at finite temperature. From
large scale Langevin simulations, we establish that the nucleation rate is
proportional to the square of the equilibrium density of kinks. We identify two
annihilation time scales: one due to kink-antikink pair recombination after
nucleation, the other from non-recombinant annihilation. We introduce a
mesoscopic model of diffusing kinks based on ``paired'' and ``survivor''
kinks/antikinks. Analytical predictions for the dynamical time scales, as well
as the corresponding length scales, are in good agreement with the simulations.


title:
Universal Long-Time Relaxation on the Lattices of Classical Spins:
  Markovian Behavior on non-Markovian Timescales

abstract:
  The long-time behavior of certain fast-decaying infinite temperature
correlation functions on one-, two- and three-dimensional lattices of classical
spins with various kinds of nearest-neighbor interactions is studied
numerically, and evidence is presented that the functional form of this
behavior is either simple exponential or exponential multiplied by cosine. Due
to the fast characteristic timescale of the long-time decay, such a
universality cannot be explained on the basis of conventional Markovian
assumptions. It is suggested that this behavior is related to the chaotic
properties of the spin dynamics.


title:
Long-Time Relaxation on Spin Lattice as Manifestation of Chaotic
  Dynamics

abstract:
  The long-time behavior of the infinite temperature spin correlation functions
describing the free induction decay in nuclear magnetic resonance and
intermediate structure factors in inelastic neutron scattering is considered.
These correlation functions are defined for one-, two- and three-dimensional
infinite lattices of interacting spins both classical and quantum. It is shown
that, even though the characteristic timescale of the long-time decay of the
correlation functions considered is non-Markovian, the generic functional form
of this decay is either simple exponential or exponential multiplied by cosine.
This work contains (i) summary of the existing experimental and numerical
evidence of the above asymptotic behavior; (ii) theoretical explanation of this
behavior; and (iii) semi-empirical analysis of various factors discriminating
between the monotonic and the oscillatory long-time decays. The theory is based
on a fairly strong conjecture that, as a result of chaos generated by the spin
dynamics, a Brownian-like Markovian description can be applied to the long-time
properties of ensemble average quantities on a non-Markovian timescale. The
formalism resulting from that conjecture can be described as ``correlated
diffusion in finite volumes.''


title: Anisotropy in the Compressible Quantum Hall State

abstract:
  Using a mean field theory on the von Neumann lattice, we study compressible
anisotropic states around $\nu=l+1/2$ in the quantum Hall system. The
Hartree-Fock energy of the UCDW are calculated self-consistently. In these
states the unidirectional charge density wave (UCDW) seems to be the most
plausible state. We show that the UCDW is regarded as a collection of the
one-dimensional lattice fermion systems which extend to the uniform direction.
The kinetic energy of this one-dimensional system is induced from the Coulomb
interaction term and the self-consistent Fermi surface is obtained.


title:
Director precession and nonlinear waves in nematic liquid crystals under
  elliptic shear

abstract:
  Elliptic shear applied to a homeotropically oriented nematic above the
electric bend Freedericks transition (FT) generates slow precession of the
director. The character of the accompanying nonlinear waves changes from
diffusive phase waves to dispersive ones exhibiting spirals and spatio-temporal
chaos as the FT is approached from above. An exact solution of the flow
alignment equations captures the observed precession and predicts its reversal
for non-flow aligning materials. The FT transforms into a Hopf bifurcation
opening the way to understand the wave phenomena.


title:
Resonances in a two-dimensional electron waveguide with a single
  delta-function scatterer

abstract:
  We study the conductance properties of a straight two-dimensional electron
waveguide with an s-like scatterer modeled by a single delta-function potential
with a finite number of modes. Even such a simple system exhibits interesting
resonance phenomena. These resonances are explained in terms of quasi-bound
states both by using a direct solution of the Schroedinger equation and by
studying the Green's function of the system. Using the Green's function we
calculate the survival probability as well as the power absorption and show the
influence of the quasi-bound states on these two quantities.


title: Acoustoelectric current for composite fermions

abstract:
  The acoustelectric current for composite fermions in a two-dimensional
electron gas (2DEG) close to the half-filled Landau level is calculated in the
random phase approximation. The Boltzmann equation is used to find the
nonequilibrium distribution of composite fermions to second order in the
acoustic field.It is shown that the oscillating Chern-Simons field created by
the induced density fluctuations in the 2DEG is important for the
acoustoelectric current. This leads to a violation of the Weinreich relation
between the acoustoelectric current and acoustic intensity. The deviations from
the Weinreich relation can be detected by measuring the angle between the
longitudinal and the Hall components of the acoustoelectric current. This
departure from the Weinreich relation gives additional information on the
properties of the compostite fermion fluid.


title: Topology and phase transitions: a paradigmatic evidence

abstract:
  We report upon the numerical computation of the Euler characteristic \chi (a
topologic invariant) of the equipotential hypersurfaces \Sigma_v of the
configuration space of the two-dimensional lattice $\phi^4$ model. The pattern
\chi(\Sigma_v) vs. v (potential energy) reveals that a major topology change in
the family {\Sigma_v}_{v\in R} is at the origin of the phase transition in the
model considered. The direct evidence given here - of the relevance of topology
for phase transitions - is obtained through a general method that can be
applied to any other model.

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


title:
Lattice effects on the current-voltage characteristics of
  superconducting arrays

abstract:
  The lattice effects on the current-voltage characteristics of two-dimensional
arrays of resistively shunted Josephson junctions are investigated. The lattice
potential energies due to the discrete lattice structure are calculated for
several geometries and directions of current injection. We compare the energy
barrier for vortex-pair unbinding with the lattice pinning potential, which
shows that lattice effects are negligible in the low-current limit as well as
in the high-current limit. At intermediate currents, on the other hand, the
lattice potential becomes comparable to the barrier height and the lattice
effects may be observed in the current-voltage characteristics.


title:
The Raman coupling function in amorphous silica and the nature of the
  long wavelength excitations in disordered systems

abstract:
  New Raman and incoherent neutron scattering data at various temperatures and
molecular dynamic simulations in amorphous silica, are compared to obtain the
Raman coupling coefficient $C(\omega)$ and, in particular, its low frequency
limit. This study indicates that in the $\omega \to 0$ limit $C(\omega)$
extrapolates to a non vanishing value, giving important indications on the
characteristics of the vibrational modes in disordered materials; in particular
our results indicate that even in the limit of very long wavelength the local
disorder implies non-regular local atomic displacements.


title:
Damping of low-energy excitations of a Bose-condensed gas in the
  hydrodynamic regime

abstract:
  We develop a theory to describe the damping of elementary excitations of a
Bose-condensed gas in the hydrodynamic regime for the thermal cloud. We discuss
second sound in a spatially homogeneous gas and the lowest excitations of a
trapped condensate.


title: Anisotropic magnetoresistance of GaAs two-dimensional holes

abstract:
  Experiments on high-quality GaAs (311)A two-dimensional holes at low
temperatures reveal a remarkable dependence of the magnetoresistance, measured
with an in-plane magnetic field ($B$), on the direction of $B$ relative to both
the crystal axes and the current direction. The magnetoresistance features, and
in particular the value of $B$ above which the resistivity exhibits an
insulating behavior, depend on the orientation of $B$. To explain the data, the
anisotropic band structure of the holes and a re-population of the
spin-subbands in the presence of $B$, as well as the coupling of the orbital
motion to $B$, need to be taken into account.


title: Exact Density Functionals in One Dimension

abstract:
  We propose a new and general method for deriving exact density functionals in
one dimension for lattice gases with finite-range pairwise interactions.
Corresponding continuum functionals are derived by applying a proper limiting
procedure. The method is based on a generalised Markov property, which allows
us to set up a rather transparent scheme that covers all previously known exact
functionals for one-dimensional lattice gas or fluid systems. Implications for
a systematic construction of approximate density functionals in higher
dimensions are pointed out.


title: Magneto-impedance of glass-coated Fe-Ni-Cu microwires

abstract:
  The magneto-impedance (MI) of glass-coated Fe-Ni-Cu microwires was
investigated for longitudinal radio-frequency (RF) currents up to a frequency
of 200 MHz using an RF lock-in amplifier method. The MI, defined as DZ/Z =
[Z(H)-Z(H=0.3T)]/Z(H=0.3T), displays a peak structure (negative MI) at zero
field for RF currents with frequencies less than 20MHz and this crosses over to
a sharp dip (positive MI) at higher frequencies. This crossover behavior is
ascribed to the skin-depth-limited response primarily governed by the
field-dependence of the permeability. Large saturation fields (300 to 600 Oe)
and other anomalies indicate the possible influence of giant magneto-resistance
(GMR) on the MI.


title:
Dynamic Radio-Frequency Transverse Susceptibility in Magnetic
  Nanoparticle Systems

abstract:
  A novel resonant method based on a tunnel-diode oscillator (TDO) is used to
study the dynamic transverse susceptibility in a Fe nanoparticle system. The
magnetic system consists of an aggregate of nanometer-size core (Au)-shell (Fe)
structure, synthesized by reverse micelle methods. Static and dynamic
magnetization measurements carried out in order to characterize the system
reveal a superparamagnetic behavior at high temperature. The field-dependent
transverse susceptibility at radio-frequencies (RF), for different temperatures
reveals distinct peak structure at characteristics fields (H_k, H_c) which
changes with temperature. It is proposed that relaxation processes could
explain the influence of the temperature on the field dependence of the
transverse susceptibility on the MI.


title: Statistics of Coulomb blockade peak spacings for a partially open dot

abstract:
  We show that randomness of the electron wave functions in a quantum dot
contributes to the fluctuations of the positions of the conductance peaks. This
contribution grows with the conductance of the junctions connecting the dot to
the leads. It becomes comparable with the fluctuations coming from the
randomness of the single particle spectrum in the dot while the Coulomb
blockade peaks are still well-defined. In addition, the fluctuations of the
peak spacings are correlated with the fluctuations of the conductance peak
heights.


title: Spectral problem on graphs and L-functions

abstract:
  The scattering process on multiloop infinite p+1-valent graphs (generalized
trees) is studied. These graphs are discrete spaces being quotients of the
uniform tree over free acting discrete subgroups of the projective group
$PGL(2, {\bf Q}_p)$. As the homogeneous spaces, they are, in fact, identical to
p-adic multiloop surfaces. The Ihara-Selberg L-function is associated with the
finite subgraph-the reduced graph containing all loops of the generalized tree.
We study the spectral problem on these graphs, for which we introduce the
notion of spherical functions-eigenfunctions of a discrete Laplace operator
acting on the graph. We define the S-matrix and prove its unitarity. We present
a proof of the Hashimoto-Bass theorem expressing L-function of any finite
(reduced) graph via determinant of a local operator $\Delta(u)$ acting on this
graph and relate the S-matrix determinant to this L-function thus obtaining the
analogue of the Selberg trace formula. The discrete spectrum points are also
determined and classified by the L-function. Numerous examples of L-function
calculations are presented.


title: Quantum Computation and Spin Electronics

abstract:
  In this chapter we explore the connection between mesoscopic physics and
quantum computing. After giving a bibliography providing a general introduction
to the subject of quantum information processing, we review the various
approaches that are being considered for the experimental implementation of
quantum computing and quantum communication in atomic physics, quantum optics,
nuclear magnetic resonance, superconductivity, and, especially, normal-electron
solid state physics. We discuss five criteria for the realization of a quantum
computer and consider the implications that these criteria have for quantum
computation using the spin states of single-electron quantum dots. Finally, we
consider the transport of quantum information via the motion of individual
electrons in mesoscopic structures; specific transport and noise measurements
in coupled quantum dot geometries for detecting and characterizing
electron-state entanglement are analyzed.


title:
Low-temperature anomalous specific heat without tunneling modes: a
  simulation for a-Si with voids

abstract:
  Using empirical potential molecular dynamics we compute dynamical matrix
eigenvalues and eigenvectors for a 4096 atom model of amorphous silicon and a
set of models with voids of different size based on it. This information is
then employed to study the localization properties of the low-energy
vibrational states, calculate the specific heat C(T) and examine the
low-temperature properties of our models usually attributed to the presence of
tunneling states in amorphous silicon. The results of our calculations for C(T)
and "excess specific heat bulge" in the C(T)/T^3 vs. T graph for voidless a-Si
appear to be in good agreement with experiment; moreover our investigation
shows that the presence of localized low-energy excitations in the vibrational
spectrum of our models with voids strongly manifests itself as a sharp peak in
C(T)/T^3 dependence at T < 3K. To our knowledge this is the first numerical
simulation that provides adequate agreement with experiment for the very
low-temperature properties of specific heat in disordered systems within the
limits of harmonic approximation.


title:
Collective Excitation of a Hybrid Atomic/Molecular Bose Einstein
  Condensate$^{\ast}$

abstract:
  The collective excitation of hybrid atomic-molecular condensate are studied
using variational method. The dipole response of the system is studied in
detail. We found that the out-of-phase dipole oscillation frequency increasing
slowly with the detuning parameter and is only few times large than the trap
frequency making the experimental observation of this mode of vibration
feasible.


title: Topological Scenario for Stripe Formation in Manganese Oxides

abstract:
  The spin-charge-orbital complex structures of manganites are studied using
topological concepts. The key quantity is the "winding number" w associated
with the Berry-phase connection of an e_g electron parallel-transported through
Jahn-Teller centers, along zigzag one-dimensional paths in an antiferromagnetic
environment of t_{2g} spins. From these concepts, it is shown that the
"bi-stripe" and "Wigner-crystal" states observed experimentally have different
w's. Predictions for the spin structure of the charge-ordered states for
heavily doped manganites are discussed.


title:
Applications of the Stell-Hemmer Potential to Understanding Second
  Critical Points in Real Systems

abstract:
  We consider the novel properties of the Stell-Hemmer core-softened
potentials. First we explore how the theoretically predicted second critical
point for these potentials is related to the occurrence of the experimentally
observed solid-solid isostructural critical point. We then discuss how this
class of potentials can generate anomalies analogous to those found
experimentally in liquid water.


title: Effect of small-scale architecture on polymer mobility

abstract:
  Processes on different length scales affect the dynamics of chain molecules.
In this work, we focus on structures on the scale of a monomer and investigate
polyolefins, i.e. hydrocarbon chains with different small scale architectures.
We present an exact enumeration scheme for the simulation of interactions and
relative motion of two short chain sections on a lattice and employ it to
deduce the probability for segmental motion for polymers of four different
architectures in the melt. The probability for segmental motion is inversely
proportional to the monomeric friction coefficient and hence the viscosity of a
polymer. Combining our simulation results with an equation of state for the
thermodynamic properties of the polymers, we are able to make predictions about
the variation of the friction coefficient with temperature, pressure, and small
scale architecture. To compare our results with experimental data, we have
determined monomeric friction coefficients from experimental viscosity data for
the four polyolefins considered in this work. For temperatures well above the
glass transition temperature, we find that our simple approach gives a good
qualitative representation of the variation of the friction coefficient with
chain architecture, temperature and pressure.


title: Coulomb screening in mesoscopic noise: a kinetic approach

abstract:
  Coulomb screening, together with degeneracy, is characteristic of the
metallic electron gas. While there is little trace of its effects in transport
and noise in the bulk, at mesoscopic scales the electronic fluctuations start
to show appreciable Coulomb correlations. Within a strictly standard Boltzmann
and Fermi-liquid framework, we analyze these phenomena and their relation to
the mesoscopic fluctuation-dissipation theorem, which we prove. We identify two
distinct screening mechanisms for mesoscopic fluctuations. One is the
self-consistent response of the contact potential in a non-uniform system. The
other couples to scattering, and is an exclusively non-equilibrium process.
Contact-potential effects renormalize all thermal fluctuations, at all scales.
Collisional effects are relatively short-ranged and modify non-equilibrium
noise. We discuss ways to detect these differences experimentally.


title:
Stability of bicontinuous cubic phases in ternary amphiphilic systems
  with spontaneous curvature

abstract:
  We study the phase behavior of ternary amphiphilic systems in the framework
of a curvature model with non-vanishing spontaneous curvature. The amphiphilic
monolayers can arrange in different ways to form micellar, hexagonal, lamellar
and various bicontinuous cubic phases. For the latter case we consider both
single structures (one monolayer) and double structures (two monolayers). Their
interfaces are modeled by the triply periodic surfaces of constant mean
curvature of the families G, D, P, C(P), I-WP and F-RD. The stability of the
different bicontinuous cubic phases can be explained by the way in which their
universal geometrical properties conspire with the concentration constraints.
For vanishing saddle-splay modulus $\bar \kappa$, almost every phase considered
has some region of stability in the Gibbs triangle. Although bicontinuous cubic
phases are suppressed by sufficiently negative values of the saddle-splay
modulus $\bar \kappa$, we find that they can exist for considerably lower
values than obtained previously. The most stable bicontinuous cubic phases with
decreasing $\bar \kappa < 0$ are the single and double gyroid structures since
they combine favorable topological properties with extreme volume fractions.


title:
On the possibility of a superfluid transition in a Fermi-gas of neutral
  particles at ultra-low temperatures

abstract:
  We predict a possibility of triplet Cooper pairing in a Fermi-gas of neutral
particles in a confined geometry of magnetic traps at ultra-low temperatures.
We evaluate a superfluid transition temperature and analyze the difference
between pairing in free space and in confined geometry. We also consider in
details a case of fermionic 6Li.


title:
Coulomb induced positive current-current correlations in normal
  conductors

abstract:
  In the white-noise limit current correlations measured at different contacts
of a mesoscopic conductor are negative due to the antisymmetry of the wave
function (Pauli principle). We show that current fluctuations at capacitive
contacts induced via the long range Coulomb interaction as consequence of
charge fluctuations in the mesoscopic sample can be {\it positively}
correlated. The positive correlations are a consequence of the extension of the
wave-functions into areas near both contacts. As an example we investigate in
detail a quantum point contact in a high magnetic field under conditions in
which transport is along an edge state.


title: Strong-coupling approach for strongly correlated electron systems

abstract:
  A perturbation theory scheme in terms of electron hopping, which is based on
the Wick theorem for Hubbard operators, is developed. Diagrammatic series
contain single-site vertices connected by hopping lines and it is shown that
for each vertex the problem splits into the subspaces with ``vacuum states''
determined by the diagonal Hubbard operators and only excitations around these
vacuum states are allowed. The rules to construct diagrams are proposed. In the
limit of infinite spatial dimensions the total auxiliary single-site problem
exactly splits into subspaces that allows to build an analytical
thermodynamically consistent approach for a Hubbard model. Some analytical
results are given for the simple approximations when the two-pole
(alloy-analogy solution) and four-pole (Hartree-Fock approximation) structure
for Green's function is obtained. Two poles describe contribution from the
Fermi-liquid component, which is dominant for small electron and hole
concentrations (``overdoped case'' of high-$T_c$'s), whereas other two describe
contribution from the non-Fermi liquid and are dominant close to half-filling
(``underdoped case'').


title: Geometry effects at conductance quantization in quantum wires

abstract:
  The transmission through quantum wires is studied within a noninteracting
model taking into account the geometry of cleaved edge overgrowth devices. We
show that the junction between wide contact and narrow wire has almost no
influence on the transmission through these devices while a potential step
between contact and wire strongly effects the conduction. Our results are
compared with recent experimental findings.


title: Phase Transitions in "Small" systems

abstract:
  Traditionally, phase transitions are defined in the thermodynamic limit only.
We discuss how phase transitions of first order (with phase separation and
surface tension), continuous transitions and (multi)-critical points can be
seen and classified for small systems. Boltzmann defines the entropy as the
logarithm of the area W(E,N)=e^S(E,N) of the surface in the mechanical N-body
phase space at total energy E. The topology of the curvature determinant D(E,N)
of S(E,N) allows the classification of phase transitions without taking the
thermodynamic limit. The first calculation of the entire entropy surface S(E,N)
for the diluted Potts model (ordinary (q=3)-Potts model plus vacancies) on a
50*50 square lattice is shown. The regions in {E,N} where D>0 correspond to
pure phases, ordered resp. disordered, and D<0 represent transitions of first
order with phase separation and ``surface tension''. These regions are bordered
by a line with D=0. A line of continuous transitions starts at the critical
point of the ordinary (q=3)-Potts model and runs down to a branching point P_m.
Along this line \nabla D vanishes in the direction of the eigenvector v_1 of D
with the largest eigen-value \lambda_1\approx 0. It characterizes a maximum of
the largest eigenvalue \lambda_1. This corresponds to a critical line where the
transition is continuous and the surface tension disappears. Here the
neighboring phases are indistinguishable. The region where two or more lines
with D=0 cross is the region of the (multi)-critical point. The micro-canonical
ensemble allows to put these phenomena entirely on the level of mechanics.


title:
Optical Absorption of CuO$_3$ antiferromagnetic chains at finite
  temperatures

abstract:
  We use a high-statistic quantum Monte Carlo and Maximum Entropy
regularization method to compute the dynamical energy correlation function
(DECF) of the one-dimensional (1D) $S=1/2$ antiferromagnetic Heisenberg model
at finite temperatures. We also present a finite temperature analytical ansatz
for the DECF which is in very good agreement with the numerical data in all the
considered temperature range. From these results, and from a finite temperature
generalisation of the mechanism proposed by Lorenzana and Sawatsky [Phys. Rev.
Lett. {\bf 74}, 1867 (1995)], we compute the line shape for the optical
absorption spectra of multimagnon excitations assisted by phonons for quasi 1D
compounds. The line shape has two contributions analogous to the Stokes and
anti-Stokes process of Raman scattering. Our low temperature data is in good
agreement with optical absorption experiments of CuO$_3$ chains in
Sr$_2$CuO$_3$. Our finite temperature results provide a non trivial prediction
on the dynamics of the Heisenberg model at finite temperatures that is easy to
verify experimentally.


title:
Dynamic modulation of electron correlation by intramolecular modes in
  charge transfer compounds

abstract:
  Electron-phonon and electron-electron interactions are in competition in
determining the properties of molecular charge transfer conductors and
superconductors. The direct influence of phonons on the electron-electron
interaction was not before considered and in the present work the coupling of
intramolecular modes to electron-electron interaction (U-vib interaction) is
investigated.The effect of this coupling on the frequency of the normal modes
of a dimer model is obtained and it is shown that frequency shifts of the Raman
active modes are directly related to this coupling. The results are used to
obtain the values of the U-vib coupling constants of intramolecular modes of a
representative molecule of charge transfer conductors, like
tetramethyltetratiafulvalene. Consequences of this coupling on the electron
pairing are also suggested.


title: Whispering Vortices

abstract:
  Experiments indicating the excitation of whispering gallery type
electromagnetic modes by a vortex moving in an annular Josephson junction are
reported. At relativistic velocities the Josephson vortex interacts with the
modes of the superconducting stripline resonator giving rise to novel
resonances on the current-voltage characteristic of the junction. The
experimental data are in good agreement with analysis and numerical
calculations based on the two-dimensional sine--Gordon model.


title: A Microscopic Look at Liquid Helium: the 3He Impurity Case

abstract:
  The description of the properties of liquid Helium is a challenge for any
microscopic many-body theory. In this context, we study the ground state and
the excitation spectrum of one $^3$He impurity in liquid $^4$He at T=0 with the
aim of illustrating the power of the correlated basis function formalism in
describing heavily correlated systems. The strong interatomic interaction and
the large density require the theory to be pushed to a high degree of
sophistication. A many-body correlation operator containing explicit two- and
thre-particle correlation functions is needed to obtain a realistic ground
state wave function, whereas a perturbative expansion including up to two
phonon correlated states must be enforced to study the impurity excitation
energies. The theory describes accurately the experimental spectrum along all
the available momentum range. As empirically shown by the experiments, a marked
deviation from the quadratic Landau-Pomeranchuck behavior is found and the
momentum dependent effective mass of the impurity increases of $\sim50~%$ at
$q\sim1.7~\AA^{-1}$ with respect to its q=0 value. Although the main emphasis
is given to the Correlated Basis Function theory, we present also comparisons
with other methods, as diffusion Monte Carlo, variational Monte Carlo with
shadow wave functions and time dependent correlations.


title: The generalized Kramers' theory for an external noise driven bath

abstract:
  We consider a system-reservoir model where the reservoir is modulated by an
external noise. Both the internal noise of the reservoir and the external noise
are stationary, Gaussian and are characterized by arbitrary decaying
correlation functions. Based on a relation between the dissipation of the
system and the response function of the reservoir driven by external noise we
derive the generalized Kramers' rate for this nonequilibrium open system.


title:
Electronic stabilization of amorphous and quasicrystalline metals:
  Importance of quantum correlations

abstract:
  Numerous experimental indications suggest that the Hume-Rothery mechanism
plays an important role in stabilizing quasicrystalline and amorphous phases.
However, the exponential damping of the conventional Friedel oscillations at
the relevant, elevated temperatures $T$ poses a severe challenge to the HR
stabilization. In order to resolve this problem it is shown using a Feynman
diagram technique that quantum correlations in the electron sea, arising from
the interplay of Coulomb interaction and impurity scattering, can strongly
enhance the Friedel oscillations in these systems even at elevated temperature.
The resulting corrections to the Friedel potential are in agreement with
available experimental results on amorphous HR alloys. It is proposed to
include the enhancement of the Friedel amplitude derived in the present work
into pseudopotentials through the local field factor.


title: Bose condensation in a model microcavity

abstract:
  We study the equilibrium properties of a system of dipole-active excitons
coupled to a single photon mode at fixed total excitation. Treating the
presence or absence of a trapped exciton as a two-level system produces a model
that is exactly soluble. It gives a simple description of the physics of
polariton condensation in optical cavities beyond the low-density bosonic
regime.


title:
Instabilities Toward Charge Density Wave and Paired Quantum Hall State
  of Half-Filled Landau Levels

abstract:
  We study the stability of spin-resolved Landau levels at electron filling
factor $\nu=n+1/2$, where n is a positive integer. Representing the half-filled
topmost Landau level by fermions and the n filled inner Landau levels by n
bosons, coupled to the Chern-Simons gauge fields, we show that the ground
states exhibit charge density wave order for $n(n+1/2)>15.54 \pi(a_B^*)^2 n_e$,
where $n_e$ is the 2D carrier density and $a_B^*$ is the effective Bohr radius.
We find that the pairing interaction mediated by the fluctuating gauge field is
enhanced near the charge density wave instability such that p-wave pairing of
the Chern-Simons fermions prevails for
$3.49(\pi{\ab}^2)n_e<n^2(n+1/2)<20.22\pi(a_B^*)^2 n_e$. The competition between
charge density wave order and paired quantum Hall state is discussed in
connection to recent experiments.


title: Electron self-trapping in intermediate-valent SmB6

abstract:
  SmB6 exhibits intermediate valence in the ground state and unusual behaviour
at low temperatures. The resistivity and the Hall effect cannot be explained
either by conventional sf-hybridization or by hopping transport in an impurity
band. At least three different energy scales determine three temperature
regimes of electron transport in this system. We consider the ground state
properties, the soft valence fluctuations and the spectrum of band carriers in
n-doped SmB6. The behaviour of excess conduction electrons in the presence of
soft valence fluctuations and the origin of the three energy scales in the
spectrum of elementary excitations is discussed. The carriers which determine
the low-temperature transport in this system are self-trapped electron-polaron
complexes rather than simply electrons in an impurity band. The mechanism of
electron trapping is the interaction with soft valence fluctuations.


title: On Coherency-Induced Ordering in Substitutional Alloys: I. Analytical

abstract:
  As pointed out by Linus Pauling in his classic work on the relationship
between crystal packing and ionic radius ratio, a difference in atomic size can
be accommodated more readily by an ordered structure than by a disordered one.
Because of mathematical complexity, however, very few works have been reported
for substitutional alloys. In this work, coherency-induced ordering in
substitutional alloys is examined through a simple model based on a
two-dimensional square lattice. Within the assumption of nearest neighbor
interactions on a square lattice, both modified Bragg-Williams and Onsager
approaches show that coherency strain arising due to atomic mismatch can exert
profound effects on order-disorder transitions in substitutional alloys. If the
alloy system is elastically homogeneous and Vegard's law is obeyed, the
order-disorder transition is of a second-order kinetics. If the atomic
mismatches significantly deviate from Vegard's law, however, the transition may
become a first-order kinetics, as the configurational free energy surface is
composed of double wells. At the transition of a first-order kinetics, the
lattice parameter can either increase or decrease upon heating, i.e., the
lattice parameter of an ordered state can be less or greater than that of a
disordered state. The results of Onsager's approach are independently confirmed
with those of the Discrete Atom Method, a Monte Carlo technique predicated upon
the combination of statistical mechanics and linear elasticity.


title:
Inhomogeneous Low Frequency Spin Dynamics in
  La_{1.65}Eu_{0.2}Sr_{0.15}CuO_4

abstract:
  We report Cu and La nuclear magnetic resonance (NMR) measurements in the
title compound that reveal an inhomogeneous glassy behavior of the spin
dynamics. A low temperature peak in the La spin lattice relaxation rate and the
``wipeout'' of Cu intensity both arise from these slow electronic spin
fluctuations that reveal a distribution of activation energies. Inhomogeneous
slowing of spin fluctuations appears to be a general feature of doped lanthanum
cuprate.


title:
Comparative review of aging properties in spin glasses and other
  disordered materials

abstract:
  Aging phenomena have been studied in very different materials like polymers,
supercooled liquids or disordered orientational crystals. We recall here the
main features of aging in spin glasses, and use this example of magnetic
systems as a guideline for the description of the others. A particular
attention is put on the sensitivity of aging to the cooling rate and to
temperature variations. This allows us to point out differences between
temperature specific processes, yielding ``rejuvenation and memory effects'' as
known in spin glasses, and domain growth processes, giving cumulative
contributions at different temperatures. The relevance of wall depinning
processes to rejuvenation and memory effects is discussed at the light of
recent results on disordered ferromagnets.


title: Trapping with biased diffusion species

abstract:
  We analyze a trapping reaction with a single penetrable trap, in a one
dimensional lattice, where both species (particles and trap) are mobile and
have a drift velocity. We obtain the density as seen from a reference system
attached to the trap and from the laboratory frame. In addition we study the
nearest neighbor distance to the trap. We exploit a stochastic model previously
developed, and compare the results with numerical simulations, resulting in an
excellent agreement.


title: Magnetic order in ferromagnetically coupled spin ladders

abstract:
  A model of coupled antiferromagnetic spin-1/2 Heisenberg ladders is studied
with numerical techniques. In the case of ferromagnetic interladder coupling we
find that the dynamic and static structure factor has a peak at $(\pi,\pi/2)$
where the first (second) direction is along (transversal) to the ladders.
Besides, we suggest that the intensity of this peak and the spin-spin
correlation at the maximum distance along the ladder direction remain finite in
the bulk limit for strong enough interladder coupling. We discuss the relevance
of these results for magnetic compounds containing ladders coupled in a trellis
lattice and for the stripe scenario in high-T$_c$ superconducting cuprates.


title:
Spin Dynamics in the LTT Phase of ~1/8 Doped Single Crystal
  La_{1.67}Eu_{0.2}Sr_{0.13}CuO_4

abstract:
  We present La and Cu NMR relaxation measurements in single crystal
La_{1.67}Eu_{0.2}Sr_{0.13}CuO_4. A strong peak in the La spin-lattice
relaxation rate observed in the spin ordered state is well-described by the BPP
mechanism[1] and arises from continuous slowing of electronic spin fluctuations
with decreasing temperature; these spin fluctuations exhibit XY-like anisotropy
in the ordered state. The spin pseudogap is enhanced by the static
charge-stripe order in the LTT phase.


title: d_c=4 is the upper critical dimension for the Bak-Sneppen model

abstract:
  Numerical results are presented indicating d_c=4 as the upper critical
dimension for the Bak-Sneppen evolution model. This finding agrees with
previous theoretical arguments, but contradicts a recent Letter [Phys. Rev.
Lett. 80, 5746-5749 (1998)] that placed d_c as high as d=8. In particular, we
find that avalanches are compact for all dimensions d<=4, and are fractal for
d>4. Under those conditions, scaling arguments predict a d_c=4, where
hyperscaling relations hold for d<=4. Other properties of avalanches, studied
for 1<=d<=6, corroborate this result. To this end, an improved numerical
algorithm is presented that is based on the equivalent branching process.


title:
Dynamics of fractal dimension during phase ordering of a geometrical
  multifractal

abstract:
  A simple multifractal coarsening model is suggested that can explain the
observed dynamical behavior of the fractal dimension in a wide range of
coarsening fractal systems. It is assumed that the minority phase (an ensemble
of droplets) at $t=0$ represents a non-uniform recursive fractal set, and that
this set is a geometrical multifractal characterized by a $f(\alpha)$-curve. It
is assumed that the droplets shrink according to their size and preserving
their ordering. It is shown that at early times the Hausdorff dimension does
not change with time, whereas at late times its dynamics follow the $f(\alpha)$
curve. This is illustrated by a special case of a two-scale Cantor dust. The
results are then generalized to a wider range of coarsening mechanisms.