title:
In-Plane Magnetolumnescence of Modulation-Doped GaAs/AlGaAs Coupled
  Double Quantum Wells

abstract:
  In-plane magnetic field photoluminescence spectra from a series of
GaAs/AlGaAs coupled double quantum wells show distinctive doublet structures
related to the symmetric and antisymmetric states. The magnetic field behavior
of the upper transition from the antisymmetric state strongly depends on sample
mobility. In lower mobility samples, the transition energy shows an $\cal
N$-type kink with fields (namely a maximum followed by a minimum), whereas
higher mobility samples have a linear dependence. The former is due to a
homogeneous broadening of electron and hole states and the results are in good
agreement with theoretical calculations.


title: Enumerations of plane meanders

abstract:
  A closed plane meander of order n is a closed self-avoiding loop intersecting
an infinite line 2n times. Meanders are considered distinct up to any smooth
deformation leaving the line fixed. We have developed an improved algorithm,
based on transfer matrix methods, for the enumeration of plane meanders. This
allows us to calculate the number of closed meanders up to n=24. The algorithm
is easily modified to enumerate various systems of closed meanders,
semi-meanders or open meanders.


title: Vortices Freeze like Window Glass: the Vortex Molasses Scenario

abstract:
  We overview several recent experimental and numerical observations, which are
at odds with the Vortex Glass theory of the freezing of disordered vortex
matter. To reinvestigate the issue, we performed numerical simulations of the
overdamped London - Langevin model, and use finite size scaling to analyze the
data. Upon approaching the transition the initial Vortex Glass type criticality
is arrested at some crossover temperature. Below this temperature the
timescales continue growing very quickly, consistent with the Vogel-Fulcher
form, while the spatial correlation length xi stops exhibiting any observable
divergence. We call this mode of freezing the Vortex Molasses scenario.


title: Nonlinear Spin Polarized Transport Through a Quantum Dot

abstract:
  We present a theoretical analysis of the nonlinear bias and temperature
dependence of current-voltage characteristics of a spin-valve device which is
formed by connecting a quantum dot to two ferromagnetic electrodes whose
magnetic moments orient at an angle $\theta$ with respect to each other. The
theory is based on nonequilibrium Green's function approach and focused on
current perpendicular to plane geometry. Coulomb interaction has been taken
into account explicitly at the Hartree level. We derive a formula in closed
form for current flowing through the device in general terms of bias and
temperature. In the wideband limit we report exact results for the TMR junction
nonlinear I-V curve as a function of $\theta$. We also report the conductance
slope at zero bias as a function of temperature for which experimental results
reported an anomalous behavior.


title:
Manifestations of the Charged Stripes in the Magnetoresistance of
  Heavily Underdoped YBa_2Cu_3O_{6+x}

abstract:
  We present a study of the in-plane and out-of-plane magnetoresistance (MR) in
heavily-underdoped, antiferromagnetic YBa_2Cu_3O_{6+x}, which reveals a variety
of striking features. The in-plane MR demonstrates a "d-wave"-like anisotropy
upon rotating the magnetic field H within the ab plane. With decreasing
temperature below 20-25 K the system acquires memory: exposing a crystal to the
magnetic field results in a persistent in-plane resistivity anisotropy. The
overall features can be explained by assuming that the CuO_2 planes contain a
developed array of stripes accommodating the doped holes, and that the MR is
associated with the field-induced topological ordering of the stripes.


title:
Charge ordering, phase separation and charge pairing in layered 3D
  systems

abstract:
  The processes of Coulomb gas ordering in 3D layered system are studied by
means of Brownian dynamics approach. It is found that at different densities of
the carriers the 3D lattice of charges as well as new specific structures are
possible in the system. In particular, at some small density the particles
inside the layers can associate into droplets that collectively repulse between
neighbouring layers. These droplets possess local stripe structure which orders
spontaneously along arbitrary chosen direction. At higher densities a specific
ordering of the charges into the tetragonal-like or hexagonal-like structures
is observed visually and described numerically. Specific ''pairing'' of the
charges from different layers plays an essential role in formation of all above
structures. It is essentially many-body effect in 3D system which can be a
reason for unusual properties of layered crystals.


title: A Spin-1/2 Model for CsCuCl_3 in an External Magnetic Field

abstract:
  CsCuCl_3 is a ferromagnetically stacked triangular spin-1/2 antiferromagnet.
We discuss models for its zero-temperature magnetization process. The models
range from three antiferromagnetically coupled ferromagnetic chains to the full
three-dimensional situation. The situation with spin-1/2 is treated by
expansions around the Ising limit and exact diagonalization. Further,
weak-coupling perturbation theory is used mainly for three coupled chains which
are also investigated numerically using the density-matrix renormalization
group technique. We find that already the three-chain model gives rise to the
plateau-like feature at one third of the saturation magnetization which is
observed in magnetization experiments on CsCuCl_3 for a magnetic field
perpendicular to the crystal axis. For a magnetic field parallel to the crystal
axis, a jump is observed in the experimental magnetization curve in the region
of again about one third of the saturation magnetization. In contrast to
earlier spinwave computations, we do not find any evidence for such a jump with
the model in the appropriate parameter region.


title:
Metal-insulator transitions: Influence of lattice structure, Jahn-Teller
  effect, and Hund's rule coupling

abstract:
  We study the influence of the lattice structure, the Jahn-Teller effect and
the Hund's rule coupling on a metal-insulator transition in AnC60 (A= K, Rb).
The difference in lattice structure favors A3C60 (fcc) being a metal and A4C60
(bct) being an insulator, and the coupling to Hg Jahn-Teller phonons favors
A4C60 being nonmagnetic. The coupling to Hg (Ag) phonons decreases (increases)
the value Uc of the Coulomb integral at which the metal-insulator transition
occurs. There is an important partial cancellation between the Jahn-Teller
effect and the Hund's rule coupling.


title: Charge transport along the c-axis in high-T_c cuprates

abstract:
  Using 61-T pulsed magnetic fields, the normal-state \rho_{ab} and \rho_{c}
are measured in Bi-2201 system down to 0.66 K, and the coexistence of the
"metallic" \rho_{ab} and the "semiconducting" \rho_{c}, usually called the
charge confinement behavior, was confirmed to extend far below T_c. Recent
measurement of the c-axis magnetoresistance under 16 T dc magnetic field in
heavily underdoped Y-123 crystals revealed that the peculiar c-axis charge
transport, and thus the charge confinement, is fundamentally related to the
antiferromagnetic spin fluctuations.


title:
Crossover from interaction induced localization to delocalization in
  disordered electron systems

abstract:
  We numerically investigate the transport properties of interacting spinless
electrons in disordered systems. We use an efficient method which is based on
the diagonalization of the Hamiltonian in the subspace of the many-particle
Hilbert space which is spanned by the low-energy Slater states. Low-energy
properties can be calculated with an accuracy comparable to that of exact
diagonalization but for larger system sizes. The method works well in the
entire parameter space, and it can handle long-range as well as short-range
interactions. Using this method we calculate the combined effect of disorder
and interactions on the Kubo-Greenwood conductance and on the sensitivity of
the ground state energy to a twist in the boundary conditions. We find that the
influence of the interactions on the transport properties is opposite for large
and small disorder. In the strongly localized regime (small kinetic energy,
large disorder) interactions increase the transport whereas for weak disorder
(large kinetic energy) interactions decrease the transport.


title: Optical absorption of spin ladders

abstract:
  We present a theory of phonon-assisted optical two-magnon absorption in
two-leg spin-ladders. Based on the strong intra-rung-coupling limit we show
that collective excitations of total spin S=0, 1 and 2 exist outside of the
two-magnon continuum. It is demonstrated that the singlet collective state has
a clear signature in the optical spectrum.


title: Spin glass overlap barriers in three and four dimensions

abstract:
  For the Edwards-Anderson Ising spin-glass model in three and four dimensions
(3d and 4d) we have performed high statistics Monte Carlo calculations of those
free-energy barriers $F^q_B$ which are visible in the probability density
$P_J(q)$ of the Parisi overlap parameter $q$. The calculations rely on the
recently introduced multi-overlap algorithm. In both dimensions, within the
limits of lattice sizes investigated, these barriers are found to be
non-self-averaging and the same is true for the autocorrelation times of our
algorithm. Further, we present evidence that barriers hidden in $q$ dominate
the canonical autocorrelation times.


title:
A semiclassical approach to the ground state and density oscillations of
  quantum dots

abstract:
  A semiclassical Thomas-Fermi method, including a Weizs\"acker gradient term,
is implemented to describe ground states of two dimensional nanostructures of
arbitrary shape. Time dependent density oscillations are addressed in the same
spirit using the corresponding semiclassical time-dependent equations. The
validity of the approximations is tested, both for ground state and density
oscillations, comparing with the available microscopic Kohn-Sham solutions.


title: Topological properties of quantum periodic Hamiltonians

abstract:
  We consider periodic quantum Hamiltonians on the torus phase space
(Harper-like Hamiltonians). We calculate the topological Chern index which
characterizes each spectral band in the generic case. This calculation is made
by a semi-classical approach with use of quasi-modes. As a result, the Chern
index is equal to the homotopy of the path of these quasi-modes on phase space
as the Floquet parameter (\theta) of the band is varied. It is quite
interesting that the Chern indices, defined as topological quantum numbers, can
be expressed from simple properties of the classical trajectories.


title:
Dynamics of Excited Electrons in Copper and Ferromagnetic Transition
  Metals: Theory and Experiment

abstract:
  Both theoretical and experimental results for the dynamics of photoexcited
electrons at surfaces of Cu and the ferromagnetic transition metals Fe, Co, and
Ni are presented. A model for the dynamics of excited electrons is developed,
which is based on the Boltzmann equation and includes effects of
photoexcitation, electron-electron scattering, secondary electrons (cascade and
Auger electrons), and transport of excited carriers out of the detection
region. From this we determine the time-resolved two-photon photoemission
(TR-2PPE). Thus a direct comparison of calculated relaxation times with
experimental results by means of TR-2PPE becomes possible. The comparison
indicates that the magnitudes of the spin-averaged relaxation time \tau and of
the ratio \tau_\uparrow/\tau_\downarrow of majority and minority relaxation
times for the different ferromagnetic transition metals result not only from
density-of-states effects, but also from different Coulomb matrix elements M.
Taking M_Fe > M_Cu > M_Ni = M_Co we get reasonable agreement with experiments.


title: Quasi-particle characteristics of the one-dimensional polaron

abstract:
  The main quasi-particle characteristics of the one-dimensional polaron are
estimated within and beyond the most general Gaussian approximation at
arbitrary electron-phonon coupling. We have derived explicitly the ground-state
energy and the effective mass in the weak- and strong-coupling regimes. For
arbitrary coupling, the Gaussian leading-order term of the polaron self energy
improves the corresponding Feynman estimate and represents the lowest upper
bound available. We have calculated the next non-Gaussian corrections. Taking
into account systematically higher-order corrections does not perturb
considerably the obtained results.


title: Quantized Skyrmion Fields in 2+1 Dimensions

abstract:
  A fully quantized field theory is developped for the skyrmion topological
excitations of the O(3) symmetric CP$^1$-Nonlinear Sigma Model in 2+1D. The
method allows for the obtainment of arbitrary correlation functions of quantum
skyrmion fields. The two-point function is evaluated in three different
situations: a) the pure theory; b) the case when it is coupled to fermions
which are otherwise non-interacting and c) the case when an electromagnetic
interaction among the fermions is introduced. The quantum skyrmion mass is
explicitly obtained in each case from the large distance behavior of the
two-point function and the skyrmion statistics is inferred from an analysis of
the phase of this function. The ratio between the quantum and classical
skyrmion masses is obtained, confirming the tendency, observed in semiclassical
calculations, that quantum effects will decrease the skyrmion mass. A brief
discussion of asymptotic skyrmion states, based on the short distance behavior
of the two-point function, is also presented.


title:
A superfluid hydrodynamic model for the enhanced moments of inertia of
  molecules in liquid 4He

abstract:
  We present a superfluid hydrodynamic model for the increase in moment of
inertia, $\Delta I$, of molecules rotating in liquid $^4$He. The static
inhomogeneous He density around each molecule (calculated using the Orsay-Paris
liquid $^4$He density functional) is assumed to adiabatically follow the
rotation of the molecule. We find that the $\Delta I$ values created by the
viscousless and irrotational flow are in good agreement with the observed
increases for several molecules [ OCS, (HCN)$_2$, HCCCN, and HCCCH$_3$ ]. For
HCN and HCCH, our model substantially overestimates $\Delta I$. This is likely
to result from a (partial) breakdown of the adiabatic following approximation.


title: Lattice-switch Monte Carlo

abstract:
  We present a Monte Carlo method for the direct evaluation of the difference
between the free energies of two crystal structures. The method is built on a
lattice-switch transformation that maps a configuration of one structure onto a
candidate configuration of the other by `switching' one set of lattice vectors
for the other, while keeping the displacements with respect to the lattice
sites constant. The sampling of the displacement configurations is biased,
multicanonically, to favor paths leading to `gateway' arrangements for which
the Monte Carlo switch to the candidate configuration will be accepted. The
configurations of both structures can then be efficiently sampled in a single
process, and the difference between their free energies evaluated from their
measured probabilities. We explore and exploit the method in the context of
extensive studies of systems of hard spheres. We show that the efficiency of
the method is controlled by the extent to which the switch conserves correlated
microstructure. We also show how, microscopically, the procedure works: the
system finds gateway arrangements which fulfill the sampling bias
intelligently. We establish, with high precision, the differences between the
free energies of the two close packed structures (fcc and hcp) in both the
constant density and the constant pressure ensembles.


title: Effect of Dephasing on Charge-Counting Statistics in Chaotic Cavities

abstract:
  We calculate the cumulants of the charge transmitted through a chaotic cavity
in the limit that the two openings have a large number of scattering channels.
The shot noise, which is the second cumulant, is known to be insensitive to
dephasing in this limit. Unexpectedly, the fourth and higher cumulants are
found to depend on dephasing: A semiclassical theory and a quantum mechanical
model with strong dephasing give a different result than a fully phase-coherent
quantum mechanical theory.


title: Emergence of scaling in random networks

abstract:
  Systems as diverse as genetic networks or the world wide web are best
described as networks with complex topology. A common property of many large
networks is that the vertex connectivities follow a scale-free power-law
distribution. This feature is found to be a consequence of the two generic
mechanisms that networks expand continuously by the addition of new vertices,
and new vertices attach preferentially to already well connected sites. A model
based on these two ingredients reproduces the observed stationary scale-free
distributions, indicating that the development of large networks is governed by
robust self-organizing phenomena that go beyond the particulars of the
individual systems.

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


title: Reply to Comment on "Ising Spin Glasses in a Magnetic Field"

abstract:
  The problem of the survival of a spin glass phase in the presence of a field
has been a challenging one for a long time. To date, all attempts using
equilibrium Monte Carlo methods have been unconclusive. In their comment to our
paper, Marinari, Parisi and Zuliani use out-of-equilibrium measurements to test
for an Almeida-Thouless line. In our view such a dynamic approach is not based
on very solid foundations in finite dimensional systems and so cannot be as
compelling as equilibrium approaches. Nevertheless, the results of those
authors suggests that there is a critical field near B=0.4 at zero temperature.
In view of this quite small value (compared to the mean field value), we have
reanalyzed our data. We find that if finite size scaling is to distinguish
between that small field and a zero field, we would need to go to lattice sizes
of about 20x20x20.


title:
Nanomechanical resonators operating as charge detectors in the nonlinear
  regime

abstract:
  We present measurements on nanomechanical resonators machined from
Silicon-on-Insulator substrates. The resonators are designed as freely
suspended Au/Si beams of lengths on the order of 1 - 4 um and a thickness of
200 nm. The beams are driven into nonlinear response by an applied modulation
at radio frequencies and a magnetic field in plane. The strong hysteresis of
the magnetomotive response allows sensitive charge detection by varying the
electrostatic potential of a gate electrode.


title:
The role of single-ion excitations in the mixed-spin quasi-1-dimensional
  quantum antiferromagnet Nd_2BaNiO_5

abstract:
  Inelastic neutron scattering experiments on Nd2BaNiO5 single-crystals and
powder samples are used to study the dynamic coupling of 1-dimensional
Haldane-gap excitations in the S=1 Ni2+ -chains to local crystal-field
transitions, associated with the rare earth ions. Substantial interference
between the two types of excitations is observed even in the 1-dimensional
paramagnetic phase. Despite that, the results provide solid justification for
the previously proposed ``static staggered field'' model for R2BaNiO5
nickelates. The observed behaviour is qualitatively explained by a simple
chain-Random-Phase-Approximation (chain-RPA) model.


title: Numerical Method for Zero-Temperature Vortex-Line Phase Diagrams

abstract:
  A numerical method to calculate equilibrium vortex-line configurations in
bulk anisotropic type-II superconductors, at zero temperature, placed in an
external magnetic field is introduced and applied to two physical problems. The
method is designed to search for the minimum of the Gibbs free-energy in the
London approximation and assumes only that the vortex lines are straight and
arranged in a periodic lattice. Based on these assumptions a simulated
annealing algorithm is developed to find the vortex-line-lattice unit cell
shape, and vortex-line positions within it. This algorithm is made fast and
accurate by the use of a rapidly converging series to calculate the lattice
sums entering the vortex-vortex interaction energy. The method's accuracy is
illustrated by calculating the magnetic induction versus applied field curve
for an isotropic superconductor. The method is applied to a superconductor with
a square lattice of columnar defects to study selected regions of the
zero-temperature-phase diagram where vortex-line-lattices commensurate with the
columnar defect lattice exist.


title:
Two stage superconducting quantum interference device amplifier in a
  high-Q gravitational wave transducer

abstract:
  We report on the total noise from an inductive motion transducer for a
gravitational-wave antenna. The transducer uses a two-stage SQUID amplifier and
has a noise temperature of 1.1 mK, of which 0.70 mK is due to back-action noise
from the SQUID chip. The total noise includes thermal noise from the transducer
mass, which has a measured Q of 2.60 X 10^6. The noise temperature exceeds the
expected value of 3.5 \mu K by a factor of 200, primarily due to voltage noise
at the input of the SQUID. Noise from flux trapped on the chip is found to be
the most likely cause.


title:
Cumulant ratios and their scaling functions for Ising systems in strip
  geometries

abstract:
  We calculate the fourth-order cumulant ratio (proposed by Binder) for the
two-dimensional Ising model in a strip geometry L x oo. The Density Matrix
Renormalization Group method enables us to consider typical open boundary
conditions up to L=200. Universal scaling functions of the cumulant ratio are
determined for strips with parallel as well as opposing surface fields.


title: Persistence exponents in a 3D symmetric binary fluid mixture

abstract:
  The persistence exponent, theta, is defined by N_F sim t^theta, where t is
the time since the start of the coarsening process and the "no-flip fraction",
N_F, is the number of points that have not seen a change of "color" since t=0.
Here we investigate numerically the persistence exponent for a binary fluid
system where the coarsening is dominated by hydrodynamic transport. We find
that N_F follows a power law decay (as opposed to exponential) with the value
of theta somewhat dependent on the domain growth rate (L sim t^alpha, where L
is the average domain size), in the range theta=1.23 +-0.1 (alpha = 2/3) to
theta=1.37 +-0.2 (alpha=1). These alpha values correspond to the inertial and
viscous hydrodynamic regimes respectively.


title: Doping Dependence of the Superconducting Leading Edge Gap in Bi2212

abstract:
  The paper compares the K-dependence of the superconducting gap in different
doping ranges. The fine behavior of the leading edge gap indicates that the
pairing susceptibility is peaked at special regions on the Fermi surface. These
hot regions are found to be centered away from nominal "hot spots". This
behavior is attributed to a feedback effect on the pairing boson.
Identification is made through comparison with neutron diffraction results.


title:
Interlayer Quasiparticle Transport in the Vortex State of Josephson
  Coupled Superconductors

abstract:
  We calculate the dependence of the interlayer quasiparticle conductivity,
$\sigma_q$, in a Josephson coupled d-wave superconductor on the magnetic field
B||c and the temperature T. We consider a clean superconductor with resonant
impurity scattering and a dominant coherent interlayer tunneling. When pancake
vortices in adjacent layers are weakly correlated at low T the conductivity
increases sharply with B before reaching an extended region of slow linear
growth, while at high T it initially decreases and then reaches the same linear
regime. For correlated pancakes $\sigma_q$ increases much more strongly with
the applied field.


title: Collective Mode in a d_{x^2-y^2} + id_xy Superconductor

abstract:
  We consider a superconducting state with a mixed symmetry order parameter
components, e.g. d+is or d+id' with d'= d_{xy}. We argue for the existence of
the new orbital magnetization mode which corresponds to the oscillations of
relative phase \phi between two components around an equilibrium value of \phi
= \pi/2. It is similar to the so called ``clapping'' mode in superfluid 3He-A.
We estimate the frequency of this mode \omega_0(B,T) depending on the field and
temperature for the specific case of magnetic field induced d'= d_{xy} state.
We find that this mode is tunable with an applied magnetic field with
\omega_0(B,T) \propto B D_0, where D_0 is the magnitude of the d-wave order
parameter. We also estimate the velocity s(B,T) of this mode.


title:
Effective action and collective modes in quasi-one-dimensional
  spin-density-wave systems

abstract:
  We derive the effective action describing the long-wavelength low-energy
collective modes of quasi-one-dimensional spin-density-wave (SDW) systems,
starting from the Hubbard model within weak coupling approximation. The
effective action for the spin-wave mode corresponds to an anisotropic
non-linear sigma model together with a Berry phase term. We compute the spin
stiffness and the spin-wave velocity. We also obtain the effective action for
the sliding mode (phason) taking into account the density fluctuations from the
outset and in presence of a weak external electromagnetic field. This leads to
coupled equations for the phase of the SDW condensate and the charge density
fluctuations. We also calculate the conductivity and the density-density
correlation function.


title: Diffusion and Transport Coefficients in Synthetic Opals

abstract:
  Opals are structures composed of the closed packing of spheres in the size
range of nano-to-micro meter. They are sintered to create small necks at the
points of contact. We have solved the diffusion problem in such structures. The
relation between the diffusion coefficient and the termal and electrical
conductivity makes possible to estimate the transport coefficients of opal
structures. We estimate this changes as function of the neck size and the
mean-free path of the carriers. The theory presented is also applicable to the
diffusion problem in other periodic structures.


title: Carrier density change in Colossal Magnetoresistive Pyrochlore Tl2Mn2O7

abstract:
  Hall resistivity and magneto-thermopower have been measured for colossal
magnetoresistive Tl2Mn2O7 over wide temperature and magnetic-field ranges.
These measurements revealed that a small number of free electron-like carriers
is responsible for the magneto-transport properties. In contrast to perovskite
CMR materials, the anomalous Hall coefficient is negligible even in the
ferromagnetic state due to negligibly small skew scattering. The characteristic
feature in Tl2Mn2O7 is that the carrier density changes with temperature and
the magnetic field. The carrier density increases around TC as the temperature
is lowered or as the magnetic field is increased, which explains the CMR of
this material. The conduction-band-edge shift, which is caused by the strong
s-d interaction between localized Mn moments and s-like conduction electrons,
is a possible mechanism for the carrier density change.


title: Orbital character of O 2p unoccupied states near the Fermi level in CrO2

abstract:
  The orbital character, orientation, and magnetic polarization of the O 2$p$
unoccupied states near the Fermi level ($E_F$) in CrO$_2$ was determined using
polarization-dependent X-ray absorption spectroscopy (XAS) and X-ray magnetic
circular dichroism (XMCD) from high-quality, single-crystal films. A sharp peak
observed just above $E_F$ is excited only by the electric field vector ($\bf
E$) normal to the tetragonal $c$-axis, characteristic of a narrow band
($\approx$ 0.7 eV bandwidth) constituted from O 2$p$ orbitals perpendicular to
$c$ (O 2$p_y$) hybridized with Cr 3$d_{xz-yz}$ $t_{2g}$ states. By comparison
with band-structure and configuration-interaction (CI) cluster calculations our
results support a model of CrO$_2$ as a half-metallic ferromagnet with large
exchange-splitting energy ($\Delta_{exch-split}$ $\approx$ 3.0 eV) and
substantial correlation effects.


title: Spectral Shape of Relaxations in Silica Glass

abstract:
  Precise low-frequency light scattering experiments on silica glass are
presented, covering a broad temperature and frequency range (9 GHz < \nu < 2
THz). For the first time the spectral shape of relaxations is observed over
more than one decade in frequency. The spectra show a power-law low-frequency
wing of the relaxational part of the spectrum with an exponent $\alpha $
proportional to temperature in the range 30 K < T < 200 K. A comparison of our
results with those from acoustic attenuation experiments performed at different
frequencies shows that this power-law behaviour rather well describes
relaxations in silica over 9 orders of magnitude in frequency. These findings
can be explained by a model of thermally activated transitions in double well
potentials.


title: Electrochemical capacitance of a leaky nano-capacitor

abstract:
  We report a detailed theoretical investigation on electrochemical capacitance
of a nanoscale capacitor where there is a DC coupling between the two
conductors. For this ``leaky'' quantum capacitor, we have derived general
analytic expressions of the linear and second order nonlinear electrochemical
capacitance within a first principles quantum theory in the discrete potential
approximation. Linear and nonlinear capacitance coefficients are also derived
in a self-consistent manner without the latter approximation and the
self-consistent analysis is suitable for numerical calculations. At linear
order, the full quantum formula improves the semiclassical analysis in the
tunneling regime. At nonlinear order which has not been studied before for
leaky capacitors, the nonlinear capacitance and nonlinear nonequilibrium charge
show interesting behavior. Our theory allows the investigation of crossover of
capacitance from a full quantum to classical regimes as the distance between
the two conductors is changed.


title:
Pseudogap due to antiferromagnetic fluctuations and the phase diagram of
  the high temperature oxide superconductors

abstract:
  A reduction of the density of states near the Fermi energy in the normal
state (pseudogap) of high-temperature oxide superconductors is examined on the
basis of the two-dimensional tight-binding model with effective interactions
due to antiferromagnetic fluctuations. By using antiferromagnetic correlation
lengths which are phenomenologically assumed, the doping dependence of the
pseudogap is obtained. The superconducting transition temperature decreases and
eventually vanishes due to the pseudogap as the hole concentration is reduced.


title:
Comment on ''Minimal Surfaces, Screw Dislocations, and Twist Grain
  Boundaries''

abstract:
  In a recent letter, (Phys. Rev. Lett. 82, 2892(1999); cond-mat/9808306)
Kamien and Lubensky calculated the energy of the surface constructed via a
linear superposition of screw dislocations in SmA phase, and obtained the
positive (repulsive) sign of the long-range interaction between dislocations.
We would like to object that this interaction is attractive. This conclusion
can be important to explain the discontinuity of the TGB - SmA phase transition
in chiral liquid crystals.


title:
Fluctuation-induced first order transition in a nonequilibrium steady
  state

abstract:
  We present the first example of a phase transition in a nonequilibrium
steady-state that can be argued analytically to be first order. The system of
interest is a two-species reaction-diffusion problem whose control parameter is
the total density \rho. Mean-field theory predicts a second-order transition
between two stationary states at a critical density \rho=\rho_c. We develop a
phenomenological picture that, instead, below the upper critical dimension
d_c=4, predicts a first-order transition. This picture is confirmed by
hysteresis found in numerical simulations, and by the study of a
renormalization-group improved equation of state. The latter approach is
inspired by the Weinberg-Coleman mechanism in QED.


title:
Evidences Against Temperature Chaos in Mean Field and Realistic Spin
  Glasses

abstract:
  We discuss temperature chaos in mean field and realistic 3D spin glasses. Our
numerical simulations show no trace of a temperature chaotic behavior for the
system sizes considered. We discuss the experimental and theoretical
implications of these findings.


title: Ordering in a spin glass under applied magnetic field

abstract:
  Torque, torque relaxation, and magnetization measurements on a AuFe spin
glass sample are reported. The experiments carried out up to 7 T show a
transverse irreversibility line in the (H,T) plane up to high applied fields,
and a distinct strong longitudinal irreversibility line at lower fields. The
data demonstrate for that this type of sample, a Heisenberg spin glass with
moderately strong anisotropy, the spin glass ordered state survives under high
applied fields in contrast to predictions of certain "droplet" type scaling
models. The overall phase diagram closely ressembles those of mean field or
chiral models, which both have replica symmetry breaking transitions.


title: Stochastic Resonance in Ion Channels Characterized by Information Theory

abstract:
  We identify a unifying measure for stochastic resonance (SR) in voltage
dependent ion channels which comprises periodic (conventional), aperiodic and
nonstationary SR. Within a simplest setting, the gating dynamics is governed by
two-state conductance fluctuations, which switch at random time points between
two values. The corresponding continuous time point process is analyzed by
virtue of information theory. In pursuing this goal we evaluate for our
dynamics the tau-information, the mutual information and the rate of
information gain. As a main result we find an analytical formula for the rate
of information gain that solely involves the probability of the two channel
states and their noise averaged rates. For small voltage signals it simplifies
to a handy expression. Our findings are applied to study SR in a potassium
channel. We find that SR occurs only when the closed state is predominantly
dwelled. Upon increasing the probability for the open channel state the
application of an extra dose of noise monotonically deteriorates the rate of
information gain, i.e., no SR behavior occurs.

categories: cond-mat.stat-mech cond-mat.soft physics.bio-ph physics.data-an


title:
Electrons on rotationally symmetric nanoparticles under a strong
  magnetic field

abstract:
  The energy spectrum of an electron confined to an arbitrary surface of
revolution in an external magnetic field, parallel to the symmetry axis, is
studied analitycally and numerically. The problem is reduced via conformal
mapping to one on the surface of a sphere. The case of a spheroid is considered
in details, and the dependence on parameters is discussed. In the high magnetic
field limit a regular structure in the energy spectrum, resembling the Landau
levels, is obtained. Level statistics is discussed.


title: Exact trimer ground states on a spin-1 chain

abstract:
  We construct a new spin-1 model on a chain. Its ground state is determined
exactly which is three-fold degenerate by breaking translational invariance.
Thus we have trimerization. Excited states cannot be obtained exactly, but we
determine a few low-lying ones by using trial states, among them solitons.


title: Statistical mechanics of random two-player games

abstract:
  Using methods from the statistical mechanics of disordered systems we analyze
the properties of bimatrix games with random payoffs in the limit where the
number of pure strategies of each player tends to infinity. We analytically
calculate quantities such as the number of equilibrium points, the expected
payoff, and the fraction of strategies played with non-zero probability as a
function of the correlation between the payoff matrices of both players and
compare the results with numerical simulations.


title:
Confinement of two-dimensional excitons in a non-homogeneous magnetic
  field

abstract:
  The effective Hamiltonian describing the motion of an exciton in an external
non-homogeneous magnetic field is derived. The magnetic field plays the role of
an effective potential for the exciton motion, results into an increment of the
exciton mass and modifies the exciton kinetic energy operator. In contrast to
the homogeneous field case, the exciton in a non-homogeneous magnetic field can
also be trapped in the low field region and the field gradient increases the
exciton confinement. The trapping energy and wave function of the exciton in a
GaAs two-dimensional electron gas for specific circular magnetic field
configurations are calculated. The results show than excitons can be trapped by
non-homogeneous magnetic fields, and that the trapping energy is strongly
correlated with the shape and strength of the non-homogeneous magnetic field
profile.


title:
Driven Tunneling Dynamics: Bloch-Redfield Theory versus Path Integral
  Approach

abstract:
  In the regime of weak bath coupling and low temperature we demonstrate
numerically for the spin-boson dynamics the equivalence between two widely used
but seemingly different roads of approximation, namely the path integral
approach and the Bloch-Redfield theory. The excellent agreement between these
two methods is corroborated by a novel efficient analytical high-frequency
approach: it well approximates the decay of quantum coherence via a series of
damped coherent oscillations. Moreover, a suitably tuned control field can
selectively enhance or suppress quantum coherence.


title: Interfacial Energy and Fine Defect Structures for Incoherent Films

abstract:
  This note summarizes recent results in which modern techniques of the
calculus of variations are used to obtain qualitative features of
film-substrate interfaces for a broad class of interfacial energies. In
particular, we show that the existence of a critical thickness for incoherency
and the formation of interfacial dislocations depend strongly on the convexity
and smoothness of the interfacial energy function.


title:
Evidence of anisotropic magnetic polarons in
  la$_{0.94}$Sr$_{0.06}$MnO$_3$ by neutron scattering and comparison with
  Ca-doped manganites

abstract:
  Elastic and inelastic neutron scattering experiments have been performed in a
La$_{0.94}$Sr$_{0.06}$MnO$_3$ untwinned crystal, which exhibits an
antiferromagnetic canted magnetic structure with ferromagnetic layers.
  The elastic small q scattering exhibits a modulation with an anisotropic
q-dependence. It can be pictured by ferromagnetic inhomogeneities or polarons
with a platelike shape, the largest size ($\approx17\AA$) and largest
inter-polaron distance ($\approx$ 38$\AA$) being within the ferromagnetic
layers. Comparison with observations performed on Ca-doped samples, which show
the growth of the magnetic polarons with doping, suggests that this growth is
faster for the Sr than for the Ca substitution. Below the gap of the spin wave
branch typical of the AF layered magnetic structure, an additional spin wave
branch reveals a ferromagnetic and isotropic coupling, already found in
Ca-doped samples. Its q-dependent intensity, very anisotropic, closely reflects
the ferromagnetic correlations found for the static clusters. All these results
agree with a two-phase electronic segregation occurring on a very small scale,
although some characteristics of a canted state are also observed suggesting a
weakly inhomogeneous state.


title: Electron-phonon coupling and Peierls transition in metallic nanotubes

abstract:
  We reexamine the putative Peierls transition in a (5,5) metallic nanotube. We
show that the conduction electrons at the Fermi level do not couple to the
longitudinal acoustic phonon but rather to a folded-in graphene zone edge
phonon having the proper Kekule modulation symmetry. The calculation for the
mean-field transition temperature gives 15 K, a value comparable to previous
estimates. We discuss the significance of this transition temperature.


title:
Fluctuations in random $RL-C$ networks: non-linear $\sigma-$ model
  description

abstract:
  Disordered $RL-C$ networks are known to be an adequate model for describing
fluctuations of electric fields in a random metal-dielectric composite. We show
that under appropriate conditions the statistical properties of such a system
can be studied in the framework of the Efetov's non-linear $\sigma-$ model.
This fact provides a direct link to the theory of Anderson localization.


title: Ion-ion correlations: an improved one-component plasma correction

abstract:
  Based on a Debye-Hueckel approach to the one-component plasma we propose a
new free energy for incorporating ionic correlations into Poisson-Boltzmann
like theories. Its derivation employs the exclusion of the charged background
in the vicinity of the central ion, thereby yielding a thermodynamically stable
free energy density, applicable within a local density approximation. This is
an improvement over the existing Debye-Hueckel plus hole theory, which in this
situation suffers from a "structuring catastrophe". For the simple example of a
strongly charged stiff rod surrounded by its counterions we demonstrate that
the Poisson-Boltzmann free energy functional augmented by our new correction
accounts for the correlations present in this system when compared to molecular
dynamics simulations.


title: Unsupervised learning of binary vectors: a Gaussian scenario

abstract:
  We study a model of unsupervised learning where the real-valued data vectors
are isotropically distributed, except for a single symmetry breaking binary
direction $\bm{B}\in\{-1,+1\}^{N}$, onto which the projections have a Gaussian
distribution. We show that a candidate vector $\bm{J}$ undergoing Gibbs
learning in this discrete space, approaches the perfect match $\bm{J}=\bm{B}$
exponentially. Besides the second order ``retarded learning'' phase transition
for unbiased distributions, we show that first order transitions can also
occur. Extending the known result that the center of mass of the Gibbs ensemble
has Bayes-optimal performance, we show that taking the sign of the components
of this vector leads to the vector with optimal performance in the binary
space. These upper bounds are shown not to be saturated with the technique of
transforming the components of a special continuous vector, except in
asymptotic limits and in a special linear case. Simulations are presented which
are in excellent agreement with the theoretical results.


title: Fractal Droplets in Two Dimensional Spin Glasses

abstract:
  The two-dimensional Edwards-Anderson model with Gaussian bond distribution is
investigated at T=0 with a numerical method. Droplet excitations are directly
observed. It turns out that the averaged volume of droplets is proportional to
l^D with D = 1.80(2) where l is the spanning length of droplets, revealing
their fractal nature. The exponent characterizing the l dependence of the
droplet excitation energy is estimated to be -0.42(4), clearly different from
the stiffness exponent for domain wall excitations.


title:
Low-Energy Quasiparticles in Cuprate Superconductors: A Quantitative
  Analysis

abstract:
  A residual linear term is observed in the thermal conductivity of
optimally-doped Bi-2212 at very low temperatures whose magnitude is in
excellent agreement with the value expected from Fermi-liquid theory and the
d-wave energy spectrum measured by photoemission spectroscopy, with no
adjustable parameters. This solid basis allows us to make a quantitative
analysis of thermodynamic properties at low temperature and establish that
thermally-excited quasiparticles are a significant, perhaps even the dominant
mechanism in suppressing the superfluid density in cuprate superconductors
Bi-2212 and YBCO.


title:
Weak localisation, hole-hole interactions and the "metal"-insulator
  transition in two dimensions

abstract:
  A detailed investigation of the metallic behaviour in high quality
GaAs-AlGaAs two dimensional hole systems reveals the presence of quantum
corrections to the resistivity at low temperatures. Despite the low density
($r_{s}>10$) and high quality of these systems, both weak localisation
(observed via negative magnetoresistance) and weak hole-hole interactions
(giving a correction to the Hall constant) are present in the so-called
metallic phase where the resistivity decreases with decreasing temperature. The
results suggest that even at high $r_{s}$ there is no metallic phase at T=0 in
two dimensions.


title: A Ternary Lattice Boltzmann Model for Amphiphilic Fluids

abstract:
  A lattice Boltzmann model for amphiphilic fluid dynamics is presented. It is
a ternary model, in that it conserves mass separately for each chemical species
present (water, oil, amphiphile), and it maintains an orientational degree of
freedom for the amphiphilic species. Moreover, it models fluid interactions at
the microscopic level by introducing self-consistent forces between the
particles, rather than by positing a Landau free energy functional. This
combination of characteristics fills an important need in the hierarchy of
models currently available for amphiphilic fluid dynamics, enabling efficient
computer simulation and furnishing new theoretical insight. Several
computational results obtained from this model are presented and compared to
existing lattice-gas model results. In particular, it is noted that lamellar
structures, which are precluded by the Peierls instability in two-dimensional
systems with kinetic fluctuations, are not observed in lattice-gas models, but
are easily found in the corresponding lattice Boltzmann models. This points out
a striking difference in the phenomenology accessible to each type of model.

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


title: Cooling-rate effects in a model of (ideal?) glass

abstract:
  Using Monte Carlo simulations we study cooling-rate effects in a
three-dimensional Ising model with four-spin interaction. During coarsening,
this model develops growing energy barriers which at low temperature lead to
very slow dynamics. We show that the characteristic zero-temperature length
increases very slowly with the inverse cooling rate, similarly to the behaviour
of ordinary glasses. For computationally accessible cooling rates the model
undergoes an ideal glassy transition, i.e., the glassy transition for very
small cooling rate coincides a thermodynamic singularity. We also study cooling
of this model with a certain fraction of spins fixed. Due to such heterogeneous
crystalization seeds the final state strongly depends on the cooling rate.Only
for sufficiently fast cooling rate does the system end up in a glassy state
while slow cooling inevitably leads to a crystal phase.


title: Dynamics of Vibrated Granular Monolayers

abstract:
  We study statistical properties of vibrated granular monolayers using
molecular dynamics simulations. We show that at high excitation strengths, the
system is in a gas state, particle motion is isotropic, and the velocity
distributions are Gaussian. As the vibration strength is lowered the system's
dimensionality is reduced from three to two. Below a critical excitation
strength, a gas-cluster phase occurs, and the velocity distribution becomes
bimodal. In this phase, the system consists of clusters of immobile particles
arranged in close-packed hexagonal arrays, and gas particles whose energy
equals the first excited state of an isolated particle on a vibrated plate.


title: Kondo effect in crossed Luttinger liquids

abstract:
  We study the Kondo effect in two crossed Luttinger liquids, using Boundary
Conformal Field Theory. We predict two types of critical behaviors: either a
two-channel Kondo fixed point with a nonuniversal Wilson ratio, or a new theory
with an anomalous response identical to that found by Furusaki and Nagaosa (for
the Kondo effect in a single Luttinger liquid). Moreover, we discuss the
relevance of perturbations like channel anisotropy, and we make links with the
Kondo effect in a two-band Hubbard system modeled by a channel-dependent
Luttinger Hamiltonian. The suppression of backscattering off the impurity
produces a model similar to the four-channel Kondo theory.


title: High-resolution Ce 3d-edge resonant photoemission study of CeNi_2

abstract:
  Resonant photoemission (RPES) at the Ce 3d -> 4f threshold has been performed
for alpha-like compound CeNi_2 with extremely high energy resolution (full
width at half maximum < 0.2 eV) to obtain bulk-sensitive 4f spectral weight.
The on-resonance spectrum shows a sharp resolution-limited peak near the Fermi
energy which can be assigned to the tail of the Kondo resonance. However, the
spin-orbit side band around 0.3 eV binding energy corresponding to the f_{7/2}
peak is washed out, in contrast to the RPES spectrum at the Ce 3d -> 4f RPES
threshold. This is interpreted as due to the different surface sensitivity, and
the bulk-sensitive Ce 3d -> 4f RPES spectra are found to be consistent with
other electron spectroscopy and low energy properties for alpha-like
Ce-transition metal compounds, thus resolves controversy on the interpretation
of Ce compound photoemission. The 4f spectral weight over the whole valence
band can also be fitted fairly well with the Gunnarsson-Schoenhammer
calculation of the single impurity Anderson model, although the detailed
features show some dependence on the hybridization band shape and (possibly) Ce
5d emissions.


title:
Moment analysis of the probability distributions of different sandpile
  models

abstract:
  We reconsider the moment analysis of the Bak-Tang-Wiesenfeld and the Manna
sandpile model in two and three dimensions. In contrast to recently performed
investigations our analysis turns out that the models are characterized by
different scaling behavior, i.e., they belong to different universality
classes.


title: Physics of glassy systems

abstract:
  In this talk I present some of the recent theoretical results that have been
obtained on glassy systems like spin glasses or structural glasses. The
physical principles at the basis of the theory are explained in a simple
language (without using replicas) and the results are compared with large scale
numerical simulations. Finally we introduce the generalized fluctuation
dissipation relation that can be directly tested in experiments with the
present day technology.


title: Speculative trading: the price multiplier effect

abstract:
  During a speculative episode the price of an item jumps from an initial level
p_1 to a peak level p_2 before more or less returning to level p_1. The ratio
p_2/p_1 is referred to as the amplitude A of the peak. This paper shows that
for a given market the peak amplitude is a linear function of the logarithm of
the price at the beginning of the speculative episode; with p_1 expressed in
1999 euros the relationship takes the form:
  $ A=a\ln p_1 +b $; the values of the parameter a turn out to be relatively
independent of the market considered: $ a \simeq 0.5 $, the values of the
parameter b are more market-dependent, but are stable in the course of time for
a given market. This relationship suggests that the higher the stakes the more
"bullish" the market becomes. Possible mechanisms of this "risk affinity"
effect are discussed.


title:
Ground-State Degeneracy of Potts Antiferromagnets on Two-Dimensional
  Lattices: Approach Using Infinite Cyclic Strip Graphs

abstract:
  The q-state Potts antiferromagnet on a lattice $\Lambda$ exhibits nonzero
ground state entropy $S_0=k_B \ln W$ for sufficiently large q and hence is an
exception to the third law of thermodynamics. An outstanding challenge has been
the calculation of W(sq,q) on the square (sq) lattice. We present here an exact
calculation of W on an infinite-length cyclic strip of the square lattice which
embodies the expected analytic properties of W(sq,q). Similar results are given
for the kagom\'e lattice.


title: Phase transitions in Ising magnetic films and superlattices

abstract:
  Within the framework of mean field theory, we examine the phase transitions
in Ising magnetic films and superlattices. By transfer matrix method, we derive
two general nonlinear equations for phase transition temperatures of Ising
magnetic films and superlattices, respectively. The equations can be applied to
the films and superlattices with arbitrary exchange interaction constants and
arbitrary layer number. Numerical results for phase transition temperatures as
a function of exchange interaction constants are presented.


title: Pseudo-contact angle due to superfluid vortices in $^{4}$He

abstract:
  We have investigated spreading of superfluid $^{4}$He on top of polished
MgF$_2$ and evaporated SiO$_{2}$ substrates. Our results show strongly varying
contact angles of 0 - 15 mrad on the evaporated layers. According to our
theoretical calculations, these contact angles can be explained by a spatially
varying distribution of vortex lines, the unpinning velocity of which is
inversely proportional to the liquid depth.


title: Coulomb interactions and delocalization in quantum Hall constrictions

abstract:
  We study a geometry-dependent effect of long-range Coulomb interactions on
quantum Hall (QH) tunneling junctions. In an X-shaped geometry, duality relates
junctions with opening angles alpha and (pi - alpha). We prove that duality
between weak tunneling and weak backscattering survives in the presence of long
range interactions, and that their effects are precisely cancelled in the self
dual geometry alpha=pi/2. Tunneling exponents as a function of alpha, the
interaction strength chi, and the filling fraction nu are calculated. We find
that Coulomb interaction induces localization in narrow channels (large alpha),
and delocalization for sharply pinched constrictions (small alpha).
Consequently, an insulator to metal transition happens at an angle
alpha_c(chi,nu) <=pi/2. We discuss the implications of our results for
tunneling experiments in QH-constriction and cleaved-edge geometries.


title: A New Type of Intensity Correlation in Random Media

abstract:
  A monochromatic point source, embedded in a three-dimensional disordered
medium, is considered. The resulting intensity pattern exhibits a new type of
long-range correlations. The range of these correlations is infinite and their
magnitude, normalized to the average intensity, is of order $1/k_0 \ell$, where
$k_0$ and $\ell$ are the wave number and the mean free path respectively.


title:
Muon-Spin Rotation Spectra in the Mixed Phase of High-T_c
  Superconductors : Thermal Fluctuations and Disorder Effects

abstract:
  We study muon-spin rotation (muSR) spectra in the mixed phase of highly
anisotropic layered superconductors, specifically Bi_2+xSr_2-xCaCu_2O_8+delta
(BSCCO), by modeling the fluid and solid phases of pancake vortices using
liquid-state and density functional methods. The role of thermal fluctuations
in causing motional narrowing of muSR lineshapes is quantified in terms of a
first-principles theory of the flux-lattice melting transition. The effects of
random point pinning are investigated using a replica treatment of liquid state
correlations and a replicated density functional theory. Our results indicate
that motional narrowing in the pure system, although substantial, cannot
account for the remarkably small linewidths obtained experimentally at
relatively high fields and low temperatures. We find that satisfactory
agreement with the muSR data for BSCCO in this regime can be obtained through
the ansatz that this ``phase'' is characterized by frozen short-range
positional correlations reflecting the structure of the liquid just above the
melting transition. This proposal is consistent with recent suggestions of a
``pinned liquid'' or ``glassy'' state of pancake vortices in the presence of
pinning disorder. Our results for the high-temperature liquid phase indicate
that measurable linewidths may be obtained in this phase as a consequence of
density inhomogeneities induced by the pinning disorder. The results presented
here comprise a unified, first-principles theoretical treatment of muSR spectra
in highly anisotropic layered superconductors in terms of a controlled set of
approximations.


title: Bethe Ansatz for the Spin-1 XXX Chain with Two Impurities

abstract:
  By using algebraic Bethe ansatz method, we give the Hamitonian of the spin-1
XXX chain associated with $sl_2$ with two boundary impurities.


title:
Integrability of the Heisenberg Chains with Boundary Impurities and
  Their Bethe Ansatz

abstract:
  In this paper, we show the integrability of spin-1/2 XXZ Heisenberg chain
with two arbitrary spin boundary Impurities. By using the fusion method, we
generalize it to the spin-1 XXZ chain. Then the eigenvalues of Hamiltonians of
these models are obtained by the means of Bethe ansatz method.


title:
Stability of Ferromagnetism in Hubbard models with degenerate
  single-particle ground states

abstract:
  A Hubbard model with a N_d-fold degenerate single-particle ground state has
ferromagnetic ground states if the number of electrons is less or equal to N_d.
It is shown rigorously that the local stability of ferromagnetism in such a
model implies global stability: The model has only ferromagnetic ground states,
if there are no single spin-flip ground states. If the number of electrons is
equal to N_d, it is well known that the ferromagnetic ground state is unique if
and only if the single-particle density matrix is irreducible. We present a
simplified proof for this result.


title: The Thermodynamics of the XXZ Heisenberg Chain with Impurities

abstract:
  In this paper, we discuss the effect of the arbitrary spin impurities to the
spin-1/2 and spin-1 XXZ model. The effect of ground state, the free energy, the
magnetic susceptibility, the specific heat and the Kondo temperature are given
by using the thermodynamic Bethe ansatz equation.


title:
Aging in glassy systems: new experiments, simple models, and open
  questions

abstract:
  We review the most striking experimental results on aging in a variety of
disordered systems, which reveal similar features but also important
differences. We argue that a generic model that reproduce many of these
features is that of {\it pinned defects} in a disordered environment, hopping
between different metastable states. The fact that energy barriers grow with
the size of the reconforming regions immediately leads to a strong hierarchy of
time scales. In particular, long wavelength glassy, aging modes and short
wavelength equilibrated modes coexist in the system and offer a simple
mechanism to explain the rejuvenation/memory effect, which does not rely on
`chaos' with temperature. These properties can be discussed within simplified
models (such as the Sinai model) where the dynamics of the whole pinned object
is reduced to that of its center of mass in a disordered potential. While the
experiments in random ferromagnet-like systems can be satisfactorily accounted
for, the correct picture for the aging dynamics of spin-glasses is still
missing. Following recent ideas of Houdayer and Martin, we propose the idea
that the ordered phase a spin-glass contains a large number of pinned, zero
tension walls. Finally, we briefly discuss the status of the mean-field
theories of aging.


title:
Spin injection and magnetoresistance in
  ferromagnet/superconductor/ferromagnet tunnel junctions

abstract:
  We theoretically study the spin-dependent transport in a
ferromagnet/superconductor/ferro-magnet double barrier tunnel junction. The
spin-polarized tunneling currents give rise to spin imbalance in the
superconductor. The resulting nonequilibrium spin density suppresses the
superconductivity with increase of the tunneling currents. We focus on the
effect of asymmetry in the double tunnel junction, where the barrier height of
the tunnel junction and the spin-polarization of the ferromagnets are
different, on spin injection, and discuss how the superconductivity is
suppressed in the asymmetric junction. Our results explain recent experimental
results on the critical current suppression in high-$T_c$ SCs by spin
injection.


title:
Propagation of solitons of the magnetization in magnetic nano-particle
  arrays

abstract:
  It is clarified for the first time that solitons originating from the dipolar
interaction in ferromagnetic nano-particle arrays are stably created. The
characteristics can be well controlled by the strength of the dipolar
interaction between particles and the shape anisotropy of the particle. The
soliton can propagate from a particle to a neighbor particle at a clock
frequency even faster than 100 GHz using materials with a large magnetization.
Such arrays of nano-particles might be feasible in an application as a signal
transmission line.


title:
Absence of aging in the remanent magnetization in Migdal-Kadanoff spin
  glasses

abstract:
  We study the non-equilibrium behavior of three-dimensional spin glasses in
the Migdal-Kadanoff approximation, that is on a hierarchical lattice. In this
approximation the model has an unique ground state and equilibrium properties
correctly described by the droplet model. Extensive numerical simulations show
that this model lacks aging in the remanent magnetization as well as a maximum
in the magnetic viscosity in disagreement with experiments as well as with
numerical studies of the Edwards-Anderson model. This result strongly limits
the validity of the droplet model (at least in its simplest form) as a good
model for real spin glasses.


title:
Charge and magnetic order in two-orbital double-exchange model for
  manganites

abstract:
  Phase diagram of half-doped perovskite manganites is studied within the
extended double-exchange model. To demonstrate the role of orbital degrees of
freedom both one- and two-orbital models are examined. A rich phase diagram is
obtained in the mean-field theory at zero temperature as a function of J
(antiferromagnetic (AFM) superexchange interaction) and V (intersite Coulomb
repulsion). For the one-orbital model a charge-ordered (CO) state appears at
any value of V with different types of magnetic order which changes with
increasing J from ferromagnetic (F) to AFM ones of the types A, C and G. The
orbital degeneracy results in appearance of a new CE-type spin order that is
favorable due to opening of the "dimerization" gap at the Fermi surface. In
addition, the CO state appears only for V>V_c for F and CE states while C-type
AFM state disappears and A-type AFM state is observed only at small values of V
as a charge disordered one. The relevance of our results to the experimental
data are dicussed.


title: Correlated atomic motion, phonons and spin-ordering in bcc $^{3}$He

abstract:
  We propose a new way to treat nuclear magnetism of solid $^{3}$He. We argue
that the magnetic interaction arises indirectly as a consequence of correlated
zero-point motion of the ions. This motion lowers the energy of the ground
state, and results in a coherent state of oscillating electric dipoles.
Distortion of the electronic wavefunctions leads to hyperfine magnetic
interactions with the nuclear spin.
  Our model describes both the phonon spectra and the nuclear magnetic ordering
of bcc $^{3}$He using a single parameter, the dipolar interaction energy
$E_{0}$. The model yields correctly both the u2d2 symmetry of the ordered phase
and the volume dependence of the magnetic interaction.


title: Finite-size versus Surface effects in nanoparticles

abstract:
  We study the finite-size and surface effects on the thermal and spatial
behaviors of the magnetisation of a small magnetic particle. We consider two
systems: 1) A box-shaped isotropic particle of simple cubic structure with
either periodic or free boundary conditions. This case is treated analytically
using the isotropic model of D-component spin vectors in the limit $D\to
\infty$, including the magnetic field. 2) A more realistic particle ($\gamma
$-Fe$_{2}$O$_{3}$) of ellipsoidal (or spherical) shape with open boundaries.
The magnetic state in this particle is described by the anisotropic classical
Dirac-Heisenberg model including exchange and dipolar interactions, and bulk
and surface anisotropy. This case is dealt with by the classical Monte Carlo
technique. It is shown that in both systems finite-size effects yield a
positive contribution to the magnetisation while surface effects render a
larger and negative contribution, leading to a net decrease of the
magnetisation of the small particle with respect to the bulk system. In the
system 2) the difference between the two contributions is enhanced by surface
anisotropy. The latter also leads to non saturation of the magnetisation at low
temperatures, showing that the magnetic order in the core of the particle is
perturbed by the magnetic disorder on the surface. This is confirmed by the
profile of the magnetisation.


title: Conductance quantisation in metallic point contacts

abstract:
  The electrical transport properties of atomic-scale conductors are reviewed,
with an emphasis on the relations of this problem with studies on quantum size
effects in metallic clusters. A brief introduction is given of the natural
formalism for discussing electron transport in ballistic conductors: the
Landauer theory. After introducing the experimental techniques, which are used
for studying ballistic point contacts in metals, the experimental observations
for the conductance of atomic-scale contacts are presented. In order to obtain
a full description in terms of the quantum modes for conductance, several
recently developed techniques are reviewed, which go beyond straightforward
measurement of the conductance. A brief discussion is given of an unusual
atomic geometry for gold contacts, which evolve into a chain of freely
suspended atoms. Then shell filling effects in sodium nanowires are discussed
in the context of the influence of the conductance modes on the total energy of
the system. The chapter ends with an outlook on promising new developments.


title: A strong-coupling theory of superfluid ${^4}$He

abstract:
  We propose a new theoretical approach to the excitation spectrum of
superfluid ${^4}He$. It is based on the assumption that, in addition to the
usual Feynman density fluctuations, there exist localized modes which describe
the short range behaviour in the liquid associated with microscopic cores of
quantiz ed vortices. We describe in a phenomenological way the hybridization of
those two kinds of excitations and we compare the resulting energy spectrum
with experimental data, e.g. the structure factor and the cross section for
sing le quasi-particle excitations.
  We also predict the existence of another type of excitation interpreted as a
vortex loop. The energy of this mode agrees both wit h critical velocity
experiments and high energy neutron scattering. In addition we derive a
relation between the condensate fraction and the roton e nergy and we calculate
the reduction of the ground-state energy due to the superfluid order.


title: Fine Structure in Energy Spectra of Ultrasmall Au Nanoparticles

abstract:
  We have studied tunneling into individual Au nanoparticles of estimated
diameters 2-5 nm, at dilution refrigerator temperatures. The I-V curves
indicate resonant tunneling via discrete energy levels of the particle. Unlike
previously studied normal metal particles of Au and Al, in these samples we
find that the lowest energy tunneling resonances are split into clusters of
2-10 subresonances. Such effects appear to be increasingly important in smaller
grains, as might be expected from the larger characteristic energies.


title: On the nature of Thermal Diffusion in binary Lennard-Jones liquids

abstract:
  The aim of this study is to understand deeper the thermal diffusion transport
process (Ludwig-Soret effect) at the microscopic level. For that purpose, the
recently developed reverse nonequilibrium molecular dynamics method was used to
calculate Soret coefficients of various systems in a systematic fashion. We
studied binary Lennard-Jones (LJ) fluids near the triple point (of one of the
components) in which we separately changed the ratio of one of the LJ
parameters mass, atomic diameter and interaction strength while keeping all
other parameters fixed and identical. We observed that the magnitude of the
Soret coefficient depends on all three ratios. Concerning its sign we found
that heavier species, smaller species and species with higher interaction
strengths tend to accumulate in the cold region whereas the other ones
(lighter, bigger or weaker bound) migrate to the hot region of our simulation
cell. Additionally, the superposition of the influence of the various
parameters was investigated as well as more realistic mixtures. We found that
in the experimentally relevant parameter range the contributions are nearly
additive and that the mass ratio often is the dominating factor.


title: Exactly solvable model of inergodic spin glass

abstract:
  A mean-field model of Ising spin glass with the Hamiltonian being a sum of
the infinite-range ferromagnetic and random antiferromagnetic interactions is
studied. It is shown that this model has phase transition in external magnetic
field into inergodic spin glass phase with a number of metastable states. The
thermodynamic properties of metastable states are studied at T=0 and near the
transition. The relations between the characteristics of slow nonequilibrium
processes in spin glass phase (such as hysteresis loop form, thermo-remanent
and isothermal remanent magnetizations, field-cooled and zero-field-cooled
thermodynamic quantities) and thermodynamic parameters of metastable states are
established.


title: Coulomb Blockade Resonances in Quantum Wires

abstract:
  The conductance through a quantum wire of cylindrical cross section and a
weak bulge is solved exactly for two electrons within the Landauer-Buettiker
formalism. We show that this 'open' quantum dot exhibits spin-dependent Coulomb
blockade resonances resulting in two anomalous structure on the rising edge to
the first conductance plateau, one near 0.25(2e^2/h), related to a singlet
resonance, and one near 0.7(2e^2/h), related to a triplet resonance. These
resonances are generic and robust, occurring for other types of quantum wire
and surviving to temperatures of a few degrees.


title:
Systematic Study of Short Range Antiferromagnetic Order and The
  Spin-Glass State in Lightly Doped La2-xSrxCuO4

abstract:
  Systematic measurements of the magnetic susceptibility were performed on
single crystals of lightly doped La2-xSrxCuO4 (x=0.03, 0.04 and 0.05). For all
samples the temperature dependence of the in-plane magnetic susceptibility
shows typical spin-glass features with spin-glass transition temperatures Tg of
6.3K, 5.5K and 5.0K for x=0.03, 0.04 and 0.05, respectively. The canonical
spin-glass order parameter extracted from the in-plane susceptibility of all
the samples follows a universal scaling curve. On the other hand, the
out-of-plane magnetic susceptibility deviates from Curie law below a
temperature Tdv, higher than Tg. Comparing with previous neutron scattering
results with an instrumental energy resolution of 0.25 meV from Wakimoto et
al., the x-dependence of Tdv is qualitatively the same as that of Tel, the
temperature below which the elastic magnetic scattering develops around (pi,
pi). Thus, a revised magnetic phase diagram in the lightly doped region of
La2-xSrxCuO4 is proposed. The Curie constants calculated from the in-plane
susceptibility are independent of the Sr concentration. On the basis of the
cluster spin-glass model, this fact might reflect an inhomogeneous distribution
of doped holes in the CuO2 plane, such as in a stripe structure.


title: Theory of plastic flows of CDWs in application to the current conversion

abstract:
  We suggest a theoretical picture for distributions of plastic deformations
experienced by a sliding Charge Density Wave in the course of the conversion
from the normal current at the contact to the collective one in the bulk.
Several mechanisms of phase slips via creation and proliferation of
dislocations are compared. The results are applied to space resolved X-ray,
multi-contact and optical studies. Numerical simulations are combined with
model independent relations.


title:
Tunneling driven tilt modes of the O octahedra in La{2-x}Sr{x}CuO{4}:
  strong dependence on doping

abstract:
  The anelastic spectrum of La{2-x}Sr{x}CuO{4} (x = 0, 0.008, 0.019, 0.032) has
been measured down to 1.5 K, in order to see the effect of doping on the
intrinsic lattice fluctuations already found in stoichiometric La{2}CuO{4}, and
identified with tunneling driven tilt modes of the O octahedra. Slight doping
with Sr causes a drastic increase of the transition rates and relaxation
strength of the tunneling systems. The influence of doping on the relaxation
rate is interpreted in terms of direct coupling between between the tilts of
the octahedra and the hole excitations. However, the observed fast dependence
of the rate on temperature cannot be explained in terms of the ususal models of
coupling between a tunneling system and the conduction electrons.


title:
Electron-phonon and spin-phonon coupling in NaV$_{2}$O$_{5}$: charge
  fluctuations effect

abstract:
  We show that the asymmetric crystal environment of the V site in the ladder
compound NaV$_{2}$O$_{5}$ leads to a strong coupling of vanadium 3d electrons
to phonons. This coupling causes fluctuations of the charge on the V ions, and
favors a transition to a charge-ordered state at low temperatures. In the low
temperature phase the charge fluctuations modulate the spin-spin superexchange
interaction, resulting in a strong spin-phonon coupling.


title: Heavy Fermion Behaviors in LiV_{2}O_{4}

abstract:
  Experiments of various types on the metallic transition metal oxide compound
LiV_{2}O_{4} with the fcc normal-spinel structure are reviewed. The
low-temperature T < 10 K data consistently indicate heavy fermion (HF)
behaviors characteristic of those of the heaviest-mass f-electron HF compounds.
A crossover is observed above about 50 K in the magnetic susceptibility, ^{7}Li
spin-lattice relaxation rate and neutron magnetic scattering function to a
magnetic state variously described as an antiferromagnetically-coupled
local-moment metal or, at the opposite extreme, as an itinerant
nearly-ferromagnetic metal. Recent theoretical investigations to understand
these properties of LiV_{2}O_{4} are discussed.


title:
A Generalized Smoluchowsky Equation: The Hydrodynamical and
  Thermodynamical Picture of Brownian Motion

abstract:
  We present a systematic expansion of Kramers equation in the high friction
limit. The latter is expanded within an operator continued fraction scheme. The
relevant operators include both temporal and spatial derivatives and a
covariant derivate or gauge like operator associated to the potential energy.
Trivially, the first order term yields the Smoluchowsky equation. The second
order term is readily obtained, known as the corrected Smoluchowsky equation.
Further terms are computed in compact and straightforward fashion. As an
application, the nonequilibrium thermodynamics and hydrodynamical schemes for
the one dimensional Brownian motion is presented.


title: Universal Critical Behavior of Aperiodic Ferromagnetic Models

abstract:
  We investigate the effects of geometric fluctuations, associated with
aperiodic exchange interactions, on the critical behavior of $q$-state
ferromagnetic Potts models on generalized diamond hierarchical lattices. For
layered exchange interactions according to some two-letter substitutional
sequences, and irrelevant geometric fluctuations, the exact recursion relations
in parameter space display a non-trivial diagonal fixed point that governs the
universal critical behavior. For relevant fluctuations, this fixed point
becomes fully unstable, and we show the apperance of a two-cycle which is
associated with a novel critical behavior. We use scaling arguments to
calculate the critical exponent $\alpha$ of the specific heat, which turns out
to be different from the value for the uniform case. We check the scaling
predictions by a direct numerical analysis of the singularity of the
thermodynamic free-energy. The agreement between scaling and direct
calculations is excellent for stronger singularities (large values of $q$). The
critical exponents do not depend on the strengths of the exchange interactions.


title:
Orthogonality Catastrophe and Spontaneous Symmetry Breaking in
  Double-layer Fermi-liquid-like States

abstract:
  The double-layer electron system with total filling factor $\nu=1/2$ can be
regarded as two separate Fermi-liquid-like states with $\nu=1/4$ when the layer
separation is sufficiently large and there is no tunneling. The weak tunneling
in this state suffers an orthogonality catastrophe and it becomes irrelevant.
Using the symmetric and antisymmetric combinations of layer indices as the
pseudospin degrees of freedom, we show that there exists the first order
transition from the above pseudospin unpolarized state to the pseudospin
polarized Fermi-liquid-like state with $\nu=1/2$ as the tunneling strength
becomes sufficiently large.


title: Overlooked Contribution to the Hall Effect in Ferromagnetic Metals

abstract:
  It is pointed out that in ferromagnetic metals a contribution to the Hall
voltage arises when a non-zero spin current exists, which is generally the case
in the presence of a charge current. This contribution is independent of any
scattering effects and exists down to zero temperature. The sign of the
resulting Hall coefficient may be either equal or opposite to the one of the
ordinary Hall coefficient depending on the band filling. This effect seems to
have been left out in previous analyses of the Hall effect in ferromagnetic
metals.


title: Spin fractionalization of an even number of electrons in a Quantum dot

abstract:
  An experiment is proposed of non perturbative tunneling in a Quantum dot
connected to leads in a pillar configuration, which would shed light on the
physics of the mesoscopic Kondo problem. We propose for the first time that
what is coupled to the leads in the case of even number of electrons on the dot
is not just the electrons on the QD, with their own spins, but the very total
spin of the dot macroscopic state itself, that displays features of the Kondo
physics, that leads to fractionalization of the spin in the dot, that is to a
"spinon box".


title: Geometric and magnetic properties of Co/Pd system

abstract:
  We measured geometric and magnetic properties of Co films on the Pd(111)
surface by x-ray photoelectron diffraction (XPD), x-ray magnetic circular
dichroism (MCD) at the Co L_{2,3} edge, and the surface magneto-optical Kerr
effect (SMOKE) measurements. Co thin films are found to grow incoherently with
fcc island structure on the smooth Pd(111) substrate. Comparison of MCD and
SMOKE measurements of Co thin films grown on rough and smooth Pd(111) surfaces
suggests that perpendicular remanent magnetization and Co orbital moment are
enhanced by the rough interface. Pd capping layer also induces perpendicular
orbital moment enhancement. These observations indicate the influence of
hybridization between Co 3d and Pd 4d at the interface on the magnetic
anisotropy.


title:
Depletion of density of states near Fermi energy induced by disorder and
  electron correlation in alloys

abstract:
  We have performed high resolution photoemission study of substitutionally
disordered alloys Cu-Pt, Cu-Pd, Cu-Ni, and Pd-Pt. The ratios between alloy
spectra and pure metal spectra are found to have dips at the Fermi level when
the residual resistivity is high and when rather strong repulsive
electron-electron interaction is expected. This is in accordance with Altshuler
and Aronov's model which predicts depletion of density of states at the Fermi
level when both disorder and electron correlation are present.


title: Measuring effective temperatures in out-of-equilibrium driven systems

abstract:
  We introduce and solve a model of a thermometric measurement on a driven
glassy system in a stationary state. We show that a thermometer with a
sufficiently slow response measures a temperature higher than that of the
environment, but that the measured temperature does not usually coincide with
the effective temperature related to the violation of the
Fluctuation-Dissipation Theorem.


title:
Flux pinning in a superconducting film by a regular array of magnetic
  dots

abstract:
  We have studied flux pinning in thin superconducting Pb films covering a
triangular array of submicron magnetic Au/Co/Au dots with in-plane
magnetization. The rectangular dots can have two possible in-plane magnetic
states: a single-domain state where the magnetization lies along the long axis
of the dot and a multi-domain state. In this paper, we report on measurements
of the field dependence of the critical current Ic(B) of the superconducting Pb
film evaporated on top of this magnetic array. We observe a clear influence of
the magnetic state of the dots on the pinning properties of the magnetic array,
showing that there is an important pinning contribution due to the stray fields
of the magnetic dots.


title: Numerical Study of Aging Phenomena in Short-Ranged Spin Glasses

abstract:
  Aging phenomena of short-range Ising spin glass models have been investigated
using Monte Carlo simulations. It is found that in the low-temperature
spin-glass phase the mean domain size exhibits a crossover from a power-law
growth associated with the critical fluctuation at the transition temperature
to slower growth inherent in the low-temperature phase. The temperature
dependence of the growth law of the domain size can be almost explained by this
crossover. We also find that the spin-autocorrelation function in the
quasi-equilibrium regime follows the expected scaling behavior from the droplet
theory expressed in terms of the mean domain size and a characteristic length
scale of droplet excitations.


title:
Possible mechanism of the fractional conductance quantization in a
  one-dimensional constriction

abstract:
  As it is well known there may arise situations when an interaction between
electrons is attractive. A weak attraction should manifest itself strongly in
1D systems, since it can create two-electron bound states. This paper
interprets the 0.7 $(2e^2/h)$ conductance structure, observed recently in a
one-dimensional constriction, as a manifestation of two-electron bound states
formed in a barrier saddle-point. The value 0.75 $(2e^2/h)$ follows naturally
from the 3:1 triplet-singlet statistical weight ratio for the two-electron
bound states, if the triplet energy is lower. Furthermore, the value 0.75 has
to be multiplied by the probability T of the bound state formation during
adiabatic transmission of two electrons into 1D channel ($T\simeq 1$). If the
binding energy is larger than the sub-band energy spacing the 0.7 structure can
be seen even when the integer steps are smeared away by the temperature. Bound
states of several electrons, if they exist, may give different steps at 1/2,
5/16, 3/16 etc in the conductance. The latter results are sensitive to the
length of 1D system and the electron density at the barrier. It is not excluded
that the fractional conductance quantization may also appear for a repulsive
interaction between electrons. In this case the electron level splitting is due
to the exchange interaction with two nearest neighbors in a 1D Wigner crystal.


title: The N-steps Invasion Percolation Model

abstract:
  A new kind of invasion percolation is introduced in order to take into
account the inertia of the invader fluid. The inertia strength is controlled by
the number N of pores (or steps) invaded after the perimeter rupture. The new
model belongs to a different class of universality with the fractal dimensions
of the percolating clusters depending on N. A blocking phenomenon takes place
in two dimensions. It imposes an upper bound value on N. For pore sizes larger
than the critical threshold, the acceptance profile exhibits a permanent tail.


title:
A transient network of telechelic polymers and microspheres : structure
  and rheology

abstract:
  We study the structure and dynamics of a transient network composed of
droplets of microemulsion connected by telechelic polymers. The polymer induces
a bridging attraction between droplets without changing their shape. A
viscoelastic behaviour is induced in the initially liquid solution,
characterised in the linear regime by a stretched exponential stress
relaxation. We analyse this relaxation in the light of classical theories of
transient networks. The role of the elastic reorganisations in the deformed
network is emphasized. In the non linear regime, a fast relaxation dynamics is
followed by a second one having the same rate as in the linear regime. This
behaviour, under step strain experiments, should induce a non monotonic
behaviour in the elastic component of the stress under constant shear rate.
However, we obtain in this case a singularity in the flow curve very different
from the one observed in other systems, that we interpret in terms of fracture
behaviour.


title:
Zeeman and Orbital Effects of an in-Plane Magnetic Field in Cuprate
  Superconductors

abstract:
  We discuss the effects of a magnetic field applied parallel to the Cu-O
($ab$) plane of the high $T_c$ cuprate superconductors. After briefly reviewing
the Zeeman effect of the field, we study the orbital effects, using the
Lawrence-Doniach model for layered superconductors as a guide to the physics.
We argue that the orbital effect is qualitatively different for in-plane and
inter-layer mechanisms for superconductivity. In the case of in-plane
mechanisms, interlayer couplings may be modeled as a weak interlayer Josephson
coupling, whose effects disappear as $H\to\infty$; in this case Zeeman
dominates the effect of the field. In contrast, in the inter-layer mechanism
the Josephson coupling {\em is} the driving force of superconductivity, and we
argue that the in-plane field suppresses superconductivity and provides an
upper bound for $H_{c2}$ which we estimate very crudely.


title: Electron Momentum Distribution Function in the t-t'-J Model

abstract:
  We study the electron momentum distribution function (EMDF) for the
two-dimensional t-t'-J model doped with one hole on finite clusters by the
method of twisted boundary conditions. The results quantitatively agree with
our analytical results for a single hole in the antiferromagnetic background,
based on the self-consistent Born approximation (SCBA). Moreover, within the
SCBA an anomalous momentum dependence of EMDF is found, pointing to an emerging
large Fermi surface. The analysis shows that the presence of
next-nearest-neighbor (NNN) hopping terms changes EMDF only quantitatively.


title: Melting of a Wigner Crystal in an Ionic Dielectric

abstract:
  The melting of a Wigner Crystal of electrons placed into a host polar
material is examined as a function of the density and the temperature. When the
coupling to the longitudinal optical modes of the host medium is turned on, the
WC is progressively transformed into a polaronic Wigner Crystal. We estimate
the critical density for crystal melting at zero temperature using the Lindeman
criterion. We show that above a certain critical value of the Frohlich
electron-phonon coupling, the melting towards a quantum liquid of polarons is
not possible, and the insulator-to-metal transition is driven by the ionization
of the polarons (polaron dissociation). The phase diagram at finite temperature
is obtained by making use of the same Lindeman criterion. Results are also
provided in the case of an anisotropic electron band mass, showing that the
scenario of polaron dissociation can be relevant in anisotropic compounds such
as the superconducting cuprates at rather moderate e-ph couplings.


title:
Theory of Spin-Resolved Auger-Electron Spectroscopy from Ferromagnetic
  3d-Transition Metals

abstract:
  CVV Auger electron spectra are calculated for a multi-band Hubbard model
including correlations among the valence electrons as well as correlations
between core and valence electrons. The interest is focused on the
ferromagnetic 3d-transition metals. The Auger line shape is calculated from a
three-particle Green function. A realistic one-particle input is taken from
tight-binding band-structure calculations. Within a diagrammatic approach we
can distinguish between the \textit{direct} correlations among those electrons
participating in the Auger process and the \textit{indirect} correlations in
the rest system. The indirect correlations are treated within second-order
perturbation theory for the self-energy. The direct correlations are treated
using the valence-valence ladder approximation and the first-order perturbation
theory with respect to valence-valence and core-valence interactions. The
theory is evaluated numerically for ferromagnetic Ni. We discuss the
spin-resolved quasi-particle band structure and the Auger spectra and
investigate the influence of the core hole.


title: Fractals from genomes: exact solutions of a biology-inspired problem

abstract:
  This is a review of a set of recent papers with some new data added. After a
brief biological introduction a visualization scheme of the string composition
of long DNA sequences, in particular, of bacterial complete genomes, will be
described. This scheme leads to a class of self-similar and self-overlapping
fractals in the limit of infinitely long constotuent strings. The calculation
of their exact dimensions and the counting of true and redundant avoided
strings at different string lengths turn out to be one and the same problem. We
give exact solution of the problem using two independent methods: the
Goulden-Jackson cluster method in combinatorics and the method of formal
language theory.


title:
Effects of Core-Hole Screening on Spin-Polarised Auger Spectra from
  Ferromagnetic Nickel

abstract:
  We calculate the spin- and temperature-dependent local density of states for
ferromagnetic Ni in the presence of a core hole at a distinguished site in the
lattice. Correlations among the valence electrons and between valence and core
electrons are described within a multi-band Hubbard model which is treated by
means of second-order perturbation theory around the Hartree-Fock solution. The
core-hole potential causes strong screening effects in the Ni valence band. The
local magnetic moment is found to be decreased by a factor 5-6. The
consequences for the spin polarisation of CVV Auger electrons are discussed.


title:
Theory of composite-band Wannier states and order-N electronic-structure
  calculations

abstract:
  From the order-N electronic-structure formulation, a Hamiltonian is derived,
of which lowest eigen state is the generalized or composite-band Wannier state.
This Hamiltonian maps the locality of the Wannier state to that of a virtual
impurity state and to a perturbation from a bonding orbital. These theories are
demonstrated in the diamond-structure solids, where the Wannier states are
constructed by a practical order-N algorithm with the Hamiltonian. The results
give a prototypical picture of the Wannier states in covalent-bonded systems.


title:
Invasion Percolation with Temperature and the Nature of SOC in Real
  Systems

abstract:
  We show that the introduction of thermal noise in Invasion Percolation (IP)
brings the system outside the critical point. This result suggests a possible
definition of SOC systems as ordinary critical systems where the critical point
correspond to set to 0 one of the parameters. We recover both IP and EDEN
model, for $T \to 0$, and $T \to \infty$ respectively. For small $T$ we find a
dynamical second order transition with correlation length diverging when $T \to
0$.


title: Ground state properties of antiferromagnetic Heisenberg spin rings

abstract:
  Exact ground state properties of antiferromagnetic Heisenberg spin rings with
isotropic next neighbour interaction are presented for various numbers of spin
sites and spin quantum numbers. Earlier work by Peierls, Marshall, Lieb,
Schultz and Mattis focused on bipartite lattices and is not applicable to rings
with an odd number of spins. With the help of exact diagonalization methods we
find a more general systematic behaviour which for instance relates the number
of spin sites and the individual spin quantum numbers to the degeneracy of the
ground state. These numerical findings all comply with rigorous proofs in the
cases where a general analysis could be carried out. Therefore it can be
plausibly conjectured that the ascertained properties hold for ground states of
arbitrary antiferromagnetic Heisenberg spin rings.


title:
Diluted Networks of Nonlinear Resistors and Fractal Dimensions of
  Percolation Clusters

abstract:
  We study random networks of nonlinear resistors, which obey a generalized
Ohm's law, $V\sim I^r$. Our renormalized field theory, which thrives on an
interpretation of the involved Feynman Diagrams as being resistor networks
themselves, is presented in detail. By considering distinct values of the
nonlinearity r, we calculate several fractal dimensions characterizing
percolation clusters. For the dimension associated with the red bonds we show
that $d_{\scriptsize red} = 1/\nu$ at least to order ${\sl O} (\epsilon^4)$,
with $\nu$ being the correlation length exponent, and $\epsilon = 6-d$, where d
denotes the spatial dimension. This result agrees with a rigorous one by
Coniglio. Our result for the chemical distance, $d_{\scriptsize min} = 2 -
\epsilon /6 - [ 937/588 + 45/49 (\ln 2 -9/10 \ln 3)] (\epsilon /6)^2 + {\sl O}
(\epsilon^3)$ verifies a previous calculation by one of us. For the backbone
dimension we find $D_B = 2 + \epsilon /21 - 172 \epsilon^2 /9261 + 2 (- 74639 +
22680 \zeta (3))\epsilon^3 /4084101 + {\sl O} (\epsilon^4)$, where $\zeta (3) =
1.202057...$, in agreement to second order in $\epsilon$ with a two-loop
calculation by Harris and Lubensky.


title: Simulated Coevolution in a Mutating Ecology

abstract:
  The bit-string Penna Model is used to simulate the competition between an
asexual parthenogenetic and a sexual population sharing the same environment. A
new-born of either population can mutate and become a part of the other with
some probability. In a stable environment the sexual population soon dies out.
When an infestation by fastly mutating genetically coupled parasites is
introduced however, sexual reproduction prevails, as predicted by the so-called
Red Queen hypothesis for the evolution of sex.


title:
Spin dynamics simulations of the magnetic dynamics of RbMnF$_3$ and
  direct comparison with experiment

abstract:
  Spin-dynamics techniques have been used to perform large-scale simulations of
the dynamic behavior of the classical Heisenberg antiferromagnet in simple
cubic lattices with linear sizes $L\leq 60$. This system is widely recognized
as an appropriate model for the magnetic properties of RbMnF$_3$.
Time-evolutions of spin configurations were determined numerically from coupled
equations of motion for individual spins using a new algorithm implemented by
Krech {\it etal}, which is based on fourth-order Suzuki-Trotter decompositions
of exponential operators. The dynamic structure factor was calculated from the
space- and time-displaced spin-spin correlation function. The crossover from
hydrodynamic to critical behavior of the dispersion curve and spin-wave
half-width was studied as the temperature was increased towards the critical
temperature. The dynamic critical exponent was estimated to be $z=(1.43\pm
0.03)$, which is slightly lower than the dynamic scaling prediction, but in
good agreement with a recent experimental value. Direct, quantitative
comparisons of both the dispersion curve and the lineshapes obtained from our
simulations with very recent experimental results for RbMnF$_3$ are presented.


title:
On the determination of the Fermi surface in high-Tc superconductors by
  angle-resolved photoemission spectroscopy

abstract:
  We study the normal state electronic excitations probed by angle resolved
photoemission spectroscopy (ARPES) in Bi2201 and Bi2212. Our main goal is to
establish explicit criteria for determining the Fermi surface from ARPES data
on strongly interacting systems where sharply defined quasiparticles do not
exist and the dispersion is very weak in parts of the Brillouin zone.
Additional complications arise from strong matrix element variations within the
zone. We present detailed results as a function of incident photon energy, and
show simple experimental tests to distinguish between an intensity drop due to
matrix element effects and spectral weight loss due to a Fermi crossing. We
reiterate the use of polarization selection rules in disentangling the effect
of umklapps due to the BiO superlattice in Bi2212. We conclude that, despite
all the complications, the Fermi surface can be determined unambiguously: it is
a single large hole barrel centered about (pi,pi) in both materials.


title: Mode-coupling theory for heteropolymers

abstract:
  We study the Langevin dynamics of a heteropolymer by means of a mode-coupling
approximation scheme, giving rise to a set of coupled integro-differential
equations relating the response and correlation functions. The analysis shows
that there is a regime at low temperature characterized by out-of-equilibrium
dynamics, with violation of time-translational invariance and of the
fluctuation-dissipation theorem. The onset of ageing dynamics at low
temperatures gives new insight into the nature of the slow dynamics of a
disordered polymer. We also introduce a renormalization-group treatment of our
mode-coupling equations, which supports our analysis, and might be applicable
to other systems.