title:
Thermodynamics of hexagonal-close-packed iron under Earth's core
  conditions

abstract:
  The free energy and other thermodynamic properties of hexagonal-close-packed
iron are calculated by direct {\em ab initio} methods over a wide range of
pressures and temperatures relevant to the Earth's core. The {\em ab initio}
calculations are based on density-functional theory in the generalised-gradient
approximation, and are performed using the projector augmented wave (PAW)
approach. Thermal excitation of electrons is fully included. The Helmholtz free
energy consists of three parts, associated with the rigid perfect lattice,
harmonic lattice vibrations, and anharmonic contributions, and the technical
problems of calculating these parts to high precision are investigated. The
harmonic part is obtained by computing the phonon frequencies over the entire
Brillouin zone, and by summation of the free-energy contributions associated
with the phonon modes. The anharmonic part is computed by the technique of
thermodynamic integration using carefully designed reference systems. Detailed
results are presented for the pressure, specific heat, bulk modulus, expansion
coefficient and Gr\"{u}neisen parameter, and comparisons are made with values
obtained from diamond-anvil-cell and shock experiments.


title: Ground State Theory of delta-Pu

abstract:
  Correlation effects are important for making predictions in the delta phase
of Pu. Using a realistic treatment of the intra-atomic Coulomb correlations we
address the long-standing problem of computing ground state properties. The
equilibrium volume is obtained in good agreement with experiment when taking
into account Hubbard U of the order 4 eV. For this U, the calculation predicts
a 5f5 atomic-like configuration with L=5, S=5/2, and J=5/2 and shows a nearly
complete compensation between spin and orbital magnetic moments.


title: Cluster renormalization in the Becker-Doring equations

abstract:
  We apply ideas from renormalization theory to models of cluster formation in
nucleation and growth processes. We study a simple case of the Becker-Doring
system of equations and show how a novel coarse-graining procedure applied to
the cluster aggregation space affects the coagulation and fragmentation rate
coefficients. A dynamical renormalization structure is found to underlie the
Becker-Doring equations, nine archetypal systems are identified, and their
behaviour is analysed in detail. These architypal systems divide into three
distinct groups: coagulation-dominated systems, fragmentation-dominated systems
and those systems where the two processes are balanced. The dynamical behaviour
obtained for these is found to be in agreement with certain fine-grained
solutions previously obtained by asymptotic methods. This work opens the way
for the application of renormalization ideas to a wide range of non-equilibrium
physicochemical processes, some of which we have previously modelled on the
basis of the Becker-Doring equations.


title:
Theory of Dielectric Absorption Line Shape in Dielectrics and
  Ferroelectrics

abstract:
  We present the theory of dielectric spectra allowing for homogeneous and
inhomogeneous broadening mechanisms contributions. The dielectric absorption
line shape for oscillators with memory and Debye type relaxors with
distribution of, respectively, resonant frequencies and relaxation times (due
to random electric fields distribution) has been calculated. The conditions of
hole burning observation are discussed. It is shown that the investigations of
the inhomogeneously broadened line shape and width as well as hole burning can
give valuable information about the distribution of random fields and
relaxation times. The latter seems to be extremely important for the disordered
systems like supercooled liquids and relaxor ferroelectrics.


title: Real-time renormalization group and charge fluctuations in quantum dots

abstract:
  We develop a perturbative renormalization-group method in real time to
describe nonequilibrium properties of discrete quantum systems coupled linearly
to an environment. We include energy broadening and dissipation and develop a
cutoff-independent formalism. We present quantitatively reliable results for
the nonlinear conductance in the mixed-valence and empty-orbital regime of the
nonequilibrium Anderson impurity model with finite on-site Coulomb repulsion.


title:
Universal Tc depression by irradiation defects in underdoped and
  overdoped cuprates

abstract:
  We report on a study of the influence of defects introduced in the CuO$_{2}$
planes of cuprates in a wide range of hole dopings n. T$_{c}$ and electrical
resistivity $\rho (T)$ measurements have been performed on electron irradiated
YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ and Tl$_{2}$Ba$_{2}$CuO$_{6+x}$ single
crystals. A universal scaling between the decrease in T$_{c}$ and
$\UNICODE[m]{0x394}\rho_{2D}\UNICODE[m]{0xd7}n$, where $\UNICODE[m]{0x394}\rho
_{2D}$ is the increase of the 2D-resistance induced by the defects, is found
for all the samples investigated here. This demonstrates that n is the relevant
parameter to describe the transport properties all over the phase diagram, in
contradiction with a recent suggestion of a change in the number of carriers
from n to 1-n at the optimal doping. Moreover, the analysis of our data
suggests that strong scattering persists on the overdoped side.


title:
Two-dimensional charged magneto-excitons: Magnetic translations and
  localization

abstract:
  For composite complexes -- quasi-two-dimensional (2D) charged
magneto-excitons X- -- we propose a new exact classification of states, which
is based on magnetic translations. We consider implications of this symmetry
for magneto-optical transitions. It is shown, in particular, that in a
translationally-invariant system with a simple valence band, the ground triplet
X- state is dark in interband transitions at finite magnetic fields B. This
exact result calls for the re-interpretation of several previous theoretical
studies. We consider the symmetry-breaking effects of spatial lateral
confinement on internal transitions of charged complexes.


title:
Pressure dependence and non-universal effects of microscopic couplings
  on the spin-Peierls transition in CuGeO_3

abstract:
  The theory by Cross and Fisher (CF) is by now commonly accepted for the
description of the spin-Peierls transition within an adiabatic approach. The
dimerization susceptibility as the essential quantity, however, is approximated
by means of a continuum description. Several important experimental
observations can not be understood within this scope. Using density matrix
renormalization group (DMRG) techniques we are able to treat the spin system
exactly up to numerical inaccuracies. Thus we find the correct dependence of
the equation of state on the spin-spin interaction constant J, still in an
adiabatic approach. We focus on the pressure dependence of the critical
temperature which is absent in the CF theory as the only energy scale with
considerable pressure dependence is J which drops out completely. Comparing the
theoretical findings to the experimentally measured pressure dependence of the
spin-Peierls temperature we obtain information on the variation of the
frustration parameter with pressure. Furthermore, the ratio of the spectral gap
and the transition temperature is analyzed.


title:
Electron Correlations in an Electron Bilayer at Finite Temperature:
  Landau Damping of the Acoustic Plasmon

abstract:
  We report angle-resolved Raman scattering observations of the temperature
dependent Landau damping of the acoustic plasmon in an electron bilayer system
realised in a GaAs double quantum well structure. Corresponding calculations of
the charge-density excitation spectrum of the electron bilayer using forms of
the random phase approximation (RPA), and the static local field formalism of
Singwi, Tosi, Land and Sj\"{o}lander (STLS) extended to incorporate non-zero
electron temperature $T_{\rm e}$ and phenomenological damping, are also
presented. The STLS calculations include details of the temperature dependence
of the intra- and inter-layer local field factors and pair-correlation
functions. Good agreement between experiment and the various theories is
obtained for the acoustic plasmon energy and damping for $T_{\rm e} \lesssim
T_{\rm F}/2$, the Fermi temperature. However, contrary to current expectations,
all of the calculations show significant departures from our experimental data
for $T_{\rm e} \gtrsim T_{\rm F}/2$. From this, we go on to demonstrate
unambiguously that real local field factors fail to provide a physically
accurate description of exchange correlation behaviour in low dimensional
electron gases. Our results suggest instead that one must resort to a
{\em{dynamical}} local field theory, characterised by a {\em{complex}} field
factor to provide a more accurate description.


title: Vortex Reconnection as the Dissipative Scattering of Dipoles

abstract:
  We propose a phenomenological model of vortex tube reconnection at high
Reynolds numbers. The basic picture is that squeezed vortex lines, formed by
stretching in the region of closest approach between filaments, interact like
dipoles (monopole-antimonopole pairs) of a confining electrostatic theory. The
probability of dipole creation is found from a canonical ensemble spanned by
foldings of the vortex tubes, with temperature parameter estimated from the
typical energy variation taking place in the reconnection process. Vortex line
reshuffling by viscous diffusion is described in terms of directional
transitions of the dipoles. The model is used to fit with reasonable accuracy
experimental data established long ago on the symmetric collision of vortex
rings. We also study along similar lines the asymmetric case, related to the
reconnection of non-parallel vortex tubes.


title: Electronic Raman Scattering in Superconducting Cuprates

abstract:
  We show that the novel features observed in Raman experiments on optimally
doped and underdoped Bi-2212 compounds in $B_{1g}$ geometry can be explained by
a strong fermionic self-energy due to the interaction with spin fluctuations.
We compute the Raman intensity $R(\omega)$ both above and below $T_c$, and show
that in both cases $R(\omega)$ progressively deviates, with decreasing doping,
from that in a Fermi-gas due to increasing contribution from the fermionic
self-energy. We also show that the final state interaction increases with
decreasing doping and gradually transforms the $2\Delta$ peak in the
superconducting state into a pseudo resonance mode below $2\Delta$. We argue
that these results agree well with the experimental data for Bi-2212.


title: Temperature enhanced persistent currents and "$\phi_0/2$ periodicity"

abstract:
  We predict a non-monotonous temperature dependence of the persistent currents
in a ballistic ring coupled strongly to a stub in the grand canonical as well
as in the canonical case. We also show that such a non-monotonous temperature
dependence can naturally lead to a $\phi_0/2$ periodicity of the persistent
currents, where $\phi_0$=h/e. There is a crossover temperature $T^*$, below
which persistent currents increase in amplitude with temperature while they
decrease above this temperature. This is in contrast to persistent currents in
rings being monotonously affected by temperature. $T^*$ is parameter-dependent
but of the order of $\Delta_u/\pi^2k_B$, where $\Delta_u$ is the level spacing
of the isolated ring. For the grand-canonical case $T^*$ is half of that for
the canonical case.


title: Instantaneous Normal Mode Analysis of Supercooled Water

abstract:
  We use the instantaneous normal mode approach to provide a description of the
local curvature of the potential energy surface of a model for water. We focus
on the region of the phase diagram in which the dynamics may be described by
the mode-coupling theory. We find, surprisingly, that the diffusion constant
depends mainly on the fraction of directions in configuration space connecting
different local minima, supporting the conjecture that the dynamics are
controlled by the geometric properties of configuration space. Furthermore, we
find an unexpected relation between the number of basins accessed in
equilibrium and the connectivity between them.


title: Dynamic exponent in Extremal models of Pinning

abstract:
  The depinning transition of a front moving in a time-independent random
potential is studied. The temporal development of the overall roughness w(L,t)
of an initially flat front, $w(t)\propto t^\beta$, is the classical means to
have access to the dynamic exponent. However, in the case of front propagation
in quenched disorder via extremal dynamics, we show that the initial increase
in front roughness implies an extra dependence over the system size which comes
from the fact that the activity is essentially localized in a narrow region of
space. We propose an analytic expression for the $\beta$ exponent and confirm
this for different models (crack front propagation, Edwards-Wilkinson model in
a quenched noise, ...).


title: Phase Diagram of the 1D Kondo Lattice Model

abstract:
  We determine the boundary of the fully polarized ferromagnetic ground state
in the one dimensional Kondo lattice model at partial conduction electron band
filling by using a newly developed infinite size DMRG method which conserves
the total spin quantum number. The obtained paramagnetic to ferromagnetic phase
boundary is below $J \approx 3.5$ for the whole range of band filling. By this
we solve the controversy in the phase diagram over the extent of the
ferromagnetic region close to half filling.


title: Dynamics of simulated water under pressure

abstract:
  We present molecular dynamics simulations of the SPC/E model of water to
probe the dynamic properties at temperatures from 350 K down to 190 K and
pressures from 2.5GPa (25kbar) down to -300MPa (-3kbar). We compare our results
with those obtained experimentally, both of which show a diffusivity maximum as
a function of pressure. We find that our simulation results are consistent with
the predictions of the mode-coupling theory (MCT) for the dynamics of weakly
supercooled liquids -- strongly supporting the hypothesis that the apparent
divergences of {\it dynamic} properties observed experimentally may be
independent of a possible thermodynamic singularity at low temperature. The
dramatic change in water's dynamic and structural properties as a function of
pressure allows us to confirm the predictions of MCT over a much broader range
of the von Schweidler exponent values than has been studied for simple atomic
liquids. We also show how structural changes are reflected in the wave-vector
dependence of dynamic properties of the liquid along a path of nearly constant
diffusivity. For temperatures below the crossover temperature of MCT (where the
predictions of MCT are expected to fail), we find tentative evidence for a
crossover of the temperature dependence of the diffusivity from power-law to
Arrhenius behavior, with an activation energy typical of a strong liquid.


title: Spin relaxation in low-dimensional systems

abstract:
  We review some of the newest findings on the spin dynamics of carriers and
excitons in GaAs/GaAlAs quantum wells. In intrinsic wells, where the optical
properties are dominated by excitonic effects, we show that exciton-exciton
interaction produces a breaking of the spin degeneracy in two-dimensional
semiconductors. In doped wells, the two spin components of an optically created
two-dimensional electron gas are well described by Fermi-Dirac distributions
with a common temperature but different chemical potentials. The rate of the
spin depolarization of the electron gas is found to be independent of the mean
electron kinetic energy but accelerated by thermal spreading of the carriers.


title: Addition spectra of quantum dots: The role of dielectric mismatch

abstract:
  Using atomistic pseudopotential wave functions we calculate the electron and
hole charging energies of InAs quantum dots. We find that the charging energies
depend strongly on the dielectric constant epsilon_out of the surrounding
material, and that when the latter is smaller than the dielectric constant of
the dot the electron-electron and hole-hole interactions are dominated by
surface-polarization effects. We predict the addition energies and the
quasi-particle gap as a function of size and epsilon_out. We find excellent
agreement with recent single-dot tunneling spectroscopy data for epsilon_out=6.


title:
Viscoelasticity and Surface Tension at the Defect-Induced First-Order
  Melting Transition of a Vortex Lattice

abstract:
  We show that thermally activated interstitial and vacancy defects can lead to
first order melting of a vortex lattice. We obtain good agreement with
experimentally measured melting curve, latent heat, and magnetization jumps for
YBCO and BSCCO. The shear modulus of the vortex liquid is frequency dependent
and crosses over from zero at low frequencies to a finite value at high
frequencies. We also find a small surface tension between the vortex line
liquid and the vortex lattice.


title:
Temperature dependent scattering rate and optical mass of ferromagnetic
  metallic manganites

abstract:
  We report on the optical properties of the hole-doped manganites
La_0.7Ca_0.3MnO_3 and La_0.7Ca_0.3MnO_3. Transmission and reflection of thin
films are measured in the infrared at temperatures from 10 - 150 K using
Fourier-transform spectroscopy. The scattering rate and optical mass are
obtained by fitting the far-infrared transmission to a Drude model. The
scattering rate shows a T^2 dependence with temperature. The optical mass
enhancement differs only slightly from specific heat results. In addition, we
compare the infrared spectral weight to band structure calculations [M. Quijada
et al., Phys. Rev. B 58, 16093 (1998)].


title: Path ensembles averages in systems driven far-from-equilibrium

abstract:
  The Kawasaki nonlinear response relation, the transient fluctuation theorem,
and the Jarzynski nonequilibrium work relation are all expressions that
describe the behavior of a system that has been driven from equilibrium by an
external perturbation. In contrast to linear response theory, these expressions
are exact no matter the strength of the perturbation, or how far the system has
been driven away from equilibrium. In this paper I show that these three
relations (and several other closely related results) can all be considered
special cases of a single theorem. This expression is explicitly derived for
discrete time and space Markovian dynamics, with the additional assumptions
that the single time step dynamics preserve the appropriate equilibrium
ensemble, and that the energy of the system remains finite.


title:
An integrable model for the integer quantum Hall transition I: The
  vertex model

abstract:
  In this study, an integrable vertex model based on the quantum affine
superalgebra $U_q\bigl(\hat{gl}(2|2)\bigr)$ is constructed. The model is
characterized by a particular assignment of spectral parameters and lowest as
well as highest weight $U_q\bigl(gl(2|2) \bigr)$ modules to its lattice links.
Solutions of the corresponding intertwining conditions yield the Boltzmann
weights. The set of mutually commuting charges contains a quantity equivalent
to the super spin chain Hamiltonian proposed for the description of the integer
quantum Hall transition.


title:
On the problem of an electron scattering in an arbitrary one-dimensional
  potential field

abstract:
  Recurrent representations for an electron transmission and reflection
amplitudes for a one-dimensional chain are obtained. The linear differential
equations for scattering amplitudes of an arbitrary potential are found.


title: Domain Wall Motion in the Presence of Nuclear Spins

abstract:
  We investigate the motion of a domain wall in the presence of a dynamical
hyperfine field. At temperature T high compared to the hyperfine coupling, the
nuclear spins create a spatially random potential landscape, with dynamics
dictated by the nuclear relaxation time $T_2$. The distribution of the domain
wall relaxation times (both in the thermal and quantum regimes) can show a long
tail, characteristic of stochastic processes where rare events are important.
Here, these are due to occasional strong fluctuations in the nuclear spin
polarisation.


title: Quantum Measurement Operations with Nanomagnets and SQUIDs

abstract:
  The low-energy behaviour of 2 coupled nanomagnets or 2 coupled SQUIDs,
interacting with their environment, can be described by the model of a ``Pair
or Interacting Spins Coupled to an Environmental Sea'' (PISCES). These physical
systems can then be used for a measurement operation in which system, apparatus
and environment are all treated quantum mechanically. We design a
``Bell/Coleman-Hepp'' measuring system, and show that in principle one may
design a situation in which quantum interference between system and apparatus
can upset the usual measurement operation.


title:
Thermodynamics of the s=1/2 transverse XX chain with
  Dzyaloshinskii-Moriya interaction in the presence of correlated Lorentzian
  disorder

abstract:
  The spin-1/2 transverse XY chain with correlated disorder may exhibit the
nonzero averaged transverse magnetization at zero averaged transverse field. We
discuss this peculiarity connected with asymmetry in the density of states and
induced by the correlated disorder analysing the moments of the density of
states.


title:
Adsorption assisted translocation of a chain molecule through a pore in
  a spherical vesicle

abstract:
  We analyze the free energy for translocation of a polymer from the outside of
a spherical vesicle to the inside. The process is assumed to be driven by the
adsorption of the polymer on the inner surface of the vesicle. We argue that in
the case where the polymer is adsorbed on the outer surface too, the entropic
barrier for translocation is absent. We analyze the adsorption energy and find
the free energy profile for the process. We argue that the motion corresponds
to a polymer crossing a region with a change in free energy per segment. Based
upon our earlier analsis of the behaviour of kinks in such a problem, we
conclude that the translocation can occur with a crossing time $t_{cross}\sim
N$.


title: The Upper Critical Field in Disordered Two-Dimensional Superconductors

abstract:
  We present calculations of the upper critical field in superconducting films
as a function of increasing disorder (as measured by the normal state
resistance per square). In contradiction to previous work, we find that there
is no anomalous low-temperature positive curvature in the upper critical field
as disorder is increased. We show that the previous prediction of this effect
is due to an unjustified analytical approximation of sums occuring in the
perturbative calculation. Our treatment includes both a careful analysis of
first-order perturbation theory, and a non-perturbative resummation technique.
No anomalous curvature is found in either case. We present our results in
graphical form.


title: Reentrant Condensation of DNA induced by Multivalent Counterions

abstract:
  A theory of condensation and resolubilization of a dilute DNA solution with
growing concentration of multivalent cations, N is suggested. It is based on a
new theory of screening of a macroion by multivalent cations, which shows that
due to strong cation correlations at the surface of DNA the net charge of DNA
changes sign at some small concentration of cations N_0. DNA condensation takes
place in the vicinity of N_0, where absolute value of the DNA net charge is
small and the correlation induced short range attraction dominates the Coulomb
repulsion. At N > N_0 positive DNA should move in the oppisite direction in an
electrophoresis experiment. From comparison of our theory with experimental
values of condensation and resolubilization thresholds for DNA solution
containing Spe$^{4+}$, we obtain that N_0 = 3.2 mM and that the energy of DNA
condensation per nucleotide is $0.07 k_B T$.


title:
Anomalous microwave response of high-temperature superconducting
  thin-film microstrip resonator in weak dc magnetic fields

abstract:
  We have studied an anomalous microwave (mw) response of superconducting
YBa_{2}Cu_{3}O_{7-delta} (YBCO) microstrip resonators in the presence of a weak
dc magnetic field, H_{dc}. The surface resistance (R_{s}) and reactance (X_{s})
show a correlated non-monotonic behaviour as a function of H_{dc}. R_{s} and
X_{s} were found to initially decrease with elevated H_{dc} and then increase
after H_{dc} reaches a crossover field, H_{c}, which is independent of the
amplitude and frequency of the input mw signal within the measurements. The
frequency dependence of R_{s} is almost linear at fixed H_{dc} with different
magnitudes (<H_{c}, =H_{c} and >H_{c}). The impedance plane analysis
demonstrates that r_{H}, which is defined as the ratio of the change in
R_{s}(H_{dc}) and that in X_{s}(H_{dc}), is about 0.6 at H_{dc}<H_{c} and 0.1
at H_{dc}>H_{c}. The H_{dc} dependence of the surface impedance is
qualitatively independent of the orientation of H_{dc}.


title: Phase ordering and shape deformation of two-phase membranes

abstract:
  Within a coupled-field Ginzburg-Landau model we study analytically phase
separation and accompanying shape deformation on a two-phase elastic membrane
in simple geometries such as cylinders, spheres and tori. Using an exact
periodic domain wall solution we solve for the shape and phase ordering field,
and estimate the degree of deformation of the membrane. The results are
pertinent to a preferential phase separation in regions of differing curvature
on a variety of vesicles.


title: Spin-polarized tunneling of La0.67Sr0.33MnO3/YBa2Cu3O7-d junctions

abstract:
  The transport properties between ferromagnets and high-Tc superconductors are
investigated in La0.67Sr0.33MnO3/YBa2Cu3O7-d (LSMO/YBCO)junctions in the
geometry of cross-strip lines. The conductance spectra show zero-bias
conductance peaks (ZBCP), reflecting the charge transport in the ab-plane. When
an external magnetic field is applied to the junctions, the conductance spectra
show two notable features, i.e., an increase of background conductance and an
asymmetric ZBCP splitting whose amplitude responds nonlinearly to the applied
field. It is shown that the magnetic field response are consistent with a
theoretical prediction of tunneling spectroscopy when the presence of a
ferromagnetic barrier between a spin-polarized ferromagnet and a d-wave
superconductor is assumed.


title:
Large fluctuations in multi-attractor systems and the generalized
  Kramers problem

abstract:
  The main subject of the paper is an escape from a multi-well metastable
potential on a time-scale of a formation of the quasi-equilibrium between the
wells. The main attention is devoted to such ranges of friction in which an
external saddle does not belong to a basin of attraction of an initial
attractor. A complete rigorous analysis of the problem for the most probable
escape path is presented and a corresponding escape rate is calculated with a
logarithmic accuracy. Unlike a conventional rate for a quasi-stationary flux,
the rate on shorter time-scales strongly depends on friction, moreover, it may
undergo oscillations in the underdamped range and a cutoff in the overdamped
range.
  A generalization of the results for inter-attractor transitions in stable
potentials with more than two wells is also presented as well as a splitting
procedure for a phenomenological description of inter-attractor transitions is
suggested.
  Applications to such problems as a dynamics of escape on time-scales shorter
than an optimal fluctuation duration, prehistory problem, optimal control of
fluctuations, fluctuational transport in ratchets, escapes at a periodic
driving and transitions in biased Josephson junctions and ionic channels are
briefly discussed.


title:
Local spin resonance and spin-Peierls-like phase transition in a
  geometrically frustrated antiferromagnet

abstract:
  Using inelastic magnetic neutron scattering we have discovered a localized
spin resonance at 4.5 meV in the ordered phase of the geometrically frustrated
cubic antiferromagnet $\rm ZnCr_2O_4$. The resonance develops abruptly from
quantum critical fluctuations upon cooling through a first order transition to
a co-planar antiferromagnet at $T_c=12.5(5)$ K. We argue that this transition
is a three dimensional analogue of the spin-Peierls transition.


title:
Amplification Effects on the Transmission and Reflexion Phases in 1D
  Periodic Systems

abstract:
  We investigate the localization observed recently for locally non-hermitian
Hamiltonians by studying the effect of the amplification on the scaling
behavior of the transmission and reflection phases in 1D periodic chains of
$\delta$-potentials. The amplification here is represented by an imaginary term
added to the on-site potential. It is found that both phases of the
transmission and reflection amplitudes are strongly affected by the
amplification term. In particular, the phases in the region of amplification
become independent of the length scale while they oscillate strongly near the
maximum transmission (or reflection). The interference effects on the phase in
passive systems are used to interpret those observed in the presence of
amplification. The phases of the transmission and reflection are found to
oscillate in passive systems whith increasing periods in the allowed band for
the transmission phase while for the reflection phase, its initial value is
always less than $\pi /2$ in this band.


title: Scaling of the distribution of shortest paths in percolation

abstract:
  We present a scaling hypothesis for the distribution function of the shortest
paths connecting any two points on a percolating cluster which accounts for
{\it (i)} the effect of the finite size of the system, and {\it (ii)} the
dependence of this distribution on the site occupancy probability $p$. We test
the hypothesis for the case of two-dimensional percolation.


title:
Direct Observation of Antiferro-Quadrupolar Ordering - Resonant X-ray
  Scattering Study of DyB2C2

abstract:
  Antiferroquadrupolar (AFQ) ordering has been conjectured in several
rare-earth compounds to explain their anomalous magnetic properties. No direct
evidences for AFQ ordering, however, have been reported. Using the resonant
x-ray scattering technique near the Dy L_III absorption edge, we have succeeded
in observing the AFQ order parameter in DyB2C2 and analyzing the energy and
polarization dependence. Much weaker coupling between orbital degrees of
freedom and lattice in 4f electron systems than in 3d compounds provides an
ideal platform to study orbital interactions originated from electronic
mechanisms.


title:
Motional narrowing effect in one-dimensional Frenkel chains with
  configurational disorder

abstract:
  We analyze the peculiarities of the motional narrowing effect in disordered
one-dimensional Frenkel chains with off-diagonal disorder, induced by
uncorrelated Gaussian fluctuations in the positions of the host units. A clear
difference in the scaling laws with respect to the magnitude of the positional
disorder and the chain size is found, as compared to those for uncorrelated
diagonal disorder as well as for off-diagonal disorder modeled by uncorrelated
randomness in the nearest-neighbor couplings. The origin of such a difference
is discussed in detail.


title:
Linear optical properties of one-dimensional Frenkel exciton systems
  with intersite energy correlations

abstract:
  We analyze the effects of intersite energy correlations on the linear optical
properties of one-dimensional disordered Frenkel exciton systems. The
absorption line width and the factor of radiative rate enhancement are studied
as a function of the correlation length of the disorder. The absorption line
width monotonously approaches the seeding degree of disorder on increasing the
correlation length. On the contrary, the factor of radiative rate enhancement
shows a non-monotonous trend, indicating a complicated scenario of the exciton
localization in correlated systems. The concept of coherently bound molecules
is exploited to explain the numerical results, showing good agreement with
theory. Some recent experiments are discussed in the light of the present
theory.


title: Metastable States in High Order Short-Range Spin Glasses

abstract:
  The mean number <N> of metastable states in higher order short-range spin
glasses is estimated analytically using a variational method introduced by
Tanaka and Edwards for very large coordination numbers. For lattices with small
connectivities, numerical simulations do not show any significant dependence on
the relative positions of the interacting spins on the lattice, indicating thus
that these systems can be described by a few macroscopic parameters. As an
extremely anisotropic model we consider the low autocorrelated binary spin
model and we show through numerical simulations that its landscape has an
exceptionally large number of local optima.


title: Berry phase and persistent current in disordered mesoscopic rings

abstract:
  A novel quantum interference effect in disordered quasi-one-dimensional rings
in the inhomogeneous magnetic field is reported. We calculate the canonical
disorder averaged persistent current using the diagrammatic perturbation
theory. It is shown that within the adiabatic regime the average current
oscillates as a function of the geometric flux which is related to the Berry
phase and the period becomes half the value of the case of a single
one-dimensional ring. We also discuss the magnetic dephasing effect on the
averaged current.


title:
Renormalization group approach to the one-dimensional 1/4-filled Hubbard
  model with alternating on-site interactions

abstract:
  The one-dimensional Hubbard model with different on-site interactions is
investigated by renormalization group technique. In the case of a 1/4-filled
band the dynamical nonequivalence of sites leads to the appearance of Umklapp
processes in the system and to the dynamical generation of a gap in the charge
excitation spectrum for $U_{a}\not=U_{b}$, $U_{a}>0$ or $U_{b}>0$. The
ground-state phase diagram is obtained in the limit of second order
renormalization. Depending on the sign and relative values of the bare coupling
constants, there is a gap in the spin or charge excitation spectrum and the
model system tends to superconducting or antiferromagnetic order at T=0, with
doubled period. The role of interaction between particles on nearest and
next-nearest neighbor sites is also considered.


title:
From an antiferromagnet to a heavy-fermion system: CeCu5Au under
  pressure

abstract:
  The electrical resistivity rho(T) of single crystalline CeCu_5Au under
pressure was measured in the temperature range 30mK<T<300K. Pressure suppresses
the antiferromagnetic order (T_N=2.35K at ambient pressure) and drives the
system into a non-magnetic heavy-fermion state above P_c=4.1(3)GPa. The
electrical resistivity shows a deviation from a T^2 dependence of a
Fermi-liquid in the pressure range 1.8GPa<=P<=5.15GPa. The rho(T)-curves can be
compared with those of CeCu_{6-x}Au_x at different Au concentrations. Just
before the long-range magnetic order vanishes, a possibly superconducting phase
(at T_c=0.1K and P=3.84GPa) occurs, pointing to a coexistence of
antiferromagnetic order and superconductivity. This new phase is only seen in a
narrow pressure interval Delta P=0.4GPa.


title:
Nonequilibrium Phase Transition in a Model of Diffusion, Aggregation and
  Fragmentation

abstract:
  We study the nonequilibrium phase transition in a model of aggregation of
masses allowing for diffusion, aggregation on contact and fragmentation. The
model undergoes a dynamical phase transition in all dimensions. The steady
state mass distribution decays exponentially for large mass in one phase. On
the contrary, in the other phase it has a power law tail and in addition an
infinite aggregate. The model is solved exactly within a mean field
approximation which keeps track of the distribution of masses. In one
dimension, by mapping to an equivalent lattice gas model, exact steady states
are obtained in two extreme limits of the parameter space. Critical exponents
and the phase diagram are obtained numerically in one dimension. We also study
the time dependent fluctuations in an equivalent interface model in (1+1)
dimension and compute the roughness exponent $\chi$ and the dynamical exponent
z analytically in some limits and numerically otherwise. Two new fixed points
of interface fluctuations in (1+1) dimension are identified. We also generalize
our model to include arbitrary fragmentation kernels and solve the steady
states exactly for some special choices of these kernels via mappings to other
solvable models of statistical mechanics.


title:
Effects of Confinement on Critical Adsorption: Absence of Critical
  Depletion for Fluids in Slit Pores

abstract:
  The adsorption of a near-critical fluid confined in a slit pore is
investigated by means of density functional theory and by Monte Carlo
simulation for a Lennard-Jones fluid. Our work was stimulated by recent
experiments for SF_6 adsorbed in a mesoporous glass which showed the striking
phenomenon of critical depletion, i.e. the adsorption excess "Gamma" first
increases but then decreases very rapidly to negative values as the bulk
critical temperature T_c is approached from above along near-critical
isochores. By contrast, our density functional and simulation results, for a
range of strongly attractive wall-fluid potentials, show Gamma monotonically
increasing and eventually saturating as the temperature is lowered towards T_c
along both the critical (rho=rho_c) and sub-critical isochores (rho<\rho_c).
Such behaviour results from the increasingly slow decay of the density profile
away from the walls, into the middle of the slit, as T->T_c. For rho < rho_c we
find that in the fluid the effective bulk field, which is negative and which
favours desorption, is insufficient to dominate the effects of the surface
fields which favour adsorption. We compare this situation with earlier results
for the lattice gas model with a constant (negative) bulk field where critical
depletion was found. Qualitatively different behaviour of the density profiles
and adsorption is found in simulations for intermediate and weakly attractive
wall-fluid potentials but in no case do we observe the critical depletion found
in experiments. We conclude that the latter cannot be accounted for by a single
pore model.


title: A Fluctuation-Dissipation Process without Time Scale

abstract:
  We study the influence of a dissipation process on diffusion dynamics
triggered by fluctuations with long-range correlations. We make the assumption
that the perturbation process involved is of the same kind as those recently
studied numerically and theoretically, with a good agreement between theory and
numerical treatment. As a result of this assumption the equilibrium
distribution departs from the ordinary canonical distribution. The distribution
tails are truncated, the distribution border is signalled by sharp peaks and,
in the weak dissipation limit, the central distribution body becomes identical
to a truncated Levy distribution.


title: Finite temperature properties of the 2D Kondo lattice model

abstract:
  Using recently developed Lanczos technique we study finite-temperature
properties of the 2D Kondo lattice model at various fillings of the conduction
band. At half filling the quasiparticle gap governs physical properties of the
chemical potential and the charge susceptibility at small temperatures. In the
intermediate coupling regime quasiparticle gap scales approximately linearly
with Kondo coupling. Temperature dependence of the spin susceptibility reveals
the existence of two different temperature scales. A spin gap in the
intermediate regime leads to exponential drop of the spin susceptibility at low
temperatures. Unusual scaling of spin susceptibility is found for temperatures
above 0.6 J. Charge susceptibility at finite doping reveals existence of heavy
quasiparticles. A new low energy scale is found at finite doping.


title: Dicke-superradiance in electronic systems coupled to reservoirs

abstract:
  We present theoretical results for superradiance, i.e. the collective
coherent decay of a radiating system, in semiconductor structures. An optically
active region can become superradiant if a strong magnetic field is applied.
Pumping of electrons and holes at a rate T through coupling to external
`reservoirs' leads to a novel kind of oscillations with frequency $\sim
\sqrt{T}$.}


title: Dicke Superradiance in a magnetoplasma

abstract:
  We present theoretical results for superradiance, i.e. the collective
coherent decay of a radiating system, in a semiconductor heterostructure under
a strong quantizing magnetic field. We predict a strong peak (`Dicke-peak') in
the emission intensity as a function of time, which should be observable after
a short initial excitation of electrons into the conduction band. This peak has
a characteristic dependence on the magnetic field and should be observable on
sub-picosecond time scales. Furthermore, pumping of electrons and holes into
the systems at a rate T leads to a novel kind of oscillations with frequency
$\sim \sqrt{T}$ in the limit of the lowest Landau level.


title:
Alternating chain with Hubbard-type interactions: renormalization group
  analysis

abstract:
  The canonical transformation diagonalizing the one-particle tight binding
Hamiltonian for an alternating chain with two non-equivalent sites per unit
cell has been used to introduce the Hubbard-like interactions (on-site,
inter-site, bond-site and intra-bond) in the corresponding two-band model. The
considerations have been restricted to the particular case, suggested by the
undistorted 3/4-filled $CuO_{3}$ chain, where the alternating structure comes
only from non-equal on-site atomic energies and the gap between the two bands
is sufficiently large. The renormalization group method has been then applied
to the upper band of this model and the corresponding phase diagram has been
analyzed in terms of some renormalized (density dependent) Hubbard-type
couplings for arbitrary filling of the upper band. The particular case of
$CuO_{3}$ chain is also discussed and the corresponding phase diagram has been
drawn in terms of the original (and realistic) coupling constants.


title: "Quantum phase transitions" in classical nonequilibrium processes

abstract:
  Diffusion limited reaction of the Lotka-Volterra type is analyzed taking into
account the discrete nature of the reactants. In the continuum approximation,
the dynamics is dominated by an elliptic fixed-point. This fixed-point becomes
unstable due to discretization effects, a scenario similar to quantum phase
transitions. As a result, the long-time asymptotic behavior of the system
changes and the dynamics flows into a limit cycle.
  The results are verified by numerical simulations.


title:
Annular Vortex Solutions to the Landau-Ginzburg Equations in Mesoscopic
  Superconductors

abstract:
  New vortex solutions to the Landau-Ginzburg equations are described. These
configurations, which extend the well known Abrikosov and giant magnetic vortex
ones, consist of a succession of ring-like supercurrent vortices organised in a
concentric pattern, possibly bound to a giant magnetic vortex then lying at
their center. The dynamical and thermodynamic stability of these annular
vortices is an important open issue on which hinges the direct experimental
observation of such configurations. Nevertheless, annular vortices should
affect indirectly specific dynamic properties of mesoscopic superconducting
devices amenable to physical observation.


title: Large Skyrmions in an Al(0.13)Ga(0.87)As Quantum Well

abstract:
  We report tilted-field magnetotransport measurements of two-dimensional
electron systems in a 200 Angstrom-wide Al(0.13)Ga(0.87)As quantum well. We
extract the energy gap for the quantum Hall state at Landau level filling \nu
=1 as a function of the tilt angle. The relatively small effective Lande
g-factor (g ~ 0.043) of the structure leads to skyrmionic excitations composed
of the largest number of spins yet reported (s ~ 50). Although consistent with
the skyrmion size observed, Hartree-Fock calculations, even after corrections,
significantly overestimate the energy gaps over the entire range of our data.


title:
Susceptibility amplitude ratios in the two-dimensional Potts model and
  percolation

abstract:
  The high-temperature susceptibility of the $q$-state Potts model behaves as
$\Gamma|T-T_c|^{-\gamma}$ as $T\to T_c+$, while for $T\to T_c-$ one may define
both longitudinal and transverse susceptibilities, with the same power law but
different amplitudes $\Gamma_L$ and $\Gamma_T$. We extend a previous analytic
calculation of the universal ratio $\Gamma/\Gamma_L$ in two dimensions to the
low-temperature ratio $\Gamma_T/\Gamma_L$, and test both predictions with Monte
Carlo simulations for $q=3$ and 4. The data for $q=4$ are inconclusive owing to
large corrections to scaling, while for $q=3$ they appear consistent with the
prediction for $\Gamma_T/\Gamma_L$, but not with that for $\Gamma/\Gamma_L$. A
simple extrapolation of our analytic results to $q\to1$ indicates a similar
discrepancy with the corresponding measured quantities in percolation. We point
out that stronger assumptions were made in the derivation of the ratio
$\Gamma/\Gamma_L$, and our work suggests that these may be unjustified.


title:
Vortices Clustering: The Origin of the Second Peak in the Magnetisation
  Loops of High Temperature Superconductors

abstract:
  We study vortex clustering in type II Superconductors. We demonstrate that
the ``second peak'' observed in magnetisation loops may be a dynamical effect
associated with a density driven instability of the vortex system. At the
microscopic level the instability shows up as the clustering of individual
vortices at (rare) preferential regions of the pinning potential. In the limit
of quasi-static ramping the instability is related to a phase transition in the
equilibrium vortex system.


title: Metastable States in Spin Glasses and Disordered Ferromagnets

abstract:
  We study analytically M-spin-flip stable states in disordered short-ranged
Ising models (spin glasses and ferromagnets) in all dimensions and for all M.
Our approach is primarily dynamical and is based on the convergence of a
zero-temperature dynamical process with flips of lattice animals up to size M
and starting from a deep quench, to a metastable limit. The results (rigorous
and nonrigorous, in infinite and finite volumes) concern many aspects of
metastable states: their numbers, basins of attraction, energy densities,
overlaps, remanent magnetizations and relations to thermodynamic states. For
example, we show that their overlap distribution is a delta-function at zero.
We also define a dynamics for M=infinity, which provides a potential tool for
investigating ground state structure.


title:
Interplay Between the Electric Field and the Nonlinear Interaction in
  Ordered and Disordered Chains

abstract:
  A simple Kronig-Penney model is used to study the effect of nonlinear
interactions on the electronic properties of ordered and disordered electrified
chains. In the case of ordered potentials, we found that the nonlinearity
suppresses the Wannier-Stark effect caused by the electric field. In the case
of disordered potentials, the nonlinearity gives rise to a transition from
superlocalized to weakly localized states.


title:
First order phase transition of the vortex lattice in twinned YBa2Cu3O7
  single crystals in tilted magnetic fields

abstract:
  We present an exhaustive analysis of transport measurements performed in
twinned YBa2Cu3O7 single crystals which stablishes that the vortex solid-liquid
transition is first order when the magnetic field H is applied at an angle
theta away from the direction of the twin planes. We show that the resistive
transitions are hysteretic and the V-I curves are non-linear, displaying a
characteristic s-shape at the melting line Hm(T), which scales as
epsilon(theta)Hm(T,theta). These features are gradually lost when the critical
point H*(theta) is approached. Above H*(theta) the V-I characteristics show a
linear response in the experimentally accessible V-I window, and the transition
becomes reversible. Finally we show that the first order phase transition takes
place between a highly correlated vortex liquid in the field direction and a
solid state of unknown symmetry. As a consequence, the available data support
the scenario for a vortex-line melting rather than a vortex sublimation as
recently suggested [T.Sasagawa et al. PRL 80, 4297 (1998)].


title:
Excitation Spectrum for Quantum Spin Systems with Ladder, Plaquette and
  Mixed-Spin Structures

abstract:
  By using the series expansion techniques, we study the excitation spectrum
for the two-dimensional quantum spin systems with ladder, plaquette and
mixed-spin structures. We calculate the spin excitation gap and thus determine
the phase boundary between the spin-gap phase and the magnetically ordered
phase rather precisely. It is found that the phase diagram obtained improves
fairly well the one previously obtained via the ground-state susceptibility.


title:
Microwave-induced constant voltage steps in surface junctions of
  Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta} single crystals

abstract:
  We have observed the zero-crossing steps in a surface junction of a mesa
structure micro-fabricated on the surface of a Bi-2212 single crystal. With the
application of microwave of frequencies 76 and 94 GHz, the current-voltage
characteristics show clear voltage steps satisfying the ac Josephson relation.
Increasing the microwave power, the heights of the steps show the
Bessel-function behavior up to step number n=4. We confirm that the intrinsic
surface junction meets the criterion for the observation of zero-crossing
steps.


title:
NMR Detection of Temperature-Dependent Magnetic Inhomogeneities in
  URu2Si2

abstract:
  We present 29Si-NMR relaxation and spectral data in URu2Si2. Our echo-decay
experiments detect slowly fluctuating magnetic field gradients. In addition, we
find that the echo-decay shape (time dependence) varies with temperature T and
its rate behaves critically near the Neel temperature TN, indicating a
correlation between the gradient fluctuations and the transition to
small-moment order. T-dependent broadening contributions become visible below
100 Kelvin and saturate somewhat above TN, remaining saturated at lower
temperatures. Together, the line width and shift suggest partial lattice
distortions below TN. We propose an intrinsic minority phase below $T_{\rm N}$
and compare our results with one of the current theoretical models.


title: Transition Matrix Monte Carlo

abstract:
  Although histogram methods have been extremely effective for analyzing data
from Monte Carlo simulations, they do have certain limitations, including the
range over which they are valid and the difficulties of combining data from
independent simulations. In this paper, we describe an complementary approach
to extracting information from Monte Carlo simulations that uses the matrix of
transition probabilities. Combining the Transition Matrix with an N-fold way
simulation technique produces an extremely flexible and efficient approach to
rather general Monte Carlo simulations.


title: Landau quantization and equatorial states on a surface of a nanosphere

abstract:
  The Landau quantization for the electron gas on a surface of sphere is
considered. We show that in the regime of strong fields the lowest energy
states are those with magnetic quantum numbers m of order of Phi/Phi_0, the
number of magnetic flux quanta piercing the sphere. For the electron gas of low
density (semiconducting situation), it leads to the formation of the electronic
stripe on the equator of the sphere in high fields.


title:
Model for crystallization kinetics: Deviations from
  Kolmogorov-Johnson-Mehl-Avrami kinetics

abstract:
  We propose a simple and versatile model to understand the deviations from the
well-known Kolmogorov-Johnson-Mehl-Avrami kinetics theory found in metal
recrystallization and amorphous semiconductor crystallization. We analyze the
kinetics of the transformation and the grain size distribution of the product
material, finding a good overall agreement between our model and available
experimental data. The information so obtained could help to relate the
mentioned experimental deviations due to preexisting anisotropy along some
regions, to certain degree of crystallinity of the amorphous phases during
deposition, or more generally to impurities or roughness of the substrate.


title: Linear spin and orbital wave theory for undoped manganites

abstract:
  We present a linear spin and orbital wave theory to account for the spin and
orbital orderings observed experimentally in undoped manganite. It is found
that the anisotropy of the magnetic structure is closely related to the orbital
ordering, and the Jahn-Teller effect stabilizes the orbital ordering. The phase
diagram and the low energy excitation spectra for both spin and orbital
orderings are obtained. The calculated critical temperatures can be
quantitatively comparable to the experimental data.


title:
Spin resolved Andreev reflection in ferromagnet-superconductor junctions
  with Zeeman splitting

abstract:
  Andreev reflection in ferromagnet-superconductor junctions is derived in a
regime in which Zeeman splitting dominates the response of the superconductor
to an applied magnetic field. Spin-up and spin-down Andreev reflections are
shown to be resolved as voltage is increased. In the metallic limit, the
transition from Andreev to tunnel conductivity in the spin-up channels has a
non trivial behavior when spin polarization is increased. The conductance is
asymmetric in a voltage reversal.


title: Magnetomechanics of mesoscopic wires

abstract:
  We have studied the force in mesoscopic wires in the presence of an external
magnetic field along the wire using a free electron model. We show that the
applied magnetic field can be used to affect the force in the wire. The
magnetic field breaks the degeneracy of the eigenenergies of the conduction
modes, resulting in more structure in the force as a function of wire length.
The use of an external magnetic field is an equilibrium method to control the
number of transporting channels. Under the least favorable circumstances (on
the middle of a low conduction step) one needs about 1.3 T, for a mesoscopic
Bismuth wire, to see an abrupt change in the force, at fixed wire length.


title: Metallic sulphur. "Electronic" mechanism of superconductivity?

abstract:
  It is shown that the rapid increase of the superconducting transition
temperature $T_c$ of sulphur with increasing pressure above 93 GPa does not
contradict with some hypothetical ``electronic'' mechanism of superconductivity
with participation of the electron interaction energy fluctuations. Such
``electronic'' mechanism is supposed to be intrinsic property of the molecular
condensates and corresponds to very high $T_c$. The low $T_c$ of sulphur (10
-17)K is likely connected with the magnetic properties of the sulphur atoms and
molecules. The equation of state for sulphur is obtained. The molar volume of
sulphur at metallization is 10 cm}$^3${/mol. The principal difference between
the ''physical'' and the ''chemical'' type bonds are discussed. Under some
pressure one bond type is changed by another and}${T}_c${may have an extremum
(transition from the Bose condensation to the BCS superconductivity).


title: Dissipative Dynamics of an Open Bose Einstein Condensate

abstract:
  As an atomic Bose Einstein condensate (BEC) is coupled to a source of
uncondensed atoms at the same temperature and to a sink (extraction towards an
atom laser) the idealized description in terms of a Gross-Pitaevsky equation
(GP) no longer holds. Under suitable physical assumptions we show that the
dissipative BEC obeys a Complex Ginzburg Landau equation (CGL) and for some
parameter range it undergoes a space time patterning. As a consequence, the
density of BEC atoms within the trap displays non trivial space time
correlations, which can be detected by monitoring the density profile of the
outgoing atom laser. The patterning condition requires a negative scattering
length, as e.g. in $^7$Li. In such a case we expect a many domain collapsed
regime, rather than a single one as reported for a closed BEC.


title:
Distribution functions and current-correlations in
  normal-metal--superconductor hetero-structures

abstract:
  We introduce electron-like and hole-like distribution functions, which
determine the currents and the fluctuation spectra of the currents measured at
a normal-conductor--superconductor hetero-structure. These distribution
functions are expressed with the help of newly defined partial densities of
states for hetero-structures. Voltage measurements using a weakly coupled
contact on such a structure show the absence of a contact resistance to the
superconducting reservoir and illustrate how the interface to the
superconductor acts as an Andreev mirror. We also discuss the current-current
correlations measured at two normal contacts and argue that the appearance of
positive correlations is a purely mesoscopic effect, which vanishes in the
limit of a large number of channels and in the average over an ensemble.


title:
Creation of persistent current and vortex in a Bose-Einstein condensate
  of alkali-metal atoms

abstract:
  It is shown theoretically that a persistent current can be continuously
created in a Bose-Einstein condensate (BEC) of alkali atoms confined in a
multiply connected region by making use of a spin-degree of freedom of the
order parameter of a BEC. We demonstrate that this persistent current is easily
transformed into a vortex. Relaxation processes of these BEC after the
confining field is turned off are also studied so that our analyses are
compared with time of flight experiments. The results are shown to clearly
reflect the existence of a persistent current.


title: Renormalization group analysis of the 2D Hubbard model

abstract:
  Salmhofer [Commun. Math. Phys. 194, 249 (1998)] has recently developed a new
renormalization group method for interacting Fermi systems, where the complete
flow from the bare action of a microscopic model to the effective low-energy
action, as a function of a continuously decreasing infrared cutoff, is given by
a differential flow equation which is local in the flow parameter. We apply
this approach to the repulsive two-dimensional Hubbard model with nearest and
next-nearest neighbor hopping amplitudes. The flow equation for the effective
interaction is evaluated numerically on 1-loop level. The effective
interactions diverge at a finite energy scale which is exponentially small for
small bare interactions. To analyze the nature of the instabilities signalled
by the diverging interactions we extend Salmhofers renormalization group for
the calculation of susceptibilities. We compute the singlet superconducting
susceptibilities for various pairing symmetries and also charge and spin
density susceptibilities. Depending on the choice of the model parameters
(hopping amplitudes, interaction strength and band-filling) we find
commensurate and incommensurate antiferromagnetic instabilities or d-wave
superconductivity as leading instability. We present the resulting phase
diagram in the vicinity of half-filling and also results for the density
dependence of the critical energy scale.


title:
Electron delocalization and multifractal scaling in electrified random
  chains

abstract:
  Electron localization property of a random chain changing under the influence
of a constant electric field has been studied. We have adopted the multifractal
scaling formalism to explore the possible localization behavior in the system.
We observe that the possible localization behavior with the increase of
electric field is not systematic and shows strong instabilities associated with
the local probability variation over the length of the chain. The multifractal
scaling study captures the localization aspects along with strong instability
when the electric field is changed by infinitesimal steps for a reasonably
large system size.


title: Dynamic scaling of I-V data for the neutral 2D Coulomb gas

abstract:
  The value of the dynamic critical exponent z has been studied for
experimental two-dimensional superconducting and Josephson Junction array
systems in zero magnetic field via the Fisher-Fisher-Huse dynamic scaling
analysis. We found z~5.6, a relatively large value indicative of non-diffusive
dynamics. We extend this work here to simulational I-V curves that are also
found to be characterized by the same large value of z.


title:
Theta-point behavior of diluted polymer solutions: Can one observe the
  universal logarithmic corrections predicted by field theory?

abstract:
  In recent large scale Monte-Carlo simulations of various models of
Theta-point polymers in three dimensions Grassberger and Hegger found
logarithmic corrections to mean field theory with amplitudes much larger than
the universal amplitudes of the leading logarithmic corrections calculated by
Duplantier in the framework of tricritical O(n) field theory. To resolve this
issue we calculate the universal subleading correction of field theory, which
turns out to be of the same order of magnitude as the leading correction for
all chain lengths available in present days simulations. Borel resummation of
the renormalization group flow equations also shows the presence of such large
corrections. This suggests that the published simulations did not reach the
asymptotic regime. To further support this view, we present results of
Monte-Carlo simulations on a Domb-Joyce like model of weakly interacting random
walks. Again the results cannot be explained by keeping only the leading
corrections, but are in fair accord with our full theoretical result. The
corrections found for the Domb-Joyce model are much smaller than those for
other models, which clearly shows that the effective corrections are not yet in
the asymptotic regime. All together our findings show that the existing
simulations of Theta-polymers are compatible with tricritical field theory
since the crossover to the asymptotic regime is very slow. Similar results were
found earlier for self avoiding walks at their upper critical dimension d=4.


title: Flux Lattice Melting and the onset of H_c2 fluctuations

abstract:
  The flux lattice melting temperature in optimally doped YBCO has been shown
to be very close to that of the onset of fluctuations around H_{c2}(T). Here,
we present a theoretical argument in support of the idea that this occurs
because the increased strength of the fluctuations as a function of magnetic
field pushes away the first order flux lattice melting transition. The argument
is based on hydrodynamic considerations (the Hansen-Verlet freezing criterion).
It is not specific to high-temperature superconductors and can be generalized
to other systems.


title:
Propylene Carbonate Reexamined: Mode-Coupling $\beta$ Scaling without
  Factorisation ?

abstract:
  The dynamic susceptibility of propylene carbonate in the moderately viscous
regime above $T_{\rm c}$ is reinvestigated by incoherent neutron and
depolarised light scattering, and compared to dielectric loss and solvation
response. Depending on the strength of $\alpha$ relaxation, a more or less
extended $\beta$ scaling regime is found. Mode-coupling fits yield consistently
$\lambda=0.72$ and $T_{\rm c}=182 $K, although different positions of the
susceptibility minimum indicate that not all observables have reached the
universal asymptotics.


title:
A reliable Pade analytical continuation method based on a high accuracy
  symbolic computation algorithm

abstract:
  We critique a Pade analytic continuation method whereby a rational polynomial
function is fit to a set of input points by means of a single matrix inversion.
This procedure is accomplished to an extremely high accuracy using a novel
symbolic computation algorithm. As an example of this method in action we apply
it to the problem of determining the spectral function of a one-particle
thermal Green's function known only at a finite number of Matsubara frequencies
with two example self energies drawn from the T-matrix theory of the Hubbard
model. We present a systematic analysis of the effects of error in the input
points on the analytic continuation, and this leads us to propose a procedure
to test quantitatively the reliability of the resulting continuation, thus
eliminating the black magic label frequently attached to this procedure.


title:
Quantitative Analysis of Experimental and Synthetic Microstructures for
  Sedimentary Rock

abstract:
  A quantitative comparison between the experimental microstructure of a
sedimentary rock and three theoretical models for the same rock is presented.
The microstructure of the rock sample (Fontainebleau sandstone) was obtained by
microtomography. Two of the models are stochastic models based on correlation
function reconstruction, and one model is based on sedimentation, compaction
and diagenesis combined with input from petrographic analysis. The porosity of
all models closely match that of the experimental sample and two models have
also the same two point correlation function as the experimental sample. We
compute quantitative differences and similarities between the various
microstructures by a method based on local porosity theory. Differences are
found in the degree of anisotropy, and in fluctuations of porosity and
connectivity. The stochastic models differ strongly from the real sandstone in
their connectivity properties, and hence need further refinement when used to
model transport.


title: Self-energy of image states on copper surfaces

abstract:
  We report extensive calculations of the imaginary part of the electron
self-energy in the vicinity of the (100) and (111) surfaces of Cu. The
quasiparticle self-energy is computed by going beyond a free-electron
description of the metal surface, either within the GW approximation of
many-body theory or with inclusion, within the GW$\Gamma$ approximation, of
short-range exchange-correlation effects. Calculations of the decay rate of the
first three image states on Cu(100) and the first image state on Cu(111) are
also reported, and the impact of both band structure and many-body effects on
the electron relaxation process is discussed.


title:
Exact solution of a modified El Farol's bar problem: Efficiency and the
  role of market impact

abstract:
  We discuss a model of heterogeneous, inductive rational agents inspired by
the El Farol Bar problem and the Minority Game. As in markets, agents interact
through a collective aggregate variable -- which plays a role similar to price
-- whose value is fixed by all of them. Agents follow a simple
reinforcement-learning dynamics where the reinforcement, for each of their
available strategies, is related to the payoff delivered by that strategy. We
derive the exact solution of the model in the ``thermodynamic'' limit of
infinitely many agents using tools of statistical physics of disordered
systems. Our results show that the impact of agents on the market price plays a
key role: even though price has a weak dependence on the behavior of each
individual agent, the collective behavior crucially depends on whether agents
account for such dependence or not. Remarkably, if the adaptive behavior of
agents accounts even ``infinitesimally'' for this dependence they can, in a
whole range of parameters, reduce global fluctuations by a finite amount. Both
global efficiency and individual utility improve with respect to a ``price
taker'' behavior if agents account for their market impact.

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


title:
Exact Solution of a Model for Crowding and Information Transmission in
  Financial Markets

abstract:
  An exact solution is presented to a model that mimics the crowding effect in
financial markets which arises when groups of agents share information. We show
that the size distribution of groups of agents has a power law tail with an
exponential cut-off. As the size of these groups determines the supply and
demand balance, this implies heavy tails in the distribution of price
variation. The moments of the distribution are calculated, as well as the
kurtosis. We find that the kurtosis is large for all model parameter values and
that the model is not self-organizing.


title: Quantization of the free charge carriers on InSb at room temperature

abstract:
  A new method of self-consistent quantum calculation of the density of the
space charge near the surface of a crystal is carried out for the semiconductor
with nonparabolic (Kane) dispersion law of bands. The remarkable feature is the
solution of the Schroedinger equation for electrons and holes in the energy
range, including both bound energy states and states in the continuum.
Theoretical voltage-capacitance dependence is calculated and coincides with
experimental data. The dependence of the electron mass and surface mobility
from the value of surface potential are analyzed.


title:
Four-fold basal plane anisotropy of the nonlocal magnetization of
  YNi2B2C

abstract:
  Studies of single crystal YNi2B2C have revealed a four-fold anisotropy of the
equilibrium magnetization in the square crystallographic basal plane. This pi/2
periodicity occurs deep in the superconductive mixed state. In this crystal
symmetry, an ordinary superconductive mass anisotropy (as in usual London
theory) allows only a constant, isotropic response. In contrast, the
experimental results are well described by generalized London theory
incorporating non-local electrodynamics, as needed for this clean,
intermediate-k superconductor.


title: Critical Quantum Chaos in 2D Disordered Systems with Spin-Orbit Coupling

abstract:
  We examine the validity of the recently proposed semi-Poisson level spacing
distribution function P(S), which characterizes `critical quantum chaos', in 2D
disordered systems with spin-orbit coupling. At the Anderson transition we show
that the semi-Poisson P(S) can describe closely the critical distribution
obtained with averaged boundary conditions, over Dirichlet in one direction
with periodic in the other and Dirichlet in both directions. We also obtain a
sub-Poisson linear number variance $\Sigma_{2}(E)\approx \chi_{0}+ \chi E$,
with asymptotic value $\chi\approx0.07$. The obtained critical statistics,
intermediate between Wigner and Poisson, is relevant for disordered systems and
chaotic models.


title: Comment on "Density Functional Simulation of a Breaking Nanowire"

abstract:
  In a recent Letter, Nakamura et al. [Phys. Rev. Lett. 82, 1538 (1999)]
described first principles calculations for a breaking Na nanocontact. Their
system consists of a periodic one-dimensional array of supercells, each of
which contains 39 Na atoms, originally forming a straight, crystalline wire
with a length of 6 atoms. The system is elongated by increasing the length of
the unit cell. At each step, the atomic configuration is relaxed to a new local
equilibrium, and the tensile force is evaluated from the change of the total
energy with elongation. Aside from a discontinuity of the force occuring at the
transition from a crytalline to an amorphous configuration during the early
stages of elongation, they were unable to identify any simple correlations
between the force and the number of electronic modes transmitted through the
contact. An important question is whether their model is realistic, i.e.,
whether it can be compared to experimental results obtained for a single
nanocontact between two macroscopic pieces of metal. In this Comment, we
demonstrate that with such a small unit cell, the interference effects between
neighboring contacts are of the same size as the force oscillations in a single
nanocontact.


title: Probing Pairing Symmetry in Sr$_2$RuO$_4$

abstract:
  We study the spin dynamics in a p-wave superconductor at the nesting vector
associated with $\alpha$ and $\beta$ bands in Sr$_2$RuO$_4$. We find a
collective mode at the nesting vector in the superconducting phase identified
as the odd-parity pairing state which breaks time reversal symmetry. This mode
in the spin channel only exists in the p-wave superconductor, not in s- or
d-wave superconductors. We propose that probing this mode would clarify the
pairing symmetry in Sr$_2$RuO$_4$. The possibility of second superconducting
phase transition is also discussed.


title:
Localization and Absorption of Light in 2D Composite Metal-Dielectric
  Films at the Percolation Threshold

abstract:
  We study in this paper the localization of light and the dielectric
properties of thin metal-dielectric composites at the percolation threshold and
around a resonant frequency where the conductivities of the two components are
of the same order. In particular, the effect of the loss in metallic components
are examined. To this end, such systems are modelized as random $L-C$ networks,
and the local field distribution as well as the effective conductivity are
determined by using two different methods for comparison: an exact resolution
of Kirchoff equations, and a real space renormalization group method. The
latter method is found to give the general behavior of the effective
conductivity but fails to determine the local field distribution. It is also
found that the localization still persists for vanishing losses. This result
seems to be in agreement with the anomalous absorption observed experimentally
for such systems.


title:
Possible Spin-Singlet Superconductivity in (TMTSF)2X: Superconducting
  Transition Temperature in a Magnetic Field

abstract:
  We study the transition temperature Tc(H) of a quasi-one-dimensional (Q1D)
superconductivity which is derived from the quantum effect of an electron
motion in a strong magnetic field. We calculate Tc(H) of both isotropic and
anisotropic superconductivity by taking account of the optimal momentum of the
Cooper pairs and the effect of higher harmonic terms along second conducting
axis in the tight-binding model. We find that, although Tc(H) of the
spin-singlet superconductivity is suppressed strongly by the Zeeman effect, the
suppression of Tc(H) is not very severe if we take the optimal pair-momentum
and the higher harmonic terms into account. The obtained Tc(H) for the
spin-singlet superconductivity is consistent with the experimental results in
TMTSF salts.


title: Scanning Tunneling Microscopy of a Luttinger Liquid

abstract:
  Explicit predictions for Scanning Tunneling Microscopy (STM) on interacting
one-dimensional electron systems are made using the Luttinger liquid formalism.
The STM current changes with distance from an impurity or boundary in a
characteristic way, which reveals the spin-charge separation and the
interaction strength in the system. The current exhibits Friedel-like
oscillations, but also carries additional modulated behavior as a function of
voltage and distance, which shows the spin-charge separation in real space.
Moreover, very close to the boundary the current is strongly reduced, which is
an indication of the interaction strength in the system.


title:
Spin-polarized transport and Andreev reflection in
  semiconductor/superconductor hybrid structures

abstract:
  We show that spin-polarized electron transmission across
semiconductor/superconductor (Sm/S) hybrid structures depends sensitively on
the degree of spin polarization as well as the strengths of potential and
spin-flip scattering at the interface. We demonstrate that increasing the Fermi
velocity mismatch in the Sm and S regions can lead to enhanced junction
transparency in the presence of spin polarization. We find that the Andreev
reflection amplitude at the superconducting gap energy is a robust measure of
the spin polarization magnitude, being independent of the strengths of
potential and spin-flip scattering and the Fermi velocity of the
superconductor.


title:
Interaction Round a Face DMRG method applied to rotational invariant
  quantum spin chains

abstract:
  An interaction-round-a-face density-matrix renormalization-group (IRF-DMRG)
method is developed for higher integer spin chain models which are rotational
invariant. The expressions of the IRF weights associated with the
nearest-neighbor spin-S interaction ${\bf S}_{i} \cdot {\bf S}_{i+1}$ are
derived explicitly. Using the IRF-DMRG with these IRF weights, the Haldane gaps
$\Delta$ and the ground state energy densities $e_{0}$ for both S=1 and S=2
isotropic antiferromagnetic Heisenberg quantum spin chains are calculated by
keeping up to only m=90 states.


title: Many-body approach to the dynamics of batch learning

abstract:
  Using the cavity method and diagrammatic methods, we model the dynamics of
batch learning of restricted sets of examples, widely applicable to general
learning cost functions, and fully taking into account the temporal
correlations introduced by the recycling of the examples.


title:
Critical Properties of One-Dimensional Electron Systems with
  $1/r^{\alpha}$-type Long-Range Interactions

abstract:
  We study the critical properties of one-dimensional electron systems with the
$1/r^{\alpha}$-type long-range interaction ($\alpha \ge 1$). Using bosonization
methods, we discuss how the 1D system exhibits a crossover from the
Tomonaga-Luttinger liquid to the Wigner crystal when $\alpha \to 1$. In
particular, a detailed analysis is given to the Fermi-edge singularity
phenomena when a {\it mobile} core hole is created suddenly in the valence
band. It is found that the effect due to the core-hole motion on the Fermi-edge
singularity is gradually suppressed as the system approaches the Wigner
crystal, and its critical behavior is dominated by the ordinary impurity
forward-scattering inherent in a static hole.


title:
Competing frustration and dilution effects on the antiferrromagnetism in
  La(2-x)Sr(x)Cu(1-z)Zn(z)O(4)

abstract:
  The combined effects of hole doping and magnetic dilution on a lamellar
Heisenberg antiferromagnet are studied in the framework of the frustration
model. Magnetic vacancies are argued to remove some of the frustrating bonds
generated by the holes, thus explaining the increase in the temperature and
concentration ranges exhibiting three dimensional long range order. The
dependence of the Neel temperature on both hole and vacancy concentrations is
derived quantitatively from earlier renormalization group calculations for the
non--dilute case, and the results reproduce experimental data with no new
adjustable parameters.


title: Zero-field spin splitting in InAs-AlSb quantum wells revisited

abstract:
  We present magnetotransport experiments on high-quality InAs-AlSb quantum
wells that show a perfectly clean single-period Shubnikov-de Haas oscillation
down to very low magnetic fields. In contrast to theoretical expectations based
on an asymmetry induced zero-field spin splitting, no beating effect is
observed. The carrier density has been changed by the persistent photo
conductivity effect as well as via the application of hydrostatic pressure in
order to influence the electric field at the interface of the electron gas.
Still no indication of spin splitting at zero magnetic field was observed in
spite of highly resolved Shubnikov- de Haas oscillations up to filling factors
of 200. This surprising and unexpected result is discussed in view of other
recently published data.


title: Excitation spectra of the two dimensional Kondo insulator

abstract:
  We present a Quantum Monte Carlo study of the temperature dependent dynamics
of the two-dimensional Kondo insulator. Working at the so-called symmetrical
point allows to perform minus-sign free QMC simulations. Study of the
temperature dependence of the single-particle Green's function and the
dynamical spin correlation function provides evidence for two characteristic
temperatures, which we associate with the Kondo and coherence temperature,
whereby the system shows a metal-insulator transition at the coherence
temperature. The data shows evidence for two distinct types of spin excitations
and we show that despite strong antiferromagnetic ordering at low temperature
the system cannot be described by spin-density-wave theory.


title: Studying Self-Organized Criticality with Exactly Solved Models

abstract:
  This is a somewhat expanded version of the notes of a series of lectures
given at Lausanne and Stellenbosch in 1998-99. They are intended to provide a
pedagogical introduction to the abelian sandpile model of self-organized
criticality, and its related models : the q=0 state Potts model, Takayasu
aggregation model, the voter model, spanning trees, Eulerian walkers model etc.
It provides an overview of the known results, and explains the equivalence of
these models. Some open questions are discussed in the concluding section.


title: Time Delay Correlations and Resonances in 1D Disordered Systems

abstract:
  The frequency dependent time delay correlation function $K(\Omega)$ is
studied analytically for a particle reflected from a finite one-dimensional
disordered system. In the long sample limit $K(\Omega)$ can be used to extract
the resonance width distribution $\rho(\Gamma)$. Both quantities are found to
decay algebraically as $\Gamma^{-\nu}$, and $\Omega^{-\nu}$, $\nu\simeq 1.25$
in a large range of arguments. Numerical calculations for the resonance width
distribution in 1D non-Hermitian tight-binding model agree reasonably with the
analytical formulas.


title:
Tunneling measurements of the coulomb pseudogap in a two-dimensional
  electron system in a quantizing magnetic field

abstract:
  We study the Coulomb pseudogap for tunneling into the two-dimensional
electron system of high-mobility (Al,Ga)As/GaAs heterojunctions subjected to a
quantizing magnetic field at filling factor $\nu \leq 1$.
  Tunnel current-voltage characteristics show that for the double maximum
observed in the tunnel resistance at $\nu \approx 1$ the pseudogap is linear in
energy with a slope that depends on filling factor, magnetic field, and
temperature. We give a qualitative account of the filling factor dependence of
the pseudogap slope and we confirm the recently reported appearance of another
relaxation time for tunneling at $\nu \approx 1$. For the tunnel resistance
peaks at $\nu=1/3$ and 2/3 a completely different behaviour of the
current-voltage curves is found and interpreted as manifestation of the
fractional gap.


title:
Forest-fire models as a bridge between different paradigms in
  Self-Organized Criticality

abstract:
  We turn the stochastic critical forest-fire model introduced by Drossel and
Schwabl (PRL 69, 1629, 1992) into a deterministic threshold model. This new
model has many features in common with sandpile and earthquake models of
Self-Organized Criticality. Nevertheless our deterministic forest-fire model
exhibits in detail the same macroscopic statistical properties as the original
Drossel and Schwabl model. We use the deterministic model and a related
semi-deterministic version of the model to elaborate on the relation between
forest-fire, sandpile and earthquake models.


title:
Polyethylene under tensile load: strain energy storage and breaking of
  linear and knotted alkanes probed by first-principles molecular dynamics
  calculations

abstract:
  The mechanical resistance of a polyethylene strand subject to tension and the
way its properties are affected by the presence of a knot is studied using
first-principles molecular dynamics calculations. The distribution of strain
energy for the knotted chains has a well-defined shape that is very different
from the one found in the linear case. The presence of a knot significantly
weakens the chain in which it is tied. Chain rupture invariably occurs just
outside the entrance to the knot, as is the case for a macroscopic rope.


title: Limits to Sympathetic Evaporative Cooling of a Two-Component Fermi Gas

abstract:
  We find a limit cycle in a quasi-equilibrium model of evaporative cooling of
a two-component fermion gas. The existence of such a limit cycle represents an
obstruction to reaching the quantum ground state evaporatively. We show that
evaporatively the \beta\mu ~ 1. We speculate that one may be able to cool an
atomic fermi gas further by photoassociating dimers near the bottom of the
fermi sea.


title:
Exact Study of the Effect of Level Statistics in Ultrasmall
  Superconducting Grains

abstract:
  The reduced BCS model that is commonly used for ultrasmall superconducting
grains has an exact solution worked out long ago by Richardson in the context
of nuclear physics. We use it to check the quality of previous treatments of
this model, and to investigate the effect of level statistics on pairing
correlations. We find that the ground state energies are on average somewhat
lower for systems with non-uniform than uniform level spacings, but both have
an equally smooth crossover from the bulk to the few-electron regime. In the
latter, statistical fluctuations in ground state energies strongly depend on
the grain's electron number parity.


title: Two-state shear diagrams for complex fluids in shear flow

abstract:
  The possible ``phase diagrams'' for shear-induced phase transitions between
two phases are collected. We consider shear-thickening and shear-thinning
fluids, under conditions of both common strain rate and common stress in the
two phases, and present the four fundamental shear stress vs. strain-rate
curves and discuss their concentration dependence. We outline how to construct
more complicated phase diagrams, discuss in which class various experimental
systems fall, and sketch how to reconstruct the phase diagrams from rheological
measurements.


title: Modification of 1D Ballistic Transport using an Atomic Force Microscope

abstract:
  We have used the scanning charged tip of an Atomic Force Microscope (AFM) to
produce images of the conductance variation of a quantised 1D ballistic
channel. The channel was formed using electron beam defined 700 nm wide split
gate surface electrodes over a high mobility GaAs/AlGaAs heterostructure with a
two dimensional electron gas (2DEG) 98 nm beneath the surface. We operate the
AFM at 1.5 K and 4.2 K in magnetic fields up to 2 T to observe several
phenomena. With a dc voltage on the AFM tip we have produced conduction images
of the tip potential perturbation, as the channel is a sensitive probe of the
electrostatic potential. We have also performed gate sweeps with the tip at a
series of points across the width of the channel. The observed structure in
transconductance corresponds to the theoretical electron density for the first
three sub-bands. When certain gates were biased near pinch off, stable two
level switching was observed in the images. We were able to control the state
of the switch with the gate bias and tip position, and so roughly locate the
position of the switching source. One of the responsible defect systems was
located beneath the 2DEG, and due to screening of the tip potential, the image
reveals the 1D channel. By asymmetrically biasing the two gate electrodes we
have produced images showing a total channel movement of 102 nm across the
width of the channel, but accompanied by 201 nm of movement along the length of
the channel which is due to imperfections in the surface electrodes and
disorder in the doping layer.


title: The statistical mechanics of turbo codes

abstract:
  The "turbo codes", recently proposed by Berrou et. al. are written as a
disordered spin Hamiltonian. It is shown that there is a threshold Theta such
that for signal to noise ratios v^2 / w^2 > Theta, the error probability per
bit vanishes in the thermodynamic limit, i.e. the limit of infinitly long
sequences. The value of the threshold has been computed for two particular
turbo codes. It is found that it depends on the code. These results are
compared with numerical simulations.


title: Dynamic spin susceptibility in the t-J model

abstract:
  Dynamic spin susceptibility is calculated for the t-J model in the
paramagnetic phase by applying the memory function method in terms of the
Hubbard operators. A self-consistent system of equations for the memory
function is obtained within the mode coupling approximation. Both itinerant
hole excitations and localized spin fluctuations give contributions to the
memory function. Spin dynamics have a diffusive character in the hydrodynamic
limit; spin-wave-like excitations are regained in the high-frequency region.


title:
Normal Fermi Liquid Behavior of Quasiholes in the Spin-Polaron Model for
  Copper Oxides

abstract:
  Based on the t-J model and the self-consistent Born approximation, the
damping of quasiparticle hole states near the Fermi surface is calculated in a
low doping regime. Renormalization of spin-wave excitations due to hole doping
is taken into account. The damping is shown to be described by a familiar form
$\text{Im}\Sigma({\bf k}^{\prime},\epsilon)\propto (\epsilon^{2}/
\epsilon_{F})\ln(\epsilon/ \epsilon_{F})$ characteristic of the 2-dimensional
Fermi liquid, in contrast with the earlier statement reported by Li and Gong
[Phys. Rev. B {\bf 51}, 6343 (1995)] on the marginal Fermi liquid behavior of
quasiholes.


title: Three-dimensional dendrite tip morphology at low undercooling

abstract:
  We investigate the three-dimensional morphology of the dendrite tip using the
phase-field method. We find that, for low undercoolings, this morphology is
ostensibly independent of anisotropy strength except for a localized shape
distortion near the tip that only affects the value of the tip radius $\rho$
(which is crudely approximated by $\rho\approx (1-\alpha)\rho_{Iv}$ where
$\rho_{Iv}$ is the Ivantsov tip radius of an isothermal paraboloid with the
same tip velocity and $\alpha$ is the stiffness anisotropy). The universal tip
shape, which excludes this distortion, is well fitted by the form $z=-r^2/2+A_4
r^4\cos 4\phi$ where $|z|$ is the distance from the tip and all lengths are
scaled by $\rho_{Iv}$. This fit yields $A_4$ in the range $0.004-0.005$ in good
quantitative agreement with the existing tip morphology measurements in
succinonitrile [LaCombe et al., Phys. Rev. E {\bf 52}, 2778 (1995)], which are
reanalyzed here and found to be consistent with a single $\cos 4\phi$ mode
non-axisymmetric deviation from a paraboloid. Moreover, the fin shape away from
the tip is well fitted by the power law $z=-a |x|^{5/3}$ with $a\approx 0.68$.
Finally, the characterization of the operating state of the dendrite tip is
revisited in the light of these results.


title:
Partially confined excitons in semiconductor nanocrystals with a finite
  size dielectric interface

abstract:
  The combined effect of finite potential barriers and dielectric mismatch
between dot and matrix on excitonic properties of semiconductor quantum dots
has been studied. To avoid the unphysical divergence in the self-polarization
energy which arises for the simplest and profusely adopted step-like model of
the dielectric interface, we proposed a realistic (finite size) smooth profile
for the dielectric interface. We have found that the excitonic binding energy
can be either higher than the corresponding one to complete confinement by
infinite barriers or essentially zero for a wide range of dot sizes depending
on the thickness of the dielectric interface.


title: Orbital Polarization and Fluctuation in Manganese Oxides

abstract:
  In a form which include the mother compound, we studied the extended
Hubbard-type model with the orbital degeneracy for any doping concentration.
Meanfield phase diagram and the spin wave excitation in RPA are calculated. It
turned out that a large orbital polarization is essential to reproduce the
magnetic phases depending on the doping concentration. Base on this result, we
discuss the spin canting, the spin wave dispersion, and spin wave stiffness
from the standing point of the large orbital polarization. We also discuss the
orbital fluctuation which turned out to be important in the ferromagnetic
metallic region where the CMR is observed. Spin wave softening near the zone
boundary is also discussed in this context.


title:
Origin of Superconductivity in 2D-Organic Compounds and High-T_c
  Cuprates

abstract:
  A phase diagram is drawn in a parameter space of the nearly half-filled
single band two-dimensional Hubbard model with U/t, t'/t and n as the
parameters, U, t, t' and n being the on-site interaction, the nearest and
second nearest neighbor transfer matrices and the number of electrons per site,
respectively. The parameter space is chosen so as to include the models for the
high-T_c cuprates as well as 2D-organic superconductors. The superconducting
and antiferromagnetic instability surfaces are drawn from the results of
calculations by using the spin fluctuation theory within the fluctuation
exchange approximation. The metal-insulator transition surface is also
indicated in the diagram. From this phase diagram we discuss the possibility of
the spin fluctuation mechanism being the common origin of the superconductivity
in the high-T_c cuprates and 2D-organic compounds.


title:
Renormalization-group theory for rotating 4He near the superfluid
  transition

abstract:
  The influence of a uniform rotation with frequency Omega on the critical
behavior of liquid 4He near T_lambda is investigated. We apply our recently
developed approach which is a renormalization-group theory based on model F
starting with the calculation of the Green's function in Hartree approximation.
We calculate the specific heat, the correlation length, and the thermal
resistivity tensor as functions of the temperature T for fixed values of the
rotation frequency Omega. For nonzero Omega we find that all physical
quantities are smooth near T_lambda so that the superfluid transition is a
smooth crossover. We define a frequency-dependent transition temperature
T_lambda(Omega) by the maximum of the specific heat and present a power law
prediction. For T<T_lambda(Omega) we find mutual friction between the
superfluid and the normal-fluid component caused implicitly by the motion of
vortex lines and calculate the Vinen coefficients B and B'.


title:
Path-decomposition expansion and edge effects in a confined magnetized
  free-electron gas

abstract:
  Path-integral methods can be used to derive a `path-decomposition expansion'
for the temperature Green function of a magnetized free-electron gas confined
by a hard wall. With the help of this expansion the asymptotic behaviour of the
profiles for the excess particle density and the electric current density far
from the edge is determined for arbitrary values of the magnetic field
strength. The asymptotics are found to depend sensitively on the degree of
degeneracy. For a non-degenerate electron gas the asymptotic profiles are
essentially Gaussian (albeit modulated by a Bessel function), on a length scale
that is a function of the magnetic field strength and the temperature. For a
completely degenerate electron gas the asymptotic behaviour is again
proportional to a Gaussian, with a scale that is the magnetic length in this
case. The prefactors are polynomial and logarithmic functions of the distance
from the wall, that depend on the number of filled Landau levels $n$. As a
consequence, the Gaussian asymptotic decay sets in at distances that are large
compared to the magnetic length multiplied by $\sqrt{n}$.


title: Detailed Topography of the Fermi Surface of Sr2RuO4

abstract:
  We apply a novel analysis of the field and angle dependence of the
quantum-oscillatory amplitudes in the unconventional superconductor Sr2RuO4 to
map its Fermi surface in unprecedented detail, and to obtain previously
inaccessible information on the band dispersion. The three quasi-2D Fermi
surface sheets not only exhibit very diverse magnitudes of warping, but also
entirely different dominant warping symmetries. We use the data to reassess
recent results on c-axis transport phenomena.


title: Solvable models of Glass Transition

abstract:
  Simple statistical agglomeration models can provide a universal link between
the local structure and the glass transition temperature in network glasses. We
first stress the physical features of the models and the hypothesis made, and
then show how to define the glass transition temperature. The models are
applied to various types of binary, ternary and multicomponent chalcogenide
glass networks and the predictions compared to experimental data.


title:
Dephasing in semiconductor-superconductor structures by coupling to a
  voltage probe

abstract:
  We study dephasing in semiconductor-superconductor structures caused by
coupling to a voltage probe. We consider structures where the semiconductor
consists of two scattering regions between which partial dephasing is possible.
As a particular example we consider a situation with a double-barrier junction
in the normal region. For a single-mode system we study the conductance both as
a function of the position of the Fermi level and as a function of the barrier
transparency. At resonance, where the double-barrier is fully transparent, we
study the suppression of the ideal factor-of-two enhancement of the conductance
when a finite coupling to the voltage probe is taken into account.


title: Long Range Magnetic Order and the Darwin Lagrangian

abstract:
  We simulate a finite system of $N$ confined electrons with inclusion of the
Darwin magnetic interaction in two- and three-dimensions. The lowest energy
states are located using the steepest descent quenching adapted for velocity
dependent potentials. Below a critical density the ground state is a static
Wigner lattice. For supercritical density the ground state has a non-zero
kinetic energy. The critical density decreases with $N$ for exponential
confinement but not for harmonic confinement. The lowest energy state also
depends on the confinement and dimension: an antiferromagnetic cluster forms
for harmonic confinement in two dimensions.


title: Intense AC field driven superlattices with barrier width dimerization

abstract:
  The evolution of two coupled mini-bands, generated by alternating barrier
widths dimerization of a superlattice, driven by intense AC fields is
investigated. The present model delivers a useful framework for the transition
between the analytical high frequency regime and the extreme low frequency
limit described by models based on Fukuyama's {\it et al.} (Phys. Rev. B {\bf
8}, 5579 (1973)) proposal.