title:
Dynamic criticality in driven disordered systems: Role of depinning and
  driving rate in Barkhausen noise

abstract:
  We study Barkhausen noise in a diluted two-dimensional Ising model with the
extended domain wall and weak random fields occurring due to coarse graining.
We report two types of scaling behavior corresponding to (a) low disorder
regime where a single domain wall slips through a series of positions when the
external field is increased, and (b) large disorder regime, which is
characterized with nucleation of many domains. The effects of finite
concentration of nonmagnetic ions and variable driving rate on the scaling
exponents is discussed in both regimes. The universal scaling behavior at low
disorder is shown to belong to a class of critical dynamic systems, which are
described by a fixed point of the stochastic transport equation with
self-consistent disorder correlations.


title: New Tetrahedral Global Minimum for the 98-atom Lennard-Jones Cluster

abstract:
  A new atomic cluster structure corresponding to the global minimum of the
98-atom Lennard-Jones cluster has been found using a variant of the
basin-hopping global optimization algorithm. The new structure has an unusual
tetrahedral symmetry with an energy of -543.665361, which is 0.022404 lower
than the previous putative global minimum. The new LJ_98 structure is of
particular interest because its tetrahedral symmetry establishes it as one of
only three types of exceptions to the general pattern of icosahedral structural
motifs for optimal LJ microclusters. Similar to the other exceptions the global
minimum is difficult to find because it is at the bottom of a narrow funnel
which only becomes thermodynamically most stable at low temperature.


title:
Aspects of the dynamics of colloidal suspensions: Further results of the
  mode-coupling theory of structural relaxation

abstract:
  Results of the idealized mode-coupling theory for the structural relaxation
in suspensions of hard-sphere colloidal particles are presented and discussed
with regard to recent light scattering experiments. The structural relaxation
becomes non-diffusive for long times, contrary to the expectation based on the
de Gennes narrowing concept. A semi-quantitative connection of the wave vector
dependences of the relaxation times and amplitudes of the final
$\alpha$-relaxation explains the approximate scaling observed by Segr{\`e} and
Pusey [Phys. Rev. Lett. {\bf 77}, 771 (1996)]. Asymptotic expansions lead to a
qualitative understanding of density dependences in generalized Stokes-Einstein
relations. This relation is also generalized to non-zero frequencies thereby
yielding support for a reasoning by Mason and Weitz [Phys. Rev. Lett {\bf 74},
1250 (1995)]. The dynamics transient to the structural relaxation is discussed
with models incorporating short-time diffusion and hydrodynamic interactions
for short times.


title: Exact Solutions of a Model for Granular Avalanches

abstract:
  We present exact solutions of the non-linear {\sc bcre} model for granular
avalanches without diffusion. We assume a generic sandpile profile consisting
of two regions of constant but different slope. Our solution is constructed in
terms of characteristic curves from which several novel predictions for
experiments on avalanches are deduced: Analytical results are given for the
shock condition, shock coordinates, universal quantities at the shock, slope
relaxation at large times, velocities of the active region and of the sandpile
profile.


title: Impurity-induced polaritons in a one-dimensional chain

abstract:
  A detailed analytical study of an impurity induced polariton band arising
inside a spectral gap between lower and upper polariton branches is presented.
Using the microcanonical method, we calculate the density of states and
localization length of the impurity polaritons. Analytical results are compared
with numerical simulations and excellent agreement is found.


title:
Successive crossover from ordinary Born scattering to multiple
  scattering to localization - A delay time analysis in electronically random
  systems

abstract:
  We have studied the reflection delay time distribution from a one-dimensional
electronically random system of finite length. We show that the average delay
time is a non-monotonic function of the strength of the disorder and exhibits
three qualitatively different regimes. In the weak disorder limit the
scattering is dominated by the ordinary Born scattering. For the intermediate
strengths of disorder a well defined regime of multiple scattering emerges
followed by the localization domain.


title: Folding in two-dimenensional off-lattice models of proteins

abstract:
  Model off-lattice sequences in two dimensions are constructed so that their
native states are close to an on-lattice target. The Hamiltonian involves the
Lennard-Jones and harmonic interactions. The native states of these sequences
are determined with a high degree of certainty through Monte Carlo processes.
The sequences are characterized thermodynamically and kinetically. It is shown
that the rank-ordering-based scheme of the assignment of contact energies
typically fails in off-lattice models even though it generates high stability
of on-lattice sequences. Similar to the on-lattice case, Go-like modeling, in
which the interaction potentials are restricted to the native contacts in a
target shape, gives rise to good folding properties. Involving other contacts
deteriorates these properties.


title: Delineation of the Native Basin in Continuum Models of Proteins

abstract:
  We propose two approaches for determining the native basins in off-lattice
models of proteins. The first of them is based on exploring the saddle points
on selected trajectories emerging from the native state. In the second
approach, the basin size can be determined by monitoring random distortions in
the shape of the protein around the native state. Both techniques yield the
similar results. As a byproduct, a simple method to determine the folding
temperature is obtained.


title:
Comparison of perturbative expansions using different phonon bases for
  two-site Holstein model

abstract:
  The two-site single-polaron problem is studied within the perturbative
expansions using different standard phonon basis obtained through the Lang
Firsov (LF), modified LF (MLF) and modified LF transformation with squeezed
phonon states (MLFS). The role of these convergent expansions using the above
prescriptions in lowering the energy and in determining the correlation
functions are compared for different values of coupling strength. The
single-electron energy, oscillator wave functions and correlation functions are
calculated for the same system. The applicability of different phonon basis in
different regimes of the coupling strength as well as in different regimes of
hopping are also discussed.


title:
Models of competitive learning: complex dynamics, intermittent
  conversions and oscillatory coarsening

abstract:
  We present two models of competitive learning, which are respectively
interfacial and cooperative learning. This learning is outcome-related, so that
spatially and temporally local environments influence the conversion of a given
site between one of two different types. We focus here on the behavior of the
models at coexistence, which yields new critical behavior and the existence of
a phase involving a novel type of coarsening which is oscillatory in nature.


title: Time-dependent tunneling of Bose-Einstein condensates

abstract:
  The influence of atomic interactions on time-dependent tunneling processes of
Bose-Einstein condensates is investigated. In a variety of contexts the
relevant condensate dynamics can be described by a Landau-Zener equation
modified by the appearance of nonlinear contributions. Based on this equation
it is discussed how the interactions modify the tunneling probability. In
particular, it is shown that for certain parameter values, due to a nonlinear
hysteresis effect, complete adiabatic population transfer is impossible however
slowly the resonance is crossed. The results also indicate that the
interactions can cause significant increase as well as decrease of tunneling
probabilities which should be observable in currently feasible experiments.


title:
Interrelation of Superconducting and Antiferromagnetic Gaps in High-Tc
  Compounds: a Test Case for a Microscopic Theory

abstract:
  Recent angle resolved photoemission (ARPES) data, which found evidence for a
d-wave-like modulation of the antiferromagnetic gap, suggest an intimate
interrelation between the antiferromagnetic insulator and the superconductor
with its d-wave gap. This poses a new challenge to microscopic descriptions,
which should account for this correlation between, at first sight, very
different states of matter. Here, we propose a microscopic mechanism which
provides a definite correlation between these two different gap structures: it
is shown that a projected SO(5) theory, which aims at unifying
antiferromagnetism and d-wave superconductivity via a common symmetry principle
while explicitly taking the Mott-Hubbard gap into account, correctly describes
the observed gap characteristics. Specifically, it accounts for both the
dispersion and the order of magnitude difference between the antiferromagnetic
gap modulation and the superconducting gap.


title: Optical detection of a BCS transition of Lithium-6 in harmonic traps

abstract:
  We study the detection of a BCS transition within a sample of Lithium--6
atoms confined in a harmonic trap. Using the local density approximation we
calculate the pair correlation function in the normal and superfluid state at
zero temperature. We show that the softening of the Fermi hole associated with
a BCS transition leads to an observable increase in the intensity of
off--resonant light scattered from the atomic cloud at small angles.


title: Molecular ratchets - verification of the principle of detailed balance

abstract:
  We argue that the recent experiments of Kelly et. al.(Angew. Chem. Int. Ed.
Engl. 36, 1866 (1997)) on molecular ratchets, in addition to being in agreement
with the second law of thermodynamics, is a test of the principle of detailed
balance for the ratchet. We suggest new experiments, using an asymmetric
ratchet, to further test the principle. We also point out methods involving a
time variation of the temperature to to give it a directional motion.


title: Critical Hamiltonians with long range hopping

abstract:
  Critical states are studied by a real space RG in the problem with strong
diagonal disorder and long range power law hopping. The RG flow of the
distribution of coupling parameters is characterized by a family of non-trivial
fix points. We consider the RG flow of the distribution of participation ratios
of eigenstates. Scaling of participation ratios is sensitive to the nature of
the RG fix point. For some fix points, scaling of participation ratios is
characterized by a distribution of exponents, rather than by a single exponent.
  The RG method can be generalized to treat certain fermionic Hamiltonians with
disorder and long range hopping. We derive the RG for a model of interacting
two-level systems. Besides couplings, in this problem the RG includes the
density of states. The density of states is renormalized so that it develops a
singularity near zero energy.


title:
Van Hove Singularities around the Fermi level in YBa2Cu3O7: The
  importance of the chains

abstract:
  We have reproduced band structure calculations from the literature and have
used them to analyze in detail the energy landscape around the Fermi level. We
found three Van Hove singularities, two below (-230, -54 meV) and one above the
Fermi level (+27 meV). We have studied the composition of each one of them and
found that states comming from the chain do contribute in a very importan way.
The contribution from the planes are indeed important and, therefore, we find
that a 2D description includes the most important contributions. Nevertheless,
the contribution from states out of the planes (the chains and the apical
oxygen) is by no means negligable. We find that it is possible that in YBaCuO
part of the condensate lies in the chains, a fact that would agree with some
recent evidences concerning PrBaCuO. Our general conclusion is that the 2D
description of 123-compounds might be insuficient to explain all the
experimental details and that a 3D description seems compulsory to fully
account for the phenomenom of superconductivity in YBaCuO.


title:
Study of critical behavior of the supersymmetric spin chain that models
  plateau transitions in the integer quantum Hall effect

abstract:
  We use the density-matrix renormalization-group (DMRG) algorithm and
finite-size scaling to study a supersymmetric (SUSY) spin chain that models
plateau transitions in the integer quantum Hall effect. To illustrate the
method, we first present results for the S=1 antiferromagnetic spin chain.
Different scaling behavior of the dimerization operator is obtained for the
Haldane phase, the dimerized phase, and at the critical point separating the
two phases. We then investigate the supersymmetric spin chain. As the on-site
Hilbert space has infinite dimension, it is non-compact. We present DMRG
calculations for supersymmetric truncations of the on-site Hilbert space and
also for non-supersymmetric truncations. These two cases scale differently, in
accord with a Lieb-Schultz-Mattis type theorem.


title:
Abrupt appearance of the domain pattern and fatigue of thin
  ferroelectric films

abstract:
  We study the domain structure in ferroelectric thin films with a `passive'
layer (material with damaged ferroelectric properties) at the interface between
the film and electrodes within a continuous medium approximation. An abrupt
transition from a monodomain to a polydomain state has been found with the
increase of the `passive' layer thickness $d$. The domain width changes very
quickly at the transition (exponentially with $d^{-2}$). We have estimated the
dielectric response $dP/dE$ (the slope of the hysteresis loop) in the
`fatigued' multidomain state and found that it is in agreement with experiment,
assuming realistic parameters of the layer. We derive a simple universal
relation for the dielectric response, which scales as $1/d$, involving only the
properties of the passive layer. This relation qualitatively reproduces the
evolution of the hysteresis loop in fatigued samples and it could be tested
with controlled experiments. It is expected that the coercive field should
increase with decreasing lateral size of the film. We believe that specific
properties of the domain structure under bias voltage in ferroelectrics with a
passive layer can resolve the long-standing `paradox of the coercive field'.


title:
Impurity-Induced Quasiparticle Transport and Universal Limit
  Wiedemann-Franz Violation in d-Wave Superconductors

abstract:
  Due to the node structure of the gap in a d-wave superconductor, the presence
of impurities generates a finite density of quasiparticle excitations at zero
temperature. Since these impurity-induced quasiparticles are both generated and
scattered by impurities, prior calculations indicate a universal limit (\Omega
-> 0, T -> 0) where the transport coefficients obtain scattering-independent
values, depending only on the velocity anisotropy v_f/v_2. We improve upon
prior results, including the contributions of vertex corrections and Fermi
liquid corrections in our calculations of universal limit electrical, thermal,
and spin conductivity. We find that while vertex corrections modify electrical
conductivity and Fermi liquid corrections renormalize both electrical and spin
conductivity, only thermal conductivity maintains its universal value,
independent of impurity scattering or Fermi liquid interactions. Hence, low
temperature thermal conductivity measurements provide the most direct means of
obtaining the velocity anisotropy for high T_c cuprate superconductors.


title:
The Essential Interactions in Oxides and Spectral Weight Transfer in
  Doped Manganites

abstract:
  We calculate the value of the Fr\"ohlich electron-phonon interaction in
manganites, cuprates, and some other charge-transfer insulators and show that
this interaction is much stronger than any relevant magnetic interaction. A
polaron shift due to the Fr\"ohlich interaction, which is about 1 eV, suggests
that carriers in those systems are small (bi)polarons at all temperatures and
doping levels, in agreement with the oxygen isotope effect and other data. An
opposite conclusion, recently suggested in the literature, is shown to be
incorrect. The frequency and temperature dependence of the optical conductivity
of ferromagnetic manganites is explained within the framework of the bipolaron
theory.


title: Geometric Resonance in Modulated Quantum Hall Systems Near $ \nu = 1/2$

abstract:
  We propose a theory for the new effects recently observed by Willett et al
[1] in the magnetoresistance of a weakly modulated two dimensional electron gas
near filling factor 1/2. Minima in transverse magnetoresistance and maxima in
longitudinal magnetoresistance at the same magnetic field producing the new
resonance structure are reported. The structure occurs due to geometric
resonance of the composite fermion cyclotron orbits with the modulation period
of the effective magnetic field $ B_{eff} $ due to the applied density
modulation. The transverse minimum occurs due to the inhomogeneity in the field
$ B_{eff} $ in the presence of density modulations, whereas the longitudinal
maximum can arise due to a shape-effect (distortion) of the composite fermion
Fermi surface (CF-FS). Thus the minima and maxima reflect different physical
mechanisms.
  PACS numbers 71.10 Pm, 73.40 Hm, 73.20 Dx {}


title: Correlated hip motions during quiet standing

abstract:
  Kinematic measurements of two simultaneous coordinates from postural sway
during quiet standing were performed employing multiple ultrasonic transducers.
The use of accurate acoustic devices was required for the detection of the
small random noise displacements. The trajectory in the anteroposterior -
mediolateral plane of human chest was measured and compared with the trajectory
in anteroposterior direction from the upper and lower body. The latter was
statistically analyzed and appeared to be strongly anti-correlated. The
anti-correlations represent strong evidence for the dominance of hip strategy
during an unperturbed one minute stance. That the hip strategy, normally
observed for large amplitude motions, also appears in the small amplitude of a
quite stance, indicates the utility of such noise measurements for exploring
the biomechanics of human balance.


title:
On the 3-particle scattering continuum in quasi one dimensional integer
  spin Heisenberg magnets

abstract:
  We analyse the three-particle scattering continuum in quasi one dimensional
integer spin Heisenberg antiferromagnets within a low-energy effective field
theory framework. We exactly determine the zero temperature dynamical structure
factor in the O(3) nonlinear sigma model and in Tsvelik's Majorana fermion
theory. We study the effects of interchain coupling in a Random Phase
Approximation. We discuss the application of our results to recent
neutron-scattering experiments on the Haldane-gap material ${\rm CsNiCl_3}$.


title: Spatial structure of an incompressible Quantum Hall strip

abstract:
  The incompressible Quantum Hall strip is sensitive to charging of localized
states in the cyclotron gap. We study the effect of localized states by a
density functional approach and find electron density and the strip width as a
function of the density of states in the gap. Another important effect is
electron exchange. By using a model density functional which accounts for
negative compressibility of the QH state, we find electron density around the
strip. At large exchange, the density profile becomes nonmonotonic, indicating
formation of a 1D Wigner crystal at the strip edge. Both effects, localized
states and exchange, lead to a substantial increase of the strip width.


title: Quantum Hall Bilayers and the Chiral Sine-Gordon Equation

abstract:
  The edge state theory of a class of symmetric double-layer quantum Hall
systems with interlayer electron tunneling reduces to the sum of a free field
theory and a field theory of a chiral Bose field with a self-interaction of the
sine-Gordon form. We argue that the perturbative renormalization group flow of
this chiral sine-Gordon theory is distinct from the standard (non-chiral)
sine-Gordon theory, contrary to a previous assertion by Renn, and that the
theory is manifestly sensible only at a discrete set of values of the inverse
period of the cosine interaction (beta). We obtain exact solutions for the
spectra and correlation functions of the chiral sine-Gordon theory at the two
values of beta at which the electron tunneling in bilayers is not irrelevant.
Of these, the marginal case (beta^2=4) is of greatest interest: the spectrum of
the interacting theory is that of two Majorana fermions with different,
dynamically generated, velocities. For the experimentally observed bilayer 331
state at filling factor 1/2, this implies the trifurcation of electrons added
to the edge. We also present a method for fermionizing the theory at the
discrete points (integer beta^2) by the introduction of auxiliary degrees of
freedom that could prove useful in other problems involving quantum Hall
multilayers.


title: Yrast line for weakly interacting trapped bosons

abstract:
  We compute numerically the yrast line for harmonically trapped boson systems
with a weak repulsive contact interaction, studying the transition to a vortex
state as the angular momentum L increases and approaches N, the number of
bosons. The L=N eigenstate is indeed dominated by particles with unit angular
momentum, but the state has other significant components beyond the pure vortex
configuration. There is a smooth crossover between low and high L with no
indication of a quantum phase transition. Most strikingly, the energy and wave
function appear to be analytical functions of L over the entire range 2 < L <
N. We confirm the structure of low-L states proposed by Mottelson, as mainly
single-particle excitations with two or three units of angular momentum.


title:
Resistive upper critical fields and irreversibility lines of
  optimally-doped high-T_c cuprates

abstract:
  We present the resistively-determined upper critical field H^{\rho}_{c2}(T)
and the irreversibility lines H^{\rho}_{irr}(T) of various high-T_c cuprates,
deduced from measurements in 61-T pulsed magnetic fields applied parallel to
the c-axis. The SHAPE of both H^{\rho}_{c2}(T) and H^{\rho}_{irr}(T) depends
monotonically on the anisotropy of the material and none of the samples show
saturation of H^{\rho}(T) at low temperatures. The anomalous positive
curvature, d^2 H^{\rho}/dT^2 > 0, is the strongest in materials with the
largest normal-state anisotropy, regardless of whether anisotropy is varied by
changing the carrier concentration or by comparing a variety of optimally-doped
compounds.


title: Integrable Kondo impurity in one-dimensional q-deformed $t-J$ models

abstract:
  Integrable Kondo impurities in two cases of the one-dimensional q-deformed
$t-J$ models are studied by means of the boundary ${\bf Z}_2$-graded quantum
inverse scattering method. The boundary $K$ matrices depending on the local
magnetic moments of the impurities are presented as nontrivial realizations of
the reflection equation algebras in an impurity Hilbert space. Furthermore,
these models are solved by using the algebraic Bethe ansatz method and the
Bethe ansatz equations are obtained.


title: Quantum Effects in Resistance-Shunted Josephson Junctions

abstract:
  Thermodynamics and transport properties of resistance-shunted Josephson
junctions are studied theoretically in the tight-binding limit E_C/E_J<<1,
where E_C and E_J are a charging energy and a Josephson coupling energy
respectively. Based on a phenomenological harmonic-oscillator model, weak
coupling region K=R_Q/R<<1 is analytically studied, where R and R_Q=h/(2e)^2
are a shunted resistance and the quantum resistance. In addition to the
effective bandwidth, we find that this multi-level system genuinely has a novel
crossover at lower energy below which the density of states becomes strongly
degenerate. These two energy scales control the linear DC responses, optical
responses, and nonlinear I-V characteristics. The lower energy crossover
indicates the existence of a new class of strongly-correlated phenomena beyond
the framework of the Kondo problem.


title:
Long-Range Coulomb Interaction and the Crossover between Quantum and
  Shot Noise in Diffusive Conductors

abstract:
  Frequency-dependent nonequilibrium noise in quantum-coherent diffusive
conductors is calculated with account taken of long-range Coulomb interaction.
For long and narrow contacts with strong external screening the crossover
between quantum and shot noise takes place at frequencies much smaller than the
voltage drop across the contact. We also show that under certain frequency
limitations, the semiclassical and quantum-coherent approaches to shot noise
are mathematically equivalent.


title:
The possibility of a metal insulator transition in antidot arrays
  induced by an external driving

abstract:
  It is shown that a family of models associated with the kicked Harper model
is relevant for cyclotron resonance experiments in an antidot array. For this
purpose a simplified model for electronic motion in a related model system in
presence of a magnetic field and an AC electric field is developed. In the
limit of strong magnetic field it reduces to a model similar to the kicked
Harper model. This model is studied numerically and is found to be extremely
sensitive to the strength of the electric field. In particular, as the strength
of the electric field is varied a metal -- insulator transition may be found.
The experimental conditions required for this transition are discussed.


title: Single-Particle Pseudogap in Two-Dimensional Electron Systems

abstract:
  We investigate pseudogap phenomena in the 2D electron system.
  Based on the mode-mode coupling theory of antiferromagnetic (AFM) and
$d_{x^2-y^2}$-wave superconducting ($d$SC) fluctuations, single-particle
dynamics is analyzed. For the parameter values of underdoped cuprates,
pseudogap structure grows in the single-particle spectral weight $A(k,\omega)$
around the wave vector $(\pi,0)$ and $(0,\pi)$ below the pseudo-spin-gap
temperature $\TPG$ signaled by the reduction of dynamical spin correlations in
qualitative agreement with the experimental data. The calculated results for
the overdoped cuprates also reproduce the absence of the pseudogap in the
experiments. We also discuss limitations of our weak-coupling approach.


title:
Theory of spin effect on Coulomb blockade of single-electron tunneling
  in ferromagnetic system

abstract:
  Spin dependent single electron tunneling in a ferromagnetic double junction
is investigated theoretically in the limit of incoherent sequential tunneling.
The junction consists of a small nonmagnetic metallic grain with discrete
energy levels, which is connected to two ferromagnetic electrodes. We have
developed a new theoretical model taking into account charge as well as spin
degrees of freedom. The model allows us to investigate new phenomena such as
spin accumulation and spin fluctuations.


title: Disordered Systems in Phase Space

abstract:
  As a function of the disorder strength in a mesoscopic system, the electron
dynamics crosses over from the ballistic through the diffusive towards the
localized regime. The ballistic and the localized situation correspond to
integrable or regular behavior while diffusive conductors correspond to chaotic
behavior. The chaotic or regular character of single wave functions can be
inferred from phase space concepts like the Husimi distribution and the Wehrl
entropy. These quantities provide useful information about the structure of
states in disordered systems. We investigate the phase space structure of one
dimensional (1d) and 2d disordered systems within the Anderson model. The Wehrl
entropy of the eigenstates allows to detect the crossover between the
ballistic, diffusive and localized regime.


title:
On the magnetic stability at the surface in strongly correlated electron
  systems

abstract:
  The stability of ferromagnetism at the surface at finite temperatures is
investigated within the strongly correlated Hubbard model on a semi-infinite
lattice. Due to the reduced surface coordination number the effective Coulomb
correlation is enhanced at the surface compared to the bulk. Therefore, within
the well-known Stoner-picture of band ferromagnetism one would expect the
magnetic stability at the surface to be enhanced as well. However, by taking
electron correlations into account well beyond the Hartree-Fock (Stoner) level
we find the opposite behavior: As a function of temperature the magnetization
of the surface layer decreases faster than in the bulk. By varying the hopping
integral within the surface layer this behavior becomes even more pronounced. A
reduced hopping integral at the surface tends to destabilize surface
ferromagnetism whereas the magnetic stability gets enhanced by an increased
hopping integral. This behavior represents a pure correlation effect and can be
understood in terms of general arguments which are based on exact results in
the limit of strong Coulomb interaction.


title: Surface Phonons and Other Localized Excitations

abstract:
  The diatomic linear chain of masses coupled by harmonic springs is a
textboook model for vibrational normal modes (phonons) in crystals. In addition
to propagating acoustic and optic branches, this model is known to support a
``gap mode'' localized at the surface, provided the atom at the surface has
light rather than heavy mass. An elementary argument is given which explains
this mode and provides values for the frequency and localization length. By
reinterpreting this mode in different ways, we obtain the frequency and
localization lengths for three other interesting modes: (1) the surface
vibrational mode of a light mass impurity at the surface of a monatomic chain;
(2) the localized vibrational mode of a stacking fault in a diatomic chain; and
(3) the localized vibrational mode of a light mass impurity in a monatomic
chain.


title: Density Matrix Renormalisation Group Variants for Spin Systems

abstract:
  This thesis gives an extension for the Density Matrix Renormalisation Group
(DMRG) to two dimensions and described a newly developed combination of the
DMRG and a Green Function Monte Carlo simulation (GFMC).
  The first two chapters focus on the DMRG method. The properties are reviewed
and it is shown that good quality results can be obtained for two-dimensional
systems through finite-size scaling.
  In the third and fourth chapter the properties of the two-dimensional
frustrated Heisenberg model are analysed using a similar combination of DMRG
and finite-size scaling.
  The final chapter presents the combination of the DMRG and GFMC with
Stochastic Reconfiguration (Sorella, Capriotti cond-mat/9902211) We find an
intermediate phase between Neel and collinear long-range order. This phase
shows spatial inhomogeneities with a tendency towards plaquette formation.


title: Triplet Waves in a Quantum Spin Liquid

abstract:
  We report a neutron scattering study of the spin-1/2 alternating bond
antiferromagnet Cu(NO_3)_2. 2.5D_2O for 0.06<k_BT/J_1<1.5. For k_BT/J_1 << 1
the excitation spectrum is dominated by a coherent singlet-triplet mode
centered at J_1=0.442(2) meV with sinusoidal dispersion and a bandwidth of
J_2=0.106(2) meV. A complete description of the zero temperature contribution
to the scattering function from this mode is provided by the Single Mode
Approximation. At finite temperatures we observe exponentially activated band
narrowing and damping. The relaxation rate is thermally activated and wave
vector dependent with the period icity of the reciprocal lattice.


title: Spin Wave Theory of Double Exchange Ferromagnets

abstract:
  We construct the 1/S spin-wave expansion for double exchange ferromagnets at
T=0. It is assumed that the value of Hund's rule coupling, J_H, is sufficiently
large, resulting in a fully saturated, ferromagnetic half-metallic ground
state. We evaluate corrections to the magnon dispersion law, and we also find
that, in contrast to earlier statements in the literature, magnon-electron
scattering does give rise to spin wave damping. We analyse the momentum
dependence of these quantities and discuss the experimental implications for
colossal magnetoresistance compounds.


title:
Influence of uncorrelated overlayers on the magnetism in thin
  itinerant-electron films

abstract:
  The influence of uncorrelated (nonmagnetic) overlayers on the magnetic
properties of thin itinerant-electron films is investigated within the
single-band Hubbard model. The Coulomb correlation between the electrons in the
ferromagnetic layers is treated by using the spectral density approach (SDA).
It is found that the presence of nonmagnetic layers has a strong effect on the
magnetic properties of thin films. The Curie temperatures of very thin films
are modified by the uncorrelated overlayers. The quasiparticle density of
states is used to analyze the results. In addition, the coupling between the
ferromagnetic layers and the nonmagnetic layers is discussed in detail. The
coupling depends on the band occupation of the nonmagnetic layers, while it is
almost independent of the number of the nonmagnetic layers. The induced
polarization in the nonmagnetic layers shows a long-range decreasing
oscillatory behavior and it depends on the coupling between ferromagnetic and
nonmagnetic layers.


title: Quantum heat transfer through an atomic wire

abstract:
  We studied the phononic heat transfer through an atomic dielectric wire with
both infinite and finite lengths by using a model Hamiltonian approach. At low
temperature under ballistic transport, the thermal conductance contributed by
each phonon branch of a uniform and harmonic chain cannot exceed the well-known
value which depends linearly on temperature but is material independent. We
predict that this ballistic thermal conductance will exhibit stepwise behavior
as a function of temperature. By performing numerical calculations on a more
realistic system, where a small atomic chain is placed between two reservoirs,
we also found resonance modes, which should also lead to the stepwise behavior
in the thermal conductance.


title: On Measuring Condensate Fraction in Superconductors

abstract:
  An analysis of off-diagonal long-range order in superconductors shows that
the spin-spin correlation function is significantly influenced by the order if
the order parameter is anisotropic on a microscopic scale. Thus, magnetic
neutron scattering can provide a direct measurement of the condensate fraction
of a superconductor. It is also argued that recent measurements in high
temperature superconductors come very close to achieving this goal.


title:
C-axis Penetration Depth and Inter-layer Conductivity in the Thallium
  Based Cuprate Superconductors

abstract:
  The c-axis Josephson plasmon in optimally doped single-layer and bi-layer
high Tc cuprates Tl2201 and Tl2212 have been investigated using infrared
spectroscopy. We observed the plasma frequencies for these two compounds at
27.8 and 25.6 cm-1 respectively, which we interpret as a Josephson resonance
across the TlO blocking layers. No maximum in the temperature dependence of the
c-axis conductivity was observed below Tc, indicating that even in the
superconducting state a coherent quasi-particle contribution to the c-axis
conductivity is absent or very weak, in contrast to the behaviour of the
ab-plane conductivity.


title:
Equivalence of two thermostatistical formalisms based on the
  Havrda&Charvat-Daroczy-Tsallis entropies

abstract:
  We show that the latest thermostatistical formalism based on the Havrda &
Charvat-Dar\'oczy-Tsallis entropy $S_q [ \rho ] = (q-1)^{-1}(1- tr (\rho^q))$
proposed by Tsallis, Mendes and Plastino is {\em equivalent} to the first one
proposed by Tsallis in 1988. Here, equivalent means: {\em the ``equilibrium''
state predicted by either formalism using $q$ leads to the same expectation
values for all observables as that predicted by the other formalism using
$1/q$''}. We also point out once again that the basic property of {\em
transitivity of equilibrium} (e.g., the $0^{th}$ Law of Thermodynamics) fails
in these formalisms.


title: Broken Symmetries in the Reconstruction of v=1 Quantum Hall Edges

abstract:
  Spin-polarized reconstruction of the v=1 quantum Hall edge is accompanied by
a spatial modulation of the charge density along the edge. We find that this is
also the case for finite quantum Hall droplets: current spin density functional
calculations show that the so-called Chamon-Wen edge forms a ring of apparently
localized electrons around the maximum density droplet (MDD). The boundaries of
these different phases qualitatively agree with recent experiments. For very
soft confinement, Chern-Simons Ginzburg-Landau theory indicates formation of a
non-translational invariant edge with vortices (holes) trapped in the edge
region.


title: Vortices in a Bose-Einstein Condensate

abstract:
  We have created vortices in two-component Bose-Einstein condensates. The
vortex state was created through a coherent process involving the spatial and
temporal control of interconversion between the two components. Using an
interference technique, we map the phase of the vortex state to confirm that it
possesses angular momentum. We can create vortices in either of the two
components and have observed differences in the dynamics and stability.


title:
The effect of interactions on Bose-Einstein condensation in a
  two-dimensional harmonic trap

abstract:
  A dilute bose gas in a quasi two-dimensional harmonic trap and interacting
with a repulsive two-body zero-range potential of fixed coupling constant is
considered. Using the Thomas-Fermi method, it is shown to remain in the same
uncondensed phase as the temperature is lowered. Its density profile and energy
are identical to that of an ideal gas obeying the fractional exclusion
statistics of Haldane. PACS: ~03.75.Fi, 05.30.Jp, 67.40.Db, 05.30.-d


title:
Effect of in-plane magnetic field on magnetic phase transitions in nu=2
  bilayer quantum Hall systems

abstract:
  By using the effective bosonic spin theory, which is recently proposed by
Demler and Das Sarma [ Phys. Rev. Lett. 82, 3895 (1999) ], we analyze the
effect of an external in-plane magnetic field on the magnetic phase transitions
of the bilayer quantum Hall system at filling factor nu=2. It is found that the
quantum phase diagram is modified by the in-plane magnetic field. Therefore,
quantum phase transitions can be induced simply by tilting the magnetic field.
The general behavior of the critical tilted angle for different layer
separations and interlayer tunneling amplitudes is shown. We find that the
critical tilted angles being calculated agree very well with the reported
values. Moreover, a universal critical exponent for the transition from the
canted antiferromagnetic phase to the ferromagnetic phase is found to be equal
to 1/2 within the present effective theory.


title: A Stochastic Description for Extremal Dynamics

abstract:
  We show that extremal dynamics is very well modelled by the "Linear
Fractional Stable Motion" (LFSM), a stochastic process entirely defined by two
exponents that take into account spatio-temporal correlations in the
distribution of active sites. We demonstrate this numerically and analytically
using well-known properties of the LFSM. Further, we use this correspondence to
write an exact expressions for an n-point correlation function as well as an
equation of fractional order for interface growth in extremal dynamics.


title: Towards a first-principles theory of surface thermodynamics and kinetics

abstract:
  Understanding of the complex behavior of particles at surfaces requires
detailed knowledge of both macroscopic and microscopic processes that take
place; also certain processes depend critically on temperature and gas
pressure. To link these processes we combine state-of-the-art microscopic, and
macroscopic phenomenological, theories. We apply our theory to the O/Ru(0001)
system and calculate thermal desorption spectra, heat of adsorption, and the
surface phase diagram. The agreement with experiment provides validity for our
approach which thus identifies the way for a predictive simulation of surface
thermodynamics and kinetics.


title:
Phase transition between d-wave and anisotropic s-wave gaps in high
  temperature oxides superconductors

abstract:
  We study models for superconductivity with two interactions: $V^>$ due to
antiferromagnetic(AF) fluctuations and $V^<$ due to phonons, in a weak coupling
approach to the high temperature superconductivity. The nature of the two
interactions are considerably different; $V^>$ is positive and sharply peaked
at ($\pm\pi$,$ \pm\pi$) while $V^<$ is negative and peaked at ($0,0$) due to
weak phonon screening. We numerically find (a) weak BCS attraction is enough to
have high critical temperature if a van Hove anomaly is at work, (b) $V^>$ (AF)
is important to give d-wave superconductivity, (c) the gap order parameter
$\Delta({\bf k})$ is constant(s-wave) at extremely overdope region and it
changes to anisotropic s-wave as doping is reduced, (d) there exists a first
order phase transition between d-wave and anisotropic s-wave gaps. These
results are qualitatively in agreement with preceding works; they should be
modified in the strongly underdope region by the presence of antiferromagnetic
fluctuations and ensuing AF pseudogap.


title:
Pattern Formation and a Clustering Transition in Power-Law Sequential
  Adsorption

abstract:
  A new model that describes adsorption and clustering of particles on a
surface is introduced. A {\it clustering} transition is found which separates
between a phase of weakly correlated particle distributions and a phase of
strongly correlated distributions in which the particles form localized fractal
clusters. The order parameter of the transition is identified and the fractal
nature of both phases is examined. The model is relevant to a large class of
clustering phenomena such as aggregation and growth on surfaces, population
distribution in cities, plant and bacterial colonies as well as gravitational
clustering.


title: Exact results and scaling properties of small-world networks

abstract:
  We study the distribution function for minimal paths in small-world networks.
Using properties of this distribution function, we derive analytic results
which greatly simplify the numerical calculation of the average minimal
distance, $\bar{\ell}$, and its variance, $\sigma^2$. We also discuss the
scaling properties of the distribution function. Finally, we study the limit of
large system sizes and obtain some analytic results.


title:
Structural instability associated with the tilting of CuO6 octahedra in
  La2-xSrxCuO4

abstract:
  Comprehensive inelastic neutron-scattering measurements were performed to
study the soft optical phonons in La2-xSrxCuO4 at x=0.10, 0.12 and 0.18. We
found at x=0.18 that the softening of Z-point phonon, suggesting incipient
structural transition from the low-temperature orthorhombic (LTO) to
low-temperature tetragonal (LTT) phase, breaks at Tc, which is consistent with
the previous report by Lee et al. for the optimally doped x=0.15 sample. As for
x=0.10 and 0.12, on the other hand, the softening continues even below Tc. It
is thus clarified that the breaking of soft phonon is characteristic of
La2-xSrxCuO4 in the optimally and overdoped regions. In the course of studying
the soft phonons, we discovered that a central peak remains above the LTO to
high-temperature tetragonal (HTT) phase transition at Ts1 and splits into
incommensurate components along the (1 1 0)HTT direction at higher
temperatures. This is a common feature for both x=0.12 and 0.18 and their
temperature dependences of the splitting 2d can be scaled by using a
renormalized temperature T/Ts1. In the high temperature limit, d saturates
around d ~ 0.12 r.l.u., which value is close to the splitting of incommensurate
magnetic signals. This implies that the incipient lattice modulation starts
appearing at very high temperature. Details of this modulation and its
relations with other properties are, however, not yet clarified.


title: Transport properties near the Anderson transition

abstract:
  The electronic transport properties in the presence of a temperature gradient
in disordered systems near the metal-insulator transition [MIT] are considered.
The d.c. conductivity $\sigma$, the thermoelectric power $S$, the thermal
conductivity $K$ and the Lorenz number $L_0$ are calculated for the
three-dimensional Anderson model of localization using the
Chester-Thellung-Kubo-Greenwood formulation of linear response. We show that
$\sigma$, S, K and $L_0$ can be scaled to one-parameter scaling curves with a
single scaling paramter $k_BT/|{\mu-E_c}/E_c|$.


title: Towards single-electron metrology

abstract:
  We review the status of the understanding of single-electron transport (SET)
devices with respect to their applicability in metrology. Their envisioned role
as the basis of a high-precision electrical standard is outlined and is
discussed in the context of other standards. The operation principles of single
electron transistors, turnstiles and pumps are explained and the fundamental
limits of these devices are discussed in detail. We describe the various
physical mechanisms that influence the device uncertainty and review the
analytical and numerical methods needed to calculate the intrinsic uncertainty
and to optimise the fabrication and operation parameters. Recent experimental
results are evaluated and compared with theoretical predictions. Although there
are discrepancies between theory and experiments, the intrinsic uncertainty is
already small enough to start preparing for the first SET-based metrological
applications.


title:
Muonium as a hydrogen analogue in silicon and germanium; quantum effects
  and hyperfine parameters

abstract:
  We report a first-principles theoretical study of hyperfine interactions,
zero-point effects and defect energetics of muonium and hydrogen impurities in
silicon and germanium. The spin-polarized density functional method is used,
with the crystalline orbitals expanded in all-electron Gaussian basis sets. The
behaviour of hydrogen and muonium impurities at both the tetrahedral and
bond-centred sites is investigated within a supercell approximation. To
describe the zero-point motion of the impurities, a double adiabatic
approximation is employed in which the electron, muon/proton and host lattice
degrees of freedom are decoupled. Within this approximation the relaxation of
the atoms of the host lattice may differ for the muon and proton, although in
practice the difference is found to be slight. With the inclusion of zero-point
motion the tetrahedral site is energetically preferred over the bond-centred
site in both silicon and germanium. The hyperfine and superhyperfine
parameters, calculated as averages over the motion of the muon, agree
reasonably well with the available data from muon spin resonance experiments.


title:
Coexistence of solutions in dynamical mean-field theory of the Mott
  transition

abstract:
  In this paper, I discuss the finite-temperature metal-insulator transition of
the paramagnetic Hubbard model within dynamical mean-field theory. I show that
coexisting solutions, the hallmark of such a transition, can be obtained in a
consistent way both from Quantum Monte Carlo (QMC) simulations and from the
Exact Diagonalization method. I pay special attention to discretization errors
within QMC. These errors explain why it is difficult to obtain the solutions by
QMC close to the boundaries of the coexistence region.


title:
A study of supercooling of the disordered vortex phase via minor
  hysteresis loops in 2H-NbSe_2

abstract:
  We report on the observation of novel features in the minor hysteresis loops
in a clean crystal of NbSe_2 which displays a peak effect. The observed
behavior can be explained in terms of a supercooling of the disordered vortex
phase while cooling the superconductor in a field. Also, the extent of spatial
order in a flux line lattice formed in ascending fields is different from (and
larger than) that in the descending fields below the peak position of the peak
effect; this is attributed to unequal degree of annealing of the state induced
by a change of field in the two cases.


title: Multiple Andreev reflection in single atom niobium junctions

abstract:
  Single atom junctions between superconducting niobium leads are produced
using the Mechanically Controllable Break Junction technique. The
current-voltage characteristics of these junctions are analysed using an exact
formulation for a superconducting quantum point contact. For tunnelling between
two single atoms with a sufficiently large vacuum barrier, it is found that a
single channel dominates the current, and that the current-voltage
characteristic is described by the theory, without adjustable parameters. For a
contact of a single Nb atom it is shown that five conductance channels
contribute to the conductance, in agreement with the number expected based on
the number of valence orbitals for this d-metal. For each of the channels the
transmission probability is obtained from the fits and the limits of accuracy
for these numbers are discussed.


title: The spin-1 ladder : A bosonization study

abstract:
  We construct a field-theoretic description of two coupled spin-1 Heisenberg
chains, starting with the known representation of a single spin-1 chain in
terms of Majorana fermions (or Ising models). After reexamining the
bosonization rules for two Ising models, taking particular care of order and
disorder operators, we obtain a bosonic description of the spin-1 ladder. From
renormalization-group and mean-field arguments, we conclude that, for a small
interchain coupling, the spin-1 ladder is approximately described by three
decoupled, two-frequency sine-Gordon models. We then predict that, starting
with decoupled chains, the spin gap decreases linearly with interchain
coupling, both in the ferromagnetic and antiferromagnetic directions. Finally,
we discuss the possibility of an incommensurate phase in the spin-1 zigzag
chain.


title: Ordered phases in spin-Peierls systems

abstract:
  The microscopic description of spin-Peierls substances is discussed.
Particular attention is paid to the ordered (dimerised and incommensurably
modulated) phases. Important points are the adiabatic and the antiadiabatic
approach, generic soliton forms, elastic and magnetic interchain couplings. The
wealth and the accuracy of experimental information collected for the first
inorganic spin-Peierls substance CuGeO_3 motivates this work.


title: Damage in Fiber Bundle Models

abstract:
  We introduce a continuous damage fiber bundle model that gives rise to
macroscopic plasticity and compare its behavior with that of dry fiber bundles.
Several interesting constitutive behaviors are found in this model depending on
the value of the damage parameter and on the form of the disorder distribution.
In addition, we compare the behavior of global load transfer models with local
load transfer models and study in detail the damage evolution before failure.
We emphasize the analogies between our results and spinodal nucleation in
first-order phase transitions.


title: Exactly Soluble Model for Umklapp Scattering at Quantum-Hall Edges

abstract:
  We consider the low-energy, long-wave-length excitations of a reconstructed
quantum-Hall edge where three branches of chiral one-dimensional edge
excitations exist. We find that, in addition to forward scattering between the
three edge-excitation branches, Coulomb interaction gives rise to a novel
Umklapp-type scattering process that cannot be accounted for within a
generalized Tomonaga-Luttinger model. We solve the theory including Umklapp
processes exactly in the long-wave-length limit and calculate electronic
correlation functions.


title: Interplay of superconductivity and magnetism in strong coupling

abstract:
  A model is introduced describing the interplay between superconductivity and
spin-ordering. It is characterized by on-site repulsive electron-electron
interactions, causing antiferromagnetism, and nearest-neighbor attractive
interactions, giving rise to d-wave superconductivity. Due to a special choice
for the lattice, this model has a strong-coupling limit where the
superconductivity can be described by a bosonic theory, similar to the strongly
coupled negative U Hubbard model. This limit is analyzed in the present paper.
A rich mean-field phase diagram is found and the leading quantum corrections to
the mean-field results are calculated. The first-order line between the
antiferromagnetic- and the superconducting phase is found to terminate at a
tricritical point, where two second-order lines originate. At these lines, the
system undergoes a transition to- and from a phase exhibiting both
antiferromagnetic order and superconductivity. At finite temperatures above the
spin-disordering line, quantum-critical behavior is found. For specific values
of the model parameters, it is possible to obtain SO(5) symmetry involving the
spin- and the phase-sector at the tricritical point. Although this symmetry is
explicitly broken by the projection to the lower Hubbard band, it survives on
the mean-field level, and modes related to a spontaneously broken SO(5)
symmetry are present on the level of the random phase approximation in the
superconducting phase.


title: Instability of a Bose-Einstein Condensate with Attractive Interaction

abstract:
  We study the stability of a Bose-Einstein condensate of harmonically trapped
atoms with negative scattering length, specifically lithium 7. Our method is to
solve the time-dependent nonlinear Schrodinger equation numerically. For an
isolated condensate, with no gain or loss, we find that the system is stable
(apart from quantum tunneling) if the particle number N is less than a critical
number N_c. For N > N_c, the system collapses to high-density clumps in a
region near the center of the trap. The time for the onset of collapse is on
the order of 1 trap period. Within numerical uncertainty, the results are
consistent with the formation of a "black hole" of infinite density
fluctuations, as predicted by Ueda and Huang. We obtain numerically N_c
approximately 1251. We then include gain-loss mechanisms, i.e., the gain of
atoms from a surrounding "thermal cloud", and the loss due to two- and
three-body collisions. The number N now oscillates in a steady state, with a
period of about 145 trap periods. We obtain N_c approximately 1260 as the
maximum value in the oscillations.


title: Laser Excitation of Polarization Waves in a Frozen Gas

abstract:
  Laser experiments with optically excited frozen gases entail the excitation
of polarization waves. In a continuum approximation the waves are
dispersionless, but their frequency depends on the direction of the propagation
vector. An outline is given of the theory of transient phenomena that involve
the excitation of these waves by a resonant dipole-dipole transfer process.


title:
Current-driven Magnetization Reversal and Spin Wave Excitations in
  Co/Cu/Co Pillars

abstract:
  Using thin film pillars ~100 nm in diameter, containing two ferromagnetic Co
layers of different thicknesses separated by a paramagnetic Cu spacer, we
examine effects of torques due to spin-polarized currents flowing perpendicular
to the layers. In accordance with spin-transfer theory, spin-polarized
electrons flowing from the thin to the thick Co layer can switch the magnetic
moments of the layers antiparallel, while a reversed electron flow causes
switching to a parallel state. When large magnetic fields are applied, the
current no longer fully reverses the magnetic moment, but instead stimulates
spin-wave excitations.


title: Long-Range Excitation of Collective Modes in Mesoscopic Metal Clusters

abstract:
  We develop a semiclassical theory for the long range excitation of plasmon
resonances in atomic clusters, based on the doorway hypothesis. The effect of
the width of the plasmon resonance is fully taken into account. As an
application we study plasmon excitation in small Sodium clusters, in collisions
with electrons and protons.


title: Is There a Unified Description of Conductivity of Layered Cuprates ?

abstract:
  We present a novel approach to the analysis of the normal state in-plane
$\sigma_{ab}$ and out-of-plane $\sigma_{c}$ conductivities of anisotropic
layered crystals such as oxygen deficient $YBa_{2}Cu_{3}O_{x}$. It can be shown
that the resistive anisotropy is determined by the ratio of the phase coherence
lengths in the respective directions; i.e.,
$\sigma_{ab}/\sigma_c=\ell_{ab}^2/\ell_c^2$. From the idea that at all doping
levels and temperatures $T$ the out-of-plane transport in these crystals is
incoherent, follows that $\ell_c$ is T-independent, equal to the spacing
$\ell_0$ between the neighboring bilayers. Thus, the T-dependence of
$\ell_{ab}$ is given by the measured anisotropy, and $\sigma_{ab}(\ell_{ab})$
dependence is obtained by plotting $\sigma_{ab}$ vs
$\ell=(\sigma_{ab}/\sigma_c)^{1/2}\ell_0$. The analysis of several single
crystals of $YBa_{2}Cu_{3}O_{x}$ ($6.35<x<6.93$) shows that for all of them
$\sigma_{ab}(\ell)$ is described by a universal dependence
$\sigma_{ab}/\bar\sigma =f(\ell/\bar\ell)$ with doping dependent parameters
$\bar\sigma$ and $\bar\ell$.


title: Anisotropic Condensation of Helium in Nanotube Bundles

abstract:
  Helium atoms are strongly attracted to the interstitial channels within a
bundle of carbon nanotubes. The strong corrugation of the axial potential
within a channel can produce a lattice gas system where the weak mutual
attraction between atoms in neighboring channels of a bundle induces
condensation into a remarkably anisotropic phase with very low binding energy.
We estimate the binding energy and critical temperature for 4He in this novel
quasi-one-dimensional condensed state. At low temperatures, the specific heat
of the adsorbate phase (fewer than 2% of the total number of atoms) greatly
exceeds that of the host material.


title:
Magnetoplasmon excitations in arrays of circular and noncircular quantum
  dots

abstract:
  We have investigated the magnetoplasmon excitations in arrays of circular and
noncircular quantum dots within the Thomas-Fermi-Dirac-von Weizs\"acker
approximation. Deviations from the ideal collective excitations of isolated
parabolically confined electrons arise from local perturbations of the
confining potential as well as interdot Coulomb interactions. The latter are
unimportant unless the interdot separations are of the order of the size of the
dots. Local perturbations such as radial anharmonicity and noncircular symmetry
lead to clear signatures of the violation of the generalized Kohn theorem. In
particular, the reduction of the local symmetry from SO(2) to $C_4$ results in
a resonant coupling of different modes and an observable anticrossing behaviour
in the power absorption spectrum. Our results are in good agreement with recent
far-infrared (FIR) transmission experiments.


title:
Bi-Composability and Generalized Entropy Composition with Different q
  Indices

abstract:
  This paper has been withdrawn by the author and the content is merged into a
new paper: H-Theorem and Generalized Entropy Composition with Different q
Indices: cond-mat/9912454.


title:
Numerical Study of the One-Dimensional Spin-Orbit Coupled System with
  SU(2)$\otimes$SU(2) Symmetry

abstract:
  We numerically study the SU(2)$\otimes$SU(2) symmetric spin-orbit coupled
model as a lower symmetric generalization of the SU(4) exchange model. On the
symmetric line with respect to the spin and orbit, our result shows the
essentially singular gap formation in consistent with the analytic approach,
which is different from the previous numerical calculation. Furthermore, we
find new critical phases around the SU(4) point, surrounding the previously
known gapless symmetric line. In these novel phases either spin or orbital
excitations around momentum $q=\pi$ form massless continua which split from the
excitations belonging to the $[2^2]$ irreducible representation at the SU(4)
point. On the critical symmetric line, the additional coupled spin-orbit
excitations around $q=\pi/2$ originating from [$2^11^2$] become critical, too.


title:
Oscillatory screening of the dc electric field in the Si-SiO$_2$
  multiple quantum wells probed by second-harmonic generation

abstract:
  DC-electric field, being screened in 3D semiconductors, normally decays
monotonically in space. Experimental studies of the DC electric field screening
in Si-SiO$_2$ multiple quantum wells by electric field induced optical
second-harmonic generation show a non-monotonic, oscillatory-like decay. The
model of electrons localized inside quantum wells, with the first subband
occupied, allows a description of the phenomenon. Interwell Coulomb
interaction, a finite value of the electron charge and strong effective-mass
anisotropy result in a crucial difference from 3D Fermi liquid.


title:
Criterion for Dominance of Directional over Size Fluctuations in
  Destroying Order

abstract:
  For systems exhibiting a second-order phase transition with a spontaneously
broken continuous O(N)-symmetry at low temperature, we give a criterion for
judging at which temperature T_K long-range directional fluctuations of the
order field destroy the order when approaching the critical temperature from
below. The temperature T_K lies always significantly below the famous Ginzburg
temperature T_G at which size fluctuations of finite range in the order field
become important.


title:
Magnetic impurity effects in metallic carbon nanotubes: local non-Fermi
  liquid theory

abstract:
  Magnetic impurity effects on metallic carbon nanotubes are studied
theoretically. The resolvent method for the multi channel Kondo effect is
applied to the band structure of the kp perturbation hamiltonian in the limit
of the infinite onsite repulsion at the impurity site. We discuss the local
non-Fermi liquid behavior at temperatures lower than the Kondo temperature T_K.
The density of states of localized electron has a singularity |omega|^1/2 which
gives rise to a pseudo gap at the Kondo resonance in low temperatures. The
temperature dependence of the electronic resistivity is predicted as T^1/2, and
the imaginary part of dynamical susceptibilities has the |omega|^1/2
dependence. Possible experimental observations are discussed.

categories: cond-mat.mtrl-sci cond-mat.soft cond-mat.str-el physics.chem-ph quant-ph


title:
Fluctuation-driven insulator-to-metal transition in an external magnetic
  field

abstract:
  We consider a model for a metal-insulator transition of correlated electrons
in an external magnetic field. We find a broad region in interaction and
magnetic field where metallic and insulating (fully magnetized) solutions
coexist and the system undergoes a first-order metal-insulator transition. A
global instability of the magnetically saturated solution precedes the local
ones and is caused by collective fluctuations due to poles in electron-hole
vertex functions.


title: Critical Current in the High-T_c Glass model

abstract:
  The high-T_c glass model can be combined with the repulsive tt'--Hubbard
model as microscopic description of the striped domains found in the high-T_c
materials. In this picture the finite Hubbard clusters are the origin of the
d-wave pairing. In this paper we show, that the glass model can also explain
the critical currents usually observed in the high-T_c materials. We use two
different approaches to calculate the critical current densities of the
high-T_c glass model. Both lead to a strongly anisotropic critical current.
Finally we give an explanation, why we expect nonetheless a nearly perfect
isotropic critical current in the high-T_c superconductors.


title: Metastability in Josephson transmission lines

abstract:
  Thermal activation and macroscopic quantum tunneling in current-biased
discrete Josephson transmission lines are studied theoretically. The degrees of
freedom under consideration are the phases across the junctions which are
coupled to each other via the inductances of the system. The resistively
shunted junctions that we investigate constitute a system of N interacting
degrees of freedom with an overdamped dynamics. We calculate the decay rate
within exponential accuracy as a function of temperature and current. Slightly
below the critical current, the decay from the metastable state occurs via a
unique ("rigid") saddlepoint solution of the Euclidean action describing the
simultaneous decay of the phases in all the junctions. When the current is
reduced, a crossover to a regime takes place, where the decay occurs via an
"elastic" saddlepoint solution and the phases across the junctions leave the
metastable state one after another. This leads to an increased decay rate
compared with the rigid case both in the thermal and the quantum regime. The
rigid-to-elastic crossover can be sharp or smooth analogous to first- or
second- order phase transitions, respectively. The various regimes are
summarized in a current-temperature decay diagram.


title:
Search for Spontaneous Nucleation of Magnetic Flux During Rapid Cooling
  of YBCO films Through Tc

abstract:
  We describe an experimental search for spontaneous formation of flux lines
during a rapid quench of thin YBaCuO films through Tc. This effect is expected
according to the Kibble-Zurek mechanism of a creation of topological defects of
the order parameter during a symmetry breaking phase transition. Spontaneously
formed vortices were previously observed in superfluid 3He, while a similar
experiment in superfluid 4He gave negative results. Using a high Tc SQUID, we
measured both the magnetic flux in the sample during a quench with a
sensitivity of 20 phi-0/cm^2, and the field noise which one would expect from
flux lines pinned in the film. The sensitivity was sufficient to detect
spontaneous flux at a level corresponding to 10^(-3) of the prediction. Within
our resolution, we saw no evidence for this effect.


title: Disorder effects on transport near AFM quantum phase transitions

abstract:
  We discuss three different scenarios recently proposed to account for the
non-Fermi liquid behavior near antiferromagnetic (AFM) quantum critical points
in heavy-Fermion systems: (i) scattering of Fermi liquid quasiparticles by
strong spin fluctuations near the spin-density-wave instability, (ii) the
breakdown of the Kondo effect due to the competition with the RKKY interaction,
and (iii) the formation of magnetic regions due to rare configurations of the
disorder. Here we focus on the first scenario and show that it explains in some
detail the anomalous temperature dependence of the resistivity observed e.g. in
CePd_2Si_2, CeNi_2Ge_2 or CeIn_3. The interplay of strongly anisotropic
scattering due to critical spin-fluctuations and weak isotropic impurity
scattering leads to a regime with a resistivity rho = rho_0 +Delta rho(T),
Delta rho(T) =T^{3/2} f(T/rho_0) \propto T sqrt{rho_0} for sufficiently large T
and small rho_0.


title:
Analytical Calculation of the Nucleation Rate for First Order Phase
  Transitions beyond the Thin Wall Approximation

abstract:
  First order phase transitions in general proceed via nucleation of bubbles. A
theoretical basis for the calculation of the nucleation rate is given by the
homogeneous nucleation theory of Langer and its field theoretical version of
Callan and Coleman. We have calculated the nucleation rate beyond the thin wall
approximation by expanding the bubble solution and the fluctuation determinant
in powers of the asymmetry parameter. The result is expressed in terms of
physical model parameters.


title: Exact solution for core-collapsed isothermal star clusters

abstract:
  Star clusters in isothermal spheres are studied from a thermodynamic point of
view. New density profiles are presented, that describe the collapsed phase at
low temperatures. At the transition a set of binaries is formed that carries 13
% of the gravitational energy, while also a huge latent heat is generated. The
total energy of the binaries is fixed by thermodynamics. In the canonical
ensemble all specific heats are positive, while previously discussed negative
specific heat solutions are metastable. In a microcanonical ensemble the latter
remain partly dominant.


title: Edge currents in the absence of edges

abstract:
  We investigate a charged two-dimensional particle in a homogeneous magnetic
field interacting with a periodic array of point obstacles. We show that while
Landau levels remain to be infinitely degenerate eigenvalues, between them the
system has bands of absolutely continuous spectrum and exhibits thus a
transport along the array. We also compute the band functions and the
corresponding probability current.


title:
Collective motion and solid-liquid-type transitions in vibrated granular
  layers

abstract:
  From pressure and surface dilation measurements, we show that a
solid-liquid-type transition occurs at low excitation frequencies in vertically
vibrated granular layers. This transition precedes subharmonic bifurcations
from flat surface to standing wave patterns, indicating that these waves are in
fact associated with the fluid like behavior of the layer. In the limit of high
excitation frequencies, we show that a new kind of subharmonic waves can be
distinguished. These waves do not involve any lateral transfer of grains within
the layer and correspond to excitations for which the layer slightly bends
alternately in time and space. These bending waves have very low amplitude and
we observe them in a vibrated two-dimensional layer of photoelastic particles.


title:
Experiments on Ladders Reveal a Complex Interplay between a Spin-Gapped
  Normal State and Superconductivity

abstract:
  In recent years, the study of ladder materials has developed into a
well-established area of research within the general context of Strongly
Correlated Electrons. This effort has been triggered by an unusual
cross-fertilization between theory and experiments. In this paper, the main
experimental results obtained in the context of ladders are reviewed from the
perspective of a theorist. Emphasis is given to the many similarities between
the two-dimensional high-$\rm T_c$ cuprates and the two-leg ladder compounds,
including Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$ (14-24-41) which has a
superconducting phase at high pressure and a small hole density. Examples of
these similarities include regimes of linear resistivity vs temperature in
metallic ladders and a normal state with spin-gap or pseudogap characteristics.
Some controversial results in this context are also discussed. It is remarked
that the ladder 14-24-41 is the first superconducting copper-oxide material
with a non-square-lattice layered arrangement, and certainly much can be
learned from a careful analysis of this compound. A short summary of the main
theoretical developments in this field is also included, as well as a brief
description of the properties of non-copper-oxide ladders. Suggestions by the
author on possible experiments are described in the text. Overall, it is
concluded that the enormous experimental effort carried out on ladders has
already unveiled quite challenging and interesting physics that adds to the
rich behavior of electrons in transition-metal-oxides, and in addition
contributes to the understanding of the two-dimensional cuprates. However,
still considerable work needs to be carried out to fully understand the
interplay between charge and spin degrees of freedom in these materials.


title: Dissipationless transport in low density bilayer systems

abstract:
  In a bilayer electronic system the layer index may be viewed as the
z-component of an isospin-1/2. An XY isospin-ordered ferromagnetic phase was
observed in quantum Hall systems and is predicted to exist at zero magnetic
field at low density. This phase is a superfluid for opposite currents in the
two layers. At B=0 the system is gapless but superfluidity is not destroyed by
weak disorder. In the quantum Hall case, weak disorder generates a random gauge
field which probably does not destroy superfluidity. Experimental signatures
include Coulomb drag and collective mode measurements.


title:
All-dielectric one-dimensional periodic structures for total
  omnidirectional reflection and partial spontaneous emission control

abstract:
  A remarkable property of one-dimensional all-dielectric periodic structures
has recently been reported, namely a one-dimensional lattice can totally
reflect electromagnetic wave of any polarization at all angles within a
prescribed frequency region. Unlike their metallic counterpart, such
all-dielectric omnidirectional mirrors are nearly free of loss at optical
frequencies. Here we discuss the physics, design criteria and applications of
the thin-film all-dielectric omnidirectional mirror. The experimental
demonstration of the mirror is presented at optical frequencies.


title: Financial Friction and Multiplicative Markov Market Game

abstract:
  We study long-term growth-optimal strategies on a simple market with linear
proportional transaction costs. We show that several problems of this sort can
be solved in closed form, and explicit the non-analytic dependance of optimal
strategies and expected frictional losses of the friction parameter. We present
one derivation in terms of invariant measures of drift-diffusion processes
(Fokker- Planck approach), and one derivation using the Hamilton-Jacobi-Bellman
equation of optimal control theory. We also show that a significant part of the
results can be derived without computation by a kind of dimensional analysis.
We comment on the extension of the method to other sources of uncertainty, and
discuss what conclusions can be drawn about the growth-optimal criterion as
such.


title: Landau Levels from the Bethe Ansatz Equations

abstract:
  The Bethe ansatz (BA) equations for the two-dimensional Bloch electrons in a
uniform magnetic field are treated in the weak field limit. We have calculated
energies near the lower boundary of the energy spectrum up to the first
nontrivial order. It corresponds to calculating a finite size correction for
the excitation energies of the BA solvable lattice models and gives the Landau
levels in the present problem.


title:
Phase diagram of the three-dimensional Anderson model of localization
  with random hopping

abstract:
  We examine the localization properties of the three-dimensional (3D) Anderson
Hamiltonian with off-diagonal disorder using the transfer-matrix method (TMM)
and finite-size scaling (FSS). The nearest-neighbor hopping elements are chosen
randomly according to $t_{ij} \in [c-1/2, c + 1/2]$. We find that the
off-diagonal disorder is not strong enough to localize all states in the
spectrum in contradistinction to the usual case of diagonal disorder. Thus for
any off-diagonal disorder, there exist extended states and, consequently, the
TMM converges very slowly. From the TMM results we compute critical exponents
of the metal-insulator transitions (MIT), the mobility edge $E_c$, and study
the energy-disorder phase diagram.


title: Numerical results for two interacting particles in a random environment

abstract:
  Much evidence has been collected to date which shows that repulsive
electron-electron interaction can lead to the formation of particle pairs in a
one-dimensional random energy landscape. The localization length \lambda_2 of
these pair states is finite, but larger than the localization length \lambda_1
of the individual particles. After a short review of previous work, we present
numerical evidence for this effect based on an analysis of the interaction
matrix elements and an application of the decimation method. The results based
on the decimation method for a two-dimensional disordered medium support the
localization-delocalization transition of pair states predicted recently.


title: Flux penetration and expulsion in thin superconducting disks

abstract:
  Using an expansion of the order parameter over the eigenfunctions of the
linearized first Ginzburg-Landau (GL) equation, we obtain numerically the
saddle points of the free energy separating the stable states with different
number of vortices. In contrast to known surface and geometrical barrier
models, we find that in a wide range of magnetic fields below the penetration
field, the saddle point state for flux penetration into a disk does not
correspond to a vortex located nearby the sample boundary, but to a region of
suppressed superconductivity at the disk edge with no winding of the current,
and which is {\it a nucleus} for the following vortex creation. The height of
this {\it nucleation barrier}, which determines the time of flux penetration,
is calculated for different disk radii and magnetic fields.


title:
Enhanced stability of the square lattice of a classical bilayer Wigner
  crystal

abstract:
  The stability and melting transition of a single layer and a bilayer crystal
consisting of charged particles interacting through a Coulomb or a screened
Coulomb potential is studied using the Monte-Carlo technique. A new melting
criterion is formulated which we show to be universal for bilayer as well as
for single layer crystals in the case of (screened) Coulomb, Lennard--Jones and
1/r^{12} repulsive inter-particle interactions. The melting temperature for the
five different lattice structures of the bilayer Wigner crystal is obtained,
and a phase diagram is constructed as a function of the interlayer distance. We
found the surprising result that the square lattice has a substantial larger
melting temperature as compared to the other lattice structures. This is a
consequence of the specific topology of the defects which are created with
increasing temperature and which have a larger energy as compared to the
defects in e.g. a hexagonal lattice.


title: Finite size effects in the magnetization of periodic mesoscopic systems

abstract:
  We calculate the orbital magnetization of a confined 2DEG as a function of
the number of electrons in the system. Size effects are investigated by
systematically increasing the area of the confining region. The results for the
finite system are compared to an infinite one, where the magnetization is
calculated in the thermodynamic limit. In all calculations the
electron-electron interaction is included in the Hartree approximation.


title: Quantization Of Cyclotron Motion and Quantum Hall Effect

abstract:
  We present a two dimensional model of IQHE in accord with the cyclotron
motion. The quantum equation of the QHE curve and a new definition of filling
factor are also given.


title:
Current-Voltage Relations in d-wave Josephson Junctions: Effects of
  Midgap Interface States

abstract:
  We investigate the dc current-voltage characteristics of d-wave Josephson
junctions, where the barrier at the interface may have arbitrary strength.
Dividing the current into n-particle currents $I_n$ (n integer), we can
explicitly show which physical processes are responsible for the subharmonic
gap structure (SGS). For orientations where midgap states (MGS) exist, the
resonances in the n-particle processes are drastically changed, giving rise to
a strongly modified SGS. Introducing broadening in a phenomenological way, we
show that MGS may produce a current peak near zero bias and we explain which
physical processes are contributing to this peak. The agreement of our theory
with recent experiments is discussed.


title: An introduction to bosonization

abstract:
  This is an expanded version of a lecture given at the {\it Workshop on
Theoretical Methods for Strongly Correlated Fermions}, held at the {\it Centre
de Recherches Math\'ematiques}, in Montr\'eal, from May 26 to May 30, 1999.
After general comments on the relevance of field theory to condensed matter
systems, the continuum description of interacting electrons in 1D is
summarized. The bosonization procedure is then introduced heuristically, but
the precise quantum equivalence between fermion and boson is also presented.
Then the exact solution of the Tomonaga-Luttinger model is carried out. Two
other applications of bosonization are then sketched. We end with a quick
introduction to non-Abelian bosonization.


title: Spectroscopy of nanoscopic semiconductor rings

abstract:
  Making use of self-assembly techniques, we demonstrate the realization of
nanoscopic semiconductor quantum rings in which the electronic states are in
the true quantum limit. We employ two complementary spectroscopic techniques to
investigate both the ground states and the excitations of these rings. Applying
a magnetic field perpendicular to the plane of the rings, we find that when
approximately one flux quantum threads the interior of each ring, a change in
the ground state from angular momentum $\ell = 0$ to $\ell = -1$ takes place.
This ground state transition is revealed both by a drastic modification of the
excitation spectrum and by a change in the magnetic field dispersion of the
single-electron charging energy.


title: Self-Organized Criticality in the Olami-Feder-Christensen model

abstract:
  A system is in a self-organized critical state if the distribution of some
measured events (avalanche sizes, for instance) obeys a power law for as many
decades as it is possible to calculate or measure. The finite-size scaling of
this distribution function with the lattice size is usually enough to assume
that any cut off will disappear as the lattice size goes to infinity. This
approach, however, can lead to misleading conclusions. In this work we analyze
the behavior of the branching rate sigma of the events to establish whether a
system is in a critical state. We apply this method to the
Olami-Feder-Christensen model to obtain evidences that, in contrast to previous
results, the model is critical in the conservative regime only.


title:
Interpolation between Hubbard and supersymmetric t-J models.
  Two-parameter integrable models of correlated electrons

abstract:
  Two new one-dimensional fermionic models depending on two independent
parameters are formulated and solved exactly by the Bethe-ansatz method. These
models connect continuously the integrable Hubbard and supersymmetric t-J
models.


title: Inelastic neutron scattering peak in Zn substituted YBa2Cu3O7

abstract:
  The effects of nonmagnetic impurities on the neutron scattering intensity are
studied for a model of the copper oxide layers in the normal state. The
contribution to the Q=(pi,pi) neutron scattering intensity from processes
involving the scattering of the spin fluctuations from an impurity with large
momentum transfers are calculated within the random phase approximation. It is
shown that this type of scatterings could lead to a peak in the neutron
scattering intensity in the normal state.


title: Dynamics of a self-repelling random walker

abstract: This paper has been withdrawn by the authors.


title: Strongly localized anharmonic modes in perfect and imperfect crystals

abstract:
  Localized modes of large amplitudes in nonlinear lattices are considered. The
applied method allows the reduction of nonlinear problem to a linear inverse
problem of phonons scattering on a local potential. The method is efficient in
the case of strongly localized modes. Analytical description of such modes in
monatomic chain is given. Results of numerical calculations of anharmonic local
vibrations of light ions in pure and impure alkali halide crystal are
presented. It is found that, in the case of amplitudes ~0.5A, the vibrations
depend very strongly on the crystallographic directions.


title:
Why Self-Consistent Diagrammatic Perturbation Theory is `just'
  Perturbation Theory

abstract:
  In this short write-up we argue that self-consistent diagrammatic
perturbation theory(i.e. Feynman diagrams) for the one-particle Green function
is unable to capture some important qualitative features no matter how
self-consistently the Green functions are obtained. This write-up is intended
to highlight the short-comings of perturbation theory and also tout the
advantages of the sea-boson technique(hep-th/9706006).


title: Monte Carlo investigations of phase transitions: status and perspectives

abstract:
  Using the concept of finite-size scaling, Monte Carlo calculations of various
models have become a very useful tool for the study of critical phenomena, with
the system linear dimension as a variable. As an example, several recent
studies of Ising models are discussed, as well as the extension to models of
polymer mixtures and solutions. It is shown that using appropriate cluster
algorithms, even the scaling functions describing the crossover from the Ising
universality class to the mean-field behavior with increasing interaction range
can be described. Additionally, the issue of finite-size scaling in Ising
models above the marginal dimension (d*=4) is discussed.


title: Ultrasound Investigations of Orbital Quadrupolar Ordering in UPd_3

abstract:
  For a high-quality single crystal of UPd_3 we present the relevant elastic
constants and ultrasonic attenuation data. In addition to the magnetic phase
transition at T_2=4.4 +/- 0.1K and the quadrupolar transition at T_1~6.8K, we
find orbital ordering at T_0=7.6 +/- 0.1K concomitant with a symmetry change
from hexagonal to orthorhombic. A striking feature is the splitting of the
phase transition at T_1 into a second-order transition at T_{+1}=6.9 +/- 0.05K
and a first-order transition at T_{-1}=6.7 +/- 0.05K. For the four phase
transitions, the quadrupolar order parameters and the respective symmetry
changes are specified.


title:
Carrier relaxation, pseudogap, and superconducting gap in high-Tc
  cuprates: A Raman scattering study

abstract:
  We describe results of electronic Raman-scattering experiments in differently
doped single crystals of Y-123 and Bi-2212. The comparison of AF insulating and
metallic samples suggests that at least the low-energy part of the spectra
originates predominantly from excitations of free carriers. We therefore
propose an analysis of the data in terms of a memory function approach.
Dynamical scattering rates and mass-enhancement factors for the carriers are
obtained. In B2g symmetry the Raman data compare well to the results obtained
from ordinary and optical transport. For underdoped materials the dc scattering
rates in B1g symmetry become temperature independent and considerably larger
than in B2g symmetry. This increasing anisotropy is accompanied by a loss of
spectral weight in B2g symmetry in the range between the superconducting
transition at Tc and a characteristic temperature T* of order room temperature
which compares well with the pseudogap temperature found in other experiments.
The energy range affected by the pseudogap is doping and temperature
independent. The integrated spectral loss is approximately 25% in underdoped
samples and becomes much weaker towards higher carrier concentration. In
underdoped samples, superconductivity related features in the spectra can be
observed only in B2g symmetry. The peak frequencies scale with Tc. We do not
find a direct relation between the pseudogap and the superconducting gap.


title:
Resonant Raman Study of Superconducting Gap and Electron-Phonon Coupling
  in YbBa2Cu3O7-delta

abstract:
  We investigate the electronic background as well as the O2-O3 mode at 330
cm^-1 of highly doped YbBa2Cu3O7-delta in B1g symmetry. Above the critical
temperature Tc the spectra consist of an almost constant electronic background
and superimposed phononic excitations. Below Tc the superconducting gap opens
and the electronic background redistributes exhibiting a 2Delta peak at 320
cm^-1. We use a model that allows us to separate the background from the
phonon. In this model the phonon intensity is assigned to the coupling of the
phonon to inter- and intraband electronic excitations. For excitation energies
between 1.96 eV and 2.71 eV the electronic background exhibits hardly any
resonance. Accordingly, the intraband contribution to the phonon intensity is
not affected. In contrast, the interband contribution vanishes below Tc at 1.96
eV while it remains almost unaffected at 2.71 eV.


title:
Electric-Field Tuning of Spin-Dependent Exciton-Exciton Interactions in
  Coupled Quantum Wells

abstract:
  We have shown experimentally that an electric field decreases the energy
separation between the two components of a dense spin-polarized exciton gas in
a coupled double quantum well, from a maximum splitting of $\sim 4$ meV to
zero, at a field of $\sim $35 kV/cm. This decrease, due to the field-induced
deformation of the exciton wavefunction, is explained by an existing
calculation of the change in the spin-dependent exciton-exciton interaction
with the electron-hole separation. However, a new theory that considers the
modification of screening with that separation is needed to account for the
observed dependence on excitation power of the individual energies of the two
exciton components.


title: Replica field theory for a polymer in random media

abstract:
  In this paper we revisit the problem of a (non self-avoiding) polymer chain
in a random medium which was previously investigated by Edwards and Muthukumar
(EM). As noticed by Cates and Ball (CB) there is a discrepancy between the
predictions of the replica calculation of EM and the expectation that in an
infinite medium the quenched and annealed results should coincide (for a chain
that is free to move) and a long polymer should always collapse. CB argued that
only in a finite volume one might see a ``localization transition'' (or
crossover) from a stretched to a collapsed chain in three spatial dimensions.
Here we carry out the replica calculation in the presence of an additional
confining harmonic potential that mimics the effect of a finite volume. Using a
variational scheme with five variational parameters we derive analytically for
d<4 the result R~(g |ln \mu|)^{-1/(4-d)} ~(g lnV)^{-1/(4-d)}, where R is the
radius of gyration, g is the strength of the disorder, \mu is the spring
constant associated with the confining potential and V is the associated
effective volume of the system. Thus the EM result is recovered with their
constant replaced by ln(V) as argued by CB. We see that in the strict infinite
volume limit the polymer always collapses, but for finite volume a transition
from a stretched to a collapsed form might be observed as a function of the
strength of the disorder. For d<2 and for large
V>V'~exp[g^(2/(2-d))L^((4-d)/(2-d))] the annealed results are recovered and
R~(Lg)^(1/(d-2)), where L is the length of the polymer. Hence the polymer also
collapses in the large L limit. The 1-step replica symmetry breaking solution
is crucial for obtaining the above results.


title: Scattering of light and atoms in a Fermi-Dirac gas with BCS pairing

abstract:
  We theoretically study the optical properties of a Fermi-Dirac gas in the
presence of a superfluid state. We calculate the leading quantum-statistical
corrections to the standard column density result of the electric
susceptibility. We also consider the Bragg diffraction of atoms by means of
light-stimulated transitions of photons between two intersecting laser beams.
Bardeen-Cooper-Schrieffer pairing between atoms in different internal levels
magnifies incoherent scattering processes. The absorption linewidth of a
Fermi-Dirac gas is broadened and shifted. Bardeen-Cooper-Schrieffer pairing
introduces a collisional local-field shift that may dramatically dominate the
Lorentz-Lorenz shift. For the case of the Bragg spectroscopy the static
structure function may be significantly increased due to superfluidity in the
nearforward scattering.


title: Chiral Interactions and Structures

abstract:
  The chiral structure of liquid crystalline phases arises due to the intrinsic
chirality of the constituent mesogens. While it is seemingly straightforward to
quantify the macroscopic chirality by using, for instance, the cholesteric
pitch or the optical rotatory power, it is not as simple to quantify the
chirality of a single molecule. I will discuss a systematic quantification of
molecular chirality and show how the resulting chiral parameters may be used to
predict macroscopic chiral structure.


title:
Fermi edge singularities in X-ray spectra of strongly correlated
  fermions

abstract:
  We discuss the problem of the X-ray absorption in a system of interacting
fermions and, in particular, those features in the X-ray spectra that can be
used to discriminate between conventional Fermi-liquids and novel "strange
metals". Focusing on the case of purely forward scattering off the core-hole
potential, we account for the relevant interactions in the conduction band by
means of the bosonization technique. We find that the X-ray Fermi edge
singularities can still be present, although modified, even if the density of
states vanishes at the Fermi energy, and that, in general, the relationship
between the two appears to be quite subtle.


title:
Wide range dielectric spectroscopy on glass-forming materials: an
  experimental overview

abstract:
  Dielectric spectroscopy is one of the most commonly used techniques for the
investigation of the dynamic response of glass-forming materials. The
tremendous slow-down of the particle motions when approaching the glass
transition and especially the fast processes in glass-forming materials, which
have come into the focus of scientific interest recently, make the
investigation in a wide frequency range highly desirable. Recently, results
from broadband dielectric spectroscopy on glass-forming materials in their
liquid and supercooled-liquid state, covering more than 18 decades of frequency
have been reported by our group. In the present paper, we give an overview of
the various experimental setups and techniques used to collect these spectra.
As an example, spectra of the prototypical glass former glycerol are presented.


title:
Linear Field Dependence of the Normal-State In-Plane Magnetoresistance
  of Sr2RuO4

abstract:
  The transverse and longitudinal in-plane magnetoresistances in the normal
state of superconducting Sr2RuO4 single crystals have been measured. At low
temperatures, both of them were found to be positive with a linear
magnetic-field dependence above a threshold field, a result not expected from
electronic band theory. We argue that such behavior is a manifestation of a
novel coherent state characterized by a spin pseudo gap in the quasi-particle
excitation spectrum in Sr2RuO4.


title:
Observation of Magnetic Fingerprints in Superconducting Au_0.7In_0.3
  Cylinders

abstract:
  Reproducible, sample-specific magnetoresistance fluctuations (magnetic
fingerprints) have been observed experimentally in the low-temperature part of
the superconducting transition regime of disordered superconducting
Au_0.7In_0.3 cylinders. The amplitude of the fluctuation was found to exceed
that of the universal conductance fluctuation in normal metals by several
orders of magnitude. The physical origin of these observations is discussed in
the context of mesoscopic fluctuations of the superconducting condensation
energy in disordered superconductors.


title:
Transient Effect of Negative Electric Current in Irradiated
  Semiconductors

abstract:
  The peculiarities of electric current are studied occurring in semiconductors
with strongly nonuniform distribution of charge carriers. The formation of such
nonuniformities and the regulation of carrier mobilities can be realized by
means of external irradiation, for instance, charge-particle beams and laser
irradiation. The transient effect of negative electric current is shown to
arise under some specific conditions.


title: A Superconducting Persistent Current Qubit

abstract:
  We present the design of a superconducting qubit that has circulating
currents of opposite sign as its two states. The circuit consists of three
nano-scale aluminum Josephson junctions connected in a superconducting loop and
controlled by magnetic fields. The advantages of this qubit are that it can be
made insensitive to background charges in the substrate, the flux in the two
states can be detected with a SQUID, and the states can be manipulated with
magnetic fields. Coupled systems of qubits are also discussed as well as
sources of decoherence.


title:
Energy spectrum, density of states and optical transitions in strongly
  biased narrow-gap quantum wells

abstract:
  We study theoretically the effect of an electric field on the electron states
and far-infrared optical properties in narrow-gap lead salt quantum wells. The
electron states are described by a two-band Hamiltonian. An application of a
strong electric field across the well allows the control of the energy gap
between the two-dimensional (2D) states in a wide range. A sufficiently strong
electric field transforms the narrow-gap quantum well to a nearly gapless 2D
system, whose electron energy spectrum is described by linear dispersion
relations \epsilon_{\sigma} (k) ~\pm (k-k_{\sigma}), where k_{\sigma} are the
field-dependent 2D momenta corresponding to the minimum energy gaps for the
states with spin numbers \sigma. Due to the field-induced shift of the 2D
subband extrema away from k=0 the density of states has inverse-square-root
divergencies at the edges. This property may result in a considerable increase
of the magnitude of the optical absorption and in the efficiency of the
electrooptical effect.


title: K-matrices for non-abelian quantum Hall states

abstract:
  Two fundamental aspects of so-called non-abelian quantum Hall states (the
q-pfaffian states and more general) are a (generalized) pairing of the
participating electrons and the non-abelian statistics of the quasi-hole
excitations. In this paper, we show that these two aspects are linked by a
duality relation, which can be made manifest by considering the K-matrices that
describe the exclusion statistics of the fundamental excitations in these
systems.


title: Hamiltonian derivation of a detailed fluctuation theorem

abstract:
  We analyze the microscopic evolution of a system undergoing a
far-from-equilibrium thermodynamic process. Explicitly accounting for the
degrees of freedom of participating heat reservoirs, we derive a hybrid result,
similar in form to both the fluctuation theorem, and a statement of detailed
balance. We relate this result to the steady-state fluctuation theorem, and to
a free energy relation valid far from equilibrium.