5,710 research outputs found
Polaritons and Pairing Phenomena in Bose--Hubbard Mixtures
Motivated by recent experiments on cold atomic gases in ultra high finesse
optical cavities, we consider the problem of a two-band Bose--Hubbard model
coupled to quantum light. Photoexcitation promotes carriers between the bands
and we study the non-trivial interplay between Mott insulating behavior and
superfluidity. The model displays a global U(1) X U(1) symmetry which supports
the coexistence of Mott insulating and superfluid phases, and yields a rich
phase diagram with multicritical points. This symmetry property is shared by
several other problems of current experimental interest, including
two-component Bose gases in optical lattices, and the bosonic BEC-BCS crossover
problem for atom-molecule mixtures induced by a Feshbach resonance. We
corroborate our findings by numerical simulations.Comment: 4 pages, 3 figure
Modeling the elastic transmission of tidal stresses to great distances inland in channelized ice streams
Geodetic surveys suggest that ocean tides can modulate the motion of Antarctic ice streams, even at stations many tens of kilometers inland from the grounding line. These surveys suggest that ocean tidal stresses can perturb ice stream motion at distances about an order of magnitude farther inland than tidal flexure of the ice stream alone. Recent models exploring the role of tidal perturbations in basal shear stress are primarily one- or two-dimensional, with the impact of the ice stream margins either ignored or parameterized. Here, we use two- and three-dimensional finite-element modeling to investigate transmission of tidal stresses in ice streams and the impact of considering more realistic, three-dimensional ice stream geometries. Using Rutford Ice Stream as a real-world comparison, we demonstrate that the assumption that elastic tidal stresses in ice streams propagate large distances inland fails for channelized glaciers due to an intrinsic, exponential decay in the stress caused by resistance at the ice stream margins. This behavior is independent of basal conditions beneath the ice stream and cannot be fit to observations using either elastic or nonlinear viscoelastic rheologies without nearly complete decoupling of the ice stream from its lateral margins. Our results suggest that a mechanism external to the ice stream is necessary to explain the tidal modulation of stresses far upstream of the grounding line for narrow ice streams. We propose a hydrologic model based on time-dependent variability in till strength to explain transmission of tidal stresses inland of the grounding line. This conceptual model can reproduce observations from Rutford Ice Stream
Topological universality of level dynamics in quasi-one-dimensional disordered conductors
Nonperturbative, in inverse Thouless conductance 1/g, corrections to
distributions of level velocities and level curvatures in quasi-one-dimensional
disordered conductors with a topology of a ring subject to a constant vector
potential are studied within the framework of the instanton approximation of
nonlinear sigma-model. It is demonstrated that a global character of the
perturbation reveals the universal features of the level dynamics. The
universality shows up in the form of weak topological oscillations of the
magnitude ~ exp(-g) covering the main bodies of the densities of level
velocities and level curvatures. The period of discovered universal
oscillations does not depend on microscopic parameters of conductor, and is
only determined by the global symmetries of the Hamiltonian before and after
the perturbation was applied. We predict the period of topological oscillations
to be 4/(pi)^2 for the distribution function of level curvatures in orthogonal
symmetry class, and 3^(1/2)/(pi) for the distribution of level velocities in
unitary and symplectic symmetry classes.Comment: 15 pages (revtex), 3 figure
Distribution of "level velocities" in quasi 1D disordered or chaotic systems with localization
The explicit analytical expression for the distribution function of
parametric derivatives of energy levels ("level velocities") with respect to a
random change of scattering potential is derived for the chaotic quantum
systems belonging to the quasi 1D universality class (quantum kicked rotator,
"domino" billiard, disordered wire, etc.).Comment: 11 pages, REVTEX 3.
Anomalous flux-flow dynamics in layered type-II superconductors at low temperatures
Low-temperature dissipation due to vortex motion in strongly anisotropic
type-II superconductors with a moderate disorder () is shown to be determined by the Zener-type transitions between
the localized electronic states in the vortex core. Statistics of these levels
is described by the random matrix ensemble of the class C defined recently by
Atland and Zirnbauer [cond-mat/9602137], so the vortex motion leads naturally
to the new example of a parametric statistics of energy levels. The flux-flow
conductivity is a bit lower than the quasiclassical one and {\it
grows} slowly with the increase of the electric field.Comment: 4 pages, Revte
Energy absorption in time-dependent unitary random matrix ensembles: dynamic vs Anderson localization
We consider energy absorption in an externally driven complex system of
noninteracting fermions with the chaotic underlying dynamics described by the
unitary random matrices. In the absence of quantum interference the energy
absorption rate W(t) can be calculated with the help of the linear-response
Kubo formula. We calculate the leading two-loop interference correction to the
semiclassical absorption rate for an arbitrary time dependence of the external
perturbation. Based on the results for periodic perturbations, we make a
conjecture that the dynamics of the periodically-driven random matrices can be
mapped onto the one-dimensional Anderson model. We predict that in the regime
of strong dynamic localization W(t) ln(t)/t^2 rather than decays exponentially.Comment: 6 pages, 1 figur
Bose--Hubbard Models Coupled to Cavity Light Fields
Recent experiments on strongly coupled cavity quantum electrodynamics present
new directions in "matter-light" systems. Following on from our previous work
[Phys. Rev. Lett. 102, 135301 (2009)] we investigate Bose-Hubbard models
coupled to a cavity light field. We discuss the emergence of photoexcitations
or "polaritons" within the Mott phase, and obtain the complete variational
phase diagram. Exploiting connections to the super-radiance transition in the
Dicke model we discuss the nature of polariton condensation within this novel
state. Incorporating the effects of carrier superfluidity, we identify a
first-order transition between the superradiant Mott phase and the single
component atomic superfluid. The overall predictions of mean field theory are
in excellent agreement with exact diagonalization and we provide details of
superfluid fractions, density fluctuations, and finite size effects. We
highlight connections to recent work on coupled cavity arrays.Comment: 16 pages, 17 figure
Classical and Quantum Dynamics in a Random Magnetic Field
Using the supersymmetry approach, we study spectral statistical properties of
a two-dimensional quantum particle subject to a non-uniform magnetic field. We
focus mainly on the problem of regularisation of the field theory. Our analysis
begins with an investigation of the spectral properties of the purely classical
evolution operator. We show that, although the kinetic equation is formally
time-reversible, density relaxation is controlled by {\em irreversible}
classical dynamics. In the case of a weak magnetic field, the effective kinetic
operator corresponds to diffusion in the angle space, the diffusion constant
being determined by the spectral resolution of the inhomogeneous magnetic
field. Applying these results to the quantum problem, we demonstrate that the
low-lying modes of the field theory are related to the eigenmodes of the
irreversible classical dynamics, and the higher modes are separated from the
zero mode by a gap associated with the lowest density relaxation rate. As a
consequence, we find that the long-time properties of the system are
characterised by universal Wigner-Dyson statistics. For a weak magnetic field,
we obtain a description in terms of the quasi one-dimensional non-linear
-model.Comment: 16 pages, RevTe
Non-universal corrections to the level curvature distribution beyond random matrix theory
The level curvature distribution function is studied beyond the random matrix
theory for the case of T-breaking perturbations over the orthogonal ensemble.
The leading correction to the shape of the level curvature distribution is
calculated using the nonlinear sigma-model. The sign of the correction depends
on the presence or absence of the global gauge invariance and is different for
perturbations caused by the constant vector-potential and by the random
magnetic field. Scaling arguments are discussed that indicate on the
qualitative difference in the level statistics in the dirty metal phase for
space dimensionalities .Comment: 4 pages, Late
Pattern Formation as a Signature of Quantum Degeneracy in a Cold Exciton System
The development of a Turing instability to a spatially modulated state in a
photoexcited electron-hole system is proposed as a novel signature of exciton
Bose statistics. We show that such an instability, which is driven by kinetics
of exciton formation, can result from stimulated processes that build up near
quantum degeneracy. In the spatially uniform 2d electron-hole system, the
instability leads to a triangular lattice pattern while, at an electron-hole
interface, a periodic 1d pattern develops. We analyze the mechanism of
wavelength selection, and show that the transition is abrupt (type I) for the
uniform 2d system, and continuous (type II) for the electron-hole interface.Comment: 5 pages, 3 figure
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