5,177 research outputs found
Kinetics of the Phase Separation Transition in Cold-Atom Boson-Fermion Mixtures
We study the kinetics of the first order phase separation transition in
boson-fermion cold-atom mixtures. At sufficiently low temperatures such a
transition is driven by quantum fluctuations responsible for the formation of
critical nuclei of a stable phase. Based on a microscopic description of
interacting boson-fermion mixtures we derive an effective action for the
critical droplet and obtain an asymptotic expression for the nucleation rate in
the vicinity of the phase transition and near the spinodal instability of the
mixed phase. We also discuss effects of dissipation which play a dominant role
close to the transition point, and identify the regimes where quantum
nucleation can be experimentally observed in cold-atom systems.Comment: 4 pages 1 figure, typos correcte
Trigonal warping and Berry’s phase Npi in ABC-stacked multilayer graphene.
The electronic band structure of ABC-stacked multilayer graphene is studied within an effective mass approximation. The electron and hole bands touching at zero energy support chiral quasiparticles characterized by Berry’s phase Nπ for N-layers, generalizing the low-energy band structure of monolayer and bilayer graphene. We investigate the trigonal-warping deformation of the energy bands and show that the Lifshitz transition, in which the Fermi circle breaks up into separate parts at low energy, reflects Berry’s phase Nπ. It is particularly prominent in trilayers, N = 3, with the Fermi circle breaking into three parts at a relatively large energy that is related to next-nearestlayer coupling. For N = 3, we study the effects of electrostatic potentials which vary in the stacking direction, and find that a perpendicular electric field, as well as opening an energy gap, strongly enhances the trigonal-warping effect. In magnetic fields, the N = 3 Lifshitz transition is manifested as a coalescence of Landau levels into triply-degenerate levels
Generalized Lifshitz-Kosevich scaling at quantum criticality from the holographic correspondence
We characterize quantum oscillations in the magnetic susceptibility of a
quantum critical non-Fermi liquid. The computation is performed in a strongly
interacting regime using the nonperturbative holographic correspondence. The
temperature dependence of the amplitude of the oscillations is shown to depend
on a critical exponent nu. For general nu the temperature scaling is distinct
from the textbook Lifshitz-Kosevich formula. At the `marginal' value nu = 1/2,
the Lifshitz-Kosevich formula is recovered despite strong interactions. As a
by-product of our analysis we present a formalism for computing the amplitude
of quantum oscillations for general fermionic theories very efficiently.Comment: 18 pages, pdftex, 1 figure. v2: figure and few comments adde
Thermal van der Waals Interaction between Graphene Layers
The van de Waals interaction between two graphene sheets is studied at finite
temperatures. Graphene's thermal length controls
the force versus distance as a crossover from the zero temperature
results for , to a linear-in-temperature, universal regime for
. The large separation regime is shown to be a consequence of the
classical behavior of graphene's plasmons at finite temperature. Retardation
effects are largely irrelevant, both in the zero and finite temperature
regimes. Thermal effects should be noticeable in the van de Waals interaction
already for distances of tens of nanometers at room temperature.Comment: enlarged version, 9 pages, 4 figures, updated reference
Local polariton states in impure ionic crystals
We consider the dynamics of an ionic crystal with a single impurity in the
vicinity of the polariton resonance. We show that if the polariton spectrum of
the host crystal allows for a gap between polariton branches, the defect gives
rise to a novel kind of local states with frequencies within the gap. Despite
the atomic size of the impurity we find that new local states are predominated
by long-wavelength polaritons. The properties of these states are shown to be
different from the properties of the well-known vibrational local states. The
difference is due to the singular behavior of the density of states of
polaritons near the low-frequency boundary of the polariton gap. Assuming cubic
simmetry of the defect site we consider a complete set of the local states
arising near the bottom of the polariton gap.Comment: 10 pages, 3 Postscript figures, to be published in Phys. Rev. B 1998,
Vol. 57, No.
Magnetic spectrum of trigonally warped bilayer graphene - semiclassical analysis, zero modes, and topological winding numbers
We investigate the fine structure in the energy spectrum of bilayer graphene
in the presence of various stacking defaults, such as a translational or
rotational mismatch. This fine structure consists of four Dirac points that
move away from their original positions as a consequence of the mismatch and
eventually merge in various manners. The different types of merging are
described in terms of topological invariants (winding numbers) that determine
the Landau-level spectrum in the presence of a magnetic field as well as the
degeneracy of the levels. The Landau-level spectrum is, within a wide parameter
range, well described by a semiclassical treatment that makes use of
topological winding numbers. However, the latter need to be redefined at zero
energy in the high-magnetic-field limit as well as in the vicinity of saddle
points in the zero-field dispersion relation.Comment: 17 pages, 16 figures; published version with enhanced discussion of
experimental finding
The Boson Peak and its Relation with Acoustic Attenuation in Glasses
Experimental results on the density of states and on the acoustic modes of
glasses in the THz region are compared to the predictions of two categories of
models. A recent one, solely based on an elastic instability, does not account
for most observations. Good agreement without adjustable parameters is obtained
with models including the existence of non-acoustic vibrational modes at THz
frequency, providing in many cases a comprehensive picture for a range of glass
anomalies.Comment: 4 pages, 3 figures, Physical Review Letters in pres
Wave localization in strongly nonlinear Hertzian chains with mass defect
We investigate the dynamical response of a mass defect in a one-dimensional
non-loaded horizontal chain of identical spheres which interact via the
nonlinear Hertz potential. Our experiments show that the interaction of a
solitary wave with a light intruder excites localized mode. In agreement with
dimensional analysis, we find that the frequency of localized oscillations
exceeds the incident wave frequency spectrum and nonlinearly depends on the
size of the intruder and on the incident wave strength. The absence of tensile
stress between grains allows some gaps to open, which in turn induce a
significant enhancement of the oscillations amplitude. We performed numerical
simulations that precisely describe our observations without any adjusting
parameters.Comment: 4 pages, 5 figures, submitted for publicatio
Bose-Einstein Condensates in Strongly Disordered Traps
A Bose-Einstein condensate in an external potential consisting of a
superposition of a harmonic and a random potential is considered theoretically.
From a semi-quantitative analysis we find the size, shape and excitation
energy as a function of the disorder strength. For positive scattering length
and sufficiently strong disorder the condensate decays into fragments each of
the size of the Larkin length . This state is stable over a large
range of particle numbers. The frequency of the breathing mode scales as
. For negative scattering length a condensate of size
may exist as a metastable state. These finding are generalized to anisotropic
traps
Homoclinic orbits and chaos in a pair of parametrically-driven coupled nonlinear resonators
We study the dynamics of a pair of parametrically-driven coupled nonlinear
mechanical resonators of the kind that is typically encountered in applications
involving microelectromechanical and nanoelectromechanical systems (MEMS &
NEMS). We take advantage of the weak damping that characterizes these systems
to perform a multiple-scales analysis and obtain amplitude equations,
describing the slow dynamics of the system. This picture allows us to expose
the existence of homoclinic orbits in the dynamics of the integrable part of
the slow equations of motion. Using a version of the high-dimensional Melnikov
approach, developed by Kovacic and Wiggins [Physica D, 57, 185 (1992)], we are
able to obtain explicit parameter values for which these orbits persist in the
full system, consisting of both Hamiltonian and non-Hamiltonian perturbations,
to form so-called Shilnikov orbits, indicating a loss of integrability and the
existence of chaos. Our analytical calculations of Shilnikov orbits are
confirmed numerically
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