3,228 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

### Anomalous galvanomagnetism, cyclotron resonance and microwave spectroscopy of topological insulators

The surface quantum Hall state, magneto-electric phenomena and their
connection to axion electrodynamics have been studied intensively for
topological insulators. One of the obstacles for observing such effects comes
from nonzero conductivity of the bulk. To overcome this obstacle we propose to
use an external magnetic field to suppress the conductivity of the bulk
carriers. The magnetic field dependence of galvanomagnetic and electromagnetic
responses of the whole system shows anomalies due to broken time-reversal
symmetry of the surface quantum Hall state, which can be used for its
detection. In particular, we find linear bulk dc magnetoresistivity and a
quadratic field dependence of the Hall angle, shifted rf cyclotron resonance,
nonanalytic microwave transmission coefficient and saturation of the Faraday
rotation angle with increasing magnetic field or wave frequency.Comment: 5 pages, 3 figures, version as publishe

### Quantum Hall effects of graphene with multi orbitals: Topological numbers, Boltzmann conductance and Semi-classical quantization

Hall conductance $\sigma_{xy}$ as the Chern numbers of the Berry connection
in the magnetic Brillouin zone is calculated for a realistic multi band
tight-band model of graphene with non-orthogonal basis. It is confirmed that
the envelope of $\sigma_{xy}$ coincides with a semi-classical result when
magnetic field is sufficiently small.
The Hall resistivity $\rho_{xy}$ from the weak-field Boltzmann theory also
explains the overall behaviour of the $\sigma_{xy}$ if the Fermi surface is
composed of a single energy band. The plateaux of $\sigma_{xy}$ are explained
from semi-classical quantization and necessary modification is proposed for the
Dirac fermion regimes.Comment: 5pages, 3figure

### 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

### 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 ${\cal L}$. This state is stable over a large
range of particle numbers. The frequency of the breathing mode scales as
$1/{\cal L}^2$. For negative scattering length a condensate of size ${\cal L}$
may exist as a metastable state. These finding are generalized to anisotropic
traps

### 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

### 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

### Confined coherence in quasi-one-dimensional metals

We present a functional renormalization group calculation of the effect of
strong interactions on the shape of the Fermi surface of weakly coupled
metallic chains. In the regime where the bare interchain hopping is small, we
show that scattering processes involving large momentum transfers perpendicular
to the chains can completely destroy the warping of the true Fermi surface,
leading to a confined state where the renormalized interchain hopping vanishes
and a coherent motion perpendicular to the chains is impossible.Comment: 4 RevTex pages, 5 figures,final version as published by PR

### Lifshitz transitions in a heavy-Fermion liquid driven by short-range antiferromagnetic correlations in the two-dimensional Kondo lattice model

The heavy-Fermion liquid with short-range antiferromagnetic correlations is
carefully considered in the two-dimensional Kondo-Heisenberg lattice model. As
the ratio of the local Heisenberg superexchange $J_{H}$ to the Kondo coupling
$J_{K}$ increases, Lifshitz transitions are anticipated, where the topology of
the Fermi surface (FS) of the heavy quasiparticles changes from a hole-like
circle to four kidney-like pockets centered around $(\pi ,\pi)$. In-between
these two limiting cases, a first-order quantum phase transition is identified
at $J_{H}/J_{K}=0.1055$ where a small circle begins to emerge within the large
deformed circle. When $J_{H}/J_{K}=0.1425$, the two deformed circles intersect
each other and then decompose into four kidney-like Fermi pockets via a
second-order quantum phase transition. As $J_{H}/J_{K}$ increases further, the
Fermi pockets are shifted along the direction ($\pi,\pi$) to ($\pi/2,\pi/2$),
and the resulting FS is consistent with the FS obtained recently using the
quantum Monte Carlo cluster approach to the Kondo lattice system in the
presence of the antiferrmagnetic order.Comment: 4 pages, 5 figure

### 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.

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