3,988 research outputs found
Electronic energy spectra and wave functions on the square Fibonacci tiling
We study the electronic energy spectra and wave functions on the square
Fibonacci tiling, using an off-diagonal tight-binding model, in order to
determine the exact nature of the transitions between different spectral
behaviors, as well as the scaling of the total bandwidth as it becomes finite.
The macroscopic degeneracy of certain energy values in the spectrum is invoked
as a possible mechanism for the emergence of extended electronic Bloch wave
functions as the dimension changes from one to two
Traversable wormhole in the deformed Ho\v{r}ava-Lifshitz gravity
Asymptotically flat wormhole solutions are found in the deformed
Ho\v{r}ava-Lifshitz gravity. It turns out that higher curvature terms can not
play the role of exotic matters which are crucial to form a traversable
wormhole, and external exotic sources are still needed. In particular, the
exotic matter behaves like phantom energy if Kehagias-Sfetsos vacuum is
considered outside the wormhole. Interestingly, the spherically symmetric
setting makes the matter and the higher curvature contribution satisfy
four-dimensional conservation of energy in the covariant form.Comment: 13 pages, 2 figures, version published in Phys. Rev.
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
Topological phase transitions in ultra-cold Fermi superfluids: the evolution from BCS to BEC under artificial spin-orbit fields
We discuss topological phase transitions in ultra-cold Fermi superfluids
induced by interactions and artificial spin orbit fields. We construct the
phase diagram for population imbalanced systems at zero and finite
temperatures, and analyze spectroscopic and thermodynamic properties to
characterize various phase transitions. For balanced systems, the evolution
from BCS to BEC superfluids in the presence of spin-orbit effects is only a
crossover as the system remains fully gapped, even though a triplet component
of the order parameter emerges. However, for imbalanced populations, spin-orbit
fields induce a triplet component in the order parameter that produces nodes in
the quasiparticle excitation spectrum leading to bulk topological phase
transitions of the Lifshitz type. Additionally a fully gapped phase exists,
where a crossover from indirect to direct gap occurs, but a topological
transition to a gapped phase possessing Majorana fermions edge states does not
occur.Comment: With no change in text, the labels in the figures are modifie
Matter Bounce in Horava-Lifshitz Cosmology
Horava-Lifshitz gravity, a recent proposal for a UV-complete renormalizable
gravity theory, may lead to a bouncing cosmology. In this note we argue that
Horava-Lifshitz cosmology may yield a concrete realization of the matter bounce
scenario, and thus give rise to an alternative to inflation for producing a
scale-invariant spectrum of cosmological perturbations. In this scenario,
quantum vacuum fluctuations exit the Hubble radius in the pre-bounce phase and
the spectrum is transformed into a scale-invariant one on super-Hubble scales
before the bounce because the long wavelength modes undergo squeezing of their
wave-functions for a longer period of time than shorter wavelength modes. The
scale-invariance of the spectrum of curvature fluctuations is preserved during
and after the bounce. A distinctive prediction of this scenario is the
amplitude and shape of the bispectrum.Comment: 6 pages, 1 figure, a couple of minor wording change
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
Symmetry of Magnetically Ordered Quasicrystals
The notion of magnetic symmetry is reexamined in light of the recent
observation of long range magnetic order in icosahedral quasicrystals [Charrier
et al., Phys. Rev. Lett. 78, 4637 (1997)]. The relation between the symmetry of
a magnetically-ordered (periodic or quasiperiodic) crystal, given in terms of a
``spin space group,'' and its neutron diffraction diagram is established. In
doing so, an outline of a symmetry classification scheme for magnetically
ordered quasiperiodic crystals is provided. Predictions are given for the
expected diffraction patterns of magnetically ordered icosahedral crystals,
provided their symmetry is well described by icosahedral spin space groups.Comment: 5 pages. Accepted for publication in Phys. Rev. Letter
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
QCD Splitting/Joining Functions at Finite Temperature in the Deep LPM Regime
There exist full leading-order-in-alpha_s numerical calculations of the rates
for massless quarks and gluons to split and join in the background of a
quark-gluon plasma through hard, nearly collinear bremsstrahlung and inverse
bremsstrahlung. In the limit of partons with very high energy E, where the
physics is dominated by the Landau-Pomeranchuk-Migdal (LPM) effect, there are
also analytic leading-log calculations of these rates, where the logarithm is
ln(E/T). We extend those analytic calculations to next-to-leading-log order. We
find agreement with the full result to within roughly 20% for E(less) >~ 10 T,
where E(less) is the energy of the least energetic parton in the
splitting/joining process. We also discuss how to account for the running of
the coupling constant in the case that E/T is very large. Our results are also
applicable to isotropic non-equilibrium plasmas if the plasma does not change
significantly over the formation time associated with particle splitting.Comment: 20 pages, 6 figures. Changes from v3: Typos fixed in the subscripts
of various Casimir factor
Unconventional strongly interacting Bose-Einstein condensates in optical lattices
Feschbach resonances in a non-s-wave channel of two-component bosonic
mixtures can induce atomic Bose Einstein condensates with a non-zero orbital
momentum in the optical lattice, if one component is in the Mott insulator
state and the other is not. Such non-s-wave condensates break the symmetry of
the lattice and, in some cases, time-reversal symmetry. They can be revealed in
specific absorption imaging patterns.Comment: Replaced with revised version. References are adde
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