40,497 research outputs found
Beyond Mean-Field Low-Lying Excitations of Dipolar Bose Gases
We theoretically investigate various beyond mean-field effects on Bose gases
at zero temperature featuring the anisotropic and long-range dipole-dipole
interaction in addition to the isotropic and short-range contact interaction.
Within the realm of the Bogoliubov-de Gennes theory, we consider static
properties and low-lying excitations of both homogeneous and harmonically
trapped dipolar bosonic gases. For the homogeneous system, the condensate
depletion, the ground-state energy, the equation of state, and the speed of
sound are discussed in detail. Making use of the local density approximation,
we extend these results in order to study the properties of a dipolar Bose gas
in a harmonic trap and in the regime of large particle numbers. After deriving
the equations of motion for the general case of a triaxial trap, we analyze the
influence of quantum fluctuations on important properties of the gas, such as
the equilibrium configuration and the low-lying excitations in the case of a
cylinder-symmetric trap. In addition to the monopole and quadrupole oscillation
modes, we also discuss the radial quadrupole mode. We find that the latter
acquires a quantum correction exclusively due to the dipole-dipole interaction.
As a result, we identify the radial quadrupole as a reasonably accessible
source for the signature of dipolar many-body effects and stress the enhancing
character that dipolar interactions have for quantum fluctuations in the other
oscillation modes.Comment: Version published in PR
Spin and Charge Structure of the Surface States in Topological Insulators
We investigate the spin and charge densities of surface states of the
three-dimensional topological insulator , starting from the continuum
description of the material [Zhang {\em et al.}, Nat. Phys. 5, 438 (2009)]. The
spin structure on surfaces other than the 111 surface has additional complexity
because of a misalignment of the contributions coming from the two sublattices
of the crystal. For these surfaces we expect new features to be seen in the
spin-resolved ARPES experiments, caused by a non-helical spin-polarization of
electrons at the individual sublattices as well as by the interference of the
electron waves emitted coherently from two sublattices. We also show that the
position of the Dirac crossing in spectrum of surface states depends on the
orientation of the interface. This leads to contact potentials and surface
charge redistribution at edges between different facets of the crystal.Comment: Use the correct spin operator. Changes affect the surface states spin
structure, but not the spectru
Edge Mode Combinations in the Entanglement Spectra of Non-Abelian Fractional Quantum Hall States on the Torus
We present a detailed analysis of bi-partite entanglement in the non-Abelian
Moore-Read fractional quantum Hall state of bosons and fermions on the torus.
In particular, we show that the entanglement spectra can be decomposed into
intricate combinations of different sectors of the conformal field theory
describing the edge physics, and that the edge level counting and tower
structure can be microscopically understood by considering the vicinity of the
thin-torus limit. We also find that the boundary entropy density of the
Moore-Read state is markedly higher than in the Laughlin states investigated so
far. Despite the torus geometry being somewhat more involved than in the sphere
geometry, our analysis and insights may prove useful when adopting entanglement
probes to other systems that are more easily studied with periodic boundary
conditions, such as fractional Chern insulators and lattice problems in
general.Comment: 13 pages, 8 figures, published version on PR
Decoherence effects on weak value measurements in double quantum dots
We study the effect of decoherence on a weak value measurement in a paradigm
system consisting of a double quantum dot continuously measured by a quantum
point contact. Fluctuations of the parameters controlling the dot state induce
decoherence. We find that, for measurements longer than the decoherence time,
weak values are always reduced within the range of the eigenvalues of the
measured observable. For measurements at shorter time scales, the measured weak
value strongly depends on the interplay between the decoherence dynamics of the
system and the detector backaction. In particular, depending on the
postselected state and the strength of the decoherence, a more frequent
classical readout of the detector might lead to an enhancement of weak values.Comment: published version, new figures and comments added; 15 pages, 7
figure
Timescale-invariant representation of acoustic communication signals by a bursting neuron
Acoustic communication often involves complex sound motifs in which the relative durations of individual elements, but not their absolute durations, convey meaning. Decoding such signals requires an explicit or implicit calculation of the ratios between time intervals. Using grasshopper communication as a model, we demonstrate how this seemingly difficult computation can be solved in real time by a small set of auditory neurons. One of these cells, an ascending interneuron, generates bursts of action potentials in response to the rhythmic syllable-pause structure of grasshopper calls. Our data show that these bursts are preferentially triggered at syllable onset; the number of spikes within the burst is linearly correlated with the duration of the preceding pause. Integrating the number of spikes over a fixed time window therefore leads to a total spike count that reflects the characteristic syllable-to-pause ratio of the species while being invariant to playing back the call faster or slower. Such a timescale-invariant recognition is essential under natural conditions, because grasshoppers do not thermoregulate; the call of a sender sitting in the shade will be slower than that of a grasshopper in the sun. Our results show that timescale-invariant stimulus recognition can be implemented at the single-cell level without directly calculating the ratio between pulse and interpulse durations
Kinetics of the disordered Bose gas with collisions
We discuss the kinetics of the disordered interacting Bose gas using the
Boltzmann transport equation. The theory may serve as a unifying framework for
studying questions of dynamics of the expanding Bose gas at different stages of
the expansion. We show that the transport theory allows us to straightforwardly
reproduce and generalize a number of results previously obtained from
microscopic models in different formalisms. Based on estimates for the
interparticle scattering rates, we discuss the relevance of interaction effects
for the localization problem in the interacting disordered Bose gas. We argue
that, if the number of particles is large enough, the size of the expanding
cloud may exceed the localization length. We describe the spreading of the wave
packet in this regime as collision-induced diffusion and compare the obtained
rate of expansion to known results on subdiffusive spreading in nonlinear
disordered lattices.Comment: 7 page
Spiral order in the honeycomb iridate Li2IrO3
The honeycomb iridates A2IrO3 (A=Na, Li) constitute promising candidate
materials to realize the Heisenberg-Kitaev model (HKM) in nature, hosting
unconventional magnetic as well as spin liquid phases. Recent experiments
suggest, however, that Li2IrO3 exhibits a magnetically ordered state of
incommensurate spiral type which has not been identified in the HKM. We show
that these findings can be understood in the context of an extended
Heisenberg-Kitaev scenario satisfying all tentative experimental evidence: (i)
the maximum of the magnetic susceptibility is located inside the first
Brillouin zone, (ii) the Curie-Weiss temperature is negative relating to
dominant antiferromagnetic fluctuations, and (iii) significant second-neighbor
spin-exchange is involved.Comment: 5 pages, 5 figures, selected as an Editors' suggestio
Continuous-variable entanglement distillation and non-commutative central limit theorems
Entanglement distillation transforms weakly entangled noisy states into
highly entangled states, a primitive to be used in quantum repeater schemes and
other protocols designed for quantum communication and key distribution. In
this work, we present a comprehensive framework for continuous-variable
entanglement distillation schemes that convert noisy non-Gaussian states into
Gaussian ones in many iterations of the protocol. Instances of these protocols
include (a) the recursive-Gaussifier protocol, (b) the temporally-reordered
recursive-Gaussifier protocol, and (c) the pumping-Gaussifier protocol. The
flexibility of these protocols give rise to several beneficial trade-offs
related to success probabilities or memory requirements, which that can be
adjusted to reflect experimental demands. Despite these protocols involving
measurements, we relate the convergence in this protocols to new instances of
non-commutative central limit theorems, in a formalism that we lay out in great
detail. Implications of the findings for quantum repeater schemes are
discussed.Comment: published versio
On entanglement evolution across defects in critical chains
We consider a local quench where two free-fermion half-chains are coupled via
a defect. We show that the logarithmic increase of the entanglement entropy is
governed by the same effective central charge which appears in the ground-state
properties and which is known exactly. For unequal initial filling of the
half-chains, we determine the linear increase of the entanglement entropy.Comment: 11 pages, 5 figures, minor changes, reference adde
Gene expression profiling in slow-Type calf soleus muscle of 30 days space-flown mice
Microgravity exposure as well as chronic disuse are two main causes of skeletal muscle atrophy in animals and humans. The antigravity calf soleus is a reference postural muscle to investigate the mechanism of disuse-induced maladaptation and plasticity of human and rodent (rats or mice) skeletal musculature. Here, we report microgravity-induced global gene expression changes in space-flown mouse skeletal muscle and the identification of yet unknown disuse susceptible transcripts found in soleus (a mainly slow phenotype) but not in extensor digitorum longus (a mainly fast phenotype dorsiflexor as functional counterpart to soleus). Adult C57Bl/N6 male mice (n = 5) flew aboard a biosatellite for 30 days on orbit (BION-M1 mission, 2013), a sex and age-matched cohort were housed in standard vivarium cages (n = 5), or in a replicate flight habitat as ground control (n = 5). Next to disuse atrophy signs (reduced size and myofiber phenotype I to II type shift) as much as 680 differentially expressed genes were found in the space-flown soleus, and only 72 in extensor digitorum longus (only 24 genes in common) compared to ground controls. Altered expression of gene transcripts matched key biological processes (contractile machinery, calcium homeostasis, muscle development, cell metabolism, inflammatory and oxidative stress response). Some transcripts (Fzd9, Casq2, Kcnma1, Ppara, Myf6) were further validated by quantitative real-time PCR (qRT-PCR). Besides previous reports on other leg muscle types we put forth for the first time a complete set of microgravity susceptible gene transcripts in soleus of mice as promising new biomarkers or targets for optimization of physical countermeasures and rehabilitation protocols to overcome disuse atrophy conditions in different clinical settings, rehabilitation and spaceflight
- …