Comment on "Phonon Spectrum and Dynamical Stability of a Dilute Quantum Degenerate Bose-Fermi Mixture

We show that the conclusions of a recent PRL by Pu et al is incorrect.Comment: late

Casimir Force between a Small Dielectric Sphere and a Dielectric Wall

The possibility of repulsive Casimir forces between small metal spheres and a dielectric half-space is discussed. We treat a model in which the spheres have a dielectric function given by the Drude model, and the radius of the sphere is small compared to the corresponding plasma wavelength. The half-space is also described by the same model, but with a different plasma frequency. We find that in the retarded limit, the force is quasi-oscillatory. This leads to the prediction of stable equilibrium points at which the sphere could levitate in the Earth's gravitational field. This seems to lead to the possibility of an experimental test of the model. The effects of finite temperature on the force are also studied, and found to be rather small at room temperature. However, thermally activated transitions between equilibrium points could be significant at room temperature.Comment: 16 pages, 5 figure

Surface-atom force out of thermal equilibrium and its effect on ultra-cold atoms

The surface-atom Casimir-Polder-Lifshitz force out of thermal equilibrium is investigated in the framework of macroscopic electrodynamics. Particular attention is devoted to its large distance limit that shows a new, stronger behaviour with respect to the equilibrium case. The frequency shift produced by the surface-atom force on the the center-of-mass oscillations of a harmonically trapped Bose-Einstein condensate and on the Bloch oscillations of an ultra-cold fermionic gas in an optical lattice are discussed for configurations out of thermal equilibrium.Comment: Submitted to JPA Special Issue QFEXT'0

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 $(\xi_T = \hbar v / k_B T)$ controls the force versus distance $(z)$ as a crossover from the zero temperature results for $z\ll \xi_T$, to a linear-in-temperature, universal regime for $z\gg \xi_T$. 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

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

Vector and tensor perturbations in Horava-Lifshitz cosmology

We study cosmological vector and tensor perturbations in Horava-Lifshitz gravity, adopting the most general Sotiriou-Visser-Weinfurtner generalization without the detailed balance but with projectability condition. After deriving the general formulas in a flat FRW background, we find that the vector perturbations are identical to those given in general relativity. This is true also in the non-flat cases. For the tensor perturbations, high order derivatives of the curvatures produce effectively an anisotropic stress, which could have significant efforts on the high-frequency modes of gravitational waves, while for the low-frenquency modes, the efforts are negligible. The power spectrum is scale-invariant in the UV regime, because of the particular dispersion relations. But, due to lower-order corrections, it will eventually reduce to that given in GR in the IR limit. Applying the general formulas to the de Sitter and power-law backgrounds, we calculate the power spectrum and index, using the uniform approximations, and obtain their analytical expressions in both cases.Comment: Correct some typos and add new references. Version to be published in Physical Reviews

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

Kohn-Luttinger superconductivity in graphene

We investigate the development of superconductivity in graphene when the Fermi level becomes close to one of the Van Hove singularities of the electron system. The origin of the pairing instability lies in the strong anisotropy of the e-e scattering at the Van Hove filling, which leads to a channel with attractive coupling when making the projection of the BCS vertex on the symmetry modes with nontrivial angular dependence along the Fermi line. We show that the scale of the superconducting instability may be pushed up to temperatures larger than 10 K, depending on the ability to tune the system to the proximity of the Van Hove singularity.Comment: 5 pages, 3 figure

Spin polarization and effective mass: a numerical study in disordered two dimensional systems

We numerically study the magnetization of small metallic clusters. The magnetic susceptibility is enhanced for lower electronic densities due to the stronger influence of electron-electron interactions. The magnetic susceptibility enhancement stems mainly from an enhancement of the mass for commensurate fillings, while for non-commensurate fillings its a result of an enhancement of the Land\'e $g$ factor. The relevance to recent experimental measurements is discussed.Comment: 4 pages, 4 figures, accepted for publication in PR

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.