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

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

Localized modes in arrays of boson-fermion mixtures

It is shown that the mean-field description of a boson-fermion mixture with a dominating fermionic component, loaded in a one-dimensional optical lattice, is reduced to the nonlinear Schr\"{o}dinger equation with a periodic potential and periodic nonlinearity. In such system there exist localized modes having peculiar properties. In particular, for some regions of parameters there exists a lower bound for a number of atoms necessary for creation of a mode, while for other domains small amplitude gap solitons are not available in vicinity of either of the gap edges. We found that the lowest branch of the symmetric solution may either exist only for a restricted range of energies in a gap or does not exist, unlike in pure bosonic condensates. The simplest bifurcations of the modes are shown and stability of the modes is verified numerically

Magnetic field dependence of pairing interaction in ferromagnetic superconductors with triplet pairing

It is developed a microscopic description of superconductivity in ferromagnetic materials with triplet pairing triggered by the exchange of magnetic fluctuations. Instead widely used paramagnon model we work with phenomenological spectrum of fluctuations in the orthorhombic ferromagnet with strong magnetic anisotropy. Depending of the field orientation parallel or perpendicular to the direction of spontaneous magnetization the effective amplitude of pairing interaction proves to be decreasing or increasing function of magnetic field that allows to explain the drastic difference in magnitudes of upper critical field in these directions.Comment: 9 pages, no figure

Nonlocal impedances and the Casimir entropy at low temperatures

The problem with the temperature dependence of the Casimir force is investigated. Specifically, the entropy behavior in the low temperature limit, which caused debates in the literature, is analyzed. It is stressed that the behavior of the relaxation frequency in the $T\to0$ limit does not play a physical role since the anomalous skin effect dominates in this range. In contrast with the previous works, where the approximate Leontovich impedance was used for analysis of nonlocal effects, we give description of the problem in terms of exact nonlocal impedances. It is found that the Casimir entropy is going to zero at $T\to0$ only in the case when $s$ polarization does not contribute to the classical part of the Casimir force. However, the entropy approaching zero from the negative side that, in our opinion, cannot be considered as thermodynamically satisfactory. The resolution of the negative entropy problem proposed in the literature is analyzed and it is shown that it cannot be considered as complete. The crisis with the thermal Casimir effect is stressed.Comment: Accepted in Phys. Rev.

Magnetic double refraction in piezoelectrics

A new type of magneto-optical effect in piezoelectrics is predicted. A low frequency behavior of Faraday effect is found.Comment: 2 pages, to be published in Europhys. Lett

Non-Gaussianity of a single scalar field in general covariant Ho\v{r}ava-Lifshitz gravity

In this paper, we study non-Gaussianity generated by a single scalar field in slow-roll inflation in the framework of the non-relativistic general covariant Ho\v{r}ava-Lifshitz theory of gravity with the projectability condition and an arbitrary coupling constant $\lambda$, where $\lambda$ characterizes the deviation of the theory from general relativity (GR) in the infrared. We find that the leading effect of self-interaction, in contrary to the case of minimal scenario of GR, is in general of the order $\hat{\alpha}_{n} \epsilon^{3/2}$, where $\epsilon$ is a slow-roll parameter, and $\hat{\alpha}_{n} (n = 3, 5)$ are the dimensionless coupling coefficients of the six-order operators of the Lifshitz scalar, and have no contributions to power spectra and indices of both scalar and tensor. The bispectrum, comparing with the standard one given in GR, is enhanced, and gives rise to a large value of the nonlinearity parameter $f_{\text{NL}}$.We study how the modified dispersion relation with high order moment terms affects the evaluation of the mode function and in turn the bispectrum, and show explicitly that the mode function takes various asymptotic forms during different periods of its evolution. In particular, we find that it is in general of superpositions of oscillatory functions, instead of plane waves like in the minimal scenario of GR. This results in a large enhancement of the folded shape in the bispectrum.Comment: Added new references and corrected some typos. 5 figures, revtex4. Phys. Rev. D86, 103523 (2012

Universal low-temperature behavior of the CePd_{1-x}Rh_x ferromagnet

The heavy-fermion metal CePd_{1-x}Rh_x evolves from ferromagnetism at x=0 to a non-magnetic state at some critical concentration x_c. Utilizing the quasiparticle picture and the concept of fermion condensation quantum phase transition (FCQPT), we address the question about non-Fermi liquid (NFL) behavior of ferromagnet CePd_{1-x}Rh_x and show that it coincides with that of both antiferromagnet YbRh_2(Si_{0.95}Ge_{0.05})_2 and paramagnet CeRu_2Si_2 and CeNi_2Ge_2. We conclude that the NFL behavior being independent of the peculiarities of specific alloy, is universal, while numerous quantum critical points assumed to be responsible for the NFL behavior of different HF metals can be well reduced to the only quantum critical point related to FCQPT.Comment: 6 pages, 7 figure

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

Application of the Lifshitz theory to poor conductors

The Lifshitz formula for the dispersive forces is generalized to the materials, which cannot be described with the local dielectric response. Principal nonlocality of poor conductors is related with the finite screening length of the penetrating field and the collisional relaxation; at low temperatures the role of collisions plays the Landau damping. The spatial dispersion makes the theory self consistent. Our predictions are compared with the recent experiment. It is demonstrated that at low temperatures the Casimir-Lifshitz entropy disappears as $T$ in the case of degenerate plasma and as $T^2$ for the nondegenerate one.Comment: Accepted for publication in PR