Sergei V. Shulga

An atomic trap based on evanescent light waves

A new type of gradient light trap is suggested for neutral atoms, based on the evanescent fields of two laser beams experiencing total internal reflection at a dielectric- vacuum interface. A distinctive feature ofthe trap is that its bottom is close to the interface: at a distance of the order of the wavelength of light.

Thermodynamics of Two-Band Superconductors: The Case of MgB2

Thermodynamic properties of the multiband superconductor MgB2 have often been described using a simple sum of the standard BCS expressions corresponding to sigma- and pi bands. However, it is a priori not clear if this approach is working always adequately, particularly in cases of strong interband scattering. Here we compare the often used approach of a sum of two independent bands using BCS-like alpha model expressions for the specific heat, entropy, and free energy to the solution of the full Eliashberg equations. The superconducting energy gaps, the free energy, the entropy, and the heat capacity for varying interband scattering rates are calculated within the framework of two-band Eliashberg theory. We obtain good agreement between the phenomenological two-band alpha model with the Eliashberg results, which delivers the theoretical verification to use the alpha model as a useful tool for a reliable analysis of heat capacity data. For the thermodynamic potential and the entropy we demonstrate that only the sum over the contributions of the two bands has physical meaning.

Extraction of the electron-phonon interaction from tunneling data in the multigap superconductor MgB2

The direct inversion of the Eliashberg equations (EE) in case of a multiband superconductor is a mathematically ill-defined problem, because it is not possible to obtain several band splitted electron-phonon spectral functions alpha2Fij(omega) from a single function of the tunnel current. In the present work we follow another direction and calculate the tunneling density of states (DOS) of MgB2 for different tunneling directions by directly solving the two-band EE in the real-axis formulation. This procedure reveals the fine structures of the DOS due to the optical phonons. Then we show that the numeric inversion of the standard single-band EE, when applied to the two-band DOS of MgB2, underestimates the strength of certain phonon branches (e.g., the E2g) in the extracted alpha2F(omega). The fine structures produced by the two-band interaction at energies between 20 and 100 meV turn out to be clearly observable only for tunneling along the ab planes and at very low temperature. Only in this case it is possible to extract some information on the sigma-band contribution to the spectral functions. For any other tunneling direction, the pi-band contribution is dominant and almost coincides with the whole alpha2F(omega) for tunneling along the c axis. Our results are compared with recent experimental tunneling and point-contact data.

Strong-coupling effects in s-wave two-band superconductor

Abstract We have studied theoretically a two-band superconductor in which superconductivity is induced by a strong retarded pairing interaction in one of the bands and has s-wave symmetry. This picture results in strongly anisotropic order parameter. Some consequencies of the model are discussed.

Collimation of atoms localized in a spherical standing light wave

Abstract The possibility of atomic beam collimation in a truncated spherical standing light wave is considered. The main factor responsible for collimation of atoms is the gradient force. The experimental results demonstrate the presence of the effect.

One-dimensional localization of atoms in a standing spherical light wave

The localization of sodium atoms in a spherical standing single-frequency light wave was experimentally studied and numerical calculations were performed for two- and three-level sodium atom models. A qualitative agreement was obtained between the experimental and theoretical results for the three-level atoms. The interaction between sodium atoms and the single-frequency field was shown to cause an effective localization of the atoms in the vicinity of the light-wave modes. The process was found to be influenced substantially by optical pumping, giving rise to an additional cooling of the atoms.

The determination of the electron–phonon interaction from tunneling data in the two-band superconductor MgB2

We calculate the tunneling density of states (DOS) of MgB2 for different tunneling directions, by directly solving the real-axis, two-band Eliashberg equations (EE). Then we show that the numeric inversion of the standard single-band EE, if applied to the DOS of the two-band superconductor MgB2, may lead to wrong estimates of the strength of certain phonon branches (e.g. the E2g) in the extracted electron–phonon spectral function α2F(ω). The fine structures produced by the two-band interaction turn out to be clearly observable only for tunneling along the ab-planes in high-quality single crystals. The results are compared to recent experimental data.

Prediction of the properties of cuprate superconductors based upon a two-gap model with magnetic impurity effects included

We have included in our two-gap model of superconductivity in the cuprates the effect of pair breaking caused by magnetic impurity scattering. This model explains many of the observed features of the cuprates including the temperature and frequency dependence of the surface resistance and penetration depth, the anomalous temperature dependence of the upper critical magnetic field, and the zero bias anomalies observed between cuprates and normal metal contacts. This model can also explain many of the phase coherence experiments in YBaCuO on the basis of magnetic scattering at an interface or grain boundary due to strain-induced oxygen loss. We have recently extended this model to calculate the electromagnetic properties and the density of states in the presence of magnetic scattering. The latter results will be presented here.

Localization of Atoms with Light-Induced Mirrors and in a Standing Light Wave

Publisher Summary This chapter discusses laser field configurations that allow the motion of atoms and molecules to be restricted to a region of space free from radiation. Such configurations may be atomic cavities relying for their functioning on the effect of reflection of atoms from a nonuniform field and ring traps based on the channeling of atoms in a standing light wave. In an experiment described in this chapter, the reflection of atoms from a nonuniform laser field was implemented experimentally. The atomic cavity was largely analogous to an optical cavity with high concentration of photons in the modes. The atoms entering the standing wave were separated into localized and nonlocalized groups. That allows treating a laser beam with a great radius of curvature as an element of a ring trap. The optical pumping effect causes a strong enrichment of the atomic beam with slow atoms.