Yu. Kagan

Collapse and Bose-Einstein Condensation in a Trapped Bose Gas with Negative Scattering Length

Evolution and collapse of a trapped Bose condensate with negative scattering length are predetermined by 3-body recombination of Bose-condensed atoms and by feeding of the condensate from the nonequilibrium thermal cloud. The collapse, starting once the number of condensate atoms reaches the critical value, ceases and turns to expansion when the density becomes so high that the recombination losses dominate over attractive interparticle interaction. As a result, we obtain a sequence of collapses, each of them followed by dynamic oscillations of the condensate. In every collapse the 3-body recombination burns only a part of the Bose-condensed atoms.

Theory of quantum diffusion of atoms in crystals

A microscopic theory of the quantum diffusion of light interstitials in a perfect crystal is developed. The basic aspects and features of the quantum diffusion and the role of the polaron effects are revealed. The expressions of the coefficients for under-barrier coherent and noncoherent tunnelling diffusion and for over-barrier diffusion, valid at the appropriate temperatures, are derived in a unified way. The basic role of the intra-well scattering for the coherent diffusion in limiting narrow bands is elucidated. The theory permits one to consider the transition from quantum to classical diffusion. It is found that coherent diffusion predominates at the low temperatures, at least in the perfect crystal. The criteria for the observation of the coherent diffusion mechanism are given.

EVOLUTION OF CORRELATION PROPERTIES AND APPEARANCE OF BROKEN SYMMETRY IN THE PROCESS OF BOSE-EINSTEIN CONDENSATION

We consider the self-evolution of strongly nonequilibrium interacting Bose gas. Because of the mere fact of large (relative to unity) occupation numbers in the initial state the problem is directly reduced to the question of temporal evolution of the statistical matrix diagonal in the coherent-state representation. Strictly speaking, gauge symmetry is not destroyed even when the long-range ordering is completed, owing to the inevitable averaging over the ensemble. Actual symmetry breaking can occur only as a result of introducing a small term of the Hamiltonian violating conservation of particles, or quantum-mechanical measurement, also implying nonconservation of particles. {copyright} {ital 1997} {ital The American Physical Society}

On excitation of isomeric nuclear states in a crystal by synchrotron radiation

The time dependence of delayed radiation is investigated when a pulse of synchrotron radiation undergoes resonance Bragg scattering by the nuclei in a crystal that have an isomeric level with a Mossbauer transition. Instead of the usual exponential law exp(-t/ tau 0) characteristic of an isolated nucleus, a time dependence in the form ( tau 0/t)2 exp(-t/ tau 0) was discovered at small deviations alpha from the Bragg angle. The acceleration of the decay is connected with the collective nature of the excitation of the nuclei in the crystal-the nuclear exciton formation. The exponential law remains at large alpha but the intensity decreases as 1/ alpha 2. In the case of large divergence of the incident beam the law ( tau 0/t) exp(-t/ tau 0) is obeyed. The frequency distribution of the reflected pulse and the possibility of formation of a resonance structure with sufficient resolution are analysed in detail.

ROTATIONAL STRUCTURE OF MOLECULES IN QUANTUM 4HE AND 3HE LIQUIDS

It is shown that the drastic distinction in the rotational structure of molecules in the He4 and He3 liquids observed in [1] is due to the difference in the spectral density of excitations regardless of the hydrodynamic properties. The large width of rotational levels in the He3 is determined by particle-hole excitations whereas the small density of phonon excitations results only in the small broadening of levels in the He4.

Evolution and global collapse of trapped Bose condensates under variations of the scattering length

We develop the idea of selectively manipulating the condensate in a trapped Bose-condensed gas, without perturbing the thermal cloud. The idea is based on the possibility to modify the mean field interaction between atoms (scattering length) by nearly resonant incident light or by spatially uniform change of the trapping magnetic field. For the gas in the Thomas-Fermi regime we find analytical scaling solutions for the condensate wave function evolving under arbitrary variations of the scattering length a . The change of a from positive to negative induces a global collapse of the condensate, and the final stages of the collapse will be governed by intrinsic decay processes. {copyright} {ital 1997} {ital The American Physical Society}

Quantum diffusion in solids

The lectures are devoted to the problems connected with the tunneling motion of particles in a solid in the presence of strong interaction with phonon and electron excitations. We examine the quantum diffusion in insulators, metals, and superconductors. The interaction with the barrier fluctuations as well as the intrawell interaction are taken into account. Special attention is paid to the role of adiabaticity, playing an important part in tunneling phenomena, revealing the decisive influence of nonadiabatic excitations. We consider the entire temperature interval: coherent diffusion at low T, suppression of the coherent phase correlations and the transition to the incoherent quantum diffusion, and then with the further increase in T the crossover to the tunneling diffusion induced by excitations. The detailed analysis of the quantum diffusion in imperfect crystals and the problems of localization, in particular, caused by the interparticle interaction is given. We discuss the wide spectrum of experimental results displaying the quantum diffusion in different systems.

On the possibility of a superfluid transition in a Fermi gas of neutral particles at ultralow temperatures

The possibility of triplet Cooper pairing in a Fermi gas of neutral particles in magnetic traps at ultralow temperatures is predicted. Estimates are presented for the superfluid transition temperature.

INTERPLAY BETWEEN HEAVY FERMIONS AND CRYSTAL-FIELD EXCITATION IN KONDO LATTICES : LOW-TEMPERATURE THERMODYNAMICS AND INELASTIC NEUTRON SCATTERING SPEC TRA OF CENISN

The microscopic theory of interaction between the heavy fermions and the crystal field excitations in Kondo lattices is presented. It is shown that the heavy-fermion spectrum scaled by the Kondo temperature

Quantum diffusion and depolarization of muons in superconductors

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