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Quantum collective phenomena at low temperature

The theory of influence of the Kondo effect on the crystal-field states was developed for the regime of localised magnetic momenta. The theory of unconventional type of magnetoelectric interaction induced by the Kondo scattering has been constructed. The suggested model is in quantitative agreement with the results of neutron scattering experiments in CeAl3. The theory describing the critical behaviour of isotropic Heisenberg antiferromagnets was developed for temperatures above the Neel point. The critical exponents are derived for both the spin diffusion coefficient and the relaxation kinetic coefficient. The developed model of coupling modes can be used for the explanation of unusual magnetic properties of heavy-fermion compounds in Fermi-liquid and non-Fermi-liquid regimes.
A novel Quantum Monte Carlo scheme has been developed, which provides efficient calculations of the Green function at finite temperatures, as well as the study of large disordered systems. The scheme is exact (no systematic errors) and works in the grand canonical ensemble. The new Monte Carlo method has been employed for a studying zero-point superfluid-insulator transitions in pure and disordered bosonic Hubbard models in one dimension.
The self-evolution of a strongly non-equilibrium weakly interacting Bose gas has been analysed. It was demonstrated that due to large occupation numbers in the initial state the problem is reduced to the time-evolution of the statistical matrix in the coherent - state representation. Further development of the idea of selective manipulations of trapped condensates has led to the construction of the scaling theory for coherent evolution of the condensate wavefunction under arbitrary variations of the scattering length. For switching the scattering length from positive to negative a novel phenomenon - global collapse of the condensate has been revealed. The scenario of the evolution and collapse of a trapped condensate with negative scattering length has been described. The interplay between the attractive interparticle interaction and the three-body recombination is found to be such that the number of Bose-condensed atoms always remain finite. The studies of elementary excitations of a trapped Bose condensate in cylindrically symmetric traps have revealed an accidental degeneracy in the energy spectrum, which can provide new BEC signatures. The theory of finite-temperature damping of low energy excitations of a trapped Bose-condensed gas has been developed. The damping rates of the lowest excitations are in fair agreement with the recent JILA and MIT experiments. The damping of quasiclassical excitations is determined by the condensate boundary region, and the result for the damping rate is drastically different from that in a spatially homogeneous gas.

Reported by

FOM-Institute for Atomic and Molecular Pysics (AMOLF)
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