[en] A brief review of some excitonic properties is presented, with the accent put on their relevance with respect to the possibility of Bose-Einstein condensation. Recent experimental evidence is described, which supports the idea that excitonic particles may form a highly quantum fluid. In Cu₂O, the analysis of the exciton decay spectrum shows a gradual evolution of the gas from a classical regime at low densities up to a strongly degenerate one at high densities, with a chemical potential μ approximately 0. In CuCl excitons are unstable against formation of molecules (biexcitons). It is possible to generate directly a high density gas of biexcitons with momentum K approximately 0 by giant two-photon absorption. At low excitation, the observed molecular emission is in agreement with Maxwell-Boltzmann statistics. At high densities, strong deviations occur. In particular, the appearance of a sharp emission line is attributed to the presence of a Bose-Einstein condensate of excitonic molecules

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[en] This paper was published online on 23 June 2011 without some of the author’s corrections incorporated into the published article. Most of these corrections relate to text citations of Refs. [19] and [20]. The paper has been corrected as of 3 August 2011. The text is incorrect in the printed version of the journal.

[en] The study of BEC in weakly interacting system holds the promise of revealing new macroscopic quantum phenomena that can be understood from first principles, and may advance present understanding of superconductivity and superfluidity. By employing several new experimental techniques, the dark SPOT trap, rf induced evaporation, and the optically plugged magnetic trap, the authors were able to observe BEC in a gas of sodium atoms. The striking signature of Bose condensation was the sudden appearance of a bimodal velocity distribution below the critical temperature of ∼ 2 μK. The distribution consisted of an isotropic thermal distribution and an elliptical core attributed to the expansion of a dense condensate. Condensates contained up to 5 x 10⁵ atoms at densities exceeding 10¹⁴ cm^-3. Such high densities are unprecedented in cold atomic gases and open up new possibilities for studying a weakly interacting Bose gas over a broad range of densities and therefore strengths of interaction

[en] Superfluidity in liquid helium ranks as one of the most fascinating discoveries in physics this century. Helium, which liquefies at 4-5 K, behaves as a conventional liquid just below the condensation point, but undergoes a phase transition when the temperature is lowered to about 2.2 K. In this new phase, known as the superfluid phase, both the viscosity and thermal conductivity of the liquid drop to zero. These remarkable properties are thought to arise from the formation of a Bose condensate, in which all of the helium atoms become locked together in their lowest quantum state, but the proof is not yet conclusive. Now, however, Adrian Wyatt of the University of Exeter in the UK has provided new evidence for the existence of a Bose condensate in superfluid helium (Nature 1998 391 56). (author)

[en] Bose-Einstein condensation (BEC) of a finite number of noninteracting bosons confined in a finite-size container is investigated by using numerical calculation. It is revealed that the characteristics of finite-size BEC are significantly different from those for the system satisfying the condition of thermodynamic limit, and some well-known conclusions about BEC described in textbooks are not valid for a finite-size Bose system. This paper may be helpful for the further understanding of the concept of BEC and thermostatistics in small systems. It is suitable for readers including undergraduate and graduate students, general physicists and so on.

[en] Experimental efforts to produce a Bose Einstein condensed (BEC) gas of spin-polarized atomic hydrogen have been intensely pursued since 1980. Initial efforts studied hydrogen in contact with superfluid helium surfaces. In order to avoid the limiting wall recombination a static magnetic trap was developed which utilizes magnetic barriers to isolate atoms from the van der Wals walls. In this approach traps can be filled to about 10¹⁴ atoms/cm³ for which the critical temperature is in the region of 20-30 μK. However, approaching BEC conditions, a new instability dominates the behavior. The authors have developed a new type of electromagnetic trap, a microwave (mw) trap in which the trapped atoms are in the ground spin-state so that at low temperature relaxation to a barrier state is strongly suppressed and BEC should be attainable. The shallow mw trap has recently been demonstrated experimentally for the cesium atom. Because the mw trap is so shallow it is necessary to precool the atoms for loading. The authors are constructing a hybrid trap in which the atoms are cooled in a static trap and then transferred to a mw trap where they can be further evaporatively cooled in an effort to achieve BEC. This trap should have density limits in the region of 10¹⁶ to 10¹⁷/cm³ due to three-body recombination. A second, promising approach is in a 2-D geometry for which in finite sized systems BEC is allowed. Hydrogen atoms are confined to a superfluid helium surface by the absorption potential to form a 2-D gas. Although recombination is an important process on helium surfaces, recently the authors have shown that only a very small fraction of the recombination energy is coupled into the surface. With an appropriate geometry the recombination energy can be removed from the sample region to suppress heating to a negligible level. Furthermore in 2-D a magnetic field can have a maximum so that atoms can be confined to a very small area

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[en] We propose a protocol for the simultaneous controlled creation of multiple concentric ring dark solitons in a toroidally trapped flat Bose–Einstein condensate. The decay of these solitons into a vortex–antivortex necklace shows revivals of the soliton structure, but eventually becomes an example of quantum turbulence. (fast track communications)