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CeNS Colloquium

Place: Kleiner Physik-Hörsaal, Geschwister-Scholl-Platz
Date: 21.01.11, Time: 15:30 h

Exciton dynamics in potential traps at ultralow temperatures: Are we on the way towards a Bose-Einstein Condensate?

Prof. Heinrich Stolz
Universität Rostock


The condensation of Bosons into the system ground state at sufficiently low temperature in thermal equilibrium is one of the most spectacular manifestations of the quantum nature of matter and was first realized for atoms in a potential trap. Due to the general nature of the arguments, this Bose-Einstein condensation (BEC) should occur in any system of quasi-particles with integer spin, the most prominent example being excitons, bound electron-hole-pair excitations in semiconductors. Especially promising have been the yellow 1S exciton states in cuprous oxide, but, despite several experimental studies of dense exciton states in this material, none of these resulted in a clear demonstration of the existence of a Bose condensed state of excitons. Our new strategy aiming towards BEC is to collect a dense system of paraexcitons in a potential trap at ultralow bath temperatures down to 50 mK. In the experiments, the paraexcitons were created by resonant excitation of orthoexcitons by a pulsed laser with a pulse length of about 50 ns followed by ortho-para-conversion. Power and time dependent measurements of spatially resolved spectra were made from which the thermalization behaviour of the excitons could be deduced. Due to their long life time, the paraexcitons reach quasithermal equilibrium with temperatures below 200 mK, while the number of excitons under these conditions can reach some 108, which is comparable to the critical number for BEC at these subkelvin temperatures. The results will be discussed with respect to theoretical predictions of the spatio-spectral luminescence of an excitonic BEC in a confining potential that takes both the weak polariton nature of the excitons and the exciton-exciton interaction into account.