a Study of the Energy and Momentum Distribution of Excitonic Particles at High Densities in Cuprous Chloride.
Interaction between excitons at high densities leads to two known stable states: one is a gas of biexcitons (excitonic molecules) and the other is the metallic electron -hole droplets. For example, the former is found in cuprous halides like CuCl, CuBr, CuI and the latter in Ge and Si. The excitonic molecule is a bound state of two single excitons. It is like the hydrogen molecule with positive holes in place of two protons. Its mass, unlike the hydrogen molecule is on the order of the free electron mass. Since the biexciton consists of four fermions, the composite molecule behaves like a boson. For large densities and low temperatures which are experimentally accessible, one expects the occurrence of a Bose-Einstein condensation in which a fraction of the biexcitons condense into a single quantum state. The small effective mass of the biexcitons causes the transition to take place at relatively high temperatures compared with the superfluid transition in liquid helium. (DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI). Also the fact that excitonic molecules are not stable and decay radiatively permits the search for such a condensation through optical means. In this work, the energy and momentum distribution of excitonic particles at high densities has been studied and a search for evidence of a stable condensed state is conducted. Several experiments are performed using three independently tunable dye lasers. In the first series of experiments, the luminescence of the biexciton decay as a function of temperature and intensity is examined. Evaporated films of CuCl with controlled thicknesses and single crystals are used. The spatial anisotropy and polarization of the luminescence are also studied. In the second series of experiments, the effects of the optical gain and induced absorption on the luminescence of the biexcitons is investigated. In the third part of the experiment, the condensed state is generated by a pump beam tuned to the two-photon absorption resonance of biexcitons and a small concentration of biexcitons are injected at other energies and wavevectors by probe beams. Therefore, the effect of these additional particles on the biexciton distribution is examined. It is observed that the probe biexcitons are preferentially attracted into the pumped state for temperatures below which the condensate is stable. From a careful analysis of the biexciton luminescence and the results of the pump and probe measurements the occurrence of a Bose-Einstein condensation of biexcitons at high densities and low temperatures is concluded.
- Pub Date:
- March 1982
- Physics: Condensed Matter