Transport and Quantum Statistics of Excitons in Cu₂O

Abstract

Since the pioneering studies of Gross and others in the 1950fs, the semiconductor cuprous oxide (Cu₂O) has provided a classic example of a Wannier exciton. Spectroscopic studies such as optical absorption, luminescence and two-photon absorption have established the hydrogen-like energy levels and the existence of ortho- and paraexcitons. Also, recent experiments have indicated that Cu₂O is a good candidate for demonstrating Bose-Einstein condensation of excitons. Towards this end, we have conducted a variety of experiments to determine the thermodynamic and transport properties of ortho- and paraexcitons in Cu₂O. In naturally grown crystals, the paraexciton lifetime exceeds a microsecond, and its diffusivity and drift mobility can be measured by time-resolved luminescence imaging- In this way, the low-temperature regime of deformation-potential scattering has been measured. Under intense photoexcitation, the orthoexcitons display quantum statistics. Time-resolved spectroscopy and imaging reveals the evolution of the basic thermodynamic parameters: density, temperature and volume of the excitonic gas. The conditions of quantum saturation have been reached and the system is observed to follow the phase boundary for Bose-Einstein condensation, n = CT_3/2 , for over an order-of-magnitude in gas density.