Strain confinement and thermodynamics of free excitons in a direct-gap semiconductor

Long-lived paraexcitons have been confined by a strain-induced parabolic potential well in ${\mathrm{Cu}}_2$O. The excitons are found to behave as a highly diffusive classical gas at the crystal lattice temperature, as determined from their spatial and energy distributions between 1.5 and 4.2 K. Strain confinement and spatially selective resonant excitation provide a new method for examining the thermodynamics and interparticle kinetics of this direct-gap exciton.