For a semiconductor with single isotropic conduction and valence bands, the effective biexciton-biexciton interaction is derived starting from the second-quantization representation for the Hamiltonian of the system of interacting excitons and for the biexciton wave function within the framework of the adiabatic approximation. The interaction is found to be an average of the sum of effective interactions between excitons forming interacting biexcitons over the envelope functions of these biexcitons. Depending on the momentum transfer and on the difference between the momenta of interacting biexcitons, the interaction admits an analytical study when the first vanishes. In this case, the interaction has in the r space the form of a function of the interbiexciton distance consisting of a strong repulsive and a weak attractive part. At low temperatures the function describes main features in the behavior of the interaction among biexcitons, in which repulsion predominates over attraction. It is shown that while biexcitons remain stable quasiparticles they weakly attract each other for any value of the distance. A quantitative analysis of obtained results for the CuCl crystal shows that the biexciton system in this model substance is a weakly nonideal Bose gas with positive scattering length a s Ӎ3a x , which closely approaches the ideal one at excitation densities nр10 18 cm Ϫ3 . ͓S0163-1829͑97͒02211-X͔

Paraexcitons, the lowest energy exciton states in Cu 2 O, have been considered a good system for realizing exciton Bose-Einstein condensation (BEC). The fact that their BEC has not been attained so far is attributed to a collision-induced loss, whose nature remains unclear. To understand collisional properties of cold paraexcitons governing their BEC, we perform a theoretical analysis of the s-wave paraexciton-paraexciton scattering at low temperatures. We show the two-channel character of the scattering, where incoming paraexcitons are coupled to a biexciton in a closed channel. Being embedded in the paraexciton scattering continuum, the biexciton is a Feshbach resonance giving rise to a paraexciton loss and a diminution of their background scattering length. In strain-induced traps, the biexciton effects generally increase with stress. Thus the scattering length a of trapped paraexcitons decreases monotonically with stress turning its sign as stress goes beyond a critical value. In the stress range with a<0, the paraexciton loss increases with stress, whereas in that with a>0 the loss is almost stress-independent. Importantly, that in the latter case the loss rate can be reduced to such small values that it has no effects on BEC by lowering temperatures to near one Kelvin and below. Our approximate calculations give the critical value of stress in the range just above one kilobar; thus BEC of strain-confined paraexcitons might be attained under low stress at a subkelvin temperature.

The genera1 properties of the damping constants of polaritons in the two-branch and three-branch models are considered in the framework of the Green's function method. In particular, the expressions for the damping constants of the polaritons in terms of those of the excitons are estaablished.PaCCMOTpeHbI o61me CBOfkCTBa KOHCTaHT 3aTyXaHHR IIOJIRPHTOHOB B AByX H TpeXBeT-JIeHHOn MOAeJIRX B paMaKaX MeTOAa $IYHKqUHEI rpHHa. B YaCTHOCTU YCTaHOBJIeHbI HX BblpameHHR uepe3 KOHCTaHTbI 3aTyXaHHR BKCHTOHOB.

Analytical expressions for the exciton-exciton interaction potentials have been approximatelyderived in both2o and 3o materiaisexhibiting explicit dependencesonexciton momentum difference. momentum transfer, electron-hole effective mass ratio and twoexciton state spin symmetry. Numerical calculations show that the character of the excitonexciton interaction is determined by all of the above-mentioned dependences.

No abstract