The binding of two free excitons into a bi-exciton due to direct exciton-exciton interaction is studied. The process of bi-exciton formation via exciton-phonon interaction is not taken into account. By group theory methods the wave functions of two interacting excitons are constructed for a model applicable to CuCl and Cu,O crystals. Making use of these functions, the Fourier transform of exciton-exciton interaction energy is calculated. By the Green's function method the dissociation energy of the bi-exciton is obtained in the adiabatic approximation (the exciton-exciton interaction energy is much smaller than the ionization energy of the exciton). A numerical estimate and comparison with experimental values is made for CuCl crystals. M3yYeIIO 06pa30BaHHe 6H3KCHTOHOB Be3 YseTa KOCBeHHOrO B3aElMOAefiCTBHR Memny BKCHTOHBMH qepe3 @OHOHbI. MeTOAOM TeOpHEl I'pyIIII IlOCTpOeHbI BOJIHOBbIe @YHKUHll nByX B3aHMOnefiCTByEOUHX BKCHTOHOB AJIII MOAeJIH, IIpHMeHHMOfi K KPIIC-TaJIJlaM CUCl H cU,O. &kIlOJlL3yfI 3TH @YHK4HM, BblWiCJIeH @ypbe-O6pa3 3Heprkiki B3aHMOHefiCTBHR ABYX BKCHTOHOB . MeTOAOM @YHKUHfi rpHHa B a n u a 6 a~~s e c~o~ npH6JIHmeHHH (3HeprHR BBaHMOHefiCTBHH BKCHTOHOB 3HaWiTeJIbHO MeHLIUe 3HeprHil.i HOHH3aUMH 3KCHTOHa) BbIqHCJIeHa 3HeprHII nHCC0UHaUHH 6HBKCHTOHa. BbInon-HeHbI 4HCJIeHHbIe OUeHKH H CpaBHeHMe C 3KCIIepHMeHTaJILHbIMH pe3yJIbTaTaMl.3 EJIII EcpacTanna CuCI .

Belyaev's equations are generalized for the case of a non-ideal gas of excitons interacting with the phonons in the crystal at T = 0. In the first approximation of the perturbation theory the energetic spectrum of the elementary excitations is received. The change of sound velocity in some crystals is calculated for different assumptions with reference to the constants of exciton-exciton and exciton-phonon interactions.YpaBHeHacI BencIeBa 0606we~b1 Ha caysat Hemeanmoro ra3a ~KCIITOHOB B3aHMO@kTByIOlHX C 4OHOHBMH B KpHCTaJIJIe IIpH T = 0. n0nyYeH 3HepreTH-W X K I I d CIIeIETp 3JIeMeHTapHbIX BO36ymAeHHa. kI3MeHeHHe CKOPOCTH 3ByHa B He-KOTOpbIX KpIICTaJIaaX paCC4HTaHO HJIII pa3JIHsHbIX IIpe~llOJIO%eHIIfi OTHOCHTeJIbHO IiOHCTaHT 3KCIITOH-3KCIITOHHOI'O H 3KCHTOH-l$OHOHHOrO B3aHMOJXefiCTBHG.

The temperature dependence of the thermopower in Bi whiskers of different thickness (0.3 to 6 μm) is measured within the temperature range 77 to 300 K. At low temperatures the thermopower in the thinnest whiskers changes its sign and becomes positive. Theoretical calculations of the thermopower in Bi whiskers are carried out taking into account the phonon drag effect. Numerical computations prove that the change of sign of the thermopower in thin Bi whiskers at low temperature is caused by phonon drag effect.

No abstract

Using the Bogolyubov's rate equation from the theory of superfluidity the possibility of Bose condensation of phonons in biological systems and the validity of Frohlich's hypothesis has been proved. We took into account both the third and the fourth anharmonism in the rate equation. All the processes with active phonons (from one to four) of biological active modes have been investigated. Taking into account these processes the expression for the chemical potential is shown to be changed, but the conditions for Bose condensation of phonons still exist. For the first time we point out the possibility of soliton wave packet propagation in the coherent systems of phonons and photons. The possibility of Bose condensation of excitons in biological systems is also discussed.