Collective Phenomea In Highly Excited Bose Gas

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Addis Ababa University


e}wij:ed gas is meant a 90."1 in which thet'e exisi: a s.i9nificant number of exalted particles. Tlw number of ~jxcited part 1cles depends on external sources (la.ser illumination, electron beams, etc.) and can be arbitrary. In typical case" the number of excited atoms N .i" smaller than the number of pal;t.icles .in the 1 ground state. \'1e11 lmoVin the energy of interaction between pa,'ticles in the same quantum Bt_ate is proportional to lid' ~Ihere R 1s the distance between partlcles. 'I'his shortrange interaction is responslblc 1:or tho coll()ct:i,ve properties of 1:he convelltional gas Huch as condensation and pha"e transitions. '1'he int_m:action hel:vl8cn dlf ferently·-exci'ced particles 1s proportional to I/n 3 and can be 21ttnlctive as Vlell as repulsive. 'rhus, t_he of even a comparaU.vely small number of excited -paxticles can and does l(-lad to 21 signU'icant change of the propertles of th,~ gas compared \~i til the non-(~xcited gas. The properties of Huch exc:l.tec1 (jas are invest.i.gated on the basis of the second'-quantJzecl Hilmil tonian and the method of equations of IUo-tion of Green functions. 1~hiB method yields I in a most natural and direct "THY, the enor(w~spectrulU of elllmentary excitations and t:he t:hGl:l!lodynaroiGCll functions of excited gas



Collective Phenomea