Also phonon dispersion law is required to be linear and isotropic too.
2.
The method of the pumping keeps the system electro-neutral and the dispersion laws of electrons and holes are assumed to be parabolic and isotropic.
3.
In the case of zero external potential, the dispersion law of interacting Bose Einstein-condensed particles is given by so-called Bogoliubov spectrum ( for \ T = 0 ):
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