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On the polaron mass at finite temperatures

The concept of polaron mass at T not=0 is shown to be poorly defined. Various forms of temperature dependence meff(T) suggested in the literature do not coincide, their difference being due to the different ways of defining the mass at T=0. A definition of effective mass as the ratio of the impulse F dt and the mean polaron velocity (v)T is suggested and the elementary derivation of the temperature dependence of the mass spectrum in such a way in the region kT<<h(cross) omega ph is given. The results obtained for the dependence meff(T) via polaron cyclotron resonance frequency are discussed and the width of the cyclotron line is shown to be of the same order as or larger than the temperature shift of its maximum, the latter being not given by any of the expressions for meff(T) discussed earlier. The author gives an expression for the shape of the resonance line and compare it with the available data.