Probing of local electron states by laser terahertz radiation in PbTe(Ga)

Highlights: • We report on unusual photoconductivity under terahertz laser pulses in PbTe(Ga). • The kinetics and the sign of the photosignal depend on the photoexcitation level. • This is due to formation of local states in the vicinity of the quasi-Fermi level. • These local electron states may lie only on the background of an allowed energy band. - Abstract: We report on unusual photoconductivity under the action of strong terahertz laser pulses in PbTe(Ga) single crystals at low temperatures. The character and even the sign on the photosignal depends on the level of photoexcitation: the photoconductivity is positive and persistent at high degrees of excitation, whereas it is negative and non-persistent at low levels of excitation. We show that this non-trivial photoelectric behaviour is due to formation of local electron states in the close vicinity of the quasi-Fermi level. These local states may be formed only when the quasi-Fermi level lies on the background of an allowed energy band with a relatively high density of states