Pseudogap and metal-insulator transitions in doped semiconductors

The electronic spectrum of a doped semiconductor described by the Anderson-Holstein impurity model and its conductivity derived from the Kubo linear response theory are calculated. Two characteristic temperatures depending on the doping level x are found in the phase diagram, TPG and Tλ(x). The pseudogap that opens in the single-particle spectrum at low doping levels and temperatures closes at the lower one, TPG. The pseudogap state of an insulator is attributed to spin fluctuations in a doped compound. At the higher characteristic temperature Tλ(x),, spin fluctuations vanish and the doped compound becomes a paramagnetic poor metal. Two distinct metal-insulator crossovers between semiconductor-like and metallic temperature dependence of resistivity are found. An insulator-to-poor-metal transition occurs at T*(x) ≈ Tλ(x). A poor-metal-to-insulator transition at a lower temperature is attributed to the temperature dependence of density of states in the pseudogap. It is shown that both transitions are observed in La2−xSrxCUO4.