Band spectrum transformation and Tc variation in the La2-xSrxCuOy system in the underdoped and overdoped regimes

The results on the resistivity and thermopower temperature dependences for the La2-xSrxCuOy (x = 0.05-0.30) system are presented. The experimental data together with the thermopower results of other authors have been analyzed within a phenomenological narrow-band model. This model was shown to be applicable to the La2-xSrxCuOy system that gives a strong support for a narrow peak of the density-of-states in the vicinity of the Fermi level to exist in the band spectrum of high-T-c superconductors. Within the narrow-band model we have determined the main band spectrum parameters (the band filling degree, the effective bandwidth? and the degree of the charge carriers localization) and traced their variation with an increasing Sr content in the La2-xSrxCuOy system. These parameters were shown to change in a different way in the underdoped regime and the overdoped one that points to a distinction between those two regimes. Nevertheless we have observed that the critical temperature value correlates with the total effective bandwidth so that the maximal T-c at the optimal doping level corresponds to a minimum of the bandwidth. Based on the results obtained we discuss the genesis of the conduction band in the underdoped regime as well as a mechanism of its transformation in the overdoped regime. The possible reasons for the modification of the superconducting properties of the La2-xSrxCuOy system with increasing Sr content, both in the underdoped regime and the overdoped one are also considered.