Mechanisms of modification of the energy spectrum in high-temperature superconductors of the bismuth,thallium, and mercury systems upon doping and increase in the number of copper-oxygen layers

This paper reports on the results of a comparative investigation of the specific features of the electron transport and superconducting properties in chain-free high-temperature superconductors of the bismuth, thallium, and mercury systems with different levels and types of doping, as well as with different numbers of copper-oxygen layers. It has been shown that the asymmetric narrow band model provides a means for adequately describing the available data on temperature dependences of the thermopower coefficient for all the studied systems, which has proved the possibility of applying this model as a universal method for describing and analyzing the specific features of the electron transport in high-temperature superconductors of different systems. The parameters of the energy spectrum and the system of charge carriers in the normal phase have been determined, and the general regularities in the transformation of the energy spectrum due to the doping and an increase in the number of copper-oxygen layers in different systems have been revealed. The conclusions have been drawn about the character and mechanisms of influence of the parameters of the energy spectrum in the normal state on the superconducting properties of chain-free high-temperature superconductors in the optimally doped and underdoped regimes. It has been demonstrated that, in the underdoped regime, regardless of the type of doping, the dependence of the critical temperature on the effective width of the conduction band has a close-to-universal character in all the studied systems for each of the phases with different numbers of copper-oxygen layers.