Combined effect of charge carriers and crystal structure on high-T_c superconductivity in codoped Y_1-xCa_xBa_2-yLa_yCu₃O_z

Polycrystalline samples of Y_1-xCa_xBa_2-yLa_yCu₃O_z cuprate superconductors with both x and y ranging from 0 to 0.5 were investigated. The combinative energy between the perovskite block and the rock-salt block in this codoped system was calculated according to a previously proposed “block model”. By defining a new parameter γ describing the combined effects of the combinative energy (lattice degree of freedom) and the charge carrier density (charge degree of freedom), it is revealed that the tendency of γ changing with doping is well consistent with that of the superconducting transition temperature (T_c) over the whole doping range, suggesting the important roles of both the lattice and charge degrees of freedom in influencing the unconventional superconductivity.