Doping-induced variations of the Fermi level in calcium-containing Y-based HTSC and their influence on the critical temperature

• Calcium-containing Y-based HTSC systems are investigated.
• Parameters of the energy spectrum and charge-carrier system are determined.
• Variations of the Fermi level are revealed and discussed.
• The mechanism of calcium effect on Tc in doped YBa2Cu3Oy is discussed.

The superconducting and normal-state transport properties (resistivity, ρ, and thermopower, S) of three series of ceramic samples with compositions of Y1−xCaxBa1.5La0.5Cu3Oy (x = 0–0.4), Y0.85−xCa0.15PrxBa2Cu3Oy (x = 0–0.3), and Y0.85Ca0.15Ba2−xLaxCu3Oy (x = 0–0.3) have been investigated. The temperature dependences of the thermopower have been analyzed using a phenomenological narrow-band model. The mechanisms of the doping influence on the parameters of the band spectrum and charge-carriers system including the Fermi level position have been discussed. It has been demonstrated that the revealed features of the Fermi level behavior in the studied systems can be explained by the formation of an additional peak in the density-of-states function under calcium doping. The influence of the Fermi level shift under doping on the critical temperature in the studied systems is discussed.