The possibility of introducing additional states in the conduction band of YBa2Cu3Oy by Ca doping

Mechanism of a strong rise of T due to the calcium doping in Y1−Ca Ba2Cu2.8Zn0.2O

“…Such a modification of S(T) dependences can be successfully described if we take into account a possibility of existence of a slight conduction band asymmetry. The applicability of an asymmetric version of the narrowband model to the calcium-doped YBa 2 Cu 3 O y was confirmed by results obtained for Y 1Àx Ca x Ba 2 Cu 3 O y , as well as for the co-doped Y 1Àx Ca x Ba 2 Cu 3Àz Co z O y and Y 1Àx Ca x Ba 2Àz La z Cu 3 O y systems [7,8,12].…”

“…In this case, according to our conclusions [7,8,12], increasing T c is rather caused by the Fermi level, E F , movement through the conduction band. In the oxygen-rich samples E F shifts with x away from the maximum of the density-of-states, DOS, function that leads to a drop in T c , while in the oxygen-reduced ones it shifts towards this maximum that results in a rise of T c .…”

“…However, this effect of the oxygen subsystem ordering cannot be responsible for the recovery of superconducting properties observed in oxygen deficit Y 1Àx Ca x Ba 2 Cu 3 O y samples [7,8,12,13]. In this case, according to our conclusions [7,8,12], increasing T c is rather caused by the Fermi level, E F , movement through the conduction band.…”

“…However, the S(T) dependences obtained for the Y 1Àx Ca x Ba 2 Cu 3 O y system cannot be adequately described in the framework of the symmetric model because of their unusual modification with increasing x -the appearance of an extensive linear region which slope grows with increasing x [7,8,12]. Such a modification of S(T) dependences can be successfully described if we take into account a possibility of existence of a slight conduction band asymmetry.…”

“…[8] and references therein). Due to its lower valence relative to the valence of substituted yttrium, calcium doping leads to an increasing oxygen deficit [7][8][9][10][11][12], thus compensating the influence of the first impurity on the charge balance in the lattice. For this reason, the oxygen content in the Y 1Àx Ca x Ba 2Àz La z Cu 3 O y and Y 1Àx Ca x Ba 2 Cu 3Àz Co z O y systems with fixed z goes down with x towards its stoichiometric value y % 7 that induces the oxygen subsystem ordering and, as a result, an increase in the T c value [7,8].…”