Revival of superconductivity by Y3+/Ca2+ substitution in YBa2Cu2.7Co0.3O7 without reported phase transformation

Results of phase formation, resistivity (ρ), and thermoelectric power (S), are reported on Y1−xCaxBa2Cu2.7Co0.3O7 compounds with x=0.1 and 0.2. Pristine compound, i.e., without Co or Ca substitution crystallizes in orthorhombic structure with space-group P∕mmm. The Cu-site Co substituted compound, i.e., YBa2Cu2.7Co0.3O7 is tetragonal. With simultaneous doping of Ca at the Y site in Co substituted compound, i.e., Y1−xCaxBa2Cu2.7Co0.3O7 the tetragonal nature still remains. ρ(T) measurements showed superconducting transition temperature (Tc) to decrease from 90K (YBa2Cu3O7) to 33K for YBa2Cu2.7Co0.3O7, which with further Ca substitution increases from 33to53K (Y0.9Ca0.1Ba2Cu2.7Co0.3O7) and 67K for Y0.8Ca0.2Ba2Cu2.7Co0.3O7. Tc decreases first with Cu-site Co substitution by hole filling and later recovers by simultaneous hole creation by Y site Ca substitution. Room temperature thermoelectric power S (300K), which is an indirect measure of mobile carriers shows the decrease of carriers with Co doping and crea...