Revival of superconductivity by Y 3 + ∕ Ca 2 + substitution in Y Ba 2 Cu 2.7 Co 0.3 O 7 without reported phase transformation

Results of phase formation, resistivity ( ρ ) , and thermoelectric power ( S ) , are reported on Y 1 − x Ca x Ba 2 Cu 2.7 Co 0.3 O 7 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 ∕ m m m . The Cu-site Co substituted compound, i.e., Y Ba 2 Cu 2.7 Co 0.3 O 7 is tetragonal. With simultaneous doping of Ca at the Y site in Co substituted compound, i.e., Y 1 − x Ca x Ba 2 Cu 2.7 Co 0.3 O 7 the tetragonal nature still remains. ρ ( T ) measurements showed superconducting transition temperature ( T c ) to decrease from 90 K ( Y Ba 2 Cu 3 O 7 ) to 33 K for Y Ba 2 Cu 2.7 Co 0.3 O 7 , which with further Ca substitution increases from 33 to 53 K ( Y 0.9 Ca 0.1 Ba 2 Cu 2.7 Co 0.3 O 7 ) and 67 K for Y 0.8 Ca 0.2 Ba 2 Cu 2.7 Co 0.3 O 7 . T c 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 ( 300 K ) , which is an indirect measure of mobile carriers shows the decrease of carriers with Co doping and creation by Ca substitution. Our results demonstrate the hole filling by Co substitution is compensated by simultaneous Ca substitution.