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

TL;DR

This study demonstrates that Ca2+ substitution in YBa2Cu2.7Co0.3O7 restores superconductivity lost due to Co doping, without phase transformation, by balancing hole filling and creation.

Contribution

It reveals that simultaneous Ca2+ doping can recover superconductivity in Co-doped YBCO without phase change, highlighting a new doping strategy.

Findings

Superconducting Tc decreases with Co doping from 90K to 33K.

Ca substitution increases Tc from 33K to 67K.

Hole filling by Co is compensated by Ca-induced hole creation.

Abstract

Results of phase formation, resistivity (r), and thermo-electric 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. Cu-site Co substituted compound i.e. YBa2Cu2.7Co0.3O7 is tetragonal. With simultaneous doping of Ca at Y site in Co substituted compound i.e. Y1-xCaxBa2Cu2.7Co0.3O7 the tetragonal nature still remains. r(T) measurements showed superconducting transition temperature (Tc) to decrease from 90K (YBa2Cu3O7) to 33 K for YBa2Cu2.7Co0.3O7 which with further Ca substitution increases from 33K to 53K (Y0.9 Ca0.1Ba2Cu2.7Co0.3O7) and 67 K for Y0.8 Ca0.2Ba2Cu2.7Co0.3O7. 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(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.