Superconductivity in YBa2Cu3O7−d /La1−x Ca x MnO3 Bilayers (x = 0.3, 0.45, 0.55 and 0.8)

Abstract

Bilayers consisting of 4 unit cell (∼47 A) YBa2Cu3O7−d (YBCO) with 200 A La 1−x Ca x MnO3 (LCMO) on top were grown on SrTiO 3 substrates using pulsed laser deposition. The Ca concentration x of the LCMO layer was varied with x= 0.3, 0.45, 0.55 and 0.8. We observed that the superconducting transition temperature (T C ) is dependent on the Ca doping level and decreases as Ca concentration increases. Bilayers consisting of YBCO and antiferromagnetic insulating La 0.2Ca 0.8MnO 3 were not superconducting down to 4.2 K. A charge transfer mechanism cannot be responsible for this reduction of T C according to our analysis of the Fermi levels in YBCO and LCMO. It was revealed that T C is monotonically reduced as the in-plane lattice mismatch between YBCO and LCMO increases. Therefore we attribute the T C reduction in our bilayers to a structural origin, more specifically a large lattice mismatch enhances a creation of oxygen vacancies in YBCO and cation intermixing between YBCO and LCMO. A structural heterogeneity in YBCO/LCMO bilayers has an impact on its superconducting properties.