In-plane Mg doping in YBa₂Cu₃O₇: influence on the superconducting anisotropy

Melt textured single domains of Mg-doped YBa 2 Cu 3 O 7 superconductors with homogeneous cation distribution and narrow superconducting transitions have been prepared by top seeding growth. The strong reduction in the transition temperature, T c , found in Mg substituted YBa 2 (Cu 1- x Mg x ) 3 O 7 samples, suggests that Mg ions occupy the CuO 2 planes. For low doping concentrations, the decrease in T c is estimated as d T c /d x ≈-12 K/at% Mg. Instead, for high doping values, a saturation of Mg substitution occurs within the matrix, leading to MgO segregation. The maximum amount of Mg atomic substitution has been estimated as x ≈4 at%, shifting T c down to T c ≈45 K. Studies of anisotropy of the superconducting state from magnetoresistance measurements reveal that, concomitant to the decrease in T c , the mass anisotropy decreases with Mg doping. These results are interpreted in the framework of non-magnetic impurity scattering effects in d-wave superconductors, where the main distinguishing feature has been the development of new quasi-particle states within the superconducting gap.