Superconductivity in the Mg-doped CuBa₂Ca₃Cu₄O_12-y system

Abstract

Synthesis of CuBa₂Ca₃Cu₄O_12−y (Cu-1234) with Mg doping at the Ca sites, hitherto unexplored, is attempted and reported here. Different samples with 5%, 10%, 20%, and 33% Mg (in nominal stoichiometry) have been prepared in gold capsules under 3.5 GPa pressure and at about 1000°C temperature using a cubic anvil-type high-pressure machine. The samples were characterized through X-ray diffractometry, resistivity, dc susceptibility, and Hall effect measurements. The samples were then aligned along their c axis under 10 T field at room temperature. Magnetization (both in H//ab and H//c) was measured between 120 and 5 K in different field values up to 5 T. The superconducting anisotropy (defined as ξ_ab/ξ_c=H_c2^ab/H_c2^c), estimated in the reversible magnetization regime, comes out to be about 1.4, and is lower than that of the pristine sample. Improvement in the anisotropy factor obtained has been discussed in light of the observed changes in the lattice parameters and the hole concentration, emanating from Mg doping in the Cu-site vacancies in charge reservoir blocks of the 1234 structure.