Local Electronic Structure, O–T Phase Transition and Superconductivity in the Ba-site Nd-substituted YBCO System

A series of YBa_2-xNd_xCu₃O_y (x = 0–0.4) samples have been systematically studied by means of X-ray diffraction, transport property measurements and positron annihilation technology. The positron lifetime parameters show strong Nd substitution dependence. There is an obvious change of positron lifetime parameters around the O–T phase transition. The local electron density n_e and vacancy concentration C_v as a function of x were calculated from the positron lifetime results. The correlations between local electronic structure, O–T phase transition and superconductivity are discussed. The results confirmed that n_e mainly has an effect on high-T_c superconductivity by affecting the charge transfer between CuO₂ planes and Cu–O chains region or Ba–O layer. The vacancy properties in the orthorhombic phase and tetragonal phase are two intrinsic different types. Positron lifetime is very sensitive to the O–T phase transition in the YBCO systems that can be used as a useful technique to determine the O–T phase transition in these systems.