Apical oxygen, 3D-2D cross over and superconductivity in Sm_(2-x)Ce_xCuO_(4-δ)

摘要

In spite of the vast amount of experimental and theoretical articles accumulated in HTSC, the mechanism of the interaction driving charge carriers to form Cooper pairs below T_c is still unknown. The comparison of the normal state transport properties of Yba_2Cu_3O_(7-δ) and the Sm_(2-x)Ce_xCuO_(4-δ)^s[1,2] might shed some light on the microscopic origin of HTSC. In comparison to the YBCO, the apical oxygen in Sm_(2-x)Ce_xCuO_(4-δ)^s[3] destroys the superconductivity via the vertical ionic bonding which localizes the charge in the Cu-O squares, however the hole transfer by moving O(4) towards the CuO+2 planes, leads to the optimization of YBCO properties. The behaviour of C axis parameter vs the oxygen content cannot be explained by a BSC mechanism. The high amount of anisotropy ratio [4] is explained by the sheer square planes in NCCO system, i.e. without apical oxygen (SC with Tc maximum). From the data of the resistivity in the normal state, we conclude the observation of a 3D-2D cross over only in Sm_(2-x)Ce_xCuO_(4-δ)^s[2] and Nd_(2-x)Ce_xCuO_(4-δ)^s[5] which is also related to its high anisotropy. The competition between anisotropy and superconductivity destroys the superconducting state in the 2D limit even in the ground state. In this material the superconductivity cannot be enhanced at high temperature because the compound is a quasi 2D system (sheer square planes of CuO_2) and the cuprate superconductors is a genuine three-dimensional (3D) phenomenon [6]. The Josephson coupling between the different layers is S-I-S for NCCO and S-N-S for YBCO, thus the Lawrence and Doniach model (LD) [7] with neighbouring layers coupled by the Josephson tunnelling is appropriate. In summary the behaviour of apical oxygen is intrinsically different in the two kinds of cuprates.