Magnetic Phase Diagram and Superconductivity in High-T_c (La_1−xBa_x)₂CuO₄

Abstract

The magnetic phase diagram of (La_1−xBa_x)₂CuO₄ has been studied using the ¹³⁹La nuclear quadrupole resonance (NQR) technique. The ordering temperature T_N has been determined from the temperature dependence of the internal magnetic field at the La site proportional to the Cu magnetic moment. It has been found that T_N markedly decreases from T_N∼240 K for x=0, to 12 K for x=0.008, however the magnitude of the Cu magnetic moment at 1.3 K is almost constant. On the other hand, in the composition range of 0.008<x<0.025, T_N gradually decreases down to ∼1.5 K for x=0.025 where the superconductivity appears. This demonstrates that two-dimensional correlations within the CuO₂ in-plane are quite strong. The magnetic phase diagram established from a microscopical point of view suggests a strong relationship between the antiferromagnetism and the superconductivity.