Pressure effects on superconductivity and structural phase transitions in La 2−xM xCuO 4 (M  Ba, Sr)

Abstract

Relations between the superconductivity and the crystal structure of La 2− x M x CuO 4 (M  Sr, Ba) are studied under high pressures up to 20 kbar. In the Sr-doped crystal, bulk superconductivity is observed both below and above the critical pressure P c where the high-temperature tetragonal (THT) phase is stabilized down to less than 20 K, the lowest temperature examined, instead of the orthorhombic phase. The superconducting transition temperature T c of the orthorhombic phase below P c is depressed in comparison with that of the THT phase above P c. The depression of T c is caused by a tilt of the CuO 6 octahedra in the orthorhombic phase. It is found for the Ba-doped crystal that the depression of T c for x⋍0.11−0.14 is removed, except for x⋍0.125 when a structure transition at T 1∼60 K disappears under high pressure. In the vicinity of x=0.125, there exists another origin for the depression of T c in addition to the structural transition at T c.