Pressure Study on Superconductibity and Successive Structural Phase Transitions in Lanthanum Copper Oxide

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Possible relations between the superconductivity and the structural phase transitions in La_<2-x>M_xCuO_4 (M=Ba,Sr) were studied under high pressures up to 2GPa. In the Sr-doped crystal, bulk superconductivity is observed both below and above the critical pressure where the high-temperature tetragonal (THT) phase is stabilized down to less than 20K, instead of the orthorhombic phase. This result is consistent with the result of electronic specific heat which indicates the super- conductivity of the THT phase above x>0.2 is of bulk. The superconducting transition temperature Tc of the orthorhombic phase is depressed in comparison with that of the THT phase. The depression of Tc is caused by a tilt of CuO_6-octahedron in the orthorhombic phase.It was found for the Ba-doped crystal that the depression of Tc for x(]SY.simeq[)0.11-0.14 is removed except for x(]SY.simeq])0.125 when the low-temperature structural phase transitionat T<@D21@>D2(-60K) disappears under high pressures. In the vicinity of x=0.125, there exists another origin for the depression of Tc in addition to the structural transition at T<@D21@>D2. It was revealed in X-ray diffraction experiments that the CuO<@D26@>D2-octahedron tilts below T<@D21@>D2 both about (110) axsis and about (1-10) axsis coherently but their tilting angles differ slightly. This result indicates that the crystal structure is a Pccn orthorhombic below T<@D21@>D2. The difference of the tilting angles is closely related to reduction of the density of states at the Fermi level which causes the depression of T<@D2c@>D2.