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

研究了La_<2-x>M_xCuO_4 （M=Ba,Sr）在2GPa高压下的超导性与结构相变之间的可能关系。在sr掺杂晶体中，在高温四方相（THT）稳定到低于20K的情况下，在临界压力下和高于临界压力下都观察到大块超导性，而不是正交相。这一结果与电子比热的结果一致，表明x>0.2以上的THT相的超导性是块状的。与THT相相比，正交相的超导转变温度Tc降低。Tc的降低是由于cuo_6 -八面体在正交相的倾斜引起的。结果表明，当低温结构相变T<@D21@>D2（-60K）在高压下消失时，除了x(]SY.simeq])0.125处Tc的下降，x(]SY.simeq[)0.11-0.14处Tc的下降被消除。在x=0.125附近，除了T<@D21@>D2处的构造转变外，还存在另一个导致Tc下降的原因。x射线衍射实验表明，CuO<@D26@>D2-八面体在T<@D21@>D2下沿（110）轴和（1-10）轴方向相干倾斜，但倾斜角度略有不同。结果表明，在T<@D21@>D2以下，晶体结构为Pccn正交晶型。倾斜角度的差异与费米能级态密度的降低密切相关，从而导致T<@D2c@>D2的降低。

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.