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

本文<2-x>研究了La_M_xCuO_4（M=Ba，Sr）在高压下的超导电性与结构相变之间的可能关系。在Sr掺杂的晶体中，在临界压力以下和以上都观察到体超导性，其中高温四方晶系（THT）相稳定到低于20 K，而不是正交相。这一结果与电子比热的结果一致，表明在x&gt;0.2以上THT相的超导是体相超导。正交相的超导转变温度Tc与THT相相比有所降低。发现在高压下，当在T&lt;@D21@&gt;D2（-60K）的低温结构相变消失时，除了x（[SY. simeq]）0.125外，x（[SY.simeq]）0.11-0.14对Tc的降低被消除。在x=0.125附近，除了T&lt;@D21@&gt;D2处的结构转变外，还存在另一个导致Tc降低的原因。X射线衍射实验表明，CuO&lt;@D26@&gt;D2-八面体在低于T&lt;@D21@&gt;D2时，在（1 - 10）轴和（1-10）轴附近均发生相干倾斜，但倾斜角略有不同。该结果表明，晶体结构在T&lt;@D21@&gt;D2以下为Pccn正交晶系。倾斜角的差异与费米能级处态密度的降低密切相关，费米能级处态密度的降低导致T&lt;@D2c@&gt;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.