The study of electric and crystal structural changes on the strongly correlated electron oxide under high pressure

本文研究了强关联电子氧化物和合金在高压下的电学和晶体结构变化。结果解释如下。(1)我们用助熔剂法在高压氧气氛下成功地生长了PrBa_2Cu_4O_8（Pr124）单晶。用四圆衍射仪获得Pr 124单晶的结构数据，用全矩阵最小二乘结构精修法对结构进行精修。单晶和多晶Pr 124之间的所有z位置参数基本上相同。Pr 124单晶的电阻率与温度的关系至少在2 K以下不表现出超导电性。沿着B轴的电阻率显示出高导电行为，其起源归因于双链位点，因为沿着a轴和B轴的电阻率之间的大的各向异性。(2)RuSr_2GdCu_2O_8氧化物在40K左右出现超导转变，在130K左右出现铁磁有序。测量了该样品的超导和铁磁转变温度的压力依赖性。超导转变温度在压力下变化不大。铁磁转变温度随着压力的增加而增加。(3)本文对Heusler型Fe_2VAl化合物的磁化强度、电阻率和比热进行了测量。电阻率表现出类似超导体的负温度依赖性。我们认为这种化合物应该是一种在费米能级上具有赝能隙的半金属。

We have investigated the electric and crystal structural changes on the strongly correlated electron oxide and alloys under high pressure. The results are explained in the following.(1) We have successfully grown single crystals of PrBa_2Cu_4O_8 (Pr124) by a flux method under high-pressure oxygen gas. The structure data of a Pr124 single crystal were taken by using a four-circle diffractometer and the structure was refined by full matrix least-square structure refinements. All z-positional parameters are essentially the same between the single crystal and the polycrystalline Pr124. The temperature dependence of resistivity of Pr124 single crystals did not exhibit superconductivity, at least down to 2 K. Resistivity along the b- axis showed a highly conductive behavior, of which the origin is attributed to the double-chain site because of the large anisotropy between the resistivities along a- and b- axes.(2) The RuSr_2GdCu_2O_8 oxide exhibits the superconducting transition at about 40K and ferromagnetic order at about 130K. The pressure dependence of the superconducting and ferromagnetic transition temperature of this sample was measured. The superconducting transition temperature was scarcely varies under pressure. The ferromagnetic transition temperature increases with increasing pressure.(3) Magnetization, electrical resistivity and specific heat measurements have been made on Heusler-type Fe_2VAl compound. The electrical resistivity shows semiconductor-like negative temperature dependence. We suggested that this compound should be a semimetal with a pseudogap in the density of states at the Fermi level.