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

我们研究了强相关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)在高压氧气条件下，用通量法成功生长了PrBa_2Cu_4O_8 （Pr124）单晶。利用四圆衍射仪获取了Pr124单晶的结构数据，并用全矩阵最小二乘结构精化法对其进行了结构精化。单晶Pr124和多晶Pr124的所有z-位置参数基本相同。Pr124单晶电阻率的温度依赖性不表现出超导性，至少在2 K以下。沿b轴的电阻率表现出高导电性，由于a轴和b轴的电阻率具有较大的各向异性，其根源可归因于双链位置。(2) RuSr_2GdCu_2O_8氧化物在40K左右表现为超导跃迁，在130K左右表现为铁磁有序。测量了样品的超导和铁磁转变温度与压力的关系。在压力作用下超导转变温度几乎没有变化。铁磁转变温度随压力的增加而升高。(3)对heusler型Fe_2VAl化合物进行了磁化、电阻率和比热测量。电阻率表现出类似半导体的负温度依赖性。我们认为这种化合物应该是在费米能级上具有赝隙的半金属。

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.