Study of high pressure electronic state on iron-oxynictide high-T_C superconductors using as probe for thermoelectric effect

Eu_Ca_Fe_2As_2的Seebeck系数（S/T）<0.5><0.5>在超导态（SC）出现之前出现符号反转，并且在SC出现之前费米面的状态发生了显著的变化。在EuFe_2As_2的情况下，证实了SC的先驱现象，S/T在负到零之后，随着压力的进一步增加可能会变为正，并有望呈现类似于Eu_Ca_Fe_2As_2的行为<0.5><0.5>。对于EuFe_2As_2和<0.5>Eu_Ca_Fe_2As_2<0.5>，超导体出现后，电阻率的T_2依赖系数A显著降低，可以理解在临界压力P_C以上，费米能级附近的态密度显著降低。

Seebeck coefficient (S/T) of Eu_<0.5>Ca_<0.5>Fe_2As_2 showed sign inversion before appearance of superconducting state (SC) and it was found that state of Fermi surface was notably transmuted just before expression of SC. In the case of EuFe_2As_2 verifying precursor phenomenon of SC, the S/T may change positive with further increasing pressure after negative to zero of S/T and it is expected to present similar behavior of Eu_<0.5>Ca_<0.5>Fe_2As_2. For EuFe_2As_2 and Eu_<0.5>Ca_<0.5>Fe_2As_2, T_2-dependence coefficient A of resistivity was prominently decrease after emergence of SC, it is able to understand that the density of state around Fermi level is distinct decrease above critical pressure P_C in this system.