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

Fe_2As_2 <0.5>Ca_<0.5>Fe_2As_2的Seebeck系数（S/T）在超导态（SC）出现前出现反转，发现在SC表达前费米表面状态发生了明显的改变，在EuFe_2As_2验证SC前体现象的情况下，S/T在S/T为负至零后，随着压力的进一步增加，S/T可能变为正，预计与Eu_<0.5>Ca_<0.5>Fe_2As_2的行为类似。对于EuFe_2As_2和Eu_<0.5>Ca_<0.5>Fe_2As_2， SC出现后电阻率t_2依赖系数A显著降低，可以理解为该体系在临界压力P_C以上费米能级附近的态密度明显降低。

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.