Research of New type of Superconductors in Tetragonal HoCoGa5-type Crystal Structure

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The tetragonal HoCoGa_5-type crystal structure is known as a tetragonal variant of the AuCu_3-strucuture in which new superconductors were discovered. CeMIn_5 (M : Co, Rh, Ir) compounds exhibit a variety of interesting properties such as non-s-superconductivity, a first-order transition near the upper critical field, heavy-fermion behavior, pressure-induced superconductivity, etc. In the exotic heavy fermion superconductors, Cooper pairs are expected formed by the exchange of magnetic spin fluctuations, in which f-electrons which are not simply localized play an important role. A large specific heat coefficient and Curie-Weiss behavior in the normal state or low anti-ferromagnetic ordering temperatures are features of the exotic heavy fermion superconductors. PuCoGa_5 and PuRhGa_5 are known as heavy fermion "high T_c" superconductors. Pu has valence states of Pu^<3+>(5f^5), PU^<4+>(5f^4), PU^<5+>, Pu^<6+> in Pu-based compounds, while Sm has Sm^<2+>(4f^6), Sm^<3+>(4f^5). Sm^<3+>(4f^5) and Pu^<3+>(5f^5) ions have the same J=5/2 multiplet, and show the local moment behavior, i.e., obey a Curie-Weiss law. It is interesting that it depends on the particular crystal structure whether Sm compounds show exotic behavior or not.We have succeeded in growing single crystals of. SmMIn_5 (M : Co, Rh, Ir) by the flux method. There was no report of the physical properties on single crystalline SmCoIn_5. Specific heat measurement, magnetization measurements and electrical resistivity measurements were performed on SmMIn_5.SmRhIn_5 and SmIrIn5 showed the antiferromagnetic ordering and extra transitions below T_N, which is almost same as the results in the previous report. SmCoIn_5 also showed the antiferromagnetic ordering and extra transitions below T_N 11.9 K. The temperatures of these anomalies are 6, 7.6 and 11.9 K and lowest found in SmMIn_5 compounds.