Superconducting properties of a stoichiometric FeSe compound and two anomalous features in the normal state

This paper reports the superconducting behavior of the tetragonal iron-chalcogenide superconductor FeSe. The electrical resistivity and the magnetic moment measurements confirmed its superconductivity with T^zero _c and T^mag _c of 9.4 K under ambient pressure. Electron probe macro analysis indicated that the sample had a stoichiometric Fe:Se ratio of 1:1. The Seebeck coefficient, which was 12.3 μV/K at room temperature, changed to a negative value near 200 K, indicating it to be a two-carrier material. Above T_c, the ρ(T) curve revealed an 'S' shape, and dρ(T)/dT and d²ρ(T)/dT² showed two anomalous features, one near T_an = 30 K and the other near T* = 110 K. The Seebeck coefficient, S(T), also displays peculiar behavior near the T_an and T*. We also report a rather smaller critical current density, J_c = 10² ~ 10⁴ A/cm², compared to those observed for the other pnictides and doped chalcogenides.