Superconducting Properties of a Stoichiometric FeSe Compound and Two Anomalous Features in the Normal State

This paper reports the synthesis and superconducting behaviors of the tetragonal iron-chalcogenide superconductor FeSe. The electrical resistivity and magnetic moment measurements confirmed its superconductivity with a $T_c^{zero}$ and $T_c^{mag}$ at 9.4 K under ambient pressure. EPMA indicated the sample to have a stoichiometric Fe:Se ratio of 1:1 ($\pm$0.02). The Seebeck coefficient which was 12.3 $\mu$V/K at room temperature, changed to a negative value near 200 K, indicating it to be a two carriers material. Above $T_c$, the $\rho(T)$ curve revealed an 'S' shape. Hence $d\rho(T)/dT$, and $d^2\rho(T)/dT^2$ showed pseudogap-like behavior at $T^*$=110 K according to the resistivity curvature mapping (RCM) method for high $T_c$ cuprates. Moreover, the magnetoresistance $\rho_H(T)/\rho_{H=0}$ under a magnetic field and the Seebeck coefficient $S(T)$ revealed revealed pseudogap-like behavior near $T^*$. Interestingly, at the same temperature, 30 K, the sign of $S(T)$ and all signs of $d^2\rho(T)/dT^2$ changed from negative to positive above $T_c$.