Superconductivity at 44.4 K achieved by intercalating EMIM~+ into FeSe

Superconductivity with transition temperature Tc above 40 K was observed in protonated Fe Se(Hy-Fe Se) previously with the ionic liquid EMIM-BF4 used in the electrochemical process. However, the real superconducting phase is not clear until now. And detailed structural, magnetization, and electrical transport measurements are lacking. By using similar protonating technique on Fe Se single crystals, we obtain superconducting samples with Tc above 40 K. We show that the obtained superconducting phase is not Hy-Fe Se but actually an organic-ion(C_6H_(11)N_2~+ referred to as EMIM~+)-intercalated phase(EMIM)x Fe Se. By using x-ray diffraction technique, two sets of index peaks corresponding to different c-axis lattice constants are detected in the obtained samples, which belong to the newly formed phase of intercalated(EMIM)_x Fe Se and the residual Fe Se, respectively. The superconductivity of(EMIM)x Fe Se with Tc of 44.4 K is confirmed by resistivity and magnetic susceptibility measurements. Temperature dependence of resistivity with different applied magnetic fields reveals that the upper critical field H_(c2) is quite high, while the irreversibility field Hirris suppressed quickly with increasing temperature till about 20 K. This indicates that the resultant compound has a high anisotropy with a large spacing between the Fe Se layers.