Superconductivity at 44.4 K achieved by intercalating EMIM+ into FeSe

Superconductivity with transition temperature Tc above 40 K was observed in protonated FeSe (Hy-FeSe) previously with the ionic liquid of 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 FeSe single crystals, we obtained superconducting samples with Tc above 40 K. We show that the obtained superconducting phase is not Hy-FeSe but actually an organic-ion (C6H11N2+ referred to as EMIM+)-intercalated phase (EMIM)xFeSe. By using X-ray diffraction technique, two sets of index peaks corresponding with different c-axis lattice constants are detected in the protonated samples, which belong to the newly formed phase of intercalated (EMIM)xFeSe and the residual FeSe, respectively. The superconductivity of (EMIM)xFeSe 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 Hc2 is quite high, while the irreversibility field Hirr is 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 FeSe layers.