Superconductivity at 44.4 K achieved by intercalating EMIM+ into FeSe* *Project supported by the National Natural Science Foundation of China (Grant Nos. 12061131001 and 52072170) and the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB25000000).

Abstract

Superconductivity with transition temperature T c above 40 K was observed in protonated FeSe (H y -FeSe) 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 FeSe single crystals, we obtain superconducting samples with T c above 40 K. We show that the obtained superconducting phase is not H y -FeSe but actually an organic-ion (C6H11 referred to as EMIM+)-intercalated phase (EMIM) x FeSe. 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 FeSe and the residual FeSe, respectively. The superconductivity of (EMIM) x FeSe with T c 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 H irr 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.