Crystal structures and sign reversal Hall resistivities in iron-based superconductors Lix(C3H10N2)0.32FeSe )**Project supported by the National Natural Science Foundation of China (Grant Nos. 51472266, 51532010, 91422303, and 51772323), the National Key Research and Development Program of China (Grant No. 2016YFA0300301), and the Key Research Program of Frontier Sciences, Chinese Academy of Sciences (Grant No. QYZDJ-SSW-SLH013).

Abstract

Heavy electron-doped FeSe-derived materials have attracted attention due to their uncommon electronic structures with only ‘electron pockets’, and they are different from other iron-based superconductors. Here, we report the crystal structures, superconductivities and normal state properties of two new Li-doped FeSe-based materials, i.e., Li0.15(C3H10N2)0.32FeSe (P-4) and Lix(C3H10N2)0.32FeSe (P4/nmm, ) with superconducting transition temperatures ranging from 40 K to 46 K. The determined crystal structures reveal a coupling between Li concentration and the orientation of 1,3-diaminopropane molecules within the largely expanded FeSe layers. Superconducting fluctuations appear in the resistivity of the two superconductors and are fitted in terms of the quasi two-dimensional (2D) Lawrence–Doniach model. The existence of a crossing point and scaling behavior in the T-dependence of diamagnetic response also suggests that the two superconductors belong to the quasi-2D system. Interestingly, with the increase of temperature, a sign of Hall coefficient (RH) reversing from negative to positive is observed at ∼185 K in both phases, suggesting that ‘hole pockets’ emerge in these electron-doped FeSe materials. First principle calculations indicate that the increase in FeSe layer distance will lift up a ‘hole band’ associated with character and increase the hole carriers. Our findings suggest that the increase in two dimensionalities may lead to the sign-reversal Hall resistivity in Lix(C3H10N2)0.32FeSe at high temperature.