Uncovering an interplay among fermiology, superconductivity, and nematicity in FeSe by epitaxial strain

Central issues in iron-based superconductors are the origins of superconductivity and nematicity as well as their interplay. To investigate these issues, we utilize a sizable strain effect in FeSe thin films, where compressive strain drastically enhances T_c by 50%, while tensile strain kills the superconductivity and simultaneously enhances the nematicity. By angle-resolved photoemission spectroscopy, we experimentally observed a monotonic change in the energy overlap (ΔE_h−e) between the hole and electron pockets in the normal state upon variation of in-plane lattice constant [1]. The change in ΔE_h−e modifies the Fermi-surface volume, leading to a change in T_c. Furthermore, the strength of nematicity is also found to be characterized by ΔE_h−e. These results suggest that the key to understanding the phase diagram is the fermiology and interactions linked to the semimetallic band overlap.
[1] G. N. Phan et al. Phys. Rev. B 95, 224507 (2017).