Spectroscopic-imaging STM studies of Fe(Se,S)

The iron chalcogenide superconductor FeSe (Tc ~ 9 K) attracts much attention because it possesses unique features. First, FeSe is one of the few iron-based superconductors that have nodes in the superconducting gap [1,2]. The superconductivity occurs in the non-magnetic orthorhombic phase (nematic phase) that is associated with the orbital ordering [3-5]. Among other things, it should be noted that Fermi energy of FeSe is as small as the superconducting gap size. In accord with this, Fermi wave length is very long, being comparable to the coherence length. These are unprecedented situations in a superconductor, placing FeSe in the BCS-BEC crossover regime [1]. We performed low-temperature spectroscopic-imaging STM on high-quality single crystals of Fe(Se,S) to visualize the spatial electronic-state variations. S substitution to the Se site suppresses the orbital ordering, providing us with the playground to study the relation between superconductivity and nematicity. We observed unidirectional quasiparticle interference (QPI) patterns that reflect the orbital ordering [1]. The QPI patterns of S-doped samples suggest the competition between superconductivity and nematicity. We also argue the anisotropy of the superconducting gap and the spatial variation of the superconducting gap, which may be related to the BCS-BEC crossover.[1] S. Kasahara, T. Watashige et al., PNAS, 111, 16309 (2014). [2] C. -L. Song et al., Science 332, 1410 (2011). [3] K. Nakayama et al., Phys. Rev. Lett. 113, 237001 (2014). [4] T. Shimojima et al., Phys. Rev. B 90, 121111(R) (2014). [5] Y. Suzuki et al., Phys. Rev. B 92, 205117 (2015).