Dichotomy of superconductivity between monolayer FeS and FeSe

Abstract

The discovery of high-temperature (Tc) superconductivity in monolayer FeSe on SrTiO3 raised a fundamental question whether high Tc is commonly realized in monolayer iron-based superconductors. Tetragonal FeS is a key material to resolve this issue because bulk FeS is a superconductor with Tc comparable to that of isostructural FeSe. However, difficulty in synthesizing tetragonal monolayer FeS due to its metastable nature has hindered further investigations. Here we report elucidation of band structure of monolayer FeS on SrTiO3, enabled by a unique combination of in-situ topotactic reaction and molecular-beam epitaxy. Our angle-resolved photoemission spectroscopy on FeS and FeSe revealed marked similarities in the electronic structure, such as heavy electron doping and interfacial electron-phonon coupling, both of which have been regarded as possible sources of high Tc in FeSe. However, surprisingly, high-Tc superconductivity is absent in monolayer FeS. This is linked to the weak superconducting pairing in electron-doped multilayer FeS in which the interfacial effects are absent. Our results strongly suggest that the cross-interface electron-phonon coupling enhances Tc only when it cooperates with the pairing interaction inherent to the superconducting layer. This finding provides a key insight to explore new heterointerface high-Tc superconductors.

Evolution of spin excitations from bulk to monolayer FeSe

“…Moreover, any quantitative model for the spin fluctuation contribution to pairing must also account for the observed evolution of the spin dynamics. As such, the evolution of the spin dynamics from FeSe to FeSe/STO represents an essential clue to a magnetic-like pairing scenario, which was previously proposed for other Fe pnictides 1,[29][30][31][32][33][34] . The present results do not, however, rule out additional interactions such as phonons or doping from the substrate, which can contribute to the enhancement of T c 35,36 .…”

High-temperature superconductivity and its robustness against magnetic polarization in monolayer FeSe on EuTiO3

“…No further enhancement in T c compared with FeSe/STO was observed, likely because the T N of ETO is lower than the T c of FeSe film, or the interface exchange interaction is not crucial to boost the superconductivity in this system. It was reported that in contrast to FeSe/STO, high-T c superconductivity is absent in FeS/ STO 66 , even though they possess similar electron doping and spin dynamics. Recent theoretical work points out that the phonons in the surface layer of TiO 2 are responsible for pairing, which is dependent on the FeSe-TiO 2 distance 40 .…”

On the Remarkable Superconductivity of FeSe and Its Close Cousins

“…Figure 16b shows the epitaxial structure of the film used originally to obtain a transport T c four times higher than bulk FeSe (8 K); and (c) the subsequent ARPES gap closing temperature of 65 K measured on similar samples [254]. The T c enhancement in monolayer FeS films on STO is not observed [255] and the same was found when FeSe is deposited on graphene [252] or Bi 2 Se 3 [256]. Taken together, these observations point to a unique high-T c superconducting system based on FeSe where the substrate SrTiO 3 plays an essential role.…”