Electron pairing in the presence of incipient bands in iron-based superconductors

Abstract

Recent experiments on certain Fe-based superconductors have hinted at a role for paired electrons in "incipient" bands that are close to, but do not cross the Fermi level. Related theoretical works disagree on whether or not strong-coupling superconductivity is required to explain such effects, and whether a critical interaction strength exists. In this work, we consider various versions of the model problem of pairing of electrons in the presence of an incipient band, within a simple multiband weak-coupling BCS approximation. We categorize the problem into two cases: case(I) where superconductivity arises from the "incipient band pairing" alone, and case(II) where it is induced on an incipient band by pairing due to Fermi-surface based interactions. Negative conclusions regarding the importance of incipient bands have been drawn so far largely based on case(I), but we show explicitly that models under case(II) are qualitatively different, and can explain the non-exponential suppression of Tc, as well as robust large gaps on an incipient band. In the latter situation, large gaps on the incipient band do not require a critical interaction strength. We also model the interplay between phonon and spin fluctuation driven superconductivity and describe the bootstrap of electron-phonon superconductivity by spin fluctuations coupling the incipient and the regular bands. Finally, we discuss the effect of the dimensionality of the incipient band on our results. We argue that pairing on incipient bands may be significant and important in several Fe-based materials, including LiFeAs, FeSe intercalates and FeSe monolayers on strontium titanate, and indeed may contribute to high critical temperatures in some cases.

Multiband full-bandwidth anisotropic Eliashberg theory of interfacial electron-phonon coupling and high−Tc superconductivity in FeSeAperis1, Oppeneer2 2018Phys. Rev. B4110771View full textAdd to dashboardCite

“…In stark contrast to previous proposals, our here predicted deep Fermi sea Cooper pairing does not depend upon interband scattering processes mediated by bosons at large wavevectors, like e.g. antiferromagnetic spin fluctuations [27,28] or the conventional short-ranged in real space EPI [29].In the presence of a strong inhomogeneous dielectric background, the EPI develops a pronounced forward scattering peak [18,30]. Here, the interface-induced EPI is modelled by a dispersionless mode at Ω=81 meV [21] coupled to FeSe electrons via the functional form g(q) = g 0 exp (−|q|/q c ) with q c = 0.3a −1 [6], with a the FeSe lattice constant.…”

“…In stark contrast to previous proposals, our here predicted deep Fermi sea Cooper pairing does not depend upon interband scattering processes mediated by bosons at large wavevectors, like e.g. antiferromagnetic spin fluctuations [27,28] or the conventional short-ranged in real space EPI [29].…”

Ab initiostudy of cross-interface electron-phonon couplings in FeSe thin films onSrTiO3andBaTiO3

“…5 The large electron doping and the resulting Fermi surface with only electron pockets directly challenge the Fermi-surface-nesting driven, purely electronic pairing mechanism. 18,19 One way to reconcile such a scenario is by taking account of the holelike band located below the Fermi level at the Γ point, 20,21 since the band top is less than 100 meV away from the Fermi level, i.e., an "incipient" band 21 that might fall in the low-energy cutoff of the bosons mediating pairing. The substrate, however, also influences the phononic degrees of freedom and the electron-phonon (e-ph) interaction, which can lead to phonon contributions to the supercon-ductivity.…”

Bounds on nanoscale nematicity in single-layerFeSe/SrTiO3

“…The boson was initially hypothesized to be an O phonon mode and subsequently observed on bare SrTiO 3 (001) [11]. Model calculations have demonstrated that phonons can enhance spin-fluctuation-mediated pairing in FeSe [5,12,13]. Others have argued that phonons alone can account for a significant portion of the high T c [14,15].…”