Abstract
We showed that the all phonons – not only forward-scattering phonon but also local (all-momentum-scattering) phonon – contribute to boosting Tc of the S±-wave pairing state in the incipient band model. In particular, when the incipient band sinks deeper, the phonon boost effect of the local phonon increases and becomes as effective as the one of the forward-scattering phonon. Our finding implies that all interface phonons from the SrTiO3 substrate – not only the 90 meV Fuchs-Kliewer (F-K) phonon but also the 60 meV F-K phonon – as well as all intrinsic phonons of the FeSe monolayer itself, regardless of their degree of “forwardness”, should contribute to increase Tc of the FeSe/STO monolayer system. This all-phonon boost mechanism should universally work in all heavily doped (either by holes or by electrons) Iron-based superconductors.
Highlights
The discovery of the FeSe/SrTiO3 monolayer system (Tc ≈ 60–100 K)[1,2,3] is posing a serious challenge to our understanding of the Iron-based superconductors (IBS)[4,5,6]
The main point of this criticism is that it is true that there exists a F-K phonon mode of the energy ~90 meV12, but there is no evidence that this phonon is a small-momentum exchange phonon which produces the replica band by coupling to the main conduction band electrons of the FeSe layer; it may couple but via all-momentum exchange and in this case no replica band can be formed
To illustrate the consequence of this effect, let us recollect the general principle of a phonon contribution to the unconventional superconductor with a sign-changing order parameter (OP) Δ(k), in general
Summary
We showed that the all phonons – forward-scattering phonon and local (all-momentumscattering) phonon – contribute to boosting Tc of the S±-wave pairing state in the incipient band model. Our finding implies that all interface phonons from the SrTiO3 substrate – the 90 meV Fuchs-Kliewer (F-K) phonon and the 60 meV F-K phonon – as well as all intrinsic phonons of the FeSe monolayer itself, regardless of their degree of “forwardness”, should contribute to increase Tc of the FeSe/STO monolayer system This all-phonon boost mechanism should universally work in all heavily doped (either by holes or by electrons) Iron-based superconductors. The high-resolution electron energy loss spectroscopy (HREELS) measurement[12] has directly measured the 90 meV and 60 meV Fuchs-Kliewer (F-K) interface phonon modes in the FeSe/SrTiO3(STO) system[12] besides several lower energy bulk phonon modes of FeSe layer This HREELS experiment itself has no information about whether the observed F-K modes (both 90 meV and 60 meV modes) are forward-scattering or all-momentum scattering phonons.
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