Abstract
We present a novel method for embedding spin and charge fluctuations in an anisotropic, multi-band and full-bandwidth Eliashberg treatment of superconductivity. Our analytical framework, based on the random phase approximation, allows for a selfconsistent calculation of material specific characteristics in the interacting, and more specifically, the superconducting state. We apply this approach to bulk FeSe as representative for the iron-based superconductors and successfully solve for the superconducting transition temperature $T_c$, the gap symmetry and the gap magnitude. We obtain $T_c \approx 6$ K, consistent with experiment ($T_c \approx 8$ K), as well as other quantities in good agreement with experimental observations, thus supporting spin fluctuations mediated pairing in bulk FeSe. On the contrary, applying our approach to monolayer FeSe on SrTiO$_3$ we find that spin fluctuations within the full Eliashberg framework give a $d$-wave gap with $T_c\le 11$ K and therefore cannot provide an explanation for a critical temperature as high as observed experimentally ($T_c \approx 70$ K). Our results hence point towards interfacial electron-phonon coupling as the dominant Cooper pairing mediator in this system.
Highlights
Ever since the discovery of superconductivity in iron-based compounds [1,2,3,4] an enormous effort has been made to understand the prevailing mechanism responsible for Cooper pairing in these materials, both experimentally and theoretically
For most members of this family, the superconducting transition temperature is rather small compared to the high-Tc cuprates, with a few exceptions such as monolayer FeSe on SrTiO3 (FeSe/STO)
In this way the superconducting gap symmetry can presumably be obtained correctly, but other important experimental aspects, such as Tc or the gap magnitude, remain partially elusive depending on the level of approximation, hampering the unambiguous identification of the pairing mechanism
Summary
Ever since the discovery of superconductivity in iron-based compounds [1,2,3,4] an enormous effort has been made to understand the prevailing mechanism responsible for Cooper pairing in these materials, both experimentally and theoretically (see Refs. [5,6,7,8,9,10] and references therein). In addition we examine the case of monolayer FeSe using modified electronic energies [68], while neglecting any possible influence of the substrate phonon Our results for this material reveal a strong mismatch to experimental findings, in particular, a computed Tc similar only to that of bulk FeSe. Our results for this material reveal a strong mismatch to experimental findings, in particular, a computed Tc similar only to that of bulk FeSe This leads us to the conclusion that spin fluctuations play a minor role in FeSe/STO only for temperatures characteristic for superconductivity in the parent compound.
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