Charge transfer and electron-phonon coupling in monolayer FeSe on Nb-dopedSrTiO3

Abstract

Monolayer films of FeSe grown on SrTiO3 substrates are electron doped relative to bulk and exhibit a significantly higher superconducting transition temperatures. We present density functional calculations and a modified Schottky model incorporating the strong paraelectricity of SrTiO3 which demonstrate that the doping may be due to charge transfer from SrTiO3 impurity bands driven by work function mismatch. Physically relevant levels of Nb doping are shown to lead to doping of the FeSe compatible with observation. The coupling of electrons in FeSe to polar phonons in the depletion regime of the SrTiO3 is calculated. A λ ∼ 0.4 is found; the coupling to long-wavelength phonons is found to be dominant.

Superfluid density and microwave conductivity of FeSe superconductor: ultra-long-lived quasiparticles and extendeds-wave energy gap

“…FeSe is one of the simplest iron-based materials, superconducting at its stoichiometric composition and available as high quality single crystals grown using vapour transport methods [19,20]. A crucial question is to understand what the bulk, ambient superconductor is holding in reserve, that it can respond so strongly when pressurised [21] or placed in contact with a substrate [22]. Central to addressing this issue is the identification of the superconducting gap structure and, ultimately, the pairing glue [21][22][23][24][25][26][27].…”

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

“…Furthermore, angular resolved photoemission spectroscopy (ARPES) measurements [6] reveal an interfaceinduced electron-phonon interaction (EPI) between FeSe electrons and polar STO phonons that is strongly peaked at the q=0 phonon wavevector [6,[14][15][16][17][18]. There is growing experimental evidence for the pivotal role of such interfacial phonons in engineering high-T c heterostructures that involve FeSe [5,9,19] or even FeAs [20] monolayers.Although ab initio calculations confirm the existence of small-q phonons as a strictly interfacial phenomenon in FeSe/STO [12,[21][22][23] and indicate the importance of the coupling between substrate phonons and FeSe electrons [24], the estimated low value of the electron-phonon coupling constant (λ ≤ 0.4) has been widely considered insufficient to explain the impressive T c enhancement unless another, dominant pairing mechanism is at play [6]. On the other hand, Eliashberg calculations within a single band model suggest that interfacial phonons may lead to the high T c with a coupling of merely half of that estimated by experiments [25].…”

“…We determine the value of the electron-phonon scattering strength g 0 , by requiring that the ARPES replica bands are reproduced at their observed energies [6]. In this way, we circumvent the need for treating screening effects at the FeSe/STO interface explicitly [23,33] and our determined value for the EPI strength may considered as the overall strength of the resulting effective EPI. We obtain g 0 = 728 meV, which is significantly close to the ab initio calculated value for anatase TiO 2 [34], but a bit lower.…”

Superconductivity in FeSe Thin Films Driven by the Interplay between Nematic Fluctuations and Spin-Orbit Coupling

“…Although such a coupling can certainly enhance T c [20][21][22][23][24], recent experiments indicate that the STO substrate may not be essential to achieve the high-T c state. In particular, T c up to 40 K was observed in electrostatically-gated films of FeSe with different thickness grown both on STO and MgO substrates [25].…”