Enhanced superconductivity due to forward scattering in FeSe thin films on SrTiO₃substrates

Abstract

We study the consequences of an electron-phonon (e-ph) interaction that is strongly peaked in the forward scattering ( = q 0) direction in a two-dimensional superconductor using Migdal-Eliashberg theory. We find that strong forward scattering results in an enhanced T c that is linearly proportional to the strength of the dimensionless e-phcoupling constant l m in the weak coupling limit. This interaction also produces distinct replica bands in the single-particle spectral function, similar to those observed in recent angle-resolved photoemission experiments on FeSe monolayers on SrTiO 3 and BaTiO 3 substrates. By comparing our model to photoemission experiments, we infer an e-phcoupling strength that can provide a significant portion of the observed high T c in these systems.A flurry of scientific activities has been generated by the discovery of an enhanced superconductivity in FeSe monolayers grown on SrTiO 3 (STO) substrates [1-20]. On its own, bulk FeSe has a modest superconducting transition temperatureT 9 c K [21]; however, when a monolayer is grown on an STO substrate, T c is increased dramatically [1]. Most reported T c values cluster within 55-75 K, close to the boiling point of liquid nitrogen (77 K). (A surprisingly highT 107 c K has also been reported in in situ transport measurements [9].) This discovery has opened a pathway to high-T c superconductivity through interface engineering, which has already produced high-T c ʼs in systems such as FeSe on BaTiO 3 (BTO) [8] and FeTe -x 1 Se x on STO [22]. Determining the origin of the T c enhancement in these interface systems is critical. At the moment, proposals include charge transfer between the substrate and FeSe [2-4, 20], electric field [6] and strain effects due to the substrate [5, 8], and lattice related effects such as enhanced electron-phonon (e-ph) coupling in the FeSe layer [1,13,16] or across the interface [7,19]. Strong evidence for the latter has been provided by a recent angleresolved photoemission spectroscopy (ARPES) study [7], which observed replica bands in the single-particle spectral function of the FeSe monolayer. These replicas are interpreted as being produced by coupling between the FeSe d 3 electrons and an optical oxygen phonon branch in the STO substrate. Moreover, the replica bands are complete copies of the corresponding main bands, which implies that the responsible e-phinteraction is strongly peaked in the forward scattering direction (small momentum transfers). Such momentum dependence is notable because it can enhance superconductivity in most pairing channels [23][24][25][26][27][28][29][30][31]. As such, this crossinterface coupling provides at the same time a suitable mechanism for the T c enhancement in the FeSe/STO and FeSe/BTO systems [7,8].We explore this possibility here by examining the consequences of strong forward scattering in the e-phinteraction for superconductivity and the spectral properties of a two-dimensional system. By solving the momentum dependent Eliashberg equations, we show that a pronounced...

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

“…The study in Ref. 24 suggests that the coupling to the forward scattering process results in a T c that depends linearly on the coupling constant, where an estimated coupling strength ∼ 0.15-0.2 for the single mode alone can account for the total T c ∼ 70 K if the Coulomb pseudopotential µ * is neglected. Therefore, the T c enhancement due to this oxygen mode can be much larger than that expected from the conventional phonon mediated pairing mechanism.…”

“…22,25 Another possibility is that vertex corrections to the e-ph interaction, which were neglected in Ref. 24, may need to be included since they can enhance T c in the perturbative regime when the e-ph interaction is peaked at small momentum transfers. [48][49][50] Our DFT results hint that FeSe/STO is in this regime providing that a finite integrated coupling strength from the forwardfocused coupling of the oxygen mode can be obtained with an improved method.…”

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].…”

Competing phases and orbital-selective behaviors in the two-orbital Hubbard-Holstein model

“…This finding is consistent with the general notion that electron correlations can enhance e-ph interactions 16 . Moreover, the discovery of the FeSe films on oxide substrates 17 has implicated new possible lattice interactions, either across the interface 18,19 or within the FeSe film 20 . Since bulk FeSe is believed to be in the OSMP regime 21,22 , these experiments naturally raise questions about when and how e-ph interactions can influence such multi-orbital phenomena.…”