Electron-phonon coupling enhanced by the FeSe/SrTiO3 interface

We study the electron-phonon coupling strength near the interface of monolayer and bilayer FeSe thin films on SrTiO3, BaTiO3, and oxygen-vacant SrTiO3 substrates, using ab initio methods. The calculated total electron-phonon coupling strength λ = 0.2-0.3 cannot account for the high Tc ∼ 70 K observed in these systems through the conventional phonon-mediated pairing mechanism. In all of these systems, however, we find that the coupling constant of a polar oxygen branch peaks at q = 0 with negligible coupling elsewhere, while the energy of this mode coincides with the offset energy of the replica bands measured recently by angle-resolved photoemission spectroscopy experiments. But the integrated coupling strength for this mode from our current calculations is still too small to produce the observed high Tc, even through the more efficient pairing mechanism provided by the forward scattering. We arrive at the same qualitative conclusion when considering a checkerboard antiferromagnetic configuration in the Fe layer. In light of the experimental observations of the replica band feature and the relatively high Tc of FeSe monolayers on polar substrates, our results point towards a cooperative role for the electron-phonon interaction, where the crossinterface interaction acts in conjunction with a purely electronic interaction. We also discuss a few scenarios where the coupling strength obtained here may be enhanced.

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“…For the same reasons, the calculated coupling strengths and widths in q space in Ref. 26 are overestimated.…”

Section: Results and Analysismentioning

confidence: 95%

“…14 This momentum structure has been qualitatively confirmed by recent ab initio calculations for the interface e-ph coupling. 26 However, in light of the sharpness of the replica bands, the q-resolution in Ref. 26 is not in line with the sharpness of the coupling in momentum space that is necessary to explain this experiment.…”

Section: Introductionmentioning

confidence: 92%

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

et al. 2016

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“…Measuring high-resolution ARPES spectra, they observed that the electronic bands of the FeSe monolayer are replicated with an energy shift of 100 meV. They attribute this effect to a strong electronphonon coupling between the layer and the substrate [89]: a phonon mode, most probably an oxygen optical phonon, in SrTiO 3 couples to the FeSe electrons.…”

Section: Superconductivity In Atomically Thin Fese Layersmentioning

confidence: 99%

Interface superconductivity

Gariglio

Gabay

Mannhart

et al. 2015

Physica C: Superconductivity and its Applications

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Low dimensional superconducting systems have been the subject of numerous studies for many years. In this article, we focus our attention on interfacial superconductivity, a field that has been boosted by the discovery of superconductivity at the interface between the two band insulators LaAlO 3 and SrTiO 3 .We explore the properties of this amazing system that allows the electric field control and on/off switching of superconductivity. We discuss the similarities and differences between bulk doped SrTiO 3 and the interface system and the possible role of the interfacially induced Rashba type spin-orbit. We also, more briefly, discuss interface superconductivity in cuprates, in electrical double layer transistor field effect experiments, and the recent observation of a high T c in a monolayer of FeSe deposited on SrTiO 3 .

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“…To the best of our knowledge, no sign of (π, π) magnetic order has been observed in FeSe thin films with only electron pockets. First-principle calculations for the momentumindependent phonon coupling estimate a resulting T c 1 K [60], an energy scale that may be too small to significantly lift the degeneracy, since T c ≈ 40 K in FeSe thin films.…”

Section: Microscopic Modelmentioning

confidence: 99%

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

Kang

Fernandes

2016

Phys. Rev. Lett.

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The origin of the high-temperature superconducting state observed in FeSe thin films, whose phase diagram displays no sign of magnetic order, remains a hotly debated topic. Here we investigate whether fluctuations arising due to the proximity to a nematic phase, which is observed in the phase diagram of this material, can promote superconductivity. We find that nematic fluctuations alone promote a highly degenerate pairing state, in which both s-wave and d-wave symmetries are equally favored, and Tc is consequently suppressed. However, the presence of a sizable spin-orbit coupling or inversion symmetry-breaking at the film interface lifts this harmful degeneracy and selects the s-wave state, in agreement with recent experimental proposals. The resulting gap function displays a weak anisotropy, which agrees with experiments in monolayer FeSe and intercalated Li1−x(OH)xFeSe.In most iron-based superconductors (FeSC), superconductivity is found in close proximity to a magnetically ordered state, suggesting that magnetic fluctuations play an important role in binding the Cooper pairs [1][2][3][4]. Indeed, the fact that the Fermi surface of these materials is composed of small hole pockets and electron pockets separated by the magnetic ordering vector led to the proposal of a sign-changing s +− wave state, in which the gap function has different signs in the hole and in the electron pockets. However, the recent observation of superconductivity over 70 K in monolayer FeSe brought new challenges to the field [5][6][7][8][9][10][11][12][13][14]. In contrast to the standard FeSC, no long-range magnetic order is observed in thin films or even bulk FeSe [15], and the Fermi surface of monolayer FeSe consists of electron pockets only [6,7,11,16]. Since T c in monolayer FeSe is the highest among all FeSC, the elucidation of its origin is a fundamental step in the search for higher T c in these systems.One of the proposed scenarios to explain the dramatic ten-fold increase of T c in monolayer FeSe with respect to the 8 K value in bulk FeSe [17] was the strong coupling to an optical phonon mode of the SrTiO 3 (STO) substrate [16,18,19], which is manifested by replica bands observed in ARPES [16]. 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]. Similar values of T c were reported in FeSe coated with potassium [26,27] and in the bulk sample Li 1−x (OH) x FeSe [28,29], which consists of intercalated FeSe layers. In common to all these systems is the fact that their Fermi surface consists of electron pockets only, suggesting that doping by negative charge carriers plays a fundamental role in stabilizing the high-T c state.Importantly, recent experiments in K-coated bulk FeSe [26] revealed that, besides shifting the chemical potential, electron-doping also su...

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Electron-phonon coupling enhanced by the FeSe/SrTiO3 interface

Cited by 62 publications

References 19 publications

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

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

Interface superconductivity

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

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