Films of iron selenide (FeSe) one unit cell thick grown on strontium titanate (SrTiO3 or STO) substrates have recently shown1,2,3,4 superconducting energy gaps opening at temperatures close to the boiling point of liquid nitrogen (77 kelvin), which is a record for the iron-based superconductors. The gap opening temperature usually sets the superconducting transition temperature Tc, as the gap signals the formation of Cooper pairs, the bound electron states responsible for superconductivity. To understand why Cooper pairs form at such high temperatures, we examine the role of the SrTiO3 substrate. Here we report high-resolution angle-resolved photoemission spectroscopy results that reveal an unexpected characteristic of the single-unit-cell FeSe/SrTiO3 system: shake-off bands suggesting the presence of bosonic modes, most probably oxygen optical phonons in SrTiO3 (refs 5, 6, 7), which couple to the FeSe electrons with only a small momentum transfer. Such interfacial coupling assists superconductivity in most channels, including those mediated by spin fluctuations8,9,10,11,12,13,14. Our calculations suggest that this coupling is responsible for raising the superconducting gap opening temperature in single-unit-cell FeSe/SrTiO3.
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This work was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. D.-H.L. is supported by the Department of Energy, Office of Basic Energy Sciences, Division of Materials Science, under the Quantum Material programme DE-AC02-05CH11231. Measurements were performed at the Stanford Synchrotron Radiation Lightsource, a national user facility operated by Stanford University on behalf of the US Department of Energy, Office of Basic Energy Sciences.
The authors declare no competing financial interests.
Extended data figures and tables
Extended Data Figure 1 Reflection high-energy electron diffraction (RHEED) images observed during FeSe growth.
a, RHEED image of SrTiO3 substrate after degassing at 450 °C for 1 h. Red box highlights the region integrated for monitoring RHEED oscillations. b, Surface reconstruction as observed by RHEED at annealing temperatures. c, RHEED image of FeSe 1UC film showing uniform streaks typical of an atomically flat thin film. d, RHEED intensity for integration region shown in a (black). The second derivative of the intensity curve (red), with numbers indicating the number of layers grown highlights the RHEED oscillations signalling the completion of a unit cell after about 30 s.
a, f, k, p, Plots of the raw spectrum at Γ, the second derivative at Γ, the raw spectrum at M, and the second derivative at M for the 1UC film, respectively, taken at 10 K. b, g, l, q, The same plots for the 1.7UC film taken at 13 K. c, h, m, r, The same plots for the 2UC film taken at 15 K. d, i, n, s, The same plots for the 30UC film taken at 50 K. e, j, o, t, The same plots for the 30UC film taken at 140 K. Data for the 30UC film are symmetrized about the high-symmetry points (indicated by the green line), and were taken with 25-eV photons. We observe a band splitting in the 30UC film at M, at low temperature (s). This band splitting closes at higher temperature (140 K), where we now observe only one band (t).
a, Spectrum at 10 K, where we can clearly see the backbending of the replica bands, exactly like the main bands near EF. b, Spectrum at 30 K. c, Spectrum at 50 K. d, Spectrum at 70 K. e, Spectrum at 90 K. f, Spectrum at 120 K. The replica bands persist up to temperatures significantly higher than the gap-opening temperature.
a, Plot showing two different backgrounds used in the fitting. Using the blue circles as fixed points, we first modelled the background using a spline interpolation, plotted in red. The second fit used a Shirley background (see Supplementary Information), plotted in purple. b, Data and fitting with the spline background subtracted. We fitted to four Gaussian peaks, which are plotted separately for clarity. c, Data and fitting with the Shirley background subtracted. We restrict our fitting energy window to be from −0.32 eV to 0.03 eV.
a, The momentum distribution curves (MDCs) of our theoretical calculation of the main electron band and the replica electron band with normalized intensities. The MDCs of both bands are taken at the same energies with respect to their band bottoms (see inset). The replica band peaks are broadened due to the electron–phonon coupling. b, The MDCs of our data, with a momentum-independent background subtracted from the replica band MDC. The momentum-dependent background—such as contributions from the hole band—is the likely cause of the extra broadening in the data.
Extended Data Figure 6 Effects of electron-phonon coupling on different gap symmetries and Fermi surfaces.
Cartoon sketches of the various Fermi surfaces and gap symmetries found or proposed in unconventional superconductors. is the projected coupling defined by equation (18) in the Supplementary Information. a, Sketch for the copper oxides with a d-wave gap. b–e, Various scenarios for the iron-based superconductors. Only one-quarter of the first Brillouin zone is shown for clarity. The thick blue and red lines indicate the phase of the gap. The arrows show various forward-focused scattering processes. The black arrows indicate scattering processes that connect portions of the Fermi surface with the same sign gap and are therefore pair-enhancing. The green arrows show pair-breaking processes which connect regions of the Fermi surface with different signs.
a, Calculated band structure used in our determination of Tc enhancement. b, Calculated Fermi surface showing slightly split electrons pockets. c, Dispersion along the M-point showing two nearly degenerate bands indicated with red and blue arrows. d, Momentum distribution curve (MDC) at EF showing peaks from the two bands plotted in c.
The blue line represents . The red line represents the transition between different gap symmetries at J2/J1 ≈ 0.31. Above the transition one finds a gap with s-wave symmetry. Below the transition one finds a gap with d-wave symmetry. Diagrams of the two different possible symmetries are drawn in their respective region of the phase diagram as insets. The lengths of the tick lines in the inset diagrams represent the magnitude of the gap, while the colour represents the sign (red for minus, blue for plus). The two electron pockets in the figure are separated for clarity.
This file contains Supplementary Text and Data 1-8 and additional references. It includes growth and measurement methods, discussion about additional angle-resolved photoemission spectroscopy data taken on films, and detailed theoretical treatment about electron-phonon coupling and its enhancement of the superconducting transition temperature. The supplementary information references the extended data figures, whose legends are attached in the main manuscript text. (PDF 500 kb)
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Lee, J., Schmitt, F., Moore, R. et al. Interfacial mode coupling as the origin of the enhancement of Tc in FeSe films on SrTiO3. Nature 515, 245–248 (2014). https://doi.org/10.1038/nature13894
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