Phonon modes and electron–phonon coupling at the FeSe/SrTiO3 interface | Nature

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The remarkable increase in superconducting transition temperature (Tc) observed at the interface of one-unit-cell FeSe films on SrTiO3 substrates (1 uc FeSe/STO)1 has attracted considerable research into the interface effects2,3,4,5,6. Although this high Tc is thought to be associated with electron–phonon coupling (EPC)2, the microscopic coupling mechanism and its role in the superconductivity remain elusive. Here we use momentum-selective high-resolution electron energy loss spectroscopy to atomically resolve the phonons at the FeSe/STO interface. We uncover new optical phonon modes, coupling strongly with electrons, in the energy range of 75–99 meV. These modes are characterized by out-of-plane vibrations of oxygen atoms in the interfacial double-TiOx layer and the apical oxygens in STO. Our results also demonstrate that the EPC strength and superconducting gap of 1 uc FeSe/STO are closely related to the interlayer spacing between FeSe and the TiOx terminated STO. These findings shed light on the microscopic origin of the interfacial EPC and provide insights into achieving large and consistent Tc enhancement in FeSe/STO and potentially other superconducting systems.

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Raw data for key experimental results are presented in Extended Data Figs. 3 and 8. Further data can be requested from the corresponding author.

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This work was primarily supported by the US Department of Energy (DOE), Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Award No. DE-SC0014430 (H.Y. and X.P.). Further support was provided by the DOE’s National Nuclear Security Administration under grant no. GRANT13583770 (F.G. and X.P.) and the National Science Foundation through the Materials Research Science and Engineering Center programme under grant award no. DMR-2011967 (X.Y. and X.P.). Y.Z. and R.W. were supported by the US DOE, Office of Science (grant no. DE-FG02-05ER46237). P.Z. and J.R. acknowledge the support of the Swedish Research Council (grant no. 2021-03848), Olle Engkvist’s foundation (grant no. 214-0331), STINT (grant no. CH2019-8211) and Knut and Alice Wallenbergs’ foundation (grant no. 2022.0079). The electron microscopy studies were performed at the UC Irvine Materials Research Institute (IMRI) supported in part by the National Science Foundation through the Materials Research Science and Engineering Center programme (grant no. DMR-2011967). The simulations were enabled by resources provided by the National Academic Infrastructure for Supercomputing in Sweden (NAISS) at NSC Centre partially funded by the Swedish Research Council through grant agreement no. 2022-06725. H.Y. acknowledges L.D. Marks for valuable discussions on the surface structure of SrTiO3. H.Y. also thanks M. Xu for assistance with low-dose STEM imaging, as well as W. Wang and H. Wu for helping with TEM specimen preparation.

Department of Materials Science and Engineering, University of California, Irvine, CA, USA

Hongbin Yang, Xingxu Yan, Francisco Guzman & Xiaoqing Pan

Department of Physics and Astronomy, University of California, Irvine, CA, USA

Yinong Zhou & Ruqian Wu

Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing, China

Guangyao Miao, Xiaofeng Xu, Xuetao Zhu, Weihua Wang & Jiandong Guo

Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden

Ján Rusz & Paul Zeiger

Department of Chemistry, Princeton University, Princeton, NJ, USA

Xianghan Xu

Irvine Materials Research Institute (IMRI), University of California, Irvine, CA, USA

Toshihiro Aoki & Xiaoqing Pan

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X.P. conceived and directed this project. H.Y. proposed and designed the research with input from J.G., R.W. and X.P. H.Y. carried out TEM specimen preparation, STEM–EELS experiments and data analysis. Y.Z. and R.W. designed and performed DFT simulations. G.M. and Xiaofeng Xu grew FeSe/STO samples with supervision from X.Z. and J.G. G.M. performed STM–STS experiments with supervision from W.W. and J.G. J.R. did FRFPMS simulations with input from P.Z. and Y.Z. F.G. and H.Y. performed STEM image simulations. Xianghan Xu grew the FeSe bulk single crystal. X.Y. and T.A. helped with STEM–EELS experiments. The manuscript was prepared by H.Y., Y.Z., R.W. and X.P., with contributions from all other co-authors.

Correspondence to Xiaoqing Pan.

The authors declare no completing interests.

Nature thanks Louk Rademaker and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

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(a) A large-scale STM image of the 1uc FeSe film on STO before post-annealing and Te capping, with some brighter islands due to 2uc FeSe at the edges of the STO surface steps. (b) and (c) zoom-in STM image after post-annealing. (d) Low-magnification cross-section HAADF image of the same sample after Te capping, showing a typical 1uc FeSe region. (e) HAADF image of a 2uc FeSe region from the same 1uc FeSe/STO sample.

(a) Scattering geometry (b) A diffraction pattern under defocused condition showing a shadow image of the TEM specimen of amorphous carbon/Te/FeSe/STO and the interface direction. (c) A diffraction pattern with intensity shown in log scale. The electron beam focused on the STO region. The EELS entrance aperture locations for OP and IP acquisitions are shown in (b).

(a) EELS of O and Sr columns in the Sr-O layer 1. The 14 meV peak corresponds to the Sr phonons, 65 and 100 meV phonon peaks are stronger on the O column. (b) O columns in the Ti-O layer 1 (blue curve) compared with equatorial O columns in STO (orange curve). New vibrational loss peaks near 76 and 85 meV are indicated by the blue arrows, which are associated with the Ti-O bonding between Ti-O layer 1 and 0. (c) O columns in the Ti-O layer 0 (blue curve) compared with the average of both apical and equatorial O columns in STO (orange curve). All EEL spectra in (d, e, f) are from OP measurements. The EELS acquisition locations are shown with color-matching rectangles in the inserted HAADF image (dimension 1 × 2 nm). Differences between each pair of EEL spectra in (a, b, c) are plotted in (d, e, f). The spectra are multiplied with E to better visualized the EELS intensity differences as a function of energy.

(a, b) Medium energy Ti+O phonon maps from OP and IP acquisition, respectively. The bright atomic columns in the STO region correspond to the Ti + O atom columns. The images show elongation in either horizontal (41 meV and 50 meV, OP) or vertical (33 meV and 45 meV, IP) direction. This is due to the vibration of different O columns near the Ti + O columns, as illustrated with the TiO6 octahedrons in the 50 meV OP and 45 meV IP images. The oxygen vibration directions are indicated with double headed arrows. The 60 meV and 65 meV images involve all O columns surrounding the Ti + O columns, therefore show less elongations. (c, d) OP and IP interface phonons. The OP interface phonons were found between 76 and 84 meV, whereas the IP interface vibrations were observed near 50 meV and 77 meV. The vibration of the “O” columns in Ti-O layer 0 is stronger at 76 meV than at 84 meV in the OP maps, as indicated by the solid and open arrows. The two IP interface phonon maps differ from each other by their vibrational intensity of O in Ti-O layer 1. The stronger oxygen column vibrations were observed in the 77 meV map, as indicated by the arrows as well. (e, f) FRFPMS simulation of the interface phonons correspond to (c, d), which agree qualitatively with EELS data.

(a) The phonon dispersions for the structure FeSe/STO (1uc, without additional TiOx layer) with phonon linewidth γqν denoted by red circles. The right panel shows the Eliashberg spectral function α2F(ω) (black line) and cumulative frequency-dependent λ(ω) (red line). (b) The atomic displacements for the 88 meV strong-coupling modes in (a) at q = 0. (c) The atomic displacements for the HE and LE modes at 78.0 and 65.7 meV, calculated for the FeSe/Ti2O2/STO (1uc) model in Fig. 3. The additional Ti2O2 interfacial layer introduces new out-of-plane vibration modes of O atoms in Ti-O layer 1.

(a) The band structures and projected orbital contributions. The blue, yellow, and red dots represent the total orbital contributions of Ti, Fe, and O, respectively. The red shaded area in [−0.05, 0.05] eV corresponds to the energy range for the charge density plot in (b). (b) The charge density (yellow) with electronic states around the Fermi level with the isosurface values of 0.0005 e/bohr3.

(a) An as-grown 10uc FeSe/STO sample with large dFe-Ti interface. (b) A 2uc FeSe region in the 500 °C annealed sample with large dFe-Ti. (c) A different region from the same specimen in (b) but with small dFe-Ti. (d) A region in the 500 °C annealed sample featuring gradual interface structure transition from large dFe-Ti (left) to small dFe-Ti (right). (e) A region in the 460 °C annealed sample featuring sharp interface structure transition from large dFe-Ti to small dFe-Ti. The transition region has disorders in the FeSe film. The vertical positions of the Fe layer in the 1uc FeSe and the Ti-O layer 0 of STO surface, as well as the corresponding dFe-Ti are horizontal lines and double headed arrows.

From left to right: asymmetric annular dark-field (aADF) image acquired simultaneously with EELS mapping, and energy filtered dark-field EELS maps at 15 meV, 40–70, meV 99 meV, 74–86 meV. From top to bottom: sets of images with increasing number of averaged datasets, with (a), (b), (c), and (d) averaging 2, 4, 8, and 15 datasets, respectively.

This Supplementary Information file contains Discussion (1. Interface structure analysis; 2. Additional vibrational EELS analysis; 3. First-principles calculations), including Figs 1–9 and extra references.

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Yang, H., Zhou, Y., Miao, G. et al. Phonon modes and electron–phonon coupling at the FeSe/SrTiO3 interface. Nature (2024). https://doi.org/10.1038/s41586-024-08118-0

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Received: 18 January 2024

Accepted: 25 September 2024

Published: 30 October 2024

DOI: https://doi.org/10.1038/s41586-024-08118-0

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