Electron-Phonon Coupling at the FeSe/SrTiO3 Interface: Insight from Atomically Resolved Vibrational Spectroscopy

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Phonons play important roles in conventional superconductivity by providing the attractive interaction between electrons. One-unit-cell thick FeSe film grown on SrTiO3 substrate (1uc FeSe/STO) is a special superconductor, whose high-Tc has been associated with the electron-phonon coupling (EPC) at the interface [1]. However, the exact role of EPC in the Tc-enhancement is still under investigation. One key question remains unclear is the microscopic coupling mechanism, i.e., which phonon modes are involved in the EPC. Previous studies often associate the oxygen optical phonons of the STO substrate with EPC. This interpretation ignores the fact that there is a double TiOx interfacial layer between the FeSe film and the STO substrate, as imaged by scanning transmission electron microscopy (STEM) [2]. It is remains elusive how the cross-interface EPC occurs between the two materials that are spatially separated. Vibrational electron energy-loss spectroscopy (EELS) in a monochromated STEM [3] may provide new insight to the longstanding problem, as direct measurement of phonons near the 1uc FeSe/STO interface is not available by most techniques. Therefore, in this work, we study the 1uc FeSe/STO with high-resolution EELS in a monochromated Nion STEM. We acquire dark-field (DF) vibrational EELS on superconducting 1uc FeSe/STO samples by displacing the EELS entrance aperture in two different directions with respect to the interface plane, as shown in Fig. 1(a) and (b). Due to the complicated interface structure and involvement of phonon polaritons in DF-EELS [4], interpretation of interface vibrational spectra can be challenging. Here we present EELS energy-filtered maps in Fig. 1(c) to (e), which clearly reveal the atomic columns from their vibrational signal. In the STO substrate, we can see the atomic columns of Sr and Ti+O at energies around 15 and 45 meV. The O columns in STO are observed at their optical phonons near 99 meV, but with the apical and equatorial O columns observed separately in the out-of-plane (OP) and in-plane (IP) EELS maps. The interface exhibits strong vibrations between 70-90 meV, in contrast to the STO substrate and FeSe film. The EELS maps in Fig. 1(e) reveal OP Ti-O vibrations near 75 and 84 meV from the two distinct Ti+O columns in double TiOx layer region. To further understand the phonon energy loss, we analyze the EEL spectra from individual atomic columns integrated from our high signal-to-noise ratio DF-EELS mapping. Figure 2(b) shows the EELS of apical oxygen (O in Sr-O layers) from OP and IP measurements. The 99 meV optical phonon peak is significantly stronger in the OP EELS than in the IP one (indicated by filled and open arrows), because the Ti-O bonding for the apical oxygen is in the OP direction. Atomically resolved vibrational EELS for the double TiOx layers (Ti-O #0 and 1), as well as their differences with STO substrate, are shown in Fig. 2 (c) and (d). We highlight the unique interface contributions (blue and red shades) that correspond to the OP vibrations in Fig. 1(e). The energy and atomic displacements of the vibrational modes indicated by the arrows match with three phonon modes with strong EPC in our first principles calculations. In summary, our atomic scale vibrational EELS imaging reveals the energy and displacement patterns of phonon modes near the FeSe/STO interface. Combined with first principles calculations, the study provides unprecedented insight on the microscopic mechanism of the interfacial EPC [5, 6]. (a) Schematic of the STEM-EELS experiment and (b) the DF-EELS acquisition condition. The EELS entrance aperture location marked with black circles. Energy-filter image of (c, d) the bulk phonons and (e, f) interface vibrations from the out-of-plane and in-plane acquisitions, respectively. Scale bar = 5Å. (a) Structural model of 1uc FeSe/STO. (b-d) Atomic column-resolved vibrational EELS for O in Sr-O layer, the bright and dark atomic columns in Ti-O layer 1 and in Ti-O layer 0. The spectrum acquisition locations are marked in the HAADF image. Gray curves are EELS of corresponding atomic columns in the STO substrate. Scale bar = 5Å.

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