Role of Substrate Phonon in the Electron-phonon Coupling at FeSe/SrTiO3 Interface

Bulk FeSe is a superconductor with transition temperature (Tc) of 8K. Once grown on SrTiO3 substrate, monolayer FeSe exhibits Tc of 60 K or even higher[1], which makes monolayer FeSe/SrTiO3 a high temperature superconductor. The drastic enhancement of superconducting Tc has attracted great interest in understanding the mechanism. The substrate contribution to the Tc enhancement includes two main factors: electron doping and electron-phonon coupling (EPC). Electron doping of FeSe occur through the charge transfer from the SrTiO3 substrate, which eliminates the hole pocket at the Gamma point—a priori for the emergence of high Tc in FeSe. On the other hand, mechanism of the EPC has been under debate. The main evidence of such electron-phonon interaction is the replica bands in the photoemission spectrum[2]. The optical phonons of SrTiO3 near 100meV have been considered as an essential ingredient for the EPC. However, it remains unclear how the electrons in FeSe couples to phonons of SrTiO3, as the two components are spatially separated. Moreover, the interfacial structure between FeSe and SrTiO3 is highly variable[3, 4]. Structural dependence of the interfacial phonon spectrum also remains elusive. Thus, in this work, we investigate the phonons in FeSe/SrTiO3 heterostructures with atomic resolution by monochromated electron energy loss spectroscopy (EELS). Atomic resolution imaging of the interface was carried out using a JEOL Grand Arm TEM operating at 300kV. The HAADF-STEM image in Fig. 1(a) shows the FeSe/SrTiO3 interface. The SrTiO3 substrate terminates with double TiO2 layers, with the top TiOx layer having a different stoichiometry and bonding character than the bottom TiO2 layer. These interfacial layers determine the minimum distance between FeSe and SrTiO3: in this case, the bottom Se layer and the top TiOx layer is 2.7Å apart. The phonons of across the interface were measured by a monochromated Nion UltraSTEM200 operating at 60kV. We perform dark-field EELS mapping with atomic spatial resolution and 10meV energy resolution, which allows probing impact scattering across the interface while avoiding most of the delocalized phonon polariton signals. Fig. 1(b) shows the phonon EEL spectra across the interface shown in Fig. 1(a). The phonon density of states (DOS) from each atomic layers are well resolved spatially and spectrally, i.e., the modulation of EELS intensity from the SrO and TiO2 layers in SrTiO3 can be seen clearly. In the averaged phonon spectra shown in Fig. 1(c), the longitudinal optical phonon (LO3) mode of SrTiO3 can be seen near 100meV. The LO3 phonons are mainly oxygen vibrations, thus have the highest phonon energy in SrTiO3. It is the only phonon modes present in SrTiO3 above 65meV, which sets a large phonon gap. In spectrum averaged from the interfacial TiO2 layers, the phonon gap is no longer visible, possibly because the oxygen vibrations are shifted to lower energies and the lowered symmetry. The phonon modes near 100meV have been thought to be responsible for the coupling with electrons in FeSe and the enhancement of superconductivity of FeSe/SrTiO3. Our results suggest that the intensity of the 90–100meV phonons drop sharply near the double TiO2 layers at the interface. Given the spacing between FeSe and SrTiO3, only the first 1 to 2 unit cell of FeSe may experience limited influence from the LO3 phonon of SrTiO3. In contrast to the SrTiO3 spectrum, which can be well interpreted via the phonon DOS, the EEL spectra of FeSe on SrTiO3 substrate show not only its phonon peak (30meV), but also an additional excitation around 73meV, as shown in Fig. 1(c) and 2(a). The intensity of this mode is stronger in the first unit cell than the upper layers and did not diminish for the largest film thickness investigated here (>10 unit cells). The origin of this 73meV excitation is revealed by investigating FeSe films on other substrates. In FeSe films grown on SiC substrate, we also observe additional peak, but around 103 meV, as shown in Fig. 2(b). Furthermore, vibrational EELS of FeSe bulk single crystal only show the phonon peak near 30meV. From dielectric theory simulation shown in Fig. 2(c) and 2(d), we find that these unexpected high energy excitations in FeSe films at 73meV and 103meV originate from the phonon polaritons from SrTiO3 and SiC substrate, respectively. The presence of phonon polariton signals in FeSe indicates that the surface phonons from the substrate can indeed penetrate into the FeSe with large distances. This phenomenon also suggests that dipolar scatterings of keV electrons cannot be neglected even in high angle dark field conditions. In summary, our atomic resolution imaging and phonon spectroscopy reveal that phonons of both SrTiO3 and the interfacial TiOx layer may contribute to the electron-phonon coupling at the FeSe/SrTiO3 interface. We also confirm the presence of surface phonon modes in FeSe, which is likely to be a common phenomenon for polar substrates [5]. (a) Atomic resolution HAADF-STEM image of the FeSe/SrTiO3 interface. The scalebar indicates 5 Å. (b) Phonon intensity map across the interface in (a). (c) Averaged EEL spectra for electron beam position labeled on the right. (a, b) Dark-field vibrational EEL spectra of bulk FeSe (black), FeSe film on SrTiO3 (blue), and FeSe film on SiC (green) substrate. The EELS acquisition locations for FeSe films are shown as the inserts. (c, d) Surface phonon dispersions simulated for SrTiO3 and SiC, respectively. The coupling strength to electron beam is indicated by the color.