Role of ${\mathrm{SrTiO}}_3$ phonon penetrating into thin FeSe films in the enhancement of superconductivity

The significant role of interfacial coupling in the enhancement of superconductivity in FeSe films on ${\mathrm{SrTiO}}_3$ has been widely recognized, but the explicit origin of this coupling is yet to be identified. Here, by surface phonon measurements using high-resolution electron energy loss spectroscopy, we found the electric field generated by Fuchs-Kliewer (F-K) phonon modes of ${\mathrm{SrTiO}}_3$ can penetrate into FeSe films and strongly interact with the electrons therein. The mode-specific electron-phonon coupling constant for the $\sim 92$ meV F-K phonon is $\sim 0.25$ in single-layer FeSe on ${\mathrm{SrTiO}}_3$. With increasing FeSe thickness, the penetrating field intensity decays exponentially, which matches well the observed exponential decay of the superconducting gap. It is unambiguously shown that the ${\mathrm{SrTiO}}_3$ F-K phonon penetrating into FeSe is essential for interfacial superconductivity enhancement.

Figure 1

HREELS results of the 1uc-FeSe/STO sample at room temperature. (a) 2D energy-momentum mapping along the $\mathrm{\Gamma }-X$ direction. Five different phonon modes with positive energy loss are labeled by $\alpha , \beta , \sigma , \zeta$, and $\eta$, respectively. Dashed lines are provided to guide the eye. The corresponding negative energy loss features correspond to their anti-Stokes peaks, which are labeled by ${\sigma }^{\prime },{\eta }^{\prime }$, etc. (b) EDCs at the $\mathrm{\Gamma }$ point and $X$ point with the inset show the Brillouin zone. (c) Phonon dispersions from the results of (a). (d) Ionic vibrations of the $\alpha$ and $\beta$ modes [30].

Figure 2

Temperature dependence of the (a) energy and (b) full width at half maximum (FWHM) of the $\alpha$ phonon branch (blue: 1uc-FeSe/STO; red: bare STO). The dots are experimental data obtained by fitting the EDCs from the HREELS spectra at the $\mathrm{\Gamma }$ point with Gaussian peaks. The red solid line is a linear fitting for bare STO, and the blue solid lines for 1uc-FeSe/STO represent the results of the phonon-phonon decay fitting described in the text.

Figure 3

(a) EDCs at the $\mathrm{\Gamma }$ point for samples with different FeSe thicknesses. (b) Plot and exponential fitting of the peak height of the $\alpha$ mode as a function of the FeSe thickness (blue). Plot and exponential fitting of the superconducting gap size for the K-doped FeSe/STO as a function of the FeSe thickness (red), with data extracted from Ref. [25].