Enhanced Superconducting State in $\mathrm{FeSe}/{\mathrm{SrTiO}}_3$ by a Dynamic Interfacial Polaron Mechanism

The observation of substantially enhanced superconductivity of single-layer FeSe films on ${\mathrm{SrTiO}}_3$ has stimulated intensive research interest. At present, conclusive experimental data on the corresponding electron-boson interaction is still missing. Here we use inelastic electron scattering spectroscopy and angle resolved photoemission spectroscopy to show that the electrons in these systems are dressed by the strongly polarized lattice distortions of the ${\mathrm{SrTiO}}_3$, and the indispensable nonadiabatic nature of such a coupling leads to the formation of dynamic interfacial polarons. Furthermore, the collective motion of the polarons results in a polaronic plasmon mode, which is unambiguously correlated with the surface phonons of ${\mathrm{SrTiO}}_3$ in the presence of the FeSe films. A microscopic model is developed showing that the interfacial polaron-polaron interaction leads to the superconductivity enhancement.

Figure 1

Phonon and polaronic plasmon modes of the STO with and without FeSe capping. (a) HREELS spectra of the single-layer $\mathrm{FeSe}/\mathrm{STO}$ measured at 35 K with the incident electron energy of 50 eV. The FeSe phonon branches are labeled by $\eta$, $\sigma$, and $\xi$, while the $\beta$, $\alpha$, and $\rho$ modes originated from the STO are described in the main text. (b) Temperature dependence of the energy loss spectra at the $\overline{\mathrm{\Gamma }}$ point from the single-layer $\mathrm{FeSe}/\mathrm{STO}$. (c) and (d) Corresponding results of the STO without capping, with the incident electron energy of $\sim 110\mathrm{eV}$. (e) and (f) Comparisons of the energies of the polaronic plasmon and $\alpha$ mode at $\overline{\mathrm{\Gamma }}$ point at different temperatures with and without FeSe, respectively. The error bars are from different fitting functions and different background subtraction methods used in the peak fitting.

Figure 2

Electronic band structure and characteristics of the interfacial EBI. (a) Band structure of single-layer $\mathrm{FeSe}/\mathrm{STO}$ measured at 35 K by ARPES, with the bands above the Fermi energy extracted from Ref. [41]. The solid and dashed lines are guides to the eye. (b) Imaginary part of the Lindhard response function, $\mathrm{Im}[\chi (q,\hslash {\omega }_B)]$, calculated from the band structure in (a) and dispersion of the α phonon in Fig. 1, see the main text for details. The lower panel displays the line profile along the arrow indicated in the upper panel. (c) Full width at half maximum (FWHM) of the α phonon of single-layer $\mathrm{FeSe}/\mathrm{STO}$ and undoped-STO at 35 K.

Figure 3

Illustrations of the nonadiabatic EBI and strengthened pairing. (a) Nonadiabatic and (b) adiabatic EBI, with the size of polaron highlighted by the shaded areas. (c) Enhanced pairing of the dynamic interfacial polarons in single-layer $\mathrm{FeSe}/\mathrm{STO}$. The yellow solid spheres indicate the electrons. The curved lines indicate the penetrating polarized field from the STO into the FeSe. The original and enhanced pairing of the electrons are represented by the red and blue dashed lines, respectively.