Interface Ferroelectric Transition near the Gap-Opening Temperature in a Single-Unit-Cell FeSe Film Grown on Nb-Doped ${\mathrm{SrTiO}}_3$ Substrate

We report findings of strong anomalies in both mutual inductance and inelastic Raman spectroscopy measurements of single-unit-cell FeSe film grown on Nb-doped ${\mathrm{SrTiO}}_3$, which occur near the temperature where the superconductinglike energy gap opens. Analysis suggests that the anomaly is associated with a broadened ferroelectric transition in a thin layer near the $\mathrm{FeSe}/{\mathrm{SrTiO}}_3$ interface. The coincidence of the ferroelectric transition and gap-opening temperatures adds credence to the central role played by the film-substrate interaction on the strong Cooper pairing in this system. We discuss scenarios that could explain such a coincidence.

Figure 1

[(a) and (b)] The spectra of 1-UC FeSe/NbSTO. [(c) and (d)] The spectra of 2-UC FeSe/NbSTO. The locations of the moment cuts are illustrated in (e). In (a)–(d), the bottom panel is the second derivative of the raw spectrum. The white dotted lines indicate the Fermi level. The yellow solid curve in (c) is a fit of the dispersion of the electron pocket showing opening of a gap. All spectra are taken at temperature of 10 K. (e) Illustration of the Brillouin zone. (f) The extracted energy gaps as a function of temperature. The solid line is a mean-field fitting of the 1-UC gap.

Figure 2

[(a) and (b)] Temperature dependence of the transimpedance $Z$ measured by mutual inductance on three samples as labeled with (a) the reactive [$\mathrm{Im}(Z)$] and (b) dissipative [$\mathrm{Re}(Z)$] components. The transimpedance is taken by dividing the pickup voltage with the excitation current. The sign convention is such that an increase in the reactive (dissipative) component corresponds to an increase in the magnetic screening (dissipation) in the sample. [(c) and (d)] Normalized (c) reactive [$\mathrm{Im}(Z)/f^2$] and (d) dissipative [$\mathrm{Re}(Z)/f^2$] components of the mutual inductance measured in frequency range from 10 to 40 kHz in a 25 nm Se/1-UC FeSe/NbSTO. A linear background is subtracted in (d). (e) The dissipative components of FeSe films with different thicknesses and capping layers grown on NbSTO. The thicknesses of all capping layers are 25 nm. (f) The dissipative components of control samples without FeSe films.

Figure 3

(a) Temperature dependence of the Raman spectra. Each spectrum is fitted with two Lorentzians plus a linear background. The fitting parameters are plotted as a function of temperature in (b) Raman shift and (c) half width at half maximum (HWHM) [18].

Figure 4

(a) Below 105 K, the STO undergoes an AFD transition where the ${\mathrm{TiO}}_6$ octahedra rotate by a small amount. The blue circles indicate the micro-ordered ferroelectric regions induced by the Nb dopants. The dark blue regions indicate the surface dipole layers where excess carriers pile up due to the interface electric field between FeSe and NbSTO. The black arrows represent the induced ferroelectric dipole moments. (b) Calculated longitudinal (perpendicular to interface) and transverse (parallel with interface) electric susceptibilities in STO. (c) Calculated 2D conductivity. [(d) and (e)] Calculated transimpedance signal based on the model developed in Ref. [28] for (d) the imaginary component and (e) the dissipative component.