Capping layer influence and isotropic in-plane upper critical field of the superconductivity at the $\mathrm{FeSe}/{\mathrm{SrTiO}}_3$ interface

Understanding the superconductivity at the interface of $\mathrm{FeSe}/{\mathrm{SrTiO}}_3$ is a problem of great contemporary interest due to the significant increase in critical temperature $\left(T_{\mathrm{c}}\right)$ compared to that of bulk FeSe, as well as the possibility of an unconventional pairing mechanism and topological superconductivity. We report a study of the influence of a capping layer on superconductivity in thin films of FeSe grown on ${\mathrm{SrTiO}}_3$ using molecular beam epitaxy. We used in vacuo four-probe electrical resistance measurements and ex situ magnetotransport measurements to examine the effect of three capping layers that provide distinct charge transfer into FeSe: insulating FeTe, nonmetallic Te, and metallic Zr. Our results show that FeTe provides an optimal cap that barely influences the inherent $T_{\mathrm{c}}$ found in pristine $\mathrm{FeSe}/{\mathrm{SrTiO}}_3$, while the transfer of holes from a nonmetallic Te cap completely suppresses superconductivity and leads to insulating behavior. Finally, we used ex situ magnetoresistance measurements in FeTe capped FeSe films to extract the angular dependence of the in-plane upper critical magnetic field. Our observations reveal an almost isotropic in-plane upper critical field, providing insight into the symmetry and pairing mechanism of high-temperature superconductivity in FeSe.

Figure 1

Capping layer influence of superconductivity in FeSe films grown on STO. (a) Normalized temperature-dependent resistance of a FeSe film from in situ and ex situ electrical transport measurements. The FeTe capping layer decreases the $T_{\mathrm{c}}$ of FeSe by around 2 K. All samples have FeSe thickness in the range of three to four unit cells. (b) Ex situ measurements of Hall resistance of an FeSe film with a Zr cap ($n=6.9\times {10}^{18}{\mathrm{m}}^{-2}$), a FeTe cap ($n=1.25\times {10}^{20}{\mathrm{m}}^{-2}$), and a Te cap ($p=8.3\times {10}^{17}{\mathrm{m}}^{-2}$), respectively. It is measured at 50 K for FeTe capping and Zr capping, and at 30 K for Te capping. The FeTe capped sample has FeSe thickness of 7.5 UC. The other two samples have FeSe thickness in the range of three to four unit cells. The data are antisymmetrized in magnetic field.

Figure 2

Atomic resolution HAADF STEM images [(a) and (c)] and EDS mapping [(b) and (d)] of oxygen of FeSe film with Zr and Te. (a),(b) FeSe capped with Zr, (c),(d) FeSe capped with Te. The EDS results are on the same scale and the same region as the corresponding STEM images. The Zr layer has an amorphous structure and it partially destroys the crystalline structure of FeSe. The red-circled area does not have as high crystalline quality as other areas. The blue arrows indicate the interface between FeSe film and STO. The red arrows indicate the interface between FeSe film and the capping layer.

Figure 3

Absence of anisotropy of the in-plane upper critical field of FeSe/STO. (a) Temperature-dependent resistance curve of an FeTe capped FeSe film, showing the superconductivity transition with onset $T_{\mathrm{c}}$ around 45 K. The thickness of the film is estimated to be three unit cells. (b) Angle dependence of the in-plane magnetoresistance at various temperatures. (c) Comparison of the angle dependence of the magnetoresistance after rotating and remounting the sample in-plane through ${45}^{\circ }$ taken at 18 K and 9 T. The angle position of the twofold symmetry shifts ${70}^{\circ }$, and the amplitude becomes smaller, which indicates that the twofold symmetry is not an intrinsic in-plane property of the FeSe film but rather arises from the out-of-plane misalignment. Insets are the schematics of the measurement setup. $\theta$ is the in-plane angle position, and $\alpha$ is the out-of-plane misalignment angle. The magnetoresistance is normalized to the minimum value of the magnetoresistance. (d) Angle dependence of the critical field extracted from resistance vs field curves at various angles at 25 K. The angle dependence (black dots are the experimental date) can be fit with the 2D Tinkham formula (red curve) by considering a fixed out-of-plane misalignment angle $\alpha$, confirming that the twofold symmetry is from the out-of-plane misalignment. Therefore, the in-plane critical field is isotropic within the resolution limit of the measurement.