Observation of high-$T_c$ superconductivity in rectangular $\text{FeSe}/{\mathrm{SrTiO}}_3\left(110\right)$ monolayers

We design a monolayer film of FeSe on ${\mathrm{SrTiO}}_3$(110) substrate [FeSe/STO(110)], with a ${\mathrm{C}}_2$ symmetry induced by epitaxial strain. Compared to FeSe on ${\mathrm{SrTiO}}_3$(001) substrate [FeSe/STO(001)], one of the in-plane lattice constants is reduced by 6%, to $\sim 3.67$ Å. FeSe/STO(110) exhibits a large nearly isotropic superconducting gap of 16 meV filled around 60 K, similar to those obtained on FeSe/STO(001) films. Our results strongly suggest that the lattice ${\mathrm{C}}_4$ symmetry is not essential for reaching high $T_c$'s in Fe-based superconductors and pose a new challenge to theory.

Figure 1

(a) RHEED pattern image of a FeSe/STO(110) film. (b) Scanning-tunneling-microscopy tomography image of an FeSe/STO(110) film. (c) LEED pattern of FeSe/STO(110) at 170 K. The Brillouin zone (BZ) of the FeSe film is indicated by solid lines, while dashed lines indicate the BZ of the STO substrate projected on the (110) surface. Red circles correspond to well-known $3\times 1$ surface reconstruction on STO(110) surface [13]. (d) Crystal structure of an FeSe monolayer on top of a STO(110) substrate. Dashed lines indicate the possible match between the FeSe film and the STO(110) substrate.

Figure 2

Top and bottom rows correspond to results obtained on monolayer films of FeSe/STO(110) and FeSe/STO(001), respectively. Data are recorded at 35 K with a He discharge lamp. (a, f) FS. Blue and green dots are maxima of the intensity map along the vertical and horizontal directions, respectively. Light-blue curves are elliptical fittings of these dots. (b, g) ARPES intensity cut along a cut passing through the $\mathrm{\Gamma }$ point. (c, h) Momentum distribution curves (MDCs) of (b) and (g), respectively. The dotted blue line is a guide for the eye for the main band dispersion. (d, i) Same as (b) and (g), but for a cut passing through M. The red curve in (d) is the MDC at $E_F$. (e, j) MDCs of (d) and (i), respectively.

Figure 3

(a1–a3) Band structure of FeSe/STO(110) at M recorded at different temperatures. (b1–b3) Same as (a1)–(a3), but divided by the Fermi-Dirac function. (c, f) Temperature evolution of the symmetrized energy distribution curves (EDCs) at $k_F$ of FeSe/STO(110) and FeSe/STO(001), respectively. Tick red curves are fitting results using a model described in the text. (d, g) Corresponding symmetrized EDCs after subtraction of the high-temperature background. Shaded areas correspond to the difference between the background and the data, as illustrated in the inset in (e). (e, h) Gap size and spectral weight loss as a function of temperature in FeSe/STO(110) and FeSe/STO(001), respectively. The gap is obtained from the fits shown in (c) and (f). (i) Momentum distribution of the EDCs recorded at 17 K on the FS of FeSe/STO(110); (j) the corresponding polar representation of the SC gap amplitude. The inset in (i) shows the momentum locations of the EDCs.

Figure 4

Superconductivity in the different forms of FeSe (only Fe atoms shown), with their corresponding FSs. The relative $T_c$ is given by the background color. Dashed color lines indicate which Fe-Fe bondings are equivalent.