Role of double $\mathrm{Ti}{\mathrm{O}}_2$ layers at the interface of FeSe/$\mathrm{SrTi}{\mathrm{O}}_3$ superconductors

We determine the surface reconstruction of $\mathrm{SrTi}{\mathrm{O}}_3$ used to achieve superconducting FeSe films in experiments, which is different from the $1\times 1\mathrm{Ti}{\mathrm{O}}_2$-terminated $\mathrm{SrTi}{\mathrm{O}}_3$ assumed by most previous theoretical studies. In particular, we identify the existence of a double $\mathrm{Ti}{\mathrm{O}}_2$ layer at the FeSe/$\mathrm{SrTi}{\mathrm{O}}_3$ interface that plays two important roles. First, it facilitates the epitaxial growth of FeSe. Second, ab initio calculations reveal a strong tendency for electrons to transfer from an oxygen deficient $\mathrm{SrTi}{\mathrm{O}}_3$ surface to FeSe when the double $\mathrm{Ti}{\mathrm{O}}_2$ layer is present. The double layer helps to remove the hole pocket in the FeSe at the Γ point of the Brillouin zone and leads to a band structure characteristic of superconducting samples. The characterization of the interface structure presented here is a key step towards the resolution of many open questions about this superconductor.

Figure 1

Identification of 2 ML $\mathrm{Ti}{\mathrm{O}}_2$ on STO and the epitaxial growth of $1\times 1$ FeSe. (a) (2,0,$\ell$) crystal truncation rod (CTR) of a treated STO substrate (blue circles). Red line is a fit using a 2 ML surface model. (b) and (c) Low-energy electron diffraction (LEED) and reflection high-energy electron diffraction (RHEED) of STO substrates showing the $\sqrt{13}\times \sqrt{13} R33.7^{\circ }$ reconstruction. (d) RHEED of 1-u.c. FeSe on STO showing the $1\times 1$ structure of the FeSe.

Figure 2

1-u.c. FeSe/STO with 2 ML $\mathrm{Ti}{\mathrm{O}}_2$ at the interface. (a) Crystal truncation rod along (2/13,10/13,$\ell$) of FeSe/STO. Inset: Reciprocal space mapping around the (2/13,10/13,0.8) peak. Ranges of $h$ and $k$ in the inset are 0.1 with a unit of 13 r.l.u. Both plots indicate that the $\sqrt{13}\times \sqrt{13}$ reconstruction of STO remains beneath FeSe. (b) ARPES intensity near the $M$ point at the corner of the Brillouin zone. To highlight the gap at the Fermi level, the data above the Fermi level are the mirror images of the data below the Fermi level. (c) Symmetrized energy distribution curves along the portion indicated by the red box and arrow in (b).

Figure 3

Orbitally resolved band structures for $\mathrm{Fe}3d$ for four models of 1-u.c. FeSe/STO: (a) with stoichiometric 1 ML $\mathrm{Ti}{\mathrm{O}}_2$ termination, (b) with stoichiometric 2 ML termination, (c) with 1 ML termination and $50\%$ oxygen vacancies, and (d) with 2 ML termination and $50\%$ oxygen vacancies. Thin black lines represent the electron energy bands for the entire heterostructure; the red overlay represents projections of the Fe $3d$ orbital, where the color intensity is proportional to the projection amount. Fermi level is at zero in all cases. Energy scale is in eV. Insets in (c) and (d): Zoom-in on the Fe bands around the Γ point.