Exploring High Transition Temperature Superconductivity in a Freestanding or ${\mathrm{SrTiO}}_3$-Supported CoSb Monolayer

As a two-dimensional entity, FeSe has been widely explored to harbor high transition temperature (high-$T_c$) superconductivity in diverse physical settings; yet to date, the underlying superconducting mechanisms are still under active debate. Here we use first-principles approaches to identify a chemically different yet structurally identical counterpart of FeSe, namely, monolayered CoSb, which is shown to be an attractive candidate to harbor high-$T_c$ superconductivity as well. We first show that a freestanding CoSb monolayer can adopt the FeSe-like layered structure, even though its known bulk phase has no resemblance to layering. Next, we demonstrate that such a CoSb monolayer possesses superconducting properties comparable with or superior to FeSe, a striking finding that can be attributed to the isovalency nature of the two systems. More importantly, the layered CoSb structure can be stabilized on ${\mathrm{SrTiO}}_3(001)$, offering appealing alternative platforms for realizing high-$T_c$ superconductivity beyond the well-established Cu- and Fe-based superconducting families. $\mathrm{CoSb}/{\mathrm{SrTiO}}_3(001)$ also exhibits distinctly different magnetic properties from $\mathrm{FeSe}/{\mathrm{SrTiO}}_3(001)$, which should provide a crucial new angle to elucidate the microscopic mechanisms of superconductivity in these and related systems.

Figure 1

Schematic atomic structures of the (a) NiAs-type bulk CoSb and (b) PbO-type monolayered CoSb. In (b), the upper and lower panel shows the top and side view, respectively. (c) Electronic structure (left panel) and density of states (DOS) (right panel) of a freestanding CoSb monolayer, with the Fermi level set at zero. The inset shows the first Brillouin zone, with the high symmetry points of $\mathrm{\Gamma }$, $M$, and $X$ indicated. The two hole pockets around the $\mathrm{\Gamma }$ point and two electron pockets around the $M$ point are colored in red and blue, respectively.

Figure 2

(a) Phonon spectra of a freestanding CoSb monolayer calculated by using DFPT, with the magnitudes of the phonon linewidths indicated by the data sizes in red. (b) The corresponding phonon density of states (PHDOS). (c) Eliashberg function ${\alpha }^{2}F(\omega )$ (black) and frequency-dependent electron-phonon coupling $\lambda (\omega )$ (blue) of the system.

Figure 3

Atomic structures of a CoSb monolayer on (a) ideal and (c) surface-oxygen-deficient ${\mathrm{SrTiO}}_3(001)$ substrates. (b),(d) Electronic structures (left panels) and DOS (right panels) of the corresponding systems. The Fermi level is set at zero. The spectral weights contributed by the CoSb overlayers are indicated by the data sizes in red. The inset in (b) shows the Fermi surface in the first Brillouin zone.

Figure 4

(a) Four commonly considered magnetic configurations of $\mathrm{CoSb}/\mathrm{STO}$ or $\mathrm{FeSe}/\mathrm{STO}$. (b),(c) Dependences of the magnetic moment $M$ (red) and relative energy $\mathrm{\Delta }E$ (blue) of the four magnetic configurations as functions of the on-site Hubbard $U$ for the two systems.