Spin-orbit-coupled ferroelectric superconductivity and multiorbital effects in SrTiO₃

Motivated by recent discoveries of the coexistent phenomena of superconductivity and ferroelectricity in doped SrTiO₃ [1,2], we investigated the feasibility of ferroelectric (FE) superconductivity in which superconductivity coexists with a FE-like order. By analyzing an electron-lattice coupled model for a two-dimensional superconductor, we show that the FE superconductivity is realized through two different mechanisms that rely on the intrinsic spin-orbit coupling [3]. First, the FE superconducting state is stabilized by the ferroelectricity-induced Lifshitz transition in low carrier density regimes. Second, the FE superconducting state is stabilized under a Zeeman magnetic field owing to the suppression of the Pauli depairing effect in the FE phase. Furthermore, we show that the FE superconductivity in SrTiO₃ is strongly affected by the multiorbital effect of t_2g electrons [4]. Then, we predict a topological Weyl superconducting state in the FE superconducting phase of bulk SrTiO₃.

[1] C. W. Rischau et al., Nat. Phys. 13, 643 (2017).
[2] R. Russell et al., Phys. Rev. Materials 3, 091401(R) (2019).
[3] S. Kanasugi and Y. Yanase, Phys. Rev. B 98, 024521 (2018).
[4] S. Kanasugi and Y. Yanase, Phys. Rev. B 100, 094504 (2019).