Possible two-component pairings in electron-doped Bi${}_2$Se${}_3$ based on a tight-binding model

Abstract

Recent experiments show the spontaneous breaking of rotational symmetry in the superconducting topological insulators M${}_x$Bi${}_2$Se${}_3$ (M represents Cu, Sr, or Nd), suggesting that the pairing belongs to a two-dimensional representation of the $D_{3d}$ symmetry group of the crystal. Motivated by these progresses, we construct an exhaustive list of possible two-component pairings of the M${}_x$Bi${}_2$Se${}_3$ superconductors, both for the odd-parity $E_u$ representation and for the even-parity $E_g$ representation. Starting from a tight-binding model for the normal phase of Bi${}_2$Se${}_3$ and M${}_x$Bi${}_2$Se${}_3$, we firstly construct the pairing channels in the spin-orbital basis, up to second-nearest-neighbor pairing correlations in the basal plane. We then infer the properties of these pairings by transforming them to the band (pseudospin) basis for the conduction band. A comparison with the key experimental consensuses on M${}_x$Bi${}_2$Se${}_3$ superconductors shows that the true pairings should also be multichannel. Besides a nematic and time-reversal symmetric pairing combination, the other pairings that we have identified are chiral and nematic at the same time, which may be nonunitary and have a spontaneous magnetization. A complementary set of experiments are proposed to identify the true pairing symmetries of these superconductors and their evolution with the doping concentration $x$.

15 pages, 5 tables

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