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Phonon-induced rotation of the electronic nematic director in superconducting Bi$_{2}$Se$_{3}$

The doped topological insulator $A_{x}\mathrm{Bi_{2}Se_{3}}$, with $A=\{\mathrm{Cu},\mathrm{Sr},\mathrm{Nb}\}$, becomes a nematic superconductor below $T_{c}\sim3-4\,\mathrm{K}$. The associated electronic nematic director is described by an angle $α$ and is experimentally manifested in the elliptical shape of the in-plane critical magnetic field $H_{c2}$. Because of the threefold rotational symmetry of the lattice, $α$ is expected to align with one of three high-symmetry directions corresponding to the in-plane nearest-neighbor bonds, consistent with a $Z_{3}$-Potts nematic transition. Here, we show that the nematic coupling to the acoustic phonons, which makes the nematic correlation length tend to diverge along certain directions only, can fundamentally alter this phenomenology in trigonal lattices. Compared to hexagonal lattices, the former possesses a sixth independent elastic constant $c_{14}$ due to the fact that the in-plane shear strain doublet $(ε_{xx}-ε_{yy},-2ε_{xy})$ and the out-of-plane shear strain doublet $(2ε_{yz},-2ε_{xz})$ transform as the same irreducible representation. We find that, when $c_{14}$ overcomes a threshold value, which is expected to be the case in doped $\mathrm{Bi_{2}Se_{3}}$, the nematic director $α$ unlocks from the high-symmetry directions due to the competition between the quadratic phonon-mediated interaction and the cubic nematic anharmonicity. This implies the breaking of the residual in-plane twofold rotational symmetry ($C_{2x}$), resulting in a triclinic phase. We discuss the implications of these findings to the structure of nematic domains and to the shape of the in-plane $H_{c2}$ in $A_{x}\mathrm{Bi_{2}Se_{3}}$, and to presence of nodes inside the superconducting state.