Evidence for singular-phonon-induced nematic superconductivity in a topological superconductor candidate SrBiSe

Wang, J.*; Ran, K.*; Li, S.*; Ma, Z.*; Bao, S.*; Cai, Z.*; Zhang, Y.*; Nakajima, Kenji  ; Kawamura, Seiko   ; ermk, P.*; Schneidewind, A.*; Savrasov, S. Y.*; Wan, X.*; Wen, J.*

Superconductivity mediated by phonons is typically conventional, exhibiting a momentum-independent -wave pairing function, due to the isotropic interactions between electrons and phonons along different crystalline directions. Here, by performing inelastic neutron scattering measurements on a superconducting single crystal of SrBiSe, a prime candidate for realizing topological superconductivity by doping the topo-logical insulator BiSe, we found that there exist singular phonons, with the linewidths of the acoustic phonons increasing substantially at long wavelengths, but only for those along the [001] direction. This observation indicates a large and singular electron-phonon coupling at small momenta, which we propose to give rise to the exotic -wave nematic superconducting pairing in the MxBiSe (M = Cu, Sr, Nb) superconductor family. Therefore, we show that these superconductors may be the first examples where electron-phonon interaction can induce more exotic superconducting pairing than the -wave.