Optical probes of two-component pairing states in transition metal dichalcogenides

Abstract

Signatures of unconventional superconductivity have been recently observed in certain transition metal dichalcogenides (TMDs), including 4H${}_b$-TaS${}_2$ and monolayer 2H-NbSe${}_2$. While the pairing channel remains unknown, it has been argued that spin fluctuations can stabilize pairing in the two-component $E^{\prime }$ channel, a $p$-wave spin-triplet state which could be consistent with some of the reported signatures. Exploiting the particular multi-orbital character of the Fermi surface and the presence of Ising spin-orbit coupling, which enable finite optical conductivity in the clean limit, in this work we predict clear-cut optical signatures to detect and distinguish the chiral and nematic ground states of the $E^{\prime }$ pairing. We quantify how nematic $E^{\prime }$ states produce a diagonal anisotropy ${\sigma }_{xx}\ne {\sigma }_{yy}$ due to the broken threefold symmetry ($C_3$), while chiral $E^{\prime }$ states yield a finite optical Hall conductivity ${\sigma }_{xy}^H$ due to broken time-reversal symmetry, and find both signals could be detected in current experiments. For instance, for realistic gaps in the meV range, we predict a relative anisotropy $\Delta \sigma /\sigma \sim 10^{-5}$ in the nematic states, and a polar Kerr rotation of ${\theta }_K\sim 10^{-5}$ rad in the chiral states. These symmetry fingerprints provide a practical route to distinguish nematic and chiral superconducting order in TMD superconductors.

7 pages, 5 figures