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

TL;DR

This paper predicts optical signatures, such as anisotropy and Kerr rotation, to distinguish chiral and nematic two-component pairing states in transition metal dichalcogenide superconductors, aiding experimental identification.

Contribution

It introduces a method to detect and differentiate chiral and nematic $E'$ pairing states in TMDs using optical conductivity measurements, based on their symmetry properties.

Findings

Nematic $E'$ states produce diagonal anisotropy in optical conductivity.

Chiral $E'$ states generate finite optical Hall conductivity and Kerr rotation.

Predicted signals are detectable with current experimental techniques.

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 $E'$ 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 $E'$ pairing. We quantify how nematic $E'$ states produce a diagonal anisotropy $\sigma_{xx}\!\neq\!\sigma_{yy}$ due to the broken threefold symmetry ($C_3$), while chiral $E'$ 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 $\Delta\sigma/\sigma\sim10^{-5}$ 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.