Unconventional superconductivity and anomalous response in hole-doped transition metal dichalcogenides

Two-dimensional transition metal dichalcogenides entwine interaction, spin-orbit coupling, and topology. Hole-doped systems lack spin degeneracy: states are indexed with spin and valley specificity. This unique structure offers new possibilities for correlated phases and phenomena. We realize an unconventional superconducting pairing phase which is an equal mixture of a spin singlet and the $m=0$ spin triplet. It is stable against large in-plane magnetic fields, and its topology allows quasiparticle excitations of net nonzero Berry curvature via pair-breaking circularly polarized light.

Figure 1

Energy bands for ${\mathrm{WSe}}_2$ as given by Eq. (2) with $at=3.939\mathrm{eV}{\AA }^{-1},E_g=1.60\mathrm{eV}$, and $E_{\text{soc}}=0.23\mathrm{eV}$. Each valley is centered at $\pm \mathbf{K}$ relative to the center of the Brillouin zone. The energy for a given band depends only on the distance $k$ measured from the valley center.

Figure 2

Optical transition rate matrix elements ${\left|P_{\pm }\right|}^2$ in the superconducting phase as a function of the ratio of the quasiparticle energy ${\lambda }_{\mathbf{k}}$ to the superconducting gap ${\mathrm{\Delta }}_{\mathbf{k}}$. Material parameters for ${\mathrm{MoSe}}_2,{\mathrm{WS}}_2$, and ${\mathrm{WSe}}_2$ are given in Ref. [26] and a gap of ${\mathrm{\Delta }}_{\mathbf{k}}=7.5\text{meV}$ is chosen for illustrative purposes. The order-of-magnitude contrast between ${\left|P_{+}\right|}^2$ and ${\left|P_{-}\right|}^2$ causes the optical-valley selectivity.

Figure 3

Pair-breaking by right circularly polarized light leads to an electron in the conduction band of the right valley and a partner in the valence band of the left valley. The valleys interchange for left circularly polarized light.