Unconventional superconductivity in monolayer transition metal dichalcogenides

TL;DR

The paper tackles unconventional superconductivity in monolayer Ising superconductors by developing a multiorbital spin- and charge-fluctuation mediated pairing framework and solving the linearized gap equation on TaS2, with tight-binding parameters derived from DFT. Using RPA to compute orbital-resolved spin and charge susceptibilities, it identifies a dominant two-dimensional $E'$ gap in the presence of Ising SOC, featuring substantial even–odd parity mixing that evolves with Coulomb interactions. The model reproduces key experimental observations: a nodal-like density of states in STM, an enhanced in-plane upper critical field beyond the Pauli limit, and a twofold gap anisotropy in magnetoresistance, and it explains the magnetic-field–driven splitting of the $E'$ ground state into a twofold symmetric state, offering a natural route to the observed anisotropy. The study also shows how conventional electron-phonon coupling can compete with spin fluctuations, tuning the leading pairing channel and aligning TaS2 results with those in NbSe2, thereby providing a unified picture of Ising superconductivity in monolayer dichalcogenides.

Abstract

A variety of experimental observations in monolayer transition metal dichalcogenide superconductors with Ising spin-orbit coupling suggest the presence of an unconventional superconducting pairing mechanism. Some of these experiments include observation of Leggett modes and a nodal superconducting gap in STM experiments, a large in-plane upper critical field compared to the Pauli limit, and the observation of a two-fold gap anisotropy in magnetoresistance measurements. Here, we propose a superconducting pairing mechanism mediated by spin and charge fluctuations and identify the dominant superconducting instability relevant to monolayer TaS$_2$. We then explore the effect of an additional electron-phonon pairing contribution, and compare our results with recent experimental findings. In particular, our theory stabilizes a superconducting ground state with nodal-like density of states that agrees with STM experiments. The theory obtains a large in-plane upper critical field due to a combination of Ising spin-orbit coupling and even-odd parity mixing in the superconducting state. Further, we find that an in-plane magnetic field splits the degeneracy of the superconducting ground state, and the resulting two-fold symmetric superconducting order parameter could explain the gap anisotropy observed in magnetoresistance experiments. Overall, the proposed theoretical pairing model can reconcile diverse experimental observations and remains consistent with observations on other dichalcogenide superconductors such as monolayer NbSe$_2$.

Figures (14)

• Figure 1: (a) shows the low-energy orbital-resolved electronic structure of monolayer TaS$_2$. The black dashed line denotes the low-energy band in the absence of Ising SOC. (b) presents the spin-resolved Fermi surface in the presence of Ising SOC. The results in both panels are obtained from a tight-binding model derived from relativistic DFT calculations.
• Figure 2: Spin susceptibility in units of $1/\mathrm{eV}$ at $\omega=0$ along the high-symmetry path $\Gamma\text{--}M\text{--}K\text{--}\Gamma$. (a) and (c) show the noninteracting and corresponding RPA susceptibilities, respectively, calculated without SOC, while (b) and (d) display the corresponding results in the presence of Ising SOC. In both cases, the RPA paramagnetic susceptibility exhibits a peak at a wave vector $\mathbf{q}_{\mathrm{max}} \sim 0.34\,\Gamma\text{--}M$. For the RPA results we used $U=0.6$ eV and $J=U/4$.
• Figure 3: Solutions of the linearized gap equation $\Delta_{k_{F}}$ plotted on the Fermi surface for the six leading superconducting instabilities with corresponding eigenvalue $\lambda$ and Irreps. Here $\eta = \pm 1$ corresponds to the pure even (odd) parity solutions. The non interacting Hamiltonian used here ignores the Ising SOC. Additional parameters: $U = 0.6$ eV, $J = U/4$.
• Figure 4: The evolution of the eigenvalues $\lambda/\lambda_{max}$, corresponding to specific Irreps, is presented as a function of $U$ with $J=U/4$ in the absence of Ising SOC. The four largest eigenvalues are plotted. Here, $J = U/4$.
• Figure 5: Superconducting gap functions plotted on Fermi surface in the presence of Ising SOC with $U = 0.6$ eV and $J = U/4$. Here, $\lambda$ on the plots denote the eigenvalues of the corresponding solutions of the gap equation and $\eta$ the even-odd parity mixing. (a) and (b) display the degenerate ground state solution.