Low-temperature anomaly and anisotropy of critical magnetic fields in transition-metal dichalcogenide superconductors

TL;DR

This paper explains the persistence of spin-singlet superconductivity in monolayer transition-metal dichalcogenides under high magnetic fields through the concept of Ising protection, involving magnetic potentials and spin-triplet pairing.

Contribution

It provides an analytical framework showing how even-frequency spin-triplet pairs stabilize superconductivity beyond the Pauli limit in these materials.

Findings

Odd-frequency triplet pairs destabilize superconductivity.

Even-frequency triplet pairs counteract this instability.

Superfluid weight analysis confirms the role of triplet pairs in high-field superconductivity.

Abstract

We clarify why spin-singlet superconductivity persists in monolayer transition-metal dichalcogenides even in high magnetic fields beyond the Pauli limit. The phenomenon called Ising protection is caused by two magnetically active potentials: a Zeeman field and an Ising spin-orbit interaction. These potentials induce two spin-triplet pairing correlations in a spin-singlet superconductor. One belonging to odd-frequency symmetry class arises solely from a Zeeman field and always makes the superconducting state unstable. The other belonging to even-frequency symmetry class arise from the interaction between the two magnetic potentials and eliminate the instability caused by odd-frequency pairs. The presence or absence of even-frequency spin-triplet pairs explains the anisotropy of the Ising protection. The analytical expression of the superfluid weight enables us to conclude that even-frequency spin-triplet Cooper pairs support spin-singlet superconductivity in high Zeeman fields.