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Friday, September 22nd 2017

12:20 pm:

NbSe_2 monolayers have been observed to superconduct in presence of relatively large (~35T) in-plane magnetic fields (as compared to the naïve Pauli limit estimate), an effect attributed to large out-of-plane spin orbit coupling (referred to in the literature as Ising SOC) in the niobium atom that pins electron spins perpendicular to the NbSe_2 plane. This lead some theorists to speculate about the effect of such magnetic fields on the nature of the superconducting gap function, which due to the SOC may be topologically non-trivial. In particular, it has been suggested that for fields above the Pauli limit, NbSe_2 becomes a nodal topological superconductor. In order to investigate this claim, I perform an RG analysis (without SOC) and solve the linearized gap equation using a simplified single band model that incorporates SOC. It turns out that while there exists a self-consistent solution to the gap equation in which an in-plane magnetic field induces a phase transition from a fully gapped phase to a nodal one, such a solution is neither favored by RG nor likely to occur in a real system due to in-plane (Rashba) SOC caused by ripples in the monolayer, which leads instead to a topologically trivial gap.

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