Electron-phonon coupling and spin fluctuations in the Ising superconductor NbSe$_{2}$

TL;DR

This paper uses first-principles calculations to analyze electron-phonon coupling and spin fluctuations in NbSe₂, revealing anisotropic EPC, the suppression of superconductivity by SF, and the potential for a Leggett mode in specific channels.

Contribution

It provides a quantitative analysis of EPC and SF in NbSe₂, clarifying their roles and ruling out triplet symmetry, while identifying conditions for Leggett modes.

Findings

EPC is strongly anisotropic, mainly from K-K' scattering.

Spin fluctuations weaken superconductivity but do not eliminate anisotropy.

A Leggett mode may exist in the $s_{++}$ - $s_\pm$ channel.

Abstract

Ising superconductivity, observed experimentally in NbSe$_{2}$ and similar materials, has generated tremendous interest. Recently, attention was called to the possible role that spin fluctuations (SF) play in this phenomenon, in addition to the dominant electron-phonon coupling (EPC); the possibility of a predominantly-triplet state was discussed and led to a conjecture of viable singlet-triplet Leggett oscillations. However, these hypotheses have not been put to a quantitative test. In this paper, we report first principle calculations of the EPC and also estimate coupling with SF, including full momentum dependence. We find that: (1) EPC is strongly anisotropic, largely coming from the K-K' scattering, and therefore excludes triplet symmetry even as an excited state; (2) superconductivity is substantially weakened by SF, but anisotropy remains as above; and, (3) we do find the possibility of a Leggett mode, not in a singlet-triplet but in an $s_{++}$ - $s_\pm$ channel.