Enhancement of Ising superconductivity in monolayer NbSe$_2$ via surface fluorination

TL;DR

This study demonstrates that surface fluorination of monolayer NbSe₂ significantly enhances its Ising superconductivity by increasing the critical temperature and critical magnetic field through symmetry preservation, charge density wave suppression, and phonon mode modifications.

Contribution

The paper introduces a novel surface fluorination method to boost Ising superconductivity in monolayer NbSe₂, improving $T_c$ and $B_{c2}$ without losing Ising pairing.

Findings

Fluorination maintains mirror symmetry and locks spins out-of-plane.

Charge density wave suppression and van Hove singularity increase carrier density.

Fluorine-related phonon modes enhance electron-phonon coupling.

Abstract

Recently discovered Ising superconductors have garnered considerable interest due to their anomalously large in-plane upper critical fields ($B_{c2}$). However, the requisite strong spin-orbital coupling in the Ising pairing mechanism generally renders these superconductors heavy-element dominant with notably low superconducting transition temperatures ($T_c$). Here, based on the Migdal-Eliashberg theory and the mean-field Bogoliubov-de Gennes Hamiltonian, we demonstrate a significant enhancement of Ising superconductivity in monolayer NbSe$_2$ through surface fluorination, as evidenced by concomitant improvements in $T_c$ and $B_{c2}$. This enhancement arises from three predominant factors. Firstly, fluorine atoms symmetrically and stably adhere to both sides of the monolayer NbSe$_2$, thereby maintaining the out-of-plane mirror symmetry and locking carrier spins out-of-plane. Secondly, fluorination suppresses the charge density wave in monolayer NbSe$_2$ and induces a van Hove singularity in the vicinity of the Fermi level, leading to a marked increase in the number of carriers and, consequently, strengthening the electron-phonon coupling (EPC). Lastly, the appearance of fluorine-related, low-frequency phonon modes further augments the EPC. Our findings suggest a promising avenue to elevate $T_c$ in two-dimensional Ising superconductors without compromising their Ising pairing.