Competition between Charge Density Wave and Superconductivity in a Janus MXene Mo2NF2

TL;DR

This study investigates the competition between charge density wave order and superconductivity in Janus MXene Mo2NF2, revealing how strain can suppress CDW and enhance superconductivity, providing insights into lattice control of electronic phases.

Contribution

It provides the first comprehensive first-principles analysis of CDW and superconductivity interplay in Mo2NF2, highlighting strain as a tuning parameter for electronic phases in Janus MXenes.

Findings

CDW instability driven by strong electron-phonon coupling, not Fermi-surface nesting

Strain suppresses CDW and enhances superconductivity, increasing Tc from 1 K to 4 K

Structural transition to a CDW phase involves bond-length modulations among Mo, N, and F atoms.

Abstract

Charge-density-wave (CDW) order and superconductivity often compete in low-dimensional materials, yet their interplay in Janus MXenes remains largely unexplored. Here, we present a comprehensive first-principles investigation of the structural, vibrational, and electronic properties of Mo2NF2. Phonon calculations reveal an unstable soft phonon mode at the M point in the high-symmetry structure, signaling a CDW instability. Analysis of phonon linewidths and the real and imaginary parts of the bare electronic susceptibility demonstrates that the CDW is not driven by simple Fermi-surface nesting but instead originates from strong momentum-dependent electron-phonon coupling. Structural relaxation yields a commensurate CDW phase characterized by bond-length modulations involving the Mo, N, and F sublattices. We further show that charge doping alone is insufficient to stabilize the soft phonon, whereas compressive biaxial strain exceeding -3 percent completely suppresses the CDW instability. Electron-phonon coupling calculations indicate that the CDW phase exhibits a reduced coupling constant lambda = 0.40 and logarithmic phonon frequency omega_log = 219 K, leading to a low superconducting transition temperature Tc about 1 K. In contrast, the strain-stabilized high-symmetry phase shows enhanced coupling (lambda = 0.53, omega_log = 272 K) and a higher Tc about 4 K. Our results establish Mo2NF2 as a strain-tunable platform where superconductivity emerges upon suppression of a competing CDW phase, highlighting the crucial role of lattice control in Janus MXenes.