[Paper Review] Competition between Charge Density Wave and Superconductivity in a Janus MXene Mo2NF2

Jakkapat Seeyangnok, Udomsilp Pinsook+1 more|arXiv (Cornell University)|Mar 6, 2026

MXene and MAX Phase Materials0 citations

TL;DR

A first-principles study showing a momentum-dependent CDW instability in Mo2NF2 Mo-based Janus MXene, which is suppressed by compressive strain to enhance superconductivity (Tc up to ~4 K).

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.

Motivation & Objective

• Investigate the structural, vibrational, and electronic properties of Mo2NF2 to identify any charge-density-wave (CDW) instabilities.
• Determine the origin of CDW and its relationship with superconductivity in this Janus MXene.
• Assess how external perturbations (charge doping and biaxial strain) influence the CDW and superconducting properties.

Proposed method

• Perform density functional theory (DFT) calculations with GGA-PBE exchange-correlation and norm-conserving pseudopotentials.
• Compute phonon dispersions via density functional perturbation theory (DFPT) to identify dynamical instabilities.
• Analyze phonon linewidths and real/imaginary parts of the bare electronic susceptibility to discriminate between nesting-driven and lattice-driven CDW origins.
• Relax structures to obtain the CDW phase and characterize the associated bond-length modulations across Mo, N, and F sublattices.
• Evaluate electron–phonon coupling using the Eliashberg function α^2F(ω) to extract λ and ω_log, and estimate Tc via the Allen–Dynes formula.

Figure 1: (a) Side and (b) top views of the optimized crystal structure of the Mo 2 NF monolayer. Orange, yellow, and green spheres represent Mo, N, and F atoms, respectively.

Experimental results

Research questions

• RQ1 Is the CDW in Mo2NF2 driven by Fermi-surface nesting or by momentum-dependent electron–phonon coupling?
• RQ2 Can charge doping or strain stabilize the high-symmetry phase and suppress the CDW?
• RQ3 How does the CDW affect electronic structure and the potential for superconductivity, and can strain enhance Tc?

Key findings

• A soft phonon mode at the M point signals a commensurate CDW instability in the high-symmetry Mo2NF2 structure.
• The CDW is driven by strong momentum-dependent electron–phonon coupling rather than simple Fermi-surface nesting.
• Relaxation yields a commensurate CDW with bond-length modulations among Mo, N, and F in a multi-sublattice distortion.
• Charge doping alone does not stabilize the soft phonon, while compressive biaxial strain >~3% suppresses the CDW and stabilizes the high-symmetry phase.
• In the CDW phase, the electron–phonon coupling constant λ = 0.40 and ω_log = 219 K yield Tc ~ 1 K; under -3% strain, λ = 0.53 and ω_log = 272 K yield Tc ~ 4 K.
• Strain-tuned suppression of CDW enhances phonon-mediated superconductivity, establishing Mo2NF2 as a platform for strain-controlled quantum phases.

Figure 2: (a) Phonon dispersion along the high-symmetry path $\Gamma$ – $K$ – $M$ – $\Gamma$ . The black solid lines correspond to the unstrained structure ( $\varepsilon=0\%$ ), while the black dashed line ( $\nu=2$ ) indicates an unstable soft phonon mode at the $M$ point. The blue curves show the