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

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.

Figures (6)

• 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.
• 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 phonon dispersion under compressive strain ($\varepsilon = -3\%$), where the soft mode is fully stabilized. (b) Phonon linewidth $\gamma(\mathbf{q}, \nu = 2)$, exhibiting a pronounced enhancement near the $M$ point, indicative of strong electron--phonon coupling. (c) Real part of the static electronic susceptibility, $\mathrm{Re}[\chi_{0}(\mathbf{q})]$, showing enhanced responses at high-symmetry wave vectors. (d) Imaginary part of the electronic susceptibility, $\mathrm{Im}[\chi_{0}(\mathbf{q})]$, revealing pronounced features associated with Fermi-surface nesting.
• Figure 3: Evolution of the phonon frequency of the unstable mode at the $M$ point as a function of biaxial strain and charge doping. The bottom horizontal axis shows the applied strain, while the top axis indicates the corresponding electron (positive) and hole (negative) doping per unit cell.
• Figure 4: Atomic-scale structural characteristics of the charge-density-wave (CDW) phase in Mo$_2$NF$_2$. (a) Side view of the CDW structure, showing lattice distortions involving the Mo, N, and F sublattices. (b) Top view highlighting the shortened Mo--Mo bonds in the CDW phase, with a representative bond length of 2.70 Å. (c) Top view illustrating the N--N bond modulation induced by the CDW, with a characteristic bond length of 2.72 Å. (d) Top view showing the corresponding F--F bond modulation in the CDW phase, also characterized by a bond length of 2.72 Å. Orange, yellow, and green spheres represent Mo, N, and F atoms, respectively.
• Figure 5: Electronic structure of Mo$_2$NF$_2$ in the high-symmetry and charge-density-wave (CDW) phases. (a) High-symmetry structure: orbital-projected electronic band structure along the high-symmetry path $\Gamma$--$K$--$M$--$\Gamma$, corresponding electronic density of states (EDOS), and Fermi surface. (b) CDW structure: orbital-projected band structure, EDOS, and Fermi surface of the relaxed CDW phase. The bands are colored according to the dominant Mo $d$-orbital contributions ($d_{z^2}$, $d_{zx}/d_{zy}$, and $d_{x^2-y^2}/d_{xy}$), while the EDOS is decomposed into Mo $d$, N $p$, and F $p$ states. The transition from the high-symmetry phase to the CDW phase leads to pronounced band reconstruction and a substantial modification of the Fermi surface topology, reflecting the lattice distortion and Brillouin-zone folding associated with the CDW ordering.