High-Tc Superconductivity in Functionalized Out-of-Plane Ordered Double Transition Metal MXenes

TL;DR

This study uses first-principles methods to screen 128 surface-functionalized out-of-plane ordered double-transition-metal MXenes (o-MXenes) and identify a subset that are mechanically, dynamically, and thermodynamically stable with superconducting transition temperatures up to $T_c \approx 52$ K. Key findings include Mo$_2$ScN$_2$O$_2$ as the leading candidate exhibiting anisotropic two-gap superconductivity, underpinned by flat-band and Van Hove singularity features that enhance electron–phonon coupling; however, anharmonic lattice dynamics modestly reduce $T_c$ via a decrease in $\lambda$ and phonon softening. The superconductivity is analyzed with isotropic and anisotropic Migdal-Eliashberg theory in a full-bandwidth framework, revealing that low-frequency phonons dominate pairing while FS anisotropy shapes the gap, which remains continuous rather than split. The results underscore the pivotal roles of surface termination, orbital hybridization, flat-band physics, and anisotropy in tuning EPC and $T_c$ in 2D MXenes, and provide design guidelines for synthesizing MXene-based superconductors.

Abstract

Two-dimensional (2D) superconductors attracted growing interest in condensed-matter physics research. In this work, we explore the superconducting properties of surface-functionalized, out-of-plane ordered double transition-metal MXenes (o-MXenes), which exhibit distinctive structural and electronic characteristics. Using first-principles calculations, we investigate the effects of electronic structure, electron-phonon coupling (EPC), anharmonicity, and anisotropy effect in superconductivity properties of o-MXenes. We examine a wide range of o-MXene systems, M$_{2}$M$^\prime$X$_{2}$T$_{2}$ (M = Mo, W; M$^\prime$ = Sc, Ti, V, Mo, Zr, Nb, Ta; X = C, N), functionalized with F, O, Cl, and H groups. Out of 128 candidates, 32 compounds are found to be mechanically, dynamically, and thermodynamically stable, exhibiting superconducting transition temperatures (T$_{c}$) from 0.1 K to 52 K. Notably, the Mo$_{2}$ScN$_{2}$O$_{2}$ compound achieves the highest T$_{c}$ of 52 K, with a superconducting gap of $\sim$10 meV. Solving the anisotropic Eliashberg equation reveals that Mo$_{2}$ScN$_{2}$O$_{2}$ is an anisotropic two-gap superconductor, and incorporating anharmonic effects decreases its T$_{c}$ slightly. We further analyze flat-band-induced EPC enhancement and present EPC matrix elements as functions of phonon wavevector q for distinct vibrational modes that show anharmonic behavior of these materials.

Figures (6)

• Figure 1: a) The total free energy as a function of ab initio molecular dynamics (AIMD) simulation time for a 2$\times$2$\times$1 supercell at 300K and 400K for Mo$_{2}$ScN$_{2}$O$_{2}$. The insets display corresponding structure snapshots at 0 and 5 ps, respectively.b-g) Six distinct configurations with different placements of surface chemical groups for each M$_{2}$M$^\prime$C$_{2}$ compound.
• Figure 2: Stability screening of o-MXenes and a map of the superconducting critical temperature (T$_{c}$), calculated using the McMillan equation with a Coulomb repulsion parameter $\mu$=0.1, is shown for all o-MXenes. The horizontal axis represents the transition metals M and M$^\prime$, while the vertical axis corresponds to the functional groups and X atoms. Phases identified as dynamically unstable from phonon calculations are marked with $'$X$'$.
• Figure 3: a) The 3D view of the flat band that has a major role in EPC and the saddle-type Van Hove singularity.b) Fermi surface, the color drawn indicates the relative Fermi velocity v$_{F}$. c) partial electronic band structure, and partial density of state of Mo$_{2}$ScN$_{2}$O$_{2}$.The Fermi energy is set to 0 eV. d) Crystal orbital hamiltonian population (COHP) of Mo$_{2}$ScN$_{2}$O$_{2}$.
• Figure 4: A 3D representation of the electronic bands crossing the Fermi level in Mo$_{2}$ScN$_{2}$O$_{2}$ and cross section of that at 0 eV, along with the corresponding electron–phonon coupling (EPC) matrix elements as functions of the phonon wavevector q, shown for representative vibrational modes—the lowest acoustic and highest optical branches.
• Figure 5: a) Phonon dispersion, b) total phonon density of states (PhDOS), c) isotropic Eliashberg spectral function $\alpha^2F(\omega)$ (solid line) and cumulative electron-phonon coupling strength $\lambda(\omega)$ (dashed lines), and d) distribution of the electron-phonon coupling strength $\lambda_{\textbf{k}}$ for both harmonic (black) and anharmonic (red) phonons in Mo$_{2}$ScN$_{2}$O$_{2}$. (e) Momentum-resolved electron-phonon coupling $\lambda_{\textbf{k}}$ mapped onto the Fermi surface (shown for the harmonic case).