[Paper Review] Predicting Stable Phase Monolayer Mo$_2$C (MXene), a Superconductor with Chemically-Tunable Critical Temperature

Jincheng Lei, Alex Kutana+1 more|arXiv (Cornell University)|Feb 11, 2017

MXene and MAX Phase Materials4 citations

TL;DR

This study predicts that monolayer 2H MXene-Mo₂C is a stable, metallic 2D superconductor with a tunable critical temperature (Tc) ranging from 0 to 13 K, depending on surface termination (O, H, OH). Using first-principles calculations and electron-phonon coupling analysis within BCS theory, it demonstrates that hydrogen termination enhances Tc to 13 K, making it a promising platform for tuning low-dimensional superconductivity via chemical functionalization.

ABSTRACT

Two-dimensional (2D) superconductors have attracted great attention in recent years due to the possibility of new phenomena in lower dimensions. With many bulk transition metal carbides being well-known conventional superconductors, here we perform first-principles calculations to evaluate the possible superconductivity in a 2D monolayer Mo$_2$C. Three candidate structures (monolayer alpha-Mo$_2$C, 1T MXene-Mo$_2$C, and 2H MXene-Mo$_2$C) are considered and the most stable form is found to be the 2H MXene-Mo$_2$C. Electronic structure calculations indicate that both unpassivated and passivated 2H forms exhibit metallic properties. We obtain the phonon frequencies and electron-phonon couplings using density-functional perturbation theory, and based on the BCS theory and McMillan equation, estimate the critical temperatures to be in the ~0-13 K range, depending on the species of the surface termination (O, H and OH). The most interesting termination group is H, which can increase the electron-phonon coupling and bring the critical temperature to 13 K. This shows a rather high critical temperature, tunable by surface termination, making this 2D carbide an interesting test bed for low-dimensional superconductivity.

Motivation & Objective

• To identify stable two-dimensional phases of Mo₂C that could exhibit superconductivity.
• To evaluate the electronic and vibrational properties of candidate monolayer Mo₂C structures using first-principles methods.
• To determine the critical temperature (Tc) of the most stable phase and assess its tunability via surface termination.
• To explore the potential of 2D MXenes as a platform for low-dimensional superconductivity with controllable Tc.

Proposed method

• First-principles density functional theory (DFT) calculations to evaluate the stability and electronic structure of three candidate monolayer Mo₂C phases: α-Mo₂C, 1T MXene-Mo₂C, and 2H MXene-Mo₂C.
• Density-functional perturbation theory (DFPT) to compute phonon frequencies and electron-phonon coupling constants.
• Application of the McMillan equation within the BCS framework to estimate the critical temperature (Tc) from electron-phonon coupling and phonon spectra.
• Systematic variation of surface termination (O, H, OH) to assess their impact on electron-phonon coupling and Tc.
• Comparison of total energies and phonon dispersion to identify the most thermodynamically stable structure.

Experimental results

Research questions

• RQ1 Which monolayer phase of Mo₂C is thermodynamically stable among the three candidate structures?
• RQ2 Does the most stable monolayer Mo₂C phase exhibit metallic behavior and superconducting characteristics?
• RQ3 What is the estimated critical temperature (Tc) of the stable 2H MXene-Mo₂C phase, and how does it vary with different surface terminations?
• RQ4 Can surface functionalization (O, H, OH) chemically tune the electron-phonon coupling and thus the Tc in 2D Mo₂C?
• RQ5 Is 2H MXene-Mo₂C a viable candidate for studying tunable low-dimensional superconductivity?

Key findings

• The 2H MXene-Mo₂C phase is identified as the most thermodynamically stable monolayer structure among the three candidates studied.
• Both unpassivated and passivated 2H MXene-Mo₂C exhibit metallic electronic structure, enabling superconductivity.
• The critical temperature (Tc) of 2H MXene-Mo₂C is predicted to range from 0 K (with O-termination) to 13 K (with H-termination).
• Hydrogen termination significantly enhances electron-phonon coupling, leading to the highest Tc of 13 K among the studied terminations.
• The study confirms that surface termination provides a viable chemical knob to tune Tc in 2D MXene superconductors.
• The results position monolayer 2H MXene-Mo₂C as a promising system for exploring tunable, low-dimensional superconductivity.