Enhancing superconductivity in MXenes through hydrogenation

Hydrogenation induced high-temperature superconductivity in two-dimensional W2C3

The discovery of highly crystalline two-dimensional (2D) superconductors provides a new alluring branch for exploring the fundamental significances. Based on first-principles calculations, we predict a new kind of 2D stable material W2C3, which is a semimetal but not a superconductor because of the weak electron-phonon coupling (EPC) strength. After hydrogenation, W2C3H2 possesses the intrinsic metallic properties with a large density of states (DOS) at the Fermi energy (EF). More interestingly, the EPC strength is greatly enhanced after hydrogenation and the calculated critical temperature (Tc) is 40.5 K. Furthermore, the compressive strain can obviously soften the low-frequency phonons and enhance the EPC strength. Then, the Tc of W2C3H2 can be increased from 40.5 K to 49.1 K with -4% compressive strain. This work paves the way for providing a new platform for 2D superconductivity.

Superconductivity in functionalized niobium-carbide MXenes

We detail the effects of Cl and S functionalization on the superconducting properties of layered (bulk) and monolayer niobium carbide (Nb₂C) MXene crystals, based on first-principles calculations combined with Eliashberg theory. For bulk layered Nb₂CCl₂, the calculated superconducting transition temperature (T_c) is in very good agreement with the recently measured value of 6 K. We show that T_c is enhanced to 10 K for monolayer Nb₂CCl₂, due to an increase in the density of states at the Fermi level, and the corresponding electron-phonon coupling. We further demonstrate feasible gate- and strain-induced enhancements of T_c for both bulk-layered and monolayer Nb₂CCl₂ crystals, resulting in T_c values of around 38 K. In the S-functionalized Nb₂CCl₂ crystals, our calculations reveal the importance of phonon softening in understanding their superconducting properties. Finally, we predict that Nb₃C₂S₂ in bulk-layered and monolayer forms is also superconducting, with a T_c of around 28 K. Considering that Nb₂C is not superconducting in pristine form, our findings promote functionalization as a pathway towards robust superconductivity in MXenes.