Superconductivity in o-MAX phases

In recent years, MAX phases and their two-dimensional counterparts, MXenes, have emerged as significant subjects of interest in the fields of science and engineering, owing to their varied geometries, compositions, and extensive range of applications. This research employs first-principles calculations to explore the geometrical structures, electronic characteristics, phonon dispersions, dynamic stability, electron-phonon coupling (EPC), and superconducting properties of 27 out-of-plane ordered double transition metal carbides, referred to as o-MAX phases, characterized by the general formula M₂M'AlC₂ (where M = Nb, Mo, W and M' = Sc, Ti, Zr, Hf, V, Nb, Ta, Mo, W). We have identified 16 superconducting o-MAX phases, with four specific compounds W₂VAlC₂, W₂NbAlC₂, W₂TaAlC₂, and Mo₂NbAlC₂ exhibiting a critical temperature (T_c) that surpasses 10 K, representing the highest T_c reported experimentally for MAX phases thus far. The calculated EPC constants for these materials are 0.98, 0.99, 1.02, and 0.74, correlating with T_c values of 17.9, 14.8, 14.5, and 11 K, respectively. Remarkably, the predicted transition temperature of 17.9 K stands as the highest T_c theoretically anticipated for any MAX phase to date. We conduct a thorough analysis of the specific mechanisms that facilitate superconductivity in these o-MAX systems. Our findings suggest that the presence of Kohn anomalies in low-frequency modes enhances electron-phonon interactions, resulting in increased superconducting transition temperatures (T_c). Additionally, our results indicate that Nb₂M'AlC₂ compounds do not display superconducting behavior.