Distinct superconducting properties and hydrostatic pressure effects in 2D α- and β-Mo2C crystal sheets

Abstract

Recently, 2D Mo₂C, a new member of the MXene family, has attracted much attention due to the exotic superconducting properties discovered in 2D α-Mo₂C. Here, not only 2D α-Mo₂C but also 2D β-Mo₂C crystal sheets with distinct disordered carbon distributions were successfully grown. 2D β-Mo₂C shows a much stronger superconductivity than 2D α-Mo₂C, and their superconductivities have different hydrostatic pressure responses. The superconducting transition temperature T_c of 2D α-Mo₂C shows a dome-shaped profile under pressure, implying the existence of two competing effects arising from phononic and electronic properties, while for 2D β-Mo₂C, T_c decreases monotonically with increasing pressure, possibly due to phonon stiffening. These results indicate that the electronic properties have a more important influence on the superconductivity in 2D α-Mo₂C compared to 2D β-Mo₂C. The ordered and disordered carbon distributions in 2D α-Mo₂C and β-Mo₂C, respectively, may be the underlying origin for their different electronic and superconducting properties.

It is still a mystery whether disorders could be beneficial for the superconductivity or not. In this work, it is surprising to find out that the carbon disordered 2D β-Mo₂C crystal sheet shows a much stronger superconductivity than the carbon ordered 2D α-Mo₂C crystal sheet, and the in-situ order-disorder transition from α-Mo₂C to β-Mo₂C induced by e-beam irradiation results in an enhanced superconductivity. Especially, the T_c variation trends of α-Mo₂C and β-Mo₂C are different under hydrostatic pressures. These results highlight the important role of disorders in the superconducting properties owing to the carbon distributions in Mo₂C.