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

Recently, 2D Mo 2 C, a new member of the MXene family, has attracted much attention due to the exotic superconducting properties discovered in 2D α -Mo 2 C. Here, not only 2D α -Mo 2 C but also 2D β -Mo 2 C crystal sheets with distinct disordered carbon distributions were successfully grown. 2D β -Mo 2 C shows a much stronger superconductivity than 2D α -Mo 2 C, and their superconductivities have different hydrostatic pressure responses. The superconducting transition temperature T c of 2D α -Mo 2 C shows a dome-shaped profile under pressure, implying the existence of two competing effects arising from phononic and electronic properties, while for 2D β -Mo 2 C, 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 -Mo 2 C compared to 2D β -Mo 2 C. The ordered and disordered carbon distributions in 2D α -Mo 2 C and β -Mo 2 C, 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 -Mo 2 C crystal sheet shows a much stronger superconductivity than the carbon ordered 2D α -Mo 2 C crystal sheet, and the in-situ order-disorder transition from α -Mo 2 C to β- Mo 2 C induced by e-beam irradiation results in an enhanced superconductivity. Especially, the T c variation trends of α -Mo 2 C and β -Mo 2 C are different under hydrostatic pressures. These results highlight the important role of disorders in the superconducting properties owing to the carbon distributions in Mo 2 C. Superconductors: 2D nanomaterials thrive under pressure and e-beam irradiation Two-dimensional (2D) molybdenum carbide (Mo 2 C) sheets, an ultrathin crystal system, have attracted much attention due to the exotic superconducting properties. Now, Li’s group from the University of Science and Technology of China in Hefei and colleagues report that 2D Mo 2 C can be synthesized into two crystal forms that exhibit distinctly different superconducting behaviors when compressed in a pressure cell. The team demonstrated that the disordered ‘beta’ crystal phase of Mo 2 C becomes superconductive at higher temperatures than the more ordered ‘alpha’ form. The in-situ order-disorder transition from ‘alpha’ to ‘beta’ Mo 2 C induced by e-bea