Role of surface functional groups to superconductivity in Nb2C-MXene: Experiments and density functional theory calculations

The recently discovered surface-group-dependent superconductivity in Nb2C-MXene fabricated by the molten salts method is attracting wide attention. However, regarding the superconductivity of Nb2C-MXene with functional F groups (Nb2CFx), there were some conflicting results in experimental and theoretical studies. Herein, we systematically carried out experimental and theoretical investigations on the superconductivity in Nb2C-MXene with the Cl functional group (Nb2CClx) and Nb(2)CFx. The experimental results of the Meissner effect and zero resistivity have proved that Nb2CClx is superconducting with the transition temperature (T-c) -5.2 K. We extract its superconducting parameters from the temperature dependence of resistivity and the field dependence of the magnetization. The Ginzburg-Landau parameter kappa GL is estimated to be 2.41, indicating that Nb2CClx is a typical type-II superconductor. Conversely, both magnetic and electrical transport measurements demonstrate that Nb2CFx is not superconducting. The first-principles density functional theory (DFT) calculations show that the T-c of Nb2CClx is -5.2 K, while Nb2CFx is dynamically unstable with imaginary frequency in phonon spectrum, which is in good agreement with the experimental results. Our studies not only are useful for clarifying the present inconsistency but also offer referential significance for future investigations on the superconductivity of MXenes.