Role of surface functional groups to superconductivity in Nb$_2$C-MXene: Experiments and density functional theory calculations

TL;DR

This study combines experiments and density functional theory calculations to investigate how surface functional groups affect superconductivity in Nb$_2$C-MXene, confirming superconductivity with Cl groups and explaining the non-superconductivity with F groups.

Contribution

It provides systematic experimental and theoretical evidence clarifying the role of surface groups in Nb$_2$C-MXene superconductivity, especially distinguishing the effects of Cl and F groups.

Findings

Nb$_2$CCl$_x$ is superconducting with Tc ~ 5.2 K

Nb$_2$CF$_x$ is not superconducting and dynamically unstable

Theoretical Tc matches experimental results for Nb$_2$CCl$_x$

Abstract

The recently discovered surface-group-dependent superconductivity in Nb$_2$C-MXene fabricated by the molten salts method is attracting wide attention. However, regarding the superconductivity of Nb$_2$C-MXene with functional F groups (Nb$_2$CF$_x$), there were some conflicting results in experimental and theoretical studies. Herein, we systematically carried out experimental and theoretical investigations on the superconductivity in Nb$_2$C-MXene with the Cl functional group (Nb$_2$CCl$_x$) and Nb$_2$CF$_x$. The experimental results of the Meissner effect and zero resistivity have proved that Nb$_2$CClx is superconducting with the transition temperature (Tc) ~ 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 K$_G$$_L$ is estimated to be 2.41, indicating that Nb$_2$CClx is a typical type-II superconductor. Conversely, both magnetic and electrical transport measurements demonstrate that Nb$_2$CF$_x$ is not superconducting. The first-principles density functional theory (DFT) calculations show that the Tc of Nb$_2$Cl$_x$ is ~ 5.2 K, while Nb$_2$CF$_x$ 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.