Effect of Carbon and Nitrogen Concentrations on the Superconducting Properties of (NbMoTaW)1CxNy Carbonitride Films

We report about the effect of nitrogen and carbon concentration on the superconducting transition temperature <i>T_C</i> of (NbMoTaW)₁C_xN_y carbonitride films deposited using reactive DC magnetron sputtering. By measuring the temperature dependence of electrical resistance and magnetization of these carbonitrides, with 0.20 ≤ <i>x</i> ≤ 1.17 and 0 ≤ <i>y</i> ≤ 0.73, we observe a <i>T_C</i> enhancement that occurs especially at high (<i>x</i> ≥ 0.76) carbon concentrations, with the largest <i>T_C</i> = 9.6 K observed in the over-doped fcc crystal structure with <i>x</i> = 1.17 and <i>y</i> = 0.41. The reason why the largest <i>T_C</i> appears at high C concentrations is probably related to the lower atomic mass of carbon compared to nitrogen and to the increase in the electron-phonon interaction due to different bonding of carbon (compared to nitrogen) to the Nb-Mo-Ta-W metallic sublattice. However, for concentrations where <i>y</i> > 0.71 and <i>x</i> + <i>y</i> > 1.58, two structural phases begin to form. Additionally, the proximity to structural instability may play a role in the observed <i>B</i>_<i>C</i>2 enhancement. Further measurements in a magnetic field show that the upper critical fields <i>B</i>_<i>C</i>2 of (NbMoTaW)₁C_xN_y carbonitrides provide <i>B</i>_<i>C</i>2/<i>B</i>_<i>C</i>2 < 2 T/K, which falls within the weak-coupling pair breaking limit.