Correlation between structural and superconducting properties of nano-granular disordered Nb thin films

We report on low temperature transport measurements on nano-granular Nb thin films deposited on Si (1 0 0) substrates using DC magnetron sputtering. The superconducting transition temperature (T_c) is found to decrease monotonically with the increase of the lattice parameter (a) irrespective of its thickness and grain size. The superconducting transition temperature is found to depend only on the lattice parameter whereas the normal state resistivity depends both on lattice parameter and the details of the sample morphology. We have modeled this T_c variation with lattice expansion in terms of Debye temperature reduction using Morse potential as the interatomic potential in Nb.