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Localization and pair breaking parameter in superconducting molybdenum nitride thin films

We have investigated the superconductor-insulator transition in molybdenum nitride films prepared by deposition onto MgO substrates. It is indicated that the Tc depression from ≈6.6 K for thick films with increase of the normal state sheet resistance RsqN was well explained by the Finkel’stein formula from the localization theory. Present analysis suggests that the superconducting-insulator transition occurs at a critical sheet resistance Rc≈2 kΩ. It is found that the Rsq(T) above Rc shows different characteristics of Rsq(T)=Rsq,0−AlnT and Rsq(T)∝exp[(T0/T)1/2] in the regions RcRsqN>RQ, respectively, where Rsq,0 is the classical residual resistance and A is a constant. The excess conductance σ′(T) due to thermal fluctuation has been analyzed by the sum of the Aslamazov–Larkin and Maki–Thompson correction terms with use of the pair breaking parameter δ in the latter term. The sum agrees well with the data, although the experimental results of the RsqN dependence of δ, that is, δ∝(RsqN)≈1.7 shows the disagreement with a linear relation δ∝(RsqN) derived from the localization theory.