A far IR study of NbN in the normal and superconductive state

A thin film of NbN (thickness t = 300 Angstrom), has been deposited on an M gO and a Si wafer. Both samples have been studied by transmission from 10 o r 20 to 120 cm(-1), and have exhibited one maximum of transmission at a giv en frequency like the classical superconductors as Pb, Sn or Hg in the supe rconductive state. From the Far IR experimental data, the characteristic te mperature theta(c), and the gap frequency (nu(gap) (theta) = 2 Delta (theta ), Delta (theta) being the energy gap) are immediately obtained (for instan ce for the NbN / MgO sample, theta(c) = 15.5 K; nu(g) (5 K)= 39.7 cm(-1)), and it is seen that 2 Delta(4 K)/k(b) theta(c) approximate to 3.5 as expect ed from the BCS theory for a weak coupling. To fit the data we had to adjus t only two additional parameters: collision and plasma frequency, nu(c) (th eta) and nu(p) (including all carriers). At theta = 5 K, the best fit for t he NbN / MgO sample is obtained with nu(c) = 371 cm(-1) and nu(p) = 12,600 cm(-1).