Far-Infrared Absorption Spectra of Thick Superconducting Films

Abstract

We have examined the absorption spectra of thick evaporated films of Pb, Sn, In, V, Ta, and Nb in the superconductive energy-gap region by a calorimetric detection technique. The intensity available from our far-infrared monochromator, together with the improved sensitivity of the detection system, made it possible to employ modulation techniques to obtain the derivatives of the absorption spectra of our samples. Multiple energy gaps were found for a Sn film and two thick Pb films, while a thinner Pb film and the other samples had single gaps. We suggest that these are the first far-infrared measurements to reveal multiple absorption edges in films. The energy gaps found for the two thick Pb films were $(4.18\mathrm{}\pm 0.04)k{T}_c$ and $(4.46\mathrm{}\pm 0.04)k{T}_c$ for one, and $(4.28\mathrm{}\pm 0.04)k{T}_c$ and $(4.65\mathrm{}\pm 0.04)k{T}_c$ for the other. For the Sn film, the gaps were $(3.58\mathrm{}\pm 0.02)k{T}_c$ and $(3.86\mathrm{}\pm 0.04)k{T}_c$. The gaps found for the other samples, in units of $k{T}_c$, were: for the thin Pb film, 4.34±0.02; for In, 3.69±0.04; for V, 3.4±0.1; for Ta, 3.5±0.2; and for Nb, 3.6±0.2. These values are more precise than the earlier far-infrared results; in addition, the anomalously low values of $\sim 3k{T}_c$ obtained in previous far-infrared experiments for the Ta and Nb gaps are not reproduced in this work. The observed absorption edges are steeper than those predicted by either the extreme local or extreme anomalous limits of skin-effect theory. The energy gap of a thick Pb sample was measured as a function of magnetic field, this being the first such measurement by far-infrared calorimetric techniques. The gap was found to decrease by approximately 7% in going from zero field to the critical field for the bulk material ($H_{\mathrm{cb}}$), in fair agreement with the Ginzburg-Landau theory. No gap was observed above $H_{\mathrm{cb}}$, in agreement with previous observations on the superconducting surface sheath.