Magnetic Field Dependence of Far-Infrared Absorption in Thin-Film Superconductors

Abstract

We have measured the absorption of far-infrared radiation in a cavity by superconducting lead films as a function of magnetic field up to the critical field. The experiments were conducted over a frequency range of 8 to 60 ${\mathrm{cm}}^{-1\mathrm{}}$ on films approximately 100 Å thick in a magnetic field nearly parallel to the film plane. Even for fields well below the critical value, we observed substantial absorption below the gap frequency and apparently extending down to very low frequencies. This low-frequency absorption is much stronger than we can account for in terms of a spatially uniform order parameter being depressed by the field. Rather, we postulate that vortices with normal cores resulting from the unavoidable, though small, perpendicular-field component play an essential role. In this way we can qualitatively interpret our data, but at present no available theory is capable of giving a quantitative account of them.