Deviations from the conventional BCS behavior in the penetration depth of LuNi₂B₂C films at terahertz frequencies

We have measured the temperature and frequency-dependant transmission and phase shift through LuNi₂B₂C thin films on MgO substrates at terahertz frequencies. From the measured data, we could accurately determine the complex dielectric constant, epsilon, the complex optical conductivity, sigma, and the penetration depth, lambda. Comparing our measured results with theory, we find strong deviations from the standard one-band BCS predictions. These deviations can be attributed to the multiband nature of the superconducting state in LuNi₂B₂C.