Optical conductivity of LuNi₂B₂C in the terahertz range

Using a backward-wave-oscillator-based setup in a Mach-Zehnder interferometer arrangement, we have measured the temperature and frequency-dependent transmission and phase-shift spectra of LuNi₂B₂C films on MgO substrates in the range 200 GHz - 1.4 THz. From the measured spectra, we have directly calculated the complex optical conductivity. We observe a clear signature of the superconducting energy gap in the spectra. In the talk, a comparison of the experimentally obtained spectra with theoretical predictions for a multi-band superconductor will be given.