Study of the optical gap in novel superconductors by coherent THz radiation

We show how coherent synchrotron radiation (CSR) allows one to measure the slight increase in the reflectivity of a BCS superconductor as it is cooled below T-c. In fact, by CSR coupled to a conventional interferometric apparatus, one can obtain a signal-to-noise ratio similar to 10(3) in the sub-THz range. We apply this technique to the measurement of the optical gap in boron-doped diamond and in CaAlSi, a superconductor isostructural to MgB2. In the latter compound we are also able to determine a slight anisotropy between the gap in the hexagonal planes and that along the orthogonal c axis. (c) 2007 Elsevier B.V. All rights reserved.