Far-infrared optical conductivity gap in superconducting MgB2 films

We report the first study of the optical conductivity of MgB2 covering the range of its superconducting energy gap. Terahertz time-domain spectroscopy is utilized to determine the complex, frequency-dependent conductivity of thin films. The imaginary part reveals an inductive reponse due to the emergence of the superconducting condensate. The real part exhibits a strong depletion of oscillator strength near 5 meV resulting from the opening of a superconducting energy gap. The gap ratio of 2Delta/kTc = 1.9 is well below the weak-coupling value, pointing to complex behavior in this novel superconductor