Far-infrared optical conductivity gap in superconducting MgB2 films -: art. no. 027003

We report the first study of the optical conductivity of MgG(2) covering the range of its lowest-energy superconducting gap. Terahertz time-domain spectroscopy is utilized to determine the complex, frequency-dependent conductivity sigma(omega)) of thin films. The imaginary part reveals an inductive response 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(0)/k(B)T(C) approximate to 1.9 is well below the weak-coupling value, pointing to complex behavior in this novel superconductor.