We report the first study of the optical conductivity of $\mathrm{MgB}{}_2$ covering the range of its lowest-energy superconducting gap. Terahertz time-domain spectroscopy is utilized to determine the complex, frequency-dependent conductivity $\sigma \left(\omega \right)$ 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 $2{\Delta }_0{/k}_B{T}_C\approx 1.9$ is well below the weak-coupling value, pointing to complex behavior in this novel superconductor.

• Received 20 June 2001

DOI:https://doi.org/10.1103/PhysRevLett.88.027003