The complex conductivity of a ${\mathrm{MgB}}_2$ film has been investigated in the frequency range $4<\mathrm{}\nu <30\mathrm{}{\mathrm{cm}}^{-1\mathrm{}}$ and for temperatures $2.7<T<\mathrm{}300\mathrm{}\mathrm{K}$. The overall temperature dependence of both components of the complex conductivity is reminiscent of BCS-type behavior, although a detailed analysis reveals a number of discrepancies. A peak in the temperature dependence of the real part of the conductivity is detected for frequencies below $9{\mathrm{cm}}^{-1\mathrm{}}$. The superconducting penetration depth follows a $T^2$ behavior at low temperatures.

• Received 17 April 2001

DOI:https://doi.org/10.1103/PhysRevLett.87.097003