The complex conductivity of a MgB2 film has been investigated in the frequency range 4 cm^{-1}< nu < 30 cm^{-1} and for temperatures 2.7 K < T <300 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. No characteristic feature of the isotropic BCS gap temperature evolution is observed in the conductivity spectra in the superconducting state. A peak in the temperature dependence of the real part of the conductivity is detected for frequencies below 9 cm^{-1}. The superconducting penetration depth follows a T^2 behavior at low temperatures.
@article{arxiv.cond-mat/0104291,
title = {Optical conductivity and penetration depth in MgB2},
author = {A. V. Pronin and A. Pimenov and A. Loidl and S. I. Krasnosvobodtsev},
journal= {arXiv preprint arXiv:cond-mat/0104291},
year = {2009}
}