Complicated nature of the gap in MgB2: magnetic field-dependent optical   studies
  We investigated the magnetic field (H)-dependent optical conductivity spectra of MgB2 thin film in the far-infrared region. The H-dependences can be explained by the increase of normal metallic regions embedded in the superconducting background. The area fraction of the normal metallic region increases rather quickly at low field, but slowly at high field. It follows neither H- nor H^{1/2}-dependences. The results suggest the complicated nature of the superconducting gap in MgB2. 
H. J. Lee, J. H. Jung, K. W. Kim, M. W. Kim, T. W. Noh, Y. J. Wang, W.   N. Kang, Eun-Mi Choi, Hyeong-Jin Kim, and Sung-Ik Lee