Complicated nature of the gap in MgB 2 : Magnetic-field-dependent optical studies

We investigated the magnetic-field- (H) dependent optical conductivity spectra of ${\mathrm{MgB}}_{2}$ 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 ${\mathrm{MgB}}_{2}.$