Magneto-optical reflectivity of superconducting MgB2 single crystals

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We present magneto-optical reflectivity results in the basal-plane of the hexagonal MgB₂. The data were collected on a mosaic of MgB₂ single crystals with Tc=38 K from the ultraviolet down to the far infrared as a function of temperature and magnetic field oriented along the c-axis. In the far infrared there is a clear signature of the superconducting gap with a gap-ratio 2Δ/kBTc 1.2, well below the weak-coupling value. The gap is suppressed in an external magnetic field, which is a function of temperature. We extract the upper critical field Hc₂ along the c-axis. The temperature dependence of Hc₂ is compatible with the Helfand-Werthamer behaviour.