Two-band effects in the angular dependence of H_c2 of MgB ₂ single crystals

The angular dependence of the upper critical field H_c2 of MgB₂ single crystals is studied at various temperatures by means of specific-heat and transport measurements in magnetic fields up to 17 T. Clear deviations from Ginzburg-Landau behavior are observed at all temperatures and are explained by two-band effects. The angular dependence and temperature dependence of the deviations are in qualitative agreement with theoretical predictions based on band-structure calculations. Quantitative agreement is obtained with an interband coupling slightly stronger than the calculated one, enabling band-structure anisotropies and interband coupling strength to be experimentally estimated. This provides a pathway to the study of disorder and doping effects in MgB₂.