Two-band Effects in the Angular Dependence of H_c2 of MgB2 Single Crystals

We have studied the angular dependence of the upper critical field of MgB₂ single crystals at various temperatures by means of specific heat and transport measurements in fields up to 17 T. Deviations from Ginzburg-Landau behavior are observed and explained by two-band effects. Their angular- and temperature dependence are in excellent agreement with theoretical two-band predictions. Quantitative agreement is obtained with slightly modified band anisotropies and interband coupling strength, enabling these parameters to be experimentally estimated. The results are in good agreement with an enlarged vortex core and give a reasonable gap anisotropy Δ_π/Δ_σ ≈ 0.38.