Temperature dependence and anisotropy of the bulk upper critical field H_c2 of MgB₂

The bulk upper critical field H_{c2}(T) of superconducting MgB_2 and its anisotropy are established by analyzing experimental data on the temperature and magnetic field dependences of the ab-plane thermal conductivity of a single-crystalline sample in external magnetic fields oriented both parallel (H_{c2}^c) and perpendicular (H_{c2}^{ab}) to the c axis of the hexagonal lattice. From numerical fits we deduce the anisotropy ratio \gamma_0 = H_{c2}^{ab}(0) / H_{c2}^c(0)=4.2 at T=0 K. Both the values and the temperature dependences of H_{c2}^c and H_{c2}^{ab} are distinctly different from previous claims based on measurements of the electrical resistivity.