Andreev reflection spectroscopy in MgB2

Our Andreev reflection measurements (Phys. Rev. Lett. 87 (2001) 137005) along with other experiments have led to a general consensus that MgB2 is a multiband superconductor with two main superconducting gaps closing at the same Tc. Here we show the behavior of the small gap as a function of the temperature and magnetic field. This gap is isotropic with Tc of the bulk material but with a specific small (crossover) critical magnetic field of about 1 T much lower than the real Hc2. The latter field is anisotropic and is rather governed by the large gap and strongly anisotropic Fermi surface of the material