Reversible magnetization measurement of the anisotropy of the London   penetration depth in MgB2 single crystals
  We have studied the anisotropy of the London penetration depth, which was obtained from reversible magnetization measurements with the magnetic field both parallel and perpendicular to the c-axis. The anisotropy of the London penetration depth has a smaller magnitude than the anisotropy of the upper critical field and increases with temperature while that of the upper critical field decreases as reported earlier. This behavior is in sharp contrast with the behaviors of superconductors with one superconducting energy gap. The temperature dependence of the anisotropies of the London penetration depth and of the upper critical field can be well explained within the theory of two-gap superconductivity in MgB2. 
Heon-Jung Kim, Byeongwon Kang, Min-Seok Park, Kyung-Hee Kim, Hyun Sook   Lee, and Sung-Ik Lee