Anisotropy of the upper critical field in superconductors with   anisotropic gaps. Anisotropy parameters of MgB2
  The upper critical field Hc2 is evaluated for weakly-coupled two-band superconductors. By modeling the actual bands and the gap distribution of MgB2 by two Fermi surface spheroids with average parameters of the real material, we show that H_{c2,ab}/H_{c2,c} increases with decreasing temperature in agreement with available data. 
P. Miranovic, K. Machida, V. G. Kogan