Anisotropic superconducting properties of MgB2 single crystals probed by   in-plane electrical transport measurements
  We report on study of the electronic anisotropy of the newly discovered MgB2 superconductor using the in-plane resistivity measurements in magnetic field applied perpendicular and parallel to Mg and B planes of MgB2 single crystals. The results show temperature dependent anisotropy of the upper critical field with anisotropy ratio increasing from 2.2 close to Tc up to about 3 below 30K. Our estimation of the in-plane and out-of-plane coherence length of about 68A and 23A and electronic mean-free path 240A and 60A respectively indicates MgB2 single crystal as approaching clean limit type-II superconductor. 
Yu. Eltsev, S. Lee, K. Nakao, N. Chikumoto, S. Tajima, N. Koshizuka,   M. Murakami