Anisotropic superconducting properties of MgB2 single crystals

In-plane electrical transport properties of MgB2 single crystals grown under a high pressure of 4-6 GPa and temperature of 1400-1700 degreesC in the Mg-B-N system have been measured. For all specimens we found a sharp superconducting transition around 38.1-38.3 K with DeltaT(c)(10-90%) within 0.2-0.3 K. Estimated resistivity value at 40 K is about 1 muOmegacm and the resistivity ratio p(273 K)/p(40 K) = 4.9 +/- 0.3. Results of measurements in magnetic field up to 5.5 T perpendicular to Mg and B planes (H-perpendicular to) and up to 9 T in parallel orientation (H-\\) show temperature dependent anisotropy of the upper critical field with anisotropy ratio gamma = H-c2\\/H-c2perpendicular to increasing from 2.2 close to T-c up to about 3 below 30 K. Strong deviation of the angular dependence of H-c2 from the anisotropic mass model has been also found. (C) 2002 Elsevier Science B.V. All rights reserved.