Anisotropic superconducting properties of MgB2 single crystals

Abstract In-plane electrical transport properties of MgB2 single crystals grown under a high pressure of 4–6 GPa and temperature of 1400–1700 °C 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 ΔTc(10–90%) within 0.2–0.3 K. Estimated resistivity value at 40 K is about 1 μ Ω cm and the resistivity ratio ρ(273 K )/ρ(40 K )=4.9±0.3 . Results of measurements in magnetic field up to 5.5 T perpendicular to Mg and B planes (H⊥) and up to 9 T in parallel orientation (H∥) show temperature dependent anisotropy of the upper critical field with anisotropy ratio γ=Hc2∥/Hc2⊥ increasing from 2.2 close to Tc up to about 3 below 30 K. Strong deviation of the angular dependence of Hc2 from the anisotropic mass model has been also found.