Growth, Structure Analysis and Anisotropic Superconducting Properties of MgB2 Single Crystals

Here we report the growth of sub-millimeter MgB 2 single crystals of various shapes under high pressure in Mg–B–N system. Structure refinement using a single-crystal X-ray diffraction analysis gives lattice parameters a =3.0851(5) A and c =3.5201(5) A with small reliability factors ( R w =0.025, R =0.018), which enables us to analyze the Fourier and Fourier difference maps. The maps clearly show the B sp 2 orbitals and covalency of the B–B bonds. The sharp superconducting transitions at T c =38.1–38.3 K were obtained in both magnetization (Δ T c =0.6 K) and resistivity (Δ T c <0.3 K) measurements. Resistivity measurements with magnetic fields applied parallel and perpendicular to the Mg and B sheets reveal the anisotropic nature of this compound, with upper critical field anisotropy ratio of about 2.7.