Single crystals, with sizes of up to 1.5 x 0.9 x 0.2mm3, were grown using a high-pressure cubic anvil technique. The crystal growth process was unique, and involved an intermediate nitride: MgNB9. Single crystals of BN and MgB2 grew simultaneously via the peritectic decomposition of MgNB9. Magnetic measurements using SQUID magnetometry, in fields of 1 to 5Oe, revealed sharp transitions to the superconducting state at 37 to 38.6K, with a width of ~0.5K. The high quality of the crystals permitted the accurate determination of magnetic, transport (electric and heat) and optical properties. Investigations of the crystals, using torque magnetometry, showed that H||cC2 for high-quality crystals was very low (24kOe at 15K) and saturated with decreasing temperature, while H||abC2 increased up to 140kOe at 15K. The upper critical field anisotropy, γ = H||abC2/H||cC2, was found to be temperature-dependent (decreasing from γ  6 at 15K to 2.8 at 35K). The effective anisotropy, γeff, as calculated from reversible torque data near to TC, was field-dependent (increasing almost linearly from γeff  2 in zero field to 3.7 in 10kOe). The temperature and field dependence of the anisotropy could be related to the double-gap structure of MgB2, with a large 2-dimensional gap and small 3-dimensional gap; the latter being rapidly suppressed in a magnetic field. Torque magnetometry investigations also revealed a pronounced peak effect, which indicated an order–disorder phase transition of vortex matter. Decoration experiments and STS revealed an hexagonal vortex lattice. The STS spectra in zero field exhibited 2 gaps, 3meV and 6meV, with a weight that depended upon the tunnelling direction. Magneto-optical investigations in the far-infrared region, with H||c revealed a clear signature of the smaller of the 2 superconducting gaps; which disappeared completely only in fields higher than H||cC2.

MgB2 Single Crystals - High Pressure Growth and Physical Properties. J.Karpinski, M.Angst, J.Jun, S.M.Kazakov, R.Puzniak, A.Wisniewski, J.Roos, H.Keller, A.Perucchi, L.Degiorgi, M.R.Eskildsen, P.Bordet, L.Vinnikov, A.Mironov: Superconductor Science and Technology, 2003, 16, 221-30