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Single crystals of MgB2 with a size up to 1.5x0.9x0.2 mm3 have been grownwith a high pressure cubic anvil technique. The crystal growth process is verypeculiar and involves an intermediate nitride, namely MgNB9. Single crystals ofBN and MgB2 grow simultaneously by a peritectic decomposition of MgNB9.Magnetic measurements in fields of 1-5 Oe show sharp transitions to thesuperconducting state at 37-38.6 K with width of ~0.5 K. The high quality ofthe crystals allowed the accurate determination of magnetic, transport andoptical properties as well as scanning tunnelling spectroscopy (STS) anddecoration studies. Investigations of crystals with torque magnetometry showthat Hc2//c is very low (24 kOe at 15 K), while Hc2//ab increases up to 140 kOeat 15 K. The upper critical field anisotropy gamma = Hc2//ab/ Hc2//c was foundto be temperature dependent (decreasing from 6 at 15 K to 2.8 at 35 K). Theeffective anisotropy gamma_eff, as calculated from reversible torque data nearTc, is field dependent (increasing roughly linearly from 2 in zero field to 3.7in 10 kOe). The temperature and field dependence of the anisotropy can berelated to the double gap structure of MgB2 with a large two-dimensional gapand small three-dimensional gap, the latter being rapidly suppressed in amagnetic field. Torque magnetometry investigations show a pronounced peakeffect, indicating an order-disorder transition of vortex matter. Decorationexperiments and STS visualise a hexagonal vortex lattice. STS spectra evidencetwo gaps (3 meV/6 meV) with direction dependent weight. Magneto-opticinvestigations with H//c show a clear signature of the smaller of the two gaps,disappearing in fields higher than Hc2//c.Comment: 17 pages pdf only, 15 figures integrated (higher resolution  photographs available on request); submitted to Supercond. Sci. Technol.  (Proceedings of Boromag conference

MgB2single crystals: high pressure growth and physical properties

MgB₂single crystals: high pressure growth and physical properties