MgB2 single crystals:: high pressure growth and physical properties

Single crystals of MgB2 with a size up to 1.5 x 0.9 x 0.2 mm(3) have been grown with a high pressure cubic anvil technique. The crystal growth process is very peculiar and involves an intermediate nitride, namely MgNB9. Single crystals of BN and MgB2 grow simultaneously by a peritectic decomposition of MgNB9. Magnetic measurements with SQUID magnetometry in fields of 1-5 Oe show, sharp transitions to the superconducting state at 37-38.6 K with a width of similar to0.5 K. The high quality of the crystals allowed the accurate determination of magnetic, transport (electric and heat) and optical properties as well as scanning tunnelling spectroscopy (STS) and decoration studies. Investigations of crystals with torque magnetometry show that H-c2(parallel toc) for high quality crystals is very low (24 kOe at 15 K) and saturates with decreasing temperature, while H-c2(parallel toab) increases up to 140 kOe at 15 K. The upper critical field anisotropy gamma = H-c2(parallel toab)/H-c2(parallel toc) was found to be temperature dependent (decreasing from gamma congruent to 6 at 15 K to 2.8 at 35 K). The effective anisotropy gamma(eff), as calculated from reversible torque data near T, is field dependent (increasing roughly linearly from gamma(eff) congruent to 2 in zero field to 3.7 in 10 kOe). The temperature and field dependence of the anisotropy can be related to the double gap structure of MgB2 with a large two-dimensional gap and small three-dimensional gap, the latter of which is rapidly suppressed in a magnetic field. Torque magnetometry investigations also show a pronounced peak effect, which indicates an order-disorder phase transition of vortex matter. Decoration experiments and STS visualize a hexagonal vortex lattice. STS spectra in zero field evidence two gaps 3 meV and 6 meV with a weight depending on the tunnelling direction. Magneto-optic investigations in the far-infrared region with Hparallel toc show a clear signature of the smaller of the two superconducting gaps, completely disappearing only in fields higher than H-c2(parallel toc).