MgB2 and Mg1-xAlxB2 single crystals:: high pressure growth and physical properties

Single crystals of MgB2 have been grown with a high pressure cubic anvil technique. They grow via the peritectic decomposition of the MgNB9 ternary nitride. The crystals are of a size up to 2 x 1 x 0.1 mm(3) with a weight up to 230 mug. Typically they have transition temperatures between 38 and 38.6 K with a width of 0.3-0.5 K. Investigations of the P-T phase diagram prove that the MgB2 phase is stable at least up to 2190 degreesC at high hydrostatic pressure in the presence of Mg vapor under high pressure. Substitution of aluminum for magnesium in single crystals leads to stepwise decrease of T-c. This indicates a possible appearance of superstructures or phases with different T-c's. The upper critical field decreases with Al doping. (C) 2004 Elsevier B.V. All rights reserved.