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

TL;DR

This paper reports the high-pressure growth of MgB2 and Mg1-xAlxB2 single crystals, analyzing their structural stability, superconducting transition temperatures, and effects of aluminum substitution on their physical properties.

Contribution

It introduces a high-pressure cubic anvil technique for growing MgB2 crystals and investigates the effects of Al doping on their superconducting properties.

Findings

MgB2 crystals grow via peritectic decomposition of MgNB9.

Superconducting transition temperature (Tc) is around 38-38.6 K.

Aluminum substitution decreases Tc and upper critical field.

Abstract

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 2x1x0.1mm3 with a weight up to 230 micrograms. 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 2190C 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 Tc. This indicates a possible appearance of superstructures or phases with different Tc's. The upper critical field decreases with Al doping.