Dependence of the superconducting transition temperature of single and polycrystalline MgB2 on hydrostatic pressure

The dependence of T-c for MgB2 on purely hydrostatic or nearly hydrostatic pressure has been determined to 29 GPa for single-crystal line and to 32 GPa for polycrystalline samples, and found to be in good agreement. T, decreases from 39 K at ambient pressure to 15 K at 32 GPa with an initial slope dT(c)/dP similar or equal to -1.11(2) K/GPa. Evidence is presented that the differing values of dT(c)/dP reported in the literature may result primarily from shear-stress effects in nonhydrostatic pressure media rather than from differences in the samples. Although comparison of these results with theory supports phonon-mediated superconductivity, a critical test of theory must await calculations based on the solution of the anisotropic Eliashberg equations as a function of the lattice parameters. (C) 2002 Elsevier Science B.V. All rights reserved.