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

The dependence of Tc for MgB2 on purely hydrostatic or nearly hydrostatic pressure has been determined to 29 GPa for single-crystalline and to 32 GPa for polycrystalline samples, and found to be in good agreement. Tc decreases from 39 K at ambient pressure to 15 K at 32 GPa with an initial slope dTc=dP ’ 1:11ð2Þ K/GPa. Evidence is presented that the differing values of dTc=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. 2002 Elsevier Science B.V. All rights reserved. PACS: 74.62.Fj; 74.70.Ad; 74.62.Dh