  The dependence of Tc for MgB2 on purely hydrostatic or nearly hydrostatic
pressure has been determined to 23 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 =
-1.11(2) K/GPa. Evidence is presented that the differing values of dTc/dP
reported in the literature result primarily from shear-stress effects in
nonhydrostatic pressure media and not differences in the samples. Although
comparison of these results with theory supports phonon-mediated
superconductivity, a critical test of theory must await volume-dependent
calculations based on the solution of the anisotropic Eliashberg equations.
