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 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. 
S. Deemyad, T. Tomita, J. J. Hamlin, B. R. Beckett, J. S. Schilling,   D. G. Hinks, J. D. Jorgensen, S. Lee, S. Tajima