Appearance of pressure-induced superconductivity in BaFe2As2 under hydrostatic conditions and its extremely high sensitivity to uniaxial stress

We have performed electrical resistivity measurements on single-crystal BaFe2As2 under high pressure P up to 16 GPa with a cubic anvil apparatus, and up to 3 GPa with a modified Bridgman anvil cell. The samples were obtained from the same batch, which was grown with a self-flux method. A cubic anvil apparatus provides highly hydrostatic pressure, and a modified Bridgman anvil cell, which contains liquid-pressure transmitting medium, provides quasihydrostatic pressure. For highly hydrostatic pressure, the crystal phase and magnetic-transition temperature decreases robustly with P and disappears at around 10 GPa. The superconducting phase appears adjacent to magnetic phase in narrow pressure region between 11 and 14 GPa. The tiny difference in hydrostaticity between the cubic anvil apparatus and modified Bridgman anvil cell induces a drastic effect on the phase diagram of BaFe2As2. This result indicates that small uniaxial stress along c axis strongly suppresses the structural/antiferromagnetic ordering and stabilizes superconductivity at much lower pressure.