Pressure effects on the structural phase transitions and superconductivity of ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ba}}_{\mathit{x}}$${\mathrm{CuO}}_4$ (x=0.125)

Neutron-diffraction experiments have been performed to study the effects of hydrostatic pressure on the structural phase transitions of ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Ba}}_{\mathit{x}}$ ${\mathrm{CuO}}_4$ with x=0.125. The phase transition to the low-temperature tetragonal (LTT) structure is greatly suppressed by the application of pressure. The LTT transition temperature, observed at around 80 K at ambient pressure, decreases at the rate of about -135 K/GPa, and this LTT phase disappears under a pressure of around 0.6 GPa. The superconducting properties of this compound under high pressure have also been investigated in detail by measurments of the ac magnetic susceptibility. The bulk superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ remains constant at pressures of up to 0.5 GPa, and then increases with pressure at the rate of 10 K/GPa up to about 15 K at 1.5 GPa. From these results it is concluded that there is a clear correlation between the disappearance of the LTT phase and the enhancement of superconductivity. This conclusion supports earlier suggestions that there is a strong relationship between the crystal structure and superconductivity and that, in particular, the LTT phase is detrimental to the superconductivity in the ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Ba}}_{\mathit{x}}$ ${\mathrm{CuO}}_4$ system.