Pressure dependence of ${\mathit{T}}_{\mathit{c}}$ in ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ at hydrostatic pressures to 6 GPa

We report the results of experiments carried out on single crystals of ${\mathrm{Tl}}_2$ ${\mathrm{Ba}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$ under nearly hydrostatic pressure conditions to 6 GPa. The critical temperature, ${\mathit{T}}_{\mathit{c}}$, is observed to possess a nonlinear pressure dependence. While the increase of ${\mathit{T}}_{\mathit{c}}$ with increasing oxygen concentration in ${\mathrm{YBa}}_2$ ${\mathrm{Ca}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ is consistent with the observation of increasing ${\mathit{T}}_{\mathit{c}}$ with increasing pressure, the ${\mathrm{Tl}}_2$ ${\mathrm{Ba}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$ system demonstrates an increasing ${\mathit{T}}_{\mathit{c}}$ with decreasing oxygen content, but has a positive ${\mathit{dT}}_{\mathit{c}}$/dP at low pressures. This behavior is clearly inconsistent with the picture applied to the Y-Ba-Cu-O system. In all the present measurements the curvature is downward and, in fact, for one sample it actually decreases. A theoretical explanation based upon the strong-coupling Eliashberg form of the BCS theory is put forth to explain this effect.