High-pressure transport phenomena in polycrystalline (Pb(1+x)/2Cu(1−x)/2)Sr2(Y1−xCax)Cu2O7+δ

The effect of pressure on ${\mathit{T}}_{\mathit{c}}$ for polycrystalline ‘‘Pb 1:2:1:2’’-phase material is investigated up to 2 GPa using a piston-cylinder apparatus. The pressure coefficient of ${\mathit{T}}_{\mathit{c}}$ is positive and depends strongly on the ambient-pressure ${\mathit{T}}_{\mathit{c}}$. We discuss the relation between d ln${\mathit{T}}_{\mathit{c}}$/dp and ${\mathit{T}}_{\mathit{c}}$. The general trend that d ln${\mathit{T}}_{\mathit{c}}$/dp increases with decreasing ${\mathit{T}}_{\mathit{c}}$ can be explained by two phenomenological assumptions: a parabolic relation between ${\mathit{T}}_{\mathit{c}}$ and the carrier concentration n in the ${\mathrm{CuO}}_2$ planes and a linear increase of n with increasing pressure. An unusually large logarithmic pressure coefficient of ${\mathit{T}}_{\mathit{c}}$ is reported (d ln${\mathit{T}}_{\mathit{c}}$/dp=0.65 ${\mathrm{GPa}}^{\mathrm{-}1}$, ${\mathit{T}}_{\mathit{c}}$=16.8 K at ambient pressure). This result suggests that there is another mechanism for the increase in the carrier concentration with an increase in pressure, besides that due to charge transfer. We propose that this large d ln${\mathit{T}}_{\mathit{c}}$/dp is correlated with the pressure-induced delocalization of holes.