High-pressure study of the anomalous isotope effect in ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathit{A}}_{\mathit{x}}$${\mathrm{CuO}}_4$ with A=Sr or Ba

The superconducting transition temperature (${\mathit{T}}_{\mathit{c}}$) of well-characterized samples of ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{A}}_{\mathit{x}}$ ${\mathrm{CuO}}_4$ (LSCO or LBCO) with A=Sr or Ba at and close to x=0.12, where the oxygen isotrope effect displays a maximum, has been determined under hydrostatic pressures up to 16 kbar. In spite of the chemical similarity between the two compound systems, ${\mathit{dT}}_{\mathit{c}}$/dP at x∼0.12 exhibits a minimum in LSCO in contrast to an expected maximum similar to that in LBCO, which undergoes a low-temperature structural transformation at this x. The results suggest that the anomalous isotope effect previously reported in LSCO and LBCO cannot be simply attributed to structural instabilities associated with the compound systems, but is consistent with an electronic transition of the Lifshitz type in these compounds.