Bulk superconductivity at 84 K in the strongly overdoped regime of cuprates

By means of magnetization, specific heat, and muon-spin relaxation measurements, we investigate newly synthesized high-pressure oxidized Cu_0.75Mo_0.25Sr₂YCu₂O_7.54, in which overdoping is achieved up to p ˜ 0.46 hole/Cu, well beyond the T_c-p superconducting dome of cuprates, where Fermi-liquid behavior is expected. Surprisingly, we find bulk superconductivity with T_c = 84 K and superfluid density similar to those of optimally doped YBa₂Cu₃O_7-δ. On the other hand, specific heat data display a large electronic contribution at low temperature, comparable to that of nonsuperconducting overdoped La_2−xSr_xCuO₄. These results point at an unusual high-T_c phase with a large fraction of unpaired holes. Further experiments may assess the Fermi-liquid properties of the present phase, which would put into question the paradigm that the high T_c of cuprates originates from a non-Fermi-liquid ground state.