Evolution of a bosonic mode across the superconducting dome in the high-T_c cuprate Pr_2-xCe_xCuO_4-δ

We report a detailed spectroscopic study of the electron-doped cuprate superconductor Pr_2-xCe_xCuO_4-δ using point contact junctions for x=0.125 (underdoped), x=0.15 (optimally doped), and x=0.17 (overdoped). From our conductance measurements, we are able to identify bosonic resonances for each doping. These excitations disappear above the critical magnetic field. We find that the energy of the bosonic excitations decreases with doping, which excludes lattice vibrations as the paring glue. We conclude that the bosonic mediator for these cuprates is more likely to be spin excitations.