A distinct bosonic mode in an electron-doped high-transition-temperature superconductor

摘要

Despite recent advances in understanding high-transition-temperature (high-Tc) superconductors, there is no consensus on the origin of the superconducting'glue': that is, the mediator that binds electrons into superconducting pairs.The main contenders are lattice vibrations (phonons) and spin-excitations withthe additional possibility of pairing without mediators. In conventionalsuperconductors, phonon-mediated pairing was unequivocally established by datafrom tunnelling experiments. Proponents of phonons as the high-T c glue weretherefore encouraged by the recent scanning tunnelling microscopy experimentson hole-doped Bi2Sr2CaCu2O8-delta (BSCCO) that reveal an oxygen latticevibrational mode whose energy is anticorrelated with the superconducting gapenergy scale. Here we report high-resolution scanning tunnelling microscopymeasurements of the electron-doped high-T c superconductor Pr0.88LaCe0.12CuO4(PLCCO) (T c = 24 K) that reveal a bosonic excitation (mode) at energies of10.5 plus/minus 2.5 meV. This energy is consistent with both spin-excitationsin PLCCO measured by inelastic neutron scattering (resonance mode) and alow-energy acoustic phonon mode, but differs substantially from the oxygenvibrational mode identified in BSCCO. Our analysis of the variation of thelocal mode energy and intensity with the local gap energy scale indicates anelectronic origin of the mode consistent with spin-excitations rather thanphonons.