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

Abstract

Despite recent advances in understanding high-transition-temperature (high-T(c)) 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, with the additional possibility of pairing without mediators. In conventional superconductors, phonon-mediated pairing was unequivocally established by data from tunnelling experiments. Proponents of phonons as the high-T(c) glue were therefore encouraged by the recent scanning tunnelling microscopy experiments on hole-doped Bi2Sr2CaCu2O8-delta (BSCCO) that reveal an oxygen lattice vibrational mode whose energy is anticorrelated with the superconducting gap energy scale. Here we report high-resolution scanning tunnelling microscopy measurements of the electron-doped high-T(c) superconductor Pr0.88LaCe0.12CuO4 (PLCCO) (T(c) = 24 K) that reveal a bosonic excitation (mode) at energies of 10.5 +/- 2.5 meV. This energy is consistent with both spin-excitations in PLCCO measured by inelastic neutron scattering (resonance mode) and a low-energy acoustic phonon mode, but differs substantially from the oxygen vibrational mode identified in BSCCO. Our analysis of the variation of the local mode energy and intensity with the local gap energy scale indicates an electronic origin of the mode consistent with spin-excitations rather than phonons.

Electron-spin excitation coupling in an electron-doped copper oxide superconductor

“…Here we report advances made by a combined neutron scattering and scanning tunneling spectroscopy (STS) studies on nominally identical electron-doped superconducting Pr 0.88 LaCe 0.12 CuO 4- (PLCCO) samples with different T c 's obtained through the oxygen annealing process 13,14 . We find that spin excitations detected by neutron scattering 15,16 have two distinct modes that evolve with T c in a remarkably similar fashion to the electron tunneling modes 17 in STS. Spatial mapping of the modes shows nanoscale regions of coexisting AF and superconducting order in the lower T c samples.…”

“…Spatial mapping of the modes shows nanoscale regions of coexisting AF and superconducting order in the lower T c samples. Since the annealing process is not expected to change lattice (phonon) properties 2,13,14,18 , these results demonstrate that antiferromagnetism and superconductivity compete locally and coexist spatially on nanometer length scales, and the dominant electron-boson coupling at low energies originates from the electron-spin excitations 17 rather than electron-phonon interactions 19 .…”

“…Our first task is to obtain the energy of these bosonic modes from the STS spectra. In a superconductor, a bosonic excitation at energy Ω results in a feature at energy (E) offset by the gap in the tunneling spectroscopy 17,29 . The local bosonic mode energy can therefore be determined from the STS spectrum by subtracting the local gap energy scale, i.e., Ω = E -Δ.…”

“…In general, the 21 K gaps have muted coherence peaks (Figs. 3b and 4d) compared to the 24 K samples 17 . The height and width of the coherence peak are a measure of the scattering rate (), which contains a temperature dependent contribution from phonons and electronic excitations as well as a temperature independent contribution from impurity scattering ( imp ) (refs.…”

Strong-coupling analysis of scanning tunneling spectra in Bi2Sr2Ca2CuBerthod1, Fasano2, Maggio-Aprile3 et al. 2013Phys. Rev. B

“…This interaction affects the one-electron spectra and is observable in the single-electron spectroscopies. Although the strength of this interaction has been a matter of controversy, 31,32 there is evidence that peculiar signatures observed in photoemission, [33][34][35][36] tunneling, [37][38][39][40][41] and optical conductivity 42,43 result from this interaction. Yet, a firm consensus has not been reached: Optical phonons often exist in the cuprates at similar energies, and distinguishing the effects of the two kinds of excitations has proven difficult.…”