Energy-Dependent Enhancement of the Electron-Coupling Spectrum of the Underdoped Bi2Sr2CaCu2O8+δ Superconductor

We have determined the electron-coupling spectrum of superconducting Bi2Sr2CaCu2O8+delta from high-resolution angle-resolved photoemission spectra by two deconvolution-free robust methods. As hole concentration decreases, the coupling spectral weight at low energies less than or similar to 15 meV shows a twofold and nearly band-independent enhancement, while that around similar to 65 meV increases moderately, and that in greater than or similar to 130 meV decreases leading to a crossover of dominant coupling excitation between them. Our results suggest the competition among multiple screening effects, and provide important clues to the source of sufficiently strong low-energy coupling, lambda(LE) approximate to 1, in an underdoped system.