Energy-dependent enhancement of the electron-coupling spectrum of the underdoped Bi2Sr2CaCu2O(8+δ) superconductor.

We have determined the electron-coupling spectrum of superconducting Bi2Sr2CaCu2O(8+δ) from high-resolution angle-resolved photoemission spectra by two deconvolution-free robust methods. As hole concentration decreases, the coupling spectral weight at low energies ≲15  meV shows a twofold and nearly band-independent enhancement, while that around ∼65  meV increases moderately, and that in ≳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, λ(LE)≈1, in an underdoped system.