Rapid enhancement of nodal quasiparticle mass with heavily underdoping in Bi2212

Abstract

We report substantial advance of our low-energy angle-resolved photoemission study of nodal quasiparticles in Bi2Sr2CaCu2O8+δ. The new data cover the samples from underdoped down to heavily underdoped levels. We also present the nodal Fermi velocities that determined by using an excitation-photon energy of hν=7.0 eV over a wide doping range. The consistency between the results with hν=8.1 and 7.0eV allows us to rule out the effect of photoemission matrix elements. In comparison with the data previously reported, the nodal effective mass increases by a factor of ∼1.5 in going from optimally doped to heavily underdoped levels. We find a rapid enhancement of the nodal quasiparticle mass at low doping levels near the superconductor-to-insulator transition. The effective coupling spectrum, λ(ω), is extracted directly from the energy derivatives of the quasiparticle dispersion and scattering rate, as a causal function of the mass enhancement factor. A steplike increase in Reλ(ω) around ∼ 65meV is demonstrated clearly by the Kramers-Kronig transform of Imλ(ω). To extract the low-energy renormalization effect, we calculated a simple model for the electron-boson interaction. This model reveals that the contribution of the renormalization at |ω|≤15meV to the quasiparticle mass is larger than that around 65meV in underdoped samples.