Doping-Dependent Nodal Fermi Velocity of the High-Temperature Superconductor Bi2Sr2CaCu2O8+δ Revealed Using High-Resolution Angle-Resolved Photoemission Spectroscopy

The improved resolution of laser-based angle-resolved photoemission spectroscopy (ARPES) allows reliable access to fine structures in the spectrum. We present a systematic, doping-dependent study of a recently discovered low-energy kink in the nodal dispersion of Bi2Sr2CaCu2O8+delta (Bi-2212), which demonstrates the ubiquity and robustness of this kink in underdoped Bi-2212. The renormalization of the nodal velocity due to this kink becomes stronger with underdoping, revealing that the nodal Fermi velocity is nonuniversal, in contrast with assumed phenomenology. This is used together with laser ARPES measurements of the gap velocity (nu(2)) to resolve discrepancies with thermal conductivity measurements.