Low-Energy (<10 meV) Feature in the Nodal Electron Self-Energy and Strong Temperature Dependence of the Fermi Velocity in Bi2Sr2CaCu2O8+δ

Using low photon energy angle-resolved photoemission, we study the low-energy dispersion along the nodal (pi, pi) direction in Bi2Sr2CaCu2O8+delta as a function of temperature. Less than 10 meV below the Fermi energy, the high-resolution data reveal a novel kinklike feature in the electron self-energy that is distinct from the larger well-known kink roughly 70 meV below E-F. This new kink is strongest below the superconducting critical temperature and weakens substantially at higher temperatures. A corollary of this finding is that the Fermi velocity nu(F), as measured in this low-energy range, varies rapidly with temperature-increasing by almost 30% from 70 to 110 K. The behavior of v(F)(T) appears to shift as a function of doping, suggesting a departure from simple universality in the nodal Fermi velocity of cuprates.