Low-Energy (<10 meV) Feature in the Nodal Electron Self-Energy and Strong Temperature Dependence of the Fermi Velocity inBi2Sr2CaCu2O8+δ

Using low photon energy angle-resolved photoemission, we study the low-energy dispersion along the nodal ($\ensuremath{\pi},\ensuremath{\pi}$) direction in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ as a function of temperature. Less than 10 meV below the Fermi energy, the high-resolution data reveal a novel ``kinklike&#39;&#39; 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 ${v}_{F}$, 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&#39;&#39; in the nodal Fermi velocity of cuprates.