Abstract
Using low photon energy angle-resolved photoemission, we study the
low-energy dispersion along the nodal (π,π) direction in Bi2Sr2CaCu2O8+δ
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 EF. 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 vF, as measured in this low-energy range, varies rapidly
with temperature—increasing by almost 30% from 70 to 110 K. The behavior
of vF(T) appears to shift as a function of doping, suggesting a departure
from simple “universality” in the nodal Fermi velocity of cuprates.
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