  Recent data from angle-resolved photoemission experiments published by Zhou
et al. [Nature, Vol. 423, 398 (2003)] concerning a number of hole-doped
copper-oxide-based high-temperature superconductors reveal that in the nodal
directions of the underlying square Brillouin zones (i.e. the directions along
which the d-wave superconducting gap is vanishing) the Fermi velocities for
some finite range of k inside the Fermi sea and away from the nodal Fermi
wavevector k_F are to within an experimental uncertainty of approximately 20%
the same both in all the compounds investigated and over a wide range of doping
concentrations and that, in line with earlier experimental observations, at
some characteristic wavevector k_* away from k_F the Fermi velocities undergo a
sudden change, with this change (roughly speaking, a finite discontinuity)
being the greatest (smallest) in the case of underdoped (overdoped) compounds.
In this paper we present a rigorous analysis concerning the implications of
these observations. [Short abstract]
