  ARPES is a priori a technique of choice to measure the Fermi velocities vF in
metals. In correlated systems, it is interesting to compare this experimental
value to that obtained in band structure calculations, as deviations are
usually taken as a good indicator of the presence of strong electronic
correlations. Nevertheless, it is not always straightforward to extract vF from
ARPES spectra. We study here the case of layered cobaltates, an interesting
family of correlated metals. We compare the results obtained by standard
methods, namely the fit of spectra at constant momentum k (energy distribution
curve, EDC) or constant binding energy omega(momentum distribution curve, MDC).
We find that the difference of vF between the two methods can be as large as a
factor 2. The reliability of the 2 methods is intimately linked to the degree
of k- and omega-dependence of the electronic self-energy. As the k-dependence
is usually much smaller than the omega dependence for a correlated system, the
MDC analysis is generally expected to give more reliable results. However, we
review here several examples within cobaltates, where the MDC analysis
apparently leads to unphysical results, while the EDC analysis appears
coherent. We attribute the difference between the EDC and MDC analysis to a
strong variation of the photoemission intensity with the momentum k. This
distorts the MDC lineshapes but does not affect the EDC ones. Simulations
including a k dependence of the intensity allow to reproduce the difference
between MDC and EDC analysis very well. This momentum dependence could be of
extrinsic or intrinsic. We argue that the latter is the most likely and
actually contains valuable information on the nature of the correlations that
would be interesting to extract further.
