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ARPES is a priori a technique of choice to measure the Fermi velocities vF inmetals. In correlated systems, it is interesting to compare this experimentalvalue to that obtained in band structure calculations, as deviations areusually taken as a good indicator of the presence of strong electroniccorrelations. Nevertheless, it is not always straightforward to extract vF fromARPES spectra. We study here the case of layered cobaltates, an interestingfamily of correlated metals. We compare the results obtained by standardmethods, namely the fit of spectra at constant momentum k (energy distributioncurve, 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 afactor 2. The reliability of the 2 methods is intimately linked to the degreeof k- and omega-dependence of the electronic self-energy. As the k-dependenceis usually much smaller than the omega dependence for a correlated system, theMDC analysis is generally expected to give more reliable results. However, wereview here several examples within cobaltates, where the MDC analysisapparently leads to unphysical results, while the EDC analysis appearscoherent. We attribute the difference between the EDC and MDC analysis to astrong variation of the photoemission intensity with the momentum k. Thisdistorts the MDC lineshapes but does not affect the EDC ones. Simulationsincluding a k dependence of the intensity allow to reproduce the differencebetween MDC and EDC analysis very well. This momentum dependence could be ofextrinsic or intrinsic. We argue that the latter is the most likely andactually contains valuable information on the nature of the correlations thatwould be interesting to extract further.

Measuring Fermi velocities with ARPES in narrow band systems. The case of layered cobaltates