  We report on the first direct observation of a transition from a
Tomonaga-Luttinger liquid to a Fermi liquid behavior in potassium intercalated
mats of single wall carbon nanotubes (SWCNT). Using high resolution
photoemission spectroscopy an analysis of the spectral shape near the Fermi
level reveals a Tomonaga-Luttinger liquid power law scaling in the density of
states for the pristine sample and for low dopant concentration. As soon as the
doping is high enough to fill bands of the semiconducting tubes a distinct
transition to a bundle of only metallic SWCNT with a scaling behavior of a
normal Fermi liquid occurs. This can be explained by a strong screening of the
Coulomb interaction between charge carriers and/or an increased hopping matrix
element between the tubes.
