  We address the quality of electrical contact between carbon nanotubes and
metallic electrodes by performing first-principles calculations for the
electron transmission through ideal 2- and 3-terminal junctions, thus revealing
the physical limit of tube-metal conduction. The structural model constructed
involves surrounding the tube by the metal atoms of the electrode as in most
experiments; we consider metallic (5,5) and n-doped semiconducting (10,0) tubes
surrounded by Au or Pd. In the case of metallic tubes, the contact conductance
is shown to approach the ideal 4e^2/h in the limit of large contact area. For
three-terminals, the division of flux among the different transmission channels
depends strongly on the metal material. A Pd electrode has nearly perfect
tube-electrode transmission and therefore turns off the straight transport
along the tube. Our results are in good agreement with some recent experimental
reports and clarify a fundamental discrepancy between theory and experiment.

Materials Science