  In an armchair carbon nanotube pn junction the p- and n- regions are
separated by a region of a Mott insulator, which can backscatter electrons only
in pairs. We predict a quantum-critical behavior in such a pn junction.
Depending on the junction's built-in electric field E, its conductance G scales
either to zero or to the ideal value G=4e^2/h as the temperature T is lowered.
The two types of the G(T) dependence indicate the existence, at some special
value of E, of an intermediate quantum critical point with a finite conductance
G<4e^2/h. This makes the pn junction drastically different from a simple
barrier in a Luttinger liquid.
