  A quasi-exciton condensate is a phase characterized by quasi-long range order
of an exciton (electron-hole pair) order parameter. Such a phase can arise
naturally in a system of two parallel oppositely doped quantum wires, coupled
by repulsive Coulomb interactions. We show that the quasi-exciton condensate
phase can be stabilized in an extended range of parameters, in both spinless
and spinful systems. For spinful electrons, the exciton phase is shown to be
distinct from the usual quasi-long range ordered Wigner crystal phase
characterized by power-law density wave correlations. The two phases can be
clearly distinguished through their inter-wire tunneling current-voltage
characteristics. In the quasi-exciton condensate phase the tunneling
conductivity diverges at low temperatures and voltages, whereas in the Wigner
crystal it is strongly suppressed. Both phases are characterized by a divergent
Coulomb drag at low temperature. Finally, metallic carbon nanotubes are
considered as a special case of such a one dimensional setup, and it is shown
that exciton condensation is favorable due to the additional valley degree of
freedom.
