  The electronic properties of interfaces between two different solids can
differ strikingly from those of the constituent materials. For instance,
metallic conductivity, and even superconductivity, have been recently
discovered at interfaces formed by insulating transition metal oxides. Here we
investigate interfaces between crystals of conjugated organic molecules, which
are large gap undoped semiconductors, i.e. essentially insulators. We find that
highly conducting interfaces can be realized with resistivity ranging from 1 to
30 kOhm square, and that, for the best samples, the temperature dependence of
the conductivity is metallic. The observed electrical conduction originates
from a large transfer of charge between the two crystals that takes place at
the interface, on a molecular scale. As the interface assembly process is
simple and can be applied to crystals of virtually any conjugated molecule, the
conducting interfaces described here represent the first examples of a new
class of electronic systems.
