  Nonequlibrium phase transition of an open Takayama-Lin Liu-Maki chain coupled
with two reservoirs is investigated by combining a mean-field approximation and
a formula characterizing nonequlibrium steady states, which is obtained from
the algebraic field approach to nonequlibrium statistical mechanics. When the
bias voltage is chosen to be a control parameter, the phase transition between
ordered and normal phases is found to be first or second order. Then, the
voltage-current characteristics is S-shaped in some parameter region. In
contrast, when the current is chosen to be a control parameter, all the
non-trivial solutions of the self-consistent equation are found to become
stable. In this case, the phase transition between the ordered and normal
phases is always second order and negative differential conductivity appears at
low temperature.
