  We demonstrate that soliton-plasmon bound states appear naturally as
propagating eigenmodes of nonlinear Maxwell's equations for a
metal/dielectric/Kerr interface. By means of a variational method, we give an
explicit and simplified expression for the full-vector nonlinear operator of
the system. Soliplasmon states (propagating surface soliton-plasmon modes) can
be then analytically calculated as eigenmodes of this non-selfadjoint operator.
The theoretical treatment of the system predicts the key features of the
stationary solutions and gives physical insight to understand the inherent
stability and dynamics observed by means of finite element numerical modeling
of the time independent nonlinear Maxwell equations. Our results contribute
with a new theory for the development of power-tunable photonic nanocircuits
based on nonlinear plasmonic waveguides.
