  The spatio-temporal evolution of nonlinear oscillations in multi-species
plasma is revisited to provide more insight into the physics of phase mixing by
constructing two sets of nonlinear solutions up to the second order. The first
solution exhibits perfect oscillations in the linear regime and phase mixing
appears only nonlinearly in the second order as a response to the ponderomotive
forces. This response can be both direct and indirect. The indirect
contribution of the ponderomotive forces appears through self-consistently
generated low frequency fields. Furthermore, the direct and indirect
contributions of the ponderomotive forces on the phase mixing process is
explored and it is found that the indirect contribution is negligible in an
electron-ion plasma and it disappears in the case of electron-positron plasma,
yet represents an equal contribution in the electron-positron-ion plasma.
However, the second solution does not exhibit any phase mixing due to the
absence of ponderomotive forces but results in an undistorted nonlinear
traveling wave. These investigations have relevance for
laboratory/astrophysical multi-species plasma.
