  Applying the separated spin evolution quantum hydrodynamics to
two-dimensional electron gas in plane samples and nanotubes located in external
magnetic fields we find new kind of wave in electron gas, which is called the
spin-electron acoustic wave. Separate spin-up electrons and spin-down electrons
evolution reveals in replacement of the Langmuir wave by the couple of hybrid
waves. One of two hybrid waves is the modified Langmuir wave. Another hybrid
wave is the spin-electron acoustic wave. We study dispersion of these waves in
two dimensional structures of electrons. We also consider dependence of
dispersion properties on spin polarisation of electrons in external magnetic
field.
