  It is shown that the thermodynamic instability at the half filling of Ll
leads to the vortex lattice formation with the electronic spectrum analogous to
that of graphene with two Dirac Fermi points on Brillouin cell boundary. This
result is used for the explanation of the observed current generated by SAW in
the heterostructure on the surface of piezoelectric GaAs. Using the existence
of two Fermi points instead Fermi surface suggested in the previous theoretical
works, permit the explanation of the experimental results.
