  We study the magnetoplasmon collective mode excitations of integer quantum
Hall systems in a parabolically confined quantum well nanostructure in the
presence of a tilted magnetic field by using the time-dependent Hartree-Fock
approximation. For even integer filling, we find that the dispersion of a spin
density mode has a magneto-roton minimum at finite wavevectors, at a few times
10^6 cm^{-1} for parallel fields of order 1-10 Tesla, only in the direction
perpendicular to the in-plane magnetic field, while the mode energy increases
monotonously with wavevector parallel to the in-plane magnetic field. When the
in-plane magnetic field is strong enough (well above 10 Tesla),we speculate
that this roton minimum may reach zero energy, suggesting a possible second
order phase transition to a state with broken translational and spin
symmetries. We discuss the possibility for observing such parallel
field-induced quantum phase transitions. We also derive an expression for the
dielectric function within the time-dependent Hartree-Fock approximation and
include screening effects in our magnetoplasmon calculation. We discuss several
exotic symmetry-broken phases that may be stable in finite parallel fields, and
propose that the transport anisotropy, observed recently in parallel field
experiments, may be due to the formation of a skyrmion stripe phase predicted
in our theory. Our predicted anisotropic finite wavevector suppression, perhaps
even a mode-softening leading to the quantum phase transition to the
anisotropic phase, in the collective spin excitation mode of the wide well
system in the direction transverse to the applied parallel magnetic field
should be directly experimentally observable via the inelastic light scattering
spectroscopy.
