  We analyze the quasinormal modes of the D2-D8' model of 2+1-dimensional,
strongly-coupled, charged fermions in a background magnetic field and at
nonzero density. The model is known to include a quantum Hall phase with
integer filling fraction. As expected, we find a hydrodynamical diffusion mode
at small momentum and the nonzero-temperature holographic zero sound, which
becomes massive above a critical magnetic field. We confirm the
previously-known thermodynamic instability. In addition, we discover an
instability at low temperature, large mass, and in a charge density and
magnetic field range near the quantum Hall phase to an inhomogeneous striped
phase.
