The Quantum Hall Effect of Field Induced Spin Density Wave Phases is
accounted for within a weak coupling theory which assumes that in the relevant
low temperature part of the phase diagram the quasi one dimensional conductor
is well described by Fermi liquid theory. Recent experimental results show that
sign inversion of the Hall Plateaux takes place all the way down from the
instability line of the normal state. The Quantum Nesting model, when it takes
into account small perturbations away from perfect nesting, describes well not
only the usual sequence of Hall plateaux, but also the anomalies connected with
sign inversion of the Hall Effect. Experimental observation of de-doubling of
sub-phase to sub-phase transition lines suggests that superposition of SDW
order parameters occurs in some parts of the phase diagram. The collective
excitations of the Ultra Quantum Crystal have a specific magneto-roton
structure. The SDW case exhibits,apart from the usual spin waves, topological
excitations which are either skyrmions or half skyrmions. It is suggested that
magneto-rotons may have been observed some years ago in specific heat
experiments.Comment: 17 pages, latex, 6 figures, available on request at
[email protected]