Two layers of electrons or holes trapped at the adjacent interfaces of
a gallium arsenide heterostructure can interact through the Coulomb i
nteraction; this leads to a rich phase diagram of ground states, some
of which are inhomogeneous in density. The cause of this is associated
with each layer's acting as a polarisable background for the other, m
aking it much easier for inhomogeneous configurations to be stable. Ev
en in the uniform liquid phase the presence of a second layer can qual
itatively change the nature of the low lying excitation spectrum and l
ead to large many-body effects in the spectrum, even at very long wave
lengths.