  The unusual properties of two-dimensional electron systems that give rise to
the quantum Hall effect have prompted the development of new microscopic models
for electrical conduction. The bulk properties of the quantum Hall effect have
also been studied experimentally using a variety of probes including transport,
photoluminescence, magnetization, and capacitance measurements. However, the
fact that two-dimensional electron systems typically exist some distance (about
100 nm) beneath the surface of the host semiconductor has presented an
important obstacle to more direct measurements of microscopic electronic
structure in the quantum Hall regime. Here we introduce a cryogenic
scanning-probe technique-- subsurface charge accumulation imaging-- that
permits very high resolution examination of systems of mobile electrons inside
materials. We use it to image directly the nanometer-scale electronic
structures that exist in the quantum Hall regime.
