  We measure the quantum capacitance and probe thus directly the electronic
density of states of the high mobility, Dirac type of two-dimensional electron
system, which forms on the surface of strained HgTe. Here we show that observed
magneto-capacitance oscillations probe, in contrast to magnetotransport,
primarily the top surface. Capacitance measurements constitute thus a powerful
tool to probe only one topological surface and to reconstruct its Landau level
spectrum for different positions of the Fermi energy.
