  In this report, we study ultra-high-mobility two-dimensional (2D)
electron/hole systems with high precision capacitance measurement. We find that
the capacitance charge appears only at the fringe of the gate at high magnetic
field when the 2D conductivity decreases significantly. At integer quantum Hall
effects, the capacitance vanishes and forms a plateau at high temperatures
$T\gtrsim 300$ mK, which surprisingly, disappears at $T\lesssim 100$ mK. This
anomalous behavior is likely a manifestation that the dilute
particles/vacancies in the top-most Landau level form Wigner crystal, which has
finite compressibility and can host polarization current.
