Tiny impurities cause random fluctuations inside a two-dimensional electron gas.
At the interface of two materials, under the right conditions an effectively 2-dimensional gas of electrons can form. As the probe moved over the sample, charges flowed onto or off of the bubble depending on the electric potential at that location. In response, the charge on the probe also varied, and by measuring this the researchers were able to obtain a map of the electric potential inside the gas. The photograph shows a contour map of this potential. The region shown is about 2 by 4 ten-thousandths of a centimeter. .