Topographic mapping of the quantum hall liquid using a few-electron bubble

A scanning probe technique was used to obtain a high-resolution map of the random electrostatic potential inside the quantum Hall liquid. A sharp meta l tip, scanned above a semiconductor surface, sensed charges in an embedded two-dimensional (2D) electron gas. Under quantum Hall effect conditions, a pplying a positive voltage to the tip locally enhanced the 2D electron dens ity and created a "bubble" of electrons in an otherwise unoccupied Landau L evel. As the rip scanned along the sample surface, the bubble followed unde rneath. The tip sensed the motions of single electrons entering or leaving the bubble in response to changes in the Local 2D electrostatic potential.