Imaging coherent electron flow in a two-dimensional electron gas

Scanning probe microscopy (SPM) has been used to obtain images of electron flow through a two-dimensional electron gas from a quantum point contact (QPC) inside a GaAs/AlGaAs heterostructure at liquid He temperatures. A negatively charged SPM tip depletes the electron gas immediately below and decreases the conductance of the QPC by backscattering electrons. Images of electron flow are obtained by recording the conductance, as the tip is scanned across the structure. At distances less than 1 μm from the QPC, the electron flow shows angular lobes that are characteristic of the quantum modes of the QPC. At distances greater than 1 μm, well-defined branches of electron flow are observed that are caused by the cumulative effects of small angle scattering by ionized donor and impurity atoms. Interference fringes spaced by half the Fermi wavelength decorate all of the images of electron flow; their spacing gives a spatial profile of the electron density.