Imaging Coherent Electron Flow from a Quantum Point Contact

Abstract

Scanning a charged tip above the two-dimensional electron gas inside a gallium arsenide/aluminum gallium arsenide nanostructure allows the coherent electron flow from the lowest quantized modes of a quantum point contact at liquid helium temperatures to be imaged. As the width of the quantum point contact is increased, its electrical conductance increases in quantized steps of 2 e(2)/h, where e is the electron charge and h is Planck's constant. The angular dependence of the electron flow on each step agrees with theory, and fringes separated by half the electron wavelength are observed. Placing the tip so that it interrupts the flow from particular modes of the quantum point contact causes a reduction in the conductance of those particular conduction channels below 2 e(2)/h without affecting other channels.

Imaging magnetic focusing of coherent electron waves

“…Images of flow were obtained by displaying the QPC conductance as the tip was scanned in a plane above the sample. 10,11 However, this backscattering imaging technique cannot be used in a perpendicular magnetic field, because the electrons follow curved paths and no longer reverse. Magnetic fields are important for spintronics and quantum information processing, [19][20][21] so it is important to understand electron flow with a field present.…”

“…When the tip backscatters by θ = π , the fringe spacing is λ F /2 as has been seen in previous experiments. 10,11 When using a weak tip withη < 1, the maximum possible scattering angle is θ < π , because it cannot backscatter. This implies that the minimum fringe spacing is Closeup surface plots in the yellow rectangles of (d,f) that show the regularity and consistency of the quantum fringe structure.…”

“…[5][6][7][8][9][10][11][12][13][14][15][16][17] Pathways for electron waves emerging from a quantum point contact (QPC) were revealed 10 and found to have a dramatic branched form. 11 The branching was shown to be the real-space consequence of small-angle scattering by charged donor atoms -the well known mechanism that limits the electron mobility.…”

Tuning the electron transport at single donors in zinc oxide with a scanning tunnelling microscope

“…Being a two-terminal device STM lacks a gate electrode for tuning the energy levels of nanostructures. To avoid this limitation, lithography has been used to fabricate a back gate 15,16 or source and drain contacts with the STM tip acting as a moveable gate [17][18][19] .…”

Unexpected features of branched flow through high-mobility two-dimensional electron gases

“…We also observe two unanticipated phenomena in high-mobility samples. In our highest-mobility sample we observe an almost complete absence of sharp impurity or defect scattering, indicated by the complete suppression of quantum coherent interference fringes.Also, branched flow through the chaotic potential of a high-mobility sample remains stable to significant changes to the initial conditions of injected electrons.Scanning gate microscopy (SGM) images of electron flow in two-dimensional electron gases (2DEGs) [8][9][10][11][12][13][14][15][16][17][18] provide direct spatial information not available in conventional electrical transport measurements. Our SGM studies show how varying disorder affects electron flow, and enable us to infer information about the disorder potential in our different samples.…”