Alexey Kozikov, ETH Zurich

Interference of electrons in backscattering through nanostructures in a GaAs heterostructure

co-authors: C. Rössler, T. Ihn, K. Ensslin, C. Reichl, and W. Wegscheider

Solid State Physics Laboratory, ETH Zurich

Scanning gate microscopy (SGM) is a scanning probe technique, in which the conducting tip of an atomic force microscope acts as a movable gate changing the electron density beneath it. It allows us to investigate the quantum mechanical wave nature of electrons with spatial resolution.
SGM is used to study local electron transport in an ultra high-mobility electron gas formed in a GaAs/AlGaAs heterostructure. By applying a negative voltage to metal gates deposited on top of the GaAs surface one can electrostatically define nanostructures: quantum point contacts (QPC) and a ballistic stadium.
A negatively biased tip of an atomic force microscope depletes electron gas beneath it and acts as a backscatterer. Electron waves leaving a QPC are scattered off the tip-induced potential back to the constriction. If the tip is placed in the way of the flow from a QPC mode, the transmission of this mode decreases. This is seen as a change in the conductance across the sample, which is measured as a function of tip position.
SGM experiments are performed at a base temperature of 300 mK using a home-built atomic force microscope in a He-3 system [1]. We observe branches caused by electron backscattering decorated by interference fringes similar to previous observations by Topinka et al. [2].
We study the behavior of the branches at different experimental conditions, such as the gate and tip voltage, tip-surface distance, magnetic field, temperature and source-drain bias. The tip bias and tip-surface dependences are need to determined optimal conditions for observing the branches and interference fringes. We observe that the most dramatic changes of the branching pattern occur at the low energy side of the QPC conductance plateaus. By laterally shifting the QPC we show that the branches are fixed in space and related to the disorder potential landscape in the two-dimensional electron gas. The importance of backscattering is confirmed in the low magnetic field dependence of the branching behavior.
We extend our studies to a stadium defined by two QPCs. Its number of open modes can be tuned by changing appropriate gate voltages. The measurements are performed at different QPC transmissions, stadium sizes and at high magnetic fields.

References:
[1] T. Ihn “Electronic Quantum Transport in Mesoscopic Semiconductor Structures”, Springer Tracts in Mod. Phys. 192, (2004)
[2] M. A. Topinka et al. Science 289, 2323 (2000).