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QUARMA Report Summary

Project ID: 515239
Funded under: FP6-MOBILITY
Country: France

Final Activity Report Summary - QUARMA (Quantum ring mapping)

The fascinating dual nature of electrons, both waves and particles, is particularly apparent when they are confined in nanoscale structures where transport becomes coherent and ballistic. Traditionally, the understanding of coherent transport, i.e. when the interference pattern of electron waves governs the conductance, and the ballistic propagation of electrons bouncing around obstacles, relied on the measurement of macroscopic properties such as the magnetoconductance. While powerful when coupled to statistical theories, this approach cannot provide a detailed image of 'how electrons behave down there'.

Ideally, a complete understanding of the transport properties would rather require tracking each electron inside the low-dimensional system. Significant advances towards this goal were obtained by combining Scanning probe microscopy (SPM) with transport measurements. In our project, we use SPM to probe the coherent and ballistic electron behaviour inside a model system: an open quantum ring, laterally confined from a two-dimensional electron system (2DES), and exhibiting the Aharonov-Bohm interference effect. Conductance maps recorded while scanning the biased tip of a cryogenic 'Atomic force microscope' (AFM) above the quantum ring and its vicinity exhibit well developed patterns with two types of fringes. The first ones, concentric around the ring, are observed as the tip scans outside the ring area; they sign the presence of coherent transport: the electrostatic Aharonov-Bohm interference effect. The second type of fringes, observed inside the ring, is mostly radial. We show evidence that they originate from ballistic effects, as confirmed by quantum mechanical simulations, and are directly related to the electron probability density within the nano-device.

Our results demonstrate that SPM techniques combined with transport measurements are well suited to study nano-electronic devices whose working principle relies on new concepts such as real-space manipulation of electron interferences and/or ballistic semiclassical trajectories.


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