Spatiotemporal Measurements of the Kondo Cloud

Stanford University, Stanford CA

Investigators

Abstract

Non-Technical Abstract Materials at the focus of modern physics research often feature competition among several ways electrons can organize. This is both a blessing and a curse, opening rich possibilities but also hindering basic understanding of these materials. What if one could build an experimental system which displays the key features believed to drive behavior of a material of interest, while discarding other effects which might obscure the fundamental mechanism? Modern technology such as transistors and lasers depends on the fact that adding even a small amount of impurities to a material can drastically alter its properties. Another example of such sensitivity is the Kondo effect: a few parts per million of magnetic impurities added to a metal drastically change its low-temperature electrical behavior. Such an impurity creates a "cloud" of electrons around it, which shields the magnetic moment of the impurity. This project aims to nanofabricate a device to measure the spatial extent of this cloud, and to study the competition between impurity-cloud and impurity-impurity interactions when two such impurities are brought together. This will shed light on a class of materials known as heavy fermion metals, in which exactly this competition is suspected to be key, by realizing the competition in a simplified and highly-controllable system. In addition to the research component, both the principal investigator and the students have a track record and plans for active involvement in outreach regarding nanoscience, including short programs for middle school teachers, and involving high school and college students in research. Technical Abstract This project aims to determine the spatial extent of the Kondo screening cloud and study the Kondo-RKKY competition using a carefully-engineered model system based on gate-defined quantum dots in a GaAs/AlGaAs heterostructure. While the Kondo effect has been seen and studied in many different systems, a clear measurement of its spatial extent is still lacking. In the proposed device, the competition between the inter-impurity interaction and the impurity-bath interaction, suspected to be key to the behavior of several heavy-fermion compounds, is gate-tunable. Realizing the effect in a much more controllable nanostructure may elucidate the rich behavior occurring in those materials, including a quantum phase transition which could play a key role. As a complement to precision transport studies, a scanning gate experiment, where a metallic tip is scanned over the surface of a nanopatterned bilayer heterostructure, can perturb the Kondo cloud at a controlled location, providing a direct mapping of the spatial extent of a quantum many-body state.