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Vendredi 17 octobre 2014 à 14h00,
Salle Nevill Mott, D420

Orateur : Boris BRUN
"Electron interactions in mesoscopic physics : scanning gate microscopy and interferometry at a quantum point contact"

Abstract

In this thesis, we studied the effect of electron electron interactions in quantum point contacts (QP Cs). Quantum point contacts are small quasi-one dimensional channels, designed on a high mobility two-dimensional electron gas (2DEG). A negative voltage applied on a pair of metallic split gates above the sample surface allows to open or close the QPC. As a QPC opens, more and more electronic modes are allowed to cross the QPC, and its conductance increases by discrete steps, separated by a conductance quantum. This can be understood from a single-particle picture in one-dimensional transport, as each transverse mode carries a conductance quantum. But from their first realization 25 years ago, quantum point contacts have shown deviations from this picture, attributed to electron electron interactions. The most well known are a shoulder below the first plateau, around 0.7*2e^2/h, called the "0.7 anomaly", and a peak in the differential conductance that arises at low temperature : the zero bias anomaly (ZBA). The tool we used to study these interaction effects is a scanning gate microscope (SGM). It consists in perturbing the device’s conductance with the polarized tip of an atomic force microscope (AFM), and record the changes in conductance as a function of the tip position. By performing this technique at very low temperature, we showed that we can modulate the conductance anomalies of QPCs. We interpret our result as the signature of a small electrons crystal forming spontaneously at low density in the QPC due to the Coulomb repulsion : a Wigner crystal. We can modify the number of crystallized electrons by approaching the tip, and obtain signatures of the parity of the localized electrons number in transport features. Depending on this parity, the Wigner crystal has a different spin state, and screening of this spin by the surrounding electrons through the so-called Kondo effect leads alternatively to a single peak or a split zero bias anomaly. This discovery brings a significant advance in this field, that has attracted research efforts of many important groups in the world over the past 15 years. We then performed interferometric measurements thanks to the scanning gate microscope by creating in-situ interferometers in the 2DEG. We obtained signatures of a universal phase shift accumulated by the electrons at the crossing of a Kondo singlet. We attribute this effect to the Kondo nature of the zero bias anomaly, reinforcing its debated link with Kondo physics. Finally, we adapted the SGM technique to the study of thermoelectric transport in QPCs, and imaged interferences of electrons driven by a temperature difference.

Contact : boris.brun@neel.cnrs.fr

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