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Interactions électroniques en physique mésoscopique, microscopie à effet de grille local et interférométrie sur un contact ponctuel quantique

Abstract : In this thesis, we studied the effect of electron electron interactions in quantum pointcontacts (QPCs). Quantum point contacts are small quasi-one dimensional channels,designed on a high mobility two-dimensional electron gas (2DEG). A negative voltageapplied on a pair of metallic split gates above the sample surface allows to open or closethe QPC. As a QPC opens, more and more electronic modes are allowed to cross theQPC, and its conductance increases by discrete steps, separated by a conductance quantum2e2/h. This can be understood from a single-particle picture in one-dimensionaltransport, as each transverse mode carries a conductance quantum.But from their first realization 25 years ago, quantum point contacts have shown deviationsfrom this picture, attributed to electron electron interactions. The most well knownare a shoulder below the first plateau, around 0.7×2e2/h, called the "0.7 anomaly", and apeak 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 by changing locally the device’s potential with the polarized tip of an atomicforce microscope (AFM), and record the changes in conductance as a function of the tipposition. By performing this technique at very low temperature, we showed that we canmodulate the conductance anomalies of QPCs. We interpret our result as the signatureof a small electrons crystal forming spontaneously at low density in the QPC due to theCoulomb repulsion: a Wigner crystal. We can modify the number of crystallized electronsby approaching the tip, and obtain signatures of the parity of the localized electrons numberin transport features. Depending on this parity, the Wigner crystal has a differentspin state, and screening of this spin by the surrounding electrons through the so-calledKondo effect leads alternatively to a single peak or a split ZBA. This discovery bringsa significant advance in this field, that has attracted research efforts of many importantgroups in the world over the past 15 years.We then performed interferometric measurements thanks to the scanning gate microscopeby creating in-situ interferometers in the 2DEG. We obtained signatures of an additionalphase shift accumulated by the electrons in the ZBA regime. We attribute this effect tothe universal phase shift that electrons accumulate when crossing a Kondo singlet, reinforcingthat the debated origin of the ZBA lies in Kondo physics.Finally, we adapted the SGM technique to the study of thermoelectric transport in QPCs,and for the first time imaged interferences of electrons driven by a temperature difference.
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Submitted on : Tuesday, March 31, 2015 - 10:12:06 AM
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  • HAL Id : tel-01137642, version 1

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Boris Brun. Interactions électroniques en physique mésoscopique, microscopie à effet de grille local et interférométrie sur un contact ponctuel quantique. Autre [cond-mat.other]. Université de Grenoble, 2014. Français. ⟨NNT : 2014GRENY049⟩. ⟨tel-01137642⟩

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