Electron-electron interactions in quantum point contacts

Scanning gate investigation of the conductance anomalies

Staff : Hermann Sellier, Serge Huant (team NOF)
PhD : Boris Brun (2011-2014) now postdoc in Belgium

Context :
We investigate the effect of electron-electron interactions in Quantum Point Contacts (QPC) which give rise to conductance anomalies below the quantized plateau at 2e²/h, called "0.7 anomaly" and "zero-bias anomaly". These anomalies result from a complex physics, involving Coulomb interactions, electron spins, screening, and possibly disorder. Despite intensive research on this topic, the microscopic origin of these effects remains highly debated.

Objectives :
The objective of this research project is to bring new information to this open problem Our approach is based on the Scanning Gate Microscopy (SGM) technique which allows us to manipulate in-situ the potential landscape in the QPC region and probe the properties of the conductance anomalies in an orginal way.

Scanning Gate Microscopy :
SGM experiments have been carried out at very low temperature (50 mK) in collaboration with UCL/IMCN in Belgium. We observe a repetitive splitting of the zero-bias anomaly when the tip approaches the QPC and tune the length of the quasi-one-dimensional channel. We interpret this result as a signature for the formation of a chain of localized electrons within the constriction, which is a sort of 1D Wigner crystal [1].

Scanning Gate Interferometry :
SGM-based interferometric experiments have been done by moving the tip along an electron path coming out of the QPC. The interference pattern reveals a phase shift of the electrons in the bias range of the zero-bias anomaly. We interpret this shift as a signature of localized states in the channel with a possible Kondo effect [2].

Thermoelectric Scanning Gate Microscopy :
We introduce a new technique derived from SGM which relies on the measurement of the thermoelectric voltage across the QPC instead of the conductance. This technique, called Thermoelectric Scanning Gate Microscopy (TSGM), is very sensitive to phenomena appearing at very low conductance. Scanning the tip in front of the QPC shows the same interference pattern in the thermoelectric signal as in the conductance, but an additional phase shift which is only visible by TSGM [3].

Interferometers :
Standard transport measurements have also been done on devices with a narrow gate coming close to the QPC and forming a fixed scattering center (no SGM). These scattering experiments at very low temperature have been conducted in collaboration with CEA/INAC and reveal a pattern of interference fringes, studied as a function of temperature, magnetic field, source-drain bias, and split-gate asymmetry. A phase shift of the interference appear in the voltage range of the conductance anomaly, similarly as in the SGM experiments.

Publications :

[1] Wigner and Kondo physics in Quantum Point Contacts revealed by Scanning Gate Microscopy, B. Brun, F. Martins, S. Faniel, B. Hackens, G. Bachelier, A. Cavanna, C. Ulysse, A. Ouerghi, U. Gennser, D. Mailly, S. Huant, V. Bayot, M. Sanquer, and H. Sellier, Nature Communications 5, 4290 (2014)

[2] Electron Phase Shift at the Zero-Bias Anomaly of Quantum Point Contacts, B. Brun, F. Martins, S. Faniel, B. Hackens, A. Cavanna, C. Ulysse, A. Ouerghi, U. Gennser, D. Mailly, P. Simon, S. Huant, V. Bayot, M. Sanquer, and H. Sellier, Phys. Rev. Lett. 116, 136801 (2016)

Interference features in scanning gate conductance maps of quantum point contacts with disorder, K. Kolasinski, B. Szafran, B. Brun, and H. Sellier, Phys. Rev. B 94, 075301 (2016)

[3] Thermoelectric scanning-gate interferometry on a quantum point contact, B. Brun, F. Martins, S. Faniel, A. Cavanna, C. Ulysse, A. Ouerghi, U. Gennser, D. Mailly, P. Simon, S. Huant, M. Sanquer, H. Sellier, V. Bayot, and B. Hackens, Phys. Rev. Applied 11, 034069 (2019)

Collaborations :

- Benoit Hackens, Vincent Bayot (IMCN, Louvain, Belgium)
- Ulf Gennser, Dominique Mailly (C2N, Marcoussis, France)
- Marc Sanquer (INAC, Grenoble, France)
- Dietmar Weinmann, Rodolfo Jalabert (IPCMS, Strasbourg, France)
- Bartlomiej Szafran (AGH, Krakow, Poland)
- Jean-Louis Pichard (SPEC, Saclay, France) Jean-Louis passed away on May 30, 2017. We dedicate this work to him.

Fundings :

- ANR ITEM-EXP (2011-2015) Interaction and transport at the mesoscopic scale

Former SGM studies of semiconductor nanostructures :

- Formation of quantum dots in the potential fluctuations of 2DEGs
- Mesoscopic analog of the Braess paradox in a congested network
- Aharonov-Bohm effect and local density of state in quantum rings

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