Interaction and Transport at a Mesoscopic Scale - Experiments – ITEM-Exp

Electron interactions are probed with the addition of another gate at the exit of the quantum point contact in order to induce electron interferences and modify the electron density in the contact. We expect a modulation of the conductance anomalies in the electrical transport measurements. This method is also applied with an original experimental technique using the mobile gate electrode of a scanning probe microscope.

An unexpected result has been obtained by scanning gate microscopy. We have revealed the presence of a localized state in the contact with a different behavior if the electron number has an odd or even parity. This represents an important result since this localized state called Wigner crystal corresponds to an old theoretical prediction but this state was not evidenced yet experimentally.

We observed an anomalous behavior of the interferences when the quantum point contact shows the anomalies induced by the interactions. This is a first step towards the final objective of the project.

Our results have been presented to several international conferences and in an article available on the website arxiv.org/abs/1307.8328

This project investigates experimentally the physics of low-density electron systems where strongly correlated states are expected at low temperature. We focus on Quantum Point Contacts (QPC), at conductance near the quantum value 2e²/h and below, where electron-electron interactions give rise to a specific phenomenon called “0.7 anomaly”. This anomaly in the conductance quantization below the first plateau results from a complex physics, involving Coulomb interactions, electron spins, impurities, disorder. Several groups already studied this anomaly, but always with the same approach based on standard transport measurements, and the microscopic origin of this phenomenon remains elusive. The ambition of this project is to bring a significant contribution in this field by exploring a new experimental approach based on a combination of a QPC and a local scattering site coupled coherently to the QPC. This scattering center reflects back the electrons coming out of the QPC into itself, affects the electron density in the QPC through a “non-local” action, and thus changes the amount of interactions in the QPC. It is a new way of testing the properties of the correlated state formed at low density in the QPC, without changing capacitively the size of the constriction.

This project will use the unique possibility to move the scattering site by moving the sharp tip of a Scanning Gate Microscope that creates a local depletion in the electron system. These experiments will be conducted down to very low temperature and up to high in-plane magnetic fields to investigate the spin contributions. Similar scattering experiments will be conducted by standard transport measurements in the very quiet environment of a dilution fridge dedicated to very sensitive mesoscopic devices. For this purpose, the project plans to study devices with a QPC and a fixed scattering center induced by a narrow gate coming close to the QPC, and also devices with a QPC and a small quantum ring in order to control the scattering with a perpendicular magnetic field, decoupled from the capacitive control of the QPC confinement. The project involves three partners : NEEL, INAC, LPN. All are experts in the field of low temperature mesoscopic physics, with complementary competences that give a good chance to make these new experimental approaches successful.

The expected results include the determination of the interaction strength at low electron density in a QPC and the microscopic origin of the 0.7 anomaly observed below the first conductance plateau. We should be able to discriminate between the theoretical models proposed to describe this electron state at low density, including a spin polarisation, a Wigner crystal, or a Kondo like state. More generally, this project will provide a better understanding of the QPC properties, which are building blocks of more complex electronic circuits such as electron interferometers in the quantum Hall regime.