Laboratoire de Physique Théorique
Toulouse - UMR 5152

In addition to quantized conductance plateaus at multiples of 2e²/h, quantum point contacts in 2DEGs often show a shoulder at 0.7(2e²/h), which cannot be explained by single-particle theories. At very low temperature, this 0.7 anomaly shades off and a zero-bias anomaly emerges. Different theoretical models have been proposed but none is fully satisfactory. Here we present new experimental results using a scanning gate microscope to change in-situ the potential landscape with a sharp polarized tip. Approaching the tip towards the point contact produces an oscillatory splitting of the zero-bias anomaly correlated with large modulations of the 0.7 anomaly. These oscillations cannot be explained by interference effects, and are interpreted as the signature of a many-body localized state induced by strong Coulomb interactions at low electron density. This state is revealed by the Kondo screening from the leads, giving a conductance peak at zero (resp. finite) bias for an odd (resp. even) number of electrons that is tuned by the tip position. Our interpretation is supported by electrostatic simulations showing that the electron density in the contact meets the criterion for 1D Wigner crystallization below the first plateau. It points in the same direction as a recent experiment with six surface gates that gave indications for the presence of emergent localized states. Our result contributes to resolving a complex mesoscopic problem involving strongly interacting electrons in a non-uniform potential landscape. In a second experiment, we probe the transmission phase-shift of the QPC in the regime of zero-bias anomaly using the SGM tip to create a Fabry-Perot interferometer. A shift of the interference pattern by about half a period is observed in the voltage range of the zero-bias conductance peak. This shift might be related to the universal pi/2 phase-shift of electrons crossing a quantum dot in the Kondo regime, giving further support to the claim that a spontaneous charge localization occurs in QPCs at low density.