Nature Commun. 5, 4290 (2014)

A quantum point contact is a constriction in a two-dimensional (2D) electron gas formed by electrostatic gating. Applying a negative voltage to a pair of split gates depletes the electron gas underneath, resulting in the confinement of the carriers to a (quasi) 1D channel. The conductance of the channel is quantized in multiple values of the quantum G0 = 2e2/h, where e is the electron charge and h is the Planck constant. However, there are anomalies in the transport characteristics whose origin is still debated. A feature is observed at a conductance of 0.7G0, and a zero-bias peak is observed in the differential conductance. Contrasting models have been put forward to explain these anomalies. Now, using scanning gate microscopy, Hermann Sellier and colleagues have shown that the presence of localized charges in the channel can explain both of the anomalies.

The researchers — who are based at the University Grenoble Alpes, the Institute Néel in Grenoble and other institutions in France — use the tip of the microscope to locally alter the electrostatic potential of the point contact and study its transport features, at a temperature of 20 mK. As they vary the tip–sample distance, they observe the oscillatory appearance of the 0.7 anomaly and at the same time a splitting of the zero-bias peak. They explain these results with the formation of a chain of localized electrons, called a 1D Wigner crystal, which forms as a result of the Coulomb interaction between electrons when their density is sufficiently low. The odd or even number of electrons in the chain gives rise to two different types of Kondo effect, whose signatures are found in the scanning gate microscopy measurements.