Electron Phase Shift at the Zero-Bias Anomaly of Quantum Point Contacts

Abstract

The Kondo effect is the many-body screening of a local spin by a cloud of electrons at very low temperature. It has been proposed as an explanation of the zero-bias anomaly in quantum point contacts where interactions drive a spontaneous charge localization. However, the Kondo origin of this anomaly remains under debate, and additional experimental evidence is necessary. Here we report on the first phase-sensitive measurement of the zero-bias anomaly in quantum point contacts using a scanning gate microscope to create an electronic interferometer. We observe an abrupt shift of the interference fringes by half a period in the bias range of the zero-bias anomaly, a behavior which cannot be reproduced by single-particle models. We instead relate it to the phase shift experienced by electrons scattering off a Kondo system. Our experiment therefore provides new evidence of this many-body effect in quantum point contacts. Quantum point contacts1,2 (QPCs) are small constrictions in high-mobility two-dimensional electron gases (2DEGs) controlled by a metallic split gate at the surface of a semiconductor heterostructure. Despite their apparent simplicity, they reveal complex many-body phenomena which defy our understanding. When these quasione-dimensional ballistic channels are sufficiently open, electrons are perfectly transmitted via each available transverse mode 3 , and the conductance is quantized in units of the conductance quantum 2e 2 /h. Below the first conductance plateau however, this single-particle picture fails due to the increasing importance of many-body effects. An additional shoulder shows up in the linear conductance curve around 0.7 × 2e 2 /h, called the 0.7 anomaly 4 , and a narrow peak of enhanced conductance appears around zero bias in the non-linear conductance curves at low enough temperature, called the zero-bias anomaly 7 (ZBA). The peak behavior versus temperature and magnetic field was shown to share strong similarities with the Kondo effect in quantum dots 6,7 (QDs), i.e. the many-body screening of a local spin by conduction electrons below a characteristic temperature 5,8,9 . However, deviations of the ZBA from the established Kondo effect have been reported [6][7][8]11 , and the occurrence of this effect in QPCs remains a debated issue [14][15][16]

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