Scanning gate thermoelectric microscopy of interaction effects in quantum point contacts

We introduce a new scanning probe technique derived from scanning gate microscopy (SGM) in order to investigate thermoelectric transport in two-dimensional semiconductor devices. The scanning gate thermoelectric microscopy (SGTM) consists in measuring the thermoelectric voltage induced by a temperature difference across a device, while scanning a polarized tip that locally changes the potential landscape. We apply this technique to investigate electron-interaction effects in a quantum point contact (QPC). Remarkably large differences between SGM and SGTM signals arise in the low density regime of the QPC, where the electron interactions are strong and the Mott's law relating the thermopower to the conductance fails. In particular, a large phase jump appears in the interference fringes recorded by SGTM, which is not visible in SGM. This observation could help to elucidate the origin of the puzzling conductance anomalies in QPCs.