Thermoelectric Scanning-Gate Interferometry on a Quantum Point Contact

Abstract

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 thermoelectric scanning gate microscopy (TSGM) 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 perform interferometry of the thermoelectric transport in a quantum point contact (QPC). We observe an interference pattern both in SGM and TSGM images, and evidence large differences between the two signals in the low density regime of the QPC. In particular, a large phase jump appears in the interference fringes recorded by TSGM, which is not visible in SGM. We discuss this difference of sensitivity using a microscopic model of the experiment, based on the contribution from a resonant level inside or close to the QPC. This work demonstrates that combining scanning gate microscopy with thermoelectric measurements offers new information as compared to SGM, and provides a direct access to the derivative of the device transmission with respect to energy, both in amplitude and in phase.

Extrinsic thermoelectric response of coherent conductors

“…[74]. Scanning gate microscopes [87,89,90] were also recently shown to induce interference fringes in the local thermopower of a quantum point contact [91]. Phase-dependent thermopower oscillations were as well reported in hybrid normal-superconducting interferometers [92,93], albeit of very different nature [94,95].…”

Readout of quantum screening effects using a time-dependent probe

“…We use this to put forward a proposal for the direct readout of -until now elusive -quantum screening effects, from tunable modifications of linearresponse coefficients. We expect presently available ex-perimental techniques [33][34][35][36] to allow for the proposed readout of quantum screening effects. For the same parameters as in Fig.…”

Scanning probe-induced thermoelectrics in a quantum point contact

“…QPCs are prototypical mesoscopic systems, whose twoterminal thermoelectric response was the subject of numerous theoretical [32][33][34][35][36][37][38][39][40] and experimental [40][41][42][43][44][45] works. In particular, the local thermopower of a QPC was recently 44 imaged by scanning gate microscopy (SGM) at low temperature (25 mK).…”