Theory of scanning gate microscopy

TL;DR

This paper develops a comprehensive scattering theory for non-invasive scanning gate microscopy, clarifying what conductance measurements reveal about quantum structures and deriving explicit formulas for conductance changes.

Contribution

It introduces a systematic scattering approach to interpret conductance measurements in scanning gate microscopy, including explicit expressions for conductance corrections and their relation to local current density.

Findings

First order correction dominates at conductance steps.

Second order correction dominates on conductance plateaus.

Analytic expressions obtained for abrupt quantum point contacts.

Abstract

A systematic theory of the conductance measurements of non-invasive (weak probe) scanning gate microscopy is presented that provides an interpretation of what precisely is being measured. A scattering approach is used to derive explicit expressions for the first and second order conductance changes due to the perturbation by the tip potential in terms of the scattering states of the unperturbed structure. In the case of a quantum point contact, the first order correction dominates at the conductance steps and vanishes on the plateaus where the second order term dominates. Both corrections are non-local for a generic structure. Only in special cases, such as that of a centrally symmetric quantum point contact in the conductance quantization regime, can the second order correction be unambiguously related with the local current density. In the case of an abrupt quantum point contact we are able to obtain analytic expressions for the scattering eigenfunctions and thus evaluate the resulting conductance corrections.