Abstract
The discovery of the quantum spin Hall (QSH) state, and topological insulators in general, has sparked strong experimental efforts. Transport studies of the quantum spin Hall state have confirmed the presence of edge states, showed ballistic edge transport in micron-sized samples, and demonstrated the spin polarization of the helical edge states. While these experiments have confirmed the broad theoretical model, the properties of the QSH edge states have not yet been investigated on a local scale. Using scanning gate microscopy to perturb the QSH edge states on a submicron scale, we identify well-localized scattering sites which likely limit the expected nondissipative transport in the helical edge channels. In the micron-sized regions between the scattering sites, the edge states appear to propagate unperturbed, as expected for an ideal QSH system, and are found to be robust against weak induced potential fluctuations.
- Received 16 November 2012
DOI:https://doi.org/10.1103/PhysRevX.3.021003
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Published by the American Physical Society
Popular Summary
Recently, a novel class of materials called topological insulators (TIs) was predicted and experimentally discovered. A key characteristic of these materials is that current flow at the boundaries of the material—on the surface of a three-dimensional TI or the edge of a two-dimensional TI—should be protected against backscattering. This characteristic suggests that TIs may be an interesting platform for possible applications, such as low-resistance interconnects in computer chips. In real samples, deviations from this “perfect” behavior have been observed but not yet investigated on a local scale. In a study of the two-dimensional version of a TI, the quantum spin Hall (QSH) system, we find that current flow in the one-dimensional edge channels of the QSH state is affected by small puddles of electrons that lift the protection against backscattering.
Using a technique called scanning gate microscopy, we locally perturb the QSH edge states and monitor the effect on current flow through those edges. We identify individual well-localized sites that control the current flow in the one-dimensional edge states. These scattering sites occur with a separation of around and are present even in the absence of our external perturbation. Thus, they help us understand scattering observed in earlier devices, where ballistic transport occurred only on length scales up to a few microns. In the regions between the inherent scattering sites, the edge states are rather robust against perturbations.
Our experiments provide a first spatially resolved study of the QSH state. Similar future studies might provide a more complete picture of scattering mechanisms in TI systems.
