Imaging currents in HgTe quantum wells in the quantum spin Hall regime
Counter-propagating spin-polarized edge channels at the sample boundaries are a key feature of the quantum spin Hall (QSH) state, which was predicted  and experimentally demonstrated  to exist in HgTe quantum wells. The existence of the edge channels has been inferred from transport measurements on sufficiently small devices, which find local  and non-local  conductance values close to the quantized values expected for ideal edge channels and recently signatures of the spin polarization . Devices with edges longer than several micrometer show significant deviations from the ideal behavior. The robustness of the edge channel in large devices and the interplay between the edge channels and a conducting bulk are experimentally largely unexplored and are difficult to assess via transport measurements. I report images of the transport current in large Hall bars by probing the magnetic field generated by the current using a scanning superconducting quantum interference device (SQUID).
We directly visualize that the current flows along the edge of the device in the QSH regime. We show that an identifiable edge channel exists even in the presence of considerable bulk conduction as the device is gated and its temperature is raised. Our results establish a versatile method for the characterization of new quantum spin Hall materials systems and confirm both the existence and the robustness of the predicted edge channels.
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