The quantum spin Hall (QSH) state is a state of matter characterized by a non-trivial topology of its band structure, and associated conducting edge channels. The QSH state was predicted and experimentally demonstrated to be realized in HgTe quantum wells. The existence of the edge channels has been inferred from local and non-local transport measurements in sufficiently small devices. Here we directly confirm the existence of the edge channels by imaging the magnetic fields produced by current flowing in large Hall bars made from HgTe quantum wells. These images distinguish between current that passes through each edge and the bulk. On tuning the bulk conductivity by gating or raising the temperature, we observe a regime in which the edge channels clearly coexist with the conducting bulk, providing input to the question of how ballistic transport may be limited in the edge channels. Our results represent a versatile method for characterization of new QSH materials systems.
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We thank S. C. Zhang, X. L. Qi and M. R. Calvo for valuable discussions, J. A. Bert and H. Noad for assistance with the experiment, G. Stewart for rendering Fig. 1a and M. E. Huber for assistance in SQUID design and fabrication. This work was financially supported by the Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under contract DE-AC02-76SF00515 (sample fabrication and scanning SQUID imaging of the QSH state in HgTe Hall bars), by the DARPA Meso project under grant no. N66001-11-1-4105 (MBE growth of the HgTe heterostructures) and by the Center for Probing the Nanoscale, an NSF NSEC, supported under grant no. PHY-0830228 (development of the scanning SQUID technique). The work at Würzburg was also supported by the German research foundation DFG (SPP 1285 Halbleiter Spintronik and DFG-JST joint research program Topological Electronics) and by the EU through the ERC-AG program (project 3-TOP). B.K. acknowledges support from FENA.