Yue Ma, Worasom Kundhikanjana, Jing Wang, M. Reyes Calvo, Biao Lian, Yongliang Yang, Keji Lai, Matthias Baenninger, Markus König, Christopher Ames, Christoph Brüne, Hartmut Buhmann, Philip Leubner, Qiaochu Tang, Kun Zhang, Xinxin Li, Laurens W. Molenkamp, Shou-Cheng Zhang, David Goldhaber-Gordon, Michael A. Kelly, Zhi-Xun Shen
To establish the central role of the helical edge states for the recently-discovered quantum spin Hall (QSH) system, direct imaging of those edge states is an important milestone. Employing a unique cryogenic microwave impedance microscope, we directly image QSH edges in a HgTe quantum well. The edge states emerge prominently when the Fermi level is tuned into the bulk gap, with the spatial width of edge conduction increasing monotonically across the bulk gap from the p-type side through the Dirac point into n-type. This monotonic evolution of edge width is counterintuitive at first glance but is in good agreement with the underlying particle-hole asymmetry of HgTe band structure. The observed dependence of edge state width on magnetic field is not expected from single-electron Landau level physics but may be understood by including band bending at the edge. Detailed agreement between theory and the imaging experiment supports the notion of HgTe being a model system for the QSH effect.
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http://arxiv.org/abs/1212.6441
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