Experimental observation of the quantum spin Hall state in HgTe quantum wells

Spin-orbit interaction in semiconductors causes many interesting and potentially useful transport effects, such as e.g. the presently very topical spin-Hall effect[1]. So far no direct evidence for a ballistic, intrinsic SHE (i.e. resulting from the band structure) has been obtained by transport experiments. Here, we demonstrate that in specially designed nanostructures[2], which are based on narrow gap HgTe type-III quantum wells, a detection of the spin signal is possible via non-local voltage measurements. Recently, it was pointed out that such HgTe quantum wells, that exhibit an inverted band structure where the ordering of electron- and hole-like states is interchanged, are topologically non-trivial insulators[3], in which the quantum spin Hall insulator state[4] should occur. In this novel quantum state of matter, a pair of spin polarized helical edge channels develops when the bulk of the material is insulating, leading to a quantized conductance. I will present transport data provide very direct evidence for the existence of this third quantum Hall effect: when the bulk of the material is insulating, we observe a quantized electric conductance[5]. Finally, we demonstrate how a combination of the techniques used in the above experiments allows us to verify that the transport in the quantum spin Hall insulator state indeed occurs through spin-polarized helical edge channels. \\[4pt] [1] S. Murakami et al., Science 301 (2003) 1348; J. Sinova et al., Phys. Rev. Lett. 92 (2004) 126603; Y. Kato et al., Science 306 (2004) 1910. \\[0pt] [2] E.M. Hankiewicz, et al., Phys. Rev. B 70 (2004) 241301(R).\\[0pt] [3] B.A. Bernevig et al., Science 314 (2006) 1757. \\[0pt] [4] C.L. Kane and E.J. Mele, Phys. Rev. Lett. 95 (2005) 146802. \\[0pt] [5] M. K\"{o}nig et al., Science 318, 766 (2007).