Recent theory predicted that the Quantum Spin Hall Effect, a fundamentally novel quantum state of matter that exists at zero external magnetic field, may be realized in HgTe/(Hg,Cd)Te quantum wells. We have fabricated such sample structures with low density and high mobility in which we can tune, through an external gate voltage, the carrier conduction from n-type to the p-type, passing through an insulating regime. For thin quantum wells with well width d < 6.3 nm, the insulating regime shows the conventional behavior of vanishingly small conductance at low temperature. However, for thicker quantum wells (d > 6.3 nm), the nominally insulating regime shows a plateau of residual conductance close to 2e^2/h. The residual conductance is independent of the sample width, indicating that it is caused by edge states. Furthermore, the residual conductance is destroyed by a small external magnetic field. The quantum phase transition at the critical thickness, d = 6.3 nm, is also independently determined from the magnetic field induced insulator to metal transition. These observations provide experimental evidence of the quantum spin Hall effect.
@article{arxiv.0710.0582,
title = {Quantum Spin Hall Insulator State in HgTe Quantum Wells},
author = {Markus Koenig and Steffen Wiedmann and Christoph Bruene and Andreas Roth and Hartmut Buhmann and Laurens W. Molenkamp and Xiao-Liang Qi and Shou-Cheng Zhang},
journal= {arXiv preprint arXiv:0710.0582},
year = {2007}
}