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Originally published in Science Express on 20 September 2007
Science 2 November 2007:
Vol. 318. no. 5851, pp. 766 - 770
DOI: 10.1126/science.1148047
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Research Articles

Quantum Spin Hall Insulator State in HgTe Quantum Wells

Markus König,1 Steffen Wiedmann,1 Christoph Brüne,1 Andreas Roth,1 Hartmut Buhmann,1 Laurens W. Molenkamp,1* Xiao-Liang Qi,2 Shou-Cheng Zhang2

Recent theory predicted that the quantum spin Hall effect, a fundamentally new quantum state of matter that exists at zero external magnetic field, may be realized in HgTe/(Hg,Cd)Te quantum wells. We fabricated such sample structures with low density and high mobility in which we could tune, through an external gate voltage, the carrier conduction from n-type to p-type, passing through an insulating regime. For thin quantum wells with well width d < 6.3 nanometers, the insulating regime showed the conventional behavior of vanishingly small conductance at low temperature. However, for thicker quantum wells (d > 6.3 nanometers), the nominally insulating regime showed a plateau of residual conductance close to 2e2/h, where e is the electron charge and h is Planck's constant. The residual conductance was independent of the sample width, indicating that it is caused by edge states. Furthermore, the residual conductance was destroyed by a small external magnetic field. The quantum phase transition at the critical thickness, d = 6.3 nanometers, was also independently determined from the magnetic field–induced insulator-to-metal transition. These observations provide experimental evidence of the quantum spin Hall effect.

1 Physikalisches Institut (EP III), Universität Würzburg, D-97074 Würzburg, Germany.
2 Department of Physics, McCullough Building, Stanford University, Stanford, CA 94305–4045, USA.

* To whom correspondence should be addressed. E-mail: molenkmp{at}physik.uni-wuerzburg.de

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