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Published Online September 20, 2007
Science DOI: 10.1126/science.1148047

Research Articles

Submitted on June 19, 2007
Accepted on September 10, 2007

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 Zhang 2

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.
Laurens W. Molenkamp , E-mail: molenkmp{at}physik.uni-wuerzburg.de

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 2e2/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.


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