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Quantum Spin Hall Effect and Quantum Anomalous Hall Effect in Semiconductors with Inverted Bandstructure

Speaker:Dr. Chaoxing Liu
Affiliation:Department of Physics and Center for Advanced Study, Tsinghua University, China
Date:November 12, 2008 (Wed)
Time:4:00-5:00 p.m.
Venue:Room 522, 5/F, Chong Yuet Ming Physics Building, HKU


The quantum spin Hall (QSH) state is a topologically nontrivial state of quantum matter which preserves time-reversal symmetry; it has an energy gap in the bulk, but topologically robust gapless states at the edge. Recently, this novel effect has been predicted and observed in HgTe/CdTe quantum wells. HgTe has the inverted band structure, which is quite different from the ordinary semiconductor and is shown to be quite crucial for the existence of QSH effect. However, the band inversion phenomena is not limited to several particular materials and it can also exist in some artificial hetero-structures. Here we show that type-II semiconductor quantum wells made from InAs/GaSb/AlSb is an example. In this quantum wells, although InAs and GaSb both are ordinary semiconductor, there still exists an 'inverted' phase due to the large band offset between these two materials, which is a QSH state when Fermi level lies inside the gap. Remarkably, the topological quantum phase transition between the conventional insulating state and the quantum spin Hall state can be continuously tuned by the gate voltage, enabling quantitative investigation of this novel phase transition. In the second part of this talk, I would like to talk about the relation between QSH effect and quantum anomalous Hall (QAH) effect. For the QAH effect, quantized Hall conductance can exist without any external magnetic field and also the associated Landau level. We show that such phenomena can be realized in HgMnTe quantum wells, which arises purely from the spin polarization of the Mn atoms. The quantized Hall conductance is predicted for a range of quantum well thickness and the concentration of the Mn atoms. This effect enables dissipationless charge current in spintronics devices.

Coffee and tea will be served 20 minutes prior to the seminar.