Current projects:


The Quantum Spin Hall Effect


Conventional matter exists in three familiar forms. But under special circumstances, quantum theory predicts exotic states of matter, such as superconductivity, Bose-Einstein condensation and the quantum Hall effect. In this work, we theoretically predict a new state of matter in semiconductors, which displays the quantum spin Hall effect. The quantum Hall effect is one of the most profound phenomena in condensed matter physics, however, because of the high magnetic field required, the effect has so far found little practical applications. In the quantum spin Hall effect, proposed to be realized in the HgTe/CdTe quantum wells without any external magnetic field, the system has a finite energy gap in the bulk, but has gapless excitations at the edge of the sample. Such a system is topologically distinct from a trivial band insulator which does not have any robust edge states.


Theory of High Tc Superconductivity


Understanding the microscopic mechanism of high Tc superconductivity is a major problem in physics. In the past few years, we have developed a theory of high Tc superconductivity which unifies antiferromagnetism with superconducitivity. Some major predictions of the theory has recently been confirmed experimentally.


The Quantum Hall Effect


Quantized Hall effect is the manifestation of a new quantum state of matter, realized at the interface between semiconductors. We have developed a topological field theory to describe the novel properties of this quantum matter. A global phase diagram for the quantum Hall effect has been constructed based on this theory, with many experimental predictions which have been confirmed. More recently, we generalized this effect to higher dimensions. Collective excitations at the boundary of this quantum Hall liquid share many properties similar to relativistic particles in 3+1 dimensions.


Quantum spintronics


Conventional electronic devices are based on Ohm's law, discovered almost two centuries ago, which describes the inevitable dissipation that accompanies an electric current flow in response to an applied voltage. Scientists are currently investigating a new class of so-called spintronic devices, which use the spin, rather than the charge, of the electron to carry energy and information. We discovered the equivalent of a new "Ohm's law" for spintronics. Our theory predicts theoretically that spin current flows in response to the applied electric field; but in contrast to the conventional electric current, the quantum spin current can flow without any dissipation even at room temperature, in materials already widely used by the electronic industry.