#### 物理学院“博约学术论坛”系列报告第48期

报告人：吴从军 教授(美国加州大学)

时 间：2014年8月21日（周四）上午10点；

ABSTRACT

Different from graphene which is usually orbitally inactive, the p$_{x,y}$-orbital band in the 2D honeycomb lattice is orbitally active which exhibits a variety of novel topological and strong correlation physics. The interplay between the orbital structure and spin-orbit coupling gives rise to the 2D quantum spin Hall state and quantum anomalous Hall state with large values of topological gaps. The gap values are equal to the atomic level spin-orbit coupling strength, and thus are much larger than those based on the mechanism of the

*s-p*band inversion. The energy spectra and eigen-wavefunctions are solved analytically based on Clifford algebra, which greatly facilitates the topological analysis. Flat bands also naturally arise and the consequential non-perturbative physics includes Wigner crystallization and ferromagnetism. In the Mott-insulating state, orbital exchange is highly frustrated described by a quantum 120$^/circ$ model in a similar form to the celebrated Kitaev model. We will show that an f-wave Cooper pairing arises if the band is filled with spinless fermions exhibiting boundary Majorana modes. Although the pairing mechanism is conventional, the unconventional pairing symmetry is driven by the non-trivial band structure. We will also show that the above physics in optical lattices is closely connected to the recent progress in several classes of solid state materials including organic materials, fluoridated tin film, germanene. BiX/SbX(X=H, F, Cl, Br), etc.

报告人简历

Congjun Wu received his Ph.D. in physics from Stanford University in 2005, and did his postdoctoral research at the Kavli Institute for Theoretical Physics, University of California, Santa Barbara, from 2005 to 2007. He became an assistant professor in the Department of Physics at the University of California, San Diego (UCSD) in 2007, and an associate professor at UCSD in 2011. His research interests include quantum magnetism, superconductivity, orbital physics, and topological states in condensed-matter and cold-atom systems.

Selected References

1. Gu-Feng Zhang, Yi Li, Congjun Wu, ,The honeycomb lattice with multi-orbital structure: topological and quantum anomalous Hall insulators with large gaps ,Phys. Rev. B 90, 075114 (2014) .

2. Wei-cheng Lee, Congjun Wu, and S. Das Sarma, "$F$-wave pairing of cold atoms in optical lattices", Phys. Rev. A 82, 053611 (2010).

3. Congjun Wu, "Orbital analogue of quantum anomalous Hall effect in $p$-band systems", Phys. Rev. Lett. 101, 186807 (2008).

4. Congjun Wu, "Orbital orderings and frustrations of p-band systems in optical lattices", Phys. Rev. Lett. 100, 200406 (2008).

5. Congjun Wu, Doron Bergman, Leon Balents, and S. Das Sarma, "Flat bands and Wigner crystallization in the honeycomb optical lattice", Phys. Rev. Lett. 99, 70401 (2008).

邀请人：姚玉贵教授

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