Dear colleagues, this week we are happy to have Congjun Wu from UCSD at our ITAMP's weekly lunch seminar. Kind regards, Richard Schmidt and Swati Singh ITAMP Topical Lunch Discussion Date: Friday, November 21th Time: 12:00-1:30 pm Pizza will be served. Location: B-106 @ Center for Astrophysics (60 Garden Street) Directions: after entering the lobby of the CfA, turn right to enter the hallway of the B building. In the hallway, turn right again, and B-106 is there. Speaker: Congjun Wu, University of California, San Diego Title: Topological and strong correlation physics in the px/py-orbital bands of the honeycomb lattice – from solid states to optical lattices Abstract: Different from graphene which is orbitally inactive, the $p_x/p_y$-orbital bands in the 2D honeycomb lattice are orbitally active, which apply to both optical lattices and several classes of solid state systems including organic materials, fluoridated tin film, BiX/SbX(X=H, F, Cl, Br). 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 topological gaps. The gap magnitudes are equal to the spin-orbit coupling strength at the atomic level, and thus are much larger than those based on the s-p band inversion. The energy spectra and eigen-wavefunctions are solved analytically based on the 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 which is similar to but different from the Kitaev model. An f-wave Cooper pairing arises if the band is filled with spinless fermions exhibiting boundary zero energy Andreev modes. Although the pairing mechanism is conventional, the unconventional pairing symmetry is driven by the non-trivial band structure. Reference: 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). --------------------------- Dr. Richard Schmidt Institute for Theoretical Atomic, Molecular, and Optical Physics (ITAMP) Harvard-Smithsonian Center for Astrophysics MS-14 60 Garden St. Cambridge, MA 02138 U.S.A. richard.schmidt(a)cfa.harvard.edu Tel. +1 (617) 496-7610 Fax +1 (617) 496-7668