时 间: 2011年6月28日（周二）下午3:00

地 点: 清华大学高等研究院 科学馆322报告厅

题 目: Novel orbital physics in the $p$-band

报 告 人: Congjun Wu, Assistant Professor

University of California, San Diego

报告摘要: Orbitals play important roles in magnetism and superconductivity in transition metal oxides. In this talk, we will show that the rapid progress of cold atom physics has opened up an opportunity to study novel features of orbital physics, which do not appear usual solid state systems. For bosons in high orbital bands, they exhibit unconventional BECs whose condensate wavefunctions are complex-valued, thus are beyond the "no-node" paradigm of conventional BECs. For fermions, the p$_{x;y}$-orbital system of the honeycomb lattice exhibits amazingly rich and fundamentally different behavior from that in the $p_z$-orbital system of graphene. Its flat band structure dramatically amplifies interaction effects, providing a natural way to study non-perturbative strong correlation phenomena such as Wigner crystallization, and ferromagnetism. Furthermore, in the Mott-insulating states, the orbital degree of freedom enables superexchange interactions as spin does. We will show how spatial anisotropy generates frustration in such systems, which leads to a promising way to the exciting orbital liquid states. At last, we will present that a topological insulating phase occurs in the presence of the lattice rotation, as an orbital analogy of the quantum anomalous Hall effect of electron systems.