Congjun Wu
Department of Physics
University of California, San Diego

Abstract

Orbital Phases of Cold Atoms Unconventional BEC, ferromagnetism, and unconventional Cooper pairing

Orbital is a degree of freedom independent of charge and spin, which is characterized by spatial anisotropy and orbital degeneracy. It plays important roles in magnetism, superconductivity, and transport in transition metal oxides. Recently, cold atom optical lattices have provided a new opportunity to investigate orbital physics. In this talk, we will present many novel features of orbital physics that are not easily accessible in transition metal oxides as below.

Bosons, as recently demonstrated in experiments, can be pumped into high orbital bands and stay with a long life time. We will show that such meta-stable states of bosons exhibit a class of novel superfluid states with complex-valued wavefunctions spontaneously breaking time reversal symmetry. These states are beyond Feynman's celebrated "no-node" theory which applies to most of the well-known ground states of bosons. For fermions, we will focus on the p(x,y) orbital system of the honeycomb lattice, which exhibits fundamentally different behavior from that in the p(z) honeycomb system of graphene. The interesting physics here includes the flat band structure and the consequential non-perturbative strong correlation effects (e.g. Wigner crystal and ferromagnetism), the frustration in orbital exchange, and the orbital analogy of the quantum anomalous Hall effect. In particular, we will show that an f-wave Cooper pairing naturally arises as driven by non-trivial band structure but with conventional attractions. Since no strong correlation mechanism is involved, this theoretical analysis is under control.

Please come meet the speaker over coffee and refreshments from 3:45-4:10 pm in the foyer on floor G above the lecture hall.   All Welcome.  Host: Dr. Chuanwei Zhang