Abstract: Orbital is a degree of freedom independent of charge and spin. It plays an important role in physical properties of transition-metal-oxides. The recent developments of cold atom systems in optical lattices have opened up an opportunity to study novel features of orbital physics that are not easily accessible in solid state systems. We predicted that cold bosons, when pumped into high orbital bands of optical lattices, exhibit a class of novel superfluid states spontaneously breaking time-reversal symmetry. In analogy to unconventional superconductivity, their complex-valued condensate wavefunctions possess unconventional symmetries beyond the scope of “no-node” theorem for most states of bosons. This class of unconventional Bose-Einstein condensations have been experimentally realized by a few prominent experimental groups. On the other hand, itinerant ferromagnetism (FM), i.e., FM based on Fermi surfaces instabilities of mobile electrons (fermions), is a hard-core problem of strong correlation physics. The well-known Stoner criterion overestimates the FM tendency by neglecting correlation effects. Furthermore, the paramagnetic metal phase above the Curie temperature, i.e., the Curie-Weiss metal state, is a long-standing challenge. It exhibits a dichotomic nature: The spin channel is incoherent, i.e., local moment-like, while the charge channel remains coherent. In spite of these difficulties, we proved a series of theorems setting up the ground state FM phase in the p-orbital bands. The Curie-Weiss metal phase and the critical scalings of the FM transitions are studied via the sign-problem free quantum Monte-Carlo simulations at high numerical precisions. These results also shed light on the mechanism of itinerant FM and Curie-Weise metal in solid state orbital systems.

Biography: 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 in 2007, an Associate Professor in 2011, and a Professor in 2017. In 2021, he became a Chair Professor at School of Science, Westlake University.  He was selected as a New Cornerstone Investigator in 2023, elected to a Fellow of American Physical Society in 2018, and awarded the Sloan Research fellowship in 2008. His research interests are exploring new states of matter and reveling their organizing principles, including quantum magnetism, superconductivity, topological states, mathematical physics, and the numerical method of quantum Monte Carlo simulations.