Title：Novel Orbital Physics – Unconventional Bose-Einstein Condensation and itinerant Fer-romagnetism in Optical lattices
Speaker：Prof. Congjun Wu
Host：Prof. Hai-Bing Wu
Time：03:00 pm, December 15, 2017
Venue：Room A814, Science Building, ECNU Zhongbei Campus
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 (UCSD) in 2007, an associate professor and a professor at UCSD in 2011 and 2017, respectively. His research interests include quantum magnetism, superconductivity, orbital physics, and topological states in condensed-matter and cold-atom systems.
Abstract of the talk：
Orbital is a degree of freedom independent of charge and spin. It plays an important role in physical properties of transition-metal-oxides including superconductivity and magnetism. 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. A p-wave condensate of orbital bosons has been experimentally realized by Hemerich’s group at Hamburg University, and its unconventional symmetry has been verified through the matter-wave interference measurements. On the other hand, itinerant ferromagnetism (FM), i.e., FM based on Fermi surfaces instabilities of mobile electrons (fermions) rather than the ordering of local spin moments, is a hard-core problem of strong correlation physics. The well-known Stoner criterion overestimates the FM tendency by neglecting correlation effects. Even under very strong repulsions, electrons in solids usually remain paramagnetic. 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 proposed the existence of itinerant FM phases of fermions with high Curie temperatures in the p-orbital bands. A series of theorems are proved setting up the ground state FM phases over a large region of fermion fillings. 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 may also apply to certain types of d-orbital transition-metal-oxides in solid state systems such as the LaAlO3/SrTiO3 interface.
1) Congjun Wu, Unconventional Bose-Einstein Condensations Beyond the ``No-node'' Theorem Mod. Phys. Lett. 23, 1 (2009), a brief review.
2) Yi Li, E. H. Lieb, Congjun Wu, Exact Results on Itinerant Ferromagnetism in Multi-orbital Systems on Square and Cubic Lattices Phys. Rev. Lett. 112, 217201(2014).
3) Shenglong Xu, Yi Li, Congjun Wu, Thermodynamic properties of a 2D itinerant ferromagnet - a sign-problem free quantum Monte Carlo study Phys. Rev. X 5, 021032, (2015).
