Congjun Wu
University of California, San Diego

报告时间: 2014年8月15日15:00-16:00
报告地点: 唐仲英楼A213

Itinerant ferromagnetism (FM) is one of the major challenges of condensed matter physics. FM is not only a strong-correlation phenomenon but also a highly non-perturbative problem. Even at U=\infinity, FM is not guaranteed. For example, the Lieb-Mattis theorem proved that itinerant electrons with the nearest neighboring hopping in 1D can never be FM no matter how strong interaction is. Exact theorems are, therefore, indispensable for understanding the mechanism of FM. Previously known examples of FM in 2D and 3D usually fall into one of the two categories: the 'Nagaoka FM' as a result of coherent hopping of a single hole in lattices under U=\infinity, or, the 'flat-band FM' on line graphs, like the Kagome lattice, where zero penalty from kinetic energy greatly assists the development of FM.

In this talk, we present our study on itinerant FM in multiorbital Hubbard models in certain two-dimensional square and three-dimensional cubic lattices. In the strong coupling limit where doubly occupied orbitals are prohibited, we prove that the fully spin-polarized states are the unique ground states, apart from the trivial spin degeneracies. Compared to the Nagaoka FM, our theorems apply to a large region of filling factors, and thus establish a stable thermodynamic phase of itinerant FM. Possible applications to p-orbital bands with ultracold fermions in optical lattices, and electronic 3d-orbital bands in transition-metal oxides such as the SrTiO3/LaAlO3 interfaces, are discussed. The ferromagnetic phases established by this theorem is free of the quantum Monte-Carlo sign problem, and thus a lot of open problems of thermodynamic properties of itinerant ferromagnetism can be addressed at a high numerical accuracy.

Ref.
1 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).
2. Shenglong Xu, Yi Li, Congjun Wu, "Sign problem free QMC simulation on thermodynamic properties of itinerant ferromagnetism," in preparation.