[LA-SiGMA] LASiGMA Seminar at Tulane today at 3:00 pm - Congjun Wu

[shelley lee]

Dear All,

As a reminder, Dr. Congjun Wu will be giving a talk today at 3:00 pm in the
Lindy Boggs Building, room 600, Tulane University. The title of the talk is
"Exact Results on Itinerant Ferromagnetism in Multi-orbital Systems on
Square and Cubic Lattices."

Please see below for more details.

Regards,
Shelley Lee
Project Coordinator
225-578-0465

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Subject: [LA-SiGMA-pd] LASiGMA Seminar at Tulane, Wednesday, October 22, at
3:00 pm - Congjun Wu

Dear All,

Dr. Congjun Wu of the University of California, San Diego will be giving a
talk, Wednesday, October 22nd, at 3:00 pm in the Lindy Boggs Building, room
600, Tulane University. The title of his talk is "Exact Results on Itinerant
Ferromagnetism in Multi-orbital Systems on Square and Cubic Lattices."

Abstract:
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. 

References:
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. 
   
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, and an associate professor at UCSD
in 2011. His research interests include quantum magnetism,
superconductivity, orbital physics, and topological states in
condensed-matter and cold-atom systems.

UNO - 234, Liberal Arts Building
LATech - 122, Nethken Hall
SUBR - 211 J.B. Moore Hall
Xavier - 226 Qatar Pavillion
LSU - 1008B Digital Media Center

Please note, this seminar will ONLY be accessible through HD
videoconferencing. Adobe connect will NOT be offered.

Regards,
Shelley Lee
Project Coordinator
225-578-0465



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