报告题目: |
Non-perturbative Results for Itinerant Ferromagnetism in Multi-orbital Systems |
报告人: |
Yi Li |
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Princeton University
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报告时间: |
2015-08-21 15:00 |
报告地点: |
Conference Hall 322, Science Building, Tsinghua |
主办单位: |
高等研究院 |
简介: |
Although ferromagnetism (FM) was discovered more than 2000 years ago, it remains one of the major challenges of contemporary condensed matter physics. Different from superconductors of which the BCS theory usually provides a controllable weak-coupling description, itinerant FM is intrinsically a strong-correlation phenomenon and needs non-perturbative study. On the other hand, most FM materials are orbitally active with prominent Hund’s coupling. However, Hund’s rule is local physics, which can only polarize electrons on the same site, and it is a longstanding problem under what conditions Hund’s coupling can lead to the global FM coherence.
In this talk, I will present non-perturbative studies on a class of 2D and 3D multi-orbital Hubbard models in which Hund’s coupling plays the essential role, including exact theorems proving itinerant FM ground states and high precision quantum Monte-Carlo (QMC) simulations on thermodynamic properties. These exact results set up stable phases of itinerant FM in a large region of electron densities. Hence, they serve as a reference point for further explorations and can be applied to a wide class of systems ranging from the transition-metal-oxide hetero-structure of the LaAlO3/SrTiO3 interface, and to the p-orbital band systems of ultra-cold fermions in optical lattices. Their connection to the 3D FM material in the SrRuO3 will also be discussed. The results provide helpful guidance for the searching of novel 2D and 3D itinerant FM materials.
Furthermore, the magnetic phase transition is another long-standing problem in itinerant FM due to the lack of non-perturbative treatment on the strong magnetic fluctuations with electron itineracy. QMC simulations are ideal non-perturbative numeric methods. However, they usually suffer from the notorious sign problem for fermions. In this talk, our itinerant FM systems will be shown free of the sign problem. The QMC results of thermodynamic properties will be presented.
References:
1) YL, E. H. Lieb, C. Wu, Phys. Rev. Lett. 112, 217201 (2014).
2) YL, Phys. Rev. B 91, 115112 (2015).
3) S. Xu, YL, C. Wu, Phys. Rev. X 5, 021032 (2015). |
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