报告题目 (Title)：Computational methods for strongly correlated electronic systems: nonequilibrium dynamical mean-field & two-particle self-consistent theories（强关联电子系统计算方法：非平衡态动力学平均场与二粒子自洽理论）

报告人 (Speaker)：严佳伟（瑞士弗里堡大学物理系）

报告时间 (Time)：2024年4月2日(周二) 10:30

报告地点 (Place)：校本部 E106

邀请人 (Inviter)：任伟 教授

主办部门：量子科技研究院/理学院物理系

摘要 (Abstract)：

Strongly correlated materials refer to systems where electron interactions are significantly compared to their kinetic energies, causing traditional band theory to fail. The exponentially growing dimensionality of the Hilbert space has prompted the development of various computational methods to address this challenge. In this talk, I will firstly introduce dynamical mean-field theory (DMFT), known for its effectiveness in predicting Mott transitions and its easy integration with density functional theory for real material simulations. We extended the theory to time-dependent nonequilibrium conditions using the three-branch Kadanoff-Baym contour, as well as to disordered systems such as the Hubbard-Anderson model. However, due to the locality of the self-energy, DMFT often predicts spurious phase transitions in low-dimensional system. To mitigate this issue, we have further developed a nonequilibrium two-particle self-consistent theory that adheres to the Mermin-Wagner theorem and various sum rules. As an intriguing application, we studied the behavior of the bi-layer Hubbard model under a perpendicular electric field, revealing a transition in spin correlations from anti-ferromagnetic to ferromagnetic alignment, which is a phenomenon arising entirely from nonequilibrium effects.