题　目：Chiral phonons in 2D systems
报告人：张力发教授（南京师范大学物理学院江苏特聘教授）
时　间：12月15日（周二），上午10:00-11:00
地　点：南校区第一实验楼423会议室

报告摘要：

  Recently, a remarkable phenomenon of the phonon Hall effect was observed in a paramagnetic insulator, which is indeed a surprise since phonons as neutral quasiparticles cannot directly couple to magnetic field via Lorentz force. The following theoretical studies showed that through Raman spin-phonon interaction the magnetic field can have an effective force to distort phonon transport, and thus drive a circulating heat  Inspired by the phonon Hall effect, very recently we found chiral phonons in systems that break time reversal or spatial inversion symmetries.

  In magnetic systems, where time reversal symmetry is broken, phonons generally carry a nonzero angular momentum .  At zero temperature, a phonon has a zero-point angular momentum in addition to a zero-point energy. With increasing temperature, the total phonon angular momentum diminishes and approaches zero in the classical limit. The nonzero phonon angular momentum can have a significant impact on the Einstein–de Haas effect. 

  In non-magnetic crystals with inversion symmetry breaking, we find chiral phonons with valley contrasting circular polarization. At valley centers, there is a three-fold rotational symmetry endowing phonons with a quantized pseudo angular momentum, which includes spin and orbital parts. The chiral valley phonons are verified and the selection rules are predicted in monolayer Molybdenum disulfide. Due to valley contrasting phonon Berry curvature,  a valley phonon Hall effect can also be observed.

[1] L. Zhang, J. Ren, J.-S. Wang, and B. Li, Phys. Rev. Lett. 105, 225901 (2010).

[2] L. Zhang and  Q. Niu, Phys. Rev. Lett. 112, 085503 (2014).

[3] L. Zhang and  Q. Niu, Phys. Rev. Lett. 115, 115502 (2015).

个人简介：

    张力发，教授，博士生导师，1977年12月出生于江苏泰兴。2012年1月毕业于新加坡国立大学获博士学位。 先后在新加坡国立大学，美国得州大学奥斯汀分校从事博士后研究。2015年7月起， 任职于南京师范大学物理系, 为江苏特聘教授。主要从事量子热传导与能量科学研究， 主要研究方向有声子角动量理论与应用，声子、磁振子霍尔效应，界面热传导与热整流现象。 先后在Phys. Rev.Lett.，Phys. Rev. B，Appl. Phys.Lett. 等国际一流期刊发表论文20余篇，担任Phys. Rev.Lett. 等国际期刊审稿人。将于2018年担任声子学国际会议Phonons 2018 会议主席。