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Chiral phonons in 2D systems

Speaker: 
Lifa Zhang (Nanjing Normal University, China)
Date: 
Wed, 03/02/2016 - 11:00am to 12:00pm
Location: 
Physics Conference Room (S11-02-07)
Event Type: 
Other Seminars

Abstract

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)

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