Experimental observation of chiral phonons in monolayer WSe2

Hanyu Zhu; Jun Yi; Ming-Yang Li; Jun Xiao; Lifa Zhang; Chih-Wen Yang; Sui Yang; Robert A. Kaindl; Lain-Jong Li; Yuan Wang; Xiang Zhang

doi:10.1117/12.2510760

27 February 2019 Experimental observation of chiral phonons in monolayer WSe₂

Hanyu Zhu, Jun Yi, Ming-Yang Li, Jun Xiao, Lifa Zhang, Chih-Wen Yang, Sui Yang, Robert A. Kaindl, Lain-Jong Li, Yuan Wang, Xiang Zhang

Author Affiliations +

Proceedings Volume 10916, Ultrafast Phenomena and Nanophotonics XXIII; 109160C (2019) https://doi.org/10.1117/12.2510760
Event: SPIE OPTO, 2019, San Francisco, California, United States

Abstract

Chirality characterizes an object that is not identical to its mirror image. In condensed matter physics, Fermions have been demonstrated to obtain chirality through structural and time-reversal symmetry breaking. These systems display unconventional electronic transport phenomena such as the quantum Hall effect and Weyl semimetals. However, for bosonic collective excitations in atomic lattices, chirality was only theoretically predicted and has never been observed. We experimentally show that phonons can exhibit intrinsic chirality in monolayer tungsten diselenide, whose lattice breaks the inversion symmetry and enables inequivalent electronic K and -K valley states. The time-reversal symmetry is also broken when we selectively excite the valley polarized holes by circularly polarized light. Brillouin-zone-boundary phonons are then optically created by the indirect infrared absorption through the hole-phonon interactions. The unidirectional intervalley transfer of holes ensures that only the phonon modes in one valley are excited. We found that such photons are chiral through the transient infrared circular dichroism, which proves the valley phonons responsible to the indirect absorption has non-zero pseudo-angular momentum. From the spectrum we further deduce the energy transferred to the phonons that agrees with both the first principle calculation and the double-resonance Raman spectroscopy. The chiral phonons have significant implications for electron-phonon coupling in solids, lattice-driven topological states, and energy efficient information processing.

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Hanyu Zhu, Jun Yi, Ming-Yang Li, Jun Xiao, Lifa Zhang, Chih-Wen Yang, Sui Yang, Robert A. Kaindl, Lain-Jong Li, Yuan Wang, and Xiang Zhang "Experimental observation of chiral phonons in monolayer WSe₂", Proc. SPIE 10916, Ultrafast Phenomena and Nanophotonics XXIII, 109160C (27 February 2019); https://doi.org/10.1117/12.2510760

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