Richard L. Greene Dissertation Award in Experimental Condensed Matter or Materials Physics (2020): Probing Chiral Excitations with Raman Scattering*

The properties of quantum materials are dominated by electronic correlations, which often leads to novel emergent phenomena and spontaneous symmetry breaking at low temperatures or material interfaces. These underlying correlations are encoded in the collective excitations, which are sometimes hidden from experimental probes. Here, we use polarization-resolved Raman scattering to directly probe the collective modes with the symmetry of a pseudovector. In the heavy-fermion metal, URu₂Si₂, the A_2g collective mode uniquely couples to the reflection symmetry breaking ground state in the low-temperature phase -- a chirality density wave that was hidden from other spectroscopic tools in the past [1,2]. Whereas on the surface of the 3D topological insulator, due to the strong Rashba spin-orbit coupling, the electrons acquire chiral spin textures that result in novel collective modes and composite particles. In the example of Bi₂Se₃, we observed an A₂ mode as the collective spin excitation from the surface states [3], and circularly polarized photoluminescence from chiral excitons [4].
Main References:
[1] H.-H. Kung, R. E. Baumbach, E. D. Bauer, V. K. Thorsmølle, W.-L. Zhang, K. Haule, J. A. Mydosh and G. Blumberg, “Chirality density wave of the ‘hidden order’ phase in URu₂Si₂”, Science 347, 1339 (2015).
[2] H.-H. Kung, S. Ran, N. Kanchanavatee, V. Krapivin, A. Lee, J. A. Mydosh, K. Haule, M. B. Maple, G. Blumberg, “Analogy Between the ‘Hidden Order’ and the Orbital Antiferromagnetism in URu2Si2”, Phys. Rev. Lett. 117, 227601 (2016).
[3] H.-H. Kung, S. Maiti, X. Wang, S.-W. Cheong, D.L. Maslov and G. Blumberg, “Chiral Spin Mode on the Surface of a Topological Insulator”, Phys. Rev. Lett. 119, 136802 (2017).
[4] H.-H. Kung, A.P. Goyal, D.L. Maslov, X. Wang, A. Lee, A.F. Kemper, S.-W. Cheong, and G. Blumberg, “Observation of Chiral Surface Excitons in a Topological Insulator Bi₂Se₃”, Proc. Natl. Acad. Sci. 116, 4006 (2019).