Richard L. Greene Dissertation Award Talk: Probing Hidden Symmetries with Raman Scattering*

The properties of quantum materials are dominated by electronic correlations, which often lead to novel emergent phenomena and spontaneous symmetry breaking at low temperatures or material interfaces. The underlying correlations are encoded in the collective excitations out of the ground state, which are sometimes unfortunately hidden from most experimental techniques. One example is the collective modes that transform as a pseudovector, e.g. the A_2g representation of the D_4h and D_3d groups. Here, we use polarization resolved Raman scattering to directly probe the collective modes with A_2g symmetry in two examples. 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 results in novel collective modes and composite particles. In the example of Bi₂Se₃, we observed an A_2g 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, 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, Phys. Rev. Lett. 117, 227601 (2016).
[3] H.-H. Kung, S. Maiti, X. Wang, S.-W. Cheong, D.L. Maslov and G. Blumberg, 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, Proc. Natl. Acad. Sci. 116, 4006 (2019).