Charge dynamics in elemental bismuth measured with momentum-resolved EELS

The topology of the quantum mechanical wavefunction has been a core interest of the condensed matter physics and material science community over the past decade. Bismuth-based compounds have played a significant role in this regard owing to the large spin-orbit coupling, even though elemental bismuth was believed to be topologically trivial. Recent studies have proposed that bismuth is likely a higher order topological insulator [1] and could also be a potential candidate for a first order topological crystalline insulator [2]. Here we have studied the charge dynamics in bismuth single crystals using momentum resolved electron energy loss spectroscopy (m-EELS). Signatures of a strongly temperature dependent plasmon are observed. While the plasmon frequency at q = 0 is consistent with infrared spectroscopy [3], the lifetime appears to be significantly shorter. We measure the dispersion of the plasmon excitation and compare it to the calculated dynamical charge susceptibility using Lindhard theory in the RPA. The results shed light on the collective excitations in the proposed topological surface states in bismuth.

[1] Schindler, Frank, et al. Nature physics 14.9: 918-924 (2018).

[2] Hsu, Chuang-Han, et al. Proceedings of the National Academy of Sciences 116.27: 13255-13259 (2019).

[3] Tediosi, Riccardo, et al. Physical review letters 99.1 : 016406 (2007).