2018/01/04(Thu) 10:30 -12:00 五樓第一會議室 5F, 1st Meeting Room
Title
Raman Scattering from the Surface States of Bi2Se3
Speaker
孔祥曦 博士 (加拿大溫哥華英屬哥倫比亞大學量子物質研究所) Dr. Hsiang-Hsi Kung (Quantum Matter Institute, University of British Columbia, Vancouver, BC V6T1Z4, Canada)Abstract
We study the Raman scattering and photoluminescence spectra induced by interband transitions on the surface of the prototypical 3D topological insulator Bi2Se3. We selectively enhance the surface signal over bulk by tuning the excitation photon energy in resonance with optical transition between surface states. With 1.8 eV excitation energy, we observe the emergence of a gapped continuum and a sharp collective mode at about 150 meV Raman shift that survives even at room temperature. Both features are only present in the pseudovector A2 symmetry channel of the C6v point group, which is forbidden in dipole transitions, and therefore invisible in the spectra of infrared absorption and photoemission. The observed A2 mode is identified as a transverse collective spin wave, which arises due to the spin-flip transitions between the lower and upper surface Dirac cones. The self-energy and lifetime of the spin mode are fully consistent with a simple model considering finite spin interaction between the surface electrons. However, the mode intensity decreases with increasing temperature, indicating additional decay channels outside our toy model, such as spin-phonon interaction with the surface phonon modes. This is the first observation of a surface collective mode purely in the spin channel in a topological insulator, which is different than the charge excitations such as the Dirac plasmon and surface plasmons. Our discovery is a first direct measurement of the spin correlation between the Dirac fermions, which in principle can be applied to other strong spin orbit coupled systems.
References:
[1] Kung et al., “Chiral Spin Mode on the Surface of a Topological Insulator”, Phys. Rev. Lett. 119, 136802 (2017)
[2] Kung et al., “Surface vibrational modes of the topological insulator Bi2Se3 observed by Raman spectroscopy”, Phys. Rev. B 95, 245406 (2017)
CV
Language
演講語言 (Language): in English
Hosted by