Chiral Spin Mode on the Surface of a Topological Insulator.

TLDR: Using polarization-resolved resonant Raman spectroscopy, a sharp peak at 150 meV is identified as the transverse collective spin mode of surface Dirac fermions, analogous to a spin wave in a partially polarized Fermi liquid, with spin-orbit coupling playing the role of an effective magnetic field.

Abstract: Using polarization-resolved resonant Raman spectroscopy, we explore collective spin excitations of the chiral surface states in a three dimensional topological insulator, ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$. We observe a sharp peak at 150 meV in the pseudovector ${A}_{2}$ symmetry channel of the Raman spectra. By comparing the data with calculations, we identify this peak as the transverse collective spin mode of surface Dirac fermions. This mode, unlike a Dirac plasmon or a surface plasmon in the charge sector of excitations, is analogous to a spin wave in a partially polarized Fermi liquid, with spin-orbit coupling playing the role of an effective magnetic field.