Collective modes of a helical liquid

S. Raghu, Suk Bum Chung, Xiao Liang Qi, Shou Cheng Zhang

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190 Scopus citations


We study low energy collective modes and transport properties of the "helical metal" on the surface of a topological insulator. At low energies, electrical transport and spin dynamics at the surface are exactly related by an operator identity equating the electric current to the in-plane components of the spin degrees of freedom. From this relation it follows that an undamped spin wave always accompanies the sound mode in the helical metal-thus it is possible to "hear" the sound of spins. In the presence of long range Coulomb interactions, the surface plasmon mode is also coupled to the spin wave, giving rise to a hybridized "spin-plasmon" mode. We make quantitative predictions for the spin-plasmon in Bi2Se3, and discuss its detection in a spin-grating experiment.

Original languageEnglish
Article number116401
JournalPhysical Review Letters
Issue number11
StatePublished - 17 Mar 2010


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