Observation of Dirac Charge-Density Waves in Bi2Te2Se

While the hexagonal Fermi level contours, often found at the surfaces of topological insulators (TIs) would imply "strong" nesting conditions, the existence of charge density waves (CDWs) - periodic modulations of the electron density - has not been verified up to now. Here, we report the observation of a CDW at the surface of the TI Bi2Te2Se(111), below ≈350K by helium atom scattering, and thus experimental evidence of a CDW involving Dirac topological electrons. Deviations of the order parameter observed below 200K and a low temperature break of time reversal symmetry suggest the onset of a spin density wave with the same period as the CDW in presence of a prominent electron-phonon interaction originating from the Rashba spin-orbit coupling.