Observation of Dirac Charge Density Waves in Bi$_2$Te$_2$Se

TL;DR

This paper reports the first experimental observation of a charge density wave involving Dirac electrons on the surface of Bi2Te2Se, revealing a complex interplay of electron-phonon interactions and potential spin density wave formation.

Contribution

It provides the first direct evidence of a surface CDW in a topological insulator involving Dirac electrons, linked to electron-phonon interactions and possible spin order.

Findings

Observation of a surface CDW below 350 K

Deviation of order parameter below 180 K indicating additional order

Possible spin density wave formation at low temperature

Abstract

While parallel segments in the 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 Bi$_2$Te$_2$Se(111), below $\approx 350\,$K by helium atom scattering, and thus experimental evidence of a CDW involving Dirac topological electrons. Deviations of the order parameter observed below $180\,$K 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.