Observation of non-adiabatic Kohn anomalies on Bi(111) - Inducing spin-flip electronic transitions using helium Atom Scattering

The unique property of Bismuth showing metallic behavior at the surfaces but semimetallic and even insulating behavior in the bulk promise applications using nano-scale structures. This surface specific metallic behavior is caused by the strong spin-orbit splitting present in bismuth. Earlier assumptions of the formation of a charge-density wave on the Bi(111) surface [1] were discarded, since the nesting vectors touching the Fermi surface at kF = 0.0532 ∓ 0.0003 Å-1 and -kF respectively, as suggested in [1] are spin-opposing states, as predetermined by time-reversal symmetry [2]. Recent inelastic Helium Atom Scattering (iHAS) experiments revealed strongly pronounced narrow dips in the phonon dispersion relation of the Bi(111) surface. The position of these dips indicates a Kohn-like anomalous interaction with the very same nesting vector as Ast and Höchst suggested. Such an anomaly is only possible considering the coupling of non-adiabatic electronic transitions to the lattice vibrational movements. Recently a very similar situation was found on the surface of the topological insulator Bi2Se3 [3], where the spin disparity between kF and -kF given by timereversal symmetry yields the striking properties of this materials class to “prohibit electronic U-turns” [4]. Those non-adiabatic Kohn anomalies provide a fundamental difference between iHAS measurements and theoretically predicted surface phonon energies, since nonadiabatic effects are not accessible using standard DFPT-methods.