Indication of phonon-assisted electron-hole relaxations on Sb(111) and Bi(111) in iHAS measurements

Full text: Approximations are a fundamental part of scientific work. Without them the systems under investigation would be far too complex and complicated to obtain results. The more important it is to know where these approximations begin to fail. Recently, a strong Kohn-like anomaly was found in the phononic dispersion relation of Bi2Se3. The supposed nesting vectors connect spin-opposing states on both of the sides of the Dirac cone which are non-equivalent. Thus the electron needs to undergo a non-adiabatic transition to make the softening of the phononic mode possible. This is even more problematic, since transitions that break the adiabatic approximation are not accessible by standard DFPT methods. Recent studies on the (111) surfaces of the semimetals antimony and bismuth revealed deep and narrow strongly pronounced features. The positions of those apparent sudden phonon softenings correspond to an electronic relaxation between the opposite sides of the inner Rashba-split cone. The necessary helicity of 1 which the electron needs to complete this transition can be supplied by a phonon. The helicity exchange between a phonon and an electron is only possible in materials with a strong spin-orbit interaction. Complete measurements of the anomalies found in Sb(111) and Bi(111) will be presented and compared with the relevant electronic data. (author)