Karl-Heinz Ernst

Affiliation: Empa, Swiss Federal Laboratories for Materials Science and Technology

Research Interests: Surface Science, Chirality, Molecules at Surfaces

Title: Magneto-chiral selectivity, spin-filtering, Kondo physics and topological self-assembly of helicenes on metal surfaces

Abstract: Soon after his seminal discovery of molecular chirality Pasteur suggested physical fields as its origin and assumed that magnetic fields must be the source of chirality in the universe. Lord Kelvin refuted Pasteur’s suggestions and made clear that magnetism has no chirality. In 1894, Pierre Curie proposed that parallel and antiparallel alignments of electric and magnetic fields will induce chirality, but such chiral influence must vanish under conditions of thermodynamic equilibrium. We report that single helical aromatic hydrocarbons, so-called helicenes, undergo enantioselective adsorption on ferromagnetic cobalt surfaces. Spin- and chirality sensitive scanning tunneling microscopy (STM) reveals that molecules of opposite handedness prefer adsorption onto cobalt islands with opposite out-of-plane magnetization. As mobility ceases in the final chemisorbed state, it is concluded that enantioselection must occur in a physisorbed transient precursor state. Such observation suggests electron spin-depend van-der-Waals forces. Simultaneous measurements of the tunneling current through both enantiomers on a given Co nanoisland yields a magneto-chiral specific conductance of up to 50% for single helicene molecules, thus refuting previously proposed ensemble effects as origin of chiralityinduced spin selectivity. Our results open the opportunity towards new single-molecule spinvalve devices and shine light into the origin of the so-called chirality-induced spin selectivity (CISS).