Too Fast for Spin Flipping: Absence of Chirality-Induced Spin Selectivity in Coherent Electron Transport through Single-Molecule Junctions.

Chirality-induced spin selectivity (CISS), which refers to the ability of chiral molecules to preferentially select spins during electron transfer, has attracted great attention during the past two decades. However, the theoretical and experimental understanding of the CISS effect remains preliminary. In this study, we demonstrate that there is no distinguishable CISS effect in the case of coherent electron transport through single chiral molecular junctions for a set of four molecule studied here. Our conclusion is based on statistical evaluations of thousands of single-molecule junctions across four different molecules with different origins of chirality measured by the scanning tunneling microscope-based break-junction technique. The experimental results for all molecules show no dependence on external magnetic field or chirality in both conductance and current-voltage measurements. In addition, ab initio Hartree-Fork calculations combined with the nonequilibrium Green's function method reveal that the spin-orbit coupling within chiral junctions bound to a few gold atoms is generally too weak to induce detectable spin polarizations from spin flipping or spin filtering during the ultrafast electron-transport time scale. The absence of an observable CISS effect in the coherent electron-transport regime suggests that the effect may only be found in other electron-transfer regimes and requires further experimental and theoretical efforts to achieve a comprehensive understanding.