Confirming the Polarizing Effect of Chiral Molecules
A quarter of a century ago, researchers observed that a film of molecules with the same chirality is more conductive to electrons with one spin direction than the other. An understanding of the mechanism underlying this chirality-induced spin selectivity (CISS) is still lacking, in part, because of the ambiguity of previous experiments. Now Lorenz Meyer at Ilmenau University of Technology in Germany and colleagues have found clear evidence for CISS by measuring the spin-dependent current across single-molecule junctions [1].
CISS is usually studied by sandwiching a chiral film between two electrodes, one of which is ferromagnetic. The dominant spin of the electrons injected through the ferromagnetic electrode depends on an external magnetic field. If the film filters spins, reversing the field direction should change the current. But reversing the field could conceivably alter the layers and the interfaces between them, introducing a difference in current even without CISS.
Meyer and colleagues coated a surface with alternating single-chirality domains of heptahelicene, a helix-shaped organic molecule. They probed the domains using the superconducting tip of a scanning tunneling microscope on which they had affixed a cluster of manganese atoms. These atoms added a spin-split energy band within the probe’s superconductivity gap, causing oppositely polarized currents to show up at two characteristic voltages. Scanning the tip across the surface, the researchers measured a current that depended on the instrument’s voltage and the chirality of the local domain—a clear sign of CISS. Although the mechanism behind the effect remains uncertain, Meyer and colleagues suggest that the heptahelicene molecules don’t merely filter electrons by spin but actively polarize them.
–Marric Stephens
Marric Stephens is a Corresponding Editor for Physics Magazine based in Bristol, UK
References
- L. Meyer et al., “Single-enantiomer spin polarizers in superconducting junctions,” Phys. Rev. Lett. 136, 226201 (2026).