  The interplay between chirality and magnetism has been a source of
fascination among scientists for over a century. In recent years,
chirality-induced spin selectivity (CISS) has attracted renewed interest. It
has been observed that electron transport through layers of homochiral
molecules leads to a significant spin polarization of several tens of percent.
Despite the abundant experimental evidence gathered through mesoscopic
transport measurements, the exact mechanism behind CISS remains elusive. In
this study, we report spin-selective electron transport through single helical
aromatic hydrocarbons that were sublimed in vacuo onto ferromagnetic cobalt
surfaces and examined with spin-polarized scanning tunneling microscopy
(SP-STM) at a temperature of 5 K. Direct comparison of two enantiomers under
otherwise identical conditions revealed magnetochiral conductance asymmetries
of up to 50% when either the molecular handedness was exchanged or the
magnetization direction of the STM tip or Co substrate was reversed.
Importantly, our results rule out electron-phonon coupling and ensemble effects
as primary mechanisms responsible for CISS.
