Nonreciprocal Nature and Magnetochiral Charge Polarization in Chiral Molecular Devices

Abstract The chirality--spin interaction is a fascinating topic for physicists and chemists. For example, chiral molecules like DNA generate giant spin polarization in nanodevices characterized by large magnetoresistance (MR). This phenomenon, called chirality-induced spin selectivity (CISS), paves pathways for unconventional spintronic devices and enantiomer separation. Different from ordinary transport, CISS MR violates Onsager's reciprocal relation and its physical mechanism is elusive and debated. In this work, we propose that the CISS MR is intimately related to the electric magnetochiral anisotropy (EMCA) while EMCA respects Onsager's relation. In a molecular device including a ferromagnetic electrode, the chiral molecule, as a spin polarizer, leads to EMCA in the second order response to the electric field. However, EMCA further generates extra charge accumulation in the device given the insulating nature of the molecule. Here, reversing either electrode magnetization or molecule chirality changes the charge accumulation, which we term magnetochiral charge polarization (MCCP). Then MCCP modifies the tunneling barrier, alters the tunneling resistance sensitively, and thus, leads to a higher-order MR that violates Onsager's reciprocity. Our model reveals the deep connection between EMCA and CISS MR and explains the unusually large MR ratio and nonequilibrium nature of CISS. We predict that the molecular spin valve device exhibits a crossover from CISS-MR to EMCA when the chiral molecule turns more metallic because of losing the accumulated charge. We further anticipate CISS MR may appear in chiral molecular devices without ferromagnetic electrodes but in an external magnetic field in case EMCA and charge accumulation co-exist.