  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. EMCA generates extra charge trapping in
the device and modifies the electron tunneling barrier that alters the
tunneling resistance sensitively, circumventing the Onsager's reciprocity. Our
theory reveals the intimate connection between EMCA and CISS MR and explains
the unusually large MR ratio by the nonlinear tunneling transport, which is
related to the induced spin polarization indirectly. We provide a tunneling
barrier model to extract the barrier modulation from available experimental
results. Our work provides a deep understanding of the CISS transport by
characterizing the tunneling barrier and further linking it to another quantity
measured in recent experiments, the surface potential.
