Chirality induced spin selectivity through time-reversal symmetric helical molecular junctions

Time-reversal symmetric charge and spin transport through a molecule comprising two-orbital channels and connected to two leads is analyzed. It is demonstrated that spin-resolved currents are generated when spin-flip processes are accompanied by a flip of the orbital channels. This finding does not contradict Bardarson's theorem [J. Phys. A 41, 405203 (2008)]: The transmission eigen values are doubly degenerate. The spin-filtering effect is explicitly demonstrated for a two-terminal chiral molecular junction, modeled by a two-orbital helical tight-binding chain with intra-atomic spin-orbit interactions (SOI). In the context of transport through DNA, this effect is termed chirality-induced spin selectivity (CISS). The model exhibits spin splitting without breaking time-reversal symmetry: the intra-atomic SOI induces concomitant spin and orbital flips. Examining these transitions from the point of view of the Bloch states in an infinite molecule, it is shown that they cause shifts in the Bloch wave numbers, of the size of the reciprocal single turn, whose directions depend on the left and right handedness of the helix. As a result, spin-up and -down states propagate in the opposite directions, leading to the CISS effect [Utsumi, Entin-Wohlman, and Aharony, PRB 102, 035445 (2020)].