Detection of spin-polarized currents in chiral molecules

Sammanfattning

Detecting spin-polarized currents in chiral molecules unambiguously is an unsolved problem. The standard approach with magneto-resistance (MR) through ferromagnets results in a conflation of molecular spin selectivity and electrode magnetization. When the ferromagnet is removed, the MR signal vanishes, regardless of whether the molecule would spin-polarize the current. In addition, a nonzero MR signal is not conclusive evidence of spin-polarized transport. The root cause of this ambiguity is addressed here: the local spin density of a chiral molecule coupled to a reservoir contains a static contribution that exists independently of any current flow and contaminates every direct measurement. This constraint is identified and exploited through constructing a voltage-inversion protocol that cancels the static contribution identically. By coupling a witness spin to the molecule via the Heisenberg exchange interaction and measuring the difference in initial torque between positive and negative bias voltages, an observable is obtained. Provided that the model assumption holds, this is a ferromagnet-free, background-free measure of the longitudinal spin polarization of the current. Moreover, an extension to the detection protocol with a falsification criterion via enantiomer exchange is added and presented numerically.