Transmission Eigenvalues in Chiral Molecular Junctions

Chiral-induced spin selectivity (CISS) is caused by the interplay between the chirality of a system and electron magnetic moments in nonequilibrium. CISS manifests itself in two-terminal junctions (e.g., electrode-molecule(s)-electrode) in magnetoresistance. For a given magnetization orientation of the ferromagnetic electrode, l- and d-enantiomers show different current-voltage behavior. Reversing the magnetization reverses the current response of l and d, respectively. The fact that this happens near zero bias in the linear regime has prompted a discussion on the consequences of time-reversal symmetry for the possible mechanisms underlying CISS, which are often based on simplifying the junction to an effectively one-dimensional system or to an idealized helix with two orbitals per site. This motivates us to explore the electron transport characteristics of realistic chiral molecular junctions as typically studied in experiments. As a measure of how strongly the junctions deviate from idealized cases, we focus on the number of nonzero transmission eigenvalues. We find that all systems considered exhibit at least two transmission eigenvalues that lie clearly above the noise threshold. This is most pronounced for unsubstituted [6]-helicene, thio-[6]-helicene, [6]-carboxyhelicene, and a pentapeptide α-helix with thiolated terminal groups and less pronounced for helicenes with amine, bromine, or thiadiazole anchoring groups and for short linear peptides. We therefore propose a comparative study of molecules with differently pronounced second transmission eigenvalues, under identical experimental conditions, to assess the relevance of these transmission eigenvalues for the CISS effect.