ABSTRACT
Spin-orbit coupling gives rise to a range of spin-charge interconversion phenomena in nonmagnetic systems where certain spatial symmetries are reduced or absent. Chirality-induced spin-selectivity (CISS), a term that generically refers to a spin-dependent electron transfer in nonmagnetic chiral systems, is one such case, appearing in a variety of seemingly unrelated situations ranging from inorganic materials to molecular devices. In particular, the origin of CISS in molecular junctions is a matter of an intense current debate. Here, we derive a set of geometrical conditions for this effect to appear, hinting at the fundamental role of symmetries beyond otherwise relevant quantitative issues. Our approach, which draws on the use of point-group symmetries within the scattering formalism for transport, shows that electrode symmetries are as important as those of the molecule when it comes to the emergence of a spin-polarization and, by extension, to the possible appearance of CISS. It turns out that standalone metallic nanocontacts can exhibit spin-polarization when relative rotations which reduce the symmetry are introduced. As a corollary, molecular junctions with achiral molecules can also exhibit spin-polarization along the direction of transport, provided that the whole junction is chiral in a specific way. This formalism also allows the prediction of qualitative changes of the spin-polarization upon substitution of a chiral molecule in the junction with its enantiomeric partner. Quantum transport calculations based on density functional theory corroborate all of our predictions and provide further quantitative insight within the single-particle framework.
ABSTRACT
In the field of molecular electronics, especially in quantum transport experiments, determining the geometrical configurations of a single molecule trapped between two electrodes can be challenging. To address this challenge, we employed a combination of molecular dynamics (MD) simulations and electronic transport calculations based on density functional theory to determine the molecular orientation in our break-junction experiments under ambient conditions. The molecules used in this study are common solvents used in molecular electronics, such as benzene, toluene (aromatic), and cyclohexane (aliphatic). Furthermore, we introduced a novel criterion based on the normal vector of the surface formed by the cavity of these ring-shaped monocyclic hydrocarbon molecules to clearly define the orientation of the molecules with respect to the electrodes. By comparing the results obtained through MD simulations and density functional theory with experimental data, we observed that both are in good agreement. This agreement helps us to uncover the different geometrical configurations that these molecules adopt in break-junction experiments. This approach can significantly improve our understanding of molecular electronics, especially when using more complex cyclic hydrocarbons.
ABSTRACT
Our analysis of the contact formation processes undergone by Au, Ag, and Cu nanojunctions reveals that the distance at which the two closest atoms on a pair of opposing electrodes jump into contact is, on average, 2 times longer for Au than either Ag or Cu. This suggests the existence of a longer-range interaction between those two atoms in the case of Au, a result of the significant relativistic energy contributions to the electronic structure of this metal, as confirmed by ab initio calculations. Once in the contact regime, the differences between Au, Ag, and Cu are subtle, and the conductance of single-atom contacts for metals of similar chemical valence is mostly determined by geometry and coordination.
