Enhanced Magnetoresistance in Chiral Molecular Junctions.

Chirality-induced spin selectivity (CISS) is a recently discovered effect, whose precise microscopic origin has not yet been fully elucidated; it seems, however, clear that spin-orbit interaction plays a pivotal role. Various model Hamiltonian approaches have been proposed, suggesting a close connection between spin selectivity and filtering and helical symmetry. However, first-principles studies revealing the influence of chirality on the spin polarization are missing. To clearly demonstrate the influence of the helical conformation on the spin polarization properties, we have carried out spin-dependent Density-Functional Theory (DFT) based transport calculations for a model molecular system. It consists of α-helix and ß-strand conformations of an oligo-glycine peptide, which is bonded to a nickel electrode and to a gold electrode in a two-terminal setup, similar to a molecular junction or a local probe, for example, in STM or AFM configurations. We have found that the α-helix conformation displays a spin polarization, calculated through the intrinsic magneto-resistance of the junction, about 100-1000 times larger than the linear ß-strand, clearly demonstrating the crucial role played by the molecular helical geometry on the enhancement of spin polarization associated with the CISS effect.

Chirality-Dependent Electron Spin Filtering by Molecular Monolayers of Helicenes.

The interaction of low-energy photoelectrons with well-ordered monolayers of enantiopure helical heptahelicene molecules adsorbed on metal surfaces leads to a preferential transmission of one longitudinally polarized spin component, which is strongly coupled to the helical sense of the molecules. Heptahelicene, composed of only carbon and hydrogen atoms, exhibits only a single helical turn but shows excess in longitudinal spin polarization of about P Z = 6 to 8% after transmission of initially balanced left- and right-handed spin polarized electrons. Insight into the electronic structure, that is, the projected density of states, and the spin-dependent electron scattering in the helicene molecule is gained by using spin-resolved density functional theory calculations and a model Hamiltonian approach, respectively. Our results support the semiclassical picture of electronic transport along a helical pathway under the influence of spin-orbit coupling induced by the electrostatic molecular potential.

Spin-orbit coupling in nearly metallic chiral carbon nanotubes: a density-functional based study.

Spin-orbit interaction in carbon nanotubes has been under debate for several years and a variety of theoretical calculations and experimental results have been published. Here, we present an accurate implementation of spin-orbit interactions in a density-functional theory framework including both core and valence orbital contributions, thus using the full potential of the system. We find that the spin-splitting of the frontier bands of armchair nanotubes is of the order of several µeV and does not strongly depend on the diameter of the nanotube. We also provide a systematic analysis of the band splitting in chiral nanotubes as a function of the diameter and the chiral angle. Very good agreement with previous theoretical studies and experimental results is overall found. In particular, our approach can be of great relevance in view of the recently discovered chirality-induced spin selectivity, since it allows us to include not only atomic contributions to the spin-orbit interaction, but more importantly, global contributions to the potential arising from the geometric structure (topology) of the system. Our methodology can thus encode effects such as helical symmetry in a straightforward way.

Real-time study of the modification of the peptide bond by atomic calcium.

Strong chemical interaction between bacterial surface protein layers and calcium atoms deposited in situ on top was revealed by means of photoemission spectroscopy. The interaction appears to mainly happen at the oxygen site of the peptide bonds and involves a large charge transfer from Ca 4s states into the peptide backbone. Chemical kinetics of this reaction was characterized using time-dependent valence band photoemission, and the reaction rate constant was determined.

Unoccupied electronic states in an organic semiconductor probed with x-ray spectroscopy and first-principles calculations.

High-quality films of copper phthalocyanine (CuPc) prepared in situ were used as a model to characterize unoccupied states of organic molecular semiconductors. We demonstrate that a combination of high-resolution near-edge x-ray absorption together with first-principles calculations constitutes a reliable tool for the detection and identification of particular molecular orbitals.

Organometallic benzene-vanadium wire: A one-dimensional half-metallic ferromagnet.

Using density functional theory we perform theoretical investigations of the electronic properties of a freestanding one-dimensional organometallic vanadium-benzene wire. This system represents the limiting case of multidecker Vn(C6H6)(n+1) clusters which can be synthesized with established methods. We predict that the ground state of the wire is a 100% spin-polarized ferromagnet (half-metal). Its density of states is metallic at the Fermi energy for the minority electrons and shows a semiconductor gap for the majority electrons. We find that the half-metallic behavior is conserved up to 12% longitudinal elongation of the wire. Ab initio electron transport calculations reveal that finite size vanadium-benzene clusters coupled to ferromagnetic Ni or Co electrodes will work as nearly perfect spin filters.

Structural and electronic properties of Li(2)b(4)O(7).

The reliability of various quantum-chemical approaches for the calculation of bulk properties of lithium tetraborate Li(2)B(4)O(7) was examined. Lattice parameters and the electronic structure obtained with density-functional theory (DFT), with DFT-Hartree-Fock (HF) hybrid methods, and with the semiempirical method MSINDO were compared to available experimental data. We also compared the results at DFT level using different wave functions, either based on linear combinations of atom-centered orbitals (LCAO), or on plane waves, as implemented in the crystalline orbital programs CRYSTAL and VASP. The basis set dependence of calculated properties was investigated for the LCAO method. In the plane wave approach ultrasoft pseudopotentials (US PP), and projector-augmented wave (PAW) potentials were used to represent the core electrons. For all methods under consideration, the calculated Li(2)B(4)O(7) structure parameters are close to each other and agree within a few percent with measured values. A more pronounced method dependence was found for the band structure, the band gap and the cohesive energy. Closest agreement between theoretical and experimental results for the band gap was obtained with the DFT-HF hybrid methods while pure DFT methods underestimate and HF based methods overestimate the measured value. It was found that the calculated band gap strongly depends on the atomic basis set in the LCAO approach. The description of the core electrons considerably affects the cohesive energy obtained with the plane wave approach. Atomic charges based on a Mulliken analysis were compared to effective charges obtained from Raman spectroscopy. Electron density maps are used to analyze the character of B-O and Li-O interactions.