MgN: a new promising material for spintronic applications

Density functional theory calculations demonstrate that rocksalt MgN is a magnetic material at the verge of half-metallicity, with an electronic structure robust against strong correlations and spin-orbit interaction. Furthermore the calculated heat of formation describes the compound as metastable and suggests that it can be fabricated by tuning the relative Mg and N abundance during growth. Intriguingly the equilibrium lattice constant is close to that of MgO, so that MgN is likely to form as an inclusion during the fabrication of N-doped MgO. We then speculate that the MgO/MgN system may represent a unique materials platform for magnetic tunnel junctions not incorporating any transition metals

Simulating STM transport in alkanes from first principles

Simulations of scanning tunneling microscopy measurements for molecules on surfaces are traditionally based on a perturbative approach, most typically employing the Tersoff-Hamann method. This assumes that the STM tip is far from the sample so that the two do not interact with each other. However, when the tip gets close to the molecule to perform measurements, the electrostatic interplay between the tip and substrate may generate non-trivial potential distribution, charge transfer and forces, all of which may alter the electronic and physical structure of the molecule. These effects are investigated with the ab initio quantum transport code SMEAGOL, combining non-equilibrium Green's functions formalism with density functional theory. In particular, we investigate alkanethiol molecules terminated with either CH3 or CF3 end-groups on gold surfaces, for which recent experimental data are available. We discuss the effects connected to the interaction between the STM tip and the molecule, as well as the asymmetric charge transfer between the molecule and the electrodes.Comment: 10 pages, 18 figure

Theoretical studies of spin-dependent electrical transport through carbon nanotbes

Spin-dependent coherent quantum transport through carbon nanotubes (CNT) is studied theoretically within a tight-binding model and the Green's function partitioning technique. End-contacted metal/nanotube/metal systems are modelled and next studied in the magnetic context, i.e. either with ferromagnetic electrodes or at external magnetic fields. The former case shows that quite a substantial giant magnetoresistance (GMR) effect occurs ($\pm 20$) for disorder-free CNTs. Anderson-disorder averaged GMR, in turn, is positive and reduced down to several percent in the vicinity of the charge neutrality point. At parallel magnetic fields, characteristic Aharonov-Bohm-type oscillations are revealed with pronounced features due to a combined effect of: length-to-perimeter ratio, unintentional electrode-induced doping, Zeeman splitting, and energy-level broadening. In particular, a CNT is predicted to lose its ability to serve as a magneto-electrical switch when its length and perimeter become comparable. In case of perpendicular geometry, there are conductance oscillations approaching asymptotically the upper theoretical limit to the conductance, $4 e^2/h$. Moreover in the ballistic transport regime, initially the conductance increases only slightly with the magnetic field or remains nearly constant because spin up- and spin down-contributions to the total magnetoresistance partially compensate each other.Comment: 15 pages, 6 figures (to apppear in Semicond. Sci. Technol.

Efficient atomic self-interaction correction scheme for non-equilibrium quantum transport

Density functional theory calculations of electronic transport based on local exchange and correlation functionals contain self-interaction errors. These originate from the interaction of an electron with the potential generated by itself and may be significant in metal-molecule-metal junctions due to the localized nature of the molecular orbitals. As a consequence, insulating molecules in weak contact with metallic electrodes erroneously form highly conducting junctions, a failure similar to the inability of local functionals of describing Mott-Hubbard insulators. Here we present a fully self-consistent and still computationally undemanding self-interaction correction scheme that overcomes these limitations. The method is implemented in the Green's function non-equilibrium transport code Smeagol and applied to the prototypical cases of benzene molecules sandwiched between gold electrodes. The self-interaction corrected Kohn-Sham highest occupied molecular orbital now reproduces closely the negative of the molecular ionization potential and is moved away from the gold Fermi energy. This leads to a drastic reduction of the low bias current in much better agreement with experiments.Comment: 4 pages, 5 figure