We investigated the origin of an inflection behavior appearing in Fowler-Nordheim (F-N) plot of current-voltage characteristics for molecular junctions using two different levels of calculation methods: nonequilibrium Green’s-function technique combined with the density-functional theory and tight-binding approximation. Although the inflection has so far been interpreted from the naive model that the charge transport mechanism transits from a direct to the F-N tunneling, our results indicated that the inflection does not necessarily mean the transition between the two regimes. We found from the close examination of the relation between the behavior of the F-N curve and the transmission function that the inflection takes place when the molecular level responsible for electric currents approaches to the edge of the bias window. While our interpretation for the inflection drastically differ from the conventional model, the F-N plots obtained from our calculations showed closely similar behavior as those from the recent experiments. DOI: http://dx.doi.org/10.1103/PhysRevB.81.235114 3 More Received 24 November 2009Revised 9 February 2010Published 15 June 2010© 2010 The American Physical Society

Field emission (FE) and the electronic-states origin of graphitic nanostructures were investigated by first-principles calculations based on time-dependent density-functional theory. We find that the FE current from graphitic ribbons changes remarkably depending on the hydrogen termination and the direction of the applied electric field. Also, the FE current from graphene sheets shows a dramatic increase around vacancy defects. We verified, through the analysis of local electronic structures and energy distributions of emitted electrons, that the dangling-bond (or $\sigma$) character is responsible for these results and governs the nature of the FE of graphitic nanostructures.

We studied GeTe structures in superlattice phase change memories (superlattice PCMs) with a [GeTe/Sb₂Te₃] stacked structure by X-ray diffraction (XRD) analysis. We examined the electrical characteristics of superlattice PCMs with films deposited at different temperatures. It was found that XRD spectra differed between the films deposited at 200 °C and 240 °C; the differences corresponded to the differences in the GeTe sequences in the films. We applied first-principles calculations to calculate the total energy of three different GeTe sequences. The results showed the Ge-Te-Ge-Te sequence had the lowest total energy of the three and it was found that with this sequence the superlattice PCMs did not run.

The composition dependence of the tunnel magnetoresistance (TMR) effect in Mn-Ga/MgO/CoFe magnetic tunnel junctions (MTJs) for Mn₅₄Ga₄₆, Mn₆₂Ga₃₈, and Mn₇₁Ga₂₉ (at. %) electrodes was investigated. An MTJ with a Mn₆₂Ga₃₈ electrode showed a maximum TMR ratio of 23% at 10 K and high annealing endurance up to 375 °C. The bias voltage dependence of the TMR ratio was distinct among MTJs with different Mn-Ga compositions. Here, we discuss this dependence on the basis of the difference in the Δ₁ band dispersions for Mn-Ga alloys calculated by first principles.