23 research outputs found
Semi-Empirical Model for Nano-Scale Device Simulations
We present a new semi-empirical model for calculating electron transport in
atomic-scale devices. The model is an extension of the Extended H\"uckel method
with a self-consistent Hartree potential. This potential models the effect of
an external bias and corresponding charge re-arrangements in the device. It is
also possible to include the effect of external gate potentials and continuum
dielectric regions in the device. The model is used to study the electron
transport through an organic molecule between gold surfaces, and it is
demonstrated that the results are in closer agreement with experiments than ab
initio approaches provide. In another example, we study the transition from
tunneling to thermionic emission in a transistor structure based on graphene
nanoribbons.Comment: 8 pages, 8 figures. Submitted to PR
Charging induced asymmetry in molecular conductors
We investigate the origin of asymmetry in various measured current-voltage
(I-V) characteristics of molecules with no inherent spatial asymmetry, with
particular focus on a recent break junction measurement. We argue that such
asymmetry arises due to unequal coupling with the contacts and a consequent
difference in charging effects, which can only be captured in a self-consistent
model for molecular conduction. The direction of the asymmetry depends on the
sign of the majority carriers in the molecule. For conduction through highest
occupied molecular orbitals (i.e. HOMO or p-type conduction), the current is
smaller for positive voltage on the stronger contact, while for conduction
through lowest unoccupied molecular orbitals (i.e. LUMO or n-type conduction),
the sense of the asymmetry is reversed. Within an extended Huckel description
of the molecular chemistry and the contact microstructure (with two adjustable
parameters, the position of the Fermi energy and the sulphur-gold bond length),
an appropriate description of Poisson's equation, and a self-consistently
coupled non-equilibrium Green's function (NEGF) description of transport, we
achieve good agreement between theoretical and experimental I-V
characteristics, both in shape as well as overall magnitude.Comment: length of the paper has been extended (4 pages to 6 pages), two new
figures have been added (3 figures to 5 figures), has been accepted for PR
Metallic and nonmetallic double perovskites: A case study of AFeReO (A= Ca, Sr, Ba)
We have investigated the structure and electronic properties of ferrimagnetic
double perovskites, A2FeReO6 (A= Ca, Sr, Ba). The A=Ba phase is cubic (Fm3m)
and metallic, while the A=Ca phase is monoclinic (P21/n) and nonmetallic.
57Fe Mossbauer spectroscopy shows that iron is present mainly in the
high-spin (S=5/2) Fe3+ state in the Ca compound, while it occurs in an
intermediate state between high-spin Fe2+ and Fe3+ in the Ba compound. It is
argued that a direct Re t2g - Re t2g interaction is the main cause for the
metallic character of the Ba compound; the high covalency of Ca-O bonds and the
monoclinic distortion (which lifts the degeneracy of t2g states) seem to
disrupt the Re-Re interaction in the case of the Ca compound, making it
non-metallic for the same electron count.Comment: 1 eps fil
Chrysomelidial in the Opisthonotal Glands of the Oribatid Mite, Oribotritia berlesei
Gas chromatographic–mass spectrometric analyses of whole body extracts of Oribotritia berlesei, a large-sized soil-dwelling oribatid mite, revealed a consistent chemical pattern of ten components, probably originating from the well-developed opisthonotal glands. The three major components of the extract were the iridoid monoterpene, (3S,8S)-chrysomelidial (about 45% of the extract), the unsaturated hydrocarbon 6,9-heptadecadiene, and the diterpene β-springene (the latter two, each about 20–25% of the extract). The remaining minor components (together about 10% of the extract) included a series of hydrocarbons (tridecene, tridecane, pentadecene, pentadecane, 8-heptadecene, and heptadecane) and the tentatively identified 9,17-octadecadienal. In contrast, analysis of juveniles showed only two compounds, namely a 2:1 mixture of (3S,8S)-chrysomelidial and its epimer, epi-chrysomelidial (3S,8R-chrysomelidial). Unexpectedly, neither adult nor juvenile secretions contained the so-called astigmatid compounds, which are considered characteristic of secretions of oribatids above moderately derived Mixonomata. The chrysomelidials, as well as β-springene and octadecadienal, are newly identified compounds in the opisthonotal glands of oribatid mites and have chemotaxonomic potential for this group. This is the first instance of finding chrysomelidials outside the Coleoptera