167 research outputs found
Conductance quantization in graphene nanoconstrictions with mesoscopically smooth but atomically stepped boundaries
We present the results of million atom electronic quantum transport
calculations for graphene nanoconstrictions with edges that are smooth apart
from atomic scale steps. We find conductances quantized in integer multiples of
2e2/h and a plateau at ~0.5*2e2/h as in recent experiments [Tombros et al.,
Nature Physics 7, 697 (2011)]. We demonstrate that, surprisingly, conductances
quantized in integer multiples of 2e2/h occur even for strongly non-adiabatic
electron backscattering at the stepped edges that lowers the conductance by one
or more conductance quanta below the adiabatic value. We also show that
conductance plateaus near 0.5*2e2/h can occur as a result of electron
backscattering at stepped edges even in the absence of electron-electron
interactions.Comment: 5 pages, 4 figure
Nonlinear conductance quantization in graphene ribbons
We present numerical studies of non-linear conduction in graphene nanoribbons
when a bias potential is applied between the source and drain electrodes. We
find that the conductance quantization plateaus show asymmetry between the
electron and hole branches if the potential in the ribbon equals the source or
drain electrode potential and strong electron (hole) scattering occurs. The
scattering may be at the ends of a uniform ballistic ribbon connecting wider
regions of graphene or may be due to defects in the ribbon. We argue that, in
ribbons with strong defect scattering, the ribbon potential is pinned to that
of the drain (source) for electron (hole) transport. In this case symmetry
between electron and hole transport is restored and our calculations explain
the upward shift of the conductance plateaus with increasing bias that was
observed experimentally by Lin et al. [Phys. Rev. B 78, 161409 (2008)].Comment: 6 pages, 3 figure
Identification of the Atomic Scale Structures of the Gold-Thiol Interfaces of Molecular Nanowires by Inelastic Tunneling Spectroscopy
We examine theoretically the effects of the bonding geometries at the
gold-thiol interfaces on the inelastic tunneling spectra of propanedithiolate
(PDT) molecules bridging gold electrodes and show that inelastic tunneling
spectroscopy combined with theory can be used to determine these bonding
geometries experimentally. With the help of density functional theory, we
calculate the relaxed geometries and vibrational modes of extended molecules
each consisting of one or two PDT molecules connecting two gold nanoclusters.
We formulate a perturbative theory of inelastic tunneling through molecules
bridging metal contacts in terms of elastic transmission amplitudes, and use
this theory to calculate the inelastic tunneling spectra of the gold-PDT-gold
extended molecules. We consider PDT molecules with both trans and gauche
conformations bound to the gold clusters at top, bridge and hollow bonding
sites. Comparing our results with the experimental data of Hihath et al. [Nano
Lett. 8, 1673 (2008)], we identify the most frequently realized conformation in
the experiment as that of trans molecules top-site bonded to both electrodes.
We find the switching from the 42 meV vibrational mode to the 46 meV mode
observed in the experiment to be due to the transition of trans molecules from
mixed top-bridge to pure top-site bonding geometries. Our results also indicate
that gauche molecular conformations and hollow site bonding did not contribute
significantly to the experimental inelastic tunneling spectra. For pairs of PDT
molecules connecting the gold electrodes in parallel we find total elastic
conductances close to twice those of single molecules bridging the contacts
with similar bonding conformations and small splittings of the vibrational mode
energies for the modes that are the most sensitive to the molecule-electrode
bonding geometries.Comment: 14 pages, 8 figures, 1 table. arXiv admin note: significant text
overlap with arXiv:1103.2378;
http://jcp.aip.org/resource/1/jcpsa6/v136/i1/p014703_s
Magnetic edge states of impenetrable stripe
The electron motion in a strong perpendicular magnetic field close to the
impenetrable stripe is considered by making use of the singular integral
equation technique. The energy spectrum is calculated and compared with the
energy spectrum of the round antidot.Comment: REVTeX4 format, 9 pages with 9 figures (*.eps
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