1,788 research outputs found
Role of friction in multidefect ordering
We use continuum simulations to study the impact of friction on the ordering
of defects in an active nematic. Even in a frictionless system, +1/2 defects
tend to align side-by-side and orient antiparallel reflecting their propensity
to form, and circulate with, flow vortices. Increasing friction enhances the
effectiveness of the defect-defect interactions, and defects form dynamically
evolving, large scale, positionally and orientationally-ordered structures
which can be explained as a competition between hexagonal packing, preferred by
the -1/2 defects, and rectangular packing preferred by the +1/2 defects
Computational physics
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Three Phases in the 3D Abelian Higgs Model with Nonlocal Gauge Interactions
We study the phase structure of the 3D nonlocal compact U(1) lattice gauge
theory coupled with a Higgs field by means of Monte-Carlo simulations. The
nonlocal interactions among gauge variables are along the temporal direction
and mimic the effect of local coupling to massless particles. We found that in
contrast to the 3D local abelian Higgs model which has only one phase, the
present model exhibits the confinement, Higgs, and Coulomb phases separated by
three second-order transition lines emanating from a triple point. This result
is quite important for studies on electron fractionalization phenomena in
strongly-correlated electron systems. Implications to them are discussed
Formation and properties of metal-oxygen atomic chains
Suspended chains consisting of single noble metal and oxygen atoms have been
formed. We provide evidence that oxygen can react with and be incorporated into
metallic one-dimensional atomic chains. Oxygen incorporation reinforces the
linear bonds in the chain, which facilitates the creation of longer atomic
chains. The mechanical and electrical properties of these diatomic chains have
been investigated by determining local vibration modes of the chain and by
measuring the dependence of the average chain-conductance on the length of the
chain. Additionally, we have performed calculations that give insight in the
physical mechanism of the oxygen-induced strengthening of the linear bonds and
the conductance of the metal-oxygen chains.Comment: 10 pages, 9 fig
Interference enhanced thermoelectricity in quinoid type structures
Quantum interference (QI) effects in molecular junctions may be used to
obtain large thermoelectric responses. We study the electrical conductance G
and the thermoelec- tric response of a series of molecules featuring a quinoid
core using density functional theory (DFT), as well as a semi-empirical
interacting model Hamiltonian describing the {\pi}-system of the molecule which
we treat in the GW approximation. Molecules with a quinoid type structure are
shown to have two distinct destructive QI features close to the frontier
orbital energies. These manifest themselves as two dips in the transmission,
that remain separated, even when either electron donating or withdraw- ing side
groups are added. We find that the position of the dips in the transmission and
the frontier molecular levels can be chemically controlled by varying the
electron donating or withdrawing character of the side groups as well as the
conjugation length inside the molecule. This feature results in a very high
thermoelectric power factor S^2G and figure of merit ZT, where S is the Seebeck
coefficient, making quinoid type molecules potential candidates for efficient
thermoelectric devices.Comment: 22 pages, 11 figure
Electric-field control of interfering transport pathways in a single-molecule anthraquinone transistor
It is understood that molecular conjugation plays an important role in charge
transport through single-molecule junctions. Here, we investigate electron
transport through an anthraquinone based single-molecule three-terminal device.
With the use of an electric-field induced by a gate electrode, the molecule is
reduced resulting into a ten-fold increase in the off-resonant differential
conductance. Theoretical calculations link the change in differential
conductance to a reduction-induced change in conjugation, thereby lifting
destructive interference of transport pathways.Comment: Nano Letters (2015
Oxygen enhanced atomic chain formation
We report experimental evidence for atomic chain formation during stretching
of atomic-sized contacts for gold and silver, that is strongly enhanced due to
oxygen incorporation. While gold has been known for its tendency to form atomic
chains, for silver this is only observed in the presence of oxygen. With oxygen
the silver chains are as long as those for gold, but the conductance drops with
chain length to about 0.1 conductance quantum. A relation is suggested with
previous work on surface reconstructions for silver (110) surfaces after
chemisorption of oxygen.Comment: 4 pages, 4 figure
Opportunities and limitations of transition voltage spectroscopy: a theoretical analysis
In molecular charge transport, transition voltage spectroscopy (TVS) holds
the promise that molecular energy levels can be explored at bias voltages lower
than required for resonant tunneling. We investigate the theoretical basis of
this novel tool, using a generic model. In particular, we study the length
dependence of the conducting frontier orbital and of the 'transition voltage'
as a function of length. We show that this dependence is influenced by the
amount of screening of the electrons in the molecule, which determines the
voltage drop to be located at the contacts or across the entire molecule. We
observe that the transition voltage depends significantly on the length, but
that the ratio between the transition voltage and the conducting frontier
orbital is approximately constant only in strongly screening (conjugated)
molecules. Uncertainty about the screening within a molecule thus limits the
predictive power of TVS. We furthermore argue that the relative length
independence of the transition voltage for non-conjugated chains is due to
strong localization of the frontier orbitals on the end groups ensuring binding
of the rods to the metallic contacts. Finally, we investigate the
characteristics of TVS in asymmetric molecular junctions. If a single level
dominates the transport properties, TVS can provide a good estimate for both
the level position and the degree of junction asymmetry. If more levels are
involved the applicability of TVS becomes limited.Comment: 8 pages, 12 figure
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