1,261 research outputs found

### 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

### 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

### 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

### 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

### Quantum Monte Carlo study of a magnetic-field-driven 2D superconductor-insulator transition

We numerically study the superconductor-insulator phase transition in a model
disordered 2D superconductor as a function of applied magnetic field. The
calculation involves quantum Monte Carlo calculations of the (2+1)D XY model in
the presence of both disorder and magnetic field. The XY coupling is assumed to
have the form -J\cos(\theta_i-\theta_j-A_{ij}), where A_{ij} has a mean of zero
and a standard deviation \Delta A_{ij}. In a real system, such a model would be
approximately realized by a 2D array of small Josephson-coupled grains with
slight spatial disorder and a uniform applied magnetic field. The different
values \Delta A_{ij} then corresponds to an applied field such that the average
number of flux quanta per plaquette has various integer values N: larger N
corresponds to larger \Delta A_{ij}. For any value of \Delta A_{ij}, there
appears to be a critical coupling constant K_c(\Delta
A_{ij})=\sqrt{[J/(2U)]_c}, where U is the charging energy, above which the
system is a Mott insulator; there is also a corresponding critical conductivity
\sigma^*(\Delta A_{ij}) at the transition. For \Delta A_{ij}=\infty, the order
parameter of the transition is a renormalized coupling constant g. Using a
numerical technique appropriate for disordered systems, we show that the
transition at this value of \Delta A_{ij} takes place from an insulating (I)
phase to a Bose glass (BG) phase, and that the dynamical critical exponent
characterizing this transition is z \sim 1.3. By contrast, z=1 for this model
at \Delta A_{ij}=0. We suggest that the superconductor to insulator transition
is actually of this I to BG class at all nonzero \Delta A_{ij}'s, and we
support this interpretation by both numerical evidence and an analytical
argument based on the Harris criterion.Comment: 17 pages, 23 figures, accepted for publication in Phys. Rev.

### Image effects in transport at metal-molecule interfaces

We present a method for incorporating image-charge effects into the
description of charge transport through molecular devices. A simple model
allows us to calculate the adjustment of the transport levels, due to the
polarization of the electrodes as charge is added to and removed from the
molecule. For this, we use the charge distributions of the molecule between two
metal electrodes in several charge states, rather than in gas phase, as
obtained from a density-functional theory-based transport code. This enables us
to efficiently model level shifts and gap renormalization caused by
image-charge effects, which are essential for understanding molecular transport
experiments. We apply the method to benzene di-amine molecules and compare our
results with the standard approach based on gas phase charges. Finally, we give
a detailed account of the application of our approach to porphyrin-derivative
devices recently studied experimentally by Perrin et al. [Nat. Nanotechnol. 8,
282 (2013)], which demonstrates the importance of accounting for image-charge
effects when modeling transport through molecular junctions

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