901 research outputs found
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
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
Electrical control of spin dynamics in finite one-dimensional systems
We investigate the possibility of the electrical control of spin transfer in
monoatomic chains incorporating spin-impurities. Our theoretical framework is
the mixed quantum-classical (Ehrenfest) description of the spin dynamics, in
the spirit of the s-d-model, where the itinerant electrons are described by a
tight-binding model while localized spins are treated classically. Our main
focus is on the dynamical exchange interaction between two well-separated
spins. This can be quantified by the transfer of excitations in the form of
transverse spin oscillations. We systematically study the effect of an
electrostatic gate bias V_g on the interconnecting channel and we map out the
long-range dynamical spin transfer as a function of V_g. We identify regions of
V_g giving rise to significant amplification of the spin transmission at low
frequencies and relate this to the electronic structure of the channel.Comment: 9 pages, 11 figure
Spatially Resolved Excitation of Rydberg Atoms and Surface Effects on an Atom Chip
We demonstrate spatially resolved, coherent excitation of Rydberg atoms on an
atom chip. Electromagnetically induced transparency (EIT) is used to
investigate the properties of the Rydberg atoms near the gold coated chip
surface. We measure distance dependent shifts (~10 MHz) of the Rydberg energy
levels caused by a spatially inhomogeneous electric field. The measured field
strength and distance dependence is in agreement with a simple model for the
electric field produced by a localized patch of Rb adsorbates deposited on the
chip surface during experiments. The EIT resonances remain narrow (< 4 MHz) and
the observed widths are independent of atom-surface distance down to ~20 \mum,
indicating relatively long lifetime of the Rydberg states. Our results open the
way to studies of dipolar physics, collective excitations, quantum metrology
and quantum information processing involving interacting Rydberg excited atoms
on atom chips
Two phase transitions in the fully frustrated model
The fully frustrated model on a square lattice is studied by means of
Monte Carlo simulations. A Kosterlitz-Thouless transition is found at , followed by an ordinary Ising transition at a slightly
higher temperature, . The non-Ising exponents reported by
others, are explained as a failure of finite size scaling due to the screening
length associated with the nearby Kosterlitz-Thouless transition.Comment: REVTEX file, 8 pages, 5 figures in uuencoded postscrip
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
Conditional phase shift from a quantum dot in a pillar microcavity
Large conditional phase shifts from coupled atom-cavity systems are a key
requirement for building a spin photon interface. This in turn would allow the
realisation of hybrid quantum information schemes using spin and photonic
qubits. Here we perform high resolution reflection spectroscopy of a quantum
dot resonantly coupled to a pillar microcavity. We show both the change in
reflectivity as the quantum dot is tuned through the cavity resonance, and
measure the conditional phase shift induced by the quantum dot using an ultra
stable interferometer. These techniques could be extended to the study of
charged quantum dots, where it would be possible to realise a spin photon
interface
Emulsification in binary liquids containing colloidal particles: a structure-factor analysis
We present a quantitative confocal-microscopy study of the transient and
final microstructure of particle-stabilised emulsions formed via demixing in a
binary liquid. To this end, we have developed an image-analysis method that
relies on structure factors obtained from discrete Fourier transforms of
individual frames in confocal image sequences. Radially averaging the squared
modulus of these Fourier transforms before peak fitting allows extraction of
dominant length scales over the entire temperature range of the quench. Our
procedure even yields information just after droplet nucleation, when the
(fluorescence) contrast between the two separating phases is scarcely
discernable in the images. We find that our emulsions are stabilised on
experimental time scales by interfacial particles and that they are likely to
have bimodal droplet-size distributions. We attribute the latter to coalescence
together with creaming being the main coarsening mechanism during the late
stages of emulsification and we support this claim with (direct)
confocal-microscopy observations. In addition, our results imply that the
observed droplets emerge from particle-promoted nucleation, possibly followed
by a free-growth regime. Finally, we argue that creaming strongly affects
droplet growth during the early stages of emulsification. Future investigations
could clarify the link between quench conditions and resulting microstructure,
paving the way for tailor-made particle-stabilised emulsions from binary
liquids.Comment: http://iopscience.iop.org/0953-8984/22/45/455102
Phase transition in ultrathin magnetic films with long-range interactions: Monte Carlo simulation of the anisotropic Heisenberg model
Ultrathin magnetic films can be modeled as an anisotropic Heisenberg model
with long-range dipolar interactions. It is believed that the phase diagram
presents three phases: An ordered ferromagnetic phase I, a phase characterized
by a change from out-of-plane to in-plane in the magnetization II, and a
high-temperature paramagnetic phase III. It is claimed that the border lines
from phase I to III and II to III are of second order and from I to II is first
order. In the present work we have performed a very careful Monte Carlo
simulation of the model. Our results strongly support that the line separating
phases II and III is of the BKT type.Comment: 7 page
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