272 research outputs found
Inelastic effects in electron transport studied with wave packet propagation
A time-dependent approach is used to explore inelastic effects during
electron transport through few-level systems. We study a tight-binding chain
with one and two sites connected to vibrations. This simple but transparent
model gives insight about inelastic effects, their meaning and the
approximations currently used to treat them. Our time-dependent approach allows
us to trace back the time sequence of vibrational excitation and electronic
interference, the ibrationally introduced time delay and the electronic phase
shift. We explore a full range of parameters going from weak to strong
electron-vibration coupling, from tunneling to contact, from one-vibration
description to the need of including all vibrations for a correct description
of inelastic effects in transport. We explore the validity of single-site
resonant models as well as its extension to more sites via molecular orbitals
and the conditions under which multi-orbital, multi-vibrational descriptions
cannot be simplified. We explain the physical meaning of the spectral features
in the second derivative of the electron current with respect to the bias
voltage. This permits us to nuance the meaning of the energy value of dips and
peaks. Finally, we show that finite-band effects lead to electron
back-scattering off the molecular vibrations in the regime of high-conductance,
although the drop in conductance at the vibrational threshold is rather due to
the rapid variation of the vibronic density of states.Comment: 38 pages, 14 figure
Relation between inelastic electron tunneling and vibrational excitation of single adsorbates on metal surfaces
We analyse theoretically a relation between the vibrational generation rate
of a single adsorbate by tunneling electrons and the inelastic tunneling (IET)
current in scanning tunneling microscope, and the influence of the vibrational
excitations on the rate of adsorbate motions. Special attention is paid to the
effects of finite lifetime of the vibrational excitations. We show that in the
vicinity and below the IET threshold the rate of adsorbate motion deviates from
a simple power-law dependence on the bias voltage due to the effects of bath
temperature and adsorbate vibrational lifetime broadenings. The temperature
broadening appears to be confined near the threshold voltage within a narrow
region of several , whereas the lifetime broadening manifests itself in
a much wider region of applied voltages below the IET threshold.Comment: 8 pages including 4 figure
Inelastic transport theory from first-principles: methodology and applications for nanoscale devices
We describe a first-principles method for calculating electronic structure,
vibrational modes and frequencies, electron-phonon couplings, and inelastic
electron transport properties of an atomic-scale device bridging two metallic
contacts under nonequilibrium conditions. The method extends the
density-functional codes SIESTA and TranSIESTA that use atomic basis sets. The
inelastic conductance characteristics are calculated using the nonequilibrium
Green's function formalism, and the electron-phonon interaction is addressed
with perturbation theory up to the level of the self-consistent Born
approximation. While these calculations often are computationally demanding, we
show how they can be approximated by a simple and efficient lowest order
expansion. Our method also addresses effects of energy dissipation and local
heating of the junction via detailed calculations of the power flow. We
demonstrate the developed procedures by considering inelastic transport through
atomic gold wires of various lengths, thereby extending the results presented
in [Frederiksen et al., Phys. Rev. Lett. 93, 256601 (2004)]. To illustrate that
the method applies more generally to molecular devices, we also calculate the
inelastic current through different hydrocarbon molecules between gold
electrodes. Both for the wires and the molecules our theory is in quantitative
agreement with experiments, and characterizes the system-specific mode
selectivity and local heating.Comment: 24 pages, 17 figure
Perturbation Study of the Conductance through an Interacting Region Connected to Multi-Mode Leads
We study the effects of electron correlation on transport through an
interacting region connected to multi-mode leads based on the perturbation
expansion with respect to the inter-electron interaction. At zero temperature
the conductance defined in the Kubo formalism can be written in terms of a
single-particle Green's function at the Fermi energy, and it can be mapped onto
a transmission coefficient of the free quasiparticles described by an effective
Hamiltonian. We apply this formulation to a two-dimensional Hubbard model of
finite size connected to two noninteracting leads. We calculate the conductance
in the electron-hole symmetric case using the order self-energy. The
conductance shows several maximums in the dependence in some parameter
regions of , where () is the hopping matrix element in the
- (-) directions. This is caused by the resonance occurring in some of
the subbands, and is related with the dependence of the eigenvalues of the
effective Hamiltonian.Comment: 17 pages, 12 figures, to be published in J.Phys.Soc.Jpn. 71(2002)No.
Proceedings of the Salford Postgraduate Annual Research Conference (SPARC) 2011
These proceedings bring together a selection of papers from the 2011 Salford Postgraduate Annual Research Conference(SPARC). It includes papers from PhD students in the arts and social sciences, business, computing, science and engineering, education, environment, built environment and health sciences. Contributions from Salford researchers are published here alongside papers from students at the Universities of Anglia Ruskin, Birmingham City, Chester,De Montfort, Exeter, Leeds, Liverpool, Liverpool John Moores and Manchester
Mechanism for the Suppression of Intermediate-Mass Black Holes
A model for the formation of supermassive primordial black holes in galactic
nuclei with the simultaneous suppression of the formation of intermediate-mass
black holes is presented. A bimodal mass function for black holes formed
through phase transitions in a model with a "Mexican hat" potential has been
found. The classical motion of the phase of a complex scalar field during
inflation has been taken into account. Possible observational manifestations of
primordial black holes in galaxies and constraints on their number are
discussed.Comment: 12 pages, 2 figure
Virtual palaeontology: the effects of mineral composition and texture of fossil shell and hosting rock on the quality of X-ray microtomography (XMT) outcomes using Palaeozoic brachiopods
X-ray microtomography (XMT) has become a popular tool for detailed investigations of a diverse range of fossils. However, XMT has not always guaranteed a satisfactory result, as the resolution of XMT images critically depends on the contrast between the fossil and its hosting rock. In this paper, XMT was applied to 11 Palaeozoic brachiopod specimens selected from a range of sedimentary rocks in order to investigate the extent of effects of mineral composition and texture in the rock and fossil shell on the quality of XMT outcomes. Our study shows that sufficient contrast in mineral composition and texture between the brachiopod shell and its infilling material is required to reproduce high-quality XMT results. Specifically, brachiopod specimens with their original calcium carbonate shell, infilled mainly with quartz grains, appear to produce the best XMT results characterized by sharply defined shell internal structures. We also found that diagenesis is significant in determining the XMT quality. Diagenetic processes including silicification and recrystallization in the brachiopod shell and/or the infilling material generally tends to diminish the resolution of the XMT results, although this impact is considerably complicated by the degree and aspect of diagenesis. Another factor of minor significance concerns the presence of bioclasts scattered in the hosting sediment that potentially could be confused with genuine shell internal structures
Human tribbles-1 controls proliferation and chemotaxis of smooth muscle cells via MAPK signaling pathways
Migration and proliferation of smooth muscle cells are key to a number of physiological and pathological processes, including wound healing and the narrowing of the vessel wall.Previous work has shown links between inflammatory stimuli and vascular smooth muscle cell proliferation and migration through mitogen activated protein kinase (MAPK) activation, though the molecular mechanisms of this process are poorly understood.
Here we report that tribbles-1, a recently described modulator of MAPK activation controls vascular smooth muscle cell proliferation and chemotaxis via the Jun Kinase pathway. Our findings demonstrate that this regulation takes place via direct interactions between tribbles-1 and MKK4/SEK1, a Jun activator kinase. The
activity of this kinase is dependent on tribbles-1 levels, whilst the activation and the expression of MKK4/SEK1 is not. In addition, tribbles-1 expression is elevated in
human atherosclerotic arteries compared to non-atherosclerotic controls, suggesting that this protein may pay a role in disease in vivo. In summary, the data presented here suggest an important regulatory role for trb-1 in vascular smooth muscle cell biology
Nonlinear climatic sensitivity to greenhouse gases over past 4 glacial/interglacial cycles.
The paleoclimatic sensitivity to atmospheric greenhouse gases (GHGs) has recently been suggested to be nonlinear, however a GHG threshold value associated with deglaciation remains uncertain. Here, we combine a new sea surface temperature record spanning the last 360,000 years from the southern Western Pacific Warm Pool with records from five previous studies in the equatorial Pacific to document the nonlinear relationship between climatic sensitivity and GHG levels over the past four glacial/interglacial cycles. The sensitivity of the responses to GHG concentrations rises dramatically by a factor of 2-4 at atmospheric CO2 levels of >220 ppm. Our results suggest that the equatorial Pacific acts as a nonlinear amplifier that allows global climate to transition from deglacial to full interglacial conditions once atmospheric CO2 levels reach threshold levels
Intermediate-mass black holes and ultraluminous X-ray sources in the Cartwheel ring galaxy
Chandra and XMM-Newton observations of the Cartwheel galaxy show ~17 bright
X-ray sources (>~5x10^38 erg s^-1), all within the gas-rich outer ring. We
explore the hypothesis that these X-ray sources are powered by
intermediate-mass black holes (IMBHs) accreting gas or undergoing mass transfer
from a stellar companion. To this purpose, we run N-body/SPH simulations of the
galaxy interaction which might have led to the formation of Cartwheel, tracking
the dynamical evolution of two different IMBH populations: halo and disc IMBHs.
Halo IMBHs cannot account for the observed X-ray sources, as only a few of them
cross the outer ring. Instead, more than half of the disc IMBHs are pulled in
the outer ring as a consequence of the galaxy collision. However, also in the
case of disc IMBHs, accretion from surrounding gas clouds cannot account for
the high luminosities of the observed sources. Finally, more than 500 disc
IMBHs are required to produce <~15 X-ray sources via mass transfer from very
young stellar companions. Such number of IMBHs is very large and implies
extreme assumptions. Thus, the hypothesis that all the observed X-ray sources
in Cartwheel are associated with IMBHs is hardly consistent with our
simulations, even if it is still possible that IMBHs account for the few
(<~1-5) brightest ultraluminous X-ray sources (ULXs).Comment: 16 pages, 12 figures, MNRAS, in press, higher resolution version at
http://www-theorie.physik.unizh.ch/~mapelli/astroph/cartwheel_ULX2.p
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