1,319 research outputs found
Silicon - single molecule - silicon circuits
In 2020, silicon - molecule - silicon junctions were fabricated and shown to be on average one third as conductive as traditional junctions made using gold electrodes, but in some instances to be even more conductive, and significantly 3 times more extendable and 5 times more mechanically stable. Herein, calculations are performed of single-molecule junction structure and conductivity pertaining to blinking and scanning-tunnelling-microscopy (STM) break junction (STMBJ) experiments performed using chemisorbed 1,6-hexanedithiol linkers. Some strikingly different characteristics are found compared to analogous junctions formed using the metals which, to date, have dominated the field of molecular electronics. In the STMBJ experiment, following retraction of the STM tip after collision with the substrate, unterminated silicon surface dangling bonds are predicted to remain after reaction of the fresh tips with the dithiol solute. These dangling bonds occupy the silicon band gap and are predicted to facilitate extraordinary single-molecule conductivity. Enhanced junction extendibility is attributed to junction flexibility and the translation of adsorbed molecules between silicon dangling bonds. The calculations investigate a range of junction atomic-structural models using density-functional-theory (DFT) calculations of structure, often explored at 300 K using molecular dynamics (MD) simulations. These are aided by DFT calculations of barriers for passivation reactions of the dangling bonds. Thermally averaged conductivities are then evaluated using non-equilibrium Green's function (NEGF) methods. Countless applications through electronics, nanotechnology, photonics, and sensing are envisaged for this technology
Speech intelligibility impact Level (SiimpL) - Beskrivning och utvÀrdering
Bakgrund: Höga ljudnivĂ„er och bullerskador Ă€r ett vanligt problem pĂ„ arbetsplatser. MĂ„nga vĂ€ljer att inte anvĂ€nda hörselskydd dĂ„ de upplevs försvĂ„ra kommunikation. En metod som med ett enkelt mĂ„tt kan utvĂ€rdera hörselskydds pĂ„verkan av taluppfattning vore vĂ€rdefullt. Syfte: Syftet med studien var att undersöka möjligheten att med metoden Speech intelligibility impact Level (SiimpL) mĂ€ta skillnad mellan olika hörselskydd med hĂ€nsyn till deras pĂ„verkan pĂ„ taluppfattning i buller samt att utvĂ€rdera inlĂ€rningseffekter, tidsĂ„tgĂ„ng och ordmaterial. Metod: Studien genomfördes i tre experiment med normalhörande forskningspersoner. I experiment 1 deltog fem forskningspersoner (22-29 Ă„r) som utförde SiimpL fem gĂ„nger om dagen i fem dagar för att kartlĂ€gga inlĂ€rningseffekter och tidsĂ„tgĂ„ng. Testgruppen i experiment 2 bestod av 20 forskningspersoner (19-37 Ă„r) som utförde SiimpL sju gĂ„nger i olika kombinationer av brus och simulerade hörselskydd. I experiment 3 ingick sex forsknings-personer frĂ„n experiment 2 som visat stor skillnad mellan olika hörselskydd. Fyra (22-26 Ă„r) testades om i ett högfrekvent brus och tvĂ„ (19-20 Ă„r) testades i ett lĂ„gfrekvent brus. Totalt utförde forskningspersonerna i experiment 3 ytterligare 12 omgĂ„ngar för att undersöka skillnader pĂ„ individnivĂ„. Resultat: Experiment 1 visade en statistiskt signifikant förbĂ€ttring för gruppen mellan testomgĂ„ng ett och fem under den första dagen. Ăven mellan dag tvĂ„ och tre fanns en statistiskt signifikant förbĂ€ttring. I experiment 2 kunde statistiskt signifikanta skillnader pĂ„visas mellan olika hörselskydd i högfrekvent brus och med och utan hörselskydd i lĂ„gfrekvent brus. Hörselskydden resulterade genomgĂ„ende i att brusnivĂ„n som kunde tolereras innan taluppfattningen pĂ„verkades blev lĂ€gre. TidsĂ„tgĂ„ngen för en testomgĂ„ng minskades i experiment 2 med 65 sekunder jĂ€mfört med experiment 1 utan konsekvens för stabiliteten i resultatet. Resultatet i experiment 3 visade att det inte var möjligt att uppnĂ„ statistiskt signifikanta skillnader mellan hörselskydd pĂ„ individnivĂ„ Diskussion: UtifrĂ„n resultaten diskuteras förslag pĂ„ framtida studier och klinisk relevans för SiimpL
Efficient Few-Shot Learning Without Prompts
Recent few-shot methods, such as parameter-efficient fine-tuning (PEFT) and
pattern exploiting training (PET), have achieved impressive results in
label-scarce settings. However, they are difficult to employ since they are
subject to high variability from manually crafted prompts, and typically
require billion-parameter language models to achieve high accuracy. To address
these shortcomings, we propose SetFit (Sentence Transformer Fine-tuning), an
efficient and prompt-free framework for few-shot fine-tuning of Sentence
Transformers (ST). SetFit works by first fine-tuning a pretrained ST on a small
number of text pairs, in a contrastive Siamese manner. The resulting model is
then used to generate rich text embeddings, which are used to train a
classification head. This simple framework requires no prompts or verbalizers,
and achieves high accuracy with orders of magnitude less parameters than
existing techniques. Our experiments show that SetFit obtains comparable
results with PEFT and PET techniques, while being an order of magnitude faster
to train. We also show that SetFit can be applied in multilingual settings by
simply switching the ST body. Our code is available at
https://github.com/huggingface/setfit and our datasets at
https://huggingface.co/setfit
Hydrodynamical Simulations of the Lyman Alpha Forest: Model Comparisons
We investigate the properties of the Lyman alpha forest as predicted by
numerical simulations for a range of currently viable cosmological models. This
is done in order to understand the dependencies of the forest on cosmological
parameters. Focusing on the redshift range from two to four, we show that: (1)
most of the evolution in the distributions of optical depth, flux and column
density can be understood by simple scaling relations, (2) the shape of optical
depth distribution is a sensitive probe of the amplitude of density
fluctuations on scales of a few hundred kpc, (3) the mean of the b distribution
(a measure of the width of the absorption lines) is also very sensitive to
fluctuations on these scales, and decreases as they increase. We perform a
preliminary comparison to observations, where available. A number of other
properties are also examined, including the evolution in the number of lines,
the two-point flux distribution and the HeII opacity.Comment: 37 pages, 21 figures, submitted to Ap
Controlling piezoresistance in single molecules through the isomerisation of bullvalenes
Nanoscale electro-mechanical systems (NEMS) displaying piezoresistance offer unique measurement opportunities at the sub-cellular level, in detectors and sensors, and in emerging generations of integrated electronic devices. Here, we show a single-molecule NEMS piezoresistor that operates utilising constitutional and conformational isomerisation of individual diaryl-bullvalene molecules and can be switched at 850 Hz. Observations are made using scanning tunnelling microscopy break junction (STMBJ) techniques to characterise piezoresistance, combined with blinking (current-time) experiments that follow single-molecule reactions in real time. A kinetic Monte Carlo methodology (KMC) is developed to simulate isomerisation on the experimental timescale, parameterised using density-functional theory (DFT) combined with non-equilibrium Greenâs function (NEGF) calculations. Results indicate that piezoresistance is controlled by both constitutional and conformational isomerisation, occurring at rates that are either fast (equilibrium) or slow (non-equilibrium) compared to the experimental timescale. Two different types of STMBJ traces are observed, one typical of traditional experiments that are interpreted in terms of intramolecular isomerisation occurring on stable tipped-shaped metal-contact junctions, and another attributed to arise from junctionâinterface restructuring induced by bullvalene isomerisation
A Large Stellar Evolution Database for Population Synthesis Studies. III. Inclusion of the full Asymptotic Giant Branch phase and Web tools for stellar population analyses
Stellar evolution tracks and isochrones are key inputs for a wide range of
astrophysical studies; in particular, they are essential to the interpretation
of photometric and spectroscopic observations of resolved and unresolved
stellar populations. We have made available to the astrophysical community a
large, homogenous database of up-to-date stellar tracks and isochrones, and a
set of programs useful in population synthesis studies.
In this paper we first summarize the main properties of our stellar model
database (BaSTI) already introduced in Pietrinferni et al. (2004) and
Pietrinferni et al. (2006). We then discuss an important update of the
database, i.e., the extension of all stellar models and isochrones until the
end of the thermal pulses along the Asymptotic Giant Branch. This extension of
the library is particularly relevant for stellar population analyses in the
near-infrared, or longer wavelengths, where the contribution to the integrated
photometric properties by cool and bright Asymptotic Giant Branch stars is
significant. A few comparisons with empirical data are also presentend and
briefly discussed. We then present three web-tools that allow an interactive
access to the database, and make possible to compute user-specified
evolutionary tracks, isochrones, stellar luminosity functions, plus synthetic
Color-Magnitude-Diagrams and integrated magnitudes for arbitrary Star Formation
Histories. All these web tools are available at the BaSTI database official
site: http://www.oa-teramo.inaf.it/BASTI.Comment: 18 pages, 5 tables, 7 figures, accepted in The Astronomical Journal.
The BaSTI database and its web-tools are available on the web
http://www.oa-teramo.inaf.it/BASTI. The web-tools are also mirrored in the
BaSTI section of http://astro.ensc-rennes.f
Spin chirality on a two-dimensional frustrated lattice
The collective behavior of interacting magnetic moments can be strongly
influenced by the topology of the underlying lattice. In geometrically
frustrated spin systems, interesting chiral correlations may develop that are
related to the spin arrangement on triangular plaquettes. We report a study of
the spin chirality on a two-dimensional geometrically frustrated lattice. Our
new chemical synthesis methods allow us to produce large single crystal samples
of KFe3(OH)6(SO4)2, an ideal Kagome lattice antiferromagnet. Combined
thermodynamic and neutron scattering measurements reveal that the phase
transition to the ordered ground-state is unusual. At low temperatures,
application of a magnetic field induces a transition between states with
different non-trivial spin-textures.Comment: 7 pages, 4 figure
Spontaneous SâSi bonding of alkanethiols to Si(111)âH: towards SiâmoleculeâSi circuits
We report the synthesis of covalently linked self-assembled monolayers (SAMs) on silicon surfaces, using mild conditions, in a way that is compatible with silicon-electronics fabrication technologies. In molecular electronics, SAMs of functional molecules tethered to gold via sulfur linkages dominate, but these devices are not robust in design and not amenable to scalable manufacture. Whereas covalent bonding to silicon has long been recognized as an attractive alternative, only formation processes involving high temperature and/or pressure, strong chemicals, or irradiation are known. To make molecular devices on silicon under mild conditions with properties reminiscent of AuâS ones, we exploit the susceptibility of thiols to oxidation by dissolved O2, initiating free-radical polymerization mechanisms without causing oxidative damage to the surface. Without thiols present, dissolved O2 would normally oxidize the silicon and hence reaction conditions such as these have been strenuously avoided in the past. The surface coverage on Si(111)âH is measured to be very high, 75% of a full monolayer, with density-functional theory calculations used to profile spontaneous reaction mechanisms. The impact of the SiâS chemistry in single-molecule electronics is demonstrated using STM-junction approaches by forming SiâhexanedithiolâSi junctions. SiâS contacts result in single-molecule wires that are mechanically stable, with an average lifetime at room temperature of 2.7 s, which is five folds higher than that reported for conventional molecular junctions formed between gold electrodes. The enhanced âONâ lifetime of this single-molecule circuit enables previously inaccessible electrical measurements on single molecules
The ACS Nearby Galaxy Survey Treasury IX. Constraining asymptotic giant branch evolution with old metal-poor galaxies
In an attempt to constrain evolutionary models of the asymptotic giant branch
(AGB) phase at the limit of low masses and low metallicities, we have examined
the luminosity functions and number ratio between AGB and red giant branch
(RGB) stars from a sample of resolved galaxies from the ACS Nearby Galaxy
Survey Treasury (ANGST). This database provides HST optical photometry together
with maps of completeness, photometric errors, and star formation histories for
dozens of galaxies within 4 Mpc. We select 12 galaxies characterized by
predominantly metal-poor populations as indicated by a very steep and blue RGB,
and which do not present any indication of recent star formation in their
color--magnitude diagrams. Thousands of AGB stars brighter than the tip of the
RGB (TRGB) are present in the sample (between 60 and 400 per galaxy), hence the
Poisson noise has little impact in our measurements of the AGB/RGB ratio. We
model the photometric data with a few sets of thermally pulsing AGB (TP-AGB)
evolutionary models with different prescriptions for the mass loss. This
technique allows us to set stringent constraints to the TP-AGB models of
low-mass metal-poor stars (with M<1.5 Msun, [Fe/H]<~-1.0). Indeed, those which
satisfactorily reproduce the observed AGB/RGB ratios have TP-AGB lifetimes
between 1.2 and 1.8 Myr, and finish their nuclear burning lives with masses
between 0.51 and 0.55 Msun. This is also in good agreement with recent
observations of white dwarf masses in the M4 old globular cluster. These
constraints can be added to those already derived from Magellanic Cloud star
clusters as important mileposts in the arduous process of calibrating AGB
evolutionary models.Comment: To appear in ApJ, a version with better resolution is in
http://stev.oapd.inaf.it/~lgirardi/rgbagb.pd
The epochs of early-type galaxy formation as a function of environment
The aim of this paper is to set constraints of the epochs of early-type
galaxy formation through the 'archaeology' of the stellar populations in local
galaxies. Using our models of absorption line indices that account for variable
abundance ratios, we derive the stellar population parameters of 124 early-type
galaxies in high and low density environments. We find that all three
parameters age, metallicity, and alpha/Fe ratio are correlated with velocity
dispersion. We further find evidence for an influence of the environment on the
stellar population properties. Massive early-type galaxies in low-density
environments appear on average ~2 Gyrs younger and slightly more metal-rich
than their counterparts in high density environments. No offsets in the
alpha/Fe ratios, instead, are detected. We translate the derived ages and
alpha/Fe ratios into star formation histories. We show that most star formation
activity in early-type galaxies is expected to have happened between redshifts
3 and 5 in high density and between redshifts 1 and 2 in low density
environments. We conclude that at least 50 per cent of the total stellar mass
density must have already formed at z 1, in good agreement with observational
estimates of the total stellar mass density as a function of redshift. Our
results suggest that significant mass growth in the early-type galaxy
population below z 1 must be restricted to less massive objects, and a
significant increase of the stellar mass density between redshifts 1 and 2
should be present caused mainly by the field galaxy population. The results of
this paper further imply vigorous star formation episodes in massive objects at
z 2-5 and the presence of evolved ellipticals around z 1, both observationally
identified as SCUBA galaxies and EROs.Comment: 20 pages, 10 figures, plus appendix, accepted by Ap
- âŠ