102 research outputs found
Bistable hysteresis and resistance switching in hydrogen gold junctions
Current-voltage characteristics of H2-Au molecular junctions exhibit
intriguing steps around a characteristic voltage of 40 mV. Surprisingly, we
find that a hysteresis is connected to these steps with a typical time scale >
10 ms. This time constant scales linearly with the power dissipated in the
junction beyond an ofset power P_s = IV_s. We propose that the hysteresis is
related to vibrational heating of both the molecule in the junction and a set
of surrounding hydrogen molecules. Remarkably, we can engineer our junctions
such that the hysteresis' characteristic time becomes >days. We demonstrate
that reliable switchable devices can be built from such junctions.Comment: Submitted to Phys. Rev. Let
Global large carnivore conservation and international law
International cooperation, including through international legal instruments, appears important for the conservation of large carnivores worldwide. This is due to, inter alia, the worrying conservation status and population trends of many large carnivore species; the importance of large carnivores for biodiversity conservation at large; their occurrence at low densities, with many populations extending across various countries; and the international nature of particular threats. For the 31 heaviest species in the order Carnivora, this study (i) documents to what extent existing international legal instruments contribute to large carnivore conservation, and (ii) identifies ways of optimizing their contribution in this regard. From this dual perspective, it reviews all global wildlife conservation treatiesâRamsar Wetlands Convention, World Heritage Convention, Convention on Trade in Endangered Species, Convention on Migratory Species (CMS), Convention on Biological Diversity (CBD)âand selected regional instruments, using standard international law research methodology. Results indicate that a substantial body of relevant international law already exists, whereas simultaneously there is clear potential for enhancing the contribution of international law to large carnivore conservation. Avenues for pursuing this include promotion of instrumentsâ effective implementation; clarification of their precise implications for large carnivore conservation; development of formal guidance; expansion of instrumentsâ scope in terms of species, sites and countries; and creation of new instruments. The CMS and CBD hold particular potential in some of these respects. The experiences being gained under European legal instruments constitute an interesting âlaboratoryâ regarding human coexistence with expanding large carnivore populations and transboundary cooperation at the (sub)population level
The role of Joule heating in the formation of nanogaps by electromigration
We investigate the formation of nanogaps in gold wires due to
electromigration. We show that the breaking process will not start until a
local temperature of typically 400 K is reached by Joule heating. This value is
rather independent of the temperature of the sample environment (4.2-295 K).
Furthermore, we demonstrate that the breaking dynamics can be controlled by
minimizing the total series resistance of the system. In this way, the local
temperature rise just before break down is limited and melting effects are
prevented. Hence, electrodes with gaps < 2 nm are easily made, without the need
of active feedback. For optimized samples, we observe quantized conductance
steps prior the gap formation.Comment: including 7 figure
Electronic and optical properties of electromigrated molecular junctions
Electromigrated nanoscale junctions have proven very useful for studying
electronic transport at the single-molecule scale. However, confirming that
conduction is through precisely the molecule of interest and not some
contaminant or metal nanoparticle has remained a persistent challenge,
typically requiring a statistical analysis of many devices. We review how
transport mechanisms in both purely electronic and optical measurements can be
used to infer information about the nanoscale junction configuration. The
electronic response to optical excitation is particularly revealing. We briefly
discuss surface-enhanced Raman spectroscopy on such junctions, and present new
results showing that currents due to optical rectification can provide a means
of estimating the local electric field at the junction due to illumination.Comment: 19 pages, 8 figures, invited paper for forthcoming special issue of
Journal of Physics: Condensed Matter. For other related papers, see
http://www.ruf.rice.edu/~natelson/publications.htm
The mechanical response of lithographically defined break junctions
We present an experimental study on the mechanical response of lithographically defined break junctions by measuring atomic chain formation, tunneling traces and Gundlach oscillations. The calibration factor, i.e., the ratio between the electrode movement and the bending of the substrate, is found to be 2.5 times larger than expected from a simple mechanical model. This result is consistent with previous finite-element calculations. Comparing different samples, the mechanical response is found to be similar for electrode separations >4 angstrom. However, for smaller electrode separations significant sample-to-sample variations appear. These variations are ascribed to differences in the shape of the two electrodes on the atomic scale which cannot be controlled by the fabrication process
Single atom adhesion in optimized gold nanojunctions
We study the interaction between single apex atoms in a metallic contact,
using the break junction geometry. By carefully 'training' our samples, we
create stable junctions in which no further atomic reorganization takes place.
This allows us to study the relation between the so-called jump out of contact
(from contact to tunnelling regime) and jump to contact (from tunnelling to
contact regime) in detail. Our data can be fully understood within a relatively
simple elastic model, where the elasticity k of the electrodes is the only free
parameter. We find 5 < k < 32 N/m. Furthermore, the interaction between the two
apex atoms on both electrodes, observed as a change of slope in the tunnelling
regime, is accounted for by the same model
Stabilizing single atom contacts by molecular bridge formation
Gold-molecule-gold junctions can be formed by carefully breaking a gold wire
in a solution containing dithiolated molecules. Surprisingly, there is little
understanding on the mechanical details of the bridge formation process and
specifically on the role that the dithiol molecules play themselves. We propose
that alkanedithiol molecules have already formed bridges between the gold
electrodes before the atomic gold-gold junction is broken. This leads to
stabilization of the single atomic gold junction, as observed experimentally.
Our data can be understood within a simple spring model.Comment: 14 pages, 3 figures, 1 tabl
Transition Voltage Spectroscopy and the Nature of Vacuum Tunneling
Transition Voltage Spectroscopy (TVS) has been proposed as a tool to analyze
charge transport through molecular junctions. We extend TVS to Au-vacuum-Au
junctions and study the distance dependence of the transition voltage V_t(d)
for clean electrodes in cryogenic vacuum. On the one hand, this allows us to
provide an important reference for V_t(d)-measurements on molecular junctions.
On the other hand, we show that TVS forms a simple and powerful test for vacuum
tunneling models
Interpreting âfavourable conservation statusâ for large carnivores in Europe: how many are needed and how many are wanted?
The effects of pelvic ring injuries on quality of life, physical, and mental health:results of a 2-year prospective cohort study
PURPOSE: Pelvic ring injuries are known to affect the patients' daily life in terms of physical functioning and quality of life (QoL). Still, prospective studies on the patient's perception over the first 2 years of rehabilitation are lacking. Therefore, patients cannot be properly informed about whether or when they will return to their pre-existing level of physical functioning and QoL. METHODS: A prospective longitudinal cohort study was performed over a 4-year period including all consecutive patients above 18 years who sustained a pelvic ring injury in a level 1 trauma center. Validated patient-reported outcome measures (PROMs) were used to assess physical functioning (SMFA) and QoL (EQ-5D) at baseline (recalled pre-injury score), 6 weeks, 3 months, 6 months, 1 year and 2 years after the injury. It was assessed whether patients had fully recovered by comparing follow-up scores to baseline PROMs. Binary logistic regression analysis was used to identify independent predictors for patients who did not fully recover. Most experienced difficulties at 3 months and 1 year were identified by analyzing the highest reported scores on individual items of the SMFA. RESULTS: A total of 297 patients with a pelvic ring injury were identified of which 189 were eligible for follow-up and 154 (82%) responded. Median SMFA function score at 3 months, 1 and 2 years was 70, 78 and 88, respectively, compared to 96 out of 100 before the injury. Median SMFA bother score was 67, 79 and 88, respectively. Median EQ-5D score at 3 months, 1 and 2 years was 0.61, 0.81 and 0.85, respectively, compared to 1 (maximum achievable) before the injury. After 1 and 2 years of follow-up, 61% and 75% of the patients fully "recovered" in physical functioning and 52% and 71% fully recovered in terms of QoL. Female gender and high-energy trauma were independent predictors for not fully recovering after 1 year. After 3 months of follow-up, 54% of patients reported severe difficulties with recreational activities, whereas after 1 year, most experienced difficulties (31% of patients) concerned heavy house or yard work. Moreover, after 3 months and 1 year, 44% and 27% of patients reported feeling physically disabled. CONCLUSION: Pelvic ring injuries have a large impact on the patients' daily life in the first 2 years of rehabilitation. Directly after the injury, physical functioning and QoL decrease strongly but then gradually improve over a 2-year period with about 75% of patients fully recovering. Female gender and high-energy trauma are shown to be independent predictors for not fully recovering. After 3 months, patients experience difficulties with both the physical and mental effects of the injury which continue to be present after 1 year
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