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 treaties—Ramsar Wetlands Convention, World Heritage Convention, Convention on Trade in Endangered Species, Convention on Migratory Species (CMS), Convention on Biological Diversity (CBD)—and 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 instruments’ effective implementation; clarification of their precise implications for large carnivore conservation; development of formal guidance; expansion of instruments’ 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 ‘laboratory’ 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

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