High-bias stability of monatomic chains

For the metals Au, Pt and Ir it is possible to form freely suspended monatomic chains between bulk electrodes. The atomic chains sustain very large current densities, but finally fail at high bias. We investigate the breaking mechanism, that involves current-induced heating of the atomic wires and electromigration forces. We find good agreement of the observations for Au based on models due to Todorov and coworkers. The high-bias breaking of atomic chains for Pt can also be described by the models, although here the parameters have not been obtained independently. In the limit of long chains the breaking voltage decreases inversely proportional to the length.Comment: 7 pages, 5 figure

Observation of Supershell Structure in Alkali Metal Nanowires

Nanowires are formed by indenting and subsequently retracting two pieces of sodium metal. Their cross-section gradually reduces upon retraction and the diameters can be obtained from the conductance. In previous work we have demonstrated that when one constructs a histogram of diameters from large numbers of indentation-retraction cycles, such histograms show a periodic pattern of stable nanowire diameters due to shell structure in the conductance modes. Here, we report the observation of a modulation of this periodic pattern, in agreement with predictions of a supershell structure.Comment: Phys. Rev. Lett., in prin

Direct link between Coulomb blockade and shot noise in a quantum coherent structure

We analyze the current-voltage characteristic of a quantum conduction channel coupled to an electromagnetic environment of arbitrary frequency-dependent impedance. In the weak blockade regime the correction to the ohmic behavior is directly related to the channel current fluctuations vanishing at perfect transmission in the same way as shot noise. This relation can be generalized to describe the environmental Coulomb blockade in a generic mesoscopic conductor coupled to an external impedance, as the response of the latter to the current fluctuations in the former.Comment: 12 pages, 2 figures, submitted to Phys. Rev. Let

Evidence for saturation of channel transmission from conductance fluctuations in atomic-size point contacts

The conductance of atomic size contacts has a small, random, voltage dependent component analogous to conductance fluctuations observed in diffusive wires (UCF). A new effect is observed in gold contacts, consisting of a marked suppression of these fluctuations when the conductance of the contact is close to integer multiples of the conductance quantum. Using a model based on the Landauer-Buettiker formalism we interpret this effect as evidence that the conductance tends to be built up from fully transmitted (i.e., saturated) channels plus a single, which is partially transmitted.Comment: An error in Eq.(2) was corrected, where a square root was added to the factor (1-cos(gamma)). This results in a revised estimate for the mean free path of 5 nm, which is now fully consistent with the estimates from the series resistance and the thermopowe

Measurement of the conductance of a hydrogen molecule

Recent years have shown steady progress in research towards molecular electronics [1,2], where molecules have been investigated as switches [3-5], diodes [6], and electronic mixers [7]. In much of the previous work a Scanning Tunnelling Microscope was employed to address an individual molecule. As this arrangement does not provide long-term stability, more recently metal-molecule-metal links have been made using break junction devices [8-10]. However, it has been difficult to establish unambiguously that a single molecule forms the contact [11]. Here, we show that a single H2 molecule can form a stable bridge between Pt electrodes. In contrast to results for other organic molecules, the bridge has a nearly perfect conductance of one quantum unit, carried by a single channel. The H2-bridge provides a simple test system and a fundamental step towards understanding transport properties of single-molecule devices.Comment: 6 pages, 4 figure

Shot noise suppression at room temperature in atomic-scale Au junctions

Shot noise encodes additional information not directly inferable from simple electronic transport measurements. Previous measurements in atomic-scale metal junctions at cryogenic temperatures have shown suppression of the shot noise at particular conductance values. This suppression demonstrates that transport in these structures proceeds via discrete quantum channels. Using a high frequency technique, we simultaneously acquire noise data and conductance histograms in Au junctions at room temperature and ambient conditions. We observe noise suppression at up to three conductance quanta, with possible indications of current-induced local heating and $1/f$ noise in the contact region at high biases. These measurements demonstrate the quantum character of transport at room temperature at the atomic scale. This technique provides an additional tool for studying dissipation and correlations in nanodevices.Comment: 15 pages, 4 figures + supporting information (6 pages, 6 figures

Conduction channels of superconducting quantum point contacts

Atomic quantum point contacts accommodate a small number of conduction channels. Their number N and transmission coefficients {T_n} can be determined by analyzing the subgap structure due to multiple Andreev reflections in the current-voltage (IV) characteristics in the superconducting state. With the help of mechanically controllable break-junctions we have produced Al contacts consisting of a small number of atoms. In the smallest stable contacts, usually three channels contribute to the transport. We show here that the channel ensemble {T_n} of few atom contacts remains unchanged up to temperatures and magnetic fields approaching the critical temperature and the critical field, respectively, giving experimental evidence for the prediction that the conduction channels are the same in the normal and in the superconducting state.Comment: 8 pages, 5 .eps figures. To be published in Physica B 22

Sex Differences in Outcome of Trauma Patients Presented with Severe Traumatic Brain Injury:A Multicenter Cohort Study

The objective of this study was to determine whether there is an association between sex and outcome in trauma patients presented with severe traumatic brain injury (TBI). A retrospective multicenter study was performed in trauma patients aged ≥ 16 years who presented with severe TBI (Head Abbreviated Injury Scale (AIS) ≥ 4) over a 4-year-period. Subgroup analyses were performed for ages 16–44 and ≥45 years. Also, patients with isolated severe TBI (other AIS ≤ 2) were assessed, likewise, with subgroup analysis for age. Sex differences in mortality, Glasgow Outcome Score (GOS), ICU admission/length of stay (LOS), hospital LOS, and mechanical ventilation (MV) were examined. A total of 1566 severe TBI patients were included (831 patients with isolated TBI). Crude analysis shows an association between female sex and lower ICU admission rates, shorter ICU/hospital LOS, and less frequent and shorter MV in severe TBI patients ≥ 45 years. After adjusting, female sex appears to be associated with shorter ICU/hospital LOS. Sex differences in mortality and GOS were not found. In conclusion, this study found sex differences in patient outcomes following severe TBI, potentially favoring (older) females, which appear to indicate shorter ICU/hospital LOS (adjusted analysis). Large prospective studies are warranted to help unravel sex differences in outcomes after severe TBI.</p

Altered myocardial substrate metabolism is associated with myocardial dysfunction in early diabetic cardiomyopathy in rats: studies using positron emission tomography

0.05). CONCLUSION: Using PET and echocardiography, we found increases in myocardial FA oxidation with a concomitant decrease of insulin-mediated myocardial glucose utilisation in early DCM. In addition, the latter was associated with impaired myocardial function. These in vivo data expand previous in vitro findings showing that early alterations in myocardial substrate metabolism contribute to myocardial dysfunctio

Diabetic cardiomyopathy in Zucker diabetic fatty rats: the forgotten right ventricle

<p>Abstract</p> <p>Background</p> <p>In patients with myocardial infarction or heart failure, right ventricular (RV) dysfunction is associated with death, shock and arrhythmias. In patients with type 2 diabetes mellitus, structural and functional alterations of the left ventricle (LV) are highly prevalent, however, little is known about the impact of diabetes on RV characteristics. The purpose of the present study was to investigate whether LV changes are paralleled by RV alterations in a rat model of diabetes.</p> <p>Methods</p> <p>Zucker diabetic fatty (ZDF) and control (ZL) rats underwent echocardiography and positron emission tomography (PET) scanning using [¹⁸F]-2-fluoro-2-deoxy-D-glucose under hyperinsulinaemic euglycaemic clamp conditions. Glucose, insulin, triglycerides and fatty acids were assessed from trunk blood. Another group of rats received an insulin or saline injection to study RV insulin signaling.</p> <p>Results</p> <p>ZDF rats developed hyperglycaemia, hyperinsulinaemia and dyslipidaemia (all p < 0.05). Echocardiography revealed depressed LV fractional shortening and tricuspid annular plane systolic excursion (TAPSE) in ZDF vs. ZL rats (both p < 0.05). A decrease in LV and RV insulin-mediated glucose utilisation was found in ZDF vs. ZL rats (both p < 0.05). LV associated with RV with respect to systolic function (r = 0.86, p < 0.05) and glucose utilisation (r = 0.74, p < 0.05). TAPSE associated with RV MRglu (r = 0.92, p < 0.05) and <it>M</it>-value (r = 0.91, p < 0.0001) and RV MRglu associated with <it>M</it>-value (r = 0.77, p < 0.05). Finally, reduced RV insulin-stimulated phosphorylation of Akt was found in ZDF vs. ZL (p < 0.05).</p> <p>Conclusions</p> <p>LV changes were paralleled by RV alterations in insulin-stimulated glucose utilisation and RV systolic function in a rat model of diabetes, which may be attributed to ventricular interdependence as well as to the uniform effect of diabetes. Since diabetic patients are prone to develop diabetic cardiomyopathy and myocardial ischaemia, it might be suggested that RV dysfunction plays a central role in cardiac abnormalities in this population.</p