Rapid (nanosecond-scale) electrical pulsing is used to study drift-velocity saturation in graphene field-effect devices. In these experiments, high-field pulses are utilized to drive graphene’s ...carriers on time scales much faster than that on which energy loss to the underlying substrate can occur, thereby allowing the observation of the highest saturation velocities reported to date. In a dramatic departure from the behavior exhibited by conventional metals and semiconductors, as the electron or hole density is reduced toward the charge-neutrality point, the drift velocity is found to reach values comparable to the Fermi velocity itself. Corresponding current densities are as large as 109 A/cm2, similar to the values reported for carbon nanotubes and for graphene-on-diamond transistors. In essence, our approach of rapid pulsing allows us to “free” graphene from the deleterious influence of its substrate, revealing a pathway to achieve the superior electrical performance promised by this material. The usefulness of this approach is not merely limited to graphene but should extend also to a broad variety of two-dimensional semiconductors.
We demonstrate a novel form of thermally-assisted hysteresis in the transfer curves of monolayer MoS2 FETs, characterized by the appearance of a large gate-voltage window and distinct current levels ...that differ by a factor of ∼102. The hysteresis emerges for temperatures in excess of 400 K and, from studies in which the gate-voltage sweep parameters are varied, appears to be related to charge injection into the SiO2 gate dielectric. The thermally-assisted memory is strongly suppressed in equivalent measurements performed on bilayer transistors, suggesting that weak screening in the monolayer system plays a vital role in generating its strongly sensitive response to the charge-injection process. By exploiting the full features of the hysteretic transfer curves, programmable memory operation is demonstrated. The essential principles demonstrated here point the way to a new class of thermally assisted memories based on atomically thin two-dimensional semiconductors.
Long-held assumptions of poor prognoses for patients with haematological malignancies (HM) have meant that clinicians have been reluctant to admit them to the intensive care unit (ICU). We aimed to ...evaluate ICU, in-hospital, and 6 month mortality and to identify predictors for in-hospital mortality.
A cohort study in a specialist cancer ICU of adult HM patients admitted over 5 yr. Data acquired included: patient characteristics, haematological diagnosis, haematopoietic stem cell transplant (HSCT), reason for ICU admission, and APACHE II scores. Laboratory values, organ failures, and level of organ support were recorded on ICU admission. Predictors for in-hospital mortality were evaluated using uni- and multivariate analysis.
Of 199 patients, median age was 58 yr inter-quartile range (IQR) 46–66, 51.7% were emergency admissions, 42.2% post-HSCT, 51.9% required mechanical ventilation, median APACHE II was 21 (IQR 16–25), and median organ failure numbered 2 (IQR 1–4). ICU, in-hospital, and 6 month mortalities were 33.7%, 45.7%, and 59.3%, respectively. Univariate analysis revealed bilirubin >32 µmol litre−1, mechanical ventilation, ≥2 organ failures, renal replacement therapy, vasopressor support (all P<0.001), graft-vs-host disease (P=0.007), APACHE II score (P=0.02), platelets ≤20×109 litre−1 (P=0.03), and proven invasive fungal infection (P=0.04) were associated with in-hospital mortality. Multivariate analysis revealed that ≥2 organ failures odds ratio (OR) 5.62; 95% confidence interval (95% CI), 2.30–13.70 and mechanical ventilation (OR 3.03; 95% CI, 1.33–6.90) were independently associated with in-hospital mortality.
Mortality was lower than in previous studies. Mechanical ventilation and ≥2 organ failures were independently associated with in-hospital mortality. ‘Traditional' variables such as neutropenia, transplantation status, and APACHE II score no longer appear to be predictive.
Nociceptin/Orphanin FQ (N/OFQ) is the endogenous opioid agonist for the N/OFQ receptor or NOP. This receptor system is involved in pain processing but also has a role in immune regulation. Indeed, ...polymorphonuclear cells (PMNs) express mRNA for N/OFQ precursor and are a potential source for circulating N/OFQ. Current measurements are based on ELISA and RIA techniques. In this study we have designed a bioassay to measure N/OFQ release from single PMNs. Chinese Hamster Ovary (CHO) cells transfected with the human (h) NOP receptor and Gαiq5 chimera force receptor coupling in biosensor cells to increase intracellular Ca2+; this can be measured with FLUO-4 dye. If isolated PMNs from healthy human volunteers are layered next to CHOhNOPGαiq5 biosensor cells then stimulated with the chemoattractant N-formyl-methionyl-leucyl-phenylalanine (fMLP) we hypothesise that released N/OFQ will activate the biosensor. PMNs also release ATP and CHO cells express purinergic receptors coupled to elevated Ca2+. In a system where these receptors (P2Y1, P2Y2 and P2X7) are blocked with high concentrations of PPADS and oATP, PMN stimulation with fMLP increases Ca2+ in PMNs then shortly afterwards the biosensor cells. Our data therfore reports detection of single cell N/OFQ release from immune cells. This was absent when cells were preincubated with the selective NOP antagonist; SB-612111. Collectively this is the first description of single cell N/OFQ release. We will deploy this assay with further purified individual cell types and use this to further study the role of the N/OFQ-NOP system in disease; in particular sepsis where there is strong evidence for increased levels of N/OFQ worsening outcome.
Soil organic matter is extensively humified; some fractions existing for more than 1000 years. The soil microbial biomass is surrounded by about 50 times its mass of soil organic matter, but can only ...metabolize it very slowly. Paradoxically, even if more than 90% of the soil microbial biomass is killed, the mineralization of soil organic matter proceeds at the same rate as in an unperturbed soil. Here we show that soil organic matter mineralization is independent of microbial biomass size, community structure or specific activity. We suggest that the rate limiting step is governed by abiological processes (which we term the Regulatory Gate hypothesis), which convert non-bioavailable soil organic matter into bioavailable soil organic matter, and cannot be affected by the microbial population. This work challenges one of the long held theories in soil microbiology proposed by Winogradsky, of the existence of autochthonous and zymogenous microbial populations. This has significant implications for our understanding of carbon mineralization in soils and the role of soil micro-organisms in the global carbon cycle. Here we describe experiments designed to determine if the Regulatory Gate operates. We conclude that there is sufficient experimental evidence for it to be offered as a working hypothesis.
To develop a clinical classification system for age-related macular degeneration (AMD).
Evidence-based investigation, using a modified Delphi process.
Twenty-six AMD experts, 1 neuro-ophthalmologist, ...2 committee chairmen, and 1 methodologist.
Each committee member completed an online assessment of statements summarizing current AMD classification criteria, indicating agreement or disagreement with each statement on a 9-step scale. The group met, reviewed the survey results, discussed the important components of a clinical classification system, and defined new data analyses needed to refine a classification system. After the meeting, additional data analyses from large studies were provided to the committee to provide risk estimates related to the presence of various AMD lesions.
Delphi review of the 9-item set of statements resulting from the meeting.
Consensus was achieved in generating a basic clinical classification system based on fundus lesions assessed within 2 disc diameters of the fovea in persons older than 55 years. The committee agreed that a single term, age-related macular degeneration, should be used for the disease. Persons with no visible drusen or pigmentary abnormalities should be considered to have no signs of AMD. Persons with small drusen (<63 μm), also termed drupelets, should be considered to have normal aging changes with no clinically relevant increased risk of late AMD developing. Persons with medium drusen (≥ 63-<125 μm), but without pigmentary abnormalities thought to be related to AMD, should be considered to have early AMD. Persons with large drusen or with pigmentary abnormalities associated with at least medium drusen should be considered to have intermediate AMD. Persons with lesions associated with neovascular AMD or geographic atrophy should be considered to have late AMD. Five-year risks of progressing to late AMD are estimated to increase approximately 100 fold, ranging from a 0.5% 5-year risk for normal aging changes to a 50% risk for the highest intermediate AMD risk group.
The proposed basic clinical classification scale seems to be of value in predicting the risk of late AMD. Incorporating consistent nomenclature into the practice patterns of all eye care providers may improve communication and patient care.
We study temperature dependent (200 – 400 K) dielectric current leakage in high-quality, epitaxial chromia films, synthesized on various conductive substrates (Pd, Pt and V2O3). We find that ...trap-assisted space-charge limited conduction is the dominant source of electrical leakage in the films, and that the density and distribution of charge traps within them is strongly dependent upon the choice of the underlying substrate. Pd-based chromia is found to exhibit leakage consistent with the presence of deep, discrete traps, a characteristic that is related to the known properties of twinning defects in the material. The Pt- and V2O3-based films, in contrast, show behavior typical of insulators with shallow, exponentially-distributed traps. The highest resistivity is obtained for chromia fabricated on V2O3 substrates, consistent with a lower total trap density in these films. Our studies suggest that chromia thin films formed on V2O3 substrates are a promising candidate for next-generation spintronics.
We explore the contributions to the electrical resistance of monolayer and bilayer graphene, revealing transitions between different regimes of charge carrier scattering. In monolayer graphene at low ...densities, a nonmonotonic variation of the resistance is observed as a function of temperature. Such behaviour is consistent with the influence of scattering from screened Coulomb impurities. At higher densities, the resistance instead varies in a manner consistent with the influence of scattering from acoustic and optical phonons. The crossover from phonon-, to charged-impurity, limited conduction occurs once the concentration of gate-induced carriers is reduced below that of the residual carriers. In bilayer graphene, the resistance exhibits a monotonic decrease with increasing temperature for all densities, with the importance of short-range impurity scattering resulting in a "universal" density-independent (scaled) conductivity at high densities. At lower densities, the conductivity deviates from this universal curve, pointing to the importance of thermal activation of carriers out of charge puddles. These various assignments, in both systems, are made possible by an approach of "differential-conductance mapping", which allows us to suppress quantum corrections to reveal the underlying mechanisms governing the resistivity.
The high field phenomena of inter-valley transfer and avalanching breakdown have long been exploited in devices based on conventional semiconductors. In this Article, we demonstrate the manifestation ...of these effects in atomically-thin WS
field-effect transistors. The negative differential conductance exhibits all of the features familiar from discussions of this phenomenon in bulk semiconductors, including hysteresis in the transistor characteristics and increased noise that is indicative of travelling high-field domains. It is also found to be sensitive to thermal annealing, a result that we attribute to the influence of strain on the energy separation of the different valleys involved in hot-electron transfer. This idea is supported by the results of ensemble Monte Carlo simulations, which highlight the sensitivity of the negative differential conductance to the equilibrium populations of the different valleys. At high drain currents (>10 μA/μm) avalanching breakdown is also observed, and is attributed to trap-assisted inverse Auger scattering. This mechanism is not normally relevant in conventional semiconductors, but is possible in WS
due to the narrow width of its energy bands. The various results presented here suggest that WS
exhibits strong potential for use in hot-electron devices, including compact high-frequency sources and photonic detectors.
The differential conductance of graphene is shown to exhibit a zero-bias anomaly at low temperatures, arising from a suppression of the quantum corrections due to weak localization and electron ...interactions. A simple rescaling of these data, free of any adjustable parameters, shows that this anomaly exhibits a universal, temperature- (T) independent form. According to this, the differential conductance is approximately constant at small voltages (V < k
T/e), while at larger voltages it increases logarithmically with the applied bias. For theoretical insight into the origins of this behaviour, which is inconsistent with electron heating, we formulate a model for weak-localization in the presence of nonequilibrium transport. According to this model, the applied voltage causes unavoidable dispersion decoherence, which arises as diffusing electron partial waves, with a spread of energies defined by the value of the applied voltage, gradually decohere with one another as they diffuse through the system. The decoherence yields a universal scaling of the conductance as a function of eV/k
T, with a logarithmic variation for eV/k
T > 1, variations in accordance with the results of experiment. Our theoretical description of nonequilibrium transport in the presence of this source of decoherence exhibits strong similarities with the results of experiment, including the aforementioned rescaling of the conductance and its logarithmic variation as a function of the applied voltage.
PDF
