Detection of Single Electron Charging in an Individual InAs Quantum Dot by Noncontact Atomic Force Microscopy

Single electron charging in an individual InAs quantum dot was observed by electrostatic force measurements with an atomic force microscope (AFM). The resonant frequency shift and the dissipated energy of an oscillating AFM cantilever were measured as a function of the tip-back electrode voltage and the resulting spectra show distinct jumps when the tip was positioned above the dot. The observed jumps in the frequency shift, with corresponding peaks in dissipation, are attributed to a single electron tunneling between the dot and the back electrode governed by Coulomb blockade effect, and are consistent with a model based on the free energy of the system. The observed phenomenon may be regarded as the ``force version'' of the Coulomb blockade effect.Comment: 4 pages, 4 figure

Extending the theory of Owicki and Gries with a logic of progress

This paper describes a logic of progress for concurrent programs. The logic is based on that of UNITY, molded to fit a sequential programming model. Integration of the two is achieved by using auxiliary variables in a systematic way that incorporates program counters into the program text. The rules for progress in UNITY are then modified to suit this new system. This modification is however subtle enough to allow the theory of Owicki and Gries to be used without change

Changes in physical fitness of Hungarian college students in the last fifteen years

This study compared the physical fitness of Hungarian undergraduate students in 2011–2012 with the representative sample surveyed in 1997–1998. A total of 123 males and 309 females (mean age 21.19±2.19 yrs) were randomly selected in two Hungarian universities (Pécs, Kaposvár). Anthropometric data (height, weight, skinfolds, body fat percentages) were measured and the subjects performed 9 tests of the Eurofit Fitness Test Battery. The BMI, total body fat and performance in most of the fitness components (balance, agility/speed, flexibility, abdominal muscular strength, aerobic fitness of females) was higher in 1997–1998 (p<0.05) whereas the handgrip strength and performance in Bent Arm Hanging test were significantly better in 2011–2012 (p<0.001). These findings support interventions focusing on increasing regular physical activity among Hungarian youth

Competition for nutrients and light: testing advances in resource competition with a natural phytoplankton community

A key challenge in ecology is to understand how nutrients and light affect the biodiversity and community structure of phytoplankton and plant communities. According to resource competition models, ratios of limiting nutrients are major determinants of species composition. At high nutrient levels, however, species interactions may shift to competition for light, which might make nutrient ratios less relevant. The "nutrient-load hypothesis" merges these two perspectives, by extending the classic model of competition for two nutrients to include competition for light. Here, we test five key predictions of the nutrient-load hypothesis using multispecies competition experiments. A marine phytoplankton community sampled from the North Sea was inoculated in laboratory chemostats provided with different nitrogen (N) and phosphorus (P) loads to induce either single resource limitation or co-limitation of N, P, and light. Four of the five predictions were validated by the experiments. In particular, different resource limitations favored the dominance of different species. Increasing nutrient loads caused changes in phytoplankton species composition, even if the N:P ratio of the nutrient loads remained constant, by shifting the species interactions from competition for nutrients to competition for light. In all treatments, small species became dominant whereas larger species were competitively excluded, supporting the common view that small cell size provides a competitive advantage under resource-limited conditions. Contrary to expectation, all treatments led to coexistence of diatoms, cyanobacteria and green algae, resulting in a higher diversity of species than predicted by theory. Because the coexisting species comprised three phyla with different photosynthetic pigments, we speculate that niche differentiation in the light spectrum might play a role. Our results show that mechanistic resource competition models that integrate nutrient-based and light-based approaches provide an important step forward to understand and predict how changing nutrient loads affect community composition

From Ecological Stoichiometry to Biochemical Composition: Variation in N and P Supply Alters Key Biosynthetic Rates in Marine Phytoplankton

One of the major challenges in ecological stoichiometry is to establish how environmental changes in resource availability may affect both the biochemical composition of organisms and the species composition of communities. This is a pressing issue in many coastal waters, where anthropogenic activities have caused large changes in riverine nutrient inputs. Here we investigate variation in the biochemical composition and synthesis of amino acids, fatty acids (FA), and carbohydrates in mixed phytoplankton communities sampled from the North Sea. The communities were cultured in chemostats supplied with different concentrations of dissolved inorganic nitrogen (DIN) and phosphorus (DIP) to establish four different types of resource limitations. Diatoms dominated under N-limited, N+P limited and P-limited conditions. Cyanobacteria became dominant in one of the N-limited chemostats and green algae dominated in the one P-limited chemostat and under light-limited conditions. Changes in nutrient availability directly affected amino acid content, which was lowest under N and N+P limitation, higher under P-limitation and highest when light was the limiting factor. Storage carbohydrate content showed the opposite trend and storage FA content seemed to be co-dependent on community composition. The synthesis of essential amino acids was affected under N and N+P limitation, as the transformation from non-essential to essential amino acids decreased at DIN:DIP ≤ 6. The simple community structure and clearly identifiable nutrient limitations confirm and clarify previous field findings in the North Sea. Our results show that different phytoplankton groups are capable of adapting their key biosynthetic rates and hence their biochemical composition to different degrees when experiencing shifts in nutrient availability. This will have implications for phytoplankton growth, community structure, and the nutritional quality of phytoplankton as food for higher trophic levels

Energy levels of few electron quantum dots imaged and characterized by atomic force microscopy

Strong confinement of charges in few electron systems such as in atoms, molecules and quantum dots leads to a spectrum of discrete energy levels that are often shared by several degenerate quantum states. Since the electronic structure is key to understanding their chemical properties, methods that probe these energy levels in situ are important. We show how electrostatic force detection using atomic force microscopy reveals the electronic structure of individual and coupled self-assembled quantum dots. An electron addition spectrum in the Coulomb blockade regime, resulting from a change in cantilever resonance frequency and dissipation during tunneling events, shows one by one electron charging of a dot. The spectra show clear level degeneracies in isolated quantum dots, supported by the first observation of predicted temperature-dependent shifts of Coulomb blockade peaks. Further, by scanning the surface we observe that several quantum dots may reside on what topologically appears to be just one. These images of grouped weakly and strongly coupled dots allow us to estimate their relative coupling strengths.Comment: 11 pages, 6 figure

Unbalanced reduction of nutrient loads has created an offshore gradient from phosphorus to nitrogen limitation in the North Sea

Measures to reduce eutrophication have often led to a more effective decline of phosphorus (P) than nitrogen(N) concentrations. The resultant changes in riverine nutrient loads can cause an increase in the N : Pratios of coastal waters. During four research cruises along a 450 km transect, we investigated how reductionsin nutrient inputs during the past 25 yr have affected nutrient limitation patterns in the North Sea. Thisrevealed a strong offshore gradient of dissolved inorganic N : P ratios in spring, from 375 : 1 nearshoretoward 1 : 1 in the central North Sea. This gradient was reflected in high nearshore N : P and C : P ratios ofparticulate organic matter (mainly phytoplankton), indicative of severe P deficiency of coastal phytoplankton,which may negatively affect higher trophic levels in the food web. Nutrient enrichment bioassays performedon-board showed P and Si limitation of phytoplankton growth nearshore, co-limitation of N and P ina transitional region, and N limitation in the outer-shore waters, confirming the existence of an offshore gradientfrom P to N limitation. Different species were limited by different nutrients, indicating that furtherreductions of P loads without concomitant reductions of N loads will suppress colonial Phaeocystis blooms,but will be less effective in diminishing harmful algal blooms by dino- and nanoflagellates. Hence, our resultsprovide evidence that de-eutrophication efforts in northwestern Europe have led to a large imbalance in theN : P stoichiometry of coastal waters of the North Sea, with major consequences for the growth, species composition,and nutritional quality of marine phytoplankton communities

Strong coupling between single-electron tunneling and nano-mechanical motion

Nanoscale resonators that oscillate at high frequencies are useful in many measurement applications. We studied a high-quality mechanical resonator made from a suspended carbon nanotube driven into motion by applying a periodic radio frequency potential using a nearby antenna. Single-electron charge fluctuations created periodic modulations of the mechanical resonance frequency. A quality factor exceeding 10^5 allows the detection of a shift in resonance frequency caused by the addition of a single-electron charge on the nanotube. Additional evidence for the strong coupling of mechanical motion and electron tunneling is provided by an energy transfer to the electrons causing mechanical damping and unusual nonlinear behavior. We also discovered that a direct current through the nanotube spontaneously drives the mechanical resonator, exerting a force that is coherent with the high-frequency resonant mechanical motion.Comment: Main text 12 pages, 4 Figures, Supplement 13 pages, 6 Figure

The value of computed tomography in detecting distal radioulnar joint instability after a distal radius fracture

This study evaluated the value of computed tomography scans for the diagnosis of distal radioulnar joint instability. A total of 46 patients, conservatively treated for a unilateral distal radius fracture, were evaluated. Clinical instability was tested using the stress test and clunk test. A computed tomography scan of both wrists was performed in pronation and supination. Two independent observers reviewed the computed tomography scans using: the radioulnar line, subluxation ratio, epicentre and radioulnar ratio methods. Radiological distal radioulnar joint instability was assessed by comparing the measurements of the injured wrist with those of the contralateral uninjured wrists. A