1,770 research outputs found
Folding energy and kinetics of mutually interacting DNA Hairpins
Treballs Finals de Grau de Física, Facultat de Física, Universitat de Barcelona, Curs: 2020, Tutors: Marc Rico, Fèlix RitortThe Szilard engine is a Maxwell demon thought experiment where information is converted into work. We propose a novel DNA molecule with two hairpins in serial that could act as a complex four-state Szilard engine using optical tweezers to apply a force. We synthesized the molecule with identical hairpins to perform two different experiments with optical tweezers, passive mode and pulling. Only 3 different states were observed as the intermediate state is degenerated. Their energy levels were obtained from kinetic rates measured in passive mode and work measured in pulling cycles. We conclude that the molecule follows well the
uctuation theorems and the Bell-Evans theory, with the exception of a Chevron type curvature in the kinetic rates due to the degeneration. The obtained data will help in the development and study of a Szilard engine with this molecule
Modeling and performance analysis of photoconductive antennas for THz generation
Photoconductive antennas are devices capable of generating THz waves. In this thesis, we propose a model of THz wave generation in photoconductive antennas based on the Dude model. The derived equations explain the THz radiation when an optical pulse illuminates a photoconductor with an applied bias field. Furthermore, we derive an additional term appearing in the Drude equation that accounts for the generation of carriers with zero velocity. The model includes space-charge screening through a geometrical factor and radiation screening through an effective antenna impedance. The system is solved numerically using the iterative fixed-point method and Anderson acceleration. The fixed-point method convergence is limited, but Anderson acceleration can converge for a broader range of parameters. Analyzing the photocurrent at different optical powers, we find that it saturates at high optical powers due to the slowdown of the dynamics by the screening fields
Chemical composition and radiative properties of nascent particulate matter emitted by an aircraft turbofan burning conventional and alternative fuels
Aircraft engines are a unique source of carbonaceous aerosols in the upper troposphere. There, these particles can more efficiently interact with solar radiation than at ground. Due to the lack of measurement data, the radiative forcing from aircraft exhaust aerosol remains uncertain. To better estimate the global radiative effects of aircraft exhaust aerosol, its optical properties need to be comprehensively characterized. In this work we present the link between the chemical composition and the optical properties of the particulate matter (PM) measured at the engine exit plane of a CFM56-7B turbofan. The measurements covered a wide range of power settings (thrust), ranging from ground idle to take-off, using four different fuel blends of conventional Jet A-1 and hydro-processed ester and fatty acids (HEFA) biofuel. At the two measurement wavelengths (532 and 870 nm) and for all tested fuels, the absorption and scattering coefficients increased with thrust, as did the PM mass. The analysis of elemental carbon (EC) and organic carbon (OC) revealed a significant mass fraction of OC (up to 90 %) at low thrust levels, while EC mass dominated at medium and high thrust. The use of HEFA blends induced a significant decrease in the PM mass and the optical coefficients at all thrust levels. The HEFA effect was highest at low thrust levels, where the EC mass was reduced by up to 50 %–60 %. The variability in the chemical composition of the particles was the main reason for the strong thrust dependency of the single scattering albedo (SSA), which followed the same trend as the fraction of OC to total carbon (TC). Mass absorption coefficients (MACs) were determined from the correlations between aerosol light absorption and EC mass concentration. The obtained MAC values (MAC532=7.5±0.3 m2 g−1 and MAC870=5.2±0.9 m2 g−1) are in excellent agreement with previous literature values of absorption cross section for freshly generated soot. While the MAC values were found to be independent of the thrust level and fuel type, the mass scattering coefficients (MSCs) significantly varied with thrust. For cruise conditions we obtained MSC532=4.5±0.4 m2 g−1 and MSC870=0.54±0.04 m2 g−1, which fall within the higher end of MSCs measured for fresh biomass smoke. However, the latter comparison is limited by the strong dependency of MSC on the particles' size, morphology and chemical composition. The use of the HEFA fuel blends significantly decreased PM emissions, but no changes were observed in terms of EC∕OC composition and radiative properties
Mobility enhancement in graphene by in situ reduction of random strain fluctuations
Microscopic corrugations are ubiquitous in graphene even when placed on
atomically flat substrates. These result in random local strain fluctuations
limiting the carrier mobility of high quality hBN-supported graphene devices.
We present transport measurements in hBN-encapsulated devices where such strain
fluctuations can be in situ reduced by increasing the average uniaxial strain.
When of uniaxial strain is applied to the graphene, an enhancement
of the carrier mobility by is observed while the residual doping
reduces by . We demonstrate a strong correlation between the mobility
and the residual doping, from which we conclude that random local strain
fluctuations are the dominant source of disorder limiting the mobility in these
devices. Our findings are also supported by Raman spectroscopy measurements
Superconductivity in type-II Weyl-semimetal WTe2 induced by a normal metal contact
WTe is a material with rich topological properties: it is a 2D
topological insulator as a monolayer and a Weyl-semimetal and higher-order
topological insulator (HOTI) in the bulk form. Inducing superconductivity in
topological materials is a way to obtain topological superconductivity, which
lays at the foundation for many proposals of fault tolerant quantum computing.
Here, we demonstrate the emergence of superconductivity at the interface
between WTe and the normal metal palladium. The superconductivity has a
critical temperature of about 1.2 K. By studying the superconductivity in
perpendicular magnetic field, we obtain the coherence length and the London
penetration depth. These parameters correspond to a low Fermi velocity and a
high density of states at the Fermi level. This hints to a possible origin of
superconductivity due to the formation of flat bands. Furthermore, the critical
in-plane magnetic field exceeds the Pauli limit, suggesting a non-trivial
nature of the superconducting state.Comment: As accepted to Journal of Applied Physics, 7 pages, 4 figure
Global strain-induced scalar potential in graphene devices
By mechanically distorting a crystal lattice it is possible to engineer the
electronic and optical properties of a material. In graphene, one of the major
effects of such a distortion is an energy shift of the Dirac point, often
described as a scalar potential. We demonstrate how such a scalar potential can
be generated systematically over an entire electronic device and how the
resulting changes in the graphene work function can be detected in transport
experiments. Combined with Raman spectroscopy, we obtain a characteristic
scalar potential consistent with recent theoretical estimates. This direct
evidence for a scalar potential on a macroscopic scale due to deterministically
generated strain in graphene paves the way for engineering the optical and
electronic properties of graphene and similar materials by using external
strain
Effective density of aircraft engine PM revisited : effects of engine thrust, engine type, fuel, and sample conditioning
Aircraft gas turbine engines emit soot agglomerates with varying size, shape, and composition as a function of their operating condition. A useful parameter, which accounts for particle morphology, is effective density. Effective density is used to relate particle number and mass emissions in aviation PM emission models. However, measurement data of PM effective density from commercial aircraft turbine engines are very limited.
Here, we report the size‐dependent effective density of PM sampled from commercial aircraft turbine engines in an engine test cell using a standardized sampling and measurement system. We used tandem DMA‐CPMA classification as in our previous study (Durdina et al. 2014). The novelty of this work is reduced scan time from over 10 minutes down to 1 minute per sample with the same hardware configuration, wider range of particle sizes, measurement of different engines, and a larger database with better data quality. The fast method allowed us to measure various engine types during their post‐overhaul test runs with short test points. We also performed effective density measurements during two dedicated test campaigns of the same engine. These campaigns investigated the effects of an alternative fuel blend on emissions and the evolution of the exhaust plume downstream of the engine exit plane. In the latter campaign, the effective density was measured with and without the treatment with a catalytic stripper approximately 25 m downstream of the engine exit plane.
Figure 1 shows the compiled results obtained for all engines and fuels tested with exhaust samples taken at the engine exit plane and 25 m downstream with a catalytic stripper. The results confirm the thrust dependence of the effective density distributions reported previously. The most distinct differences are between the effective density distributions at idle thrust (Figure 1, a) and medium to high thrust (Figure 1, b). This trend was qualitatively the same for all engines tested. In contrast to our previous report, the effective densities at medium and high thrust did not follow the mass‐mobility relationship determined previously. The best fit of the data is an exponential function.
The fit functions determined have potential applications in aircraft PM emissions modeling and measurement. The size‐dependent densities can be used to estimate PM mass concentration from particle size distributions measured using mobility particle sizers. The density functions can be used to improve particle loss correction models in sampling systems for aircraft engine emissions
Compact SQUID realized in a double layer graphene heterostructure
Two-dimensional systems that host one-dimensional helical states are exciting
from the perspective of scalable topological quantum computation when coupled
with a superconductor. Graphene is particularly promising for its high
electronic quality, versatility in van der Waals heterostructures and its
electron and hole-like degenerate 0 Landau level. Here, we study a compact
double layer graphene SQUID (superconducting quantum interference device),
where the superconducting loop is reduced to the superconducting contacts,
connecting two parallel graphene Josephson junctions. Despite the small size of
the SQUID, it is fully tunable by independent gate control of the Fermi
energies in both layers. Furthermore, both Josephson junctions show a skewed
current phase relationship, indicating the presence of superconducting modes
with high transparency. In the quantum Hall regime we measure a well defined
conductance plateau of 2 an indicative of counter propagating edge
channels in the two layers. Our work opens a way for engineering topological
superconductivity by coupling helical edge states, from graphene's
electron-hole degenerate 0 Landau level via superconducting contacts.Comment: 38 pages, 12 figure
Out-of-plane corrugations in graphene based van der Waals heterostructures
Two dimensional materials are usually envisioned as flat, truly 2D layers.
However out-of-plane corrugations are inevitably present in these materials. In
this manuscript, we show that graphene flakes encapsulated between insulating
crystals (hBN, WSe2), although having large mobilities, surprisingly contain
out-of-plane corrugations. The height fluctuations of these corrugations are
revealed using weak localization measurements in the presence of a static
in-plane magnetic field. Due to the random out-of-plane corrugations, the
in-plane magnetic field results in a random out-of-plane component to the local
graphene plane, which leads to a substantial decrease of the phase coherence
time. Atomic force microscope measurements also confirm a long range height
modulation present in these crystals. Our results suggest that phase coherent
transport experiments relying on purely in-plane magnetic fields in van der
Waals heterostructures have to be taken with serious care
Nutritional strategies for correcting low glucose values in patients with postbariatric hypoglycaemia: A randomized controlled three-arm crossover trial.
AIM
To evaluate the efficacy of nutritional hypoglycaemia correction strategies in postbariatric hypoglycaemia (PBH) after Roux-en-Y gastric bypass (RYGB).
MATERIALS AND METHODS
In a randomized, controlled, three-arm crossover trial, eight post-RYGB adults (mean [SD] 7.0 [1.4] years since surgery) with PBH ingested a solid mixed meal (584 kcal, 85 g carbohydrates, 21 g fat, 12 g protein) to induce hypoglycaemia on three separate days. Upon reaching plasma glucose of less than 3.0 mmol/L, hypoglycaemia was corrected with 15 g of glucose (G15), 5 g of glucose (G5) or a protein bar (P10, 10 g of protein) in random order. The primary outcome was percentage of time spent in the target plasma glucose range (3.9-5.5 mmol/L) during 40 minutes after correction.
RESULTS
Postcorrection time spent in the target glucose range did not differ significantly between the interventions (P = .161). However, postcorrection time with glucose less than 3.9 mmol/L was lower after G15 than P10 (P = .007), whereas time spent with glucose more than 5.5 mmol/L, peak glucose and insulin 15 minutes postcorrection were higher after G15 than G5 and P10 (P < .001). Glucagon 15 minutes postcorrection was higher after P10 than after G15 and G5 (P = .002 and P = .003, respectively). G15 resulted in rebound hypoglycaemia (< 3.0 mmol/L) in three of eight cases (38%), while no rebound hypoglycaemia occurred with G5 and P10.
CONCLUSIONS
Correcting hypoglycaemia with 15 g of glucose should be reconsidered in post-RYGB PBH. A lower dose appears to sufficiently increase glucose levels outside the critical range in most cases, and complementary nutrients (e.g. proteins) may provide glycaemia-stabilizing benefits.
REGISTRATION NUMBER OF CLINICAL TRIAL
NTC05250271 (ClinicalTrials.gov)
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