597 research outputs found
A quantum-dot heat engine operating close to the thermodynamic efficiency limits
Cyclical heat engines are a paradigm of classical thermodynamics, but are
impractical for miniaturization because they rely on moving parts. A more
recent concept is particle-exchange (PE) heat engines, which uses energy
filtering to control a thermally driven particle flow between two heat
reservoirs. As they do not require moving parts and can be realized in
solid-state materials, they are suitable for low-power applications and
miniaturization. It was predicted that PE engines could reach the same
thermodynamically ideal efficiency limits as those accessible to cyclical
engines, but this prediction has not been verified experimentally. Here, we
demonstrate a PE heat engine based on a quantum dot (QD) embedded into a
semiconductor nanowire. We directly measure the engine's steady-state electric
power output and combine it with the calculated electronic heat flow to
determine the electronic efficiency . We find that at the maximum power
conditions, is in agreement with the Curzon-Ahlborn efficiency and that
the overall maximum is in excess of 70 of the Carnot efficiency
while maintaining a finite power output. Our results demonstrate that
thermoelectric power conversion can, in principle, be achieved close to the
thermodynamic limits, with direct relevance for future hot-carrier
photovoltaics, on-chip coolers or energy harvesters for quantum technologies
Development of laminar flow control wing surface porous structure
It was concluded that the chordwise air collection method, which actually combines chordwise and spanwise air collection, is the best of the designs conceived up to this time for full chord laminar flow control (LFC). Its shallower ducting improved structural efficiency of the main wing box resulting in a reduction in wing weight, and it provided continuous support of the chordwise panel joints, better matching of suction and clearing airflow requirements, and simplified duct to suction source minifolding. Laminar flow control on both the upper and lower surfaces was previously reduced to LFC suction on the upper surface only, back to 85 percent chord. The study concludes that, in addition to reduced wing area and other practical advantages, this system would be lighter because of the increase in effective structural wing thickness
Pattern transfer of sub-micrometre-scaled structures into solid copper by laser embossing
Laser embossing allows the micron and submicron patterning of metal substrates that is of great interest in a wide range of applications. This replication process enables low-cost patterning of metallic materials by non-thermal, high-speed forming which is driven by laser-induced shock waves. In this study the surface topography characteristics as well as the material structure at laser embossing of sub-micrometre gratings into solid copper is presented. The topography of the laser-embossed copper pattern is analysed with atomic force microscopy (AFM) in comparison to the master surface. The height of the embossed structures and the replicated pattern fidelity increases up to a laser fluence of F ∼ 10 J/cm2. For higher laser fluences the height of the embossed structures saturates at 75% of the master pattern height and the shape is adequate to the master. Structural modifications in the copper mono crystals after the laser embossing process were investigated with transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). Almost no modifications were detected. The residual stress after laser embossing of 32 MPa (F = 30 J/cm2) has only a limited influence on the surface pattern formation
Analyzing capacitance-voltage measurements of vertical wrapped-gated nanowires
The capacitance of arrays of vertical wrapped-gate InAs nanowires are
analyzed. With the help of a Poisson-Schr"odinger solver, information about the
doping density can be obtained directly. Further features in the measured
capacitance-voltage characteristics can be attributed to the presence of
surface states as well as the coexistence of electrons and holes in the wire.
For both scenarios, quantitative estimates are provided. It is furthermore
shown that the difference between the actual capacitance and the geometrical
limit is quite large, and depends strongly on the nanowire material.Comment: 15 pages, 6 Figures included, to appear in Nanotechnolog
Thermoelectric characterization of the Kondo resonance in nanowire quantum dots
We experimentally verify hitherto untested theoretical predictions about the
thermoelectric properties of Kondo correlated quantum dots (QDs). The specific
conditions required for this study are obtained by using QDs epitaxially grown
in nanowires, combined with a recently developed method for controlling and
measuring temperature differences at the nanoscale. This makes it possible to
obtain data of very high quality both below and above the Kondo temperature,
and allows a quantitative comparison with theoretical predictions.
Specifically, we verify that Kondo correlations can induce a polarity change of
the thermoelectric current, which can be reversed either by increasing the
temperature or by applying a magnetic field
Measuring Temperature Gradients over Nanometer Length Scales
When a quantum dot is subjected to a thermal gradient, the temperature of
electrons entering the dot can be determined from the dot's thermocurrent if
the conductance spectrum and background temperature are known. We demonstrate
this technique by measuring the temperature difference across a 15 nm quantum
dot embedded in a nanowire. This technique can be used when the dot's energy
states are separated by many kT and will enable future quantitative
investigations of electron-phonon interaction, nonlinear thermoelectric
effects, and the effciency of thermoelectric energy conversion in quantum dots.Comment: 6 pages, 5 figure
GaAs:Mn nanowires grown by molecular beam epitaxy of (Ga,Mn)As at MnAs segregation conditions
GaAs:Mn nanowires were obtained on GaAs(001) and GaAs(111)B substrates by
molecular beam epitaxial growth of (Ga,Mn)As at conditions leading to MnAs
phase separation. Their density is proportional to the density of catalyzing
MnAs nanoislands, which can be controlled by the Mn flux and/or the substrate
temperature. Being rooted in the ferromagnetic semiconductor (Ga,Mn)As, the
nanowires combine one-dimensional properties with the magnetic properties of
(Ga,Mn)As and provide natural, self assembled structures for nanospintronics.Comment: 13 pages, 6 figure
Coherent Single Charge Transport in Molecular-Scale Silicon Nanowire Transistors
We report low-temperature electrical transport studies of molecule-scale
silicon nanowires. Individual nanowires exhibit well-defined Coulomb blockade
oscillations characteristic of charge addition to a single nanostructure with
length scales up to at least 400 nm. Further studies demonstrate coherent
charge transport through discrete single particle quantum levels extending the
whole device, and show that the ground state spin configuration follows the
Lieb-Mattis theorem. In addition, depletion of the nanowires suggests that
phase coherent single-dot characteristics are accessible in a regime where
correlations are strong.Comment: 4 pages and 4 figure
GaN and InN nanowires grown by MBE: a comparison
Morphological, optical and transport properties of GaN and InN nanowires
grown by molecular beam epitaxy (MBE) have been studied. The differences
between the two materials in respect to growth parameters and optimization
procedure was stressed. The nanowires crystalline quality has been investigated
by means of their optical properties. A comparison of the transport
characteristics was given. For each material a band schema was shown, which
takes into account transport and optical features and is based on Fermi level
pinning at the surface.Comment: 5 pages, 5 figure
Fatal poisoning in drug addicts in the Nordic countries in 2017
This study is the seventh report on fatal poisonings among drug addicts in the Nordic countries. In this report, we analyse data from the five Nordic countries: Denmark, Finland, Iceland, Norway and Sweden. Data on gender, number of deaths, places of deaths, age, main intoxicants and substances detected in blood were recorded to obtain national and comparable Nordic data, and to allow comparison with earlier studies conducted in 1984, 1991, 1997, 2002, 2007 and 2012. The death rate (number of deaths per 100,000 inhabitants) was highest in Iceland (6.58) followed closely by Sweden (6.46) and then lowest in Denmark (4.29). The death rate increased in Finland (5.84), Iceland and Sweden and decreased in Denmark compared to earlier studies. The death rate in Norway, which has decreased since 2002, has stabilised around 5.7 as of 2017. Women accounted for 7-23% of the fatal poisonings. The percentage was lowest in Iceland and highest in Finland and Norway. The age range was 14-70 years. The median age (41 years) was highest in Denmark and Norway. The other countries had a median age between 33 and 35 years. Opioids were the main cause of death. Methadone remained the main intoxicant in Denmark, while heroin/morphine was still the main intoxicant in Norway, as was buprenorphine in Finland. However, the picture has changed in Sweden compared to 2012, where heroin/morphine caused most deaths in 2017. Sweden also experienced the highest number of deaths from fentanyl analogues (67 deaths) and buprenorphine (61 deaths). Deaths from fentanyl analogues also occurred in Denmark, Finland and Norway, but to a smaller extent. Over the years, the proportion of opioid deaths has decreased in all countries except Sweden, which has experienced an increase. This decline has been replaced by deaths from CNS stimulants like cocaine, amphetamine and methylenedioxymethamphetamine (MDMA). Cocaine deaths have occurred in all countries but most frequently in Denmark. MDMA deaths have increased in all countries but mostly in Finland. Poly-drug use was widespread, as seen in the earlier studies. The median number of detected drugs per case varied from 4-6. Heroin/morphine, methadone, buprenorphine, cocaine, amphetamine, methamphetamine, MDMA, tetrahydrocannabinol (THC) and benzodiazepines were frequently detected. Pregabalin and gabapentin were detected in all countries, especially pregabalin, which was detected in 42% of the Finnish cases. New psychoactive substances (NPS) occurred in all countries except Iceland. (C) 2020 Elsevier B.V. All rights reserved.Peer reviewe
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