220 research outputs found
Optimal power and efficiency of single quantum dot heat engines: theory and experiment
Quantum dots (QDs) can serve as near perfect energy filters and are therefore
of significant interest for the study of thermoelectric energy conversion close
to thermodynamic efficiency limits. Indeed, recent experiments in [Nat. Nano.
13, 920 (2018)] realized a QD heat engine with performance near these limits
and in excellent agreement with theoretical predictions. However, these
experiments also highlighted a need for more theory to help guide and
understand the practical optimization of QD heat engines, in particular
regarding the role of tunnel couplings on the performance at maximum power and
efficiency for QDs that couple seemingly weakly to electronic reservoirs.
Furthermore, these experiments also highlighted the critical role of the
external load when optimizing the performance of a QD heat engine in practice.
To provide further insight into the operation of these engines we use the
Anderson impurity model together with a Master equation approach to perform
power and efficiency calculations up to co-tunneling order. This is combined
with additional thermoelectric experiments on a QD embedded in a nanowire where
the power is measured using two methods. We use the measurements to present an
experimental procedure for efficiently finding the external load which
should be connected to the engine to optimize power output. Our theoretical
estimates of show a good agreement with the experimental results, and we
show that second order tunneling processes and non-linear effects have little
impact close to maximum power, allowing us to derive a simple analytic
expression for . In contrast, we find that the electron contribution to
the thermoelectric efficiency is significantly reduced by second order
tunneling processes, even for rather weak tunnel couplings
Quantum-Dot Heat Engines
This thesis explores the possibilities of using quantum dots (QDs) in nanoscale energy har- vesters converting heat into electrical energy, i.e. heat engines. From a theory perspective, these possibilities have been investigated for almost two decades, and interest in them seem to continuously increase over time. However, a high degree of experimental control over the manufacturing and operation of QD engines have only recently been achieved. This opens up the possibility of verifying the theory predictions and brings new questions to be answered, which is where this thesis aims at making a contribution. The author’s contributions to the work that the thesis builds upon are theoretical, but are often used together with experimental results for synergistic effects.The thesis starts with an introduction to relevant concepts in classical thermodynamics and a quantum mechanical description of electron states in QDs. This is followed by a discus- sion of electron transport in QDs, as well as an introduction to the master equation based approaches used to model the relevant experimental devices.There are three studies included in the thesis, all of which have been peer-reviewed and published in scientific journals. The details of the physics relevant for each one are presen- ted together with a summary of the studies. The first is an investigation of the performance limits of an experimental implementation of a steady-state QD heat engine, in which the Curzon-Ahlborn efficiency is observed at maximum power and the highest efficiency was in excess of 70% of the Carnot efficiency. This is the first verification that QDs can be used in high efficiency heat engines. The second study investigates how to practically optimize the output power of similar devices, and quantifies how high efficiency one can hope to reach in other implementations of QD engines. The third study proposes an experimental quantum engine based on a double QD where entangled singlet spin states are used to drive the engine. This can be viewed as entanglement acting as the engine’s fuel
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
Cadmium in insects after ash fertilization
Ash fertilization of forests returns nutrients to forest ecosystems and has a
positive effect on soil pH, but it also may elevate Cd concentrations of forest biota. Cadmium
concentrations of some forest insects (Formica ants, carabids and Coleopteran larvae from
decaying wood) were investigated in southern Finland, where two plots were fertilized with
wood ash, while two other plots represented unfertilized control plots. In ants, mean Cd
concentration was 3.6 ± 1.4 mg/kg, with nest workers having significantly higher concentrations
than workers trapped in pitfall traps. Concentrations at fertilized and unfertilized
plots were similar. In carabid beetles, the average Cd concentration of Carabus glabratus
was 0.44 ± 0.36 mg/kg, with no significant difference between control plots and fertilized
plots. In another carabid beetle, Pterostichus niger, mean Cd concentration was higher at
fertilized plots compared to control plots. We conclude that the variation of Cd concentrations
in the insects studied is more efficiently controlled by species-specific differences than
fertilization history of the forest floor
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Informal Caregivers' Experiences and Needs When Caring for a Relative With Heart Failure: An Interview Study
Background: Informal caregivers play an important role for persons with heart failure in strengthening medication adherence, encouraging self-care, and identifying deterioration in health status. Caring for a relative with heart failure can affect informal caregivers' well-being and cause caregiver burden.
Objective: The objective of this study was to explore informal caregivers' experiences and needs when caring for a relative with heart failure living in their own home.
Methods: The study has a qualitative design with an inductive approach. Interviews were conducted with 14 informal caregivers. Data were analyzed using qualitative content analysis.
Results: Two themes emerged: "living in a changed existence" and "struggling and sharing with healthcare." The first theme describes informal caregivers' experiences, needs, and ways of moving forward when living in a changed existence with their relative. Informal caregivers were responsible for the functioning of everyday life, which challenged earlier established roles and lifestyle. They experienced an ever-present uncertainty related to the relative's impending sudden deterioration and to lack of knowledge about the condition. Incongruence was expressed between their own and their relative's understanding and acceptance of the heart failure condition. They also expressed being at peace with their relative and managed to restore new strength and motivation to care. The second theme describes informal caregivers' experiences, needs, and ways in which they handled the healthcare. They felt counted upon but not accounted for, as their care was taken for granted while their need to be seen and acknowledged by healthcare professionals was not met. Informal caregivers experienced an ever-present uncertainty regarding their lack of involvement with healthcare. The lack of involvement with healthcare had a negative impact on the relationship between informal caregivers and their relative due to the mutual loss of important information about changes in medication regimens and the relative's symptoms and well-being. Another cause of negative impact was the lack of opportunity to talk with healthcare professionals about the emotional and relational consequences of heart failure. Healthcare professionals had provided them neither with knowledge on heart failure nor with information on support groups in the municipality. Informal caregivers captured their own mandate through acting as deputies for their relative and claiming their rights of involvement in their relative's healthcare. They also felt confident despite difficult circumstances. The direct access to the medical clinic was a source of relief and they appreciated the contacts with the registered nurses specialized in heart failure. Informal caregivers' own initiatives to participate in meetings were positively received by healthcare professionals.
Conclusions: Informal caregivers' daily life involves decisive changes that are experienced as burdensome. They handled their new situations using different strategies to preserve a sense of "self" and of "us." Informal caregivers express a need for more involvement with healthcare professionals, which may facilitate informal caregivers' situation and improve the dyadic congruence in the relation with their relative
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
Heat driven transport in serial double quantum dot devices
Studies of thermally induced transport in nanostructures provide access to an
exciting regime where fluctuations are relevant, enabling the investigation of
fundamental thermodynamic concepts and the realization of thermal energy
harvesters. We study a serial double quantum dot formed in an InAs/InP nanowire
coupled to two electron reservoirs. By means of a specially designed local
metallic joule-heater, the temperature of the phonon bath in the vicinity of
the double quantum dot can be enhanced. This results in phonon-assisted
transport, enabling the conversion of local heat into electrical power in a
nano-sized heat engine. Simultaneously, the electron temperatures of the
reservoirs are affected, resulting in conventional thermoelectric transport. By
detailed modelling and experimentally tuning the interdot coupling we
disentangle both effects. Furthermore, we show that phonon-assisted transport
gives access to the energy of excited states. Our findings demonstrate the
versatility of our design to study fluctuations and fundamental
nanothermodynamics.Comment: 11 pages, 4 figures + S
Platelet function in brown bear (Ursus arctos) compared to man
The article can also be located here: http://www.thrombosisjournal.com/content/8/1/11Background: Information on hemostasis and platelet function in brown bear (Ursus arctos) is of importance for
understanding the physiological, protective changes during hibernation.
Objective: The study objective was to document platelet activity values in brown bears shortly after leaving the den
and compare them to platelet function in healthy humans.
Methods: Blood was drawn from immobilized wild brown bears 7-10 days after leaving the den in mid April. Blood
samples from healthy human adults before and after clopidogrel and acetylsalicylic acid administration served as
control. We analyzed blood samples by standard blood testing and platelet aggregation was quantified after
stimulation with various agonists using multiple electrode aggregometry within 3 hours of sampling.
Results: Blood samples were collected from 6 bears (3 females) between 1 and 16 years old and from 10 healthy
humans. Results of adenosine diphosphate, aspirin, and thrombin receptor activating peptide tests in bears were all
half or less of those in humans. Platelet and white blood cell counts did not differ between species but brown bears
had more and smaller red blood cells compared with humans.
Conclusion: Using three different tests, we conclude that platelet function is lower in brown bears compared to
humans. Our findings represent the first descriptive study on platelet function in brown bears and may contribute to
explain how bears can endure denning without obvious thrombus building. However, the possibility that our findings
reflect test-dependent and not true biological variations in platelet reactivity needs further studies
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