| 1. |
- Chen, I. Ju, et al.
(författare)
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Thermoelectric Power Factor Limit of a 1D Nanowire
- 2018
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Ingår i: Physical Review Letters. - 0031-9007. ; 120:17
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Tidskriftsartikel (refereegranskat)abstract
- In the past decade, there has been significant interest in the potentially advantageous thermoelectric properties of one-dimensional (1D) nanowires, but it has been challenging to find high thermoelectric power factors based on 1D effects in practice. Here we point out that there is an upper limit to the thermoelectric power factor of nonballistic 1D nanowires, as a consequence of the recently established quantum bound of thermoelectric power output. We experimentally test this limit in quasiballistic InAs nanowires by extracting the maximum power factor of the first 1D subband through I-V characterization, finding that the measured maximum power factors conform to the theoretical limit. The established limit allows the prediction of the achievable power factor of a specific nanowire material system with 1D electronic transport based on the nanowire dimension and mean free path. The power factor of state-of-the-art semiconductor nanowires with small cross section and high crystal quality can be expected to be highly competitive (on the order of mW/m K2) at low temperatures. However, they have no clear advantage over bulk materials at, or above, room temperature.
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| 2. |
- Dorsch, Sven, et al.
(författare)
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Heat Driven Transport in Serial Double Quantum Dot Devices
- 2021
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 21:2, s. 988-994
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Tidskriftsartikel (refereegranskat)abstract
- 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 nanosized heat engine. Simultaneously, the electron temperatures of the reservoirs are affected, resulting in conventional thermoelectric transport. By detailed modeling and experimentally tuning the interdot coupling, we disentangle both effects. Furthermore, we show that phonon-assisted transport is sensitive to excited states. Our findings demonstrate the versatility of our design to study fluctuations and fundamental nanothermodynamics.
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| 3. |
- Josefsson, Martin, et al.
(författare)
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A quantum-dot heat engine operating close to the thermodynamic efficiency limits
- 2018
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Ingår i: Nature Nanotechnology. - : Springer Science and Business Media LLC. - 1748-3387 .- 1748-3395. ; 13:10, s. 920-924
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Tidskriftsartikel (refereegranskat)abstract
- 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 reservoirs1,2. 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 engines3–6, 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 η. We find that at the maximum power conditions, η is in agreement with the Curzon–Ahlborn efficiency6–9 and that the overall maximum η 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 photovoltaics10, on-chip coolers or energy harvesters for quantum technologies.
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| 4. |
- Josefsson, Martin, et al.
(författare)
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Optimal power and efficiency of single quantum dot heat engines : Theory and experiment
- 2019
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Ingår i: Physical Review B. - 2469-9950. ; 99:23
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Tidskriftsartikel (refereegranskat)abstract
- 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)1748-338710.1038/s41565-018-0200-5] 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 RP which should be connected to the engine to optimize power output. Our theoretical estimates of RP show good agreement with the experimental results, and we show that second order tunneling processes and nonlinear effects have little impact close to maximum power, allowing us to derive a simple analytic expression for RP. 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.
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| 5. |
- Svilans, Artis, et al.
(författare)
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Experiments on the thermoelectric properties of quantum dots
- 2016
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Ingår i: Comptes Rendus. Physique. - : Elsevier BV. - 1631-0705. ; 17:10, s. 1096-1108
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Tidskriftsartikel (refereegranskat)abstract
- Quantum dots (QDs) are good model systems for fundamental studies of mesoscopic transport phenomena using thermoelectric effects because of their small size, electrostatically tunable properties and thermoelectric response characteristics that are very sensitive to small thermal biases. Here we provide a review of experimental studies on thermoelectric properties of single QDs realized in two-dimensional electron gases, single-walled carbon nanotubes and semiconductor nanowires. A key requirement for such experiments is to have some methods for nanoscale thermal biasing at one's disposal. We briefly review the main techniques used in the field, namely, heating of the QD contacts, side heating and top heating, and touch upon their relative advantages. The thermoelectric response of a QD as a function of gate potential has a characteristic oscillatory behavior with the same period as is observed for conductance peaks. Much of the existing literature focuses on the agreement between experiments and theory, particularly for amplitude and line-shape of the thermovoltage Vth. A general observation is that the widely used single-electron tunneling approximation for QDs has limited success in reproducing measured Vth. Landauer-type calculations are often found to describe measurement results better, despite the large electron–electron interactions in QDs. More recently, nonlinear thermoelectric effects have moved into the focus of attention, and we offer a brief overview of the experiments done so far. We conclude by discussing open questions and avenues for future work, including the role of asymmetries in tunnel- and capacitive couplings in the thermoelectric behavior of QDs.
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| 6. |
- Svilans, Artis, et al.
(författare)
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Nonlinear thermoelectric response due to energy-dependent transport properties of a quantum dot
- 2016
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Ingår i: Physica E: Low-Dimensional Systems and Nanostructures. - : Elsevier BV. - 1386-9477. ; 82, s. 34-38
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Tidskriftsartikel (refereegranskat)abstract
- Quantum dots are useful model systems for studying quantum thermoelectric behavior because of their highly energy-dependent electron transport properties, which are tunable by electrostatic gating. As a result of this strong energy dependence, the thermoelectric response of quantum dots is expected to be nonlinear with respect to an applied thermal bias. However, until now this effect has been challenging to observe because, first, it is experimentally difficult to apply a sufficiently large thermal bias at the nanoscale and, second, it is difficult to distinguish thermal bias effects from purely temperature-dependent effects due to overall heating of a device. Here we take advantage of a novel thermal biasing technique and demonstrate a nonlinear thermoelectric response in a quantum dot which is defined in a heterostructured semiconductor nanowire. We also show that a theoretical model based on the Master equations fully explains the observed nonlinear thermoelectric response given the energy-dependent transport properties of the quantum dot.
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| 7. |
- Svilans, Artis, et al.
(författare)
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Thermoelectric Characterization of the Kondo Resonance in Nanowire Quantum Dots
- 2018
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Ingår i: Physical Review Letters. - 0031-9007. ; 121:20
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 8. |
- Svilans, Artis
(författare)
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Thermoelectric experiments on nanowire-based quantum dots
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis experimentally investigates the possibilities of using quantum effects in semiconductor nanostructures for engineering their thermoelectric properties. More specifically, heterostructured InAs/InP nanowires are used to create short InAs quantum dots (QDs) with electronic state structure resembling that found in atoms. Recently developed top-heater architecture is used to apply a temperature differential across the QDs. The nanowire-based QD devices are used for studies of thermoelectric effects at the nanoscale and for experimental demonstration of particle-exchange heat engines.This thesis first gives an overview of the most important physical effects governing the behavior of quantum dots (QDs). The Master equation approach to model the electronic transport in QDs is introduced in the sequential electron tunneling approximation. It is used to illustrative the transport behavior of QDs. The Landauer-Büttiker approach is also introduced as a reference and the differences with the sequential electron tunneling approximation are discussed. A summary of the most important literature on the thermoelectric properties of single QDs is given and discussed to provide the context for the experimental studies in this thesis. Finally, a description of the experimental methods used in this thesis is given.There are three studies included in this thesis. The first investigates the nonlinear thermoelectric response of a QD with an applied thermal bias. A strongly nonlinear behavior is observed which can be fully explained by the interplay between different QD electronic states contributing to thermocurrent in opposing directions. The second study experimentally demonstrates efficient particle-exchange heat engines based on QDs for the first time. The analysis of the heat engines' power and efficiency indicate heat-to-electric conversion efficiencies up to 70% of Carnot efficiency. The third study investigates the thermoelectric response of QDs in the presence of Kondo correlations. It verifies a previous theoretical prediction that the sign of the thermoelectric signature in QDs inverts due to the Kondo correlations.The experiments presented in this thesis have been successful in filling a gap between theory and experiments on several fronts. Future experiments could, for example, study Kondo-correlated QDs in the nonlinear thermoelectric response regime in the presence of magnetic field, where theory predictions are harder to obtain, or could employ thermoelectric characterization techniques to study entropy of various different QD states.
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