| 1. |
- Aydin, Alhun, et al.
(författare)
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Discrete and Weyl density of states for photonic dispersion relation
- 2019
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Ingår i: Physica Scripta. - : IOP Publishing. - 0031-8949 .- 1402-4896. ; 94:10
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Tidskriftsartikel (refereegranskat)abstract
- The current density of states (DOS) calculations do not take into account the essential discretenessof the state space, since they rely on the unbounded continuum approximation. Recently, discrete DOS based on the quantum-mechanically allowable minimum energy interval has been introducedfor quadratic dispersion relation. In this work, we consider systems exhibiting photonic (photon-like) dispersion relation and calculate the related density and number of states (NOS). Also, a Weyl's conjecture-based DOS function is calculated for photons and acoustic phonons at low frequency limit,by considering the bounded continuum approach. We show that discrete DOS function reducesto expressions of bounded and unbounded continua in the appropriate limits. The uctuationsin discrete DOS completely disappear under accumulation operators. It's interesting that relativeerrors of NOS and DOS functions with respect to discrete ones have exactly the same character withthe ones of quadratic dispersion relation. Furthermore, the application of discrete and Weyl DOS for the calculation of internal energy of a photon gas is presented and importance of discrete DOSis discussed. It's shown that discrete DOS function given in this work needs to be used wheneverthe low energy levels of a physical system are heavily occupied.
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| 2. |
- Aydin, Alhun, et al.
(författare)
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Landauer’s Principle in a Quantum Szilard Engine without Maxwell’s Demon
- 2020
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Ingår i: Entropy. - : MDPI AG. - 1099-4300. ; 22:3
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Tidskriftsartikel (refereegranskat)abstract
- Quantum Szilard engine constitutes an adequate interplay of thermodynamics, information theory and quantum mechanics. Szilard engines are in general operated by a Maxwell’s Demon where Landauer’s principle resolves the apparent paradoxes. Here we propose a Szilard engine setup without featuring an explicit Maxwell’s demon. In a demonless Szilard engine, the acquisition of which-side information is not required, but the erasure and related heat dissipation still take place implicitly. We explore a quantum Szilard engine considering quantum size effects. We see that insertion of the partition does not localize the particle to one side, instead creating a superposition state of the particle being in both sides. To be able to extract work from the system, particle has to be localized at one side. The localization occurs as a result of quantum measurement on the particle, which shows the importance of the measurement process regardless of whether one uses the acquired information or not. In accordance with Landauer’s principle, localization by quantum measurement corresponds to a logically irreversible operation and for this reason it must be accompanied by the corresponding heat dissipation. This shows the validity of Landauer’s principle even in quantum Szilard engines without Maxwell’s demon.
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| 3. |
- Aydin, Alhun, et al.
(författare)
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Origin of the quantum shape effect
- 2023
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Ingår i: Physical review. E. - : American Physical Society. - 2470-0045 .- 2470-0053. ; 108:2
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Tidskriftsartikel (refereegranskat)abstract
- The quantum size and shape effects are often considered difficult to distinguish from each other because oftheir coexistence. Essentially, it is possible to separate them and focus solely on the shape effect by consideringa size-invariant shape transformation, which changes the discrete energy spectra of strongly confined systemsand causes the quantum shape effects. The size-invariant shape transformation is a geometric technique oftransforming shapes by preserving the boundary curvature, topology, and the Lebesgue measure of a boundeddomain. The quantum shape effect is a quite different phenomenon from quantum size effects, as it can havethe opposite influence on the physical properties of nanoscale systems. While quantum size effects can usuallybe obtained via bounded continuum approximation, the quantum shape effect is a direct consequence of theenergy quantization in specifically designed confined geometries. Here, we explore the origin of the quantumshape effect by theoretically investigating the simplest system that can produce the same physics: quantumparticles in a one-dimensional box separated by a moving partition. The partition moves quasistatically fromone end of the box to the other, allowing the system to remain in equilibrium with a reservoir throughout theprocess. The partition and the boundaries are impenetrable by particles, forming two effectively interconnectedregions. The position of the partition becomes the shape variable.We investigate the quantum shape effect on thethermodynamic properties of confined particles considering their discrete spectrum. In addition, we applied ananalytical model based on dimensional transitions to predict thermodynamic properties under the quantum shapeeffect accurately. A fundamental understanding of quantum shape effects could pave the way for employing themto engineer physical properties and design better materials at the nanoscale.
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| 4. |
- Aydin, Alhun, et al.
(författare)
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Quantum shape oscillations in the thermodynamic properties of confined electrons in core–shell nanostructures
- 2021
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Ingår i: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 0953-8984 .- 1361-648X. ; 34:2
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Tidskriftsartikel (refereegranskat)abstract
- Quantum shape effect appears under the size-invariant shape transformations of stronglyconfined structures. Such a transformation distinctively influences the thermodynamicproperties of confined particles. Due to their characteristic geometry, core–shellnanostructures are good candidates for quantum shape effects to be observed. Here weinvestigate the thermodynamic properties of non-interacting degenerate electrons confined incore–shell nanowires consisting of an insulating core and a GaAs semiconducting shell. Wederive the expressions of shape-dependent thermodynamic quantities and show the existenceof a new type of quantum oscillations due to shape dependence, in chemical potential, internalenergy, entropy and specific heat of confined electrons.We provide physical understanding ofour results by invoking the quantum boundary layer concept and evaluating the distributions ofquantized energy levels on Fermi function and in state space. Besides the density, temperatureand size, the shape per se also becomes a control parameter on the Fermi energy of confinedelectrons, which provides a new mechanism for fine tuning the Fermi level and changing thepolarity of semiconductors.
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| 5. |
- Aydin, Alhun, et al.
(författare)
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Thermodefect voltage in graphene nanoribbon junctions
- 2022
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Ingår i: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 0953-8984 .- 1361-648X. ; 34:19
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Tidskriftsartikel (refereegranskat)abstract
- Thermoelectric junctions are often made of components of different materials characterized by distinct transport properties. Single material junctions, with the same type of charge carriers, have also been considered to investigate various classical and quantum effects on the thermoelectric properties of nanostructured materials. We here introduce the concept of defect-induced thermoelectric voltage, namely, thermodefect voltage, in graphene nanoribbon (GNR) junctions under a temperature gradient. Our thermodefect junction is formed by two GNRs with identical properties except the existence of defects in one of the nanoribbons. At room temperature the thermodefect voltage is highly sensitive to the types of defects, their locations, as well as the width and edge configurations of the GNRs. We computationally demonstrate that the thermodefect voltage can be as high as 1.7 mV K-1 for 555-777 defects in semiconducting armchair GNRs. We further investigate the Seebeck coefficient, electrical conductance, and electronic thermal conductance, and also the power factor of the individual junction components to explain the thermodefect effect. Taken together, our study presents a new pathway to enhance the thermoelectric properties of nanomaterials.
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| 6. |
- Aydin, Alhun, et al.
(författare)
-
Thermosize voltage induced in a ballistic graphene nanoribbon junction
- 2019
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Ingår i: Journal of Applied Physics. - : American Institute of Physics (AIP). - 0021-8979 .- 1089-7550. ; 126:10
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Tidskriftsartikel (refereegranskat)abstract
- A thermoelectric voltage is induced in a junction, constituted of two dissimilar materials under a temperature gradient. Similarly, a thermosize voltage is expected to be induced in a junction made by the same material but having differentsizes, so-called thermosize junction. This is a consequence of dissimilarity in Seebeck coefficients due to differencesin classical and/or quantum size effects in the same materials with different sizes. The studies on thermosize effectsin literature are mainly based on semi-classical models under relaxation time approximation or even simpler localequilibrium ones where only very general ideas and results have been discussed without considering quantum transport approaches and specific materials. To make more realistic predictions for a possible experimental verification, here,we consider ballistic thermosize junctions made by narrow and wide (n-w) pristine graphene nanoribbons with perfectarmchair edges and calculate the electronic contribution to the thermosize voltage, at room temperature, by using the Landauer formalism. The results show that the maximum thermosize voltage can be achieved for semiconducting nanoribbons and it is about an order of magnitude larger than that of metallic nanoribbons. In the semiconducting case, the thermosize voltage forms a characteristic plateau for a finite range of gating conditions. We demonstrate, throughnumerical calculations, that the induced thermosize voltage per temperature difference can be in the scale of mV/K,which is high enough for experimental measurements. Owing to their high and persistent thermosize voltage values,graphene nanoribbons are expected to be good candidate for device applications of thermosize effects.
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| 7. |
- Aydin, Murat, et al.
(författare)
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Flow-controlled thermal response test and its comparison with the conventional test methods
- 2024
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Ingår i: Geothermics. - : Elsevier. - 0375-6505 .- 1879-3576. ; 120
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Tidskriftsartikel (refereegranskat)abstract
- A novel flow-controlled (FC) thermal response test (TRT) system is introduced to resolve the recently addressed inconsistency between the constant heat flux (CHF) and constant temperature (CT) TRTs. FC-TRT allows us to keep both inlet and outlet temperatures constant and improve the accuracy of CT-TRT. Using the FC-TRT system, four types of TRT experiments are performed, providing CT, CHF, and constant inlet temperature conditions, besides the novel one keeping both temperature and heat flux constant. Thermal conductivities from these TRT measurements are compared, and a good agreement is observed. FC-TRT offers higher accuracy and various TRT applications in one platform.
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| 8. |
- Ekmekci, Ece, et al.
(författare)
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Collective behavior of boreholes and its optimization to maximize BTES performance
- 2023
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Ingår i: Applied Energy. - : Elsevier BV. - 0306-2619 .- 1872-9118. ; 343
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Tidskriftsartikel (refereegranskat)abstract
- Borehole layout strongly affects the behavior of borehole heat exchangers (BHEs) and changes the performance of a borehole thermal energy storage (BTES). This study investigates the existence and importance of the optimum collective behavior of BHEs to maximize the performance of BTES. Charge benefit ratio, storage efficiency and configurational benefit factor are proposed as performance indicators for better and finer performance evaluations of BTES systems. A small-scale BTES consisting of ten boreholes arranged on a concentric double-ring layout is considered as an application. Performance variations with the inner and the outer radii of the borehole field are analyzed for the first five years of operation. The temperature fields of different configurations show the transition from collective to individual behavior of boreholes, which leads to an optimal radial configuration maximizing the performance indicators. It is seen that the indicators strongly depend on both inner and outer radii and they reach their maximums for the same distinct radial configuration. The optimum arrangement can almost double the thermal performance indicators. It is thus of great importance to optimize collective behavior to maximize the usage of stored thermal energy. The results are qualitatively general and represent the common behavioral patterns of BTES systems.
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| 9. |
- Ekmekci, Ece, et al.
(författare)
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Very high temperature BTES : A potential for operationally cost-free and emission-free heating
- 2024
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Ingår i: Applied Energy. - : Elsevier. - 0306-2619 .- 1872-9118. ; 360
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Tidskriftsartikel (refereegranskat)abstract
- In cold climates, the heating load is much higher than the cooling load and borehole thermal energy storage (BTES) systems offer an opportunity for high -efficient heating. Heat energy from different sources is stored in the ground to use in wintertime. Higher storage temperatures lower the operating cost of heating and the size of the BTES field. BTES systems, up to 90°C, are studied in the literature. In this study, we consider the charge temperatures beyond 100°C to analyze the possibility of free -heating for Nordic countries. The system is called here very high -temperature BTES (VHT-BTES). A residence field of 25 houses, 125m2 each, is chosen as a mid -scale target and concentrated solar collectors (CSC) are used to charge VHT-BTES up to 140°C. A double -ring layout of ten boreholes is optimized to minimize the heat pump consumption. Free -heating and heat pump modes are activated for high and low borehole temperatures, respectively. The actual meteorological data for Uppsala/Sweden is used. The gradually increasing very high seasonal coefficient of performance values (SCOP), 8-23, are achieved using free -heating and heat pump modes together for the first four years. From the fifth year, all of the heating demand is basically met by the stored energy (free -heating). The results show that VHT-BTES provides practically operationally cost-free and emission -free heating even for a Nordic country. The return of investment is calculated as ten to fourteen years, depending on the cost of the additional land for CSC.
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| 10. |
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