Search

In this study, the thermal transport properties for various geometries of monolayer h-BN/h-AlN superlattice nanoribbons are investigated using non-equilibrium molecular dynamics simulations. In this context, the lattice thermal conductivities of the superlattice nanoribbons are obtained for different period lengths, geometries, sample lengths, and temperatures. Results reveal that a decrease in the thermal conductivities of superlattice nanoribbons when compared with those of the pristine nanoribbons, the lattice thermal conductivities decrease with decreasing sample lengths and increasing temperatures, also the formation of the extremum points resulting from the competition between wave-like and particle-like phonon transport in the thermal conductivity of superlattice nanoribbons with the change of the period lengths. Moreover, superlattice nanoribbons with different geometries are created to connect the h-BN/h-AlN interface, and it is observed that there is a difference between the thermal conductivities calculated in the reverse directions. This difference leads to thermal rectification in the superlattice structures. As the asymmetry between thermal contact areas increases especially at low temperatures, it is found out the thermal rectification ratio increases.

In order to characterize thermal dependent physical properties of materials, potentially to be used in technological applications, an accurate interatomic-potential parameter set is a must. In general, conjugate-gradient methods and more recently, metaheuristics such as genetic algorithms are employed in determining these interatomic potentials, however, especially the use of metaheuristics specifically designed for optimization of real valued problems such as particle swarm and evaluation strategies are limited in the mentioned problem. In addition, some of these parameters are conflicting in nature, for which multi objective optimization procedures have a great potential for better understanding of these conflicts. In this respect, we aim to present a widely used interatomic potential parameter set, the Stillinger–Weber potential, obtained through three different optimization methods (particle swarm optimization, PSO, covariance matrix adaptation evolution strategies, CMA-ES, and non-dominated sorting genetic algorithm, NSGA-III) for two-dimensional materials MoS2, WS2, WSe2, and MoSe2. These two-dimensional transition metal dichalcogenides are considered as a case mainly due to their potential in a variety of promising technologies for next generation flexible and low-power nanoelectronics, (such as photonics, valleytronics, sensing, energy storage, and optoelectronic devices) as well as their excellent physical properties (such as electrical, mechanical, thermal, and optical properties) different from those of their bulk counterparts. The results show that the outputs of all optimization methods converge to ideal values with sufficiently long iterations and at different trials. However, when we consider the results of the statistical analyses of different trials under similar conditions, we observe that the method with the lowest error rate is the CMA-ES.

Efficient thermal transport control is a fundamental issue for electronic device applications such as information, communication, and energy storage technologies in modern electronics in order to achieve desired thermal conditions. Structural defects in materials provide a mechanism to adjust the thermal transport properties of these materials on demand. In this context, the effect of structural defects on lattice thermal conductivities of two-dimensional hexagonal binary group-III nitride (XN, X = B, Al, and Ga) semiconductors is systematically investigated by means of classical molecular dynamics simulations performed with recently developed transferable inter-atomic potentials accurately describing defect energies. Here, two different Green-Kubo based approaches and another approach based on non-equilibrium molecular dynamics are compared in order to get an overall understanding. Our investigation clearly shows that defect concentrations of 3% decrease the thermal conductivity of systems containing these nitrites up to 95%. Results hint that structural defects can be used as effective adjustment parameters in controlling thermal transport properties in device applications associated with these materials. Published under an exclusive license by AIP Publishing.

Direct experimental measurement of thermal expansion coefficient without substrate effects is a challenging task for two-dimensional (2D) materials, and its accurate estimation with large-scale ab initio molecular dynamics is computationally very expensive. Machine learning-based interatomic potentials trained with ab initio data have been successfully used in molecular dynamics simulations to decrease the computational cost without compromising the accuracy. In this study, we investigated using Gaussian approximation potentials to reproduce the density functional theory-level accuracy for graphene within both lattice dynamical and molecular dynamical methods, and to extend their applicability to larger length and time scales. Two such potentials are considered, GAP17 and GAP20. GAP17, which was trained with pristine graphene structures, is found to give closer results to density functional theory calculations at different scales. Further vibrational and structural analyses verify that the same conclusions can be deduced with density functional theory level in terms of the reasoning of the thermal expansion behavior, and the negative thermal expansion behavior is associated with long-range out-of-plane phonon vibrations. Thus, it is argued that the enabled larger system sizes by machine learning potentials may even enhance the accuracy compared to small-size-limited ab initio molecular dynamics.

The structure and thermal boundary conductance of the wurtzite GaN/AlN (0001) interface are investigated using molecular dynamics simulation. Simulation results with three different empirical interatomic potentials have produced similar misfit dislocation networks and dislocation core structures. Specifically, the misfit dislocation network at the GaN/AlN interface is found to consist of pure edge dislocations with a Burgers vector of 1/3(1 (2) over bar 10) and the misfit dislocation core has an eight-atom ring structure. Although different interatomic potentials lead to different dislocation properties and thermal conductance values, all have demonstrated a significant effect of misfit dislocations on the thermal boundary conductance of the GaN/AlN (0001) interface. Published under an exclusive license by AIP Publishing.

We present here a new interatomic potential parameter set to predict the thermal conductivity of zirconium trisulfide monolayers. The generated Tersoff-type force field is parameterized using data collected with first-principles calculations. We use non-equilibrium molecular dynamics simulations to predict the thermal conductivity. The generated parameters result in very good agreement in structural, mechanical, and dynamical parameters. The room temperature lattice thermal conductivity (κ) of the considered crystal is predicted to be κxx = 25.69 W m−1 K−1 and κyy = 42.38 W m−1 K−1, which both agree well with their corresponding first-principles values with a discrepancy of less than 5%. Moreover, the calculated κ variation with temperature (200 and 400 K) are comparable within the framework of the accuracy of both first-principles and molecular dynamics simulations, We acknowledge the financial support received from the European Union’s Horizon 2020 Research and Innovation Program under Grant Agreement No. 793726 (TELIOTES—Thermal and electronic transport in inorganic–organic thermoelectric superlattices) and the support of the Supercomputing Center of Galicia (CESGA), where the calculations have been made. We also acknowledge financial support by the Ministerio de Economía, Industria y Competitividad (MINECO) under Grant No. FEDER-MAT2017-90024-P, the Severo Ochoa Centres of Excellence Program under Grant No. SEV-2015-0496, and the Generalitat de Catalunya under Grant No. 2017 SGR 1506. The authors acknowledge the fruitful discussions with Dr. Jesús Carrete Montaña at TU Wien.

In this study, by means of classical molecular dynamics simulations, we investigate the thermal-transport properties of hexagonal single-layer, zinc-blend, and wurtzite phases of $\mathrm{BN}$, $\mathrm{Al}\mathrm{N}$, and $\mathrm{Ga}\mathrm{N}$ crystals, which are very promising for the application and design of high-quality electronic devices. With this in mind, we generate fully transferable Tersoff-type empirical interatomic potential parameter sets by utilizing an optimization procedure based on particle-swarm optimization. The predicted thermal properties as well as the structural, mechanical, and vibrational properties of all materials are in very good agreement with existing experimental and first-principles data. The impact of isotopes on thermal transport is also investigated and between approximately $10$ and 50% reduction in phonon thermal transport with random isotope distribution is observed in $\mathrm{BN}$ and $\mathrm{Ga}\mathrm{N}$ crystals. Our investigation distinctly shows that the generated parameter sets are fully transferable and very useful in exploring the thermal properties of systems containing these nitrides.

WOS: 000376213000005, We investigate the effects of Rashba spin-orbit interaction on the optical absorption coefficients and refractive index changes associated with transitions between the first two lower-lying electronic levels in double quantum wire. The wire system represented by a symmetric, double quartic-well confinement potential is subjected to a perpendicular magnetic field. The analytical expressions of the linear and third-order nonlinear optical absorption coefficients and refractive index changes are obtained by using the compact density matrix formalism and iterative scheme. Optical properties are investigated as a function of structural parameter, magnetic field, Rashba spin-orbit interaction and photon energies. Numerical results reveal that competing effects between spin-orbit interaction and magnetic field modify strongly the optical properties and can be altered by these parameters. (C) 2016 Elsevier Ltd. All rights reserved., Scientific and Technological Research Council of Turkey [TUBITAK BIDEB 2211], The authors Y. Karaaslan and B. Gisi would like to thank the Scientific and Technological Research Council of Turkey (TUBITAK BIDEB 2211) for doctoral scholarship.

WOS: 000372559600045, We investigate the electronic structure, spin and transport properties of double quantum wires formed by a symmetric, double quartic-well potential subjected to an in-plane magnetic field by taking into account Rashba and Dresselhaus spin-orbit couplings. The energy dispersion relation of the system is analyzed for different strengths of spin-orbit interactions, magnitude and direction of magnetic field. Our numerical results reveal that the existence of aforementioned parameters modifies strongly the energy band structure, introduces a wave vector dependence to energies and also leads to crossings and anticrossings between subbands. This complex structure of energy dispersion gives rise to the appearance of square-wave like oscillations in the conductance. The spin-orbit couplings, magnetic field, potential profile and Fermi energy of an electron significantly affect the depth and width of conductance steps. Moreover, we found that the competing effect between spin-orbit couplings and magnetic field leaves its marks on the spin texturing. (C) 2015 Elsevier Ltd. All rights reserved., Scientific and Technological Research Council of Turkey [TUBITAK BIDEB 2211], The authors B. Gisi and Y. Karaaslan would like to thank the Scientific and Technological Research Council of Turkey (TUBITAK BIDEB 2211) for doctoral scholarship. This work has been completed at the Dokuz Eylul University, Graduate School of Natural and Applied Sciences and is the subject of the forthcoming Ph.D. Thesis of Bircan Gisi.

WOS: 000423886800011, We study the influence of electric field on the electronic energy band structure, zero-temperature ballistic conductivity and optical properties of double quantum wire. System described by double well anharmonic confinement potential is exposed to a perpendicular magnetic field and Rashba and Dresselhaus spin-orbit interactions. Numerical results show up that the combined effects of internal and external agents cause the formation of crossing, anticrossing, camel-back/anomaly structures and the lateral, downward/upward shifts in the energy dispersion. The anomalies in the energy subbands give rise to the oscillation patterns in the ballistic conductance, and the energy shifts bring about the shift in the peak positions of optical absorption coefficients and refractive index changes. (C) 2017 Elsevier B.V. All rights reserved., Scientific and Technological Research Council of Turkey [TUBITAK BIDEB 2211], The authors Y. Karaaslan and B. Gisi would like to thank the Scientific and Technological Research Council of Turkey (TUBITAK BIDEB 2211) for doctoral scholarship.

Bu tezde, biz çift kuantum telinin elektronik enerji band yapısı ve sıfır-sıcaklık balistik iletkenliği üzerine enlemesine bir elektrik alan ve dik bir manyetik alanın etkisini çalışıyoruz. Çift-kuyu anharmonik hapsetme potansiyeli ile tanımlanan sistem için Rashba ve Dresselhaus spin-yörünge etkileşimlerinin etkisi hesaba katılır. Sonuçlar, enerji band spektrumunun elektrik alan, manyetik alan ve Rashba ve Dresselhaus spin-yörünge etkileşim kuvvetlerinin yanısıra hapsetme potansiyelinin yapısal parametresi tarafından değiştirilebileceğini ortaya koyar. Dahası, manyetik ve elektrik alanlara maruz kalmış telin enerji dağılımındaki spin-yörünge indüklü değişimlere oldukça duyarlı olan balistik manyetik-iletkenlik salınımlarını elde ediyoruz.Ayrıca, dış bir manyetik alanın varlığında Rashba ve Dresselhaus spin-yörünge etkileşimli bir yarı-bir-boyutlu iletkende balistik elektron taşınımını araştırıyoruz. Üç bölmeli iletken sisteminin her bir bölgesine ait teller harmonik hapsetme potansiyeli ile tanımlanır. Transfer matris yöntemi kullanılarak Fermi enerjisinin bir fonksiyonu olarak toplam ve mod-bağımlı iletkenlikler sıfır-sıcaklıkta elde edilir. İletkenliklerin iç ve dış parametrelere önemli ölçüde bağımlı olduğu gözlemlenir. Bunun yanında, iletkenlik bileşenleri manyetik alan, Rashba spin-yörünge etkileşim sabiti ve kuantum telinin uzunluğunun bir fonksiyonu olarak hesaplanır. İletkenlik bileşenleri sistem üzerine etki eden parametre setine bağlı olarak düzenli veya düzensiz salınım desenleri gösterir. In this thesis, we study the influence of a transverse electric field and a perpendicular magnetic field on the electronic energy band structure and zero-temperature ballistic conductivity of double quantum wire. Effects of Rashba and Dresselhaus spin-orbit interactions are taken into account for a system described by double-well anharmonic confinement potential. The results put forth that the energy band spectrum can be changed by structural parameters of confinement potential as well as strengths of electric field, magnetic field and Rashba and Dresselhaus spin-orbit interactions. Moreover, we obtain that the ballistic magnetoconductance oscillations are quite susceptible to spin-orbit-induced modifications in the wire energy dispersion subjected to magnetic and electric fields.Furthermore, we investigate the ballistic electron transport in a quasi-one-dimensional conductor with Rashba and Dresselhaus spin-orbit interactions in the presence of the external magnetic field. The wires belonging to each region of the three-region conductor system are described by harmonic confinement potential. Using transfer matrix method, total and the mode-dependent conductances as a function of the Fermi energy are obtained at zero-temperature. It is observed that they depend substantially on internal and external parameters. Besides, the conductivity components are calculated as a function of the magnetic field, Rashba spin-orbit interaction constant and length of the quantum wire. They show the regular or irregular oscillation patterns by depending on the parameter set acting on the system. 103

WOS: 000377228700010, In this work, we investigate the intersubband optical absorption coefficients and refractive index changes for transitions between the lower-lying electronic levels of double quantum wires formed by a symmetric, double quartic-well potential. The system is subjected to an external in-plane magnetic field and Rashba and Dresselhaus spin-orbit couplings are taken into account. The analytical expressions of the linear and nonlinear absorption coefficients and refractive index changes are obtained by using the compact density-matrix approach and iterative method. The dependence of the optical characteristics on the magnetic field, spin-orbit interactions, quantum wire radius, structural parameter and photon energies has been examined. Numerical results exhibit that the optical properties are considerably sensitive to the strength and orientation of magnetic field as well as to the spin-orbit couplings and thus can be controlled by these parameters. (C) 2016 Elsevier B.V. All rights reserved., Scientific and Technological Research Council of Turkey (TUBITAK BiDEB) [2211], The authors B. Gisi and Y. Karaaslan would like to thank the Scientific and Technological Research Council of Turkey (TUBITAK BiDEB 2211) for doctoral scholarship.

WOS: 000359873200042, We investigate the effects of Rashba spin-orbit interaction on the electronic energy dispersion and zero-temperature ballistic conductance of double quantum wire under the influence,of perpendicular magnetic field. The wire system is represented by a symmetric, double quartic-well confinement potential. Numerical results reveal that competing effects between spin-orbit interaction and magnetic field modify strongly the energy subband structure and introduce anticrossings between subbands. Moreover, it is found that the conductance character of a quasi-one-dimensional ballistic conductor, which is closely related to the energy-dispersion spectrum, is very sensitive to the shape of potential profile, magnetic field and Rashba spin-orbit coupling. (C) 2015 Elsevier Ltd. All rights reserved., Scientific and Technological Research Council of Turkey [TUBITAK BIDEB 2211], One of the authors (Y. Karaaslan) would like to thank the Scientific and Technological Research Council of Turkey (TUBITAK BIDEB 2211) for partial financial support. This work has been completed at the Dokuz Eylul University, Graduate School of Natural and Applied Sciences and is the subject of the forthcoming Ph.D. Thesis of Yenal Karaaslan.

The effect of the off-diagonal Onsager rate coefficient on the relaxation process of iron-group dihalides has been investigated within the framework of statistical equilibrium theory and the thermodynamics of irreversible processes. For this purpose linearized kinetic equations of the total and staggered magnetization is utilized to study the effect of the off-diagonal kinetic coefficient gamma on the temperature dependence of the relaxation times near the first order, critical and multicritical points of the spin-1/2 metamagnetic Ising model. In accordance with the previous results in the literature, we have observed a non-critical maxima of one the relaxation times below the critical temperature according to the values of off-diagonal coefficient. (C) 2011 Elsevier B.V. All rights reserved.