Materials Science and Engineering / Malzeme Bilimi ve Mühendisliği
Permanent URI for this collectionhttps://hdl.handle.net/11147/4719
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Article Citation - WoS: 4Citation - Scopus: 4Indirect Exchange Interaction in Two-Dimensional Materials With Quartic Dispersion(American Physical Society, 2022) Canbolat, Ahmet Utku; Sevinçli, Haldun; Sevinçli, Haldun; Çakır, Özgür; 03.09. Department of Materials Science and Engineering; 04.05. Department of Pyhsics; 03. Faculty of Engineering; 04. Faculty of Science; 01. Izmir Institute of TechnologyWe investigate the indirect magnetic exchange interaction between two magnetic moments in a two-dimensional semiconductor with quartic dispersion, featuring a singularity at the band edge. We obtain the Green's functions analytically to calculate the magnetic exchange interaction at zero temperature. We show that the singularity in the density of states (DOS) for quartic dispersion gives rise to an enhancement in the amplitude of the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction as the Fermi energy is swept toward the band edge. Furthermore, a region of finite exchange interaction arises, with a range increasing as the Fermi energy approaches the band edge. The results lay the possibility of an electrical/chemical control over the exchange interactions.Article Citation - WoS: 14Citation - Scopus: 17Enhancement of Thermoelectric Efficiency of T-Hfse2 Via Nanostructuring(American Physical Society, 2021) Sevinçli, Haldun; Senger, Ramazan Tuğrul; Sevinçli, Haldun; 03.09. Department of Materials Science and Engineering; 04.05. Department of Pyhsics; 03. Faculty of Engineering; 04. Faculty of Science; 01. Izmir Institute of TechnologyIn this work, ab initio calculations based on density functional theory and the Landauer formalism are carried out to investigate ballistic thermoelectric properties of T-HfSe2 nanoribbons (NRs). The zigzag-edged NRs are metallic, and they are not included in this study. The armchair NRs possess two types of edge symmetries depending on the number of atoms present in a row; odd-numbered NRs have mirror symmetry, whereas the even-numbered NRs have glide reflection symmetry. The armchair-edged NRs are dynamically stable and show semiconducting properties with varying band gap values in the infrared and visible regions. Detailed transport analyses show that the n-type Seebeck coefficient and the power factor differ because of the structural symmetry, whereas the p-type thermoelectric coefficients are not significantly influenced. It is shown that the phonon thermal conductance is reduced to a third of its two-dimensional value via nanostructuring. The p-type Seebeck coefficient and the power factor for T-phase HfSe(2 )are enhanced in NRs. We report that the p-type ZT value of HfSe2 NRs at 300 and 800 K are enhanced by factors of 4 and 3, respectively.Article Citation - WoS: 32Citation - Scopus: 33Ballistic Thermoelectric Transport Properties of Two-Dimensional Group Iii-Vi Monolayers(American Physical Society, 2021) Çınar, Mustafa Neşet; Sevinçli, Haldun; Özbal Sargın, Gözde; Sevim, Koray; Sevim, Koray; Özdamar, Burak; Kurt, Gizem; Sevinçli, Haldun; 03.09. Department of Materials Science and Engineering; 04.05. Department of Pyhsics; 03. Faculty of Engineering; 04. Faculty of Science; 01. Izmir Institute of TechnologyBallistic transport and thermoelectric properties of group III-VI compounds (XY: X = B, Al, Ga, In, Tl; Y = O, S, Se, Te, Po) are investigated based on first-principles calculations and Landauer formalism. This large family is composed of 25 compounds which stands out with their unique electronic band structures. Mexican hat shaped valence band, which exhibits quartic energy-momentum relation gives rise to a sharp peak in the density of states as well as a steplike electronic transmission spectrum near the valence band edge. The intriguing electronic band structure and transport properties motivate us to explore thermoelectric properties of group III-VI monolayers. We find that, in addition to the stepwise transmission at the band edge, flat bands, valley degeneracy, and band degeneracy are the factors that enhance thermoelectric efficiencies. For heavier compounds, better thermoelectric efficiencies are possible for both n-type and p-type carriers.Article Citation - WoS: 1Citation - Scopus: 1First-Principles Investigation of Photoisomeric Switching of Vibrational Heat Current Across Molecular Junctions(American Physical Society, 2020) Kurt, Gizem; Sevinçli, Haldun; Sevinçli, Haldun; 03.09. Department of Materials Science and Engineering; 03. Faculty of Engineering; 01. Izmir Institute of TechnologyPhotoisomeric molecules rearrange their structure when exposed to light, which alters their chemical, electronic, mechanical, as well as vibrational properties. The present study explores the possibilities to tune the thermal transport across molecular junctions by using photoisomeric molecules. The effect of isomeric switching on phonon transport through single-molecule junctions linking two macroscopic reservoirs is investigated using density-functional-theory-based tight-binding calculations and Green-function formalism. The junctions are built using azobenzene and its derivatives (azobiphenyl and azotriphenyl) that display photoisomeric behavior. Effects of system setup on the heat current and the switching coefficient are studied systematically. Dependence on the molecular species, the choice of reservoir, as well as the type of linkers that bind the molecules to the reservoir are investigated with calculating the phonon-transmission spectra and temperature-dependent thermal conductance values. The results show that thermal conductance can be altered significantly by switching the molecule from trans- to cis-configuration since all molecules yield higher conductances in trans-configurations than their cis-configurations at temperatures higher than 50 K. In the low-temperature range, results reveal considerable switching coefficients exceeding 50%. At room temperature, the switching coefficient can be as high as 20%. It is shown that the effect is robust under the variation of both the molecular species and the linkers. © 2020 American Physical Society.Article Citation - WoS: 5Citation - Scopus: 5Collapse of the Vacuum in Hexagonal Graphene Quantum Dots: a Comparative Study Between Tight-Binding and Mean-Field Hubbard Models(American Physical Society, 2020) Polat, Mustafa; Sevinçli, Haldun; Sevinçli, Haldun; Güçlü, Alev Devrim; Güçlü, Alev Devrim; Polat, Mustafa; 04.05. Department of Pyhsics; 03.09. Department of Materials Science and Engineering; 03. Faculty of Engineering; 04. Faculty of Science; 01. Izmir Institute of TechnologyIn this paper, we perform a systematic study on the electronic, magnetic, and transport properties of the hexagonal graphene quantum dots (GQDs) with armchair edges in the presence of a charged impurity using two different configurations: (1) a central Coulomb potential and (2) a positively charged carbon vacancy. The tight-binding and the half-filled extended Hubbard models are numerically solved and compared with each other in order to reveal the effect of electron interactions and system sizes. Numerical results point out that off-site Coulomb repulsion leads to an increase in the critical coupling constant to beta(c) = 0.6 for a central Coulomb potential. This critical value of beta is found to be independent of the GQD size, reflecting its universality even in the presence of electron-electron interactions. In addition, a sudden downshift in the transmission peaks shows a clear signature of the transition from subcritical beta < beta(c) to the supercritical beta > beta(c) regime. On the other hand, for a positively charged vacancy, collapse of the lowest bound state occurs at beta(c) = 0.7 for the interacting case. Interestingly, the local magnetic moment, induced by a bare carbon vacancy, is totally quenched when the vacancy is subcritically charged, whereas the valley splittings in electron and hole channels continue to exist in both regimes.