Physics / Fizik
Permanent URI for this collectionhttps://hdl.handle.net/11147/6
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Research Project Grafen Yapılarda Elektronik Fısıldayan Galeri Modları ve Güçlenmiş Rkky Etkileşimleri(2019) Çakır, Özgür; Sevinçli, HaldunRKKY(Ruderman-Kittel-Kasuya-Yosida) etkileşimi manyetik momentler arasında Fermi denizini oluşturan elektronlar vasıtasıyla ortaya çıkan indirekt etkileşimlerdir. Bu etkileşimler nadir toprak elementlerinde ortaya çıkan manyetizmada, seyreltilmiş manyetik yarıiletkenlerde rol oynamaktadır. Farklı kuantum noktacıklarda elektron spinlerinin RKKY tipi etkileşimler vasıtasıyla kuantum eş-evreli bir şekilde kontrol edilebileceği deneysel olarak gösterilmiştir[1]. RKKY etkileşimleri vasıtasıyla birbirinden ferromanyetik etkileşim altında birbirine uzak elektronik spinlerin temel durumları dolanık bir duruma denk gelmektedir. Spintronik uygulamalarında, manyetizmada ve elektronik spinlerin eşevreli kontrol edilebilmeleri konularında RKKY etkileşimi önemli bir fiziksel mekanizma olarak öne çıkmaktadır. Projede ilk olarak RKKY etkileşimlerin dış potansiyeller altında RKKY etkileşiminin davranışı incelenmiştir. Dış potansiyeller varlığında, belirli enerjilerde yüksek durum yoğunluğuna sahip uzaysal olarak uzun erimli elektronik durumların varlığı elektronik çınlaç etkisine yol açmaktadır. Grafende Klein tünellemesi olmasına rağmen, potansiyel uygulanan bölgede yüksek genliğe ve durum yoğunluğuna sahip yarı bağlı durumlar(quasi bound states) mevcuttur. Dairesel potansiyeller altında grafen ve benzeri malzemelerde fısıldayan galeri modları olarak adlandırılan elektronik durumlar vasıtasıyla manyetik momentlerin uzak mesafelerde dahi birbirleriyle güçlü bir şekilde etkileşebildiği gösterilmiştir. Fermi enerjisinin QBS enerji değerleri ile çakıştığı durumda uzun erimli, güçlenmiş manyetik etkileşimler ortaya çıktığı numerik ve teorik olarak gösterilmiştir. Yığınsal grafende manyetik atomların bulundukları alt-örgüye bağlı olarak ya ferromanyetik ya da antiferromanyetik karakterli etkileşimler oluşmaktadır. Ancak dairesel potansiyel varlığında bu etkileşimin uzak mesafelerde, manyetik momentlerin bulundukları örgüden bağımsız olarak, ferromanyetik bir karaktere büründüğü gösterilmiştir. Grafen yüzeyinde bulunan adatomlar arasında Fermi denizindeki elektronlar vasıtasıyla, elektronik Casimir etkisi olarak adlandırılan etkileşimler ortaya çıkmaktadır. Numerik sonuçlar sonucunda dış potansiyeller varlığında, uzak erimli kuvvetlerin varlığı öngörülmüştür. RKKY etkileşimi enerji momentumu dağınımı momentumun dördüncü kuvvetine bağlı olarak değişim gösteren iki boyutlu altıgen yapılarda sıkı bağlanma modeli kullanılarak ve Green fonksiyonları kullanılarak analitik olarak hesaplanmıştır. Bu malzemeler Grup 15 elementlerinin oluşturdukları iki boyutlu yapılar olup, bant kenarlarında enerji E ? (k^2 ? k_c^ 2)^2 şeklinde bir dağınım göstermektedir. Buna bağlı olarak bant kenarında bir tekillik ortaya çıkmakta ve boşluk/elektron katkılaması altında, Fermi enerjisi bant kenarına yaklaştıkça RKKY etkileşimlerinin büyüklüğü ve menzilinin artış gösterdiği görülmüştür. Yakın mesafelerde ferromanyetik olan etkileşim Fermi falga boyunda salınım göstermekte, Fermi enerjisi bant kenarına yaklaştıkça, RKKY etkileşimlerinin hem büyüklüğünün hem de menzilinin artış göstereceği hesaplanmıştır.Article Citation - WoS: 14Citation - Scopus: 17Enhancement of Thermoelectric Efficiency of T-Hfse2 Via Nanostructuring(American Physical Society, 2021) Ünsal, Elif; Senger, Ramazan Tuğrul; Sevinçli, HaldunIn 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: 85Citation - Scopus: 91Ballistic Thermoelectric Properties of Monolayer Semiconducting Transition Metal Dichalcogenides and Oxides(American Physical Society, 2019) Özbal, Gözde; Senger, Ramazan Tuğrul; Sevik, Cem; Sevinçli, HaldunCombining first-principles calculations with Landauer-Mittiker formalism, ballistic thermoelectric transport properties of semiconducting two-dimensional transition metal dichalcogenides (TMDs) and oxides (TMOs) (namely MX2 with M = Cr, Mo, W, Ti, Zr, Hf; X = O, S, Se, Te) are investigated in their 2H and 1T phases. Having computed structural, as well as ballistic electronic and phononic transport properties for all structures, we report the thermoelectric properties of the semiconducting ones. We find that 2H phases of four of the studied structures have very promising thermoelectric properties, unlike their 1T phases. The maximum room temperature p-type thermoelectric figure of merit (ZT) of 1.57 is obtained for 2H-HfSe2, which can be as high as 3.30 at T = 800 K. Additionally, 2H-ZrSe2, 2H-ZrTe2, and 2H-HfS2 have considerable ZT values (both nand p-type), that are above 1 at room temperature. The 1T phases of Zr and Hf-based oxides possess relatively high power factors, however their high lattice thermal conductance values limit their ZT values to below 1 at room temperature.Article Citation - WoS: 1Citation - Scopus: 1Directed Growth of Hydrogen Lines on Graphene: High-Throughput Simulations Powered by Evolutionary Algorithm(American Physical Society, 2018) Özbal, Gözde; Falkenberg, J. T.; Brandbyge, M.; Senger, Ramazan Tuğrul; Sevinçli, HaldunWe set up an evolutionary algorithm combined with density functional tight-binding calculations to investigate hydrogen adsorption on flat graphene and graphene monolayers curved over substrate steps. During the evolution, candidates for the new generations are created by adsorption of an additional hydrogen atom to the stable configurations of the previous generation, where a mutation mechanism is also incorporated. Afterwards a two-stage selection procedure is employed. Selected candidates act as the parents of the next generation. The evolutionary algorithm predicts formation of lines of hydrogen atoms on flat graphene. In curved graphene, the evolution follows a similar path except for a new mechanism, which aligns hydrogen atoms on the line of minimum curvature. The mechanism is due to the increased chemical reactivity of graphene along the minimum radius of curvature line (MRCL) and to sp(3) bond angles being commensurate with the kinked geometry of hydrogenated graphene at the substrate edge. As a result, the reaction barrier is reduced considerably along the MRCL and hydrogenation continues like a mechanical chain reaction. This growth mechanism enables lines of hydrogen atoms along the MRCL, which has the potential to overcome substrate or rippling effects and could make it possible to define edges or nanoribbons without actually cutting the material.Article Citation - WoS: 9Structural, Electronic, and Magnetic Properties of Point Defects in Polyaniline (c3n) and Graphene Monolayers: a Comparative Study(American Institute of Physics, 2020) Sevim, Koray; Sevinçli, HaldunThe newly synthesized two-dimensional polyaniline (C3N) is structurally similar to graphene and has interesting electronic, magnetic, optical, and thermal properties. Motivated by the fact that point defects in graphene give rise to interesting features, like magnetization in an all carbon material, we perform density functional theory calculations to investigate vacancy and Stone-Wales type point defects in monolayer C3N. We compare and contrast the structural, electronic, and magnetic properties of these defects with those in graphene. While monovacancies and Stone-Wales defects of C3N result in reconstructions similar to those in graphene, divacancies display dissimilar geometrical features. Different from graphene, all vacancies in C3N have metallic character because of altered stoichiometry; those that have low-coordinated atoms have finite magnetic moments. We further investigate the robustness of the reconstructed structures and the changes in the magnetic moments by applying tensile and compressive biaxial strain. We find that, with the advantage of finite bandgap, point defects in C3N are qualified as good candidates for future spintronics applications.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; Güçlü, Alev DevrimIn 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.Article Citation - WoS: 2Citation - Scopus: 2Tuning Thermal Transport in Graphene Via Combinations of Molecular Antiresonances(Elsevier Ltd., 2018) Sevim, Koray; Sevinçli, HaldunWe propose a method to engineer the phonon thermal transport properties of low dimensional systems. The method relies on introducing a predetermined combination of molecular adsorbates, which give rise to antiresonances at frequencies specific to the molecular species. Despite their dissimilar transmission spectra, thermal resistances due to individual molecules remain almost the same for all species. On the other hand, thermal resistance due to combinations of different species are not additive and show large differences depending on the species. Using a toy model, the physics underlying the violation of resistance summation rule is investigated. It is demonstrated that equivalent resistance of two scatterers having the same resistances can be close to the sum of the constituents or ∼ 70% of it depending on the relative positions of the antiresonances. The relative positions of the antiresonances determine the net change in transmission, therefore the equivalent resistance. Since the entire spectrum is involved in phonon spectrum changes in different parts of the spectrum become important. Performing extensive first-principles based computations, we show that these distinctive attributes of phonon transport can be useful to tailor the thermal transport through low dimensional materials, especially for thermoelectric and thermal management applications.Article Citation - WoS: 134Citation - Scopus: 137Structural, Vibrational, and Electronic Properties of Single-Layer Hexagonal Crystals of Group Iv and V Elements(American Physical Society, 2018) Özdamar, Burak; Özbal, Gözde; Çınar, Mustafa Neşet; Sevim, Koray; Kurt, Gizem; Kaya, Birnur; Sevinçli, HaldunUsing first-principles density functional theory calculations, we investigate a family of stable two-dimensional crystals with chemical formula A2B2, where A and B belong to groups IV and V, respectively (A=C, Si, Ge, Sn, Pb; B=N, P, As, Sb, Bi). Two structural symmetries of hexagonal lattices P6m2 and P3m1 are shown to be dynamically stable, named as α- and β -phases correspondingly. Both phases have similar cohesive energies, and the α phase is found to be energetically favorable for structures except CP, CAs, CSb, and CBi, for which the β phase is favored. The effects of spin-orbit coupling and Hartree-Fock corrections to exchange correlation are included to elucidate the electronic structures. All structures are semiconductors except CBi and PbN, which have metallic character. SiBi, GeBi, and SnBi have direct band gaps, whereas the remaining semiconductor structures have indirect band gaps. All structures have quartic dispersion in their valence bands, some of which make the valence band maximum and resemble a mexican-hat shape. SnAs and PbAs have purely quartic valence band edges, i.e., E-αk4, a property reported for the first time. The predicted materials are candidates for a variety of applications. Owing to their wide band gaps, CP, SiN, SiP, SiAs, GeN, GeP can find their applications in optoelectronics. The relative band positions qualify a number of the structures as suitable for water splitting, where CN and SiAs are favorable at all pH values. Structures with quartic band edges are expected to be efficient for thermoelectric applications.