Scopus İndeksli Yayınlar Koleksiyonu / Scopus Indexed Publications Collection
Permanent URI for this collectionhttps://hdl.handle.net/11147/7148
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Article Citation - WoS: 3Citation - Scopus: 3Atomic Collapse in Disordered Graphene Quantum Dots(American Physical Society, 2020) Polat, Mustafa; Güçlü, Alev Devrim; Güçlü, Alev Devrim; Polat, Mustafa; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of TechnologyIn this paper, we numerically study a Coulomb impurity problem for interacting Dirac fermions restricted in disordered graphene quantum dots. In the presence of randomly distributed lattice defects and spatial potential fluctuations, the response of the critical coupling constant for atomic collapse is mainly investigated by local density of states calculations within the extended mean-field Hubbard model. We find that both types of disorder cause an amplification of the critical threshold. As a result, up to a 34% increase in the critical coupling constant is reported. This numerical result may explain why the Coulomb impurities remain subcritical in experiments, even if they are supercritical in theory. Our results also point to the possibility that atomic collapse can be observed in defect-rich samples such as Ar+ ion bombarded, He+ ion irradiated, and hydrogenated graphene.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.Article Citation - WoS: 7Citation - Scopus: 7Effects of Random Atomic Disorder on the Magnetic Stability of Graphene Nanoribbons With Zigzag Edges(American Physical Society, 2018) Çakmak, Korhan Ertan; Altıntaş, Abdulmenaf; Güçlü, Alev Devrim; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of TechnologyWe investigate the effects of randomly distributed atomic defects on the magnetic properties of graphene nanoribbons with zigzag edges using an extended mean-field Hubbard model. For a balanced defect distribution among the sublattices of the honeycomb lattice in the bulk region of the ribbon, the ground-state antiferromagnetism of the edge states remains unaffected. By analyzing the excitation spectrum, we show that while the antiferromagnetic ground state is susceptible to single spin-flip excitations from edge states to magnetic defect states at low defect concentrations, its overall stability is enhanced with respect to the ferromagnetic phase.Article Citation - WoS: 7Citation - Scopus: 7Effects of Interedge Scattering on the Wigner Crystallization in Graphene Nanoribbons(American Physical Society, 2017) Modarresi, Mohsen; Güçlü, Alev Devrim; Güçlü, Alev Devrim; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of TechnologyWe investigate the effects of coupling between the two zigzag edges of graphene nanoribbons on the Wigner crystallization of electrons and holes using a combination of tight-binding, mean-field Hubbard and many-body configuration interaction methods. We show that the thickness of the nanoribbon plays a crucial role in the formation of Wigner crystal. For ribbon widths smaller than 16 Å, increased kinetic energy overcomes the long-range Coulomb repulsion and suppresses the Wigner crystallization. For wider ribbons up to 38 Å wide, strong Wigner localization is observed for an even number of electrons, revealing an even-odd effect also found in the Coulomb-blockade addition spectrum. Interedge correlations are found to be strong enough to allow simultaneous crystallization on both edges, although an applied electric field can decouple the two edges. Finally, we show that Wigner crystallization can also occur for holes, albeit weaker than for electrons.Article Citation - WoS: 8Citation - Scopus: 8Effects of Long-Range Disorder and Electronic Interactions on the Optical Properties of Graphene Quantum Dots(American Physical Society, 2017) Altıntaş, Abdulmenaf; Çakmak, K. E.; Güçlü, Alev Devrim; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of TechnologyWe theoretically investigate the effects of long-range disorder and electron-electron interactions on the optical properties of hexagonal armchair graphene quantum dots consisting of up to 10 806 atoms. The numerical calculations are performed using a combination of tight-binding, mean-field Hubbard, and configuration interaction methods. Imperfections in the graphene quantum dots are modeled as a long-range random potential landscape, giving rise to electron-hole puddles. We show that, when the electron-hole puddles are present, the tight-binding method gives a poor description of the low-energy absorption spectra compared to mean-field and configuration interaction calculation results. As the size of the graphene quantum dot is increased, the universal optical conductivity limit can be observed in the absorption spectrum. When disorder is present, the calculated absorption spectrum approaches the experimental results for isolated monolayers of graphene sheets.Article Citation - WoS: 16Citation - Scopus: 16Magnetic Phases of Graphene Nanoribbons Under Potential Fluctuations(American Physical Society, 2016) Özdemir, Hakan Ulaş; Altıntaş, Abdulmenaf; Güçlü, Alev Devrim; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of TechnologyWe investigate the effects of long-range potential fluctuations and electron-electron interactions on the electronic and magnetic properties of graphene nanoribbons with zigzag edges using an extended mean-field Hubbard model. We show that electron-electron interactions make the edge states robust against potential fluctuations. When the disorder is strong enough, the presence of electron-hole puddles induces a magnetic phase transition from antiferromagnetically coupled edge states to ferromagnetic coupling, in agreement with recent experimental results.Article Citation - WoS: 4Citation - Scopus: 4Wigner Crystallization at Graphene Edges(American Physical Society, 2016) Güçlü, Alev Devrim; Güçlü, Alev Devrim; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of TechnologyUsing many-body configuration interaction techniques, we show that Wigner crystallization occurs at the zigzag edges of graphene at surprisingly high electronic densities up to 0.8nm-1. In contrast with one-dimensional electron gas, the flatband structure of the edge states makes the system interaction dominated, facilitating electronic localization. The resulting Wigner crystal manifests itself in pair-correlation functions, and evolves smoothly as the edge electron density is lowered. We also show that the crystallization affects the magnetization of the edges. While the edges are fully polarized when the system is charge neutral (i.e., high density), above the critical density, the spin-spin correlations between neighboring electrons go through a smooth transition from antiferromagnetic to magnetic coupling as the electronic density is lowered.Article Citation - WoS: 10Citation - Scopus: 10Spin-Spin Correlations of Magnetic Adatoms on Graphene(American Physical Society, 2015) Güçlü, Alev Devrim; Bulut, Nejat; Bulut, Nejat; Güçlü, Alev Devrim; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of TechnologyWe study the interaction between two magnetic adatom impurities in graphene using the Anderson model. The two-impurity Anderson Hamiltonian is solved numerically by using the quantum Monte Carlo technique. We find that the interimpurity spin susceptibility is strongly enhanced at low temperatures, significantly diverging from the well-known Ruderman-Kittel-Kasuya-Yoshida result which decays as R-3.Article Citation - WoS: 58Citation - Scopus: 60Microscopic Theory of the Optical Properties of Colloidal Graphene Quantum Dots(American Physical Society, 2014) Özfidan, Işıl; Güçlü, Alev Devrim; Güçlü, Alev Devrim; Mcguire, John A.; Hawrylak, Pawel; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of TechnologyWe present a microscopic theory of electronic and optical properties of colloidal graphene quantum dots (CGQDs). The single-particle properties are described in the tight-binding model based on the pz carbon orbitals. Electron-electron screened Coulomb direct, exchange, and scattering matrix elements are calculated using Slater pz orbitals. The many-body ground state and excited states are constructed as a linear combination of a finite number of excitations from the Hartree-Fock (HF) ground state (GS) by exact diagonalization techniques. HF ground states corresponding to semiconductor, Mott-insulator, and spin-polarized phases are obtained as a function of the strength of the screened interaction versus the tunneling matrix element. In the semiconducting phase of a triangular CGQD, the top of the valence band and the bottom of the conduction band are found to be degenerate due to rotational symmetry. The singlet and triplet exciton spectra from the HF GS are obtained by solving the Bethe-Salpeter equation. The low-energy exciton spectrum is predicted to consist of two bright-singlet exciton states corresponding to two circular polarizations of light and a lower-energy band of two dark singlets and 12 dark triplets. The robustness of the bright degenerate singlet pair against correlations in the many-body state is demonstrated as well as the breaking of the degeneracy by the lowering of symmetry of the CGQD. The band-gap renormalization, electron-hole attraction, fine structure, oscillator strength, and polarization of the exciton are analyzed as a function of the size, shape, screening, and symmetry of the CGQD. The theoretical results are compared with experimental absorption spectra.Article Citation - WoS: 26Citation - Scopus: 27Electron-Electron Interactions and Topology in the Electronic Properties of Gated Graphene Nanoribbon Rings in Möbius and Cylindrical Configurations(American Physical Society, 2013) Güçlü, Alev Devrim; Güçlü, Alev Devrim; Hawrylak, Pawel; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of TechnologyWe present a theory of the electronic properties of gated graphene nanoribbon rings with zigzag edges in Möbius and cylindrical configurations. The finite width opens a gap and nontrivial topology of the Möbius ring leads to a single edge with edge states with an induced, effective gauge field, in analogy to topological insulators. The single zigzag edge leads to a shell of degenerate states at the Fermi level and a ferromagnetic (FM) ground state at half-filling, i.e., at charge neutrality, due to electron-electron interactions. For fractional fillings, both the magnetic moment and the energy gap are found to oscillate as a function of the shell filling. In cylindrical rings, the two edges lead to AF ground state at half-filling but FM ground state at fractional fillings.