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: 4Citation - Scopus: 3Quantum Monte Carlo Study of Semiconductor Artificial Graphene Nanostructures(AMER PHYSICAL SOC, 2023) Öztarhan, Gökhan; Güçlü, Alev Devrim; Kul, E. Bulut; Okçu, Emre; Guclu, A. D.Semiconductor artificial graphene nanostructures where the Hubbard model parameter U/t can be of the order of 100, provide a highly controllable platform to study strongly correlated quantum many-particle phases. We use accurate variational and diffusion Monte Carlo methods to demonstrate a transition from antiferromagnetic to metallic phases for an experimentally accessible lattice constant a = 50 nm in terms of lattice site radius rho, for finite-sized artificial honeycomb structures nanopatterned on GaAs quantum wells containing up to 114 electrons. By analyzing spin-spin correlation functions for hexagonal flakes with armchair edges and triangular flakes with zigzag edges, we show that edge type, geometry, and charge nonuniformity affect the steepness and the crossover rho value of the phase transition. For triangular structures, the metal-insulator transition is accompanied with a smoother edge polarization transition.Article Citation - WoS: 4Citation - Scopus: 4Atomic Collapse in Graphene Quantum Dots in a Magnetic Field(Elsevier, 2022) Eren, İsmail; Güçlü, Alev DevrimWe investigate finite size and external magnetic field effects on the atomic collapse due to a Coulomb impurity placed at the center of a hexagonal graphene quantum dot within tight binding and mean-field Hubbard approaches. For large quantum dots, the atomic collapse effect persists when the magnetic field is present, characterized by a series of Landau level crossings and anticrossings, in agreement with previous bulk graphene results. However, we show that a new regime arises if the size of the quantum dot is comparable to or smaller than the magnetic length: While the lowest bound states cross the Fermi level at a lower value of coupling constant β<0.5, a size independent critical coupling constant βc∗>0.5 emerges in the local density of states spectrum, which increases with the applied magnetic field. These effects are found to be persistent in the presence of electron–electron interactions within mean-field Hubbard approximation.Article Citation - WoS: 3Citation - Scopus: 3Spin-Orbit Coupling and Optical Detection of Spin Polarisation in Triangular Graphene Quantum Dots(Inderscience Enterprises Ltd., 2015) Potasz, P.; Güçlü, Alev Devrim; Özfidan, I.; Hawrylak, P.We present a theory of the effect of spin-orbit coupling on optical properties of triangular graphene quantum dots (TGQD). TGQDs with zigzag edges exhibit a degenerate band of states at the Fermi level. For the charge neutral TGQD, the shell is expected to be half-filled by spin polarised electrons leading to finite magnetisation. Using four-band tight-binding and effective Kane-Mele models, we show that, if the TGQD is spin polarised, the low energy optical absorption spectrum reveals two distinct peaks corresponding to left and right circularly polarised light while the unpolarised TGQD shows only one peak. This allows optical detection of spin polarisation, its direction and the strength of spin-orbit coupling in TGQDs.Article Citation - WoS: 3Citation - Scopus: 3Atomic Collapse in Disordered Graphene Quantum Dots(American Physical Society, 2020) Polat, Mustafa; Güçlü, Alev DevrimIn 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: 12Citation - Scopus: 13Electronic and Magnetic Properties of Graphene Quantum Dots With Two Charged Vacancies(Elsevier, 2020) Kul, Erdoğan Bulut; Polat, Mustafa; Güçlü, Alev DevrimElectronic and magnetic properties of a system of two charged vacancies in hexagonal shaped graphene quantum dots are investigated using a mean-field Hubbard model as a function of the Coulomb potential strength ? of the charge impurities and the distance R between them. For ?=0, the magnetic properties of the vacancies are dictated by Lieb's rules where the opposite (same) sublattice vacancies are coupled antiferromagnetically (ferromagnetically) and exhibit Fermi oscillations. Here, we demonstrate the emergence of a non-magnetic regime within the subcritical region: as the Coulomb potential strength is increased to ??0.1, before reaching the frustrated atomic collapse regime, the magnetization is strongly suppressed and the ground state total spin projection is given by Sz=0 both for opposite and same sublattice vacancy configurations. When long-range electron–electron interactions are included within extended mean-field Hubbard model, the critical value for the frustrated collapse increases from ?cf?0.28 to ?cf?0.36 for R<27Å. © 2020 Elsevier LtdArticle 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: 5Citation - Scopus: 5Defect Induced Anderson Localization and Magnetization in Graphene Quantum Dots(Elsevier, 2018) Altıntaş, Abdulmenaf; Güçlü, Alev DevrimWe theoretically investigate the effects of atomic defect related short-range disorders and electron-electron interactions on Anderson type localization and the magnetic properties of hexagonal armchair graphene quantum dots using an extended mean-field Hubbard model and wave packet dynamics for the calculation of localization lengths. We observe that randomly distributed defects with concentrations between 1 and 5% of the total number of atoms leads to localization alongside magnetic puddle-like structures. Although the localization lengths are not affected by interactions, staggered magnetism and localization are found to be enhanced if the defects are distributed unevenly between the sublattices of the honeycomb lattice.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 DevrimWe 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: 29Citation - Scopus: 31Wigner Crystallization in Topological Flat Bands(IOP Publishing Ltd., 2018) Jaworowski, Blazej; Güçlü, Alev Devrim; Kaczmarkiewicz, Piotr; Kupczynski, Michal; Potasz, Pawel; Wójs, ArkadiuszWe study the Wigner crystallization on partially filled topological flat bands of kagome, honeycomb and checkerboard lattices. We identify the Wigner crystals (WCs) by analyzing the Cartesian and angular Fourier transform of the pair correlation density of the many-body ground state obtained using exact diagonalization. The crystallization strength, measured by the magnitude of the Fourier peaks, increases with decreasing particle density. The Wigner crystallization observed by us is a robust and general phenomenon, existing in all three lattice models for a broad range of filling factors and interaction parameters. The shape of the resulting WCs is determined by the boundary conditions of the chosen plaquette. It is to a large extent independent on the underlying lattice, including its topology, and follows the behavior of classical point particles.Conference Object Graphene-Based Integrated Electronic, Photonic and Spintronic Circuit(SPIE, 2013) Potasz, P.; Güçlü, Alev Devrim; Özfidan, Işıl; Korkusinski, Marek; Hawrylak, PawelTo create carbon-based nanoscale integrated electronic, photonic, and spintronic circuit one must demonstrate the three functionalities in a single material, graphene quantum dots (GQDs), by engineering lateral size, shape, edges, number of layers and carrier density. We show theoretically that spatial confinement in GQDs opens an energy gap tunable from UV to THz, making GQDs equivalent to semiconductor nanoparticles. When connected to leads, GQDs act as single-electron transistors. The energy gap and absorption spectrum can be tuned from UV to THz by size and edge engineering and by external electric and magnetic fields. The sublattice engineering in, e.g., triangular graphene quantum dots (TGQDs) with zigzag edges generates a finite magnetic moment. The magnetic moment can be controlled by charging, electrical field, and photons. Addition of a single electron to the charge-neutral system destroys the ferromagnetic order, which can be restored by absorption of a photon. This allows for an efficient spin-photon conversion. These results show that graphene quantum dots have potential to fulfill the three functionalities: electronic, photonic, and spintronic, realized with different materials in current integrated circuits, as well as offer new functionalities unique to graphene.