Physics / Fizik
Permanent URI for this collectionhttps://hdl.handle.net/11147/6
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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: 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: 10Citation - Scopus: 10The Comparison of Transient Photocurrent Spectroscopy Measurements of Pulsed Electron Deposited Zno Thin Film for Air and Vacuum Ambient Conditions(Elsevier, 2019) Özdoğan, Mehmet; Yiğen, Serap; Çelebi, Cem; Utlu, GökhanPhotoconduction mechanism of ZnO thin films that produced by Pulsed Electron Deposition method is systematically investigated by taking Transient Photocurrent Spectroscopy measurements for different atmospheres including high vacuum and air environments. Response and recovery rates of photocurrent in the air are faster than the rates in high vacuum condition. The results in the presented work clearly indicate that the photoconduction of ZnO thin films with high surface-area-to-volume ratio are surface-related and mostly governed by adsorption/desorption of oxygen and water molecules in the atmosphere. Therefore, the high surface interaction tendency of ZnO surface with the atmosphere inevitably leads to charge transfer from surface to adsorbates and/or vice versa.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.