Physics / Fizik

Permanent URI for this collectionhttps://hdl.handle.net/11147/6

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Author: search.filters.author.Polat, MustafaScopus Q: search.filters.scopusQuartile.Q2

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Now showing 1 - 3 of 3
  • Article
    Citation - WoS: 3
    Citation - Scopus: 3
    Atomic Collapse in Disordered Graphene Quantum Dots
    (American Physical Society, 2020) Polat, Mustafa; Güçlü, Alev Devrim
    In 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: 5
    Citation - Scopus: 5
    Collapse 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 Devrim
    In 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: 4
    Citation - Scopus: 5
    Identifying Threading Dislocations in Cdte Films by Reciprocal Space Mapping and Defect Decoration Etching
    (American Institute of Physics, 2018) Polat, Mustafa; Bilgilisoy, Elif; Arı, Ozan; Öztürk, Orhan; Selamet, Yusuf
    We study threading dislocation (TD) density of high-quality cadmium telluride (CdTe) layers grown on a (211) oriented GaAs substrate by molecular beam epitaxy. High-resolution X-ray diffraction was performed to calculate the density of screw-type TDs by measuring the broadening of the asymmetrical (511) Bragg reflections of CdTe epilayers. In addition, total TD densities were determined by the Everson-etching method and were compared with screw TDs. Our results show that the total TD densities in CdTe films were dominated by those with screw character. The screw component TDs are estimated to account for more than 90% of the total TD density. CdTe layers grown at a thickness of less than 3.0 μm typically exhibit the screw TD densities in the 106 cm-2 and 107 cm-2 range. It can be noted that as the nucleation temperature increases, i.e., ≥222 °C, both the area density of TDs with the screw component of the CdTe films and the total TD density are roughly four times larger than those of the epilayer grown at the nucleation temperature of 215 °C. Furthermore, we discuss the influence of the II/VI flux ratio on the density of threading dislocations. The contribution of screw TDs to the total TD density showed a significant decrease in roughly 30% in the case of a high II/VI flux ratio. We further examine the reciprocal space maps in the vicinity of the (422) reflections.