Physics / Fizik
Permanent URI for this collectionhttps://hdl.handle.net/11147/6
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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: 26Citation - Scopus: 26Theory of Optical Properties of Graphene Quantum Dots(John Wiley and Sons Inc., 2016) Özfidan, Işıl; Güçlü, Alev Devrim; Korkusinski, Marek; Hawrylak, Pawel; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of TechnologyWe present here a theory of the optical properties of graphene quantum dots (GQDs) with tunable band gaps by lateral size confinement, from UV to THz. Starting from the Hartree-Fock ground state, we construct the correlated many-body ground and excited states of GQDs as a linear combination of a finite number of electron-hole pair excitations. We discuss the evolution of the band gap with size and its renormalization by self-energy and excitonic effects. We calculate and analyze the dipole moments of graphene quantum dots that possess a degenerate valence and conduction band edge, and construct a characteristic exciton and biexciton spectrum. We find an exciton band consisting of a pair of robust, spin singlet bright exciton states and a band of dark, spin singlet and spin triplet, exciton states at lower energies. We predict a characteristic band of biexciton levels at the band edge, discuss the Auger processes and identify a biexciton-exciton cascade. Our theoretical results are compared with experimental linear absorption and non-linear transient absorption spectra of colloidal GQDs. We next discuss the optical properties of triangular GQDs with zigzag edges whose magnetic moment can be controlled by gates. The control over the magnetic moment through carrier density manipulation results in optical spin blockade and gate tunable optical properties over a wide range of photon energies.Article Citation - WoS: 2Citation - Scopus: 2Muon Anomalous Magnetic Moment Constraints on Supersymmetric U(1)?? Models(American Physical Society, 2010) Cincioğlu, Elif; Kırca, Zerrin; Sert, Hale; Solmaz, Saime; Solmaz, Levent; Hiçyılmaz, Yaşar; 01. Izmir Institute of TechnologyWe study the anomalous magnetic moment of the muon in supersymmetric E 6 models and generic U(1)′ models to probe the model reactions and to find constraints on the large parameter space of these models. For future searches, by imposing the existing bounds coming from collider searches and theoretical considerations upon the U(1)′ model parameters, we examine the lightest Higgs boson mass mh and the mass of the additional Z boson mZ2 in such singlet extensions of the MSSM. We observed that not only supersymmetric E6 models but also generic U(1)′ models are sensitive to the imposition of the considered bounds. Indeed, without the muon anomaly constraints E6 models and generic U(1)′ models can predict mh as large as ∼150GeV and ∼180GeV, respectively. However, in addition to the mentioned constraints when a 1σ range for the anomalous magnetic moment of the muon is considered, we observe that generic U(1)′ models do not favor the mass of the lightest Higgs boson to be larger than 140 GeV; it should be smaller than 135 GeV in E6 models. © 2010 The American Physical SocietyArticle Citation - WoS: 69Citation - Scopus: 72Spintronic Properties of Zigzag-Edged Triangular Graphene Flakes(American Institute of Physics, 2010) Şahin, Hasan; Şahin, Hasan; Senger, Ramazan Tuğrul; Senger, Ramazan Tuğrul; Çıracı, Salim; 04.04. Department of Photonics; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of TechnologyWe investigate quantum transport properties of triangular graphene flakes with zigzag edges by using first principles calculations. Triangular graphene flakes have large magnetic moments which vary with the number of hydrogen atoms terminating its edge atoms and scale with its size. Electronic transmission and current-voltage characteristics of these flakes, when contacted with metallic electrodes, reveal spin valve and remarkable rectification features. The transition from ferromagnetic to antiferromagnetic state under bias voltage can, however, terminate the spin polarizing effects for specific flakes. Geometry and size dependent transport properties of graphene flakes may be crucial for spintronic nanodevice applications. © 2010 American Institute of Physics.