Phd Degree / Doktora
Permanent URI for this collectionhttps://hdl.handle.net/11147/2869
Browse
3 results
Search Results
Doctoral Thesis Electronic, Magnetic and Transport Properties of Graphene Quantum Dots With Charged Impurities(Izmir Institute of Technology, 2020) Polat, Mustafa; Güçlü, Alev DevrimIn this thesis, electronic, magnetic, and transport properties of armchair edged hexagonal and zigzag edged triangular graphene quantum dots (GQDs) are investigated in the presence of charged impurities. In this manner, a special attention has been paid to the Coulomb impurity problem in these structures. The collapse of the wave functions starting from the 1S$_{1/2}$ state is studied in the presence of not only the Coulomb impurity but also in the presence of a Coulomb charged vacancy with the help of tight-binding and extended mean-field Hubbard (MFH) models. Here, we report an interaction induced renormalization of the critical coupling constant ($\beta_{c}$). In addition, our results suggest that the induced charge for the interacting fermions is smaller than that of the non-interacting fermions. Furthermore, the transport coefficients reveal two different characteristics of the subcritical ($\beta$ $<$ $\beta_{c}$) and supercritical ($\beta$ $>$ $\beta_{c}$) regimes. As for the charged vacancy, the bare carbon vacancy induces a local magnetic moment in the hexagonal GQDs, but it is suppressed when the vacancy is charged with the subcritical Coulomb potential. Except the pristine cases of the GQDs, we numerically study a Coulomb impurity problem for the interacting fermions restricted in disordered hexagonal GQDs. In the presence of randomly distributed lattice defects and spatial potential fluctuations induced by Gaussian impurities, the response of $\beta_{c}$ for atomic collapse is mainly investigated by local density of states (LDOS) calculations within the MFH model. We find that both types of disorder cause an amplification of the critical threshold. As for the zigzag edged triangular GQDs, in the presence of the bare vacancy, we exactly obtain the spin splitting with the help of LDOS calculations in the energy spectrums, which are dominated by the edge states around the Fermi level. Similar to the hexagonal GQDs, if the vacancy is charged, the local magnetic moment disappears in these GQDs.Doctoral Thesis Physics of Higher Spin Fields(Izmir Institute of Technology, 2020) Sargın, Ozan; Güçlü, Alev DevrimSpin-3/2 fields are the next spin multiplet we look for in the general particle search. Although these fields can be either fundamental vector-spinors or just excited leptons and quarks we assume that they are fundamental throughout this thesis. These higher-spin fields, described by the Rarita-Schwinger equations have to obey certain constraints to have correct degrees of freedom when they are on the physical shell. \par In the first chapter after the introduction, we introduce these spinor-vector fields to the reader by first going through the different representations that can be employed to describe them. We then recapitulate some facts on the most general free lagrangian and the propagator for these fields. \par In the next chapters we investigate different phenomenological implications. We start out in chapter \ref{chap:1} with a massive spin-3/2 field hidden in the standard model (SM) spectrum thanks to the form of the special interaction that vanishes when the field falls into the mass shell. Different collider signatures are investigated through analytical computations and numerical predictions. \par In chapter \ref{chap:2}, we assume that the Higgs boson stays stable via a finely tuned hidden sector which involves a spin-3/2 field that is split from the SM and whose sole contact with it at the renormalizable level is through the neutrino portal. Then, the total mass correction to the Higgs mass is used as a constraint to calculate the mass scale of the spin-3/2 field. \par Lastly, we investigate the possible role that a spin-3/2 field could play in leptogenesis. Our model incorporates a spin-3/2 field in addition to the type-I see-saw fields in inducing the CP asymmetry and mitigating the naturalness problem of the Higgs boson. We investigate the plausibility in regard to successful leptogenesis with no side effects, specifically the naturalness of the Higgs boson and correct prediction of the active neutrino masses.Doctoral Thesis Electronic, Magnetic and Optical Properties of Disordered Graphene Quantum Dots(Izmir Institute of Technology, 2018) Altıntaş, Abdulmenaf; Güçlü, Alev DevrimIn this thesis, we theoretically investigate electronic, magnetic and optical properties of disordered graphene quantum dots. The numerical calculations are performed using a combination of tight-binding, mean-field Hubbard and configuration interaction methods. We focus on the effects of long-range disorder and electron-electron interactions on the optical properties and 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. For the case of long-range disorder, we show that, when the electron-hole puddles are present, tight-binding method gives a poor description of the low-energy absorption spectra compared to meanfield 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, calculated absorption spectrum approaches the experimental results for isolated monolayer of graphene sheet. On the other hand, for the case of short-range related disorder, we observe that randomly distributed defects with concentrations between 1-5% of the total number of atoms leads to electronic localization alongside magnetic puddle-like structures. We show that localization length is not affected by magnetization if there is an even distribution of defects between the two sublattices of the honeycomb lattice. However, for an uneven distributions, localization is found to be significantly enhanced.