Master Degree / Yüksek Lisans Tezleri
Permanent URI for this collectionhttps://hdl.handle.net/11147/3008
Browse
Search Results
Master Thesis Rkky Interaction and Its Control in Graphene and Related Materials(Izmir Institute of Technology, 2019) Canbolat, Ahmet Utku; Çakır, Özgür; Çakır, Özgür; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of TechnologyGraphene got dramatic attention and lead the two-dimensional material physics after its first successful synthesis in 2004. Its unique electronic properties contain great potential for both scientific and technological applications. RKKY (Ruderman-Kittel-Kasuya Yosida) is an indirect exchange interaction mediated by conduction electrons. In graphene, the interaction strength decay as 1/R³ where R is the distance between the magnetic moments. In the first part of this work, we calculated that applying circular potential on a graphene sheet forms quasi-bound states in the potential region. Via these states, the RKKY interaction is enhanced between magnetic moments on the edge of the potential well. This can be thought of an electronic analog of the Purcell effect. We showed that the interaction strength is even more enhanced if the Fermi level is in resonance with the energies of the quasi-bound states. In the second part, we considered zigzag edged hexagonal nanoflakes. It is known that zigzag edged flakes have zero-energy edge-states. It is also known that the states with closer energies contribute more to RKKY interaction. Thus, we calculated that there is an enhancement between these edge-states. In the third part, we investigated the behavior of RKKY interaction for two dimensional materials with quartic dispersion. An energy dispersion is said to be quartic if it is of the form E = α(k² - kc² )². Here, α and kc are material dependent constants. There are many materials exhibiting the quartic dispersion such as nitrogene, phosphorene, and arsenene. These materials are also sharing two-dimensional hexagonal lattice structure with graphene. What makes quartic dispersion special is that it has van-Hove singularity in its density of states near the band-edge. RKKY interaction is sensitive to the density of states because it depends on the number of electrons contributing spin exchange. Thus, the larger the number of electrons, the stronger the coupling. In this part, we tuned the Fermi level so that it lies on the DOS singularity and then we calculated the interaction strength as a function of R. We found a slowly decaying RKKY interaction for quartic dispersion. If the energy dispersion is pure quartic (i.e. E = ak4), we found the interaction strength depends on 1/(kf R) instead of 1/R which makes the RKKY interaction long range for arbitrarily small Fermi level.Master Thesis Spin-Spin Interactions of Magnetic Impurities in Graphene Nanoribbons(Izmir Institute of Technology, 2019) Kolay, Anıl; Güçlü, Alev Devrim; Güçlü, Alev Devrim; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of TechnologyIn this thesis, we investigate the interaction between two impurity adatoms with high magnetic moment which are located on zigzag graphene nanoribbons that consist of 10516 atoms. The magnetic adatoms communicate with other trough the host electrons such as Ruderman-Kittel-Kasuya-Yoshida (RKKY) interactions. Firstly, in order to numerically calculate the two impurity Anderson model, we use quantum Monte Carlo technique. When the impurity adatoms are located far from edges, the results we obtained are consistent whit the bulk graphene results in the literature. On the other hand, the specific location and orientation of adatoms on the sublattices, significantly affects the spin-spin correlations of the two impurities. However, we observe that while the adatoms approach to the edges, significant differences occur due to the edge effect of zigzag graphene nanoribbon. As a results of this, we found that the magnetic correlations can be also enhanced if the adatoms belong to the same sublattice as the edga atoms, since the states of the adatoms hybridize with edge states. Moreover, we show that chaning chemical potential can crucially affect the magnitude of the correlations of the adatoms, and may lead to aphase transitions from ferromagnetic to antiferromagnetic or vice versa. Besides, we observe that when the width of the zigzag graphene nanoribbons is decreased, the spin-spin correlations are affected.On the other hand, we also calculated spin-spin correlations using mean-field approximation for themean-field Anderson model. We found that results significantly differ from quantum Monte Carlo results. In addition, when the electron-electron interations of he host atoms are taken into account, crucial differences are obtained at the impurity correlations.