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- Haldar, Soumyajyoti, 1986-
(author)
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Influence of defects and impurities on the properties of 2D materials
- 2016
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Doctoral thesis (other academic/artistic)abstract
- Graphene, the thinnest material with a stable 2D structure, is a potential alternative for silicon-based electronics. However, zero band gap of graphene causes a poor on-off ratio of current thus making it unsuitable for logic operations. This problem prompted scientists to find other suitable 2D materials. Creating vacancy defects or synthesizing hybrid 2D planar interfaces with other 2D materials, is also quite promising for modifying graphene properties. Experimental productions of these materials lead to the formation of possible defects and impurities with significant influence in device properties. Hence, a detailed understanding of the effects of impurities and defects on the properties of 2D systems is quite important.In this thesis, detailed studies have been done on the effects of impurities and defects on graphene, hybrid graphene/h-BN and graphene/graphane structures, silicene and transition metal dichalcogenides (TMDs) by ab-initio density functional theory (DFT). We have also looked into the possibilities of realizing magnetic nanostructures, trapped at the vacancy defects in graphene, at the reconstructed edges of graphene nanoribbons, at the planar hybrid h-BN graphene structures, and in graphene/graphane interfaces. A thorough investigation of diffusion of Fe adatoms and clusters by ab-initio molecular dynamics simulations have been carried out along with the study of their magnetic properties. It has been shown that the formation of Fe clusters at the vacancy sites is quite robust. We have also demonstrated that the quasiperiodic 3D heterostructures of graphene and h-BN are more stable than their regular counterpart and certain configurations can open up a band gap. Using our extensive studies on defects, we have shown that defect states occur in the gap region of TMDs and they have a strong signature in optical absorption spectra. Defects in silicene and graphene cause an increase in scattering and hence an increase in local currents, which may be detrimental for electronic devices. Last but not the least, defects in graphene can also be used to facilitate gas sensing of molecules as well as and local site selective fluorination.
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| 2. |
- Grånäs, Oscar, 1979-
(author)
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Theoretical Studies of Magnetism and Electron Correlation in Solids
- 2012
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Doctoral thesis (other academic/artistic)abstract
- This work presents new development and applications of ab-initio simulation tools for material science. Focus lies on materials with strong electronic correlation and strong spin-orbit coupling. Improvements on methods for solving the impurity problem in LDA+DMFT is presented, as well as a reliant method for charge self-consistency in a LMTO based electronic structure code. A new adaptive scheme for Brillouin zone integration is developed, where we show a strong reduction of numerical noise compared to standard techniques. A reformulation of the standard LDA+U method aiming to reduce the number of free parameters is introduced. Fast and realistic reduction of the number of free parameters provides the possibility of high throughput calculations and enabled us to study a large number of compounds. An analysis method for polarization in terms of coupled multipoles, and their corresponding energy contributions is developed and applied. This led to the formulation of Katt's rules, a set of rules complementary to Hund's rules. Katt's rules applies for occupying the orbitals of an electronic shell with strong spin-orbit coupling. The analysis is also used to investigate the unconventional Uranium based superconductors URu2Si2, UPt3, UPd2Al3 and UNi2Al3, as well as the high temperature superconductor LaOFeAs. We also investigate the non-magnetic delta-phase of Plutonium, providing insight to the electronic structure and the branching ratios of 4d to 5f transitions seen in photo emission spectra.The influence of surface reconstruction on the magneto crystalline anisotropy is investigated in multilayer Fe/ZnSe, showing that Fe deposited on an unreconstructed interface strongly reduces the uniaxial component of the MAE. We provide a detailed understanding of the magnetic properties of Fe2P, opening possible routes for enhancing the MAE in this system. A general route to strong MAE in nano-laminates is presented, we apply this to propose a candidate with extremely strong anisotropy energy density, 5Fe/2W1-xReX for x=[0.6-0.8].
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| 3. |
- Lindström, Anna, 1987-
(author)
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Defects and Impurities in CdTe : An ab Initio Study
- 2015
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Doctoral thesis (other academic/artistic)abstract
- In this thesis defects and impurities in CdTe have been studied with ab initio methods. CdTe is a II-VI semiconductor with many important applications such as γ- and X-ray detectors, solar cells and medical imaging. Even though CdTe has been studied for more than 70 years, some of its properties connected with defects and impurities, are still shrouded in mystery. Todays experimental techniques are highly developed and can provide rather detailed data, but require elaborate theoretical analysis. Here ab initio modelling comes into play and in particular density functional theory (DFT). When reviewing different theoretical studies of defects and impurities in CdTe, one finds a vast number of discrepancies between experiment and theory. Mismatches appear even between different theoretical studies. Although many problems, such as, for example, the semiconductor band gap underestimation or the spurious interaction between charged defects, are avoided by employing corrections or implementing new functionals, some of them still remain. Employing the hybrid functional HSE06, the following topics were studied in this thesis:- Te antisites: Experimental data predict the defect state to appear in the middle of the band gap, thus "pinning" the Fermi level. In contrast, our calculations show that Te antisite alone cannot be the reason for the Fermi level pinning, since it does not form a defect level in the middle of the band gap. Instead we propose that charge compensation between Te antisites in a (+2) state and Cd vacancies in a (-2) state explains the Fermi level pinning.- Cd vacancy: Electron paramagnetic resonance experiments clearly show the existence of a hole polaron for the (-1) charged vacancy. But DFT studies report a completely delocalised hole. In our studies, for the first time, this state was found in its proper geometrical configuration with a hole localisation stabilised by a Jahn-Teller distortion, thereby removing the discrepancy between experiment and theory.- Cd chalcogenides: Additionally, with particular focus on the hole localisation problem, the series of isovalent compounds (CdTe, CdSe and CdS) was studied to understand the mechanism of hole polaron formation. We explain the trend of the hole localisation in terms of Coulomb interaction, explicitly showing that the effect of electron correlation is negligible.- Cl-doped CdTe: The formation of a Cl - Cd vacancy complex explains the selfcompensation and selfpurification mechanism. We find Cl to annihilate the hole polaron.- Te antisite under deformation: In an attempt to tailor the energy position of the Te antisite defect level in the CdTe band gap, we studied CdTe under different deformations. It is shown that by a carefully chosen deformation the defect levels can be pushed closer to the valence and/or conduction band and hence the CdTe detector performance may be improved.
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| 4. |
- Wang, Duo
(author)
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Ab initio studies of advanced functional materials with complex magnetism
- 2022
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Doctoral thesis (other academic/artistic)abstract
- For centuries, magnetism of materials has been an inevitable part of human civilization. Only in the last century, the mysteries of magnetism started to unfold thanks to the development of quantum theory of solids. Nevertheless, even today, new exotic phenomena related to magnetism keep on surprising us and provide an enormous playground for theoreticians and experimentalists to unravel the complexities. In this thesis, the magnetic properties of materials are studied from different aspects by using first-principle density functional theory. Specifically, we investigated the substituted quadruple perovskite compounds ACu2Fe2Re2O12 (A=Ca, Sr, Ba, Pb, Sc, Y, La). Seven different A-site doped structures are studied, including divalent and trivalent charge substitutions. We found that all these compounds are half-metallic ferrimagnets with large magnetization and high transition temperatures (above 405K). Interestingly, the trivalent atom doping at the A-site can significantly increase the transition temperature. The exchange mechanism is explained by the super-exchange in the Re-Cu and Re-Fe pairs. Moreover, we investigated three different two-dimensional magnets, CrI3, FeS2, and CrO. For the first project, we studied stacking dependent magnetic properties of CrI3. It was found that the magnetic ground state can be tuned by the stacking sequences. In the second project, we studied the monolayer FeS2. The results show that the structures with FM and AFM configuration are close in energy. By performing further spin-spiral calculations, we found that the ground state magnetic configurations are different with different crystal structures. This structure dependent magnetic property indicates the existence of spin-lattice coupling in this material. In the third project, we predicted a monolayer CrO, which is a Weyl semimetal with antiferromagnetism up to room temperature. Finally, a heterostructure structure with G-type SrMnO3 supported on SrTiO3 substrate is investigated. We found that with a 2.9% tensile strain introduced by the substrate, the SrMnO3 keeps as G-type AFM. Moreover, oxygen vacancy intends to stay at the surface. Interestingly, this vacancy induces the AFM-FM transition on the specific layer due to the double exchange mechanism.
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