"Nanostructures Theory and Modelling" Seminar
2019-01-24 (Thursday)
room 1.02, Pasteura 5 at 17:15 
Dr Kamil Czelej (Warsaw University of Technology)Ab initio calculations of point defects and their complexes in diamond
To understand the impact of certain point defects on the electronic structure, electrical, magnetic, and optical properties of diamond, I performed theoretical calculations based on advanced hybrid density functional theory.One of the most relevant problems in the field of diamond–based high–power electronics is relatively high donor activation energy of phosphorus–doped diamond. The calculations performed in this thesis indicate that arsenic and antimony can theoretically generate shallower donor level than substitutional P, however, due to very high formation energy, the fabrication of As– or Sb–doped diamond with reasonable concentration of these elements at substitutional site might be very challenging.The investigation of yet unidentified H-vacancy and H-P-vacancy complexes in diamond has been carried out employing the extended Hubbard model Hamiltonian with fully ab initio derived parameters was introduced to properly describe their electronic structure. Some of the investigated defects were found to be promising for quantum information processing application. Further the Ti-N and Ti-vacancy-N complexes has been studied.
2018-12-20 (Thursday)
room 1.02, Pasteura 5 at 17:15
Marta Brzezińska (University of Zurich and Wroclaw University of Science and Technology)Topological states in fractal lattices
Existing classi fication of topological phases rely on the symmetries as well as on the number of spatial dimensions being an integer. However, equipped with a notion of locality and the possibility to take a thermodynamic limit, the classi fication schemes can be extended to be suitable for quantum states on general graphs. In particular, one can consider fractal geometries characterized by (non-integer) Hausdor ff dimensionand rami fication number. Here, we investigate two fractal lattices, Sierpinski carpet and gasket, exposed to an external magnetic eld. By examining spectral and localization properties, together with the Chern number calculations and level spacings analysis in the presence of disorder, we identify states with non-trivial topology that exhibit featuressimilar to the quantum Hall e ffect.
2018-12-13 (Thursday)
room 1.02, Pasteura 5 at 17:15
dr Magdalena Birowska (IFT UW)Chemical phase separation as origin of nematic properties of alloys
In plane rotational symmetry breaking commonly occurs for many materials, such as unconventional superconductors (e. g. CuxBi2Se3), ferromagnetic semiconductors (e. g. Ga1-xMnxAs), or quantum wells with e. g. AlxGa1-xAs. These nematic properties are usually attributed to spontaneous breaking of symmetry, i.e. a phase transition induced by correlations. I will present the thesis that the frozen anisotropy of the distribution of alloy components, which arises during the crystal growth, corresponds for the breaking of rotational symmetry, as illustrated by the example of two alloys In In1-xFexAs and Ga1-xMnxAs. In this seminar I will present the results of the ab initio calculations concerning the anisotropic chemical phase separation of these materials. I will show that the frozen anisotropic distribution of alloy components is responsible for magnetic or magnetic transportproperties, which will be discussed on the example of magnetoresistance anisotropic (AMR) In1-xFexAs and Ga1-xMnxAs.
2018-11-29 (Thursday)
room 1.02, Pasteura 5 at 17:15
mgr inż. Maciej Polak (Department of Experimental Physics, Wroclaw University of Science and Technology)The electronic band structure and defect properties of of highly mismatched III-V-Bi alloys
In this presentation an entirely original and thorough first-principle study of the electronic properties of III-V-Bi alloys is presented. All III-V zinc-blende semiconductors alloyed with bismuth were studied, i.e. AlPBi, AlAsBi, AlSbBi, GaPBi, GaAsBi, GaSbBi, InPBi, InAsBi and InSbBi. State-of-the-art DFT methods were employed in order to calculate the electronic band structure of those materials in the whole Brillouin zone, and parameters describing the changes in the band structures were obtained and presented. The band structure calculations were followed by the investigation of defect properties in these systems. In this case, native as well as Bi-related defects in form of point or pair defects were studied in the low Bi composition (diluted regime). Hybrid functionals together with the Freysoldt correction scheme for charged defects were used in order to obtain the most accurate results of formation energies, binding energies and charge-state transition defect levels. In order to obtain the values for every studied defect, a regression model based on the obtained results was built, allowing for accurate prediction of the values without employing computationally expensive tools.The chemical trend for the materials were then discussed in terms of the applicability of the materials in optoelectronics.
2018-11-22 (Thursday)
room 1.02, Pasteura 5 at 17:15
Dr Karolina Jurkiewicz (Department of Biophysics and Molecular Physics, Institute of Physics, University of Silesia)Simulations of the atomic structure for carbon materials and nanomaterials and their experimental verification using diffraction method
Detailed knowledge of the atomic scale structure is crucial factor for understanding properties of materials. Diffraction is one of the most powerful techniques used for determination of the atomic structure and in the case of fully periodic – crystalline materials allows for robust and quantitative description of atomic positions. However, complex systems such as carbon materials and nanomaterials are intractable with using standard crystallographic methods, because they cannot be regarded as crystalline materials. A different approach combining modelling studies and their experimental verification by diffraction data can tackle the structural issues in carbons to a high degree of accuracy. This approach focuses on the atomic pair distribution function (PDF) analysis. During the talk I will present how the combination of the PDF approach and computer simulations using density-functional theory, semi-empirical methods or classical molecular dynamics can be used for establishing features of the atomic structure for different carbon materials such as disordered carbons, fullerenes, graphene. I will demonstrate the process of structure refinement and reconstruction of 3D fragments of material in the form of Cartesian coordinates of atoms based on computer modeling.
2018-11-15 (Thursday)
room 1.02, Pasteura 5 at 17:15
Jacek A. Majewski (IFT UW)Nano-materials for non-metallic catalysts
First, I will provide some introduction to the catalysis processes, just explaining the most important concepts and stressing the meaning of the field in science and economy. Next, I will describe the present activities searching for non-metallic catalysts. Finally, I will describe our ab initio molecular dynamic studies in the framework of the density functional theory on catalytic role of graphene for the chemical reactions leading to the dehydrogenation of methane. The energetics of the studied processes will be compared to analogous reactions on the surface of copper and nickel. I conclude this talk by demonstrating how our findings explain the growth of graphene monolayer on metals in chemical vapour deposition growth process.
2018-10-25 (Thursday)
room 1.02, Pasteura 5 at 17:15
dr Manel Mabrouk (IFT UW)Electronic and magnetic properties of two-dimensional TM-Pc and TM-TCNB monolayers
We investigate the electronic and magnetic properties of two-dimensional metal-organic planar networks TM-Pc and TM-TCNB (where TM means a transition metal of the 3d series: Ti, V, Cr, …, or Zn, Pc - Phthalocyanine, and TCNB - Tetracyanobenzene) using first-principles calculations on the basis of spin-polarized generalized gradient approximation with the Hubbard-like Coulomb term (SGGA+U). It is found that the TM-Pc network is more stable than the TM-TCNB one. Our results demonstrate that all the TM-Pc frameworks have an insulating behavior with the exception of Mn-Pc which is half-metallic and favor antiferromagnetic order in the case of our magnetic systems except for V-Pc which is ferromagnetic. In contrast, the TM-TCNB networks are metallic at least in one spin direction and exhibit long-range ferromagnetic coupling in case for magnetic structures, which represent ideal candidates and an interesting prospect of unprecedented applications in spintronics. These results may shed light to achieve a new pathway on further experimental research in molecular spintronics.
2018-10-11 (Thursday)
room 1.02, Pasteura 5 at 17:15
Agnieszka Jamróz (IFT UW)Hexagonal carbon-boron-nitride layered alloys: ordering phenomena and electronic properties from empirical simulations
We employ Monte Carlo method and valence force field (VFF) reactive Tersoff”s potential to describe interaction between atoms in order to find equilibrium distribution of carbon, boron, and nitrogen which constitute the alloy over the honeycomb lattice. We analyze the morphology and order in the alloys and connect the distribution of atoms to the experimental growth conditions of these systems. Further, we employ semi empirical tight-binding method to calculate the electronic structure of the alloys. This sheds light on the physical mechanisms leading to the particular distribution patterns.