Density-Functional Theory+Dynamical Mean-Field Theory Study of the Magnetic Properties of Transition-Metal Nanostructures

In this thesis, Density Functional Theory (DFT) and Dynamical Mean-Field Theory (DMFT) approaches are applied to study the magnetic properties of transition metal nanosystems of different sizes and compositions. In particular, in order to take into account dynamical electron correlation effects (time-resolved local charge interactions), we have adopted the DFT+DMFT formalism and made it suitable for application to nanostructures. Preliminary application of this DFT+DMFT approach, using available codes, to study the magnetic properties of small (2 to 5-atom) Fe and FePt clusters provide meaningful results: dynamical effects lead to a reduction of the cluster magnetic moment as compared to that obtained from DFT or DFT+U (U being the Coulomb repulsion parameter). We have subsequently developed our own nanoDFT+DMFT code and applied it to examine the magnetization of iron particles containing10-147 atoms. Our results for the cluster magnetic moments are in a good agreement with experimental data. In particular, we are able to reproduce the oscillations in magnetic moment with size as observed in the experiments. Also, DFT+DMFT does not lead to an overestimation of magnetization for the clusters in the size range of 10-27 atoms found with DFT and DFT+U. On application of the nanoDFT+DMFT approach to systems with mixed geometry – Fe2O3 film, which are periodic (infinitely extended), in two directions, and finite in the third. Similar to DFT+U, we find that the surface atom magnetic moments are smaller compared to the bulk. However, the absolute values of the surface atoms magnetic moments are smaller in DFT+DMFT. In parallel, we have carried out a systematic study of magnetic anisotropy in bimetallic L10 FePt nanoparticles (20-484 atoms) by using two DFT-based approaches: direct and the torque method. We find that the magnetocrystalline anisotropy (MCA) of FePt clusters is larger than that of the pure Fe and Pt ones. We explain this effect by a large hybridization of 3d Fe- and 5d Pt-atom orbitals, which lead to enhancement of the magnetic moment of the Pt atom, and hence to a larger magnetic anisotropy because of large spin-orbit coupling of Pt atoms. In addition, we find that particles whose (large) central layer consists of Pt atoms, rather than Fe, have larger MCA due to stronger hybridization effects. Such \u27protected\u27 MCA, which does not require protective cladding, can be used in modern magnetic technologies

Modelling local scour around bridge piers using TELEMAC

Scour at bridge crossings is a major cause of bridge failure. There are several different types of scour such as general scour, constriction scour and local scour. One of the most serious types is local scour which occurs as a result of vortex formation around bridge piers and abutments (Hoffmans & Verheij, 1997; Raudkivi, 1998; Melville & Coleman, 2000; Richardson & Davis, 2001; Armitage & McGahey, 2003). Local scour is also one of the most difficult to predict accurately. If not adequately designed for, local scour of a riverbed at a bridge pier may become deep enough to undermine the pier foundation and eventually cause the bridge to collapse. Complete protection against scour is expensive and therefore not a favourable design option. It is generally cheaper to ensure that the foundation lies below the maximum expected scour depth. Traditionally, the maximum scour depth is predicted from empirical equations derived from simple laboratory tests without much regard for local conditions. Alternatively, smaIlscale hydraulic models, which are laborious and time intensive, are widely used. In view of the above, increasing attention is being paid to the use of Computational Fluid Dynamics (CFD) based modelling for the prediction of local scour and its opposite, local deposition. The ever-improving capabilities of computers and the increasing availability of powerful and flexible CFD codes have further assisted in this process. This study is a contribution in this direction

Magnetic anisotropy of FePt nanoparticles

We carry out a systematic theoretical investigation of Magneto Crystalline Anisotropy (MCA) of L10 FePt clusters with alternating Fe and Pt planes along the (001) direction. We calculate the structural relaxation and magnetic moment of each cluster by using ab initio spin-polarized density functional theory (DFT), and the MCA with both spin-polarized DFT (including spin-orbit coupling self-consistently) and the torque method. We find that the MCA of any composite structure of a given size is enhanced with respect to that of the same-sized pure Pt or pure Fe cluster as well as to that of any pair of Fe and Pt atoms in bulk L10 FePt. This enhancement results from the hybridization we observe between the 3d orbital of the Fe atoms and the 5d orbital of their Pt neighbors. This hybridization, however, affects the electronic properties of the component atoms in significantly different ways. While it somewhat increases the spin moment of the Fe atoms, it has little effect on their orbital moment; at the same time, it greatly increases both the spin and orbital moment of the Pt atoms. Given the fact that the spin-orbit coupling (SOC) constant of Pt is about 7 times greater than that of Fe, this Fe-induced jump in the orbital moment of the Pt atoms produces the increase in MCA of the composite structures over that of their pure counterparts. That any composite structure exhibits higher MCA than bulk L10 FePt results from the lower coordination of Pt atoms in the cluster, whether Fe or Pt predominates within it. We also find that bipyramidal clusters whose central layer is Pt have higher MCA than their same-sized counterparts whose central layer is Fe. This results from the fact that Pt atoms in such configurations are coordinated with more Fe atoms than in the latter. By thus participating in more instances of hybridization, they contribute higher orbital moments to the overall MCA of the unit

The Role of Energy in an Energy Constrained Economy

The thesis consists of three individual papers on energy and economic development focusing on the energy constrained economies. The first paper examines how energy subsidies influence the relationship between economic growth and electricity consumption. The study constructs a panel data set consisting of 172 countries to see how the level of economic development, energy availability and energy subsidies affect electricity consumption. To address dynamic panel bias, the study uses the panel estimation technique, generalized method of moments. The findings suggest that in energy subsidized economics, electricity consumption augments economic growth in general but is evident in middle income and energy constrained countries. This relationship, however, is reversed for the high-income countries where economic output and price of electricity influence power consumption. These results suggest that energy subsidies as a policy for transfer payments, especially in middle income and energy deficit countries, has no upward pressure on electricity consumption; rather, it acts as an impetus for economic development thereby endorsing the growth hypothesis in energy economics. Conversely, energy subsidies in high-income countries determine how energy-income or inter-fuel substitution affects environmental outcomes. The second paper investigates how an energy constrained economy responds to energy shocks by taking Bangladesh as an example. To explore the long run relationship, Autoregressive Distributed Lag bounds testing approach is used followed by an error correction model estimation. The results appear both in the short and long run, whereby output growth precipitates energy consumption, supporting the energy conservation hypothesis. The results, however, do not confirm that energy consumption supports output growth or the growth hypothesis. We also find that, in the time of lower energy consumption, the relationship between energy and output is very weak, but in higher energy consumption periods, output growth increases energy uptake. The argument of the paper is that in an energy constrained economy, energy conservation policies, such as increasing fuel price or reducing oil supply, may potentially destabilize the socio-political environment. The third paper focuses on oil supply shocks to transportation prices of an emerging economy. The motivation for the analysis is stemmed from the fact that in Bangladesh, despite the combined price of all the other goods and services fluctuating across both the directions, the transport price trends positively only. The study hypothesizes that due to oil supply shocks, the transport price shows this particulate behaviour. Using time series models (ARMA, GARCH and EGARCH), the study finds the evidence of negative impact of oil shocks on transportation. This analysis also concludes that transport price volatility has no impact on its average price, and the volatility itself is not explosive but rather bounded. In tackling the increasing transportation price, importing more oil may be one option but from the environmental perspective, fuel switching or energy efficiency gain would be more sustainable policy options.Thesis (Ph.D.) -- University of Adelaide, School of Economics and Public Policy, 202

Climate Change Perception and Adaptation Strategies of Southwest Coastal Bangladesh

Bangladesh is cited as one of the most vulnerable countries to the impacts of climate change. Climate change poses serious impacts on agricultural sustainability, food security, natural resources and rural livelihood pattern. The study investigates farmers’ perception to climate change and their agricultural adaptation in the coastal area of Bangladesh. Two hundred household survey were conducted in Satkhira and Barguna district. Study revealed that farmers were well aware of climate change and they observed an increased temperature, rainfall, number of cyclones, flood intensity etc. over the years in the study area. Farmers’ thought that weed and pest infestation, disease outbreak and pesticide use have been increased due to the change in temperature and rainfall. Water logging, cyclone, river erosion and salinity were identified as the major environmental problems in the study area. However, the study identified 28 adaptation strategies that have been adopted by the farmers to reduce the impact of climate change. Crop diversification, introduction of new crops that can resist climatic stress, crop rotation, mix cropping, change in planting and harvesting date, shortening growing season, homestead gardening, application of organic fertilizers and pesticide, increased use of irrigation, different soil conservation techniques and income diversification were found as the most common adaptation measures. The results of the regression analysis showed that socioeconomic characteristics of the farmers (age, education, farming status and experience, farm income etc.) and their perception to climate change influenced the farmers in choosing different adaptation strategies. The adaptation measures were economically profitable as well as agriculturally sustainable though lack of experience and knowledge, agricultural extension services, availability of inputs and lack of credit facilities were identified as the major challenges in the area

Acacia Catechu Trees in Rice Fields: A Traditional Agroforestry System of Northern Bangladesh

Growing Acacia catechu trees on rice fields is one traditional crop-land Agroforestry system of Rajshahi region in Bangladesh. Farming system was explored with detailed information on farm operations and cropping calendar including system outputs. System dynamics was also evaluated. PRA exercises were conducted for a biophysical assessment. The information was collected on informant wise and cross-checked. Best growth of trees observed under rain-fed conditions. Higher density of trees found in un-irrigated fields. Trees were better managed in small plots. In general, density of khoir trees was found higher in small holdings (less than 2 ha) with secured land tenure. Trees on farms were of uneven age indicating the khoir + rice system biologically sustainable. The ease of establishment of Khoir seedlings, the low cost of its maintenance, and less vulnerability to any serious pest or disease, easy marketability of products rated high in the farmer’s preference for khoir. They appreciate the versatility of the wood for a variety of farm uses though its prime economic use to them is for production of lali for katha. They also get pitch khoir as by product of katha (red dyestuff for textiles and paper) production. The multiple products and services offered and the ease of managing the trees on crop fields without causing any immediate or long-term reduction in crop yield seemed to be the most important factors that encourage the farmers to continue this traditional practice. If farmers could be motivated to follow appropriately the silvicultural practices, production could further be increased

Dynamical mean-field theory for molecules and nanostructures

Dynamical mean-field theory (DMFT) has established itself as a reliable and well-controlled approximation to study correlation effects in bulk solids and also two-dimensional systems. In combination with standard density-functional theory (DFT), it has been successfully applied to study materials in which localized electronic states play an important role. It was recently shown that this approach can also be successfully applied to study correlation effects in nanostructures. Here, we provide some details on our recently proposed DFT+DMFT approach to study the magnetic properties of nanosystems [V. Turkowski, A. Kabir, N. Nayyar, and T. S. Rahman, J. Phys.: Condens. Matter 22, 462202 (2010)] and apply it to examine the magnetic properties of small FePt clusters. We demonstrate that DMFT produces meaningful results even for such small systems. For benchmarking and better comparison with results obtained using DFT+U, we also include the case of small Fe clusters. As in the case of bulk systems, the latter approach tends to overestimate correlation effects in nanostructures. Finally, we discuss possible ways to further improve the nano-DFT+DMFT approximation and to extend its application to molecules and nanoparticles on substrates and to nonequilibrium phenomena