First-principles investigation of transport and magnetism

Study of the quantum mechanical nature of a material provides invaluable understanding of its underlying mechanisms governing their fascinating novel properties. Density functional based first-principles methods provide us with the necessary tools to approximate the Schrodinger's equation for many-electron periodic solids. Starting from the atomic position of their constituent atoms, using one of the most accurate methods of all-electron calculations, here in this thesis, I present my investigations of several such different materials in light of these novel phenomena. In the first chapter, I have discussed the basics of this tool and other theoretical concepts that work in the background in order to obtain reliant and consistent results. The results on each of these materials have been arranged around the following six chapters. The second chapter, in its two sections, I demonstrated these methods via two materials: a) the widely known industrial compound TiO2 where I addressed the long standing theory versus experiment disparity of the energy ordering of its two most used polymorphs. Our results, like most of the previous theoretical studies gave anatase as its ground state. In the next section, I investigated the recently synthesized layered monoclinic material: NaSbSe2. The results on its superior electronic and transport property shows its potential as a thermoelectric (TE) candidate. The investigation of TE properties in next chapter focusses on the Lorenz number where a certain widely used prescription for its approximation has been closely examined. Comparing against our first-principles based transport results on few well-known TEs as well as the ideal single parabolic band model, I found that for some materials the prescription works well within acceptable deviations. However, for TEs with complex band structure the deviations are too big which suggests precaution to its use since efficient TEs are often marked by such complex electronic structures only. The following chapters explore magnetism. Starting with the discussion of the pervoskite compound MnSeO3, we found our results to be predicting its true magnetic ground state order. The study of its energetics and electronic structure, in comparison to its non-magnetic analogue ZnSeO3, its magnetic nature was determined to be of local moment nature. Showing unconventional structural properties for a pervoskite compound, doping and spin-wave dispersion investigations will probably be useful. In the next two chapters, I focus on the novel material Sr3Ru2O7. Widely considered as a classic quantum critical material, I discuss why it is important to understand the nature of fluctuations associated with its quantum critical properties. For this purpose, it is important to know the low-energy metastable states in competition with its ground state. The first-principles investigation based survey yielded the striped E-type antiferromagnetic state that lies closest to the ground state. The magnetic-energy ordering in combination to its electronic structure properties, e.g. the density of states suggest its magnetism to be of itinerant nature. My results on the electronic transport indicates that only this striped E-type ordered state carries a distinct anisotropy among its in-plane conductivity components. This result is particularly important since the material Sr3Ru2O7 is experimentally known to display a similar transport anisotropy of the same order under specific magnetic field.Includes bibliographical references (pages 131-166)

On the ground state of TiO

The energy ordering of rutile and anatase forms of TiO2 are reported with several density functionals as obtained using all electron calculations. Careful convergence tests were performed. Results obtained both with and without structure optimization are reported. In each case anatase is found to be more stable than rutile, in contrast to the experimental expectation. Upon full relaxation, the lattice parameters of both forms are similar to experiment regardless of the functional used. GGA functionals, which are generally regarded to be more reliable than the LDA for energies, yield a more strongly stable anatase ground state. The possibility of a poor approximation to the Ti core-electrons in pseudopotential methods can be excluded as a source of this long standing puzzle. The results are discussed in relation to experiment