Date of Degree
PhD (Doctor of Philosophy)
Michael E. Flatté
The emerging field of spintronics relies on the manipulation of electron spin in order to use it in spin-based electronics. Such a paradigm change has to tackle several challenges including finding materials with sufficiently long spin lifetimes and materials which are efficient in generating pure spin currents. This thesis predicts that two types of material families could be a solution to the aforementioned challenges: complex oxides and bismuth based materials.
We derived a general approach for constructing an effective spin-orbit Hamiltonian which is applicable to all nonmagnetic materials. This formalism is useful for calculating spin-dependent properties near an arbitrary point in momentum space. We also verified this formalism through comparisons with other approaches for III-V semiconductors, and its general applicability is illustrated by deriving the spin-orbit interaction and predicting spin lifetimes for strained SrTiO3 and a two-dimensional electron gas in SrTiO3 (such as at the LaAIO3/SrTiO3 interface). Our results suggest robust spin coherence and spin transport properties in SrTiO3 related materials even at room temperature.
In the second part of the study we calculated intrinsic spin Hall conductivities for bismuth-antimony Bi1-xSbx semimetals with strong spin-orbit couplings, from the Kubo formula and using Berry curvatures evaluated throughout the Brillouin zone from a tight-binding Hamiltonian. Nearly crossing bands with strong spin-orbit interaction generate giant spin Hall conductivities in these materials, ranging from 474 ((ћ/e)Ω-1cm-1) for bismuth to 96((ћ/e)Ω-1cm-1) for antimony; the value for bismuth is more than twice that of platinum. The large spin Hall conductivities persist for alloy compositions corresponding to a three-dimensional topological insulator state, such as Bi0.83Sb0.17. The spin Hall conductivity could be changed by a factor of 5 for doped Bi, or for Bi0.83Sb0.17, by changing the chemical potential by 0.5 eV, suggesting the potential for doping or voltage tuned spin Hall current. We have also calculated intrinsic spin Hall conductivities of Bi2Se3 and Bi2Te3 topological insulators from an effective tight-binding Hamiltonian including two nearest-neighbor interactions. We showed that both materials exhibit giant spin Hall conductivities calculated from the Kubo formula in linear response theory and the clean static limit. We conclude that bismuth-antimony alloys and bismuth chalcogenides are primary candidates for efficiently generating spin currents through the spin Hall effect.
Spintronics is an emerging field of condensed matter physics which aims to change the functioning principles of electronic devices. Spintronics offers a variety of opportunities for electron and nuclear spin, which are intrinsic properties of subatomic particles, to carry information and perform computations. This new paradigm for computer chips and memories promises more efficient devices with much higher speed, less power consumption, nonvolatility and even quantum computation. Recent progress in physics and materials engineering holds the promise of building such spintronic devices shortly. However, there are still several challenges that should be addressed. Two types of materials that are studied in this thesis aim to help solve two fundamental problems of spintronics: bismuth based alloys and topological insulators that can efficiently generate a spin current and complex perovskite oxides which can retain the polarization of spins for long times.
Complex oxides with a variety of properties and design flexibility are of particular interest for spintronic applications. In this thesis, we derive an effective spin-orbit Hamiltonian and calculate spin lifetimes of LaAlO3/SrTiO3 interfaces and SrTiO3 crystals. Our calculations show that these oxides have exceptionally long spin lifetimes even at room temperature. The second part of this work focuses on bismuth-based materials, such as bismuth-antimony alloys and bismuth chalcogenides, and proves that they are excellent materials for the generation of currents with full spin polarization. In conclusion, both SrTiO3 heterostructures and bismuth-related alloys and chalcogenides are promising candidates for materials to be used in spintronic transistors.
publicabstract, bismuth-antimony, complex oxides, spin Hall conductivity, spin lifetime, spintronics, topological insulator
xii, 134 pages
Includes bibliographical references (pages 117-134).
Copyright 2015 Cuneyt Sahin