Band theory has proven to be one of the most successful developments in condensed matter theory. It is the basis of our current understanding of crystalline solids, describing complex electronic behavior in terms of a single quasi-particle that moves in some effective field of the crystal lattice environment and other particles. In recent years topological and geometrical considerations opened a fundamentally new branch of research in band theory. One of the major advances in this field came with the realization that insulating band structures can be classified according to the values of some topological invariants associated with the occupied single-particle states. Insulators that correspond to non-trivial values of these topological invariants realize new states of matter with properties drastically different from those attributed to an ordinary insulator. In this work we address questions that arise in the context of band theory in the presence of topologically non-trivial bands. Part of the thesis is aimed at the actual determination of the presence of non-trivial band topology. We develop a method to distinguish an ordinary insulator from a topological one in the presence of time-reversal symmetry. The method is implemented within the density functional theory framework and is illustrated with applications to real materials in {it ab initio} calculations. Another question considered in this work is that of a real-space representation of topological insulators, and in particular, the construction of Wannier functions -- localized real-space wavefunctions. Wannier functions form one of the most powerful tools in band theory, and it is very important to understand how to implement Wannier function techniques in the presence of topological bands. In some cases bands with non-trivial topology do not allow for the construction of exponentially localized Wannier functions. While previous work has shown that in the presence of time-reversal symmetry such a construction should be possible in principle, it has remained unclear how to do it in practice. We present an explicit construction of a Wannier representation for a particular model of a time-reversal invariant topological insulator. This construction is very different from the one used for ordinary band insulators. We then proceed to develop a procedure that allows for such a construction in the general case, without any reference to a particular model. Our work provides a basis for extending Wannier function techniques to topologically non-trivial band structures.
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Physics and Astronomy
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Rutgers University Electronic Theses and Dissertations
Rutgers University. Graduate School - New Brunswick
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