京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第22237号 / 理博第4551号 / 新制||理||1654(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 柳瀬 陽一, 教授 川上 則雄, 教授 松田 祐司 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

京都大学 / 新制・課程博士 / 博士(理学) / 甲第25102号 / 理博第5009号 / 新制||理||1715(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 佐藤 昌利, 教授 柳瀬 陽一, 准教授 吉田 恒也 / 学位規則第4条第1項該当 / Doctor of Agricultural Science / Kyoto University / DGAM

Dirac fermions provide a prototypical description of topological insulators and their gapless boundary states, which are predicted by the bulk-boundary correspondence. Motivated by the unusual physical properties of these states, we study them in two different Hermitian quantum systems. In non-Hermitian systems, we investigate the failure of the bulk-boundary correspondence and show that non-Hermitian topological invariants impact a system’s bulk response. First, we study electronic topological insulators in three dimensions with time-reversal symmetry. These can be characterized by a quantized magnetoelectric coefficient in the bulk, which, however, does not yield an experimentally observable response. We show that the signature response of a time-reversal-invariant topological insulator is a nonlinear magnetoelectric effect, which in the presence of a small electric field leads to the appearance of half-integer charges bound to a magnetic flux quantum. Next, we consider topological superconducting nanowires. These feature Majorana zero modes at their ends, which combine nonlocally into a single electronic state. An electron tunneling through such a state will be transmitted phase-coherently from one end of the wire to the other. We compute the transmission phase for nanowires with broken time-reversal symmetry and confirm that it is independent of the wire length. Turning to non-Hermitian systems, we consider planar optical microcavities with an anisotropic cavity material, which may feature topological degeneracies known as excep- tional points in their complex frequency spectrum. We present a quantitative method to extract an effective non-Hermitian Hamiltonian for the eigenmodes, and describe how a pair of exceptional points arises from a Dirac point due to the cavity loss. Finally, we investigate generalized topological invariants that can be defined for non- Hermitian systems, but which have no counterpart (i.e. vanish) in Hermitian systems, for example the so-called non-Hermitian winding number in one dimension. Contrary to Hermitian systems, the bulk-boundary correspondence breaks down: Comparing Green functions for periodic and open boundary conditions, we find that in general there is no correspondence between topological invariants computed for periodic boundary con- ditions, and boundary eigenstates observed for open boundary conditions. Instead, we prove that the non-Hermitian winding number in one dimension signals a topological phase transition in the bulk: It implies spatial growth of the bulk Green function, which we define as the response of a gapped system to an external perturbation on timescales where the induced excitations have not propagated to the boundary yet. Since periodic systems cannot accommodate such spatial growth, they differ from open ones.