Floquet Topological Magnons

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Abstract

We show that in the presence of a circularly polarized laser field, the underlying charge-neutral magnons of a quantum magnetic system acquire a time-dependent Aharonov-Casher phase. We show that the floquet theory generates a tunable synthetic Dzyaloshinskii-Moriya (DM) interaction in this system. Thus, Dirac magnons and magnon nodal rings in two-dimensional (2D) and three-dimensional (3D) quantum magnets can be tuned to magnon Chern insulators and Weyl magnons respectively under circularly polarized laser field. The floquet formalism also results in a tunable intrinsic DM interaction for insulating quantum magnets without an inversion center. We observe a finite thermal Hall conductivity, which can be tuned on or off by the laser field.

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Topological Insulators

, (2011)

Topological insulators are electronic materials that have a bulk band gap like an ordinary insulator, but have protected conducting states on their edge or surface. The 2D topological insulator is a quantum spin Hall insulator, which is a close cousin of the integer quantum Hall state. A 3D topological insulator supports novel spin polarized 2D Dirac fermions on its surface. In this Colloquium article we will review the theoretical foundation for these electronic states and describe recent experiments in which their signatures have been observed. We will describe transport experiments on HgCdTe quantum wells that demonstrate the existence of the edge states predicted for the quantum spin Hall insulator. We will then discuss experiments on Bi_{1-x}Sb_x, Bi_2 Se_3, Bi_2 Te_3 and Sb_2 Te_3 that establish these materials as 3D topological insulators and directly probe the topology of their surface states. We will then describe exotic states that can occur at the surface of a 3D topological insulator due to an induced energy gap. A magnetic gap leads to a novel quantum Hall state that gives rise to a topological magnetoelectric effect. A superconducting energy gap leads to a state that supports Majorana fermions, and may provide a new venue for realizing proposals for topological quantum computation. We will close by discussing prospects for observing these exotic states, a well as other potential device applications of topological insulators.