Implementation of Axion Electrodynamics in Topological Films and Devices

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We propose to exploit the axion magnetoelectric response of topological materials – including topological insulators (TIs) and Weyl semimetals (WSMs) – to generate new routes to couple electric and magnetic degrees of freedom in materials and devices. The specific research objectives include: (1) theoretical analysis of novel topological materials and structures for strong axion coupling and (2) modeling of device applications for unique functionality. In the first task, the axionic properties of the TI based structures and magnetic WSMs will be systematically examined through a combination of model Hamiltonian treatments and first-principles calculations. The focus will be on multi-layered structures of TI and magnetic thin films that can realize axion insulators with a strong magnetoelectric response such as a large topological surface bandgap or charge polarization for room temperature operation. The investigation will also be extended to magnetic WSMs including those with antiferromagnetic ordering. The physical systems and properties identified in the analysis will then be exploited for potential device applications with unique functionality beyond the non-axionic counterparts. The concepts under consideration include transistor-like switches based on topological phase transition for steep turn-on/turn-off characteristics, tunable THz waveguide/modulator, and WSM based spintronics such as low-power nonvolatile domain-wall memory. The device exploration will start from the study of enabling physical mechanisms, followed by numerical studies for the feasibility demonstration and performance estimate. A multiscale approach will be applied as appropriate by using a suite of analytical and numerical treatments including micromagnetic simulations, Green’s functions, and finite difference time domain method.