Exploring quantized axion electrodynamics in magnetic topological insulator multilayer heterostructures

Exploring Quantized Axion Electrodynamics in FerromagneticTopological Insulator Multilayer Heterostructures PI: Moses Chan; Co-PIs: Cui-Zu Chang, Chao-Xing Liu Department of Physics, Pennsylvania State UniversityAxion is a hypothetical elementary particle (like an electron), predicted more than forty years ago to resolve a puzzle in particle physics. It was also proposed that the as-yet- unobserved dark matter in the universe might be made up of axions. While direct detection of axions is proven to be elusive, the elegant physics of axion, known as 'axion electrodynamics', was recently found in a condensed matter physics system, specifically 'topological insulators'. The surfaces of topological insulators are electrically conducting but the interior of the material is insulating. When the surface of a topological insulator is properly decorated with magnetic ions and becomes ferromagnetic, axion electrodynamics can be induced and give rise to a variety of topological quantum phenomena. These phenomena are normally unaffected by the continuous change in the shape or size of the sample and can provide precise measurements of fundamental physical constants. The topological magnetoelectric (TME) effect and the quantum anomalous Hall (QAH) effect are two examples and will be explored in this project.Interestingly, the current flowing along the edges of a sample in the QAH state encounters no electrical resistance and loses no energy in the process and thus provides a unique scheme to reduce energy consumption in electronic and spintronic devices. Although electric current flowing through a QAH device does not cost energy, resistance still exists at the contacts between the metal electrodes and the QAH devices. The contact resistance, however, can be substantially reduced by stacking multiple QAH samples to increase the number of conducting paths for the electrical current. The stacking of the QAH samples into a single device is known as realizing a high-Chern-number QAH state.This project aims to confirm and elucidate two topological phenomena, the TME effect and the high-Chern-number QAH effect, in ferromagnetic topological insulator-based multilayer samples aka heterostructures. These samples will be synthesized using state-of-the-art molecular beam epitaxy (MBE) and characterized by in-situ probes such as reflection high energy electron diffraction (RHEED) and angle-resolved photoemission spectroscopy (ARPES). A custom-built superconducting quantum interference device (SQUID) setup will be employed for detecting and quantifying the TME effect. Systematic ex-situ electrical transport measurements will be carried out to search for the high-Chern-number QAH effect. Theoretical simulations on TME and high-Chern-number QAH samples will be carried out to guide the experimental efforts in optimizing the sample configurations and assist in the interpretation of experimental observations. In addition to making contact with particle physics, the exploring of 'axion electrodynamics' in TME and high-Chern-number QAH effects may also reveal insights on 'anyon' physics built on topological principles.