Research
Molecular beam epitaxy growth of quantum materials
Molecular beam epitaxy is one of the most powerful methods to synthesize high-quality thin films of crystalline solids, where the source elements are vaporized from heated effusion cells or electron-beam evaporators and then crystallized on substrates layer-by-layer under a high vacuum condition (~10-7 Pa). The fine controllabilities of the growth rate and chemical composition allow us to create heterostructures which cannot be obtained in nature. So far, we have explored various chalcogenide layered compounds, such as 3D topological insulators (e.g., Sb2Te3, Bi2Te3 doped with transition metal elements), 2D materials (e.g., Cr2Ge2Te6, Fe3GeTe2), and their novel heterostructures.
Quantum anomalous Hall effect and axion electrodynamics in 3D topological insulators
High-temperature quantum anomalous Hall effect by magnetic modulation doping
The quantum anomalous Hall effect is a zero-magnetic-field version of the quantum Hall effect, where a non-dissipative chiral edge state propagates along the sample edge. This effect has been firstly discovered in a magnetically doped topological insulator (Bi,Sb)2Te3 (BST). While the observable temperature had been limited to the dilution fridge temperature (~50 mK) in the initial experimental studies, we developed a magnetic modulation doping method that the magnetic ions are intensively doped near the surface, which dramatically enhances the observable temperature up to 2 K.
Creation of axion insulator state
The axion insulator, which may exhibit an exotic magnetoelectric effect, is a new magnetic quantum phase predicted for the magnetic topological insulator. This state is expected to show up when the magnetization points inwards or outwards from the surface. In this study, we designed and synthesized magnetic sandwich heterostructures (i.e., Cr-doped BST/BST/Cr-doped BST and Cr-doped BST/BST/V-doped BST). We verified the axion insulator state via magneto-transport experiments, exhibiting giant magnetoresistance and a new zero-Hall conductivity plateau associated with on/off switching of the chiral edge channel.
Spintronic functionalities of proximity-coupled topological surface states
Large anomalous Hall effect in a topological insulator/ferromagnetic insulator structure
Besides magnetic doping, stacking a ferromagnet on topological insulators is expected to be an ideal approach to make the topological surface state ferromagnetic uniformly via magnetic proximity effect. The induced surface ferromagnetism may lead to the realization of robust topological magnetic states. In this study, we fabricated heterostructures consisting of BST and a ferromagnetic insulator Cr2Ge2Te6, which form a well-ordered, sharp interface with BST through van der Waals forces. We observed a large anomalous Hall state associated with the emergence of the magnetically induced gap in the surface state, of which the origin is the magnetic proximity effect as confirmed by combining cross-sectional atomic and magnetic characterizations including scanning transmission electron microscopy, and x-ray/polarized neutron reflectometry.
Current-induced switching of ferromagnetic surface states
Generation and application of non-dissipative topological currents are one of the central interests for the realization of energy-efficient electronics. The proximity-induced ferromagnetic surface state is an ideal arena to electrically control the anomalous Hall current via current-induced magnetization switching. In this study, we demonstrate efficient current-induced switching of the surface ferromagnetism in a Cr2Ge2Te6/BST hetero-bilayer. The results may provide novel controllability of topological quantum states in a highly energy-efficient manner.
