Research

PhD student Xu Zhang (on the left) and his advisor Dr Zi Yang Meng from the Department of Physics at HKU.

A team of international researchers led by The University of Hong Kong (HKU) and The University of Science and Technology (HKUST) has made a significant discovery in the field of quantum materials, uncovering the controllable nonlinear Hall effect in twisted bilayer graphene. The findings, published as an Editors' Suggestion article in the prestigious physics journal Physical Review Letters, shed new light on the unique properties of two-dimensional quantum moiré materials and hold promise for a wide range of applications in industries such as new materials and quantum information to achieve terahertz detection with ultra-high sensitivity at room temperature.

The team, composed of PhD student Xu ZHANG and his advisor Dr Zi Yang MENG from the Department of Physics at HKU; Professor Ning WANG from the Department of Physics at HKUST and his postdoctoral researchers Meizhen HUANG and Zefei WU (currently an Associate Researcher at the University of Manchester); as well as Professor Kai SUN from The University of Michigan, conducted in-depth research using a combination of theory, computation, and experiments. They discovered that by adjusting the dispersion of the topological flat bands in twisted bilayer graphene, the Berry curvature dipole moments, which play a crucial role in the Hall effect (details can be found in the Supplementary Note), can be easily controlled and manipulated.

Using a vertically applied electric field, the researchers found the dispersion of the flat bands in twisted graphene can be easily tuned and observed a clear nonlinear voltage response in the longitudinal direction when a transverse driving current was applied. The response varied significantly with the adjustment of the applied field, strain and twist angles, exhibiting increases, decreases, and changes in direction. These experimental observations confirmed the sensitivity of the nonlinear transport behaviour to the sliding of the Berry curvature hotspots in the topological flat bands, perfectly explained by their theoretical calculations (details can be found in the Supplementary Note).

The researchers also investigated the role of the moiré potential and twist angle in the controllable nonlinear Hall effect of twisted bilayer graphene. They found that the strength of the moiré potential played a crucial role in determining the magnitude of the observed nonlinear response. By varying the twist angle between the layers of graphene, the researchers were able to manipulate the moiré potential and consequently control the nonlinear transport behaviour.

The controllable nonlinear Hall effect demonstrated in twisted bilayer graphene holds great potential for the realisation of quantum Hall materials and nonlinear Hall effects in new experimental platforms. Unlike traditional electronic devices, the nonlinear Hall effect in graphene, driven by low-frequency currents, does not have voltage threshold or transition time limitations. This opens up possibilities for applications in frequency multiplication and rectification using low-frequency currents, especially in the terahertz frequency range with significant response and ultra-high sensitivity at room temperature (details can be found in the Supplementary Note).

This discovery of the controllable nonlinear Hall effect in twisted bilayer graphene represents a significant advancement in the field of quantum materials. It paves the way for further exploration and applications in condensed matter physics, new materials, and quantum information. This collaborative research between top institutions also underscores the importance of interdisciplinary cooperation in pushing the boundaries of scientific knowledge.

This research was supported by the Area of Excellence Scheme (AoE 2D materials) and the Collaborative Research Fund (CRF many-body paradigm in quantum moiré material research) of the Hong Kong Research Grants Council, highlighting the forward-looking perspective and support of the Hong Kong government in the research of two-dimensional quantum materials, especially quantum moiré materials such as twisted graphene. The large-scale numerical calculations conducted in this study were performed on the High-Performance Computing Platform HPC2021 at the Information Technology Services, HKU, and the ‘Blackbody’ supercomputer at the Department of Physics at HKU.