Unconventional Quantum Effect Achievable Without Edges, Demonstrates Experiments

April 18, 2023

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RIKEN physicists have successfully created an exotic quantum state in a disk-shaped device, for the first time, without requiring edges. This breakthrough discovery facilitates the realization of new, innovative electronic behaviors. Their findings have been published in Nature Physics.

Over time, physics has progressed beyond the classic three states of matter, namely, solid, liquid, and gas. A better theoretically understood quantum effects in crystals coupled with the development of advanced experimental tools to measure and probe them have led to the discovery of a myriad of exotic states of matter.

A significant embodiment of such states is the topological insulator, which refers to a type of crystalline solid exhibiting starkly different properties on its surface compared to the rest of the material. For example, topological insulators conduct electricity on their surfaces but are insulating in their interiors.

Another example being the quantum anomalous Hall effect that has been known for over a century. The conventional Hall effect arises when an electric current in a conductor gets deflected from its straight line by a magnetic field applied perpendicular to it, resulting in a voltage across the conductor and electrical resistance.

However, some magnetic materials demonstrate the anomalous Hall effect, which causes this phenomenon even without applying a magnetic field. 'The anomalous Hall resistance can become very large in topological insulators,' says Minoru Kawamura of the RIKEN Center for Emergent Matter Science. 'At low temperatures, the anomalous Hall resistance increases and reaches a fundamental value, while the resistance along the current direction becomes zero.' This is the quantum anomalous Hall effect, first recognized in the laboratory nearly a decade ago.

Kawamura and his team have now demonstrated the Laughlin charge pumping effect in a quantum anomalous Hall insulator. They fashioned a donut-shaped disk using multiple layers of magnetic topological insulators and measured the reaction of the electric current circulating through it to an alternate magnetic field generated by metal electrodes. The electrodes were placed on the donut's inner and outer curves.

The researchers discovered that this field caused the accumulation of electric charge at the cylinder's ends, resulting in Laughlin charge pumping. Previous displays of quantum anomalous Hall insulators used rectangular devices that had edges linking the electrodes, and it was assumed that these edges' electronic states were essential for supporting the quantum anomalous Hall insulator.

However, the team's discovery negates this assumption. 'Our demonstration of Laughlin charge pumping in a quantum anomalous Hall insulator uses a disk-shaped device without edge channels connecting the two electrodes,' Kawamura states. 'Our result raises the possibility that other exciting electronic phenomena can be realized in quantum anomalous Hall materials.'

Journal information: Nature Physics

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