Scientists at RIKEN have successfully demonstrated an extraordinary quantum state called the quantum anomalous Hall effect in a disk-shaped device, providing evidence that edge states are not essential for this phenomenon. Through their study, the team showcased Laughlin charge pumping in a quantum anomalous Hall insulator using a layered, donut-shaped disk composed of different magnetic topological insulators. This breakthrough expands the potential for discovering further electronic phenomena in similar materials.
Contrary to expectations, experimental results indicate that edges are not necessary to realize this peculiar quantum effect.
For the first time, RIKEN physicists have generated an exotic quantum state within a device featuring a disk-like structure, thus proving that edges are not a prerequisite. This demonstration paves the way for the realization of other exceptional electronic behaviors.
The field of physics has long progressed beyond the conventional three states of matter—solid, liquid, and gas. With an improved theoretical comprehension of quantum effects in crystals and the advancement of sophisticated experimental tools to explore and measure them, a plethora of extraordinary states of matter have been unveiled.
One prominent example is the topological insulator, a type of crystalline solid that exhibits vastly distinct properties on its surfaces compared to its bulk material. The most well-known manifestation of this is the ability of topological insulators to conduct electricity on their surfaces while remaining insulating within their interiors.
Another manifestation is the renowned “quantum anomalous Hall effect.”
The conventional Hall effect has been known for over a century and occurs when an electric current flowing through a conductor is deflected from its straight path by a magnetic field perpendicular to the current. This deflection induces a voltage across the conductor, resulting in electrical resistance.
Figure 1: The structure of the device used in experiments illustrating Laughlin charge pumping without edges. Credit: © 2023 RIKEN Center for Emergent Matter Science
In certain magnetic materials, this phenomenon can arise even without the presence of an applied magnetic field, giving rise to the anomalous Hall effect.
“The anomalous Hall resistance can become extremely large in topological insulators,” explains Minoru Kawamura from 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 phenomenon is known as the quantum anomalous Hall effect and was first observed in laboratories nearly a decade ago.
Now, Kawamura and his colleagues have demonstrated an effect called Laughlin charge pumping in a quantum anomalous Hall insulator.
The research team fabricated a donut-shaped disk composed of layers of distinct magnetic topological insulators (Fig. 1). They then measured the response of the electric current passing through the device to an alternating magnetic field generated by metal electrodes positioned on the inner and outer curves of the donut.
The researchers observed that this field led to the accumulation of electric charge at the ends of the cylinder, known as Laughlin charge pumping.
Previous demonstrations of quantum anomalous Hall insulators utilized rectangular devices that included edges connecting the electrodes. It was previously believed that electronic states in these edges were essential for supporting the quantum anomalous Hall insulator.
However, the team’s findings have overturned this assumption. “Our demonstration of Laughlin charge pumping in a quantum anomalous Hall insulator employs a disk-shaped device without edge channels connecting the two electrodes,” states Kawamura. “This result suggests the possibility of realizing other fascinating electronic phenomena in quantum anomalous Hall materials.”
Reference: “Laughlin charge pumping in a quantum anomalous Hall insulator” by Minoru Kawamura, Masataka Mogi, Ryutaro Yoshimi, Takahiro Morimoto, Kei S. Takahashi, Atsushi Tsukaz
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Frequently Asked Questions (FAQs) about quantum anomalous Hall effect
What is the quantum anomalous Hall effect?
The quantum anomalous Hall effect is a phenomenon observed in certain materials, known as topological insulators, where they exhibit unique electrical properties on their surfaces. It refers to the occurrence of a large Hall resistance and zero longitudinal resistance in the presence of a magnetic field, even in the absence of an applied magnetic field.
How did RIKEN prove that edges are unnecessary for the quantum anomalous Hall effect?
RIKEN physicists conducted experiments using a disk-like device composed of different magnetic topological insulators. They demonstrated Laughlin charge pumping, a characteristic effect of the quantum anomalous Hall insulator, in this edge-free device. This finding challenges the previous belief that edge states were crucial for supporting the quantum anomalous Hall effect.
What are the implications of RIKEN’s discovery?
RIKEN’s discovery expands the possibilities for exploring new electronic phenomena in quantum anomalous Hall materials. By demonstrating the quantum anomalous Hall effect without relying on edges, it suggests that other exciting electronic behaviors can be realized. This finding opens up avenues for further research and potential applications in the field of quantum physics and materials science.
How does the quantum anomalous Hall effect differ from the conventional Hall effect?
The conventional Hall effect occurs when an electric current flowing through a conductor is deflected by a perpendicular magnetic field, resulting in a voltage across the conductor. In the quantum anomalous Hall effect, the anomalous Hall resistance can become very large in topological insulators, while the resistance along the current direction becomes zero. This unique behavior arises due to the topological properties of the materials involved and does not require the application of an external magnetic field.
More about quantum anomalous Hall effect
- Nature Physics: “Laughlin charge pumping in a quantum anomalous Hall insulator”
- RIKEN Center for Emergent Matter Science
- Topological insulators
- Quantum Hall effect
- RIKEN
