**Contrary to expectations, experiments show that edges are not needed to achieve an unusual quantum effect.**

RIKEN physicists have created an exotic quantum state for the first time in a device with a disk-like geometry, proving that edges are unnecessary. This demonstration paves the way for the realization of other new electronic behaviors.

Physics has long since moved on from the three classic states of matter: solid, liquid and gaseous. A better theoretical understanding of quantum effects in crystals and the development of advanced experimental tools to probe and measure them has revealed a whole range of exotic states of matter.

A striking example of this is the topological insulator: a kind of crystalline solid that exhibits very different properties on its surfaces than the rest of the material. The best-known manifestation of this is that topological insulators conduct electricity on their surfaces but insulate inside.

Another manifestation is the so-called quantum anomalous Hall effect.

Known for over a century, the conventional Hall effect occurs when an electric current flowing through a conductor is deflected out of a straight line by a magnetic field applied perpendicular to the current. This deflection produces a voltage across the conductor (and a corresponding electrical resistance).

In some magnetic materials, this phenomenon can occur even when no magnetic field is applied, which is called the anomalous Hall effect.

The anomalous Hall resistance can become very large in topological insulators, explains 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 direction of the current becomes zero. This is the quantum anomalous Hall effect, and it was first observed in the laboratory nearly a decade ago.

Now, Kawamura and his colleagues have demonstrated an effect known as Laughlin charge pumping in a quantum anomalous Hall insulator.

The team fabricated a donut-shaped disk made up of layers of different magnetic topological insulators (Fig. 1). They then measured how the electric current through the device responded to an alternating magnetic field generated by metal electrodes on the donut’s inner and outer curves.

The researchers observed that this field led to the accumulation of electric charge at the ends of the cylinder. This is Laughlin charge pumping.

Previous demonstrations of quantum anomalous Hall insulators used rectangular devices that included edges connecting the electrodes. And the electronic states in these edges were thought to be crucial to supporting the quantum anomalous Hall insulator.

But the discovery of the teams overturns this hypothesis. Our demonstration of Laughlin charge pumping in a quantum anomalous Hall insulator uses a disk-shaped device with no edge channels connecting the two electrodes, says Kawamura. Our result raises the possibility that other exciting electronic phenomena could be realized 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 Tsukazaki, Naoto Nagaosa, Masashi Kawasaki, and Yoshinori Tokura, January 19, 2023,

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