Unusual Quantum Effect Can Be Achieved Without Edges, Experiments Reveal.

RIKEN physicists have created an exotic quantum state in a device with a disc-like geometry for the first time, showing that edges are not required. This demonstration opens the way to realizing yet another new electronic behavior. Their findings have been published in nature physics.

Physics has long moved away from the three classical states of matter: solid, liquid and gas. A better theoretical understanding of quantum effects in crystals and the development of advanced experimental tools to examine and measure them have revealed a whole host of exotic states of matter.

One notable example of this is a topological insulator: a type 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 are insulators on the inside.

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

The classic Hall effect known more than a century ago arises when an electric current flowing through a conductor is deflected from a straight line by a magnetic field applied at right angles to the current. This deviation results in a voltage across the conductor (and corresponding electrical resistance).

In some magnetic materials, this phenomenon can arise 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 Emerging Matter Science. “At lower temperatures, the anomalous Hall resistance increases and reaches a fundamental value, while the resistance along the current direction becomes zero.” This is an anomalous quantum Hall effect, and it was first observed in the laboratory nearly a decade ago.

Now, Kawamura and colleagues have demonstrated an effect known as Laughlin charge pumping into a quantum anomaly Hall insulator.

The team made a circular disk made of layers of different magnetic topological insulators. They then measured how the electric current through the device responded to an alternating magnetic field generated by the metal electrodes on the inner and outer curves of the donut.

The researchers noticed that this field led to a buildup of electrical charges at the ends of the cylinder. This is Laughlin’s infusion charge.

Previous demonstrations of quantum anomaly Hall insulators have used rectangular devices containing edges connecting electrodes. The electronic states at these edges were thought to be necessary to support the quantum anomalous Hall insulator.

But the team’s discovery overturns that assumption. “Our demonstration of Laughlin charge infusion into a quantum anomalous Hall insulator uses a disk-shaped device without edge channels connecting the two electrodes,” says Kawamura. “Our result raises the possibility of realizing other exciting electronic phenomena in quantum anomalous Hall materials.”