Zero-field plasmon splitter: Controlled interaction between quantum anomalous Hall edge states

In the world of electronics, we are used to controlling the flow of electricity with switches, wires, and magnetic fields. But there is a special class of materials called magnetic topological insulators that behave differently. Inside these materials, electricity can flow along the very edges without any resistance, creating a one-way street for electrons that is determined by the material's own internal magnetism rather than an external magnet. This phenomenon, known as the quantum anomalous Hall effect, is fascinating because it promises to make faster, more efficient electronic devices. However, a major hurdle has been that once these electron streams are flowing, scientists have found it very difficult to steer them or make them interact with one another without using complex magnetic switches or forcing the electrons to jump across gaps.

A team of researchers at the University of Cologne and RWTH Aachen University has now found a way to guide and mix these electron streams using only the invisible force of electric fields. By placing a tiny, floating piece of metal between two parallel paths where the electrons travel in opposite directions, they created a device that acts like a splitter for waves of charge. Instead of forcing electrons to collide or jump, they used the metal island to let the two streams "feel" each other across a small gap. This interaction causes the waves traveling along the edges to mix and change direction in a predictable way, depending on the speed of the signal. The result is a new tool for controlling these exotic electron streams, opening the door to building complex circuits that operate without the need for external magnetic fields.

The researchers built their experiment using a thin film of a material called vanadium-doped bismuth antimony telluride. They carved a narrow trench into this film, effectively splitting it into two separate bars. On top of this trench, they placed a small, rectangular piece of metal that was not connected to any power source, making it "floating." This metal island sat just above the gap, separated by a very thin insulating layer. When they sent microwave signals into one end of the device, they created ripples of charge, known as plasmons, that traveled along the edges of the two bars. Because the bars were so close to the metal island, the electric field from the ripples on one side could reach across the gap and influence the ripples on the other side, even though the electrons themselves never crossed the trench.

When the team measured the signals coming out of the other end, they discovered that the floating metal island acted as a powerful mixer. The microwave signal injected into one edge did not simply continue straight ahead; instead, it split and reappeared at different output ports. What made this behavior remarkable was that the amount of signal going to each output changed as they adjusted the frequency of the microwave. At some frequencies, most of the signal went to one side, while at others, it shifted to the opposite side. This shifting pattern created a rhythmic oscillation, proving that the two edge streams were interacting strongly through the metal island. Without the island, this mixing effect was barely noticeable, confirming that the metal was the key to engineering this interaction.

To understand exactly what was happening, the scientists developed a model based on how electric fields spread through space. They found that the presence of the floating metal island drastically slowed down the speed of the charge waves. In the open sections of the material, these waves travel at speeds between 300 and 500 kilometers per second. However, once they entered the region under the metal island, their speed dropped to just 12 kilometers per second. This massive slowdown occurred because the metal island acted like a shield, screening the electric fields and making it harder for the waves to move. This slowing effect was crucial because it compressed the wavelength of the waves, allowing the researchers to observe the mixing and splitting effects within the range of frequencies their equipment could measure.

The study also revealed that the strength of this interaction depended on the length of the metal island. By testing devices with islands of different sizes, the team showed that the time it took for the waves to travel through the interaction zone increased linearly with the length of the island. This confirmed that the mixing was a cumulative process that happened along the entire length of the metal piece. The researchers also noted that at very high frequencies, the clear rhythmic splitting began to fade, and the signal split evenly between the two outputs. This happened because the waves lost some of their energy as they traveled, a process called dissipation, which smoothed out the sharp differences between the frequencies.

This work demonstrates a new method for controlling collective electron excitations in magnetic topological insulators without needing to move the electrons themselves or apply external magnetic fields. By simply designing the shape and position of a floating metal component, scientists can now tune how these edge states interact. The ability to slow down these waves and mix them in a controlled way provides a fundamental building block for future technologies. It suggests a path toward creating chiral plasmonic circuits and interferometers that operate at zero magnetic field, which could be essential for developing new types of quantum information devices. The researchers have shown that with the right engineering of the electrostatic environment, the behavior of these exotic quantum states can be precisely manipulated, turning a difficult-to-control phenomenon into a versatile tool for future electronics.