Axionic tunneling from a topological Kondo insulator

For decades, physicists have been fascinated by a strange idea: that the empty space between stars, known as the vacuum, might not be truly empty but filled with invisible fields that dictate how particles behave. In the world of solid materials, scientists have discovered that certain crystals can mimic these exotic properties of the vacuum, creating a new frontier where the rules of quantum mechanics and relativity seem to merge. Among these materials, a class known as topological insulators stands out. These are substances that act as electrical insulators in their interior but conduct electricity effortlessly along their surfaces. A particularly intriguing member of this family is samarium hexaboride, a material that has long puzzled researchers because its surface conducts electricity in a way that suggests a deep, hidden connection between electric and magnetic forces.

The central mystery involves a theoretical concept called the axion. In the realm of high-energy physics, axions are hypothetical particles that could explain why the universe behaves the way it does. In solid materials, an "axion-like" effect predicts that if you apply an electric field to the surface of a topological insulator, it should generate a magnetic response, and vice versa. This is a profound link between electricity and magnetism that goes beyond the standard behavior of magnets. However, detecting this effect has been notoriously difficult because it is subtle and easily masked by other, more common magnetic signals. The question remained: could this elusive axion-like behavior be observed in a real, tangible experiment?

A team of researchers has now provided compelling evidence for this phenomenon using a technique called scanning tunneling microscopy. Instead of looking at the material from a distance, they used an incredibly sharp needle, tipped with a tiny wire made of samarium hexaboride, to probe the surface of a magnetic crystal called iron telluride. The goal was to see how the tip reacted when they applied a small voltage to it. In a standard magnetic experiment, one might expect the magnetic properties of a tip to remain constant regardless of whether the voltage is positive or negative. However, the researchers observed something far more unusual. When they reversed the direction of the voltage, the magnetic signature of the tip flipped. A millivolt of positive voltage created a specific magnetic orientation, while the same amount of negative voltage created the exact opposite orientation.

This voltage-induced magnetism is the key finding. The researchers measured that a tiny voltage, roughly 30 millivolts, was enough to generate a measurable magnetic moment at the tip of the wire, equivalent to about 0.1 times the magnetic strength of a single electron. Crucially, this magnetic effect was not static; it was directly controlled by the electric field. The team ruled out the possibility that this was simply a result of the tip picking up stray magnetic atoms from the surface or a standard change in the material's electronic structure. They confirmed this by comparing their results with a conventional magnetic tip made of chromium, which did not show this flipping behavior. The fact that the effect vanished when the temperature rose above roughly 10 Kelvin further suggested that it was a unique property of the samarium hexaboride tip itself, rather than a feature of the magnetic surface it was probing.

The explanation for this behavior lies in the unique topology of the samarium hexaboride. The researchers propose that the electric field from the voltage creates a specific kind of interaction at the surface of the tip, one that is predicted by the theory of axions. In this scenario, the electric field induces a magnetic polarization that is odd under time reversal, meaning it behaves differently when the direction of time or the direction of the field is flipped. This is distinct from the usual magnetic properties of materials. The study suggests that the surface of the samarium hexaboride acts as a boundary where the rules of the material change, allowing this electric-to-magnetic conversion to happen. The effect is so sensitive that it can be turned on and off with the flick of a voltage switch, a level of control that was previously thought to be difficult to achieve in such systems.

While the researchers are careful to note that this is an interpretation of their data rather than a final proof of the existence of axion particles, the evidence strongly points toward an axion-like magnetoelectric response. The work demonstrates that the surface of a topological insulator can indeed generate a magnetic field in response to an electric field, a phenomenon that had been predicted but never clearly observed in this specific context. This discovery opens a new window into understanding how quantum materials can emulate the fundamental forces of the universe. It suggests that by simply adjusting the voltage in a tiny electronic junction, scientists can manipulate spin and magnetic properties in ways that could eventually lead to new types of electronic devices. The ability to control magnetism with such small electrical signals offers a fresh path for exploring the deep connections between electricity, magnetism, and the topology of matter.