Probing the anomalous symmetry-breaking in kagome material CsV3Sb5 via third-order nonlinearity

In the world of materials science, some crystals are built like intricate, repeating patterns found in nature, such as the arrangement of leaves on a stem or the tiles on a floor. Among these, a specific geometric shape known as a "kagome" lattice—named after a traditional Japanese basket weave—has captured the imagination of physicists. These materials are not just pretty patterns; their unique atomic arrangement forces electrons, the tiny particles that carry electricity, to move in ways that are impossible in ordinary metals. This strange movement creates a playground for exotic physics, where electrons can behave like waves, get trapped in flat energy zones, or form new states of matter that defy standard rules. One family of these materials, made from cesium, vanadium, and antimony, has recently become a focal point because it hosts a mix of superconductivity, where electricity flows with zero resistance, and a "charge density wave," a state where electrons organize themselves into a static pattern. However, within this complex dance of electrons, there is a mysterious moment where the material's internal symmetry breaks, creating a new, hidden order that scientists have struggled to pin down.

Researchers have now turned to a powerful new way of looking at this material, not by simply measuring how well it conducts electricity, but by watching how it reacts when the electrical current is pushed harder. In a recent study, a team of scientists investigated a thin flake of this kagome material, measuring how the voltage changed as they increased the strength of the electrical current. They discovered that the material responds with a massive, third-order nonlinear signal, a type of electrical reaction that grows much faster than the current itself. This effect is so strong that it persists all the way up to room temperature, a remarkable feat for such delicate quantum phenomena. More importantly, the strength of this signal acts like a sensitive thermometer for the material's internal order. As the researchers cooled the sample, they watched the signal change dramatically at two specific temperatures: first at 77 Kelvin, where the electrons form their charge density wave, and again at 39 Kelvin, where the mysterious symmetry-breaking occurs.

The team found that the material produces two distinct types of these nonlinear signals: one that flows straight through the material and another that appears sideways, similar to how a magnetic field can push a current to the side. The straight-flowing signal was more than four times stronger than the sideways one. By carefully analyzing how these signals changed with temperature and electrical resistance, the scientists were able to separate the causes. They determined that above the 77 Kelvin mark, the effect is driven entirely by how electrons scatter off impurities in the crystal, a messy, external process. But once the material cools below that point, something deeper takes over. The signals are then governed by a combination of this scattering and a fundamental, intrinsic property of the electrons' quantum geometry—a built-in twist in their energy landscape that exists even in a perfect crystal.

This discovery provides a clear, reliable method to detect the elusive symmetry-breaking that happens around 39 Kelvin. Previous studies had hinted at this transition, noting strange changes in how the material conducts electricity or responds to magnetic fields, but the exact nature of the event remained debated. The new research confirms that this transition is linked to a change in the material's electronic nematicity, a state where the electrons choose a preferred direction, breaking the rotational symmetry of the crystal. The fact that the third-order nonlinear signal spikes precisely at this temperature suggests that this electrical measurement is a direct probe of that hidden order. The researchers also ruled out several alternative explanations, such as the signals being caused by heat or faulty connections, confirming that the effect is a genuine property of the material's quantum state.

By mapping out these responses, the study reveals that the kagome material is a rich environment where different physical forces compete and cooperate. The giant nonlinear response observed is not just a curiosity; it serves as a distinct fingerprint for the complex interplay between the material's geometry and its electronic behavior. The work shows that even in a material that looks simple under a microscope, the electrons are engaged in a sophisticated dance of quantum mechanics, and by listening to the right electrical frequencies, scientists can finally hear the steps of that dance. This approach opens a new door for understanding how symmetry breaks in quantum materials, offering a tool that works at room temperature and could help identify similar hidden orders in other exotic substances.