Scientists solve mystery of loop current switching in kagome metals

(Press-News.org) Quantum metals are metals where quantum effects—behaviors that normally only matter at atomic scales—become powerful enough to control the metal's macroscopic electrical properties. 

Researchers in Japan have explained how electricity behaves in a special group of quantum metals called kagome metals. The study is the first to show how weak magnetic fields reverse tiny loop electrical currents inside these metals. This switching changes the material's macroscopic electrical properties and reverses which direction has easier electrical flow, a property known as the diode effect, where current flows more easily in one direction than the other.  

Notably, the research team found that quantum geometric effects amplify this switching by about 100 times. The study, published in Proceedings of the National Academy of Sciences, provides the theoretical foundation that could eventually lead to new electronic devices controlled by simple magnets. 

Scientists had observed this strange magnetic switching behavior in experiments since around 2020 but could not explain why it happened and why the effect was so strong. This study provides the first theoretical framework explaining both. 
 
When frustrated electrons cannot settle 

The name "kagome metal" comes from the Japanese word "kagome," meaning "basket eyes" or "basket pattern," which refers to a traditional bamboo weaving technique that creates interlocking triangular designs. 

These metals are special because their atoms are arranged in this unique basket-weave pattern that creates what scientists call "geometric frustration"—electrons cannot settle into simple, organized patterns and are forced into more complex quantum states that include the loop currents.  

When the loop currents inside these metals change direction, the electrical behavior of the metal changes. The research team showed that loop currents and wave-like electron patterns (charge density waves) work together to break fundamental symmetries in the electronic structure. They also discovered that quantum geometric effects—unique behaviors that only occur at the smallest scales of matter—significantly enhance the switching effect. 

“Every time we saw the magnetic switching, we knew something extraordinary was happening, but we couldn't explain why,” Hiroshi Kontani, senior author and professor from the Graduate School of Science at Nagoya University, recalled. 

“Kagome metals have built-in amplifiers that make the quantum effects much stronger than they would be in ordinary metals. The combination of their crystal structure and electronic behavior allows them to break certain core rules of physics simultaneously, a phenomenon known as spontaneous symmetry breaking. This is extremely rare in nature and explains why the effect is so powerful.” 

The research method involved cooling the metals to extremely low temperatures of about -190°C. At this temperature, the kagome metal naturally develops quantum states where electrons form circulating currents and create wave-like patterns throughout the material. When scientists apply weak magnetic fields, they reverse the direction these currents spin, and as a result, the preferred direction of current flow in the metal changes. 

New materials meet new theory 

This breakthrough in quantum physics was not possible until recently because kagome metals were only discovered around 2020. While scientists quickly observed the mysterious electrical switching effect in experiments, they could not explain how it worked.  

The quantum interactions involved are very complex and require advanced understanding of how loop currents, quantum geometry, and magnetic fields work together—knowledge that has only developed in recent years. These effects are also very sensitive to impurities, strain, and external conditions, which makes them difficult to study. 

“This discovery happened because three things came together at just the right time: we finally had the new materials, the advanced theories to understand them, and the high-tech equipment to study them properly. None of these existed together until very recently, which is why no one could solve this puzzle before now," Professor Kontani added. 

“The magnetic control of electrical properties in these metals could potentially enable new types of magnetic memory devices or ultra-sensitive sensors. Our study provides the fundamental understanding needed to begin developing the next generation of quantum-controlled technology,” he said. 

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