The Mysterious 1/9 Fraction: How Magnetic Fields Unravel the Secrets of a Quantum Magnet express
A handful of copper coins scattered on a triangular grid freezes into a strange order when a current is passed through them. That's roughly how tiny magnetic arrows, electron spins, behave in a crystal with the rare name Y-capellasite. Its copper atoms are arranged in a kagome pattern (a woven basket of triangles and hexagons), where each spin wants to point opposite to its neighbor, but the geometry prevents them from agreeing. This is called frustration — the magnetic equivalent of an argument where no one can give in.
When such a crystal is placed in a strong magnetic field, its magnetization doesn't increase smoothly but in steps, like water freezing in layers. Particularly puzzling is the step at 1/9 of the maximum: theory predicted it, but it had been seen extremely rarely. The authors grew single crystals of Y-capellasite and hit them with short but powerful field pulses. In addition, they used NMR on chlorine nuclei — a method akin to eavesdropping on the spins' conversations through their influence on neighboring atoms. It turned out that at the 1/9 plateau, the spins align into a clear pattern, and their jitter almost vanishes, as if a dance suddenly turned into a photograph.
This "freezing" of spins is not just a curiosity. It shows that in the quantum world, a collective of particles can find a common rhythm even where the rules are contradictory. Understanding these rhythms is a step toward controlling magnetic states at the microscopic level, which could one day be useful for ultra-dense memory or spin-based computing.
The kagome lattice gets its name from a Japanese basket-weaving technique: its pattern of triangles and hexagons indeed resembles a traditional bamboo mesh.
In Greg Egan's novel 'Despair', quantum states of matter with unusual magnetic properties are explored, reminiscent of frustrated magnets where information is stored in collective spin patterns.