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.
Original abstract
as written by the authors arXiv:2605.08067
Fractional magnetization plateaus provide a sensitive probe of many-body spin states in frustrated quantum magnets, yet their microscopic origin in kagome antiferromagnets remains unresolved. This is particularly true of the mysterious $$1/9$$ plateau, which is predicted by theory but infrequently observed in experiment. Here, we investigate this problem in the $$S = 1/2$$ anisotropic kagome antiferromagnet Y-kapellasite, Y$$_3$$Cu$$_9$$(OH)$$_{19}$$Cl$$_8$$, using pulsed-field magnetization measurements on single crystals and high-field $$^{35}$$Cl NMR. We identify a hierarchy of field-induced fractional features, including $$1/3$$ and $$1/9$$ plateaus, as well as a weaker low-field feature. Analysis of the NMR spectra and the magnetic susceptibility across the $$1/9$$ plateau demonstrate that it is accompanied by an ordered local spin configuration, a strong suppression of low-energy spin fluctuations and activated behavior, consistent with a gapped fractional state. These features differ from those in the only other material YCu$$_3$$(OH)$$_6$$Br$$_2$$[Br$$_{1-y}$$(OH)$$_y$$] in which this plateau is observed, implying a surprising robustness of the $$1/9$$ state to the details of the underlying magnetism.
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.