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arXiv:2506.08112 · Nature Communications

Sharp spectroscopic fingerprints of disorder in an incompressible magnetic state

Chaebin Kim, Sumedh Rathi, Naipeng Zhang, Arnab Seth, Nikolai V. Simonov, Aya Rutherford, Long Chen, Haidong D. Zhou, Cheng Peng, Mingyu Xu, Weiwei Xie, A. D. Vira, Mengkun Tian, Mykhaylo Ozerov, Itamar Kimchi, Martin Mourigal, Dmitry Smirnov, Zhigang Jiang

Published Dec 6, 2025• 1 citation
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Abstract

Disorder significantly impacts the electronic properties of conducting quantum materials by inducing electron localization and thus altering the local density of states and electric transport. In insulating quantum magnetic materials, the effects of disorder are less understood and can drastically impact fluctuating spin states like quantum spin liquids. In the absence of transport tools, disorder is typically characterized using chemical methods or by semi-classical modeling of spin dynamics. This requires high magnetic fields that may not always be accessible. Here, we show that magnetization plateaus—incompressible states found in many quantum magnets—provide an exquisite platform to uncover small amounts of disorder, regardless of the origin of the plateau. Using optical magneto-spectroscopy on the Ising-Heisenberg triangular-lattice antiferromagnet $${{{{\rm{K}}}}}_{2}{{{\rm{Co}}}}{({{{{\rm{SeO}}}}}_{3})}_{2}$$ exhibiting a 1/3 magnetization plateau, we identify sharp spectroscopic lines, the fine structure of which serves as a hallmark signature of disorder. Through analytical and numerical modeling, we show that these fingerprints not only enable us to quantify minute amounts of disorder but also reveal its nature—as dilute vacancies. Remarkably, this model explains all details of the thermomagnetic response of our system, including the existence of multiple plateaus. Our findings provide a new approach to identifying disorder in quantum magnets. Disorder in quantum magnetic materials can mimic the behaviour of quantum spin liquids, however, quantifying disorder is challenging, often requiring magnetic fields large enough to fully polarize the system. Here, Kim, Rathi and coauthors show how the fine spectroscopic structure of magnetization plateaus can be used to quantify disorder in magnetic insulators.

DOI
10.1038/s41467-025-67394-0

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Plateaus can be observed in the zero-temperature magnetization curve of quantum spin systems at rational values of the magnetization. In one dimension, the appearance of a plateau is controlled by a quantization condition for the magnetization which involves the length of the local spin and the volume of a translational unit cell of the ground state. We discuss examples of geometrically frustrated quantum spin systems with large (in general unbounded) periodicities of spontaneous breaking of translational symmetry in the ground state. In two dimensions, we discuss the square, triangular and Kagomé lattices using exact diagonalization (ED) for up to N = 40 sites. For the spin-1/2 XXZ model on the triangular lattice we study the nature and stability region of a plateau at one third of the saturation magnetization. The Kagomé lattice gives rise to particularly rich behaviour with several plateaus in the magnetization curve and a jump due to local magnon excitations just below saturation.
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