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arXiv · 2508.07687

Proximate spin-liquid behavior in the double trillium lattice antiferromagnet K$_2$Co$_2$(SO$_4$)$_3$

A. Magar·K. Somesh·M. P. Saravanan·J. Sichelschmidt·Y. Skourski·M. T. F. Telling·V. A. Ginga·A. A. Tsirlin·R. Nath
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Abstract

We report proximate quantum spin liquid behavior in K$_2$Co$_2$(SO$_4$)$_3$ with the magnetic Co$^{2+}$ ions embedded on a highly frustrated three-dimensional double trillium lattice. Single-crystal and high-resolution synchrotron powder x-ray diffraction experiments reveal a structural phase transition at $T_{\rm t} \simeq 125$ K from high-temperature cubic to low-temperature monoclinic phase with the three-fold superstructure. Magnetization and heat capacity consistently show the formation of the $J_{\rm eff} =1/2$ state of Co$^{2+}$ below 50 K. In zero field, K$_2$Co$_2$(SO$_4$)$_3$ shows signatures of static magnetic order formed below $T^* \simeq 0.6$ K, but muon spin relaxation experiments reveal a large fluctuating component that persists down to at least 50 mK, reminiscent of quantum spin liquid (QSL). Static order is completely suppressed in the small magnetic field of $\sim 1$ T, and low-temperature heat capacity demonstrates the $T^2$ behavior above this field, another fingerprint of QSL. Ab initio calculations show a competition of several antiferromagnetic couplings that render K$_2$Co$_2$(SO$_4$)$_3$ a promising pseudospin-$\frac12$ material for studying quantum magnetism in the double trillium lattice geometry.

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A. Magar, K. Somesh, M. P. Saravanan, J. Sichelschmidt, Y. Skourski, M. T. F. Telling, V. A. Ginga, A. A. Tsirlin, R. Nath. 2025-08-11. Proximate spin-liquid behavior in the double trillium lattice antiferromagnet K$_2$Co$_2$(SO$_4$)$_3$. https://arxiv.org/abs/2508.07687

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