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

TL;DR

This study reveals that K$_2$Co$_2$(SO$_4$)$_3$ exhibits proximate quantum spin liquid behavior due to its highly frustrated double trillium lattice, with experimental and theoretical evidence of persistent spin fluctuations and suppressed magnetic order.

Contribution

It provides the first detailed investigation of quantum spin liquid signatures in a three-dimensional double trillium lattice antiferromagnet, combining experimental data and ab initio calculations.

Findings

Presence of static magnetic order below 0.6 K

Persistent spin fluctuations down to 50 mK

Suppression of order and T^2 heat capacity in small magnetic field

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.