Evidence for Mobile Gapless Spinons in a Honeycomb Lattice

Abstract One important issue in current condensed matter physics is the search of quantum spin liquid (QSL), an exotic magnetic state with strongly-fluctuating and highly-entangled spins down to zero temperature without static order. However, there is no consensus on the existence of a QSL state in any real material so far, due to inevitable disorder and intricate competing exchange interactions on frustrated spin lattices. Here we report systematic heat transport measurements on a honeycomb-lattice compound BaCo₂(AsO₄)₂, which manifests magnetic order in zero field. In a narrow in-plane field range after the magnetic order is nearly suppressed, in both perpendicular and parallel to the zigzag direction, a finite residual linear term of thermal conductivity is clearly observed, which is attributed to mobile fermionic excitations. In addition, the spin-phonon scattering rate exhibits a T-linear behavior when the order disappears. These observations suggest a partial QSL state with gapless spinon excitations in BaCo₂(AsO₄)₂, that emerges when a portion of the spins remains ordered, and vanishes as the spins become progressively polarized.