# Rearrangement of uncorrelated valence bonds evidenced by low-energy spin excitations in YbMgGaO4

## TL;DR

This study reveals low-energy spin excitations in YbMgGaO4 are due to valence bond rearrangements, supporting its classification as a quantum spin liquid, and suggests a universal excitation hierarchy in such states.

## Contribution

It demonstrates the nature of low-energy excitations in YbMgGaO4 as valence bond rearrangements and extends this understanding to other quantum spin liquids like herbertsmithite.

## Key findings

- Low-energy excitations disappear above certain temperatures.
- High-energy excitations persist at high temperatures.
- Valence bond rearrangements drive low-energy spin dynamics.

## Abstract

DC-magnetization data measured down to 40 mK speak against conventional freezing and reinstate YbMgGaO$_4$ as a triangular spin-liquid candidate. Magnetic susceptibility measured parallel and perpendicular to the $c$-axis reaches constant values below 0.1 and 0.2 K, respectively, thus indicating the presence of gapless low-energy spin excitations. We elucidate their nature in the triple-axis inelastic neutron scattering experiment that pinpoints the low-energy ($E$ $\leq$ $J_0$ $\sim$ 0.2 meV) part of the excitation continuum present at low temperatures ($T$ $<$ $J_0$/$k_B$), but \emph{completely} disappearing upon warming the system above $T$ $\gg$ $J_0$/$k_B$. In contrast to the high-energy part at $E$ $>$ $J_0$ that is rooted in the breaking of nearest-neighbor valence bonds and persists to temperatures well above $J_0$/$k_B$, the low-energy one originates from the rearrangement of the valence bonds and thus from the propagation of unpaired spins. We further extend this picture to herbertsmithite, the spin-liquid candidate on the kagome lattice, and argue that such a hierarchy of magnetic excitations may be a universal feature of quantum spin liquids.