dc-magnetization data measured down to 40 mK speak against conventional freezing and reinstate ${\mathrm{YbMgGaO}}_4$ 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 ($E\le J_0\sim 0.2\mathrm{meV}$) part of the excitation continuum present at low temperatures ($T<J_0/k_B$), but 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.

• Received 23 July 2018

DOI:https://doi.org/10.1103/PhysRevLett.122.137201