Observation of plaquette fuctuations in the spin-1/2 honeycomb lattice YbBr3

Quantum spin liquid materials are widely studied to improve our understanding of quantum entangled spin correlations in the absence of magnetic long-range order at T = 0 K. However, it is a challenging task to unambiguously identify a quantum spin liquid since it is difficult to measure quantum entanglement directly. Fortunately, neutron scattering is capable to measure fractionalized excitations that are a characteristic of quantum spin liquids. We studied YbBr3 by neutron scattering whose magnetism is a realization of an effective S = 1/2 Heisenberg honeycomb lattice where nearest and next-nearest neighbor interactions compete. YbBr3 avoids order down to at least T = 100 mK and features a dynamic spin-spin correlation function with broad continuum scattering typical of quantum spin liquids near a quantum critical point. The continuum in the spin spectrum is consistent with plaquette type fluctuations. Our study is the experimental demonstration that strong quantum fluctuations can exist on the honeycomb lattice even in the absence of Kitaev-type interactions, and opens a new perspective on quantum spin liquids.