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High-Energy Continuum of Magnetic Excitations in the Two-Dimensional Quantum Antiferromagnet Sr2CuO2Cl2

We have measured the magnetic excitation spectrum of the model square-lattice spin-1/2 antiferromagnet $\rm Sr_2CuO_2Cl_2$ over a broad range of energy and momentum using high-resolution inelastic neutron scattering (INS). The magnon dispersion along the zone boundary was accurately measured to be a 43 meV between (1/2,0) and (3/4,1/4) indicating the importance of coupling beyond nearest neighbors in the spin Hamiltonian. We observe a strong momentum dependent damping of the zone-boundary magnons at (1/2,0) revealing a high energy continuum of magnetic excitations. A direct comparison between our INS measurements and resonant inelastic X-ray scattering (RIXS) measurements shows that the RIXS spectrum contains significant contributions from higher energy excitations not previously considered. Our observations demonstrate that this high-energy continuum of magnetic fluctuations is a ubiquitous feature of insulating monolayer cuprates, apparent in both inelastic neutron and light scattering measurements.