| Spin excitations in a magnet with ordered moments are magnons, quantized spin waves that behave like particles with integer spin and Bose-Einstein statistics. A magnet lacking long-range order cannot support spin waves and may have unusual magnetic excitations with noninteger spin and different statistics. The Heisenberg antiferromagnet with spins of length S=1/2 on kagome and hyperkagome lattices has emerged as a leading candidate for novel quantum physics. Here we show that magnetic excitations in this system are spinons, quasiparticles with spin 1/2 and Fermi-Dirac statistics. The ground state of the system is a quantum liquid or solid of small, heavy S=0 pairs of spinons with a large density of nonmagnetic (Delta S=0) excitations at low energies. A magnetic (Delta S=1) excitation amounts to breaking up a pair into two spinons, a process with a threshold energy of 0.06 exchange constants. |
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