  Bulk magnetism in solids is fundamentally quantum mechanical in nature. Yet
in many situations, including our everyday encounters with magnetic materials,
quantum effects are masked, and it often suffices to think of magnetism in
terms of the interaction between classical dipole moments. Whereas this
intuition generally holds for ferromagnets, even as the size of the magnetic
moment is reduced to that of a single electron spin (the quantum limit), it
breaks down spectacularly for antiferromagnets, particularly in low dimensions.
Considerable theoretical and experimental progress has been made in
understanding quantum effects in one-dimensional quantum antiferromagnets, but
a complete experimental description of even simple two-dimensional
antiferromagnets is lacking. Here we describe a comprehensive set of neutron
scattering measurements that reveal a non-spin-wave continuum and strong
quantum effects, suggesting entanglement of spins at short distances in the
simplest of all two-dimensional quantum antiferromagnets, the square lattice
Heisenberg system.
