Experimental observation of Bethe strings

The spin of electrons is what gives most materials their magnetic properties, and these spins are often treated as if they are located at fixed points in a crystal lattice. When a single spin flips in a one-dimensional system, it can cause neighbouring spins to flip and thus propagate the flip along the chain. This type of magnetic excitation can be described as an elementary quasiparticle known as a magnon. In the 1930s, Hans Bethe predicted that multiple spins could flip and that the multi-spin flip could propagate, giving rise to complex multi-magnon states that look like strings. Using terahertz spectroscopy, Zhe Wang and colleagues measure such 'Bethe strings' directly in a one-dimensional Heisenberg–Ising antiferromagnet, and show that they dominate the quantum spin dynamics when the system is close to a quantum phase transition.