Quasiparticle Interactions in the $S = 1/2$ Heisenberg Antiferromagnetic Chain CuPzN
We investigate spinon--spinon interactions in the quasi-one-dimensional $S=1/2$ Heisenberg antiferromagnet Cu(C$_4$H$_4$N$_2$)(NO$_3$)$_2$ (CuPzN) using inelastic neutron scattering. At zero field, the spectrum exhibits the canonical multi-spinon continuum of the Heisenberg chain. With increasing field we demonstrate t...