Theoretical aids to neutron scattering searches for exotic quantum magnetism

The search for exotic quantum magnetism in models and real materials has been a very active area of research over the last few decades. We argue that multiparticle excitation spectra are crucial in identifying exotic features, such as spin fractionalization, spin-liquid behavior, interplay of continuum and bound states, division of spectral weights between single-particle, multi-particle states, etc. Neutron scattering is clearly the method of choice for finding these, but given the expectation of weak and diffuse spectra in many cases, quantitative theoretical calculations are essential. We discuss our recently developed cluster expansion methods to calculate many-particle spectra and spectral weights of quantum spin-models and illustrate their unprecedented accuracy by presenting results for the alternating Heisenberg chain.