Symmetry-Induced Universal Momentum-Transfer Dependencies for Inelastic Neutron Scattering on Anisotropic Spin Clusters

Abstract: Inelastic neutron scattering (INS) is a key method for studying magnetic excitations in spin systems, including molecular spin clusters. The method has significantly advanced in recent years and now permits to probe the scattering intensity as a function of the energy transfer and the momentum-transfer vector Q. It was recently shown that high molecular symmetry facilitates the analysis of spectra. Point-group symmetry imposes selection rules in isotropic as well as anisotropic spin models. Furthermore, the Q-dependence of the INS intensity may be completely determined by the point-group symmetry of the states involved in a transition, thereby affording a clear separation of dynamics (energies, transition strengths) and geometrical features (Q-dependencies). The present work addresses this issue for anisotropic spin models. We identify a number of cases where the Q-dependence is completely fixed by the point-group symmetry. For six- and eight-membered planar spin rings and two polyhedra (the cube and the icosahedron) we tabulate and plot the corresponding powder-averaged universal intensity functions. The outlined formalism straightforwardly applies to other highly-symmetric systems and should be useful for future analyses of INS spectra by focusing on those features that contain information on either spin dynamics or the point-group symmetry of states.