Rotational bands in open antiferromagnetic rings: A neutron spectroscopy study of Cr8Zn

Low-lying excitations in closed antiferromagnetic rings can be described in terms of rotational bands, which reflect the peculiar internal spin structure of the corresponding eigenstates. Here we use inelastic neutron-scattering measurements on the Cr8Zn molecule to investigate the validity of the rotational band picture in magnetically open rings. We determine the parameters of the microscopic spin Hamiltonian describing each Cr8Zn molecule and we analyze how the spin dynamics changes with respect to the parent closed-ring compound Cr-8. Qualitative differences can be found not only in the energy-level diagrams but also in the internal structure of the spin eigenstates, probed through the wave-vector transfer dependence of the neutron cross section. The usual rotational bands picture fails because quantum fluctuations of the total-spin length of the two sublattices do not occur jointly due to the ring opening.