Properties of the depleted Heisenberg spin ladder material series $({\mathrm{C}}_7{\mathrm{H}}_{10}\mathrm{N}{)}_2{\mathrm{Cu}}_{1-z}{\mathrm{Zn}}_z{\mathrm{Br}}_4$ have been studied by the combination of magnetic measurements and neutron spectroscopy. Disorder-induced degrees of freedom lead to a specific magnetic response, described in terms of emergent strongly interacting “spin island” objects. The structure and dynamics of the spin islands is studied by high-resolution inelastic neutron scattering. This allows us to determine their spatial shape and to observe their mutual interactions, manifested by strong spectral in-gap contributions.

• Received 1 March 2016

DOI:https://doi.org/10.1103/PhysRevLett.116.257203