Emergent interacting spin islands in a depleted strong-leg Heisenberg ladder

Properties of the depleted Heisenberg spin ladder material series (C$_7$H$_{10}$N)$_2$Cu$_{1-z}$Zn$_{z}$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 to determine their spatial shape and to observe their mutual interactions, manifested by strong spectral in-gap contributions.