Anomalous diffusion and localization in a positionally disordered quantum spin array

Abstract: Disorder in quantum systems can lead to the disruption of long-range order in the ground state and to the localization of the elementary excitations - famous examples thereof being the Bose glass of interacting bosons in a disordered or quasi-periodic environment, or the localized phase of spin chains mapping onto fermions. Here we present a two-dimensional quantum Ising model - relevant to the physics of Rydberg-atom arrays - in which positional disorder of the spins induces a randomization of the spin-spin couplings and of an on-site longitudinal field. This form of disorder preserves long-range order in the ground state, while it localizes the elementary excitations above it, faithfully described as spin waves: the spin-wave spectrum is partially localized for weak disorder (seemingly exhibiting mobility edges between localized and extended, yet non-ergodic states), while it is fully localized for strong disorder. The regime of partially localized excitations exhibits a very rich non-equilibrium dynamics following a low-energy quench: correlations and entanglement spread with a power-law behavior whose exponent is a continuous function of disorder, interpolating between ballistic and arrested transport. Our findings expose a stark dichotomy between static and dynamical properties of disordered quantum spin systems, which is readily accessible to experimental verification using quantum simulators of closed quantum many-body systems.