Perovskite*

Here, we demonstrate experimentally and confirm by theoretical calculations that rare-earth perovskite YbAlO₃ is an excellent experimental realization of an effective spin-1/2 quantum XXZ chain. Inelastic neutron scattering spectra taken on YbAlO₃ provide evidence for unconventional low-energy magnetic excitations. The spectrum at temperatures above antiferromagnetic ordering T_N=0.8K is characterized by a gapless continuum with dispersive boundaries in one direction and bandwidth of ΔE~0.6meV at the zone center. Cooling down through the T_N, a different spectrum emerges. A gap is opened abruptly and the excitation spectrum is dominated by a high-intensity sharp mode. The observed excitations have negligible dispersion along the other directions, proving our conclusion that the Yb moments form weakly coupled spin chains despite the three-dimensional perovskite structure. We also argue that the series of rare-earth based perovskites could be an exceptional candidate for low dimensional S=1/2 quantum chain with tunable intrinsic parameters to explore quantum magnetism.