Structure of End States for a Haldane Spin Chain

Inelastic neutron scattering was used to probe edge states in a quantum spin liquid. The experiment was performed on finite length antiferromagnetic spin-1 chains in ${\mathrm{Y}}_2{\mathrm{B}\mathrm{a}\mathrm{N}\mathrm{i}}_{1-x}{\mathrm{M}\mathrm{g}}_x{\mathrm{O}}_5$. At finite fields, there is a Zeeman resonance below the Haldane gap. The wave-vector dependence of its intensity provides direct evidence for staggered magnetization at chain ends, which decays exponentially towards the bulk [$\xi =8(1)$ at $T=0.1\mathrm{K}$]. Continuum contributions to the chain-end spectrum indicate interchain segment interactions. We also observe a finite size blueshift of the Haldane gap.