Y89NMR Imaging of the Staggered Magnetization in the Doped Haldane ChainY2BaNi1−xMgxO5

We present a static, model-independent, experimental determination of the spin-spin correlation length $\xi$ in a quantum spin system. This is achieved in the doped Haldane (i.e., $S=1\mathrm{}$ Heisenberg antiferromagnetic) chain ${\mathrm{Y}}_2{\mathrm{BaNi}}_{1-x}{\mathrm{Mg}}_x{\mathrm{O}}_5$ by ${}^{89}\mathrm{Y}$ NMR imaging of the staggered magnetization induced around the Mg impurities (i.e., chain boundaries) by a magnetic field. The magnitude of this magnetization is found to decay exponentially, with $\xi$ equal to the theoretical prediction for an infinite $S=1\mathrm{}$ chain and the staggered magnetic moment at the edge site showing the Curie behavior of an effective $S=1\mathrm{}/2$ spin.