${\mathrm{Y}}_2$BaNi${\mathrm{O}}_5$: A nearly ideal realization of the $S=1\mathrm{}$ Heisenberg chain with antiferromagnetic interactions

We report an inelastic-neutron-scattering experiment on single crystals of the one-dimensional spin-one antiferromagnet ${\mathrm{Y}}_2$BaNi${\mathrm{O}}_5$. The data show that this compound is a nearly ideal material for studying the Haldane conjecture in the Heisenberg limit. In particular, the Haldane gap at $\tilde{q}=\pi$ is almost isotropic, taking on the values of 7.5(1), 8.6(1), and 9.6(1) meV for polarizations parallel to the three principal orthorhombic axes respectively. Interchain coupling along the edges of the orthorhombic unit cell is found to be very weak ($\frac{J^{\prime }}{J}<~5\times {10}^{-4\mathrm{}}$). Finally, we show that defects in our sample at the 1% level cause visible broadening of the excitation at $\tilde{q}=\pi$.