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Coexistence of Haldane gap excitations and long-range order in R_2BaNiO_5 ($R$=rare earth)

$R_2BaNiO_5$ ($R=$ rare earth) quasi-1-D antiferromagnets are structurally equivalent to the well-studied 1-D S=1 Haldane-gap compound Y_2BaNiO_5. Unlike the Y-nickelate though, these materials undergo 3-D magnetic ordering at finite temperatures. Recent inelastic neutron scattering studies of Pr_2BaNiO_5 and (Nd_{x}Y_{1-x})_2BaNiO_5 revealed purely 1-dimensional gap excitations that propagate exclusively on the Ni-chains and are strikingly similar to Haldane gap modes in Y_2BaNiO_5. In the ordered phase these excitations survive and actually coexist with conventional spin waves. Below $T_{N}$ the gap energy increases and scales as the square of the ordered moment on the Ni sites. The results suggest that the Haldane singlet ground state of the Ni-chains is not fully destroyed by Néel ordering.