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

TL;DR

This study investigates R_2BaNiO_5 compounds, revealing coexistence of Haldane gap excitations with long-range magnetic order, showing that the Haldane state persists despite 3D magnetic ordering.

Contribution

It demonstrates that Haldane gap excitations coexist with 3D magnetic order in R_2BaNiO_5, indicating the Haldane state is not fully destroyed by Ne9el ordering.

Findings

Haldane gap excitations survive in the ordered phase.

Gap energy scales with the square of the ordered Ni moment.

Haldane singlet ground state persists despite magnetic ordering.

Abstract

$R_2BaNiO_5$ ($R=$ 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 N\'{e}el ordering.