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Abstract:
Effects of interchain couplings and anisotropy on a Haldane chain have
been investigated by single-crystal inelastic neutron scattering and
density functional theory (DFT) calculations on the model compound
SrNi2V2O8. Significant effects on low-energy excitation spectra are
found where the Haldane gap (Delta(0) approximate to 0.41J, where J is
the intrachain exchange interaction) is replaced by three energy minima
at different antiferromagnetic zone centers due to the complex
interchain couplings. Further, the triplet states are split into two
branches by single-ion anisotropy. Quantitative information on the
intrachain and interchain interactions as well as on the single-ion
anisotropy is obtained from the analyses of the neutron scattering
spectra by the random-phase approximation method. The presence of
multiple competing interchain interactions is found from the analysis of
the experimental spectra and is also confirmed by the DFT calculations.
The interchain interactions are two orders of magnitude weaker than the
nearest-neighbor intrachain interaction J = 8.7 meV. The DFT
calculations reveal that the dominant intrachain nearest-neighbor
interaction occurs via nontrivial extended superexchange pathways
Ni-O-V-O-Ni involving the empty d orbital of V ions. The present
single-crystal study also allows us to correctly position SrNi2V2O8 in
the theoretical D-J(perpendicular to) phase diagram [T. Sakai and M.
Takahashi, Phys. Rev. B 42, 4537 (1990)], showing where it lies within
the spin-liquid phase.