Consequences of critical interchain couplings and anisotropy on a Haldane chain

TL;DR

This study investigates how interchain couplings and anisotropy affect the magnetic excitation spectra of a Haldane chain compound, revealing complex interactions and their impact on the spin-liquid phase using neutron scattering and DFT calculations.

Contribution

It provides detailed experimental and theoretical analysis of interchain couplings and anisotropy effects in a Haldane chain, including quantitative interaction parameters and phase diagram positioning.

Findings

Interchain couplings significantly modify low energy excitation spectra.

Haldane gap replaced by multiple energy minima due to complex interactions.

Dominant intrachain interaction occurs via extended superexchange pathways.

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 SrNi$_2$V$_2$O$_8$. Significant effects on low energy excitation spectra are found where the Haldane gap ($\Delta_0 \approx 0.41J$; 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 are obtained from the analyses of the neutron scattering spectra by the random phase approximation (RPA) 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-neighbour 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 SrNi$_2$V$_2$O$_8$ in the theoretical $D$-$J_{\perp}$ phase diagram [T. Sakai and M. Takahashi, Phys. Rev. B 42, 4537 (1990)] showing where it lies within the spin-liquid phase.