$S$ = 1/2 ferromagnetic-antiferromagnetic alternating Heisenberg chain in a zinc-verdazyl complex

Abstract

We successfully synthesized the zinc-verdazyl complex [Zn(hfac)${}_2$]$\cdot$($o$-Py-V) [hfac = 1,1,1,5,5,5-hexafluoroacetylacetonate; $o$-Py-V = 3-(2-pyridyl)-1,5-diphenylverdazyl], which is an ideal model compound with an $S$ = 1/2 ferromagnetic-antiferromagnetic alternating Heisenberg chain (F-AF AHC). $Ab$ $initio$ molecular orbital (MO) calculations indicate that two dominant interactions $J_{\mathrm{F}}$ and $J_{\mathrm{AF}}$ form the $S=1/2$ F-AF AHC in this compound. The magnetic susceptibility and magnetic specific heat of the compound exhibit thermally activated behavior below approximately 1 K. Furthermore, its magnetization curve is observed up to the saturation field and directly indicates a zero-field excitation gap of 0.5 T. These experimental results provide evidence for the existence of a Haldane gap. We successfully explain the results in terms of the $S=1/2$ F-AF AHC through quantum Monte Carlo calculations with $|J_{\mathrm{AF}}/J_{\mathrm{F}}|$ = 0.22. The $ab$ $initio$ MO calculations also indicate a weak AF interchain interaction $J^{\prime }$ and that the coupled F-AF AHCs form a honeycomb lattice. The $J^{\prime }$ dependence of the Haldane gap is calculated, and the actual value of $J^{\prime }$ is determined to be less than 0.01$|J_{\mathrm{F}}|$.

arXiv comment17 pages, 5 figures

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• Quantum critical phenomena in a spin-1/2 frustrated square lattice with spatial anisotropy
• Observation of thermodynamics originating from a mixed-spin ferromagnetic chain
• Gapped ground state in a spin-1/2 frustrated square lattice
• Field-induced quantum magnetism in the verdazyl-based charge-transfer salt $[$$o$-MePy-V-($p$-Br)${}_2]$FeCl${}_4$
• Spin-Peierls transition to a Haldane phase
• Ground state properties of the bond alternating spin-$\frac{1}{2}$ anisotropic Heisenberg chain
• Field-induced quantum phase in a frustrated zigzag-square lattice
• Zero-temperature study of a tetrameric spin-$1/2$ chain in a transverse magnetic field