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Two types of alternating spin-$\frac12$ chains and their field-induced transitions in $\varepsilon$-LiVOPO$_4$

Thermodynamic properties, $^{31}$P nuclear magnetic resonance (NMR) measurements, and density-functional band-structure calculations for $\varepsilon$-LiVOPO$_4$ are reported. This quantum magnet features a singlet ground state and comprises two types of alternating spin-$\frac12$ chains that manifest themselves by the double maxima in the susceptibility and magnetic specific heat, and by the two-step magnetization process with an intermediate $\frac12$-plateau. From thermodynamic data and band-structure calculations, we estimate the leading couplings of $J_1\simeq 20$ K and $J_2\simeq 60$ K and the alternation ratios of $α_1=J_1'/J_1\simeq 0.6$ and $α_2=J_2'/J_2\simeq 0.3$ within the two chains, respectively. The zero-field spin gap $Δ_0/k_{\rm B}\simeq 7.3$ K probed by thermodynamic and NMR measurements is caused by the $J_1$-$J_1'$ spin chains and can be closed in the applied field of $μ_{0}H_{\rm c1}\simeq 5.6$ T, giving rise to a field-induced long-range order. The NMR data reveal predominant three-dimensional spin-spin correlations at low temperatures. Field-induced magnetic ordering transition observed above $H_{c1}$ is attributed to the Bose-Einstein condensation of triplons in the sublattice formed by the $J_1$-$J_1'$ chains with weaker exchange couplings.