Two types of alternating spin-$\frac{1}{2}$ chains and their field-induced transitions in $\epsilon$-LiVOPO${}_4$

Abstract

Thermodynamic properties, ${}^{31}$P nuclear magnetic resonance (NMR) measurements, and density-functional band-structure calculations for $\epsilon$-LiVOPO${}_4$ are reported. This quantum magnet features a singlet ground state and comprises two types of alternating spin-$\frac{1}{2}$ 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 $\frac{1}{2}$-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 ${\alpha }_1=J_1^{\prime }/J_1\simeq 0.6$ and ${\alpha }_2=J_2^{\prime }/J_2\simeq 0.3$ within the two chains, respectively. The zero-field spin gap ${\Delta }_0/k_{\mathrm{B}}\simeq 7.3$ K probed by thermodynamic and NMR measurements is caused by the $J_1$-$J_1^{\prime }$ spin chains and can be closed in the applied field of ${\mu }_0{H}_{c1}\simeq 5.6$ 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 $J_1$-$J_1^{\prime }$ chains with weaker exchange couplings.

14 pages, 14 figures