Two types of alternating spin-1/2 chains and their field-induced transitions in ε-LiVOPO₄

Thermodynamic properties, ³¹P nuclear magnetic resonance (NMR) measurements, and density-functional band-structure calculations for ε-LiVOPO₄ are reported. This quantum magnet features a singlet ground state and comprises two types of alternating spin- 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 1/2 -plateau. From thermodynamic data and band-structure calculations, we estimate the leading couplings of J1 � 20 K and J2 � 60 K and the alternation ratios of α₁ = J '₁/J₁ ≈ 0.6 and of α₂ = J '<span style="font-size:10.8333px">2</span>/J<span style="font-size:10.8333px">2</span> ≈ 0.3 within the two chains, respectively. The zero-field spin gap △_⁰/k_B ≈ 7.3 K probed by thermodynamic and NMR measurements is caused by the J₁-J'₁ spin chains and can be closed in the applied field of μ₀H_c1 ≈ 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 Hc1 is attributed to the Bose-Einstein condensation of triplons in the sublattice formed by the J₁-J'₁ chains with weaker exchange couplings.