Two Types Of Alternating Spin-1/2 Chains And Their Field-Induced Transitions In Epsilon-Livopo4

Thermodynamic properties, P-31 nuclear magnetic resonance (NMR) measurements, and density-functional band-structure calculations for epsilon-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 J(1) similar or equal to 20 K and J(2) similar or equal to 60 K and the alternation ratios of alpha(1) = J(1)'/J(1) similar or equal to 0.6 and alpha(2) = J(2)'/J(2) similar or equal to 0.3 within the two chains, respectively. The zero-field spin gap Delta(0)/k(B) similar or equal to 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 mu H-0(c1) similar or equal to 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.