Quenching of the Haldane gap in LiVSi<Subscript>2</Subscript>O<Subscript>6</Subscript> and related compounds

We report results of susceptibility χ and <Superscript>7</Superscript>Li NMR measurements on LiVSi<Subscript>2</Subscript>O<Subscript>6</Subscript>. The temperature dependence of the magnetic susceptibility χ(T) exhibits a broad maximum, typical for low-dimensional magnetic systems. Quantitatively it is in agreement with the expectation for an S=1 spin chain, represented by the structural arrangement of V ions. The NMR results indicate antiferromagnetic ordering below T<Subscript>N</Subscript>=24 K. The intra- and interchain coupling J and J<Subscript>p</Subscript> for LiVSi<Subscript>2</Subscript>O<Subscript>6</Subscript>, and also for its sister compounds LiVGe<Subscript>2</Subscript>O<Subscript>6</Subscript>, NaVSi<Subscript>2</Subscript>O<Subscript>6</Subscript> and NaVGe<Subscript>2</Subscript>O<Subscript>6</Subscript>, are obtained via a modified random phase approximation which takes into account results of quantum Monte Carlo calculations. While J<Subscript>p</Subscript> is almost constant across the series, J varies by a factor of 5, decreasing with increasing lattice constant along the chain direction. The comparison between experimental and theoretical susceptibility data suggests the presence of an easy-axis magnetic anisotropy, which explains the formation of an energy gap in the magnetic excitation spectrum below T<Subscript>N</Subscript>, indicated by the variation of the NMR spin-lattice relaxation rate at T≪T<Subscript>N</Subscript>. Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 2007