Quenching of the Haldane gap in LiVSi2O6 and related compounds

TL;DR

This study investigates the magnetic properties of LiVSi2O6 and related compounds, revealing antiferromagnetic ordering, exchange interactions, and magnetic anisotropy effects through susceptibility and NMR measurements.

Contribution

It provides detailed experimental and theoretical analysis of exchange couplings and magnetic anisotropy in LiVSi2O6 and related compounds, highlighting the variation of intra-chain coupling and the presence of an energy gap.

Findings

Antiferromagnetic ordering below 24 K.

Exchange coupling J varies significantly across compounds.

Presence of an energy gap explained by magnetic anisotropy.

Abstract

We report results of susceptibility chi and 7Li NMR measurements on LiVSi2O6. The temperature dependence of the magnetic susceptibility chi(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_N=24 K. The intra- and interchain coupling J and J_p for LiVSi2O6, and also for its sister compounds LiVGe2O6, NaVSi2O6 and NaVGe2O6, are obtained via a modified random phase approximation which takes into account results of quantum Monte Carlo calculations. While J_p 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_N, indicated by the variation of the NMR spin-lattice relaxation rate at T << T_N.