Zero-point fluctuation in quasi one-dimensional antiferromagnets: theory and experiment

Abstract

A systematic comparison is given of the quantum reduction in the ordered spin of quasi one-dimensional Heisenberg antiferromagnets as predicted by spin-wave theory and as measured in ionic systems with known spin-wave parameters. Simple linear theory yields a leading term (1/(2 pi ))ln(C mod J/J' mod )-1/2 where further terms can be neglected for practical purposes if mod J'/J mod <10-2; such expressions are listed for ferromagnetic and antiferromagnetic square and triangular lattices of antiferromagnetic chains with zero anisotropy. For half-integer-spin systems with 1/2<or=S<or=5/2, excellent agreement with experiment is found over the range 10-2> mod J'/J mod >10-4 when a known correction for 'kinematical interactions' is applied. It should thus be possible to identify covalency reductions in excess of 10%. For the S=1 system CsNiCl3, however, the measured spin reduction may indicate extra quantum fluctuations related to the Haldane effect. From the susceptibility behaviour, the quantum ground state for progressively smaller mod J'/J mod is argued to increasingly resemble a singlet separated from other states by an energy gap.