TEST OF THE HALDANE CONJECTURE IN A 1D-HEISENBERG (S = 1) ANTIFERROMAGNET BY OPTICAL LINEAR BIREFRINGENCE J. Ferré, J. P. Jamet, C. P. Landee, K. A. Reza and J. P. Renard J. Phys. Colloques, 49 C8 (1988) C8-1441-C8-1442 DOI: 10.1051/jphyscol:19888661