Direct observation of the quantum energy gap in tetragonal cuprate antiferromagnetsK. Katsumata, M. Hagiwara, Z. Honda, J. Satooka, A. Aharony, R. J. Birgeneau, F. C. Chou, O. Entin-Wohlman, A. B. Harris, M. A. Kastner, Y. J. Kim and Y. S. LeeEurophys. Lett., 54 4 (2001) 508-514DOI: https://doi.org/10.1209/epl/i2001-00276-4