Direct Observation of the Quantum Energy Gap in S = 1/2 Tetragonal   Cuprate Antiferromagnets
  Using an electron spin resonance spectrometer covering a wide range of frequency and magnetic field, we have measured the low energy excitations of the S=1/2 tetragonal antiferromagnets, Sr_{2}CuO_{2}Cl_{2} and Sr_{2}Cu_{3}O_{4}Cl_{2}. Our observation of in-plane energy gaps of order 0.1 meV at zero external magnetic field are consistent with a spin wave calculation, which includes several kinds of quantum fluctuations that remove frustration. Results agree with other experiments and with exchange anisotropy parameters determined from a five band Hubbard model. 
K. Katsumata, M. Hagiwara, Z. Honda, J. Satooka, Amnon Aharony, R. J.   Birgeneau, F. C. Chou, O. Entin-Wohlman, A. B. Harri...