Direct Observation of the Quantum Energy Gap in S = ½ 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 = ½ tetragonal antiferromagnets, Sr2CuO2Cl2 and Sr2Cu3O4Cl2. Our observations 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. Such gaps vanish for classical spins, and were too small to be observed by other techniques. Results agree with other experiments and with exchange anisotropy parameters determined from a five-band Hubbard model.