We applied the high-field ESR technique to study the field-dependent behavior of two quantum spin chains. In the half-integer (S = 1/2) antiferromagnet Cu-benzoate, we observed rather unusual temperature- and field-dependences of the shift and width of the resonance line. These results are discussed in the context of the recent theoretical description by Oshikawa and Affleck, which predicts, for this compound, the opening of a field-induced energy gap at low temperature. For the integer (S = 1) antiferromagnet Ni(C5H14N2)2N3(ClO 4), we observed various ESR modes both in the nonmagnetic Haldane phase (H < Hc) and in the magnetic high-field phase (H > Hc) where Hc is the critical field at which the Haldane gap closes. From the frequency-field diagram and the temperature dependence of the signal intensity, we discuss the possible origin of these ESR modes.
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