High-field, high-frequency ESR in the one-dimensional quantum spin systems

We applied the high-field, high-frequency electron spin resonance (ESR) technique to study the held-dependent behaviors of three representative examples of quantum spin chain. (1) In the half-integer (S = 1/2) antiferromagnet Cu-benzoate, Cu(C6H5COO)(2). 3H(2)O, we observed rather unusual temperature- and field-dependences of the shift and width of the resonance line. These results are discussed in terms of the breather excitations predicted by Oshikawa and Affleck for this compound. (2) For the integer (S = 1) antiferromagnet NDMAZ, Ni(C5H14N2)(2)N-3 (ClO4), we observed various ESR modes both in the nonmagnetic Haldane phase (H < H-c) and in the magnetic high-field phase (H > H-c) where H-c 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. (3) For S = 1/2 trimerized chain, 3CuCl(2). 2dioxane, we observed unique features of a composite spin system, such as a dynamical process of the formation of S = 3/2 trimerized units from three S = 1/2 spins, as indicated by the signal splitting into 3 lines at most depending on the frequency and gradual change of the resonance fields with decreasing temperature.