ESR Study of a novel elementary excitation : Breather in quantum spin chains

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A non-linear excitation : breather has been studied in a quantum antiferromagnetic chain CuBenzoate by means of ESR. The anisotropy and the field dependence of the field induced gap has been investigated by a high resolution ESR. In low field region, the exponent of the gap on the magnetic field found to be very close to theoretical value of 2/3. A dynamical crossover between the gapped soliton-breather regime and the gapless spinon regime has been found as the anomaly of ESR line shape and line width. By using a Voight configuration, the energy of breather mode has been determined very precisely without a correction of Zeeman term. In the high field regime, a decrease of the exponent is found. A coupling constant of a sign-Gordon model has been experimentally obtained from the accurate field dependence of the breather mass. High field zero field muon spin rotation experiments have been performed down to 16 mK and no evidence of a long range order is found. It shows the extremely good one-dimensionality of Cubenzoate. Several different types of higher order breather excitations have also found.