A new elementary-excitation, the so called "breather excitation", is observed directly by millimetersubmillimeter wave electron spin resonance (ESR) in the Heisenberg quantum spin-chain Cu benzoate, in which a field-induced gap is found recently by specific heat and neutron scattering measurements. Distinct anomalies were found in line width and in resonance field around the "dynamical crossover" regime between the gap-less spinon-regime and the gapped breather-regime. When the temperature becomes sufficiently lower than the energy gap, a new ESR-line with very narrow linewidth is found, which is the manifestation of the breather excitation. The non-linear field dependence of the resonance field agrees well with the theoretical formula of the first breather-excitation proposed by Oshikawa and Affleck. The present work establishes experimentally for the first time that a sine-Gordon model is applicable to explain spin dynamics in a S = 1/2 Heisenberg spin chain subjected to staggered field even in high fields.PACS numbers:42.50. Md, 75.40.Gb A magnetic field has been recognized as a unique handling-parameter to control the quantum-critical phenomena in various low-dimensional spin systems. An example of the drastic change of magnetic excitation in high magnetic fields has been found recently in Cu benzoate Cu(C 6 H 5 COO) 2 ·3H 2 O. For a very long time, this compound had been regarded as a good representative of S = 1/2 Heisenberg quantum spin chain (HQSC), with an exchange coupling J = 8.6 K.[1] More recently, however, intensive measurements performed below 1 Kspecific heat, neutron scattering and susceptibility [2,3]-revealed rather unexpected features. Besides the dynamical incommensurability expected in high fields, an unexpected energy-gap E g (H) in the magnetic excitation spectrum was observed to develop as a function of the applied magnetic field H. Based on a field theoretical approach, a description was proposed by Oshikawa and Affleck (OA) [4,5] and, subsequently by Essler and Tsvelik. [6,7] They claimed that these effects were caused by the staggered fields acting between neighboring spins in a chain.(Note that some aspects of the field-induced gap has been discussed theoretically by several authors, [8]) Their most striking theoretical proposal for Cu benzoate is that the particle-like "breather" excitation appears besides solitons, in the extreme low-temperature limit, where the temperature is smaller than the gap.The "breather" is the soliton-antisoliton bound-state and one of the elementary excitations in a quantum sineGordon model. For conventional S = 1/2 HQSC, the gap-less spinon excitation develops due to the short-range correlation when the temperature T is much lower than J. In case of Cu benzoate subjected to the staggered field, for further decrease of the temperature, a drastic change in spin-fluctuation spectrum arises due to the presence of the field-induced energy gap; i.e. the dynamical crossover takes place between the spinon-regime and the gapped breather-regime. In the gapped b...

An S = 9 Heisenberg antiferromagnet on a triangular lattice CuCr02, in which stacking of the triangular lattice of magnetic Cr atoms forms a layered rhombohedral antiferromagnet, is studied by neutron powder diffraction. In the paramagnetic phase the powder diffraction pattern shows asymmetry, which proves a two-dimensional character. In the ordered phase, magnetic Bragg scattering has large width, indicating that the scattering is distributed on a line ($4 b) with peaks where 5 takes integer values. Although the magnetic long-range order is established in the c plane, correlation in the c direction is finite or the modulation vector is distributed on the line. Intensity of magnetic reflections is consistent with the 120" structure in the a-c plane with moment (3.1 2 0 . 2 ) ~~.