The Dzyaloshinski-Moriya interaction partially lifts the magnetic frustration of the spin-1/2 oxide SrCu2(BO3)2. It explains the fine structure of the excited triplet state and its unusual magnetic field dependence, as observed in previous ESR and new neutron inelastic scattering experiments. We claim that it is mainly responsible for the dispersion. We propose also a new mechanism for the observed ESR transitions forbidden by standard selection rules, that relies on an instantaneous Dzyaloshinski-Moriya interaction induced by spin-phonon couplings. PACS numbers:Strontium Copper Borate (SrCu 2 (BO 3 ) 2 ) is a new example of a magnetic oxide with a spin gap [1], with a ground state well described as simply a product of magnetic dimers in two dimensions on the bonds giving the strongest magnetic exchange [2]. The weaker exchanges are frustrated by the geometry and, as shown by Shastry and Sutherland [3], the ground state of the isotropic Hamiltonian is independent of the value of the weaker exchange, up to a critical value. The excitations, however, are not purely local and cannot be explicitly given. Recent experiments by ESR [4] and neutron inelastic scattering presented here show how in fact there are spin anisotropies needed for an accurate description of the dynamics. We shall show the corrections to the ground state are needed that, while small, will be necessary to understand many physical properties. For example at finite external magnetic field SrCu 2 (BO 3 ) 2 appears to exhibit a number of finite magnetization plateaux [1,5], and the anisotropies will determine the observability of plateaux in different field directions. Furthermore SrCu 2 (BO 3 ) 2 is believed to be close in parameter space to a quantum critical point whose nature is somewhat controversial, and while the anisotropies are small they may be essential to its nature.For spin 1 2 the leading anisotropic terms are of form Dzyaloshinski-Moriya [6] and exchange anisotropy. The former is particularly relevant, since it may not be frustrated even if the isotropic exchange is. While a small Dzyaloshinski-Moriya interaction should not destroy a gap generated by larger isotropic interactions it modifies the pure locality of the ground state correlations and delocalizes the first triplet excitation. This is because it appears in lower order in perturbation theory than the frustrated isotropic interactions. In this paper we predict the Dzyaloshinski-Moriya interactions that should be expected in SrCu 2 (BO 3 ) 2 from the structure, and show that they do indeed explain new features of the excitations observed with ESR and neutron inelastic scattering experiments. Miyahara and Ueda [2] have introduced the frustrated Shastry-Sutherland modelfor SrCu 2 (BO 3 ) 2 , with S = 1/2 and where nn stands for nearest neighbor spins and nnn for next nearest neighbors. The lattice is shown in fig. 1. J = 85K and J ′ = 54K are antiferromagnetic interactions estimated from the susceptibility and the gap [2]. The spectrum of spin excitations has several interesting f...

review of recent experimental results obtained on the inorganic spin-Peierls compound CuGe03 is proposed. As the spin-Peierls phenomenon results from trie interplay between lattice and magnetic (quantum) fluctuations, the presentation focuses on the diflerent dynarnical aspects of the problem. With CuGe03, many points have been clarified and new resultsacqmred.However, it remains open questions which, due to the high quality of the available crystals, can be expected to be solved in the future.

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...

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