Magnetic excitation of S = 1/2 antiferromagnetic spin chain Cu benzoate in high magnetic fields

Magnetic excitation of S = 1/2 antiferromagnetic spin chain (AFC) Cu benzoate has been studied by means of high-frequency high-field electron spin resonance (ESR). A field-induced gap caused by staggered fields is directly probed by ESR and a distinct nonlinearity is found. Below the half of the saturation field, the magnetic field dependence of the gap can be interpreted quantitatively by using a sine-Gordon (SG) model with some renormalization of parameters. In higher field regions, the deviation from the model grows gradually and the gap even decreases towards the saturation field. Such behaviour illustrates the breakdown of the SG model. A cause of the breakdown is discussed by considering the large distortion of magnetic dispersion. Above the saturation field, another excitation gap is found to develop. It is interpreted as the excitation in the fully polarized spin state. The present study gives the first experimental result on the magnetic excitation of S = 1/2 AFC exposed to staggered fields over the whole range of the magnetic field.