Low-Temperature Electron Spin Resonance Theory for Half-Integer Spin Antiferromagnetic Chains

A theory of low-temperature ( $T$) electron spin resonance (ESR) in half-integer spin antiferromagnetic chains is developed using field theory methods and avoiding previous approximations. It is compared to experiments on Cu benzoate. Power laws are predicted for the linewidth broadening due to various types of anisotropy. At $T\to 0$, zero width absorption peaks occur in some cases. The second ESR peak in Cu benzoate, observed at $T<\mathrm{}0.76\mathrm{}\mathrm{K}$, is argued not to indicate Néel order as previously claimed, but to correspond to a sine-Gordon “breather” excitation.