  The ESR (Electron Spin Resonance) spectrum of the XXZ spin chain with finite
length shows a double-peak structure at high temperatures around the EPR
(Electron Paramagnetic Resonance) resonance frequency. This fact has been
pointed out by direct numerical methods (S. El Shawish, O. Cepas and S.
Miyashita; H. Ikeuchi, H. De Raedt, S. Bertaina and S. Miyashita). On the other
hand, at low temperatures the spectrum has a single peak with a finite shift
from the frequency of EPR as predicted by the analysis of field theoretical
works (M. Oshikawa and I. Affleck). We study how the spectrum changes with the
temperature, and also we study the size-dependence of the line shape including
the even-odd effect. In order to understand those dependences, we introduce a
decomposition of the spectrum into contributions from transitions specified by
magnetization, and we characterize the structure of the spectrum by individual
contributions. Applying the moment method introduced by M. Brockman et al., to
each component, we analyze the size-dependence of the structure of the
spectrum, which supports the numerical observation that separation of the
double-peak structure vanishes inversely with the size.
