arXiv2012
We analyze the absorption of microwaves by the Heisenberg-Ising chain combining exact calculations, based on the integrability of the model, with numerical calculations. Within linear response theory the absorbed intensity is determined by the imaginary part of the dynamical susceptibility. The moments of the normalized intensity can be used to define the shift of the resonance frequency induced by the interactions and the line width independently of the shape of the spectral line. These moments can be calculated exactly as functions of temperature and strength of an external magnetic field, as long as the field is directed along the symmetry axis of the chain. This allows us to discuss the line width and the resonance shift for a given magnetic field in the full range of possible anisotropy parameters. For the interpretation of these data we need a qualitative knowledge of the line shape which we obtain from fully numerical calculations for finite chains. Exact analytical results on the line shape are out of reach of current theories. From our numerical work we could extract, however, an empirical parameter-free model of the line shape at high temperatures which is extremely accurate over a wide range of anisotropy parameters and is exact at the free fermion point and at the isotropic point. Another prediction of the line shape can be made in the zero-temperature and zero magnetic field limit, where the sufficiently anisotropic model shows strong absorption. For anisotropy parameters in the massive phase we derive the exact two-spinon contribution to the spectral line. From the intensity sum rule it can be estimated that this contribution accounts for more than 80% of the spectral weight if the anisotropy parameter is moderately above its value at the isotropic point.