2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/23025We formulate a direct and systematic perturbation theory on the shift of the main paramagnetic peak in Electron Spin Resonance, and derive a general expression up to second order. It is applied to one-dimensional XXZ and transverse Ising models in the high field limit, to obtain explicit results including the polarization dependence for arbitrary temperature.5 pages (no figures) in REVTEXStatistical MechanicsStrongly Correlated Electronstext