The field and temperature dependence of the line shape of ESR for strongly interacting quantum spin systems would provide various important informations for the magnetic interaction. We have introduced a method of direct numerical estimation of the dynamical susceptibility with the Kubo formula and applied it to spin systems of various lattice structures. There the ESR line shape shows significant dependence on the strength of the field, and also on the geometrical situation among the directions of the fields and lattice axes.KEYWORDS: ESR, Kubo formula, Quantum spin system, Dynamical Shift, Antiferromagnetic Resonance, Paramagnetic Resonance, Nanoscale molecular magnets §1. IntroductionElectron spin resonance (ESR) has been one of the most powerful tool to study the magnetic properties of the material.1-4) In low dimensional strongly interacting spin systems, the ESR line shape depends on the geometrical structure among the lattice structure of the magnetic interaction, the static field and the oscillating So far, extensive studies have been done on the various ground state phases of the Heisenberg magnets with so-called spin gap, inspired by discovery of the Haldane phase of the spin 1 Heisenberg antiferromagnets.
5)The various phases are characterized in the view point of the VBS picture.
6, 7)However, from the view point of the ESR, as far as the Hamiltonian consists of the isotropic Heisenberg couplings, all of these systems give the same response, that the Zeeman term, which causes the paramagnetic resonance. In order to have a change in the line shape of ESR from the paramagnetic resonance (electron paramagnetic resonance, EPR), the system needs some perturbations which violate the SU(2) symmetry of the Heisenberg model and do not commute with the total magnetization.As a typical example, the effect of the dipole-dipole interaction on the shift of resonant field in the onedimensional Heisenberg antiferromagnet has been investigated by Nagata and Tazuke 8) with the effective mode method proposed by Kanamori-Tachiki.9) The resonant frequency shifts depending on the angle between the chain axis and the static field H 0 . The difference between the cases whether the static field and the chain axis are parallel or perpendicular is clarified. Furthermore the shift also depends on the direction of the oscillating field, as has been pointed out as the dynamical * E-mail address: ogasa@spin.t.u-tokyo.ac.jp * * E-mail address: miya@spin.t.u-tokyo.ac.jp shift.
10, 11)In order to study such dependence of the shift in the line shape and also of the width of the resonant peak in more general cases, we have introduced a direct numerical method to estimate the ESR line shape by studying the Kubo formula 4) with the full Hamiltonian of the spin system and the field. We have applied this method to Heisenberg antiferromagnetic chain and confirmed that we can reproduce the characteristic temperature and angle dependence.13) We also applied the method to antiferromagnetic clusters with easy-axis anisotropy and found that the ...