LATTICE-SPIN AND SPIN-POLARONIC DEFECTS IN ANTIFERROMAGNETIC HALOGEN-BRIDGED TRANSITION-METAL CHAIN COMPOUNDS

The results of many-body modeling are reported for polaronic defect excitations in the nickel-based halogen-bridged chain compounds, exemplified by NiBr. One-electron band structures and Raman scattering amplitudes, for electron and hole polarons and bipolarons, triplet excitons, negative, neutral, and positive solitons, are qualitatively described. A singlet exciton is found to be unstable with our model parameters, due to the large on-site Hubbard repulsion. Most significantly, the local defect excitations exhibit a substantial lattice distortionrelative to the undistorted antiferromagnetic spin background. The stoichiometric lattice symmetry is therefore broken locally, resulting in Paman activity, which is unexpected on the basis of the stoichiometric ground-state symmetry.