Polarons near Van Hove points in 2D charge or spin density waves

We study theoretically the effects of impurities and of gap distortions in binding and self-trapping of particles added or excited over the insulating state of an antiferromagnetic Mott insulator. Wave functions of single particles or of bound excitons are affected by interactions with collective degrees of freedom, as shown by ARPES and optics experiments. The resulting self-localized state lowers the total particle energy, enhances its effective mass and splits off the electronic level below the nominal insulating gap. Most of the features measured experimentally in lightly n-doped cuprates are related to electronic states originated by anti-nodal points (π,0) of the Brillouin zone. In these points the van Hove singularity plays a major role by enhancing bound states of the electron with neutral and charged impurities and of inter-gap excitons.