Continuous canonical transformation for the double exchange model

The method of continuous canonical transformation is applied to the double exchange model with a purpose to eliminate the interaction term responsible for non conservation of magnon number. Set of differential equations for the effective Hamiltonian parameters is derived. Within the lowest order (approximate) solution we reproduce results of the standard (single step) canonical transformation. Results of the selfconsistent numerical treatment are compared with the other known studies for this model.Comment: 21 pages, 9 figures. Eur. Phys. J B (accepted for publication

Nature of correlations in the atomic limit of the boson fermion model

Using the equation of motion technique for Green's functions we derive the exact solution of the boson fermion model in the atomic limit. Both (fermion and boson) subsystems are characterised by the effective three level excitation spectra. We compute the spectral weights of these states and analyse them in detail with respect to all possible parameters. Although in the atomic limit there is no true phase transition, we notice that upon decreasing temperature some pairing correlations start to appear. Their intensity is found to be proportional to the depleted amount of the fermion nonbonding state. We notice that pairing correlations behave in a fashion observed for the optimally doped and underdoped high $T_{c}$ superconductors. We try to identify which parameter of the boson fermion model can possibly correspond to the actual doping level. This study c larifies the origin of pairing correlations within the boson fermion model and may elucidate how to apply it for interpretation of experimental data.Comment: 5 pages, 3 figures; to appear in Eur. Phys. J.

Quasiparticle states driven by a scattering on the preformed electron pairs

We analyze evolution of the single particle excitation spectrum of the underdoped cuprate superconductors near the anti-nodal region, considering temperatures below and and above the phase transition. We inspect the phenomenological self-energy that reproduces the angle-resolved-photoemission-spectroscopy (ARPES) data and we show that above the critical temperature, such procedure implies a transfer of the spectral weight from the Bogoliubov-type quasiparticles towards the in-gap damped states. We also discuss some possible microscopic arguments explaining this process.Comment: 11 pages, 7 figure