The Green Function Variational Approximation: Significance of Physical Constraints |

K. Bieniasz
^{a,b,c}, M. Berciu^{b,c} and A.M. Oleś^{a,d}^{a}Marian Smoluchowski Institute of Physics, Jagiellonian University, S. Łojasiewicza 11, PL-30-348 Kraków, Poland
^{b}Department of Physics and Astronomy, University of British Columbia, Vancouver BC, Canada V6T 1Z1
^{c}Stewart Blusson Quantum Matter Institute, University of British Columbia, Vancouver BC, Canada V6T 1Z4
^{d}Max-Planck-Institut für Festkörperforschung, Heisenbergstr. 1, D-70569 Stuttgart, Germany |

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We present a calculation of the spectral properties of a single charge doped at a Cu(3d) site of the Cu-F plane in KCuF_{3}. The problem is treated by generating the equations of motion for the Green function by means of subsequent Dyson expansions and solving the resulting set of equations. This method, dubbed the variational approximation, is both very dependable and flexible, since it is a systematic expansion with precise control over elementary physical processes. It allows for deep insight into the underlying physics of polaron formation as well as for inclusion of many physical constraints, such as excluding crossing diagrams and double occupation constraint, which are not included in the self-consistent Born approximation. Here we examine the role and importance of such constraints by analyzing various spectral functions obtained in second order variational approximation. |

DOI: 10.12693/APhysPolA.130.659 PACS numbers: 75.25.Dk, 03.65.Ud, 75.10.Lp, 79.60.-i |