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Polaron States in a CuO Chain

100%
Bieniasz K., Oleś A.
Acta Physica Polonica A
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2015
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vol. 127
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issue 2
269-271
EN
We introduce a one-dimensional model for a CuO chain, with holes and S=1/2 spins localized in 3d_{x²-y²} orbitals, and p_σ oxygen orbitals without holes in the ground state. We consider a single hole doped at an oxygen site and study its propagation by spin-flip processes. We develop the Green function method and treat the hole-spin coupling in the self-consistent Born approximation, similar to that successfully used to study polarons in the regular t-J model. We present an analytical solution of the problem and investigate whether the numerical integration is a good approximation to this solution.
2
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Quantum versus Classical Polarons in a Ferromagnetic CuO_3-Like Chain

100%
Bieniasz K., Oleś A.
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vol. 126
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issue 4a
A-80-A-84
EN
We present an exact solution for an itinerant hole added into the oxygen orbitals of a CuO_{3}-like ferromagnetic chain. Using the Green function method, the quantum polarons obtained for the Heisenberg SU(2) interaction between localized Cu spins are compared with the polarons in the Ising chain. We find that magnons with large energy are favorable towards quasiparticle existence, even in the case of relatively modest electron-magnon coupling. We observe two quasiparticle states with dispersion ~2t each, which emerge from the incoherent continuum when the exchange coupling J increases. Quantum fluctuations in the spin system modify the incoherent part of the spectrum and change the spectral function qualitatively, beyond the bands derived from the perturbation theory.
3
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The Green Function Variational Approximation: Significance of Physical Constraints

81%
Bieniasz K., Berciu M., Oleś A.
Acta Physica Polonica A
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2016
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vol. 130
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issue 2
659-663
EN
We present a calculation of the spectral properties of a single charge doped at a Cu(3d) site of the Cu-F plane in KCuF₃. 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.
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