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538.945 (13) 
Condensed matter physics. Solid state physics (225) 
SM ISO690:2012 DOHOTARU, Leonid. A diagram approach to the strong coupling in the twofolddegenerate Anderson impurity model. In: Materials Science and Condensed Matter Physics. Ediția a 9a, 2528 septembrie 2018, Chișinău. Chișinău, Republica Moldova: Institutul de Fizică Aplicată, 2018, p. 86. 
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Materials Science and Condensed Matter Physics Ediția a 9a, 2018 

Conferința "International Conference on Materials Science and Condensed Matter Physics" Chișinău, Moldova, 2528 septembrie 2018  


CZU: 538.945  
Pag. 8686  


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It is well known that orbital degeneracy plays an essential role in the Mott metalinsulator transition. Here, we study the role of the Hundrule coupling in an orbitally degenerate model using a diagram approach and taking the intraatomic Coulomb interactions of two electrons with opposite spins occupying the same or different orbitals into account on an equal footing with the intraatomic exchange. The investigations are based on our diagram theory for strongly correlated electron systems previously developed for the nondegenerate [18] and twofold degenerate [9] models. Our approximation includes only local selfenergy terms. It is well known that such an approximation is well justified for a large coordination number. The nonlocal terms [10] corresponding to a higherorder approximation in the inverse coordination number are neglected here. We first determined all the energy eigenfunctions and eigenvalues of the localized delectron part of the Hamiltonian. We obtained their dependence on the intra and interorbital Coulomb interactions and on the Hundrule coupling constant. We developed the perturbation theory around the atomic limit and determine the Matsubara Green‘s functions in the normal state. We obtained a Dysontype equation for these functions and discussed its analytical solutions in detail. Because the main elements of our diagram technique are the irreducible Green‘s functions, we calculated the simplest twoparticle irreducible Green‘s function and determined its dependence on the spin and orbital quantum numbers. This quantity, found only in the lowtemperature limit, was approximated by taking contributions of statistical weight into account assuming that the ground state of our system is the twoparticle triplet state . Knowing this quantity and summing a specific class of diagrams, we obtained the correlation function. We found two solutions for the renormalized Green‘s functions of the electrons and determined their spectral weight. We proved that orbital degeneracy gives an additional contribution to the ―metallization‖ of the impurity states, i.e. enhances the transfer of spectral weight to the Fermi level. 

