35 research outputs found
A diagram approach to the strong coupling in the single-impurity Anderson model
No full textAccess full text – https://doi.org/10.1007/s11232-008-0077-9We propose a diagram theory around the atomic limit for the single-impurity Anderson model in which the strongly correlated impurity electrons hybridize with free (uncorrelated) conduction electrons. Using this diagram approach, we prove a linked-cluster theorem for the vacuum diagrams and derive Dyson-type equations for localized and conduction electrons and the corresponding equations for mixed propagators. The system of equations can be closed by summing an infinite series of ladder diagrams containing irreducible Green’s functions. The result allows discussing resonances associated with quantum transitions at the impurity site
Many-cloud phonon propogators in the case of acoustical phonons
No full textIn the High Temperature Superconductors we consider the system of correlated electrons which interect with phonons. In the theory of such sistems an effective way to describe correlation effects of electrons is based on the use of Green’s functions metod. A new diagram technique for the Green’s functions of the strong correlated electrons was proposed by V. A. Moskalenko et al. in. This technique involves simpler creation and annihilation operators for electrons at all intermediate stages of the theory and Hubbard operators only when evaluating final expressions. In the case when the electrons interect whith the acoustical phonons the new elements of the diagram technique appear. The aim of the present paper is to propose the mode how to calculate the such new elements of the diagram technique as the phonon-cloud propagators
Diagrammatic approach for the twofold degenerate Anderson impurity model
No full textThe twofold degenerate Anderson impurity model is investigated in normal and superconducting states, and the strong electronic correlations of d-electrons of impurity ion are taken into account by elaborating a suitable diagram technique. We discuss the properties of the SlaterKanamori model of d-impurity electrons. After finding the eigenfunctions and eigenvalues of all 16 local states, we determine the local one-particle propagator. Then we construct the perturbation theory around the atomic limit of the impurity ion and obtain the Dyson type equations between impurity electron propagators and normal and anomalous correlation functions. By summing infinite series of ladder diagrams, the approximation for correlation functions is established. The criterion for appearance of a superconducting state of the model is discussed
Diagram theory for the twofold-degenerate Anderson impurity model
No full textAccess full text – https://doi.org/10.1007/s11232-014-0133-6We develop a diagram technique for investigating the twofold-degenerate Anderson impurity model in the normal state with the strong electronic correlations of d electrons of the impurity ion taken into account. We discuss the properties of the Slater-Kanamori model of d electrons. After finding the eigenfunctions and eigenvalues of all 16 local states, we determine the local one-particle propagator. We construct the perturbation theory around the atomic limit of the impurity ion and obtain a Dyson-type equation establishing the relation between the impurity electron propagator and the normal correlation function. As a result of summing infinite series of ladder diagrams, we obtain an approximation for the correlation function
The theory of nonequilibrium Anderson impurity model for strongly correlated electron systems
No full textAccess full text – https://doi.org/ https://doi.org/10.1063/1.4919378The nonequilibrium theory of strongly correlated systems is proposed theory which is grounded on the generalized Wick theorem. This theorem is employed for calculation of the thermal averages of the contour arranged products of electron operators by generalizing Keldysh formalism. Perturbation expansion is realized for Anderson impurity model in which we consider the Coulomb interaction of the impurity electrons as a main parameter of the model and the mixing interaction between impurity and conduction electrons as a perturbation. The first two approximations are used and is obtained the value of the current between one of the leads and central region of interacting electrons. The contribution of the strong correlations and of irreducible diagrams is analyzed
Diagrammatic theory for twofold degenerate Anderson impurity model
No full textSursa: Conferința –"Materials Science and Condensed Matter Physics",
Chișinău, Moldova, 11-14 septembrie 2012.→ https://ibn.idsi.md/collection_view/483We study the influence of the intra-atomic Coulomb interactions of the two electrons with opposite spins situated on the same or different orbitals and intra-atomic exchange is analyzed. We discuss the properties of the Slater-Kanamori model [1-3] of d-impurity electrons. After finding the eigenfunctions and eigenvalues of all 16 local states, we determine the local one-particle propagator. We have formulated the diagram theory for the two orbital degenerate Anderson impurity model by generalizing the perturbating theory formulated for strongly correlated non degenerated electronic systems. Perturbation theory was formulated around atomic limit of impurity localized electrons. We determined the correlation functions as a sum of strongly connected irreducible diagrams and have formulated Dyson type equation for the full propagators of conduction and impurity electrons. Special diagrammatic approximation was discussed and summation of diagram has been considered
Theory of high-temperature superconductivity in a two-layer system: FeSe/SrTiO3
No full textThe proposed model can qualitatively describe the occurrence of high-temperature superconductivity in FeSe/SrTiO3. The Hamiltonian of a two-layer system is reduced to a twoband Hamiltonian, taking into account all possible pairing of electrons inside one energy band and electrons in the different energy bands. The superconductivity mechanism can be both phononic and electronic. The main factor is the presence of a large number of constants of the electron–electron interactions that contribute to the formation of Cooper pairs and also the possibility of increasing the ratio of densities of electronic energy bands N2/N1 by doping the systems or in some other way. A method for experimental verification of the dominance of interband interactions over intraband interactions is proposed
