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ONIC PROPERTIES OF SOLID

On Superconductivity of HighSpin Transition Metal Compounds R. O. Zaitsev Moscow Institute of Physics and Technology (State University), Dolgoprudnyi, Moscow oblast, 141700 Russia email: [email protected] Received July 21, 2010; in final form, January 21, 2011

Abstract—The possibility of Cooper instability in transition metal compounds is established based of the concept of the strong interaction in the same unit cell. The multicomponent scattering amplitude of excita tions is calculated. The superconductivity equations are derived for compounds of 3d transition metals. It is shown that in the pole approximation, the superconductivity equations can be reduced to the multicompo nent superconductivity equations with preset BCS constants. A method is developed for calculating one orbital constants and constants with different orbitals as functions of the total spin. The concentration ranges of superconducting ordering are obtained for oneorbital equations. DOI: 10.1134/S1063776111130152

2. HAMILTONIAN AND ZERO GREEN FUNCTION

1. INTRODUCTION After the discovery of hightemperature supercon ductivity in copper compounds [1] and many years later in iron compounds [2], it has become clear that this phenomenon must also exists in compounds of all other 3d transition elements. A general property of electrons in an underfilled shell of a transition element is the strong intraatomic electron–electron interac tion attaining a value of 10 eV [3]. The computation of the scattering amplitude for electron excitations in the lattice with allowance of the strong shortrange repul sion indicates that beginning from a certain value of relative energy, the scattering amplitude may change its sign. This circumstance has made it possible to explain the emergence and existence of hightemper ature superconductivity in a finite interval of the hole and electron concentrations [4–6]. In LaFeAsO1 – xFx and αFe(Te,Se) compounds exhibiting anomalous superconductivity, the same regularities as in cuprates are observed: a high super conducting transition temperature (Tc), anomalous isotopic effect, and the strong dependence of Tc on the degree of underfilling of the lower Hubbard subband. For this reason, the superconductivity theory will be constructed here using the same Hubbard model, but with allowance for the possible dependence of the ini tial Hamiltonian on the quantum numbers character izing the lowest highspin state. We will consider sequential filling of the 3d hole shell taking into account the strong electron–electron intraatomic interaction assuming that its value is the largest energy parameters.

The Hamiltonian of the system considered here can be written in terms of the creation and annihila tion operators; in the simplest case of transitions to the nearest neighbors, the Hamiltonian has the form

∑

ˆ = H

† aˆ i, σ ( r 1 )aˆ k, σ ( r 2 )t i, k ( r 1 – r 2 )

i, k, r 1, r 2, σ

–μ

(1)

∑ aˆ

† ˆ k, σ ( r )a λ, σ ( r ).

k, r, σ

We will henceforth assume that hopping integral t