Implementation of the concept of spin polaron in cuprate superconductors within the diagram technique

The spectral properties of an ensemble of spin-polaron quasiparticles have been studied within the spin–fermion model of cuprate superconductors using the method combining the Feynman diagram technique and the diagram technique for spin operators. It has been shown that strong spin–charge coupling results in the formation of the lower spin-polaron band separated by a wide energy gap from the band of bare holes. It has been shown that the spin-polaron band has a local minimum near the $(\pi/2, \pi/2)$ point of the Brillouin zone. A class of diagrams for the self-energy part that have a fundamental significance for the description of the main features of the spin-polaron spectrum has been determined.