Electronic spectrum and superconductivity in the tt-JJ model on the honeycomb lattice

A microscopic theory of electronic spectrum and superconductivity within the $t$-$J$ model on the honeycomb lattice is formulated. The Dyson equation for the normal and anomalous Green functions for the two-band model in terms of the Hubbard operators is derived by applying the Mori-type projection technique. The self-energy is evaluated in the self-consistent Born approximation for electron scattering on spin and charge fluctuations induced by the kinematical interaction for the Hubbard operators. Superconducting pairing mediated by the antiferromagnetic exchange and spin fluctuations is discussed.Comment: 11 pages, 2 figures. arXiv admin note: text overlap with arXiv:1803.0314

Superconductivity in the t-J model

A comparison of microscopic theories of superconductivity in the limit of strong electron correlations is presented. We consider results for the two-dimensional t-J model obtained within the projection technique for the Green functions in terms of the Hubbard operators and the slave-fermion representation for the RVB state. It is argued that the latter approach resulting in the odd-symmetry p-wave pairing for fermions is inadequate.Comment: 11 pages, RevTex, 1 figure, to appear in Condensed Matter Physics v.5, No.4 (2002)(Lviv, Ukraine) v.2: corrected typo

Spin fluctuations and high-temperature superconductivity in cuprates

To describe the cuprate superconductors, models of strongly correlated electronic systems, such as the Hubbard or t-J models, are commonly employed. To study these models, projected (Hubbard) operators have to be used. Due to the unconventional commutation relations for the Hubbard operators, a specific kinematical interaction of electrons with spin and charge fluctuations emerges. The interaction is induced by the intraband hopping with a coupling parameter of the order of the kinetic energy of electrons W which is much larger than the antiferromagnetic exchange interaction J induced by the interband hopping. This review presents a consistent microscopic theory of spin excitations and superconductivity for cuprates where these interactions are taken into account within the Hubbard operator technique. The low-energy spin excitations are considered for the t-J model, while the electronic properties are studied using the two-subband extended Hubbard model where the intersite Coulomb repulsion V and electron-phonon interaction are taken into account.Comment: 26 pages, 33 figures, 130 references. arXiv admin note: text overlap with arXiv:1301.4347, arXiv:1402.493

Optical and dc conductivities of cuprates: Spin-fluctuation scattering in the t-J model

A microscopic theory of the electrical conductivity $\sigma(\omega)$ within the t-J model is developed. An exact representation for $\sigma(\omega)$ is obtained using the memory-function technique for the relaxation function in terms of the Hubbard operators, and the generalized Drude law is derived. The relaxation rate due to the decay of charge excitations into particle-hole pairs assisted by antiferromagnetic spin fluctuations is calculated in the mode-coupling approximation. Using results for the spectral function of spin excitations calculated previously, the relaxation rate and the optical and dc conductivities are calculated in a broad region of doping and temperatures. The reasonable agreement of the theory with experimental data for cuprates proves the important role of spin-fluctuation scattering in the charge dynamics.Comment: 13 pages,15 figures, v.2, publication referenc