A. A. Vladimirov

Dynamic spin susceptibility in the t-J model

The dynamic spin susceptibility is calculated for the t-J model in the paramagnetic phase using the memory function method in terms of the Hubbard operators. A self-consistent system of equations for the memory function is obtained in the mode-coupling approximation. Both itinerant hole excitations and localized spin fluctuations contribute to the memory function. The spin dynamics have a diffusive character in the hydrodynamic limit; spin-wave-like excitations are regained in the high-frequency region.

Dynamic spin susceptibility of superconducting cuprates: A microscopic theory of the magnetic resonance mode

A microscopic theory of the dynamic spin susceptibility (DSS) in the superconducting state within the t-J model is presented. It is based on an exact representation for the DSS obtained by applying the Mori-type projection technique for the relaxation function in terms of Hubbard operators. The static spin susceptibility is evaluated by a sum-rule-conserving generalized mean-field approximation, while the self-energy is calculated in the mode-coupling approximation. The spectrum of spin excitations is studied in the underdoped and optimally doped regions. The DSS reveals a resonance mode (RM) at the antiferromagnetic wave vector Q = \pi(1,1) at low temperatures due to a strong suppression of the damping of spin excitations. This is explained by an involvement of spin excitations in the decay process besides the particle-hole continuum usually considered in random-phase-type approximations. The spin gap in the spin-excitation spectrum at Q plays a dominant role in limiting the decay in comparison with the superconducting gap which results in the observation of the RM even above

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

T_c

Magnetic order and spin excitations in layered Heisenberg antiferromagnets with compass-model anisotropies

in the underdoped region. A good agreement with inelastic neutron-scattering experiments on the RM in YBCO compounds is found.

Spin excitations and thermodynamics of the antiferromagnetic Heisenberg model on the layered honeycomb lattice

A microscopic theory of the electrical conductivity

Spin excitations and thermodynamics of the t-J model on the honeycomb lattice

\sigma(\omega)

Magnetic order and spin excitations in the Kitaev–Heisenberg model on a honeycomb lattice

within the t-J model is developed. An exact representation for

Dynamical Spin Susceptibility in the t-J Model: The Memory Function Method

\sigma(\omega)

Magnetic susceptibility and short-range order in iron pnictides: Anisotropic J1-J2 Heisenberg model

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.

STATIC SPIN SUSCEPTIBILITY IN THE t-J MODEL

The spin-wave excitation spectrum, magnetization, and Néel temperature for the quasi-two-dimensional spin-1/2 antiferromagnetic Heisenberg model with the compass-model interaction in the plane proposed for iridates are calculated in the random phase approximation. The spin-wave spectrum agrees well with data of Lanczos diagonalization. We find that the Néel temperature is enhanced by the compass-model interaction and is close to the experimental value for Ba2IrO4.