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Experimental data on the spin susceptibility of HTSC cuprates are reproduced on the basis of a spherically symmetric approach in the frustrated Heisenberg model. The inclusion of real and imaginary renormalizations in spin Green’s functions makes it possible to explain the evolution of spin excitation spectrum ω( q) and susceptibility spectrum χ( q, ω) in the range from insulator to optimal doping. In the low-frustration limit corresponding to the weakly doped mode, the saddle singularity of ω( q) and scaling of χ2D(ω) =∫ d q Im χ( q, ω) are reproduced and an analytic expression is derived for the scaling function. In the strong frustration (optimal doping) mode, the stripe scenario is demonstrated; this leads to a peak of χ2D (ω) in the region of ω∼60 meV. [ABSTRACT FROM AUTHOR]

Abstract: A theory of the dynamical spin susceptibility for 2D quantum frustrated Heisenberg model is developed to treat the experiment on magnetic response scaling. The approach is based on a spherically-symmetrical self-consistent scheme for the Green''s function, which allows to determine the explicit form of the spin excitation spectrum. It is shown, that scaling can be explained when the temperature dependence of the excitations damping is taken into account. Damping is deduced form the formally exact expression for the Green''s function. Its temperature dependence is determined by the best fit for the scaling. [Copyright &y& Elsevier]

The conductivity and the Hall coefficient of a doped 2D antiferromagnet in the normal state are considered using the Kondo lattice model in the multimoment approximation. The anomalous temperature dependence of the kinetic coefficients is explained by the strong anisotropic charge-carrier scattering from the spin subsystem and found to be in qualitative agreement with the experimental data for the normal state of high-Tc superconductors. [ABSTRACT FROM AUTHOR]

For the one-dimensional t–J model the optical conductivity σ(ω,T) is examined in the limit of small ω and T. The treatment is based on string operators appropriate for description of spin–charge separation. In the limit of small hole doping the inverse relaxation time Γ(ω,T) is proportional to ω2ln&z.sfnc;max(ω,T)/ω&z.sfnc;. Such a behaviour of Γ(ω,T) manifests the presence of a finite Drude weight D(T) at T>0. [Copyright &y& Elsevier]

Using the spherically symmetric self-consistent Green's function method, we consider thermodynamic properties of the S = 1 / 2 J 1 – J 2 Heisenberg model on the 2D square lattice. We calculate the temperature dependence of the spin–spin correlation functions c r = 〈 S 0 z S r z 〉 , the gaps in the spin excitation spectrum, the energy E and the heat capacity C V for the whole J 1 – J 2 -circle, i.e. for arbitrary φ , J 1 = cos ( φ ) , J 2 = sin ( φ ) . Due to low dimension there is no long-range order at T ≠ 0 , but the short-range holds the memory of the parent zero-temperature ordered phase (antiferromagnetic, stripe or ferromagnetic). E ( φ ) and C V ( φ ) demonstrate extrema “above” the long-range ordered phases and in the regions of rapid short-range rearranging. Tracts of c r ( φ ) lines have several nodes leading to nonmonotonic c r ( T ) dependence. For any fixed φ the heat capacity C V ( T ) always has maximum, tending to zero at T → 0 , in the narrow vicinity of φ = 155 ° it exhibits an additional frustration-induced low-temperature maximum. We have also found the nonmonotonic behaviour of the spin gaps at φ = 270 ° ± 0 and exponentially small antiferromagnetic gap up to ( T ≲ 0.5 ) for φ ≳ 270 ° . [ABSTRACT FROM AUTHOR]