Spin transport in the anisotropic easy-plane two-dimensional Heisenberg antiferromagnet

Spin transport in the easy-plane two-dimensional anisotropic Heisenberg antiferromagnet with S=1 is studied. Regular part of spin conductivity is calculated, at zero temperature, using a self-consistent harmonic approximation and the Kubo formalism. Three magnon processes provide the dominant contribution to the spin conductivity. Furthermore, the transport is ballistic and characterized by finite Drude weight.     

Spin flop in one-dimensional Heisenberg antiferromagnet KCuF3

Following Yamada and Kato [J. Phys. Soc. Japan 63 (1994) 289], we have calculated the frequency－field diagram of antiferromagnetic resonance of KCuF_3 using an eight-sublattice model and mean field approximation. A spin flop is found to take place from the [110]_p to a direction perpendicular to [110]_p in the c-plane at H\simeq 42172A/m, which is in good agreement with the data obtained by measuring magnetic susceptibility.     

Spin transport in a one-dimensional anisotropic Heisenberg model

We analytically and numerically study spin transport in a one-dimensional Heisenberg model in linear-response regime at infinite temperature. It is shown that as the anisotropy parameter Δ is varied spin transport changes from ballistic for Δ<1 to anomalous at the isotropic point Δ=1, to diffusive for finite Δ>1, ending up as a perfect isolator in the Ising limit of infinite Δ. Using perturbation theory for large Δ a quantitative prediction is made for the dependence of diffusion constant on Δ.     

Spin transport in the two-dimensional anisotropic XY model coupled to phonons

In this paper, we study spin currents, generated by a magnetic field gradient, in the two-dimensional XY model with single-ion anisotropy and coupled to phonons in the large D phase. The spin conductivity is calculated, at zero temperature, using the Kubo formula.     

Spin correlations near the surface of a three-dimensional Heisenberg antiferromagnet

Nearest-neighbor spin correlations are considered near the surface of a semi-infinite spin- Heisenberg antiferromagnet on a simple cubic lattice. In the spin-wave approximation, the excitation spectrum of this model involves bulk modes – standing spin waves and a quasi-two-dimensional mode of surface spin waves. These latter excitations eject the bulk excitations from the surface region thus dividing the antiferromagnet into two parts with different excitations. As a result absolute values of the spin correlations near the surface exceed the bulk value. In the surface region, the pattern of spin correlations resembles the comb structure recently obtained for the two-dimensional case.     

Spin pump effects on the spin current through two coupled quantum dots

We theoretically study the spin pump effects of the rotating magnetic field on the spin current through two coupled quantum dots. Owing to the interdot coupling, two molecular states with different bands can be formed, resulting asymmetric spin current peaks. The possibility of manipulating the spin current is explored by tuning the strength, the frequency, and the direction of the rotating magnetic field. The number and location of the spin current peaks can be controlled by making use of various tunings. Furthermore, the normal 2π period of the spin current with respect to the magnetic flux can be destroyed by the interdot coupling.     

Spin dynamics and spin correlations in the spinS=1 two-dimensional square-lattice Heisenberg antiferromagnet La2NiO4

The static and dynamic spin fluctuations in the spinS=1, two-dimensional (2D) square-lattice antiferromagnet La₂NiO₄ have been studied over a wide temperature range using neutron scattering techniques. The spin correlations in La₂NiO₄ exhibit a crossover from two- to three-dimensional (3D) behavior as the Néel temperature is approached from above. Critical slowing down of the low-energy spin fluctuations is also observed just aboveT _N . The correlation length, (T), and the static structure factor,S(0), have been measured and are compared with recent theoretical calculations for the quantum 2D Heisenberg antiferromagnet using microscopic parameters determined from previous spin-wave measurements. Good agreement for (T) is found with the exact low-temperature result of Hasenfratz and Niedermeyer provided that 2 p _s is renormalized by 20% from the spin-wave value.     

Elementary spin excitations in a Heisenberg S=1/2 antiferromagnet with Skyrmions

The elementary spin excitations in two-dimensional Heisenberg antiferromagnets with spin S=1/2 in a metastable, spatially inhomogeneous state are investigated. The energy spectrum of the excitations, the local order parameter, and the temperature dependence of the spin correlation length are found. It is shown that the results obtained can be used to explain the experimental data on neutron scattering in La₂CuO₄ at temperatures T>T _N. Fiz. Tverd. Tela (St. Petersburg) 39, 656–659 (April 1997)     

Spin correlations in the two-dimensional spin-5/2 Heisenberg antiferromagnet

We report a neutron scattering study of the instantaneous spin correlations in the two-dimensional spin S =5/2 square-lattice Heisenberg antiferromagnet Rb₂MnF₄. The measured correlation lengths are quantitatively described, with no adjustable parameters, by high-temperature series expansion results and by a theory based on the quantum self-consistent harmonic approximation. Conversely, we find that the data, which cover the range from about 1 to 50 lattice constants, are outside of the regime corresponding to renormalized classical behavior of the quantum non-linear model. In addition, we observe a crossover from Heisenberg to Ising critical behavior near the Néel temperature; this crossover is well described by a mean-field model with no adjustable parameters. Received: 3 March 1998 / Received in final form: 4 May 1998 / Accepted: 19 May 1998     

Residual entropy of the one-dimensional Ising antiferromagnet with general spin

We have calculated the ground-state degeneracy and the concominant residual entropy of the one-dimensional Ising antiferromagnet in a critical magnetic field. We demonstrate that the results obtained can be related to the ground-state properties of the q-state Potts antiferromagnet in the corresponding external field.