Spin transport in the Neel and collinear antiferromagnetic phase of the   two dimensional spatial and spin anisotropic Heisenberg model on a square   lattice
  We analyze and compare the effect of spatial and spin anisotropy on spin conductivity in a two dimensional S=1/2 Heisenberg quantum magnet on a square lattice. We explore the model in both the Neel antiferromagnetic (AF) phase and the collinear antiferromagnetic (CAF) phase. We find that in contrast to the effects of spin anisotropy in the Heisenberg model, spatial anisotropy in the AF phase does not suppress the zero temperature regular part of the spin conductivity in the zero frequency limit - rather it enhances it. We also explore the finite temperature effects on the Drude weight in the AF phase for various spatial and spin anisotropy parameters. We find that the Drude weight goes to zero as the temperature approaches zero. At finite temperatures (within the collision less approximation) enhancing spatial anisotropy increases the Drude weight value and increasing spin anisotropy decreases the Drude weight value. In the CAF phase (within the non-interacting approximation) the zero frequency spin conductivity has a finite value for non-zero values of the spatial anisotropy parameter. In the CAF phase increasing the spatial anisotropy parameter suppresses the regular part of the spin conductivity response at zero frequency. Furthermore, we find that the CAF phase displays a spike in the spin conductivity not seen in the AF phase. Inclusion of the smallest amount of spin anisotropy causes a gap to develop in the spin conductivity response of both the AF and CAF phase. Based on these studies we conclude that materials with spatial anisotropy are better spin conductors than those with spin anisotropy both at zero and finite temperatures. We utilize exchange parameter ratios for real material systems as inputs to the computation of spin conductivity. 
Zewei Chen, Trinanjan Datta, Dao-Xin Yao
cond-mat.str-el cond-mat.mtrl-sci