Anisotropic exchange integrals in the two-leg spin ladderLaCuO2.5

The magnetic excitation in a two-leg $S=1/2\mathrm{}$ antiferromagnetic spin ladder is strongly dependent on the anisotropy in exchange integrals. The exchange integrals along the leg $J_{\Vert }$ and the rung $J_{\perp }$ were obtained separately by polarized micro-Raman scattering in single domains of polycrystalline ${\mathrm{LaCuO}}_{2.5}.$ The exchange integrals are $J_{\Vert }=1012\mathrm{}\hspace{0.5em}{\mathrm{cm}}^{-1}$ and $J_{\perp }=957\mathrm{}\hspace{0.5em}{\mathrm{cm}}^{-1}$ after correcting the quantum spin effects. The anisotropy is found to be small as $J_{\perp }{/J}_{\Vert }=\mathrm{0.95.}\mathrm{}$