Diffusion of Frenkel excitons in anisotropic crystals

The temperature dependence of the diffusion tensor D ik (T) of coherent excitons in anisotropic molecular crystals is considered. The influence of such exciton energetic band peculiarities as effective mass tensor anisotropy and nonanalyticity of exciton energy on the dependence D ik (T) is analyzed. It is shown that the dependence D ∼ T −1/2 , which is characteristic of isotropic crystals, is retained only at weak anisotropy of the effective mass tensor m max /m min ≈ 1 with no regard for nonanalyticity. At the same time, at great values of the nonanalytic part of the exciton energy the diffusion tensor component dependences vary from D aa ≈ D cc ∼ T −1 ≪ D bb ∼ T −2 at weak anisotropy of the effective mass tensor to D aa ≈ D cc ≈ const ≪ D bb ∼ T −3/4 at strong anisotropy.