Anisotropy of the hole drift mobility in barium titanate

The static dielectric tensor is highly anisotropic in the tetragonal phase of barium titanate: the static dielectric constant perpendicular to the c axis, epsilon(perpendicular to), is 40 times that parallel to the c axis, epsilon(parallel to), at T=20 degrees C, with this ratio decreasing to similar to 20 times at T=90 degrees C. Using both holographic and bulk measurements we show that the hole drift mobility has comparable anisotropy: the hole mobility perpendicular to the c axis, mu(perpendicular to), is 20 times larger than that parallel to the c axis, mu(parallel to), at T=20 degrees C, with this ratio decreasing to 8 times at T=90 degrees C. We also show that mu(parallel to)/mu(perpendicular to)similar to 2 epsilon(parallel to)/epsilon(perpendicular to) independent of temperature between T=20 degrees C and T=90 degrees C, suggesting that the hole drift mobility in barium titanate crystals scales with the static dielectric constant in this temperature range.