Anisotropy of the hole drift mobility in barium titanate

Abstract

The static dielectric tensor is highly anisotropic in the tetragonal phase of barium titanate: the static dielectric constant perpendicular to the c axis, ${\mathrm{\ensuremath{\varepsilon}}}_{\mathrm{\ensuremath{\perp}}}$, is 40 times that parallel to the c axis, ${\mathrm{\ensuremath{\varepsilon}}}_{\mathrm{\ensuremath{\parallel}}}$, at T=20 \ifmmode^\circ\else\textdegree\fi{}C, with this ratio decreasing to \ensuremath{\sim}20 times at T=90 \ifmmode^\circ\else\textdegree\fi{}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, ${\mathrm{\ensuremath{\mu}}}_{\mathrm{\ensuremath{\perp}}}$, is 20 times larger than that parallel to the c axis, ${\mathrm{\ensuremath{\mu}}}_{\mathrm{\ensuremath{\parallel}}}$, at T=20 \ifmmode^\circ\else\textdegree\fi{}C, with this ratio decreasing to 8 times at T=90 \ifmmode^\circ\else\textdegree\fi{}C. We also show that ${\mathrm{\ensuremath{\mu}}}_{\mathrm{\ensuremath{\parallel}}}$/${\mathrm{\ensuremath{\mu}}}_{\mathrm{\ensuremath{\perp}}}$\ensuremath{\sim}2${\mathrm{\ensuremath{\varepsilon}}}_{\mathrm{\ensuremath{\parallel}}}$/${\mathrm{\ensuremath{\varepsilon}}}_{\mathrm{\ensuremath{\perp}}}$ independent of temperature between T=20 \ifmmode^\circ\else\textdegree\fi{}C and T=90 \ifmmode^\circ\else\textdegree\fi{}C, suggesting that the hole drift mobility in barium titanate crystals scales with the static dielectric constant in this temperature range. \textcopyright{} 1996 The American Physical Society.