Electric field dependence of the thermal conductivity of quantum paraelectrics

Thermal conductivity measurements have been carried out on single crystals of ${\mathrm{SrTiO}}_{3}$ and ${\mathrm{KTaO}}_{3}$ in the presence of electric fields applied either perpendicular or parallel to the heat current. In the case of ${\mathrm{SrTiO}}_{3},$ the response is surprisingly large: a 7 kV/cm parallel field increases the thermal conductivity by up to $400%.$ A pronounced anisotropy in both materials, which becomes apparent above nearly the same temperature, is also observed. These effects can be interpreted qualitatively using a simple model in which TA phonons in an energy band near the soft TO mode scatter off of either impurities, ferroelectric microregions, or quantum ferroelectric fluctuations.