Field Effect on the Cochran Modes in SrTiO3and KTaO3

Abstract

In connection with the Cochran theory of ferroelectricity, the thermal conductivity has been investigated in SrTi${\mathrm{O}}_3$ between 5 and 300°K and in KTa${\mathrm{O}}_3$ between 2.5 and 300°K. For temperatures above its maximum value, the conductivity shows an anomaly in SrTi${\mathrm{O}}_3$ which is absent in KTa${\mathrm{O}}_3$. This behavior is explained in terms of the degeneracy between the transverse optic (Cochran modes) and the longitudinal acoustic phonon branches. Measurements were also taken in electric fields up to 55 kV/cm, applied perpendicular to the temperature gradient. A considerable increase in thermal conductivity is observed in SrTi${\mathrm{O}}_3$ below 50°K, while the effect is much smaller in KTa${\mathrm{O}}_3$. An interpretation of this effect is given by assuming the transverse optic mode at $q=0\mathrm{}$ to shift in frequency upon application of an electric field. An analysis of the experimental data in terms of a simple model is presented which leads to quantitative values for the frequency shifts in SrTi${\mathrm{O}}_3$. The results obtained for KTa${\mathrm{O}}_3$ indicate the presence of a phase transition of the nonferroelectric type at 4.5°K, in analogy to the 103°K transition known for SrTi${\mathrm{O}}_3$.