Reduction of thermal conductivity in ferroelectric SrTiO3 thin films

Bulk ${\mathrm{SrTiO}}_{3}$ is a quantum paraelectric in which an antiferrodistortive distortion below $\ensuremath{\approx}105$ K and quantum fluctuations at low temperature preclude the stabilization of a long-range ferroelectric state. However, biaxial mechanical stress, impurity doping, and Sr nonstoichiometry, among other mechanisms, are able to stabilize a ferroelectric or relaxor ferroelectric state at room temperature, which develops into a longer-range ferroelectric state below 250 K. In this paper, we show that epitaxial ${\mathrm{SrTiO}}_{3}$ thin films grown under tensile strain on ${\mathrm{DyScO}}_{3}$ exhibit a large reduction of thermal conductivity, of $\ensuremath{\approx}60%$ at room temperature, with respect to identical strain-free or compressed films. The thermal conductivity shows a further reduction below 250 K, a temperature concurrent with the peak in the dielectric constant [J. H. Haeni et al., Nature (London) 430, 758 (2004)]. These results suggest that strain gradients in the relaxor and ferroelectric phase of ${\mathrm{SrTiO}}_{3}$ are very effective phonon scatterers, limiting the thermal transport in this material.