Temperature and electric field tuning of the ferroelectric soft mode in a strained ${\text{SrTiO}}_3/{\text{DyScO}}_3$ heterostructure

We studied THz dielectric spectra of ${\text{SrTiO}}_3/{\text{DyScO}}_3$ epitaxial multilayers with four 50-nm-thick layers of each compound deposited on ${\text{DyScO}}_3$ substrate as a function of temperature and electric field. At room temperature, ${\text{SrTiO}}_3$ is in the paraelectric phase and, due to the in-plane tensile strain within the films, the soft-mode frequency is shifted down to $47{\text{cm}}^{-1}$. The ferroelectric phase transition occurs in the strained ${\text{SrTiO}}_3$ layers around 270 K and in the ferroelectric phase the soft-mode hardening upon temperature decrease is observed. The soft mode is linearly coupled to an excitation of relaxation type at $10{\text{cm}}^{-1}$ at all temperatures. This relaxation is silent in the paraelectric phase, but it becomes coupled to the polarization below the ferroelectric transition temperature and its strength progressively increases. The recently proposed universal model of soft-mode behavior in strained ${\text{SrTiO}}_3$ films [C. Kadlec et al., J. Phys.: Condens. Matter 21, 115902 (2009)] was generalized to describe the observed spectra at all temperatures and applied fields.

Figure 1

(Color online) Complex permittivity spectra (without bias) of samples $4\times 50\text{A}$ (symbols) and $4\times 50\text{B}$ (lines) as a function of temperature.

Figure 2

(Color online) Complex THz spectra for sample $4\times 50\text{A}$ for several temperatures and bias field. Symbols: experimental data; lines: fit with Eq. (4).

Figure 3

Temperature dependence of the (a) soft-mode frequency and (b) relaxation strength for both samples studied. Inset: estimation of the temperature dependence of the spontaneous polarization.

Figure 4

(Color online) Soft-mode eigenfrequency as a function of the electric bias at several temperatures for sample $4\times 50\text{A}$. Symbols: experiment; lines: fit with Eq. (8), yielding $P_s$. Crosses represent an estimation after room-temperature measurements at 10 GHz of a 50-nm-thick STO/DSO in Ref. 12.