Doping-induced ferroelectric phase transition in strontium titanate: Observation of birefringence and coherent phonons under ultraviolet illumination

The doping-induced ferroelectric phase transition in Ca-doped ${\text{SrTiO}}_3$ is investigated by observing the birefringence and coherent phonons. The structural phase-transition temperature is determined by the birefringence measurement. Coherent phonons of the soft modes are studied by using ultrafast polarization spectroscopy. Two phonon modes are observed to be softened toward the ferroelectric phase-transition point at 28 K and other two phonon modes are observed to be softened toward the structural phase-transition point at 180 K. Another structural deformation at 25 K is found below the ferroelectric phase-transition temperature. From the temperature dependence of phonon frequencies under an ultraviolet (UV) illumination, a shift of the ferroelectric phase-transition point toward the lower temperature side is found. A decrease in phonon frequencies after the UV illumination is also found.

Figure 1

Temperature dependences of the change in birefringence between 4.5 and 250 K in ${\text{SrTiO}}_3$ and Ca-doped ${\text{SrTiO}}_3$. The polarization plane of the probe light for ${\text{SrTiO}}_3$ is along the [110] axis (STO [110], solid circles). That for Ca-doped ${\text{SrTiO}}_3$ is along the [110] axis (SCTO [110], solid squares) and along the [100] axis (SCTO [100], open triangles).

Figure 2

(a) Coherent phonon signals in Ca-doped ${\text{SrTiO}}_3$ at 50 K observed for the $0°$, $25°$, and $45°$ pumping, where the angle between the [100] axis of the crystal and the polarization direction of the pump pulse is changed. (b) Fourier transform of the coherent phonon signals in (a).

Figure 3

Temperature dependence of the coherent phonon signal in Ca-doped ${\text{SrTiO}}_3$ observed for the $45°$ pumping.

Figure 4

(a) Temperature dependence of the coherent phonon signal in Ca-doped ${\text{SrTiO}}_3$ observed for the $25°$ pumping below the ferroelectric phase-transition temperature $T_{c2}$. (b) Fourier transform of the coherent phonon signals in (a).

Figure 5

(a) Temperature dependence of the coherent phonon signal in Ca-doped ${\text{SrTiO}}_3$ observed for the $25°$ pumping above the ferroelectric phase-transition temperature $T_{c2}$. (b) Fourier transform of the coherent phonon signals in (a).

Figure 6

Temperature dependence of the phonon frequencies obtained from the coherent phonon signals below $T_{c1}$. The solid circles are the oscillation frequency for the $A_{1g}$ mode and the solid squares are that for the $E_g$ mode. The broken curves describe a temperature dependence of the form $\omega \propto {\left(T_c-T\right)}^n$, where $T_c=180\text{K}$ and $n=0.5$ for both modes. The open triangles are that for modes 1, 2, and 3 related to the ferroelectric phase transition.

Figure 7

Temperature dependence of the phonon frequencies (a) under dark (1, 2, 3) and under UV $\left(1^{\prime },2^{\prime },3^{\prime }\right)$ illumination below $T_{c2}$, which correspond the phonon modes related to the ferroelectric phase transition, and (b) under dark (1, 2, 3) and after UV $\left(1^{″},2^{″},3^{″}\right)$ illumination, where the intensity of the UV illumination is $7\text{mW}/{\text{mm}}^2$.