Dynamic Tilting of Ferroelectric Domain Walls Caused by Optically Induced Electronic Screening

Optical excitation perturbs the balance of phenomena selecting the tilt orientation of domain walls within ferroelectric thin films. The high carrier density induced in a low-strain ${\mathrm{BaTiO}}_3$ thin film by an above-band-gap ultrafast optical pulse changes the tilt angle that $90°a/c$ domain walls form with respect to the substrate-film interface. The dynamics of the changes are apparent in time-resolved synchrotron x-ray scattering studies of the domain diffuse scattering. Tilting occurs at 298 K, a temperature at which the $a/b$ and $a/c$ domain phases coexist but is absent at 343 K in the better ordered single-phase $a/c$ regime. Phase coexistence at 298 K leads to increased domain-wall charge density, and thus a larger screening effect than in the single-phase regime. The screening mechanism points to new directions for the manipulation of nanoscale ferroelectricity.

Figure 1

(a) BTO thin film with arrangement and atomic structure of $a/c$ domain pattern. (b) Scattered x-ray intensity in the $Q_x-Q_z$ section of reciprocal space at $Q_y=0$. The BTO 002 reflection is at $Q_z=3.13{\mathrm{\AA }}^{-1}$. Intensity oscillations corresponding to the BTO thickness are along $Q_z$ at $Q_x=0$. The intensity distribution along $Q_x$ (inset) is obtained by integrating the intensity with respect to $Q_z$. The $\pm 1$ and $\pm 2$ orders of domain scattering appear at $Q_x=\pm 0.008{\mathrm{\AA }}^{-1}$ and $\pm 0.016{\mathrm{\AA }}^{-1}$. (c) Intensity profiles of $-1$ and $+1$ orders of domain scattering before optical excitation ($t<0$) and at $t=1\mathrm{ns}$ for $F_{\mathrm{abs}}=2.4\mu \mathrm{J}/{\mathrm{cm}}^2$. The maximum-intensity values of $Q_z$ are indicated with arrows at each time.

Figure 2

Time dependence of the intensities of the (a) $-1$, (b) $+1$, (c) $-2$, and (d) $+2$ orders of domain scattering as a function of $\mathrm{\Delta }{Q}_z/Q_z$ following optical excitation at $F_{\mathrm{abs}}=2.4\mu \mathrm{J}/{\mathrm{cm}}^2$. $\mathrm{\Delta }{Q}_z/Q_z=0$ corresponds to wave vectors of the intensity maxima before optical excitation. Intensities are normalized to values before optical excitation for each order of domain scattering.

Figure 3

Time dependence of fractional change of the wave vector $Q_z$ of $-1$ and $+1$ orders of domain scattering at (a) 298 K for $F_{\mathrm{abs}}=2.4\mu \mathrm{J}/{\mathrm{cm}}^2$ and (b) 343 K for $F_{\mathrm{abs}}=6.5\mu \mathrm{J}/{\mathrm{cm}}^2$. (c) $\mathrm{\Delta }\alpha (t=1\mathrm{ns})$ at $T=298\mathrm{K}$ and 343 K as a function of $F_{\mathrm{abs}}$.

Figure 4

Diffracted x-ray intensity distributions in the $Q_x-Q_y$ section at (a) 298 K and (b) 343 K. Intensity profiles of $-1$ and $+1$ orders of domain scattering along (c) $Q_x$ and (d) $Q_y$ at 298 and 343 K. The $Q_x-Q_y$ sections are obtained by integrating diffracted intensity distributions from $Q_z=3.105$ to $3.16{\mathrm{\AA }}^{-1}$.