Localized Excited Charge Carriers Generate Ultrafast Inhomogeneous Strain in the Multiferroic ${\mathrm{BiFeO}}_3$

We apply ultrafast x-ray diffraction with femtosecond temporal resolution to monitor the lattice dynamics in a thin film of multiferroic ${\mathrm{BiFeO}}_3$ after above-band-gap photoexcitation. The sound-velocity limited evolution of the observed lattice strains indicates a quasi-instantaneous photoinduced stress which decays on a nanosecond time scale. This stress exhibits an inhomogeneous spatial profile evidenced by the broadening of the Bragg peak. These new data require substantial modification of existing models of photogenerated stresses in ${\mathrm{BiFeO}}_3$: the relevant excited charge carriers must remain localized to be consistent with the data.

Figure 1

(a) Rocking curves of the 002 pseudocubic Bragg reflections from the BFO layer and STO substrate measured by the PXS. The solid black line is a simulation of the static rocking curve. (b) Measured transient rocking curves (diffracted intensity in logarithmic scale). The green dashed line indicates the extracted center of the BFO peak.

Figure 2

Shift $\mathrm{\Delta }{q}_z$ of the 002 pseudocubic BFO Bragg peak and corresponding average strain ${⟨\epsilon (t)⟩}_z$ measured as a function of fluence at different delays. The dashed and dotted lines represent linear fits.

Figure 3

The transient shift $\mathrm{\Delta }{q}_z$ and corresponding average strain ${⟨\epsilon (t)⟩}_z$ (a) as well as the relative change of the width $\mathrm{\Delta }w/w_0$ (b) of the 002 pseudocubic BFO Bragg peak. The APS data were measured at an absorbed fluence of $F=2.4\mathrm{mJ}/{\mathrm{cm}}^2$ and the PXS data was scaled according to the fluence calibration. The black solid line represents an exponential fit of the APS data.

Figure 4

The black circles in (b) show the transient shift of the 002 pseudocubic BFO Bragg peak extracted from the data shown in Fig. 1. The black solid line represent the simulated peak shift for an instantaneous stress. The simulated peak shift for a semi-instantaneous stress (gray dashed line) and a fully diffusivelike stress (gray dotted line) both with a time constant ${\tau }_{\mathrm{rise}}=2\mathrm{ps}$, see text. The corresponding stress amplitudes $A(t)$ are plotted in panel (a). The gray Gaussian at $t=0$ in panel (b) indicates the temporal resolution of 200 fs of the PXS setup.