Photoinduced phase transitions over three phases in ${\mathbf{Ba}}_{0.9}{\mathbf{Sr}}_{0.1}{\mathbf{V}}_{13}{\mathbf{O}}_{18}$

${\mathrm{Ba}}_{0.9}{\mathrm{Sr}}_{0.1}{\mathrm{V}}_{13}{\mathrm{O}}_{18}$ exhibits two ordered phases caused by the charge/orbital ordering of the $d$ electrons on the V ions. We have succeeded in observing photoinduced phase transitions over three phases in this compound: a transition from the low-temperature phase to the high-temperature phase within 0.3 ps, followed by a transition to the intermediate-temperature phase within 2 ps. The relationship between the symmetry at each of the three phases plays a key role in the dynamics of the photoinduced phase transitions in such a compound.

Figure 1

(a) Schematic diagram of the crystal structure for ${\mathrm{Ba}}_{1-x}{\mathrm{Sr}}_x{\mathrm{V}}_{13}{\mathrm{O}}_{18}$ at each of the three phases, the high-temperature (HT), intermediate-temperature (IT), and low-temperature (LT) phases, and the orbital ordering in the V tetramer (upper left) and V trimer (lower left). The upper-right panel shows the resistivity and the magnetic susceptibility for ${\mathrm{Ba}}_{0.9}{\mathrm{Sr}}_{0.1}{\mathrm{V}}_{13}{\mathrm{O}}_{18}$. (b) Reflectivity spectrum at each phase for ${\mathrm{Ba}}_{0.9}{\mathrm{Sr}}_{0.1}{\mathrm{V}}_{13}{\mathrm{O}}_{18}$. (c) Optical conductivity spectrum at each phase for ${\mathrm{Ba}}_{0.9}{\mathrm{Sr}}_{0.1}{\mathrm{V}}_{13}{\mathrm{O}}_{18}$.

Figure 2

(a) Photoinduced change in the reflectivity spectra 0.3 ps, 2 ps, and 40 ps after a pump pulse is applied to ${\mathrm{Ba}}_{0.9}{\mathrm{Sr}}_{0.1}{\mathrm{V}}_{13}{\mathrm{O}}_{18}$ at 50 K (in the LT phase) (b) Photoinduced change in reflectivity as a function of the delay time up to 4 ps at various values of $\hslash \omega$ for a probe pulse at 50 K. (c) Difference spectra in reflectivity with temperature for ${\mathrm{Ba}}_{0.9}{\mathrm{Sr}}_{0.1}{\mathrm{V}}_{13}{\mathrm{O}}_{18}$. (d) Photoinduced change in reflectivity up to 40 ps.

Figure 3

(a) Photoinduced change in the reflectivity spectra 0.3 ps, 2 ps, and 40 ps after a pump pulse is applied to ${\mathrm{Ba}}_{0.9}{\mathrm{Sr}}_{0.1}{\mathrm{V}}_{13}{\mathrm{O}}_{18}$ at 100 K (in the IT phase) for. (b) Photoinduced change in reflectivity as a function of the delay time up to 4 ps at various values of $\hslash \omega$ for the probe laser at 100 K. (c) Schematic of the process of the photoinduced phase transitions in ${\mathrm{Ba}}_{1-x}{\mathrm{Sr}}_x{\mathrm{V}}_{13}{\mathrm{O}}_{18}$.

Figure 4

(a) Photoinduced change in reflectivity as a function of the delay time at $\hslash \omega =1.2$ eV at 50 K for various intensities of a pump pulse. The circles are the experimental data and the solid lines are the fitting curves (see text.) (b) Dependence of $A$ (immediate response) and $B$ (delayed response) on the pump pulse intensity. The inset shows a schematic functional form for the fitting (without the convolution). [(c) and (d)] Photoinduced change in the reflectivity spectra for various intensities of a pump pulse at (c) 0.3 ps and (d) 40 ps.