Temperature dependence of Photoinduced dynamics in the orbital-ordered state of $A$V$_{10}$O$_{15}$ ($A$ = Ba, Sr)

In $A$V$_{10}$O$_{15}$ ($A$ = Ba, Sr), the V ions with mixed-valence states, V$^{2+}$/V$^{3+}$ ($3d^{3}$/$3d^{2}$), are located on the triangular lattice. BaV$_{10}$O$_{15}$ exhibits a structural phase transition with V trimerization caused by the orbital ordering of V ions at $T_{c}$ = 123 K, whereas SrV$_{10}$O$_{15}$ does not exhibit such a phase transition. We performed a femtosecond pump-probe reflection spectroscopy on BaV$_{10}$O$_{15}$ and SrV$_{10}$O$_{15}$ to clarify their photoinduced dynamics. For $A$ = Ba, a photoinduced melting of V trimerization, i.e. a photoinduced phase transition, was observed at 10 K ($< T_{c}$). At $T$ = 200 K ($>> T_{c}$), the photoinduced reflectivity change ($\Delta R/R$) for $A$ = Ba shows an oscillation with the period of several tens picoseconds, similarly to the behavior for $A$ = Sr at 10 K. This oscillation can be explained by assuming that the photoinduced state at the sample surface propagates into the inside of the sample. At $T$ = 135 K, immediately above $T_{c}$, we found that $\mid\Delta R/R\mid$ for BaV$_{10}$O$_{15}$ increases with time, suggesting that the area of the photoinduced state on the sample surface increases with time.