Charge-gap formation in the insulating states ofAV10O15(A= Ba, Sr)

We measured the optical conductivity spectra of SrV${}_{10}$O${}_{15}$ with V${}^{2.8+}$, which exhibits an insulating behavior without any phase transition, and compared the results with those of BaV${}_{10}$O${}_{15}$, which shows orbital ordering below ${T}_{c}=123$ K. We found that a soft gap opens at the lowest temperature but it collapses with increasing temperature in SrV${}_{10}$O${}_{15}$, in contrast to a finite charge gap ($\ensuremath{\sim}0.3$ eV) in BaV${}_{10}$O${}_{15}$ below ${T}_{c}$. We also found that the difference of the conductivity spectra between SrV${}_{10}$O${}_{15}$ and BaV${}_{10}$O${}_{15}$ evolves even above ${T}_{c}$ of Ba, indicating that the fluctuation of orbital ordering produces a pseudogap in BaV${}_{10}$O${}_{15}$.