Charge-gap formation in the insulating states of AV ₁₀O ₁₅ (A = Ba, Sr)

We measured the optical conductivity spectra of SrV ₁₀O ₁₅ with V2.8 ⁺, which exhibits an insulating behavior without any phase transition, and compared the results with those of BaV ₁₀O 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 ₁₀O ₁₅, in contrast to a finite charge gap (∼0.3 eV) in BaV ₁₀O ₁₅ below T _c. We also found that the difference of the conductivity spectra between SrV ₁₀O ₁₅ and BaV ₁₀O ₁₅ evolves even above T _c of Ba, indicating that the fluctuation of orbital ordering produces a pseudogap in BaV ₁₀O ₁₅.