Revealing the pseudogap in Sr 3 ( Ru 0.985 Fe 0.015 ) 2 O 7 by optical spectroscopy

We report resistivity, magnetization, and optical spectroscopy study on single-crystal sample of ${\mathrm{Sr}}_{3}{({\mathrm{Ru}}_{0.985}{\mathrm{Fe}}_{0.015})}_{2}{\mathrm{O}}_{7}$. An upturn is observed in resistivity at about 30 K. Below 30 K, the dip in resistivity $R(\ensuremath{\omega}$), the suppression in scattering rate $1/\ensuremath{\tau}(\ensuremath{\omega})$, the peaklike feature in optical conductivity ${\ensuremath{\sigma}}_{1}(\ensuremath{\omega})$, and the remainder of spectral weight all suggest the formation of a pseudogap. In addition, one phonon peak at about $600\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ is distinguished at all temperatures, which has asymmetric line shape. Such asymmetric line shape can be fit by a Fano function, and the resulting Fano factor $1/{q}^{2}$ and linewidth $\ensuremath{\gamma}$ show significant increases below 30 K, giving further evidence for the formation of a pseudogap, which might originate from the partial $k$-space gap opening due to density wave instability.