Charge-stripe fluctuations in Nd4Ni3O8 as evidenced by optical spectroscopy

We present an investigation into the optical properties of ${\mathrm{Nd}}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{8}$ at different temperatures from 300 down to 5 K over a broad frequency range. The optical conductivity at 5 K is decomposed into infrared-active phonons, a far-infrared band $\ensuremath{\alpha}$, a mid-infrared band $\ensuremath{\beta}$, and a high-energy absorption edge. By comparing the measured optical conductivity to first-principles calculations and the optical response of other nickelates, we find that ${\mathrm{Nd}}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{8}$ features evident charge-stripe fluctuations. The $\ensuremath{\beta}$ band is attributed to electronic transitions between the gapped Ni-${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ bands due to fluctuating charge stripes, while the high-frequency absorption edge corresponds to the onset of transitions involving other high-energy bands. Furthermore, an analysis of the temperature-dependent optical spectral weight reveals a ${T}^{2}$ law, which is likely to originate from strong correlation effects.