Observation of momentum-resolved charge fluctuations proximate to the charge-order phase using resonant inelastic X-ray scattering

In strongly correlated electron systems, enhanced fluctuations in the proximity of the ordered states of electronic degrees of freedom often induce anomalous electronic properties such as unconventional superconductivity. While spin fluctuations in the energy-momentum space have been studied widely using inelastic neutron scattering, other degrees of freedom, i.e., charge and orbital, have hardly been explored thus far. Here, we use resonant inelastic X-ray scattering to observe charge fluctuations proximate to the charge-order phase in transition metal oxides. In the two-leg ladder of Sr_14-xCa_xCu₂₄O₄₁, charge fluctuations are enhanced at the propagation vector of the charge order (q_CO) when the order is melted by raising temperature or by doping holes. In contrast, charge fluctuations are observed not only at q_CO but also at other momenta in a geometrically frustrated triangular bilayer lattice of LuFe₂O₄. The observed charge fluctuations have a high energy (∼1 eV), suggesting that the Coulomb repulsion between electrons plays an important role in the formation of the charge order.