Form factor dispersion at La M5,4 edges and average density of resonant atoms

Ultrafast time-resolved x-ray scattering reveals diffusive charge order dynamics in La2-x Ba x CuO4

Charge order is universal among high-T _c cuprates, but its relation to superconductivity is unclear. While static order competes with superconductivity, dynamic order may be favorable and even contribute to Cooper pairing. Using time-resolved resonant soft x-ray scattering at a free-electron laser, we show that the charge order in prototypical La_2-x Ba _x CuO₄ exhibits transverse fluctuations at picosecond time scales. These sub-millielectron volt excitations propagate by Brownian-like diffusion and have an energy scale remarkably close to the superconducting T _c. At sub-millielectron volt energy scales, the dynamics are governed by universal scaling laws defined by the propagation of topological defects. Our results show that charge order in La_2-x Ba _x CuO₄ exhibits dynamics favorable to the in-plane superconducting tunneling and establish time-resolved x-rays as a means to study excitations at energy scales inaccessible to conventional scattering techniques.

Dynamical Effects in Resonant X-Ray Diffraction

Using resonant magnetic diffraction at the Ni L_{2,3} edge in a LaNiO_{3} superlattice, we show that dynamical effects beyond the standard kinematic approximation can drastically modify the resonant scattering cross section. In particular, the combination of extinction and refraction convert maxima to minima in the azimuthal-angle dependence of the diffracted intensity, which is commonly used to determine orbital and magnetic structures by resonant x-ray diffraction. We provide a comprehensive theoretical description of these effects by numerically solving Maxwell's equations in three dimensions. The understanding and description of dynamical diffraction enhances the capabilities of resonant x-ray scattering as a probe of electronic ordering phenomena in solids.

Sequence of hole resonances in complex oxide heterostructures

Resonant soft x-ray scattering measurements at the O K edge on Sr2CuO4-ν/La2NiO4+δ (SCO/LNO) complex oxide superlattices show resonances for holes in the two constituent layers, in a sequence of energy levels. The observation of well defined resonances, on a superlattice with layers one unit cell thick, indicates that the resonance energy is largely unaffected by atoms outside a cluster extending half a unit cell along the c axis, consistent with calculations for bulk materials. Comparison to measurements on related superlattices confirms that the order of resonances at the O K edge reflects the order of hole ground-state energies in the heterostructure buried layers. For the SCO/LNO superlattices, the measurements show that the ground-state energies remain different in very thin SCO and LNO layers, which is a contributing factor when considering electronic reconstruction at interfaces, in addition to the areal density of ionic charges in the atomic planes. Different hole energy levels in the SCO/LNO superlattice also imply that holes do not spread into SCO from LNO layers.