Anisotropy of the core-hole relaxation in x-ray-absorption spectroscopy as probed in square planar cuprates

Effects of strain relaxation in Pr0.67Sr0.33MnO3 films probed by polarization dependent X-ray absorption near edge structure

The Mn K edge X-ray absorption near edge structure (XANES) of Pr_0.67Sr_0.33MnO₃ films with different thicknesses on (001) LaAlO₃ substrate was measured, and the effects of strain relaxation on film properties were investigated. The films showed in-plane compressive and out-of-plane tensile strains. Strain relaxation occurred with increasing film thickness, affecting both lattice constant and MnO₆ octahedral rotation. In polarization dependent XANES measurements using in-plane (parallel) and out-of-plane (perpendicular) geometries, the different values of absorption resonance energy E_r confirmed the film anisotropy. The values of E_r along these two directions shifted towards each other with increasing film thickness. Correlating with X-ray diffraction (XRD) results it is suggested that the strain relaxation decreased the local anisotropy and corresponding probability of electronic charge transfer between Mn 3d and O 2p orbitals along the in-plane and out-of-plane directions. The XANES results were used to explain the film-thickness dependent magnetic and transport properties.

The 1s x-ray absorption pre-edge structures in transition metal oxides

We develop a general procedure to analyse the pre-edges in 1s x-ray absorption near edge structure (XANES) of transition metal oxides and coordination complexes. Transition metal coordination complexes can be described from a local model with one metal ion. The 1s 3d quadrupole transitions are calculated with the charge-transfer multiplet program. Tetrahedral coordination complexes have more intense pre-edge structures due to the local mixing of 3d and 4p states, implying a combination of 1s 3d quadrupole and 1s 4p dipole transitions. Divalent transition metal oxides can be described similar to coordination complexes, but for trivalent and tetravalent oxides, additional structures are visible in the pre-edge region due to non-local dipole transitions. The 1s 4p dipole transitions have large cross section at the 3d-band region due to the strong metal-metal interactions, which are oxygen mediated. This yields large intensity in the 3d-band region but at a different energy than the local 1s 3d quadrupole transitions because of smaller core-hole effects due to the delocalization of the excited electron.

Localized and delocalized excitons: resonant inelastic x-ray scattering in La(2-x)Sr(x)NiO4 and La(2-x)Sr(x)CuO4

The dynamics of doped charge in an antiferromagnetic lattice is central to the description of the insulator-metal transition that occurs on doping the parent high T(c) compounds. In this work we use high resolution resonant inelastic x-ray scattering to investigate the dynamics of the charge-transfer exciton by measuring its energy dispersion in two prototype compounds, La2CuO4 and La2NiO4. We show that this behavior is radically different in the cuprate with respect to a system known to exhibit strong polaronic behavior, namely, the nickelate: the exciton is mobile in the cuprate while it is well localized in the nickelate. Using a simple Wannier-Mott model we can estimate the total hole plus electron effective mass in the cuprate to be 3.5 +/- 0.3 m(e) which would exclude strong localization in the undoped cuprate.

Polarized resonant inelastic x-ray scattering as an ultrafine probe of excited States of La2CuO4

X-ray absorption is the standard method to probe the unoccupied density of states at a given edge. Here we show that polarized resonant inelastic x-ray scattering in La2CuO4 at the Cu L edge is extremely sensitive to the environment of the Cu atom and the fine structure in the Cu 4p density of states. Combined ab initio and many-body cluster calculations, used for the first time in such a context, show remarkable agreement with experiment. In particular, we identify a nonlocal effect, namely, a transition to off-site Cu 3d states.