L. Lu

Incident energy and polarization-dependent resonant inelastic x-ray scattering study of La2CuO4

We present a detailed Cu K-edge resonant inelastic X-ray scattering (RIXS) study of the Mott insulator La2CuO4 in the 1-7 eV energy transfer range. As initially found for the high-temperature superconductor HgBa2CuO4+d, the spectra exhibit a multiplet of weakly-dispersive electron-hole excitations, which are revealed by utilizing the subtle dependence of the cross section on the incident photon energy. The close similarity between the fine structures for in-plane and out-of-plane polarizations is indicative of the central role played by the 1s core hole in inducing charge excitations within the CuO2 planes. On the other hand, we observe a polarization dependence of the spectral weight, and careful analysis reveals two separate features near 2 eV that may be related to different charge-transfer processes. The polarization dependence indicates that the 4p electrons contribute significantly to the RIXS cross section. Third-order perturbation arguments and a shake-up of valence excitations are then applied to account for the final-energy resonance in the spectra. As an alternative scenario, we discuss fluorescence-like emission processes due to 1s ->4p transitions into a narrow continuum 4p band.

Charge-transfer excitations in the model superconductor HgBa2CuO4+δ

We report a Cu -edge resonant inelastic x-ray scattering (RIXS) study of charge-transfer excitations in the 2-8 eV range in the structurally simple compound HgBa(2)CuO(4+delta) at optimal doping (T(c)=96.5 K). The spectra exhibit a significant dependence on the incident photon energy which we carefully utilize to resolve a multiplet of weakly dispersive (<0.5 eV) electron-hole excitations, including a mode at 2 eV. The observation of this 2 eV excitation suggests the existence of a remnant charge-transfer gap deep in the superconducting phase. Quite generally, our results, which include additional data for the Mott insulator La(2)CuO(4), demonstrate the importance of exploring the incident photon-energy dependence of the RIXS cross section.

Quasistationary criticality of the order parameter of the three-dimensional random-field Ising antiferromagnet Fe0.85Zn0.15F2 : A synchrotron x-ray scattering study

The critical exponent beta =0.17(1) for the three-dimensional random-field Ising model (RFIM) order parameter upon zero-field cooling (ZFC) has been determined using extinction-free magnetic x-ray scattering techniques for Fe(0.85)Zn(0.15)F2. This result is consistent with other exponents determined for the RFIM in that Rushbrooke scaling is satisfied. Nevertheless, there is poor agreement with equilibrium computer simulations, and the ZFC results do not agree with field-cooling (FC) results. We present details of hysteresis in Bragg scattering amplitudes and line shapes that help elucidate the effects of thermal cycling in the RFIM, as realized in dilute antiferromagnets in an applied field. We show that the ZFC critical-like behavior is consistent with a second-order phase transitions, albeit quasi-stationary rather than truly equilibrium in nature, as evident from the large thermal hysteresis observed near the transition.