Soft X-Ray Resonant Diffraction Study of Magnetic and Orbital Correlations
in a Manganite Near Half Doping
K. J. Thomas, J. P. Hill, S. Grenier, Y-J. Kim, P. Abbamonte, L.Venema, A. Rusydi, Y. Tomioka, Y. Tokura,D. F. McMorrow, G. Sawatzky, and M. van Veenendaal (6.2004)
Phys. Rev. Lett. 92, 237204 (2004).
We have utilized resonant x-ray diffraction at the Mn LII/III edges in order to directly compare magnetic and orbital correlations in Pr0.6Ca0.4MnO
. Comparing the widths of the magnetic and orbital diffraction peaks, we find that the magnetic correlation length exceeds that of the orbital order by nearly a factor of 2. Furthermore, we observe a large (~3 eV) spectral weight shift between the magnetic and orbital resonant line shapes, which cannot be explained within the classic Goodenough picture of a charge-ordered ground state. To explain the shift, we calculate the orbital and magnetic resonant diffraction line shapes based on a relaxed charge-ordered model.
Comparison between the calculated and measured resonance line shapes. In (a) the magnetic scattering is dominated by the |3; 4> configuration while in (b) the orbital scattering is dominated by the |4; 3> configuration.
Resonant Inelastic X-ray Scattering has been increasingly used to study strongly correlated systems. Zhang et al. [Phys. Rev. Lett. 88, 077401 (2002)] used RIXS to study valence excitations in the ladder compound NaV2O5. We have shown the importance of including the core-hole potential and polarization effects in the interpretation of their experiments. We find that the feature, Zhang et al. ascribe to an excitation across the Hubbard gap is in fact not the lowest excitation state and should be ascribed to a local dd-transition.
(a) The solid line shows the XAS spectrum for a four-site cluster. The letters a-ƒ indicate the various incident energies. RIXS data on an energy-loss scale in the π → πσ and σ → πσ configurations are shown in (b) and (c), respectively. Also shown in (c) are the RIXS spectra with the elastic contribution removed (dotted line).
We show that inclusion of the competition between quasiparticle and local-moment behavior in CrO2 is necessary to obtain good agreement between the calculated and experimentally observed spin polarization. By going beyond a single Slater determinant description, we find a spin polarization of close to 100% near the Fermi level reflecting quasi-particle behavior. At energies higher than 0.1-0.2 eV above the Fermi level, the local moment character dominates and the spin polarization is reduced to approximately 50%.
The upper half shows the spin polarization with (blue) and without (green) the inclusion of the Sz=1/2 component of the quartet states. A comparison is made with the experimentally obtained spin polarization (triangles and diamonds). The lower half shows the spin-resolved density of states in the region of the Fermi level broadened with a Lorentzian with a width of 0.25 eV.
An explanation is offered for the surprising effect of Resonant Magnetic Scattering
at nonmagnetic ions, such as Ga in UGa3 and As in UAs. It is shown that the
excited electron probes the neighboring magnetic ions and is thereby sensitive
of the local Uranium moment.
The polarization dependence of x-ray emission spectroscopy (XES) is studied by employing the angle dependence of incident and emitted x-ray. The Kramers-Heisenberg formula is used to describe the optical process. It is shown that the quantum mechanical interference effect is directly observable in magnetic circular dichroism spectra in a special geometrical configuration. It is also shown that by making use of the linearly polarized x-ray, information on the symmetry of ground states of materials can be directly determined from simple selection rules. Potential possibilities of x-ray spectroscopy with polarized X-rays are demonstrated.