Jun-ichi Igarashi

Pre-K-edge structure on anomalous X-ray scattering in LaMnO3

We study the pre-K-edge structure of the resonant X-ray scattering for forbidden reflections (anomalous scattering) in LaMnO 3 , using the band structure calculation based on the local density approximation. We find a two-peak structure with an intensity approximately 1/100 that of the main peak. This originates from a mixing of 4 p states of Mn to 3 d states of neighboring Mn sites. The effect is enhanced by an interference with the tail of the main peak. The effect of the quadrupole transition is found to be one order of magnitude smaller than that of the dipole transition, slightly modifying the azimuthal-angle dependence.

Resonant X-Ray Scattering from the Quadrupolar Ordering Phase of CeB6

We theoretically investigate the origin of the resonant X-ray scattering (RXS) signal near the Ce L III absorption edge in the quadrupolar ordering phase of CeB 6 , considering the intersite interaction between the Γ 8 states in the initial state. The anisotropic charge distribution of the 4f states modulates the 5d states through the intra-atomic Coulomb interaction and thereby generates a large RXS superlattice intensity. The temperature and magnetic field dependence indicates that the induced dipolar and octupolar orders have little influence on the RXS spectra, in good agreement with the recent experimental results.

Mechanism of resonant x-ray magnetic scattering in NiO

We study the resonant x-ray magnetic scattering (RXMS) around the K edge of Ni in the antiferromagnet NiO, by treating the 4p states of Ni as a band and the 3d states as localized states. We propose a mechanism that the 4p states are coupled to the magnetic order through the intra-atomic Coulomb interaction between the 4p and the 3d states and through the p-d mixing to the 3d states of neighboring Ni atoms. These couplings induce the orbital moment in the 4p band, and thereby give rise to the RXMS intensity at the K edge in the dipolar process. It is found that the spin-orbit interaction in the 4p band has negligibly small contribution to the RXMS intensity. The present model reproduces well the experimental spectra. We also discuss the azimuthal angle dependence of the intensity.

Resonant X-Ray Magnetic Scattering from CoO

We analyze the recent experiment [W. Neubeck et al. : Phys. Rev. B 60 (1999) R9912] for the resonant x-ray magnetic scattering (RXMS) around the K edge of Co in the antiferromagnet CoO. We propose a mechanism of the RXMS that makes the 4 p states couple to the magnetic order: the intraatomic exchange interaction between the 4 p and the 3 d states and the p - d mixing to the 3 d states of neighboring Co atoms. These couplings induce the orbital moment in the 4 p states and make the scattering tensor antisymmetric. Using a cluster model, we demonstrate that this modification gives rise to a large RXMS intensity in the dipole process, in good agreement with the experiment. We also find that the pre-edge peak is generated by the transition to the 3 d states in the quadrupole process, with negligible contribution of the dipole process. We also discuss the azimuthal angle dependence of the intensity.

Kondo lattice model with correlated conduction electrons

We investigate a Kondo lattice model with correlated conduction electrons. Within dynamical mean-field theory the model maps onto an impurity model where the host has to be determined self-consistently. This impurity model can be derived from an Anderson-Hubbard model both by equating the low-energy excitations of the impurity and by a canonical transformation. On the level of dynamical mean-field theory this establishes the connection of the two lattice models. The impurity model is studied numerically by an extension of the noncrossing approximation to a two-orbital impurity. We find that with decreasing temperature the conduction electrons first form quasiparticles unaffected by the presence of the lattice of localized spins. Then, reducing the temperature further, the particle-hole symmetric model turns into an insulator. The quasiparticle peak in the one-particle spectral density splits and a gap opens. The size of the gap increases when the correlations of the conduction electrons become stronger. These findings are similar to the behavior of the Anderson-Hubbard model within dynamical mean-field theory and are obtained with much less numerical effort.

1/S expansion for dynamical structure factors in a two-dimensional Heisenberg antiferromagnet at zero temperature

The author calculates the dynamical structure factors of a two-dimensional Heisenberg antiferromagnet at T=0, to order 1/(2S)2, taking careful account of the umklapp processes. It is shown that, for the transverse spin part, the terms of order 1/(2S)2 give rise to a broad sideband peak of three-magnon excitations, whose intensity is small but not negligible. For the longitudinal spin part, the terms of order 1/(2S)2 modify noticeably the spectral shape of the continuum of two-magnon excitations as a function of frequency. These results could be observed in future neutron scattering experiments.

Three-body scattering theory for electronic excitations in transition metal oxides

Abstract Developing a local three-body scattering theory on a multi-orbital tight-binding model, we study the excitation spectra of the antiferromagnetic transition metal oxides, MnO, FeO, CoO and NiO. The multiplet structures of three-particle states are fully taken into account. It is shown that the self-energy correction drastically improves the excitation spectra, leading to the correct value of the band gap and the satellite structure in agreement with photoemission experiments. The quasiparticle dispersions are obtained in good agreement with the angle-resolved photoemission data of CoO and NiO.

ORBITAL MOMENTS AND MAGNETIC CIRCULAR DICHROISM AT THE K EDGE IN 3D TRANSITION METALS

We calculate the orbital moment and the magnetic circular dichroism (MCD) spectra at the K edge for ferromagnetic Fe, Co and Ni, on the basis of a tight-binding model including the spin-orbit interaction. We find that both quantities are enhanced by the intra-atomic Coulomb interaction, in good agreement with the experiments. We also find that the MCD spectra are mainly induced by the orbital moment of the 3d states on the neighbor sites through the p-d hybridization.

Pre-K-Edge structure on resonant X-ray scattering in LaMnO3

Abstract We study the pre-K-edge structure of the resonant X-ray scattering (RXS) for forbidden reflections in LaMnO 3 , using the band structure calculation based on the local density approximation. We find a two-peak structure with an intensity of approximately 1% of the main peak in the pre-K edge region in the RXS. This originates from the p symmetric states of Mn mixing with the 3d states of neighboring Mn sites in the intermediate states of the dipole process. We also calculate the X-ray absorption spectra in the pre-K-edge region, which were in good agreement with the recent experiment.

Local approach to electronic state of CuO

Using a multiorbital tight-binding model for CuO, we calculate the single-particle excitation spectra by taking account of the local three-particle correlations. The self-energy correction to the Hartree-Fock approximation is carried out. It is shown that the excitation spectra are greatly improved, in good agreement with XPS and O Kα emission experiments.