[en] We show that high-resolution Resonant Inelastic X-ray Scattering (RIXS) provides direct, element-specific and momentum-resolved information on the electron-phonon (e-p) coupling strength. Our theoretical analysis indicates how the e-p coupling can be extracted from RIXS spectra by determining the differential phonon scattering cross-section. An alternative manner to extract the coupling is to use the one- and two-phonon loss ratio, which is governed by the e-p coupling strength and the core-hole lifetime. This allows the determination of the e-p coupling on an absolute energy scale.
Journal
[en] We express the cross-section for indirect resonant inelastic X-ray scattering in terms of an intrinsic dynamic correlation function of the system that is studied with this technique. We find that the cross-section is a linear combination of the charge response function and the dynamic longitudinal spin density correlation function of the electrons that couple to the core-hole. This result is asymptotically exact for both strong and weak local core-hole potentials. The relative charge and spin contribution to the inelastic spectral weight can be changed by varying the incident photon energy. (authors)
Journal
[en] Based on exact diagonalization results for small clusters we discuss the effect of intersite Coulomb repulsion in Mott-Hubbard or charge transfers insulators. Whereas the exchange constant J for direct exchange is enhanced by intersite Coulomb interaction, that for superexchange is suppressed. The enhancement of J in the single-band models holds up to the critical value for the charge density wave (CDW) instability, thus opening the way for large values of J. Single-band Hubbard models with sufficiently strong intersite repulsion to be near a CDW instability thus may provide physical realizations of t-J-like models with the open-quote open-quote unphysical close-quote close-quote parameter ratio J/t≅1. copyright 1996 The American Physical Society
Journal
[en] Recent experiments reveal indirect resonant inelastic X-ray scattering (RIXS) to be a new probe of spin dynamics. Here I derive the cross-section for magnetic RIXS and determine the momentum-dependent four-spin correlation function that it measures. These results show that this technique offers information on spin dynamics that is complementary to, e.g., neutron scattering. As an example the RIXS spectrum of Heisenberg antiferromagnets is calculated by considering a half filled single band Hubbard model at strong coupling and zero temperature. It turns out that only scattering processes that involve at least two magnons are allowed. Other selection rules imply that the scattering intensity vanishes for specific transferred momenta q, in particular for q = 0. All results agree very well with recent experimental data. (author)
Journal
[en] We consider the influence of a nearest-neighbor Coulomb interaction in an extended Hubbard model and introduce an interaction term which simulates atomic polarizabilities. The inclusion of atomic polarizabilities in the model has the effect of screening the on-site Coulomb interaction for charged excitations, unlike a neighbor Coulomb interaction which reduces energies of neutral local excitations, leaving the conductivity gap unchanged. This is supported by an exact diagonalization study on small, mainly one-dimensional, extended Mott-Hubbard systems. For large V, the system undergoes a phase transition from a spin-density wave (SDW) to a charge-density wave (CDW). It is shown that the SDW/CDW phase transition cannot be described correctly when using effective parameters, but that screening effects have to be included explicitly. Also other phases, one of which is ferroelectric, appear when atomic polarizabilities are included in the model Hamiltonian. We study C60 in terms of the degenerate extended Hubbard model with screening effects
Journal
[en] Using x-ray absorption and resonant inelastic x-ray scattering, charge dynamics at and near the Fe L edges is investigated in Fe pnictide materials, and contrasted tothat measured in other Fe compounds. It is shown that the XAS and RIXS spectra for 122 and 1111 Fe pnictides are each qualitatively similar to Fe metal. Cluster diagonalization, multiplet, and density-functional calculations show that Coulomb correlations are much smaller than in the cuprates, highlighting the role of Fe metallicity and strong covalency in these materials. Best agreement with experiment is obtained using Hubbard parameters U ∼< 2eV and J ∼ 0.8eV.
Journal
Physical Review. B, Condensed Matter and Materials Physics; ISSN 1098-0121; 
; v. 80(1); p. 10
; v. 80(1); p. 10
[en] Phonons, the quantum mechanical representation of lattice vibrations, and their coupling to the electronic degrees of freedom are important for understanding thermal and electric properties of materials. For the first time, phonons have been measured using resonant inelastic x-ray scattering (RIXS) across the Cu K-edge in cupric oxide (CuO). Analyzing these spectra using an ultra-short core-hole lifetime approximation and exact diagonalization techniques, we can explain the essential inelastic features. The relative spectral intensities are related to the electron-phonon coupling strengths.
Journal