We investigate the polaronic ground state of anatase TiO2 by bulk-sensitive resonant inelastic x-ray spectroscopy (RIXS) at the Ti L3 edge. We find that the formation of the polaron cloud involves a single 95 meV phonon along the c axis, in addition to the 108 meV ab-plane mode previously identified by photoemission. The coupling strength to both modes is the same within error bars, and it is unaffected by the carrier density. These data establish RIXS as a directional bulk-sensitive probe of electron-phonon coupling in solids.

We report an angle-resolved photoemission study of the charge stripe ordered La1.6−xNd0.4SrxCuO4 system.A comparative and quantitative line shape analysis is presented as the system evolves from the overdoped regime into the charge ordered phase. On the overdoped side (x = 0.20), a normal state anti-nodal spectral gap opens upon cooling below 80 K. In this process spectral weight is preserved but redistributed to larger energies. A correlation between this spectral gap and electron scattering is found. A different lineshape is observed in the antinodal region of charge ordered Nd-LSCO x = 1/8. Significant low-energy spectral weight appears to be lost. These observations are discussed in terms of spectral weight redistribution and gapping originating from charge stripe ordering.

High-resolution resonant inelastic x-ray scattering (RIXS) at the oxygen K edge has been used to study the orbital excitations of Ca 2 RuO 4 and Sr 2 RuO 4 . In combination with linear dichroism x-ray absorption spectroscopy, the ruthenium 4d-orbital occupation and excitations were probed through their hybridization with the oxygen p orbitals. These results are described within a minimal model, taking into account crystal field splitting and a spin-orbit coupling λ so = 200 meV. The effects of spin-orbit interaction on the electronic structure and implications for the Mott and superconducting ground states of (Ca,Sr) 2 RuO 4 are discussed.

No abstract

Resonant inelastic x-ray scattering (RIXS) experiments performed at the oxygen K edge on the iridate perovskites Sr 2 IrO 4 and Sr 3 Ir 2 O 7 reveal a sequence of well-defined dispersive modes over the energy range up to ∼0.8 eV. The momentum dependence of these modes and their variation with the experimental geometry allows us to assign each of them to specific collective magnetic and/or electronic excitation processes, including single and bimagnons, and spin-orbit and electron-hole excitons. We thus demonstrate that dispersive magnetic and electronic excitations are observable at the O K edge in the presence of the strong spin-orbit coupling in the 5d shell of iridium and strong hybridization between Ir 5d and O 2p orbitals, which confirm and expand theoretical expectations. More generally, our results establish the utility of O K-edge RIXS for studying the collective excitations in a range of 5d materials that are attracting increasing attention due to their novel magnetic and electronic properties. Especially, the strong RIXS response at O K edge opens up the opportunity for investigating collective excitations in thin films and heterostructures fabricated from these materials. DOI: 10.1103/PhysRevB.97.041102 Characterizing the elementary excitations in correlated electron systems is an essential prerequisite for obtaining a complete understanding of the underlying electronic interactions and therefore crucial for revealing the origin of emergent phases [1,2]. Resonant inelastic x-ray scattering (RIXS) has become established in the past decade as a powerful tool for studying the momentum dependence of electronic and magnetic excitations [2]

No abstract

We investigate by resonant inelastic x-ray scattering the magnetic excitations in thin films of tetragonal CuO. We identify a spin wave excitation, dispersing on two cupratelike antiferromagnetic sublattices. Its energy at the boundary of the Brillouin zone (220 meV), is significantly lower than typical values (E ∼ 300 meV) found in two-dimensional cuprates. A spin wave expansion starting from an extended Hubbard model suggests two possible scenarios for this energy lowering. DOI: 10.1103/PhysRevB.92.140404 PACS number(s): 74.72.Cj, 74.25.Jb, 75.30.Ds, 78.70.Ck Magnetism in the undoped, insulating quasi-twodimensional (2D) cuprates is determined by the 180• Cu-O-Cu bonds of their corner-sharing copper oxide layers [ Fig. 1(a) [3]. Phenomenologically, these systems can be described by extended spin-1/2 Heisenberg models with antiferromagnetic coupling on a square lattice [4,5], in agreement with secondorder superexchange theory. However, to make contact with other spectroscopies one needs to adopt a more microscopic description in terms of an extended Hubbard model. To be quantitative, it is necessary to go beyond second-order perturbation theory and to consider the effective spin model generated by the one-band Hubbard model up to fourth order in hopping. Such a procedure renormalizes the two-spin interactions and also leads to four-spin interactions. This approach yields more realistic values for the on-site Coulomb repulsion U , is consistent with higher-energy resonant inelastic x-ray scattering (RIXS) and angle-resolved photoemission spectroscopy (ARPES) data, and provides good fits to the experimentally observed spin excitation spectra [6,7].A recent exciting development was the discovery of a new tetragonal form of the simple binary oxide CuO, with an alternative structure to that of the cuprates. Tetragonal CuO (T-CuO) can be grown epitaxially on a SrTiO 3 (001) substrate up to a thickness of several unit cells [8][9][10]. In this material CuO 6 octahedra give rise to infinite CuO layers, consisting of edge-sharing CuO 4 plaquettes, stacked along the c axis. At variance with the CuO 2 cuprate layers, oxygen ions do not bridge nearest-neighbor (NN) but next-NN copper ions. Electronically, the edge-sharing structure is well described by two interpenetrating corner-sharing sublattices [ Fig. 1(b) • Cu-O-Cu bonds, the coupling could be either ferromagnetic or antiferromagnetic but, in any case, it introduces frustration. In the classical limit, the system would still develop independent AFM Heisenberg order on each sublattice. However, if quantum fluctuations are taken into account, J d is expected to lock the relative orientation of the two sublattices and to break the fourfold rotational C 4 symmetry, leading to an additional Ising order parameter [12][13][14].To examine these speculations, we have studied the magnetic excitation spectrum of T-CuO. Since inelastic neutron scattering is inadequate due to the limited film thickness, we employ RIXS at the Cu L 3 (Cu 2p 3/2 → 3d) edge [15]. We find a magnon...

We use resonant inelastic x-ray scattering (RIXS) at the Cu L3 edge to measure the charge and spin excitations in the "half-stuffed" Cu-O planes of the cuprate antiferromagnet Ba2Cu3O4Cl2. The RIXS line shape reveals distinct contributions to the dd excitations from the two structurally inequivalent Cu sites, which have different out-of-plane coordinations. The low-energy response exhibits magnetic excitations. We find a spin-wave branch whose dispersion follows the symmetry of a CuO2 sublattice, similar to the case of the "fully-stuffed" planes of tetragonal CuO (T-CuO). Its bandwidth is closer to that of a typical cuprate material, such as Sr2CuO2Cl2, than it is to that of T-CuO. We interpret this result as arising from the absence of the effective four-spin inter-sublattice interactions that act to reduce the bandwidth in T-CuO. arXiv:1709.03427v1 [cond-mat.str-el]