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We report a combined valence band photoemission and Auger spectroscopy study of single crystalline Ca(Fe,Co)2As2 and Ba(Fe,TM)2As2 with transition metal TM = Ni or Cu. The valence band photoemission data show directly that the TM states move to higher binding energies with increasing atomic number,contnbuting less and less to the states close to the Fermi level. Furthermore, the 3d8 final state of the LV V Auger decay, which is observed for Ni and Cu, unambiguously reveals the accumulation of charge at these impurities. We also show that the onsite Coulomb interaction on the impurity strongly increases when moving from Co over Ni to Cu. Our results quantify the impurity potentials and imply that the superconducting state is rather robust against impurity scattering. [ABSTRACT FROM AUTHOR]

We assess the capabilities of magnetic resonant inelastic x-ray scattering (RIXS) at the O K edge in undoped cuprates by taking La2CuO4 as a benchmark case, based on a series of RIXS measurements that we present here. By combining the experimental results with basic theory we point out the fingerprints of bimagnon excitation in the O K edge RIXS spectra. These are a dominant peak around 450 meV, the almost complete absence of dispersion both with π and &sgr; polarization, and the almost constant intensity vs the transferred momentum with &sgr; polarization. This behavior is quite different from Cu L3 edge RIXS giving a strongly dispersing bimagnon tending to zero at the center of the Brillouin zone. This is clearly shown by RIXS measurements at the Cu L3 edge that we present. The Cu L3 bimagnon spectra and those at the Cu K edge--both from the literature and from our data--however, have the same shape. These similarities and differences are understood in terms of different sampling of the bimagnon continuum. This panorama points out the unique possibilities offered by O K RIXS in the study of magnetic excitations in cuprates near the center of the BZ. [ABSTRACT FROM AUTHOR]

We present RIXS data at the O K edge from La2-xSrxCuO4 vs doping between x =0.10 and x = 0.22 with attention to the magnetic excitations in the mid-infrared region. The sampling done by RIXS is the same as in the uiidoped cuprates provided the excitation is at the first prepeak induced by doping. Note that this excitation energy is about 1.5 eV lower than that needed to see bimagnons in the parent compound. This approach allows the study of the upper region of the bimagnon continuum around 450 meV within about one third of the Brillouin zone around &Ggr;. The results show the presence of damped bimagnons and of higher even order spin excitations with almost constant spectral weight at all the dopings explored here. The implications for high-Tc studies are briefly addressed. [ABSTRACT FROM AUTHOR]

Resonant inelastic x-ray scattering spectra excited in the immediate vicinity of the core-level ionization thresholds of N2 have been recorded. Final states of well-resolved symmetry-selected Rydberg series converging to valence-level ionization thresholds with vibrational excitations are observed. The results are well described by a quasi-two-step model which assumes that the excited electron is unaffected by the radiative decay. This threshold dynamics simplifies the interpretation of resonant inelastic x-ray scattering spectra considerably and facilitates characterization of low-energy excited final states in molecular systems. [ABSTRACT FROM AUTHOR]

The cuprate high temperature superconductors exhibit a pronounced trend in which the superconducting transition temperature Tc increases with the number of CuC2planes n in the crystal structure. We compare the magnetic excitation spectrum of Bi2+xSr2-xCuO6+δ (Bi-2201) and Bi2Sr2Ca2CuO10+δ (Bi-2223), with n = 1 and 3, respectively, using Cu L3-edge resonant inelastic x-ray scattering. Near the antinodal zone boundary we find the paramagnon energy in Bi-2223 is substantially higher than that in Bi-2201, indicating that multilayer cuprates host stronger effective magnetic exchange interactions, providing a possible explanation for the Tc vs n scaling. In contrast, the nodal direction exhibits very strongly damped, almost nondispersive excitations. We argue that this implies that the magnetism in the doped cuprates is partially itinerant in nature. [ABSTRACT FROM AUTHOR]

Oxygen Is → 2p (K-edge) x-ray absorption spectra mimic the density of metalliclike states of transition metal oxides, thus shedding light on the electronic band structure near Fermi energies of these compounds. We take here advantage of x-ray linear dichroism measurements at the O K edge of different (SrMn03)3(LaMn03)2n superlattices, of SrMnO3 and of LaMn03 thin films. X-ray linear dichroism, in the pre-edge region of the spectra, provides evidence about the specific symmetry of the Mn 3d orbitals hybridized with the oxygen ions, thus allowing us to disentangle the pure interface contribution from those related to strain effect and charge leakage among the superlattices' constituent blocks. [ABSTRACT FROM AUTHOR]

The hole distribution in Sr14Cu24CX41 is studied by low-temperature polarization-dependent O K near-edge x-ray absorption fine-structure measurements and state-of-the-art electronic structure calculations that include core-hole and correlation effects in a mean-field approach. Contrary to all previous analysis, based on semiempirical models, we show that correlations and antiferromagnetic ordering favor the strong chain-hole attraction. For the remaining small number of holes accommodated on ladders, leg sites are preferred to rung sites. The small hole affinity of rung sites explains naturally the one-dimensional to two-dimensional crossover in the phase diagram of (La,Y,Sr,Ca)14Cu24041. [ABSTRACT FROM AUTHOR]

The discovery of superconductivity in a d9-δ nickelate has inspired disparate theoretical perspectives regarding the essential physics of this class of materials. A key issue is the magnitude of the magnetic superexchange, which relates to whether cuprate-like high-temperature nickelate superconductivity could be realized. We address this question using Ni L-edge and O K-edge spectroscopy of the reduced d9-1/3 trilayer nickelates R4Ni3O8 (where R=La, Pr) and associated theoretical modeling. A magnon energy scale of ∼80  meV resulting from a nearest-neighbor magnetic exchange of J=69(4)  meV is observed, proving that d9-δ nickelates can host a large superexchange. This value, along with that of the Ni-O hybridization estimated from our O K-edge data, implies that trilayer nickelates represent an intermediate case between the infinite-layer nickelates and the cuprates. Layered nickelates thus provide a route to testing the relevance of superexchange to nickelate superconductivity. [ABSTRACT FROM AUTHOR]

Charge density wave (CDW) correlations are prevalent in all copper-oxide superconductors. While CDWs in conventional metals are driven by coupling between lattice vibrations and electrons, the role of the electron-phonon coupling (EPC) in cuprate CDWs is strongly debated. Using Cu L3 edge resonant inelastic x-ray scattering, we study the CDW and Cu-O bond-stretching phonons in the stripe-ordered cuprate La1.8-xEu0.2SrxCuO4+δ. We investigate the interplay between charge order and EPC as a function of doping and temperature and find that the EPC is enhanced in a narrow momentum region around the CDW ordering vector. By detuning the incident photon energy from the absorption resonance, we extract an EPC matrix element at the CDW ordering vector of M≃0.36  eV, which decreases to M≃0.30  eV at high temperature in the absence of the CDW. Our results suggest a feedback mechanism in which the CDW enhances the EPC which, in turn, further stabilizes the CDW. [ABSTRACT FROM AUTHOR]

The discovery of charge-density-wave-related effects in the resonant inelastic x-ray scattering spectra of cuprates holds the tantalizing promise of clarifying the interactions that stabilize the electronic order. Here, we report a comprehensive resonant inelastic x-ray scattering study of La2-xSrxCuO4 finding that charge-density wave effects persist up to a remarkably high doping level of x=0.21 before disappearing at x=0.25. The inelastic excitation spectra remain essentially unchanged with doping despite crossing a topological transition in the Fermi surface. This indicates that the spectra contain little or no direct coupling to electronic excitations near the Fermi surface, rather they are dominated by the resonant cross section for phonons and charge-density-wave-induced phonon softening. We interpret our results in terms of a charge-density wave that is generated by strong correlations and a phonon response that is driven by the charge-density-wave-induced modification of the lattice. [ABSTRACT FROM AUTHOR]

The magnetic correlations within the cuprates have undergone intense scrutiny as part of efforts to understand high-temperature superconductivity. We explore the evolution of the magnetic correlations along the nodal direction of the Brillouin zone in La2-xSrxCuO4, spanning the doping phase diagram from the antiferromagnetic Mott insulator at x=0 to the metallic phase at x=0.26. Magnetic excitations along this direction are found to be systematically softened and broadened with doping, at a higher rate than the excitations along the antinodal direction. This phenomenology is discussed in terms of the nature of the magnetism in the doped cuprates. Survival of the high-energy magnetic excitations, even in the overdoped regime, indicates that these excitations are marginal to pairing, while the influence of the low-energy excitations remains ambiguous. [ABSTRACT FROM AUTHOR]

Superconductivity in infinite layer nickelates Nd_{1-x}Sr_{x}NiO_{2} has so far been achieved only in thin films, raising questions on the role of substrates and interfaces. Given the challenges associated with their synthesis it is imperative to identify their intrinsic properties. We use resonant inelastic x-ray scattering to investigate the influence of the SrTiO_{3} capping layer on the excitations of Nd_{1-x}Sr_{x}NiO_{2} (x=0 and 0.2). Spin excitations are observed in parent and 20% doped Nd_{1-x}Sr_{x}NiO_{2} regardless of capping, proving that magnetism is intrinsic to infinite-layer nickelates and appears in a significant fraction of their phase diagram. In parent and superconducting Nd_{1-x}Sr_{x}NiO_{2}, the spin excitations are slightly hardened in capped samples compared to the noncapped ones. Additionally, a weaker Ni-Nd charge transfer peak at ∼0.6  eV suggests that the hybridization between Ni 3d and Nd 5d orbitals is reduced in capped samples. From our data, capping induces only minimal differences in Nd_{1-x}Sr_{x}NiO_{2} and we phenomenologically discuss these differences based on the reconstruction of the SrTiO_{3}-NdNiO_{2} interface and other mechanisms such as crystalline disorder.

The discovery of superconductivity in a d^{9-δ} nickelate has inspired disparate theoretical perspectives regarding the essential physics of this class of materials. A key issue is the magnitude of the magnetic superexchange, which relates to whether cuprate-like high-temperature nickelate superconductivity could be realized. We address this question using Ni L-edge and O K-edge spectroscopy of the reduced d^{9-1/3} trilayer nickelates R_{4}Ni_{3}O_{8} (where R=La, Pr) and associated theoretical modeling. A magnon energy scale of ∼80  meV resulting from a nearest-neighbor magnetic exchange of J=69(4)  meV is observed, proving that d^{9-δ} nickelates can host a large superexchange. This value, along with that of the Ni-O hybridization estimated from our O K-edge data, implies that trilayer nickelates represent an intermediate case between the infinite-layer nickelates and the cuprates. Layered nickelates thus provide a route to testing the relevance of superexchange to nickelate superconductivity.

Charge density wave (CDW) correlations are prevalent in all copper-oxide superconductors. While CDWs in conventional metals are driven by coupling between lattice vibrations and electrons, the role of the electron-phonon coupling (EPC) in cuprate CDWs is strongly debated. Using Cu L_{3} edge resonant inelastic x-ray scattering, we study the CDW and Cu-O bond-stretching phonons in the stripe-ordered cuprate La_{1.8-x}Eu_{0.2}Sr_{x}CuO_{4+δ}. We investigate the interplay between charge order and EPC as a function of doping and temperature and find that the EPC is enhanced in a narrow momentum region around the CDW ordering vector. By detuning the incident photon energy from the absorption resonance, we extract an EPC matrix element at the CDW ordering vector of M≃0.36  eV, which decreases to M≃0.30  eV at high temperature in the absence of the CDW. Our results suggest a feedback mechanism in which the CDW enhances the EPC which, in turn, further stabilizes the CDW.

The discovery of charge-density-wave-related effects in the resonant inelastic x-ray scattering spectra of cuprates holds the tantalizing promise of clarifying the interactions that stabilize the electronic order. Here, we report a comprehensive resonant inelastic x-ray scattering study of La_{2-x}Sr_{x}CuO_{4} finding that charge-density wave effects persist up to a remarkably high doping level of x=0.21 before disappearing at x=0.25. The inelastic excitation spectra remain essentially unchanged with doping despite crossing a topological transition in the Fermi surface. This indicates that the spectra contain little or no direct coupling to electronic excitations near the Fermi surface, rather they are dominated by the resonant cross section for phonons and charge-density-wave-induced phonon softening. We interpret our results in terms of a charge-density wave that is generated by strong correlations and a phonon response that is driven by the charge-density-wave-induced modification of the lattice.

Fractionalization of an electronic quasiparticle into spin, charge, and orbital parts is a fundamental and characteristic property of interacting electrons in one dimension. However, real materials are never strictly one dimensional and the fractionalization phenomena are hard to observe. Here we studied the spin and orbital excitations of the anisotropic ladder material CaCu_{2}O_{3}, whose electronic structure is not one dimensional. Combining high-resolution resonant inelastic x-ray scattering experiments with theoretical model calculations, we show that (i) spin-orbital fractionalization occurs in CaCu_{2}O_{3} along the leg direction x through the xz orbital channel as in a 1D system, and (ii) no fractionalization is observed for the xy orbital, which extends in both leg and rung direction, contrary to a 1D system. We conclude that the directional character of the orbital hopping can select different degrees of dimensionality. Using additional model calculations, we show that spin-orbital separation is generally far more robust than the spin-charge separation. This is not only due to the already mentioned selection realized by the orbital hopping, but also due to the fact that spinons are faster than the orbitons.

Taking spinon excitations in the quantum antiferromagnet CaCu2O3 as an example, we demonstrate that femtosecond dynamics of magnetic electronic excitations can be probed by direct resonant inelastic x-ray scattering (RIXS). To this end, we isolate the contributions of single and double spin-flip excitations in experimental RIXS spectra, identify the physical mechanisms that cause them, and determine their respective time scales. By comparing theory and experiment, we find that double spin flips need a finite amount of time to be generated, rendering them sensitive to the core-hole lifetime, whereas single spin flips are, to a very good approximation, independent of it. This shows that RIXS can grant access to time-domain dynamics of excitations and illustrates how RIXS experiments can distinguish between excitations in correlated electron systems based on their different time dependence.

We report a high-resolution resonant inelastic soft x-ray scattering study of the quantum magnetic spin-chain materials Li(2)CuO(2) and CuGeO(3). By tuning the incoming photon energy to the oxygen K edge, a strong excitation around 3.5 eV energy loss is clearly resolved for both materials. Comparing the experimental data to many-body calculations, we identify this excitation as a Zhang-Rice singlet exciton on neighboring CuO(4) plaquettes. We demonstrate that the strong temperature dependence of the inelastic scattering related to this high-energy exciton enables us to probe short-range spin correlations on the 1 meV scale with outstanding sensitivity.

We probe the collective magnetic modes of La2CuO4 and underdoped La2-xSrxCuO4 (LSCO) by momentum resolved resonant inelastic x-ray scattering (RIXS) at the Cu L3 edge. For the undoped antiferromagnetic sample, we show that the single magnon dispersion measured with RIXS coincides with the one determined by inelastic neutron scattering, thus demonstrating that x rays are an alternative to neutrons in this field. In the spin dynamics of LSCO, we find a branch dispersing up to approximately 400 meV coexisting with one at lower energy. The high-energy branch has never been seen before. It indicates that underdoped LSCO is in a dynamic inhomogeneous spin state.

By resonant inelastic x-ray scattering in the soft x-ray regime we probe the dynamical multiple-spin correlations in the antiferromagnetic cuprates La2CuO4 and CaCuO2. High resolution measurements at the copper L3 edge allow the clear observation of dispersing bimagnon excitations. Theory based on the ultrashort core-hole lifetime expansion fits the data on these coherent spin excitations without free parameters.

In 2004, Ohtomo and Hwang discovered that an electron gas is created at the interface between insulating LaAlO3 and SrTiO3 compounds. Here we show that the generation of a conducting electron gas is related to an orbital reconstruction occurring at the LaAlO3/SrTiO3 interface. Our results are based on extensive investigations of the electronic properties and of the orbital structure of the interface using x-ray absorption spectroscopy. In particular, we find that the degeneracy of the Ti 3d states is fully removed and that the Ti 3d xy levels become the first available states for conducting electrons.

Linear dichroism (LD) in x-ray absorption, diffraction, transport, and magnetization measurements on thin La(0.7)Sr(0.3)MnO(3) films grown on different substrates, allow identification of a peculiar interface effect, related just to the presence of the interface. We report the LD signature of preferential 3d-e(g)(3z(2)-r(2)) occupation at the interface, suppressing the double exchange mechanism. This surface orbital reconstruction is opposite to that favored by residual strain and is independent of dipolar fields, the chemical nature of the substrate and the presence of capping layers.