In the past decade, Resonant Inelastic X-ray Scattering (RIXS) has made remarkable progress as a spectroscopic technique. This is a direct result of the availability of highbrilliance synchrotron X-ray radiation sources and of advanced photon detection instrumentation. The technique's unique capability to probe elementary excitations in complex materials by measuring their energy-, momentum-, and polarization-dependence has brought RIXS to the forefront of experimental photon science. We review both the experimental and theoretical RIXS investigations of the past decade, focusing on those determining the low-energy charge, spin, orbital and lattice excitations of solids. We present the fundamentals of RIXS as an experimental method and then review the theoretical state of affairs, its recent developments and discuss the different (approximate) methods to compute the dynamical RIXS response. The last decade's body of experimental RIXS data and its interpretation is surveyed, with an emphasis on RIXS studies of correlated electron systems, especially transition metal compounds. Finally, we discuss the promise that RIXS holds for the near future, particularly in view of the advent of x-ray laser photon sources.

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Orbital reconstructions and covalent bonding must be considered as important factors in the rational design of oxide heterostructures with engineered physical properties. We have investigated the interface between high-temperature superconducting (Y,Ca)Ba(2)Cu3O7 and metallic La(0.67)Ca(0.33)MnO3 by resonant x-ray spectroscopy. A charge of about -0.2 electron is transferred from Mn to Cu ions across the interface and induces a major reconstruction of the orbital occupation and orbital symmetry in the interfacial CuO2 layers. In particular, the Cu d(3z(2)-r(2)) orbital, which is fully occupied and electronically inactive in the bulk, is partially occupied at the interface. Supported by exact-diagonalization calculations, these data indicate the formation of a strong chemical bond between Cu and Mn atoms across the interface. Orbital reconstructions and associated covalent bonding are thus important factors in determining the physical properties of oxide heterostructures.

X-ray absorption spectroscopy studies of the magnetic-insulating ground state of Sr2IrO4 at ambient pressure show a clear deviation from a strong spin-orbit (SO) limit J(eff)=1/2 state, a result of local exchange interactions and a nonzero tetragonal crystal field mixing SO split J(eff)=1/2, 3/2 states. X-ray magnetic circular dichroism measurements in a diamond anvil cell show a magnetic transition at a pressure of ∼17  GPa, where the "weak" ferromagnetic moment is quenched despite transport measurements showing insulating behavior to at least 40 GPa. The magnetic transition has implications for the origin of the insulating gap and the nature of exchange interactions in this SO coupled system. The expectation value of the angular part of the SO interaction, , extrapolates to zero at ∼80-90  GPa where an increased bandwidth strongly mixes J(eff)=1/2, 3/2 states and SO interactions no longer dominate the electronic ground state of Sr2IrO4.

The electronic structure and magnetism of Ir 5d5 states in nonmetallic, weakly ferromagnetic BaIrO3 are probed with x-ray absorption techniques. Contrary to expectation, the Ir 5d orbital moment is found to be ~1.5 times larger than the spin moment. This unusual, atomiclike nature of the 5d moment is driven by a strong spin-orbit interaction in heavy Ir ions, as confirmed by the nonstatistical large branching ratio at Ir L(2,3) absorption edges. As a consequence, orbital interactions cannot be neglected when addressing the nature of magnetic ordering in BaIrO3. The local moment behavior persists even as the metallic-paramagnetic phase boundary is approached with Sr doping or applied pressure.

We show that high resolution Resonant Inelastic X-ray Scattering (RIXS) provides direct, elementspecific and momentum-resolved information on the electron-phonon (e-p) coupling strength. Our theoretical analysis demonstrates that the e-p coupling can be extracted from RIXS spectra by determining the differential phonon scattering cross section. An alternative, very direct 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 life-time. This allows measurement of the e-p coupling on an absolute energy scale. PACS numbers: 78.70.CkOften novel electronic properties of a material can be understood by systematically unravelling the interaction between its electrons and phonons. Tunable electric transport properties in molecular crystals, for instance, are explained by the presence of a strong electronphonon (e-p) coupling [1]. The dressing of electrons by phonons is also responsible for the colossal magnetoresistance effect in manganites [2]. More delicate is the role that the e-p interaction plays in high T c superconducting cuprates -topic of a persisting debate [3][4][5]. The lack of a technique to measure the e-p coupling strength perpetuates this controversy. Here we show that high resolution Resonant Inelastic X-ray Scattering (RIXS) can fill this void as it gives direct, element-specific and momentumresolved information on the coupling between electrons and phonons. We provide the theoretical framework required to distill e-p interaction strengths from RIXS, particularly in strongly correlated transition metal oxides such as the High T c cuprates.In RIXS experiments one scatters high energy, x-ray photons inelastically off a material [6]. The energy of the incident photons is chosen such that it coincides, and thus resonates, with an intrinsic electronic excitation of the material under study -one of the materials' x-ray absorption edges. At present the highest energy resolutions are reached at the L-edge of transition metal oxides, where an incident photon launches a 2p electron out of the atomic core into an empty 3d state around the Fermi-level. This highly unstable intermediate state decays rapidly, typically within 1-2 femtoseconds, so that the 2p core-state is refilled and an outgoing photon emitted. The state-of-the-art resolution is such that photon energy loss features on an energy scale of 25 meV can be distinguished at a copper or nickel L 3 -edge [7-9].This resolution has brought phonons within the energy window of observation and indeed last year for the first time phonon loss features were resolved in RIXS [8-10].To put this achievement in perspective, one should realize that the incident photons at the Cu L-edge have an energy of around 930 eV, implying experiments have a resolving power better than 10 4 . Advanced instrumentation will drive this up further. Here we show how the progress in accuracy allows the extraction of a number of characteristics of the e-p interaction directly from RIXS, including spatial...

Element-and orbital-selective x-ray absorption and magnetic circular dichroism measurements are carried out to probe the electronic structure and magnetism of Ir 5d electronic states in double perovskite Sr2MIrO6 (M=Mg, Ca, Sc, Ti, Ni, Fe, Zn, In) and La2NiIrO6 compounds. All the studied systems present a significant influence of spin-orbit interactions in the electronic ground state. In addition, we find that the Ir 5d local magnetic moment shows different character depending on the oxidation state despite the net magnetization being similar for all the compounds. Ir carries an orbital contribution comparable to the spin contribution for Ir 4+ (5d 5 ) and Ir 5+ (5d 4 ) oxides, whereas the orbital contribution is quenched for Ir 6+ (5d 3 ) samples. Incorporation of a magnetic 3d atom allows getting insight into the magnetic coupling between 5d and 3d transition metals. Together with previous susceptibility and neutron diffraction measurements the results indicate that Ir carries a significant local magnetic moment even in samples without a 3d metal. The size of the (small) net magnetization of these compounds is a result of predominant antiferromagnetic interactions between local moments coupled with structural details of each perovskite structure.

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