Inelastic X-ray Scattering by Electronic Excitations in Solids at High Pressure

Investigating electronic structure and excitations under extreme conditions gives access to a rich variety of phenomena. High pressure typically induces behavior such as magnetic collapse and the insulator-metal transition in 3d transition metals compounds, valence fluctuations or Kondo-like characteristics in $f$-electron systems, and coordination and bonding changes in molecular solids and glasses. This article reviews research concerning electronic excitations in materials under extreme conditions using inelastic x-ray scattering (IXS). IXS is a spectroscopic probe of choice for this study because of its chemical and orbital selectivity and the richness of information it provides. Being an all-photon technique, IXS has a penetration depth compatible with high pressure requirements. Electronic transitions under pressure in 3d transition metals compounds and $f$-electron systems, most of them strongly correlated, are reviewed. Implications for geophysics are mentioned. Since the incident X-ray energy can easily be tuned to absorption edges, resonant IXS, often employed, is discussed at length. Finally studies involving local structure changes and electronic transitions under pressure in materials containing light elements are briefly reviewed.Comment: submitted to Rev. Mod. Phy

Light-induced ultrafast dynamics of spin crossovers in LaCoO3

Ultrafast quantum dynamics relaxation of a photoexcited state in a strongly correlated spin crossover system LaCoO3 under a sudden perturbation is considered with the density matrix generalized master equation. The magnetization and cobalt-oxygen bond length oscillations were found. The evolution of the electronic band structure during relaxation is calculated in the framework of the LDA+GTB method

Bulk sensitive Photoelectron Spectroscopy of strongly correlated transition metal oxides

Modern solid state research has focused to a considerable extent on the physics of strongly correlated electron systems. Being such systems, the transition metal oxides represent model systems for the development of theoretical descriptions. At the same time, the physical properties found in these materials are invaluable for technological applications. Photoelectron spectroscopy is a well established tool to investigate the electronic structure of solids and solid surfaces. A key characteristic of this technique is its inherent surface sensitivity. This problem is most acute for strongly correlated materials, since their electronic structure depends strongly on their local environment. We report in chapter 2 on the different contributions from surface and bulk of LaCoO3 in the range from 65 - 300 K, using photon energies between 450 eV and 6 keV and varying degrees of surface sensitivity. We observe a prominent low binding energy peak, which is characteristic of a Co3+ low spin state. The intensity of the peak is reduced in going from low to high temperature. By contrast, previously published spectra do not show a pronounced peak in this energy region, and no temperature dependence up to room temperature either. We can show that the literature results are not representative for the bulk system. By analyzing the emission angle dependence we have been able to separate the bulk and surface contributions. We observe a temperature dependent, predominantly low-spin bulk spectrum. At the same time, the surface is temperature independent in a high spin state. In chapter 3, we present a study of the valence and conduction bands of VO2 across the metal-insulator transition using bulk-sensitive photoelectron and O-K x-ray absorption spectroscopies. Our measurements reveal a giant transfer of spectral weight. Particular spectral features signal that the transition is not of the standard Peierls nor single-band Mott-Hubbard type. The valence band spectrum of the metallic phase is characterized by two structures in the V 3d contribution which can be identified with the coherent and incoherent parts of the spectrum. The symmetry and energies of the bands are discussed in connection to the recently determined orbital occupation and to the importance of the k-dependence of the self-energy correction. This analysis reveals the decisive role of the V 3d orbital degrees of freedom. The orbital switching is the key for opening a band gap that is much larger than the energy scale of the transition temperature. Comparison to recent realistic many body calculations within the dynamical mean field theory using a two-site cluster, shows that much of the k-dependence of the self-energy correction can be cast within a dimer model. The results of the bulk-sensitive photoemission study of the low temperature insulating phase of Ti2O3 using both soft and hard x-ray photons are presented in chapter 4. We find in both the valence band and the Ti 2p spectra structures that have not been reported yet. The Ti 3d spectral weight displays a two peak structure in the insulating state, which can be identified with the bonding- and antibonding contributions associated with the c axis Ti-dimers. Also in the Ti 2p core level spectra we observe additional satellites at lower binding energies which can be explained qualitatively in terms of non-local screening effects from the hydrogen molecule model. Finally, in chapter 5 GdBaCo2O5.5 is presented. The Co ions possess the same 3d6 configuration as in LaCoO3, however the distinct crystalline environment leads to considerably different properties. We find that the gap remains finite up to 380 K, implying that the high temperature phase above the so-called metal-insulator transition is actually not metallic. We show furthermore that a dominating low-spin contribution commonly claimed for the octahedral site at low temperatures, does not show up in the valence band spectra. From the line shape and the weak temperature dependence in comparison with LaCoO3, we can derive an upper bound for the low spin contribution of 25% at 80 K. These findings cast doubts on most of the current models proposed to explain the complex magnetic and transport behaviour of the layered cobaltates. Our results of bulk-sensitive photoelectron spectroscopy on selected correlated materials demonstrate that indeed there is a need for this kind of investigations. A large number of systems yet remains to be studied in this respect for the future

Effects of spin and orbital correlations on the optical spectral weights of transition-metal oxides

Within the scope of this thesis different transition-metal oxides with open d shells are investigated by means of spectroscopic ellipsometry in the energy range from 0.75eV to 5.5eV for temperatures ranging from 15K to 490K. The focus is on spin and orbital degrees of freedom and their impact on the optical spectra. The multipeak structures observed in the optical conductivity show a pronounced dependence on both temperature and polarization. We analyze the spectra in terms of multiplets which form the upper Hubbard band. Our optical analysis of the multi-orbital Mott-Hubbard insulators YVO3, GdVO3, and CeVO3 yields a consistent description of the observed absorption bands in terms of 3d3 excited states which constitute the upper Hubbard band and thereby solves the discrepancies of the optical spectra of YVO3 reported in the literature. The temperature and polarization dependence of the optical spectra reflects the complex spin and orbital ordering phase diagram of RVO3 (R=Y, rare earth ion). A comparison of our data with theoretical predictions based on either rigid orbital order or strong orbital fluctuations leads us to the conclusion that orbital fluctuations cannot be strong in RVO3. The line shape and temperature dependence of a feature observed in the optical conductivity at around 2eV gives evidence for an excitonic Mott-Hubbard resonance, i.e. not a truly bound state below the gap but a resonance within the absorption band, and demonstrates the important role played by the kinetic energy for exciton formation in orbitally ordered Mott-Hubbard insulators. Due to the layered structure of the correlated insulator LaSrFeO4 the optical spectra strongly depend on polarization. This anisotropy in combination with their different spectral weights offers an efficient tool to disentangle Mott-Hubbard excitations, corresponding to an electron transfer between neighboring Fe(3+) sites, and charge-transfer excitations, corresponding to an electron transfer from the oxygen 2p band to the Fe 3d band. We arrive at a consistent peak assignment and find that the lowest dipole-allowed excitation, which contributes to the in-plane optical conductivity only, is of Mott-Hubbard type. This result is rather unexpected at first sight as the 3d5 electron configuration of Fe(3+) is particularly stable due to the intra-atomic Hund exchange. We argue that the Fe 3d - O 2p hybridization and particularly the large splitting of the eg level originating from the tetragonal structure justify our result. The temperature dependence of the Mott-Hubbard excitations is only weak. This finding is in accordance with the fact that variations of nearest-neighbor spin-spin and orbital-orbital correlations are not strong below room temperature in LaSrFeO4 with a Néel temperature of 366K. A further concern of this work lies on the correlated insulators LaCoO3 and EuCoO3 with Co(3+) 3d6 electron configuration, which have attracted a lot of interest because of the spin-state degree of freedom. The low-spin state, the intermediate-spin state, and the high-spin state, lie energetically close in these pseudocubic perovskites. It is well established that a thermal population of the high-spin state takes place from the low-spin ground state at T>25K in LaCoO3. This so-called spin-state transition is shifted to much higher temperatures in the sister compound EuCoO3. In addition, we present optical data of the single-layered perovskites La(2-x)SrxCoO4 (x=0, 0.33, 0.45, 0.5, 0.9) and La1.5Ca0.5CoO4, which contain both Co(3+) 3d6 and Co(2+) 3d7 ions. In agreement with claims of the literature for a doping-induced spin-state transition from a Co(3+) low-spin state being realized in La(2-x)Sr(x)CoO4 for doping concentrations x<0.8 to a state of mixed Co(3+) low-spin and high-spin ions in LaSrCoO4, our optical spectra of La1.1Sr0.9CoO4 differ considerably from the spectra of the compounds with smaller doping concentrations. We assign the observed absorption bands to charge-transfer excitations from the oxygen 2p bands to the upper Hubbard bands. Surprisingly, we observe only small changes in the optical spectra of LaCoO3 across the spin-state transition temperature, our spectra of LaCoO3 resemble the spectra of La1.1Sr0.9CoO4 already at low temperatures

Analyzing the Local Electronic Structure of Co3_3O4_4 Using 2p3d Resonant Inelastic X-ray Scattering

We present the cobalt 2p3d resonant inelastic X-ray scattering (RIXS) spectra of Co$_3$O$_4$. Guided by multiplet simulation, the excited states at 0.5 and 1.3 eV can be identified as the $^4$T$_2$ excited state of the tetrahedral Co$^{2+}$ and the $^3$T$_{2g}$ excited state of the octahedral Co$^{3+}$, respectively. The ground states of Co$^{2+}$ and Co$^{3+}$ sites are determined to be high-spin $^4$A$_2$(T$_d$) and low-spin $^1$A$_{1g}$(O$_h$), respectively. It indicates that the high-spin Co$^{2+}$ is the magnetically active site in Co$_3$O$_4$. Additionally, the ligand-to-metal charge transfer analysis shows strong orbital hybridization between the cobalt and oxygen ions at the Co$^{3+}$ site, while the hybridization is weak at the Co$^{2+}$ site

Optical study of orbital excitations in transition-metal oxides

The orbital excitations of a series of transition-metal compounds are studied by means of optical spectroscopy. Our aim was to identify signatures of collective orbital excitations by comparison with experimental and theoretical results for predominantly local crystal-field excitations. To this end, we have studied TiOCl, RTiO3 (R=La, Sm, Y), LaMnO3, Y2BaNiO5, CaCu2O3, and K4Cu4OCl10, ranging from early to late transition-metal ions, from t_2g to e_g systems, and including systems in which the exchange coupling is predominantly three-dimensional, one-dimensional or zero-dimensional. With the exception of LaMnO3, we find orbital excitations in all compounds. We discuss the competition between orbital fluctuations (for dominant exchange coupling) and crystal-field splitting (for dominant coupling to the lattice). Comparison of our experimental results with configuration-interaction cluster calculations in general yield good agreement, demonstrating that the coupling to the lattice is important for a quantitative description of the orbital excitations in these compounds. However, detailed theoretical predictions for the contribution of collective orbital modes to the optical conductivity (e.g., the line shape or the polarization dependence) are required to decide on a possible contribution of orbital fluctuations at low energies, in particular in case of the orbital excitations at about 0.25 eV in RTiO3. Further calculations are called for which take into account the exchange interactions between the orbitals and the coupling to the lattice on an equal footing.Comment: published version, discussion of TiOCl extended to low T, improved calculation of orbital excitation energies in TiOCl, figure 16 improved, references updated, 33 pages, 20 figure

HAXPES on Transition Metal Oxides:: new insights into an effective use of the photoionization cross-sections

Materials containing transition metals and rare earth elements have continued to attract attention due to many fascinating properties that emerge from the intricate interplay between the electron correlation effects, which arise from the strong Coulomb interactions often present in d and f orbitals, with the band formations in the periodic structure of the solids. The mathematical description of such systems, however, is highly complex and results in unsolvable sets of equations. Thus, an appropriate model must be chosen on each case and tested. Experimental input is thus needed as a verification, and also as guidance to make better models. Amongst the wide range of experimental techniques available to determine the electronic structure, photoelectron spectroscopy is special due to the close relation between the spectra that is measured with these techniques and the one particle Green's functions, providing very direct information content. Photoelectron spectroscopy is a very well established experimental technique, but when used on bulk materials, it can have one major issue: the surface sensitivity. The electronic structure of the surface is not the same as in the bulk, and in correlated systems, these differences can lead into a major alteration of the electronic structure due to the delicate balance of different interaction strengths, and so it is crucial that spectra representative of the bulk can be obtained. The most reliable way in which the surface contributions can be minimised for photoelectron spectroscopy is by using its high photon energy variant also known as Hard X-ray Photoelectron Spectroscopy (HAXPES). HAXPES is more bulk sensitive and also has several further advantages with respect to its lower energy counterparts such as the possibility to measure buried interfaces, reach deeper core levels, or greater polarisation dependence effects, to name a few examples. Despite all these advantages, HAXPES is still not very widely used for the study of valence bands due to several challenges in the interpretation of the spectra. In this thesis, we show that while often considered to be a minor detail or even neglected, understanding the photo-ionization cross-sections is crucial for the correct interpretation of its spectra. We study many of the different ways in which the cross-sections can affect the HAXPES experimental spectra, in order to learn how to make an effective use of them in our favour to get the information that is most relevant to us in each case. In order to achieve this, we study several carefully selected transition metal oxides with HAXPES as well as with other PES variants. We start by solving an apparent contradiction which prevented a proper understanding of the HAXPES valence band spectra of compounds containing transition metals and rare earths, and could have been one of the reasons why there are not so many studies with HAXPES on the valence band in the literature. We will show that due to the cross-section relations, we cannot neglect contributions from e.g. the La 5p, which are typically considered almost core-like and irrelevant for the physics studied in transition metal oxides. We will focus on LaCoO3 as a representative example, but also provide a few more examples and provide a guide to see in which cases it may be necessary to make such considerations. We make use of the very high cross-sections of 5d materials in HAXPES to study the class of double perovskite iridates, which have recently attracted interest as candidates for Kitaev physics. Due to the high Ir 5d cross-sections, the valence band spectra is completely dominated by the iridium spectral weight, making it possible to directly compare the spectra of many different iridates despite containing a wide range of ions in their composition. By fitting our calculations parameters to the experimental spectra, we conclude that the double perovskite iridates are highly covalent systems with essentially zero charge transfer, which would result in long-ranged interactions limiting the extent to which the Kitaev model can materialise. Two different compounds are studied by complementing the information from the element-specific core level spectra and the valence band spectra, which has other contributions mixed but is more sensitive to parameters such as the hybridization strength: First, we present a temperature dependence study of the LaCoO3 HAXPES combined with O-K X-ray absorption spectroscopy to investigate the gradual low-to-high spin transition observed with increased temperature. Our results suggest a scenario in which paramagnetic LaCoO3 should be considered as an inhomogeneous mixed spin-state system. Then, we present a comprehensive photoemission study of CaCu3Ru4O12, a very rare system with 3d transition metal ions that according to some claims display Kondo behaviour. The HAXPES spectra is used to tune the parameters for LDA+DMFT calculations. Additional photoemission measurements are also performed with a wide range of energies to change the ratio of the multiple contributions, allowing us to find a small resonance peak in the Cu 3d as predicted by the calculations. From the calculations and their good agreement with the experimental studies, we conclude that CaCu3Ru4O12 is a Kondo material, but with a very high Kondo temperature, finding a compromise between both sides of the literature and concluding a long standing debate. Finally, we experimentally show that the initial state dependence, as predicted by Fadley et al. from the cross-section formulas, can be observed by measuring a single crystal sample with HAXPES on different orientations. We show, using the example of ReO3, that a clear orientation dependence is present in many of the valence band feature. By applying the appropriate corrections, the shape of the atomic orbitals is recovered in the angular intensity plot. Other samples are also measured to prove that this technique can be used in a wide range of compounds

Mechanism of spin crossover in LaCoO3 resolved by shape magnetostriction in pulsed magnetic fields

In the scientific description of unconventional transport properties of oxides (spin-dependent transport, superconductivity etc.), the spin-state degree of freedom plays a fundamental role. Because of this, temperature- or magnetic field-induced spin-state transitions are in the focus of solid-state physics. Cobaltites, e.g. LaCoO3, are prominent examples showing these spin transitions. However, the microscopic nature of the spontaneous spin crossover in LaCoO3 is still controversial. Here we report magnetostriction measurements on LaCoO3 in magnetic fields up to 70 T to study the sharp, field-induced transition at Hc ~ 60 T. Measurements of both longitudinal and transversal magnetostriction allow us to separate magnetovolume and magnetodistortive changes. We find a large increase in volume, but only a very small increase in tetragonal distortion at Hc. The results, supported by electronic energy calculations by the configuration interaction cluster method, provide compelling evidence that above Hc LaCoO3 adopts a correlated low spin/high spin state.Comment: 14 pages, 4 figure