Bremsstrahlung Isochromat Spectroscopy Research Articles

Thorium model and weak 5f delocalization

Recently, it was demonstrated that an empirical model based on bremsstrahlung isochromat spectroscopy of elemental thorium (Th) could be used to explain the experimental results and unoccupied 5f electronic structure for simple localized actinide systems with n = ⅔, 2, 3, and 5, where n is the 5f occupancy. Here, the thorium model will be extended to provide an understanding of the observed behavior and unoccupied 5f electronic structure in the uranium monochalcogenide systems of uranium sulfide and uranium telluride, in terms of weak 5f delocalization.

Underlying simplicity of 5f unoccupied electronic structure

Using a simple empirical model based upon the bremsstrahlung isochromat spectroscopy of elemental Th, it is possible to explain the recent high energy resolution fluorescence detection measurements of UF4 (n = 2) and UCd11 (n = 3) as well as the new inverse photoelectron spectroscopy of Pu2O3 (n = 5), where n is the 5f occupation number. A critical issue in this analysis is the assumption that the Th 5f states are essentially empty, which will be confirmed both experimentally and computationally. Thus, for 5f systems, this simple model provides a unified and consistent picture of 5f unoccupied density of states in simple, localized systems, as the 5f occupation varies in the early part of the series, for n = 0, 2, 3, and 5.

Ab initio self-energy embedding for the photoemission spectra of NiO and MnO

The accurate ab-initio simulation of periodic solids with strong correlations is one of the grand challenges of condensed matter. While mature methods exist for weakly correlated solids, the ab-initio description of strongly correlated systems is an active field of research. In this work, we show results for the single particle spectral function of the two correlated $d$-electron solids NiO and MnO from self-energy embedding theory. Unlike earlier work, the theory does not use any adjustable parameters and is fully ab-initio, while being able to treat both the strong correlation and the non-local screening physics of these materials. We derive the method, discuss aspects of the embedding and choices of physically important orbitals, and compare our results to x-ray and angle-resolved photoemission spectroscopy as well as bremsstrahlung-isochromat spectroscopy.

Separate measurement of the 5f5/2 and 5f7/2 unoccupied density of states of UO2

Coupling High Energy Resolution Fluorescence Detection (HERFD) with electric dipole selection rules, the uranium 5f5/2 and 5f7/2 Unoccupied Densities of States (UDOS) have been determined in UF4, UO2 and UCd11. Striking changes were observed between UF4 (5f2) to UCd11 (5f3), consistent with the Intermediate Coupling Model. Results for UO2 were confirmed by Bremstrahlung Isochromat Spectroscopy (BIS). This new approach can be a powerful tool for shedding light on the hidden mysteries of Actinides 5f-electrons properties.

Spectrum for nonmagnetic mott insulators from power functional within reduced density matrix functional theory.

We demonstrate that reduced density matrix functional theory (RDMFT), in conjunction with the power functional, can successfully treat the nonmagnetic insulating state of the transition metal oxides NiO and MnO, finding for both a gapped single particle spectrum. While long-range spin order is thus not necessary for qualitative agreement with experiment, we find that it is required for good agreement with the X-ray photoemission spectroscopy and Bremsstrahlung isochromat spectroscopy data. We further examine the nature of the natural orbitals in the materials, finding that they display significant Hubbard localization and are, as a consequence, very far from the corresponding Kohn-Sham orbitals. This contrasts with the case of the band insulator Si, in which the Kohn-Sham orbitals are found to be very close to the RDMFT natural orbitals.

Ab initiocalculations of the HubbardUfor the early lanthanides using the constrained random-phase approximation

In this work we have calculated $U$ from first principles for the early lanthanides (Ce-Gd) using the constrained random phase approximation (cRPA). We compare the static limit of $U$ with experimental estimations from x-ray photoemission spectroscopy (XPS) and bremsstrahlung isochromat spectroscopy (BIS) spectra and also discuss the frequency dependence of $U$ across the series.The localized subspace is constructed using maximally localized Wannier functions (MLWFs). We discuss the choice of parameters used to construct the Wannier functions and show that in the case of Eu and Gd, where the local density approximation does not yield the correct position of the $4f$ bands, a physically motivated choice gives better agreement with experiment.

Splittings, Satellites and Fine Structure in the Soft X-ray Spectroscopy of the Actinides

Perhaps the most demanding and powerful actinide spectroscopy is that using soft X-ray and VUV photons. Because of the relatively low energy and fairly small sampling depths of these photons and the corresponding electrons, it is necessary to use un-encapsulated samples with highly cleaned and well-prepared surfaces. This causes a myriad of sample containment problems for these radioactive materials. Despite these hindrances and difficulties, the soft-X-ray and ultra-violet spectroscopy of the actinides can provide an amazing level of detailed information, particularly having to do with 5f electronic structure. In this paper, the splittings, satellites and fine structure of the following actinide soft X-ray spectroscopies will be discussed: X-ray photoelectron spectroscopy; X-ray absorption spectroscopy; and inverse photoelectron spectroscopy, including Bremstrahlung isochromat spectroscopy and resonant inverse photoelectron spectroscopy.

Multi-electronic effects in uranium dioxide from X-ray Emission Spectroscopy

X-ray Emission Spectroscopy (XES) and Bremstrahlung Isochromat Spectroscopy (BIS), two electron-in/photon-out measurements, have been used to probe multi-electronic effects in the important actinide compound, uranium dioxide (UO2). The new observables in the XES are a distinct satellite, a core-to-core-transition, and the lack of a spin-orbit-split satellite peak. New XES data for 4f and 4p derived transitions are presented and discussed. Electron shielding in the 4d5/2 main and satellite XES peaks is discussed in light of the experimental observations.

An instrument for the investigation of actinides with spin resolved photoelectron spectroscopy and bremsstrahlung isochromat spectroscopy

A new system for spin resolved photoelectron spectroscopy and bremsstrahlung isochromat spectroscopy has been built and commissioned at Lawrence Livermore National Laboratory for the investigation of the electronic structure of the actinides. Actinide materials are very toxic and radioactive and therefore cannot be brought to most general user facilities for spectroscopic studies. The technical details of the new system and preliminary data obtained therein will be presented and discussed.

Confirmation of sample quality: X-ray and ultraviolet photoelectron spectroscopies of uranium dioxide

X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy have been utilized to demonstrate the sample quality of a UO2 specimen. This specimen is to be used in further studies with bremsstrahlung isochromat spectroscopy and Fano spectroscopy.

Electronic Structure of Strongly Correlated Systems

The article reviews the rich phenomena of metal-insulator transitions, anomalous metalicity, taking as examples iron and titanium oxides. The diverse phenomena include strong spin and orbital fluctuations, incoherence of charge dynamics, and phase transitions under control of key parameters such as band filling, bandwidth, and dimensionality. Another important phenomena presented in the article is a valence fluctuation which occur often in rare-earth compounds. We consider some Ce, Sm, Eu, Tm, and Yb compounds such as Ce, Sm and Tm monochalcogenides, Sm and Yb borides, mixed-valent and charge-ordered Sm, Eu and Yb pnictides and chalcogenides R4X3and R3X4(R = Sm, Eu, Yb; X = As, Sb, Bi), intermediate-valence YbInCu4and heavy-fermion compounds YbMCu4(M = Cu, Ag, Au, Pd). Issues addressed include the nature of the electronic ground states, the metal-insulator transition, the electronic and magnetic structures. The discussion includes key experiments, such as optical and magneto-optical spectroscopic measurements, x-ray photoemission and x-ray absorption, bremsstrahlung isochromat spectroscopy measurements as well as x-ray magnetic circular dichroism.

Dynamical coherent-potential approximation approach to excitation spectra in3dtransition metals

First-principles dynamical CPA (Coherent-Potential Approximation) for electron correlations has been developed further by taking into account higher-order dynamical corrections with use of the asymptotic approximation. The theory is applied to the investigations of a systematic change of excitation spectra in $3d$ transition metals from Sc to Cu at finite temperatures. It is shown that the dynamical effects damp main peaks in the densities of states (DOS) obtained by the local density approximation to the density functional theory, reduce the band broadening due to thermal spin fluctuations, create the Mott-Hubbard type bands in the case of fcc Mn and fcc Fe, and create a small hump corresponding to the `6 eV' satellite in the case of Co, Ni, and Cu. Calculated DOS explain the X-ray photoelectron spectroscopy data as well as the bremsstrahlung isochromat spectroscopy data. Moreover, it is found that screening effects on the exchange energy parameters are significant for understanding the spectra in magnetic transition metals.

Narrowing the range of possible solutions to the Pu electronic structure problem: Developing a new Bremstrahlung Isochromat Spectroscopy capability

Bremstrahlung Isochromat Spectroscopy (BIS) has a proven record for the probing of unoccupied electronic structure, including feasibility studies of actinides such as U and Th. A description of the BIS or Inverse Photoelectron Spectroscopy (IPES) process and of our reasoning concerning the potential importance of extending the application of BIS related techniques to the actinides, will be provided. To put this into the proper context, our past successes, using soft x-ray techniques to interrogate the electronic structure of Pu, will be briefly reviewed. Finally, the initial results of the calibration of the BIS device, including preliminary spectra from of the surrogate system CeOxide, will be presented.

Inverse photoemission of uranium oxides

Understanding the itinerant-localised bonding role of the 5 f electrons in the light actinides will afford an insight into their unusual physical and chemical properties. In recent years, the combination of core and valance band electron spectroscopies with theoretic modelling have already made significant progress in this area. However, information of the unoccupied density of states is still scarce. When compared to the forward photoemission techniques, measurements of the unoccupied states suffer from significantly less sensitivity and lower resolution. In this paper, we report on our experimental apparatus, which is designed to measure the inverse photoemission spectra of the light actinides. Inverse photoemission spectra of UO 2 and UO 2.2 along with the corresponding core and valance electron spectra are presented in this paper. UO 2 has been reported previously, although through its inclusion here it allows us to compare and contrast results from our experimental apparatus to the previous Bremsstrahlung Isochromat Spectroscopy and Inverse Photoemission Spectroscopy investigations.

Electron-spectroscopic investigation of the metal-insulator transition inSr2Ru1−xTixO4(x=0–0.6)

We investigate the nature and origin of the metal-insulator transition in ${\mathrm{Sr}}_{2}{\mathrm{Ru}}_{1\ensuremath{-}x}{\mathrm{Ti}}_{x}{\mathrm{O}}_{4}$ as a function of increasing Ti content $(x)$. Employing detailed core, valence, and conduction band studies with x-ray and ultraviolet photoelectron spectroscopies along with Bremsstrahlung isochromat spectroscopy, it is shown that a hard gap opens up for Ti content $\ensuremath{\geqslant}0.2$, while compositions with $x<0.2$ exhibit finite intensity at the Fermi energy, ${E}_{F}$. This establishes that the metal-insulator transition in this homovalent substituted series of compounds is driven by Coulomb interaction leading to the formation of a Mott gap, in contrast to transitions driven by disorder effects or band filling.

Competition between delocalization and spin-orbit splitting in the actinide5fstates

Synchrotron-radiation-based x-ray absorption, electron energy-loss spectroscopy in a transmission electron microscope, multielectronic atomic spectral simulations, and improved first-principles calculations (generalized gradient approximation in the local density approximation) have been used to investigate the electronic structure of the light actinides: $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Th}$, $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{U}$, and $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Pu}$. It will be shown that the spin-orbit interaction can be used as a measure of the degree of localization of valence electrons in a material. The spin-orbit interaction in the light actinide metals $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Th}$, $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{U}$, and $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Pu}$, has been determined using the branching ratio of the white line peaks of the ${N}_{4,5}$ edges, which correspond to $4d\ensuremath{\rightarrow}5f$ transitions. Examination of the branching ratios and spin-orbit interaction shows that the apparent spin-orbit splitting is partially quenched in $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{U}$, but is strongly dominant in $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Pu}$. These results are fully quantified using the sum rule. This picture of the actinide $5f$ electronic structure is confirmed by comparison with the results of electronic structure calculations for $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Th}$, $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{U}$, and $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Pu}$, which in turn are supported by a previous bremsstrahlung isochromat spectroscopy experiment.

Experimental Benchmarking of Pu Electronic Structure

AbstractThe standard method to determine the band structure of a condensed phase material is to (1) obtain a single crystal with a well defined surface and (2) map the bands with angle resolved photoelectron spectroscopy (occupied or valence bands) and inverse photoelectron spectroscopy (unoccupied or conduction bands). Unfortunately, in the case of Pu, the single crystals of Pu are either nonexistent, very small and/or having poorly defined surfaces. Furthermore, effects such as electron correlation and a large spin-orbit splitting in the 5f states have further complicated the situation. Thus, we have embarked upon the utilization of unorthodox electron spectroscopies, to circumvent the problems caused by the absence of large single crystals of Pu with well-defined surfaces. Our approach includes the techniques of resonant photoelectron spectroscopy [1], x-ray absorption spectroscopy [1,2,3,4], electron energy loss spectroscopy [2,3,4], Fano Effect measurements [5], and Bremstrahlung Isochromat Spectroscopy [6], including the utilization of micro-focused beams to probe single-crystallite regions of polycrystalline Pu samples. [2,3,6]

Using Nano-focussed Bremstrahlung Isochromat Spectroscopy (nBIS) to Determine the Unoccupied Electronic Structure of Pu

AbstractThe investigation of the actinides is of great interest because of their unique electronic structure. At the pivotal point of the behavior of the electronic structure of the actinide series is plutonium. Pu has the most complex phase diagram of all metals, both with regard to the intricacy of the crystal structures and the number of different phases. While there are a number of ongoing experimental efforts directed at determining the occupied electronic structure of Pu, there is essentially no experimental data on the unoccupied electronic structure of Pu. We aim to determine the conduction band (unoccupied) electronic structure of Pu and other actinides in a phase specific fashion and emphasizing bulk contributions by using Nano-focussed Bremstrahlung Isochromat Spectroscopy (nBIS). BIS is the high-energy variant of inverse photoelectron spectroscopy (IPES: electron in, photon out), which is essentially the time reversal of photoelectron spectroscopy (photon in, electron out). IPES can be used to follow the dispersion of electronic states in ordered samples. Owing to its low energies, IPES is usually very surface sensitive. However, by working at higher energies, we will sample preferentially for bulk properties, downgrading the impact of surface effects. Thus, from BIS, we would have a direct measure of the conduction band or unoccupied electronic structure of the bulk Pu. By using a nano-focused electron source associated with a SEM, we hope to gather phase specific information from crystallites within polycrystalline Pu samples. We will discuss the experimental arrangement required to carry out such an experiment and our progress in building such a system.

Photoemission study of electronic structure and metal-semiconductor transition in pyrochlore-typeTl2Ru2O7

The electronic structure of pyrochlore-type ${\mathrm{Tl}}_{2}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7},$ which undergoes a metal-semiconductor transition at ${T}_{t}\ensuremath{\sim}120\mathrm{K},$ has been studied by ultraviolet photoemission spectroscopy (UPS) and inverse-photoemission spectroscopy [bremsstrahlung isochromat spectroscopy (BIS)]. The temperature dependence of the UPS intensity around the Fermi level ${(E}_{F}),$ which is largely due to the Ru $4d$ states, reflects the transition. In the BIS spectra, changes occur across the metal-semiconductor transition not only near ${E}_{F}$ but also several eV above ${E}_{F}$ where the Ru $4d$ ${e}_{g}$ states and the Tl $6s$ states are dominant. It is therefore concluded that the transport properties across the metal-semiconductor transition in ${\mathrm{Tl}}_{2}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ are mainly determined by the Ru ${4dt}_{2g}$ states around ${E}_{F}$ but that the changes in the Tl $6s$ states also make contributions to the transition.

Electronic structure and magneto-optical Kerr effect of Tm monochalcogenides

The optical and magneto-optical (MO) spectra of Tm monochalcogenides are investigated theoretically from first principles, using the fully relativistic Dirac linear combination of muffin-tin orbitals band structure method. The electronic structure is obtained with the local-spin-density approximation (LSDA), as well as with the so-called $\mathrm{LSDA}+U$ approach. In contrast to LSDA, where the stable solution in TmTe is a metal, the $\mathrm{LSDA}+U$ gave an insulating ground state. $\mathrm{LSDA}+U$ theory predicts the thulium ion in TmTe to be in an integer divalent state. It also shows a gradual decreasing of the energy gap with reducing of the lattice constant. $\mathrm{LSDA}+U$ theoretical calculations produce a similar energy band structure in TmS and TmSe, with twelve $4f$ bands fully occupied and hybridized with chalcogenide p states. The 14th f hole level was found to be completely unoccupied and well above the Fermi level and a hole 13th f level is partly occupied and pinned at the Fermi level. The occupation number of the 13th f level is equal to 0.12 and 0.27 in TmS and TmSe, respectively (valence 2.88+ and 2.73+). Such an energy band structure of thulium monochalcogenides describes well their measured bremsstrahlung isochromat spectroscopy (BIS), and x-ray and ultraviolet photoemission spectra as well as the optical and MO spectra. The origin of the Kerr rotation realized in the compounds is examined.