Particle-hole cumulant approach for inelastic losses in x-ray spectra

Kas, J. J.; Rehr, J. J.; Curtis, J. B.

doi:10.1103/physrevb.94.035156

Cited by 41 publications

(35 citation statements)

References 49 publications

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“…A similar situation seems to be present in other transition metal oxides like SrTiO 3 83 and its interfaces. 84,85 Furthermore, extrinsic loss that can be large for plasmonic satellites 86 , is not included in our calculations. We propose that the difference in spectral weight between the photoemssion experiments and our ab initio results in the inset of Fig.…”

Section: Spectral Functionsmentioning

confidence: 99%

Multitier self-consistent GW+EDMFT

Nilsson

Boehnke

Werner

et al. 2017

Phys. Rev. Materials

102

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We discuss a parameter-free and computationally efficient ab initio simulation approach for moderately and strongly correlated materials, the multitier self-consistent GW +EDMFT method. This scheme treats different degrees of freedom, such as high-energy and low-energy bands, or local and nonlocal interactions, within appropriate levels of approximation, and provides a fully self-consistent description of correlation and screening effects in the solid. The ab initio input is provided by a one-shot G 0 W 0 calculation, while the strong-correlation effects originating from narrow bands near the Fermi level are captured by a combined GW plus extended dynamical mean-field (EDMFT) treatment. We present the formalism and technical details of our implementation and discuss some general properties of the effective EDMFT impurity action. In particular, we show that the retarded impurity interactions can have non-causal features, while the physical observables, such as the screened interactions of the lattice system, remain causal. As a first application, we present ab initio simulation results for SrMoO3, which demonstrate the existence of prominent plasmon satellites in the spectral function not obtainable within LDA+DMFT, and provide further support for our recent re-interpretation of the satellite features in the related cubic perovskite SrVO3. We then turn to stretched sodium as a model system to explore the performance of the multitier self-consistent GW +EDMFT method in situations with different degrees of correlation. While the results for the physical lattice spacing a0 show that the scheme is not very accurate for electron-gas like systems, because nonlocal corrections beyond GW are important, it does provide physically correct results in the intermediate correlation regime, and a Mott transition around a lattice spacing of 1.5a0. Remarkably, even though the Wannier functions in the stretched compound are less localized, and hence the bare interaction parameters are reduced, the self-consistently computed impurity interactions show the physically expected trend of an increasing interaction strength with increasing lattice spacing.

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Section: Spectral Functionsmentioning

confidence: 99%

Multitier self-consistent GW+EDMFT

Nilsson

Boehnke

Werner

et al. 2017

Phys. Rev. Materials

102

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“…The cumulant approach has been successful at zero T in elucidating correlation effects beyond the GW approximation of MBPT in a variety of contexts [27][28][29][30][31][32]. For example, in contrast to GW , the approach explains the multiple-plasmon satellites observed in x-ray photoemission spectra (XPS) [27,[33][34][35][36].…”

mentioning

confidence: 99%

Finite Temperature Green’s Function Approach for Excited State and Thermodynamic Properties of Cool to Warm Dense Matter

Kas

Rehr

2017

Phys. Rev. Lett.

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We present a finite-temperature extension of the retarded cumulant Green's function for calculations of exited-state and thermodynamic properties of electronic systems. The method incorporates a cumulant to leading order in the screened Coulomb interaction W and improves excited state properties compared to the GW approximation of many-body perturbation theory. Results for the homogeneous electron gas are presented for a wide range of densities and temperatures, from cool to warm dense matter regime, which reveal several hitherto unexpected properties. For example, correlation effects remain strong at high T while the exchange-correlation energy becomes small. In addition, the spectral function broadens and damping increases with temperature, blurring the usual quasi-particle picture. Similarly Compton scattering exhibits substantial many-body corrections that persist at normal densities and intermediate T . Results for exchange-correlation energies and potentials are in good agreement with existing theories and finite-temperature DFT functionals. Finite temperature (FT) effects in electronic systems are both of fundamental interest and practical importance. These effects vary markedly depending on whether the temperature T is larger or smaller than the Fermi temperature T F (typically a few eV). At "cool" temperatures, where T is much smaller than T F , electrons are nearly degenerate, and Fermi factors and excitations such as phonons dominate the thermal behavior [1][2][3]. In contrast thermal occupations become nearly semi-classical and electronic excitations such as plasmons become important in the warm-dense-matter (WDM) regime, where T is of order T F or larger, and condensed matter is partially ionized. Recently there has been considerable interest in both experimental and theoretical investigations of WDM for applications ranging from laser-shocked systems and inertial confinement fusion to astrophysics [4,5]. Many of these studies focus on equilibrium thermodynamic properties, e.g., using the FT generalization of density functional theory (DFT) [6][7][8]. Although in principle, FT DFT is exact [9][10][11], practical applications require exchange-correlation functionals which must be approximated, e.g., by constrained fits [12] to theoretical electron gas calculations [8,[13][14][15][16][17]. However, these approaches have various limitations. First, many materials properties such as optical and x-ray spectra depend on quasi-particle or excited-state effects. For example, band-gaps depend on quasi-particle energies, and calculations of x-ray spectra [18] and Compton scattering require correlation corrections [19]. Although methods like quantum Monte-Carlo and the random phase approximation (RPA) can provide accurate correlation energies, they are not directly applicable to such excited state properties. Secondly, currently available exchangecorrelation functionals can exhibit unphysical behavior outside the range of validity of theoretical data [20]. On the other hand Green's function (GF) methods within...

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“…We also note that multi-electron excitation effects appear to be quite significant, judging from the loss of resolution of features above about 30 eV, just as related effects are significant in photoemission and give rise to features beyond quasiparticle peaks. 52 The theoretical reproduction of fine structure is particularly appealing when comparing the indices of refraction and absorption as in Figs. 3–4.…”

Section: Resultsmentioning

confidence: 99%

Quantitative first-principles calculations of valence and core excitation spectra of solidC60

et al. 2017

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We present calculated valence and C 1s near-edge excitation spectra of solid C60 and experimental results measured with high-resolution electron energy-loss spectroscopy. The near-edge calculations are carried out using three different methods: solution of the Bethe-Salpeter equation (BSE) as implemented in the OCEAN suite (Obtaining Core Excitations with ab initio methods and the NIST BSE solver), the excited-electron core-hole approach (XCH), and the constrained-occupancy method using the Stockholm-Berlin core-excitation code, StoBe. The three methods give similar results and are in good agreement with experiment, though the BSE results are the most accurate. The BSE formalism is also used to carry out valence level calculations using the NIST Bethe-Salpeter Equation solver (NBSE). Theoretical results include self-energy corrections to the band gap and band widths, lifetime-damping effects, and Debye-Waller effects in the core-excitation case. A comparison of spectral features to those observed experimentally illustrates the sensitivity of certain features to computational details, such as self-energy corrections to the band structure and core-hole screening.

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Particle-hole cumulant approach for inelastic losses in x-ray spectra

Cited by 41 publications

References 49 publications

Multitier self-consistent GW+EDMFT

Multitier self-consistent GW+EDMFT

Finite Temperature Green’s Function Approach for Excited State and Thermodynamic Properties of Cool to Warm Dense Matter

Quantitative first-principles calculations of valence and core excitation spectra of solidC60

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