LDA+DMFT Spectral Functions and Effective Electron Mass Enhancement in Superconductor LaFePO

In this Letter we report the first LDA+DMFT results (method combining Local Density Approximation with Dynamical Mean-Field Theory) for spectral properties of superconductor LaFePO. Calculated {\bf k}-resolved spectral functions reproduce recent angle-resolved photoemission spectroscopy (ARPES) data [D. H. Lu {\it et al}., Nature {\bf 455}, 81 (2008)]. Obtained effective electron mass enhancement values $m^{*}/m\approx$ 1.9 -- 2.2 are in good agreement with infrared and optical studies [M. M. Qazilbash {\it et al}., Nature Phys. {\bf 5}, 647 (2009)], de Haas--van Alphen, electrical resistivity, and electronic specific heat measurements results, that unambiguously evidence for moderate correlations strength in LaFePO. Similar values of $m^{*}/m$ were found in the other Fe-based superconductors with substantially different superconducting transition temperatures. Thus, the dynamical correlation effects are essential in the Fe-based superconductors, but the strength of electronic correlations does not determine the value of superconducting transition temperature.Comment: 4 pages, 3 figure

Resonant photoemission at the absorption edge of Mn and Ti and electronic structure of 1T-Mn0.2_{0.2}TiSe2_2

Resonant valence-band X-ray photoelectron spectra (ResPES) excited near 2p$_{3/2}$ core level energies, 2p X-ray photoelectron spectra (XPS) and L$_{3,2}$ X-ray absorption spectra (XAS) of Ti and Mn in single crystal of 1T-Mn$_2$TiSe$_2$ were studied for the first time. The ionic-covalent character of bonds formed by Mn atoms with the neighboring Se atoms in the octahedral coordination is established. From the XPS and XAS measurements compared with results of atomic multiplet calculations of Ti and Mn L$_{3,2}$ XAS it is found that Ti atoms are in ionic state of 4+ and Mn atoms are in the state of 2+. In ResPES of Mn$_{0.2}$TiSe$_2$ excited near Ti 2p$_{3/2}$ and Mn 2p$_{3/2}$ absorption edges the Ti 3d and Mn 3d bands at binding energies just below the Fermi level are observed. According to band structure calculations E$(\textbf{k})$ the Ti 3d states are localized in the vicinity of $\mathsf{\Gamma}$ point and the Mn 3d states are localized along the direction K-$\mathsf{\Gamma}$-M in the Brillouin zone of the crystal.Comment: 18 pages (preprint), 9 figure

Band structure approach to the resonant x-ray scattering

We study the resonance behaviour of the forbidden 600 and 222 x-ray Bragg peaks in Ge using LDA band structure methods. These Bragg peaks remain forbidden in the resonant dipole scattering approximation even taking into account the non local nature of the band states. However they become allowed at resonance if the eigenstates of the unoccupied conduction band involve a hybridization of p like and d like atomic states. We show that the energy dependence of the resonant behaviour, including the phase of the scattering, is a direct measure of this p-d hybridization.and obtain quantitative agreement with experiment. A simple physical picture involving a product of dipole and quadrupolar transition matrix elements explains this behaviour and shows that it should be generally true for cases where the resonating atom is not at an inversion center. This has strong implications for the description of the resonance behavior of x-ray scattering in materials where the resonant atom is not at an inversion center such as V2O3 and in ferro and antiferro electric and piezo electric materials in general.Comment: 4 pages, 5figure

The nature of the ferromagnetic ground state in the Mn4 molecular magnet

Using ab initio band structure and model calculations we studied magnetic properties of one of the Mn$_4$ molecular magnets (Mn4(hmp)6), where two types of the Mn ions exist: Mn3+ and Mn2+. The direct calculation of the exchange constants in the GGA+U approximation shows that in contrast to a common belief the strongest exchange coupling is not between two Mn3+ ions (J_{bb}), but along two out of four exchange paths connecting Mn3+ and Mn2+ ions (J_{wb}). The microscopic analysis performed within the perturbation theory allowed to establish the mechanism for this largest ferromagnetic exchange constant. The charge ordering of the Mn ions results in the situation when the energy of the excited state in the exchange process is defined not by the large on-site Coulomb repulsion U, but by much smaller energy V, which stabilizes the charge ordered state. Together with strong Hund's rule coupling and specific orbital order this leads to a large ferromagnetic exchange interaction for two out of four Mn2+ --Mn3+ pairs.Comment: 12 pages, 10 figure

Correlation effects in Ni 3d states of LaNiPO

The electronic structure of the new superconducting material LaNiPO experimentally probed by soft X-ray spectroscopy and theoretically calculated by the combination of local density approximation with Dynamical Mean-Field Theory (LDA+DMFT) are compared herein. We have measured the Ni L2,3 X-ray emission (XES) and absorption (XAS) spectra which probe the occupied and unoccupied the Ni 3d states, respectively. In LaNiPO, the Ni 3d states are strongly renormalized by dynamical correlations and shifted about 1.5 eV lower in the valence band than the corresponding Fe 3d states in LaFeAsO. We further obtain a lower Hubbard band at -9 eV below the Fermi level in LaNiPO which bears striking resemblance to the lower Hubbard band in the correlated oxide NiO, while no such band is observed in LaFeAsO. These results are also supported by the intensity ratio between the transition metal L2 and L3 bands measured experimentally to be higher in LaNiPO than in LaFeAsO, indicating the presence of the stronger electron correlations in the Ni 3d states in LaNiPO in comparison with the Fe 3d states in LaFeAsO. These findings are in accordance with resonantly excited transition metal L3 X-ray emission spectra which probe occupied metal 3d-states and show the appearance of the lower Hubbard band in LaNiPO and NiO and its absence in LaFeAsO.Comment: 6 pages, 5 figure

Deep learning-based denoising for improved dose efficiency in EDX tomography of nanoparticles

Computer Science

Classification of the electronic correlation strength in the Fe-pnictides: The case of the parent compound BaFe2As2

Electronic correlations in the Fe-pnictide BaFe2As2 are explored within LDA+DMFT, the combination of density functional theory with dynamical mean-field theory. While the correlated band structure is substantially renormalized there is only little transfer of spectral weight. The computed k-integrated and k-resolved spectral functions are in good agreement with photoemission spectroscopy (PES) and angular resolved PES experiments. Making use of a general classification scheme for the strength of electronic correlations we conclude that BaFe2As2 is a moderately correlated system.Comment: 4 pages, 3 figure

LDA+DMFT Spectral Functions and Effective Electron Mass Enhancement in the Superconductor LaFePO

In this paper we report LDA+DMFT results (method combining local-density approximation with dynamical mean-field theory) for spectral properties of superconductor LaFePO. Calculated k -resolved spectral functions reproduce recent angle-resolved photoemission spectroscopy data. Obtained effective electron mass enhancement values m/≈1.9-2.2 are in good agreement with infrared and optical studies, de Haas-van Alphen, electrical resistivity, and electronic specific-heat measurements results that unambiguously evidence for moderate correlations strength in LaFePO. Similar values of m /m were found in the other Fe-based superconductors with substantially different superconducting transition temperatures. Thus, the dynamical correlation effects are essential in the Fe-based superconductors, but the strength of electronic correlations does not determine the value of superconducting transition temperature. © 2010 The American Physical Society