190 research outputs found
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 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 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-MnTiSe
Resonant valence-band X-ray photoelectron spectra (ResPES) excited near
2p core level energies, 2p X-ray photoelectron spectra (XPS) and
L X-ray absorption spectra (XAS) of Ti and Mn in single crystal of
1T-MnTiSe 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 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 MnTiSe excited near Ti 2p and Mn 2p
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 the Ti 3d states are localized in the vicinity of
point and the Mn 3d states are localized along the direction
K--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 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
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