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1.
J Phys Condens Matter ; 27(27): 275501, 2015 Jul 15.
Article in English | MEDLINE | ID: mdl-26086296

ABSTRACT

We propose a new scenario for the magnetic collapse under pressure in ferropericlase (FP) (Fe(1/4)Mg(3/4))O without the presence of intermediate spin state, which contradicts the mechanism proposed in (2013 Phys. Rev. B 87 165113). This scenario is supported by results of combined local density approximation + dynamical mean-field theory method calculations for the paramagnetic phase at ambient and high pressures. At ambient pressure, calculation gave (Fe(1/4)Mg(3/4))O as an insulator with Fe 3d-shell in high-spin state. Experimentally observed high-spin to low-spin state transition of the Fe(2+) ion in the pressure range of 35-75 GPa is successfully reproduced in our calculations. The spin crossover is characterized by coexistence of Fe(2+) ions in high and low spin state but intermediate spin state is absent in the whole pressure range. Moreover, the probability of Fe ion d(7) onfiguration with S = 1 grows with pressure due to shortening of metal-oxygen distance. Also, no metal-insulator transition was obtained up to the pressure 140 GPa in agreement with experiment.


Subject(s)
Ferrous Compounds/chemistry , Magnesium Oxide/chemistry , Magnetics , Pressure , Quantum Theory , Electron Spin Resonance Spectroscopy , Models, Molecular , Spectroscopy, Mossbauer
2.
J Phys Condens Matter ; 27(4): 045502, 2015 Feb 04.
Article in English | MEDLINE | ID: mdl-25566766

ABSTRACT

Recently in iron free arsenide compound CaCo(2)As(2) a 7(1)% of vacancies on the Co sites was detected (Quirinale D G et al 2013 Phys. Rev. B 88 174420). Here we report the investigation of electronic structure and magnetic properties of CaCo(1.86)As(2) within the coherent potential approximation (CPA). First, the CPA calculations are performed on the base of the local spin density approximation. Second, the possible role of Coulomb correlations is checked within the CPA scheme developed recently for strongly correlated systems. Then the spin-orbit coupling, which could be essential for Co, is also taken into account within the CPA calculation. The A type antiferromagnetic ground state and the value of magnetic moment obtained within the CPA approximation are in good agreement with experiment.

3.
J Phys Condens Matter ; 26(11): 115501, 2014 Mar 19.
Article in English | MEDLINE | ID: mdl-24589676

ABSTRACT

A method of electronic structure calculations for strongly correlated disordered materials is developed employing the basic idea of the coherent potential approximation. The evolution of the electronic structure and spin magnetic moment value with the concentration x in strongly correlated Ni1-xZnxO solid solutions is investigated in the framework of this method. The values of the energy gap and magnetic moment obtained are in agreement with the available experimental data.


Subject(s)
Electrons , Magnetics , Nickel/chemistry , Zinc Oxide/chemistry , Quantum Theory , Solutions
4.
J Phys Condens Matter ; 22(49): 495501, 2010 Dec 15.
Article in English | MEDLINE | ID: mdl-21406785

ABSTRACT

The electronic structures of bcc Np, fcc Pu, Am, and Cm pure metals under pressure have been investigated employing the LDA + U method with spin-orbit coupling (LDA + U + SO). The magnetic state of the actinide ions was analyzed in both LS and jj coupling schemes to reveal the applicability of corresponding coupling bases. It was demonstrated that whereas Pu and Am are well described within the jj coupling scheme, Np and Cm can be described appropriately neither in a {mσ}, nor in a {jmj} basis, due to intermediate coupling scheme realization in these metals that requires some finer treatment. The LDA + U + SO results for the considered transuranium metals reveal band broadening and gradual 5f electron delocalization under pressure.

5.
Phys Rev Lett ; 102(14): 146402, 2009 Apr 10.
Article in English | MEDLINE | ID: mdl-19392460

ABSTRACT

The local density approximation combined with dynamical mean-field theory is applied to study the paramagnetic and magnetically ordered phases of hematite Fe2O3 as a function of volume. As the volume is decreased, a simultaneous first-order insulator-metal and high-spin to low-spin transition occurs close to the experimental value of the critical volume. The high-spin insulating phase is destroyed by a progressive reduction of the spectral gap with increasing pressure, upon closing of which the high-spin phase becomes unstable. We conclude that the transition in Fe2O3 at approximately 50 GPa can be described as an electronically driven volume collapse.

6.
J Phys Condens Matter ; 21(7): 075602, 2009 Feb 18.
Article in English | MEDLINE | ID: mdl-21817332

ABSTRACT

The LDA+DMFT (local density approximation combined with dynamical mean-field theory) computation scheme has been used to calculate spectral properties of LaFeAsO-the parent compound of the new high-T(c) iron oxypnictides. The average Coulomb repulsion [Formula: see text] and Hund's exchange J parameters for iron 3d electrons were calculated using the first-principles constrained density functional theory scheme in the Wannier functions formalism. Resulting values strongly depend on the number of states taken into account in the calculations: when the full set of O-2p, As-4p and Fe-3d orbitals and the corresponding bands are included, the interaction parameters [Formula: see text] eV and J = 0.8 eV are obtained. In contrast, when the basis set is restricted to the Fe-3d orbitals and bands only, the calculation gives much smaller values of [Formula: see text] eV, J = 0.5 eV. Nevertheless, DMFT calculations with both parameter sets and the corresponding basis sets result in a weakly correlated electronic structure that is in agreement with the experimental x-ray and photoemission spectra.

7.
J Phys Condens Matter ; 21(43): 435702, 2009 Oct 28.
Article in English | MEDLINE | ID: mdl-21832444

ABSTRACT

Two different structural models for non-stoichiometric FeSe(x) are examined and compared with soft x-ray spectroscopy findings for FeSe(x) (x = 0.85, 0.50). A structural model of tetragonal FeSe with excess interstitial Fe gives better agreement with experiment than a structural model of tetragonal FeSe with Se vacancies. This interstitial Fe increases the number of 3d states at the Fermi level. We find evidence that large non-stoichiometric ratios of Fe:Se, such as that of FeSe(0.50), yield clusters of pure Fe in the crystal structure.

8.
Phys Rev Lett ; 95(19): 196404, 2005 Nov 04.
Article in English | MEDLINE | ID: mdl-16384003

ABSTRACT

We found direct experimental evidence for an orbital switching in the V 3d states across the metal-insulator transition in VO2. We have used soft-x-ray absorption spectroscopy at the V L2,3 edges as a sensitive local probe and have determined quantitatively the orbital polarizations. These results strongly suggest that, in going from the metallic to the insulating state, the orbital occupation changes in a manner that charge fluctuations and effective bandwidths are reduced, that the system becomes more one dimensional and more susceptible to a Peierls-like transition, and that the required massive orbital switching can only be made if the system is close to a Mott insulating regime.

9.
Phys Rev Lett ; 93(14): 146404, 2004 Oct 01.
Article in English | MEDLINE | ID: mdl-15524820

ABSTRACT

Charge and orbital ordering in the low-temperature monoclinic structure of magnetite (Fe3O4) is investigated using the local spin density approximation with Coulomb interaction correction method. While the difference between t(2g) minority occupancies of Fe(2+)(B) and Fe(3+)(B) cations is large and gives direct evidence for charge ordering, the screening is so effective that the total 3d charge disproportion is rather small. The charge order has a pronounced [001] modulation, which is incompatible with the Anderson criterion. The orbital order agrees with the Kugel-Khomskii theory.

10.
Phys Rev Lett ; 89(25): 257203, 2002 Dec 16.
Article in English | MEDLINE | ID: mdl-12484913

ABSTRACT

The contrasting ground states of isoelectronic, isostructural FeSi and FeGe are explained within an extended local density approximation scheme (LDA+U) by an appropriate choice of the Coulomb repulsion U on the Fe sites. A minimal two-band model with interband interactions leads to a phase diagram for the alloys FeSi1-xGex. A mean field approximation gives a first-order transition between a small gap semiconductor and a ferromagnetic metal as a function of magnetic field, temperature, and concentration x. Unusually the transition from metal to insulator is driven by broadening, not narrowing, the bands and it is the metallic state that shows magnetic order.

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