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ABSTRACT: The electronic structure of the recently discovered LaCo2B2 was studied from first-principles calculations. Our results indicate that the hybridization between Co-3d and B-2p is much stronger than that between Fe-3d and As-4p in LaOFeAs and BaFe2As2, which removes the magnetic ordering of Co ions. Therefore, the ground state of LaCo2B2 is nonmagnetic. At the Fermi level, the density of state of La-5d is very high and here are four bands cutting across the Fermi level, which are mostly derived from dz2−3r2 and dyz/zx orbitals. Such results are different with both LaOFeAs and BaFe2As2. The substitution of La by Y atoms would not change the ground state, while substitution of La with Sc atoms might induce a magnetic moment on the Co atom. Comparing with LuNi2B2C2, we classify LaCo2B2 as a BCS superconductor.
EPL (Europhysics Letters) 06/2011; 95(1):17001. · 2.17 Impact Factor
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ABSTRACT: By systematic first-principles calculations, we study the electronic structure and magnetic property of SrCrO$_3$. Our results suggest that SrCrO$_3$ is a weakly correlated antiferromagnetic (AF) metal, a very rare situation in transition-metal oxides. Among various possible AF states, the C-type spin ordering with small amount of orbital polarization (dxy orbital is more occupied than the d_{yz/zx} orbital) is favored. The detailed mechanism to stabilize the C-type AF state is analyzed based on the competition between the itinerant Stoner instability and superexchange, and our results suggest that the magnetic instability rather than the orbital or lattice instabilities plays an important role in this system. The experimentally observed compressive tetragonal distortion can be naturally explained from the C-type AF state. By applying the LDA+$U$ method to study this system, we show that wrong ground state will be obtained if $U$ is large. Comment: 17 pages 15 figures
07/2010;
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ABSTRACT: We obtained wave velocities of FeSiO3 within perovskite and post-perovskite phases at 130 GPa by first-principles calculations in order to understand the abrupt reduction in seismic velocity at the core–mantle boundary. Our results proved that high iron density significantly reduces the seismic velocity. The elastic anisotropy, electronic structure and chemical bonding of FeSiO3 in perovskite and post-perovskite phases are extensively explored to illustrate the variation of mechanical and magnetic properties with pressure. Magnetic collapse was predicted in the perovskite phase, which is attributed to the pressure-induced broadening of 3d valence bands of iron.
High Pressure Research 06/2010; 30(2):292-300. · 0.78 Impact Factor
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ABSTRACT: The electronic structures of FeAs compounds are sensitive to FeAs bonding, which is described unsuccessfully by the local density approximation (LDA). Treating the multiorbital fluctuations from ab inito LDA+Gutzwiller method, we can now predict the correct FeAs bond length and bonding strength, which will explain the observed "soft phonon." The bands are narrowed by a factor of 2 from their LDA widths. The d{3z{2}-r{2}} orbital is pushed up to cross the Fermi level, forming a three-dimensional Fermi surface, which reduces the anisotropy. The interorbital Hund's coupling J rather than U plays a crucial role in obtaining these results.
Physical Review Letters 01/2010; 104(4):047002. · 7.37 Impact Factor
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ABSTRACT: Inspired by the experience in CuO-based superconductor that a larger spacing distance between CuO planes induced a higher T(C), some researchers synthesized (Sr(3)Sc(2)O(5))Fe(2)As(2) and related compounds with spacing distances between FeAs planes as large as 15 Å and expected a higher T(C). Our density functional calculations indicate that the Fermi surface of (Sr(3)Sc(2)O(5))Fe(2)As(2) is very similar to that of LaOFeAs, while the projected band structure shows some differences. From Fermi surface nesting and the calculated Lindhard response function χ(q), we predict that a spin density wave (SDW) and stripe antiferromagnetism (AF) may exist in the undoped compound and that electron or hole doping will suppress this SDW state. Similar to LaFeAsO, both the stabilization energy and the moment are very sensitive to the As atom positions. Because of the considerable similarity to LaFeAsO, (Sr(3)Sc(2)O(5))Fe(2)As(2) is expected to become a superconductor with electron or hole doping.
Journal of Physics Condensed Matter 10/2009; 21(41):415702. · 2.55 Impact Factor
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ABSTRACT: Inspired by the experience in CuO-based superconductor that a larger spacing distance between CuO planes induced a higher TC, some researchers synthesized (Sr3Sc2O5)Fe2As2 and related compounds with spacing distances between FeAs planes as large as 15 Å and expected a higher TC. Our density functional calculations indicate that the Fermi surface of (Sr3Sc2O5)Fe2As2 is very similar to that of LaOFeAs, while the projected band structure shows some differences. From Fermi surface nesting and the calculated Lindhard response function χ(q), we predict that a spin density wave (SDW) and stripe antiferromagnetism (AF) may exist in the undoped compound and that electron or hole doping will suppress this SDW state. Similar to LaFeAsO, both the stabilization energy and the moment are very sensitive to the As atom positions. Because of the considerable similarity to LaFeAsO, (Sr3Sc2O5)Fe2As2 is expected to become a superconductor with electron or hole doping.
Journal of Physics Condensed Matter 09/2009; 21(41):415702. · 2.55 Impact Factor
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ABSTRACT: By the first-principles calculations, we studied the structure, electronic and magnetic properties of LaOMnSe. The band structure and Fermi surface of LaOMnSe are very similar to those of LaOFeAs, where there are three hole-like Fermi surfaces around Γ-point and two electron-like Fermi surfaces around M-point. The hole-like Fermi surfaces will strongly overlap with electron-like Fermi surfaces if they are shifted by the q vector (π,π,0). Such Fermi surfaces nesting will induce magnetic instability and spin density wave (SDW), which is similar to LaOFeAs. Because of so much similarity to LaOFeAs, LaOMnSe is expected to become superconductor with electron or hole doping.
Physics Letters A - PHYS LETT A. 01/2009; 374(2):351-354.
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ABSTRACT: The ground states of Na$_x$CoO$_2$ ($0.0<x<1.0$) is studied by the LDA+Gutzwiller approach, where charge transfer and orbital fluctuations are all self-consistently treated {\it ab-initio}. In contrast to previous studies, which are parameter-dependent, we characterized the phase diagram as: (1) Stoner magnetic metal for $x>0.6$ due to $a_{1g}$ van-Hove singularity near band top; (2) correlated non-magnetic metal without $e_g^{\prime}$ pockets for $0.3<x<0.6$; (3) $e_g^{\prime}$ pockets appear for $x<0.3$, and additional magnetic instability involves. Experimental quasi-particle properties is well explained, and the $a_{1g}$-$e_g^{\prime}$ anti-crossing is attributed to spin-orbital coupling. Comment: 4 pages, 3 figures
01/2008;
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ABSTRACT: Inspired by the experience in CuO-based superconductor that larger spacing distance between CuO planes induced higher superconductivity transition temperature (TC), some researchers synthesized (Sr2ScO3)2Fe2As2 and (Sr2VO3)2Fe2As2 with the spacing distance between FeAs layers as large as 15.66 Å and found a TC of 37.2 K in the latter compound. Our density-functional calculations indicate that the ground states of (Sr2ScO3)2Fe2As2 and (Sr2VO3)2Fe2As2 are stripe antiferromagnetic and checkerboard antiferromagnetic, respectively. The band structure and Fermi surface of (Sr2ScO3)2Fe2As2 are similar to those of LaOFeAs, while those of (Sr2VO3)2Fe2As2 show significant difference. In (Sr2VO3)2Fe2As2, both Fe 3d and V 3d states contribute to the Fermi surfaces, which implies that the V 3d states may play important roles in the superconductivity.
Phys. Rev. B. 80(18).
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ABSTRACT: The electronic structure of Sr2CuMn2As2O2 and Sr2CuFe2As2O2 are studied by the first-principle calculations. These compounds have a body-centered-tetragonal crystal structure that consists of the CuO2 layers similar to those in the high-Tc cuprate superconductor, and intermetallic MAs (M = Mn, or Fe) layers similar to the FeAs layers in high-Tc pnictides. Such special structure makes them as interesting candidates for new type of superconductor since they have two types of superconducting layers. However, our calculations indicate that the states in the range from −2.0 eV to +2.0 eV are dominated by Mn-3d or Fe-3d states, while the states of Cu-3d are far away from the Fermi level (in the range from −3.0 eV to −1.0 eV). Such results are significantly different with the Cu-based superconductor, like La2CuO4, where the states around Fermi level are dominated by Cu-3d states. Besides, we find that the mean-field magnetic ground state is the checkerboard antiferromagnetic in Cu sublattice and the stripe antiferromagnetic in Fe (or Mn) sublattice.
Physics Letters A. 374(46):4727-4731.
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ABSTRACT: The electronic, magnetic, and orbital structures of KCrF3 in its recently identified crystallographic phases (tetragonal and cubic) [ S. Margadonna and G. Karotsis J. Am. Chem. Soc. 128 16436 (2006)] are studied by the first-principles method. In the tetragonal phase, both the generalized gradient approximation (GGA) and the generalized gradient approximation plus Hubbard parameter U (GGA + U) calculations show that the ground state is the A-type antiferromagnetic (A-AFM) configuration with G-type orbital ordering pattern. Our calculations show that the orbital structures and the magnetic configurations can be measured by the optical conductivity. In the cubic state, the GGA calculations show that the ground state is a ferromagnetic half-metal state, while the GGA + U (Ueff=3.0 eV) calculations show that the A-AFM insulator phase is the ground state. Our calculations indicate that the electron-electron interactions rather than the electron-phonon interactions are the driving forces behind the orbital ordering.
Phys. Rev. B. 84(4).
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ABSTRACT: The magnetism in SrFe2As2 can be suppressed by electron doping with a small substitution of Fe by Co or Ni, giving the way to superconductivity. Using the virtual-crystal approximation, we find the isoelectronic substitution of Fe with Ru suppresses the spin-density-wave characteristic of SrFe2As2 by decreasing the Stoner enhancement and increasing the bandwidth due to the more extended bandwidth of Ru 4d compared with that of Fe 3d. Although Ru is isoelectronic with Fe, such substitution changes the Fermi surface and band structure greatly. From the first-principles calculation, we find the phase diagram of SrFe2−xRuxAs2 can be sorted as three different phases: (I) the stripe antiferromagnetic state (0.0≤x<0.6); (II) the low spin state (0.6≤x<1.0), and (III) the nonmagnetic state (1.0≤x≤2.0). In phase II, the spin-density wave is suppressed greatly, accompanying with the emergence of superconductivity.
Phys. Rev. B. 81(1).
