R. Vidya

Theoretical investigation of magnetoelectric behavior in BiFeO3

The magnetoelectric behavior of BiFeO3 has been explored on the basis of accurate density functional calculations. We are able to predict structural, electronic, magnetic, and ferroelectric properties of BiFeO3 correctly without including any strong correlation effect in the calculation. Unlike earlier calculations, the equilibrium structural parameters are found to be in very good agreement with the experimental findings. In particular, the present calculation correctly reproduced experimentally-observed elongation of cubic perovskite-like lattice along the [111] direction. At high pressure we predicted a pressure-induced structural transition from rhombohedral (R3c) to an orthorhombic (Pnma) structure. The total energy calculations at expanded lattice show two lower energy ferroelectric phases (with monoclinic Cm and tetragonal P4mm structures), closer in energy to the ground state phase. Spin-polarized band-structure calculations show that BiFeO3 will be an insulator in Aand G-type antiferromagnetic phases and a metal in C-type antiferromagnetic, ferromagnetic configurations, and in the nonmagnetic state. Chemical bonding in BiFeO3 has been analyzed using partial density-of-states, charge density, charge transfer, electron localization function, Born-effective-charge tensor, and crystal orbital Hamiltonian population analyses. Our electron localization function analysis shows that stereochemically active lone-pair electrons are present at the Bi sites which are responsible for displacements of the Bi atoms from the centro-symmetric to the noncentrosymmetric structure and hence the ferroelectricity. A large ferroelectric polarization of 88.7 μC/cm is predicted in accordance with recent experimental findings, but differing by an order of magnitude from earlier experimental values. The strong spontaneous polarization is related to the large values of the Born-effective-charges at the Bi sites along with their large displacement along the [111] direction of the cubic perovskite-type reference structure. Our polarization analysis shows that partial contributions to polarization from the Fe and O atoms almost cancel each other and the net polarization present in BiFeO3 mainly (> 98%) originates from Bi atoms. We found that the large scatter in experimentally reported polarization values in BiFeO3 is associated with the large anisotropy in the spontaneous polarization.

Detailed electronic structure studies on superconducting MgB 2 and related compounds

Our recent electronic structure studies on series of transition metal diborides indicated that the electron phonon coupling constant is much smaller in these materials than in superconducting intermetallics. However experimental studies recently show an exceptionally large superconducting transition temperature of 40 K in MgB2. In order to understand the unexpected superconducting behavior of this compound we have made electronic structure calculations for MgB2 and closely related systems. Our calculated Debye temperature from the elastic properties indicate that the average phonon frequency is very large in MgB2 compared with other superconducting intermetallics and the exceptionally high Tc in this material can be explained through BCS mechanism only if phonon softening occurs or the phonon modes are highly anisotropic. We identied a doubly-degenerate quasi-two dimensional key-energy band in the vicinity ofEF along -A direction of BZ (having equal amount of B px and py character) which play an important role in deciding the superconducting behavior of this material. Based on this result, we have searched for similar kinds of electronic feature in a series of isoelectronic compounds such as BeB2, CaB2 ,S rB 2 ,L iBC and MgB 2C2 and found that MgB2C2 is one potential material from the superconductivity point of view. We have also investigated closely related compound MgB4 and found that its EF is lying in a pseudogap with a negligibly small density of states at EF which is not favorable for superconductivity. There are contradictory experimental results regarding the anisotropy in the elastic properties of MgB2 ranging from isotropic, moderately anisotropic to highly anisotropic. In order to settle this issue we have calculated the single crystal elastic constants for MgB2 by the accurate full-potential method and derived the directional dependent linear compressibility, Young’s modulus, shear modulus and relevant elastic properties from these results. We have observed large anisotropy in the elastic properties consistent with recent high-pressure measurements. Our calculated polarized optical dielectric tensor shows highly anisotropic behavior even though it possesses isotropic transport property. MgB2 possesses a mixed bonding character and this has been veried from density of states, charge density and

Magnetic properties of Ca-doped SrRuO3 from full-potential calculations

Abstract We have carried out accurate generalized-gradient-corrected fully-relativistic full-potential calculations for Sr 1− x Ca x RuO 3 ( x =0, 0.25, 0.5, 0.75, and 1) in para-, ferro-, and A -, C -, and G -type antiferromagnetic configurations. We have performed electronic structure calculations for the experimentally observed orthorhombic structure as well as the hypothetical cubic structure. Our results are analyzed with the help of total, site-, spin-, and orbital-projected density of states. The total-energy studies show that CaRuO 3 stabilizes in the G -type antiferromagnetic state. The octahedral tilting owing to the relatively small radius of Ca 2+ leads to weak hybridization between Ru 4 d and O 2 p . This weak hybridization along with exchange splitting causes a pseudogap-like feature close to the Fermi level, which should stabilize G -type antiferromagnetic ordering in CaRuO 3 . However, powder neutron diffraction data on CaRuO 3 taken at 8 and 298 K do not show any magnetic peaks, implying that CaRuO 3 exhibits a spin-glass-like state with dominant short-range antiferromagnetic interaction. The calculated magnetic ground state of Sr 1− x Ca x RuO 3 is found to be consistent with the experimental findings. We have also calculated optical spectra as well as X-ray and ultra-violet photoemission spectra and Ru and O K -edge X-ray absorption spectra for G -type CaRuO 3 and found good agreement with available experimental spectra.

Ab-initio studies on Li doping, Li-pairs, and complexes between Li and intrinsic defects in ZnO

First-principles density functional calculations have been performed on Li-doped ZnO using all-electron projector augmented plane wave method. Li was considered at six different interstitial sites (Lii), including anti-bonding and bond-center sites and also in substitutional sites such as at Zn-site (Lizn) and at oxygen site (Lio) in the ZnO matrix. Stability of LiZn over Lii is shown to depend on synthetic condition, viz., LiZn is found to be more stable than Lii under O-rich conditions. Hybrid density functional calculations performed on LiZn indicate that it is a deep acceptor with (0/-) transition taking place at 0.74 eV above valence band maximum. The local vibrational frequencies for Li-dopants are calculated and compared with reported values. In addition, we considered the formation of Li-pair complexes and their role on electronic properties of ZnO. Present study suggests that at extreme oxygen-rich synthesis condition, a pair of acceptor type LiZn-complex is found to be stable over the compensati...

Spin, charge, and orbital ordering in the ferrimagnetic insulator YBaMn 2 O 5

The oxygen-decient (double) perovskite YBaMn2O5, containing cornerlinked MnO5 square pyramids, is found to exhibit ferrimagnetic ordering in its ground state. In the present work we report generalized-gradient-corrected, relativistic rst-principles full-potential density-functional calculations performed on YBaMn2O5 in the nonmagnetic, ferromagnetic and ferrimagnetic states. The charge, orbital and spin orderings are explained with site-, angular momentum- and orbital-projected density of states, charge-density plots, electronic structure and total energy studies. YBaMn2O5 is found to stabilize in a G-type ferrimagnetic state in accordance with experimental results. The experimentally observed insulating behavior appears only when we include ferrimagnetic ordering in our calculation. We observed signicant optical anisotropy in this material originating from the combined eect of ferrimagnetic ordering and crystal eld splitting. In order to gain knowledge about the presence of dierent valence states for Mn in YBaMn2O5 we have calculated K-edge x-ray absorption near-edge spectra for the Mn and O atoms. The presence of the dierent valence states for Mn is clearly established from the x-ray absorption near-edge spectra, hyperne eld parameters and the magnetic properties study. Among the experimentally proposed structures, the recently reported description based on P 4/nmm is found to represent the stable structure.

Structural and electronic properties of transparent conducting delafossite: a comparison between the AgBO2 and CuBO2 families (B = Al, Ga, In and Sc, Y)

The Ag-based delafossite transparent conducting oxides (TCO) are potential p-type materials for transparent electronics. However, they have attracted less attention compared with the Cu-based delafossite systems due to their difficult synthetic chemistry and relatively low conductivity. We present here a complete comparison of structural and electronic properties of these two families based on the results obtained from the periodic density functional calculation. The equilibrium structural parameters are obtained with the Perdew Burke Ernzerhof (PBE) exchange correlation functional in the calculation, while the electronic structure is investigated by using the screened hybrid functionals proposed by Heyd, Scuseria and Ernzerhof (HSE06). The structural stabilities of these two families of compounds are similar, being completely determined by the B-site metal ions. The density of states plots show that the valence band is relatively broader for the Ag-compounds. The Ag-4d orbitals are narrow and much lower in energy than the O p states. Therefore holes created at the oxygen site are highly localized and consequently have low mobility. The computed band gaps values are found to be in excellent agreement with the corresponding experimentally observed band gap values from optical measurements. The effective mass analysis suggests that for the Cu-compounds the conductivity follows the following trend: Sc > Ga ≈ Y > Al > In, in excellent agreement with the experimental observations. However, the calculated effective masses of the carriers suggest that the conductivity of the Ag-based compounds follows the following trend: Sc > Y > Ga > In > Al.

Electronic and Magnetic Structures of Hole Doped Trilayer La4–xSrxNi3O8 from First-Principles Calculations

The magnetic and electronic properties of trilayer La4Ni3O8, similar to hole-doped cuprates, are investigated by performing full-potential linearized augmented plane wave method-based spin-polarized calculations with LDA and GGA functionals including Hubbard U parameters to account for strong correlation effects. On the basis of these calculations, we found that La4Ni3O8 is a C-type anti-ferromagnetic (C-AFM) Mott insulator in agreement with previous experimental and theoretical observations. Our calculations suggest that the two crystallographically nonequivalent nickel atoms Ni1 and Ni2 are found to be in high-spin state with an average valency of +1.33. Intermediate band-gap states are originated from dz2 electrons of both types of Ni ions after including the strong correlation effects. To understand the role of hole doping on electronic structure, phase stability, and magnetic properties of La4Ni3O8, similar calculations were performed for La4-xSrxNi3O8 as a function of x, using the supercell approach. We found that the hole doping brings insulator-to-metal transition without changing the C-AFM ordering, though the magnetic moment is enhanced at both Ni sites. Moreover, these Ni atoms are always in an average valence state irrespective of hole doping or volume change. So the electronic properties of hole-doped La4Ni3O8 cannot be compared with hole-doped cuprates that are high-TC superconductors.