First principles study of the electronic and magnetic structures of the tetragonal and orthorhombic phases of Ca3Mn2O7

On the basis of density functional theory electronic band structure calculations using the augmented spherical wave method, the electronic and magnetic properties of the orthorhombic and tetragonal phases of Ca3Mn2O7 were investigated and the spin exchange interactions of the orthorhombic phase were analyzed. Our calculations show that the magnetic insulating states are more stable than the non-magnetic metallic state for both polymorphs of Ca3Mn2O7, the orthorhombic phase is more stable than the tetragonal phase, and the ground state of the orthorhombic phase is antiferromagnetic. The total energies calculated for the three spin states of the orthorhombic phase of Ca3Mn2O7 led to estimates of the spin exchange interactions Jnn = -3.36 meV and Jnnn = -0.06 meV. The accuracy of these estimates were tested by calculating the Curie-Weiss temperature within the mean-field approximation.Comment: 11 pages, 7 figure

Charge order, orbital order, and electron localization in the Magneli phase Ti4O7

The metal-insulator transition of the Magneli phase Ti4O7 is studied by means of augmented spherical wave (ASW) electronic structure calculations as based on density functional theory and the local density approximation. The results show that the metal-insulator transition arises from a complex interplay of charge order, orbital order, and singlet formation of those Ti 3d states which mediate metal-metal bonding inside the four-atom chains characteristic of the material. Ti4O7 thus combines important aspects of Fe3O4 and VO2. While the charge ordering closely resembles that observed at the Verwey transition, the orbital order and singlet formation appear to be identical to the mechanisms driving the metal-insulator transition of vanadium dioxide.Comment: 11 pages, 4 figures, more information at http://www.physik.uni-augsburg.de/~eyert

Octahedral Tilting in ACu3Ru4O12 (A=Na,Ca,Sr,La,Nd)

The perovskite-like compounds ACu3Ru4O12 (A=Na,Ca,Sr,La,Nd) are studied by means of density functional theory based electronic structure calculations using the augmented spherical wave (ASW) method. The electronic properties are strongly influenced by covalent type bonding between transition metal d and oxygen p states. The characteristic tilting of the RuO6 octahedra arises mainly from the Cu--O bonding, allowing for optimal bond lengths between these two atoms. Our results provide a deeper understanding of octahedral tilting as a universal mechanism, applicable to a large variety of multinary compounds.Comment: 9 pages, 4 eps figure

The Metal-Insulator Transition of the Magneli phase V_4O_7: Implications for V_2O_3

The metal-insulator transition (MIT) of the Magneli phase V_4O_7 is studied by means of electronic structure calculations using the augmented spherical wave method. The calculations are based on density functional theory and the local density approximation. Changes of the electronic structure at the MIT are discussed in relation to the structural transformations occuring simultaneously. The analysis is based on a unified point of view of the crystal structures of all Magneli phase compounds V_nO_2n-1 (3 =< n =< 9) as well as of VO_2 and V_2O_3. This allows to group the electronic bands into states behaving similar to the dioxide or the sesquioxide. In addition, the relationship between the structural and electronic properties near the MIT of these oxides can be studied on an equal footing. For V_4O_7, a strong influence of metal-metal bonding across octahedral faces is found for states both parallel and perpendicular to the hexagonal c_hex axis of V_2O_3. Furthermore, the structural changes at the MIT cause localization of those states, which mediate in-plane metal-metal bonding via octahedral edges. This band narrowing opens the way to an increased influence of electronic correlations, which are regarded as playing a key role for the MIT of V_2O_3.Comment: 7 pages, 3 figures, more information at http://www.physik.uni-augsburg.de/~eyert

Orbital ordering in the two-dimensional ferromagnetic semiconductor Rb_2CrCl_4

We present the results of electronic structure calculations for the two-dimensional ferromagnet Rb_2CrCl_4. They are obtained by the augmented spherical wave method as based on density functional theory and the local density approximation. In agreement with experimental data Rb_2CrCl_4 is found to be semiconducting and displays long-range ferromagnetic order of the localized Cr 3d moments. The magnetic properties are almost independent of the structural modifications arising from the Jahn-Teller instability, which leads from the parent body-centered tetragonal K_2NiF_4 structure to a side-centered orthorhombic lattice. In contrast, our calculations give evidence for a strong response of the optical band gap to the corresponding structural changes.Comment: 7 pages, 4 figures, for more information see http://www.physik.uni-augsburg.de/~eyert

Extended moment formation and magnetic ordering in the trigonal chain compound Ca3Co2O6

The results of electronic structure calculations for the one-dimensional magnetic chain compound Ca3Co2O6 are presented. The calculations are based on density functional theory and the local density approximation and used the augmented spherical wave (ASW) method. Our results allow for deeper understanding of recent experimental findings. In particular, alternation of Co 3d low- and high-spin states along the characteristic chains is related to differences in the oxygen coordination at the inequivalent cobalt sites. Strong hybridization of the d states with the O 2p states lays ground for polarization of the latter and the formation of extended localized magnetic moments centered at the high-spin sites. In contrast, strong metal-metal overlap along the chains gives rise to intrachain ferromagnetic exchange coupling of the extended moments via the d_{3z^2-r^2} orbitals of the low-spin cobalt atoms.Comment: 10 pages, 4 figures more information at http://www.physik.uni-augsburg.de/~eyert

The Metal-Insulator Transition of NbO2: an Embedded Peierls Instability

Results of first principles augmented spherical wave electronic structure calculations for niobium dioxide are presented. Both metallic rutile and insulating low-temperature NbO2, which crystallizes in a distorted rutile structure, are correctly described within density functional theory and the local density approximation. Metallic conductivity is carried to equal amounts by metal t_{2g} orbitals, which fall into the one-dimensional d_parallel band and the isotropically dispersing e_{g}^{pi} bands. Hybridization of both types of bands is almost negligible outside narrow rods along the line X--R. In the low-temperature phase splitting of the d_parallel band due to metal-metal dimerization as well as upshift of the e_{g}^{pi} bands due to increased p-d overlap remove the Fermi surface and open an optical band gap of about 0.1 eV. The metal-insulator transition arises as a Peierls instability of the d_parallel band in an embedding background of e_{g}^{pi} electrons. This basic mechanism should also apply to VO2, where, however, electronic correlations are expected to play a greater role due to stronger localization of the 3d electrons.Comment: 4 pages, revtex, 6 eps figures, additional material avalable at http://www.physik.uni-augsburg.de/~eyert

On the strong impact of doping in the triangular antiferromagnet CuCrO2

Electronic band structure calculations using the augmented spherical wave method have been performed for CuCrO2. For this antiferromagnetic (T_N = 24 K) semiconductor crystallizing in the delafossite structure, it is found that the valence band maximum is mainly due to the t_2g orbitals of Cr^3+ and that spin polarization is predicted with 3 mu_B per Cr^3+. The structural characterizations of CuCr1-xMgxO2 reveal a very limited range of Mg^2+ substitution for Cr^3+ in this series. As soon as x = 0.02, a maximum of 1% Cr ions substituted by Mg site is measured in the sample. This result is also consistent with the detection of Mg spinel impurities from X-ray diffraction for x = 0.01. This explains the saturation of the Mg^2+ effect upon the electrical resistivity and thermoelectric power observed for x > 0.01. Such a very weak solubility limit could also be responsible for the discrepancies found in the literature. Furthermore, the measurements made under magnetic field (magnetic susceptibility, electrical resistivity and Seebeck coefficient) support that the Cr^4+ "holes", created by the Mg^2+ substitution, in the matrix of high spin Cr^3+ (S = 3/2) are responsible for the transport properties of these compounds.Comment: 9 pages, 11 figures, more information at http://www.physik.uni-augsburg.de/~eyert

Optical conductivity in A3C60 (A=K, Rb)

We study the optical conductivity in A3C60 (A =K, Rb). The effects of the electron-phonon interaction is included to lowest order in the coupling strength lambda. It is shown that this leads to a narrowing of the Drude peak by a factor 1+lambda and a transfer of weight to a mid-infrared peak at somewhat larger energies than the phonon energy. Although this goes in the right direction, it is not sufficient to describe experiment.Comment: 5 pages, revtex, 2 figures more information at http://librix.mpi-stuttgart.mpg.de/docs/ANDERSEN/fullerene