Local Density Approximation Band Structure Research Articles

Physical properties and electronic structure of a new barium titanate suboxide Ba1+δTi13−δO12 (δ = 0.11)

The structure, transport, thermodynamic properties, x-ray absorption spectra (XAS), and electronic structure of a new barium titanate suboxide, Ba1+δTi13−δO12 (δ = 0.11), are reported. It is a paramagnetic poor metal with hole carriers dominating the transport. Fermi liquid behavior appears at low temperature. The oxidization state of Ti obtained by the XAS is consistent with the metallic Ti2+ state. Local density approximation band structure calculations reveal the material is near the Van Hove singularity. The pseudogap behavior in the Ti-d band and the strong hybridization between the Ti-d and O-p orbitals reflect the characteristics of the building blocks of the Ti13 semi-cluster and the TiO4 quasi-squares, respectively.

Deviating band symmetries and many-body interactions in a model hole-doped iron pnictide superconductor

We present a polarization resolved study of the low energy band structure in the optimally doped iron pnictide superconductor Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ (T$_c$=37K) using angle resolved photoemission spectroscopy. Polarization-contrasted measurements are used to identify and trace all three low energy hole-like bands predicted by local density approximation (LDA) calculations. The photoemitted electrons reveal an inconsistency with LDA-predicted symmetries along the $\Gamma$-X high symmetry momentum axis, due to unexpectedly strong rotational anisotropy in electron kinetics. We evaluate many-body effects such as Mott-Hubbard interactions that are likely to underlie the anomaly, and discuss how the observed deviations from LDA band structure affect the energetics of iron pnictide Cooper pairing in the hole doped regime.

Frustrated Magnetism in Vanadium Oxides

The effect of frustration in various localized and itinerant vanadium oxide compounds is discussed within next nearest neighbors Heisenberg and spin fluctuation models, respectively. In the localized moment case the S = 1/2 J1−J2-model on a square lattice exhibits a rich phase diagram with magnetic as well as exotic hidden order phases due to the interplay of frustration and quantum fluctuations. Their signatures in the high field magnetization and in magnetocaloric quantities are surveyed. The possible quantum phase transitions are discussed and applied to layered vanadium oxides of the type AA′VO(PO4)2 where A, A′ = Pb, Zn, Sr, Ba, Cd. In itinerant electron systems magnetic frustration may emerge as a result of electron correlations on a geometrically frustrated lattice. This mechanism causes enhanced spin fluctuations in a large region of momentum space and therefore can lead to a heavy fermion state at low temperatures as in the 3d spinel compound LiV2O4. The evidence from neutron scattering and NMR experiments is discussed within self-consistent renormalization theory based on local density approximation band structure calculations.

Dynamical correlations across the spin-state transition inLaCoO3

The electronic properties of $\mathrm{La}\mathrm{Co}{\mathrm{O}}_{3}$ across the spin-state transition are studied using the $\mathrm{LDA}+\mathrm{DMFT}$ method. Combining the local density approximation band structure of the $\mathrm{Co}\phantom{\rule{0.2em}{0ex}}3d$ orbitals in the low-spin state with multiorbital dynamical mean field theory for $U=5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, we investigate the evolution of the single-particle spectra at different spin states. We show that small differences in the orbital occupation can induce a smooth spin-state crossover due to large dynamical renormalizations of the energy splitting between the ${t}_{2g}$ and ${e}_{g}$ manifolds. We find large changes in the one-particle spectra that are unique fingerprints of each of the possible spin states. The key signature of the intermediate- and high-spin states is the presence of Hubbard satellites in the ${t}_{2g}$ spectral density. Further, our results for the paramagnetic metallic phase shows Kondo-like resonance in the ${t}_{2g}$ sector, indicating the role of multiorbital Kondo screening processes in the high-spin state. These results provide a theoretical basis for physics of room-temperature thermoelectric materials based on cobalt oxides.

In situ photoemission study of LaNiO 3 thin films grown by pulsed laser deposition

We investigate the electronic structure of high-quality single-crystal LaNiO 3(LNO) thin films using in situ photoemission spectroscopy (PES) in order to reveal the intrinsic electronic structure of LNO. The O 1s X-ray absorption (XAS) spectrum, which have much deeper probing depth than that of PES measurement, is in good agreement with previous studies on polycrystalline LNO surfaces. The in situ valence band PES spectrum shows well-resolved Ni 3d-derived t 2g and e g features, while earlier X-ray photoemission studies on polycrystals showed a single band peak. The narrow e g -derived feature exhibits enhanced intensity compared to local density approximation band structure calculations. The results are consistent with a renormalization of electronic states at E F , in terms of the known enhanced effective mass.

Bulk band structure and Fermi surface of nickel: A soft x-ray angle-resolved photoemission study

We study the bulk band structure and Fermi surface of nickel metal by soft x-ray angle-resolved photoemission spectroscopy (SX ARPES). SX ARPES, using tunable photons from $h\ensuremath{\nu}\ensuremath{\sim}300\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}800\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, facilitates depth-sensitive in-plane band mapping of Ni(100). Horizontal- and vertical-polarization-dependent studies are used to selectively enhance dipole-allowed transitions. While low-temperature $(50\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ results provide band dispersions consistent with the direct transition model, room-temperature $(300\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ studies confirm and quantify significant intensity loss due to nondirect transitions. The band maps provide band dispersions and identify all the bands in the $\ensuremath{\Gamma}\text{\ensuremath{-}}X\text{\ensuremath{-}}W\text{\ensuremath{-}}W\text{\ensuremath{-}}X\text{\ensuremath{-}}\ensuremath{\Gamma}$ quadrant in momentum space. In particular, the results show that a hole pocket derived from the ${X}_{2\ensuremath{\downarrow}}$ down-spin band exists in bulk Ni. This is in contrast to results of surface-sensitive ultraviolet ARPES studies but consistent with other bulk-sensitive measurements. The ${Z}_{1\ensuremath{\downarrow}}$ band is also shown to have depth-sensitive band dispersion and Fermi surface crossings. In addition, the magnetically active ${Z}_{2\ensuremath{\downarrow}}$ down-spin band shows nearly flatband behavior. The Fermi surface and band dispersions determined by the present ARPES measurements are in good agreement with local density approximation band structure calculations. SX ARPES is thus a valuable probe of the intrinsic momentum-resolved electronic structure of solids.

Insulator-metal transition in the doped3d1transition metal oxideLaTiO3

The doping induced insulator-metal transition in ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Ti}{\mathrm{O}}_{3}$ is studied using the ab initio local density approximation (LDA) and DMFT method. Combining the LDA bandstructure for the actual, distorted structure found recently with multiorbital DMFT to treat electronic correlations, we find (i) ferro-orbital order in the Mott insulating state without orbital degeneracy, (ii) a continuous filling induced transition to the paramagnetic metal (PM) with $x$, and (iii) excellent quantitative agreement with published photoemission data for the case of 6% doping. Our results imply that this system can be described as a Mott-Hubbard system without orbital (liquid) degeneracy.

ELECTRONIC STRUCTURE AND EXCHANGE IN Li2VOSiO4

Exchange couplings are calculated for Li2VOSiO4 using local density approximation (LDA). From a two band tight-binding model fitted to the LDA band structure and mapped to a Heisenberg model, the in-plane and inter-plane exchange integrals are obtained. While the sum of in-plane couplings J1+J2=9.5±1.5 K and the inter-plane coupling J⊥ ~ 0.2-0.3 K agree with recent experimental data, the ratio J2/J1 ~ 12 exceeds the reported value by an order of magnitude. Using geometrical considerations, high temperature expansions and perturbative mean field theory, we show that the LDA derived exchange constants lead to a remarkably accurate description of the properties of these materials including specific heat, susceptibility and Neél temperatures.

STUDY ON THE ELECTRONIC STRUCTURE OF HIGH-TC SUPERCONDUCTOR MgB2

We report a first-principles local density approximation band structure calculation on the electronic structure of high-TC superconductor MgB2.It is revealed that the band structure of magnesium diboride is of metallic type with a wide valence band.The density of states of electrons at Fermi energy is only about 0.72 (states/eV).We conclude that the present calculation excludes the possibility of BCS mechanism in MgB2.

Quasiparticle bands in the realistic bilayer cuprates

We calculate the dispersion relation of the single hole for the realistic bilayer $t$-$J$ model within the self-consistent Born approximation taking into account both many-body effects and the local-density approximation band structure. We show that the many-body effects strongly reduce the band splitting. The large excitation energy arising at $(\ensuremath{\pi},0),$ compared with the pure $t$-$J$ model, results from the specific band structure where the function of in-plane hopping terms are dominant. Our results are similar to the angle-resolved photoemission data of the single-layer antiferromagnetic insulator Sr${}_{2}$CuO${}_{2}$Cl${}_{2}$, and suggests that in the presence of strong many-body effects the role of the interlayer couplings in forming the quasiparticle bands is significantly suppressed.

Local-density-derived semiempirical pseudopotentials.

Transferable screened atomic pseudopotentials were developed 30 years ago in the context of the empirical pseudopotential method (EPM) by adjusting the potential to reproduce observed bulk electronic energies. While extremely useful, such potentials were not constrained to reproduce wave functions and related quantities, nor was there a systematic way to assure transferability to different crystal structures and coordination numbers. Yet, there is a significant contemporary demand for accurate screened pseudopotentials in the context of electronic structure theory of nanostructures, where local-density-approximation (LDA) approaches are both too costly and insufficiently accurate, while effective-mass band approaches are inapplicable when the structures are too small. We can now improve upon the traditional EPM by a two-step process: First, we invert a set of self-consistently determined screened LDA potentials for a range of bulk crystal structures and unit cell volumes, thus determining spherically symmetric and structurally averaged atomic potentials (SLDA). These potentials reproduce the LDA band structure to better than 0.1 eV, over a range of crystal structures and cell volumes. Second, we adjust the SLDA to reproduce observed excitation energies. We find that the adjustment represents a reasonably small perturbation over the SLDA potential, so that the ensuing fitted potential still reproduces a >99.9% overlap with the original LDA pseudowave functions despite the excitation energies being distinctly non-LDA. We apply the method to Si and CdSe in a range of crystal structures, finding excellent agreement with the experimentally determined band energies, optical spectra ${\mathrm{\ensuremath{\epsilon}}}_{2}$(E), static dielectric constants, deformation potentials, and, at the same time, LDA-quality wave functions.

Absence of metal clusters and appearance of new electron states in Na6C60.

We calculate the structural and electronic properties of the ${\mathrm{Na}}_{6}$${\mathrm{C}}_{60}$ fulleride using the Car-Parrinello method as well as standard local density approximation band structure calculations. In contrast to previous claims we find that metal clusters do not form but, like the potassium and rubidium fullerides, sodium intercalation gives rise to a conventional ionic compound. Unlike the case of heavier alkali fullerides, a characteristic new kind of low-lying electron state appears that is not the ${\mathrm{C}}_{60}$ state. Effects of temperature on the electronic nature of the compound are discussed as well as consequences of these findings for higher fullerides.