Fermi surface evolution and Luttinger theorem in Na(x)CoO2: a systematic photoemission study

We report a systematic angle-resolved photoemission study on Na(x)CoO2 for a wide range of Na concentrations (0.3 < or = x < or = 0.72). In all the metallic samples at different x, we observed (i) only a single holelike Fermi surface centered around gamma and (ii) its area changes with x according to the Luttinger theorem. We also observed a surface state that exhibits a larger Fermi surface area. The e'(g) band and the associated small Fermi surface pockets near the K points predicted by band calculations are found to sink below the Fermi energy in a manner almost independent of the doping and temperature.     

Electronic structures of RTe2 (R = La,Ce): a clue to the pressure-induced superconductivity in CeTe1.82

Electronic structures of RTe2 (R=La,Ce) have been investigated by using the local spin density approximation (LSDA) and the LSDA + U (U: on-site Coulomb interaction) band methods. Both LaTe2 and CeTe2 show the very similar Fermi surface nesting features along the [100] direction, which drive the charge-density wave (CDW) instability in the Te(1) sheets. The contribution near E(F) from Ce 4f states is negligible in agreement with the measured ARPES spectra. In the semimetallic CDW-distorted RTe2, both Te vacancy and pressure induce the charge transfer from Te(1) 5p to R 5d states, producing the enhanced density of states at E(F). We suggest that these increased self-doped Te(1) 5p hole carriers are responsible for the pressure-induced superconductivity in nonstoichiometric CeTe1.82.     

Reconstructed Fermi surface of underdoped Bi2Sr2CaCu2O(8+δ) cuprate superconductors

The Fermi surface topologies of underdoped samples of the high-T(c) superconductor Bi2Sr2CaCu2O(8+δ) have been measured with angle resolved photoemission. By examining thermally excited states above the Fermi level, we show that the observed Fermi surfaces in the pseudogap phase are actually components of fully enclosed hole pockets. The spectral weight of these pockets is vanishingly small at the magnetic zone boundary, creating the illusion of Fermi "arcs." The area of the pockets as measured in this study is consistent with the doping level, and hence carrier density, of the samples measured. Furthermore, the shape and area of the pockets is well reproduced by phenomenological models of the pseudogap phase as a spin liquid.     

Electron correlation and fermi surface topology of NaxCoO2

The electronic structure of NaxCoO2 revealed by recent photoemission experiments shows important deviations from band theory predictions. The six small Fermi surface pockets predicted by local-density approximation calculations have not been observed as the associated e'(g) band fails to cross the Fermi level for a wide range of sodium doping concentration x. In addition, significant bandwidth renormalizations of the t(2g) complex have been observed. We show that these discrepancies are due to strong electronic correlations by studying the multiorbital Hubbard model in the Hartree-Fock and strong-coupling Gutzwiller approximation. The quasiparticle dispersion and the Fermi surface topology obtained in the presence of strong local Coulomb repulsion are in good agreement with experiments.     

A comparative study of a Heusler alloy Co2FeGe using LSDA and LSDA+U

We have calculated the on-site Coulomb repulsion (U) for the transition elements Co and Fe. To study the impact of Hubbard potential or on-site Coulomb repulsion (U) on structural and electronic properties the calculated values of U were added on GGA and LSDA. We performed the structure optimization of Co₂FeGe based on the generalized gradient approximation (GGA and GGA+U). The calculation of electronic structure was based on the full potential linear augmented plane wave (FP-LAPW) method and local spin density approximation (LSDA) as well as exchange correlation LSDA+U. The Heusler alloy Co₂FeGe fails to give the half-metallic ferromagnetism (HMF) when treated with LSDA. The LSDA+U gives a good result to prove that Co₂FeGe is a HMF with a large gap of 1.10 eV and the Fermi energy (E_F) lies at the middle of the gap of minority spin. The calculated density of states (DOS) and band structure show that Co₂FeGe is a HMF when treated with LSDA+U.     

Raising Bi-O bands above the Fermi energy level of hole-doped Bi2Sr2CaCu2O8+delta and other cuprate superconductors

The Fermi surface (FS) of Bi2Sr2CaCu2O8+delta (Bi2212) predicted by band theory displays Bi-related pockets around the (pi, 0) point, which have never been observed experimentally. We show that when the effects of hole doping either by substituting Pb for Bi or by adding excess O in Bi2212 are included, the Bi-O bands are lifted above the Fermi energy (E(F)) and the resulting first-principles FS is in remarkable accord with measurements. With decreasing hole doping the Bi-O bands drop below and the system self-dopes below a critical hole concentration. Computations on other Bi- as well as Tl- and Hg-based compounds indicate that lifting of the cation-derived band with hole doping is a general property of the electronic structures of the cuprates.     

Effect of Coulomb correlations on the electronic structure of PuCoGa(5)

We investigate the effect of strong Coulomb correlations on the electronic structure of the Pu-based superconductor PuCoGa5 by employing the relativistic local spin density approximation+ Hubbard U (LSDA+U) method. The inclusion of intra-atomic Coulomb U and exchange J parameters leads to a significant reconstruction of the f states electronic structure over that given by the LSDA approach. At variance with the LSDA, the LSDA+U suggests "jj"-like coupling for the Pu 5f manifold.     

Angle-resolved photoemission study of the cobalt oxide superconductor Na(x)CoO(2) x yH(2)O: observation of the Fermi surface

The cobalt oxide superconductor Na(x)CoO(2) x yH(2)O is studied by angle-resolved photoemission spectroscopy. We report the Fermi surface (FS) topology and electronic structure near the Fermi level (E(F)) in the normal state of Na(x)CoO(2) x yH(2)O. Our result indicates the presence of the hexagonal FS centered at the Gamma point, while the small pocket FSs along Gamma-K direction are absent, similar to Na(x)CoO(2). The top of the e(g)(') band, which is expected in band calculations to form the small pocket FSs, extends to within approximately 30 meV below E(F), closer to E(F) than in Na(x)CoO(2). We discuss its possible role in superconductivity, comparing with other experimental and theoretical results.     

Shubnikov-de Haas effect in the metallic state of Na0.3CoO2

Shubnikov-de Haas oscillations for two well-defined frequencies, corresponding, respectively, to areas of 0.8 and 1.36% of the first Brillouin zone, were observed in single crystals of Na(0.3)CoO2. The existence of Na superstructures in Na0.3CoO2, coupled with this observation, suggests the possibility that the periods are due to the reconstruction of the large Fermi surface around the Gamma point. An alternative interpretation in terms of the long sought-after epsilon'(g) pockets is also considered but found to be incompatible with existing specific heat data.     

Can disorder alone destroy the e(')g hole pockets of NaxCoO2? A Wannier function based first-principles method for disordered systems

We investigate from first principles the proposed destruction of the controversial e(')g pockets in the Fermi surface of Na(x)CoO(2) due to Na disorder, by calculating its k-dependent configuration-averaged spectral function . To this end, a Wannier function-based method is developed that treats the effects of disorder beyond the mean field. Remarkable spectral broadenings of order ～1 eV are found for the oxygen orbitals, possibly explaining their absence in the experiments. In contradiction with the current claim, however, the e(')g pockets remain almost perfectly coherent. The developed method is expected to also generate exciting opportunities in the study of the countless functional materials that owe their important electronic properties to disordered dopants.     

Understanding STM images and EELS spectra of oxides with strongly correlated electrons: a comparison of nickel and uranium oxides

Using a theoretical approach combining the local spin density approximation (LSDA) of density functional theory and the Hubbard U term (LSDA + U), we analyse the connection between the experimentally observed electron energy loss spectra and elevated temperature scanning tunnelling images of surfaces of semiconducting nickel monoxide NiO and uranium dioxide UO2. We show that a combination of electron energy loss spectroscopy, atomic-resolution tunnelling imaging and first-principles ab initio calculations provides a powerful tool for studying electronic and structural properties of surfaces of transition metal and actinide oxides.     

Effect of dynamical coulomb correlations on the fermi surface of Na0.3CoO2

The t(2g) quasiparticle spectra of Na(0.3)CoO(2) are calculated within the dynamical mean field theory. It is shown that as a result of dynamical Coulomb correlations charge is transferred from the nearly filled e(g(')) subbands to the a(1g) band, thereby reducing orbital polarization among Co t(2g) states. Dynamical correlations therefore stabilize the small e(g(')) Fermi surface pockets, in contrast to angle-resolved photoemission data, which do not reveal these pockets.     

Ni5Nd2B4的电子结构和磁性能研究

采用LSDA（Local spin-density approximation）近似及LSDA+U（在位库伦势）近似模拟金属间化合物Ni5Nd2B4的磁性能对于R-M-B合金特性的研究具有重要意义。研究结果显示,LSDA近似下,Ni5Nd2B4具备金属导体性质,晶体结构中最紧邻Ni、B原子间杂化成键,最紧邻Ni-Ni共价成键,Nd、B原子形成成键分子轨道作用,Ni原子间存在自旋消弱现象;LSDA+U近似下,Nd原子磁矩提供体系磁性来源,由于自旋排斥作用Ni原子电子与Nd原子电子自旋方向相反,体系在U值约为6.35eV的作用下能较理想的处理体系电子作用。     

Ni5Nd2B4的电子结构和磁性能研究

采用LSDA（Local spin-density approximation）近似及LSDA+U（在位库伦势）近似模拟金属间化合物Ni5Nd2B4的磁性能对于R-M-B合金特性的研究具有重要意义。研究结果显示,LSDA近似下,Ni5Nd2B4具备金属导体性质,晶体结构中最紧邻Ni、B原子间杂化成键,最紧邻Ni-Ni共价成键,Nd、B原子形成成键分子轨道作用,Ni原子间存在自旋消弱现象;LSDA+U近似下,Nd原子磁矩提供体系磁性来源,由于自旋排斥作用Ni原子电子与Nd原子电子自旋方向相反,体系在U值约为6.35eV的作用下能较理想的处理体系电子作用。     

ARPES on Na0.6CoO2: Fermi surface and unusual band dispersion

The electronic structure of single crystals Na0.6CoO2, which are closely related to the superconducting Na0.3CoO2.yH(2)O (T(c) approximately 5 K), is studied by angle-resolved photoelectron spectroscopy. While the measured Fermi surface (FS) is consistent with the large FS enclosing the Gamma point from the band theory, the predicted small FS pockets near the K points are absent. In addition, the band dispersion is found to be highly renormalized, and anisotropic along the two principal axes (Gamma-K, Gamma-M). Our measurements also indicate that an extended flatband is formed slightly above E(F) along Gamma-K.     

Electron-phonon renormalization in cuprate superconductors

Electron-phonon (e-ph) renormalization effects in a model cuprate system CaCuO2 are studied by employing density functional theory based methods. Whereas calculations based on the local spin-density approximation (LSDA) predicts negligible e-ph coupling effects of the half-breathing Cu-O bond stretching mode, the inclusion of a screened on-site Coulomb interaction (U) in the LSDA+U calculations greatly enhances the e-ph coupling strength of this mode. The full-breathing mode, on the other hand, shows a much weaker e-ph renormalization effect.     

Charge and spin order on the triangular lattice: NaxCoO2 at x=0.5

The nature of electronic states due to strong correlation and geometric frustration on the triangular lattice is investigated in connection to the unconventional insulating state of NaxCoO2 at x=0.5. We study an extended Hubbard model using a spatially unrestricted Gutzwiller approximation. We find a new class of charge and spin ordered states at x=1/3 and x=0.5 where antiferromagnetic (AFM) frustration is alleviated via weak charge inhomogeneity. At x=0.5, we show that the square root of 3a x 2a off-plane Na dopant order induces weak square root of 3a x 1a charge order in the Co layer. The symmetry breaking enables successive square root of 3a x 1a AFM and 2a x 2a charge- or spin-ordering transitions at low temperatures. The Fermi surface is truncated by the 2a x 2a hexagonal zone boundary into small electron and hole pockets. We study the phase structure and compare to recent experiments.     

Large orbital moments and internal magnetic fields in lithium nitridoferrate(I)

The iron nitridometalates Li2[(Li(1-x)Fe(I)(x))N] display ferromagnetic ordering and spin freezing. Large magnetic moments up to 5.0mu(B)/Fe are found in the magnetization. In M?ssbauer effect studies huge hyperfine magnetic fields up to 696 kOe are observed at specific Fe sites. These extraordinary fields and moments originate in an unusual ligand field splitting for those Fe species leading [within local spin density approximation (LSDA)] to a localized orbitally degenerate doublet. Including spin-orbit interaction and strong intra-atomic electron correlation (LDA+SO+U) gives rise to a large orbital momentum.     

Role of electronic correlations in the Fermi surface formation of Na<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CoO<Subscript>2</Subscript>

Band structure of metallic sodium cobaltate Na _x CoO₂ (x = 0.33, 0.48, 0.61, 0.72) has been investigated by local density approximation + Hubbard U (LDA+U) method and within Gutzwiller approximation for the Co-t _2g manifold. Correlation effects being taken into account results in suppression of the e′ _g hole pockets at the Fermi surface in agreement with recent angle-resolved photoemission spectroscopy (ARPES) experiments. In the Gutzwiller approximation the bilayer splitting is significantly reduced due to the correlation effects. The formation of high spin (HS) state in Co d-shell was shown to be very improbable.     

Magnetically induced phonon anisotropy in ZnCr2O4 from first principles

We have studied the influence of magnetic order on the optical phonons of the geometrically frustrated spinel ZnCr2O4 within density-functional theory. By mapping local spin density approximation plus Hubbard U (LSDA + U) phonon calculations onto a Heisenberg-like model, we developed a method to calculate exchange derivatives and subsequently the spin-phonon coupling parameter from first principles.