Band Picture Research Articles

Transport properties of high-Tc superconducting oxides

The transport and magnetic properties of the (La1–xAx)2CuO4 system are investigated systematically over the composition range from x = 0 to 0.1. The change in the resistivity and magnetic susceptibility with alkaline metals and alkali earth metals substitution seems to be consistent with the band picture with nesting Fermi surface. However, the result for the Seebeck coefficient suggests the possibility of a strongly correlated electron system, casting a question about the simple SDW picture. The similar feature in the Seebeck coefficient is also observed in the Ba2YCu3O7-y system.

Itinerant magnetic character in 3d transition-metal intercalation compounds MxTiS2

The magnetization and specific heat data, together with the transport and photoemission data, of 3d metal intercalation compounds MxTiS2 with the 1T-CdI2 type layered structure, show that an itinerant electron or band picture is valid in this material system, in agreement with band calculations using a self-consistent APW method.

Electrical and thermal properties of intercalation compound FexTiS2

Intercalation phenomena of guest Fe atoms into van der Waals gaps of the host 1T - TiS2 layered crystal have been studied by transport measurements. These experimental results, together with other data, are indicative of the presence of spin-up and spin-down bands produced by the host-guest interactions and understood in terms of an itinerant band picture, in agreement with recent band calculations.

Frequency dependence of the large, electronic χ(3) in polyacetylene

Abstract The third order optical susceptibilities ( χ (3) ) of trans - and cis - (CH) x have been determined in the spectral region below the gap by measuring the third harmonic generation efficiency. The magnitude of χ (3) parallel to the polymer chains in trans -(CH) x is in excess of 10 −9 esu for wavelengths larger than 1.3 μm, and the spectrum of χ (3) in trans -(CH) x has a sharp two-photon resonance at an energy corresponding to half the semiconducting gap. Both the off resonance and the two-phonon resonance enhanced values of χ (3) are explained by a simple band picture.

Magnetic properties of intercalation compounds M xTiS 2 (M = 3d transition metal)

Magnetization measurements have been made on 3d transition-metal intercalation compounds of M x TiS 2 (M = V, Cr, Mn, Fe, Co and Ni; 0 ⩽ x ⩽ 1) at 4.2, 77 and 300 K in static fields up to 20 kOe and in pulsed fields up to 160 kOe. The ac magnetic susceptibility measurements in dc fields have also been made for Fe x TiS 2. We have identified various types of magnetic phases in M x TiS 2, such as paramagnetic, spin-glass, cluster-glass or mictomagnetic, and ferromagnetic ones, depending on the kind of guest 3d metals and their concentrations. Magnetic properties of these materials can be understood in terms of an itinerant or band picture rather than a localized or rigid band model.

Photo-emission studies of AgxTaS2

The authors present angle-integrated photo-emission spectra of the Ta 4f levels of pure 2H-TaS2 and of AgxTaS2, obtained at high resolution using synchrotron radiation, and of the valence-band regions. The data throw light on the band picture of the intercalated samples, and on the extent of the charge transfer from the Ag+ intercalant to the host lattice. The samples are shown to be inhomogeneous, with some regions including no Ag+ ions while in others the d(z2) conduction band of the TaS2 host appears to be virtually filled due to charge transfer from the intercalant.

Fermi Surface in La2CuO4-σ Determine by Positron 2D-ACAR

ABSTRACTThe topology of the Fermi surface in La2CuO4-σ is determined by two dimensional angular correlation measurements of annihilation radiations (2D-ACAR) at room temperature. The determined Fermi surface is two dimensional and has a slender electron pillar along ΓZ and two kinds of hole pillars along PX and along NN direction parallel to ΓZ, respectively, in the Bradley and Cracknell notation modified by Yu, Freeman and Xu. It is concluded that the Fermi surface is not a simple half filled one and this compound should be metallic at least at room temperature in the band picture.

High Temperature Superconductors: Electronic Structure Changes due to Replacement of La with Ba and Sr in the Cu–O-Based Systems

The replacement of some La in La2CuO4 with divalent atoms has been found to lead to high temperature superconductivity with Tc=30-40 K. These superconductors have a bet layer perovskite structure with large anisotropies in their electronic band structure. Using first principles local density methods applied to both layer perovskite and the related cubic perovskite structures, we find that alloying on the La site leads to large changes in the chemical bonding of both structures, except near the Fermi level where a rigid band picture is reasonable. Large changes in the spectral density near the “dopant” atom occurs on both the nearby O and Cu atoms, and is not simply described by a Cu(2+)-Cu(3+) valence change. These results are consistent with recent x-ray absorption studies by Tranquada et al.

One- and two-hole excitation spectra of the Hubbard model in the non-half-filled case.

Exact results for the one- and two-hole excitation spectra of the one- dimensional Hubbard model for filling larger than one electron per site are presented. We consider short chains of up to eight atoms and use the so-called modified periodic boundary conditions, with which we are able to extrapolate the results up to infinite chains. The one-hole excitation spectra demonstrate a gradual transition from the band picture to the spectra characterized by new satellite structures, with increasing electron-electron interaction U. The weight of the satellite part of these spectra increases with increasing U, approaching the atomic-limit value. In the ferromagnetic case, a substantial reduction of the exchange splitting is found as compared with the independent-electron approximation. The two-hole excitation spectra exhibit characteristic atomic lines at large values of U, while at intermediate values of U they are much broader in the non-completely-filled chain than the respective ones of the filled chain. The distance between the satellite and the band part is found to be larger than the value of U for small and intermediate values of the interaction. The results obtained support those found with second-order perturbation theory and the t-matrix approximation.

X-Ray Spectroscopic Study of CrSe2

The K absorption edges of chromium and selenium in CrSe 2 have been recorded using a Cauchois type bent crystal spectrograph of diameter 0.4 m. The shifts in the positions of these edges with respect to those in the pure elements show that chromium is a cation and selenium is an anion; and it exhibits the d 2 sp 3 type of bonding in CrSe 2 . The various features of the near edge structure have been assigned to transitions of the core state electron to the available unoccupied states in the conduction band. Based on these transitions a band picture is proposed. The EXAFS associated with the chromium and selenium edges have been analysed in the light of the graphical method proposed by Lytle, Sayers and Stern.

Optical transitions in ZrS2and ZrSe2from thermoreflectance measurements

Thermoreflectance spectra of single-crystal ZrS2 and ZrSe2 were obtained at 60K. A Kramers-Kronig analysis was carried out and fits of experimental curves calculated. The resulting parameters are interpreted on the basis of existing band-structure and spin-orbit splitting calculations. A simplified band picture is presented.

Model supercell local-density calculations of the 3d excitation spectra in NiO.

The local-density supercell impurity approach is used to calculate the excitation spectra due to the emission and absorption of a $3d$ electron in the Mott insulator NiO. Good agreement is found with both photoemission (PES) and inverse photoemission [bremsstrahlung isochromat spectroscopy (BIS)] data. In particular, a BIS excitation gap of 4.1 eV is found compared to 4.3 eV experimentally. The predicted values for the screened Coulomb interaction $U$ is 7.9 eV. These results indicate that there is no conflict between the band picture of Terakura et al. for the ground-state properties of NiO and the excited-state properties derived from PES and BIS by Sawatzky and Allen and by McKay and Henrich.

D-Band filling in Ni-P metallic glasses

We present the first realistic computations on the electronic structure of Ni-P metallic glasses. Our results show strong deviations from the commonly assumed rigid band picture for the electronic structure of this classic metal-metalloid system and consistently explain a variety of experiments. The Ni- d holes are found to persist over the entire glass forming range. We show nevertheless how Faber-Ziman and Nagel-Tauc type arguments concerning the properties of this glass may remain intact in our theory.

Electrical and photoelectrical studies of plasma polymerized acrylonitrile

The electrical and photoelectrical properties of plasma polymerized acrylonitrile (PAN) are reported. The polymer films were studied in silver-PAN-silver systems. Electrical conductivity at room temperature was of the order of 10−11 ohm−1 cm−1. The space charge limited current (SCLC) studies at room temperature and thermally stimulated current studies (TSC) over a temperature range of 290–500°K led to a clear understanding of carrier concentration, carrier mobility, trapping levels and activation energies. Photoelectric measurements were used to draw a band picture in plasma depositedPAN.

Neutron inelastic scattering and X-ray structural study of the charge-density-wave state in K0.3MoO3

The Peierls transition in the quasi-one-dimensional conductor K0.3MoO3 has been studied to clarify its dynamical nature and the electron correlation problem. A giant Kohn anomaly and the central peak in S(q, omega ) were observed. The amplitude below the transition temperature Tc was also measured. An asymmetry of the elastic diffuse scattering above Tc around the reciprocal point which corresponds to the superlattice point can be explained by taking into account the structure factor of the charge-density wave. A detailed type of modulation of Mo atoms is presented by the X-ray study. No evidence for the correlation effect was observed and the transition of K0.3MoO3 seems to be explained by a simple band picture.

Transition-metal monoxides: Band or Mott insulators? (invited; abstract)

We argue that the widely held view that the transition-metal monoxides are Mott insulators is incorrect. Self-consistent spin-polarized energy-band calculations indicate that MnO and NiO are ordinary band insulators, and that FeO and CoO, while conducting in the local-spin-density approximation, satisfy the condition required for insulating behavior due to the development of an orbital magnetic moment. Our picture of the monoxides is supported by a variety of experimental data. The fundamental question raised by this work is the importance of charge fluctuations, or equivalently, of ionic configurations. The distinction between our band picture and the conventional picture based on ionic configurations (the energy price for which is the Coulomb integral ‘‘U’’) is particularly clear for the variation of the Néel temperature across the monoxide series. Perturbation theory indicates that T(Néel) varies inversely with U in the conventional theory, but inversely with the exchange splitting in the band picture. Since the exchange splitting tends linearly toward zero across the series, whereas U is constant or increases, the increase by a factor of five of T(Néel) across the series is more naturally explained by the band picture.

Electronic conduction in bulk Se1−x Te x glasses at high pressures and at low temperatures

The electrical resistivity of bulk Se1−x Te x glasses is reported as a function of pressure (up to 8 GPa) and temperature (down to 77K). The activation energy for electronic conduction has been calculated at different pressures. The samples with 0⩽x⩽0·06 show a single activation energy throughout the temperature range of investigations. On the other hand samples with 0·08⩽x⩽0·3 show two activation energies in the different regions of temperature. The observed behaviour has been explained on the basis of band picture of amorphous semiconductors.

Transition-metal monoxides: Itinerant versus localized picture of superexchange

We demonstrate that energy-band theory based on the local-spin-density-functional treatment of exchange and correlation accounts well for the insulating behavior of MnO, MnS, and NiO. This is true, however, only if the magnetization is allowed to vary in the [111] direction, as is observed experimentally. As a further test of the itinerant picture of these materials, the ‘‘exchange integrals’’ entering a Heisenberg model are calculated, using muffin-tin CPA theory to describe the interaction between two magnetized atoms embedded in a medium of disordered spins. These parameter-free calculations of the coupling strength are the first nonempirical estimates to be in qualitative agreement with measurements. The numerical overestimate of the coupling strength by approximately a factor of three is consistent with the fact that both of the fundamental approximations underlying the estimate act to overestimate the coupling strength; these are: (1) the neglect of magnetic order in the CPA calculations and (2) the neglect of screening effects caused by the response of the electrons to the spin-wave-like perturbations described by a Heisenberg Hamiltonian. The band-theoretic description of superexchange differs qualitatively from the traditional localized-electron picture. Two of the most interesting of these differences concern the observed sharp increase in the coupling strength at the end of the transition series, and the ferromagnetic coupling of nearest Ni neighbors in NiO. The band picture, provides a natural explanation for both observations, whereas the localized picture does not.

Model Calculations on One-Dimensional (1D) Poly-Decker Sandwich Compounds. A Crystal Orbital Investigation Based on the Tight-Binding Formalism

Abstract The band structures of 11 one-dimensional (ID) poly-decker sandwich compounds with dif­ferent transition metal centers M (M = Mn, Fe, Co, Ni, Cu, Zn) and a variety of fivemembered π ligands L from the cyclopentadienyl moiety (C5H5) to the pure boron ring B5H5 have been studied by means of a semiempirical crystal orbital procedure based on the INDO approxima­tion in order to allow a priori predictions on possible semiconducting or conducting low-dimen­sional materials composed by ML fragments. To determine the (numerically) different self­energy corrections (i.e. long-range and short-range “correlations”) in the transition metal 3d spines and the ligand backbones approximate quasi-particle shifts have been employed for the correction of the Hartree-Fock (HF) band energies. The band structure properties (e.g., dispersion curves, density of states distributions, effective mass parameters, propagation times of charge carriers) are discussed in the light of the semiempirical tight-binding approach. It is shown that the forbidden band gaps are reduced with an increasing number of B atoms in the π ligands. The gap in the Mn(C5H5) stack amounts to 8.27 eV, while overlapping dispersion curves are predicted in the Zn(B5H5) derivative. This model polymer is the only intrinsic conductor in the series of the studied ID metallocenes; all other compounds require injected charge carriers (electrons or holes) in order to achieve partially filled bands. Injected holes in the Mn or Fe backbones lead to ID materials with conducting 3d spines; the charge transfer in this regime is best described as some type of hopping motion. The remaining poly-decker strands belong to the class of organic metals (injected carriers) with conductive pathways that are formed by diffuse ligand states leading to transfer processes that can be rationalized in terms of a band picture. The rotational profiles and the magnitudes of intracell and intercell interactions are also studied. The band structure properties (band gaps, characters of the valence and conduction bands) depend critically on the mutual orientations between neighbouring unit cells in the case of ligands with low spatial symmetries. General rules and strategies for synthetic approaches to organometallic ID materials containing 3d centers with small or vanishing band gaps are formulated.

Preparation and chemical and physical properties of the new layered phasesLixTi1−yMyS2withM=V,Cr,orFe

We have prepared the layered compounds $\mathrm{Li}{\mathrm{Ti}}_{1\ensuremath{-}y}{M}_{y}{\mathrm{S}}_{2}$ ($M=\mathrm{V},\phantom{\rule{0ex}{0ex}}\mathrm{C}\mathrm{r},\phantom{\rule{0ex}{0ex}}\mathrm{F}\mathrm{e}$) by direct synthesis from a mixture of ${\mathrm{Li}}_{2}$S and the appropriate elements. The structure of these compounds is $1T$ for $M=\mathrm{V}\mathrm{and}\mathrm{Cr}$, and $3R$ for $M=\mathrm{Fe}$. In the Fe case two phases are obtained for $y>0.66$. Magnetic and M\"ossbauer data indicate that Fe is divalent and in the high-spin state for $y<0.5$ (effective magnetic moment \ensuremath{\sim}4.6 Bohr magnetons), that Cr is trivalent (effective magnetic moment \ensuremath{\sim}3.4 Bohr magnetons), and that V is nonmagnetic. The lithium can be removed by reaction with ${\mathrm{I}}_{2}$ in an acetonitrile solution at room temperature, producing layered compounds containing oxidized sulfur if $M=\mathrm{Cr}\mathrm{or}\mathrm{Fe}$. The Cr and Fe compounds are metastable, decomposing above 300\ifmmode^\circ\else\textdegree\fi{}C into Ti${\mathrm{S}}_{2}$ and ${\mathrm{Cr}}_{2}$${\mathrm{S}}_{3}$ or Fe${\mathrm{S}}_{2}$. In ${\mathrm{Ti}}_{1\ensuremath{-}y}{\mathrm{V}}_{y}{\mathrm{S}}_{2}$ the V is in the 4+ state and has a local magnetic moment at low concentrations. Cr and Fe are trivalent, and the latter is in low-spin state. Consequently, the valences are assigned as follows: ${{\mathrm{Ti}}^{4+}}_{1\ensuremath{-}y}{{M}^{3+}}_{y}{({\mathrm{S}}^{(\frac{2\ensuremath{-}y}{2})\ensuremath{-}})}_{2}$. In the Cr case the compounds are poor electrical conductors, indicating that the $y$ holes are localized, either by the cation disorder or by forming stable monovalent and divalent sulfur atoms (the monovalent species might form ${\mathrm{S}}_{2}$ pairs in this case, but they dissociate when lithium is reintercalated). Since the Fe compounds remain conducting, the holes are not localized and a band picture with the Fermi level moved well into the valence band to produce $y$ holes applies. Electrochemical measurements in nonaqueous lithium cells showed behavior similar to that of Ti${\mathrm{S}}_{2}$. Those compounds containing large amounts of Fe could be intercalated with more than one lithium per transition metal, but the reaction beyond $x=1$ was irreversible.