Nature Of Electronic States Research Articles

Photoluminescence in polyamide/mica and polyethylene/ MgO nanocomposites induced by ultraviolet photons

Nature of electronic states in the energy band is investigated by means of optical absorption and photoluminescence induced by vacuum ultraviolet photons for polyamide-6/mica and low-density polyethylene/MgO nanocomposites. The nanofiller loading does not affect the absorption spectra of polyamide samples. Contrary to this, two absorption bands are induced at around 5.0 and 6.2 eV by the nanofiller loading in polyethylene samples, but they are due to absorption by the fillers. A luminescence band is observed at around 3.0 eV in polyamide samples, whereas three luminescence bands are observed at around 4.3, 3.7, and 2.9 eV in polyethylene samples. However, for all the luminescence bands, neither emission nor excitation energies change by the addition of nanofillers. Decay profiles of all the luminescence bands are essentially unchanged by the nanofiller loading. Moreover, no new PL bands are induced in the observed wavelength range. These results indicate that localized states, at least as far as the ones that can emit luminescence photons with intensities more than the sensitivity of the present research, are not induced by the nanofiller loading.

Nature of electron states and symmetry breaking in quantum point contacts according tothe local spin density approximation

Electron states and local magnetization in quantum point contacts (QPCs) with differentgeometries and applied gate voltages are examined for a model GaAs/AlGaAs device.Using the local spin density approximation (LSDA) we recover ferromagnetic spatially splitsolutions in the pinch-off regime as well as antisymmetric solutions that occur withdecreasing gate voltage. These kinds of spin states, which may appear in a repeated fashionin the few-electron regime, are precursors to an extended ferromagnetic statethat may be associated with the 0.7 conductance anomaly. We briefly commenton some recent experiments indicating the presence of bound states (Yoon et al 2007 Phys. Rev. Lett. 99 136805). We have not found any indication of suchstates but suggest that the accumulations of spin and charge at the two endsof a QPC and associated singlet and triplet states are relevant in this context.

The nature of electron states in AlN andα-Al2O3

Both α-alumina and aluminium nitride are insulators. They are widely applied as tunnel barriers.On the basis of first-principles calculations, it is shown here that the conduction band ofthese two compounds is of fundamentally different origin than generally assumed. Thebottom of the conduction band of both compounds is primarily derived fromoxygen/nitrogen 3s states with an admixture of a small amount of aluminium s characteronly. The presence of the anion 3s states is of importance for the size of the band gap:without them they would be significantly larger. The consequences of these differences arediscussed.

Nature of electronic states at the Fermi level of metallicβ−PbO2revealed by hard x-ray photoemission spectroscopy

The nature of electronic states at the Fermi energy in metallic $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Pb}{\mathrm{O}}_{2}$ has been identified by hard x-ray photoemission spectroscopy (HXPS) combined with x-ray emission spectroscopy and density-functional theory calculations. $\mathrm{Pb}\phantom{\rule{0.2em}{0ex}}6s$ states are strongly enhanced in HXP spectra, allowing us to locate the Fermi level above the top of the valence band. The occupied conduction-band states have significant $\mathrm{Pb}\phantom{\rule{0.2em}{0ex}}6s$ character, although the major $6s$ contribution to the density of states is found $9\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ below the Fermi energy. The enormous potential of HXPS for the study of $s$ states in oxides is emphasized.

The nature of electronic states and chemical bonding in lithium and potassium carbonates

In the context of the density functional theory the calculation of electron energy and space distribution in (LixK1−x )2CO3 (x = 0, 0.5, 1) have been made employing the pseudopotential method in the basis of numerical pseudo-orbitals. The electron structure of alkali metal carbonates is shown to be essentially different from alkaline-earth ones, this being connected with the change in the charge state of both the anion as a whole and the atoms composing it. The mechanisms of chemical bond formation were investigated by the sublattice method and it was shown that its peculiarities were due to the presence of nonequivalent oxygen and cation sublattices. In Li2CO3, anion atoms are covalently bonded, while in LiKCO3 and K2CO3 the ion constituent presents.

Electronic limitations of lithium diffusibility. From layered and spinel toward novel olivine type cathode materials

The ability and efficiency of lithium intercalation into transition metal compounds have been found to depend strongly on their electronic structure. This work is a brief review of physicochemical properties of intercalated transition metal compounds with layered, spinel or olivine type structure in order to correlate their microscopic electronic properties, i.e. the nature of electronic states with the efficiency of lithium intercalation process that is controlled by the chemical diffusion coefficient of lithium. The data concerning cell voltages and character of discharge curves for various materials are correlated with the nature of chemical bonding and electronic structure following it. The nature of the metallic type conductivity of doped phospho-olivine is discussed and some fundamental arguments against the bulk nature of the observed high electronic conductivity are presented.

Electronic transport properties of carbon nanotube based metal/semiconductor/metalintramolecular junctions

The electronic structure and the conductance of a carbon nanotube basedmetal/semiconductor/metal intramolecular junction is investigated numerically. The natureof electronic states at the interfaces and in the semiconductor section is analysed. Thequantum conductance of the system is calculated in the coherent regime and its variationswith energy and length are shown to be related to contributions from different kinds ofelectronic state.

Time dependence and excitation spectra of the photoluminescence emission at 1.54μm in Si-nanocluster and Er co-doped silica

Abstract The risetime of the 1.54 μm luminescence emission in Er:Si-nc co-doped glasses provides information on the mechanisms leading to population of the luminescent level, i.e. on the energy transfer time. In this paper we present a detailed study of this risetime in silica glasses after excitation in the wavelength region 360–860 nm by a continuously tunable pulsed Ti:Al 2 O 3 laser. The emission risetime turns out to be dependent on the sample fabrication method used, i.e. implantation or co-sputtering and on the annealing temperature used to precipitate the silicon aggregates. The fastest energy transfer time observed in our samples is τ tr ⩽ 2 μs. The risetime was found to be weakly dependent on the excitation wavelength up to around 700 nm where a sudden increase is observed. This increase is more pronounced in samples where the annealing temperature was such to produce amorphous aggregates rather than crystalline ones. The difference in the nature of electronic states excited by near-infrared pumping in the two types of samples containing crystalline or amorphous aggregates is also evidenced by CW photoluminescence excitation (PLE) measurements which reflect the absorption coefficient of the sensitizing centers. In samples containing amorphous aggregates we observe an exponential dependence of the PL emission intensity versus excitation energy, which is indicative of an Urbach tail due to the disordered structure of aggregates. In the sample containing crystalline aggregates we observe an indirect interband transition behavior, with an energy gap of 1.56 eV.

Growth and Formation of Fullerene Clusters

Theoretical studies on the stability and electronic structure of small carbon clusters assuming chain, ring, bowl, and fullerene structures have been carried out using a linear combination of atomic orbitals molecular orbital approach within a density functional formalism. Our studies on clusters containing between 12 and 60 atoms indicate three regimes for the growth and formation of carbon clusters. In clusters containing less than 20 atoms, the most stable geometry is the ring arrangements. Between 20 and 28 atoms, clusters with very different geometry have comparable energies. For clusters with larger than 30 atoms, the fullerene structures are the most stable structures. An analysis of the electronic structure shows a distinct correlation between the geometry and the nature of electronic states.

Theoretical calculations of magnetic order and anisotropy energies in molecular magnets

We present theoretical electronic structure calculations on the nature of electronic states and the magnetic coupling in the Mn12O12 free cluster and the Mn12O12(RCOO)16(H2O)4 molecular magnetic crystal. The calculations have been performed with the all-electron full-potential NRLMOL code. We find that the free Mn12O12 cluster relaxes to an antiferromagnetic cluster with no net moment. However, when coordinated by sixteen HCOO ligands and four H2O groups, as it is in the molecular crystal, we find that the ferrimagnetic ordering and geometrical and magnetic structure observed in the experiments is restored. Local Mn moments for the free and ligandated molecular magnets are presented and compared to experiment. We identify the occupied and unoccupied electronic states that are most responsible for the formation of the large anisotropy barrier and use a recently developed full-space and full-potential method for calculating the spin–orbit coupling interaction and anisotropy energies. Our calculated second-order anisotropy energy is in excellent agreement with experiment.

Magnetic anisotropy barrier for spin tunneling inMn12O12molecules

Electronic structure calculations on the nature of electronic states and the magnetic coupling in Mn-acetate $[{\mathrm{Mn}}_{12}{\mathrm{O}}_{12}(R\mathrm{COO}{)}_{16}({\mathrm{H}}_{2}\mathrm{O}{)}_{4}]$ molecules have been been carried out within the generalized gradient approximation to the density functional formalism. Our studies on this 100-atom molecule illustrate the role of the nonmagnetic carboxyl host in stabilizing the ferrimagnetic ${\mathrm{Mn}}_{12}{\mathrm{O}}_{12}$ core and provide estimates of the local magnetic moment at the various sites. We provide a first density-functional-based prediction of the second-order magnetic anisotropy energy of this system. Results are in excellent agreement with experiment. To perform these calculations we introduce a simplified exact method for spin-orbit coupling and magnetic anisotropy energies in multicenter systems. This method is free of shape approximations and has other advantages as well. First, it is valid for periodic boundary conditions or finite systems and is independent of basis set choice. Second, the method does not require the calculation of electric field. Third, for applications to systems with a finite energy gap between occupied and unoccupied electronic states, a perturbative expansion allows for a simple determination of the magnetic anisotropy energy.

Multifractal scaling of electronic transmission resonances in perfect and imperfect Fibonacci [formula omitted]-function potentials

We present here a detailed multifractal scaling study for the electronic transmission resonances with the system size for an infinitely large one-dimensional perfect and imperfect quasiperiodic system represented by a sequence of δ-function potentials. The electronic transmission resonances in the energy minibands manifest more and more fragmented nature of the transmittance with the change of system size. We claim that when a small perturbation is randomly present at a few number of sites or layers, the nature of electronic states will change and this can be understood by studying the electronic transmittance with the change of system size. We report the different critical states manifested in the size variation of the transmittance corresponding to the resonant energies for both perfect and imperfect cases through multifractal scaling study for few of these resonances.

Metal-Insulator Transitionlike Behavior In Several Icosahedral Phases

AbstractThe highly non-metallic electron transport properties in icosahedral Al70.5Pd21(Re,Mn)8.5 phases and their dependence on alloy composition and sample preparation are reviewed and fuirther studied. The study has led to the highest resistivity ratios ρ(4.2K)/ρ(295K)˜280 reported so far on i-AIPdRe alloys. Similar investigation has also resulted in enhanced resistivity ratios in i-AlCuOs. Evidences for the metal-insulator transition in these i-alloys are documented and discussed in light of the new results. Electron diffraction studies (article by Beeli, Poon, Guo, and Haussler in this Conference Proceedings) have provided evidence for electron localization induced by phason disorder. These findings may provide the basis for understanding the nature of electronic states and effects of disorder on electron localization in quasiperiodic systems.

The nature of electronic states in anodic Zirconium oxide films part 2: Photoelectrochemical characterization

Coupling between the electronic and crystallographic defect structures of anodic ZrO 2 films has been explored using the recently developed photoelectrochemical impedance technique. With this technique, the dynamic photocurrent response for ultra-violet excitation was analyzed. Additionally, photo-stimulated capacitance transients were also examined. Our studies indicate that, upon UV irradiation, photo-generated holes are transported to the oxide/solution interface by a diffusion/migration mechanism. Coincidentally with this electronic response, oxygen ion vacancies also migrate through anodic zirconia films upon UV irradiation. This leads to photo-stimulated formation of ZrO 2. The rate-limiting step in this film formation process is the transport of oxygen ion vacancies in the film by a diffusion/migration mechanism. Both the photo-electric response and the “photo-ionic” response of anodic zirconia films can be explained by the point defect model for anodic films.

The nature of electronic states in anodic zirconium oxide films part 1: The potential distribution

In order to derive the potential distribution in anodic films formed on zirconium in 1 M H 3PO 4, various optical and electrochemical techniques have been employed. All of the results demonstrate the existence of a linear relationship between the film thickness and the applied voltage. Once the films are formed, neither chemical nor electrochemical dissolution of the metal oxide can be observed in our electrolyte. The applied potential proves to be distributed over a space charge region, presumably located at the metal/film interface, and the neutral oxide, as can be concluded from capacitance-voltage ( cv) profiles. A band model is proposed in which the energies of the conduction and valence bands are compared with the work function of Zr. Also the space charge region at the metal/film interface is included. This model not only fully accounts for the cv profiles, but also explains the observed strongly rectifying current-voltage ( i- V) behavior, in the cathodic and anodic regions.

The multifractal character of the electronic states in disordered two-dimensional systems

The nature of electronic states in disordered two-dimensional (2D) systems is investigated. With this aim, we present our calculations of both density of states and d.c. conductivity for square lattices modelling the Anderson Hamiltonian with on-site energies randomly chosen from a box distribution of width W. For weak disorder (W), the eigenfunctions calculated by means of the Lanczos diagonalization algorithm display spatial fluctuations reflecting their (multi)fractal behaviour. For increasing disorder the observed increase of the curdling of the wavefunction reflects its stronger localization. However, as a function of energy, the eigenstates at energy mod E mod /V approximately=1.5 are found to be the least localized over the band. Our d.c. conductivity results suggest a critical fractal dimension dc*=1.48+or-0.17 to discriminate between me exponentially and the power-law-localized states. Consequences of the localization for transport properties are also discussed.

Nature of electronic states in slabs.

Electronic states in metallic slabs ordinarily involve multiple interplanar couplings even in a nearest-neighbor-atom-coupling approximation, except for special cases such as (100) and (111) face-centered-cubic surfaces. A tight-binding study with nonzero multiple interplanar couplings shows that there can be many, depending upon the orientation of the slab, exponentially decaying states within the energy band. Their existence is needed to satisfy the boundary conditions at the slab surface. The effect of multiple interplanar coupling is to cause phase shifts in the plane-wave states in the bulk of material. These phase shifts produce an effective free-electron surface plane, independent of energy to the extent that the bands are parabolic, and affect quantization rules and the energy spectrum in slabs. The corresponding effects must also occur in more accurate representations of the electron states.

Anomalous below-gap absorption by bulk high z. sbnd; Tc superconductors

Optical absorption spectra for various high z. sbnd; T c superconductors have been measured by using an optically pumped far-infrared laser at the temperature region between 12 and 160 K, and the nature of electronic states is discussed.

Spectroscopic properties and potential energy curves of I 2 and I 2+

Complete active space MCSCF followed by first-order configuration interaction, second-order configuration interaction, and relativistic configuration interaction calculations are carried out on 30 electronic states of I 2 and 13 electronic states of I 2 +, among which the spectroscopic properties of the bound states are calculated. The potential curves of the calculated low-lying states are reported. These calculations confirm earlier assignments of the X, A, A′, B, B′, B″, a, a′, C, D′, E, G, G′, and H states of I 2, and the X, A, and B states of I 2 +. In addition, spectroscopic properties of many states of I 2 + and a few states of I 2 are predicted which are yet to be observed. The spin-orbit effects are found to be significant for many states of I 2 and I 2 +. The nature of electronic states as a function of internuclear distance is also provided. The experimentally predicted maximum in the potential curve of the B state of I 2 + is explained by our theoretical calculations.

Polarized X-Ray Absorption Fine Structure of La2CuO4-ySingle Crystal

Polarized Cu K X-ray absorption fine structure (XAFS) has been measured on a single crystal of undoped La 2 CuO 4- y for the first time, using a fluorescence detection technique. The polarized near-edge structure and EXAFS were strongly anisotropic, demonstrating the quasi-two-dimensional nature of electron states and local structure. The polarization dependence of the near-edge structure confirms that the Cu atoms take 2+ valence or d 9 configuration with a strong fourfold Cu–O1 bonds within the CuO 2 plane and weak twofold Cu–O2 bonds along the c -axis, which is consistent with the anisotropic local structure suggested by the Fourier transform of polarized EXAFS and a crystal structure.