Local Electronic Density Of States Research Articles

Single‐Walled Carbon Nanotubes

Single-walled carbon nanotubes (SWNTs) are ideal systems for investigating fundamental properties in one-dimensional electronic systems and have the potential to revolutionize many aspects of nano/molecular electronics. Scanning tunneling microscopy (STM) has been used to characterize the atomic structure and tunneling density of states of individual SWNTs. Detailed spectroscopic measurements showed one-dimensional singularities in the SWNT density of states for both metallic and semiconducting nanotubes. The results obtained were compared to and agree well with theoretical predictions and tight-binding calculations. SWNTs were also shortened using the STM to explore the role of finite size, which might be exploited for device applications. Segments less than 10 nm exhibited discrete peaks in their tunneling spectra, which correspond to quantized energy levels, and whose spacing scales inversely with length. Finally, the interaction between magnetic impurities and electrons confined to one dimension was studied by spatially resolving the local electronic density of states of small cobalt clusters on metallic SWNTs. Spectroscopic measurements performed on and near these clusters exhibited a narrow peak near the Fermi level that has been identified as a Kondo resonance. In addition, spectroscopic studies of ultrasmall magnetic nanostructures, consisting of small cobalt clusters on short nanotube pieces, exhibited features characteristic of the bulk Kondo resonance, but also new features due to their finite size.

Electronic Structure and X-Ray Spectra of the System SiC–(Al, Ti, C)

The method of local coherent potential within the framework of the multiple scattering theory has been applied to calculate the electronic structure of solid solutions Si1—xCRx (x = 0.25; R = C, Al, Ti) with zinc-blende lattice. Within the same approximation the comparison of the electronic structure of the considered solid solutions with one of the binary analogues is carried out. The calculated partial local densities of electronic states are compared with experimental X-ray spectra of Si in the compounds. The analysis of the electronic structures of the investigated solid solutions testifies the delocalisation of resonance states inherent to the binary compounds, that can be an evidence of the weakening of chemical bonds in the solid solutions in question.

Electronic Structure and X-Ray Spectra of the System SiC-(Al, Ti, C)

The method of local coherent potential within the framework of the multiple scattering theory has been applied to calculate the electronic structure of solid solutions Si 1-x CR x (x = 0.25; R = C, Al, Ti) with zinc-blende lattice. Within the same approximation the comparison of the electronic structure of the considered solid solutions with one of the binary analogues is carried out. The calculated partial local densities of electronic states are compared with experimental X-ray spectra of Si in the compounds. The analysis of the electronic structures of the investigated solid solutions testifies the delocalisation of resonance states inherent to the binary compounds, that can be an evidence of the weakening of chemical bonds in the solid solutions in question.

Scanning tunneling spectroscopy of a-C:H and a-C:(H, Cu) films prepared by magnetron sputtering

Scanning tunneling spectroscopy was used to study a-C:H and a-C:(H, Cu) films under atmospheric conditions; these films were formed on semiconductor (Si) and metallic (Cr/Si) substrates using dc magnetron sputtering of graphite or graphite/copper targets. The local density of electron states was determined from normalized differential tunneling conductance with the aim of probing the individual sp 2-phase clusters. The well-defined valence-band edge and the varying (i.e., dependent on the scanning coordinate) shape of the distribution of the density of electron states within the conduction band are characteristic of the a-C:H films; also, the largest experimental value of the band gap in these films is ∼3 eV; finally, the tendency towards the stable position of the Fermi level at a level of ∼1 eV above the valence-band top is observed in a-C:H films. The a-C:(H, Cu) films are homogeneous with respect to the local density of electron states, which is accounted for by the formation of a homogeneous surface layer in the course of growth.

An empirical study of the Mn partial density of states in icosahedral Al70Pd20Mn10

Abstract Using the line-shape analysis of the Mn 2p3/23d3d Auger transitions and by observing the Mn 3p→3d transitions in electron energy-loss spectroscopy, we have studied the local density of electronic states of the crystalline cubic and the quasicrystalline icosahedral phases of Al70Pd20Mn10. We find that, in the cubic phase the Mn 3d states coincide with the Fermi level EF, but in the quasicrystalline state the Mn 3d band is fully occupied and shifted to lower binding energies causing a significant reduction in the density of states at EF.

Local electronic density of states at the surface of the bulk quasicrystal Al 70Pd 20Mn 10

At the surface of the bulk Al 70Pd 20Mn 10 quasicrystal, different surface phases can be induced upon changing the chemical composition. Using combined Auger electron and X-ray photoelectron spectroscopy, information was obtained on the local density of states of the alloy components. In the quasicrystalline state the Mn 3d band moves away from the Fermi level thus lowering the density of states and opening the pseudogap.

Density functional study of the Σ3(111) [10] symmetrical tilt grain boundary in SrTiO3

Density functional supercell calculations were carried out by amixed-basis pseudopotential method for the Σ3 (111)[11̄0] symmetrical tilt grain boundary in strontium titanate,SrTiO3, as a prototype planar defect in an electro-ceramicmaterial. A low grain boundary energy of 0.52 J m-2 wasobtained. Minor structural relaxations compared to the coincidencesite lattice model are obtained close to the grain boundary plane bytheory and previous experiment. Within the limits of the rathersmall supercell model employed in the calculations, the calculatedgeometric structure agrees well with the experimental one. Ananalysis of calculated local electron densities of states shows thatthe perturbation due to the grain boundary is localized in the nearvicinity of the boundary plane. From the present results for thesmall supercell model, there is no evidence for the build-up of aspace-charge layer at this highly ordered and undoped twin boundaryin SrTiO3.

A fast-scanning, low- and variable-temperature scanning tunneling microscope

The design and performance of a fast-scanning, low- and variable-temperature, scanning tunneling microscope (STM) incorporated in an ultrahigh vacuum system is described. The sample temperature can be varied from 25 to 350 K by cooling the sample using a continuous flow He cryostat and counter heating by a W filament. The sample temperature can be changed tens of degrees on a time scale of minutes, and scanning is possible within minutes after a temperature change. By means of a software implemented active drift compensation the drift rate can be as low as 1 nm/day. The STM is rigid, very compact, and of low weight, and is attached firmly to the sample holder using a bayonet-type socket. Atomic resolution on clean metal surfaces can be achieved in the entire temperature range. The performance of the instrument is further demonstrated by images of adsorbed hexa-tert-butyl-decacyclene molecules on Cu(110), by STM movies, i.e., sequential STM images with a time resolution down to 1 s/image (100×100 Å2 with 256×256 pixels), of the mobility of these molecules, and finally by constant current images of standing waves in the electronic local density of states on Cu(110).

Optical Properties of Amorphous Co‐Containing Alloys in the IR Region of the Spectrum and Their Magnetoresistive Characteristics

The optical properties of amorphous and nanocrystalline tapes of CoFeNiSiB alloys and the ac magnetoresistive effect in them have been investigated. It is shown that the magnetoresistive effect in these alloys increases by about a factor of two after low‐temperature treatment and subsequent laser annealing of these tapes in an external magnetic field. Correlation between the anomalous behavior of the dielectric function of these alloys in the IR region of the spectrum and their magnetoresistive characteristics after thermal annealings has been established. The interrelation between the density of localized electronic states near the Fermi level and the value of the magnetoresistive effect has been revealed. The influence of the induced magnetic anisotropy on the transverse permeability in amorphous and nanocrystalline CoFeNiSiB alloys and, consequently, also on the magnetoresistive effect has been analyzed.

Magnetic clusters on single-walled carbon nanotubes: the Kondo effect in a one-dimensional host.

Single-walled carbon nanotubes are ideal systems for investigating fundamental properties and applications of one-dimensional electronic systems. The interaction of magnetic impurities with electrons confined in one dimension has been studied by spatially resolving the local electronic density of states of small cobalt clusters on metallic single-walled nanotubes with a low-temperature scanning tunneling microscope. Spectroscopic measurements performed on and near these clusters exhibit a narrow peak near the Fermi level that has been identified as a Kondo resonance. Using the scanning tunneling microscope to fabricate ultrasmall magnetic nanostructures consisting of small cobalt clusters on short nanotube pieces, spectroscopic studies of this quantum box structure exhibited features characteristic of the bulk Kondo resonance, but also new features due to finite size.

Structural Dependence of Electronic and Magnetic Properties of the Cr13 Cluster**The project supported by National Natural Science Foundation of China

A tight-binding effective potential has been employed, in conjunction with a genetic algorithm, to fully optimize the Cr13 cluster geometry without imposing any symmetry constraints. The minimum energy structure of this cluster is found to be a slightly distorted icosahedron. Based on the optimized structure and three assumed geometries (icosahedron,bcc-like and fcc-like), the structural dependence of electronic and magnetic properties of the Cr13 cluster is discussed by using a d-band Hubbard-like Hamiltonian in the unrestricted Hartree-Fock approximation. Results are given for the average magnetic moment and local magnetic moments. It is found that for all considered geometries the Cr13 cluster exhibits antiferromagnetic behavior. Results are also given for the cohesive energy, average coordination number, and local electronic densities of states. The results indicate that the average coordination number per atom in the cluster geometry is a significant factor to affect the magnetism.Also, the local density of states is a sensitive function of geometry.

Electronic indices from semi-empirical calculations to identify carcinogenic activity of polycyclic aromatic hydrocarbons

Recently we have shown that it is possible to group and identify the carcinogenic activity of the polycyclic aromatic hydrocarbons (PAHs) using very simple rules derived from simple Hückel calculations. In this work we have carried out similar calculations using semi-empirical methods in order to investigate whether the derived rules are method dependent. We present PM3 (Parametric Method 3) and ZINDO-CI (Zerner Intermediate Neglect of Differential Overlap-Configuration Interaction) study on the electronic structure of PAHs. Very similar rules, based on the concept of electronic local density of states over specific molecular regions, are derived from PM3 calculations showing that they are essentially method independent. The analysis of the ZINDO-CI results for the intensity of the threshold transitions (first optical transitions) and the composition of the CI contributions also show a differentiated behavior for the strong carcinogenic molecules and the inactive ones.

Valence-Band Structure Features of Nickel Disilicide Films

The present paper reports the results of theoretical electronic structure study of nickel disilicide films. Such spectral characteristics as the total and local densities of electronic states, X-ray emission-band spectra of different series for all nonequivalent film atoms and photoemission spectra for series of quantum energies of exciting radiation were calculated. The calculation was carried out in terms of the film linearized augmented plane wave method. The comparison of theoretically obtained results with the known experimental data allowed to interpret the peculiarities of spectral properties of this material and in particular to explain qualitatively the transformation of L 2.3 spectrum of silicon at the transition from the bulk sample to the thin film one.

Strain effect in aGaAs−In0.25Ga0.75As−Al0.5Ga0.5Asasymmetric quantum wire

We report a theoretical investigation of the strain effects on the electronic energy band in a ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}\ensuremath{-}\mathrm{I}\mathrm{n}}_{0.25}{\mathrm{Ga}}_{0.75}{\mathrm{A}\mathrm{s}\ensuremath{-}\mathrm{A}\mathrm{l}}_{0.5}{\mathrm{Ga}}_{0.5}\mathrm{As}$ asymmetric quantum wire formed in a V-grooved substrate. Our model is based on the ${\mathrm{sp}}^{3}{s}^{*}$ tight-binding model. It includes different spatial distributions of the lattice-mismatch-induced strain. We solve numerically the tight-binding Hamiltonian through the local Green's function from which the electronic local density of states (LDOS) is obtained. The detailed energy band structure (discrete localized states and energy bands of extended states) and the spatial distribution of the eigenfunctions (wave function amplitude of nondegenerate states or sum of the wave function amplitudes of degenerate states) are directly reflected in the LDOS. Spatial mapping of the LDOS's shows a reduction of the lowest excitation energies in different regions of the system when the local lattice structure of the ${\mathrm{In}}_{0.25}{\mathrm{Ga}}_{0.75}\mathrm{As}$ layer relaxes from completely strained to completely relaxed. By comparing the calculated results with photoluminescence measurement data, we conclude that the strain in the ${\mathrm{In}}_{0.25}{\mathrm{Ga}}_{0.75}\mathrm{As}$ layer relaxes linearly from the heterointerface with the ${\mathrm{Al}}_{0.5}{\mathrm{Ga}}_{0.5}\mathrm{As}$ buffer layer to the heterointerface with the top GaAs layer.

Insulator-superconductor-metal transitions induced by spin fluctuations in the paramagnetic phase of doped insulators

AbstractThe band structure of cuprates as a doped 2D insulator is modeled assuming that the excess charge carriers are associated with the corresponding substitution atoms, and the phase diagram of the paramagnetic states as a function of the degree x of doping at zero temperature is studied. The Hamiltonian contains electronic correlations on impurity orbitals and hybridization between them and the initial band states of the insulator. It is shown that the change in the electronic structure of a doped compound includes the formation of impurity bands of distributed and localized electronic states in the initial insulator gap. It is established that in the case of one excess electron per substitution atom the spin fluctuations (1) give rise to an insulator state of the doped compound for x xthr, 2 increases further, and the doped compound transforms into a paramagnetic state of a “poor” metal with a high density of localized electronic states at the Fermi level.

Atomic and electronic structure and interatomic potentials at a polar ceramic/metal interface:{222}MgO/Cu

Local density functional theory (LDFT) calculations, within the plane-wave-pseudopotential framework, are performed for the ${222}\mathrm{M}\mathrm{g}\mathrm{O}/\mathrm{C}\mathrm{u}$ polar interface, with the objective of elucidating the atomic and electronic structure of the interface, as well as interface interatomic potentials. Calculations are performed for both coherent interfaces and semicoherent interfaces that approximate the lattice constant mismatch of the true system. Calculations of local electronic density of states and adhesive energies are performed primarily for coherent interfaces. The density of electronic states at the interface for the oxygen-terminated configuration exhibits a peak in the bulk MgO energy gap that results from $\mathrm{O}(2p)\ensuremath{-}\mathrm{Cu}(3d)$ hybridization. The calculated interface adhesive energies for coherent interfaces as a function of the interface spacing and translation state are well reproduced by a simple analytical expression that combines an attractive Rydberg-function term and a repulsive pairwise Born-Mayer potential across the interface. Calculations are performed for a semicoherent interface with $5\ifmmode\times\else\texttimes\fi{}5\mathrm{Cu}$ layer unit cells opposite $4\ifmmode\times\else\texttimes\fi{}4$ MgO layer unit cells, an approximation to the true system with lattice constant ratio of 7/6, to investigate the relaxation at the interface in the presence of misfit. The terminating oxygen layer as well as the interface Cu layer exhibits warping albeit on a scale of less than 0.1 \AA{}.

On the electronic structure of surface Pt–Sn alloys

The electronic structures of Pt 3Sn(111),Pt 3Sn/Pt(111) and Pt 2Sn/Pt(111) surfaces are studied using the linear-muffin-tin-orbital tight-binding method in the atomic-sphere approximation. Both ideal and rumpled surface terminations are considered. The hybridization between Pt d- and Sn p-electrons, respectively, leads to a lowering of the local density of electronic states at the Fermi level and to a downward shift of the Pt local d-band, which accounts for the lower reactivity of the Pt–Sn surfaces. The effect is more pronounced for rumpled surfaces. Generally, the situation is similar to that of bimetallic transition-metal surfaces. The initial-state approximation is used to predict Pt(4f) core-level shifts. We find moderate shifts of either sign, but the d-band centre of gravity moves to higher binding energies, as compared to Pt(111), in most cases. The correlation between the surface reactivity and core-level shifts, respectively, seems to be less favourable than at bimetallic transition-metal surfaces

Identifying carcinogenic activity of methylated polycyclic aromatic hydrocarbons (PAHs)

Polycyclic aromatic hydrocarbons (PAHs) are a class of planar molecules that can induce chemical carcinogenesis. Their carcinogenic power vary in a large range, from the very strong carcinogens to the inactive ones. Recently a new methodology was proposed (using the topological simple Hückel theory) to group and identify PAHs carcinogenic activity in terms of very simple rules based on the concept of electronic local density of states over specific molecular regions. In the present work we have extended this study to include methylated compounds. Our results show that the previous methodology can also be simplified and successfully used to predict the carcinogenic activity of methylated PAHs.

Predictions of Enhanced Chemical Reactivity at Regions of Local Conformational Strain on Carbon Nanotubes: Kinky Chemistry

Simulations that model the effects of conformational strain on the chemical reactivity of single-walled carbon nanotubes suggest a method for significantly enhancing their reactivity locally by controlled deformations. The chemisorption of hydrogen atoms is predicted to be enhanced by as much as 1.6 eV at regions of high conformational deformation, suggesting that local reactivity will be significantly enhanced. Analysis of the local electronic density of states suggests the introduction of radical p orbital character to the sites that are locally deformed, consistent with the heightened reactivity and large pyramidalization angles at these sites. Preliminary experimental data consistent with this predicted heightened reactivity is also presented.

Structure–activity relationship study of some inhibitors of HIV-1 integrase

Recently a new and simple methodology has been proposed to correlate electronic indices to the biological activity of a class of carcinogenic polycyclic aromatic hydrocarbons (PAHs). This methodology was based on the energy separation values between frontier orbitals and in their relative contribution to the local density of electronic states over specific molecular regions. In this work we adapted the methodology to the study of some new HIV-1 Integrase inhibitors. Integrase is an HIV-1 enzyme essential for effective viral replication. Our calculations were carried out using the semi-empirical Parametric Method 3 (PM3). Our results show that, similar to the PAHs, it is possible to derive very simple rules correlating inhibitory integrase activity to electronic indices.