Local Electronic Density Of States Research Articles

Covalence of chemical bonds and white-line intensity of an L 3-edge X-ray absorption near-edge structure of rare earth elements embedded in glass

The covalence of chemical bonds of rare earth ions embedded in glasses is estimated in terms of local structural parameters obtained from an extended X-ray absorption fine structure analysis and interpreted in connection with L 3-edge white-line intensity of the rare earth ions, which, according to some suggestions, is proportional to the covalence. However, the Dy L 3-edge white-line intensity is stronger in Dy-doped sulfide glasses, even though the covalence of the Dy-S bonds is decreased. Moreover, the intensity variations of the Ho L 3-edge white lines in Ho-containing glasses and crystalline compounds reveal no clear correlation with the covalence of the Ho bonds. The amorphous nature of atomic arrangements affects the multiple scattering of photoelectrons and the local electronic density of states. These effects may need to be considered, in addition to the previously suggested screening effect on the electronic transition, when evaluating the L 3-edge white-line intensity and optical properties of rare earth elements doped in glasses.

A Combined Experimental and Theoretical Investigation of Atomic-Scale Defects Produced on Graphite Surfaces by Dielectric Barrier Discharge Plasma Treatment

The characteristics and nature of atomic-scale defects produced on graphite surfaces by dielectric barrier discharge (DBD) plasma oxidation have been investigated, both experimentally and theoretically. Two main types of defect visualized by scanning tunneling microscopy (STM) were studied: protrusions ∼1-5 nm in diameter and smooth circular depressions 5-7 nm wide, the latter constituting a novel type of defect on carbon surfaces that was only very recently reported for the first time. STM and atomic force microscopy (AFM) experiments indicated that both the protrusions and the depressions are not associated to topographical features on the graphite surface and that their observation by STM should be related to electronic effects. The thermal behavior of the protrusions, which could only be removed at a temperature of ∼900 °C, as well as their reactivity toward molecular oxygen, allowed their identification as multiatomic vacancies. In comparison, the depressions displayed a higher thermal stability (they could be eliminated only at ∼1200 °C) and a lower reactivity toward oxidation. Density functional theory (DFT) calculations suggested that the depressions are associated with two-dimensional clusters of interstitial oxygen formed by the agglomeration of migrating oxygen atoms. Such clusters induce a lowering in the local density of electronic states on the graphite surface and are therefore detected as a depression by STM. Taken as a whole, the findings reported here provide a consistent picture of the basic mechanism underlying the modification of graphitic surfaces by this type of plasma, which is driven by physical processes (i.e., ion bombardment).

Graphene on gold: Electron density of states studies by scanning tunneling spectroscopy

Graphene devices require electric contacts with metals, particularly with gold. Scanning tunneling spectroscopy studies of electron local density of states performed on mono-, bi-, and trigraphene layer deposited on metallic Au/Cr/SiO2/Si substrate shows that gold substrate causes the Fermi level shift downwards which means that holes are donated by metal substrate to graphene which becomes p-type doped. These experimental results are in good accordance with recently published density function theory calculations.

Quasiparticle interference and Landau level spectroscopy in graphene in the presence of a strong magnetic field

We present a calculation of the modulation in the Local Density Of electronic States (LDOS) caused by an impurity in graphene in the presence of external magnetic field. We focus on the spatial Fourier Transform (FT) of this modulation around the impurity. The FT due to the low energy quasiparticles are found to be nonzero over the reciprocal lattice corresponding to graphene. At these lattice spots the FT exhibits well-defined features at wavevectors that are multiples of the inverse cyclotron orbit diameter (see Figure 2) and is cut off at the wavevector corresponding to the energy of observation. Scanning Tunneling Spectroscopy (STS) on graphene and the energy-resolved FT fingerprint obtained therefrom may be used to observe the quasiparticle interference of Dirac particles in graphene in the presence of magnetic field.

Negative impurity magnetic susceptibility and heat capacity in a Kondo model with narrow peaks in the local density of electron states

Temperature dependences of the impurity magnetic susceptibility, entropy, and heat capacity have been obtained by the method of numerical renormalization group and exact diagonalization for the Kondo model with peaks in the electron density of states near the Fermi energy (in particular, with logarithmic Van Hove singularities). It is shown that these quantities can be negative. A new effect has been predicted (which, in principle, can be observed experimentally), namely, the decrease in the magnetic susceptibility and heat capacity of a nonmagnetic sample upon the addition of magnetic impurities into it.

Fabrication of a quantum well heterostructure based on plasma polymerized aniline and its characterization using STM/STS

Two-dimensional electronic systems play a crucial role in modern electronics and offer a multitude of opportunities to study the fundamental phenomena at low dimensional physics. A quantum well heterostructure based on polyaniline (P) and iodine doped polyaniline (I) thin films were fabricated using radio frequency plasma polymerization on indium tin oxide coated glass plate. Scanning probe microscopy and scanning electron microscopy studies were employed to study the morphology and roughness of the polymer thin films. Local electronic density of states (LDOS) of the P–I–P heterostructures is probed using scanning tunnelling spectroscopy (STS). A step like LDOS is observed in the P–I–P heterostructure and is attributed to the quantum well confinement of electrons in the polymer heterostructure.

Scanning tunneling microscopy at theNbSe3surface: Evidence for interaction betweenq1andq2charge density waves in the pinned regime

We have investigated both ${\mathbf{q}}_{\mathbf{1}}$ and ${\mathbf{q}}_{\mathbf{2}}$ charge density wave (CDW) states taking place in ${\text{NbSe}}_{3}$ by means of low-temperature scanning tunneling microscopy (STM) under ultrahigh vacuum on the in situ cleaved $(\mathbit{b},\mathbit{c})$ surface. High-resolution topographical images with atomic lattice resolution were obtained in the temperature range between 5 and 140 K. The careful and thorough analysis of the dependence of the STM images on bias polarity, energy, and temperature allowed us to identify unambiguously the three different types of chains composing the ${\text{NbSe}}_{3}$ unit cell at all temperatures, resolving contradictions from previous STM results. From two-dimensional Fourier transform of the STM images, we show that at the surface plane both CDW's wave vectors are in very good agreement with bulk reported values projected on the $(\mathbit{b},\mathbit{c})$ plane. The ${\mathbf{q}}_{\mathbf{1}}$ CDW has, for wave vector, ${\mathbf{q}}_{\mathbf{1}}=0.24{\mathbit{b}}^{\ensuremath{\ast}}$. Spatially, the ${\mathbf{q}}_{\mathbf{1}}$ modulation is essentially developed on type III chains with a weak contribution on type II neighboring chains. The ${\mathbf{q}}_{\mathbf{2}}$ CDW has, for wave vector, the projected value ${\mathbf{q}}_{\mathbf{2}\mathbit{p}}=0.26{\mathbit{b}}^{\ensuremath{\ast}}+0.5{\mathbit{c}}^{\ensuremath{\ast}}$. This modulation is mainly developed on type I chains but surprisingly has an important contribution on type III chains with an amplitude similar to the ${\mathbf{q}}_{\mathbf{1}}$ contribution on these chains. This simultaneous double modulation on chain III leads to a beating phenomenon between the ${\mathbf{q}}_{\mathbf{1}}$ and ${\mathbf{q}}_{\mathbf{2}\mathbit{p}}$ periodicities and gives rise to a new domain superstructure developed along the chain axis which is characterized by the vector $\mathbit{u}=2\ifmmode\times\else\texttimes\fi{}({\mathbf{q}}_{\mathbf{2}\mathbit{p}}\ensuremath{-}{\mathbf{q}}_{\mathbf{1}})\ensuremath{-}{\mathbit{c}}^{\ensuremath{\ast}}=2\ifmmode\times\else\texttimes\fi{}(0.26\ensuremath{-}0.24){\mathbit{b}}^{\ensuremath{\ast}}$. We believe that these new features give a clue of the coupling between the ${\mathbf{q}}_{\mathbf{1}}$ and ${\mathbf{q}}_{\mathbf{2}}$ CDWs in the pinned regime. Whereas most studies investigated the various characteristics of both CDWs by probing the Nb atoms properties, our results are consistent with the interpretation according to which the electronic local density of states probed by STM is mostly that of the surface Se atoms.

Electronic properties of H on vicinal Pt surfaces: First-principles study

In this work, we use the first-principles density-functional approach to study the electronic structure of a H atom adsorbed on the ideal Pt(111) and vicinal Pt(211) and Pt(331) surfaces. Full three-dimensional potential-energy surfaces (3D PES's) as well as local electronic density of states on various adsorption sites are obtained. The results show that the steps modify the PES considerably. The effect is nonlocal and extends into the region of the (111) terraces. We also find that different type of steps have different kind of influence on the PES when compared to the one of the ideal Pt(111) surface. The full 3D PES's calculated in this work provide a starting point for the theoretical study of vibrational and diffusive dynamics of H adatoms adsorbed on these vicinal surfaces.

Spatially resolved spectroscopy of MgO–Fe(100)–MgO(100) structure

Using scanning tunneling spectroscopy and microscopy, the local density of electronic states was correlated with the topography of Fe on MgO and MgO on Fe. The growth mode of Fe on MgO is by island formation that leads to an electronically inhomogeneous surface. Fe (100) surface states were observed on flat terraces that diminish at the step edges, while bandgaps were observed in some deep trenches that separate the islands. MgO on Fe (100), on the other hand, grows by the simpler layer-by-layer mode. The electronic states evolved as a function of thickness from less than one to several monolayers. The bulklike MgO behavior with more than 6 V bandgap was found for the thickest films (∼1 nm). However, the spectra for very thin MgO depended upon whether it was measured on top of Fe atoms or on the MgO grains. At one monolayer, the measured density of states was different from either very thick or very thin MgO, which suggests the possibility of an interfacial layer that is distinct from the Fe (100) and MgO (100) surfaces. The observed local inhomogeneity may account for the reduced tunneling magnetoresistance of these systems.

Local Electronic Properties of Titanocene Chloride Dimer Molecules on a Metal Surface

We have examined the local electronic behavior of titanocene chloride dimer molecules, [Cp2TiCl]2 (Cp = C5H5), on Au(111) using both scanning tunneling microscopy (STM) and density functional theory (DFT). Isolated dimeric molecules are seen to decorate gold step edges at low surface coverages and two coexisting monolayer phases are observed at higher coverages. Differential conductance STM spectroscopy shows a variety of structure in the electronic local density of states for single molecules and for the two monolayer phases. DFT calculations modeling an isolated titanocene chloride dimer do not reproduce the wide variations seen in spectral density, suggesting that a fraction of titanocene dimers break apart into metallocene monomers on the Au(111) surface and display magnetic behavior.

Conductance through atomic point contacts between fcc(100) electrodes of gold

Electrical conductance through various nanocontacts between electrodes is studied by using the density functional theory, scalar-relativistic pseudopotentials, generalized gradient approximation for the exchange-correlation energy and the recursion-transfer-matrix method along with channel decomposition. The nanocontact is modeled with pyramidal fcc(100) tips and 1 to 5 atoms between the tips. Upon elongation of the contact by adding atoms between the tips, the conductance at Fermi energy E_F evolves from G ~ 3 G_0 to G ~ 1 G_0 (G_0 = 2e/h^2). Formation of a true one-atom point contact, with G ~ 1 G_0 and only one open channel, requires at least one atom with coordination number 2 in the wire. Tips that share a common vertex atom or tips with touching vertex atoms have three partially open conductance channels at E_F, and the symmetries of the channels are governed by the wave functions of the tips. The long 5-atom contact develops conductance oscillations and conductance gaps in the studied energy range -3 < E-E_F < 5 eV, which reflects oscillations in the local density of electron states in the 5-atom linear gold between the electrodes, and a weak coupling of this molecule to the tips.

Comparison of chlorine and oxygen adsorption on the ferromagnetic Fe(0 0 1) surface: Density-functional theory study

We compare chemisorption of Cl and O on the ferromagnetic Fe(0 0 1) surface by using density-functional theory calculation. p( 2 × 2 ) and p( 1 × 1 ) adsorbate overlayers are considered. The chlorine adsorption resembles in most respects the previously thoroughly studied oxygen deposition, although the Cl atom is well separated from the subsurface-layer Fe atom and, at low coverage, its induced magnetic moment is very small: 0.06 μ B . For the p( 2 × 2 ) structure, the spin-resolved local density of electronic states is dominated by exchange-splitted peaks of O(2p) or Cl(3p) character. The calculations are employed to discuss the data obtained recently by the new spin-polarized ion-scattering experimental technique [T.T. Suzuki et al., Surf. Sci. 602 (2008) 1688] for oxygen and chlorine adsorption on Fe(0 0 1). It remains to show whether the experimentally found significant difference between the signal from oxygen and chlorine, respectively, can be explained by the comparatively little differing electronic properties of the two adsorbates, or whether the signal from O does not rather reflect partly properties of the surface iron atoms. For chlorine, the latter mechanism might be suppressed due to its large atomic radius.

Site-Dependent 13C Chemical Shifts of CO Adsorbed on Pt Electrocatalysts

13C NMR and hydrogen-region cyclic voltammetry are used to parse the distribution of adsorbed CO on Pt electrocatalysts into two different types of sites. Trends in the NMR shift data show that 13CO adsorbed on so-called weakly bound H sites show larger Knight shifts as compared to 13CO adsorbed onto strongly bound H sites, and thus experience greater back-bonding from the Pt conduction band. These results are discussed in the context of local electron densities of states and the varying oxidation reactivities associated with these sites on the Pt surface.

The local electronic structure of α-Li3N

New theoretical and experimental investigations of the occupied and unoccupied local electronic densities of states (DOS) are reported for α-Li3N. Band-structure and density-functional theory calculations confirm the absence of covalent bonding character. However, real-space full-multiple-scattering (RSFMS) calculations of the occupied local DOS find less extreme nominal valences than have previously been proposed. Nonresonant inelastic x-ray scattering, RSFMS calculations, and calculations based on the Bethe–Salpeter equation are used to characterize the unoccupied electronic final states local to both the Li and N sites. There is a good agreement between experiment and theory. Throughout the Li 1s near-edge region, both experiment and theory find strong similarities in the s-and p-type components of the unoccupied local final DOS projected onto an orbital angular momentum basis (l-DOS). An unexpected, significant correspondence exists between the near-edge spectra for the Li 1s and N 1s initial states. We argue that both spectra are sampling essentially the same final DOS due to the combination of long core-hole lifetimes, long photoelectron lifetimes, and the fact that orbital angular momentum is the same for all relevant initial states. Such considerations may be generally applicable for low atomic number compounds.

D-Wave Superconductivity Orbital Magnetism and Unidirectional Charge Order in the t-J Model

Recent scanning tunneling microscopy in the superconducting regime of two different cuprate families revealed unidirectional bond-centered modulation in the local electronic density of states. Motivated by this result we investigate the emergence of modulated d-wave superconductivity coexisting with charge domains that form along one of the crystal axes. While detailed stripe profiles depend on the used form of the Gutzwiller factors, the tendency towards a valence bond crystal remains robust. We also find closely related stripe phase originating from the staggered flux phase, a candidate for the pseudogap phase of lightly doped cuprates.

A novel method achieving ultra-high geometrical resolution in scanning tunnelling microscopy

We report a new contrast mechanism in which scanning tunnelling micrographs of a certain class of molecules resemble chemists' structure formulae. The method is based on adding molecular hydrogen below its condensation temperature to the tunnelling junction of a low-temperature scanning tunnelling microscope. In the presence of hydrogen, the scanning tunnelling microscope contrast can be switched between the conventional mapping of the electronic local density of states and the new geometric imaging by selecting the appropriate bias voltage. Scanning tunnelling spectroscopy suggests that the coupling of the electron tunnelling current to an internal degree of freedom in the tunnelling junction is responsible for the geometric contrast. The new scanning tunnelling hydrogen microscopy (STHM) allows the chemical identification of certain molecular species by their structure.

Scanning Tunneling Spectroscopy of Individual PbSe Quantum Dots and Molecular Aggregates Stabilized in an Inert Nanocrystal Matrix

The electronic local density of states (LDOS) of single PbSe quantum dots (QDs) and QD molecules is explored using low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS). Both individual PbSe QDs and molecular aggregates of PbSe QDs (dimers, trimers, etc.) are mechanically stabilized in a two-dimensional superlattice of wide band gap CdSe QDs acting as an inert matrix. The LDOS measured at individual QDs dispersed in the matrix is identical to that of single isolated QDs chemically linked to a substrate. We investigate the degree of quantum mechanical coupling between the PbSe QDs in molecular aggregates by comparing the LDOS measured at each site in the aggregates to that of an individual PbSe QD. We observe a variable broadening of the resonances indicating a spatially dependent degree of electron delocalization in the molecular aggregates.

Valence bond solid order near impurities in two-dimensional quantum antiferromagnets

Recent scanning tunnelling microscopy (STM) experiments on underdoped cuprates have displayed modulations in the local electronic density of states which are centered on a Cu-O-Cu bond (Kohsaka et. al., cond-mat/0703309). As a paradigm of the pinning of such bond-centered ordering in strongly correlated systems, we present the theory of valence bond solid (VBS) correlations near a single impurity in a square lattice antiferromagnet. The antiferromagnet is assumed to be in the vicinity of a quantum transition from a magnetically ordered Neel state to a spin-gap state with long-range VBS order. We identify two distinct classes of impurities: i) local modulation in the exchange constants, and ii) a missing or additional spin, for which the impurity perturbation is represented by an uncompensated Berry phase. The `boundary' critical theory for these classes is developed: in the second class we find a `VBS pinwheel' around the impurity, accompanied by a suppression in the VBS susceptibility. Implications for numerical studies of quantum antiferromagnets and for STM experiments on the cuprates are noted.

Direct Measurement of the Binding Energy and Bohr Radius of a Single Hydrogenic Defect in a Semiconductor Quantum Well

Low-temperature scanning tunneling spectroscopy under ultrahigh vacuum was used to study donor point defects located at the epitaxial surface of an In(0.53)Ga(0.47)As quantum well. The electronic local density of states was measured with nanoscale resolution in the vicinity of single defects. In this way, both the binding energy and the Bohr radius of the defects could be determined. The binding energy and the Bohr radius were found to be functions of the quantum well thickness, in quantitative agreement with variational calculations of hydrogenic impurity states.

Comparison of c(2 × 2)N/Fe(001) and Fe4N(002) surfaces: a density-functional theory study

By using first-principles density-functional theory calculations, we compare the properties ofc(2 × 2)N/Fe(001) with theFe4N(002) surfacepossessing Fe2N stoichiometry. We observe a number of similarities as far as the geometry, bond lengths, localdensity of electronic states and magnetic moments in the surface region are concerned. However, forc(2 × 2)N/Fe(001) the shortest interatomic distance is between N and subsurface Fe atoms, whereas forFe4N(002) the shortest bond is formed between N and surface Fe atoms, which leads to someimportant differences. In particular, the magnetic moments are higher for thec(2 × 2)N/Fe(001) surface Fe atomsthan for the Fe4N(002) ones, and the opposite is true for the subsurface Fe atoms.