One-electron Density Of States Research Articles

Scanning tunneling spectroscopy on oxidized surfaces of highly resistive quasicrystalline alloys

We report on a detailed investigation of the one-electron density of states on oxidized surfaces of highly resistive quasicrystalline alloys. Scanning tunneling spectroscopy measurements were performed at low temperatures (resolution around 5 meV) on icosahedral $(i$-)AlPdMn, i-AlCuFe, and i-AlPdRe. The tunneling spectra depend largely on the scanning tunneling microscope tip location on the oxidized surfaces. Beside spectra characteristic for the Coulomb blockade, we have identified a zero bias dip intrinsic to our quasicrystalline samples. This single feature increases as a square root of the bias voltage up to $\ensuremath{\simeq}100\mathrm{mV}.$ A natural explanation comes from the combined effects of electron-electron interactions and disorder on the one-electron density of states. In this respect, highly resistive icosahedral alloys behave similar to disordered systems, for which such singularity has been predicted and widely observed. We have found no evidence for the quasicrystal specific energy dependence which was expected from anomalous diffusion laws predicted by quasiperiodic models. The predicted multiple gap structure has not been found either. We discuss which density of states is actually measured in tunneling spectroscopy to help clarify the controversial results obtained in this field.

Hypo–hyper-d-electronic interactive nature of interionic synergism in catalysis and electrocatalysis for hydrogen reactions

The nature, causes and consequences of the existence of volcano plots along transition series with resulting theoretical conclusions have been extended on the Brewer hypo–hyper-d-electronic combinations of both intermetallic phases and interionic composites. It has been pointed out that every such phase diagram behaves as the part of the Periodic Table between initial constituents, and thereby features the Gschneidner type of volcano curve for the hydrogen electrode reactions (helr), in common with the well-known electrocatalytic volcano plots of H. Kita and M. H. Miles for the hydrogen evolution reaction (her) upon individual transition metals. Several typical Brewer-type intermetallic systems (Ti–Ni, Zr–Ni, Mo–Ni, Mo–Pt) were investigated and their comparable electrocatalytic and hydridic properties were revealed from volcanic plots along the corresponding phase diagrams. In the light of such conclusions, it has been pointed out that catalytic volcano plots represent in their essence the main criterion and guidance leading to both optimal and synergetic effects in broader (electro)catalytic sense for the helr. Electrocatalysis and its synergetic effect appear as the hypo–hyper-d -electronic interactive effect of corresponding transition element composites either within metallic lattice, or their ions upon the substrate surface. The d-band both as the bonding and adsorptive orbital for intermediates in the rate determining step (rds) has been inferred to be decisive in (electro)catalytic hydrogen reactions, while electronic density, in addition, defines the overall kinetics and reaction rate. Such statements have been supported by the novel self-consistent calculated functional density of one-electron states (DOS) of adsorbed H-adatom, wherefrom the Fermi level relative to the antibonding peak plays decisive role in electrocatalysis for the helr. The surface state of a given interionic bulk structure reflects the latter and appears decisive in its mutual electronic configuration for the overall electrocatalytic effect and resulting synergism in the helr.

The use of Auger photoelectron coincidence spectroscopy to deconvolute the M45N45N45 AES of Palladium

Palladium (Pd) has a complex Auger spectrum. It is difficult to fully describe the M45N45N45 Auger spectra of its surface atoms because of the many possible cascade processes placing intensity in this energy region. Weightman et al. described this Auger peak from a multiplet-splitting point of view and we used this model as a starting point. The model was compared to Auger photoelectron coincidence spectroscopy (APECS) data of the same peak and intensity was found in the M4N45N45 APECS spectra that could not be accounted for by Weightman's model. Thus new model curves for each component of the Auger peak were created by applying the Cini-Sawatzky Theory to the convoluted one-electron density of states. Experimental APECS data was used to verify the model curves and the final composite picture was compared to a high resolution AES spectrum. There was a good fit between the final model, the APECS data and high resolution AES data. The model also provided evidence for an M4-M5N45-N45N45(N45) Coster-Kronig process.

Difference between the one-electron small gap and the wide optical gap in 1-, 2-, and 3-dimensional insulating states coming from the strong Coulomb repulsion

Abstract We calculate one-electron density of states (DOS) and light absorption spectra for 1-, 2-, and 3-dimensional half-filled Hubbard systems by means of a quantum Monte-Carlo simulation. We will show that the DOS has only a small gap while the light absorption spectrum has a relatively wide gap. Though the one-electron gap in the DOS slightly increases with dimension, it is always smaller than the two-electron gap in the light absorption spectrum. Our result indicates that the gap given by the photoemission (or the inverse photoemission) spectrum is smaller than the optical gap.

Statistics of the charging spectrum of a two-dimensional Coulomb-glass island

The fluctuations of capacitance of a two-dimensional island are studied in the regime of low electron concentration and strong disorder, when electrons can be considered classical particles. The universal capacitance distribution is found, with the dispersion being of the order of the average. This distribution is shown to be closely related to the shape of the Coulomb gap in the one-electron density of states of the island. Behavior of the capacitance fluctuations near the metal-insulator transition is discussed.

Superconductivity in a model with overlapping wide and flat bands

The density of one-electron states at the Fermi level with allowance for weak Coulomb correlations between the band electrons is calculated in a superconductivity model having overlapping wide and flat bands. The dependence of the superconducting transition temperature on the doping level is obtained.

Quantum Monte Carlo study of the Su-Schrieffer-Heeger model in the high-level-doping concentration regime: Metal-insulator transition.

We investigate the effects of quantum lattice fluctuations in the half-filled-band ground state and in the high-level-doping regime using the quantum Monte Carlo method. In particular, we find that quantum lattice fluctuations generate a nonzero one-electron density of states into the classical gap and in the heavily doped regime the fluctuations close the energy gap between conducting and soliton levels. Thus we conclude that the fluctuations can produce the Fermi surface and explain the metallic properties experimentally observed.

Spin flux and magnetic solitons in an interacting two-dimensional electron gas: Topology of two-valued wave functions.

It is suggested that an interacting many-electron system in a two-dimensional lattice may condense into a topological magnetic state distinct from any discussed previously. This condensate exhibits local spin-1/2 magnetic moments on the lattice sites but is composed of a Slater determinant of single-electron wave functions which exist in an orthogonal sector of the electronic Hilbert space from the sector describing traditional spin-density-wave or spiral magnetic states. These one-electron spinor wave functions have the distinguishing property that they are antiperiodic along a closed path encircling any elementary plaquette of the lattice. This corresponds to a 2\ensuremath{\pi} rotation of the internal coordinate frame of the electron as it encircles the plaquette. The possibility of spinor wave functions with spatial antiperiodicity is a direct consequence of the two-valuedness of the internal electronic wave function defined on the space of Euler angles describing its spin. This internal space is the topologically, doubly-connected, group manifold of SO(3). Formally, these antiperiodic wave functions may be described by passing a flux which couples to spin (rather than charge) through each of the elementary plaquettes of the lattice. When applied to the two-dimensional Hubbard model with one electron per site, this new topological magnetic state exhibits a relativistic spectrum for charged, quasiparticle excitations with a suppressed one-electron density of states at the Fermi level.For a topological antiferromagnet on a square lattice, with the standard Hartree-Fock, spin-density-wave decoupling of the on-site Hubbard interaction, there is an exact mapping of the low-energy one-electron excitation spectrum to a relativistic Dirac continuum field theory. In this field theory, the Dirac mass gap is precisely the Mott-Hubbard charge gap and the continuum field variable is an eight-component Dirac spinor describing the components of physical electron-spin amplitude on each of the four sites of the elementary plaquette in the original Hubbard model. Within this continuum model we derive explicitly the existence of hedgehog Skyrmion textures as local minima of the classical magnetic energy. These magnetic solitons carry a topological winding number \ensuremath{\mu} associated with the vortex rotation of the background magnetic moment field by a phase angle 2\ensuremath{\pi}\ensuremath{\mu} along a path encircling the soliton. Such solitons also carry a spin flux of \ensuremath{\mu}\ensuremath{\pi} through the plaquette on which they are centered. The \ensuremath{\mu}=1 hedgehog Skyrmion describes a local transition from the topological (antiperiodic) sector of the one-electron Hilbert space to the nontopological sector. We derive from first principles the existence of deep level localized electronic states within the Mott-Hubbard charge gap for the \ensuremath{\mu}=1 and 2 solitons. The spectrum of localized states is symmetric about E=0 and each subgap electronic level can be occupied by a pair of electrons in which one electron resides primarily on one sublattice and the second electron on the other sublattice. It is suggested that flux-carrying solitons and the subgap electronic structure which they induce are important in understanding the physical behavior of doped Mott insulators.

Quantum-chemical self-consistent-field X alpha scattered-wave investigation of La2CuO4 vacant surface electron states.

The vacant electron states of the ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ surface, formed by the cleft between two La-O layers, have been studied by the self-consistent-field X\ensuremath{\alpha} scattered-wave method. It was shown that the vacant electronic states of the ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ surface differ slightly from those of the bulk. This is explained by the fact that in spite of appreciable changes in the density of one-electron states near the Fermi level in the thin film modeled, spatial distribution of the total electronic charge remains practically unchanged.

Approximate thermodynamical treatment of the Coulomb gap

Abstract In disordered systems, in which transport occurs by hopping between localized states, both Pollak and Efros and Shklovskii have pointed out that conduction could be greatly affected by the electron-electron repulsion, opening the so-called Coulomb gap in the one-electron density of states, N(E). The aim of this paper is to represent the effect of the Coulomb gap on the free energy, F. F is expressed in terms of an unknown correlation length, r0. Minimization of F with respect to r0 yields r 0 ∝ T −1, divergent at zero temperature, T. Owing to this singularity in the correlation length, models for transport must be based on finite temperature structure, as finite temperature conditions are not representable as small departures from those at zero temperature. The density of states at the Fermi level obtained by this treatment is in agreement with Efros and with recently published results of Mogilyanski.

Hardening of the Coulomb gap by electronic polarons.

A one-electron density of states for a system of interacting electrons is obtained in the limit of strong localization. The method is based on making the ground state stable with respect to electronic polaron excitations. The resulting density of states presents a relatively small hard gap of the order of a fifth of the Coulomb gap.

On the direct determination of constrained pure state one-electron density matrices: II. A modified algorithm and its applications

An algorithm recently proposed by us for the direct determination of pure state one-electron density matrices (P) under externally imposed constraints has been remodelled. The modified algorithm is applied to the construction of ground state potential energy curve of lithium hydride molecule usingknown values of dipole moments of LiH at various internuclear distances as theexternal constraint. The equilibrium internuclear distances (R c) is seen to be unaffected by the constraint, but the force constant improves remarkably. The relevant features of the constrained density vis-a-vis those of the unconstrained one are analyzed revealing some of the improved features of the constrained density.

On the direct determination of constrained pure state one-electron density matrices: a new algorithm

An algorithm based on a “constrained variational principle” is suggested for the direct determination of constrained one-electron density matrices in a self-consistent manner. The algorithm uses both penalty-function and Lagrangian multiplier methods of incorporating equality constraints and can tackle any number of constraints. Results of preliminary calculations are presented.

Effective-Pair-Interactions from Supercell Total Energy Calculations: Al-Transition Metal Alloys

AbstractEffective-pair-interactions (EPI) are computed for alloys of Al with transition metals, Li, and Zn, using a method in which concentration-independent cluster interactions are resummed to obtain the concentration-dependent EPI. The method includes alloy fluctuations in the interatomic charge transfer, enables one to transcend the muffin-tin approximation and thus treat surfaces and layered structures, and allows the inclusion of lattice strain effects. The calculated EPI have a large magnitude when d-bonding effects are important. For transition metals the EPI are strongly concentration-dependent. In Ni-Al, results for bcc and fcc lattices are similar and exhibit a quick decay of the EPI with interatomic separations. The concentration dependence of the transition metalEPI exhibits rapid oscillations with the number of valence electrons. The concentration-averaged EPI varies less dramatically. The oscillations in the concentration dependence of the EPI are interpreted in terms of the position of the Fermi level relative to peaksand valleys in the one-electron density of states.

On the direct determination of constrained pure state one-electron density matrices: Part I. A new theoretical model for closed-shell systems

A new method based on the penalty-function way of satisfying equality constraints is proposed for the determination of constrained pure state one-electron density matrices for closed-shell many-electron systems. The algorithm suggested can handle many constraints simultaneously. Certain interesting features of the proposed algorithm are discussed with numerical examples.

One-electron density of states in chalcogenide glasses

Abstract A model for the electronic structure of chalcogenide glasses is presented. The valence and the conduction bands have broad exponential tails associated with doubly and singly occupied states. There is no overlap between the tails because of strong electron correlations.

CarbonKVVAuger line shapes of graphite and stage-one cesium and lithium intercalated graphite

A symmetry-resolved one-electron density of states (DOS) for graphite is extracted from existent x-ray photoelectron and x-ray emission spectroscopy (XES) data and used in conjunction with empirical atomic matrix elements to construct a carbon $\mathrm{KVV}$ Auger line shape for graphite which is in substantial agreement with experiment. For the intercalated graphites, an additional peak is added to the graphite one-electron DOS near the Fermi energy. In the absence of available XES data for ${\mathrm{C}}_{8}$Cs and ${\mathrm{C}}_{6}$Li, the energy and intensity of this peak are determined by fitting the C $\mathrm{KVV}$ Auger spectra. The enhanced intensities found for this peak, relative to the ultraviolet photoelectron spectroscopy data, are interpreted as being due to the screening charge surrounding the C $1s$ core. The measure of this enhancement corresponds to 0.5 and 0.6 electrons (intercalant charge donation plus screening) on the core-ionized carbon atom for the case of ${\mathrm{C}}_{8}$Cs and ${\mathrm{C}}_{6}$Li, respectively. No evidence is found for the selection rule or large matrix-element effects postulated in previous work.

Lower and upper bounds to the decay of the ground state one-electron density of helium-like systems

The asymptotic behaviour of the ground state one-electron density rho (x) of two-electron atoms is investigated. Lower and upper bounds to rho (x) showing 'almost' the same decay are derived.

MANY-BODY EFFECTS ON DEEP LEVEL SPECTRA OF METALS

Many-body effects in metals due to a change in the core electron occupation number appear in a large number of spectroscopies. The possible importance of the core hole has been realized and discussed for a very long time. Mahan's idea of excitons in metals gave the topic strong stimulus. First the interest was focused on threshold singularities in simple metals. The topic was thoroughly discussed in Mahan's 1974 review and declared solved and completed. This declaration was strongly challenged by Dow and collaborators. This has led to a deeper and more detailed understanding. In particular the importance of static core hole effects, of phonons, of electron bandstructure, of exchange coupling (the Onodera effect) and of spindependent phaseshifts has been realized. Thus the longstanding puzzle of the lithium edge now seems to have found its solution in terms of phonons and bandstructure and not in the MND-effect - the broad emission edge can be explained by incomplete phonon relaxation and the broad absorption edge by the one-electron density of states. (Less)

X-ray photoionization cross sections ofd-band metals

It is shown that the charge-density overlap between $d$ orbitals on adjacent atomic sites in $d$-band metals leads to a dependence of the photoionization cross section on the initial-state energy, which accounts for most of the differences observed by Smith et al. between the one-electron density of states and the corresponding x-ray photoemission spectra.