Local Electronic Density Of States Research Articles

Onset of itinerant electron magnetism inCo(Ga1−xNix)andCo(Ga1−xCux)alloys

Onset of the magnetic order in $\mathrm{Co}({\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x})$ and $\mathrm{Co}({\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{x})$ alloys caused by Ni and Cu substitutions is studied in ab initio framework using density functional theory and the coherent potential approximation. It is found that Ni and Cu atoms energetically prefer Ga sites and, contrary to the earlier interpretation, the magnetism develops on the Co sublattice rather then being induced by magnetic Co antisites atoms moved to the Ga sublattice. The changes in the local electronic density of states of Co caused by Ni and Cu substitutions on Ga sublattice lead to the Stoner instability at some critical alloy composition and weak itinerant ferromagnetism, similar to that observed in ${\mathrm{ZnZn}}_{2}$ and ${\mathrm{Ni}}_{3}\mathrm{Al}$ compounds, develops.

Scanning tunneling microscopy and spectroscopy of the electronic local density of states of graphite surfaces near monoatomic step edges

We measured the electronic local density of states (LDOS) of graphite surfaces near monoatomic step edges, which consist of either the zigzag or armchair edge, with the scanning tunneling microscopy (STM) and spectroscopy (STS) techniques. The STM data reveal that the $(\sqrt{3} \times \sqrt{3}) R 30^{\circ}$ and honeycomb superstructures coexist over a length scale of 3-4 nm from both the edges. By comparing with density-functional derived nonorthogonal tight-binding calculations, we show that the coexistence is due to a slight admixing of the two types of edges at the graphite surfaces. In the STS measurements, a clear peak in the LDOS at negative bias voltages from -100 to -20 mV was observed near the zigzag edges, while such a peak was not observed near the armchair edges. We concluded that this peak corresponds to the graphite "edge state" theoretically predicted by Fujita \textit{et al.} [J. Phys. Soc. Jpn. {\bf 65}, 1920 (1996)] with a tight-binding model for graphene ribbons. The existence of the edge state only at the zigzag type edge was also confirmed by our first-principles calculations with different edge terminations.

Surface Reactivity and Quantum-Size Effects on the Electronic Density Decay Length of Ultrathin Metal Films

The origin of the correlation between surface reactivity and quantum-size effects, observed in recent experiments on the oxidation of ultrathin magnesium films, is addressed by means of ab initio calculations and model predictions. We show that the decay length in vacuum of the electronic local density of states at the Fermi energy exhibits systematic oscillations with film thickness, with local maxima induced when a quantum-well state at crosses the Fermi energy. The predicted changes in the decay length are expected to have a major impact on the electron transfer rate by tunneling, which has been proposed to control the initial sticking of in the oxidation process.

Local Superconducting Density of States ofErNi2B2C

We present local tunneling microscopy and spectroscopy measurements at low temperatures in single crystalline samples of the magnetic superconductor ErNi2B2C . The electronic local density of states shows a striking departure from s-wave BCS theory with a finite value at the Fermi level, which amounts to half of the normal phase density of states.

Electronic structures of size-selected single-layered platinum clusters on silicon(111)-7×7 surface at a single cluster level by tunneling spectroscopy

Tunneling spectra of size-selected single-layered platinum clusters (size range of 5-40) deposited on a silicon(111)-7x7 surface were measured individually at a temperature of 77 K by means of a scanning tunneling microscope (STM), and the local electronic densities of states of individual clusters were derived from their tunneling spectra measured by placing an STM tip on the clusters. In a bias-voltage (V(s)) range from -3 to 3 V, each tunneling spectrum exhibits several peaks assignable to electronic states associated with 5d states of a constituent platinum atom and an energy gap of 0.1-0.6 eV in the vicinity of V(s)=0. Even when platinum cluster ions having the same size were deposited on the silicon(111)-7x7 surface, the tunneling spectra and the energy gaps of the deposited clusters are not all the same but can be classified in shape into several different groups; this finding is consistent with the observation of the geometrical structures of platinum clusters on the silicon(111)-7x7 surface. The mean energy gap of approximately 0.4 eV drops to approximately 0.25 eV at the size of 20 and then decreases gradually as the size increases, consistent with our previous finding that the cluster diameter remains unchanged, but the number density of Pt atoms increases below the size of 20 while the diameter increases, but the density does not change above it. It is concluded that the mean energy gap tends to decrease gradually with the mean cluster diameter. The dependence of the mean energy gap on the mean Pt-Pt distance shows that the mean energy gap decreases sharply when the mean Pt-Pt distance exceeds that of a platinum metal (0.28 nm).

Chemical fingerprinting at the atomic level with scanning tunneling spectroscopy

Scanning tunneling spectroscopy (STS) based on scanning tunneling microscopy (STM) makes it possible to map the local electronic density of states for clean surfaces and for those with adsorbates. We have developed a protocol that allows us to obtain the spectral fingerprints of halogen atoms on Si(0 0 1), and we use those fingerprints to distinguish between adatom species for surfaces with Cl and Br mixed adsorbates. The key to the process is the energy distribution of the antibonding states that depend on the halogen species.

Atomic-level strain-relieving mechanism and local electronic structure of a wetting film

The strain-relieving mechanism and local electronic density of states of a wetting film, was studied in the Ag∕W system using scanning tunneling microscopy and spectroscopy. In the Ag wetting film, a periodic bright ridge structure was observed along the two equivalent directions, relieving mixed compressive-tensile strain. Two unoccupied electronic states were observed between the ridges, while the other two occupied electronic states were observed at the ridges. The Ag atoms occupying the bridge sites contribute to relieve the elastic strain and to induce the occupied electronic states.

Short-range ferromagnetism and transport properties of amorphous (Gd, Y)xSi1−x alloys

We present a theoretical description and electrical conductivity measurements for amorphous (Gd, Y)xSi1−x alloys with 0.1 < x < 0.2. In our model, we take into account the strong topological disorder in the system, causing the appearance of regions with higher electron density (electron “ drops”) around nanoscale structural defects enriched with rare-earth ions (“clusters”). We calculate the local density of electron states in the drops and in the matrix and establish the criterion for local instability to ferromagnetism. In the framework of the “local phase transition” approach, we find that short-range ferromagnetic order is more favorable inside the drops than in the matrix and exists in a wide temperature range. We analyze recent measurements of the temperature and magnetic-field dependence of the electrical conductivity in these systems and show that the spin polarization of the electron states in the drops enhances the tendency towards the metal-insulator transition.

Resonant states of a double-barrier junction

Abstract We study resonant properties of an SIS′IS junction with quantized Andreev bound states (ABS); here S, S′ and I denote a superconductor, a superconductor with a smaller energy gap and an insulating barrier, respectively. Using the quasiclassical approach we compute the local electron density of states, spatial extent of the quantized states and inelastic electron-phonon recombination rate versus junction transparency, non-magnetic impurity concentration, and the thickness of the middle layer. We find that the local electron density of states in a low-transparency limit has sharp peaks at the ABS level positions while the inelastic electron-phonon recombination time, τe−p (e), associated with an ABS level E0 is a sharp function of energy variable e and diverges at e = E0.

Local electronic density of states of a semiconducting carbon nanotube interface

The local electronic structure of semiconducting single-wall carbon nanotubes was studied with scanning tunneling microscopy. We performed scanning tunneling spectroscopy measurement at selected locations on the center axis of carbon nanotubes, acquiring a map of the electronic density of states. Spatial oscillation was observed in the electronic density of states with the period of atomic lattice. Defect induced interface states were found at the junctions of the two semiconducting nanotubes, which are well-understood in analogy with the interface states of bulk semiconductor heterostructures. The electronic leak of the van Hove singularity peaks was observed across the junction, due to inefficient charge screening in a one-dimensional structure.

Charge-density oscillation on graphite induced by the interference of electron waves

We report on a pronounced redistribution of the local electronic density of states at the graphite surface, which is induced by the presence of low energy hydrogen-ion induced point defects. Scanning tunneling microscopy reveals standing waves in the local density of states, which are due to backscattering of electron wave functions at individual point defects. The superstructure thereby formed is directly related to the pointlike structure of the Fermi surface of graphite. For high defect density interference patterns are observed which sensitively change structure on the relative positions of the defects. These patterns could be reproduced by tight binding simulations of various defect distributions.

Interpretation of orientational contrast in STM images of GaAs(1 1 0) cleavage surface

The electronic structure of GaAs(1 1 0) surface is analyzed using Density Functional Theory (DFT-GGA) in atomic orbital basis (LCAO). The surface orbitals and the corresponding local density of electronic states (LDOS) are calculated for purposes of interpreting STM images. We show how local atomic orbitals of surface atoms are related to tunneling channels for electrons in STM imaging. A destructive interference between orbitals of two neighbouring atoms increases the contrast between the two atoms, and this is reflected in directionality of STM patterns of GaAs(1 1 0) surfaces. We also discuss how the basic formalism of Tersoff–Hamann approach to STM simulation can be reformulated to reveal the role of phase difference between tunneling channels.

Capacitance response due to recharging of extended defects attached to the edge of the space charge region of the Schottky diode

Extended defects with a high local density of electronic states like quantum wells, quantum dots, dislocations and precipitates build an own space charge region (SCR) of finite sizes in their close vicinity. When the defects are situated close to the Schottky diode SCR the overlapping of the potentials of the diode and of the defects results in the creation of a potential minimum. In this work, a new theoretical treatment of this situation based on an analytical/numerical solution of Poisson equation taking into account the presence of free electrons is presented. It is shown that small but noticeable lowering of the extended defect states occupancy with electrons is accompanied with unexpectedly large capacitance changes that exhibit a maximum at a certain distance between the defect and the diode SCR. The magnitude of the capacitance changes at the maximum is practically the same as the changes within the diode SCR due to complete refilling of the defect from its equilibrium occupancy to zero. The origin of the capacitance maximum can be qualitatively explained as an effect of “touching/detouching” of the SCR of the defect and that of the diode. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Local electronic edge states of graphene layer deposited on Ir(1 1 1) surface studied by STM/CITS

Scanning tunnelling microscopy and current imaging tunnelling spectroscopy were used to observe electronic structure of the edges of monolayer graphite film deposited on the Ir(1 1 1) surface. The electronic structure derived from the tunnelling spectra revealed peak in electron local density of states very close to the Fermi level. This electronic state was interpreted in terms of localised edge state caused by the topology of the π electrons networks typical for the zig-zag edges. The observed maximum of local density of states at about 0.2 eV above the Fermi level was ascribed to the presence of resonant state caused by the appearance of disclinations centres in the vicinity of the graphite edges.

Scanning tunneling spectroscopy of a semiconducting heterojunction nanotube on the Au(1 1 1) surface

The atomic and electronic structures of a single-wall carbon nanotube prepared on the Au(1 1 1) surface was studied. We performed scanning tunneling spectroscopy measurement in a semiconducting nanotube heterojunction, obtaining the local electronic density of states. An interface state was observed within the energy gaps of the semiconducting nanotubes. The interface state is spatially localized within a few nm scales and decays into the two semiconducting CNTs with unequal screening lengths. The interface state is donor-like, possibly originating from a heptagonal atomic structure. The interface state is well-understood, in analogy with that of a conventional semiconductor heterojunction.

Theoretical Study of Main-Group Metal−Borazine Sandwich Complexes

Using density functional theory within the generalized gradient approximation, we have theoretically studied the formation of neutral metal-aromatic complexes R1-M and R1-M-R2, where M is either neutral lithium, calcium, or gallium and R1 or R2 is benzene or borazine. We first find that calcium atom is an effective mediator for cooperative formation of a sandwich complex with borazine, while others are not. When benzene and borazine are mixed in the presence of calcium, a 1:2:1 mixture of benzene-calcium-benzene, borazine-calcium-benzene, and borazine-calcium-borazine is expected. An "A"-shaped structure is predicted for homo- and heterocomplexes of borazine with partial B-B and B-C bonds, while two rings are planar in the case of homocomplexes of benzene. Our analysis of the electron density distributions in HOMO-1 to LUMO in terms of orbital symmetry in conjunction with analysis of l,m-projected electronic local density of states shows that this correlates with the charge transfer and the interaction of pi states of the rings mediated by empty d-states of Ca, which is ultimately related to the polarity of the B-N bond. We find that there is a large accumulation of electron density on particular atoms upon complex formation, predicting characteristic behavior in electron-transfer reaction and nucleophilic reaction different from those for pure benzene or borazine molecule. The hetero-sandwich complex is of particular interest due to its asymmetrical distribution of excess electrons.

Density of localized electronic states in a-Se from electron time-of-flight photocurrent measurements

Electron time-of-flight transient photocurrents have been investigated in stabilized a-Se as a function of electric field, annealing, aging (relaxation), and alloying with As and doping with Cl. The distribution of localized states (DOS) in stabilized a-Se has been investigated by comparing the measured and calculated transient photocurrents. The samples were prepared by conventional vacuum deposition techniques. The theoretical analysis of multiple-trapping transport has been done by the discretization of a continuous DOS and the use of Laplace transform formalism. The resulting DOS has distinct features: A first peak at ∼0.30eV below Ec with an amplitude ∼1017eV−1cm−3, a second small peak (or shoulder) at 0.45–0.50 eV below Ec with an amplitude 1014–1015eV−1cm−3, and deep states with an integral concentration 1011–1014cm−3 lying below 0.65 eV, whose exact distribution could not be resolved over the time scale of present experiments. The influence of doping, aging, annealing, and substrate temperature on the DOS distribution has been investigated. The doping with Cl does not affect the amplitudes of the first and second peaks while the concentration of deep states increases dramatically. The alloying with As reduces the density of deep states and seems to increase the amplitude of first and second peaks. The aging substantially reduces the deep states density and the amplitude of the second peak while the amplitude of the first one remains practically unchanged. The results have been interpreted primarily in terms of thermodynamic and intrinsic structural defects in the chalcogenide glass structure.

Detecting surface resonance states ofSi(111)−3×3−Agwith a scanning tunneling microscope

We have used the scanning tunneling microscopy and first-principles theoretical simulations to spatially image the energy-resolved local density of electronic states on the $\mathrm{Si}(111)\text{\ensuremath{-}}\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\text{\ensuremath{-}}\mathrm{Ag}$ surface. The $dI∕dV$ maps for different electronic states on this surface, including two new surface resonance states that had not been observed before, reveal the distribution of these states in real space. The results show that the scanning tunneling microscopy combined with the theoretical simulation can discover and distinguish various surface states, especially surface resonance states. Our studies also suggest that the correct information about surface states must be acquired via adopting a slab model with sufficient Si double layers.

Periodic coherence-peak height modulations in superconductingBi2Sr2CaCu2O8+δ

In this paper we analyze, using scanning tunneling spectroscopy (STS), the local density of electronic states (LDOS) in nearly optimally doped BSCCO in zero field. We see both dispersive and non-dispersive spatial LDOS modulations as a function of energy in our samples. Moreover, a spatial map of the superconducting coherence peak heights shows the same structure as the low energy LDOS. This suggests that these non-dispersive LDOS modulations originate from an underlying charge-density modulation which interacts with superconductivity.

Scanning tunneling microscopy and spectroscopy studies of graphite edges

We studied experimentally and theoretically the electronic local density of states (LDOS) near single-step edges at the surface of exfoliated graphite. In scanning tunneling microscopy measurements, we observed the ( 3 × 3 ) R 30 ° and honeycomb superstructures extending over 3–4 nm both from the zigzag and armchair edges. Calculations based on a density-functional-derived non-orthogonal tight-binding model show that these superstructures can coexist if the two types of edges admix each other in real graphite step edges. Scanning tunneling spectroscopy measurements near the zigzag edge reveal a clear peak in the LDOS at an energy below the Fermi energy by 20 meV. No such a peak was observed near the armchair edge. We concluded that this peak corresponds to the “edge state” theoretically predicted for graphene ribbons, since a similar prominent LDOS peak due to the edge state is obtained by the first principles calculations.