Scanning gate microscopy in graphene nanostructures

A systematic theory of the conductance measurements of noninvasive (weak probe) scanning gate microscopy is presented that provides an interpretation of what precisely is being measured. A scattering approach is used to derive explicit expressions for the first- and second-order conductance changes due to the perturbation by the tip potential in terms of the scattering states of the unperturbed structure. In the case of a quantum point contact, the first-order correction dominates at the conductance steps and vanishes on the plateaux where the second-order term dominates. Both corrections are nonlocal for a generic structure. Only in special cases, such as that of a centrally symmetric quantum point contact in the conductance quantization regime, can the second-order correction be unambiguously related with the local current density. In the case of an abrupt quantum point contact, we are able to obtain analytic expressions for the scattering eigenfunctions and thus evaluate the resulting conductance corrections.

In this study, we investigated the effect of impurities—carbon (C), nitrogen (N), and fluorine—on the electronic properties of a Si/SiO2 interface by measuring the atomic layer-resolved partial local density of states using first-principles simulations. Our results show that C and N impurity atoms with dangling bonds (DBs) induce midgap states at the interface. If DBs are terminated by hydrogen atoms, the formation of midgap states is effectively suppressed even in the presence of impurity atoms at the interface. If DBs are terminated by an oxygen (O) atom, both π bonding and π∗ antibonding orbitals of C=O are occupied and such occupied antibonding orbital forms a midgap state at the interface, increasing the leakage current. Based on these observations, we propose that preparation of the SiO2 substrate with oxygen-rich conditions prior to Si deposition is important in realizing a defect-tolerant interface. In addition, hydrogen treatment after Si deposition recovers the pure band structure by terminating the midgap states, thereby preventing leakage current.

Self-consistent PAW band-structure calculations were performed for B2 FeTi, CoTi and NiTi. Using the resulting electron and difference electron densities together with partial local densities of states and band charges, the covalent bonding contribution is shown to increase distinctly in the series NiTi-CoTi-FeTi. Structure factors are extracted from the calculated electron densities and they can be compared with measured X-ray diffraction data as soon as the latter become available. X-ray emission and absorption spectra as well as angle-integrated photoelectron and bremsstrahlung isochromat spectra are calculated from the APW eigenvalues and wavefunctions and compared with measured spectra from the literature, if available. For the XPS of NiTi some discrepancies between theory and experiment can be eliminated by applying an estimated self-energy correction to the eigenvalues before the calculation of the spectrum.

The energy eigenvalue results obtained by a linear KKR method for B2 FeAl and an hypothetical B2 AlAl of the same lattice spacing are used to calculate the local partial DOS (densities of states) as well as the corresponding valence‐electron numbers per muffin‐tin sphere (MTS) of these two systems. The comparison of the two different local partial Al DOS indicates that the Al states in the FeAl compound are strongly affected by the antiresonance effect caused by the presence of the resonant Fe scatterer in the neighbourhood. The charge transfer is also discussed in connection with the antiresonance effect.

We analyse the properties of the partitioned densities of states inmesoscopic transport problems. In terms of an imaginary-potentialmodel, a concise derivation of the local partial density of states (LPDOS)is given. Using a numerical method to calculate the functionalderivative of the scattering matrix, a detailed study of different kindsof decomposed densities of states in double-barrier structures iscarried out. We find that the dynamical local density of statesdnαα(x)/dE and the global partialdensity of states (GPDOS) dNαα/dE in the nonclassical regime cannot be neglected in full quantumcalculations.

New experiments which measure the low-frequency shot-noise spectrum at local tunneling contacts on mesoscopic structures are proposed. The current fluctuation spectrum at a single tunneling tip is determined by local partial densities of states. The current-correlation spectrum between two tunneling tips is sensitive to the nondiagonal density of states elements which are expressed in terms of products of scattering states of the conductor. Such an experiment permits the investigation of correlations of electronic wave functions. We present specific results for a clean wire with a single barrier and for metallic diffusive conductors.

Electronic structure and 89Y spin-lattice relaxation for YH 2

A self-consistent LAPW band structure calculation for TiO0.75 has been performed, assuming long-range order of vacancies on the oxygen sublattice. The calculation is based on a hypothetical model structure which can be described as Ti 3 [4] Ti[6]O3□o, where □o denotes an oxygen vacancy. In the model structure two types of titanium atoms occur: Ti[4] atoms which have two vacancies as neighbours and are quadratically surrounded by four oxygen atoms, and Ti[6] atoms which are octahedrally surrounded by six oxygen atoms. The calculated density of states (DOS) and the local partial densities of states are compared with the respective values for stoichiometric TiO containing no vacancies. A characteristic difference is the appearance of two sharp peaks in the DOS curve below the Fermi energy which are caused by the vacancies. These “vacancy states” exhibit a considerable amount of charge in the vacancy muffin-tin sphere and are found to be derived from Ti 3d states extending into the vacancy sphere. The introduction of vacancies also leads to a lowering of the Fermi energy indicating a stabilizing effect. The bonding situation in TiO0.75 as compared to TiO as well as the changes in chemical bonding in the series TiC0.75−TiN0.75−TiO0.75 are discussed on the basis of electron density plots. The loss of Ti[4]−O bonds (as compared to TiO) is compensated by the formation of Ti[4]−□o−Ti[4] bonds across the vacancy and by an increase of the bond strength of the Ti[4]−O bonds. On the other hand, the Ti[4]−Ti[4] and particularly the Ti[4]−Ti[6] bonds are weakened by the introduction of vacancies.

The electronic structure of cobalt disilicide film has been investigated theoretically. The calculations are performed within the framework of the linearized augmented plane wave method. A combined analysis of the total and local partial densities of states and the photoelectron and x-ray emission spectra of different series for all the nonequivalent atoms of the film makes it possible to interpret the main features of the spectral characteristics of this material.

Interpretation of the x-ray emission spectra of a-SiO x

Using first-principles calculations within density functional theory, we explore the electronic structure and ground state properties of new hypothetical III–V compounds formed by the group III elements (B, Al, Ga, In, and Tl) and superheavy element of group V: Ununpentium (Uup). We find that wurtzite structure is the ground state phase of BUup; whereas AlUup, GaUup, InUup and TlUup prefer tetragonal PbO phase. Omission of spin-orbit interaction leads to minor changes in ground state structural parameters, but without affecting the ground state phase stability. In contrast, disregarding relativistic effects is instead dramatic: the wurtzite structure becomes in such a case the ground state phase of all the III-Uup compounds. Our studies reveal that the proposed hypothetical new IIIUup family divulges surprising features compared with other III–V semiconductors. Specifically, all compounds are found metallic, the partial density of states display strong s-d coupling, strong presence of cation-s states, and exhibit an almost equal contribution of cation and anion(Uup) p-states in the vicinity of Fermi level. This suggests the possibility of strong covalent bonds between III elements and Uup. Furthermore, the calculated partial phonon density of states shows the dynamical stabilities of the predicted ground state phase of all III-Ununpentium compounds.

The first-principle Density Functional Theory (DFT) approach was used for the optimization of nanostructured ZnO Buckyball (ZnO-B) and Zig-Zag Graphene nanoribbon (Z-Graphene). Different investigations were carried out to study its structural, electronic, and optical properties, for better understanding. Initially, ZnO-B and Z-Graphene structures were investigated separately, and finally, the interaction between the two of them was observed. The acquired Bandgap of ZnO-B and Z-Graphene structure was around 1.5 eV and 0.7 eV, respectively. The detailed study included the observation of structures, bond length, and the density of states, the partial density of states, absorbance, refractive index, and reflectance. This DFT study produced good comparative results with solid-state physics, showing improved electronic and optical properties. The outcomes may be useful for various applications in the field of optoelectronic devices.

We report results of first-principles molecular orbital calculations using the discrete-variational method on a model cluster of -quartz . Self-consistent calculations of the cluster which include a half-filled core hole, i.e. at Slater's transition state, are made in order to evaluate the effect arising from the presence of a core hole associated with the electron energy-loss process. The presence of a half-filled Si-2p core hole does not change the unoccupied Si-3s/3d partial density of states (PDOS) significantly. On the other hand, a remarkable core hole effect appears on O-2p PDOS when a half-filled O-1s hole is introduced. The calculated PDOS at the transition states is in good agreement with the electron energy-loss near-edge structures (ELNES) at both the edge and O-K edge. Absolute transition energies at both edges are also reproduced to within an error of 3 eV when spin-polarization during the transition is taken into account. The photoabsorption cross sections (PACS) are calculated from first principles and are compared with the PDOS for the corresponding electric dipole transitions. The difference between the PACS and the PDOS is found to be small at both the ground state and the Slater's transition state in the present calculation.

In this thesis, we present transport and scanning-gate microscopy experiments on graphene nanostructures at cryogenic temperatures. Moreover, an analytic expression for the quantum states of a circular graphene quantum dot in a perpendicular magnetic field is derived. Graphene is a semi-metal showing the electric field effect and thus is an interesting material for the mesoscopic semiconductor community. In graphene quantum dots, we observed clear indications of transport through confinementinduced excited states. These showed up in resonant sequential tunneling and in inelastic co-tunneling in the Coulomb-blockaded regime using tunneling spectroscopy. To deepen our understanding of the confining potential, we looked at the magnetic-field dependence and identified a transition from confinement-induced to magnetic-field induced behavior. Motivated by these experimental results, we could derive an analytic mathematical expression which qualitatively describes both the confinement-induced and the magnetic-field induced effects as well as the transition between both. As a side-effect, a strategy for determining experimentally the electron-hole crossover in graphene quantum dots was developed. A second graphene quantum dot was studied with our dilution refrigerator scanning-gate microscope. We could clearly resolve Coulomb resonances of the quantum dot. More than 35 Coulomb resonances were recorded in some measurements where a spacing of around 20 nm of adjacent Coulomb resonances could clearly be resolved. This proved the exceptional quality of both the sample and the scanning sensor. Our measurements also gave direct proof of the existence of localized states in graphene nanostructures: We could image and locate a single localized puddle in each constriction connecting the quantum dot to source and drain. Furthermore, our technique enabled us to deduce a radius of around 10 to 13 nm for such a localized state. Despite the good spatial resolution, we could not resolve quantum confinement effects in the quantum dot or in one of the localized puddles. Finally, we analyzed experimentally how a change in the electrostatic environment can influence the outcome of a scanning-gate measurement. We identified

Surface analysis of the stress-induced, impurity driven face centered cubic titanium phase and the ranging lattice parameter sizes