Electron Localization Effects Research Articles

Modified adiabatic approximation: Charge asymmetry in HD+ and HD

The modified adiabatic approximation is discussed, in which the interaction of electrons with nuclei is partitioned between the electronic and nuclear Hamiltonian, in order to simulate the finite nuclear mass effect. The proposed formalism is universal and can be used in calculations for molecules of any size. The effect of electron localization on the deuteron in vibrationally excited states of HD(+) and the permanent dipole moment of HD, typically both explained in terms of nonadiabatic couplings between g and u states, are well reproduced with this method.

Additive and non-additive information channels in orbital communication theory of the chemical bond

The molecular communication system in the resolution of the basis functions χ = {χ X} contributed by the molecule constituent atoms {X}, the key concept of the Orbital Communication Theory (OCT) of the chemical bond, is introduced and its information-theoretic (IT) bond descriptors are summarized. The additive and non-additive components of these molecular information channels are identified. The former involve only the internal (one-center) communications {X → X} between the basis functions χ X of each bonded atom X, determined by the associated (diagonal) block P(χ X|χ X) of the molecular conditional probabilities, which are responsible for the intra-atom promotion to its effective valence state. The latter accordingly involve only the external (two-center) communications between the contributed AO of each pair of bonded atoms, {Y → X and X → Y}, generated by the off-diagonal blocks of conditional probabilities P(χ X|χ Y) and P(χ Y|χ X),X ≠ Y, respectively, which are responsible for the inter-atomic bonding effects in the molecule. Both these probability scatterings ultimately determine the resultant multiplicities of the system chemical bonds. The input-ensemble average value of the channel conditional-entropy, which measures its communication “noise” due to electron delocalization via all chemical bonds, measures the IT-covalency in the molecule, while the complementary descriptor of the ensemble average value of the network mutual-information (information-capacity) reflects the electron localization effects and measures the system IT-ionic component. The illustrative example of the localized chemical bond originating from the interaction between two atomic orbitals is reexamined in some detail and the bond-weighted ensemble approach to chemical interactions in diatomic molecular fragments is discussed within the standard Restricted Hartree-Fock theory. In diatomic systems such treatment exactly reproduces the familiar bond index of Wiberg and provides its resolution into the complementary IT-covalent and IT-ionic components. The operator formulation of the probability-scattering phenomena in molecules is given and the probability-amplitude channel defined by the first-order density matrix is introduced. The AIM internal and external eigenvalue problems of this Charge-and-Bond-Order matrix are introduced and a similar approach to probability propagation matrices/operators is suggested.

Nonequilibrium Green’s function study ofPd4-cluster-functionalized carbon nanotubes as hydrogen sensors

Pd-cluster-functionalized carbon nanotubes (CNTs) have been shown experimentally to be effective hydrogen sensors. Semiconducting CNTs exhibit much higher sensitivity than ensemble (mixed) ones. Using the nonequilibrium Green's function method combined with the density-functional theory, we simulate and contrast the (8,0) semiconducting and the (5,5) metallic CNT model systems. We find that the electron localization effect plays a crucial role in determining electron transport. Pd clusters and hydrogen adsorption cause opposite effects on electron localization in the CNT backbone for the semiconducting CNT-based systems. Consequently Pd functionalization dramatically increases the conductance, but then it is strongly suppressed by hydrogen absorption. For the metallic CNT-based systems, there is a tiny shift of the transmission peak near the Fermi energy. These results offer a consistent explanation for the experiments.

Electronic structures and optical properties of neutral substituted fluorene-based cyclometalated platinum(II)–acetylide complexes: A DFT exploration

We report a combinational DFT and TD-DFT study of the electronic and optical properties of several tridentate cyclometalated mononuclear [Pt(C^N^N)(CCR)] (1–3), [Pt(C^N^N)(CCRCCH)] (4), and dinuclear [Pt(C^N^N)(CCRCC)Pt(C^N^N)] (5 (C2 symmetry) and 5′ (Cs symmetry)) platinum(II) complexes with σ-acetylide ligand bearing fluorene substituents, where HC^N^N = 6-aryl-2,2′-bipyridine, R = fluorene-2,7-diyl 1, 4, 5 and 5′, R = 9,9-dimethylfluorene-2,7-diyl 2, R = 9,9-diethylfluorene-2,7-diyl 3. The structural and electronic properties of the ground- and lowest triplet state and the EA and IP values of the complexes are discussed. It is found that all of the lowest-lying absorptions are categorized as the LLCT combined with the MLCT transitions. The oscillator strengths of the lowest energy absorptions get a remarkable enhancement for the dinuclear complexes 5 and 5′compared to 1–4 due to the increase of electronic delocalization on the more planar molecular geometry. In general, the phosphorescent emissions of these complexes in CH2Cl2 are the reverse process of their lowest energy absorption transitions, except that of 4 is assigned as 3[π∗−π]/3MLCT transition because of the strengthened electronic localization effect and the interaction with the solvent in the lowest triplet state. In addition, these complexes hold promise as a new kind of nonlinear optical material owing to their large static first hyperpolarizabilities (β0). The β0 value has increased in the dinuclear complexes in contrast to those of the mononuclear ones owing to their larger transition moment and smaller transition energy.

Experimental evidence for electron localization on Au upon photo-activation of Au/anatase catalysts

Time resolved microwave conductivity (TRMC) measurements show that the presence of Au on anatase Hombikat UV100 significantly reduces the lifetime of mobile electrons formed by photo-excitation of this photocatalyst at 300 nm, providing evidence for the widely acclaimed electron localization effect of Au in promoting TiO(2) photocatalysts. Electron localization efficiency of Au was even higher for Au-promoted Hombikat calcined at 400 degrees C, explained by enlargement of the anatase particle size and the associated, relatively larger fraction of anatase particles in direct contact with Au.

Möbius and twisted graphene nanoribbons: Stability, geometry, and electronic properties

Results of classical force field geometry optimizations for twisted graphene nanoribbons with a number of twists N(t) varying from 0 to 7 (the case N(t)=1 corresponds to a half-twist Möbius nanoribbon) are presented in this work. Their structural stability was investigated using the Brenner reactive force field. The best classical molecular geometries were used as input for semiempirical calculations, from which the electronic properties (energy levels, HOMO, LUMO orbitals) were computed for each structure. CI wavefunctions were also calculated in the complete active space framework taking into account eigenstates from HOMO-4 to LUMO+4, as well as the oscillator strengths corresponding to the first optical transitions in the UV-VIS range. The lowest energy molecules were found less symmetric than initial configurations, and the HOMO-LUMO energy gaps are larger than the value found for the nanographene used to build them due to electronic localization effects created by the twisting. A high number of twists leads to a sharp increase of the HOMO-->LUMO transition energy. We suggest that some twisted nanoribbons could form crystals stabilized by dipolar interactions.

Nanoscale molecular surface electron attachment

In this work we have considered the ability of nanoscale molecular surfaces (around 2 nm in width) in trapping excess electrons. As previously reported, we suggested that molecular surfaces with hydrogen bonding networks (consisting of OH groups) on one side of the surface and hydrogen atoms on the opposite side were capable of forming stable dipole-bound anions. The increased dipole moments generated by the OH groups coupled to the partial positive charge of the hydrogen atoms creates charge pockets that are can trap excess electrons. We have extended the size of the surface to study the effect of electron localization on molecular surfaces.

Absence of weak electron localization in (Co/Pt)n-multilayer-nanowires with perpendicular anisotropy

We report on low temperature resistance and magnetoresistance (MR) measurements for various (Co/Pt)n-multilayer-nanowires with perpendicular magnetic anisotropy and different widths, designed as model systems for the experimental observation of weak electron localization effects in ferromagnetic systems as proposed by Dugaev et al. [Phys. Rev. B 64, 144423 (2001)]. The low temperature MR is found to originate from both the anisotropic MR and domain wall scattering. The temperature dependence of the resistance shows a logarithmic resistance increase with decreasing temperature, as it is typical for quantum corrections in two dimensional systems. Upon application of a perpendicular magnetic field, however, the slope of the logarithmic resistance increase does not change in magnitude, which proofs that weak localization effects are not present, at least within the accuracy of our measurements. Instead, the observed quantum corrections of the resistance are well explained by enhanced electron-electron interaction effects in two dimensions. The results are discussed with regard to recent experimental as well as theoretical works.

Absence of weak electron localization in (Co/Pt)n‐multilayer‐nanowires with perpendicular anisotropy

AbstractWe report on low temperature resistance and magnetoresistance (MR) measurements for various (Co/Pt)n‐multilayer‐nanowires with perpendicular magnetic anisotropy and different widths, designed as model systems for the experimental observation of weak electron localization effects in ferromagnetic systems as proposed by Dugaev et al. [Phys. Rev. B 64, 144423 (2001)]. The low temperature MR is found to originate from both the anisotropic MR and domain wall scattering. The temperature dependence of the resistance shows a logarithmic resistance increase with decreasing temperature, as it is typical for quantum corrections in two dimensional systems. Upon application of a perpendicular magnetic field, however, the slope of the logarithmic resistance increase does not change in magnitude, which proofs that weak localization effects are not present, at least within the accuracy of our measurements. Instead, the observed quantum corrections of the resistance are well explained by enhanced electron‐electron interaction effects in two dimensions. The results are discussed with regard to recent experimental as well as theoretical works.

Synthesis and characterization of polyaniline–polyamidoamine dendrimer blends

AbstractA novel conductive blend of polyaniline (PANI) with polyamidoamine dendrimer (PAMAM (G 2.0)) was prepared by different blending procedure. The PANI‐PAMAM blended polymers were characterized by UV–vis, FTIR, and electron paramagnetic resonance (EPR) spectra. The effect of varying the blending procedure on structure and EPR properties of PANI‐PAMAM blended polymers was investigated. Varying the blending procedure and temperature has a direct effect on the structure and EPR parameters (ΔHPP, g factor, NS, T2, and A/B ratio). EPR spectroscopic studies suggested the presence of chemical interaction between PANI and PAMAM. Electron localization effects in PANI‐PAMAM blended polymers can therefore be studied using the technique of EPR. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1–8, 2006

Microwave-Induced Suppression of Dissipative Conductivity and Its Shubnikov–de Haas Oscillations in Two-Dimensional Electron Systems: Effects of Dynamic Electron Localization and Plasma Reflection

We present a model for microwave photoconductivity in two-dimensional electron systems (2DESs) in a magnetic field at microwave frequencies lower than the electron cyclotron frequency when the intra-Landau level (LL) transitions dominate. Using this model, we explain the effect of the decrease in the 2DES dissipative conductivity (and resistivity) and smearing of its Shubnikov–de Haas oscillations caused by microwave radiation observed recently. The model invokes the concept of suppression of elastic impurity scattering of electrons by the microwave electric field. We calculated the dependence of the 2DES conductivity associated with intra-LL transitions as a function of the radiation and cyclotron frequencies and microwave power. We take into account the effect of plasma reflection of microwaves from 2DES resulting in a distinction between the ac microwave electric field acting on electrons and that generated by a microwave source. The obtained dependences are consistent with the results of recent experimental observations.

Small polaron hopping in spinel manganates

The temperature dependence of small polaron hopping conduction in ceramic spinel NiMn_(2)O_(4+δ) thermistor material has been investigated. We used a theoretical framework based on a random resistor network model to describe small polaron nearest-neighbor hopping (NNH) and variable range hopping (VRH), following the principles of Shklovskii and Efros. We find that in printed thick films and in pressed pellets resistivity is described best by a VRH model of the form ρ~T^(2p) exp(T_(0)/T)^(p), whereas in thin NiMn_(2)O_(4+δ) films resistivity is better described by NNH, ρ~T exp(T_(0)/T). Steady state dc resistance vs temperature measurements for NiMn_(2)O_(4+δ) thick films, thin films, and pellets have been carried out and the parameters p and T_(0) determined. For thick films p was found to be ~0.5, indicating VRH with an approximately parabolic distribution of the density of states (DOS) around Fermi level. For pellets p was ~0.65, and in thin films ~1 indicating NNH. The increase of p was interpreted as an increase of disorder in the system, leading to strong electron localization effects and narrowing Mn^(3+)/Mn^(4+) bandwidth. In thick films and pellets the DOS was determined by a parametrization related to the p value, giving 10^(20)– 10^(21) eV^(−1) cm^(−3). The characteristic temperature T_(0) was in the range of 2X10^(5) K in thick films, 3X10^(4) K in pellets, and 5X10^(3) K in thin films.

Transport properties of moderately disorderedUCu4Pd

We present a detailed study on the (magneto)transport properties of as-cast and heat treated material UCu$_4$Pd. We find a pronounced sample dependence of the resistivity $\rho$ of as-cast samples, and reproduce the annealing dependence of $\rho$. In our study of the Hall effect we determine a metallic carrier density for all samples, and a temperature dependence of the Hall constant which is inconsistent with the Skew scattering prediction. The magnetoresistive response is very small and characteristic for spin disorder scattering, suggesting that overall the resistivity is controlled mostly by nonmagnetic scattering processes. We discuss possible sources for the temperature and field dependence of the transport properties, in particular with respect to quantum criticality and electronic localization effects.

Superconductor–insulator transition induced by over-deposited Ge in the insulating ultrathin a-Nb film

We have observed the superconductor–insulator (S–I) transition induced by over-deposited Ge in the insulating ultrathin amorphous Nb (a-Nb) film. The experiments are performed by depositing Ge onto the insulating a-Nb film with a thickness of 1.04 nm. For d Ge > 0.3 nm , where d Ge was a thickness for Ge film, the reduction of electrical sheet resistances was seen at low temperature region, suggesting superconductor. The normal sheet resistance at 8 K decreased monotonically with increasing d Ge . The simplest explanation of this S–I transition is that the electron localization effect is weakened due to the addition of Ge film.

Electron transport in magnetic nanostructures

Single cobalt-nanowires with in-plane anisotropy and (Co/Pt)n-multilayer nanowires with out-of-plane anisotropy are prepared by electron beam lithograhpy (EBL). The structural properties of the nanowires are investigated by transmission electron microscopy (TEM), secondary electron microscopy (SEM) and atomic force microscopy (AFM). The magnetic properties of single nanowires are imaged by magnetic force microscopy (MFM). Resistance measurements are carried out mainly at low temperatures to explore the temperature dependence of the resistance with high precision and the magnetoresistance (MR) for various orientations between the current direction and the applied external magnetic field. We find that the longitudinal MR displays the nucleation behavior of magnetic domains, whereas the transversal MR can be explained by coherent rotation processes. Applying magnetic fields perpendicular to the film plane quantum transport phenomena have been investigated. We find that internal magnetic fields destroy effects caused by weak electron localization (WEL), whereas effects based on electron–electron-interactions (EEI) survive in magnetic nanostructures. Furthermore, we show that (Co/Pt)n-mul- tilayers with out-of-plane anisotropy are suitable to determine the domain wall resistance (DWMR) when applying external magnetic fields in-plane and perpendicular to the wire axis. We find that the DWMR is positive in accordance to the absence of weak electron localization (dephasing) effects. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Joint effect of lattice interaction and potential fluctuation in colossal magnetoresistive manganites

Taking into account both the Jahn-Teller lattice distortion and the on-site electronic potential fluctuations in the orbital-degenerated double-exchange model, in which both the core-spin and the lattice distortion are treated classically, we investigate theoretically the metal-insulator transition (MIT) in manganites by considering the electronic localization effect. An inverse matrix method is developed for calculation in which we use the inverse of the transfer matrix to obtain the localization length. We find that within reasonable range of parameters, both the lattice effect and the potential fluctuation are responsible to the occurrence of the MIT. The role of the orbital configuration is also discussed.

Low-Energy Excitations in Resonant Inelastic X-Ray Scattering ofα′-NaV2O5

This thesis addresses the electronic structure of vanadium and copper oxides using soft X-ray absorption (SXA) spectroscopy and resonant inelastic X-ray scattering (RIXS) at high brightness synchrotron radiation sources. In RIXS incident photons, tuned to the energy of specific absorption resonances, are inelastically scattered leaving behind a low energy valence excitation in the system studied. Effects of electron localization are reflected by the occurrence of low-energy excitations in form of dd- and charge-transfer excitations that are modelled by cluster calculations. Band-like states are dominating when the intermediate core excited state is delocalized.RIXS at V 2p and O 1s resonances has been used to study the electronic structure of the monovalent vanadium oxides VO2 and V2O3, and of the mixed valence compounds, NaV2O5 and V6O13. For NaV2O5 and V6O13 significant contributions from localized low-energy excitations reflect the partly localized character of their valence band electronic structure, whereas VO2 and V2O3 appear mostly as band-like. Effects of carrier doping are addressed for the case of Mo doping into VO2 and reveal a quasi-rigid band behavior. In the cases of VO2 and V6O13 the temperature dependent metal-insulator transition could be monitored by following the spectral evolution of bands originating from V 3d and V 3d - O2p hybridized states. For Na2V3O7 nanotubes it was possible to selectively probe states from the apical and the basal oxygen sites of VO5 pyramids that constitute these nanotubes. Furthermore, the RIXS technique has been demonstrated to be highly valuable in characterizing the charge transfer processes that accompany lithium insertion into vanadium oxide battery cathodes. Finally, for insulating cuprates RIXS at O 1s, Cu 3p and Cu 3s resonances has been recorded at high-resolution for the detailed investigation of crystal field excitations.

Energy splitting of eg orbitals in Fe‐doped bilayer manganites

Abstract57Fe Mössbauer spectra measurements of Fe‐doped La2–2xSr1+2xMn1.88Fe0.12O7 (x = 0.30, 0.40, and 0.55) compounds were employed to estimate the energy splitting of the Mn3+ ion eg orbital. A large splitting energy ES, about 1.0 eV, was found for each sample. This suggests that there is a strong electron localization effect in these Fe‐doped bilayer manganites. The transport behaviour can be explained on the basis of the effect of ES on the hopping possibility of Mn3+ eg electrons. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electron localization in the strongly correlated organic systemκ−(BEDT−TTF)2Xprobed with nuclear magnetic resonanceC13−NMR

$\ensuremath{\kappa}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}X$ salts exhibit a wide range of properties from superconductivity to magnetic ordering. The phase diagram of this system has been understood as the competition between anti-ferromagnetic insulating behavior and superconductivity. The $\ensuremath{\kappa}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}X$ system has been well explained using a parameter, U/W, as well as the high ${T}_{c}$ cuprates. $\ensuremath{\kappa}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}{\mathrm{Cu}}_{2}{(\mathrm{CN})}_{3}$, however, behaved as an insulator without magnetic ordering. This salt cannot be described by the phase diagram described by the competition between superconductivity and magnetism. It is interesting to explore the nature of this insulating phase with an eye toward understanding the universal phase diagram of the $\ensuremath{\kappa}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}X$ system. Magnetic frustration due to the triangular spin lattice has recently been reported for $\ensuremath{\kappa}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}{\mathrm{Cu}}_{2}{(\mathrm{CN})}_{3}$ from $^{1}\mathrm{H}$-nuclear magnetic resonance (NMR) study. To investigate the insulating properties of this salt, we characterized it using $^{13}\mathrm{C}\text{\ensuremath{-}}\mathrm{NMR}$. In contrast to previous $^{1}\mathrm{H}\text{\ensuremath{-}}\mathrm{NMR}$ studies, at low temperatures, we observed line broadenings that were proportional to the hyperfine coupling constants, which suggested inhomogeous electron localization. We also observed antiferromagnetic fluctuations which, as is the case with other salts, intensified with decreasing temperature. To understand the electronic properties of $\ensuremath{\kappa}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}X$ system, in addition to the U/W, the electron localization effect from the relatively narrow bandwidth is important.

Electron transport properties in Nb and NbN cluster-assembled films produced by a plasma–gas–condensation cluster source

Nb and NbN cluster-assembled films were produced by a plasma–gas–condensation cluster deposition apparatus and examined by transmission electron microscopy, electrical resistivity, and ultraviolet photoemission spectroscopy. The electron diffraction patterns of the Nb and NbN clusters displayed body-centered-cubic and NaCl-type diffraction rings, respectively. The electrical sheet resistance, R□, of both Nb and NbN cluster-assembled films, however, showed no superconductivity down to 2 K. We found a linear relation in the log R□ versus T−1/4 plot for the Nb cluster assembly, suggesting an electron localization effect. For the NbN cluster-assembly, on the other hand, R□ showed a semiconductor type temperature dependence, which is consistent with the valence electron spectra.