Electron Localization Effects Research Articles

Description of light-element magnetic systems via density functional theory plus U with an example system of fluorinated boron nitride: An efficient alternative to hybrid functional approach

It is well known that for light-element magnetic materials density functional theory (DFT) in the local density approximation or generalized gradient approximation (LDA or GGA) underestimates the electron localization effects and tends to give misleading results. Hybrid functionals such as Heyd-Scuseria-Ernzerhof (HSE) perform much better while being computationally expensive, especially for extended systems. In order to go beyond semi-local DFT without needing to calculate the expensive Fock exchange, here we explore the performance of the more efficient GGA plus the Hubbard U correction (GGA+U) approach to light-element magnetic materials by considering fluorinated boron nitride (F-BN) sheets and nanotubes as model systems. By applying the Hubbard U correction to the N-2p orbitals with the value of U determined by fitting the HSE results in a particular F-BN sheet, it is found that the GGA+U approach shows a great improvement to GGA in describing the magnetic properties of F-BN systems with an accuracy close to that of the HSE hybrid functional approach. It indicates the possibility of using the ad hoc correction approach as an efficient alternative to study light-element magnetic materials, especially for large systems where calculations based on hybrid functionals become cost-demanding.

Direct evidence of traps controlling the carriers transport in SnO2 nanobelts

This work reports on direct evidence of localized states in undoped SnO2 nanobelts. Effects of disorder and electron localization were observed in Schottky barrier dependence on the temperature and in thermally stimulated currents. A transition from thermal activation to hopping transport mechanisms was also observed. The energy levels found by thermally stimulated current experiments were in close agreement with transport data confirming the role of localization in determining the properties of devices.

Detection of phase separation and nano-droplet states in La_033Pr_034Ca_033MnO_3 nanowires via near-infrared reflection experiments

We present an optical investigation on La0.33Pr0.34Ca0.33MnO3 (LPCMO) nanowires by using standard near-infrared (NIR) reflection experiments. It is found that the samples can respond strongly to the NIR radiation so that the temperature dependence of the reflectivity can be measured accordingly. Interestingly, the temperature induced phase separation and nano-droplet states in the nanowire samples with different diameters can be identified and observed optically without making ohmic contact electrodes on the samples. The onset temperature of the nano-droplet state measured optically is in line with that obtained from previous transport measurements. Importantly, via examining the dependence of the NIR reflectivity upon radiation wavelength, we find that the electronic localization effect exists in the LPCMO nanowires in both electronic states. Such information cannot be directly obtained from electric transport and magneto-transport experiments. The interesting and important experimental findings from this study demonstrate that the simple and contactless NIR reflection measurements can be employed to conveniently study and characterize the basic physical properties of perovskite manganites based nanostructure systems.

Study of spinel LiTi2O4 superconductors via near-infrared reflection experiments.

We present an optical study on high-quality and single-phase LiTi2O4 (LTO) superconductor thin films grown on MgAl2O4 substrates by pulsed laser deposition. The near infrared (NIR) reflectivity is measured for samples with (001) and (111) lattice orientations. The temperature-induced metal-superconductor transition can be observed, and the superconducting transition temperature can be measured for both samples. We find that the NIR reflection experiment can reflect rightly the basic features of LTO superconductor thin films. Furthermore, the results obtained from this simple optical measurement suggest that the photo-induced electronic localization effect can be present in LTO thin films in a metallic state. Such information cannot be obtained directly from conventional transport and magneto-transport measurements. These interesting and important findings demonstrate that the NIR reflection experiment is a powerful optical technique for contactless characterizations and investigations of superconductor materials.

First-principles calculations of Ti3SiC2 and Ti3AlC2 with hydrogen interstitial

In this paper, the effects of hydrogen interstitial defect on the structural stability of two kinds of MAX materials (Ti3SiC2 and Ti3AlC2) were investigated by first-principles calculations. The results indicated that the hydrogen interstitial energetically prefers to reside at the 2Ti3Si site for Ti3SiC2 and 3TiAl site for Ti3AlC2, respectively, and the latter has much lower formation energy. Both of these MAX phases are slightly hardened and the elastic anisotropy is reduced appreciably after the introduction of hydrogen interstitial. The hydrogen interstitial in Ti3SiC2 and Ti3AlC2 leads to an electronic localization effect on the Si/Al atom and the effect is more remarkable in Ti3AlC2. The interlayer bonding strength of Ti3AlC2 is more weakened by hydrogen interstitials than that of Ti3SiC2. As a result, the interatomic bonding between Si/Al and Ti atom layers is deteriorated and their structural stabilities degrade subsequently.

Effects of Stone-Wales defect on the electronic and transport properties of bilayer armchair graphene nanoribbons

We report a first principles study on the electronic and transport properties of bilayer armchair graphene nanoribbons (BLAGNRs) containing Stone-Wales (SW) defect. It is shown that in the presence of SW defect in BLAGNRs, some electron localization occurs in defect atoms and degradation of transmission is observed in specific energy regions. The strength of electron localization is dependent on the symmetry of SW defect. In case of symmetric SW defect, stronger electron localization leads to sharper dip in its transmission spectrum in comparison with the broad dip in the transmission spectrum of the BLAGNR containing asymmetric SW defect. The effect of electron localization is also evident from the calculated I-V characteristics of pristine and defected structures which shows the reduced current in defected structures with respect to pristine structure. Most current reduction is observed in symmetric SW defected BLAGNR due to the stronger electron localization in symmetric SW defect.

Large linear magnetoresistance in heavily-doped Nb:SrTiO3 epitaxial thin films.

Interaction between electrons has long been a focused topic in condensed-matter physics since it has led to the discoveries of astonishing phenomena, for example, high-Tc superconductivity and colossal magnetoresistance (CMR) in strongly-correlated materials. In the study of strongly-correlated perovskite oxides, Nb-doped SrTiO3 (Nb:SrTiO3) has been a workhorse not only as a conducting substrate, but also as a host possessing high carrier mobility. In this work, we report the observations of large linear magnetoresistance (LMR) and the metal-to-insulator transition (MIT) induced by magnetic field in heavily-doped Nb:STO (SrNb0.2Ti0.8O3) epitaxial thin films. These phenomena are associated with the interplay between the large classical MR due to high carrier mobility and the electronic localization effect due to strong spin-orbit coupling, implying that heavily Nb-doped Sr(Nb0.2Ti0.8)O3 is promising for the application in spintronic devices.

Understanding molecular harmonic emission at relatively long intense laser pulses: Beyond the Born-Oppenheimer approximation

The underlying physics behind the molecular harmonic emission in relatively long sin$^2$-like laser pulses is investigated. We numerically solved the full-dimensional electronic time-dependent Schr\"{o}dinger equation beyond the Born-Oppenheimer approximation for simple molecular ion H$_2^+$. The occurrence and the effect of electron localization, non-adiabatic redshift and spatially asymmetric emission are evaluated to understand better complex patterns appearing in the high-order harmonic generation (HHG) spectrum. Results show that the complex patterns in the HHG spectrum originate mainly from a non-adiabatic response of the molecule to the rapidly changing laser field and also from a spatially asymmetric emission along the polarization direction. The effect of electron localization on the HHG spectrum was not observed as opposed to what is reported in the literature.

Anomalous Hall effect in ZnxFe3-xO4: Universal scaling law and electron localization below the Verwey transition

We show that the well-established universal scaling σxyAHE ∼ σxx1.6 between anomalous Hall and longitudinal conductivities in the low conductivity regime (σxx < 104 Ω−1 cm−1) transforms into the scaling σxyAHE ∼ σxx2 at the onset of strong electron localization. The crossover between the two relations is observed in magnetite-derived ZnxFe3-xO4 thin films where an insulating/hopping regime follows a bad metal/hopping regime below the Verwey transition temperature Tv. Our results demonstrate that electron localization effects come into play in the anomalous Hall effect (AHE) modifying significantly the scaling exponent. In addition, the thermal evolution of the anomalous Hall resistivity suggests the existence of spin polarons whose size would decrease below Tv.

Effect of electron (de)localization and pairing in the electrochemistry of polyoxometalates: study of Wells-Dawson molybdotungstophosphate derivatives.

Polyoxometalates (POMs) are inorganic entities featuring extensive and sometimes unusual redox properties. In this work, several experimental techniques as well as density functional theory (DFT) calculations have been applied to identify and assess the relevance of factors influencing the redox potentials of POMs. First, the position of the Mo substituent atom in the Wells-Dawson structure, α1- or α2-P2W17Mo, determines the potential of the first 1e(-) reduction wave. For P2W(18-x)Mox systems containing more than one Mo atom, reduction takes place at successively more positive potentials. We attribute this fact to the higher electron delocalization when some Mo oxidizing atoms are connected. After having analyzed the experimental and theoretical data for the monosubstituted α1- and α2-P2W17Mo anions, some relevant facts arise that may help to rationalize the redox behavior of POMs in general. Three aspects concern the stability of systems: (i) the favorable electron delocalization, (ii) the unfavorable e(-)-e(-) electrostatic repulsion, and (iii) the favorable electron pairing. They explain trends such as the second reduction wave occurring at more positive potentials in α1- than in α2-P2W17Mo, and also the third electron reduction taking place at a less negative potential in the case of α2, reversing the observed behavior for the first and the second waves. In P2W17V derivatives, the nature of the first "d" electron is more localized because of the stronger oxidant character of V(V). Thus, the reduction potentials as well as the computed reduction energies (REs) for the second reduction of either isomer are closer to each other than in Mo-substituted POMs. This may be explained by the lack of electron delocalization in monoreduced P2W17V(IV) systems.

The effect of electron localization on the electronic structure and migration barrier of oxygen vacancies in rutile

By applying the on-site Coulomb interaction (Hubbard term U) to the Ti d orbital, the influence of electron localization on the electronic structure as well as the transport of oxygen vacancies (V O) in rutile was investigated. With U = 4.5 eV, the positions of defect states in the bandgap were correctly reproduced. The unbonded electrons generated by taking out one neutral oxygen atom are spin parallel and mainly localized on the Ti atoms near V O, giving rise to a magnetic moment of 2 μB, in agreement with the experimental finding. With regard to the migration barrier of V O, surprisingly, we found that U = 4.5 eV only changed the value of the energy barrier by ±0.15 eV, depending on the diffusion path. The most probable diffusion path (along ) is the same as that calculated by using the traditional GGA functional. To validate the GGA + U method itself, a hybrid functional with a smaller supercell was used, and the trend of the more probable diffusion path was not changed. In this regard, the traditional GGA functional might still be reliable in the study of intrinsic-defect transportation in rutile. Analyzing the atomic distortion and density of states of the transition states for different diffusion paths, we found that the anisotropy of the diffusion could be rationalized according to the various atomic relaxations and the different positions of the valence bands relative to the Fermi level of the transition states.

Trapping of electrons in troughs of self generated electromagnetic standing waves in a bounded plasma column

Observations and measurements are reported on electron trapping in troughs of self-generated electromagnetic standing waves in a bounded plasma column confined in a minimum-B field. The boundaries are smaller than the free space wavelength of the waves. Earlier work of researchers primarily focused upon electron localization effects induced by purely electrostatic perturbation. We demonstrate the possibility in the presence of electromagnetic standing waves generated in the bounded plasma column. The electron trapping is verified with electrostatic measurements of the plasma floating potential, electromagnetic measurements of the wave field profile, and optical intensity measurements of Argon ionic line at 488 nm. The experimental results show a reasonably good agreement with predictions of a Monte Carlo simulation code that takes into account all kinematical and dynamical effects in the plasma in the presence of bounded waves and external fields.

Bound states, electron localization and spin correlations in low-dimensional GaAs/AlGaAs quantum constrictions

We analyze the occurrence of local magnetization and the effects of electron localization in different models of quantum point contacts (QPCs) using spin-relaxed density functional theory (DFT/LSDA) by means of numerical simulations. In the case of soft confinement potentials the degree of localization is weak and we therefore observe only traces of partial electron localization in the middle of the QPC. In the pinch-off regime there is, however, distinct accumulation at the QPC edges. At the other end, strong confinement potential, low-electron density in the leads and top or implant gates favor electron localization. In such cases one may create a variety of electron configurations from a single localized electron to more complex structures with multiple rows and Wigner lattices.

MOCVD-grown heterostructures with GaAs/AlGaAs Superlattices: Growth features and optical and transport characteristics

The results of studies of MOCVD growth regularities of GaAs/AlGaAs superlattices with narrow forbidden minibands are presented. The spectra of photoluminescence and X-ray diffraction are measured, the concentration distribution profiles of components are determined by secondary-ion mass spectrometry, and the concentration distribution of charge carriers are determined by the method of capacitance profiling. The relation of the growth modes of heterostructures to their crystalline characteristics and luminescent and electrical properties are studied. The photoluminescence measurements indicate the high quality of the superlattices. X-ray diffraction and the data on secondary ions confirm the high periodicity of the superlattices grown in the optimized modes. The nonlinear current-voltage characteristics with a region of negative differential conductivity at moderate voltages and subsequent current increase at higher voltages because of tunneling between the minibands is found for the superlattices grown in the optimal growth modes. Current oscillations at frequencies of ∼60 MHz were observed in the region of negative differential conductivity. The negative differential conductivity and oscillations confirm the presence of the electron localization effect in moderate electric fields in the first conductivity miniband, which emerges due to the Bragg reflection of carriers in the superlattice.

Electronic transport and quantum localization effects in organic semiconductors

We explore the charge transport mechanism in organic semiconductors based on a model that accounts for the thermal intermolecular disorder at work in pure crystalline compounds, as well as extrinsic sources of disorder that are present in current experimental devices. Starting from the Kubo formula, we develop a theoretical framework that relates the time-dependent quantum dynamics of electrons to the frequency-dependent conductivity. The electron mobility is then calculated through a relaxation time approximation that accounts for quantum localization corrections beyond Boltzmann theory, and allows us to efficiently address the interplay between highly conducting states in the band range and localized states induced by disorder in the band tails. The emergence of a "transient localization" phenomenon is shown to be a general feature of organic semiconductors, that is compatible with the bandlike temperature dependence of the mobility observed in pure compounds. Carrier trapping by extrinsic disorder causes a crossover to a thermally activated behavior at low temperature, which is progressively suppressed upon increasing the carrier concentration, as is commonly observed in organic field-effect transistors. Our results establish a direct connection between the localization of the electronic states and their conductive properties, formalizing phenomenological considerations that are commonly used in the literature.

Adsorption, Dissociation, and Hydrogenation of CO2 on WC(0001) and WC-Co Alloy Surfaces Investigated with Theoretical Calculations

We applied periodic density functional theory to investigate the adsorption of CO2 on WC(0001) and various WC-Co alloy surfaces and discuss the reaction trend of dissociation of CO2 and hydrogenation on these surfaces. We employed an electron localization function (ELF) to investigate the effect of electron localization or delocalization on various WC-Co alloy surfaces with a varied Co coverage; the partial-delocalization surfaces (WC-Co (0.25 ML) surface) exhibit the largest energy of adsorption of CO2 (−1.61 eV) on all calculated surfaces. When we increased the Co ratio to form a WC-2Co (0.50 ML) surface, the activation energy for the dissociation CO2→CO+O decreased to 0.57 eV. The energy of CO2 hydrogenation to form formate (CO2+H → HCOO) decreases with Co coverage due to increased electron delocalization.

Magnetic and transport properties of CePt3Ge Kondo lattice in crystalline and sub-micron state

We have studied a ternary phase of the CePt3±xGe composition, with the maximum values of x=0.2. A series of samples was prepared by arc melting, Czochralski method and splat cooling, and characterized by scanning electron microscopy and X-ray diffraction. The arc-melted samples are well-crystalline and single-phase with slightly varying stoichiometry close to the nominal CePt3Ge, while the splat-cooled samples form an amorphous or a nanocrystalline constitution of the ∼CePt3Ge composition. Detailed measurements of magnetization, specific heat and electrical resistivity were performed in the temperature range 0.40–300K, and magnetic fields up to 9T. In addition, a delicate neutron diffraction experiment down to 70mK was done, which excluded long-range magnetic order in the CePt3Ge compound. The specific heat data were tested to anisotropic Kondo model, and the resistivity behavior has been treated by means of electron localization effects. The experiments evidenced that the CePt3Ge compound can be considered as a Kondo lattice with low-temperature magnetic correlations, and the size effect seems to have minor role in control of the Kondo scattering phenomena in this compound.

Effect of electron localization on the edge-state spins in a disordered network of nanographene sheets

The magnetism and its dynamical behavior is investigated in relation to electron-localization effect for the edge-state spins of three-dimensional randomly networked nanographene sheets which interact weakly with each other. The electron transport is governed by Coulomb-gap variable-range hopping between nanographene sheets. At high temperatures, the electron spin resonance (ESR) signal with a feature of homogeneous spin system reveals the bottleneck effect in the spin relaxation to the lattice for a strongly coupled system of edge-state spins and conduction $\ensuremath{\pi}$ electrons, in a given nanographene sheet. Below 20 K, a discontinuous ESR line broadening accompanied by hole-burning proves the formation of an inhomogeneous spin state, indicating a static spatial distribution of on-resonance fields. This inhomogeneity originates from a distribution of the strengths of the ferrimagnetic moments on the individual nanographene sheets, taking into account that the constituent nanographene sheets with their shapes randomly varying have different strengths of ferrimagnetic moments. Strong electron localization below 20 K in the internanographene electron hopping is responsible for the crossover from the homogeneous spin state to the inhomogeneous one, in the latter of which ferrimagnetic short-range ordering is evident in the edge-state spin system.

Unoccupied electronic structure of ball-milled graphite

Changes in electronic and vibrational structure of well characterised macrocrystalline graphite milled by a planetary ball-mill are investigated by Raman spectroscopy and Near Edge X-ray Absorption Fine structure (NEXAFS) measurements at the C K-edge. The electronic structure changes at the surface and in the sub-surface of the particles are examined by comparing two-different NEXAFS detection modes: total fluorescence yield (TFY) and partial electron yield (PEY) respectively. When the in-plane crystallite sizes of graphite are decreased to nanosized (from approximately 160 nm to approximately 9 nm), a new spectral structure appears in TFY at 284.1 eV which is not present in the macrocrystalline graphite. This feature is assigned to electronic states associated with zigzag edges. Further the TFY shows a shift of the main graphite pi* band from 285.5 to 285.9 eV, attributed to breaking the conjugation and hence the electron localization effect during milling, The TFY spectra also show strong spectral features at 287.5 and 288.6 eV, which suggest that the local environment of carbon atoms changes from sp(2) to more sp(3) due to physical damage of the graphite sheets and formation of structures other than aromatic hexagons. Complementary Raman spectroscopic measurements demonstrate an up-shift of the graphite G band from 1575 to 1583 cm(-1)en route to nanosize. The changes in TFY NEXAFS and Raman spectra are attributed to modification of the sub-surface electronic structure due to the presence of defects in the graphite crystal produced during milling. The discovery of the strong spectral feature at 284.1 eV in nanographite and the 0.4 eV up-shift of the pi* band may open up possibilities to influence the electronic transport properties of graphite by manipulation of defects during the preparation of the nanographite.

The electronic structure of UCoGe by ab initio calculations and XPS experiment

The crystal and electronic structures of the orthorhombic compound UCoGeare presented and discussed. It has been either refined by the x-ray diffractionon a single crystal or computed within the local spin density functional theory,employing the fully relativistic version of the full-potential local-orbital band structurecode, respectively. We particularly give our attention to investigating the Fermisurface and de Haas–van Alphen quantities of UCoGe. The calculated electronicdensity is then examined by x-ray photoelectron spectroscopy (XPS). Fairly goodagreement is achieved between theoretical and experimental XPS results in theparamagnetic state. A small difference in the position (in energy scale) of the U 5fbands is caused by the electron localization effect observed in the experimentalXPS. There is also some discrepancy for the Co 3d electron contributions belowEF. The Fermi surface in the non-magnetic state is of a semimetallic typewhile that in the ferromagnetic state, with the ordered moment of−0.47 μB/f.u. alongthe c axis, is more metallic, with nesting properties that may favour superconductivity.