Energy Gap Region Research Articles

FIRST-PRINCIPLES CALCULATION OF MAGNETO-ELECTRONIC STRUCTURE OF MOLECULE-FERROMAGNET HYBRID TUNNEL JUNCTIONS

In this paper, using first-principles calculations the electronic and magnetic structure of trans- and cis-polyacetylene based magnetic tunnel junctions is investigated. Energy minimization calculations are performed to obtain the equilibrium bonding length at the metal/polymer interfaces. Magnetic proximity-induced spin polarization across the polymeric chains is calculated and it is shown that irrespective of the parallel or anti-parallel magnetic configuration of the electrodes the carbon atoms attaching the Fe electrodes get oppositely polarized. Local density of states calculations reveal that, as a result of being attached to the ferromagnetic leads, states are induced in the energy gap region of molecule's px and py orbitals which infers their contribution in electronic transmission of the device.

Higgs Amplitude Mode in the BCS SuperconductorsNb1−xTixNInduced by Terahertz Pulse Excitation

Ultrafast responses of BCS superconductor Nb(1-x)Ti(x)N films in a nonadiabatic excitation regime were investigated by using terahertz (THz) pump-THz probe spectroscopy. After an instantaneous excitation with the monocycle THz pump pulse, a transient oscillation emerges in the electromagnetic response in the BCS gap energy region. The oscillation frequency coincides with the asymptotic value of the BCS gap energy, indicating the appearance of the theoretically anticipated collective amplitude mode of the order parameter, namely the Higgs amplitude mode. Our result opens a new pathway to the ultrafast manipulation of the superconducting order parameter by optical means.

Localized electronic states induced by oxygen vacancies on anatase TiO2 (101) surface

A theoretical study about the influence of surface oxygen vacancies on the structural and electronic properties of anatase TiO2 (101) surface is presented. Calculations were performed within the density functional theory using the self-consistent Hubbard U correction to take into account the electron repulsion increase and the localization of additional electrons in the reduced anatase. The oxygen vacancies yield surface Ti3+ species, producing localized electronic states in the energy gap of the anatase TiO2 (101). By increasing the oxygen vacancies, the electron concentration also increases in the energy gap region and some of the localized electronic states approaching the conduction band minimum. Thus reduced anatase TiO2 (101) behaves as an n-type semiconductor, and the surface Ti3+ species presence improves its photocatalytic properties.

Observation of a highly spin-polarized topological surface state in GeBi2Te4

Spin polarization of a topological surface state for GeBi$_2$Te$_4$, the newly discovered three-dimensional topological insulator, has been studied by means of the state of the art spin- and angle-resolved photoemission spectroscopy. It has been revealed that the disorder in the crystal has a minor effect on the surface state spin polarization and it exceeds 75% near the Dirac point in the bulk energy gap region ($\sim$180 meV). This new finding for GeBi$_{2}$Te$_{4}$ promises not only to realize a highly spin polarized surface isolated transport but to add new functionality to its thermoelectric and thermomagnetic properties.

Surface Scattering via Bulk Continuum States in the 3D Topological InsulatorBi2Se3

We have performed scanning tunneling microscopy and differential tunneling conductance (dI/dV) mapping for the surface of the three-dimensional topological insulator Bi(2)Se(3). The fast Fourier transformation applied to the dI/dV image shows an electron interference pattern near Dirac node despite the general belief that the backscattering is well suppressed in the bulk energy gap region. The comparison of the present experimental result with theoretical surface and bulk band structures shows that the electron interference occurs through the scattering between the surface states near the Dirac node and the bulk continuum states.

Surface reconstruction of W2C(0001)

A single crystal surface of ditungsten carbide,W2C(0001) was investigated using low-energy (LEED) and high-energy electron diffraction, Augerelectron spectroscopy, x-ray photoelectron spectroscopy (XPS), and high-resolution electronenergy loss spectroscopy (HREELS). A new reconstruction, , was found as a clean surface structure after annealing theW2C at > 1900 K.The surface carbon content is shown as larger than that in the bulk. Our preliminary results showedthat the same structure is realized also on WC(0001). The same surface periodicity is described foran Mo2C(0001) LEED pattern in the literature. This reconstruction phase is presumablycommon on the (0001) surface of hexagonal group-6 transition-metalcarbides. In the off-specular HREELS, an atomic vibration of 44.8 meV (361 cm − 1) appeared within the gap energy region of the bulk phonon bands, which was assigned to asurface carbon vibration perpendicular to the surface. One possible explanation of the lowvibrational frequency is very low adsorption height of the surface carbon atoms.

Fabrication and Characterization of RF Plasma Polymerized Thin Films from 3,7-Dimethyl-1,6-octadien-3-ol for Electronic and Biomaterial Applications

Poly(linalool) thin films were fabricated using RF plasma polymerisation. All films were found to be smooth, defect-free surfaces with average roughness of 0.44 nm. The FTIR analysis of the polymer showed a notable reduction in –OH moiety and complete dissociation of C=C unsaturation compared to the monomer, and presence of a ketone band absent from the spectrum of the monomer. Poly(linalool) were characterised by chain branching and a large quantity of short polymer chains. Films were optically transparent, with refractive index and extinction coefficient of 1.55 and 0.001 (at 500 nm) respectively, indicating a potential application as an encapsulating (protective) coating for circuit boards. The optical band gap was calculated to be 2.82 eV, which is in the semiconducting energy gap region.

A DFT study on the interaction of Co with an anatase TiO 2 (001)-(1×4) surface

The substitution/adsorption structures of Co on an anatase TiO 2 (001)-(1×4) surface are investigated using the DFT/local density approximation (LDA) method. Theoretical calculation shows that the Co ion prefers to be adsorbed on the surface of anatase TiO 2. The density of states (DOS) analysis finds that the Co 3dis located mainly in the energy gap region. The Co 3dpartial density of states (PDOS) indicates that there is a substantial degree of hybridization between O 2s and Co3din valence band (VB) regions in the substitution models. The conclusion is that the mode of substitution is more active when the catalyst is a higher-energy surface.

Electronic structure of defects in a boron nitride monolayer

We investigate, using first-principles calculations, the electronic structure of substitutional and vacancy defects in a boron nitride monolayer. We found that the incorporation of a substitutional carbon atom induces appreciable modification on the electronic properties, when compared to a non-defective boron nitride sheet. The incorporation of substitutional carbon impurity also induces a significant reduction of the work function. In addition, we found that defects introduce electronic states in the energy-gap region, with strong impact on the optical properties of the material. The calculation results indicate that spin polarization is obtained when substitutional impurities or vacancy defects are introduced in the structure

Piezoelectric theory of detection of surface states

This paper presents a developed theory of computations and analysis of piezoelectric photothermal amplitude and phase spectra of semiconductors with surface defects states on one or both surfaces of the sample exhibiting energies in the energy gap region of a semiconductor. The theory comprises considerations of the front and rear experimental configurations. The presented theory is the extension of the Jackson–Amer model for the case of many optically generated and spatially separated heat sources.

Pseudogap Dependence of the Optical Conductivity Spectra ofCa3Ru2O7: A Possible Contribution of the Orbital Flip Excitation

Optical spectra of a double-layered perovskite ruthenate Ca3Ru2O7 show a pseudogap opening around 200 cm(-1) below 50 K, which is attributable to the partial k-space gap opening due to the density wave instability. Unlike most other density wave materials, Ca3Ru2O7 has spectral weight redistributions, not near the energy gap region, but at a much higher energy region around 800 cm(-1). As a possible origin of these intriguing features, we discuss the orbital flip excitation in the density wave ground state.

Free energy landscapes of electron transfer system in dipolar environment below and above the rotational freezing temperature

Electron transfer reaction in a polar solvent is modeled by a solute dipole surrounded by dipolar molecules with simple rotational dynamics posted on the three-dimensional distorted lattice sites. The interaction energy between the solute and solvent dipoles as a reaction coordinate is adopted and free energy landscapes are calculated by generating all possible states for a 26 dipolar system and by employing Wang-Landau sampling algorithm for a 92 dipolar system. For temperatures higher than the energy scale of dipole-dipole interactions, the free energy landscapes for the small reaction coordinate region have quadratic shape as predicted by Marcus [Rev. Mod. Phys. 65, 599 (1993)] whereas for the large reaction coordinate region, the landscapes exhibit a nonquadratic shape. When the temperature drops, small notched structures appear on the free energy profiles because of the frustrated interactions among dipoles. The formation of notched structure is analyzed with statistical approach and it is shown that the amplitude of notched structure depend upon the segment size of the reaction coordinate and is characterized by the interaction energy among the dipoles. Using simulated free energy landscapes, the authors calculate the reaction rates as a function of the energy gap for various temperatures. At high temperature, the reactions rates follow a bell shaped (inverted parabolic) energy gap law in the small energy gap regions, while it becomes steeper than the parabolic shape in a large energy gap regions due to the nonquadratic shape of the free energy landscape. The peak position of parabola also changes as the function of temperature. At low temperature, the profile of the reaction rates is no longer smooth because of the many local minima of the free energy landscape.

Numerical Analysis of Piezoelectric Spectra of Zn1-x-yBexMnySe Mixed Crystals

This article presents the numerical analysis of the amplitude and phase piezoelectric spectra of a group of mixed crystals of the Zn1-x-yBexMnySe type. After growth treatment of mixed crystals, such as grinding, polishing, and etching, some surface defects have been created whose energy levels are in the region of the band gap of the material. The lower the concentration of these defects is, the smaller amplitude of the photothermal signal in this energy region is expected and the higher the quality of such a crystal surface is. This article focused on the study of the application of photothermal piezoelectric method for investigation of the surface energy levels and their corresponding absorption bands in the energy-gap region of the material.

(0 0 1) Ideal-surface band structure for the series of Cu-based [formula omitted] chalcopyrites

We use the surface Green's function matching method and a tight-binding Hamiltonian to calculate the (0 0 1)-surface electronic band structure and local density of states for the series of Cu-based A I B III C 2 VI chalcopyrites. We find four surface states in the optical gap energy region of s–p character and three surface states in the conduction band region of p-character. We show the trend of different characteristics within the series and analyze their quantitative behavior. We did not find frontier-induced semi-infinite medium states of non-dispersive character in the studied range of energy within the valence band as we found in the case of the (1 1 2) surface electronic band structure for CuInSe 2 .

Niobium nitride Josephson junctions: Experiment and computer simulations

Highly asymmetric Josephson NbN–NbN*–Nb oxide–Pb junctions have been fabricated. A prominent feature, known as a ‘knee’, has been observed in the energy-gap region of quasiparticle current–voltage curves. It is discussed and explained within a conventional scattering approach to the phase-coherent charge transport in a quasiballistic double-barrier S1–I1–N–I2–S2 heterostructure with an extremely great difference between the two barrier transparencies.

Transparent p-type ZnO films obtained by oxidation of sputter-deposited Zn 3N 2

We report on the fabrication of thin ZnO:N film by thermal oxidation of zinc nitride layers sputter-deposited on quartz, sapphire, GaN, and ZnO surfaces. Through optimising several crucial technological steps we have achieved p-type conductivity with the carrier concentration in mid 10 17 cm −3 range and mobility of ∼10 cm 2/Vs. Rich photoluminescence spectra have been observed in the region corresponding to the fundamental energy gap region with peaks that can be related to acceptor bound exciton transitions. The transmittance of p-ZnO:N in the whole visible spectrum is ∼80%.

Electronic structure of protonic conductor SrCeO 3 by soft-X-ray spectroscopy

The electronic structure of protonic conductor SrCe 1− x Y x O 3− δ ( x=0, 0.05) has been studied by resonant-photoemission spectroscopy (RPES) and X-ray absorption spectroscopy (XAS) in the soft-X-ray region. The RPES spectra exhibit that the O 2p state strongly hybridizes with Ce 4f state in the valence band. The valence band consists of the mixed-valent states of the 4f 0 (Ce 4+) and 4f 1 L (Ce 3+) configurations. The XAS spectra in the band gap energy region show a hole state at the top of the valence band and an acceptor-induced level just above the Fermi level ( E F). The energy position of the hole state accords with that of the Ce 3+ state. The energy separation between the hole state and E F is in good agreement with the activation energy.

Structural and optical properties of evaporated Zn oxide, Ti oxide and Zn-Ti oxide films

Zn oxide, Ti oxide and Zn-Ti oxide thin films were prepared by vacuum evaporation. Their structural and optical properties have been obtained by X-ray diffractometry (XRD), the energy-dispersive X-ray fluorescence (EDXRF) method and spectrophotometry. The EDXRF method was used to study the stoichiometry of the deposited Zn-Ti oxide films. The XRD patterns show that the prepared ZnO and Zn-Ti oxide films were polycrystalline, while Ti oxide films were amorphous. Spectroscopic optical constants n(λ) and k(λ) as well as the energy gap Eg were evaluated from spectrophotometry in the interband transition energy region. It was discovered that ZnO–TiO thin films remain transparent in a shorter wavelength range than the ZnO thin films, resulting from the slight increase of their band gap. It was found that n, k and transmittance values for the mixed-oxide film vary smoothly between the values of the pure constituent oxides in the fundamental energy gap region.

Structure, refractive index dispersion and optical absorption properties of evaporated Zn–Eu oxide films

Zn oxide, Eu oxide and Zn–Eu oxide thin films were prepared by vacuum evaporation. Their structural and optical properties have been obtained by X-ray diffractometry (XRD), energy dispersion X-ray fluorescence (EDXRF) method and spectrophotometry. The EDXRF method was used to study the composition of the deposited Zn–Eu oxide films. The XRD patterns show that the prepared thin films of Zn oxide and Zn–Eu oxide with 55% ZnO were polycrystalline, while films of Eu oxide and Zn–Eu oxide with 28% ZnO were amorphous. Spectroscopic optical constants n( λ) and k( λ) as well as energy gap E g were evaluated from spectrophotometry in the interband transition energy region. It was observed that Zn–Eu oxide thin films remain transparent in shorter wavelength range than the ZnO thin films, resulting from the increase of their band-gap. The energy gap of ‘mixed’ oxide sample is controlled by the weight fraction of the constituent oxides. It was found that n, k and transmittance values for mixed-oxide film varies smoothly between the values of the pure constituent oxides in the fundamental energy gap region.

Electronic structures of protonic conductors SrTiO 3 and SrCeO 3 by O 1 s X-ray absorption spectroscopy

The electronic structures of protonic conductors SrTi 1− x Sc x O 3 and SrCe 1− x Yb x O 3 have been studied by O 1 s X-ray absorption spectroscopy (XAS). In SrTi 1− x Sc x O 3, hole state and acceptor-induced level are observed in the band gap energy region. Such a structure is also observed in the O 1 s XAS spectra of SrCe 1− x Yb x O 3. The XAS structures and their temperature dependence reflect the activation energy estimated from electrical conductivity. These facts indicate that the conductivity is achieved by deep acceptor-induced level lying in the middle of the band gap.