Filled Conduction Band Research Articles

Electronic and half-metallic behaviour of manganese doped SiC Zigzag nanotubes using density functional theory

Single walled SiC nanotubes have gain centre of attraction due to the unique behaviours and potential application in the field of optoelectronics devices, LED, storage materials, and Photovoltaic materials. Electronic and structural properties of intrinsic and Manganese (Mn) doped SiC Zigzag nanotubes verified using first principle calculations associated with Density functional theory (DFT). Band gap of SiC Zigzag nanotubes influenced the structural and optical properties which play a crucial role in semiconductor application and create new opportunities in optoelectronic components. Electronic band plot of intrinsic SiC Zigzag nanotubes indicates band gap increased with the increase diameter of the nanotubes. PDOS plot shows 2p orbitals and 3p orbitals has major contribution on mostly filled valence band and lowest filled conduction band region whereas 2 s and 3 s orbitals of C atom and Si atom has minor contribution. Band structure plot of Mn doped SiC Zigzag nanotube introduced an extra energy layer in conduction band and resulting decrease the band gap value and valence band and conduction band cross the Fermi energy level in DOS plot representing the half-metallic characteristics which has different potential applications specially in photovoltaic materials. Half-metallic materials have outstanding spin transport characteristics. Spin-polarized electrons can effectively have conducted by Sic Zigzag nanotubes that controlled transport of spins. These half-metallic properties are essential for spintronic materials where electron spin used for information storage and processing. PDOS plot of Mn doped SiC Zigzag nanotube indicate 2p and 3p states of C atom and Si atom has major contribution compared to the 2 s and 2p states of C atom and Si atom whereas 3d orbitals of dopant Mn atom has major contribution near Fermi level of both conduction and valence band. This study highlights the potential optoelectronic applications of Mn doped SiC Zigzag nanotubes.

Momentum-Space Imaging of Ultra-Thin Electron Liquids in δ-Doped Silicon.

Two-dimensional dopant layers (δ-layers) in semiconductors provide the high-mobility electron liquids (2DELs) needed for nanoscale quantum-electronic devices. Key parameters such as carrier densities, effective masses, and confinement thicknesses for 2DELs have traditionally been extracted from quantum magnetotransport. In principle, the parameters are immediately readable from the one-electron spectral function that can be measured by angle-resolved photoemission spectroscopy (ARPES). Here, buried 2DEL δ-layers in silicon are measured with soft X-ray (SX) ARPES to obtain detailed information about their filled conduction bands and extract device-relevant properties. This study takes advantage of the larger probing depth and photon energy range of SX-ARPES relative to vacuum ultraviolet (VUV) ARPES to accurately measure the δ-layer electronic confinement. The measurements are made on ambient-exposed samples and yield extremely thin (< 1nm) and dense (≈1014 cm-2 ) 2DELs. Critically, this method is used to show that δ-layers of arsenic exhibit better electronic confinement than δ-layers of phosphorus fabricated under identical conditions.

Observation of Phase-Filling Singularities in the Optical Dielectric Function of Highly Doped n-Type Ge.

Phase-filling singularities in the optical response function of highly doped (>10^{19} cm^{-3}) germanium are theoretically predicted and experimentally confirmed using spectroscopic ellipsometry. Contrary to direct-gap semiconductors, which display the well-known Burstein-Moss phenomenology upon doping, the critical point in the joint density of electronic states associated with the partially filled conduction band in n-Ge corresponds to the so-called E_{1} and E_{1}+Δ_{1} transitions, which are two-dimensional in character. As a result of this reduced dimensionality, there is no edge shift induced by Pauli blocking. Instead, one observes the "original" critical point (shifted only by band gap renormalization) and an additional feature associated with the level occupation discontinuity at the Fermi level. The experimental observation of this feature is made possible by the recent development of low-temperature, insitu doping techniques that allow the fabrication of highly doped films with exceptionally flat doping profiles.

Transport and angular resolved photoemission measurements of the electronic properties of In2O3 bulk single crystals

High quality In2O3 single crystals of bcc structure were grown by chemical vapour transport. The temperature dependence of resistivity, Hall constant, and mobility yielded an electron density of n = 1.3 × 1019 cm−3. The transport properties showed characteristics best describable by the degenerate semiconductor model. The crystals were additionally investigated by high resolution angular resolved photoelectron spectroscopy (ARPES). Emission from the valence band and the partially filled conduction band at the Γ point yielded a direct bandgap of (2.7 ± 0.1)eV. The partially filled conduction band furthermore enabled the determination of its three dimensional Fermi surface and the effective masses m* by ARPES.

Interplay between spin-singlet and spin-triplet order parameters in a model of an anisotropic superconductor with cuprate planes

Model of a two-dimensional anisotropic superconductor with a relatively wide, partially filled conduction band is considered. Attractive nearest neighbor interaction with the amplitude V1, and on-site interaction (with the amplitude V0) taken either as repulsive or attractive are included in the model, along with the tight-binding dispersion relation implying singularities in the density of states. The analytic study is based on the conformal transformation method, which can be regarded as an extension of the Van Hove Scenario, plausible for anisotropic systems such as high-Tc cuprates. Employing the Fourier harmonics as a complete and orthogonal basis for irreducible representations of the cuprate plane symmetry group (C4v), the symmetry of the superconducting order parameter is identified for various values of the model parameters. A wide range of parameters η = 2t1/t0 (the ratio of the second and the first nearest neighbor hopping integrals) and n (the carrier concentration) is studied. The obtained diagrams allow one to identify and mark the areas of stability for the superconducting spin-singlet s- and d-wave and the spin-triplet p-wave order parameters. In particular the problem of coexistence of the s-, p- and d-wave order parameters is addressed and solved for selected values of the ratio V0/V1. A possible island of stability of the d-wave order parameter in the s-wave order parameter environment for a relatively strong on-site interaction is revealed. The triple points, around which the s-, d-, and p-wave order parameters coexist, are identified on the diagrams. It is shown that results of the calculations performed for the two-dimensional tight-binding band model are dissimilar with some obtained within the standard BCS-type approximation. The influence of the on-site interaction on the stability of the s-wave order parameter is explained in detail. The developed and widely illustrated formalism can be easily applied to some more composed models.

Coexistence of spin‐singlet s‐ and d‐wave and spin‐triplet p‐wave order parameters in anisotropic superconductors

AbstractA two‐dimensional model of an anisotropic superconductor with a relatively wide partially filled conduction band and a dispersion relation implying singularities in the density of states is considered. Employing the conformal transformation method and choosing the Fourier harmonics as the complete and orthogonal basis for C4v irreducible representations the symmetry of the superconducting order parameter is identified. Detailed numerical calculations are carried out for the two‐dimensional tight‐binding band model and compared with the results obtained within the BCS‐type approximation. Diagrams of stable superconducting states in the coordinates η = 2t1/t0 and n (carrier concentration) are outlined. Symmetry relations with respect to replacing η with −η are found and discussed. The problem of coexistence of s‐, p‐ and d‐wave order parameters is addressed. The triple points are localized and supreme transition temperatures are evaluated. It is shown that the results obtained for various values of the pairing amplitude V1 coincide, whereas the tight‐binding results and some obtained within the BCS‐type approximation are dissimilar. The influence of the attractive or repulsive onsite interaction on the stability of the s‐wave order parameter is explained. The developed and widely illustrated formalism can be easily applied to some more composed models. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

X-ray edge spectra — a sea-boson perspective

The well-studied X-ray-edge problem is revisited using the sea-boson method. This approach is contrasted with the well-known theories of Mahan, Nozières and De Dominicis (MND). The present approach does not use the sudden approximation and the holes carry a momentum label unlike in the MND theory. We focus on the case of doped semiconductors rather than metals. The problem of electrons in a partially filled conduction band and holes in the initially hole-depleted valence band is recast in the sea-boson language. The resulting hamiltonian is shown to be equivalent to the electron–phonon hamiltonian with the excitons taking on the role of electrons and intra-conduction band particle–hole excitations known as ‘conductrons’ taking on the role of phonons. It is shown that the excitonic pole in the computed absorption spectra is replaced by a branch cut with a simple radical leading to a broadening of the exicton line due to these many-body effects. A critical comparison is made with the MND theory as well as with relevant experiments.

Electronic properties of the semiconductor TiSe 2

Angle-resolved photoelectron spectroscopy (ARPES) was used to investigate the electronic structure of 1 T -TiSe 2 single crystals with He I radiation ( ℏ ω = 21.22 eV) as a function of exposure to H 2O at room temperature. A significant enhancement of the Ti 3d peak near the Fermi energy E F is observed and we find a small shift for the lowest conduction band and the uppermost valence bands to lower binding energies depending on the amount of adsorbate. We are able to undoubtedly determine 1 T -TiSe 2 as a small gap semiconductor, because the H 2O adsorption onto the van der Waals-like surface causes a distinct band bending and an almost filled conduction band making the band gap now detectable for photoemission spectroscopy.

Surface states, surface potentials, and segregation at surfaces of tin-dopedIn2O3

Surfaces of ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$ and tin-doped ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$ (ITO) were investigated using photoelectron spectroscopy. Parts of the measurements were carried out directly after thin film preparation by magnetron sputtering without breaking vacuum. In addition samples were measured during exposure to oxidizing and reducing gases at pressures of up to $100\phantom{\rule{0.3em}{0ex}}\mathrm{Pa}$ using synchrotron radiation from the BESSY II storage ring. Reproducible changes of binding energies with temperature and atmosphere are observed, which are attributed to changes of the surface Fermi level position. We present evidence that the Fermi edge emission observed at ITO surfaces is due to metallic surface states rather than to filled conduction band states. The observed variation of the Fermi level position at the ITO surface with experimental conditions is accompanied by a large apparent variation of the core level to valence band maximum binding energy difference as a result of core-hole screening by the free carriers in the surface states. In addition segregation of Sn to the surface is driven by the surface potential gradient. At elevated temperatures the surface Sn concentration reproducibly changes with exposure to different environments and shows a correlation with the Fermi level position.

Effects of Electronic Correlations for Adiabatic Electrochemical Reactions of Electron Transfer in Electrode Models with Nearly Empty and Nearly Filled Conduction Bands: Adiabatic Free Energy Surfaces

Adiabatic free energy surfaces (AFES) for adiabatic electrochemical reactions of electron transfer (ARET) are computed with exact allowance for electron–electron correlation effects (EECE) in models of electrode with nearly empty and almost filled conduction bands and analyzed on the basis of a diagram of kinetic modes obtained earlier. The EECE role in ARET for an electrode with an arbitrary Fermi level in a conduction band of an arbitrary width is discussed. In the general case, allowing for EECE gives at some model parameters results other than for the Fermi level coinciding with the conduction band center (model of a surface molecule, MSM). As in the case of MSM considered previously, EECE considerably reduce activation free energies and at some model parameters give qualitatively different AFES.

Effects of Electronic Correlations for Adiabatic Electrochemical Reactions of Electron Transfer in Electrode Models with Nearly Empty and Nearly Filled Conduction Bands: Diagram of Kinetic Modes

Effects of Electronic Correlations for Adiabatic Electrochemical Reactions of Electron Transfer in Electrode Models with Nearly Empty and Nearly Filled Conduction Bands: Diagram of Kinetic Modes

Effects of Electronic Correlations for Adiabatic Electrochemical Reactions of Electron Transfer in Electrode Models with Nearly Empty and Nearly Filled Conduction Bands: General Relationships

Expressions for the calculation of the adiabatic free energy surfaces (AFES) and average number of electrons in the valence orbital of the reagent for adiabatic electrochemical reactions of electron transfer are obtained in terms of exactly solved models for a metal electrode with nearly empty and nearly filled conduction bands. The models are extreme cases of the Anderson model, which account exactly for the electron–electron correlation effects. In particular, the electrode model with a nearly filled conduction band can be applied to transition metals of Group VIII in the periodic table. Exact relationships connecting AFES and diagrams of kinetic modes (DKM) for electrodes with symmetric position of Fermi levels relative to the conduction band center are obtained. Two characteristic functions for analyzing the role of electron–electron correlation effects in the system under consideration are proposed and calculated. The results form a basis for calculating AFES and studying correlation effects in adiabatic electrochemical reactions of electron transfer and constructing DKM that would correspond to different electron transfer modes.

ON POSSIBLE FORMALISM OF ANISOTROPIC FERMI LIQUID AND BCS–TYPE SUPERCONDUCTIVITY

We prove that an arbitrary crystal with anisotropic lattice and partially filled conduction band, for which one-electron energy spectrum is known, can be considered, after some curvilinear transformation of the reciprocal lattice as the Fermi liquid of the Landau–type quasiparticles in the isotropic system. However, the additional scalar field modifying the size of quantum-mechanical states in the reciprocal space has to be included. It is shown that the established formalism can be employed for two- and three-dimensional systems. As an example one-band Hubbard model has been investigated in details. The new precise formulas for the postulated scalar field and the density of states are derived. Additionally, assuming the microscopic mechanism which leads to the pairing interaction in a crystal, the anisotropic Fermi liquid formalism is extended to superconducting system according to the BCS theory principles. The obtained results enable us to explain and understand the great success of the BCS theory.

Calculation of the first four moments of electronic energy loss of protons in aluminium

We present a novel scheme for calculating electronic energy losses and their higher moments which allows to fully account for the electronic structure of solids. Typical structures in solid materials are found in metals with their half filled conduction bands, and in insulators which exhibit a completely filled valence band, and above this a forbidden band gap. The conduction electrons in a metal resemble most closely a free electron gas which would fill a Fermi sphere in velocity space (or in momentum space) homogeneously from ν = 0 to ν = νF. The core electrons are separated from the freely available phase space by distinct band gaps. For protons slowing-down in metals, this means that conduction electrons can accept small energy transfers which are inacceptable for core electrons or valence electrons in insulators. This leads to a completely different stopping behavior of metals, particularly at low velocities. In metals we expect that all energy loss moments increase strictly proportional to νn starting at zero velocity, i.e., the energy loss proportional to ν1, the energy loss straggling proportional to ν 2 etc. We perform the stopping calculations first for the metal Al, where the theoretical input is well known, and where most measurements have been performed. The comparison of theory and experiments shows agreement within the experimental straggle.

EPR of a fullerene-molecule-derived paramagnetic center as a mesoscopic conducting object

Discussion of theg-factor value of fullerene is based on the model of itinerant electrons restricted to the surface of the fullerene molecule C60. The Ag shift, i.e., the difference between the experimentalg-factor and theg-factor of free electron Δg = g − 2.0023 for C60−1 is negative as for a very small metallic conducting particle.g-factor value is proportional to the interaction between itinerant electrons in the conduction band, thus the Δg is negative for C60−1 and C60−3 having less than half filled conduction band, while Δg is positive for C60+ where the conduction band is almost filled.

Quasi-one-dimensional response of the three-dimensional electronic systemsAC60

The stability of the metallic phase for the electronically three-dimensional $A{\mathrm{C}}_{60}$ compounds is considered theoretically. A calculation of a possible metal-insulator transition temperature as a function of the degree of imperfect nesting of the Fermi surface is performed. We find that ${\mathrm{RbC}}_{60}$ and ${\mathrm{CsC}}_{60}$ are unique systems so far where the instability of the metallic phase originates from the nearly half filled conduction band and not from a low dimensionality of the electronic properties.

Concentration dependent photoluminescence of Te-doped In0.5Ga0.5P layers grown by liquid phase epitaxy

Photoluminescence (PL) of Te-doped In0.5Ga0.5P epilayers grown by the liquid phase epitaxy technique has been investigated as a function of carrier concentration. The PL results are interpreted using a model taking into account nonparabolicity of the conduction band. Both the band filling as well as band tailing due to the Coulomb interaction of free carriers with ionized impurities and band shrinkage due to the exchange interaction between free carriers are considered in order to properly portray the observed features of the PL spectra. The theoretical calculations are in satisfactory agreement with the observed PL results. The PL line shape is well explained by a direct transition with a simple k-selection rule up to a carrier concentration of 2.0 ×1018 cm−3. Above the carrier concentration of 2.0 ×1018 cm−3, on the other hand, it is properly interpreted in terms of non-k-conserving transitions that arise from the indirect recombination of electrons in a highly filled conduction band. It was found that a concentration dependent gap shrinkage due to the exchange interaction in Te-doped In0.5Ga0.5P at 17 K is described by the relation Ece=2.34×10−8 n1/3 (eV). The concentration dependent effective mass has also been calculated using Kane’s three band model.

Electronic structures inVO2susing the periodic polarizable point-ion shell model and DV-Xα method

Electronic structures in the metallic ${\mathrm{VO}}_{2}$ phase above the metal-insulator (MI) transition temperature of ${\mathrm{T}}_{\mathrm{c}}$ and the insulating phase below ${\mathrm{T}}_{\mathrm{c}}$ have been investigated using the combination of the three-dimensional periodic shell model and the discrete-variational (DV)-X\ensuremath{\alpha} cluster method. Besides the correlation effect for ${\mathrm{d}}_{\mathrm{\ensuremath{\parallel}}}$ electrons, the Hamiltonian in the insulating phase includes the Anderson's attractive potential due to the electron-phonon interactions which stabilize the three-dimensional periodic distribution of ${\mathrm{V}}^{4+}$ -${\mathrm{V}}^{4+}$ dimers. The shell model estimates the electron-phonon coupling constant and provides direct theoretical evidence that the dimers are stable in the low-temperature phase. The DV-X\ensuremath{\alpha} cluster method calculates the electron energies in [${\mathrm{V}}_{2}$ ${\mathrm{O}}_{10}$ ${]}^{\mathrm{\ensuremath{-}}12}$ clusters and the value for the intersite repulsive nearest-neighbor d-d Coulombic interaction which quantifies the correlation effect for ${\mathrm{d}}_{\mathrm{\ensuremath{\parallel}}}$ electrons. The electron-phonon interaction effect and the correlation effect for ${\mathrm{d}}_{\mathrm{\ensuremath{\parallel}}}$ electrons are found to split d band into the empty upper and the occupied lower Hubbard bands and also to result in an obvious energy gap between these bands in the insulating phase. In the metallic phase, the nonresolved d band overlaps the \ensuremath{\pi} band and they construct a partially filled conduction band. These calculations explain well the MI transition in ${\mathrm{VO}}_{2}$ and, in particular, the electron-phonon interaction assessed by the periodic shell model is an indispensable contribution in the stabilization of the insulating phase.

Reflectance studies of soluble polypyrrole doped with dodecylbenzene sulfonic acid

Recent progress in the processing of conducting polypyrrole (PPy) from solution has made possible the spin-casting of optical quality films or varying thickness. We present the results of reflectance studies of soluble PPy films doped with dodecylbenzene sulfonic acid (DBSA) over a wide spectral range (.01 eV –6 eV) at room temperature. The reflectance exhibits a plasma resonance at 1.7 eV and increases sharply at the lowest frequencies. These are the spectral features expected for a material with a partially filled conduction band. However, the corresponding optical conductivity and the real part of the dielectric function show low-frequency features characteristic of a conductor on the insulating side of the metal-insulator (M-I) transition. We conclude that there exists a finite density of localized states at the Fermi level, typical of a Fermi glass. The processibility and stability of this material make it attractive for applications such as transparent electrodes.

Structural phase transitions initiating the Peierls transition in di-perylene hexafluorophosphate

High-resolution conduction-electron spin g-factor analysis is used in order to monitor the molecular reorientations in the high-temperature metal phase of a perylene radical cation salt with 5 8 - filled conduction band and Peierls transition at T P = 118 K. Well defined transition temperatures, but sample dependent reorientations are derived.