Browsing by Subject "Electron energy levels"

Now showing 1 - 20 of 25

Open Access
Accurate method for obtaining band gaps in conducting polymers using a DFT/hybrid approach
(American Chemical Society, 1998) Salzner, U.; Pickup, P. G.; Poirier, R. A.; Lagowski, J. B.
DFT calculations on a series of oligomers have been used to estimate band gaps, ionization potentials, electron affinities, and bandwidths for polyacetylene, polythiophene, polypyrrole, polythiazole, and a thiophene - thiazole copolymer. Using a slightly modified hybrid functional, we obtain band gaps within 0.1 eV of experimental solid-state values Calculated bond lengths and bond angles for the central ring of sexithiophene differ by less than 0.026 Å and 0.7° from those of the sexithiopnene crystal structure. IPs and EAs are overestimated by up to 0.77 eV compared to experimental bulk values. Extrapolated bandwidths agree reasonably well with bandwidths from band structure calculations.
Open Access
Balancing energy loads in wireless sensor networks through uniformly quantized energy levels-based clustering
(IEEE, 2010) Ali, Syed Amjad; Sevgi, Cüneyt; Kocyigit, A.
Clustering is considered a common and an effective method to prolong the lifetime of a wireless sensor network. This paper provides a new insight into the cluster formation process based on uniformly quantizing the residual energy of the sensor nodes. The unified simulation framework provided herein, not only aids to reveal an optimum number of clusters but also the required number of quantization levels to maximize the network's lifetime by improving energy load balancing for both homogeneous and heterogeneous sensor networks. The provided simulation results clearly show that the uniformly quantized energy level-based clustering provides improved load balancing and hence, a longer network lifetime than existing methods. © 2010 IEEE.
Open Access
Bound electron pairs in strongly correlated models of high-temperature superconductivity
(Natsional'na Akademiya Nauk Ukrainy, 1998) Boyaci, H.; Kulik, I. O.
The ground-state energy of two electrons on a ring is calculated for the one-dimensional Hubbard model with positive and negative on-site interaction and for the contraction model with additive and multiplicative interaction terms. The hc/2e periodicity of the ground-state energy with respect to a flux Φ threading the loop is derived. The periodicity may serve as an indication of superconductivity. The results are shown to be consistent with the Lieb-Wu solution for Φ=0 limit. In addition, the new states that were missing in the Lieb-Wu solution are derived.
Open Access
Collective oscillations in a two-dimensional Bose-Einstein condensate with a quantized vortex state
(The American Physical Society, 2005) Banerjee, A.; Tanatar, Bilal
We study the effect of lower dimensional geometry on the frequency splitting of the quadrupole oscillations of a harmonically trapped Bose-Einstein condensate due to the presence of a quantized vortex. To study the effect of two-dimensional geometry we consider a pancake-shaped condensate and employ various models for the coupling parameter depending on the thickness of the condensate relative to the value of the scattering length. Using these models and the sum-rule approach we obtain analytical expressions for the frequency splitting. These expressions are valid for positive scattering length and large N. We show that the frequency splitting of the quadrupole oscillations are significantly altered by the reduced dimensionality and also study the evolution of the splitting as the system makes transition from one scattering regime to the other.
Open Access
Contraction of atomic orbitals in the oxygen anion network and superconductivity in metal oxide compounds
(TÜBİTAK, 1996) Kulik, Igor Orestovich
Anion network in the CuO2 plane of metal-oxide compound is considered as an intrinsic-hole metal with holes rather than electrons comprising a Fermi liquid immersed in the background of negative O2- ions. Due to the contraction of p-orbital of oxygen as a result of occupation by a hole, hole hopping between nearest neighbor sites (i, j) is dependent upon hole occupation as tij,σ = to + Vni,-σnj,-σ = W(ni,-σ + nj,-σ). Coupling parameters W and V (additive and multiplicative "contraction interaction" terms) result in the binding of holes into singlet, on-site configuration, or into triplet, nearest-neighbor-site configuration, due to W and V respectively. In the weak coupling limit, W results in the BCS type of superconductive pairing (singlet, s-wave), whereas multiplicative contraction V provides for either singlet, d-wave, or triplet, p-wave-like pairing states. It is concluded that the latter state may result in a plausible mechanism for high-Tc superconductivity in metal oxide compounds. The superconducting p-phase is shown to be in accord with recently published symmetry tests of the order parameter in oxides.
Open Access
Density functional theory investigation of substituent effects on building blocks of conducting polymers
(Elsevier, 1999) Salzner, U.
Substituted heterocyclic dimers were calculated employing density functional theory (DFT) and analyzed with the natural bond orbits method (NBO). Substitution in 3- and 4-positions leads to parallel shifting of HOMO and LUMO but does not reduce energy gaps. For bridge dimers, HOMO-LUMO gaps correlate with π-electron densities in the carbon backbone and energy gap reduction correlate with the strength of π-π* interactions from the backbone to the bridging group. Alternating donor-acceptor groups do not reduce energy gaps and lead to systems with average HOMO and LUMO levels compared to the parent molecules.
Open Access
Does the donor-acceptor concept work for designing synthetic metals? 1. theoretical investigation of poly(3-cyano-3′-hydroxybithiophene)
(American Chemical Society, 2002) Salzner, U.
Homo- and copolymers of hydroxythiophene and cyanothiophene have been investigated by employing density functional theory with the aim of determining the effect of donor-acceptor substitution on the electronic structure. The band gap of the copolymer is 0.11 eV smaller than that of polythiophene. Bandwidths of valence and conduction bands are reduced by 0.22 and 0.36 eV compared to polybithiophene. Conductivity after p- and n-doping could therefore be less than that of polythiophene. All properties of the copolymer are averages between those of the homopolymers. The charge separation between hydroxy- and cyano-substituted rings is 0.12 e in the neutral state and 0.13 e and the dication. The ionization potential and electron affinity of poly(hydroxythiophene) are 1.78 and 1.63 eV smaller than those of poly(cyanothiophene). According to the donor-acceptor concept, a decrease in band gap and an increase in bandwidths compared to the homopolymers should have resulted: We rationalize the absence of band broadening with reduced interaction between fragments with very different energies in agreement with perturbation theory.
Open Access
Does the donor-acceptor concept work for designing synthetic metals? 2. theoretical investigation of copolymers of 4-(dicyanomethylene)-4H-cyclopenta[2, 1-b: 3, 4-b′]dithiophene and 3, 4-(ethylenedioxy)thiophene
(American Chemical Society, 2002) Salzner, U.; Köse, M. E.
Density functional theory (DFT) calculations were performed on oligomers of 3,4-(ethylenedioxy)thiophene (EDOT), 4-(dicyanomethylene)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene (CDM), and co-oligomers (CDM/ EDOT). Oligomer data were extrapolated to polymer values. Theoretical band gaps reproduce λmax from UV spectroscopy for PEDOT and are about 1 eV larger than electrochemical band gaps. λmax of PCDM/EDOT is predicted to be 0.42 eV smaller than that of PEDOT and 0.15 eV smaller than that of PCDM. PCDM/EDOT has a wide valence and an extremely narrow conduction "band". It is probably better not to refer to these localized states as a band at all. This rationalizes the mobility ratio of 500 between p-type and n-type charge carriers and the low n-type conductivity of PCDM/EDOT. The lack of dispersion of the conduction band is due to the very different EAs of EDOT and CDM.
Open Access
Dynamical screening effects in hot-electron scattering from electron-hole plasma and LO-phonon modes in quantum wires
(Elsevier, 1996) Bennett, C. R.; Tanatar, Bilal; Constantinou, N. C.
We present a fully dynamical and finite temperature study of the hot-electron momentum relaxation rate and the power loss in a coupled system of electron-hole plasma and bulk LO-phonons in a quantum wire structure. Interactions of the scattered electron with neutral plasma components and phonons are treated on an equal footing within the random-phase approximation. Coupled mode effects substantially change the transport properties of the system at low temperatures. Particularly, the "plasmon-like" and "LO-phonon-like" excitations yield comparable rates which, as a consequence of the singular nature of the ID density of states, can be large at the threshold. This is in contrast to room temperature results where only the LO-phonon mode contributes significantly to the rate. The density and temperature dependence of the power loss reveals that dynamical screening effects are important, and energy-momentum conservation cannot be satisfied above a certain density for a given initial energy.
Open Access
Effect of cross-sectional geometry on the RPA plasmons of quantum wires
(Pergamon Press, 1994) Bennett, C. R.; Tanatar, Bilal; Constantinou, N. C.; Babiker, M.
The effect of cross-sectional geometry on both the intrasubband plasmon and intersubband plasmon of a quantum wire is investigated within a two-subband RPA scheme. Exact analytical electronic wavefunctions for circular, elliptical and rectangular wires are employed within the infinite barrier approximation. It is found that for fixed cross-sectional area and linear electron concentration, the intrasubband plasmon energy is only marginally dependent on the wire geometry whereas the intersubband plasmon energy may change considerably due to its dependence on the electronic subband energy difference. © 1994.
Open Access
Electronic and magnetic properties of zinc blende half-metal superlattices
(A I P Publishing LLC, 2004) Fong, C. Y.; Qian, M. C.; Pask, J. E.; Yang, L. H.; Dag, S.
Zinc blende half-metallic compounds such as CrAs, with large magnetic moments and high Curie temperatures, are promising materials for spintronic applications. We explore layered materials, consisting of alternating layers of zinc blende half-metals, by first principles calculations, and find that superlattices of (CrAs)1(MnAs)1 and (CrAs)2(MnAs)2 are half-metallic with magnetic moments of 7.0mB and 14.0mB per unit cell, respectively. We discuss the nature of the bonding and half-metallicity in these materials and, based on the understanding acquired, develop a simple expression for the magnetic moment in such materials. We explore the range of lattice constants over which half-metallicity is manifested, and suggest corresponding substrates for growth in thin film form.
Open Access
Energy and mass of 3D and 2D polarons in the overall range of the electron-phonon coupling strength
(Institute of Physics Publishing Ltd., 1994) Ercelebi, A.; Senger, R. T.
The ground-state characterization of the polaron problem is retrieved within the framework of a variational scheme proposed previously by Devreese et al for the bound polaron. The formulation is based on the standard canonical transformation of the strong coupling ansatz and consists of a variationally determined perturbative extension serving for the theory to interpolate in the overall range of the coupling constant. Specializing our considerations to the bulk and strict two-dimensional polaron models we see that the theory yields significantly improved energy upper bounds in the strong coupling regime and, moreover, extrapolates itself successfully towards the well-established weak coupling limits for all polaron quantities of general interest.
Open Access
Energy relaxation of electrons in InGaN quantum wells
(Springer New York LLC, 2015-04) Sarikavak-Lisesivdin, B.; Lisesivdin, S. B.; Balkan, N.; Atmaca, G.; Narin, P.; Cakmak, H.; Özbay, Ekmel
In this study, electron energy relaxation mechanisms in HEMT structures with different InxGa1−xN-channel quantum well (QW) widths are investigated. Theoretical value of the inelastic scattering rates is carried out at electron temperatures between 30 K (−243 °C) < Te < 700 K (427 °C). We used both the experimentally determined and calculated electron temperatures to estimate the energy relaxation rates of non-equilibrium electrons. In wide InGaN QWs, power loss of an electron is shown to be significantly smaller than that in the narrower QWs. © 2015, The Minerals, Metals & Materials Society and ASM International.
Open Access
Infrared photoluminescence from TlGaS2 layered single crystals
(Wiley - V C H Verlag GmbH & Co., 2004) Yuksek, N. S.; Gasanly, N. M.; Aydınlı, Atilla; Ozkan, H.; Acikgoz, M.
Photolimuniscence (PL) spectra of TlGaS2 layered crystals were studied in the wavelength region 500-1400 nm and in the temperature range 15-115 K. We observed three broad bands centered at 568 nm (A-band), 718 nm (B-band) and 1102 nm (C-band) in the PL spectrum. The observed bands have half-widths of 0.221, 0.258 and 0.067 eV for A-, B-, and C-bands, respectively. The increase of the emission band half-width, the blue shift of the emission band peak energy and the quenching of the PL with increasing temperature are explained using the configuration coordinate model. We have also studied the variations of emission band intensity versus excitation laser intensity in the range from 0.4 to 19.5 W cm-2. The proposed energy-level diagram allows us to interpret the recombination processes in TlGaS2 crystals.
Open Access
Low-temperature photoluminescence spectra of layered semiconductor TlGaS2
(Pergamon Press, 1998) Gasanly, N. M.; Aydınlı, Atilla; Bek, A.; Yilmaz, I.
Photoluminescence (PL) spectra of TlGaS2 layered single crystals were studied in the wavelength region 500-860 nm and in the temperature range 9.5-293 K. We observed a total of three PL bands centered at 568 nm (2.183 eV, A-band), 718 nm (1.727 eV, B-band) and 780 nm (1.590 eV, C-band) at various temperatures. We have also studied the variations of the A- and B-band intensities vs excitation laser density in the range from 7 × 10-2 to 9 W cm-2. The A- and B-bands were found to be due to radiative transitions from the deep donor levels located at 0.362 and 0.738 eV below the bottom of the conduction band to the shallow acceptor levels at 0.005 and 0.085 eV located above the top of the valence band, respectively. The proposed energy-level diagram permits us to interpret the recombination processes in TlGaS2 layered single crystals. © 1997 Elsevier Science Ltd.
Open Access
Magnetic and electric Aharonov-Bohm effects in nanostructures
(Elsevier BV, 1996) Kulik, I. O.
The paper reviews and extends the magnetic Aharonov-Bohm effect (persistent current, resistance oscillation) in normal-metal rings including spin-independent and spin-dependent hopping, Zeeman splitting, magnetic textures and wheels, ring rotation and weak coupling, as well as the electric Aharonov-Bohm effect ("persistent charge") in small metallic contacts. We then discuss dynamical screening effects in a surface charge in a metal. Energy dissipation due to motion of the surface charge has a singularity at the velocity of motion equal to the phonon propagation velocity. Surface image of an external charge inside the metal is strongly distorted at the velocity of motion larger than the Fermi velocity.
Open Access
Magnetic-field dependence of low-temperature mobility in quasi-one-dimensional electron systems
(Institute of Physics Publishing Ltd., 1994) Tanatar, Bilal; Constantinou, N. C.
We study the mobility of a quasi-one-dimensional (Q1D) electron system in the presence of an axial magnetic field at low temperatures. We consider the mobility limits for remote-impurity scattering, homogeneous-background scattering, interface-roughness scattering, and alloy-disorder scattering mechanisms. For a system in which all carriers are in the lowest subband, the electron-impurity interaction is modelled for the above cases, and analytic expressions are derived. Calculations appropriate for a GaAs Q1D structure are presented for typical wire radius R, electron density N, impurity density Ni, and applied magnetic field B.
Open Access
Nanosecond sum-frequency generating optical parametric oscillator using simultaneous phase matching
(Optical Society of American (OSA), 2005) Figen, Z.G.; Aytür O.
We report a nanosecond sum-frequency generating optical parametric oscillator based on a single KTiOAsO4 crystal that is simultaneously phase matched for optical parametric generation and sum-frequency generation. Pumped at a wavelength of 1064 nm by a Q-switched Nd:YAG laser, this device produces 10.4-ns-long 8.3 mJ red pulses at a wavelength of 627 nm with 21% energy conversion efficiency. This device provides a simple and efficient method for converting high energy Nd:YAG lasers to a red wavelength. © 2005 Optical Society of America.
Open Access
Polarization of radiation in multipole Jaynes-Cummings model
(Taylor & Francis Ltd, 2002) Can, M.A.; Shumovsky, A.S.
We discuss the spatial properties of quantum radiation emitted by a multipole transition in a single atom. It is shown that the polarization of multipole radiation and quantum fluctuations of polarization change with distance from the source. In the case of a transition specified by a given quantum number m, the quantum noise of polarization contains contributions coming from the modes with m′ ≠ m as well.
Open Access
The role of the interface in germanium quantum dots: when not only size matters for quantum confinement effects
(Royal Society of Chemistry, 2015) Cosentino, S.; Mio, A. M.; Barbagiovanni, E. G.; Raciti, R.; Bahariqushchi, R.; Miritello, M.; Nicotra, G.; Aydınlı, Atilla; Spinella, C.; Terrasi, A.; Mirabella, S.
Quantum confinement (QC) typically assumes a sharp interface between a nanostructure and its environment, leading to an abrupt change in the potential for confined electrons and holes. When the interface is not ideally sharp and clean, significant deviations from the QC rule appear and other parameters beyond the nanostructure size play a considerable role. In this work we elucidate the role of the interface on QC in Ge quantum dots (QDs) synthesized by rf-magnetron sputtering or plasma enhanced chemical vapor deposition (PECVD). Through a detailed electron energy loss spectroscopy (EELS) analysis we investigated the structural and chemical properties of QD interfaces. PECVD QDs exhibit a sharper interface compared to sputter ones, which also evidences a larger contribution of mixed Ge-oxide states. Such a difference strongly modifies the QC strength, as experimentally verified by light absorption spectroscopy. A large size-tuning of the optical bandgap and an increase in the oscillator strength occur when the interface is sharp. A spatially dependent effective mass (SPDEM) model is employed to account for the interface difference between Ge QDs, pointing out a larger reduction in the exciton effective mass in the sharper interface case. These results add new insights into the role of interfaces on confined systems, and open the route for reliable exploitation of QC effects. © The Royal Society of Chemistry.