Browsing by Subject "Electronic density of states"

Now showing 1 - 12 of 12

Open Access
Alteration of spontaneous emission in hydrogenated amorphous silicon nitride microcavities
(Elsevier BV, 1998) Serpengüzel, A.; Aydınlı, Atilla; Bek, A.
A Fabry-Perot microcavity is used for the alteration of the spontaneous emission spectrum in hydrogenated amorphous silicon nitride. The modified photon density of states of the Fabry-Perot microcavity are responsible for the alteration of the spontaneous emission spectrum. The Fabry-Perot microcavity enhances the intensity of the spontaneous emission signal by a factor of 4 at the photon energies corresponding to the microcavity resonances. The 0.075 eV wide spontaneous emission linewidth of the Fabry-Perot microcavity resonances is 7 times smaller than the 0.5 eV wide spontaneous emission linewidth of the bulk hydrogenated amorphous silicon nitride. © 1998 Elsevier Science B.V. All rights reserved.
Open Access
Bose-Einstein condensation of noninteracting charged Bose gas in the presence of external potentials
(Elsevier Science Publishers B.V., 2001) Bayındır, Mehmet; Tanatar, Bilal
We investigate thermodynamic properties of noninteracting charged bosons in the presence of externally applied electric and magnetic fields. Using the semiclassical density of states, we obtain the condensate fraction, chemical potential, total energy, and specific heat of a system of finite number of charged Bose particles. We conclude that Bose-Einstein condensation of the charged Bose gas occurs in the crossed electric and magnetic fields.
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
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
Electronic structure of half-metallic ferromagnet Co2MnSi at high-pressure
(Springer New York LLC, 2010) Gökoǧlu, G.; Gülseren, O.
In this study, first principles calculation results of the half-metallic ferromagnetic Heusler compound Co2MnSi are presented. All calculations are based on the spin-polarized generalized gradient approximation (σ-GGA) of the density functional theory and ultrasoft pseudopotentials with plane wave basis. Electronic structure of related compound in cubic L21 structure is investigated up to 95 GPa uniform hydrostatic pressure. The half-metal to metal transition was observed around ∼70 GPa together with downward shift of the conduction band minimum (CBM) and a linear increase of direct band gap of minority spins at Γ-point with increasing pressure. The electronic density of states of minority spins at Fermi level, which are mainly due to the cobalt atoms, become remarkable with increasing pressure resulting a sharp decrease in spin polarization ratio. It can be stated that the pressure affects minority spin states rather than that of majority spins and lead to a slight reconstruction of minority spin states which lie below the Fermi level. In particular, energy band gap of minority spin states in equilibrium structure is obviously not destroyed, but the Fermi level is shifted outside the gap.
Open Access
Electronic structure of the contact between carbon nanotube and metal electrodes
(American Institute of Physics, 2003) Dag, S.; Gülseren, O.; Çıracı, Salim; Yildirim, T.
Our first-principles study of the contact between a semiconducting single-walled carbon nanotube ~s-SWNT! and metal electrodes shows that the electronic structure and potential depend strongly on the type of metal. The s-SWNT is weakly side-bonded to the gold surface with minute charge rearrangement and remains semiconducting. A finite potential barrier forms at the contact region. In contrast, the molybdenum surface forms strong bonds, resulting in significant charge transfer and metallicity at the contact. The radial deformation of the tube lowers the potential barrier at the contact and increases the state density at the Fermi level.
Open Access
Electronic structure, insulator-metal transition and superconductivity in K-ET2X salts
(1998) Ivanov V.A.; Ugolkova, E.A.; Zhuravlev, M.Ye.; Hakioǧlu, T.
The electronic structure and superconductivity of layered organic materials based on the bis(ethylenedithio)tetrathiafulvalene molecule (BEDT-TTF, hereafter ET) with essential intra-ET correlations of electrons are analysed. Taking into account the Fermi surface topology, the superconducting electronic density of states (DOS) is calculated for a realistic model of K-ET2X salts. A d-symmetry of the superconducting order parameter is obtained and a relation is found between its nodes on the Fermi surface and the superconducting phase characteristics. The results are in agreement with the measured non-activated temperature dependences of the superconducting specific heat and NMR relaxation rate of central 13C atoms in ET. © 1998 John Wiley & Sons, Ltd.
Open Access
Graphene-based optical modulators
(World Scientific Publishing Co. Pte. Ltd., 2017) Balci S.; Kocabas, C.
In this chapter, we summarize the recent progress on graphene based optical modulators. Ability to control density of high mobility electrons on large area graphene surface enables realization of new type of electrooptical modulators in optoelectronics. Due to the low electronic density of states, accumulation of charges on graphene significantly shifts the Fermi energy up to 1 eV giving rise to profound optical effects in the infrared and visible spectra. On the other hand, graphene operates as a tunable Drude metal in long wavelengths such as THz and microwave. This unique broadband activity of graphene has stimulated a great deal of interest in graphene community due to its potential use in new optoelectronic devices. After discussing the electrically tunable optical properties of graphene, we highlight the key achievements in the field.
Open Access
Many-body properties of a disordered charged Bose gas superlattice
(Academic Press, 2000) Tanatar, Bilal; Das, A. K.
We study some many-body properties of a disordered charged Bose gas (CBG) superlattice-an infinite array of CBG layers each of which containing disorder. The latter is assumed to cause collisions with the charged bosons, the effect of collisions being taken into account through a number-conserving relaxation time approximation incorporated within the random phase approximation (RPA) at T = 0. We go beyond the RPA and include a local-field correction G(q, q z) which is assumed to be collision independent, as an approximation. The resulting density-density correlation function is then used to calculate a number of many-body quantities of physical interest, e.g. (a) collective modes, (b) static structure factor, (c) energy-loss function, (d) plasmon density of states, and (e) groundstate energy. The effects of collisions on these quantities are discussed, and the results are compared with the corresponding results for an electron gas superlattice.
Open Access
A new method of probing the phonon mechanism in superconductors, including MgB2
(2001) Park, M.-A.; Savran, K.; Kim, Y.-J.
Weak localization has a strong influence on both the normal and superconducting properties of metals. In particular, since weak localization leads to the decoupling of electrons and phonons, the temperature dependence of resistance (i.e. λtr) decreases with increasing disorder, as manifested by Mooij's empirical rule. In addition, Testardi's universal correlation of Tc (i.e. λ) and the resistance ratio (i.e. λtr) follows. This understanding provides a new means to probe the phonon mechanism in superconductors, including MgB2. The merits of this method are its applicability to any superconductor and its reliability because the McMillan's electron-phonon coupling constant λ and λtr change in a broad range, from finite values to zero, due to weak localization. Karkin et al's preliminary data of irradiated MgB2 show the Testardi correlation, indicating that the dominant pairing mechanism in MgB2 is a phonon-mediated interaction.
Open Access
Regional model-based computerized ionospheric tomography using GPS measurements: IONOLAB-CIT
(Wiley-Blackwell Publishing, Inc., 2015) Tuna, H.; Arıkan, Orhan; Arikan, F.
Three-dimensional imaging of the electron density distribution in the ionosphere is a crucial task for investigating the ionospheric effects. Dual-frequency Global Positioning System (GPS) satellite signals can be used to estimate the slant total electron content (STEC) along the propagation path between a GPS satellite and ground-based receiver station. However, the estimated GPS-STEC is very sparse and highly nonuniformly distributed for obtaining reliable 3-D electron density distributions derived from the measurements alone. Standard tomographic reconstruction techniques are not accurate or reliable enough to represent the full complexity of variable ionosphere. On the other hand, model-based electron density distributions are produced according to the general trends of ionosphere, and these distributions do not agree with measurements, especially for geomagnetically active hours. In this study, a regional 3-D electron density distribution reconstruction method, namely, IONOLAB-CIT, is proposed to assimilate GPS-STEC into physical ionospheric models. The proposed method is based on an iterative optimization framework that tracks the deviations from the ionospheric model in terms of F2 layer critical frequency and maximum ionization height resulting from the comparison of International Reference Ionosphere extended to Plasmasphere (IRI-Plas) model-generated STEC and GPS-STEC. The suggested tomography algorithm is applied successfully for the reconstruction of electron density profiles over Turkey, during quiet and disturbed hours of ionosphere using Turkish National Permanent GPS Network.
Open Access
Tuning the degree of oxidation and electron delocalization of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) with solid-electrolyte
(Elsevier, 2017-10) Vempati, Sesha; Ertaş, Yelda; Çelebioğlu, Aslı; Uyar, Tamer
We report on the effects of ionic interaction on the electronic structure of PEDOT:PSS where the oxidation state of PEDOT is an import aspect for various applications. Additional ionic interactions are introduced and controlled by varying the fraction of poly(ethylene oxide) (PEO). These interactions are balanced against the inherent cohesive forces within each of the polymers constituting intertwined networks. Raman spectra evidenced a peak-shift as high as ∼14 cm−1 for C[dbnd]C vibrational region which suggested increasing degree of oxidation of PEDOT for higher PEO fractions. Changes to the single and bipolaronic absorption bands support the results from the Raman spectra. For highest PEO fraction neutral-PEDOT and lowered bipolaron density is attributed to localization of PEDOT chains within PEO matrix. Interestingly, for higher PEO fractions the electronic density of states (DOS) of HOMO and core-levels (S2p, C1s and O1s) suggested increased degree of oxidation and electron localization on PEDOT. Near and below (∼12 eV) Fermi level, contribution to the O2p and C2p atomic orbitals depicted significantly different DOS. Also we note energetic shift for O2s/C2s and bonding σCC atomic and molecular DOS, respectively. The correlation between some surface and bulk-related properties suggests the uniformity of the blend material which might be vital for the application in electrochemical devices.