Browsing by Author "Güven, K."
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Item Open Access Band-gap renormalization in quantum wire systems: dynamical correlations and multi-subband effects(Institute of Physics Publishing, 2000) Güven, K.; Tanatar, Bilal; Bennett, C. R.We study the band-gap renormalization m a model semiconductor quantum wire due to the exchange-correlation effects among the charge carriers. We construct a two-subband model for the quantum wire, and employ the GW-approximation to obtain the renormalized quasi-particle energies at the optical band edge. The renormalization is calculated as a function of electron-hole plasma density and the wire radius. Our results show that the very presence of the second subband affects the renormalization process even in the absence of occupation by the carriers. We compare the fully dynamical random-phase approximation results to the quasi-static case in order to emphasize the dynamical correlation effects. Effects of electron-phonon interaction within the two-subband model are also considered.Item Open Access Confined optical phonon effects on the band gap renormalization in quantum wire structures(Elsevier Science, 1999) Bennett, C. R.; Güven, K.; Tanatar, BilalWe consider the different approximations for the bandgap renormalization (BGR) within the random phase approximation (RPA), the quasi-static limit and the plasmon-pole approximation, and compare with the full result. We then include bulk optical phonons and also the phonon confinement using the phonons from the dielectric continuum (DC) model. We show that the results are very similar except at low densities where the quasi-static results overestimate the renormalization.Item Open Access Confined-phonon effects in the band-gap renormalization of semiconductor quantum wires(American Physical Society, 1998) Bennett, C. R.; Güven, K.; Tanatar, BilalWe calculate the band-gap renormalization in quasi-one-dimensional semiconductor quantum wires including carrier-carrier and carrier-phonon interactions. We use the quasistatic approximation to obtain the self-energies at the band edge that define the band-gap renormalization. The random-phase approximation at finite temperature is employed to describe the screening effects. We find that confined LO-phonon modes through their interaction with the electrons and holes modify the band gap significantly and produce a larger value than the static ∈0 approximation.Item Open Access Coupled plasmon-phonon mode effects on the Coulomb drag in double-quantum-well systems(American Physical Society, 1997) Güven, K.; Tanatar, BilalWe study the Coulomb drag rate for electrons in a double-quantum-well structure taking into account the electron-optical phonon interactions. The full wave vector and frequency dependent random-phase approximation (RPA) at finite temperature is employed to describe the effective interlayer Coulomb interaction. The electron-electron and electron-optical phonon couplings are treated on an equal footing. The electron-phonon mediated interaction contribution is investigated for different layer separations and layer densities. We find that the drag rate at high temperatures (i.e., T≥0.2EF) is dominated by the coupled plasmon-phonon modes of the system. The peak position of the drag rate is shifted to the low temperatures with a slight increase in magnitude, compared to the uncoupled system results in RPA. This behavior is in qualitative agreement with the recent measurements. Including the local-field effects in an approximate way we also estimate the contribution of intralayer correlations.Item Open Access Effect of disorder on magnetic resonance band gap of split-ring resonator structures(Optical Society of American (OSA), 2004) Aydın, K.; Güven, K.; Katsarakis, N.; Soukoulis, C. M.; Özbay, EkmelWe investigated the influence of periodicity, misalignment, and disorder on the magnetic resonance gap of split-ring resonators (SRRs) which are essential components of left handed-metamaterials (LHMs). The resonance of a single SRR which is induced by the split is experimentally demonstrated by comparing transmission spectra of SRR and closed ring resonator. Misaligning the SRR boards do not affect the magnetic resonance gap, while destroying the periodicity results in a narrower band gap. The disorder in SRR layers cause narrower left-handed pass band and decrease the transmission level of composite metamaterials (CMMs), which may significantly affect the performance of these LHMs. © 2004 Optical Society of America.Item Open Access Experimental demonstration of a left-handed metamaterial operating at 100 GHz(American Physical Society, 2006) Gökkavas, M.; Güven, K.; Bulu, I.; Aydın, K.; Penciu, R. S.; Kafesaki, M.; Soukoulis, C. M.; Özbay, EkmelThe existence of a left-handed (LH) transmission band in a bulk composite metamaterial (CMM) around 100 GHz is demonstrated experimentally. The CMM consists of stacked planar glass layers on which periodic patterns of micron-scale metallic wires and split-ring resonators are fabricated. The LH nature of the CMM band is proved by comparing the transmission spectra of individual CMM components. Theoretical investigation of the CMM by transmission simulations and an inversion scheme for the retrieval of the effective permeability and permittivity functions supports the existence of LH behavior. © 2006 The American Physical Society.Item Open Access Local current distribution at large quantum dots (QDs): A self-consistent screening model(Elsevier B.V., 2008) Krishna, P. M.; Siddiki, A.; Güven, K.; Hakioǧlu T.We report the implementation of the self-consistent Thomas-Fermi screening theory, together with the local Ohm's law to a quantum dot system in order to obtain local current distribution within the dot and at the leads. We consider a large dot (size > 700 nm) defined by split gates, and coupled to the leads. Numerical calculations show that the non-dissipative current is confined to the incompressible strips. Due to the non-linear screening properties of the 2DES at low temperatures, this distribution is highly sensitive to external magnetic field. Our findings support the phenomenological models provided by the experimental studies so far, where the formation of the (direct) edge channels dominate the transport.Item Open Access Phonon renormalization effects in photoexcited quantum wires(American Physical Society, 1995) Güven, K.; Tanatar, BilalWe study the effects of screening on polaronic corrections to the effective band edge in a quasi-one-dimensional GaAs quantum wire. We find that the screening effects and finite well width considerably reduce the polaron energy and oppose the polaronic band-gap renormalization. We calculate the polaronic effective mass as a function of the carrier density and temperature. Effects of the vertex corrections to the conduction- and valence-band edges are also discussed. © 1995 The American Physical Society.Item Open Access Raman scattering from confined phonons in GaAs/AlGaAs quantum wires(Academic Press, 1998) Bairamov, B. H.; Aydınlı, Atilla; Tanatar, Bilal; Güven, K.; Gurevich, S.; Mel'tser, B. Ya.; Ivanov, S. V.; Kop'ev, P. S.; Smirnitskii, V. B.; Timofeev, F. N.We report on photoluminescence and Raman scattering performed at low temperature (T = 10 K) on GaAs/Al 0.3Ga 0.7As quantum-well wires with effective wire widths of L = 100.0 and 10.9 nm prepared by molecular beam epitaxial growth followed by holographic patterning, reactive ion etching, and anodic thinning. We find evidence for the existence of longitudinal optical phonon modes confined to the GaAs quantum wire. The observed frequency at ω L10 = 285.6 cm -1 for L = 11.0 nm is in good agreement with that calculated on the basis of the dispersive dielectric continuum theory of Enderlein† as applied to the GaAs/Al 0.3Ga 0.7As system. Our results indicate the high crystalline quality of the quantum-well wires fabricated using these techniques. © 1998 Academic Press.Item Open Access Screening effects on the confined and interface polarons in cylindrical quantum wires(American Physical Society, 1996) Tanatar, Bilal; Güven, K.; Bennett, C. R.; Constantinou, N. C.We study the contribution of confined and interface phonons to the polaron energy in quantum-well wires. We use a dispersionless, macroscopic continuum model to describe the phonon confinement in quantum wires of circular cross section. Surface phonon modes of a free-standing wire and interface phonon modes of a wire embedded in a dielectric material are also considered. Polaron energy is calculated by variationally incorporating the dynamic screening effects. We find that the confined and interface phonon contribution to the polaron energy is comparable to that of bulk phonons in the density range N=105-107 cm-1. Screening effects within the random-phase approximation significantly reduce the electron-confined phonon interaction, whereas the exchange-correlation contribution tends to oppose this trend at lower densities.Item Open Access A self-consistent microscopic model of Coulomb interaction in a bilayer system as an origin of Drag Effect Phenomenon(Elsevier B.V., 2008) Güven, K.; Siddiki, A.; Krishna, P. M.; Hakioǧlu T.In this work we implement the self-consistent Thomas-Fermi model that also incorporates a local conductivity model to an electron-electron bilayer system, in order to describe novel magneto-transport properties such as the Drag Phenomenon. The model can successfully account for the poor screening of the potential within the incompressible strips and its impact on the inter-layer Coulomb interaction. An externally applied current in the active layer results in the tilting of the Landau levels and built-up of a Hall potential across the layer, which, in turn, induces a tilted potential profile in the passive layer as well. We investigate the effect of the current intensity, temperature, magnetic field, and unequal density of layers on the self-consistent density and potential profiles of the bilayer system.Item Open Access Simplified calculations of band-gap renormalization in quantum-wells(Academic Press, 1996) Güven, K.; Tanatar, BilalNon-linear optical properties of photoexcited semiconductor quantum-wells are of interest because of their opto-electronic device application possibilities. Many-body interactions of the optically created electrons and holes lead to the band-gap renormalization which in turn determines the absorption spectra of such systems. We employ a simplified approach to calculate the band-gap renormalization in quantum-well systems by considering the interaction of a single electron-hole pair with the collective excitations (plasmons). This method neglects the exchange-correlation effects but fully accounts for the Coulomb-hole term in the single-particle self-energy. We demonstrate that the density, temperature, and well-width dependence of the band-gap renormalization for GaAs quantum-wells within our model is in good agreement with the experimental results. © 1996 Academic Press Limited.Item Open Access Simulation and micro-fabrication of optically switchable split ring resonators(Elsevier BV, 2007-10) Gundogdu, T. F.; Gökkavas, M.; Güven, K.; Kafesaki, M.; Soukoulis, C. M.; Özbay, EkmelThe effect of conductivity variation as a proposed method for the investigation of photoconductive switching properties of split ring resonators (SRRs) is simulated. Three different systems that are applicable under certain fabrication and/or optical excitation conditions are described. The simulated transmission spectrum indicates that for a large range of dark conductivity values, complete switching of the SRR resonance is possible. One of the simulated systems, involving split ring resonators on Si substrate, was fabricated and characterized. The transmission spectrum of that system was measured, with the Si in its high-resistivity state, and a 60 dB dip between 108 and 115 GHz, due to SRRs magnetic resonance, was observedItem Open Access Transmission properties of various split-ring resonator systems(Optical Society of America, 2006) Aydın, Koray; Bulu I.; Güven, K.; Özbay, EkmelWe investigated the magnetic resonance of split-ring resonators (SRR) experimentally. The dependence of the geometrical parameters on the resonance frequency of SRR is studied. We further investigated the effect of disorder on performance of SRRs. © 2006 Optical Society of America.Item Open Access Variational approach for phonon renormalization effects in photoexcited quantum wires and quantum wells(Wiley - V C H Verlag, 1996) Güven, K.; Tanatar, BilalWe investigate the effects of screening on polaronic corrections to the effective band edge in photoexcited quasi-one-dimensional GaAs quantum wires and two-dimensional quantum wells. We develop a variational method to calculate the polaron energy of a two-component plasma (electrons and holes) coupled to LO-phonons. Screening effects are incorporated within a dynamical scheme. We find that the screening effects and finite well width considerably reduce the polaron energy as the plasma density increases. Many-body corrections beyond the random-phase approximation are also considered.