Physics / Fizik

Permanent URI for this collectionhttps://hdl.handle.net/11147/6

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Native and Light Induced Defect States in Wide Band Gap Hydrogenated Amorphous Silicon-Carbon (a-Si1 : H) Alloy Thin Films
(Springer, 1997) Güneş, Mehmet
In this study, wide band gap a-Si1-x C-x:H alloy thin films prepared with and without hydrogen diluation of (SiH4, CH4) were characterized using optical absorption, dark conductivity, steady-state photoconductivity, sub-bandgap absorption obtained with both photothermal deflection spectroscopy (PDS) and dual beam photoconductivity (DBP), and electron spin resonance (ESR) techniques. Experimental results of steady-state photoconductivity and sub-bandgap absorption for different generation rates were analyzed using a detailed numerical model based on Simmons-Taylor statistics. The densities, energy location and nature of the native and light induced defect states in diluted and undiluted a-Si1-xCx:H alloy thin films were derived from the best fits to the experimental data. The extracted parameters for defect states were compared with those of a-Si:H films both in the annealed and light degraded states.
Citation - WoS: 37
Citation - Scopus: 41
Differences in the Densities of Charged Defect States and Kinetics of Staebler-Wronski Effect in Undoped (nonintrinsic) Hydrogenated Amorphous Silicon Thin Films
(American Institute of Physics, 1997-04) Güneş, Mehmet; Wronski, Christopher R.
A variety of undoped (nonintrinsic) hydrogenated amorphous silicon (a-Si:H) thin films was studied in greater detail using steady-state photoconductivity, σph, subband-gap absorption, α(hν), steady-state photocarrier grating (SSPG), and electron-spin-resonance (ESR) techniques both in the annealed and stabilized light soaked states. The experimental results were self-consisiently modeled using a detailed numerical analysis. It was found that large differences in the optoelectronic properties of device quality a-Si:H thin films can only be explained using a gap slate distribution which consists of positively charged D+ defect states above the Fermi level, the neutral D0 defect states, and the negatively charged D- defect states below the Fermi level. There are large differences both in the densities of neutral and charged defect states and R ratios in different a-Si:H films in the annealed state. The densities of both neutral and charged defect states increased, however, R ratios decreased in the stabilized light soaked state. Very good agreement was obtained between the densities of neutral defect states measured by ESR and those derived from the numerical analysis in the stabilized light soaked state. The kinetics of the Staebler-Wronski effect was also investigated. There was no direct correlation between the decrease of steady-state photoconductivity and increase of subband-gap absorption. The self-consistent fits to wide range of experimental results obtained with the three Gaussian distributions of charged defect states imply that this model is much better representation of the bulk defect states in undoped hydrogenated amorphous silicon thin films.
Citation - WoS: 5
Citation - Scopus: 7
Higgs Field as the Gauge Field Corresponding To Parity in the Usual Space-Time
(World Scientific Publishing Co. Pte Ltd, 1998-02) Erdem, Recai
We find that the local character of field theory requires the parity degree of freedom of the fields to be considered as an additional discrete fifth dimension which is an artifact emerging due to the local description of space-time. Higgs field can be interpreted as the gauge field corresponding to this discrete dimension. Hence the noncommutative geometric derivation of the standard model follows as a manifestation of the local description of the usual space-time.
1/Nc Expansion and Anomaly Cancellation in the Presence of Electroweak Interactions
(IOP Publishing Ltd., 1998-03) Erdem, Recai
We study the question of a consistent formulation of the 1/Nc expansion in the presence of electroweak interactions. We show that in some cases the previous formulation leads to an unrealistic picture. We improve the scheme. We derive the corresponding hypercharge and electric charge values of fermions under the requirement that the standard model in the large-Nc limit should be free of chiral gauge anomalies. We find that the resulting hypercharge and the electric charge values for quarks are the same as for the standard model.
Citation - WoS: 15
Citation - Scopus: 14
1/F-noise Study of Undoped Intrinsic Hydrogenated Amorphous Silicon Thin Films
(American Physical Society, 1999) Güneş, Mehmet; Johanson, Robert E.; Kasap, Safa O.
Conductance fluctuations in four samples of undoped intrinsic hydrogenated amorphous silicon (a-Si:H) were measured in the temperature range of 450 K to 500 K and for frequencies from 2 Hz to 3 kHz. The noise spectra divide into two regions that each fit a 1/fα power law but with different slope parameters α and different temperature dependences. At low frequencies, α is greater than unity and increases with temperature. At high frequencies, α is near 0.6 and temperature independent, but the noise magnitude decreases rapidly with temperature. We infer from the different dependences on temperature that the noise is generated by two independent mechanisms operating simultaneously in a-Si:H. We also observe that the 1/f noise exhibits a quadratic dependence on bias current and Gaussian statistics.
Citation - WoS: 1
Citation - Scopus: 1
Modification of Al-Oxide Tunnel Barriers With Organic Self-Assembled Monolayers
(American Institute of Physics, 1999-05) Okur, Salih; Zasadzinski, John F.
Al-oxide tunneling barriers were modified by exposure to a vapor of n-octadecyltrimethoxysilane which forms self-assembled monolayers. The dynamic conductance dI/dV of the modified Al-oxide barrier between Al and Pb electrodes was measured at 4.2 K. Quasilinear conductance backgrounds are observed up to 200 mV with a strength that increases with increasing exposure time from 10 to 60 min. A saturation effect is observed around 200 mV. Beyond 200 mV the dynamic conductance shows a parabolic behavior indicative of elastic tunneling from an asymmetric barrier. The linear background is attributed to inelastic tunneling from a continuum of excitations
Citation - WoS: 4
Citation - Scopus: 4
Simultaneous Quasiparticle and Josephson Tunneling in Bscco-2212 Break Junctions
(Institute of Electrical and Electronics Engineers Inc., 1999-06) Özyüzer, Lütfi; Miyakawa, Nobuaki; Zasadzinski, John F.; Yusof, Zikri M.; Romano, Pierom; Kendziora, Christopher A.; Hinks, David G.; Gray, Kenneth E.
Tunneling measurements are reported for superconductor-insulator-superconductor (SIS) break junctions on underdoped, optimally-doped, and overdoped single crystals of tSrsCaCiOs-).,! (Bi2212). The junction I -V characteristics exhibit welldefined quasiparticle current jumps at eV = 2A as well as hysteretic Josephson currents. The quasiparticle branch has been analyzed in the framework of dxa_y2 (d-wave) superconductivity and indicates that there is preferential tunneling along the lobe directions of the d-wave gap. For overdoped Bi-2212 with TC-62 K, the Josephson current is measured as a function of junction resistance, Rn, which varied by two orders of magnitude (1 kO to 100 kO). IcRn product is proportional to the 0.47 power of /c and displays a maximum of 7.0 mV. When the hole doping is decreased from overdoped (Tc=62 K) to the underdoped regime (Tc=70 K), the average IcRn product increases as does the quasiparticle gap. The maximum IcRn is ~ 40% of the A/e at each doping level, with a value as high as 25 mV in underdoped Bi-2212.
Citation - WoS: 18
Citation - Scopus: 17
Tunneling Spectroscopy of Tl2ba2cuo6
(Elsevier Ltd., 1999-07) Özyüzer, Lütfi; Yusof, Zikri; Zasadzinski, John F.; Li, Ting-Wei; Hinks, David G.; Gray, Kenneth E.
New results from tunneling spectroscopies on near optimally doped single crystals of Tl2Ba2CuO6 (Tl-2201) junctions are presented. The superconductor-insulator-normal metal (SIN) tunnel junctions are obtained using the point-contact technique with a Au tip. The tunneling conductances reproducibly show a sharp cusp-like subgap, prominent quasiparticle peaks with a consistent asymmetry, and weakly decreasing backgrounds. A rigorous analysis of the SIN tunneling data is performed using two different models for the dx(2)-y(2) (d-wave) density of states (DOS). Based on these and earlier results, the tunneling DOS of Tl-2201 has exhibited the most reproducible data that are consistent with a d-wave gap symmetry. We show that the dip feature at 2 Δ that is, clearly seen in SIN tunneling data of Bi2Sr2CaCu2O8+δ is also present in Tl-2201, but at a weaker level. The gap values for crystals with a bulk Tc = 86 K are in the range of 19-25 meV.
Citation - WoS: 200
Citation - Scopus: 211
Predominantly Superconducting Origin of Large Energy Gaps in Underdoped Bi2sr2cacu2o8+? From Tunneling Spectroscopy
(American Physical Society, 1999-08) Miyakawa, Nobuaki; Zasadzinski, John F.; Özyüzer, Lütfi; Guptasarma, Prasenjit; Hinks, David G.; Kendziora, Christopher A.; Gray, Kenneth E.
New tunneling data are reported in Bi2Sr2CaCu2O8+δ which show quasiparticle excitation gaps, Δ, reaching values as high as 60 meV for underdoped crystals with Tc = 70 K. These energy gaps are nearly 3 times larger than those of overdoped crystals with similar Tc. Despite the large differences in gap magnitude, the tunneling spectra display qualitatively similar characteristics over the entire doping range. Detailed examination of the spectra, including the Josephson IcRn product measured in break junctions, indicates that these energy gaps are predominantly of superconducting origin.
Citation - WoS: 16
Citation - Scopus: 17
Superconducting Gap and Pseudogap From Tunneling Conductance on Bi2sr2cacu2og+δ With Various Oxygen Concentration
(Elsevier, 2000) Miyakawa, N.; Zasadzinski, J.F.; Ozyuzer, L.; Guptasarma, P.; Kendziora, C.; Hinks, D.G.; Gray, K.E.
The evolution of tunneling spectra on Bi2Sr2CaCu2Og+δ measured by SIN point contacts, SIS break junctions and STM/STS has been studied as a function of doping and temperature. The detailed examinations of the spectra show that the energy gap measured at low temperature appear to be due to superconducting pairing. The doping dependence of the superconducting gap at low temperature suggests a strong connection to the pseudogap temperature, T*, and this indicates that the pseudogap region is at least partly a consequence of some form of precursor superconductivity. In addition, dip/hump structures observed at high bias scale approximately with not only Δ but also superexchange interaction J over the entire doping range examined, indicating these features are linked to the underlying interaction responsible for superconductivity. We suggest that the hump structure may originate from short-range magnetic correlations. © 2000 Elsevier Science B.V All rights reserved.
Citation - WoS: 53
Quasiparticle and Josephson Tunneling of Overdoped Bi2sr2cacu2o8+? Single Crystals
(American Physical Society, 2000-02) Özyüzer, Lütfi; Zasadzinski, John F.; Kendziora, Christopher A.; Gray, Kenneth E.
The point contact tunneling technique is used to examine quasiparticle and Josephson currents in overdoped Bi2Sr2CaCu2O8+δ (Bi-2212) single crystals with bulk Tc values ranging from 82 K down to 62 K. Superconductor-insulator-normal-metal (SIN) tunnel junctions are formed between Bi-2212 crystals and a Au tip, which display well-resolved quasiparticle gap features including sharp conductance peaks. Reproducible superconductor-insulator-superconductor (SIS) tunnel junctions are also obtained between two pieces of the Bi-2212 crystals, resulting in simultaneous quasiparticle and Josephson currents. The dynamic conductances of both SIN and SIS junctions are qualitatively similar to those found on optimally doped Bi-2212, but with reduced gap values, e.g., Δ = 15-20 meV for Tc = 62 K. Fits to the conductance data in the gap region are obtained using a model with dx2-y2 symmetry, and it is shown that this provides a better fit than s-wave symmetry. Both SIN and SIS tunneling conductances also display dip and hump features at high bias voltages similar to those found on optimal and underdoped crystals, indicating that these are intrinsic properties of the quasiparticles. The SIS data indicate that these features appear to be part of a larger spectrum that extends out to 300-400 mV. The Josephson current has been measured for 13 SIS junctions on the 62 K crystals with resistances varying over two decades. It is found that the maximum value depends on junction resistance in a manner consistent with Ambegaokar-Baratoff theory, but with a reduced IcRn product.
Citation - WoS: 10
Citation - Scopus: 7
1/F Noise in Doped and Undoped Amorphous Silicon
(Elsevier Ltd., 2000-05) Johanson, Robert E.; Güneş, Mehmet; Kasap, Safa O.
We measured the spectrum of conductance fluctuations in n-type, p-type, and undoped hydrogenated amorphous silicon (a-Si:H) as a function of temperature. In general, the spectra can be fit to a power law, 1/fα, although in the p-type and undoped samples deviations from a strict power law occur. For n-type and p-type samples, the noise magnitude increases with temperature by approximately a factor of 5 from 295 to 450 K. The slope parameter, α, also increases with temperature in the p-type samples from near unity to 1.4 but not in the n-type sample where it remains near 1.05 independent of temperature. The undoped sample could be measured only over a limited range of elevated temperatures, but α does trend larger. The undoped and lightly doped material have similar noise levels but larger p-type doping reduces the noise by two orders of magnitude. Correlation measurements indicate the 1/f noise is Gaussian for all samples. However, intermittent random-telegraph noise is observed in n-type material.
Citation - WoS: 4
Citation - Scopus: 4
Conductance Fluctuations in Undoped Intrinsic Hydrogenated Amorphous Silicon Films Prepared Using Several Deposition Techniques
(Elsevier Ltd., 2000-05) Güneş, Mehmet; Johanson, Robert E.; Kasap, Safa O.
Coplanar conductance fluctuations in a range of device quality undoped hydrogenated amorphous silicon (a-Si:H) films prepared using different deposition systems were measured in the temperature range of 440-505 K for frequencies from 2 Hz to 3 kHz. The 1/fa type noise spectra had two different power law dependencies, one at lower frequencies with slope α1 close to unity and a second region at higher frequencies with slope α2 around 0.60. The noise power density decreases with increasing temperature in the high frequency region, but only increases much less with temperature at low frequencies. The results indicate that the noise in undoped intrinsic a-Si:H films is due to two independent noise mechanisms operating simultaneously
Citation - WoS: 14
Citation - Scopus: 14
High Energy Secondary Peak Structure in Tunneling Spectra (hump) as Possible Magnetic Pseudogap
(Elsevier Ltd., 2000-11) Zasadzinski, John F.; Özyüzer, Lütfi; Miyakawa, Nobuaki; Hinks, David G.; Gray, Kenneth E.
It is demonstrated that tunneling spectra in various high Tc cuprates display generic features. The principal conductance peaks in superconductor-insulator-normal metal (SIN) junctions indicate the superconducting gap in the density of states (DOS), Higher energy features include a dip and hump structure with a strength that is asymmetric in bias voltage. The dip and hump features follow the doping trends of the superconducting gap, Δ, with a rough scaling as ∼2Δ and ∼3Δ respectively. Tunneling spectra in underdoped Bi2Sr2CaCu2O8+δ display a more pronounced hump feature suggestive of a second gap in the DOS. It is observed that the hump feature in the tunneling density of states is consistent with other experimental observations of the so-called high energy pseudogap which may have magnetic origins
Citation - WoS: 3
Citation - Scopus: 3
Tunneling Spectroscopy of Heavily Underdoped Crystals of Bi2sr2cacu2o8-?
(Elsevier Ltd., 2000-11) Özyüzer, Lütfi; Zasadzinski, John F.; Miyakawa, Nobuaki; Kendziora, Christopher A.; Sha, J.; Hinks, David G.; Gray, Kenneth E.
Crystals of Bi2Sr2CaCu2O8+δ with optimal Tc=95 K have been underdoped using two different methods and the superconducting gaps have been obtained by tunneling. In some cases, three different tunneling geometries have been utilized: point contact, STM and break junctions. The first doping method involves control of the oxygen content by annealing in various partial pressures of oxygen. These crystals exhibit a narrow spread of gap values over a wide doping range from overdoped (Tc=56 K) to underdoped with Tc=70 K. However, for underdoped crystals with Tc midpoints in the range 25 K - 63 K, there is a dramatic increase in the spread of gap values which may signal the development of static phase separation of either chemical or electronic origin. To avoid possible chemical phase separation, we have explored another doping procedure which incorporates Dy substitution on the Ca site. These crystals exhibit a relatively narrow superconducting transition width and some preliminary tunneling spectra will be presented.
Can Cpt Be Violated Through Extended Time Reversal?
(World Scientific Publishing, 2001) Erdem, Recai; Ufuktepe, Ünal
We consider the implications of the extension of time reversal through Wigner types and group extensions. We clarify its physical content and apply the results in a toy model. Finally we point out the possibility of violation of CPT in this framework.
Citation - WoS: 149
Citation - Scopus: 6
Correlation of Tunneling Spectra in Bi2sr2cacu2o8+? With the Resonance Spin Excitation
(American Physical Society, 2001-08) Zasadzinski, John F.; Özyüzer, Lütfi; Miyakawa, Nobuaki; Gray, Kenneth E.; Hinks, David G.; Kendziora, Christopher A.
New break-junction tunneling data are reported in Bi2Sr2CaCu2O8+δ over a wide range of hole concentration from underdoped (Tc=74k) to optimal doped (Tc=95k) to overdoped (Tc=48k). The conductances exhibit sharp dips at a voltage, Ω/e, measured with respect to the superconducting gap. Clear trends are found such that the dip strength is maximum at optimal doping and that Ω scales as 4.9kTc over the entire doping range. These features link the dip to the resonance spin excitation and suggest quasiparticle interactions with this mode are important for superconductivity.
Citation - WoS: 22
Citation - Scopus: 23
Implications of Tunneling Studies on High-Tc Cuprates: Superconducting Gap and Pseudogap
(Elsevier Ltd., 2001-11) Miyakawa, Nobuaki; Zasadzinski, John F.; Oonuki, S.; Asano, M.; Henmi, D.; Kaneko, Tsutomu; Özyüzer, Lütfi; Gray, Kenneth E.
Tunneling spectra have been measured on high-Tc cuprates including single crystals Bi2Sr2-xLaxCuO6+δ (Bi2201) and Bi2Sr2CaCu2O8+δ (Bi2212) using superconductor-insulator-normal metal point contact or superconductor-insulator-superconductor break junction methods. The doping dependence of the energy gap parameter is similar in both Bi2212 and Bi2201, increasing monotonically to very large values in the underdoped regime even as Tc decreases. This doping dependence of superconducting gap is similar to that of pseudogap temperature, T*, indicating this is consistent with the scenario whereby the low-energy pseudogap is due to some type of precursor of superconductivity. The high-energy feature observed as the hump structure may be another kind of pseudogap whose energy scale is much larger than superconducting gap, and it may be magnetic in origin.
Citation - WoS: 13
Noise in Hydrogenated Amorphous Silicon
(Institute of Electrical and Electronics Engineers, 2002) Johanson, Robert E.; Güneş, Mehmet; Kasap, Safa O.
Published work on conductance fluctuations in hydrogenated amorphous silicon is surveyed. There are many reports of 1/f noise. some describing unusual features Such as non-Gaussian statistics. The relative insensitivity to doping and temperature is highlighted. In addition to the 1/f noise. random-telegraph-like noise is often reported. The successes and failures of generation-recombination models for 1 f noise and current filament models for the telegraph noise are summarised.
Citation - WoS: 4
Citation - Scopus: 5
Conductance Fluctuations in Undoped Hydrogenated Amorphous Silicon-Germanium Alloy Thin Films
(Elsevier Ltd., 2002-04) Güneş, Mehmet; Johanson, Robert E.; Kasap, Safa O.; Yang, Jeffrey C.; Guha, Subhendu
We report coplanar conductance fluctuations of device quality, undoped hydrogenated amorphous silicon-germanium alloy thin films (a-SiGe:H) measured from 430 to 490 K. The a-SiGe:H alloys produce noise power spectra similar to coplanar undoped a-Si:H films in the same temperature range. The noise power spectrum S(n) does not fit a single 1/fα power law but rather has two distinct regions, each accurately fitted by a power law, but with different slopes. The low frequency slope α1 is similar to that observed in undoped a-Si:H films varying from 1.30 to 1.46 for different Ge concentrations and shows a slight temperature dependence. At higher frequencies, the slope α2 is less than unity and temperature independent but depends on the Ge content of the film. α2 decreases from 0.60 for no Ge (pure a-Si:H) to 0.15 for 40 at.% Ge. The noise power at lower frequencies increases and at higher frequencies decreases substantially as the temperature increases from 430 to 490 K. We infer that similar noise mechanisms are operating in undoped a-SiGe:H and a-Si:H films but that the Ge content is influencing the noise, particularly the slope at higher frequencies. In addition, the noise has the expected quadratic dependence on bias current and obeys Gaussian statistics.

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