Physics / Fizik
Permanent URI for this collectionhttps://hdl.handle.net/11147/6
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Conference Object Citation - WoS: 3Citation - Scopus: 3Photometric Multi-Site Campaign on Massive B Stars in the Open Cluster Chi Persei (ngc 884)(IOP Publishing, 2008) Saesen, S.; Pigulski, A.; Carrier, F.; De Ridder, J.; Aerts, C.; Handler, G.; Drummond, R.; Kalomeni, BelindaIn 2005 a photometric observation campaign started on the open cluster chi Persei, involving 13 telescopes spread over the whole northern hemisphere. After two years we gathered almost 1200 hours of data. We present here preliminary results on the variability search, especially from the 60-cm telescope in Bialkow (Poland), which show seven confirmed beta Cephei stars, four candidate B-type pulsators and other interesting variable stars.Article Citation - WoS: 31Citation - Scopus: 40Structural and Magnetic Characterization of Plasma Ion Nitrided Layer on 316l Stainless Steel Alloy(Elsevier Ltd., 2009) Öztürk, Orhan; Okur, Salih; Riviere, Jean PaulIn this study, an FeCrNi alloy (316L stainless steel disc) was nitrided in a low-pressure R.F. plasma at 430 °C for 72 min under a gas mixture of 60% N2-40% H2. Structural, compositional and magnetic properties of the plasma nitrided layer was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and magnetic force microscopy (MFM). The magnetic behaviour of the nitrided layer was also investigated with a vibrating sample magnetometer (VSM). Combined X-ray diffraction, cross-sectional SEM, AFM and MFM, as well as VSM analyses provide strong evidence for the formation of the γN phase, [γN-(Fe, Cr, Ni)], with mainly ferromagnetic characteristics. The uniform nature of the γN layer is clearly demonstrated by the XRD, cross-sectional SEM and AFM analyses. Based on the AFM and SEM data, the thickness of the γN layer is found to be ∼6 μm. According to the MFM and VSM analyses, ferromagnetism in the γN layer is revealed by the observation of stripe domain structures and the hysteresis loops. The cross-sectional MFM results demonstrate the ferromagnetic γN phase distributed across the plasma nitrided layer. The MFM images show variation in the size and form of the magnetic domains from one grain to another.Article Citation - WoS: 18Citation - Scopus: 22Microstructural and Mechanical Characterization of Nitrogen Ion Implanted Layer on 316l Stainless Steel(Elsevier Ltd., 2009) Öztürk, OrhanNitrogen ion implantation can be used to improve surface mechanical properties (hardness, wear, friction) of stainless steels by modifying the near-surface layers of these materials. In this study, a medical grade FeCrNi alloy (316L stainless steel plate) was implanted with 85 keV nitrogen ions to a high fluence of 1 × 1018N2+ / cm2 at a substrate temperature <200 °C in an industrial implantation facility. The N implanted layer microstructures, thicknesses and strengths were studied by a combination of X-ray diffraction (XRD), conversion electron Mössbauer spectroscopy (CEMS), atomic force microscopy (AFM) and nanohardness measurements. AFM was also used for the surface roughness analysis of the implanted as well as polished materials. The CEMS analysis indicate that the N implanted layer is ∼200 nm thick and is composed of ε-(Fe,Cr,Ni)2+xN-like nitride phase with mainly paramagnetic characteristics. The nanohardness measurements clearly indicate an enhanced hardness behaviour for the N implanted layer. It is found that the implanted layer hardness is increased by a factor of 1.5 in comparison to that of the substrate material. The increased hardness resulting from nitrogen implantation is attributed to the formation of ε nitride phase.Conference Object Citation - WoS: 36Citation - Scopus: 40Thermal Management in Large Bi2212 Mesas Used for Terahertz Sources(Institute of Electrical and Electronics Engineers Inc., 2009) Kurter, Cihan; Gray, Kenneth E.; Zasadzinski, John F.; Özyüzer, Lütfi; Koshelev, A. E.; Li, Q.; Yamamoto, T.; Kadowaki, K.; Kwok, W. K.; Tachiki, M.; Welp, U.We present a thermal analysis of a patterned mesa on a Bi 2Sr2CaCu2O8 (Bi2212) single crystal that is based on tunneling characteristics of the c-axis stack of ∼800 intrinsic Josephson junctions in the mesa. Despite the large mesa volume (e.g., 40 × 300 × 1.2 μm3) and power dissipation that result in self-heating and backbending of the current-voltage curve (I-V), there are accessible bias conditions for which significant polarized THz-wave emission can be observed. We estimate the mesa temperature by equating the quasiparticle resistance, Rqp(T), to the ratio V/I over the entire I-V including the backbending region. These temperatures are used to predict the unpolarized black-body radiation reaching our bolometer and there is substantial agreement over the entire I-V. As such, backbending results from the particular R qp (T) for Bi2212, as first discussed by Fenton, rather than a significant suppression of the energy gap. This model also correctly predicts the observed disappearance of backbending above ∼60 K.Conference Object Towards the Solution of Cosmological Constant and Zero Point Energy Problems Through Metric Reversal Symmetry(IOP Publishing Ltd., 2009) Erdem, RecaiIn this talk I review my studies on metric reversal symmetry and their further implications. The talk is mainly concentrated on the relevance of the metric reversal symmetry to the solutions of the cosmological constant and zero point energies. However the use of metric reversal symmetry to hide higher Kaluza-Klein modes at the scales larger than the size of extra dimensions is also discussed, and speculations on its possible relevance to Pauli-Villars and Lee-Wick model are also briefly mentioned.Article Citation - WoS: 150Citation - Scopus: 146Direct Observation of Tetrahertz Electromagnetic Waves Emitted From Intrinsic Josephson Junctions in Single Crystalline Bi2sr2cacu2o8+?(Elsevier Ltd., 2008) Kadowaki, K.; Yamaguchi, H.; Kawamata, K.; Yamamoto, T.; Minami, H.; Kakeya, I.; Welp, U.; Özyüzer, Lütfi; Koshelev, A. E.; Kurter, Cihan; Gray, Kenneth E.; Kwok, W. K.We have observed intense, coherent, continuous and monochromatic electromagnetic (EM) emission at terahertz frequencies generated from a single crystalline mesa structure of the high-Tc superconductor Bi2Sr2CaCu2O8+δ intrinsic Josephson junction system. The mesa is fabricated by the Argon-ion-milling and photolithography techniques on the cleaved surface of Bi2Sr2CaCu2O8+δ single crystal. The frequency, ν, of the EM radiation observed from the sample obeys simple relations: ν = c/nλ = c/2nw and ν = 2eV/hN, where c is the light velocity in vacuum, n the refractive index of a superconductor, λ the wave length of the EM emission in vacuum, w the shorter width of the mesa, V the voltage applied to the mesa, N the number of layers of intrinsic Josephson junctions, e and h are the elementary charge and the Planck constant, respectively. These two relations strongly imply that the mechanism of the emission is, firstly, due to the geometrical resonance of EM waves to the mesa like a cavity resonance occuring in the mesa structure, and forming standing waves as cavity resonance modes, and secondly, due to the ac-Josephson effect, which works coherently in all intrinsic Josephson junctions. The peculiar temperature dependence of the power intensity emitted form samples shows a broad maximum in a temperature region between 20 and 40 K, suggesting that the nonequilibrium effect plays an essential role for the emission of EM waves in this system. The estimated total power is significantly improved in comparison with the previous report [L. Ozyuzer et al., Science 318 (2007) 1291, K. Kadowaki, et al., Physica C 437-438 (2006) 111, I.E. Batov, et al., Appl. Phys. Lett. 88 (2006) 262504], and reached as high as 5 μW from single mesa with w = 60 μm at 648 GHz, which enables us to use it for some of applications. So far, we succeeded in fabricating the mesa emitting EM waves up to 960 GHz in the fundamental mode in the w = 40 μm mesa, whereas the higher harmonics up to the 4-th order were observed, resulting in a frequency exceeding 2.5 THz. In sharp contrast to the previous reports [K. Kadowaki, et al., Physica C 437-438 (2006) 111, M.-H. Bae, et al., Phys. Rev. Lett. 98, (2007) 027002], all the present measurements were done in zero magnetic field. Lastly, a plausible theoretical model for the mechanism of emission is discussed.Article Citation - WoS: 21Citation - Scopus: 22Vacuum Energy as the Origin of the Gravitational Constant(Springer Verlag, 2009) Demir, Durmuş AliWe develop a geometro-dynamical approach to the cosmological constant problem (CCP) by invoking a geometry induced by the energy-momentum tensor of vacuum, matter and radiation. The construction, which utilizes the dual role of the metric tensor that it structures both the spacetime manifold and energy-momentum tensor of the vacuum, gives rise to a framework in which the vacuum energy induced by matter and radiation, instead of gravitating, facilitates the generation of the gravitational constant. The non-vacuum sources comprising matter and radiation gravitate normally. At the level of classical gravitation, the mechanism deadens the CCP yet quantum gravitational effects, if strong, can keep it existent.Conference Object Citation - WoS: 1Selection of the Best Proper Dc-Squids in a Multi-Squid Configuration(IEEE, 2007) Avcı, İlbeyi; Akram, R.; Bozbey, Ali; Tepe, Mustafa; Abukay, DoğanWe have carried out experimental investigation of multi-DC-SQUID magnetometer configuration fabricated on YBa2Cu30 7-δ thin films onto 24 degree SrTiO3 bicrystal substrates by directly coupling the pick-up loop to DC-SQUIDs. The layout of the magnetometer pick-up loop was chosen as a square washer configuration by maximizing loop effective area and minimizing loop inductance. We have used De-Magnetron Sputtering technique for deposition of the films and chemical etching process for patterning the Josephson junctions having 4 μm widths. The use of multi-SQUID configuration is related to the selection of the best proper junctions for SQUID to improve the chip sensitivity with selectivity option of choosing the squid junctions rather than multichannel operation. Selection of the best junctions compared to each other depending on the junction critical currents and noise levels caused by the fabrication process and placements of the junctions on the grain boundary enable having an increased output signal of the DC-SQUID.Conference Object Citation - WoS: 1Citation - Scopus: 1Self-Heating Effect in Intrinsic Tunneling Spectroscopy of Hgbr2 Intercalated Bi2.1sr1.4ca1.5cu 2o8+? Single Crystals(IEEE, 2007) Kurter, Cihan; Özyüzer, Lütfi; Zasadzinski, John F.; Hinks, David G.; Gray, Kenneth E.We report tunneling results in intrinsic Josephson junction (IJJ) stacks fabricated in the form of square micromesas on HgBr2 intercalated Bi2.1Sr1.4Ca1.5Cu2O 8+δ (Bi2212) single crystals using photolithography and Ar ion milling techniques. Self-heating is the most common problem encountered in interlayer tunneling and it is likely to reduce the reliability of IJJ data. Although intercalation reduces heating a hundredfold, it still needs to be minimized substantially in order to approach the authentic superconducting energy gap observed by tunneling using more conventional junctions. We report tunneling characteristics of two mesas with the same height but different sizes (5 × 5 μm2 and 10 × 10 μm2) to show that heating effects are strongly related to IJJ stack size. For the smaller mesa, we observed an energy gap close to that seen in single SIN (S: superconductor, I: insulator, N: normal metal) and SIS break junctions as well as the dip and hump structures at high bias. The subgap data of 5 × 5 μm2 mesa were successfully fit with a momentum averaged d-wave model using convenient parameters. Thus our data is consistent with the predominant pairing symmetry suggested by point contact tunneling, break junction, scanning tunneling microscopy/spectroscopy and angle resolved photoemission measurements in Bi2Sr2CaCu2O8+δ.Conference Object Citation - WoS: 2Citation - Scopus: 2Spin Polarized Current Injection Through Hgbr2 Intercalated Bi2212 Intrinsic Josephson Junctions(Institute of Electrical and Electronics Engineers Inc., 2007) Özyüzer, Lütfi; Kurter, Cihan; Özdemir, Mustafa; Zasadzinski, John F.; Gray, Kenneth E.; Hinks, David G.To investigate the effect of polarized current on tunneling characteristics of intrinsic Josephson junctions (IJJs), spin-polarized and spin-degenerate current have been injected through the c-axis of HgBr2 intercalated Bi2.1Sr1.5Ca1.4Cu2O 8+δ (Bi2212) single crystals on which 10 × 10 μm 2 mesas have been fabricated. These two spin conditions are achieved by depositing either Au (15 nm)/Co (80 nm)/Au (156 nm) multilayers or single Au film on HgBr2 intercalated Bi2212 with Tc = 74 K followed by photolithography and Ar ion beam etching. The I-V characteristics have been measured with and without a magnetic field parallel to c-axis at 4.2 K. A fine, soft Au wire is used to make a gentle mechanical contact on the top of a particular mesa in the array. Tunneling conductance characteristics were obtained and the magnetic field dependence of sumgap voltage peaks was investigated. These peaks do not change in position with increasing magnetic field for both contact configurations. In addition, the temperature dependence of tunneling characteristics of the IJJs are obtained and existence of pseudogap feature is observed above Tc for HgBr2 intercalated Bi2212.
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