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Publication Open Access A comparative study on GaSb epilayers grown on nominal and vicinal Si(100) substrates by molecular beam epitaxy(Institute of Physics (IOP) Publishing, 2020) Arpapay, Burcu; Suyolcu, Y. Eren; van Aken, Peter A.; Gülgün, Mehmet Ali; Serincan, Uğur; Çorapçıoğlu, Gülcan; n2STAR-Koç University Nanofabrication and Nanocharacterization Center for Scientifc and Technological Advanced ResearchThe direct growth of GaSb buffer layers on Si substrates is attracting considerable interest in the integration of group III-Sb based device structures on lower-cost Si substrates. Here, we present the effect of various growth steps on the defect types and defect density that are crucial for advancing high crystal quality GaSb buffer layer on nominal/vicinal Si substrate. As a growth step, the applied thermal annealing at an intermediate step provided a decrease in the threading dislocation (TD) density down to 1.72 x 10(8) cm(-2), indicating a more effective method compared to post-growth annealing. Additionally, the importance of period number and position of GaSb/AlSb superlattice layers inserted in GaSb epilayers is demonstrated. In the case of the GaSb epilayers grown on vicinal substrates, the APB density as low as 0.06 mu m(-1) and TD density of 1.98 x 10(8) cm(-2) were obtained for the sample grown on 4 degrees miscut Si(100) substrate.
Publication Metadata only A deformation-based approach to tuning of magnetic micromechanical resonators(2018) Yalçınkaya, Arda D.; Department of Mechanical Engineering; N/A; Department of Mechanical Engineering; Biçer, Mahmut; Esfahani, Mohammad Nasr; Alaca, Burhanettin Erdem; Researcher; PhD Student; Faculty Member; Department of Mechanical Engineering; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); College of Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; N/A; 115108Resonance frequency tuning in magnetic micromechanical resonators remains a primary field of study for frequency reference applications. The use of magnetic micromechanical resonators for innovative timing, oscillator and sensing applications necessitates a platform for the precise control of the resonance frequency. The present work addresses a deformation based technique for tuning the resonance frequency of nickel micromechanical resonators. Frequency response is measured through magnetic actuation and optical readout. The tuning approach is based on a combination of flexural deformation and uniaxial strain. The bending deformation is achieved by using a DC current through the microbeam. This magnetomotive mechanism reduces the resonance frequency by about 13% for a maximum DC current of 80 mA. A substrate bending method is used for applying uniaxial strain to increase the resonance frequency by about 8%. A bidirectional frequency modulation is thus demonstrated by utilizing both deformation techniques. The interpretation of results is carried out by finite element analysis and electromechanical analogy in an equivalent circuit. Using deformation techniques, this study provides a rigorous approach to control the resonance frequency of magnetic micromechanical resonators.Publication Open Access A photonic Carnot engine powered by a spin-star network(European Physical Society (EPS), 2017) Türkpençe, Deniz; Paternostro, Mauro; Department of Physics; Altıntaş, Ferdi; Müstecaplıoğlu, Özgür Esat; Researcher; Faculty Member; Department of Physics; College of Sciences; N/A; 1674We propose a spin-star network, where a central spin-(1/2), acting as a quantum fuel, is coupled to N outer spin-(1/2) particles. If the network is in thermal equilibrium with a heat bath, the central spin can have an effective temperature, higher than that of the bath, scaling nonlinearly with N. Such temperature can be tuned with the anisotropy parameter of the coupling. Using a beam of such central spins to pump a micromaser cavity, we determine the dynamics of the cavity field using a coarse-grained master equation. We find that the central-spin beam effectively acts as a hot reservoir to the cavity field and brings it to a thermal steady state whose temperature benefits from the same nonlinear enhancement with N and results in a highly efficient photonic Carnot engine. The validity of our conclusions is tested against the presence of atomic and cavity damping using a microscopic master equation method for typical microwave cavity-QED parameters. The role played by quantum coherence and correlations on the scaling effect is pointed out. An alternative scheme where the spin-(1/2) is coupled to a macroscopic spin-(N/2) particle is also discussed. Copyright (C) EPLA, 2017Publication Metadata only A profound analysis of Rb-2[PH] and Cs-2[PH] and the role of [PH](2-) ions during temperature-induced solid-solid phase transitions(Elsevier, 2014) Hochrein, Oliver; Zahn, Dirk; Department of Chemistry; Department of Chemistry; Somer, Mehmet Suat; Schnering, H. G. Von; Faculty Member; Other; Department of Chemistry; College of Sciences; College of Sciences; 178882; N/AThe temperature-induced solid-solid transformation of Rb-2[PH] and Cs-2[PH] is characterized from both experiment and theory. Neutron diffraction, IR-spectroscopy and ab-initio molecular dynamics simulations reveal an asymmetric shift of the lattice constants at 80 K. The molecular mechanism of the structural transformation as identified from IR-spectroscopy and ab-initio molecular dynamics simulations is closely connected to the orientation of the [PH](2-) moieties which undergo a partial order-disorder phase transition.Publication Metadata only Abbott-Deser energy in connection with Thirring's superpotentials(Institute of Physics (IOP) Publishing, 2018) Baykal, Ahmet; Department of Physics; Dereli, Tekin; Facuty Member; Department of Physics; College of Sciences; 201358We linearize vacuum Einstein field equations with a cosmological constant around a curved background to elaborate on the reconstruction of the Abbott-Deser charges and incorporate a spin connection into the definition using the algebra of differential forms on a given curved background spacetime. Copyright (C) EPLA, 2018Publication Metadata only Analysis of the knight shift data on Li and Zn substituted YBa2Cu3O6+x(Elsevier, 2001) Department of Physics; Bulut, Nejat; Faculty Member; Department of Physics; College of Sciences; N/AThe Knight shift data on Li and Zn substituted YBa2Cu3O6+x are analyzed using an itinerant model with short-range antiferromagnetic correlations. The model parameters, which are determined by fitting the experimental data on the transverse nuclear relaxation rate T-2(-1) of pure YBa2Cu3O6+x, are used to calculate the Knight shifts for various nuclei around a nonmagnetic impurity located in the CuO2 planes. The calculations are carried out for Li and Zn impurities substituted into optimally doped and underdoped YBa2Cu3O6+x. The results are compared with the Li-7 and Y-89 Knight shift measurements on these materials. (C) 2001 Elsevier Science B.V. All rights reserved.Publication Metadata only Bose-einstein condensate in a harmonic trap with an eccentric dimple potential(Institute of Physics (IOP) Publishing, 2008) Uncu, H.; Tarhan, D.; Demiralp, E.; Department of Physics; Müstecaplıoğlu, Özgür Esat; Faculty Member; Department of Physics; College of Sciences; 1674We investigate Bose-Einstein condensation of noninteracting gases in a harmonic trap with an offcenter dimple potential. We specifically consider the case of a tight and deep dimple potential, which is modeled by a point interaction. This point interaction is represented by a Dirac delta function. The atomic density, chemical potential, critical temperature and condensate fraction, and the role of the relative depth and the position of the dimple potential are analyzed by performing numerical calculations.Publication Metadata only Composition dependence of the Judd-Ofelt intensity parameters in TeO 2-PbF 2 : Tm 3+ glasses(Materials Research Society (MRS), 2005) Kabalcı, İdris; Özen, Gönül; Department of Physics; Department of Physics; Sennaroğlu, Alphan; Kurt, Adnan; Faculty Member; Teaching Faculty; Department of Physics; College of Sciences; College of Sciences; 23851; 194455Tm 3+ -doped glasses with the composition of (1-x)TeO 2-xPbF 2, where x=10, 15, 20, 25 mol.% were synthesized and, their thermal and absorption measurements were investigated. All the glasses were transparent. The absorption bands corresponding to the absorption of the 1G 4, 3F 2, 3 3, 3F 4, 3H 4, and 3H 4 levels from the 3H 6 ground level of the Tm 3+ ion were observed in the optical UV/VIS/NIR absorption measurements. We calculated the integrated absorption cross sections of each band except that of 3H 5 level was found to vary with composition of the PbF 2. The absorption measurements were first made to determine the spontaneous emission probabilities of the 4f-4f transitions of the Tm 3+ ions. The calculations were made by using the Judd - Ofelt theory. The Ω 2 parameter shows the strongest dependence on the host composition and it increases with the increasing PbF 2 amount. The values Ω 4 increases rather slowly while the value of Ω 6 is practically independent of the composition. The strong dependence of the parameter Ω 2 indicates that this parameter is related to the structural change and symmetry of the local environment of the Tm 3+ ions in this glass.Publication Metadata only Constrained thermal denaturation of DNA under fixed linking number(De Gruyter, 2012) Bar, Amir; Mukamel, David; Department of Physics; Kabakçıoğlu, Alkan; Faculty Member; Department of Physics; College of Sciences; 49854A DNA molecule with freely fluctuating ends undergoes a sharp thermal denaturation transition upon heating. However, in circular DNA chains and some experimental setups that manipulate single DNA molecules, the total number of turns (linking number) is constant at all times. The consequences of this additional topological invariant on the melting behaviour are nontrivial. Below, we investigate the melting characteristics of a homogeneous DNA where the linking number along the melting curve is preserved by supercoil formation in duplex portions. We obtain the mass fraction and the number of loops and supercoils below and above the melting temperature. We also argue that a macroscopic loop appears at T (c) and calculate its size as a function of temperature.Publication Metadata only Continuous-wave power performance of a 2.47-μm Cr2+:ZnSe laser: experiment and modeling(IEEE-Inst Electrical Electronics Engineers Inc, 2000) Konca, Ali Özgün; Pollock, Clifford R.; Department of Physics; Sennaroğlu, Alphan; Faculty Member; Department of Physics; College of Sciences; 23851Power performance of a continuous-wave Cr2+:ZnSe laser was investigated experimentally and numerically at 2.47 mum, In the experiments, an astigmatically compensated z-cavity with a 3% transmitting output coupler produced as high as 250 mW of output power when excited by a NaCl:OH- color-center laser at 1.58 mum. The measured absorbed power slope efficiency was 24.2%. Experimental pump absorption saturation and laser efficiency data were analyzed by using a numerical model to determine the absorption, emission, and excited-state absorption cross-sections. Simulations were then performed to investigate the dependence of the output power on crystal and resonator parameters. Results indicate that optimum power performance should be obtained with a 1.5 cm-long crystal which has a total unsaturated absorption of 86%, Finally, the optimum output coupling of the resonator was determined to be 10%.