Browsing by Subject "Oscillation"
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Item Open Access Describing droplet motion on surface-textured ratchet tracks with an inverted double pendulum model(American Chemical Society, 2021-04-27) Naji, Mayssam; Yelekli Kirici, Ecem; Javili, Ali; Erdem, Emine YeganWe describe the motion of a droplet on a textured ratchet track using a nonlinear resonator model. A textured ratchet track is composed of a semicircular pillar array that induces a net surface tension local gradient on a droplet placed on it. When a vertical vibration is applied, hysteresis is overcome, and the droplet moves toward the local lower energy barrier; however, due to the repetitive structure of texture, it keeps moving until the end of the track. The droplet motion depends on the amplitude and frequency of the vertical oscillation, and this dependence is nonlinear. Therefore, finding a fully analytic solution to represent this motion is not trivial. Consequently, the droplet motion remains poorly understood. In this study, we elaborate on the utility of a double pendulum as a basis for modeling the droplet motion on surfaces inducing asymmetric force. Similar to the droplet motion, resonators, such as a double pendulum, are simple, yet nonlinear systems. Moreover, an inverted double pendulum motion has key characteristics such as the two-phase motion and the double peak motion, which are also observed in the droplet motion. We use various data-processing methods to highlight the similarity between these two systems both qualitatively and quantitatively. After establishing this comparison, we propose a model that utilizes an inverted double pendulum mounted on a moving cart to successfully simulate the motion of a droplet on a ratchet track. This methodology will lead to the development of an accurate droplet-motion modeling approach, and we believe that it will be useful to understand droplet dynamics more deeply.Item Open Access Dynamic correlation effects on the plasmon dispersion in a two-dimensional electron gas(The American Physical Society, 2003) Yurtsever, A.; Moldoveanu, V.; Tanatar, BilalThe charge-density oscillations (plasmons) of a low-density two-dimensional uniform electron gas are studied within the framework of finite temperature and frequency dependent (dynamic) version of Singwi, Tosi, Land, and Sjölander theory and compared with the recent experimental results. The use of the Hartree-Fock approximation for the static structure factor leads to a finite temperature dynamical counterpart of the static Hubbard approximation. We observe important differences between dynamic and static local-field factors as well as between the corresponding plasmon dispersion laws. Our calculated plasmon energies that include dynamic correlations are in very-good agreement with the recent experimental results.Item Open Access Dynamic correlations in double-layer electron systems(The American Physical Society, 2001) Tanatar, Bilal; Davoudi, B.We study the effects of dynamic correlations on the ground-state properties of a double-layer two-dimensional electron gas within the quantum Singwi-Tosi-Land-Sjölander theory (STLS). The intralayer and interlayer static structure factors, the pair-correlation functions, and the wave vector and frequency-dependent local-field factors have been calculated for a range of electron densities and layer separations. We find that the local-field factors have an oscillatory frequency dependence and the magnitude of interlayer local-field factors is about an order of magnitude smaller than that of the intralayer. Our results are compared with the random-phase approximation and the static STLS approximation to assess the importance of dynamical correlations. We also calculate the dispersion relations for the optical and acoustic plasmons and the damping of these modes to compare them with other mean-field theories, and we comment on the relevance of our results to the recent experiments.Item Open Access Enhancing higher harmonics of a tapping cantilever by excitation at a submultiple of its resonance frequency(American Physical Society, 2005-03) Balantekin, M.; Atalar, AbdullahIn a tapping-mode atomic force microscope, the frequency spectrum of the oscillating cantilever contains higher harmonics at integer multiples of the excitation frequency. When the cantilever oscillates at its fundamental resonance frequency w 1, the high Q-factor damps the amplitudes of the higher harmonics to negligible levels, unless the higher flexural eigenmodes are coincident with those harmonics. One can enhance the nth harmonic by the Q factor when the cantilever is excited at a submultiple of its resonance frequency (w 1/n). Hence, the magnitude of the nth harmonic can be measured easily and it can be utilized to examine the material properties. We show theoretically that the amplitude of enhanced higher harmonic increases monotonically for a range of sample stiffness, if the interaction is dominated by elastic force.Item Open Access Glycosaminoglycan-Mimetic Signals Direct the Osteo/Chondrogenic Differentiation of Mesenchymal Stem Cells in a Three-Dimensional Peptide Nanofiber Extracellular Matrix Mimetic Environment(American Chemical Society, 2016-02) Arslan, E.; Güler, Mustafa O.; Tekinay, A. B.Recent efforts in bioactive scaffold development focus strongly on the elucidation of complex cellular responses through the use of synthetic systems. Designing synthetic extracellular matrix (ECM) materials must be based on understanding of cellular behaviors upon interaction with natural and artificial scaffolds. Hence, due to their ability to mimic both the biochemical and mechanical properties of the native tissue environment, supramolecular assemblies of bioactive peptide nanostructures are especially promising for development of bioactive ECM-mimetic scaffolds. In this study, we used glycosaminoglycan (GAG) mimetic peptide nanofiber gel as a three-dimensional (3D) platform to investigate how cell lineage commitment is altered by external factors. We observed that amount of fetal bovine serum (FBS) presented in the cell media had synergistic effects on the ability of GAG-mimetic nanofiber gel to mediate the differentiation of mesenchymal stem cells into osteogenic and chondrogenic lineages. In particular, lower FBS concentration in the culture medium was observed to enhance osteogenic differentiation while higher amount FBS promotes chondrogenic differentiation in tandem with the effects of the GAG-mimetic 3D peptide nanofiber network, even in the absence of externally administered growth factors. We therefore demonstrate that mesenchymal stem cell differentiation can be specifically controlled by the combined influence of growth medium components and a 3D peptide nanofiber environment.Item Open Access On the stabilization of periodic orbits for discrete time chaotic systems(Elsevier, 2005-02) Morgül, Ö.In this Letter we consider the stabilization problem of unstable periodic orbits of discrete time chaotic systems. We propose a novel and simple periodic delayed feedback law and present some stability results. These results show that all hyperbolic periodic orbits as well as some non-hyperbolic periodic orbits can be stabilized with the proposed method. The stability proofs also give the possible feedback gains which achieve stabilization. We will also present some simulation results.Item Open Access Orbital magnetization of single and double quantum dots in a tight-binding model(American Physical Society, 2003) Aldea, A.; Moldoveanu, V.; Niţǎ, M.; Manolescu, A.; Gudmundsson, V.; Tanatar, BilalWe calculate the orbital magnetization of single and double quantum dots coupled both by Coulomb interaction and by electron tunneling. The electronic states of the quantum dots are calculated in a tight-binding model, and the magnetization is discussed in relation to the energy spectrum and to the edge and bulk states. We identify effects of chirality of the electronic orbits and of the anticrossing of the energy levels when the magnetic field is varied. We also consider the effects of detuning the energy spectra of the quantum dots by an external gate potential. We compare our results with the recent experiments of Oosterkamp et al. [Phys, Rev. Lett. 80, 4951 (1998)].Item Open Access Persistent currents in helical structures(American Physical Society, 2004) Iskin, M.; Kulik, I. O.The recent discovery of mesoscopic electronic structures, in particular the carbon nanotubes, made necessary an investigation of what effect a helical symmetry of the conductor (metal or semiconductor) may have on the persistent current oscillations, We investigate persistent currents in helical structures which are nondecaying in time, not requiring a voltage bias, dissipationless stationary flow of electrons in a normal-metallic or semiconducting cylinder or circular wire of mesoscopic dimension. In the presence of magnetic flux along the toroidal structure, helical symmetry couples circular and longitudinal currents to each other. Our calculations suggest that circular persistent currents in these structures have two components with periods Φ0 and Φ0/s (s is an integer specific to any geometry). However, resultant circular persistent current oscillations have Φ0 period.Item Open Access Quantum effects of thermal conductance through atomic chains(2001) Ozpineci, A.; Çıracı, SalimWe present a formalism for an atomic scale study of phononic heat transfer. The expression of thermal energy current can be cast in the Landauer form and incorporates the transmission coefficient explicitly. Calculation of the thermal conductance of a monoatomic chain of N atoms between two reservoirs shows interesting quantum features. The conductance density appears as Lorentzian type resonances at the eigenfrequencies of the chain. At low-temperature limit the discrete vibrational frequency spectrum of a "soft" chain may reflect on the thermal conductance by giving rise to a sudden increase. At room temperature, the conductance through a "stiff" chain may oscillate with the number of chain atoms. The obtained quantum features are compared with similar effects found in the quantized electrical conductance.Item Open Access Resonant and coherent transport through Aharonov-Bohm interferometers with coupled quantum dots(The American Physical Society, 2005) Moldoveanu, V.; Ţolea, M.; Aldea, A.; Tanatar, BilalA detailed description of the tunneling processes within Aharonov-Bohm (AB) rings containing two-dimensional quantum dots is presented. We show that the electronic propagation through the interferometer is controlled by the spectral properties of the embedded dots and by their coupling with the ring. The transmittance of the interferometer is computed by the Landauer-Büttiker formula. Numerical results are presented for an AB interferometer containing two coupled dots. The charging diagrams for a double-dot interferometer and the Aharonov-Bohm oscillations are obtained, in agreement with the recent experimental results of Holleitner et al. [Phys. Rev. Lett. 87, 256802 (2001)] We identify conditions in which the system shows Fano line shapes. The direction of the asymetric tail depends on the capacitive coupling and on the magnetic field. We discuss our results in connection with the experiments of Kobayashi et al. [Phys. Rev. Lett. 88, 256806 (2002)] in the case of a single dot. ©2005 The American Physical Society.Item Open Access Tunneling properties of quantum dot arrays in a strong magnetic field(The American Physical Society, 2004) Moldoveanu, V.; Aldea, A.; Tanatar, BilalWe study the transport properties of coherently coupled quantum dots in the quantum Hall regime within the Landauer-Büttiker formalism which captures and explains the experimentally observed features in terms of the spectral properties of the coupled dot system. The subpeak structure of the transmittance spectrum and the charging stability diagrams are obtained and discussed. The role of the intradot and interdot Coulomb interaction are pointed out. We show the subpeak evolution with the magnetic field and predict a specific oscillatory behavior of the Hall resistance in strong magnetic field which can be experimentally tested.