Browsing by Subject "Physics"

Now showing 1 - 20 of 37

Open Access
Anti-de sitter-wave solutions of higher derivative theories
(American Physical Society, 2013) Gürses, M.; Hervik, S.; Şişman, T. Ç.; Tekin, B.
We show that the recently found anti-de Sitter (AdS)-plane and AdS-spherical wave solutions of quadratic curvature gravity also solve the most general higher derivative theory in D dimensions. More generally, we show that the field equations of such theories reduce to an equation linear in the Ricci tensor for Kerr-Schild spacetimes having type-N Weyl and type-N traceless Ricci tensors. © 2013 American Physical Society.
Open Access
Archimedes' principle as an application of the divergence theorem
(Institute of Electrical and Electronics Engineers, 1990) Altintas, A.
A derivation is provided that allows one to obtain Archimedes' principle as an application of the divergence theorem.
Restricted
Batı’yı Batı ile aşmak isteyen Fizikçi: Asım Orhan Barut’un biyografisi
(Bilkent University, 2020) Arslan, Ali; Erdem, Ege; Deveci, Fatih Efe; Doğukaya, Beliz; Dilsiz, Can
Asım Orhan Barut (1926-1994), Malatyalı bir Türk Fizikçidir. Üniversite yıllarından itibaren yurt dışında çalışmaya başlayan Barut, 1970’ler ve özellikle 1980’lerde Türkiye’de de çalışmaya başlamıştır. Kuramsal fiziğin temel problemleri, matematiksel fizik ve parçacık fiziği hakkında yaptığı çalışmaları literatürdeki önemli eksikleri doldurmuş, Batılı üniversitelerde okutulmuştur. Barut için gelişmekte olan ülkeler ancak - gelişmiş Batılı ülkeler ile iletişim halinde olmak şartıyla- bilimsel gelişmeler yapabilirse kalkınabilir ve tam bağımsız olabilirlerdi. Bu makalede Barut’un çalışmalarının ve bilim hakkındaki görüşlerinin tarihsel koşullardan etkilendiği anlatılmış, Barut’un kültürel bağımsızlığını kaybetmediği savunulmuş, her noktada Barut’un istekleri ve kararlarından da bahsedilerek determinist bir yaklaşımdan uzak durulmuştur.
Open Access
Beaming of light and enhanced transmission via surface modes of photonic crystals
(Optical Society of America, 2005-11-15) Bulu, I.; Caglayan, H.; Özbay, Ekmel
We report beaming and enhanced transmission of electromagnetic waves by use of surface corrugated photonic crystals. The modes of a finite-size photonic crystal composed of dielectric rods in free space have been analyzed by the plane-wave expansion method. We show the existence of surface propagating modes when the surface of the finite-size photonic crystal is corrugated. We theoretically and experimentally demonstrate that the transmission through photonic crystal waveguides can be substantially increased by the existence of surface propagating modes at the input surface. In addition, the power emitted from the photonic crystal waveguide is confined to a narrow angular region when an appropriate surface corrugation is added to the output surface of the photonic crystal.
Restricted
Boğaziçi Fizik Bölümü tarihi
(Bilkent University, 2022) Semiz, Fatih Gürkan; Siner, Işılay; Yüceler, Zeynep; Erdoğan, İbrahim Ozan; Sayan, Deniz Tuna
Boğaziçi Üniversitesi, Robert Koleji’nin kuruluş tarihi 1863’ten bu yana, Fizik dalında eğitim vermektedir. Fizik bölümü, 1963 yılında dört yıllık lisans programının başlatılması ile kurulmuştur. Fizik bölümü, tarihi boyunca Boğaziçi Üniversitesinde izlenen çok yönlü birey yetiştirme esasına dayalı olarak eğitim modelini uygulamıştır. Bölümün tarihini üniversitenin bir amerikan kolejinden bir devlet üniversitesine dönüşümü ve bölüme katkılarda bulunmuş isimler ile onların çalışmaları şekillendirmiştir. Bu araştırmada Robert Koleji’nin kurucusu ve burada fizik eğitimi vermiş isimlerin eserlerinden, fizik bölümünde okumuş ve akademisyenlik görevini sürdürmüş/sürdüren kişilerle yapılan röportajlardan ve konuyla ilgili diğer birincil kaynaklardan yararlanılarak Boğaziçi Fizik Bölümü’nün tarihinin anlatılması ve değerinin gösterilmesi hedeflenmektedir.
Restricted
Comparing Conceptions: Frost and Eddington, Heisenberg, and Bohr
(1987) Rotella, Guy
Open Access
Conductance of ferromagnetic nanowires
(American Physical Society, 1998) Mehrez, H.; Çıracı, Salim
The conductance distribution obtained from an ensemble of stretching ferromagnetic nanowires or point contacts do not exhibit peaks near the integer multiples of 2e2/h. This observation has been interpreted as the absence of conductance quantization. In this report, we examine various features of electron transport through the Ni nanowires and clarify the behavior of conductance distribution that is different from gold and copper nanowires. Our study concludes that the tunneling through the closely spaced states near EF that originate from crystal field and spin split 3d states of Ni prevents the plateaus of conductance from forming in the course of stretch.
Open Access
Diodelike asymmetric transmission of linearly polarized waves using magnetoelectric coupling and electromagnetic wave tunneling
(American Physical Society, 2012-05-24) Mutlu, M.; Akosman, A. E.; Serebryannikov, A. E.; Özbay, Ekmel
An asymmetric, reciprocal, diffraction-free transmission of linearly polarized waves in a new diodelike, three-layer, ultrathin, chiral structure is studied theoretically and experimentally. The exploited physical mechanism is based on the maximization of the cross-polarized transmission in one direction due to the polarization selectivity dictated by the peculiar eigenstate combination, which is efficiently controlled by the electromagnetic tunneling through the metallic subwavelength mesh sandwiched between these layers. Simulation and microwave experiment results demonstrate a nearly total intensity transmission at normal incidence in one direction and a small intensity transmission in the opposite direction.
Open Access
Effective mass calculations for two-dimensional gas of dipolar fermions
(Springer New York LLC, 2017) Seydi, I.; Abedinpour, S. H.; Tanatar, Bilal
We consider a two-dimensional system of ultracold dipolar fermions with dipole moments aligned in the perpendicular direction. We use the static structure factor information from Fermi-Hypernetted-Chain calculations to obtain the effective many-body dipole–dipole interaction and calculate the many-body effective mass of the system within the G0W approximation to the self-energy. A large cancellation between different contributions to the self-energy results in a weak dependence of the effective mass on the interaction strength over a large range of coupling constants.
Open Access
Exohydrogenated single-wall carbon nanotubes
(American Physical Society, 2001) Yildirim, T.; Gülseren, O.; Çıracı, Salim
An extensive first-principles study of fully exohydrogenated zigzag (n,0) and armchair (n,n) single-wall carbon nanotubes (CnHn), polyhedral molecules including cubane, dodecahedrane, and C60H60 points to crucial differences in the electronic and atomic structures relevant to hydrogen storage and device applications. CnHn’s are estimated to be stable up to the radius of a ~8,8! nanotube, with binding energies proportional to 1/R. Attaching a single hydrogen to any nanotube is always exothermic. Hydrogenation of zigzag nanotubes is found to be more likely than armchair nanotubes with similar radius. Our findings may have important implications for selective functionalization and finding a way of separating similar radius nanotubes from each other.
Open Access
First-principles investigation of structural and electronic properties of solid cubane and its doped derivatives
(American Physical Society, 2000) Yildirim, T.; Çıracı, Salim; Kılıç, Ç.; Buldum, A.
The electronic and structural properties of molecular and solid cubane have been studied by first-principles, self-consistent field total energy calculations. Calculated molecular properties such as equilibrium geometry and electronic and vibrational spectra are found to be in good agreement with experimental data. Structural parameters and the energetics of both the low-temperature, orientationally ordered and high-temperature, orientationally disordered or plastic phases of solid cubane are determined. The valence band of solid cubane is derived from the molecular states; the energy gap between the lowest unoccupied and highest occupied molecular orbital bands is rather large due to the saturated carbon atoms. The effect of alkali-metal-atom doping on the electronic energy bands is investigated. It is found that the metallic band of doped cubane is derived from the undoped solid cubane’s lowest conduction band with a significant contribution from the alkali-metal atom.
Open Access
First-principles study of the iron pnictide superconductor BaFe2 As2
(American Physical Society, 2009) Aktürk, E.; Çıracı, Salim
This paper presents our study on the atomic, electronic, magnetic structures, and phonon modes of the low-temperature orthorhombic phase of undoped BaFe2As2 crystal. The electronic structure is characterized by a sharp Fe-3d peak close to the Fermi level and is dominated by Fe-3d- and As-4p-hybridized states. Ba contribution occurs only at lower energies. The spin ordering of the magnetic ground state, which is determined by minimizing the total energy of different spin alignments on Fe atoms in the conventional cell, is in agreement with experimental findings but is different from the antiferromagnetic spin ordering obtained by assigning antiparallel spin directions on two Fe atoms in the primitive unit cell. Valuable information about the charge transfer and bonding is revealed through the analysis of the charge density. Electrons are transferred from Ba to Fe-As layers and also from Fe to As atoms. The magnetic phonon calculations of the ground state are carried out to predict Raman and infrared-active modes. Softening of some calculated spin-dependent phonon modes corroborates the contribution of spin-lattice coupling to the structural phase transition from I4/mmm to Fmmm.
Restricted
Gravity Models and interaction probabilities
(1975) Tellier, Luc-Normand
Open Access
How can we use nanophotonics to help in combating climate change?
(IEEE, 2008-08) Demir, Hilmi Volkan
The paper is about emerging technologies enabled by nanophotonics to combat climate change. We developed and demonstrated 1.) nanocrystal hybridized white light sources and those also embedded with plasmonic metal nanoparticles for the reduction of global energy consumption, 2.) photovoltaic nanocrystal scintillators hybridized on solar cells for the generation of efficient green energy sources, and 3.) photocatalytic nanoparticle and nanocomposite systems for massive environmental decontamination, all to contribute to the reduction of greenhouse gas amount.
Open Access
Increased quantum efficiency and reduced red-shift in polymer nanoparticle luminophors
(IEEE, 2008-11) Huyal, İlkem Özge; Özel, Tuncay; Tuncel, Dönüş; Demir, Hilmi Volkan
In this paper, using a polyfluorene derivative, increased fluorescence quantum efficiency and reduced red-shift in the film form of polymer nanoparticle luminophors is achieved, when compared to directly spin coated polymer thin films.
Open Access
Inflation and inflation uncertainty: a dynamic framework
(Elsevier BV, 2012) Berument, Hakan; Yalcin, Y.; Yildirim J.
This paper aims to investigate the direct relationship between inflation and inflation uncertainty by employing a dynamic method for the monthly country-region-place United States data for the time period 1976-2007. While the bulk of previous studies has employed GARCH models in investigating the link between inflation and inflation uncertainty, in this study Stochastic Volatility in Mean models are used to capture the shocks to inflation uncertainty within a dynamic framework. These models allow researchers to assess the dynamic effects of innovations in inflation as well as inflation volatility on inflation and inflation volatility over time, by incorporating the unobserved volatility as an explanatory variable in the mean (inflation) equation. Empirical findings suggest that innovations in inflation volatility increases inflation. This evidence is robust across various definitions of inflation and different sub-periods. © 2012 Elsevier B.V. All rights reserved.
Open Access
Laser-slicing of silicon with 3D nonlinear laser lithography
(OSA, 2017) Tokel, Onur; Turnalı, Ahmet; Çolakoğlu, T.; İlday, Serim; Borra, M. Z.; Pavlov, Ihor; Bek, A.; Turan, R.; İlday, Fatih Ömer
Recently, we have showed a direct laser writing method that exploits nonlinear interactions to form subsurface modifications in silicon. Here, we use the technique to demonstrate laser-slicing of silicon and its applications.
Open Access
A measurable force driven by an excitonic condensate
(American Institute of Physics Inc., 2014) Hakioǧlu, T.; Özgün, E.; Günay, M.
Free energy signatures related to the measurement of an emergent force (≈10-9N) due to the exciton condensate (EC) in Double Quantum Wells are predicted and experiments are proposed to measure the effects. The EC-force is attractive and reminiscent of the Casimir force between two perfect metallic plates, but also distinctively different from it by its driving mechanism and dependence on the parameters of the condensate. The proposed experiments are based on a recent experimental work on a driven micromechanical oscillator. Conclusive observations of EC in recent experiments also provide a strong promise for the observation of the EC-force. © 2014 AIP Publishing LLC.
Open Access
Metal-insulator transition induced by random dipoles
(American Physical Society, 2013) Larcher, M.; Menotti, C.; Tanatar, Bilal; Vignolo, P.
We study the localization properties of a test dipole feeling the disordered potential induced by dipolar impurities trapped at random positions in an optical lattice. This random potential is marked by correlations which are a convolution of short-range and long-range ones. We show that when short-range correlations are dominant, extended states can appear in the spectrum. Introducing long-range correlations, the extended states, if any, are wiped out and localization is restored over the whole spectrum. Moreover, long-range correlations can either increase or decrease the localization length at the center of the band, which indicates a richer behavior than previously predicted.
Restricted
Modern fiziğin bir kalbi var mı?
(2000) Öktülmüş, Buket