Browsing by Subject "Theory"

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Open Access
Approximate fourier domain expression for bloch-siegert shift
(John Wiley and Sons Inc., 2015) Turk, E. A.; Ider, Y. Z.; Ergun, A. S.; Atalar, Ergin
Purpose: In this study, a newsimple Fourier domain-based analytical expression for the Bloch-Siegert (BS) shift-based B1 mapping method is proposed to obtain |B1+| more accurately while using short BS pulse durations and small off-resonance frequencies.Theory and Methods: A new simple analytical expression for the BS shift is derived by simplifying the Bloch equations. In this expression, the phase is calculated in terms of the Fourier transform of the radiofrequency pulse envelope, and thus making the off- and on-resonance effects more easily understandable. To verify the accuracy of the proposed expression, Bloch simulations and MR experiments are performed for the hard, Fermi, and Shinner-Le Roux pulse shapes.Results: Analyses of the BS phase shift-based B1 mapping method in terms of radiofrequency pulse shape, pulse duration, and off-resonance frequency show that |B1+| can be obtained more accurately with the aid of this new expression.Conclusions: In this study, a new simple frequency domain analytical expression is proposed for the BS shift. Using this expression, |B1+| values can be predicted from the phase data using the frequency spectrum of the radiofrequency pulse. This method works well even for short pulse durations and small offset frequencies.
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Armoni
(1983) İdiz, Cem
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Batı musikisinin geleneksel sanat musikimize etkileri ve oturum sorunu
(1991) gedikli, Necati
Open Access
Charged bosons in a quasi-one-dimensional system
(American Physical Society, 2000) Tanatar, Bilal; Davoudi, B.; Kohandel, M.
The ground-state properties of a system of charged bosons in a quasi-one-dimensional model with a neutralizing background are investigated within the hypernetted-chain approximation. Strong correlation effects drive the system from a homogeneous fluid phase toward a more ordered structure akin to Wigner crystallization in higher-dimensional charged quantum systems. The ordered phase of charged bosons is signaled by the development of a peak in the static structure factor, which is analyzed as a function of the density and the lateral width of the one-dimensional structure. We also calculate the pair-distribution function, the ground-state energy, and the local-field correction, and compare our results with other theoretical approaches.
Open Access
Circuit partitioning using mean field annealing
(Elsevier, 1995) Bultan, T.; Aykanat, Cevdet
Mean field annealing (MFA) algorithm, proposed for solving combinatorial optimization problems, combines the characteristics of neural networks and simulated annealing. Previous works on MFA resulted with successful mapping of the algorithm to some classic optimization problems such as traveling salesperson problem, scheduling problem, knapsack problem and graph partitioning problem. In this paper, MFA is formulated for the circuit partitioning problem using the so called net-cut model. Hence, the deficiencies of using the graph representation for electrical circuits are avoided. An efficient implementation scheme, which decreases the complexity of the proposed algorithm by asymptotical factors is also developed. Comparative performance analysis of the proposed algorithm with two wellknown heuristics, simulated annealing and Kernighan-Lin, indicates that MFA is a successful alternative heuristic for the circuit partitioning problem. © 1995.
Open Access
Context and theorizing in the global south: challenges and opportunities for an international dialogue
(ANPAD - Associacao Nacional de Pos-Graduacao e Pesquisa em Administracao, 2019) Ger, Güliz; Suarez, M. C.; Nascimento, T. C.
Brazilian researchers have been encouraged to internationalize their research, expanding their insertions into high impact international journals. This task in turn demands thinking outside the commonly used Eurocentric prism. How can Brazilian scholars say something that would be of interest globally and have an impact on the work done in the global North? How should the researcher from the global South deal with context in the international dialogue to avoid being perceived as the exotic Other? This interview with professor Guliz Ger addresses these and other important issues related to the challenge of theorizing and publishing research from a global perspective. Professor Ger’s work inspires Brazilian researchers in their internationalization efforts by offering concrete examples of how to theorize, strategies to overcome editors’ and journals’ prejudices and convince the international audience of the value of global South, and more particularly, Brazilian research. Professor Ger’s experience as a visiting professor in numerous universities around the world including in Brazil, and her service on a number of journal editorial boards, including the Journal of Consumer Research, for which she served as Associate Editor, provided the background for this interview produced by Maribel Suarez and Thaysa Nascimento.
Open Access
Density profile of a Bose-Einstein condensate inside a pancake-shaped trap: Observational consequences of the dimensional cross-over in the scattering properties
(Elsevier Science B.V., 2002) Tanatar, Bilal; Minguzzi, A.; Vignolo, P.; Tosi, M. P.
It is theoretically well known that two-dimensionality of the scattering events in a Bose-Einstein condensate introduces a logarithmic dependence on density in the coupling constant entering a mean-field theory of the equilibrium density profile, which becomes dominant as the s-wave scattering length gets larger than the condensate thickness. We trace the regions of experimentally accessible system parameters for which the cross-over between different dimensionality behaviors in the scattering properties may become observable through in situ imaging of the condensed cloud with varying trap anisotropy and scattering length.
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, Bilal
The 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.
Open Access
Guiding, bending, and splitting of electromagnetic waves in highly confined photonic crystal waveguides
(American Physical Society, 2001) Bayındır, Mehmet; Özbay, Ekmel; Temelkuran, B.; Sigalas, M. M.; Soukoulis, C. M.; Biswas, R.; Ho, K. M.
We have experimentally demonstrated the guiding, bending, and splitting of electromagnetic (EM) waves in highly confined waveguides built around three-dimensional layer-by-layer photonic crystals by removing a single rod. Full transmission of the EM waves was observed for straight and bended waveguides. We also investigated the power splitter structures in which the input EM power could be efficiently divided into the output waveguide ports. The experimental results, dispersion relation and photon lifetime, were analyzed with a theory based on the tight-binding photon picture. Our results provide an important tool for designing photonic crystal based optoelectronic components.
Open Access
Intravascular extended sensitivity (IVES) MRI antennas
(John Wiley & Sons, Inc, 2003) Susil, R. C.; Yeung, C. J.; Atalar, Ergin
The design and application of an intravascular extended sensitivity (IVES) MRI antenna is described. The device is a loopless antenna design that incorporates both an insulating, dielectric coating and a winding of the antenna whip into a helical shape. Because this antenna produces a broad region of high SNR and also allows for imaging near the tip of the device, it is useful for imaging long, luminal structures. To elucidate the design and function of this device, the effects of both insulation and antenna winding were characterized by theoretical and experimental studies. Insulation broadens the longitudinal region over which images can be collected (i.e., along the lumen of a vessel) by increasing the resonant pole length. Antenna winding, conversely, allows for imaging closer to the tip of the antenna by decreasing the resonant pole length. Over a longitudinal region of 20 cm, the IVES imaging antenna described here produces a system SNR of approximately 40,000/r (mL-1Hz1/2), where r is the radial distance from the antenna axis in centimeters. As opposed to microcoil antenna designs, these antennas do not require exact positioning and allow for imaging over broad tissue regions. While focusing on the design of the IVES antenna, this work also serves to enhance our overall understanding of the properties and behavior of the loopless antenna design. © 2003 Wiley-Liss, Inc.
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Müzik : insanın ses doğası
(1988) Serdar, Mehmet
Open Access
Negative refraction and superlens behavior in a two-dimensional photonic crystal
(American Physical Society, 2005) Moussa, R.; Foteinopoulou, S.; Zhang, L.; Tuttle, G.; Guven, K.; Özbay, Ekmel; Soukoulis, C. M.
We experimentally and theoretically studied a left-handed structure based on a photonic crystal (PC) with a negative refractive index. The structure consists of triangular array of rectangular dielectric bars with dielectric constant 9.61. Experimental and theoretical results demonstrate the negative refraction and the superlensing phenomena in the microwave regime. The results show high transmission for our structure for a wide range of incident angles. Furthermore, surface termination within a specific cut of the structure excite surface waves at the interface between air and PC and allow the reconstruction of evanescent waves for a better focus and better transmission. The normalized average field intensity calculated in both the source and image planes shows almost the same full width at half maximum for the source and the focused beam.
Open Access
Operational research: methods and applications
(Taylor and Francis Ltd., 2023-12-27) Petropoulos, F.; Laporte, G.; Aktas, E.; Alumur, S.; Archetti, C.; Ayhan, H.; Battarra, M.; Bennell, J.; Bourjolly, J.; Boylan, J. E.; Breton, M.; Canca, D.; Charlin, L.; Chen, B.; Cicek, C.; Cox, L.; Currie, C. S.M.; Demeulemeester, E.; Ding, L.; Disney, S. M.; Ehrgott, M.; Eppler, M. J.; Erdoğan, G.; Fortz, B.; Franco, L. A.; Frische, J.; Greco, S.; Gregory, A. J.; Hämäläinen, R. P.; Herroelen, W.; Hewitt, M.; Holmström, J.; Hooker, J. N.; Işık, T.; Johnes, J.; Kara, B. Y.; Karsu, Özlem; Kent, K.; Köhler, C.; Kunc, M.; Kuo, Y.; Letchford, A. N.; Leung, J.; Li, D.; Li, H.; Lienert, J.; Ljubić, I.; Lodi, A.; Lozano, S.; Lurkin, V.; Martello, S.; McHale, I. G.; Midgley, G.; Morecroft, J. D.W.; Mutha, A.; Oğuz, C.; Petrovic, S.; Pferschy, U.; Psaraftis, H. N.; Rose, S.
Throughout its history, Operational Research has evolved to include methods, models and algorithms that have been applied to a wide range of contexts. This encyclopedic article consists of two main sections: methods and applications. The first summarises the up-to-date knowledge and provides an overview of the state-of-the-art methods and key developments in the various subdomains of the field. The second offers a wide-ranging list of areas where Operational Research has been applied. The article is meant to be read in a nonlinear fashion and used as a point of reference by a diverse pool of readers: academics, researchers, students, and practitioners. The entries within the methods and applications sections are presented in alphabetical order. The authors dedicate this paper to the 2023 Turkey/Syria earthquake victims. We sincerely hope that advances in OR will play a role towards minimising the pain and suffering caused by this and future catastrophes.
Open Access
Rydberg aggregates
(Institute of Physics Publishing, 2018) Wüster, S.; Rost, J.-M.
We review Rydberg aggregates, assemblies of a few Rydberg atoms exhibiting energy transport through collective eigenstates, considering isolated atoms or assemblies embedded within clouds of cold ground-state atoms. We classify Rydberg aggregates, and provide an overview of their possible applications as quantum simulators for phenomena from chemical or biological physics. Our main focus is on flexible Rydberg aggregates, in which atomic motion is an essential feature. In these, simultaneous control over Rydberg-Rydberg interactions, external trapping and electronic energies, allows Born-Oppenheimer surfaces for the motion of the entire aggregate to be tailored as desired. This is illustrated with theory proposals towards the demonstration of joint motion and excitation transport, conical intersections and non-adiabatic effects. Additional flexibility for quantum simulations is enabled by the use of dressed dipole-dipole interactions or the embedding of the aggregate in a cold gas or Bose-Einstein condensate environment. Finally we provide some guidance regarding the parameter regimes that are most suitable for the realization of either static or flexible Rydberg aggregates based on Li or Rb atoms. The current status of experimental progress towards enabling Rydberg aggregates is also reviewed. © 2018 IOP Publishing Ltd.
Open Access
A sketch of a system of theory and practice
(Sage Publications Ltd, 2015) Alexander, J.
Most political theorists are committed to one particular view about the relation between theory and practice. It is argued in this article that there are in fact four possible ways of relating theory and practice, which are distinguished in terms of the answers that are given to two distinct questions. Derived from this is the suggestion that all political theorists can be classified according to whether they are sanguine, phlegmatic, melancholic or choleric. The purpose of this sketch of a system is to indicate the questionable nature of much of what passes for modern political theory - especially that of the now dominant sanguine tradition, which has for several decades especially concerned itself with the 'impact' theory can have on practice. © 2015 Political Studies Association and John Wiley & Sons Ltd.
Open Access
Spin correlations in electron liquids: Analytical results for the local-field correction in two and three dimensions
(Springer, 1998) Tanatar, Bilal; Mutluay, N.
We study the spin correlations in two- and three-dimensional electron liquids within the sum-rule version of the self-consistent field approach of Singwi, Tosi, Land, and Sjölander. Analytic expressions for the spin-antisymmetric static structure factor and the corresponding local-field correction are obtained with density dependent coefficients. We calculate the spin-dependent pair-correlation functions, paramagnon dispersion, and static spin-response function within the present model, and discuss the spin-density wave instabilities in double-layer electron systems.
Open Access
Spin-dependent analysis of two-dimensional electron liquids
(The American Physical Society, 2002) Bulutay, C.; Tanatar, Bilal
Two-dimensional electron liquid (2D EL) at full Fermi degeneracy is revisited, giving special attention to the spin-polarization effects. First, we extend the recently proposed classical-map hypernetted-chain (CHNC) technique to the 2D EL, while preserving the simplicity of the original proposal. An efficient implementation of CHNC is given utilizing Lado's quadrature expressions for the isotropic Fourier transforms. Our results indicate that the paramagnetic phase stays to be the ground state until the Wigner crystallization density, even though the energy separation with the ferromagnetic and other partially polarized states become minute. We analyze compressibility and spin stiffness variations with respect to density and spin polarization, the latter being overlooked until now. Spin-dependent static structure factor and pair-distribution functions are computed; agreement with the available quantum Monte Carlo data persists even in the strong-coupling regime of the 2D EL.
Open Access
SPT-function and its properties
(2014) Gezmiş, Oğuz
In this thesis, we survey some properties of the spt-function. We start with providing some background information for q-series and the partition function p(n). Then we define the spt-function and study its generating function. Our aim is to prove parity result for the spt-function. We also obtain a congruence relation between spt-function and a certain mock theta function.
Open Access
A systematic theory of tradition
(Brill Academic Publishers, 2016) Alexander, J.
We still lack a systematic or complete theory of tradition. By referring to the works of many major figures of the last century - Arendt, Boyer, Eisenstadt, Eliot, Gadamer, Goody, Hobsbawm, Kermode, Leavis, MacIntyre, Oakeshott, Pieper, Pocock, Popper, Prickett, Shils and others - I show that a theory of tradition must include insights taken not only from the study of sociology and anthropology, but also from the study of literature and religion (and, it goes without saying here, the study of philosophy and history). The proliferation of separate academic subjects does not make it any less necessary for us to attempt to say in general what we are talking about when we talk about tradition. In this article I distinguish three elements which are found in traditions. I call these continuity, canon, and core. The argument is that traditions can be distinguished in terms of whether there is a core in addition to canon and continuity, a canon in addition to continuity, or only mere continuity. Together these form a theory of tradition which enables us to see what is necessary to all traditions and also what it is which distinguishes different types of tradition from each other. © 2016 Koninklijke Brill NV, Leiden, The Netherlands.
Open Access
Taming the boojum: Being theoretical about peculiarities of learning
(Psychonomic Society, Inc., 2022) Bowers, Robert Ian
The case of the “biological constraints” movement in mid-20th-century psychology provides a reminder of the weight of psychology’s reliance on theory and theory-driven methods. By 1980, a critical mass of demonstrations of the specifcity of learning had eroded faith in general-process approaches. A common reaction was to call for a biological orientation. However, this proved not as straightforward as it had seemed, and much of the ostensibly biological research that followed was atheoretical. The successes in this context were due to careful theoretical work by people who appreciated the aims of the involved sciences and the interdependence of the aims with methods. Michael Domjan slowed the feld’s haphazard rush into ostensible biological research, and rather urged adoption of principled biological approaches. In 1982, his positive recommendation was for comparative psychology to begin to live up to its name, and adopt principled comparative methods as practised in biology. Although lauded, few followed this recommendation. Indeed, even Domjan’s own subsequent research was mostly not comparative in the way he had described, but rather involved single species, guided by a behaviour systems approach. With reference to two major perspectives associated with Domjan—comparative methods and behaviour systems theory—I present Domjan’s challenge not as being to make our feld comparative per se, but to make it theoretical. This challenge remains current.