Ph.D. Theses

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Analytic solutions of scalar field cosmology with minimal and nonminimal coupling and deformed discrete and finite quantum systems
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) İldeş, Medine.; Turgut, Osman Teoman.; Arık, Metin.
In this thesis first, we study analytic solutions of cosmology. We investigate the most general cosmological model with real scalar field which is minimally coupled to gravity and Brans- Dicke cosmology. Field equations consist of three differential equations. We switch independent variable from time to scale factor by change of variable ˙a/a = H(a). Thus a new set of differential equations are analytically solvable with known methods. a(t) can be explicitly found as long as methods of integration techniques are available. We investigate the dynamics of the universe at early times as well as at late times in light of these formulas. We find mathematical machinery which turns on and turns off early accelerated expansion. On the other hand late time accelerated expansion is explained by cosmic domain walls. φ 4 potential is studied in Brans-Dicke Cosmology. In this thesis we also study discrete and finite quantum systems. We define a deformed kinetic energy operator for a discrete position space with a finite number of points. The structure may be either periodic or nonperiodic with well-defined end points. It is shown that for the nonperiodic case the translation operator becomes nonunitary due to the end points. This uniquely defines an algebra which has the desired unique representation. Energy eigenvalues and energy wave functions for both cases are found. In addition, we uncover the mathematical structure of the Schwinger algebra and introduce almost unitary Schwinger operators which are derived by considering translation operators on a finite lattice.
The role of trigger waves in cancer angiogenesis
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Debir, Birses.; Ünlü, Mehmet Burçin.
This study includes mathematical modeling of biological observations and numer ical solutions of these models. Our study focused on different phenomenons, including; vessel formation, intracellular calcium ion concentration, traveling wave solutions, and cytosol elasticity. Understanding signaling in diseases is essential for a proper response. For this reason, understanding the secondary messenger signaling that mechanisms of ten prefer to use and their interaction with the mechanisms enables the system’s re sponse to be better understood. This study examined the interaction of angiogenesis, a mechanism that contributes to tumor growth, and cytosolic calcium ion, an intracel lular secondary messenger. Therefore, we simulated a mathematical model involving essential angiogenesis and calcium homeostasis elements using previously used models. In our simulations, we developed two and multiple cell scenarios and examined the results of our system in distributions of different angiogenic stimulus uptake. Since angiogenesis requires the cell to move in a specific direction, we simulated the cytosolic gelation- solution mechanism, inspired by another model used in the literature. We examined the condition under which the wave direction persists in the traveling wave configuration.
Perturbative and non-perturbative physics from singularities
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022), 2022) Pazarbaşı, Cihan.; Bleeken, Dieter Van den.
A function that representing a physical quantity has singularities which con tain perturbative or non-perturbative information about the physical system under investigation. Moreover, the theory of resurgence tells us that these perturbative and non-perturbative parts are intimately connected and it is possible to use one of them to obtain the other one. In this thesis, we combine these two ideas with a focus on the functions formulated in integral representations. Specifically, first consider ing two different examples on the semi-classical expansion in quantum mechanics and the pair production problem in electromagnetic backgrounds, we will concentrate on the quantum action which we express in the Schwinger’s integral representation. We will show that the perturbative and non-perturbative information about the physical system is hidden in singularities of the propagator TrU(t). The way we obtain the non-perturbative one is very similar to the Borel method which is used to handle the divergent perturbation series. Contrary to the Borel method, by probing the singular ities of TrU(t) directly and using the iε prescription, we will be able to prevent the Borel ambiguity problem in the physical cases that it leads to the violation of the uni tarity. Later, we will turn our attention to the renormalon problem in non-relativistic quantum mechanics. After presenting the existence of the renormalon divergence in a scattering problem with a background potential consisting of 2D δ-potential perturbed with a tilted 1D δ-potential, we will argue that the Borel ambiguity in the summation of the divergent series can be prevented again by a careful application of the iε pre scription and the resulting non- perturbative contribution due to the renormalon obeys the causality condition.
Numerical modeling, design, and time-frequncy metrology applications of low-noise optical frequency combs
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Şenel, Çağrı.; Ünlü, Mehmet Burçin.
Optical frequency combs are very versatile tools, which revolutionized many ar eas, and enabled novel applications, especially in time-frequency metrology. In this thesis, I tried to convey a complete picture of the road that goes from conception through design to applications of optical frequency combs. To this end, implementation details of a free, publicly- available numerical pulse propagation simulation software, which can be used for, but not limited to, simulating and designing complete fem tosecond laser and amplifier systems, supercontinuum generation, and Raman lasers, is presented. An algorithmic design methodology, which was used for designing all of the femtosecond lasers and fiber amplifiers presented in this thesis, is introduced, which might pave the way to discovery of new laser pulse evolution regimes. Sev eral optical frequency combs were developed and utilized in time- frequency metrology applications. A fully-stabilized, robust, and transportable low-noise Yb-fiber optical frequency comb was developed and used for absolute frequency measurements of stabi lized lasers. High-harmonic mode-locking and repetition rate multiplication methods were used to generate ultra-low phase noise RF-MW signals with frequencies ranging from 78.125 MHz to 10 GHz by utilizing optical frequency combs. A femtosecond fiber laser that can be automatically mode-locked was also developed during these studies. Finally, an atomic frequency standard based on a passively mode-locked femtosecond laser was developed, for the first time to the best of my knowledge. Resulting 87Rb atomic frequency standard performed at a similar level to the commercial Rb atomic frequency standards, with much better phase noise at high frequency offsets.
Probing mechanical and chemical properties of biological materials by multiple modalities
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2022., 2022) Pesen, Medine Tuna.; Ünlü, Mehmet Burçin.
Mechanical and chemical alterations are the signature of most diseases, such as endometriosis, sickle cell, malaria, and cancer. Detecting such alterations has vital importance for early diagnosis and efficient treatments. In the first project, we aimed to establish a non-invasive diagnostic technique for endometriosis disease by detect ing chemical changes in blood serum using Raman spectroscopy and machine learning algorithms. We measured 49 endometriosis patients’ and 45 healthy women’s blood sera for this project. A diagnostic tool was built-up by applying machine learning algorithms (kNN and SVM) to the Raman spectral data. The results of this study indicated diagnosis sensitivity and specificity values of 80.5% and 89.7%, respectively. In the following study, we investigated the effect of repetitive bi-axial stretch-relaxation cycles on the individual erythrocyte cells with optical tweezers to assess their mechan ical response. According to the findings, the cells became stiffer with each stretch and became completely undeformable after several stretch-relaxation cycles. Also, we showed that stiffness changes with each stretching cycle as a sign of weak power law. Therefore, we showed that cell rheology is scale-free at a single and whole-cell level for the first time. In the last project, we studied the camel erythrocytes compared to the human erythrocytes in their mechanochemical features since camel erythrocytes have the most distinctive cell features among the vertebrates. Using optical tweezers, we quantified that camel erythrocytes are almost ten times more rigid than human ery throcytes. The chemical analysis by the Raman spectroscopy revealed the difference in the lipid:protein ratio of these two cell types.
A ramond-nevu schwarz string one end fixed
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2005., 2005.) Arapoğlu, A. Savaş.; Turgut, Osman Teoman.; Saçlıoğlu, Cihan,
We study an open string with one end free and the other fixed on a D0-brane as a qualitative guide to the spectrum of hadrons containing one very heavy quark. We first consider the bosonic degrees of freedom, then introduce the fermionic degrees offreedom through the world sheet supersymmetry. The mixed boundary conditions break half of the world sheet supersymmetry and allow only odd-moded a and even-moded d oscillators in the Ramond sector, while the Neveu-Schwarz oscillators b's become odd-integer moded. Boson-fermion masses can still be matched if space-time is 9 dimensional; thus SO(8) triality still plays a role in the spectrum, although full space-time supersymmetry does not survive. We quantize the system in a temporal-like gauge where X^0 ~ t. Although the gauge choice eliminates negative-norm states at the outset, there are still even-moded Virasoro and even(odd) moded super-Virasoro constraints to be imposed in the NS(R) sectors. The Casimir energy is now positive in both sectors; there are no tachyons. States for a' M^2 b 13/4 are explicitly constructed and found to be organized into SO(8) irreps by (super)constraints, which include a novel ``" operator in the NS and I in the R-sectors. GSO projections are not allowed. The pre-constraint states above the ground state have matching multiplicities, indicating spacetime supersymmetry is broken by the (super)constraints. A distinctive physical feature of the system is a slope twice that of the open RNS string. When both ends are fixed, all leading and subleading trajectories are eliminated, resulting in a spectrum qualitatively similar to the J/ and particles.
Multiparameter generalization of deformed particle algebras
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2004., 2004.) Arıkan, Ali Serdar.; Arık, Metin.
In this study, we generalize deformed boson and fermion algebras by considering as many deformation parameters as possible. The restriction on the number of deformation parameters emerges from the relations leaving the algebra consistent. In the bosonic part, we consider the Newton oscillator which is a one parameter generalization of the harmonic oscillator as a starting point. Then, we achieve the multiparameter generalization with d(d+1) parameters for d dimensional oscillator system. We also present the Fock representation of this oscillator. In the fermionic part, we first study the quantum group covariant two-parameter deformed fermion algebra. Then we show that if the quantum group symmetry is not preserved, then the number of deformation parameters in d dimension can be increased to 2(d-1). We study the limiting case where all deformation parameters go to zero such that the deformed fermionic oscillator reduces to the orthofermion algebra. Finally, we investigate the symmetry properties of this limiting case.
Dense molecular gas tracers in water megamaser galaxies
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2021., 2021.) Farhan, Ahlam.; Ercan, Nihal.
Since their discovery in 1979, water megamasers (H2OMMs) have been of special interest for both astrophysicists and cosmologists. The unique environment they are emitted from is interesting, being located inside accretion disks surrounding SMBHs. H2OMMs have helped astrophysicists accurately calculate SMBH masses and determine the kinematics of gas falling into the central SMBHs. More important, H2OMMs gave the golden chance to find the distance to a special kind of H2OMM galaxies, i.e., disk masers, accurately and directly, without any need to use standard candles or the cosmic microwave background (CMB) radiation. Unfortunately, detecting H2OMMs is very difficult, only 180 galaxies have been found out of more than 6000 galaxies searched for H2OMMs. The low detection rate is motivating many researchers to find a method that would help to increase the detecting rate. As H2OMMs are believed to be created inside dense gas clouds at galaxy centers, a correlation between dense gas and maser emission was suggested. Until now, this correlation has been barely studied. Only one work has been done so far in 2014 with a small sample number. In this work, we tried to study this possible correlation. A sample of HCN(J=1-0) and HCO+(J=1-0) emission lines was successfully collected and used as dense gas tracers via normalizing them by CO(J=1-0) emission lines, to get dense gas fractions. Subsequently, a sample of 30 H2OMM galaxies along with the dense gas fractions has been statistically analysed. Both of the dense gas fractions show a correlation with water maser emission, with HCO+ being stronger than HCN. Moreover, after removing (Ultra)Luminous IR Galaxies (U)LIRG sources, the HCO+ correlation becomes very tight and HCN correlation becomes stronger. Our results agree with previous works on dense gas as a favorable parameter to be used in H2OMM surveys. According to our analyses, HCO+ is preferable to HCN as a dense gas tracer in MM sources.
Quantum group structures associated with invariances of some physical algebras
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2005., 2005.) Kayserilioğlu, Ufuk.; Arık, Metin.
In this study, the anticommuting spin algebra is introduced and it is shownto be invariant under the action of the quantum group SOq=-1(3). Furthermore, itsrepresentations and Hopf algebra structure are studied and found to be closely resemble the similar results for the angular momentum algebra. The invariance propertiesof the bosonic and fermionic oscillator algebras under inhomogeneous transformationsare also studied. The bosonic inhomogeneous symplectic group, BISp(2d,R) , andthe fermionic inhomogeneous orthogonal group, FIO(2d,R) , are defined as the inhomogeneous invariance quantum groups of these algebras. The sub(quantum)groupsand contractions of these quantum groups are studied as a source for new quantumgroups. Finally, the fermionic inhomogeneous orthogonal quantum group is defined forodd number of dimensions and its sub(quantum)groups and contractions are studied.
Normal forms, nonlocal chaotic behaviour in sportt and NMR systems
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2002., 2002.) Perdahçı, Nazım Ziya.; Oğuz, Ömer.; Hacinliyan, Avadis.
Poincaré's theory of normal forms is applied to a number of simple chaotic Sprott flows that have resonant eigenvalues. It is shown that the normal form expansion can give significant information not limited to the local properties of chaotic attractors, but also, on nonlocal properties such as positive and zero Liapunov exponents for systems that have the Hopf bifurcation property. Existence of a zero Liapunov exponent is indicated if the system has hyperbolic fixed points. The method is not directly applicable where an eigenvalue of the linearized part vanishes, because of the complexity of the normal form. Rational transformations that change the eigenvalue spectrum of the linearized parts are employed on the Sprott C and E systems to obtain simpler systems. A proposel on the possible use of fractal analysis methods on functional MRI data and preliminary results on possible source of chaotic behavior inherent in nuclear spin systems are presented.
Some new cosmological applications of the energy-momentum squared gravity
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2021., 2021.) Uzun, Nebiye Merve.; Turgut, Osman Teoman.; Akarsu, Özgür.
We investigate two specific forms of a new-type of modified gravity called Energy Momentum Squared Gravity (EMSG) constructed by the addition of the term f(TµνT µν) to the Einstein-Hilbert action with a cosmological constant (Λ). First, we propose a new model of EMSG, called Energy-Momentum Log Gravity (EMLG) described by f(TµνT µν) = α ln(λ TµνT µν). This choice is made as a specific way of including new terms in the right-hand side of the Einstein field equations, resulting in constant effective inertial mass density and leading to an explicit exact solution of the dust energy density in terms of redshift. We look for viable cosmologies, in particular, an extension of standard ΛCDM model. The EMLG provides an effective dynamical dark energy passing below zero at large redshifts, accommodating a mech anism for screening Λ, in line with suggestions for alleviating some of the tensions between observational data sets prevailing within the ΛCDM model. We present a theoretical investigation and then constrain the free parameters of the model using the latest observational data, and discuss the results. Second, we simultaneously replace the spatially flat Robertson-Walker metric with its simplest anisotropic extension, and couple the CDM to the gravity via the EMSG of the form f(TµνT µν) ∝ TµνT µν. These two modifications can mutually cancel out, namely, the shear scalar can be screened completely, and reproduce mathematically the same Friedmann equation of the ΛCDM model. This evades the BBN limits on the anisotropy, and thereby provides an op portunity to manipulate the CMB quadrupole temperature fluctuation at the desired amount. We also discuss the consequences of the model on the very early times and far future of the Universe.
Measurement of heat, strain, and refractive index with multicore optical fibers
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2020., 2020.) Güvenç Kılıç, Sema.; İnci, Naci.
In this thesis, different types of multicore optical fibers are designed and employed in the optical sensing experimental setups to investigate their detection abilities of temperature, heat transfer rate, strain, and refractive index. In the first section of this comprehensive investigation, a four-core optical fiber is introduced as a calorimetric gauge for the measurements of one-dimensional heat transfer rates. Transient heat pulses from an Nd:YAG laser are sent onto Aluminum coated surface, provided that the laser pulse is focused onto one of the optical fiber cores only. As a result of this, a phase shift in the interference pattern occurs and the corresponding optical signal is recorded by a CMOS camera. In the second section, the thermal expansion property of a stainless-steel cylinder is used to generate a strain on the optical fiber cores. Multicore fiber is coiled around a solid stainless-steel cylinder to obtain a circular optical fiber loop and this cylinder is placed onto the heater. Heating the stainless-steel cylinder results in thermal expansion in radial direction and a shear strain in the optical fiber cores. Thereby, a phase shift in the transmission spectrum is observed and monitored by the camera. As a third study, a four-core fiber is proposed and demonstrated for temperature sensing and heat transfer rate measurements. Laser heat pulses from are sent onto this Aluminum coated surface, which results in a variation in the interference pattern as a result of change in the refractive index and physical lengths of the cores. In addition to this experiment, hot air vortices were sent onto Aluminum coated surface of the fiber and variations in interference pattern are recorded. In the last section, a seven-core optical fiber with two fiber Bragg gratings is employed to detect the refractive index changes of surrounding environment. A small portion of optical fiber is etched in order to further expose the outer six cores to the surrounding environment. As the outer cores are affected by both temperature and refractive index change, the center core is only affected by the temperature.
Adiabatic solutions in general relativity and boundary symmetries
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2020., 2020.) Kutluk, Emine Şeyma.; Bleeken, Dieter Van den.
We investigate adiabatic solutions to general relativity for a spacetime with spatial slices with boundary, by Manton approximation. This approximation tells us for a theory with a Lagrangian in the natural form, a motion that is described as a slow motion on the space of vacua-static solutions that minimize the energy- is a good approximate solution. To apply this to the case of general relativity we rst bring it to the natural form by splitting space and time and choosing Gaussian normal coordinates, where a spacetime is described by the metric on its spatial slices. Then following Manton we propose slow solutions such that each slice is a slowly changing di eomorphism of a reference slice, and thus each solution is described by a vector eld on the spatial slice. These solutions will have the property that the action will become a functional of the vector elds on the boundaries of the spatial slices. Moreover using the Hodge- Morrey-Friedrichs decomposition we will show that the constraints of general relativity will identify a unique solution for a given boundary value. Then we comment on the structure of the space of vacua which we show to be a (pseudo)-Riemannian homogeneous space. We illustrate our procedure for a speci c reference slice we choose: the 3d Euclidean round ball.
Light-matter interaction in photonic cavities
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2020., 2020.) Gökbulut, Belkıs.; İnci, Naci.
In this thesis, light‒matter interaction in several novel photonic platforms are investigated by time-resolved fluorescence lifetime measurements along with their theoretical and numerical analyses. Firstly, a simple leaky-mode wedge-type photonic nanocavity is proposed for the inhibition of spontaneous light emission of the CdTe/CdS quantum dots (QDs) by enforcing the suppression of optical modes under the weak coupling regime. Thus, for the first time, the inhibited spontaneous emission rate is achieved using a low Q-factor photonic structure. Secondly, a fluorescence lifetime imaging technique is employed to investigate the spontaneous transition rate of the BODIPY dye molecules, which are doped in a single PEG nanofiber. The spontaneous emission rate of the confined dye molecules is observed to inhibit and enhance periodically upon swelling of the nanofiber through its exposure to relative humidity, suggesting an efficient optical switch mechanism. In the third photonic platform, the critical role of the confined hybrid mode in a CdTe QDs-doped single nanofiber, which is partially decorated by gold nanoparticles, on the modification of the spontaneous emission dynamics of the fluorescent emitters in a low Q-factor photonic-plasmonic nanocavity is explored for the first time. This is followed by chemically growing hollow cylindrical nanocavities on the surface of a single polymer microfiber, which is uniformly doped with perovskite nanowires; the assembly is employed for the localization of light to meaningfully alter the emission rate of the fluorescent nanowires; due to interaction of the electromagnetic field of the whispering gallery modes, supported by the microfiber, with the boundaries of the cylindrical hollow nanocavities. Finally, a quasi-optical cavity, originating from the transverse Anderson localization of the light waves in an entirely three-dimensional disordered medium, formed by polymer medium with randomly positioned air bubbles, is realized as an alternative to conventional photonic cavities for the intense light localization to strongly enhance the spontaneous emission rate of the fluorescent emitters.
Frequency-resolved photoconductivity studies in amorphous chalcogenide films
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2000., 2000.) Akat, Ercüment.; Aktaş, Gülen, 1951- .
Frequency-resolved photocurrent measurements (FRPC) were carried out on amorphous chalcogenide films such as AsSeSb AsSe and AsSeTe, by a method called frequency-resolved spectroscopy (FRS) as a function of temperature (between 230 K and 395 K), of excitation intensity (between 40 and 600 ) and in the frequency range of 10 Hz to 1.1x104 Hz, yielding the lifetime distribution directly. The results indicate that the recombination is through distant pairs, since the lifetime does depend on the excitation intensity, through ; as at 295 K and at 395 K, rather than by geminate (twin) pairs. It can also be concluded that there is a continuous distribution of states in the mobility gap implied by the value of the exponent 0.72 (in ), which is between 0.5 and 1.0, the figures that according to Rose (1978), correspond respectively to monomolecular and bimolecular recombination. The temperature exponent in the relationship has turned out to be between 7 and 10 for a frequency of some (=200 Hz); and 11 and 13 for (=3322 Hz); the value of the exponent showing a diminishing behaviour as the generation rate (intensity of excitation) is elevated. Finally, the exponent in the relationship is found to depend on both temperature and the generation rate in a manner such that the photocurrent will be more sensitive (inversely) to the frequency of excitation as either one of the above variables or both are lowered.
Mechanical characterization of cells and tissues by scanning acoustic microscopy and optical tweezers
(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2020., 2020.) Demirkan, İrem.; Ünlü, Mehmet Burçin.
Cell and tissue mechanics has a vital role in several pathological and physiological processes. Mechanical perturbations produce numerous biological processes within a cell or a tissue, which can result in mechanical changes. Thus, examination of the underlying mechanical characteristics of biological surroundings is a promising and important research area for the diagnosis and prognosis of diseases, especially in cancer. To delve into this, we rst proposed the application of Scanning Acoustic Microscopy (SAM) for monitoring di usion of sodium ion, which is known to regulate bodily functions and biological processes, by time-dependent acoustic impedance measurements on the soft tissue-mimicking agarose phantom. We established a linear correlation between the alterations in the phantom concentration and its acoustic impedance distribution. The second question in this thesis was to whether establish a correlation between cumulative irradiation doses and the mechanical properties of the human teeth or not. Here, 320 MHz SAM in micrometer resolution was used to characterize the acousto-mechanical e ects on human teeth after radiation therapy application, which is applied to cure head and neck cancer patients. Apart from SAM, this thesis introduces a dual-beam optical tweezers study for the quanti cation of red blood cell deformability as a determinant factor to assess cell reactions to radiotherapy on the cell cortex level. These two imaging techniques were used to understand the mechanical variations in response to exterior stimuli in biological matter. For each experiment, this thesis revealed the advantages and disadvantages of these methods and the reasons behind the ndings from the physical and biological points of view.
Scaling solutions of N = 2 supergravity and holography
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2019., 2019.) Mirfendereski, Delaram.; Bleeken, Dieter Van den.
The 4D N = 2 SUGRA is the low energy limit of Type II string theory compactiﬁed on a Calabi-Yau three-fold. The theory contains BPS black holes with multi centered conﬁgurations consisting of bound states of dyonic single-centered black holes. These stationary solutions have an intrinsic angular momentum originating in electro magnetic interaction between charges. A subset of this family called “scaling solutions” for which the relative distances between centers are constrained up to a scale. Contrary to the asymptotically ﬂat non-scaling cases, scaling solutions have an AdS2 factor in their asymptotic geometry and their total angular momentum vanishes. We showed that the AdS2 spacetime at far inﬁnity is equipped with an S2 ﬁber called a “twist”. It can be thought of as a rotating two-sphere whose angular velocity is determined by the total charge and the total dipole moment of the black hole. In the absence of an angular momentum, the twist can be considered as a topological hair of this family of supersymmetric black holes. It provides a new asymptotic observable to distinguish diﬀerent black hole solutions. Moreover, to have a better understanding of the twist and its potential role in a quantum theory of supersymmetric black holes, we S2 reduced the asymptotic geometry of the scaling solutions. That leads to an eﬀective 2D theory with an AdS2 geometry and a single U(1) gauge ﬁeld presenting the twist. Consequently, one would expect the AdS/CFT correspondence to clarify the role of the twist as a new hair of these black holes.
Linac and damping ring designs of the future circular e+e− collider of CERN
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2019., 2019.) Oğur, Salim.; Özcan, Veysi Erkcan.; Zimmermann, F.
CERN has started the design of a 98 km circular positron-electron collider as a ﬁrst step of the global Future Circular Collider (FCC) project since 2014. The FCCe+e− is being designed to furnish high luminosities for Z, W, H, and t¯ t particles with diﬀerent energies and currents in the collider and it will restore the charge decrease due to collisions, scatterings etc. via quasi-continuous top-up injection. However, the pre injector chain will keep its present course yet cycle distinctly through the 4 operational stages in the collider. The injector complex will consist of a 6 GeV S-Band linac, a damping ring at 1.54 GeV and pre-booster ring to reach 20 GeV for injection to the main booster, which is located in the same tunnel as the collider, which will accelerate the beams to the ﬁnal energies for collisions. As an alternative option to the pre-booster ring, a 20 GeV linac is also considered. The positrons will be created by impinging onto a hybrid target inside the linac at 4.46 GeV and accelerated in the remaining part of the linac up to 1.54 GeV and get matched to be transferred into the damping ring for radiation damping. The designs of the linacs and damping ring will be the foci of this thesis, as well as the optimisation of bunch schedules of the whole injector complex up to the collider. The beam optics designs are partially applied at SuperKEKB linac of KEK by hands-on operations. The beam based misalignment studies and beam jitter source survey have taken place which are also presented in this thesis.
Spin orientations and spin-orbit torque in magnetic nanostructures
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2019., 2019.) Karakaş, Vedat.; Özatay, Özhan,
In this thesis three diﬀerent spintronic devices and their static behaviours are studied under external magnetic ﬁeld or applied current. We started to work on the static properties of magnetic antivortices, formed in a special geometry called astroid, and achieved stable antivortex nucleation, annihilation and manipulation. We also observed the response of antivortex to the external magnetic ﬁeld. Another study was the observation of magnetic radial vortices in the presence of interfacial Dzyaloshinskii-Moriya interaction (i-DMI) in Pt/CoFeB/Ti nanopillar multilayers. By tuning the magnetic anisotropy via interface engineering and postpro duction treatments, we achieved to stabilize a variety of magnetic conﬁgurations such as Ńeel skyrmions, horseshoes and radial vortices at room temperature and under zero bias ﬁeld. Lastly, we have focused on the observation of self-torque of (100) oriented IrMn3 antiferromagnet. We observed the exchange ﬁeld shift with respect to applied current. Additionally, we studied the eﬀect of spin orbit torque by measuring DC current de pendence of Hall bar resistance. The eﬃciency of diﬀerent types of heavy metals, which have diﬀerent spin Hall angles, was studied to compare their resistance variation of the device.
A study of non-cool core galaxy clusters and the feedback from their central active galactic nuclei
(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2019., 2019.) Tümer, Ayşegül.; Ercan, E. Nihal.
The study of the brightest cluster galaxy (BCG) coronae of non-cool core (NCC) galaxy clusters and their central active galactic nuclei (AGN) is crucial for the under standing of the BCG’s role on galaxy cluster evolution as well as the activation of the cooling and heating mechanism in the central regions of galaxy clusters. In this thesis, the X-ray properties of the intracluster medium (ICM) of a sample of NCC galaxy clusters and their BCG interstellar medium (ISM), along with their central AGN is investigated using archival XMM-Newton observations. For this purpose, a joint spec troscopic and imaging method is proposed and applied to the sample. It is found that, AGN plays an important role on the ICM structure and the heating of the central regions of the clusters, although at smaller spatial scales compared to that of cool core clusters. In addition, BCG coronae seem to preserve their structural integrity and are isothermal, despite the disturbed morphology and merger history of the cluster. Fur thermore, a chemically rich BCG tail structure is discovered that extends up to 40 kpc even though they are assumed to be rare in galaxy populations.

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