LEE- Fizik Mühendisliği Lisansüstü Programı

Bu topluluk için Kalıcı Uri

http://hdl.handle.net/11527/19275

Gözat

Cosmological interacting models via energy-momentum squared gravity

(Graduate School, 2024-06-24) Bulduk, Bildik ; Akarsu, Özgür ; Katırcı, Nihan Ayşe ; 509201113 ; Physics Engineering

It was recently shown in the literature that gravity models that modify the material part of the standard Einstein-Hilbert action with $f(\mathcal{L}_{\rm m})$, $f(T)$, and $f(T_{\mu\nu}T^{\mu\nu})$ terms are equivalent to general relativity, encompassing non-minimal matter interactions between the material field and its accompanying partner, uniquely formed by the function $f$. In Energy-momentum squared gravity (EMSG), the ``squared" terminology arises from the self contraction of EMT $f(T_{\mu\nu}T^{\mu\nu})$ added to Einstein Hilbert action, nontrivial interaction kernels have been obtained and these models diverge from phenomenological interacting models (constructed in ad hoc way); this is due to the function $f$ and its variations with respect to both its argument and the metric, which intricately intertwine the interaction kernel $\mathcal{Q}(f,\delta f/\delta\mathbf{T^2},\delta, \mathbf{T^2}/\delta g^{\mu\nu})$. This makes the interaction kernel as Equation of State (EoS) parameter dependent as well. Bianchi identity $\nabla^{\mu}G_{\mu\nu}=0$ implies the conservation of total energy momentum tensor (EMT of the standard source plus its EMSF partner's), $\nabla^{\mu}(T_{\mu\nu}+T_{\mu\nu}^{{\rm EMSF} })=0$, leading cosmological models having an interaction between these sectors $\nabla^{\mu}T_{\mu\nu}=\mathcal{Q}_{\nu}$ and $\nabla^{\mu}T_{\mu\nu}^{\rm EMSF}=-\mathcal{Q}_{\nu}$ where $\mathcal{Q}_{\nu}\neq0$. In this thesis, different than the literature, still consistent with the Bianchi Identity, we focus on a scenario where the sector comprising conventional fluids (standard material fields) overall interacts minimally with the sector associated with their EMSF partners, i.e., satisfying $\nabla^{\mu}T_{\mu\nu}=0=\nabla^{\mu} T_{\mu\nu}^{{\rm EMSF}}$. Specifically, we consider the case characterized by $\mathcal{Q}=0$. Accordingly, we will consider a two-fluid model (perfect fluids described by constant EoS parameters) leading to the following conservation equations, $\nabla^{\mu}\left(T_{\mu\nu,1}+T_{\mu\nu,2}\right)=0$, and $\nabla^{\mu}\left(T_{\mu\nu,1}^{\rm EMSF}+T_{\mu\nu,2}^{\rm EMSF}\right)=0$ where we name the partner arisen from EMSG corrections as ``Energy Momentum Squared Field" (EMSF). We will explore this choice in detail within the framework of scale-independent EMSG which introduces a simple interaction kernel: a kernel linear in energy density. Then, we examine alternative cosmologies wherein the sector comprising conventional fluids minimally interacts with the sector associated with their EMSF partners, represented by $\nabla^{\mu}\left(T_{\mu\nu}^1+T_{\mu\nu}^{\rm 2}\right)=-\nabla^{\mu}\left(T_{\mu\nu}^{\rm EMSF1}+T_{\mu\nu}^{\rm EMSF2}\right)=\mathcal{Q}_{\nu}$ with $\mathcal{Q}_{\nu}=0$, diverging from the more commonly studied scenarios in literature where $\mathcal{Q}_{\nu}\neq0$. We also show that this model is reminiscent of the cosmological model with energy exchange studied by Barrow and Clifton in [Phys. Rev. D 73, 103520 (2006)] where the interaction term is taken ad hoc to be proportional to energy density, $\mathcal{Q}(H\rho)$. Unlike their model, the coefficients in our work are not arbitrary constants but are dependent on the species. Moreover, with an additional sector associated with the EMSF partners of the conventional fluids in the Friedmann equation, it is possible to negate one of the fluid's contributions in the Friedmann equation via its EMSF partner for a specific choice of $\alpha$ and two sources may superpose in their energy densities in the Friedmann equation, resulting in a joint (degenerate) scale factor dependence even if $w_1 \neq w_2$ reproducing interesting cosmologies such as power-law universes where the scale factor of the universe grows as a EoS parameter dependent power of time in the presence of a perfect fluid and vacuum energy density/stiff fluid, de Sitter universe in the presence of a perfect fluid and vacuum energy density. In this thesis, we show a simple mathematical description of the exchange of energy between two standard fluids from matter modified theories within GR choosing the simplest case study, yet some non-trivial functions/behaviors are favored by observations to alleviate tensions, non-linear interactions and non-linear energy density contributions from matter-type modified theories which may work for the change of direction of energy transfer in dark sector are prospects for future research.
Ho(1-x)ErxNi2B2C yapısında gözlemlenen burgaç oluşumu

(Lisansüstü Eğitim Enstitüsü, 2022-01-11) Gündoğdu, Sultan Süleyman ; Ramazanoğlu, Mehmet Kerim ; 509171117 ; Fizik Mühendisliği

Nadir toprak elementlerinin farklı oranlardaki katkılanmaları ile Ho(1-x)ErxNi2B2C (x = 0, 0.25, 0.50, 0.75, 1) tek kristal numunelerdeki manyetik düzen gerek manyetizasyon deneyleriyle gerekse nötron difraksiyon deneyleriyle incelenmiştir. Kristal yapıda olan numunelerimizden nötron deneyleri sırasında güçlü sinyaller elde edebilmek adına elimizdeki birden çok tek kristalin birlikte yönlendirilmesi, Laue X-ışını ölçümleriyle, Kanada'da (Hamilton, Ontario) McMaster Üniversitesi bünyesinde bulunan Brockhouse Institute for Materials Research (BIMR)'de yapılmıştır. Manyetizasyon ve manyetik duygunluk ölçümleri de yine aynı enstitünün PPMS (Physical Properties Measurement Systems), yani Fiziksel Parametreler Ölçüm Düzeneği, manyetik ve Küçük Açı Nötron Saçılması KANS (Small Angle Neutron Scattering, SANS) deneyleri ise Washington DC, ABD'de kurulu bulunan National Institute of Standards and Technology (NIST) enstitüsünün nötron kısmı olan NIST Center for Neutron Research (NCNR) laboratuvarında sırasıyla BT-9, NG-7, NG-5 ve BT-7 deney mahalleri (beam-line) kullanılarak gerçekleştirilmiştir. Geçiş sıcaklığı, R2CuO4 süperiletken bileşiğinde, R'nin Er ve Ho olduğu durumlar için yaklaşık 10 K'dir. Ne var ki, bu malzemeleri asıl ilginç kılan husus, bunların, içlerinde tam da bu sıcaklıklar civarında bir manyetik düzen oluşturmalarıdır. Nadir toprak kısmın yapısına bağlı olmakla birlikte süperiletkenlik ile manyetizma arasındaki bağlaşma, yeniden girilen süperiletkenliğin oranlı ve oransız antiferromanyetizma ile eşzamanlı olarak varolmasından tutun da zayıf bir ferromanyetik düzen ile tamamıyla oransız antiferromanyetik bir spin modülasyonunun birlikte varolmasına kadar çeşitli fazların oluşmasına sebebiyet verir. Tüm bu fazlar süperiletkenlik ile eşzamanlı olarak varolurlar. "Saf" bileşiklerdeki manyetik düzen RKKY manyetik etkileşmesi ile açıklanmış olup katkılı numunelerdeki manyetik yapıyla saf numunelerin manyetik yapıları üzerinde yapılan nötron saçılması deneylerinin sonuçları da karşılaştırılmıştır. Saf Ho yapısında Er katkısının artmasıyla 1. Derece düzenli fazdan 3D XY düzen değerlerine doğru bir değişim gözlemlenmiştir. Özellikle Er katkısının oranı 0.75 olduğunda manyetik pik diğer numunelerden daha farklı bir yansıma oluşturmuş ve bu pik, daha önce başka bir araştırmada R2CuO4 (R = Nd ve Pr) kuantum mıknatısında gözlemlenmiş olan manyetik tepe profiline benzetilmiştir.
Interaction between magnetized stars and disks

(Lisansüstü Eğitim Enstitüsü, 2021) Türkoğlu, Murat Metehan ; Ekşi, Kazım Yavuz ; 709913 ; Fizik Mühendisliği

X-ray binary systems consist of a compact object, such as a neutron star, white dwarf or black hole, and a normal star that transfers mass to this compact object. X-ray binary systems are split into two groups depending on the mass of the donor star. If the mass of the donor star is Md ≤ 1M⊙, these kinds of systems are called LMXB and if the donor star mass is Md > 10M⊙, these systems are known as HMXB. The other component of the X-ray binary systems are compact stars: white dwarfs, neutron stars or black holes. The observed X-ray power of these systems originate from the gravitational potential energy released by the accretion of matter onto the compact star and depends on the compactness, M∗/R. In LMXB, matter from the outer envelope of the donor star may may be transferred to the compact star by Roche lobe overflow. In HMXB, matter from the outer envelope of the donor star may be transferred to compact star by stellar wind. In both cases because the matter transferred from the donor star has angular momentum, the matter can not accrete on to the compact object directly; instead an accretion disk forms. The physical parameters that define the interaction between a neutron star and a surrounding disk are the magnetic field and angular velocity of the of the compact star, and the mass flow rate in the disk. The interactions occur in three different stages: i-) Mass accretion stage: If the inner radius of the disk, Rm, is smaller than the corotation radius, Rco, the matter follows the magnetic field lines and flow to the polar caps of the neutron stars. ii-) Propeller stage: In this stage, Rm > Rco, the matter at the inner region of the disk meets with more rapidly rotating field lines attached to the star. A decline may occur in the observed X-ray flux because the mass accretion is centrifugally inhibited. iii-) Radio pulsar stage: If the inner radius of the disk is larger than the light cylinder radius, RL, an interaction can not occur between the neutron star and the disk. In this stage, the cause of the observed X-ray flux is the slowing down of the rotation of the neutron star. The QPO are thought to be generated in regions close to the neutron star and the inner part of the disk. Therefore, special types of QPOs provide direct evidence for disk-magnetosphere interaction. In this study, models were created by using both observational physical parameters (period, period derivative, luminosity, etc) and QPOs. The observation that the X-ray luminosity does not change significantly during transitions to the spin-down stage led to MTD of Ghosh & Lamb in 1979. In this model, magnetic field can thread the disk by instabilities between disk and magnetosphere and the presence of turbulence in the disk. The magnetic field lines slip around the disk due to the differential rotation between disk and the neutron star. According to the Ghosh & Lamb model, there is a stable region in which the twisted magnetic field balances the spread magnetic field around the disk. In this way a toroidal magnetic field is generated. However, as long as the magnetic field gets twisted around the disk, arbitrarily strong toroidal magnetic field is generated and such strong magnetic fields can destroy the disk. Because of the problems mentioned above, Ghosh & Lamb model have important inadequacies. The magnetic field lines that penetrate the disk beyond the corotation radius slow down the neutron star. The net torque acting on the neutron star is the sum of the material torque which spins up the star and the magnetic torque which slows down the star. Toroidal magnetic field is an important factor that determines the net torque. In order to understand the long-term evolution of the neutron star, it is important to specify how the torque depends on the fastness parameter, ω∗. As LMXB have weak magnetic fields, it is hard to observe the spin change of the system. Also HMXB have stellar winds that affect the torque and observed luminosity, the relation between the fastness and the torque can not be specified, sensitively. For these reasons, we choose 4U–1626 67 which has high magnetic field and accretes from a low mass donor star. 4U–1626 67 underwent two torque reversals in June 1990 and February 2008. We used the torque reversal data and explored the coherence between observational data and some torque models in the literature. It is discovered that each nearby galaxy host one or two "ultraluminous X-ray sources" (ULXs) whose luminosity exceed the Eddington limit for a solar mass object. It was initially assumed that the ULX host IMBH but later with the discovery of X-ray pulsations from some of these objects (e.g M82 X-2, ULX NGC 5907, ULX NGC 7793 P-13, NGC 300 ULX1, M51 ULX-7, NGC 1313 X-2 and Swift J0243.6+6124) showed that they at least some of them are neutron stars. Population studies indicate that the accreting neutron stars are common sources in the ULX population. In this thesis, we investigate the surface magnetic field dipole strength, beaming fraction and fastness parameter of the, PULX, taking into account the accretion flow in the super-critical regime, beaming of X-ray emission and the reduction of the scattering cross section in the presence of a strong magnetic field. We used three different methods for determining the magnetic fields of the PULX: i-) We assume the system to be near torque equilibrium. ii-) We rely on the spin-up rate and solve the torque equation. iii-) We assume the systems to be accreting at the critical rate. This critical rate depends on the electron scattering cross-section determined by the super-critical magnetic fields. The plan of the thesis is as follows: In Chapter 1, the main ideas of the star-disk interactions are given. In Chapter 2, the flux, the period and the period derivative data of 4U-1626 67 embracing the torque reversal events in June 1990 and February 2008 are analysed and compared with Ghosh-Lamb model and some other models in the literature. In Chapter 3, the magnetic fields of the pulsating X-ray sources are calculated using three different assumptions. Also, as the beaming fraction depends on the inner radius of the disc which in turn depends on the mass accretion rate, we find that the isotropic-equivalent luminosity of the source does not depend linearly on the mass accretion rate. In Chapter 4 all of the results are discussed
Modeling the magnetosphere of neutron stars with numerical simulations

(Graduate School, 2022-06-27) Çıkıntoğlu, Sercan ; Ekşi, Kazım Yavuz ; 509152107 ; Physics Engineering

In this thesis, I study the magnetosphere of a neutron star in two different contexts. Firstly, I investigate the interaction between an accretion disc and the magnetosphere of a neutron star. I perform a number of two spatial dimensional general relativistic magnetohydrodynamics simulations within the ideal magnetohydrodynamics limit by employing Black Hole Accretion Code. I vary the strength of the dipole magnetic field of the star while keeping the other parameters fixed. I initialise a thick torus around the star and trigger a magnetorotational instability to drive the disc towards the star. I determine the magnetospheric radius numerically and then investigate how it depends on the magnetic dipole moment and the mass accretion rate. I find that the magnetospheric radius is proportional to the magnetic dipole moment as in the Newtonian case, i.e., r_{msph}\propto \mu^{4/7}, but also that it depends weakly on the mass-accretion rate. Also, I calculate the mass accretion rate and the angular momentum transfer rate. I investigate the correlation between the mass accretion rate and the matter part of the angular momentum transfer rate and find that they are almost linearly correlated. On the other hand, I observe that the total angular momentum transfer rate fluctuates vividly even though the system reaches a steady-state. The amplitudes of the fluctuations are so large that the angular momentum transfer rate sometimes takes negative values. These could be associated with the spin fluctuations observed in X-ray pulsars. I observe that the discs driven by the magnetorotational instability are quite different than the constant alpha-viscosity discs. The disc quantities within the disc such as the pitch factor and the alpha-parameter exhibit fluctuations larger than their time averages. Secondly, I investigate newly born magnetars by modelling X-ray afterglow lightcurves following gamma-ray bursts. I employ the magnetic dipole torque of the plasma-filled magnetosphere and a decaying magnetic field. I find approximate analytic solutions for the torque equations. By modelling the X-ray afterglows within this model, I determine the initial period, the inclination angle, magnetic dipole moment as well as the time scale of the decay of the magnetic moment and its asymptotic value. Finally, I study fallback discs with low-angular momentum, hence short lifetime, around newly born neutron stars in the context of X-ray afterglow lightcurves following gamma-ray bursts. Some models of gamma-ray burst afterglows invoke fallback discs interacting with the magnetospheres of nascent millisecond magnetars. Initially, the accretion rate in such a disc is very high, well exceeding the rate required for the Eddington limit. Inner parts of such a disc get spherical due to the radiation pressure and the mass accretion rate within the spherization radius is regulated so that the Eddington luminosity is exceeded only logarithmically. This restrains the achievable luminosity produced by the disc-magnetosphere interaction to very low levels compared to the typical luminosities observed in the X-ray afterglow light curves. Due to the high magnetic field and the spin frequency of the magnetar, the disc cannot penetrate the light cylinder and cannot interact with the magnetosphere until the star slows down sufficiently by magnetic dipole radiation. Accordingly, the interaction of the fallback disc with the star during the first few days in the life of the star is very unlikely. Even if they interact, it would be hard to observe since the required drop in the spin frequency would lead to an abrupt drop in the X-ray luminosity which is larger than the sensitivity range of Swift's XRT telescope. We conclude that a fallback disc model can only address sources with unusually low luminosities.
Newtonian perturbation theory in cosmology: From inflation to large-scale structure

(Graduate School, 2025-01-28) Kinsiz, Rumeysa ; Arapoğlu, A. Savaş ; 509211113 ; Physics Engineering

Cosmology is the scientific study of the physical characteristics of the universe, its beginning, development and organization, based on observational outcomes and theoretical foundations. The Lambda-CDM model is currently one of the most popular theories in cosmology. This model of the universe outlines the behavior of the cosmos through the use of dark matter and energy. The cosmological constant (dark energy) is an energy density used to describe the acceleration of the expansion of the universe. From this model, it can be seen that cold dark matter and dark energy contribute greatly to the total mass-energy density of the universe. While dark matter affects the dynamics of galaxies and large-scale structures, dark energy drives the accelerated expansion of the universe. However, ongoing problems led to the formulation of "inflation theory." Inflation theory is a convincing paradigm that solves fundamental questions like the flatness problem and the horizon problem, which ask why the universe appears nearly flat and why distant parts show similar properties. Inflation hypothesis argues that the universe had a rapid expansion during its formative period, which mitigated initial anomalies and established the foundational conditions for the world we observe today. Numerous mathematical models have been introduced to advance inflation theory, including scalar field inflation, Starobinsky inflation, and Higgs inflation, which explain the dynamics of early expansion and the transformation of primordial perturbations into extensive cosmic structures. We also need observational evidence from the early cosmos to prove these theoretical hypotheses. The cosmic microwave background (CMB) and large-scale structure (LSS) are two of the most critical. CMB is described as the conditions immediately after the Big Bang and gives us a perspective on what the early universe was like, while Large Scale Structure (LSS) refers to the general arrangement of galaxies and matter throughout cosmic history. To form these structures one has to consider both the observation of them and the processes by which they are formed. The growth of cosmic structures is mainly due to gravitational collapse, which amplifies small density perturbations in the early universe. This process is also understood by using Newtonian perturbation theory, which is a useful approach to describing how early anisotropies evolve into the large scale structures we see today. The concepts of Jeans length, growth function, transfer function and power spectrum are useful tools to study the evolution of structures and distribution of matter and to generate theoretical data to compare with experimental data. However, the examination of nonlinear evolution show that the creation of xxi structures has a more complex background. Different theoretical instruments have been used to analyze this complicated structure. The spherical collapse model elucidates the evolution of overdense regions into stable entities like galaxies and galaxy clusters, whereas the idea of virialization delineates the equilibrium state of these structures, especially dark matter halos. Moreover, the Press-Schechter theory offers a statistical framework for elucidating the creation of cosmic formations. This theory provides an analytical approach to assess the mass distribution of collapsed entities. The mass function forecasts the probability of structure formation across various masses, whereas biasing delineates the correlation between observable galaxies and the fundamental density field. Comprehending the genesis and evolution of the universe necessitates a comprehensive methodology that integrates theoretical, observational, and statistical analyses. Newtonian perturbation theory is a crucial instrument for examining large-scale structures, with its validity corroborated by empirical evidence and simulations.
Non-relativistic gravity in three-dimensions

(Lisansüstü Eğitim Enstitüsü, 2021) Zorba, Utku ; Özdemir, Neşe ; 692464 ; Fizik Mühendisliği

In this thesis, we examined the non-relativistic three-dimensional $\mathcal{N}=2$ supergravity theories. These gravity theories are based on a symmetry algebra in which Lie algebra admits non-degenerate, invariant, and symmetric Killing form. We considered a supersymmetric extension of non-relativistic symmetry algebras from which we constructed Chern-Simons actions, and as a result, we have obtained their gauge transformations and field equations, and the matter couplings. In addition, we developed a framework to construct Lie algebra expansion to obtain extended Schrödinger algebra for the first time in the literature, and this result will be used for our future plan for constructing matter multiplets that transform under supersymmetric extended Schrödinger symmetries. The first chapter of the thesis presents a sufficient groundwork for the following sections. Our purpose is to elaborate on Newton-Cartan geometry, Newton-Cartan gravity, three-dimensional Einstein gravity, Chern-Simons formalism, and finally basics of spinors in three dimensions. Having collected these tools, we apply the corresponding formalism into three-dimensional non-relativistic symmetries. With the term non-relativistic symmetry we imply that all the algebras that we will consider next sections are an extension of Galilei algebra, since we designate the symmetry algebras as non-relativistic. In Ch. 2, we establish the supersymmetric extension of the extended Newton-Hooke, Lifshitz and Schrödinger algebras and construct the corresponding Chern-Simons supergravity models. The extended Newton-Hooke superalgebra admits two distinct non-degenerate invariant bi-linear forms that gives rise to two different supergravity models with the same equations of motion. These two models are particularly different in terms of the parity of the bosonic actions. In particular, we showed that there is an exotic non-relativistic model such that parity-even field equations arise from a parity-odd Lagrangian. We then showed that it is possible to improve the extended Bargmann superalgebra with dilatations (without including non-relativistic special conformal symmetry) which we called the extended Lifshitz superalgebra and also established the Chern-Simons extended Lifshitz supergravity action. In the final step, we include the nonrelativistic special conformal symmetry and establish the extended Schrödinger superalgebra and the corresponding Chern-Simons extended Schrödinger supergravity action. We consider our result as a first step to construct an off-shell formulation for the extended Bargmann supergravity and its matter couplings. In Ch. 3, we present a three-dimensional non-relativistic model of gravity that is invariant under the central extension of the symmetry group that leaves the recently constructed Newtonian gravity action invariant. In particular, we show that the three-dimensional model is the contraction of a bi-metric model that is the sum of the Einstein gravity in Lorentzian and the Euclidean signatures. Moreover, the model is distinct from the Newtonian gravity both at the level of action and the matter coupling. By choosing fields appropriately, we show that this action can be obtained by a contraction procedure. Our model is of the Chern-Simons type, which allowes us to establish the supersymmetric completion by extending the algebra with five supersymmetry generators. The supersymmetric completion of this action provides one of the very few examples of action for non-relativistic supergravity. In Ch 4, we present a Lie algebra expansion method to generate higher-order three-dimensional Schrödinger algebras. Our construction relies on a recent novel three-dimensional non-relativistic conformal Galilei algebra that we used as a core algebra. By employing the Lie algebra expansions, we first recovered the extended Schrödinger algebra and obtained a new higher-order Schrödinger algebra which we refer to as the enhanced Schrödinger algebra. We, next, truncate the non-relativistic conformal symmetry generators and find a new algebra that goes beyond the three-dimensional extended Bargmann algebra. In particular, we show that the symmetry algebra that was proposed as the symmetry algebra of action for Newtonian gravity is not uniquely defined but can be closed with three parameters. We also show that for a particular choice of these parameters the Bargmann algebra becomes a subalgebra of the extended algebra and one can introduce a mass current in a Bargmann-invariant sense to the extended theory.
Pulsar magnetospheres and intra - pulse variability by plasmoid formatio

(Graduate School, 2024-12-06) Andaç, İbrahim Ceyhun ; Ekşi, Kazım Yavuz ; 509152103 ; Physics Engineering

Pulsars are remnants of massive stars that have endured supernova explosions. They are rapidly rotating neutron stars with extraordinary magnetic fields. These celestial objects serve as natural laboratories for studying physics under extreme conditions that includes physics of dense matter, strong-field gravity, and relativistic plasma processes. Their magnetospheres are dominated by strong magnetic fields and populated by an electron-positron plasma. It exhibits complex dynamics that give rise to a broad spectrum of electromagnetic emissions which spans from radio waves to gamma-rays. Among the various phenomena observed, pulse-to-pulse variability offers critical insights into the underlying plasma processes and emission mechanisms. It also includes intra-pulse structures such as subpulses. The precise origins of this variability remain incompletely understood despite extensive research. It is posing a significant challenge in pulsar astrophysics. What is investigated in this thesis is the role of plasmoid formation induced by magnetic reconnection in pulsar magnetospheres and its impact on intra-pulse variability. Magnetic reconnection in the current sheet beyond the light-cylinder radius ($\rlc$) is a fundamental plasma process. It can lead to the formation of plasmoids—coherent structures consisting of plasma and magnetic fields. The intermittent nature of reconnection and the hierarchical merging of plasmoids can introduce significant fluctuations in the emitted radiation. It is potentially accounting for the observed pulse-to-pulse variability and subpulse phenomena. To examine this hypothesis, we employ two-dimensional particle-in-cell (PIC) simulations using the \texttt{ZELTRON} code, modeling an orthogonal pulsar magnetosphere restricted to the equatorial plane. The simulations begin with a split magnetic monopole configuration and evolve into a quasi-steady-state force-free magnetosphere filled with electron-positron plasma. Plasma is continuously injected from the neutron star surface, ensuring a sustained supply of charged particles. The wind current sheet forms as two thin Archimedean spirals extending beyond $\rlc$, where magnetic reconnection triggers the tearing instability. This instability leads to the fragmentation of the current sheet into a dynamic chain of plasmoids, which evolve through processes of growth, merging, and outward propagation. Our simulations expose that plasmoids are predominantly formed near the light cylinder and subsequently grow and merge hierarchically as they move out at relativistic speeds. The size distribution of plasmoids ranges from small-scale structures on the order of $0.02 \rlc$ to large entities comparable to the neutron star radius ($\sim 0.3 \rlc$). Statistical analysis indicates that the plasmoid size distribution follows an inverse relationship with their width. This is consistent with theoretical predictions for hierarchical merging in relativistic reconnection processes. This distribution suggests that a multitude of small plasmoids coexist with fewer large ones, contributing differently to the emission characteristics. We compute synthetic synchrotron emission profiles by tracking particles and electromagnetic fields within the simulation. Our computation focuses on the incoherent high-energy emission originating from the current sheet. The results demonstrate that plasmoid formation leads to significant intra-pulse variability. This variability is manifested as bright subpulses superimposed on the main pulse profile. The subpulses predominantly appear on the leading edge of each pulse and exhibit a broad range of fluxes and durations. A key finding is the proportionality between subpulse flux and width in pulsar phase. This correlation indicates that the larger plasmoids produce brighter and broader subpulses. This relationship arises because larger plasmoids contain more energetic particles and occupy a greater angular extent. It enhances their contribution to the observed emission. The statistical properties of the subpulses align with the plasmoid size distribution with the number of subpulses following a power-law dependence on their flux and width. This correlation suggests that the observed intra-pulse variability is directly linked to the dynamics of plasmoid formation and evolution within the magnetosphere. The power-law behavior indicates that there is a significant probability of observing exceptionally bright and wide subpulses. This corresponds to the fact that the largest plasmoids formed through merging events while most subpulses are weak and narrow. Advanced analysis shows that the intermittent nature of magnetic reconnection leads to episodes where the current sheet thickens near the light cylinder. It temporarily suppresses plasmoid formation. These quiet periods result in reduced emission and are followed by recovery phases. The intense subpulses reappear in the recovery phases and this corresponds to the delayed fragmentation of the stretched current sheet. This behavior contributes to the overall variability observed in the emission profiles. It may explain observed phenomena such as nulling and mode changes in some pulsars. Our study offers several testable predictions.
Structural and upconversion luminescence properties of polyethyl methacrylate (PEMA) polymers doped with rare earths ions

(Graduate School, 2024-04-18) Bahuar, Thami ; Aktaş Kaya, Demet ; 509122112 ; Physics Engineering

This thesis aims to analyze the spectral characteristics of nanocrystal powders containing [CdNb2O6:Er3+] and [CdNb2O6: Er3+/Yb3+] by using polyethyl methacrylate (PEMA) crosslinked networks as the host matrix. The first study involved the incorporation of CdNb2O6:Er2O3(%1,5 Er3+) powders doped with Er3+ into both bulk linear polyethyl methacrylate (PEMA) and bulk PEMA crosslinked networks (gels) with varying amounts of crosslinker. The polymerization process was achieved through free-radical crosslinking copolymerization using 0.1 EMA (weight %) at a temperature of 60 °C. The X-ray diffraction (XRD) method was used to analyze the structures of Er3+ ions contained in bulk linear PEMA and PEMA gels. The crystalline grain size was determined using the Pielazsek grain distribution and the Scherrer equation techniques. By increasing the crosslinker amounts, the average-crystalline grain sizes reduced from 75 nm to 12.50 nm for CdNb2O6:Er2O3 (%1,5 Er3+) crystalines. Thus, it was shown that the distribution grew more evenly distributed. The Fourier transform infrared (FT-IR) spectra were used to monitor the functional groups present in both linear PEMA and PEMA gel samples. The obtained results were found to be in agreement with the XRD results for the two cases. The second part of this thesis included the synthesis of linear polyethyl methacrylate (PEMA) and crosslinked PEMA gels, which were doped with CdNb2O6: Er3+/Yb3+ nanocrystal powders. The synthesis was achieved using the process of free radical crosslinking copolymerization. The X-ray diffraction method was used to analyze the architectures of Er3+/Yb3+ ions contained in PEMA polymers. The average sizes of the crystalline particles were determined by using the Pielazsek particle distribution and the Scherrer equation. The particle sizes exhibited a reduction from 60 nm to 17 nm for CdNb2O6: Er3+/Yb3+ powders upon their incorporation into PEMA polymers. The morphological changes of polymer samples doped with CdNb2O6: Er3+/Yb3+ powders were observed using transmission electron microscopy and scanning electron microscopy. Luminescence spectra were measured at room temperature to explore the optical characteristics of polymer materials. The Er3+/Yb3+ ions, which was excited with a 975 nm diode laser, produced upconversion (UC) emissions using two-photon absorptions in the visible range. The intensities of UC (upconversion) and the quantities of absorbed photons were enhanced by increasing the quantity of crosslinker. The study examined how changes in the structure and morphology of the host polymer material affected color tuning, color coordinates, and color quality.
Transition dynamic in the LSCDM model: Implications for bound cosmic structures

(Graduate School, 2024-06-27) Çam, Arman ; Akarsu, Özgür ; 509201112 ; Physics Engineering

We explore the predictions of $\Lambda_{\rm s}$CDM, a novel framework suggesting a rapid anti-de Sitter (AdS) to de Sitter (dS) vacua transition in the late Universe, on bound cosmic structures. In its simplest version, $\Lambda_{\rm s}$ abruptly switches sign from negative to positive, attaining its present-day value at a redshift of ${z_\dagger\sim 2}$ i.e., $\Lambda_{\rm s} \equiv \Lambda{\rm sgn}(z_{\dagger}-z)$. We will show that in the case of an abrupt sign-switching cosmological constant, there occurs a type II (sudden) singularity at the transition redshift, $z_{\dagger}$, where the total pressure of the universe diverges to infinity and the total energy density remains constant and finite. To avoid type II singularity, one can ``smooth-out'' the sudden sign-switch and describe it by using sigmoid functions (e.g., $\tanh$, logistic). However, since this correction would introduce an additional parameter ($\sigma$) to the model, we decided to examine the scenario in which the sign change of the cosmological constant is abrupt. This will also allow us to study the behavior of structure formation in the most extreme case without adding an extra parameter to our analysis. We will start our analysis by studying the spherical collapse model for a universe that contains dust (consisting of cold dark matter and baryons) and cosmological constant ($\Lambda$). For this universe, we will derive the equations describing the dynamics of the overdensity as a function of the background universe. Due to the shell crossing---and consequently the breakdown of the homogeneity and isotropy after the turnaround---, one cannot use the Friedmann equations (i.e., spherical collapse model) to describe the dynamics of the overdensity. Thus, we must refer to the semi-Newtonian approach and use the virialization condition to describe its dynamics. In the next step, we will extend our analysis of the spherical collapse model to include $\Lambda_{\rm s}$CDM, by incorporating the sign-switching cosmological constant ($\Lambda_{\rm s}$) into our calculations. To understand this process more clearly, we will separate our discussion into three parts. In the first part, we will study the evolution of the overdensity, if it enters turnaround under the effect of the positive cosmological constant (i.e., $\Lambda_{\rm s} \equiv +\Lambda$). In the second part, we will discuss the dynamics of the overdensity, if it enters turnaround under the effect of the negative cosmological constant (i.e., $\Lambda_{\rm s} \equiv -\Lambda$). In the third and final part, we will discuss the halos that completely virializes before the AdS-dS transition, and study the effect of the type II singularity on the bounded cosmic structures. At a first glance, it's clear that depending on the time of the transition, the overdensity will be effected differently. In summary, we can identify three primary influences which effects the structure formation in the $\Lambda_{\rm s}$CDM model: (i) the negative cosmological constant (AdS) phase for $z > z_\dagger$, (ii) the abrupt transition marked by a type II (sudden) singularity, leading to a sudden increase in the universe's expansion rate at $z=z_\dagger$, and (iii) an increased expansion rate in the late universe under a positive cosmological constant for $z < z_\dagger$, compared to $\Lambda$CDM. We find that the virialization process of cosmic structures, and consequently their matter overdensity, varies depending on whether the AdS-dS transition precedes or follows the turnaround. Specifically, structures virialize with either increased or reduced matter overdensity compared to the Planck/$\Lambda$CDM model, contingent on the timing of the transition. Despite its profound nature, the singularity exerts only relatively weak effects on such systems, thereby reinforcing the model's viability in this context.

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