LEE- Fizik Mühendisliği Lisansüstü Programı

Bu topluluk için Kalıcı Uri

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

Gözat

Application of matrix product states for few photon dynamics and quantum walks in reduced dimensions

(Graduate School, 2021-12-02) Danacı, Burçin ; Subaşı, Ahmet Levent ; 509142102 ; Physic Engineering

Numerical simulations of low-dimensional quantum many-body systems have been a very active field in recent years. New techniques have enabled the experimental realization of these systems and have shed light on both theoretical and technological developments. However, the numerical simulations of these systems have been challenging due to the exponential growth of the Hilbert space with the system size. In addition, quantum correlations such as entanglement play an important role in many-body systems. Therefore approximate methods have been developed. One of the methods to simulate such quantum systems in one dimension is the Matrix Product States (MPS) Formalism. In this thesis, we concentrate on the application of MPS to quantum optical systems and quantum walks. For this purpose, we have developed a pedagogical numerical library that consists of functions responsible for the efficient representation of the wave function and its time evolution. We have tested the efficiency of these functions for different parameters. The quantum optical system we consider is a one-dimensional coupled cavity array interacting with a two-level system. One of the techniques to simulate the long-time dynamics of a quantum many-body system in a computationally manageable grid is to impose absorbing boundary conditions. We have applied absorbing boundary conditions in the form of an imaginary potential and determined the optimum parameter intervals for efficient simulation. Another objective of this thesis is to examine the photon dynamics and the decay of the two-level system from its excited state for different interaction strengths. We have shown that in the strong interaction regime where rotating wave approximation (RWA) is applicable, the results obtained from exact diagonalization and MPS simulations are in perfect agreement. For higher interaction strengths we have used polaron transformation to lower the effective interaction and applied RWA afterward. We have discussed the differences between the results in terms of photon numbers and the excited-state population of the two-level system. As part of this thesis, we have studied two types of discrete-time quantum walks. Firstly, we have considered a quantum walk with a single-phase impurity and investigated the effects of the bound states on its spatial localization and non-Markovianity properties. In Markovian systems, there is an irreversible flow of information from the system under consideration to its environment, whereas in non-Markovian systems some of this information flows back to the system. Our findings show that there is a strong relation between localization and non-Markovianity in this model. Secondly, we turned our attention to a quantum walk coupled with a spin chain environment where there is a dynamic spin attached to each site. Using our MPS algorithm, we have studied the relationship between the quasi-energy spectrum obtained from the exact diagonalization of finite systems, dynamical localization, entanglement entropy, and spin dynamics of this walk. We have observed that due to the extensive number of conserved quantities it possesses, this model is similar to the disorder-free localization models found in literature, where disorder is induced due to the interaction between the constituents of the system.
Charge density waves in transition metal di-chalcogenides: A comparison of fermi surface nesting and electron-phonon coupling

(Graduate School, 2024-05-16) Sanga, Cem ; Hakioğlu, Tahsin Tıuğrul ; 509201104 ; Physics Engineering

Charge density waves are ordered phases of matter in condensed matter physics. It is like a standing wave floating above the lattice which also has a phenomenon called periodic lattice distortion. A charge density wave and a periodic lattice distortion might come together on a material since electrical charges make the system come to a new ground state due to lattice-driven effects and the coupling between lattice and the electrons. A thought experiment made by Peierls pave the way going to the charge density waves. According to the Peierls, a 1-dimensional half filled band will go under a transition if the system is probed with a momentum of two times its wavevector. This is called Peierls instability. After several years, another scientist named Fröchlich wrote down a microscopic theory of electron-phonon coupling, which has the similar types of instability with the charge density waves. According to Peierls, an instability pave the way for a mechanism called Fermi surface Nesting. It says that when there is a q vector connecting the opposite edges of the Fermi surface there will be a instability just like the as in the earlier idea of Peierls distortion/instability. This idea of Fermi surface nesting is feasible in 1-dimensional materials but as one uses higher dimensional materials Fermi surface nesting starts to fail. The reason for that, in two dimensional Fermi surfaces it is not easy to connect any point on the edges os the Fermi surface directly. The geometrical shapes of the Fermi surfaces are much more complex than their 1-dimensional counterparts. The amount of points that can manage the connect with CDW vectors a not enough to create an instability and hence, a CDW. A good measure of Fermi surface nesting in a material is a function called static Lindhard susceptibility. If there is a divergence in this function at some q point, that q point would be a good candidate for charge density wave instability. At higher dimension, with Fermi surface nesting not working any more, new mechanisms are searched for since the first two dimensional observation of charge density waves in 1970s. To this day, there is still not any mechanism ideal for all cases or for all materials. Still, one has to run experiments or simulations for which mechanism is stronger in a specific material. The problem of determining fundamental mechanism between two candidates, which are Fermi surface nesting and electron-phonon coupling, will be the motivation of this study. We have a candidate material called NbSe2 , a member of the transition-metal di-chalcogenides family. After neutron and x-ray scattering experiments, we know that there is a q vector of charge density wave in the path Γ-M of the Brillouin zone. We run simulations to observe the Lindhard susceptibility and electron-phonon coupling constant on this path. For these simulations we use DFT programs called Quantum Espresso and Electron-Phonon Wannier. After the calculations, we see that the electronic effects observed by searching for a peak value of Lindhard susceptibility, is not very effective or distinctive. Whereas, the electron-phonon coupling constant has a mighty peak just above the searched q vector. This gives the hint that for instabilities about that q point, electron-phonon coupling effects are much more effective, and materials NbSe2 is identified as a q-dependent electron-phonon coupling material. For some researchers this corresponds to the type 2 CDW. The judgment of the which effect has more influence on the resultant instability is decided o their behavior on the ⃗qCDW = 0.66(Γ − M). The peaks in graphs are quantitatively measured according to their elevation from the mean of their respective datasets. The peak of the electron-phonon coupling effect has showed %865 peak values, where the electronics effects which are probed with the Lindhard susceptibility has showed only %38 peak behavior.
CSA'nın P3HT VE MEH-PPV polimerlerinin fiziksel özelliklerine etkisi

(Lisansüstü Eğitim Enstitüsü, 2023-04-07) Uludağ, Cem ; Durucu Alveroğlu, Esra ; 509142110 ; Fizik Mühendisliği

Bu tez çalışmasında iki temel organik yarıiletken polimerin P3HT ve MEH-PPV kullanılması hedeflenmiştir. İlk olarak P3HT analiz edilmiştir. Daha önceki çalışmalarda, kamfor sülfonik asitin (CSA) konjuge polimerler üzerindeki doping (katkılama) etkisi belirtilmiş ve nanofilmler aracılığıyla polimer üzerinde elektriksel (dc) iletkenliklerinin artışına sebep olduğu gözlemlenmiştir. Öte yandan, bir çözücü olarak m-cresolün asitliği nedeniyle polimerlerdeki taşıyıcı yük hareketliliğini arttırdığı da öne sürülmüştür. P3HT polimerinde CSA'nın katkılama etkisi ile ilgili birçok çalışma olmasına rağmen, P3HT, CSA ve m-cresolün birlikte kombinasyonu ile ilgili bir çalışmaya rastlanmamıştır. Bu çalışmanın ilk aşamasında çözücü m-cresolün ve CSA'nın P3HT üzerindeki etkisi açıklanmıştır. P3HT ince filmlerin optik absorpsiyonunu ve dc elektriksel özelliklerini geliştirmek için katkı maddesi olarak CSA ve ayrıca çözücü olarak m-cresol kullanılmıştır. İnce film örneklerinin yapısal, optik ve elektriksel özellikleri için çözücü m-cresolde CSA'nın P3HT üzerindeki katkılama etkisi incelenmiştir. Ayrıca ilk aşamada, P3HT filmlerin iletkenlik-sıcaklık özellikleri incelenmiş ve daha sonra ince filmlerdeki CSA miktarlarının etkilerine katkı sağlanarak her bir numunenin aktivasyon enerjileri hesaplanmıştır. P3HT'nin (Poly (3-hexylthiophene)) yapısal, optik ve elektriksel özellikleri, CSA m-cresol içindeki katkılama etkisi ile incelenmiştir. P3HT ince film örneklerinin SEM görüntüleri ve optik mikroskopisi, CSA'nın P3HT yüzeyinde dağılması yoluyla morfolojik etki göstermiştir. Optik absorpsiyon analizinde, 650 nm ile 900 nm arasında CSA filmleriyle katkılı P3HT'de kırmızıya kayma ve genişleme araştırılmıştır. Ayrıca, P3HT ince filmlerin elektriksel iletkenliklerinin, 313-337 K sıcaklıkları arasında doğrusal bir artış gösterdiği ve CSA'nın P3HT'nin dc iletkenliğini Kelvin başına 7.04x10-9 S/cm'den 1.31x10-6 S/cm'ye yükselttiği ortaya konmuştur. İlk aşamanın sonunda, Arrhenius denklemi kullanılarak örneklerin aktivasyon enerjileri hesaplanmıştır. Çözücü m-cresolde CSA'nın P3HT üzerindeki artan etkisi sonucu, P3HT'nin aktivasyon enerjisi 0.605 eV'den 0.302 eV'ye düşmüş ve dolayısıyla elektriksel iletkenliğe katkısının olduğu ispatlanmıştır. P3HT polimerinin analizinin ikinci aşamasında dielektrik özellikler incelenerek polimerin kapasitif etkisine bakılmıştır. P3HT/CSA kompozitlerinin kompleks elektriksel geçirgenlik, kayıp tanjantı, kompleks elektrik modülü dahil olmak üzere frekansa bağlı elektriksel özellikleri incelenmiştir. Numunelerin yapısal özellikleri FT-IR spektroskopisi ve Taramalı Elektron Mikroskobu (SEM) görüntüleme ile gerçekleştirilirken; enerji depolama potansiyelini ortaya çıkarmak için empedans ve iletkenlik ilişkisi, 20 Hz ile 10 MHz arasında empedans analizörü kullanılarak yapılmıştır. Numunelerin kompleks elektriksel geçirgenlik analizi, CSA takviyesinin P3HT'nin hem enerji depolama kabiliyetini hem de iletkenliğini artırmada olumlu bir etkiye sahip olduğunu ortaya çıkarmıştır. Özellikle 0.011M CSA içeren kompozitin ' değerinin diğerlerine göre daha yüksek olması, daha düşük kayıp tanjantına sahip olması ve iletkenliğinin tüm numunelere göre yüksek olması bu kompoziti birçok uygulama için cazip kılmaktadır. Ayrıca P3HT'nin süperkapasitif yapısının bir göstergesi olan yüksek frekanslarda (3-10 MHz) P3HT'nin -90o'ye çok yakın bir faz farkına sahip olduğu ilk kez tespit edilmiştir. Tez çalışmasının son aşamasında MEH-PPV polimeri analiz edilmiştir. MEH-PPV/CSA kompozitleri çeşitli spektroskopik metotlarla analiz edilmiş; frekansa bağlı elektriksel özellikleri ayrıca değerlendirilmiştir, CSA'nın polimer matrisine dâhil edilmesi, FTIR, UV, PL, SEM ve AFM sonuçları ile doğrulanmış, ayrıca DC iletkenlik analizi, polimerin elektrik doğasını lineer değişimden farklı şekilde etkilediğini göstermiştir.
D = 3 string theory review and closed string spectrum

(Graduate School, 2023-07) Turhan, Şafak ; Özkan, Mehmet ; 509201102 ; Physics Engineering

String theory is a framework in which all matter and force particles are mathematically represented by tiny vibrating strings. One of the most remarkable aspects of the theory is that it is a theory of quantum gravity, in string theory gravity emerges as in the scope of the closed string spectrum. Another quite intriguing property of string theory is the fact that it "dictates" the necessity of a specific space-time dimension, namely the critical dimension, in order for the theory to preserve the Lorentz invariance. It is exactly this aspect that the thesis will build up to and offer another way out other than the renowned 26-dimensional space-time. The thesis, as is customary, will start with a brief investigation of relativistic point particle. The reason laying behind this is that the string case will be treated in a very similar fashion. The relativistic point particle action and the equations of motion will be calculated. The action of the point particle can be generalized to the p-brane action, which is simply the action of a p-dimensional membrane. By making use of the generalized action, the action of the string will then be examined. In the scope of the thesis, the focus will be on the free bosonic strings. The string motion is represented by the worldsheet of the string, the 2-dimensional space-time surface which the string sweeps throughout its motion. The parametrization will be made by specifying the string coordinates by $\sigma$ and a time parameter $\tau$. The string action is derived by considering the area of the worldsheet and is called the Nambu-Goto action, from which will be moved on to obtain the conjugate worldsheet momentum in the Hamiltonian formalism. That conjugate momentum will give rise to two constraints, which then will give rise to the final form of the action that will be examined. The action contains a constant $T$, analogous to the mass $m$ in the relativistic point particle case, which is called the string tension. The equations of motion will again be derived and then the open string boundary conditions will be analysed: Dirichlet and Neumann boundary conditions. For the closed string, the periodicity condition will be introduced together with the reparameterization invariance of the string action. After all of the above-mentioned calculations, the conserved currents and charges will be calculated and after that, a switch to the light-cone coordinates will be made. It will be the spin part of the Noether charge that will be used to check the Lorentz invariance of the theory later on. Then, the string wave equation will be calculated. Moving on from the wave equation, the Fourier mode expansion will be written. Following the calculation of the conserved currents, the mass-shell condition will be derived. The canonical quantization procedure will then be conducted and it will appear that the oscillator modes of the pre-calculated mode expansion will correspond to the annihilation and creation operators when the quantization is made. All other operators will be quantized as well. Finally, it will be possible to check the Lorentz invariance of the theory by looking at the commutation relations of Lorentz charges. This will be equivalent to examining only the commutators of the spin parts of the Lorentz charges and requiring them to be equal to zero will give rise to the critical dimension of D = 26. But also, it will be shown that for the special case of D = 3 this commutation relation also vanishes i.e. preserves the Lorentz invariance. At last, the focus can be directed on the spectrum of the D = 3 theory. Even more specifically, to the D = 3 closed string. The Poincaré invariants will be calculated and then the level-matching condition will be shown. It will then be possible to obtain the states corresponding to different levels in terms of the creation and annihilation operators from before. Eventually, a set of calculations will be conducted to find the spins of different levels and then we will end up with a set of numbers that depend on the normal-ordering constant $a$. After examining the final results, it will be apparent that the spectrum gives rise to anyonic states at some levels regardless of the choice of $a$, states which has spin $s$ where $2s$ is not an integer. An effort to evaluate this result will be made and further areas for possible contributions will be discussed.
Development of a segmented detector for reactor antineutrinos

( 2020) Kandemir, Mustafa ; Çakır, Muammer Altan ; 621467 ; Fizik Mühendisliği ; Physics Engineering
Dynamical system analysis of cosmological inflation models with axion-like-particles (ALP)

(Graduate School, 2022-01-13) Çağan, Sermet ; Arapoğlu, Savaş A. ; 509181126 ; Physics Engineering

Inflation theory, developed in 1980 by Alan Guth, solves the two biggest problems of the standard Big Bang cosmology called flatness and horizon problem. The flatness problem essentially is a fine-tuning of the initial value of the energy density problem. The name itself comes from the relation between energy density to critical energy density ratio and curvature parameter. From current observations, we know that the deviation of the ratio of the energy density of the content of the universe to the critical energy density from unity is of the order $O\left(10^{-3}\right)$. Extrapolating this deviation back in time reveals that, in order to satisfy current observations, the value of the energy density has to be in agreement with the critical energy density of the order $O\left(10^{-62}\right)$. Therefore this extreme sensitivity to initial conditions arises the flatness problem. The horizon problem is the problem regarding the inexplicability of isotropy and homogeneity in the observed cosmic microwave background radiation (CMBR/CMB). CMB is almost uniformly in agreement on temperature distribution with $T \approx 2.7\ \text{K}$. One important fact of the CMB is that it contains regions that are separated by a distance larger than the particle horizon. Particle horizon is the definition of distance that light can reach from the start of the universe until now. Thus, regions or simply points in space-time that are separated more than the particle horizon are called causally disconnected regions. Causally disconnected points can never contact each other or ever be in contact previously. Therefore, CMB having causally disconnected patches that are almost in thermal equilibrium arises the question of how are the causally disconnected patches can reach a thermal equilibrium without the possibility of information exchange. Inflation theory solves those two major problems by introducing an exponential accelerated expansion in the very early universe before the start of the Big Bang theory. This accelerated expansion eventually reveals that there is no need for extreme fine-tuning of initial conditions on the energy density. Furthermore, the theory explains the horizon problem as rapid early accelerated expansion separates regions that were actually in causal contact but now seems to be causally disconnected, by the process called shrinking Hubble radius. There is no shortage of cosmological inflation theory models in the literature, starting from the original inflation theory model called chaotic inflation with squared potential to string theory motivated axion monodromy inflation. Axions are hypothetical pseudo-Nambu-Goldstone bosons that are emerged from solution to the CP problem, introduced by R.D. Peccei and H. Quinn in 1977. Axions in cosmology are regarded as the scalar field that enjoys the shift symmetry, i.e. $\phi \rightarrow \phi + \text{const}$ which solves the UV sensitivity of slow-roll inflation models. Cosmological inflation models can be examined by employing a mathematical method called dynamical system analysis. In this thesis, we tried to work out dynamical system analysis of two main axion-like inflation theory models in the linear stability analysis framework. In linear stability analysis, one defines meaningful model variables so that the evolution of said dynamical variables can be written in terms of the defined variables, i.e. there is no explicit dependence on the independent variables of the dynamical variables. This differential equation system building is called an autonomous equation system. Solution of the autonomous equation system yields several or no critical points of the system that the behaviour of mentioned critical points in the phase space can be understood by examining the eigenvalues of the evaluated Jacobian matrix at critical points of the autonomous system. There is more advanced method to determine the behaviour of critical points that fails to be determined in linear stability analysis but the scope of this thesis does not include them and further discussion on the reason for not including them is clarified in the thesis. We started with the linear stability analysis of a single scalar field having a natural inflation potential with several couplings to the gravity sector of the model. The analysis showed that having a cosine potential form is problematic in the definition of linear stability analysis therefore, we approximated to chaotic-like one. Results showed that in most of the configurations the critical points of the phase space behaves as an unstable point and in other cases linear stability theory fails to determine its behaviour. Moreover, we continued the analysis on the non-Abelian gauge field inflation model with extra scalar introduced to the model as an axion-like particle field with several different potential settings. We omitted the couplings to the gravity sector in this model for simplicity since most of the complexity comes from those said couplings and further difficulty comes from the fact that the model now has a multi-field form by definition. In a scenario where the extra scalar field is free, i.e. zero potential, with $F^{2}$ term has the coupling with the axionic field does not provide an inflationary period and by changing the potential to different forms, i.e. exponential, chaotic and general monomial we have found that in exponential case all critical points of the autonomous equation system becomes unstable and in chaotic-like and general monomial setting, none of the points' behaviour can be determined by linear stability analysis. The final attempt of linear stability analysis to axion-like field models was made to save the zero potential case by instead of coupling axion-like field to $F^{2}$ term we coupled it to a $F^{4}$ term which automatically solves the problem of not having an inflationary period since now the extra contribution coming from the $F^{4}$ has the equation of state parameter value of minus one. Although inflationary period is saved, linear stability method suffers from the non-minimal couplings since in order to observe the effect of newly introduced term one needs to use the same dynamical variables defined in the $F^{2}$ model, and while most of the equations can be written in required form, some explicit dependence to the coupling functions makes the model non-closed therefore none examinable with the same variables. Therefore, a direct comparison between those two models can not be made without defining a new variable set. As a result, we learned that the examination of axion-like cosmological model is not viable utilizing the dynamical system analysis with linear stability analysis constraint.
Effects of solar radiation and neutron, gamma material intereaction effects on the solar cells/modules/panels

(Graduate School, 2021-12-17) Demirel Turna, İnal Begüm ; Er, Zuhal ; 509162103 ; Physics Engineering

This thesis study covers the examination of the performance and degradation conditions of photovoltaic cells/modules/panels that convert solar radiation into electrical energy in case of exposure to solar radiation and nuclear factors (neutron and gamma irradiation) in laboratory and outdoor conditions. In this context, with the beginning of this thesis, first of all, the solar radiation for the province of Istanbul, where the performance and degradation experiments of photovoltaic devices will be carried out, was examined and presented to the literature. In these studies, statistical analysis methods were used to examine the accuracy of the models and the most suitable model was determined for the region to be studied. Afterwards, the working principles of the devices are examined and detailed on the basis of physical science in order to understand the effects of both natural conditions and nuclear effects that the photovoltaic cell/module/panels are exposed to on the device performance and to examine the decay structure. There is a manufacturer's datasheet detailing the electrical characteristics of most of the produced photovoltaic cells/modules/panels at standard test condition (STC; 25℃ and 1000W/m2) and/or nominal operating cell temperature (NOCT; 20℃ and 800W/m2). By using the electrical characteristic data in this datasheet, performance and degradation modeling of photovoltaic cells/modules/panels can be performed theoretically. On the other hand, in some cases, the aforementioned data document may contain incomplete data, and even the data document of the produced device cannot be found at all. In this case, it becomes difficult to examine the performance of the device and to make performance predictions under different environmental conditions. Performance modeling of photovoltaic cells/modules/panels, which is one of the main objectives of this thesis; It was carried out by using the Single Diode Model (SDM), which is frequently used in the literature, and the Double Diode Model (DDM), which is rarely used in the literature, both in the absence of a datasheet or in the absence of a datasheet. The aforementioned theoretical study was firstly carried out by producing sub-models containing interconnected block structures on the MATLAB/Simulink platform and as a result of the merging of the sub-models, with the representation of the photovoltaic device. In the system; with the definition of the ambient conditions, which are the input data, the electrical characteristic output data that the photovoltaic device will show under these ambient conditions has been examined. Simulink study was performed for both SDM and DDM. There are electrical characteristic parameters of the photovoltaic device, which are not available in the manufacturer's datasheet, but have an important place in the evaluation of photovoltaic device performance. These parameters are; photocurrent, ideality factor, saturation current and resistance values. For performance and degradation evaluations, these parameters should be derived in line with the available data. By available data is meant manufacturer data sheet and/or experimental data. In cases where the manufacturer's datasheet is available, with the new photovoltaic device model created on the Simulink platform, firstly the electrical characteristic parameters of the device are derived, and then the electrical data (current, voltage, power) produced by the photovoltaic device under the requested ambient conditions are turned into a program output for performance and degradation analysis. The slope method and/or the simplified open method were used in cases where the manufacturer's data sheet was missing. In this case, theoretical analyzes were carried out with the performance program created with a set of codes on MATLAB. These studies have been carried out for different photovoltaic type devices. Obtained results were compared with experimental data. Both the Simulink model and the MATLAB program result documents created with this doctoral thesis showed that; the theoretically produced performance result for the determined environmental conditions is more compatible with the experimental results carried out within the scope of this thesis in the Double Diode Model compared to the Single Diode Model. As mentioned above, the performance evaluation of photovoltaic cells/modules/panels was also carried out experimentally within the scope of this thesis. The performance of 10W Monocrystalline and 7W Polycrystalline Photovoltaic Modules available in Zuhal ER's Laboratory at ITU Faculty of Arts and Sciences, Fluke Ti90_9Hz Thermal Camera also available within Zuhal ER's Laboratory, and some of the experiments were carried out with the Seaward PV200 Solar PV Test Device and Solar Survey 200R, which were purchased with the project support of the ITU Scientific Research Projects Unit, which is affiliated with the doctoral thesis, with the project number 41722. Performance and degradation effects of environmental factors on photovoltaic modules, which is one of the main objectives of this doctoral thesis, were determined with these devices. The electrical characteristics of the photovoltaic modules with the PV200 Solar PV Test Device and the solar radiation, ambient temperature, photovoltaic module surface temperature were determined with the Solar Survey 200R. It has been seen that the open circuit voltage has increased over time due to the overheating caused by the deterioration of the modules. On the other hand, the surface temperature of the photovoltaic module and extremely hot regions, which are indicators of degradation, were observed and determined with the Fluke Ti90_9Hz Thermal Camera. Performance and degradation evaluations of photovoltaic devices within five years were determined by the laboratory facilities of Zuhal Er, as well as the maximum power determination and performance experiments carried out at the Turkish Standards Institute (TSE). As a result of the experiments carried out at TSE, it was observed that the maximum power values obtained from the modules decreased with the effect of environmental conditions. Experimental studies in addition to the above-mentioned modules, performance and degradation tests of a total of 20 modules, including 1.5W monocrystalline and polycrystalline photovoltaic modules, were carried out. As can be seen, the modules have very low power values compared to the modules mentioned above. The power measurement ranges of the Seaward PV200 Solar PV Test Device, which was purchased with the project support of ITU Scientific Research Projects Unit 41722, is between 5W and 15kW. Therefore, it could not be able to use in the electrical characteristic determination of 1.5W photovoltaic modules. In this case, measuring devices have been produced for the modules that are intended to be examined within the scope of the thesis. Arduino device, which is a microcontroller used in various projects and studies in recent years, has been used for the aforementioned purpose. On the other hand, sensors that determine current, voltage, ambient humidity and temperature are used. The communication protocol called SPI (Serial Peripheral Interface) between the sensors and the Arduino, in other words, the data connection standard starts the operation of the data current sensor and the data flow is provided. Thus, the measurement is started with the Arduino signal and the obtained data is transferred to the SD card; it is recorded via the same communication protocol between the Arduino and the SD card module. Thus, electrical data is collected and recorded for each photovoltaic module. The code is processed on the Arduino in order to determine the initialization of the created measuring device, the operating range, the data collection frequency and the way the data is recorded and listed. Data are performed every five minutes and during the time the solar radiated module. On the other hand, solar radiation data is collected in line with latitude and longitude information via an online platform. Both solar radiation data and module electricity data were recorded and analyzed on both MATLAB and Excel platforms. The obtained results showed that the modules could not perform a full current-voltage curve as expected because they did not receive radiation at all hours of the day. Mostly, data is collected between the maximum power point and the open circuit voltage. On the other hand, it has been observed that modules of the same structure produce similar results. During these studies, since the system was exposed to the external environment, deteriorations such as dulling of the modules and wear at the junction of the cells were observed. In addition to the determination of the effects of environmental factors on photovoltaic cells/modules/panels, neutron and gamma radiation effects are one of the other important objectives of this doctoral thesis. Neutron and gamma rays. Dr. It was held in Isparta, Süleyman Demirel University, Faculty of Arts and Sciences, with the support of İskender Akkurt. Modules that were not exposed to either solar radiation or neutron & gamma radiation before were exposed to gamma and neutron radiation in two sets. first set; two monocrystalline and 2 polycrystalline modules were exposed to gamma radiation for 30 days. second set; Two monocrystalline and 2 polycrystalline modules were exposed to neutron radiation for 15 days. Afterwards, electricity measurements were made under solar radiation like other modules. The results showed that the modules gave voltage values around the open circuit voltage. On the other hand, while fading occurred on the front surface of both sets of modules, deterioration occurred on the rear surface of the modules exposed only to neutron radiation.
Elektron demeti buharlaştırma yöntemi ile hazırlanmış tungsten oksit ince filmler üzerine detaylı bir çalışma: Elektrokromik cihaz üretimi ve karakterizasyonları

(Lisansüstü Eğitim Enstitüsü, 2022-03-15) Evecan, Dilek ; Zayim, Esra ; 509112108 ; Fizik Mühendisliği

Son yıllarda, akademik ve endüstriyel araştırmacılar, uygulama açısından hızla gelişen elektrokromik (EC) malzeme ve cihaz teknolojilerine gün geçtikçe artan bir ilgi duymaktadır. Elektrokromik cihazlar, uygulanan düşük voltajla renklerini tersinir olarak değiştirebilirler ve bu teknoloji, akıllı camlar, parlamayı önleyici araba dikiz aynaları, akıllı güneş gözlükleri, elektrokromik enerji depolama cihazları ve ekran uygulamaları gibi birçok alanda umut verici olması nedeniyle önem kazanmaktadır. En yaygın uygulama alanlarından biri olan elektrokromik pencereler (akıllı pencereler), odaya giren güneş ışığı miktarını ayarlayarak binalarda enerji tasarrufu sağlayan önemli bir teknoloji haline gelmiştir. Enerji tüketiminin önemli bir kısmı, yaklaşık yüzde kırktan fazlası binalarda ısıtma ve aydınlatmada kullanılmaktadır. Enerji verimliliği açısından binalarda akıllı cam sistemlerinin kullanılması iç mekân konforu ve az enerji tüketimi gibi avantajlar sağlamaktadır. Gelecekte artan bir talep ile mevcut cam sistemlerinin yerini, çoğunlukla elektrokromik olmak üzere akıllı cam sistemlerine bırakması beklenmektedir. Yüksek ticarileşme potansiyeli, enerjinin verimli kullanılması gibi sebeplerden dolayı özellikle tungsten oksit tabanlı elektrokromik cihazlara olan ilgi her geçen gün artmaktadır. Elektrokromik cihazın aktif tabakası olarak kullanılan tungsten oksit, gösterdiği üstün elektrokromik performansından dolayı, üzerinde en çok çalışılan malzemelerden biri olmuştur. Tersinir olarak renk değişiminin önemli olduğu ileri teknoloji sistemlerinde kullanım için yüksek optik modülasyonlu elektrokromik cihaz üretimi daha da önem kazanmaktadır. Bu tezin amacı, ticarileşmeye başlayan, yakın gelecekte farklı uygulama alanları bulacak olan ve gelecek vadeden tungsten oksit ince film tabanlı tersinir, kısmen hızlı yanıt süresine sahip, yüksek optik kontrastlı ve yüksek verimlilikte elektrokromik cihazların üretilmesi, karakterizasyonu ve geliştirilmesidir. Ekonomik, homojen ve yüksek saflıkta elektrokromik ince film yapılarını üretmek, büyük ölçekte elektrokromik akıllı camlarda kullanımı ve gelecekteki uygulama alanları açısından önemlidir. Kaplama yöntemi ve kaplama parametreleri film kalitesini etkileyen en önemli unsurlardır. Bu çalışmada son yıllarda daha sık kullanılmaya başlanan ve fiziksel kaplama yöntemlerinden biri olan elektron demeti buharlaştırma yöntemi uygulanmıştır. Bu yöntemin diğer yaygın kullanılan fiziksel yöntemlere göre üstünlüğü, toz numunelerden (genellikle pelet halinde) yüksek saflıkta ince filmlerin üretilmesi sayılabilir. Elektron demeti buharlaştırma yöntemi, hedef malzemeleri yüksek bir sıcaklığa kadar ısıtabilmekte ve ayrıca yüksek kaplama hızlarına ulaşmaya ve yüksek erime noktalı metalleri kolayca buharlaştırmaya olanak sağlamaktadır. Bu şekilde maliyeti yüksek hedef malzemesi alımı, kompozisyonun değiştirilememesi gibi zorluklar elimine edilebilirken, yüksek saflıkta ve kalitede ince filmler üretilebilmektedir. Bu çalışmada amaç, anahtarlama hızı, renklenme etkinliği gibi parametreleri optimize edilen elektrokromik cihazlar için, elektron demeti buharlaştırma yöntemi ile homojen ve amorf tungsten oksit ince filmlerini oluşturmaktır. Kaplanan filmlerin elektriksel, optik ve elektrokimyasal ölçümleri ve katı kromojenik cihazda kullanımı incelenmiştir. İlk aşamada ticari tungsten oksit tozu kullanılarak elektron demeti buharlaştırma yöntemi ile üretilen WO3-x ince filmlerin XRD, SEM, EDS, AFM, NKD, EIS, CV ve CA ölçümleri ile yapısal, optik ve elektrokimyasal karakterizasyonları detaylı olarak incelenmiştir. X-ışını kırınım desenleri, filmlerin amorf olduğunu göstermektedir. Amorf tungsten oksit ince film yapısının elektrokromik cihazlarda kullanımı avantaj sağlamaktadır. Üretilen ince filmlerin kalınlıkları, profilometre ve SEM ölçümleri ile belirlenmiştir. Bütün karakterizasyonlarda, üç elektrotlu sistemde ve katı EC cihaz yapımında kullanılan WO3-x filmlerin kalınlığı ~200 nm'dir. SEM ölçüm sonuçlarına göre e-demeti buharlaştırma yöntemi ile kaplanan filmler yüksek homojenliktedir. Optik, elektriksel ve elektrokimyasal karakterizasyonlar, akıllı sistemlerin farklı koşullarda kullanımı için önem taşımaktadırlar. Bu nedenle homojen ve amorf tungsten oksit ince filmlerin sıcaklığa bağlı elektriksel özellikleri, spektrofotometre ve NKD analizörü ile geniş bir spektrumda optik özellikleri incelenmiştir. DC tip elektriksel ölçümlerde incelenen tüm örnekler için omik bir davranış görülmüştür. Üç elektrot sisteminde (LiClO4/PC sıvı elektroliti kullanılarak) dönüşümlü voltametri ve kronoamperometri ölçümleri ile tungsten oksit ince filmlerin elektrokromik özellikleri ayrıntılı olarak araştırılmıştır. Bu ölçümler neticesinde yapının kararlı olduğu, sırasıyla 10 ve 4 saniyelik renklenme ve saydamlaşma anahtarlama süresi gösterdiği gözlemlenmiştir. Bu sistemde yapılan ömür testlerinde 100 çevrimin sonunda elde edilen CA eğrilerinde ciddi bir değişime rastlanmamıştır. Ayrıca hafıza etkisi incelendiğinde filmin rengini açık devrede ve oda şartlarında 1,5 saat koruduğu, bu süre sonunda yaklaşık % 1 optik geçirgenlik farkı oluştuğu görülmüştür. Optiksel ölçümler sonucunda 630 nm'de % 68,7 optik geçirgenlik farkı gözlemlenmiştir. Filmlerin renklenme verimliliği, 52,6 cm2/C olarak hesaplanmıştır. Sonraki aşamada ticari tozlardan üretilen tungsten oksit filmler kullanılarak katı EC cihaz yapımına geçilmiştir. Üretilen katı cihazlar, geçirgen iletken elektrot olarak indiyum kalay oksit kaplı cam, katodik renklenen malzeme olarak tungsten oksit ve elektrolit olarak ise LiClO4-PC-PMMA bazlı iletken jel içermektedirler. Buna göre Cam /ITO/WO3-x/Jel Elektrolit/ITO/Cam sandviç yapısına sahip katı cihaz üretimi yapılmıştır. Katı cihazın elektrokimyasal ölçümleri ± 3,2 V potansiyel aralığında test edilmiştir ve 630 nm dalga boyunda optik kontrastının yaklaşık % 48,1 olduğu görülmüştür. Ardından çoklu çevrim için tasarlanan cihazın ömür testi yapılarak cihazın renk değişiminin 600. çevrimde de tersinir olarak devam ettiği gözlemlenmiştir. Sonraki aşamada Cam/ITO/WO3-x/Elektrolit/PANI/ITO konfigürasyonunda olmak üzere LiClO4-PC-PMMA bazlı iletken jel ve Nafyon elektrolit kullanılarak farklı cihaz üretimleri yapılmıştır ve sırasıyla 124 cm2/C ve 173 cm2/C renklenme verimliliği elde edilerek cihaz performansında artış olmuştur. EC cihazların yapımı için tungsten oksit tozu üretmek ekonomik açıdan önemlidir. Birçok yöntem arasında çözelti yanma sentezi (SCS) yöntemi, sahip olduğu avantajlar neticesinde öne çıkmaktadır. Bu yöntemin en büyük avantajı, nano boyutlu oksitlerin hazırlanmasındaki kolaylıktır. En fazla çalışılmış EC malzeme olan tungsten oksitin EC özellikleri üzerine kapsamlı araştırmalar yapılmış olmasına rağmen, akıllı cam uygulamaları için elektron demeti buharlaştırma yöntemi ile hazırlanmış tungsten oksit ince film çalışmaları sınırlı sayıdadır. Bu nedenle, EC cihazında kullanmak üzere hazırlanan düzgün ve yüzeye iyi tutunan tungsten oksit ince filmler, kendi ürettiğimiz tungsten oksit tozundan e-demeti buharlaştırma yöntemi ile hazırlanmıştır. Yeni tozun ince filmlerin özellikleri üzerindeki etkisini gösterebilmek için, fiziksel ve morfolojik özellikleri, ticari tungsten oksit tozu ile hazırlanan ince film ile karşılaştırılmıştır. SCS yöntemi ile sentezlenen ve ticari tungsten oksit tozlarından e-demeti buharlaştırma yöntemi ile hazırlanan ince filmler (~150-200 nm) cam ve ITO kaplı cam altlıklar üzerine kaplanmış ve SEM görüntüleri filmlerin homojen ve düzgün yapıda olduğunu göstermiştir. Filmlerin EC özellikleri incelendikten sonra sentezlenen tungsten oksit tozu ile katı EC cihaz üretimi yapılmıştır. Yüksek optik geçirgenlik modülasyonuna (550 nm'de % 63) sahip Nafyon elektrolit ile hazırlanan ve tüm katmanları katı halde EC cihazının elektrokimyasal ve optik özellikleri incelenmiştir. Bu çalışma, EC aktif tabaka için çözelti yanma sentezi yöntemiyle sentezlenen nano boyutlu, yüksek saflıkta tungsten oksit tozundan üretilen ince filmlerin ayrıntılı olarak incelenmesinde öncü çalışmalardan biridir. Böylece bu çalışma istenilen boyutta ve kompozisyonda tozların sentezlenmesine olanak sağlayan SCS yöntemiyle sentezlenmiş tungsten oksit tozunun kullanımını mümkün kılan ve e-demeti buharlaştırma yöntemi ile hazırlanan filmlerle daha ekonomik, enerji verimli EC aktif malzeme ve cihazlarının geliştirilmesi ve tasarlanması için bir temel sağlamaktadır. En son aşamada ticari tungsten oksit tozu kullanılarak elektron demeti buharlaştırma ile hazırlanan ince filmlerin dielektrik özellikleri farklı sıcaklıklarda frekansa bağlı incelenerek yapı hakkında daha detaylı araştırma yapılmıştır. Deneysel sonuçlar, kaplanan filmlerin yapılarının çok düzgün ve homojen olmasından dolayı hem katı hem de sıvı elektrolit ile hazırlanan elektrokromik cihazların başlangıçta çok saydam olduğunu ve yapılarına rahat iyon girmesi nedeniyle mükemmel optik modülasyon ve renklenme etkinliği sergilediklerini göstermiştir. Ortaya çıkan en önemli bulgulardan biri, elektron demeti buharlaştırma yöntemi ile hazırlanmış tungsten oksit ince film tabanlı elektrokromik cihazların, diğer kaplama yöntemlerine göre göreceli olarak üstün performans göstermesidir. Bu nedenle, renk değişimi davranışının mükemmel tersinir olması elektrokromik enerji depolama cihazlarında ve gelecekteki birçok uygulama alanında kullanım için önemli olmaktadır. Bu tez, elektron demeti buharlaştırma yöntemi ile kaplanmış ince, homojen ve düzgün tungsten oksit film tabanlı katı elektrokromik cihazlara yönelik kapsamlı ve ayrıntılı bir çalışmadır. Ticarileştirmeye aday ve yüksek performanslı prototip elektrokromik cihazlar üretilmiş ve detaylı olarak karakterize edilmiştir. Bu çalışma, her geçen gün ticarileşme oranı artan akıllı cihazların mevcut ve gelecekteki uygulamaları için bir temel oluşturmaktadır.
Enhancement of dispenser cathode fabrication with pre – design activation simulations and polymer doping

(Graduate School, 2023-04-05) Atmaca Angın, Nergis Yıldız ; Durucu Alveroğlu, Esra ; 509172106 ; Physics Engineering

This study includes dispenser cathode activation design including therefore cathode's peripheral parts and fabricating two different types of cathodes with some improvements. Activation which is a must for all of the dispenser cathodes is modeled for both M-type and scandate cathodes. The simulation results are verified with experimental data. This modeling study helps improve the thermal tests of cathodes. Besides the modeling studies, both cathodes are fabricated and tested. Polymer material and polymer-tungsten composition research is carried out also. The effects of polymer doping on the cathode matrix material on the surface roughness are investigated. Metal–polymer composition and its debinding process before sintering are also studied and new recipes are created. At the end of the study, it was examined that scandate cathodes operates at lower temperatures and their production has more controllable fabrication parameters. They can be used instead of M–type cathodes for the same current level at lower temperatures. Before the fabrication of the cathode, the components of the cathode are determined and designed with modeling, then polymer doping can be used to reduce surface roughness if pure tungsten is used. If there is an oxide compound in it, the polymer may not be needed. Scandate cathodes can be safely used for even its "as-sintered" form to operate at lower temperatures. Besides, the modeling study is also verified by commonly used cathodes and newly developed cathode types as well. It can be also used for any of the impregnated dispenser cathode types confidently.
Ensemble and deep learning on astronomical data with different modalities

(Graduate School, 2023-03-20) Huyal Edeş, Fatma Kuzey ; Kahya, Emre Onur ; Çataltepe, Zehra ; 509152102 ; Physics Engineering

Light curves inform us how the brightness of an astronomical object changes over time. They are obtained by subtracting successive images of astronomical objects of a photometric band. In this thesis, the first task was to focus on the classification of 14 astronomical objects in the Photometric LSST Astronomical Time-series Classification Challenge (PLAsTiCC) data set which was prepared to model future observations of the Vera Rubin Observatory. The light curve of each object was divided into two separate parts using a Haar wavelet transform. By using these two wavelet components, statistical properties such as the mean, standard deviation, and minimum were obtained for each band. The classification was performed with boosted artificial neural networks and boosted decision trees by using extracted features and also including additional features such as the photometric redshift and its uncertainty. The training set was highly biased toward low-redshifted objects. Importance weights were estimated for each training object, and more representative data were given larger weights. The combination of boosted decision trees, random forests and extremely randomized trees was found to give the best performance for type Ia supernovae which are important indicators for cosmological measurements. The role of photometric redshift was examined by excluding the redshift parameter. The performance of superluminous supernovae (SLSN-I) was significantly affected by the omission of the redshift parameter with a 10% decrease in the area under the receiver operating characteristic curve (AUC) score. The relevant features of each object class were ranked using a neural network classifier. The time-frequency images were also obtained using a continuous wavelet transform as an alternative to discrete Haar wavelets. Since all data points of the light curves were used instead of utilizing feature extraction, a powerful model was needed to estimate the missing data. Gaussian process regression was employed for that task as one of the most successful models in filling the gaps of the light curves. As discussed previously in the literature, it is very difficult to accurately estimate 14 astronomical objects with only one kernel. Therefore we limited our attention to only include type Ia and II supernovae for the classification task. One of the difficulties that will be faced in future observations is that researchers will not be able to obtain any data on some photometric bands, depending on the status of the observation. Villar et al. proposed a 2D Gaussian process regression for this situation. The missing data in this part of the thesis was estimated using this two-dimensional kernel. The convolutional neural network (CNN) structures were decomposed using a tensor decomposition method (CP decomposition). It was observed that rank selection, which changes the number of parameters, also affects the classifier performance. While the classifier performed poorly on very small ranks, an optimal rank was determined for better performance. This indicates that tensor decomposition methods may be significant enough to include in the analysis of noisy light curves. This method provides an alternative to feature engineering that applies principal component analysis (PCA) to wavelet components. Another main task that was performed in this thesis was the morphological classification of Galaxy Zoo images. The images used in the Galaxy Zoo challenge were taken from the seventh data release (DR7) of the Sloan Digital Sky Survey (SDSS). SDSS made observations in 5 photometric bands which are labeled as u, g, r, i and z. The Galaxy Zoo photos are composite photos of galaxies observed in the r, g, and i bands. As galaxy images do not have a fixed or preferred orientation, any classifier that aims to correctly classify galaxies should be expected to properly detect when two galaxies are rotated versions of each other. Although convolutional layers are successful in processing images and preserve feature maps when encountering a translated version of an object, they fail in identifying rotated objects as the same. The convolution operation does not commute with rotation and this causes the feature map to change when it is generated for different orientations of the same galaxies. To overcome the problem of not identifying rotated versions of the same object, an equivariant structure is needed for an image classifier. Convolutional layers can be excellent ingredients in defining generalized equivariant architectures. Instead of using traditional convolution layers, it is possible to define a generalized convolution operation with group operations that preserve different symmetries. A dynamic structure using the p4m group convolutional layers (p4m is the plane symmetry group of translations, reflections, and rotations by 90 degrees) is proposed in this thesis, which introduces additional angles to the architecture after the initial training of the p4m group convolutional layers.
Femtosaniye lazer ile işlenen yapıların katıhal lazerlerde uygulamaları ve üst çevrim pompalı Tm3+:KY3F10 lazerleri

(Lisansüstü Eğitim Enstitüsü, 2021-05-07) Morova, Yağız ; Esinoğlu Aksoy, Seda ; Sennaroğlu, Alphan ; 509142117 ; Fizik Mühendisliği

u tez çalışması ile yakın ve orta kızılaltı bölgede çalışan özgün katıhal lazerlerin geliştirilmesi amaçlanmış ve iki farklı deneysel yaklaşım uygulanmıştır. Bunlardan ilkinde femtosaniye lazer ile işlenen yapılar katıhal lazer tasarımlarında kullanılmıştır. Diğer yaklaşımda ise üst çevrim pompalama yöntemi kullanılarak Tm3+:KY3F10 lazeri iki farklı dalgaboyunda çalıştırılmıştır. Tezin ilk bölümünde, deneysel çalışmalarda kullanılan terimlerin ve kavramların tanıtılması hedeflenmiştir. Bu kapsamda lazer konseptine ait temel teorik altyapı ve deneysel karakterizasyon yöntemleri ile ilgili genel bilgiler sunulmuştur. Tezin ikinci bölümünde, grafen doyabilen soğurucunun femtosaniye lazer ile mikro şeritler oluşturacak şekilde yüzeyden kaldırılmasıyla elde edilen zebra desenli grafen doyabilen soğurucunun, üretimi ve bir test lazer kavitesi içerisinde kip kilitleyici olarak kullanılması sunulmuştur. Grafen doyabilen soğurucular, sahip oldukları avantajlı optik özellikler sayesinde lazer sistemlerinde yakın kızılaltı ve orta kızılaltı bölgelerini de kapayan geniş bir optik spektrum aralığında kip kilitleyici olarak kullanılmaktadırlar. Ancak sahip oldukları %2.3 küçük sinyal soğurması, bu kip kilitleyicileri kayba duyarlı düşük optik kazançlı lazer sistemleri için elverişsiz kılmaktadır. Bu kaybın azaltılmasına yönelik kimyasal katkılama, grafen tabanlı kapasitör, süper kapasitör yapıları gibi farklı deneysel yaklaşımlar literatürde gösterilmiştir. Bu yaklaşımlar genel olarak iyi sonuçlar vermesine rağmen, kimyasal katkılama yönteminde sabit Fermi seviyesi kayması, kapasitör yapılarda dielektrik bozulma riski, süper kapasitör yapılarda ise grafen doyabilen soğurucunun spektral operasyon aralığının daralması bu sistemlerin dezavantajları olarak öne çıkmaktadır. Bu nedenle bahsi geçen dezavantajları aşan ve aynı zamanda grafenin özgün özelliklerini taşıyan bir doyabilen soğurucu geliştirilmesi ihtiyacı doğmuştur. Bu kapsamda tezin ikinci bölümünde sunulan çalışmada infrasil alttaş üzerine kaplanmış tek katmanlı grafen, femtosaniye lazer ile işlenerek zebra desenli grafen doyabilen soğurucu üretilmiştir. Bu üretim süreci seçici aşındırma (ablasyon) yöntemine dayanmaktadır. Bu yöntemde lazer hüzmesinin gücü grafenin aşınma eşiğinden yüksek, alttaş olan infrasilin aşınma eşiğinden düşük tutularak grafenin alttaşa zarar vermeden yüzeyden kaldırılması amaçlanır. Bunun için 120 fs darbe süresine sahip, 1 kHz tekrarlama frekansında ve 800 nm dalgaboyundaki Ti:safir lazeri, 20X büyütmeye sahip bir objektif ile örnek yüzeyine odaklanmış ve tek katmanlı grafen yüzeyden aşındırılarak grafen mikro şeritlerden oluşan altı farklı bölge elde edilmiştir. Her bölgede yüzeyden kaldırılan şeritler arasındaki mesafe değiştirilerek farklı grafen miktarlarına sahip bölgeler elde edilmiştir. Femtosaniye lazer ile mikro işlemenin ardından geliştirilen grafen yapısı, test lazeri olarak seçilen Cr+4:forsterite lazeri içerisine eklenmiş ve kip kilitleme performansı incelenmiştir. Yapılan deney sonucunda mikro işlemeye tabi tutulmamış bölgedeki grafen kaynaklı kayıp %4.14 olarak ölçülürken en düşük grafen kaybına sahip bölgenin kaybı ise %0.21 olarak hesaplanmıştır. Bu sayede grafen kaynaklı kayıplarda belirgin bir azalma tespit edilmiştir. Ayrıca aşındırılmamış bölgede elde edilen kip kilitleme performansı incelendiğinde, 6.3 W sabit giriş gücünde 68 mW çıkış gücü elde edilirken, %0.21 grafen kaybı olan bölgede aynı giriş gücünde elde edilen çıkış gücü 114 mW olarak ölçülmüştür. Darbe süreleri karşılaştırıldığında ise 62 fs'den 48 fs'ye düşüş gözlenmiştir. Bu sonuçlar, kaybı azaltılmış zebra desenli grafen doyabilen soğurucunun geleneksel grafen doyabilen soğurucuya göre lazer performansındaki belirgin iyileşmeyi göstermektedir. Tezin üçüncü bölümünde, femtosaniye lazer ile mikro işleme yöntemi kullanılarak dalga kılavuzu lazer geliştirilmesi ve karakterizasyonu sunulmuştur. Yapılan çalışmada, Tm3+:BaY2F8 kristaline ilk defa femtosaniye lazer ile dalga kılavuzu yazılmış ve bu kristalin lazer olarak çalıştırılması gösterilmiştir. Dalga kılavuzları, 800 nm merkez dalgaboyunda 120 fs darbe süresinde ve 1 kHz tekrarlama frekansındaki lazer hüzmesi 40X objektif kullanılarak %8 (atomik) Tm+3 iyonu katkılanmış baryum yitriyum florür (BaY2F8) kristaline, yüzeyin 100 µm altında sıkıştırılmış kılıf (depressed cladding) biçiminde yazılmıştır. 7 mm uzunluğunda 28 çizgi çizilerek yaklaşık 30 µm çapında silindirik formda sıkıştırılmış kılıf dalga kılavuzu oluşturulmuştur. Dalga kılavuzu yazımını takiben ışık eşleme deneyleri yapılmıştır. Bu deneylerde dalga kılavuzuna, sürekli dalga formunda akort edilebilir Ti:safir lazeri, 3 cm odak uzaklığına sahip bir mercek ile eşlenmiştir. Eşlenen lazerin ilerleme kayıpları hesap edilirken soğurma etkisini minimize etmek amacıyla Ti:safir lazerinin dalgaboyu, Tm3+:BaY2F8 kristalinin soğurma bandı dışına alınarak 731 nm'ye ayarlanmıştır. Yapılan ölçümlerde üretilen dalga kılavuzunun ilerleme kaybının 0.32 dB/cm olduğu tespit edilmiştir. Optik eşleme deneylerini takiben pompa lazeri, 781 nm olan rezonant soğurma dalgaboyuna ayarlanmıştır ve elde edilen dalga kılavuzu, lazer rezonatörü içerisine eklenerek 1858 nm merkez dalgaboyuna sahip sürekli dalga lazer operasyonu gösterilmiştir. Geliştirilen dalga kılavuzu lazerinin, %2 çıkış aynası ile %3.3, %6 çıkış aynası ile %6.5 güç performansı veriminde çalıştığı tespit edilmiştir. Elde edilen en yüksek güç ise 553 mW giriş gücüne karşılık 34 mW olarak ölçülmüştür. En düşük eşik pompa gücü ise %2 çıkış aynası ile 23 mW olarak ölçülmüştür. Tezin dördüncü bölümünde, CeO2 nanoparçacıkların femtosaniye lazer ile işleme yöntemi ve kimyasal çökeltme yöntemi kullanılarak üretilmesi ve karakterizasyonu sunulmuştur. Lazer ile işleme yönteminde CeO2 pelet bir petri kabının içine yerleştirilerek yüzeyi yaklaşık 5 mm geçecek şekilde izopropil alkol ile doldurulmuştur. Hazırlanan örneğin yüzeyine 800 nm dalgaboyunda 1 kHz tekrarlama frekansında 120 fs darbeler üreten Ti:safir lazer hüzmesi, 50 mm odak uzaklığına sahip bir mercek ile odaklanmıştır. 5 mm uzunluğundaki çizigilerin 20 µm aralıklarla yan yana çizilmesi ile 5 mm x 5 mm alanındaki bir bölge lazer ile taranmıştır. Lazer taraması sırasında yüzeyden kopan CeO2 nanoparçacıklar izopropanol içerisinde toplanmıştır. Elde edilen parçacıklar taramalı elektron mikroskobu ile incelenerek 20 nm - 1 µm aralığında üretildiği görülmüştür. Ayrıca dinamik ışık saçılması ölçümü ile üretilen parçacıkların ortalama 333 nm çapına sahip oldukları görülmüştür. 30 dk süren bir işlem sonucunda mikro gram mertebesinde nanoparçacık üretilmiştir. Özellikle dielektrik özellikleri inceleyebilmek için parçacık üretim miktarı kimyasal çökeltme yöntemi kullanılarak artırılmıştır. Bu yöntem ile elde edilen parçacıkların boyutları 100 nm'nin altında olup CeO2 nanoparçacıklarının dielektrik sabiti oda sıcaklığında 25 olarak bulunmuştur. Üretilen ve ticari olarak satın alınan iki CeO2'nin dielektrik ölçümleri karşılaştırılmış ve CeO2 nanoparçacıkların dielektrik katsayısı daha yüksek bulunmuştur. Ayrıca Fe katkılı CeO2 nanoparçacıkları üretilmiş ve karakterize edilmiştir. Tezin beşinci bölümünde, literatürde ilk defa, 1064 nm dalgaboyunda üst çevrim pompalama yöntemi ile pompalanmış Tm3+:KY3F10 lazerinin 1.9 µm ve 2.3 µm dalgaboylarında çalıştırılması sunulmuştur. Florür tabanlı lazer kristallerinin kızılaltı bölgedeki lazer operasyonları, düşük fonon enerjileri, geniş spektral bölgedeki yüksek geçirgenlikleri gibi özellikleri nedeni ile birçok araştırma grubu tarafından incelenmiştir. Bu çalışmalarda genel olarak florür tabanlı lazer kristali grubunun bir üyesi olan Tm3+:KY3F10 kristali, konvensiyonel pompalama yöntemi olan 800 nm civarında doğrudan pompalanarak lazer olarak çalıştırılmıştır. Ancak bu kristalin 1064 nm dalgaboyuna sahip Yb:fiber lazeri ile üstçevrim pompalama yöntemi kullanılarak 1.9 µm ve 2.3 µm dalgaboyunda çalıştırılması ilk kez tez kapsamında yapılan çalışmalar ile gösterilmiştir. Bu çalışma kapsamında öncelikle %8 (atomik) Tm+3 iyonu katkılanmış KY3F10 kristalinin 1064 nm'de doğrusal olmayan soğurma özellikleri incelenmiştir. 2.8 mm uzunluğundaki Tm3+:KY3F10 kristalinin 1064 nm'deki temel seviye soğurmasının %2 olmasına rağmen pompa lazer şiddetinin artmasını takiben uyarılı seviye soğurmasının devreye girmesiyle %38 değerine ulaştığı görülmüştür. Ardından sırası ile x-kavite ve z-kavite mimarileri ile çalıştırılan 1.9 µm ve 2.3 µm lazerlerinin güç performansları incelenmiştir. 1.9 µm operasyonunda %2.3 çıkış aynası ile 1.9 W giriş gücünde en yüksek 142 mW çıkış gücü elde edilmiştir. Ayrıca kavite içerisine eklenen CaF2 ve suprasil prizmalar ile lazer dalgaboyunun 1849 nm - 1994 nm aralığında ayarlanabildiği gösterilmiştir. Bu konfigürasyon için en yüksek güç performans verimi, %5.5 çıkış aynası ile, soğurulan giriş gücüne göre %29 olarak elde edilmiştir. 2.3 µm deneylerinde ise z-kavite kullanılarak 1.6 W giriş gücünde %3 çıkış aynası kullanılarak, soğurulan pompa gücüne göre %36 verimle en yüksek 130 mW çıkış gücü elde edilmiştir. Tezin son bölümü olan sonuçlar bölümünde tez dahilinde yapılan deneysel çalışmaların sonuçları özetlenerek ileriye dönük potansiyel uygulamalarından bahsedilmiştir.
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.
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.
Nucleosynthesis in alternative theories of gravity

(Graduate School, 2022-06-22) Bulunur, İlayda ; Özdemir, Neşe ; 509191116 ; Physics Engineering

Big Bang nucleosynthesis (BBN) is one of the most reliable tools for testing standard model cosmology, as well as alternative models, well-known models are Brans-Dicke's theory of gravity, quintessence models, and higher-dimensional models. Standard BBN employs general relativity and the standard model of particle physics, thus, relying solely on one adjustable parameter; the baryon number density. Predicted primordial abundances based on SBBN are calculated with the help of BBN codes that contain well-established thermonuclear reactions network involved during the early evolution of the universe and presented as a function of the baryon number density. Observations from CMB and large-scale structure distributions indicate that the baryon number density can be restricted to a small range, allowing us to derive the basic relationship between predicted primordial abundances and new parameters emerging from alternative models of cosmology. All modifications to SBBN enforce the expansion rate of the early evolution of the universe to change, resulting in new relic abundances that differ from element abundances predicted by SBBN. Hence, we can parameterize the deviations from SBBN by introducing the $S$ parameter as $S\equiv H'/H $ where $H'$ is the modified Hubble parameter, $H$ is the Hubble parameter in the first Friedmann equation derived from the Einstein equation inserting the FRW metric. $S$ is constrained with the range of $0.85 \leq S \leq 1.15$ to obtain the simple relations between relic abundances and free parameters of the alternative models. Therefore, with this range of $S$, we can bound for free parameters of non-standard cosmological models. This thesis focuses on two models; Brans-Dicke's theory of gravity and its extensions with self-coupling potentials, and five-dimensional pure gravity which has an extra curled and compact dimension. Both theories have two free parameters. For the five-dimensional pure gravity, the parameters are the scale factor of the extra dimension, $b(t)$, and the length of the extra dimension, $l_c$ whereas the Brans-Dicke theory has parameters $w$ and $\beta$ that comes from the evolution function of the scalar field as $\phi(t) = \phi_i e^{-\beta(t-t_i)}$. To constrain these parameters, we used predicted primordial element abundances, leftover in the first three minutes of the universe, as a function of the number baryon density and expansion rate factor, $S$. In our five-dimensional model, the scale factor $b$ and the length of the extra dimension, $l_c$, directly impact on the synthesis of light elements. Since the range of $S$ is kept limited, that is, the deviation from SBBN is minimal, it is anticipated that its effect decreases as time passes. Therefore, first, it is assumed that the evolution of an extra dimension is $b(t)=b_0e^{-\beta t}$. In that case, predicted $^4 {He}$ mass fraction $Y_p$, $De$ abundance, $y_D$ and $Li$ abundance as a function of $\beta$ and $l_c$ can be obtained and compared with the data inferred from observations. The Big Bang Nucleosynthesis (BBN) bounds on the parameters of the five-dimensional theory of gravity as $\beta \sim 2$x$10^{-2}$, $10^{-7} \lesssim l_c \lesssim 10^{-2}$. It can be seen that $\beta$ works only in a limited range while $l_c$ is suitable in an extensive range. Our motivation for an extra dimension comes from the string theory, which suggests that the extra dimension should be too small to be not detected in a large scale. Hence, it can be concluded that our results are compatible with our motivation. Also, we investigate another possibility that the evolution of the scale factor of an extra dimension as $b=b_0 t^{-p}$. In that case, $p$ is restricted on $p\sim 0.5$ while the broad range of $l_c$ satisfies the theory, $10^{-7} \lesssim l_c \lesssim 10^{-2}$. For Brans-Dicke theory of gravity, first, we studied the effects of the BD scalar field in the absence of potential, $V(\phi)$, on Big Bang Nucleosynthesis. Inserting the FRW metric to the Brans-Dicke field equation, we obtained the modified Hubble parameter of the theory, which depends on various parameters $(\phi,\Dot{\phi},w,\rho)$. Therefore, these parameters can directly alter the synthesis of primordial elements. Within the allowed range of $S$, it is assumed that the effects of a scalar field diminish over time as $\phi(t)=\phi_i e^{-\beta(t-t_i)}$, where $t_i$ is the initial cosmic time. These parameters can be constrained by using $^4 {He}$ mass fraction, $De$, and $Li$ abundances. It is found that $\beta$ is limited in the range of $10^{-5}-10^{-6}$, and for $w$ is $10^{-3}-10^{-2}$. Also, we have obtained the initial value of a scalar field extremely large value as $\phi_i = 1.3$x$10^7$. Next, we looked for alternative models which include scalar field potential, $V(\phi)$, to be compatible with data from BBN. The scalar field potential is taken polynomial function as $V(\phi) = V_0 \phi^n$. In all cases, from $n=-1$ to $n=3$, the same conclusion as the previous model without scalar field potential has been achieved; the theory is highly dependent on the initial condition of the scalar field and requires a considerably large value of $\phi_i$.
Preparation and characterization of nano- and micro- particles for targeted drug delivery in cancer therapy

(Graduate School, 2023-05-12) Bekaroğlu, Maide Gökçe ; Turutoğlu İşçi, Sevim ; 509152104 ; Physics Engineering

It is well established that cancer is one of the most common and debilitating diseases with high mortality rates, and yet the most commonly used therapy method for cancer is intravenous chemotherapy and/or radiotherapy, which causes unwanted and serious side effects and can affect healthy tissues or organs. Seek for new therapy methods with higher efficiency and fewer side effects still continues to be popular amongst researchers. One of the most promising methods to optimize cancer treatment is targeted drug delivery of already available antitumor drugs, which aims to minimize systemic concentration while increasing drug concentration at the target site. Hence, reducing the side effects while increasing the therapeutic effect of the antitumor drugs. Particle-based targeted drug delivery systems have various advantages over conventional drugs and can significantly improve cancer therapy in various ways. These drug carrier particles can provide many advantages such as; high drug capasities, protection of loaded drugs, sustained and/or controlled drug release, possible variable routes of administration, ease of surface modification, precise targeting to tumor sites, etc. Although particle-based drug delivery is a popular research subject, there are still several limitations to particle-based drug delivery applications such as; possible toxicity of particles in their unmodified form, possibility, insufficient accumulation at the target site, immunogenicity, unstable particles, aggregation, etc. If these disadvantages could be eliminated and/or significantly reduced targeted drug delivery applications could be highly efficient and effective in cancer therapy. In the scope of this thesis particulate-based targeted drug delivery vehicles were produced via several techniques and characterized in order to achieve effective and efficient cancer treatment. To ensure our objective, nano- and micro-sized drug delivery particles were prepared, which depending on the size will have different physical properties, targeting strategies, working mechanisms, and administration routes. Clay minerals (montmorillonite) and iron oxide particles were used separately to produce both nano- and micro-particles for drug delivery applications. Lures of using magnetic particles such as iron oxide particles are that they are magnetically targetable, heatable under applied alternating magnetic fields, imageable, etc. And clay minerals also possess attractive properties such as; good biocompatibility, the ability to adsorb large amounts of drugs and improve drug stability, controlled and/or sustained release behavior, etc. Four different targeted drug delivery particles were produced during this thesis for different types of targeting strategies and administration routes. Iron oxide-based and clay-based microparticles were produced for direct targeting such as transarterial chemoembolization and Iron oxide-based and clay-based nanoparticles were produced for magnetic, active, and/or passive targeting. The most suitable form of each particle for the desired targeted drug delivery application was obtained via conventional, rheological, and in vitro characterization, tuning colloidal behavior, surface modification, etc. Different types of targeted drug delivery particles that were produced for this thesis comprise of montmorillonite-based microparticles, montmorillonite-based nanoparticles, iron oxide-based microparticles, and iron oxide-based nanoparticles, which were explained in chapters 2,3,4 and 5-7, respectively. Each particle is briefly summarized in the following 5 paragraphs. Drug-delivering montmorillonite-based microparticles that are also imageable were produced and characterized and the results indicated that these microparticles can be utilized for imaging or targeted drug delivery applications such as transcatheter arterial chemoembolization. Furthermore, the adsorption of antitumor drugs and/or radio-opaque contrast material onto montmorillonite influenced particle size and could be modified to the desired size for specific applications. So, these particles were suitable for embolization and can occlude tumor vasculature. The traceability of particles via CT was successful and precise. Moreover, the drug release behaviors of montmorillonite-based microparticles were sustained over a long period of time and were able to exhibit antitumor activity as the pure drug against cancerous cells. The adsorbed antitumor drug on the particles caused both devascularization and simultaneous release of the drug around the tumor site which was predicted to cause progressive shrinkage and/or necrosis of the tumor while the shrinkage of the tumor can be monitored by the imageable particles. Targeted drug delivery particles from raw and purified montmorillonite were produced and their potential to be delivery vehicles and effects of purification on the final product were analyzed. The results showed that the chemical composition, interlayer spaces, surface, and adsorption properties of raw montmorillonite were altered during purification. Both purified and raw montmorillonite Were able to load significant amounts of antitumor drugs and perform similarly to pure antitumor drugs against cancerous cells. However, In vitro drug release profiles indicated that purified and raw montmorillonite drug release behavior was different over 20 days which indicates that purification had an effect on desorption. In addition, cytotoxicity tests using normal cells revealed that raw and purified montmorillonite did not significantly affect cell viability at low concentrations, but at high concentrations, both treatments significantly decreased cell viability. Both drug loaded forms of raw and purified montmorillonite were effective as the pure antitumor drug, but it is important to take their release behavior into account before deciding which formulation is most suitable for the specific application. Iron oxide based microparticles suitable for transcatheter chemoembolization and magnetic hyperthermia were produced via utilizing both micro- and nano-iron oxide particles. Particles were linked together with a polymer and later enveloped using another polymer to obtain particle sizes suitable for embolization. Embolization particles produced from both micro- and nano-iron oxide particles exhibited different drug loading capacities and dimensional variance. Regardless of these differences, they both demonstrated favorable drug releasing behaviors, the ability to induce magnetic hyperthermia, could achieve arterial occlusion, etc. Both chemoembolic particles showed that they might be successfully utilized in chemoembolization and hyperthermia procedures. Furthermore, magnetic nanoparticles suitable for targeted drug delivery were produced using iron oxide nanoparticles, several different biopolymers, and an antitumor drug. Nanoparticles were coated with different polymers and drugs were loaded onto polymer-coated surfaces. The effect of coating and each polymer's effect on the final product was analyzed. Polymers that were chosen were each either anionic, cationic or nonionic in nature and results indicated that there were several differences in the final product properties depending on the polymer type. Although almost all polymers except anionic ones, completely covered particle surfaces, our studies indicated that cationic polymers may be more suitable for targeted delivery applications. The cationic polymer was able to completely cover particle surfaces and reduce particle toxicity to normal cells more than anionic and nonionic polymers. Furthermore, cationic polymer also allowed antitumor drug loading on the particles and drug-loaded particles were effective enough similar to pure antitumor drugs. Additionally, the magnetism of the particles was not considerably diminished by the cationic polymer coatings, allowing the particles to be guided to the appropriate location by applied magnetic fields and induction of hy hyperthermia if desired. Finally, actively targeted forms of magnetic iron oxide nanoparticles were produced using several different biopolymers (PVP and PVA), targeting agents (Folic acid or - Estradiol) and an antitumor drug in order to target folate or estrogen receptors that are highly expressed on certain breast cancers and achieve ligand-receptor binding at the target site. Results indicated that incorporation of targeting ligands onto particle surfaces stabilized the dispersions and also increased antitumor activity compared to particles without targeting ligands. Which suggests that incorporation of Folic acid or - Estradiol enhanced antitumor drug activity while stabilizing state of the dispersions. Studies carried out in this thesis indicated that the production of targeted drug delivery particles suitable for almost all desired targeting strategies (active, passive, direct, and/or magnetic) and preferred modes of drug administration can be successfully produced by exploiting the unique properties of clay minerals and/or magnetic particles. In sum, micro- and nano-particles suitable for various strategies of targeted drug delivery applications were successfully produced within the scope of this thesis. All particles produced for this study exhibited an advantageous combination of properties to be utilized as drug delivery vehicles to the target site.
Quantum corrected correlation functions and power spectra of spectator scalar and inflaton fields during inflation

(Graduate School, 2022-05-31) Karakaya, Gülay ; Önemli, Vakıf Kemal ; 509132104 ; Physics Engineering

The inflationary cosmology not only explains apparently mysterious features of the universe like the flatness, horizon, and relic particle abundance problems, but also provides a natural mechanism to produce primordial density fluctuations that eventually lead to the cosmic structure formation. The primordial density perturbations are generated spontaneously from the quantum fluctuations of a scalar field, called inflaton, that drives the inflation. The physical origin of quantum fluctuations is the uncertainty principle that manifests itself as the virtual particle pair production out of vacuum. The observed anisotropies in the cosmic microwave background (CMB) are believed to be the amplified imprints of the fluctuations of an inflaton which later grow, due to gravitational instability, to become galaxies and clusters of galaxies. Variation of the amplitudes of fields with length scales is measured by the spectral index. Predictions of the inflation models for the spectral index and its variation with scale, quantified the by running of the spectral index, can be compared with observations to discriminate between the alternative inflation models. It is, therefore, important to study the correlation function and power spectrum of the quantum fluctuations to understand the distribution and the origin of large scale structure in the universe.
Radiation damage measurement of plastic scintillators with silicon photomultipliers using collision data in the CMS hadronic calorimeter experiment

(Graduate School, 2023-05-05) Kömürcü, Yıldıray ; Çakır, M. Altan ; 509122120 ; Physics Engineering

In high energy particle collisions, plastic scintillators are commonly used to detect secondary particles and measure their energy along with silicon photomultipliers to amplify the signal. These detectors deteriorate over time due to irradiation they are exposed to and lose their capacity to detect particles efficiently. In the Compact Muon Experiment (CMS) experiment at CERN, hadronic calorimeter is equipped with passive absorbers, plastics scintillators and silicon photomultipliers. In 2017 and 2018, in the data taking periods, in Large Hadron Collider (LHC) two proton beams traveling at opposite directions at energies 6.5TeV are collided and outgoing particles are detected at two detector locations (CMS and ATLAS) along the accelerating ring. In the CMS experiment, energy response obtained from plastic scintillators are monitored in hadronic endcap calorimeter throughout the year and signal drop is measured with collision data, a method also known as in-situ measurement. Energy response of the detector can be monitored with precision in cellular level in longitudinal and azimuthal directions. Signal drop is compared with the results obtained by alternative methods such as laser method. These results not only give insights about the life time of plastic scintillators and silicon photomultipliers but also show their response to dose and dose rate effects. They also provide prospects for future upgrades. The results are used to calibrate the energy values of the collision events for precision measurements and are an integral part of the experiment. We provide the full results of our analysis.
State density in one dimensional ferromagnetic spin system with impurity

(Institute of Science and Technology, 1995) Alp, Murat ; Erzan, Ayşe ; 46467 ; Institute of Science and Technology

Spin sistemleri katıhal fiziğinin önemli çalışma alanlarından birisidir. Atomlar, katıhalde madde içinde örgü noktalarına yerleşirler ve atomların çekirdeği bir elektron bulutu ile sanlıdır., Yalıtkan maddelerde atoma bağlı elektronun potensiyelinin V(r) dönmeye göre simetrisi olmadığı için özdurumları net bir açısal momentuma sahip değildir. Açısal momentumu manyetik moment kaynağı olarak ele almadığımızda geriye elektronun iç spini kalır. Elektronlar arası etkileşmeler ile atomdaki elektronların spinleri tek bir toplam spin yapacak şekilde birleşir. Örgü noktalarındaki toplam spinier ise birbirleri ile aralarındaki uzaklığın fonksiyonu olarak etkileşirler. Yalıtkan maddelerde Hamiltonyen, spin spin etkileşmeleri türünden Heisenberg Hamiltonyeni ile her örgü köşesine yerleşmiş spinlerin bir etkileşme matris elemanının fonksiyonu olarak yazabiliriz. Etkileşmeyi sağlayan değiş tokuş integrali, elektron dalga fonksiyonlarının uzaklığa bağlı olarak üst üste kesişmesinin bir fonksiyonudur. Bu çalışmada ve benzeri çalışmalarda da genellikle en yakın komşu spin etkileşmeleri alınarak çalışılır. Hamilton denklemi en genel halde kristal simetrisinden gelebilen spinlerin uzayda ağırlıklı olarak yönelmek istedikleri yönler olabileceği durumda anizotropik Hamiltonyen denklemi alınır.

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