LEE- Fizik Mühendisliği Lisansüstü Programı

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

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A remedy for major cosmological tensions: Dark energy with an oscillating inertial mass density

(Graduate School, 2022) Kıbrıs, Cihad ; Akarsu, Özgür ; 772305 ; Department of Physics Engineering

The preponderance of observational evidence indicates that a vast portion of the energy density of the Universe today comes in dark matter and enigmatic dark energy (DE). The standard cosmological model, namely, the so-called Lambda Cold Dark Matter model ($\Lambda$CDM), resting on this dark sector as well as a small fraction of baryons has been remarkably successful in elucidating the bulk of Universe we inhabit. Though, astronomical observations improving in precision over the course of years are increasingly exposing that $\Lambda$CDM is significantly discrepant with various datasets. The direct and local measurements of the present-day expansion rate yielding $H_{0}=73.04\pm1.04 \;\, {\rm km\,s^{-1}\,Mpc}^{-1}$ are at more than $5\sigma$ tension (the Hubble $H_0$ tension) with the one $H_{0}=67.36\pm0.54 \;\, {\rm km\,s^{-1}\,Mpc}^{-1}$ inferred within the $\Lambda$CDM based on matter-baryon densities and the spacing between acoustic peaks of the CMB. The $H_0$ tension effectively propagates to the supernovae absolute magnitude $M_B$ through the distance modulus $\mu(z_i,H(z))=m_{B,i}-M_{B,i}$ where $m_{B,i}$ is the measured apparent magnitude of the supernovae observed at the redshift $z_i$, and creates a $3.4\sigma$ tension with the results calibrated by the CMB sound horizon scale. Another discrepancy regarding the expansion rate $H(z)$ within the best fit $\Lambda$CDM is the $\sim1.5\sigma$ tension between low (Galaxy BAO) and high redshift (Lyman-$\alpha$ at $z\approx2.33$) BAO data. It first emerged as a preference for smaller $H(z)$ and accompanying negative DE densities for $1.7\lesssim z\lesssim2.34$, being at $2.5\sigma$ tension with $\Lambda$CDM. In addition, $\Lambda$CDM predicts a larger weighted amplitude of matter fluctuations $S_8$ in comparison with what the independent large scale structure dynamical low-redshift probes suggest, thereby running into $2$ to $3\sigma$ tension. Given the long-standing theoretical issues such as the cosmological constant and coincidence problems related to the $\Lambda$, all these enumerated challenges and more inevitably motivate many to seek for a more complete framework either as modified theories of gravity or as minimal extensions beyond $\Lambda$CDM in the context of General Relativity (GR). In this sense, an approach that constitutes an example of the latter attempts focuses on inertial mass density $\Varrho\;=\rho+p$ parametrizations. The graduated dark energy (gDE) model proposed in Akarsu \textit{et al}. [Phys. Rev. D 101, 063528 (2020)], whose inertial mass density $\Varrho$ measures the minimum dynamical deviation $\Varrho\;\propto \rho^{\lambda}$ from the assumption of null QFT vacuum energy $\Varrho_{\Lambda}\;=0$ is one that exhibits nontrivial properties. It turns out that smaller and smaller negative values of the parameter $\lambda$ corresponds to a constant negative DE density that changes its sign from negative to positive in the past. Such a dynamical behavior would imply that $H(z)$ suppressed by the presence of a negative source at high redshifts results in an enhanced $H(z)$ at lower redshifts as the comoving angular diameter distance $D_M(z)$ to the surface of last scattering $D_M(z_*)=c\int_0^{z_*} H^{-1}(z)\d z$ is very stringently and almost model-independently constrained by the CMB for any given pre-recombination physics and should be kept unaltered. This means that if the redshift at which the sign-flip in the DE density occurs is slightly below the anomalous Ly-$\alpha$ at effective redshift $z\approx2.34$, it is quite conceivable that a dynamical DE possessing negative energy density mitigates both the $H_0$ and Ly-$\alpha$ discrepancies. The observational analysis of the gDE strongly favors a scenario in which the sign change of the gDE density is so swift it is practically identical to the cosmological constant phenomenologically flipping its sign much like the step function, except that $\Lambda<0$ in the past. It arises as a limiting case $\lambda\rightarrow-\infty$ of the gDE such that $\rho_{\rm gDE}(a)\rightarrow \rho_{\rm gDE,0}{\rm sgn}[f(a)]$ where sgn is the signum function. This limit has been comprehensively studied under the name of the $\Lambda_{\rm s}$CDM model in Akarsu \textit{et al}. [Phys. Rev. D 104, 123512 (2021)] where the late-time accelerated expansion is driven by $\Lambda_s\equiv\Lambda_{\rm s0}{\rm sgn}[z-z_{\dagger}]$ with $z_\dagger$ being the switching redshift rather than the usual $\Lambda$. The confrontation with the data sets encompassing CMB, Pantheon SNIa with and without SH0ES $M_B$ priors and BAO, shows that $\Lambda_{\rm s}$CDM simultaneously ameliorates six of the present significant tensions, namely $H_0$, $M_B$, $S_8$, Ly-$\alpha$, $t_0$ and $\omega_b$ tensions.
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.
Beyond the ACDM model: Addressing observational tensions of cosmology with negative and/or oscillating dark energy density

(Graduate School, 2024-08-29) Özülker, Emre ; Akarsu, Özgür ; Di Valentino, Eleonora ; 509202103 ; Physics Engineering

The standard cosmological paradigm, the Lambda Cold Dark Matter ($\Lambda$CDM) model, despite its prominence in explaining the existing cosmological data, encounters challenges due to inconsistencies observed across various cosmological probes over the past decade. While these observational tensions may stem from systematic errors, their increasing statistical significance with improved observations, and their recurrence through the increased diversity of observations, coupled with the inherent ambiguities surrounding dark energy (DE) and cold dark matter (CDM) in the $\Lambda$CDM model, indicate the necessity of looking for more realistic cosmological models to better elucidate current observations and refine our comprehension of the universe. This thesis explores what kind of phenomenology is necessary for an alternative model to resolve the observational tensions and what would that phenomena imply for fundamental physics; particular attention is paid to the proposal of a dynamical DE whose density attains negative values in the past before becoming positive to drive the present-day acceleration of the Universe, and/or the DE density having an oscillatory behaviour. Moreover, alternative models to $\Lambda$CDM are built and observationally analysed utilizing the state of the art statistical methods. The research conducted within the scope of this thesis extensively use analytical arguments to show in which way the features of having negative values and oscillatory behaviors in the DE density allow a better fit to the available cosmological data, and what are the properties of such a DE source. These arguments are used as a motivation to introduce a new cosmological model/paradigm that replaces the usual cosmological constant of the $\Lambda$CDM model with a sign switching one dubbed $\Lambda_{\rm s}$CDM; later this phenomenological model is promoted to a fully predictive one by embedding it in the theoretical framework of a preexisting type-II minimally modified theory of gravity, and this combination is dubbed $\Lambda_{\rm s}$VCDM. The analytical arguments are also used to introduce a new class of rampant DE models incorporating both of the negative and oscillatory features dubbed \textit{omnipotent dark energy}, and the preexisting DMS20 model in the literature that fits this classification is studied. Four cosmological models, namely, $\Lambda_{\rm s}$CDM, DMS20, $\Lambda_{\rm s}$VCDM, and a combination of $\Lambda_{\rm s}$CDM with the preexisting proposal of a time-varying electron mass, are observationally analyzed using a Bayesian approach. Broadly, the methodology used for analyzing the models consist of the following steps: the equations of the model for the relevant cosmological observables are derived, a preliminary analyses is conducted through plotting these equations with rough constraints from observations to understand the potential phenomenology of the model and how it relates to its parameters, meaningful combinations of robust cosmological data are chosen to constrain the parameter space of the model, state of the art cosmological codes are used to sample from the posterior of the parameters of the model, the parameter constraints and visualisations of the posteriors are constructed from the output samples, the results are discussed in light of the observational tensions. A very brief summary of the conclusions derived from the research conducted within the scope of this thesis is as follows. It is proved that a DE density whose density crosses from negative to positive values has a singular equation of state parameter. It is shown that, under reasonable conditions, any deviations from the Hubble radius of the $\Lambda$CDM model should oscillate in the form of an admissible wavelet in order to preserve the excellent fit of the $\Lambda$CDM model to the cosmic microwave background data. A combination of the two features in the DE density, namely, having oscillations and attaining negative values in the past as exhibited by omnipotent dark energy models, is a promising direction of research in alleviation of the cosmological tensions. Both of these features play an important role in relaxing the cosmological tensions within the DMS20 model. The $\Lambda_{\rm s}$CDM model shows remarkable success in alleviation of a multitude of cosmological tensions including the most major ones when the baryon acoustic oscillations data is not present in the constraining data set. Baryon acoustic oscillations data spoils this success by preferring an earlier time for the sign-switch of the cosmological constant contrary to the rest of the data sets. The $\Lambda_{\rm s}$VCDM model promotes the model to a fully predictive and theoretically viable one, and this new model performs even better observationally albeit this is very marginal. Combination of the $\Lambda_{\rm s}$CDM model with a promising early time modification to $\Lambda$CDM based on a time-varying electron mass do not perform better than the individual models contrary to naive expectations.
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.
Cosmological interacting models via energy-momentum squared gravity

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

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

(Lisansüstü Eğitim Enstitüsü, 2024-01-26) Yılmaz, Handan ; Reyhancan, İskender Atilla ; 509191131 ; Fizik Mühendisliği

Radyasyon kaynağı kullanılarak tahribatsız bir şekilde çevrimiçi element analizi yapan cihazlar birçok araştırma alanında kullanılmaktadır. Radyasyon kaynağı kullanılan, tahribatsız analiz sistemlerinde çevresel doz değerleri insan sağlığı açısından önemlidir. Doz değerleri için Uluslarası Radyolojik Koruma Komisyonu (URKK) ve Uluslararası Radyasyon Birimleri ve Ölçümleri Komisyonu (RBÖK) tarafından belirli sınırlamalar mevcuttur. URKK, halkın bireysel üyeleri için yıllık 1 mSv'lik doz sınırı önermektedir. Analiz sistemlerinde kullanılan radyoaktif kaynak, çevresel doz değerlendirmesi adına büyük önem arz etmektedir. Analiz cihazlarının çevresindeki radyasyon dozu etkileri, radyasyon zırhlama ile minimum seviyeye indirmek mümkündür. Radyasyon zırhlama, kaynağın etkisini azaltacak malzemeler ile sağlanır. Analiz cihazının fiziksel ortama aktarımı yapılmadan önce bilgisayar ortamında hesaplanmalıdır. Radyasyonun malzeme ile etkileşimlerinin bilgisayar ortamında modellenmesi için Monte Carlo simülasyonları kullanılır. Bu simülasyon programları EGSnrc, FLUKA, SimSet, PENELOPE, MCNP, GEANT4, GATE, GAMOS ve TOPAS'dır. Bu çalışmada, Kaliforniyum (Cf-252) radyoaktif maddesi kullanılan prototip analiz cihazının doz değerlendirmesini ele alınmıştır. Çalışmada cihaz modellemesi için kullanılan simülasyon programı GEANT4, parçacık ve madde etkileşimlerinde geniş bir enerji aralığına sahip fizik modellerini barındırdığı için tercih edilmiştir. Çalışmanın amacı, deneysel olarak dedektörlerden alınan verilerle Monte Carlo tabanlı GEANT4 kodu kullanılarak doz haritalarının karşılaştırılmasıdır. Deneysel çalışma İstanbul Teknik Üniversitesi (İTÜ) Enerji Enstitüsü'nde bulunan prototip analiz cihazının çevresinde gama ışını ve nötron doz değerleri alınmıştır. Cihaz geometrisinin simülasyon programına aktarılması için ölçüleri alınmıştır. Doz değerleri bir adet gama dedektörü (identiFINDER) ve bir adet nötron dedektörü (ATOMTEX BDKN-02) ile elde edilmiştir. Doz değerleri tekrarlı ölçümlerle kaydedilmiştir. Deneysel olarak kaydedilen bu değerlerle birlikte cihaz ölçüleri kullanılarak cihazın çevresinde gama ışını ve nötron doz haritaları oluşturulmuştur. Cihazın çevresinde beş farklı yüzeyde haritalar oluşturulmuş olup, toplamda gama ışını ve nötron için on harita oluşturulmuştur. Oluşturulan haritalarda interpolasyon analiz yöntemi kullanılmıştır. Eksik noktalardaki doz değerleri, interpolasyon yöntemi kullanılarak tahmin edilmiştir. Haritalamaların sonucunda, her bir harita yüzeyinde yirmi noktanın simülasyon programıyla karşılaştırılması için hesaplamalar yapılmış olup kaydedilmiştir. GEANT4 simülasyon programı ile deneysel ortamın birebir geometrisi oluşturularak gama ışını ve nötron doz değerlerleri hesaplanmıştır. Simülasyon çalışmasında geometrinin oluşturulması, radyasyon kaynağının tanımlanması, fizik modellerinin belirlenmesi ve harita analizleri yapılmıştır. Radyasyon kaynağı, Cf-252 enerji spektrumu Mannhart spektrum ile düzenlenmiştir. Fizik modelleri için Nötron Veri Kütüphanesi veritabanı kullanılmıştır. Haritalama analizleri, cihazın çevresindeki beş farklı yüzeydeki puanlama hacimlerinde gama ışını ve nötron doz değerleri ölçümleri için ayrı ayrı gerçekleştirilmiştir. Puanlama hacminde 27000 noktada değer kaydedilmiştir. Bu büyük verinin beş farklı yüzeyde görselleştirmesi ROOT yazılım programı ile yapılmıştır. Fiziksel laboratuvar ortamında yapılan çalışma ile bilgisayar ortamında (GEANT4) gerçekleştirilen çalışmanın karşılaştırılması, ölçüm belirsizlikleri ve istatistiksel analizler kullanılarak gerçekleştirilmiştir. İstatiksel analiz için ortalama mutlak hata (MAE), hataların ortalama karekökü (RMSE), ortalama mutlak yüzde hata (MAPE), R Kare ve Pearson korelasyon katsayısı hesaplanmıştır. GEANT4 Ölçüm belirsizliği rastgele sayı üretim fonksiyonun değiştirilmesi ile tekrarlı ölçüm alınması ve analiz edilmesi sonucunda standart sapması 0,000337 olarak hesaplanmıştır. Deneysel ölçüm belirsizliği, beş farklı yüzey için ayrı olarak hesaplanmış olup gama ışını doz hızı için standart sapma 0,03 - 0,05 aralığındadır. Aynı şekilde nötron akış hızı için 0,0185 - 0,9792 aralığındadır. Deney ve simülasyon sonuçlarının karşılaştırılması için şu kavramlara değinilmelidir: Tezde, deney çalışmasının laboratuvar ortamında yürütülmüş olup simülasyon çalışmasının ise bilgisayar ortamında yürütülerek gerçekleştirilmiştir. Deney çalışmasında laboratuvar ortamının ve prototip analiz cizahının sahip oldukları malzeme cinsi ile GEANT4 simülasyonu yürütülen çalışmada C++ yazılım dili bu çalışmadaki referans parametrelerdir. Deney ve GEANT4 sonuçlarının karşılaştırılmalı istatistiksel analizleri hesaplanmıştır. Deneysel ve GEANT4 ölçüm sonuçları doğrultusunda toplamda 10 farklı doz haritası oluşturuldu. 10 farklı yüzeydeki haritalama sonuçlarında MAE değeri, 0,0150 - 2,3345 aralığındadır. MAPE değeri, 3,0728 - 21,1023 arasındadır. RMSE değeri 0,4239 - 3,4199 aralığında olup $R^2$ değeri ise -0,0143 - 0,9966 aralığında bulunmuştur. $R^2$ değerinin, 1 değerine yakınlığı simülasyonun deneysel sonuçları doğru bir şekilde hesapladığını gösterdi. Son olarak korelasyon katsayısı, 0,4767 - 0,9982 değer aralığında bulunmuştur. Sonuç oarak deneysel ve GEANT4 ölçüm sonuçları uyumlu bulunmuştur. Aynı zamanda operatör çalışma bölgesi olan yüzey URKK doz limitlerinin içinde olup yıllık 1mSv'i geçmemektedir. Bu yüzeyin eşdeğer doz hızı yıllık 0,948 mSv olarak bulunmuştur.
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.
Design of a si-pin based gamma detector used for the assessment of environmental radioactivity

(Graduate School, 2024-06-24) Tataroğlu, Gökçen ; Özben, Cenap Şahabettin ; 509211107 ; Physics Engineering

Radiation, that individuals frequently experience in daily life, is classified into two types: ionizing and non-ionizing radiation. Non-ionizing radiation is a type of radiation that we are exposed to in daily life, such as radio waves (RF). This type of radiation that does not harm the cell structure. On the contrary, ionizing radiation is a type of radiation that we are less likely to encounter in daily life, and it can cause destruction to our cells. One of the most common examples of these types of radiation is X-rays. Radiation can be exposed in two ways, internally or externally. The human body can be exposed to harmful ionizing radiation, such as inhaling dust or smoke containing radioactive materials, consuming liquids and food contaminated with radioactivity, as well as radon gas and medical sources used for treatment and diagnostic purposes. X-rays with energies ranging from 20keV to 300 keV are used in X-ray devices in hospitals. While the patient is exposed to X-rays, the technician moves to a room where he can protect himself from the radiation exposure. Since radiation workers are trained for possible dangers of the radiation exposure and the radiation protection methods. Radiation workers can keep the external radiation dose they are exposed under control with the help of their personal dosimeters in the institutions they work for. Given the fact that not everyone, like radiation workers, will be able to carry a personal dosimeter, the importance of constantly monitoring the level of spatial and environmental radiation is obvious. Nuclear power plants, research reactors, accelerators, isotope production, irradiation and sterilization facilities are places that must always be kept under control for safety. Examples in history have shown that radiation leaks can occur as a result of errors in these plants, releasing unacceptable amounts of radioactivity into the environment. In the face of such a situation, it is necessary to constantly monitor the level of ionizing radiation in certain areas in order to make the fastest intervention. In this study, a gamma detector that can monitor both spatial and environmental radiation was designed and prototype production of this system was carried out. During the construction of this detector, its simplicity, low power consumption, ability to be built with readily available materials, ability to detect radiation with as low energies as possible, which is one of the requirements in the measurement of environmental radiation, and ability to operate in rural areas as self-sufficient in terms of power were highlighted. The device basically consists of a transmitting station where all the measurements are performed in the field and transmitted to the base-station and a receiver unit where the data is received and stored by the receiving station. The sender unit consists of ten main elements. These are microprocessor (ATmega328P), solar panels, step up and down converters, power filtering circuitry, batteries, real time clock (RTC), atmospheric sensor, LoRa and a gamma detector.
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.
Electro optical properties of liquid crystal nanocomposites

(Graduate School, 2022-11-29) Çetinkaya, Mehmet Can ; Özbek Yıldız, Sevtap ; 509132102 ; Physics Engineering

The thesis presents the results of the study on electro-optical and elastic properties of smectogen octylcyanobiphenyl (8CB) liquid crystal doped with well dispersed multi-walled carbon nanotubes (MWCNTs). The study aims to uncover the effect of MWCNT doping on electro-optical properties of smectic liquid crystal 8CB. Nanoparticle doping has been investigated as a possible way to enhance the properties of liquid crystals. Such studies focused on doping nematic liquid crystals have shown that it is possible to increase dielectric anisotropy, lower threshold voltages and increase electrical response times. One of the promising nanoparticles is carbon nanotubes. Carbon nanotubes are exotic materials with very high shape anisotropy. They are basically one atom thick carbon sheets rolled into tubes. If the nanotubes are formed from many cocentric tubes they are called multi-walled carbon nanotubes. Due to their shape anisotropy, they also exhibit anisotropic mechanical and electronic qualities. What makes them especially intriguing to study as liquid crystal dopants is their geometry. Carbon nanotubes are known to produce liquid crystalline dispersions, and once they are dispersed in a liquid crystalline medium, they are expected to impart their anisotropic qualities on this already anisotropic media and enhance them. The study is carried out on four different samples: one set dispersed with pristine, used as is, MWCNTs of concentrations 0.007 wt.\% and 0.07 wt.\% (percent by weight), and another set dispersed with -COOH functionalized MWCNTs at same concentrations. So the scope of this thesis is investigating the effects of functionalized and nonfunctionalized MWCNT doping on 8CB liquid crystal host's dielectric anisotropy, threshold voltage, birefringence and response time in Freédericksz transition. Each of said properties were measured against temperature and voltage. The doping of MWCNTs is carried out by solvent dispersion method. The 8CB and MWCNTs are mixed in high purity toluene. The mixture is heated to 43 $^{\circ}$C, where 8CB is in isotropic liquid phase. The mixture is then sonicated and magnetically stirred. Once toluene is evaporated, the remaining 8CB - MWCNT dispersion, or so-called nanocomposite, is filled into sandwich type sample cells by capillary action. Differential scanning calorimetry measurements and polarizing microscopy imaging are also conducted to confirm the existence of nematic and smectic A phases and transition temperatures. Experimental datum for the temperature dependence of birefringence, dielectric anisotropy, threshold voltage, voltage-on and voltage-off response are presented. Birefringence data is obtained by the rotating analyzer method. Dielectric data is collected by capacitance measurements. Threshold voltages and response times are obtained by light transmission measurements through crossed polarizers during voltage application on their respective sample cells. The isotropic to nematic and nematic to smectic A phase transition temperatures are obtained from birefringence measurements. The splay elastic constants and rotational viscosities of functionalized and non-functionalized MWCNT dispersed 8CB nanocomposites are calculated from these dielectric anisotropies, threshold voltages and voltage-off response times. The nematic range, temperature interval between isotropic to nematic and nematic to smectic A transition is extended in all nanocomposite samples. Non-functionalized MWCNT nanocomposites have wider nematic ranges than functionalized ones. It is observed that dielectric anisotropies are lowered in all MWCNT dispersed samples, higher concentrations having lower dielectric anisotropies. While birefringence is lowered in pristine MWCNT nanocomposites and functionalized MWCNT 0.07 wt.\% nanocomposite, compared to 8CB, functionalized 0.007 wt.\% nanocomposite's birefringence is higher than pure 8CB's. These findings suggest that while nematic order is not enhanced with MWCNT dispersions at hand, but the exception, functionalized 0.007 wt.\%, points to a possibility of better incorporation of MWCNTs in liquid crystalline media by dispersion of functionalized MWCNTs at low concentrations. The threshold voltages, with the exception of non-functionalized 0.07 wt.\%, are increased. Calculation of splay elastic constants (K$_{11}$) shows that elasticity of the nanocomposites also differ from pure 8CB. The non-functionalized MWCNT nanocomposites have lower elastic constants in the nematic range, while funtionalized MWCNT nanocomposites have higher. Also, a crossover of elastic constants is observed, with respect to temperature, as f0.007 wt.\% nanocomposite's elastic constant increases at an higher rate than f0.07 wt.\%'s elastic constant. Voltage-on and voltage-off response times are higher in all nanocomposites. Functionalized 0.007 wt.\% nanocomposite has the highest voltage-on and voltage-off response times. On the other hand, functionalized 0.07wt.\% has the closest response times to 8CB. The calculation of rotational viscosity from response times and elastic constants reveal that rate of change of rotational viscosity with respect to temperature is highest in funtionalized 0.007 wt.\% nanocomposite and rotational viscosities of all nanocomposite samples are higher than pure 8CB. With non-functionalized samples, rate of change of rotational viscosity is higher with 0.007 wt.\%. Temperature dependencies of rotational viscosity of 0.07 wt.\% and pure 8CB are similar. The same is true for functionalized 0.07 wt.\%. The study reveals the temperature dependence of various electro-optical properties of 8CB dispersed with functionalized and non-functionalized MWCNTs. With drops in birefringence and dielectric anisotropy, accompanied by increasing response times and threshold voltages, no electro-optical enhancement is observed in studied samples. However, with effect of functionalization and at optimal concentration, it would be possible to enhance a selected electro-optical property, i.e. improving birefringence while keeping response times at a reasonable level for electrical switching. The results of the study also hint at a complex web of dependencies related to functionalization and concentration, which justifies further numerical and experimental studies on smectic liquid crystal - MWCNT nanocomposites.
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.
Gravitational waves as the probe of early universe

(Graduate School, 2024-07-24) Yükselci, Ahmet Emrah ; Arapoğlu, Abdurrahman Savaş ; 509162107 ; Physics Engineering

The study of gravitational waves has recently become more and more important especially after their direct detection, since they have the potential to provide information about own sources due to their weakly interacting nature, which may allow them to remain almost intact even from processes of the early Universe and to contribute to the "Stochastic Gravitational Wave Background" today. The main purpose of this thesis is to examine the first-order cosmological phase transitions and the gravitational waves as the remnants of these processes by virtue of numerical simulations. To this end, firstly, the non-minimally coupled scalar field model is examined in the context of first-order cosmological phase transitions in order to determine the possible effect of the coupling parameter on the process. Then, the other section of the thesis is devoted to investigation of the mutual effects between the expansion of the Universe and the phase transition, together with the inspection of anisotropy through the energy density parameter of the shear scalar by implementing the Bianchi Type-I metric to the model, which contains a minimally coupled scalar field as the order parameter of the transition.
High resolution dielectric anisotropy investigation of carbon nanotube - smectic A liquid crystal dispersion

(Graduate School, 2022-12-08) Güven, Funda ; Özbek, Sevtap Yıldız ; 509181124 ; Physics Engineering

It is well-known that there are three common phases in condensed matter which are crystal, liquid, and gas. Crystalline solids are anisotropic, which means that many physical properties are different in direction. Besides, liquids are isotropic, which means they present the same properties in all directions. Liquid crystals (LCs) have physical features like an ordinary liquid, though their molecules are ordered, leading to anisotropy. Many liquid crytsalline materials which have one or more different mesophases exhibit properties between crystalline solid and isotropic liquid. Mesophases, which liquid crystals have, are mostly characterized by their properties of orientation. One of the mesophases is entitled nematic (N) phase, which has a long-range orientational order. Molecules which have orientational order tend to align along a director, which is the measure of the average direction of the molecules. The other phase is called has smectic phase (Sm), which has partial positional order in addition to the orientational order. Thereby, molecules are arranged in layers in smectic phases which are coded alphabetical according to the order of the discovery. Smectic A (SmA) phases have a wide range usage in a smectic phases. In the smectic A phase, molecules have a layered structure which is normal to the layers. It is worthwhile noting that knowledge of the phase transition is crucial to understanding the properties of the liquid crystals. According to de Gennes, a physical system which has a physical property can be a measure of the order. In nematic phase, molecules have a property of anisotropy which can be measured with a dielectric anisotropy. Therefore, dielectric measurements, which are an example of physical quantities, can be used for testing liquid crystalline behavior. Phase transition is a change of a phase from one to another by external effects like temperature or pressure. The point in phase transition where two phases cannot be distinguished from each other is defined as a critical point. Temperature refers to the critical point is called critical temperature Tc. If there is a latent heat and discontinuous change in entropy near the critical point, it is called a first order phase transition. Otherwise, it is called second order phase transition. Worthwhile recalling that critical behaviour of the liquid crystalline materials can be determined by the order parameter S(T) which is produced from dielectric anisotropy data. In this work, high resolution dielectric data have been obtained for 8CB (octylcyanobiphenyl) liquid crystals in addition to 8CB nanocomposites doped with both pristine multi-walled carbon nanotubes (p-MWCNT) and -carboxyl group (-COOH) functionalized MWCNTs (f-MWCNTs). Nematic order parameter for both the nematic-isotropic (N-I) and nematic smectic A (N-SmA) of the neat 8CB and 8CB+MWCNT has been derived from Maier-Meier theory, which can be used to examine the effect of dielectric anisotropy in the nematic phase. In 8CB doped with both p-MWCNT and f-MWCNT, the N-I and N-SmA transition temperatures shifted to a lower value compared to pure 8CB. N-I transitions for all 8CB+MWCNT nanocomposites manifests weakly first order, on the other hand, N-SmA transition remains continuous. Sufficiently far away from the SmA phase, the critical exponent β which determines the tricritical behavior of order parameter S(T) are obtained 0.238 ± 0.002 on average, which is in excellent agreement with the obtained from optical birefringence data. The obtained β value is compatible with the hypothesis that the N-I phase transition exhibits tricritical behavior. All investigated compounds present the temperature dependence of N-I and the nematic-crystalline phase transitions of the order parameter is quasi-tricritical. Previous studies show that, the critical behaviour at N-SmA transition is an important research area. For the first time, the upper limits for a latent heat ∆HNA for the neat 8CB and MWCNT doped 8CB have been derived from the detailed dielectric anisotropy data in the N-SmA transition. Besides, the ∆HNA values produced in this way appear to be in agreement when compared with the values obtained from optical birefringence data and ASC measurements. By using the power law analysis of the ∆ε(T) data, which is the temperature gradient of the nematic order parameter through the N-SmA transition, the effective specific heat capacity exponents α have been yielded for all samples. For the first time, high resolution dielectric measurements ∆ε enable to invetigate the N-SmA transition behaviour.
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.

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