LEE- Fizik Mühendisliği-Yüksek Lisans

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http://hdl.handle.net/11527/19276

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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.
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.
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.
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.
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.
Investigation of thermal conduction in microcontacts created by indentation

(Graduate School, 2022) Uluca, Ahmed ; Özer, Hakan Özgür ; 509191101 ; Physics Engineering Programme

Thermal contact conduction has been investigated on different scales for many practical and scientific motivations in the literature. Demands for engineering the interfaces are increasing for accurately managing the contact mechanics and heat transfer with miniaturization of the electronics devices. In this study, microcontacts, that are created by indentation, have been investigated with experimental, simulation, and analytical works. The spreading resistance perspective of the disc constriction case has been extended for the studied highly plastic microcontacts of indentation. Creating the microcontacts and investigating the conductance through them had been realized by indentation of metallic surfaces by specially prepared diamond micro-particles/indenters. Thermal measurements had been realized by mounting thin thermocouples on diamond tips. The experimental setup is home-built with commercial piezo, motor, DAQ utilities, and other miscellaneous devices. PC User Interface and Intercessor Microcontroller Unit had been programmed to properly manage to conduct experiments. Furthermore, to measure the resistance, we employed an oscillatory experimental procedure and lumped analysis of transient heat transfer. The application of oscillations at different indentation depths has enabled us to extract the RC behavior of the microcontacts created by high plastic deformation. Therefore, the time constant of the contacts can be obtained. Additionally, we could find an effective measure of the thermal diffusivity of the contact through the diamond tip by fitting the change of time constant to depth with the proposed modified constriction models. Moreover, to analyze and predict the change of the time constant with respect to depth and load, several simulations and calculation work had been pursued. The increase in the contact area by indenter penetration into the sample has been concerned to be suppressed by gradient occurrence along the tip-sample contact. Moreover, with help of the simulations, we deduced the effect of plasticity such as pile-up on the improvement of the indentation contact for the heat transfer can be effective. Consequently, for the first time, we conducted the periodic contact procedure for the thermal contact of single micro asperity of indentation. The periodic experimental procedure and fin efficiency application to spreading cases for single microcontact are unique parts of this work. Results with the diamond tip on three different metallic samples showed that the gradient occurrence along the indentation contact can be analyzed with the fin solutions of the literature. Experimental results were fitted properly to a unified function of conic fin and spreading resistance functions. In addition, parameters of the fits can be deduced for the conductivity and interface conductance. However, state of the results are not sufficient to exactly determine the contact and material parameters due to need for exact parameters for transient analysis and, uncertainties in the properties of the tip and samples. With help of more precise thermal measurements and indenter systems, this experimental procedure may provide further advances and ease in the investigation of the thermal contacts of many different materials and scales. In addition, for the solid-state thermal interface materials solutions, we deduce that investigation of the geometry optimization for pressure and heat transfer as indicated in this thesis would provide insights into the bottlenecks of the contact heat transfer. Specifically, the gradient occurrence and its effectivity on the overall contact heat transfer should be taken into account for the indentation contacts while improving the contact by plasticity.
Machine learning analysis of pulsar timing data

(Graduate School, 2021-12-17) Eser Hasançebi, Esma ; Çakır, Altan Muammer ; 509191108 ; Physics Engineering

In 1967, radio pulsations from a celestial body were discovered by a graduate student Jocelyn Bell and her advisor Antony Hewish. This was the first sample of about three thousand similar sources, called pulsars, to be discovered in our galaxy to date. It has been understood that pulsars are rapidly spinning, strongly magnetized neutron stars. Neutron stars are very dense objects formed by the collapse of the cores of massive stars at the end of their life. Each pulsar has its characteristic pulse shape and rotational frequency. The rotational frequency of pulsars can be measured very precisely. The rotation frequencies of pulsars are observed to decrease in time. Pulsars tap their radiative energy from their rotational kinetic energy. The mechanisms by which pulsars achieve this energy conversion is not well-understood. According to a prominent model, the pulsars convert their kinetic energy into radiation by emission of magnetic dipole radiation (MDR). However, studies with young pulsar data show that the MDR model does not fully explain the observations and there should be other mechanisms assisting the spin-down. The ejection of high-energy particles, the growth of the dipole magnetic field over time, interaction with a supernova debris disc, increasing inclination angle between the rotation and magnetic axis, and gravitational wave emission are some of the processes proposed to affect spin-evolution. Occasionally, some pulsars suffer sudden increases in their spin, also known as "glitches" which decay in the following weeks or months. When they were first discovered, it was thought that glitches result from the breaking of the crust and hence they were called "stellar quakes". Today, it is conceived that this model can only account for the smallest glitches or that it could be a triggering mechanism for the main cause of the glitches. According to the more favoured view, the glitches are caused by the dynamics of the crustal superfluid. Sometimes a new glitch occurs before the previous glitch decayed. The presence of glitches in the pulsar data complicates the understanding of the spin evolution. The aim of the thesis is to contribute to the understanding of the spin evolution of pulsars by machine learning methods. To this end, long-term time-dependent spin frequency data of Crab and Vela pulsars are used. These are the two best-known pulsars that have been studied the most. Since the frequency changes by a small fraction throughout the time-span of the observations, we have eliminated the basic trend by fitting the data with a polynomial function. By subtracting this basic trend from the data, we obtained the residuals that clearly show the complicated features of spin evolution such as glitches. We tested the performance of two machine learning methods in reproducing the evolution of the residuals. The first method is called the sparse identification of nonlinear dynamics (SINDY). Given a time-series data, SINDY can identify the governing system of ordinary differential equations. We thus used this method to find the governing equations for the evolution of the residuals. The SINDy method gave information about the order of the equations and their coefficients. In addition, we used a recurrent neural network (RNN) architecture called long short-term memory (LSTM) method on the same data sets. We found that LSTM can predict the dates of glitches in the test data. The results show that SINDy and LSTM applications can contribute to the studies on the spin evolution of pulsars and may take place more in studies related to pulsars in the future.
MEMS ile entegre mikro ısıtıcı ve IDE mikro sistemlerin fabrikasyonu ve nano kompozit yarı iletken gaz sensör uygulaması

(Lisansüstü Eğitim Enstitüsü, 2024-08-05) İlbeyiilingi, Halime ; Morova, Berna ; Taşaltın, Cihat ; 509211108 ; Fizik Mühendisliği

Günümüzde Mikro Elektro Mekanik Sistemler (MEMS) teknolojisi ile mikro ısıtıcı sistemlerin inter dijital elektrotlar (IDE) ile entegrasyonunun geliştirilmesine yönelik ihtiyaç gün be gün artmaktadır. MEMS teknolojisi, mikroskopik ölçekte mekanik ve elektronik bileşenlerin entegrasyonunu içerir. Mikrosistem mühendisliği, elektronik, kimya, biyoloji ve fizik gibi birçok farklı disiplini birleştirir. Bu entegrasyon, daha karmaşık sistemlerin ve uygulamaların geliştirilmesini mümkün kılar. Örneğin, kimyasal algılama için kullanılan sensörler, biyolojik materyallerle birleştirilerek hastalık tespitinde kullanılabilir. Bu tür çok disiplinli çalışmalar, araştırma ve geliştirme süreçlerini zenginleştirir ve bilim ile mühendislik arasındaki sınırları aşarak yenilikçi çözümler üretir. Bu teknoloji, yüksek hassasiyet ve düşük maliyet avantajları ile öne çıkarak, biyomedikal uygulamalar, tüketici elektroniği gibi bir çok alanda kullanılmaktadır. MEMS teknolojisinde kullanılan gaz sensörleri, endüstriyel süreçlerin kontrolü, hava kalitesinin izlenmesi, çevresel güvenlik ve tıbbi teshişler için hayati öneme sahip alanlarda kritik rolller üstlenmektedir. Özellikle, endüstriyel ve çevresel uygulamalarda zararlı gazların tespit edilmesi, halk sağlığı ve güvenliği açısından büyük öneme sahiptir. Bu nedenle, düşük maliyetli, yüksek duyarlılık ve hızlı yanıt süresine sahip gaz sensörlerine duyulan ihtiyaç büyüktür. Bu sensörler biyomedikal uygulamalarda, solunum yolu hastalılarının teşhisinde öneme sahip uçucu organik biyo belirteçlerin (VOC) tespitiden kanser tipine kadar geniş bir kullanım alanına sahiptir. Bu çalışmada tek bir silikon yonganın üzerine ince film biriktirme yöntemlerinde kullanılan; çok katlı foto-litografi, PVD (e-beam), PECVD, elektrokimyasal yöntemler, üst üste entegrasyon, ICP-RIE kuru aşındırma, metalizasyon gibi yöntemler kullanılarak bir çok uygulama alanında kullanılabilir platformlar üretilmiştir. Üretilen platformun çalışıp çalışmadığının kontrolü için gaz sensör uygulaması seçilmiştir. İlgili malzemelerin sentez, katkılama ve platform üzerine kaplanması için hidro termal ve damlatma metodları ile gaz sensörleri üretimi başarılı bir şekilde gerçekleştirilmiştir. Çalışma sonucunda 2.2 mm en ve 4.8 mm boy oranlarına sahip, 300 µm Si-yonga üzerine çok katlı (2 µm SiO2 / 30 nm Ti / 30 nm Au / 600 nm Pt ) mikro ısıtıcı sistemleler üretilerek, 1 dakikada max 417℃ sıcaklığa yükselen platin mikro ısıtıcılar üretilmiştir. Platin mikro ısıtıcıların sıcaklık karakterizasyonları için hem kendi oluşturduğumuz devre hem de termal kamera ile ölçümler yapılmıştır. Ölçüm sonuçlarından, sıcaklık değişimine karşı direnç değişim grafiğinden platin metali için α sabiti 0.00345 ℃-1 olarak hesaplanmıştır. Üst üste biriktirme teknolojisi sayesinde 250 nm kalınlığında Si3N4 pasivasyon malzemesi kullanılarak ve üretilen mikro ısıtıcıların 200℃ ve 400℃ de 2 saat tavlama işlemi gerçekleştirilmiştir. Çok katmanlı xxvi (100 nm Ti / 100 nm Au) IDE'lerin üretimi ve Si3N4 ara katman üzerine entegrasyonu gerçekleştirilmiştir. Bu platform için 4 çıkışlı 2 si mikro ısıtıcı, 2 si IDE sistem çıkışlı bakır PCB'ler üzerine ilk olarak mikro ısıtıcı sistemlerin ısı kaybını önlemek için 2 mm en ve 2 mm boya sahip 300 µm kalınlığında Si-yonga (wafer) takoz kesimi gerçekleştirilip üst üste yapıştırılmıştır, ardından tel bağlama (wire bonder) tekniği ile 25 µm Au teller ile bond işlemleri gerçekleştirilmişitir. Gaz sensör uygulaması için elektro aktif polimer ve metal oksitler kullanılmıştır. PANI, SnO2 malzelemelerin sentez kısımları gerçekleştirilmiş (PANI için emeraldin baz yalıtkan formu HCl ile muamele edilerek iletken hale getirilmiştir) ve ticari olarak satılan ZnO malzemesi ile 1:1 mg ve 1:5 mg gibi farklı oranlarında PANI, PANI / SnO2, PANI / ZnO nano kompozit metal oksit 3 tip gaz sensörleri üretilmiştir. Bu gaz sensörleri ile gaz sensör uygulamasının; endüstriyel süreçlerin kontrolü, hava kalitesinin izlenmesi, çevresel güvenlik için öneme sahip NO2 gazı ve solunum yolu hastalıkları için öneme sahip olan aseton, etanol, nem ve kloroform gazlarının akım-zaman yanıt grafikleri MATLAB kodu geliştirilerek analiz edilmiştir. Tüm sensörlerin saf gazlara karşı ve bu saf gazların %30, %50, %70 neme maruz bırakılmış konsantrasyonları için, oda sıcaklığında ve 55℃ sıcaklıkta ölçümler alınarak bar grafikleri elde edilmiştir. Platin metali için α sabiti 0.00345 ℃-1 olarak hesaplanması çok katmanlı mikro ısıtıcı sistemlerin doğru bir biçimde geliştirildiği, 2 mm – 2 mm (en-boy) oranlarındaki takozların sisteme yapıştırılması ısı kaybını önlemiştir ve max 417℃ sıcaklık elde edilmiştir. Üretilen 3 sensör tipinin çalışır durumda olduğu ölçüm sisteminden alınan verilerin MATLAB analizi ile çalışır durumda olduğu tespit edilmiştir.
Newtonian perturbation theory in cosmology: From inflation to large-scale structure

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

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

(Graduate School, 2022) Şenışık, Cemal Berfu ; Kahya, Emre Onur ; 770975 ; Physics Engineering Programme

Lie algebra expansion is an exciting method to obtain higher dimensional algebras and using this method one can write some interesting non-relativistic gravitational theories beginning from the Poincaré algebra. This method was first developed by Hatsuda and Sakaguchi (2003) and has been used in many other studies. In this work, we will first give a brief introduction to the gauge theories, which are seminal for understanding gravitational theories in depth, especially the algebraic structure of gravitational theories. Note that this is crucial for many gravity theories, such as supergravity. After that, we will study the general aspects of differential geometry shortly. This will give us the main mathematical framework to study gravity as a gauge theory. Thirdly, we will try to understand the theories of gravity, especially general relativity, as a gauge theory. After a simple introduction to the second-order formalism of GR, we will define the first-order formalism and its action. In the last part of this section, we will obtain GR beginning from the Poincaré algebra and by gauging this algebra. At last, we will give the definition of Newton-Cartan theory, its conditions, and its action. We will first show that this theory can be obtained from an algebraic point of view, i.e. by using Lie Algebra Expansion. We will also give the method, which is based on Ekiz et al. (2022), to obtain the same results by contraction of a multi-metric theory.
Nucleosynthesis in alternative theories of gravity

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

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

(Graduate School, 2022-06-02) Güzel, Ahmet Oğuz ; Çakır, Altan M. ; 509181120 ; Physics Engineering

Since the discovery of the Higgs boson in 2012 by the CMS and ATLAS experiments at the CERN's Large Hadron Collider in Geneva, Switzerland, physicists have tried to measure accurately its properties and to understand better the underlying electroweak symmetry breaking mechanism. In this pursuit, the search for Higgs boson pair production is crucial to test our understanding of the Higgs potential and to search for clues for the Beyond the Standard Model searches. This thesis describes the search for the Higgs boson pair production in decays to a W boson pair and a photon pair. The ττγγ channel of the Higgs boson pair decay is analysed alongside since an overlap is expected in the final states. Monte Carlo simulations of proton-proton collisions corresponding to an integrated luminosity of 3000 fb^-1 at a centre-of-mass energy of 14 TeV are used. The gluon-gluon fusion production mode of the Higgs boson pair is considered only. The Delphes fast detector simulation is used with an average pile-up of 200 per interaction with a dedicated card for Phase-2-upgraded CMS detector. A Python-based analysis library called Bamboo is used to perform the object selections and event categorisation in the data analysis of the study. Cut-flow tables reporting the number of events at each final state of interest are shown. Two binary Deep Neural Networks (DNN) are employed using the Keras API for TensorFlow machine learning library in order to increase the signal and background discrimination in the semi-leptonic final state of WWγγ and single τ final state of ττγγ channels. DNN score cuts are applied to each final state and the di-photon invariant mass distributions are obtained. The results are then used in the Higgs Combine Tool with the statistical and systematic uncertainties applied. The significance levels are reported for each final state along with their combination.
Sezyum atomik gaz hücresinin yüksek çözünürlüklü spektroskopisi

(Lisansüstü Eğitim Enstitüsü, 2024-06-05) Caklı, Berfin Muhlise ; Aktaş, Demet ; Hamid, Ramiz ; 509211104 ; Fizik Mühendisliği

Günümüzde, kuantum temelli sensör araştırmaları son derece ilgi çekici ve popüler bir alandır. Alkali metal içeren atomik gaz hücrelerinde yüksek çözünürlüklü lazer spektroskopisi, lazer-atom etkileşimi prensibine dayanan yüksek hassasiyetli atomik manyetometre, atomik jiroskop ve atomik saat gibi çeşitli teknolojilerin temelini oluşturur. Aynı zamanda referans frekansı mikrodalga osilatörleri veya lazerleri stabilize etmek için de kullanılır. Atom ve molekül fiziğinde önemli bir rol oynayan yüksek çözünürlüklü lazer spektroskopisi, modern metrolojide yeni standartların oluşturulmasında öncülük ederken, atomik ve moleküler yapıların detaylı analizine olanak sağlamaktadır. Uluslararası ölçü birim sisteminde zaman birimi olan saniye, Sezyum (Cs-133) atomunun aşırı ince yapısına dayalı tanım içermektedir. Tez kapsamında TÜBİTAK UME Atomik Sensörler Laboratuvarında üretilen sezyum atomik gaz hücresinin yüksek çözünürlüklü spektroskopisi üzerine çalışılmıştır. Sezyum alkali metalinin kullanılmasının birkaç nedeni vardır, ancak en belirgin olanı, dış kabuğunda tek bir değerlik elektronu bulunmasıdır. Bu tez çalışmasında, Cs alkali metalinin 852 nm dalga boyuna denk gelen D2 çizgisinin 6S1/2 - 6P3/2 atomik geçişleri ve Cs atomlarının soğurum spektroskopisi laboratuvarda özel olarak üretilen 10 mm çapındaki atomik gaz hücresi kullanılarak analiz edilmiştir. Bu bağlamda, farklı lazer güçlerinde, polarizasyonlarında ve atomik hücre sıcaklıklarında soğurum spektroskopisi ve atomik rezonanslar incelenmiştir. Böylelikle, spektroskopi tabanlı uygulamalardaki sensör hassasiyetlerini artırmak için Cs atomik gazının D2 enerji geçişindeki soğurum rezonanslarının çizgi genişliği ve genliği gibi parametrelerin yüksek çözünürlüklü spektroskopi teknikleri ile araştırılması yapılmıştır. Tez çalışmasında DBR lazer kullanarak sezyum alkali metali içeren atomik gaz hücresi ile deneysel lazer spektroskopisi çalışması yapılmıştır. Pompa lazer demeti atomik hücreden geçirilerek Doppler etkisi ile genişleyen soğurum spektral rezonansları gözlenmiş ve incelenmiştir (Lineer spektroskopi). Sonrasında pompa lazer demetine karşı gönderilen prob lazer demeti gönderilerek doyum soğurum spektroskopisi tekniğiyle (Lineer olmayan spektroskopi) Doppler genişlemesinin ötesinde sezyum atomunun ince geçiş çizgileri incelemiştir. Dolayısıyla Doppler bağımsız doyum spektroskopisi tekniği kullanılarak lineer spektroskopi yönteminin sınırlarını aşan lineer olmayan spektroskopi olarak isimlendirilen doyum soğurum spektroskopi yöntemiyle sezyum atomunun ince geçiş çizgileri elde edilmiştir. Lineer olmayan yüksek çözünürlüklü spektroskopi, maddeyi atomik ve moleküler seviyelerde incelemek için oldukça etkili bir araçtır. Alt Doppler rezonasların (sub Doppler veya Doppler-free) görüntülenmesi için kurulan deney düzeneği, yüksek çözünürlüklü spektroskopinin bir uygulaması olarak tek bir lazer kaynağından elde edilen aynı frekanstaki, farklı güçlere sahip iki lazer demetinin (pompa ve prob) hücre üzerinde karşıt yönlerde yönlendirilmesiyle kurulmuştur. Güçlü lazer demeti pompa lazer demeti, zayıf lazer demeti ise prob lazer demeti olarak isimlendirilir. Gaz hücresi üzerinde iki lazer demeti birbiriyle karşılaşır. Pompa lazer demeti temel enerji seviyesinde bulunan atomları uyararak bir üst enerji seviyesine taşıyacak kadar güçlü, prob lazer demeti ise rezonans doyumu yapamayacak kadar zayıftır. Soğrulan prob lazeri fotodedektöre düşürülmüştür. Fotodedektörden alınan sinyal ile osiloskop üzerinden soğurum profillerinin görüntülenmesini sağlamıştır. Hücreye gönderilen lazer ışınının frekansı, atomların geçiş frekanslarına eşit olduğunda ve aynı hız grubundaki atomlarla etkileştiklerinde, atomların Maxwell- Boltzman hız dağılımına uygun gelen genişlemiş doppler profili üzerinde rezonans frekanslarında lorentz profiline sahip atomik geçişler görünür. Elde edilen alt doppler rezonanslarının çizgi genişliğini etkileyen başlıca etken parametreler vardır. Bunlar doğal çizgi genişlemesi, doppler genişlemesi, çarpışma etkisi ve güç genişlemesinden oluşmaktadır. Doğal çizgi genişlemesi, atomun uyarılmış enerji seviyesindeki yaşam süresiyle orantılıdır. Doppler genişlemesi, hücre içindeki atomların termal hareketlerinden kaynaklanan hızları nedeniyle soğurduğu radyasyonun frekansını etkiler. Bu etkileşim sonucunda radyasyonun frekansında bir kayma olur ve bu da doppler genişlemesine yol açar. Alt Doppler rezonansının Doppler genişlemesine (𝜑𝑥𝛥𝜔D) genelde prob ve pompa lazer demetlerinin arasındaki açı (𝜑) ve demet parelleliği katkı sağlamaktadır. Çarpışma etkisi, atomlar birbirleriyle etkileşime girdiklerinde itme ve çekme kuvveti genişleme etkisi yaratır. Güç genişlemesi, doğrudan lazerin sahip olduğu ışık şiddetiyle ilgilidir. Gerçekleştirilen deneylerde Sezyum atomunun 852 nm D2 çizgisinin 6S1/2(F=3) 6P3/2(F'=2,3,4) ve 6S1/2(F=4)-6P3/2(F'=3,4,5) atomik geçişlerinde çalışılmıştır. Kurulan iki deney düzeneğinde ölçümler alınmıştır. İlk deney düzeneğinde pompa lazer demetinin farklı güçlerinde, prob lazer demeti düşük güçte sabit tutulmuş ve atomik geçişler üzerindeki etkisi incelenmiştir. Pompa lazer demet gücünün farklı değerlerinde çizgi genişlemesi hesabı yapılmış ve pompa lazer demetinin artan gücüyle çizgi genişliklerinin arttığı gözlenmiştir. Gerçekleştirilen bir diğer ölçüm hücrenin sıcaklığıyla ilgildir. Hücre sıcaklığı belirli değer aralıklarında arttırılarak, prob lazer demetinin hücre içerisindeki atomlar tarafından tamamen soğrulduğu değere kadar ölçüm alınmıştır. Sıcaklık arttırıldıkça atom yoğunluğu arttığı için atomlar arası çarpışma ve etkileşim artmıştır. Dolayısıyla gönderilen prob lazer demeti belli sıcaklık değerinde atomlar tarafından tamamen soğurulmuştur. Son olarak pompa ve probe lazer demetlerinin polarizasyonları değiştirilerek atomik geçişlerin soğurum spekturmları incelenmiştir. Uyarılmış seviyedeki atom yoğunluğu, lazer demetlerinin polarizasyonuna, lazer demetlerinin yoğunluğuna ve hücre sıcaklığına bağlı olarak değişmiştir. İkinci Deney düzeneğinde Sezyum alkali metali bulunan atomik gaz hücresi mu-metal içerisine konulmuştur. Zayıflatılmış manyetik alan altında lazer demetlerinin farklı polarizasyonlarında atomik geçişlerin soğurum spektrumları incelenmiştir. Gerçekleştirilen deneyde pompa ve prob lazer demetleri, eşit güçte olup karşı yönlerde yönlendirilmiştir. Bu lazer demetlerinin polarizasyonları, pompa lazer demeti sağ el dairesel (σ+) ve prob lazer demeti sol el dairesel (σ-) polarizasyon olacak şekilde ayarlanmıştır. Bu düzenleme ile alt Doppler rezonanslar incelenmiştir. Bu şekilde, optik pompalama telafisinin Doppler rezonansları üzerindeki etkisi incelenmiştir. Sürekli ve periyodik olarak enerji seviyeleri arasında etkileşim sağlanmış ve böylece lazer kaynağının gücü arttırıldığında sadece kapalı geçişdeki 6S1/2(F=3)-6P3/2(F'=2) resonans elde edilmiştir. Gerçekleştirilen bir diğer deney, güçlü pompa lazer demeti ve zayıf prob lazer demetinin farklı polarizasyonlar altında atomik geçişler üzerindeki etkisi incelenmiştir. Farklı polarizasyon koşullarında etkileşim özelliklerinin değiştiği gözlemlenmiştir. Atomik tabanlı sensörlerin hassasiyetlerini arttırabilmek için, yapılan deneylerden çıkarılan sonuç, dar çizgi genişliğine sahip rezonanslardan türetilen sinyallerle sensör hassasiyetleri arttırılabilir. Bundan dolayı, atomik rezonanslarının çizgi genişliği ve genliğini etkileyen parametrelerin (lazer ışınım demetlerinin şiddetleri ve polarizasyonları, atomik gazın yoğunluğu ve hücre sıcaklığı) araştırılması çok önemlidir.
Structural, optical and mechanical properties of polyacrylamide hydrogels doped with multiwalled carbon nanotubes

(Graduate School, 2022) Öztürk, Mert Can ; Aktaş Kaya, Demet ; 725977 ; Physics Engineering Programme

Hydrogels are two or multi-components systems made up of a three-dimensional network of polymer chains and water fillings the gap between macromolecules. Depending on the characteristics of the polymers utilised, and the type and density of the network joints, such structures in equilibrium can comprise varying quantities of water; in the swollen state, the weight percentage of water in a hydrogel is generally considerably larger than the weight percentage of polymer. On the other hand, carbon nanotubes (CNT) are one of the most intriguing novel materials in the last three decades. CNTs are hollow carbon tubes in nanometer sizes. They create a new form of carbon equivalent in configuration to a graphite sheet wrapped in a nanometer-sized hollow tube. It can be synthesized in sizes ranging from a few microns to several nanometers and in the thickness of many carbon layers from single-walled (SWCNT) to multi-walled (MWCNT) structures. The unique structure of these nanotubes gives them properties such as electrical and thermal conductivity, strength, hardness and toughness. CNT's mechanical properties have been extensively researched, both theoretically and experimentally. Measurements were taken using a 'nanostressing stage' inside a scanning electron microscope (SEM) indicating that the Young's modulus of MWCNT's outermost layers ranged from 270 to 950 GPa. Single nanotube tensile strengths were also determined, with values ranging from 11 to 63 GPa. CNT dispersion is critical for manufacturing high-quality nanocomposites because nanotube dispersion has such a large impact on suspension quality. CNTs readily agglomerate, bundle, and entangle, resulting in a plethora of defect sites in composites and a reduction in CNT efficiency. When all accessible CNTs are evenly dispersed in the host matrix, a good distribution is achieved. Mechanical parameters that fall short of theoretical expectations are mostly due to poor distribution and limited interfacial load transmission among CNTs and the polymer matrices. The mechanical characteristics of nanocomposites are influenced by interfaces in particular. Mechanical interlocking, chemical bonding, and the van der Waals force are three interfacial load transfer techniques that have been identified. Inadequate load transference across CNT and polymer chains may result in interfacial slippage and low mechanical strength and stiffness unless the interface is carefully built. When there is no interfacial load transmission, nonlinear stress–strain behavior is expected. CNTs have been widely explored as a novel nanomaterial because of their remarkable electrical conductivities and mechanical qualities. The inclusion of CNTs in the hydrogel matrix could result in a resilient and electrically conductive hydrogel due to their nano-reinforcement and intrinsic conductivity. The preparation procedure, however, is extremely complex due to the CNTs' inherent hydrophobic nature, restricting their application and even reducing the hydrogel's tensile strength. Studies continue on hydrophobic linkages and π-π interactions to better disperse CNTs in the hydrogel matrix. For example, adding oxidized multi-walled carbon nanotubes (oxCNTs) to the PAAm hydrogel results in a hydrogel with high sensitivity, self-recovery, mechanical strength, and stretchability. Gelatin was employed to functionalize the oxCNTs via hydrogen bonding among carboxyl groups on the oxCNTs and hydroxyl, carboxyl groups in the gelatin chain to minimize oxCNT aggregation in the hydrogel mesh. The PAAm-oxCNTs hydrogel was then created by polymerizing AAm free-radically in the presence of an initiator and a chemical crosslinker. The hydrogel's backbone was chemically cross-linked PAAm, and the oxCNTs successfully increased the mechanical characteristics via the nano-enhancement effect. Furthermore, the hydrogel's mechanical characteristics were improved by physical interplays amongst oxCNTs, gelatin, and PAAm. It has increased stretchability and tensile strength and exhibits rapid self-recovery behavior. Furthermore, the hydrogel's mechanical characteristics were improved by physical interactions between oxCNTs, gelatin, and PAAm. The oxCNTs in the hydrogel also contributed to the strain sensing activity by generating conducting routes in the hydrogel. Unlike the literature, in this thesis study, homogeneous distribution of CNTs in pure PAAm hydrogels was observed for the first time. These results are very important in terms of literature for the development of new high quality, conductive and mechanically durable composite systems. The universality class, which shares critical exponents and other universal properties, encompasses a wide range of systems and models with continuous phase transitions in composite systems. Monomers are thought to locate to the corners of a periodic lattice in percolation theory. The ends connecting these corners at any instant, namely chemical bonds, are given with the probability p. The gel point is defined by the percolation threshold p_c at the thermodynamic limit where the infinite set begins to form. Around the percolation threshold, many percolation properties obey the scaling law regardless of network structure and microscopic details. Thus, the scaling laws for the gel fraction, G(p) and mean cluster size, S(p) around the threshold value are obtained. G(p)≈(p-p_c )^β , p>p_c S(p)≈(p_c-p)^(-γ) , p
Synthesis, temperature sensing and white light production properties of the lithiumniobate and tungstenoxide modifed TeO2+Yb2O3+Er2O3 optical glasses

(Graduate School, 2022) Konca, Güliz ; Eryürek, Gönül ; 741241 ; Physics Engineering

Tellurium-based glasses doped with rare earths (RE) have been studied in recent years from various aspects due to their many important optical and physical advantages. In this study, glass materials were obtained by synthesizing Yb3+/Er3+ doped TeO2- WO3-LiNbO3 lattices with different ratios by melting method. The lattices of our glass materials were modified by increasing the Er3+ ion concentration. The up-conversion mechanism under 980 nm laser excitation, absorption properties, optical band gaps and Urbach energies in the range of 200-1100 nm, luminescence properties, thermal properties and white light parameters in the wavelength range of 400-850 nm, and the variation of rare earth ions as a function of concentration were investigated. The transitions of Er3+ ions from their ground state, which is 4I15/2, to different excited states, such as 4F3/2,5/2, 2H11/2, 4S3/2, 4F9/2, 4I11/2, were observed. At laser excitation of 400-850 nm, the emission bands 2H11/2-4I15/2, 4S3/2-4I15/2, 4F9/2-4I15/2 and 4I9/2-4I15/2 UC of Er3+ ion transitions were observed. It was found that there were small differences in the measured color parameters of the TWL glasses with increasing power. When comparing the TWL glasses as a function of concentration changes, it was found that the Er3+ concentration shifted slightly from the red to the green range with increasing power. In the optical thermometry study, measurements could be made on the TWL1, TWL2 and TWL3 glass samples. In these measurements, two green emission bands, ~527 nm and ~551 nm, were observed. It was found that the intensity at the 2H11/2→4I15/2 transition at 300 K was quite low compared to the 4S→4I transition at 573 K. On study aims to contribute to the literature on the fabrication of Er3+/Yb3+ doped TeO2-WO3 glass materials, the design of photonic devices, and the development of temperature and light sensors.
Testing spatial curvature and anisotropic expansion of the universe on top of the lambda-ACDM model

(Graduate School, 2022) Özyiğit, Maya ; Akarsu, Özgür ; 766671 ; Physics Engineering Programme

In this thesis, we explore the possible advantages of extending the standard $\Lambda$CDM model by more realistic backgrounds compared to its spatially flat Robertson-Walker (RW) spacetime assumption, while preserving the underpinning physics; in particular, by simultaneously allowing non-zero spatial curvature and anisotropic expansion on top of $\Lambda$CDM, viz., the An-$o\Lambda$CDM model. This is to test whether the latest observational data still support spatial flatness and/or isotropic expansion in this case, and, if not, to explore the roles of spatial curvature and expansion anisotropy (due to its stiff fluid-like behavior) in addressing some of the current cosmological tensions associated with $\Lambda$CDM. We first present the theoretical background and explicit mathematical construction of An-$o\Lambda$CDM. We replace, in the simplest manner, the spatially flat RW spacetime assumption of the $\Lambda$CDM model with the simplest more realistic background that simultaneously allows non-zero spatial curvature and anisotropic expansion; namely, considered the simplest anisotropic generalizations of the RW spacetime, viz., the Bianchi type I, V, and IX spacetimes (having the simplest homogeneous and flat, open, and closed spatial sections, respectively) combined in one Friedmann equation. Then we constrain the parameters of this model and its particular cases, namely, An-$\Lambda$CDM (allowing anisotropic expansion), $o\Lambda$CDM (allowing non-zero spatial curvature), and $\Lambda$CDM, by using the latest data sets from different observational probes, viz., Planck CMB(+Lens), BAO, SnIa Pantheon, and CC data, and discuss the results in detail. Ultimately, we conclude that, within the setup under consideration, (i) the observational data confirm the spatial flatness and isotropic expansion assumptions of $\Lambda$CDM, though a very small amount of expansion anisotropy cannot be excluded, e.g., $\Omega_{\sigma0}\lesssim10^{-18}$ (95\% C.L.) for An-$\Lambda$CDM from CMB+Lens data, (ii) the introduction of spatial curvature or anisotropic expansion, or both, on top $\Lambda$CDM does not offer a possible relaxation to the $H_0$ tension, and (iii) the introduction of anisotropic expansion neither affects the closed space prediction from the CMB(+Lens) data nor does it improve the drastically reduced value of $H_0$ led by the closed space.

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