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ÖgeElectro optical properties of liquid crystal nanocomposites(Graduate School, 2022-11-29)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.
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ÖgePulsar magnetospheres and intra - pulse variability by plasmoid formatio(Graduate School, 2024-12-06)Pulsars are remnants of massive stars that have endured supernova explosions. They are rapidly rotating neutron stars with extraordinary magnetic fields. These celestial objects serve as natural laboratories for studying physics under extreme conditions that includes physics of dense matter, strong-field gravity, and relativistic plasma processes. Their magnetospheres are dominated by strong magnetic fields and populated by an electron-positron plasma. It exhibits complex dynamics that give rise to a broad spectrum of electromagnetic emissions which spans from radio waves to gamma-rays. Among the various phenomena observed, pulse-to-pulse variability offers critical insights into the underlying plasma processes and emission mechanisms. It also includes intra-pulse structures such as subpulses. The precise origins of this variability remain incompletely understood despite extensive research. It is posing a significant challenge in pulsar astrophysics. What is investigated in this thesis is the role of plasmoid formation induced by magnetic reconnection in pulsar magnetospheres and its impact on intra-pulse variability. Magnetic reconnection in the current sheet beyond the light-cylinder radius ($\rlc$) is a fundamental plasma process. It can lead to the formation of plasmoids—coherent structures consisting of plasma and magnetic fields. The intermittent nature of reconnection and the hierarchical merging of plasmoids can introduce significant fluctuations in the emitted radiation. It is potentially accounting for the observed pulse-to-pulse variability and subpulse phenomena. To examine this hypothesis, we employ two-dimensional particle-in-cell (PIC) simulations using the \texttt{ZELTRON} code, modeling an orthogonal pulsar magnetosphere restricted to the equatorial plane. The simulations begin with a split magnetic monopole configuration and evolve into a quasi-steady-state force-free magnetosphere filled with electron-positron plasma. Plasma is continuously injected from the neutron star surface, ensuring a sustained supply of charged particles. The wind current sheet forms as two thin Archimedean spirals extending beyond $\rlc$, where magnetic reconnection triggers the tearing instability. This instability leads to the fragmentation of the current sheet into a dynamic chain of plasmoids, which evolve through processes of growth, merging, and outward propagation. Our simulations expose that plasmoids are predominantly formed near the light cylinder and subsequently grow and merge hierarchically as they move out at relativistic speeds. The size distribution of plasmoids ranges from small-scale structures on the order of $0.02 \rlc$ to large entities comparable to the neutron star radius ($\sim 0.3 \rlc$). Statistical analysis indicates that the plasmoid size distribution follows an inverse relationship with their width. This is consistent with theoretical predictions for hierarchical merging in relativistic reconnection processes. This distribution suggests that a multitude of small plasmoids coexist with fewer large ones, contributing differently to the emission characteristics. We compute synthetic synchrotron emission profiles by tracking particles and electromagnetic fields within the simulation. Our computation focuses on the incoherent high-energy emission originating from the current sheet. The results demonstrate that plasmoid formation leads to significant intra-pulse variability. This variability is manifested as bright subpulses superimposed on the main pulse profile. The subpulses predominantly appear on the leading edge of each pulse and exhibit a broad range of fluxes and durations. A key finding is the proportionality between subpulse flux and width in pulsar phase. This correlation indicates that the larger plasmoids produce brighter and broader subpulses. This relationship arises because larger plasmoids contain more energetic particles and occupy a greater angular extent. It enhances their contribution to the observed emission. The statistical properties of the subpulses align with the plasmoid size distribution with the number of subpulses following a power-law dependence on their flux and width. This correlation suggests that the observed intra-pulse variability is directly linked to the dynamics of plasmoid formation and evolution within the magnetosphere. The power-law behavior indicates that there is a significant probability of observing exceptionally bright and wide subpulses. This corresponds to the fact that the largest plasmoids formed through merging events while most subpulses are weak and narrow. Advanced analysis shows that the intermittent nature of magnetic reconnection leads to episodes where the current sheet thickens near the light cylinder. It temporarily suppresses plasmoid formation. These quiet periods result in reduced emission and are followed by recovery phases. The intense subpulses reappear in the recovery phases and this corresponds to the delayed fragmentation of the stretched current sheet. This behavior contributes to the overall variability observed in the emission profiles. It may explain observed phenomena such as nulling and mode changes in some pulsars. Our study offers several testable predictions.
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ÖgeStructural and upconversion luminescence properties of polyethyl methacrylate (PEMA) polymers doped with rare earths ions(Graduate School, 2024-04-18)This thesis aims to analyze the spectral characteristics of nanocrystal powders containing [CdNb2O6:Er3+] and [CdNb2O6: Er3+/Yb3+] by using polyethyl methacrylate (PEMA) crosslinked networks as the host matrix. The first study involved the incorporation of CdNb2O6:Er2O3(%1,5 Er3+) powders doped with Er3+ into both bulk linear polyethyl methacrylate (PEMA) and bulk PEMA crosslinked networks (gels) with varying amounts of crosslinker. The polymerization process was achieved through free-radical crosslinking copolymerization using 0.1 EMA (weight %) at a temperature of 60 °C. The X-ray diffraction (XRD) method was used to analyze the structures of Er3+ ions contained in bulk linear PEMA and PEMA gels. The crystalline grain size was determined using the Pielazsek grain distribution and the Scherrer equation techniques. By increasing the crosslinker amounts, the average-crystalline grain sizes reduced from 75 nm to 12.50 nm for CdNb2O6:Er2O3 (%1,5 Er3+) crystalines. Thus, it was shown that the distribution grew more evenly distributed. The Fourier transform infrared (FT-IR) spectra were used to monitor the functional groups present in both linear PEMA and PEMA gel samples. The obtained results were found to be in agreement with the XRD results for the two cases. The second part of this thesis included the synthesis of linear polyethyl methacrylate (PEMA) and crosslinked PEMA gels, which were doped with CdNb2O6: Er3+/Yb3+ nanocrystal powders. The synthesis was achieved using the process of free radical crosslinking copolymerization. The X-ray diffraction method was used to analyze the architectures of Er3+/Yb3+ ions contained in PEMA polymers. The average sizes of the crystalline particles were determined by using the Pielazsek particle distribution and the Scherrer equation. The particle sizes exhibited a reduction from 60 nm to 17 nm for CdNb2O6: Er3+/Yb3+ powders upon their incorporation into PEMA polymers. The morphological changes of polymer samples doped with CdNb2O6: Er3+/Yb3+ powders were observed using transmission electron microscopy and scanning electron microscopy. Luminescence spectra were measured at room temperature to explore the optical characteristics of polymer materials. The Er3+/Yb3+ ions, which was excited with a 975 nm diode laser, produced upconversion (UC) emissions using two-photon absorptions in the visible range. The intensities of UC (upconversion) and the quantities of absorbed photons were enhanced by increasing the quantity of crosslinker. The study examined how changes in the structure and morphology of the host polymer material affected color tuning, color coordinates, and color quality.
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ÖgeGravitational waves as the probe of early universe(Graduate School, 2024-07-24)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.
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ÖgeBeyond the ACDM model: Addressing observational tensions of cosmology with negative and/or oscillating dark energy density(Graduate School, 2024-08-29)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.
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