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ÖgeFrom swampland to cosmos: A journey through swampland conjectures via supergravity and string inflation(Graduate School, 2023-08-18)In this dissertation, we have combined insights from distinct yet interrelated areas of research to obtain a more thorough understanding of the de Sitter swampland conjectures, the trans-Planckian Censorship Conjecture, and the EFT for inflation with spontaneously broken supersymmetry. By integrating these diverse aspects of the research, we aim to provide a unified, in-depth overview of the theoretical frameworks of inflation, the swampland program, and string cosmology, as well as their connections, interdependencies, and implications for one another. Chapter 1 situates this dissertation within the current scientific landscape by providing a concise overview of various key topics, such as Inflation, Standard Model, Supersymmetry, Supergravity, String Inflation, and Swampland Conjectures. It also introduces the essential theoretical background, methodologies, and notation utilized throughout the study. The remaining chapters of the dissertation present a combination of my published research and ongoing work in the field. The research explores the interplay between single-field inflationary models and de Sitter swampland conjectures, seeking to provide a deeper understanding of the possibility of their mutual compatibility. By incorporating nilpotent superfields and examining the Trans-Planckian censorship conjecture, the scope of the research is broadened, addressing crucial aspects of supergravity and effective field theories. Chapter 2 was published in Atli and Guleryuz (2021), and investigates de Sitter swampland conjectures and their implications for single-field inflationary models with a generic potential. It revisits the de Sitter swampland conjectures by distinguishing the effective potential and inflationary potential from superconformal embedding of the superconformal action. It introduces the bounded de Sitter conjecture, which provides insight into the relationship between single-field inflationary models and de Sitter swampland conjectures. In that chapter, we showed that the stabilization of the inflaton field required the concave inflation potential and that the bounded de Sitter conjecture can be satisfied for any concave inflation potential as long as the related inflation parameter fulfills the condition $0 \lesssim -\eta \leq c' \approx \mathcal{O}(1)$. This finding offers valuable insights into the relationship between single-field inflationary models and the de Sitter swampland conjectures, providing a modest step towards understanding their possible mutual compatibility. Chapter 3 was published in Guleryuz (2021), where we focused on the trans-Planckian Censorship Conjecture (TCC) and examined its implications for non-minimal coupling terms in both Einstein and Jordan frames. It demonstrates the robustness of the TCC and the strong interplay between non-minimal coupling terms and inflationary dynamics. In that chapter, we demonstrated a general embedding of the generic non-minimal coupling term to the TCC for both frames and presented an example for the generalized non-minimal coupling case. In this example, we showed that the upper bound on the inflationary parameter $r$ becomes independent of the Jordan frame potential in the strong coupling limit of the generalized non-minimal coupling case. Moreover, the upper bound on the Jordan frame potential $V_J(\phi)$ can be eased in the strong coupling limit. These findings not only highlight the robustness of the TCC but also shows that the TCC bound might become less constraining in certain circumstances, allowing for a wider range of inflationary potentials that satisfy the conjecture. Thus, open up new avenues for future research on the compatibility of various inflationary models with the TCC, as well as the implications of non-minimal coupling terms for our understanding of the early universe's dynamics. Chapter 4 was published in Guleryuz (2023). In that chapter, we built an effective field theory for inflation with spontaneously broken supersymmetry without generating any supersymmetric AdS vacua by combining the tools offered by the nilpotent supergravity and the universal attractor mechanism. We showed that various Kähler potentials could produce the same kinetic term in flat directions without disrupting the scalar potential up to a Kähler transformation, and linearly coupled nilpotent superfields in the Kähler potential plays a crucial role in inflation, spontaneous SUSY breaking, and obtaining a de Sitter vacuum at the minimum of the potential. We also showed that one can promote an arbitrary parameter, such as in the $\alpha$-attractor models, as an overall multiplier to the total Kähler potential, capturing a unified region of the inflationary attractors, allowing us to obtain de Sitter and Minkowski vacua while avoiding AdS vacua. Additionally, we embedded the nilpotent superfields into the de Sitter swampland conjecture in an $\mathcal{N}=1$, $D=4$ SUGRA background, and expanded the consistent EFT space for the dSC. We showed that the bound on the parameter $c\sim \mathcal{O}(1)$ of the de Sitter swampland conjecture might undergo dynamic evolution during the inflationary epoch in case nilpotent superfields are incorporated into the potential gradient. The most stringent bound seems to emerge in the asymptotic limit with $4.61 \gtrsim c$. This result reveals a possible intricate interplay between the swampland conjecture and the underlying structure of the effective field theory space. Hence, the (super)universal attractors in the presence of nilpotent superfields offer a potential way out of the single-field inflation and swampland conflict by drawing an elegant picture for the swampland, which could contribute to a deeper understanding of the interrelations between string theory and cosmology. In conclusion, this study has provided a comprehensive analysis of the de Sitter swampland conjectures, the trans-Planckian Censorship Conjecture, and the effective field theory for inflation with spontaneously broken supersymmetry. Our findings offer a solid foundation for further research into the compatibility of various inflationary models with the TCC and the de Sitter swampland conjectures, as well as the implications of non-minimal coupling terms. Moreover, our results suggest that there is still much to be explored, and the insights gained from these investigations may prove instrumental in shaping our future understanding of the universe's underlying principles and the ultimate nature of reality.
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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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