No description available.

Synthesis and functionalization of large-area graphene and its structural, electrical and electrochemical properties has been investigated. First, the graphene films, grown by thermal chemical vapor deposition (CVD), contain three to five atomic layers of graphene, as confirmed by Raman spectroscopy and high-resolution transmission electron microscopy. Furthermore, the graphene film is treated with CF4 reactive-ion plasma to dope fluorine ions into graphene lattice as confirmed by X-ray photoelectron spectroscopy (XPS) and UV-photoemission spectroscopy (UPS). Electrochemical characterization reveals that the catalytic activity of graphene for iodine reduction enhanced with increasing plasma treatment time, which is attributed to increase in catalytic sites of graphene for charge transfer. The fluorinated graphene is characterized as a counter-electrode (CE) in a dye-sensitized solar cell (DSSC) which shows ~ 2.56% photon to electron conversion efficiency with ~11 mAcm−2 current density. Second, the large scale graphene film is covalently functionalized with HNO3 for high efficiency electro-catalytic electrode for DSSC. The XPS and UPS confirm the covalent attachment of C-OH, C(O)OH and NO3- moieties with carbon atoms through sp2-sp3 hybridization and Fermi level shift of graphene occurs under different doping concentrations, respectively. Finally, CoS-implanted graphene (G-CoS) film was prepared using CVD followed by SILAR method. The G-CoS electro-catalytic electrodes are characterized in a DSSC CE and is found to be highly electro-catalytic towards iodine reduction with low charge transfer resistance (Rct ~5.05 Wcm2) and high exchange current density (J0~2.50 mAcm-2). The improved performance compared to the pristine graphene is attributed to the increased number of active catalytic sites of G-CoS and highly conducting path of graphene. We also studied the synthesis and characterization of graphene-carbon nanotube (CNT) hybrid film consisting of graphene supported by vertical CNTs on a Si substrate. The hybrid film is inverted and transferred to flexible substrates for its application in flexible electronics, demonstrating a distinguishable variation of electrical conductivity for both tension and compression. Furthermore, both turn-on field and total emission current was found to depend strongly on the bending radius of the film and were found to vary in ranges of 0.8 – 3.1 V/μm and 4.2 – 0.4 mA, respectively.

京都大学 / 新制・課程博士 / 博士(理学) / 甲第22997号 / 理博第4674号 / 新制||理||1670(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)准教授 國友 浩, 教授 杉本 茂樹, 教授 田中 貴浩 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

Ce travail porte sur l’étude du processus d’instabilité de modulation dans les fibres optiques et notamment son étape nonlinéraire. Ce processus peut induire une dynamique complexe de couplage entre une onde de pompe et des bandes latérales avec notamment un, voire de multiples, retour à l’état initial s’il est amorcé activement. Ce phénomène est connu sous le nom de récurrences de Fermi-Pasta-Ulam-Tsingou. Dans cette thèse, nous décrivons la mise en place d’un montage expérimental se basant sur la détection hétérodyne d’un signal rétrodiffusé et une compensation active des pertes. Il permet une caractérisation distribuée rapide et non-invasive tout le long d’une fibre de l’amplitude et la phase des principales composantes spectrales d’une impulsion. En outre, nous détaillons une méthode de post-traitement qui nous permet de retrouver l’évolution du champ complexe dans le domaine temporel. Mettant en oeuvre ces outils, nous avons rapporté l’observation de deux récurrences de Fermi-Padta-Ulam-Tsingou et leur brisure de symétrie, à la fois dans les domaines fréquentiel et temporel. Suite à cela, nous avons quantitativement examiné l’influence des conditions initiales des trois ondes envoyées dans la fibre sur la position des récurrences, en comparaison avec de récentes prédictions théoriques. Finalement, nous avons étudié la dynamique de structures nonlinéraies d’ordre supérieur, à savoir les breathers du deuxième ordre. / This work deals with the investigation of the modulation instability process in optical fibres and in particular its nonlinear stage. This process can induce a complex coupling dynamic between the pump and sidebands waves, with a single or multiple returns to the initial state if it is seeded. This phenomenon is referred as Fermi-Pasta-Ulam-Tsingou recurrences. In this thesis, we describe the implementation of a novel experimental technique based on heterodyne optical time-domain reflectometry and active compensation of losses. It allows fast and non-invasive distributed characterisation along a fibre of the amplitude and phase of the main frequency components of a pulse. Furthermore, we detail a simple post-processing method which enable us to retrieve the complex field evolution in the time domain. Using these tools, we reported the observation of two Fermi-Pasta-Ulam-Tsingou recurrences and their symmetry-breaking nature, both in the frequency and time domain. Then, we quantitatively studied the influence of the initial three-wave input conditions on the recurrence positions, in regards with recent theoretical predictions. Finally, we investigated the dynamics of higher-order nonlinear structures, namely second-order breathers.

Thesis: M.S., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 1980 / Includes bibliographcial references. / by Farhad Hakimi. / M.S. / M.S. Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science

An analysis is made on the Flat Spectrum Radio Quasar PKS 0426-380 using two sets of data. The first set of data is the Fermi-LAT data collected over the time 54682.66 − 59317.66 in Modified Julian Date within the energy range of 100 MeV to 500 GeV. The second set of data is a multi-wavelength spectral energy distribution within the approximate frequencies of <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%0A%0A%5Cleft%5B10%5E%7B10%7D,%2010%5E%7B27%7D%20%5Cright%5D" data-classname="equation" data-title="" />. First, the Fermi-LAT data were analysed and after modelling a lightcurve over the entire available time, a period of interest was located. The next step was to obtain a multi-wavelength spectral energy distribution of said period. Then, using JetSeT modelling, the data were analysed and a model of the Synchrotron Self-Compton, External-Compton and Synchrotron curves was fitted to the data. The final model which contained the best fit provided a set of physical parameters that described the source. These parameters were finally compared to two other Flat Spectrum Radio Quasars and conclusions regarding the properties of PKS 0426-380 were eventually drawn. A discussion comparing a related work on the same source to the results in this thesis followed. With the large differences in the constrained data between the Flat Spectrum Radio Quasars as background, three predictions concluded the thesis. These are, firstly, a cautioned approach to future searches for periodicity in AGN's. Secondly, in time, local periodicity for AGN's might be more common and interesting for future research. Finally, no certain values for the physical parameters of the AGN can be assessed and the results can only be wived as indications of the actual properties. / <p>Passed</p>

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第17973号 / 理博第3917号 / 新制||理||1565(附属図書館) / 80817 / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 高橋 義朗, 教授 田中 耕一郎, 教授 石田 憲二 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

No description available.

Thesis advisor: Kevin S. Bedell / We present several results based upon non-trivial extensions of Landau-Fermi liquid theory. First proposed in the mid-20th century, the Fermi liquid approach assumes an adiabatic “switching-on” of the interaction, which allows one to describe the collective excitations of the many-body system in terms of weakly-interacting quasiparticles and quasiholes. At its core, Landau-Fermi liquid theory is often considered a perturbative approach to study the equilibrium thermodynamics and out-of-equilibrium response of weakly-correlated itinerant fermions, and therefore non-trivial extensions and consequences are usually overlooked in the contemporary literature. Instead, more emphasis is often placed on the breakdown of Fermi liquid theory, either due to strong correlations, quantum critical fluctuations, or dimensional constraints. After a brief introduction to the theory of a Fermi liquid, I will first apply the Landau quasiparticle paradigm to the theory of itinerant Majorana-like fermions. Defined as fermionic particles which are their own anti-particle, traditional Majorana zero modes found in topological materials lack a coherent number operator, and therefore do not support a Fermi liquid-like ground state. To remedy this, we will apply a combinatorical approach to build a statistical theory of self-conjugate particles, explicitly showing that, under this definition, a filled Fermi surface exists at zero temperature. Landau-Fermi liquid theory is then used to describe the interacting phase of these Majorana particles, from which we find unique signatures of zero sound in addition to exotic, non-analytic contributions to the specific heat. The latter is then exploited as a “smoking-gun” signature for Majorana-like excitations in the candidate Kitaev material Ag3LiIr2O6, where experimental measurements show good agreement with a sharply-defined, “Majorana-Fermi surface” predicted in the underlying combinatorial treatment. I will then depart from Fermi liquid theory proper to tackle the necessary conditions for the applicability of Luttinger’s theorem. In a nutshell, Luttinger’s theorem is a powerful theorem which states that the volume of phase space contained in the Fermi surface is invariant with respect to interaction strength. In this way, whereas Fermi liquid only describes fermionic excitations near the Fermi surface, Luttinger’s theorem describes the fermionic degrees of freedom throughout the entire Fermi sphere. We will show that Luttinger’s theorem remains valid only for certain frequency and momentum-dependencies of the self-energy, which correlate to the exis- tence of a generalized Fermi surface. In addition, we will show that the existence of a power-law Green’s function (a unique feature of “un-particle” systems and a proposed characteristic of the pseudo-gap phase of the cuprate superconductors) forces Luttinger’s theorem and Fermi liquid theory to be mutually exclusive for any non-trivial power of the Feynman propagator. Finally, we will return to Landau-Fermi liquid theory, and close with novel out-of-equilibrium behavior and stability in unconventional Fermi liquids. First, we will consider a perfectly two- dimensional Fermi liquid. Due to the reduction in dimension, the traditional mode expansion in terms of Legendre polynomials is modified to an expansion in terms of Chebyshev polynomials. The resulting orthogonality conditions greatly modifies the stability and collective modes in the 2D system. Second, we will look at a Fermi liquid in the presence of a non-trivial gauge field. The existence of a gauge field will effectively shift the Fermi surface in momentum space, resulting in, once again, a modified stability condition for the underlying Fermi liquid. Supplemented with a modernized version of Mermin’s condition for the propagation of zero sound, we outline the full effects a spin symmetric or anti-symmetric gauge would have on a Fermi liquid ground state. / Thesis (PhD) — Boston College, 2021. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

No description available.