1. Reasons for Facebook usage : data from 46 countriesMarta Kowal, Piotr Sorokowski, Agnieszka Sorokowska, Małgorzata Dobrowolska, Katarzyna Pisanski, Anna Oleszkiewicz, Toivo Aavik, Grace Akello, Charlotte Alm, Naumana Amjad, Maja Zupančič, Tina Kavčič, Bojan Musil, Nejc Plohl, Afifa Anjum, Kelly Asao, Chiemezie Atama, Derya Atamtürk Duyar, Richard Ayebare, Mons Bendixen, Aicha Bensafia, Boris Bizumic, Mahmoud Boussena, David M. Buss, Marina Butovskaya, Seda Can, Katarzyna Cantarero, Antonin Carrier, Hakan Cetinkaya, Daniel Conroy-Beam, Marco A. C. Varella, Rosa M. Cueto, Marcin Czub, Daria Dronova, Seda Dural, Izzet Duyar, Berna Ertugrul, Agustín Espinosa, Ignacio Estevan, Carla S. Esteves, Tomasz Frackowiak, Jorge Contreras-Graduño, Farida Guemaz, Ivana Hromatko, Chin-Ming Hui, Iskra Herak, Jas L. Jaafar, Feng Jiang, Konstantinos Kafetsios, Leif Edward Ottesen Kennair, Nicolas Kervyn, Nils C. Köbis, András Láng, Georgina R. Lennard, Ernesto León, Torun Lindholm, Giulia Lopez, Mohammad Madallh Alhabahba, Alvaro Mailhos, Zoi Manesi, Rocío Martínez, Sarah L. McKerchar, Norbert Meskó, Girishwar Misra, Hoang Moc Lan, Conal Monaghan, Emanuel C. Mora, Alba Moya Garófano, George Nizharadze, Elisabeth Oberzaucher, Mohd S. Omar Fauzee, Ike E. Onyishi, Baris Özener, Ariela F. Pagani, Vilmante Pakalniskiene, Miriam Parise, Farid Pazhoohi, Mariia Perun, Annette Pisanski, Camelia Popa, Pavol Prokop, Muhammad Rizwan, Mario Sainz, Svjetlana Salkicević, Ruta Sargautyte, Susanne Schmehl, Oksana Senyk, Rizwana Shaikh, Shivantika Sharad, Franco Simonetti, Meri Tadinac, Truong Thi Khanh Ha, Trinh Thi Linh, Karina Ugalde González, Nguyen Van Luot, Christin-Melanie Vauclair, Luis D. Vega, Gyesook Yoo, Stanislava Yordanova Stoyanova, Zainab F. Zadeh, 2020, drugi znanstveni članki Ključne besede: online social networks, Facebook, cross cultural psychology, cross cultural differences, human sex differences, age differences, motives Objavljeno v DKUM: 27.01.2025; Ogledov: 0; Prenosov: 8
Celotno besedilo (259,63 KB) Gradivo ima več datotek! Več... |
2. DIFFRACTION GRATINGS FORMED BY BENT-CORE LIQUID CRYSTALS IN THE TWIST – BEND NEMATIC PHASEMuhammad Ali, 2021, doktorska disertacija Opis: In this thesis, we study the structure and optical transmission properties of the twist-bend nematic liquid crystalline phase, made of bent dimers, confined in thin planar cells. Confinement leads to the formation of a periodic modulated structure, the formation of which is explained as follows. The twist-bend nematic phase is characterized by a heliconical modulation of the molecular long axes. Due to a short pitch of modulation (approximately 10 nm), the twist-bend nematic phase behaves as a pseudo-layered medium. At temperatures below the nematic – twist-bend nematic phase transition, the heliconical pitch and thus the thickness of the pseudo-layers reduces, which leads to a two-dimensional undulation of pseudo-layers in the direction perpendicular to the cell surfaces and along the surfaces. The undulated structure is responsible for a stripe texture observed under a polarizing microscope and acts as a diffraction grating.
We constructed theoretical models to predict the pseudo-layer structure of a confined twist-bend nematic phase and to describe the properties of light diffracted on such cells. The free energy of the two-dimensional pseudo-layer structure of the twist-bend nematic phase is expressed in terms of the nematic director field, by which we describe the direction of the heliconical axis, and a complex smectic order parameter, the gradient of which gives the direction of the layer normal. At first, we assume that pseudo-layers are perpendicular to the surfaces (bookshelf geometry) and find a stable structure by assuming an ansatz for the pseudo-layer displacement from the bookshelf geometry and then minimizing the free energy at a very strong and very weak surface anchoring. In this way a threshold condition for the onset of the modulated structure is obtained, as well as the amplitude and period of modulation. Next, we assume that, at the onset of the twist-bend nematic phase, pseudo-layers are formed at some angle (pre-tilt) with respect to the surface. We find that in both cases, the bookshelf and pre-tilted one, the calculated period of modulation far from the phase transition is always approximately twice the cell thickness, which agrees with experimental observations.
The properties of light diffracted by the spontaneously formed grating were studied both experimentally and theoretically. We measured the intensity and polarization properties of the first two orders of the diffracted light and the temperature dependence of the polarization of the second order diffraction peaks. To predict the observed properties of the diffracted light and to simplify the description of such gratings, we consider different preliminary models of a one-dimensional spatial variation of the optic axis, the direction of which is given by two angles. A transfer matrix method is used and a good agreement between the experimental and theoretical results is obtained. In a more comprehensive approach, we determine the spatial variation of the optic axis direction from the modeled structure. The electric field in the diffracted light is obtained by using the transfer matrix method and beam propagation method. In the case of a pre-tilt of the pseudo-layers and very strong surface anchoring both methods give good qualitative agreement with experimental results, only in the case of the temperature dependence of the second order diffraction peaks, a more complex beam propagation method is superior to the transfer matrix method.
The thesis is divided into three parts. In the first part, we focus on the physical properties of the twist-bend nematic phase and its structure in thin planar cells. In the second part, a continuum model is proposed and finally, the properties of diffracted light are discussed and theoretically predicted by using the beam propagation method and transfer matrix method. Ključne besede: Bent-dimer liquid crystals, twist-bend nematic phase, undulation of pseudo-layers, polarization, diffraction grating, beam propagation method, transfer matrix method. Objavljeno v DKUM: 21.10.2021; Ogledov: 1359; Prenosov: 92
Celotno besedilo (10,65 MB) |