On the possibility of superluminal energy propagation in a hyperbolic metamaterial of metal-dielectric layers

The energy propagation of electromagnetic fields in the effective medium of a one-dimensional photonic crystal consisting of dielectric and metallic layers is investigated. We show that the medium behaves like Drude and Lorentz medium, respectively, when the electric field is parallel and perpendicular to the layers. For arbitrary time-varying electromagnetic fields in this medium, the energy density formula is derived. We prove rigorously that the group velocity of any propagating mode obeying the hyperbolic dispersion must be slower than the speed of light in vacuum, taking into account the frequency dependence of the permittivity tensor. That is, it is not possible to have superluminal propagation in this dispersive hyperbolic medium consisting of real dielectric and metallic material layers. The propagation velocity of a wave packet is also studied numerically. This packet velocity is very close to the velocity of the propagating mode having the central frequency and central wave vector of the wave packet. When the frequency spread of the wave packet is not narrow enough, small discrepancy between these two velocities manifests, which is caused by the non-penetration effect of the evanescent modes. This work reveals that no superluminal phenomenon can happen in a dispersive anisotropic metamaterial medium made of real materials.Comment: 17 pages, 7 figure

Social networks, language learning and language school student sojourners: a qualitative study

This thesis investigates individual language school student sojourners’ learning experiences in the UK. It takes into account the importance of learners learning through interaction with others in the target language community and recognises how learners’ social networks affect their language learning and use. There are many studies about students who study abroad for academic purposes and immigrant learners, but not many relating to individual language learners in private language schools. In order to fill this gap, this study focused on individual language school learners. With the intention of understanding how the social networks and language learning interact over time as part of their sojourner experience, I utilised different theoretical frameworks that have been applied to other groups of language learners and concluded that these frameworks are also applicable to private language school students. I recognised that in order to understand my participants’ learning experiences I needed to interact with them and also observe how they interacted with their social world. Therefore, formal interviews (semi-structured) and informal interviews (informal group meeting or chat) were the main methods for my study together with observation of interaction in various situations. This study identified the expectations that learners had with regard to the target language community, host families and native speakers’ attitudes towards foreign students, and the realisation that these expectations were higher than what was actually encountered. It also identified the steps the individual language learners took to overcome these disappointments and how they reconstructed their relationships with the target language and community respectively. In contrast with many previous studies which only focused on learning from native speakers, my study recognised that learners sometimes can have more interpersonal contact with their fellow students than native speakers. And consequently they benefit more from these contacts, in terms of language learning, than from native speakers. This thesis also helps language learners and language educators recognise basic theoretical frameworks which could help them evaluate the benefits and problems related to learning through interpersonal contact. And with this understanding learners will be able to facilitate their autonomous learning in the target language community

New class of 3D topological insulator in double perovskite

We predict a new class of three-dimensional topological insulators (TIs) in which the spin-orbit coupling (SOC) can more effectively generate a large band gap at $\Gamma$ point. The band gap of conventional TI such as Bi$_2$Se$_3$ is mainly limited by two factors, the strength of SOC and, from electronic structure perspective, the band gap when SOC is absent. While the former is an atomic property, we find that the latter can be minimized in a generic rock-salt lattice model in which a stable crossing of bands {\it at} the Fermi level along with band character inversion occurs for a range of parameters in the absence of SOC. Thus, large-gap TI's or TI's comprised of lighter elements can be expected. In fact, we find by performing first-principle calculations that the model applies to a class of double perovskites A$_2$BiXO$_6$ (A = Ca, Sr, Ba; X = Br, I) and the band gap is predicted up to 0.55 eV. Besides, more detailed calculations considering realistic surface structure indicate that the Dirac cones are robust against the presence of dangling bond at the boundary with a specific termination.Comment: submitted; title changed and new references added; see DOI for published versio

Social networks, language learning and language school student sojourners : a qualitative study

This thesis investigates individual language school student sojourners’ learning experiences in the UK. It takes into account the importance of learners learning through interaction with others in the target language community and recognises how learners’ social networks affect their language learning and use. There are many studies about students who study abroad for academic purposes and immigrant learners, but not many relating to individual language learners in private language schools. In order to fill this gap, this study focused on individual language school learners. With the intention of understanding how the social networks and language learning interact over time as part of their sojourner experience, I utilised different theoretical frameworks that have been applied to other groups of language learners and concluded that these frameworks are also applicable to private language school students. I recognised that in order to understand my participants’ learning experiences I needed to interact with them and also observe how they interacted with their social world. Therefore, formal interviews (semi-structured) and informal interviews (informal group meeting or chat) were the main methods for my study together with observation of interaction in various situations. This study identified the expectations that learners had with regard to the target language community, host families and native speakers’ attitudes towards foreign students, and the realisation that these expectations were higher than what was actually encountered. It also identified the steps the individual language learners took to overcome these disappointments and how they reconstructed their relationships with the target language and community respectively. In contrast with many previous studies which only focused on learning from native speakers, my study recognised that learners sometimes can have more interpersonal contact with their fellow students than native speakers. And consequently they benefit more from these contacts, in terms of language learning, than from native speakers. This thesis also helps language learners and language educators recognise basic theoretical frameworks which could help them evaluate the benefits and problems related to learning through interpersonal contact. And with this understanding learners will be able to facilitate their autonomous learning in the target language community.EThOS - Electronic Theses Online ServiceGBUnited Kingdo