Strongly-correlated 2D Electron Systems in Si-MOSFETs

Si-MOSFETs are basic building blocks of present-day integrated circuits. Above a threshold gate voltage, a layer of two-dimensional electrons is induced near the silicon-silicon dioxide interface of a Si-MOSFET. According to theory for noninteracting and weakly interacting electrons, no metallic state can exist in two dimensions in zero magnetic field in the limit of zero temperature. However, in strongly interacting electron systems the observation of a resistivity that changes from metallic to insulating temperature dependence has fueled a debate over whether this signals a quantum phase transition to a metallic phase in two dimensions. In this thesis I will present the results of two detailed experimental studies performed on high mobility Si-MOSFET samples. In the first study, we find the thermopower of this low-disorder, strongly interacting 2D electron system in silicon diverges at a finite disorder-independent density, providing evidence that this IS a transition to a new phase at low densities. For the second study, we conducted measurements on I-V characteristics as well as the AC voltage generated by the sample in the insulating phase. Nonlinear I-V characteristics observed in the insulating phase have been attributed to the presence of an additional conduction channel due to a sliding electron solid (Wigner crystal). We seek to provide evidence for the presence of a zero-field Wigner solid by detecting the noise generated by the sliding crystallites

Institutional environment and total entrepreneurial activity: the comparison on nations

L26 Entrepreneurship, O17 Formal and Informal Sectors, Shadow Economy, Institutional ArrangementsPurpose - The main objective of this paper is to analyse the influence of environmental factors on entrepreneurship at the countries level, using institutional economics as the theoretical framework for the research. Design/methodology/approach -National panel data (2009-2016 period) is from the Global Entrepreneurship Monitor (GEM), specifically from the National Expert Survey (NES) for environmental conditions and Adult Population Survey (APS) for environmental conditions and entrepreneurial activity. The data relating to the division of the countries according to the economic level is from World Bank. Findings -The main findings of the study indicate that both informal and formal factors influence TEA of any income level. Research limitations/implications - The research study was limited to the period of 2009 and 2016 and hence hinders any further generalization and its application needs a careful interpretation of the data. Originality/value -The study provides a methodology to analyse the environmental factors for new firm creation at a national level, combining GEM data and institutional economics.Meta - O objetivo principal da presente tese é analisar a influência dos fatores ambientais no empreendedorismo a nível dos países, utilizando a economia institucional como o quadro teorético para o estudo. Metodologia - Estatísticas para o Painel Nacional (durante o período 2009-2016) derivam do Monitor Global do Empreendedorismo (GEM, na sigla inglesa), especialmente do Inquérito Nacional dos Especialistas para as condições ambientais e do Inquérito da População Adulta para as condições ambientais e atividades empreendedoras. As estatísticas, referentes à divisão dos países conforme o nível económico, são do Banco Mundial. Resultado - O resultado principal do presente estudo indica que tanto os fatores informais como os formais influenciam TEA do que seja o nível do rendimento. Restrição da Pesquisa - A pesquisa esteve sujeita ao período entre 2009 e 2016, pelo que impede uma generalização posterior e a sua aplicação necessita de uma interpretação acautelada das estatísticas. Originalidade/Valor - O presente estudo proporciona uma metodologia que serve para analisar os fatores ambientais na criação de uma nova empresa a nível nacional, juntando as estatísticas da GEM e a teoria económica institucional

Energy-Efficient Data Acquisition in Wireless Sensor Networks through Spatial Correlation

The application of Wireless Sensor Networks (WSNs) is restrained by their often-limited lifetime. A sensor node's lifetime is fundamentally linked to the volume of data that it senses, processes and reports. Spatial correlation between sensor nodes is an inherent phenomenon to WSNs, induced by redundant nodes which report duplicated information. In this paper, we report on the design of a distributed sampling scheme referred to as the 'Virtual Sampling Scheme' (VSS). This scheme is formed from two components: an algorithm for forming virtual clusters, and a distributed sampling method. VSS primarily utilizes redundancy of sensor nodes to get only a subset to sense the environment at any one time. Sensor nodes that are not sensing the environment are in a low-power sleep state, thus conserving energy. Furthermore, VSS balances the energy consumption amongst nodes by using a round robin method