Terahertz magnetospectroscopy of high electron mobility CdTe/CdMgTe quantum wells

The extent of Terahertz (THz) radiation applications during last two decades has grown extensively as it was found to be useful in the fields such as medical imaging, security or drugs control to name just a few. However, despite a tremendous work already done, the compact system of THz imaging operating at room temperature is still an issue. Up to now, GaAs or GaN-based heterostructures and Si electron inversion layers were mostly used in solid-state THz optoelectronics. There, such a resonant phenomena as collective oscillations in a two-dimensional (2D) electron plasma and a free electron cyclotron resonance (CR) were observed. When it comes about THz magnetospectroscopy in a high electron mobility CdTe-based QWs, not many reports in the literature can be found. Due to a polar nature of the crystal lattice bonds and relatively low optical phonon energies the influence of a polaron effect on cyclotron resonance as well as a plasmon-phonon interaction are much larger in CdTe than in the materials mentioned before. Better understanding of these fundamental effects is extremely important for the operation of the devices based on CdTe such as frequency tunable THz detectors, for example. The goal of this work was to investigate the THz radiation induced effects in 2D electron plasma in CdTe-based QWs, concentrating an attention on the 2D plasmon excitation and peculiarities resulting from the polar nature of the material. The experiments were done on a high electron mobility CdTe/CdMgTe quantum wells, modulation doped with n-type impurity at one of the barriers (closer to the surface). All measurements were done at liquid helium temperature. A set of samples with a different parameters such as doping density and quantum well width were investigated in a photocurrent experiments. Also, transmission on samples containing a photo-lithographically formed metal grid-gate on the top was measured. A photocurrent induced in a quantum point contact (QPC) device defined by an electron beam lithography was also investigated. In order to characterize the samples, magnetotransport experiments were done at first. The 2DEG concentration was found as well as electron effective mass and quantum scattering time by analyzing the oscillation period and amplitude of Shubnikov-de Haas oscillations. Also, the feature at Landau level filling factor $\nu$ =4/3 was observed which might arise due to a Fractional Quantum Hall Effect (FQHE). Photocurrent spectra as a function of magnetic field (B) and exciting the sample with a constant frequency radiation taken from THz laser were recorded. In some samples only a narrow cyclotron resonance (CR) peak was observed ($\Delta B_{\text{{FWHM}}}\sim0.2$ T), while in the others optically induced SdH oscillations were present together with CR peak. The absence of SdH oscillations can be explained by the 2DEG heating due to bias current through the sample. An interesting phenomenon was observed, when optically induced SdH oscillations were analyzed. The additional splitting at Landau level filling factors $\nu$ = 1, 2 and 4 was observed. This phenomena might arise to the breaking of an Integer Quantum Hall (IQH) state due to THz radiation induced bolometric effect. However deeper investigation of the phenomenon is needed. The next topic of the investigation was THz detection with a gated 2D electron plasma in CdTe/CdMgTe quantum wells. For this purpose a few samples with a deposited gold gate electrode allowing for the 2DEG concentration control with a voltage, were prepared. Here the SdH oscillations and a CR transition were observed and a non-monotonical dependence of CR peak magnitude on $V_{\text{g}}$ was found. In general, changing the $V_{\text{g}}$ it is possible to reduce the CR peak magnitude to the level of SdH oscillations, or to increase it to exceed SdH magnitude more than 20 times. Also, sample response at a few laser lines was measured as a function of the gate voltage at a constant magnetic field $B=B_{\text{CR}}$. The above described CR peak magnitude dependence on $V_{\text{g}}$ or, in other words, a possibility to switch on and off the resonant detection was demonstrated. Another sample used for these experiments had a Hall bar geometry with a gated conduction channel. Experimental results also showed a non-monotonical CR peak magnitude dependence on the gate polarization. However, in this case the CR magnitude dependence on $V_{\text{g}}$ showed well developed plateau regions. Photocurrent spectra as a function of $B$ were measured on a QPC sample. The device has a lateral gate and the conduction channel with the constriction of the form of a bottle-neck. Spectra were measured for a few laser frequencies and for a few gate voltage values. In both cases, CR maxima with a spectral features at low-$B$ shoulder were registered. The spectral structures were found to represent the first and higher harmonics of electron plasma oscillations in magnetic field, also known as magnetoplasmons. It was found that plasmon frequency does not depend on the gate voltage, suggesting that detected plasma resonances form at the wide part of the channel, and not at the bottle-neck. Data analysis showed that plasmons observed are of ungated type and are confined in the conduction channel (width $W=2.4 \mu$m). In transmission spectra recorded on the grid-gated samples using the Fourier interferometer, the electron effective mass increase was observed at the strong magnetic fields which arises from the resonant polaron effect. Transmission spectra recorded using the THz laser show a deep symmetric CR minimum for the reference sample (that without a metal grid). For the samples with the metal grid, on the low-$B$ shoulder of CR dip, additional features were present. They were observed for two different laser frequencies and for two metal grids of a different periodicity. According to the theoretical calculations, these features are the first four harmonics of magnetoplasmons, with wavevectors defined by the period of the grating. These plasmons were shown to be a mixture of screened plasmons existing under metal fingers and unscreened plasmons existing in the openings of the grid. The proportion of plasmons of each type in the mixture were found to be approximately equal to the geometrical aspect ratio of the grating. Also, comparing experimental results with the theory it was found that the 2D plasmon frequency in n-doped CdTe/CdMgTe QWs of high electron mobility is strongly influenced by the plasmon-phonon interaction. To sum up, resonant (cyclotron transition and 2D plasmons) and non-resonant (SdH oscillations) THz detection was demonstrated in the high electron mobility CdTe/CdMgTe QWs. Also, effects arising from polar nature of CdTe lattice were such as resonant polaron effect and plasmon-phonon interaction were shown to be important. To the best knowledge of the author 2D plasmons excited by THz radiation were demonstrated in CdTe/CdMgTe QWs for the first time

Penilaian Kinerja Keuangan Koperasi di Kabupaten Pelalawan

This paper describe development and financial performance of cooperative in District Pelalawan among 2007 - 2008. Studies on primary and secondary cooperative in 12 sub-districts. Method in this stady use performance measuring of productivity, efficiency, growth, liquidity, and solvability of cooperative. Productivity of cooperative in Pelalawan was highly but efficiency still low. Profit and income were highly, even liquidity of cooperative very high, and solvability was good

Juxtaposing BTE and ATE – on the role of the European insurance industry in funding civil litigation

One of the ways in which legal services are financed, and indeed shaped, is through private insurance arrangement. Two contrasting types of legal expenses insurance contracts (LEI) seem to dominate in Europe: before the event (BTE) and after the event (ATE) legal expenses insurance. Notwithstanding institutional differences between different legal systems, BTE and ATE insurance arrangements may be instrumental if government policy is geared towards strengthening a market-oriented system of financing access to justice for individuals and business. At the same time, emphasizing the role of a private industry as a keeper of the gates to justice raises issues of accountability and transparency, not readily reconcilable with demands of competition. Moreover, multiple actors (clients, lawyers, courts, insurers) are involved, causing behavioural dynamics which are not easily predicted or influenced. Against this background, this paper looks into BTE and ATE arrangements by analysing the particularities of BTE and ATE arrangements currently available in some European jurisdictions and by painting a picture of their respective markets and legal contexts. This allows for some reflection on the performance of BTE and ATE providers as both financiers and keepers. Two issues emerge from the analysis that are worthy of some further reflection. Firstly, there is the problematic long-term sustainability of some ATE products. Secondly, the challenges faced by policymakers that would like to nudge consumers into voluntarily taking out BTE LEI

Severe early onset preeclampsia: short and long term clinical, psychosocial and biochemical aspects

Preeclampsia is a pregnancy specific disorder commonly defined as de novo hypertension and proteinuria after 20 weeks gestational age. It occurs in approximately 3-5% of pregnancies and it is still a major cause of both foetal and maternal morbidity and mortality worldwide1. As extensive research has not yet elucidated the aetiology of preeclampsia, there are no rational preventive or therapeutic interventions available. The only rational treatment is delivery, which benefits the mother but is not in the interest of the foetus, if remote from term. Early onset preeclampsia (<32 weeks’ gestational age) occurs in less than 1% of pregnancies. It is, however often associated with maternal morbidity as the risk of progression to severe maternal disease is inversely related with gestational age at onset2. Resulting prematurity is therefore the main cause of neonatal mortality and morbidity in patients with severe preeclampsia3. Although the discussion is ongoing, perinatal survival is suggested to be increased in patients with preterm preeclampsia by expectant, non-interventional management. This temporising treatment option to lengthen pregnancy includes the use of antihypertensive medication to control hypertension, magnesium sulphate to prevent eclampsia and corticosteroids to enhance foetal lung maturity4. With optimal maternal haemodynamic status and reassuring foetal condition this results on average in an extension of 2 weeks. Prolongation of these pregnancies is a great challenge for clinicians to balance between potential maternal risks on one the eve hand and possible foetal benefits on the other. Clinical controversies regarding prolongation of preterm preeclamptic pregnancies still exist – also taking into account that preeclampsia is the leading cause of maternal mortality in the Netherlands5 - a debate which is even more pronounced in very preterm pregnancies with questionable foetal viability6-9. Do maternal risks of prolongation of these very early pregnancies outweigh the chances of neonatal survival? Counselling of women with very early onset preeclampsia not only comprises of knowledge of the outcome of those particular pregnancies, but also knowledge of outcomes of future pregnancies of these women is of major clinical importance. This thesis opens with a review of the literature on identifiable risk factors of preeclampsia

Measurement of associated W plus charm production in pp collisions at √s=7 TeV

Peer reviewe

Discutindo a educação ambiental no cotidiano escolar: desenvolvimento de projetos na escola formação inicial e continuada de professores

A presente pesquisa buscou discutir como a Educação Ambiental (EA) vem sendo trabalhada, no Ensino Fundamental e como os docentes desta escola compreendem e vem inserindo a EA no cotidiano escolar., em uma escola estadual do município de Tangará da Serra/MT, Brasil. Para tanto, realizou-se entrevistas com os professores que fazem parte de um projeto interdisciplinar de EA na escola pesquisada. Verificou-se que o projeto da escola não vem conseguindo alcançar os objetivos propostos por: desconhecimento do mesmo, pelos professores; formação deficiente dos professores, não entendimento da EA como processo de ensino-aprendizagem, falta de recursos didáticos, planejamento inadequado das atividades. A partir dessa constatação, procurou-se debater a impossibilidade de tratar do tema fora do trabalho interdisciplinar, bem como, e principalmente, a importância de um estudo mais aprofundado de EA, vinculando teoria e prática, tanto na formação docente, como em projetos escolares, a fim de fugir do tradicional vínculo “EA e ecologia, lixo e horta”.Facultad de Humanidades y Ciencias de la Educació

Antenna-coupled titanium microbolometers

An ability of lensless titanium-based antenna coupled microbolometers (Ti-µbolometers) operating at room temperature to monitor precisely radiation patterns in terahertz time-domain spectroscopy (THz-TDS) systems are demonstrated. To provide comprehensive picture, two different THz-TDS systems and Ti-µbolometers coupled with three different antennas—narrowband dipole antennas for 0.3 THz, 0.7 THz and a log-periodic antenna for wideband detection—were selected for experiments. Radiation patterns, spatial beam profiles and explicit beam evolution along the propagation axis are investigatedpolarization-sensitive properties under various THz emitter power ranges are revealed. It was found that the studied Ti-µbolometers are convenient lensless sensors suitable to discriminate and control THz radiation pattern features in various wideband THz-TDS systems