Responsivity of quantum well infrared photodetectors at terahertz detection wavelengths

A first-principles model of the photocurrent in quantum well infrared photodetectors (QWIPs) is derived. The model examines the responsivity, carrier capture probability and quantum efficiency. It is found that the QWIP sensitivity reaches a plateau below the 10 µm detection wavelength and remains nearly constant from 10 to 50 µm. © 2002 American Institute of Physics

A categorical approach to the theory of equations

AbstractWe construct a small category whose objects are monic square-free polynomials with coefficients in a field F. For a monic, irreducible, and normal polynomial, Aut (f) is the usual Galois group of f. We prove that there exists a unique topological group G such that the category of finite discrete G-sets is equivalent to the opposite of our category. We then replace categories by commutative rings and define the Burnside ring of a field, which has Burnside rings of finite groups as its building blocks. We next extend scalars to the rationals and explicity determine the algebra that results. We find all valuations of this algebra and prove that an irreducible polynomial is completely determined by its values under these valuations

Evaluating the Potential of Using 5-Azacytidine as an Epimutagen

A number of early flowering lines were induced when 5-azacytidine was applied to germinating flax (Linum usitatissimum L.) seed. The genetics of these lines indicate that the induced changes are epigenetic and probably result from demethylation of the genomic DNA at loci that affect flowering age. Although the growth and development of three stable early flowering lines are altered and the percentage of filled seed was reduced in all three lines compared with controls, measures of seed productivity demonstrated that harvest index was unaffected in two of the lines. In the third, harvest index was lower than normal and both seed set per capsule and seed mass per 100 seed were reduced. Furthermore, six generations after induction this line began to display relatively high levels of polyembryony. The late appearance of this twinning and other aspects related to working with lines induced by 5-azacytidine and using 5-azacytidine as an epimutagen are discussed

A new quantum fluid at high magnetic fields in the marginal charge-density-wave system α\alpha-(BEDT-TTF)2M_2MHg(SCN)4_4 (where M=M=~K and Rb)

Single crystals of the organic charge-transfer salts $\alpha$-(BEDT-TTF)$_2M$Hg(SCN)$_4$ have been studied using Hall-potential measurements ($M=$K) and magnetization experiments ($M$ = K, Rb). The data show that two types of screening currents occur within the high-field, low-temperature CDW$_x$ phases of these salts in response to time-dependent magnetic fields. The first, which gives rise to the induced Hall potential, is a free current (${\bf j}_{\rm free}$), present at the surface of the sample. The time constant for the decay of these currents is much longer than that expected from the sample resistivity. The second component of the current appears to be magnetic (${\bf j}_{\rm mag}$), in that it is a microscopic, quasi-orbital effect; it is evenly distributed within the bulk of the sample upon saturation. To explain these data, we propose a simple model invoking a new type of quantum fluid comprising a CDW coexisting with a two-dimensional Fermi-surface pocket which describes the two types of current. The model and data are able to account for the body of previous experimental data which had generated apparently contradictory interpretations in terms of the quantum Hall effect or superconductivity.Comment: 13 pages, 11 figure

Variable plasmid fitness effects and mobile genetic element dynamics across Pseudomonas species

Mobile genetic elements (MGE) such as plasmids and transposons mobilise genes within and between species, playing a crucial role in bacterial evolution via horizontal gene transfer (HGT). Currently we lack data on variation in MGE dynamics across bacterial host species. We tracked the dynamics of a large conjugative plasmid, pQBR103, and its Tn5042 mercury resistance transposon, in five diverse Pseudomonas species in environments with and without mercury selection. Plasmid fitness effects and stability varied extensively between host species and environments, as did the propensity for chromosomal capture of the Tn5042 mercury resistance transposon associated with loss of the plasmid. Whereas P. fluorescens and P. savastanoi stably maintained the plasmid in both environments, the plasmid was highly unstable in P. aeruginosa and P. putida, where plasmid-free genotypes with Tn5042 captured to the chromosome invaded to higher frequency under mercury selection. These data confirm that plasmid stability is dependent upon the specific genetic interaction of the plasmid and host chromosome rather than being a property of plasmids alone, and moreover imply that MGE dynamics in diverse natural communities are likely to be complex and driven by a subset of species capable of stably maintaining plasmids which would then act as hubs of HGT

Psychological and emotional needs, assessment and support post-stroke: a multi-perspective qualitative study

Background: International stroke care guidelines recommend the routine assessment and management of psychological and emotional problems post-stroke. Understanding the experiences of those delivering and receiving these services is vital to improving the provision of psychological support post-stroke. Objectives: To explore patients’, carers’ and health professionals’ experiences of psychological need, assessment and support post-stroke whilst in hospital and immediately post-discharge. Methods: Participants were recruited from seven specialist stroke services in the North of England. Qualitative semi-structured interviews and focus groups were conducted with 31 stroke patients, 28 carers and 66 health professionals. The interviews were recorded and transcribed verbatim, and analysed using thematic analysis. Results: Two central themes emerged Minding the gap: psychological expertise, and Protective factors perceived to reduce the need for formal psychological support. The lack of psychological expertise amongst healthcare professionals working on stroke units was a source of frustration and resulted in other disciplines assuming the role of a psychologist without the required skills and training. Multiple stakeholders discussed the importance of protective factors, including downward social comparison, social support, peer support, communication and information provision, that were perceived to reduce the need for formal psychological support. Discussion: Stroke patients need better access to psychological support, including information, advice and peer or social support. More research is required to establish the effectiveness of alternative options to formal psychological suppor

The RHIC project

The design and construction status of the Relativistic Heavy Ion collider (RHIC) is discussed. Those novel features of a heavy ion collider that are distinct from hadron colliders in general are noted. These features are derived from the experimental requirements of operation with a variety of ion species over a wide energy range including collisions between ions of unequal energies. The project is in the fourth year of a seven year construction cycle. A review of the superconducting magnet program is given together with progress to date on the machine construction

Genomic analysis of Anderson typing phages of Salmonella Typhimrium: towards understanding the basis of bacteria-phage interaction

The Anderson phage typing scheme has been successfully used worldwide for epidemiological surveillance of Salmonella enterica serovar Typhimurium. Although the scheme is being replaced by whole genome sequence subtyping methods, it can provide a valuable model system for study of phage-host interaction. The phage typing scheme distinguishes more than 300 definitive types of Salmonella Typhimurium based on their patterns of lysis to a unique collection of 30 specific Salmonella phages. In this study, we sequenced the genomes of 28 Anderson typing phages of Salmonella Typhimurium to begin to characterize the genetic determinants that are responsible for the differences in these phage type profiles. Genomic analysis of typing phages reveals that Anderson phages can be classified into three different groups, the P22-like, ES18-like and SETP3-like clusters. Most Anderson phages are short tailed P22-like viruses (genus Lederbergvirus); but phages STMP8 and STMP18 are very closely related to the lambdoid long tailed phage ES18, and phages STMP12 and STMP13 are related to the long noncontractile tailed, virulent phage SETP3. Most of these typing phages have complex genome relationships, but interestingly, two phage pairs STMP5 and STMP16 as well as STMP12 and STMP13 differ by a single nucleotide. The former affects a P22-like protein involved in DNA passage through the periplasm during its injection, and the latter affects a gene whose function is unknown. Using the Anderson phage typing scheme would provide insights into phage biology and the development of phage therapy for the treatment of antibiotic resistant bacterial infections

A Heuristic for General Rule Extraction From a Multilayer Perceprtron

Rule extraction from Artificial Neural Networks (ANN's) is an essential step towards the integration of ANN's and Knowledge-based Systems (KBS's). Two central questions addressed in this paper are what is a suitable format embodying ANN knowledge correctly and efficiently; and how is the knowledge extracted. A General Rule is defined in an efficient format to represent the knowledge from ANN's. General Rules are extracted from a trained Multilayer Perceptron (MLP). The inputs of the MLP correspond to the premises and the outputs , to the conclusion of the rules. Two criteria are used to ascertain the significance of input components for the construction of rules. The first criterion, the Potential Default Set (PDS) is drawn up from the weighted connections combined with the input/output correspondence of a training pattern. A subset of the inputs in the training pattern, which is possibly redundant, is defined as the PDS. The second criterion, the Feature Salient Degree (FSD) embodies the casual bond of the changes on each input bit and on each output bit. The system using both PDS and FSD is demonstrated by application to typical logic problems such as AND, OR,XOR and by interpretation for unknown data. Clinical data have also been used to assess the performance of the method in the real-world. The rules derived here have been evaluated by a domain expert and are found to conform with his view of the problem. The computational complexity is a third order polynomial of the problem size