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Search Results: 1 - 10 of 223893 matches for " R. Seguin "
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GaN/AlN Quantum Dots for Single Qubit Emitters
M. Winkelnkemper,R. Seguin,S. Rodt,A. Hoffmann,D. Bimberg
Physics , 2008, DOI: 10.1088/0953-8984/20/45/454211
Abstract: We study theoretically the electronic properties of $c$-plane GaN/AlN quantum dots (QDs) with focus on their potential as sources of single polarized photons for future quantum communication systems. Within the framework of eight-band k.p theory we calculate the optical interband transitions of the QDs and their polarization properties. We show that an anisotropy of the QD confinement potential in the basal plane (e.g. QD elongation or strain anisotropy) leads to a pronounced linear polarization of the ground state and excited state transitions. An externally applied uniaxial stress can be used to either induce a linear polarization of the ground-state transition for emission of single polarized photons or even to compensate the polarization induced by the structural elongation.
Decay dynamics of neutral and charged excitonic complexes in single InAs/GaAs quantum dots
M. Feucker,R. Seguin,S. Rodt,A. Hoffmann,D. Bimberg
Physics , 2008, DOI: 10.1063/1.2844886
Abstract: Systematic time-resolved measurements on neutral and charged excitonic complexes (X, XX, X+, and XX+) of 26 different single InAs/GaAs quantum dots are reported. The ratios of the decay times are discussed in terms of the number of transition channels determined by the excitonic fine structure and a specific transition time for each channel. The measured ratio for the neutral complexes is 1.7 deviating from the theoretically predicted value of 2. A ratio of 1.5 for the positively charged exciton and biexciton decay time is predicted and exactly matched by the measured ratio indicating identical specific transition times for the transition channels involved.
Compression and the origins of Zipf's law of abbreviation
R. Ferrer-i-Cancho,C. Bentz,C. Seguin
Computer Science , 2015,
Abstract: Languages across the world exhibit Zipf's law of abbreviation, namely more frequent words tend to be shorter. The generalized version of the law - an inverse relationship between the frequency of a unit and its magnitude - holds also for the behaviours of other species and the genetic code. The apparent universality of this pattern in human language and its ubiquity in other domains calls for a theoretical understanding of its origins. To this end, we generalize the information theoretic concept of mean code length as a mean energetic cost function over the probability and the magnitude of the types of the repertoire. We show that the minimization of that cost function and a negative correlation between probability and the magnitude of types are intimately related.
Synthèse des facteurs déterminant le succès de la rénovation de paturages par semis direct de légumineuses en Amérique du Nord
Seguin P.
Biotechnologie, Agronomie, Société et Environnement , 1998,
Abstract: Pasture renovation by legumes sod-seeding, which consist in drilling legumes seeds in a grass dominated sward, is a way of increasing both pasture productivity and quality. In North America the use of this method is increasing in popularity. However, results are variable and some problems still persistent. Factors determining the outcome of legumes sod-seeding during pasture renovation are reviewed. Particular attention is given to the role and effects of herbicides, which are generally used to suppress the competition provided by the resident grass vegetation. The use of physical sod suppression methods (eg: mowing or grazing) as an alternative to herbicides is also discussed.
Polarized emission lines from A- and B-type excitonic complexes in single InGaN/GaN quantum dots
M. Winkelnkemper,R. Seguin,S. Rodt,A. Schliwa,L. Reissmann,A. Strittmatter,A. Hoffmann,D. Bimberg
Physics , 2007, DOI: 10.1063/1.2743893
Abstract: Cathodoluminescence measurements on single InGaN/GaN quantum dots (QDs) are reported. Complex spectra with up to five emission lines per QD are observed. The lines are polarized along the orthogonal crystal directions [1 1 -2 0] and [-1 1 0 0]. Realistic eight-band k.p electronic structure calculations show that the polarization of the lines can be explained by excitonic recombinations involving hole states which are either formed by the A or the B valence band.
Polarized Emission Lines from Single InGaN/GaN Quantum Dots: Role of the Valence-band Structure of Wurtzite Group-III Nitrides
M. Winkelnkemper,R. Seguin,S. Rodt,A. Schliwa,L. Reissmann,A. Strittmatter,A. Hoffmann,D. Bimberg
Physics , 2007, DOI: 10.1016/j.physe.2007.11.033
Abstract: We present a study of the polarization properties of emission lines from single InGaN/GaN quantum dots (QDs). The QDs, formed by spinodal decomposition within ultra-thin InGaN quantum wells, are investigated using single-QD cathodoluminescence (CL). The emission lines exhibit a systematic linear polarization in the orthogonal crystal directions [1 1 -2 0] and [-1 1 0 0]--a symmetry that is non-native to hexagonal crystals. Eight-band k.p calculations reveal a mechanism that can explain the observed polarizations: The character of the hole(s) in an excitonic complex determines the polarization direction of the respective emission if the QD is slightly elongated. Transitions involving A-band holes are polarized parallel to the elongation; transitions involving B-type holes are polarized in the orthogonal direction. The energetic separation of both hole states is smaller than 10 meV. The mechanism leading to the linear polarizations is not restricted to InGaN QDs, but should occur in other wurtzite-nitride QDs and in materials with similar valence band structure.
Size-dependent fine-structure splitting in self-organized InAs/GaAs quantum dots
R. Seguin,A. Schliwa,S. Rodt,K. P?tschke,U. W. Pohl,D. Bimberg
Physics , 2006, DOI: 10.1103/PhysRevLett.95.257402
Abstract: A systematic variation of the exciton fine-structure splitting with quantum dot size in single InAs/GaAs quantum dots grown by metal-organic chemical vapor deposition is observed. The splitting increases from -80 to as much as 520 $\mu$eV with quantum dot size. A change of sign is reported for small quantum dots. Model calculations within the framework of eight-band k.p theory and the configuration interaction method were performed. Different sources for the fine-structure splitting are discussed, and piezoelectricity is pinpointed as the only effect reproducing the observed trend.
Multi-excitonic complexes in single InGaN quantum dots
R. Seguin,S. Rodt,A. Strittmatter,L. Rei?mann,T. Bartel,A. Hoffmann,D. Bimberg
Physics , 2006, DOI: 10.1063/1.1751214
Abstract: Cathodoluminescence spectra employing a shadow mask technique of InGaN layers grown by metal organic chemical vapor deposition on Si(111) substrates are reported. Sharp lines originating from InGaN quantum dots are observed. Temperature dependent measurements reveal thermally induced carrier redistribution between the quantum dots. Spectral diffusion is observed and was used as a tool to correlate up to three lines that originate from the same quantum dot. Variation of excitation density leads to identification of exciton and biexciton. Binding and anti-binding complexes are discovered.
Size-dependence of anisotropic exchange interaction in InAs/GaAs quantum dots
R. Seguin,S. Rodt,A. Schliwa,K. P?tschke,U. W. Pohl,D. Bimberg
Statistics , 2006, DOI: 10.1002/pssb.200671519
Abstract: A comprehensive study of the exchange interaction between charge carriers in self-organized InAs/GaAs quantum dots is presented. Single quantum-dot cathodoluminescence spectra of quantum dots of different sizes are analyzed. Special attention is paid to the energetic structure of the charged excited exciton (hot trion). A varying degree of intermixing within the hot trion states leads to varying degrees of polarization of the corresponding emission lines. The emission characteristics change from circularly polarized for small quantum dots to elliptically polarized for large quantum dots. The findings are explained by a change of magnitude of the anisotropic exchange interaction and compared to the related effect of fine-structure splitting in the neutral exciton and biexciton emission.
Atrial fibrillation is not just an artefact in the ICU
Philippe Seguin, Yoann Launey
Critical Care , 2010, DOI: 10.1186/cc9093
Abstract: Atrial fibrillation (AF) onset in the intensive care unit (ICU) is attracting widespread attention because of its frequency and prognostic significance. In the previous issue of Critical Care, Meierhenrich and colleagues complete the description of new-onset AF in the ICU in a selected population of patients suffering from septic shock [1]. They found that 46% of their patients developed AF and this arrhythmia was significantly associated with increased ICU length of stay without affecting mortality. Interestingly, they reported a significant and continuous increase in C-reactive protein levels the days before the occurrence of AF, corroborating previous findings on the hypothesis of an inflammatory substrate in AF onset [2].AF is the most significant arrhythmia in the ICU. The risk to develop AF in the ICU is largely superior to that of the general population but differs with regard to the type of ICU involved. Indeed, the risk is estimated to be 4% in the general population, from 4 to 9% in the general ICU and an incidence of 32% has been recently reported in a cardiac surgical ICU [3-9].How could we explain such a difference? In fact, AF is considered both a cardiac disease and a noncardiac disease. Age, essential hypertension, ischaemic heart failure and valvular heart disease are well recognized as cardiac components of AF, and on the contrary inflammation, whatever its origin, is now considered an important noncardiac trigger [4]. In this context, it is not surprising that cardiac surgery generates a higher incidence of AF, and several data support this assumption. Effectively, in cardiac surgery the risk of AF is common in the first 3 postoperative days and a strength correlation has been found between various inflammatory parameters and postoperative arrhythmia [7,10]. In a prospective double-blind study, 236 patients undergoing elective heart surgery were randomized to receive placebo or dexamethasone after the induction of anaesthesia. Patients who receive
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