Selection Rules (selection + rule)

Distribution by Scientific Domains
Distribution within Chemistry


Selected Abstracts


Effects of Practical Constraints on Item Selection Rules at the Early Stages of Computerized Adaptive Testing

JOURNAL OF EDUCATIONAL MEASUREMENT, Issue 2 2004
Shu-Ying Chen
The purpose of this study was to compare the effects of four item selection rules,(1) Fisher information (F), (2) Fisher information with a posterior distribution (FP), (3) Kullback-Leibler information with a posterior distribution (KP), and (4) completely randomized item selection (RN),with respect to the precision of trait estimation and the extent of item usage at the early stages of computerized adaptive testing. The comparison of the four item selection rules was carried out under three conditions: (1) using only the item information function as the item selection criterion; (2) using both the item information function and content balancing; and (3) using the item information function, content balancing, and item exposure control. When test length was less than 10 items, FP and KP tended to outperform F at extreme trait levels in Condition 1. However, in more realistic settings, it could not be concluded that FP and KP outperformed F, especially when item exposure control was imposed. When test length was greater than 10 items, the three nonrandom item selection procedures performed similarly no matter what the condition was, while F had slightly higher item usage. [source]


Providing care for an elderly parent: interactions among siblings?

HEALTH ECONOMICS, Issue 9 2009
Roméo Fontaine
Abstract This article is focused on children providing and financing long-term care for their elderly parent. The aim of this work is to highlight the interactions that may take place among siblings when deciding whether or not to become a caregiver. We look at families with two children using data from the Survey of Health, Ageing and Retirement in Europe; our sample contains 314 dependent elderly and their 628 adult children. In order to identify the interactions between siblings, we have specified a two-person discrete game model. To estimate this model, without invoking the ,coherency' condition, we have added an endogenous selection rule to solve the incompleteness problem arising from multiplicity or absence of equilibrium. Our empirical results suggest that the three classical effects identified by Manski could potentially explain the observed correlation between the siblings' caregiving behaviour. Correlated effects alone appear to be weak. Contextual interactions and endogenous interactions reveal cross-effects. The asymmetric character of the endogenous interactions is our most striking result. The younger child's involvement appears to increase the net benefit of caregiving for the elder one, whereas the elder child's involvement decreases the net benefit of caregiving for the younger child. Copyright © 2009 John Wiley & Sons, Ltd. [source]


A priori step size adaptation for the simulation of non-smooth systems

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN BIOMEDICAL ENGINEERING, Issue 2 2007
Ralf Uwe PfauArticle first published online: 19 JUN 200
Abstract The simulation of technical system include systems with discontinuities, jumps, discrete variables, slip,stick-changes and similar. The solution of those systems with standard ODE solvers normally leads to a break-down in the efficiency due to the changes in the systems which causes the step size selection rule to perform poorly. We present a strategy how to modify the standard step size selection with a priori information from the system to react on the upcoming change. This improves the efficiency and accuracy of the solution.Copyright © 2006 John Wiley & Sons, Ltd. [source]


An efficient gait recognition based on a selective neural network ensemble

INTERNATIONAL JOURNAL OF IMAGING SYSTEMS AND TECHNOLOGY, Issue 4 2008
Heesung Lee
Abstract The neural network ensemble is a learning paradigm where a collection of neural networks is trained for the same task. Generally, the ensemble shows better generalization performance than a single neural network. In this article, a selective neural network ensemble is applied to gait recognition. The proposed method selects some neural network based on the minimization of generalization error. Since the selection rule is directly incorporated into the cost function, we can obtain adequate component networks to constitute an ensemble. Experiments are performed with the NLPR database to show the performance of the proposed algorithm. © 2008 Wiley Periodicals, Inc. Int J Imaging Syst Technol, 18, 237,241, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). [source]


Deriving target selection rules from endogenously selected samples

JOURNAL OF APPLIED ECONOMETRICS, Issue 5 2006
Bas Donkers
The selection of the most profitable customers in a customer database for targeted activities is often done based on observed behaviour in the past. Consequently, databases arising from the responses to, for example, direct mailings in the past are not random samples. When not all heterogeneity across customers is observed, target selection will be based on unobserved heterogeneity and hence it is endogenous. We develop a method to adjust the likelihood function of latent class models to correct for this endogenous sampling process. We apply this technique to the selection of mail targets for a Dutch charity. Based on a joint model for the response rate and the amount donated, we create a target selection rule that maximizes expected revenues. Copyright © 2006 John Wiley & Sons, Ltd. [source]


Adsorption of 4,4,-thiobisbenzenethiol on silver surfaces: surface-enhanced Raman scattering study

JOURNAL OF RAMAN SPECTROSCOPY, Issue 3 2008
Yuling Wang
Abstract Adsorption of 4,4,-thiobisbenzenethiol (4,4,-TBBT) on a colloidal silver surface and a roughened silver electrode surface was investigated by means of surface-enhanced Raman scattering (SERS) for the first time, which indicates that 4,4,-TBBT is chemisorbed on the colloidal silver surface as dithiolates by losing two H-atoms of the SH bond, while as monothiolates on the roughened silver electrode. The different orientations of the molecules on both silver surfaces indicate the different adsorption behaviors of 4,4,-TBBT in the two systems. It is inferred from the SERS signal that the two aromatic rings in 4,4,-TBBT molecule are parallel to the colloidal silver surface as seen from the disappearance of ,CH band (3054 cm,1), which is a vibrational mode to be used to determine the orientation of a molecule on metals according to the surface selection rule, while on the roughened silver electrode surface they are tilted to the surface as seen from the enhanced signal of ,CH. The orientation of the C-S bond is tilted with respect to the silver surface in both cases as inferred from the strong enhancement of the ,CS. SERS spectra of 4,4,-TBBT on the roughened silver electrode with different applied potentials reveal that the enhancement of 4,4,-TBBT on the roughened silver electrode surface may be related to the chemical mechanism (CM). More importantly, the adsorption of 4,4,-TBBT on the silver electrode is expected to be useful to covalently adsorb metal nanoparticles through the free SH bond to form two- or three- dimensional nanostructures. Copyright © 2007 John Wiley & Sons, Ltd. [source]


Raman spectroscopy of optical phonon confinement in nanostructured materials

JOURNAL OF RAMAN SPECTROSCOPY, Issue 6 2007
Akhilesh K. Arora
Abstract If the medium surrounding a nano-grain does not support the vibrational wavenumbers of a material, the optical and acoustic phonons get confined within the grain of the nanostructured material. This leads to interesting changes in the vibrational spectrum of the nanostructured material as compared to that of the bulk. Absence of periodicity beyond the particle dimension relaxes the zone-centre optical phonon selection rule, causing the Raman spectrum to have contributions also from phonons away from the Brillouin-zone centre. Theoretical models and calculations suggest that the confinement results in asymmetric broadening and shift of the optical phonon Raman line, the magnitude of which depends on the widths of the corresponding phonon dispersion curves. This has been confirmed for zinc oxide nanoparticles. Microscopic lattice dynamical calculations of the phonon amplitude and Raman spectra using the bond-polarizability model suggest a power-law dependence of the peak-shift on the particle size. This article reviews recent results on the Raman spectroscopic investigations of optical phonon confinement in several nanocrystalline semiconductor and ceramic/dielectric materials, including those in selenium, cadmium sulphide, zinc oxide, thorium oxide, and nano-diamond. Resonance Raman scattering from confined optical phonons is also discussed. Copyright © 2007 John Wiley & Sons, Ltd. [source]


Adsorption of 4,4,-biphenyl diisocyanide on gold nanoparticle surfaces investigated by surface-enhanced Raman scattering

JOURNAL OF RAMAN SPECTROSCOPY, Issue 4 2003
Sang-Woo Joo
Abstract We investigated the spectral features of the ,(NC) bands when 4,4,-biphenyl diisocyanide (BPDNC) is adsorbed on gold nanoparticle surfaces by surface-enhanced Raman scattering (SERS). The mode of adsorption of BPDNC on gold nanoparticles was found to change with the bulk concentration. At low concentrations of BPDNC, only the ,(NC)bound band was conspicuous at ,2185 cm,1 and the free NC stretching band was barely detected in the SERS spectra. When the bulk concentration was increased, the ,(NC)free band became prominent at ,2123 cm,1. BPDNC was assumed to bridge two different gold particles at low concentrations, but as the concentration was increased, the bridge appeared to be broken and bonded to the gold particle only via one of the two isocyanide groups. On the basis of the electromagnetic surface selection rule, we attempted to explain the orientation of the adsorbate on Au surfaces by determining the relative enhancement factor of each vibrational band. Copyright © 2003 John Wiley & Sons, Ltd. [source]


Surface-enhanced Raman scattering of 4,4,-dicyanobiphenyl on gold and silver nanoparticle surfaces

JOURNAL OF RAMAN SPECTROSCOPY, Issue 6 2002
Chung-ro Lee
The adsorption behavior of 4,4,-dicyanobiphenyl (DCBP) on Ag and Au nanoparticle surfaces was studied by surface-enhanced Raman scattering (SERS). The absence of the ring CH band denoted a flat orientation of the biphenyl ring on Ag and Au. The red shift of the ,8a band indicated a direct ring ,-orbital interaction with the surfaces. The two benzene rings of DCBP appeared to lie on the same plane with a ,-type interaction upon adsorption on the surfaces. The ,(CN) bands were blue shifted in the Ag and Au SERS spectra by 2 and 22 cm,1, respectively from the ordinary Raman spectrum. The orientation of DCBP on the Ag and Au surfaces was analyzed by the relative enhancement factors of the vibrational bands based on the electromagnetic SERS selection rule. Copyright © 2002 John Wiley & Sons, Ltd. [source]


PhD Graduates with Post-doctoral Qualification in the Private Sector: Does It Pay Off?

LABOUR, Issue 3 2007
Isabelle Recotillet
Post-doctoral training was initially developed for PhD graduates wishing to embark on a career in the public sector. However, a large proportion of post-doctorate graduates turn to the private sector, and in particular to occupations that do not involve research. The question we raise is that of the wage premium on post-doctoral training. To control for selection bias arising in the case where unobservable elements are correlated between participation and wages, we first estimate a treatment effect model. The main finding is that when selection bias is not controlled for, post-doctoral participation increases earnings; however, when selection bias is controlled for, the participation in a post-doctoral programme has no positive effect. With regards to this finding we show that post-doctoral programmes play much more the role of a signal in the first stage of a career. This finding is also reinforced when we use a bivariate selection rule to control for the endogenous nature of having been recruited in the private sector. [source]


Multivariate Statistical Process Monitoring Using Kernel Density Estimation

ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 1-2 2005
J. Liang
In this paper, a general kernel density estimator has been introduced and discussed for multivariate processes in order to provide enhanced real-time performance monitoring. The proposed approach is based upon the concept of kernel density function, which is more appropriate to the underlying probability distribution of industrial process data in the development of a real-time monitoring scheme, to overcome the limitations of the conventional approach of defining the normal operating region based upon the assumption of normality. An optimal bandwidth selection rule is given based on the so-called mean integrated squared error index, and that is the normal operating region of process calculated using the optimal kernel density estimator before new process data are projected onto the normal operating region. The results of a case study of an industrial reheating furnace clearly demonstrates the power and advantages (e.g. decreasing the number of false alarms, identifying abnormal behaviour earlier, and reducing data sparsity) of the kernel density estimator-based approach over the conventional approach under the assumption of normality, which is still widely used. [source]


Oriented Electric Fields Accelerate Diels,Alder Reactions and Control the endo/exo Selectivity

CHEMPHYSCHEM, Issue 1 2010
Rinat Meir
Abstract Herein we demonstrate that an external electric field (EEF) acts as an accessory catalyst/inhibitor for Diels,Alder (DA) reactions. When the EEF is oriented along the "reaction axis" (the coordinate of approach of the reactants in the reaction path), the barrier of the DA reactions is lowered by a significant amount, equivalent to rate enhancements by 4,6 orders of magnitude. Simply flipping the EEF direction has the opposite effect, and the EEF acts as an inhibitor. Additionally, an EEF oriented perpendicular to the "reaction axis" in the direction of the individual molecule dipoles can change the endo/exo selectivity, favouring one or the other depending on the positive/negative directions of the EEF vis-ŕ-vis the individual molecular dipole. At some critical value of the EEF along the "reaction axis", there is a crossover to a stepwise mechanism that involves a zwitterionic intermediate. The valence bond diagram model is used to comprehend these trends and to derive a selection rule for EEF effects on chemical reactions: an EEF aligned in the direction of the electron flow between the reactants will lower the reaction barrier. It is shown that the exo/endo control by the EEF is not associated with changes in secondary orbital interactions. [source]


An Approach to the Synthesis of Silicon Carbide Nanowires by Simple Thermal Evaporation of Ferrocene onto Silicon Wafers

EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 25 2007
Jun-Jie Niu
Abstract Scales of silicon carbide nanowires (SiC-NWs) with high quality were synthesized by direct thermal evaporation of ferrocene onto silicon wafers at high temperature. Ferrocene decomposed into iron and carbon, which was subsequently treated with silicon to form SiC-NWs at high temperature. The SiC-NWs possess small diameters of , 20 nm and lengths of several ,ms. Furthermore, the samples show a uniform morphology, crystalline structure, and a very thin oxide layer. The main crystal direction of [111] was confirmed by high-resolution field-emission-transmission electron microscopy (HR-FETEM). The Raman scattering spectra showed two peaks at , 796 (TO) and , 980 cm,1 (LO) with varying intensity ratios at different positions. The band line fluctuation was contributed to the Raman selection rules. With reference to the experimental results, we suggested a tentative growth model according to the vapor,liquid,solid (VLS) mechanism. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source]


Theory of chemical bonds in metalloenzymes.

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 15 2010

Abstract Reaction mechanisms of oxygen evolution in native and artificial photosynthesis II (PSII) systems have been investigated on the theoretical grounds, together with experimental results. First of all, our previous broken-symmetry (BS) molecular orbitals (MO) calculations are reviewed to elucidate the instability of the d,-p, bond in high-valent (HV) Mn(X)O systems and the d,-p,-d, bond in HV MnOMn systems. The triplet instability of these bonds entails strong or intermediate diradical characters: ,Mn(IV)O, and ,MnOMn,; the BS MO resulted from strong electron correlation, leading to the concept of electron localizations and local spins. The BS computations have furthermore revealed guiding principles for derivation of selection rules for radical reactions of local spins. As a continuation of these theoretical results, the BS MO interaction diagrams for oxygen-radical coupling reactions in the oxygen evolution complex (OEC) in the PSII have been depicted to reveal scope and applicability of local singlet diradical (LSD) and local triplet diradical (LTD) mechanisms that have been successfully utilized for theoretical understanding of oxygenation reactions mechanisms by p450 and methane monooxygenase (MMO). The manganese-oxide cluster models examined are London, Berlin, and Berkeley models of CaMn4O4 and related clusters Mn4O4 and Mn3Ca. The BS MO interaction diagrams have revealed the LSD and/or LTD mechanisms for generation of molecular oxygen in the total low-, intermediate and high-spin states of these clusters. The spin alignments are found directly corresponding to the spin-coupling mechanisms of oxygen-radical sites in these clusters. The BS UB3LYP calculations of the clusters have been performed to confirm the comprehensive guiding principles for oxygen evolution; charge and spin densities by BS UB3LYP are utilized for elucidation and confirmation of the LSD and LTD mechanisms. Applicability of the proposed selection rules are examined in comparison with a lot of accumulated experimental and theoretical results for oxygen evolution reactions in native and artificial PSII systems. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [source]


Deriving target selection rules from endogenously selected samples

JOURNAL OF APPLIED ECONOMETRICS, Issue 5 2006
Bas Donkers
The selection of the most profitable customers in a customer database for targeted activities is often done based on observed behaviour in the past. Consequently, databases arising from the responses to, for example, direct mailings in the past are not random samples. When not all heterogeneity across customers is observed, target selection will be based on unobserved heterogeneity and hence it is endogenous. We develop a method to adjust the likelihood function of latent class models to correct for this endogenous sampling process. We apply this technique to the selection of mail targets for a Dutch charity. Based on a joint model for the response rate and the amount donated, we create a target selection rule that maximizes expected revenues. Copyright © 2006 John Wiley & Sons, Ltd. [source]


Effects of Practical Constraints on Item Selection Rules at the Early Stages of Computerized Adaptive Testing

JOURNAL OF EDUCATIONAL MEASUREMENT, Issue 2 2004
Shu-Ying Chen
The purpose of this study was to compare the effects of four item selection rules,(1) Fisher information (F), (2) Fisher information with a posterior distribution (FP), (3) Kullback-Leibler information with a posterior distribution (KP), and (4) completely randomized item selection (RN),with respect to the precision of trait estimation and the extent of item usage at the early stages of computerized adaptive testing. The comparison of the four item selection rules was carried out under three conditions: (1) using only the item information function as the item selection criterion; (2) using both the item information function and content balancing; and (3) using the item information function, content balancing, and item exposure control. When test length was less than 10 items, FP and KP tended to outperform F at extreme trait levels in Condition 1. However, in more realistic settings, it could not be concluded that FP and KP outperformed F, especially when item exposure control was imposed. When test length was greater than 10 items, the three nonrandom item selection procedures performed similarly no matter what the condition was, while F had slightly higher item usage. [source]


A novel search framework for multi-stage process scheduling with tight due dates

AICHE JOURNAL, Issue 8 2010
Yaohua He
Abstract This article improves the original genetic algorithm developed by He and Hui (Chem Eng Sci. 2007; 62:1504,1527) and proposes a novel global search framework (GSF) for the large-size multi-stage process scheduling problems. This work first constructs a comprehensive set of position selection rules according to the impact factors analysis presented by He and Hui (in this publication in 2007), and then selects suitable rules for schedule synthesis. In coping with infeasibility emerging during the search, a penalty function is adopted to force the algorithm to approach the feasible solutions. The large-size problems with tight due dates are challenging to the current solution techniques. Inspired by the gradient used in numerical analysis, we treat the deviation existing among the computational tests of the algorithm as evolutionary gradient. Based on this concept, a GSF is laid out to fully utilize the search ability of the current algorithm. Numerical experiments indicate that the proposed search framework solves such problems with satisfactory solutions. © 2010 American Institute of Chemical Engineers AIChE J, 2010 [source]


Polar phonon mode selection rules in tip-enhanced Raman scattering

JOURNAL OF RAMAN SPECTROSCOPY, Issue 10 2009
Samuel Berweger
Abstract We discuss the use of the symmetry selectivity of phonon Raman scattering to determine nanocrystallographic information of solids using tip-enhanced Raman scattering (TERS). The necessary degrees of freedom arise from the combination of the Raman selection rules reflecting crystal symmetry superimposed by the polarization and k -vector-dependent field enhancement and scattering of the scanning probe tip. The resulting phonon TERS selection rules are discussed, including the use of the crystal Raman tensor and momentum conservation for polar phonon modes. We demonstrate the selection rules for both far-field and tip-enhanced near-field Raman scattering from bulk and nanocrystalline LiNbO3. Copyright © 2009 John Wiley & Sons, Ltd. [source]


Coadsorption of trimethyl phosphine and thiocyanate on colloidal silver: a SERS study combined with theoretical calculations

JOURNAL OF RAMAN SPECTROSCOPY, Issue 4 2009
Gengshen Hu
Abstract The adsorption of trimethyl phosphine (TMP) on colloidal silver has been investigated by means of surface-enhanced Raman scattering spectroscopy (SERS). On the basis of surface selection rules, it is deduced from the SERS results that TMP adsorbs on silver surface via its P atom. The electron donor effect of TMP can be sensitively probed by the coadsorbed SCN,. The Raman wavenumber of ,CN of the adsorbed SCN, shifts to lower wavenumbers when TMP is coadsorbed with SCN, and the red shift of C,N stretching wavenumber is found to increase with increasing surface coverage of TMP. This could be explained in terms of the electron donor effect of TMP. Density functional theory (DFT) calculations further confirm the experimental results that the charge transfer is from TMP to silver surface rather than reversely. Natural bond orbital (NBO) analysis indicates that the red shift of C,N stretching mode is due the increase of electronic populations of ,* orbital of C,N bond induced by coadsorbed TMP, consequently the C,N bond is weakened, and the ,CN shifts to lower wavenumbers. An NBO analysis also indicates that the conjugated effect between S atom and C,N bond could easily make the charge transfer from silver surface to C,N bond. Copyright © 2008 John Wiley & Sons, Ltd. [source]


Raman and infrared studies of synthetic Maya pigments as a function of heating time and dye concentration

JOURNAL OF RAMAN SPECTROSCOPY, Issue 9 2007
F. S. Manciu
Abstract Maya Blue is a famous indigo-based pigment produced by the ancient Mayas. The organic/inorganic complexes inspired by Maya Blue have led to a new class of surface compounds that have novel applications to pigment industries. Materials analyzed in the present work are made by a synthetic route, and demonstrate chemical stability similar to that of the ancient Maya Blue samples. However, we have learned that stable complexes can be synthesized at much higher dye concentrations than used by the Mayas. Analysis by FT-Raman and FT-IR spectroscopy demonstrates the partial elimination of the selection rules for the centrosymmetric indigo, indicating distortion of the molecule. This distortion accounts for the observed color changes, as the molecular orbital structure is modified, allowing the complex to stabilize. The spectroscopic data also shows the disappearance of the indigo NH bonding, as the organic molecules incorporate into palygorskite material. A structural change of indigo to dehydroindigo during heating is suggested by this result. Infrared data confirm the loss of zeolitic water and a partial removal of structural water after the heating process. Evidence of bonding between cationic aluminum and dehydroindigo through oxygen and nitrogen is revealed by FT-Raman measurements at higher dye concentrations. Copyright © 2007 John Wiley & Sons, Ltd. [source]


Combining two structural techniques on the micrometer scale: micro-XAS and micro-Raman spectroscopy

JOURNAL OF SYNCHROTRON RADIATION, Issue 5 2007
V. Briois
X-ray absorption and Raman spectroscopies are complementary in the sense that both give very precise information about the local structure of a sample, both are not restricted to crystalline materials, and in both cases the volumes of the material probed are similar. The X-ray technique has the advantage of being element- and orbital-selective, and sensitive to orientational effects owing to polarization selection rules. In many cases, however, its analysis can present some ambiguity. Combining the two techniques on a micrometer scale could therefore be a very powerful method structurally. In this paper the experimental set-up developed at the LUCIA beamline and its application to a natural mineral are described. [source]


Breeding habitat selection behaviors in heterogeneous environments: implications for modeling reintroduction

OIKOS, Issue 5 2009
Jean-Baptiste Mihoub
Animal movement and habitat selection behavior are important considerations in ecology, and remain a major issue for successful animal reintroductions. However, simple rules are often used to model movement or focus only on intrinsic environmental cues, neglecting recent insights in behavioral ecology on habitat selection processes. In particular, social information has been proposed as a widespread source of information for habitat evaluation. We investigated the role of explicit breeding habitat selection strategies on the establishment pattern of reintroduced populations and their persistence. We considered local movement at the scale of a single population. We constructed a spatially-implicit demographic model that considered five breeding habitat selection rules: 1) random, 2) intrinsic habitat quality, 3) avoidance of conspecifics, 4) presence of conspecifics and 5) reproductive success of conspecifics. The impact of breeding habitat selection was examined for different release methods under various levels of environmental heterogeneity levels, for both long and short-lived monogamous species. When heterogeneity between intrinsic habitat patch qualities is high, the persistence of reintroduced populations strongly depends on habitat selection strategies. Strategies based on intrinsic quality and conspecific reproductive success lead to a lower reintroduction failure risk than random, conspecific presence or avoidance-based strategies. Conspecific presence or avoidance-based strategies may aggregate individuals in suboptimal habitats. The release of adults seems to be more efficient independent of habitat selection strategy. We emphasize the crucial role of oriented habitat selection behavior and non ideal habitat selection in movement modeling, particularly for reintroduction. [source]


Magnetotunneling into Fock,Darwin-like quantum dot states: Lateral matrix elements and the role of selection rules

PHYSICA STATUS SOLIDI - RAPID RESEARCH LETTERS, Issue 7 2010
Gerold Kießlich
Abstract We study theoretically the magnetotunneling transport through quantum dots formed by thermal diffusion of charged manganese interstitials in the vicinity of a GaAs quantum well [Phys. Rev. Lett. 101, 226807 (2008)]. In particular, we examine the lateral matrix elements between Landau subbands in the contact and Fock,Darwin-like states of an individual dot at high magnetic fields. We explicitly demonstrate the effect of spatial deformation of the dot on the wave function's overlap. The comparison with measured data suggests a selection rule similar to angular momentum conservation for tunneling into perfect Fock,Darwin states. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


Pulsed electrically detected magnetic resonance in organic semiconductors

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 11-12 2009
C. Boehme
Abstract Carbon-based materials have an intrinsically weak spin,orbit coupling which imposes spin selection rules on many electronic transitions. The spin degree of freedom of electrons and nuclei can therefore play a crucial role in the electronic and optical properties of these materials. Spin-selection rules can be studied via magnetic resonance techniques such as electron,spin resonance and optically detected magnetic resonance as well as electrically detected magnetic resonance (EDMR). The latter has progressed in recent years to a degree where the observation of coherent spin motion via current detection has become possible, providing experimental access to many new insights into the role that paramagnetic centers play for conductivity and photoconductivity. While mostly applied to inorganic semiconductor materials such as silicon, this new, often called pulsed-(p) EDMR spectroscopy, has much potential for organic (carbon-based) semiconductors. In this study, progress on the development of pEDMR spectroscopy on carbon-based materials is reviewed. Insights into materials properties that can be gained from pEDMR experiments are explained and limitations are discussed. Experimental data on radiative polaron-pair recombination in poly[2-methoxy-5-(20-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) organic light emitting diodes (OLEDs) are shown, revealing that under operating conditions the driving current of the device can be modulated by spin-Rabi nutation of the polaron spin within the charge carrier pairs. From this experimental data it becomes clear that for polaron pairs, the precursor states during exciton formation, exchange interaction is not the predominant influence on the observed pEDMR spectra. [source]


Spin polarization of a non-magnetic high g-factor semiconductor at low magnetic field

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 12 2007
J. Lee
Abstract We have studied the spin polarization of HgCdTe by measuring Shubnikov-de Haas oscillations. The magnetic field have been applied in parallel and perpendicular to the current. Relatively long spin relaxation time was observed since only spin conserved transition is allowed by selection rules. The electronic spin is completely polarized when the applied magnetic field is larger than 0.5 Tesla, which can be easily generated by micromagnets deposited on the surface of the specimen. Thus, the spin-manipulation such as spin up/down junction can be realized with this semiconductor. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


Symmetry of electron states and optical transitions in GaN/AlN hexagonal quantum dots

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 13 2004
P. Tronc
Abstract The exact symmetry of hexagonal quantum dots (QDs) made of materials with the wurtzite structure such as GaN/AlN QDs for example, is described by the C3v point group and does not depend on the existence of a wetting layer. We have determined the possible exact symmetries of electron states and vibration modes in the dots and derived the optical selection rules. The vibration modes involved in the Frölich interaction are totally symmetric with respect to the C3v group and can induce transitions only between states with the same symmetry. The not totally symmetric modes provide other channels for lowering the energy of excited carriers and excitons by connecting states with symmetries different one from another. The rapid decay of created polarons, due to the short lifetime of vibration modes, releases the carriers and excitons into ground levels. In the envelope function approximation (EFA), the symmetry of the dots is represented by the C6v point group. Interband transitions are allowed only between states whose envelope functions have the same symmetry. EFA artificially increases the number of dark exciton symmetries. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


Anisotropic polarization of non-polar GaN quantum dot emission

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue S2 2009
R. Mata
Abstract We report on experimental and theoretical studies of the polarization selection rules of the emission of non-polar GaN/AlN self-assembled quantum dots. Time-integrated and time-resolved photoluminescence measurements have been performed to determine the degree of polarization. It is found that the emission of some samples can be predominantly polarized parallel to the wurtzite c axis, in striking difference with the previously reported results for bulk GaN and its heterostructures, in which the emission was preferentially polarized perpendicular to the c axis. Theoretical calculations based on an 8-band k·p model are used to analyze the relative importance of strain, confinement and quantum dot shape on the polarization selection rules. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


Bipolariton laser emission from a GaAs microcavity

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 2 2007
L. M. Moreira
Abstract Biexciton emission properties were studied in a single GaAs quantum well (QW) semiconductor planar microcavity by photoluminescence measurements at low temperatures. At high pump intensity a bipolariton emission appears close to the lower polariton mode. This new mode appears when we detune the cavity resonance out of the lower polariton branch, showing a laser like behavior. Very small linewidths were measured, lying below 110 µeV and 150 µeV for polariton and bipolariton emission respectively. The input/output power (I/O) measurements show that the bipolariton emission has a weaker coupling efficiency compared to previous results for polariton emission. Varying the pump laser polarization, we were able to show the selection rules for the biexciton particle creation in the quantum well. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


Symmetry analysis of extinction rules in diffuse-scattering experiments

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2010
R. L. Withers
Structured diffuse-scattering intensities, whether of compositional or of pure displacive origin, static or dynamic, contain important information about the symmetry of the individual compositional and/or displacive modes responsible for the observed intensities. However, the interpretation of the experimental data is very often impeded by the lack of a symmetry-based approach to the analysis of the structured diffuse-scattering distributions. Recently, we have demonstrated the existence of systematic phonon selection rules for diffuse scattering that depend on the symmetries of the mode and the scattering vector, and not on the specific structure. Here, we show that such symmetry analysis can be successfully extended and also applied to structure-dependent diffuse scattering associated with `disordered' materials: the combination of theoretically determined, diffuse-scattering extinction conditions with the concept of non-characteristic orbits proves to be very useful in the interpretation of the observed diffuse-scattering extinctions. The utility of this approach is illustrated by the analysis of diffuse-scattering data from ThAsSe, FeOF and FeF2. The essential part of the associated calculations are performed by the computer programs NEUTRON (systematic phonon extinction rules in inelastic scattering) and NONCHAR (non-characteristic orbits of space groups) that are available on the Bilbao crystallographic server (http://www.cryst.ehu.es). [source]


Ring Currents as Probes of the Aromaticity of Inorganic Monocycles : P5,, As5,, S2N2, S3N3,, S4N3+, S4N42+, S5N5+, S42+ and Se42+

CHEMISTRY - A EUROPEAN JOURNAL, Issue 4 2004
Frank De Proft Prof.
Abstract Current-density maps were calculated by the ipsocentric CTOCD-DZ/6-311G** (CTOCD-DZ=continuous transformation of origin of current density-diamagnetic zero) approach for three sets of inorganic monocycles: S42+, Se42+, S2N2, P5, and As5, with 6 , electrons; S3N3,, S4N3+ and S4N42+ with 10 , electrons; and S5N5+ with 14 , electrons. Ipsocentric orbital analysis was used to partition the currents into contributions from small groups of active electrons and to interpret the contributions in terms of symmetry- and energy-based selection rules. All nine systems were found to support diatropic , currents, reinforced by , circulations in P5,, As5,, S3N3,, S4N3+, S4N42+ and S5N5+, but opposed by them in S42+, Se42+ and S2N2. The opposition of , and , effects in the four-membered rings is compatible with height profiles of calculated NICS (nucleus-independent chemical shifts). [source]