3,280 research outputs found
Handling missing data by re-approaching non-respondents
When handling missing data, a researcher should be aware of the mechanism underlying the missingness. In the presence of non-randomly missing data, a model of the missing data mechanism should be included in the analyses to prevent the analyses based on the data from becoming biased. Modeling the missing data mechanism, however, is a difficult task. One way in which knowledge about the missing data mechanism may be obtained is by collecting additional data from non-respondents. In this paper the method of re-approaching respondents who did not answer all questions of a questionnaire is described. New answers were obtained from a sample of these non-respondents and the reason(s) for skipping questions was (were) probed for. The additional data resulted in a larger sample and was used to investigate the differences between respondents and non-respondents, whereas probing for the causes of missingness resulted in more knowledge about the nature of the missing data patterns
Bistable hysteresis and resistance switching in hydrogen gold junctions
Current-voltage characteristics of H2-Au molecular junctions exhibit
intriguing steps around a characteristic voltage of 40 mV. Surprisingly, we
find that a hysteresis is connected to these steps with a typical time scale >
10 ms. This time constant scales linearly with the power dissipated in the
junction beyond an ofset power P_s = IV_s. We propose that the hysteresis is
related to vibrational heating of both the molecule in the junction and a set
of surrounding hydrogen molecules. Remarkably, we can engineer our junctions
such that the hysteresis' characteristic time becomes >days. We demonstrate
that reliable switchable devices can be built from such junctions.Comment: Submitted to Phys. Rev. Let
Spin Relaxation in Graphene with self-assembled Cobalt Porphyrin Molecules
In graphene spintronics, interaction of localized magnetic moments with the
electron spins paves a new way to explore the underlying spin relaxation
mechanism. A self-assembled layer of organic cobalt-porphyrin (CoPP) molecules
on graphene provides a desired platform for such studies via the magnetic
moments of porphyrin-bound cobalt atoms. In this work a study of spin transport
properties of graphene spin-valve devices functionalized with such CoPP
molecules as a function of temperature via non-local spin-valve and Hanle spin
precession measurements is reported. For the functionalized (molecular)
devices, we observe a slight decrease in the spin relaxation time ({\tau}s),
which could be an indication of enhanced spin-flip scattering of the electron
spins in graphene in the presence of the molecular magnetic moments. The effect
of the molecular layer is masked for low quality samples (low mobility),
possibly due to dominance of Elliot-Yafet (EY) type spin relaxation mechanisms
Demand-Driven Scheduling of Movies in a Multiplex
This paper describes a model that generates weekly movie schedules in a multiplex movie theater. A movie schedule specifies within each day of the week, on which screen(s) different movies will be played, and at which time(s). The model consists of two parts: (i) conditional forecasts of the number of visitors per show for any possible starting time; and (ii) an optimization procedure that quickly finds an almost optimal schedule (which can be demonstrated to be close to the optimal schedule). To generate this schedule we formulate the so-called movie scheduling problem as a generalized set partitioning problem. The latter is solved with an algorithm based on column generation techniques. We have applied this combined demand forecasting /schedule optimization procedure to a multiplex in Amsterdam where we supported the scheduling of fourteen movie weeks. The proposed model not 2 only makes movie scheduling easier and less time consuming, but also generates schedules that would attract more visitors than the current ‘intuition-based’ schedules.Integer programming;Column generation;Demand forecasting;Optimization of movie schedules
Localized states influence spin transport in epitaxial graphene
We developed a spin transport model for a diffusive channel with coupled
localized states that result in an effective increase of spin precession
frequencies and a reduction of spin relaxation times in the system. We apply
this model to Hanle spin precession measurements obtained on monolayer
epitaxial graphene on SiC(0001) (MLEG). Combined with newly performed
measurements on quasi-free-standing monolayer epitaxial graphene on SiC(0001)
our analysis shows that the different values for the diffusion coefficient
measured in charge and spin transport measurements in MLEG and the high values
for the spin relaxation time can be explained by the influence of localized
states arising from the buffer layer at the interface between the graphene and
the SiC surface.Comment: 6 pages, 3 figures, including supplementary materia
Demand-driven scheduling of movies in a multiplex
This paper describes a model that generates weekly movie schedules in a multiplex movie theater. A movie schedule specifies within each day of the week, on which screen(s) different movies will be played, and at which time(s). The model consists of two parts: (i) conditional forecasts of the number of visitors per show for any possible starting time; and (ii) an optimization procedure that quickly finds an almost optimal schedule (which can be demonstrated to be close to the optimal schedule). To generate this schedule we formulate the so-called movie scheduling problem as a generalized set partitioning problem. The latter is solved with an algorithm based on column generation techniques. We have applied this combined demand forecasting /schedule optimization procedure to a multiplex in Amsterdam where we supported the scheduling of fourteen movie weeks. The proposed model not only makes movie scheduling easier and less time consuming, but also generates schedules that would attract more visitors than the current ‘intuition-based’ schedules.column generation;integer programming;demand forecasting;optimization of movie schedules
The inhibition of tissue respiration and alcoholic fermentation at different catabolic levels by ethyl carbamate (urethan) and arsenite
1. A hypothesis is given concerning the action of urethan and arsenite on malignant growth. Two assumptionsares made:-
(a) the enzyme system responsible for energy production in malignant tumours is working at maximal rate, contrary to the corresponding enzyme system in normal tissues.
(b) a given concentration of urethan or arsenite blocks an equal part of the sensitive enzyme(s), by which a measurable effect of a small concentration of these inhibitors on the catabolism will only be obtained in case of maximal turnover rate.
2. Experiments with kidney and liver minces and with yeast have shown that indeed the inhibition caused by urethan and arsenite increases with increasing rate of respiration or fermentation.
3. If the enzyme concentration is not considered to be negligible as compared to the substrate concentration, as is done in the theory of and , the increase of inhibition with increasing turnover number can be made plausible by enzyme kinetics for the much simplified case of one single enzyme.
4. The dependence of inhibition upon the turnover number of the catabolic enzyme system in tumours is discussed in relation to other possible causes of the effect of urethan and arsenite on malignant growth
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