2,439 research outputs found
Tight-Binding model for semiconductor nanostructures
An empirical tight-binding (TB) model is applied to the
investigation of electronic states in semiconductor quantum dots. A basis set
of three -orbitals at the anions and one -orbital at the cations is
chosen. Matrix elements up to the second nearest neighbors and the spin-orbit
coupling are included in our TB-model. The parametrization is chosen so that
the effective masses, the spin-orbit-splitting and the gap energy of the bulk
CdSe and ZnSe are reproduced. Within this reduced TB-basis the
valence (p-) bands are excellently reproduced and the conduction (s-) band is
well reproduced close to the -point, i.e. near to the band gap. In
terms of this model much larger systems can be described than within a (more
realistic) -basis. The quantum dot is modelled by using the (bulk)
TB-parameters for the particular material at those sites occupied by atoms of
this material. Within this TB-model we study pyramidal-shaped CdSe quantum dots
embedded in a ZnSe matrix and free spherical CdSe quantum dots (nanocrystals).
Strain-effects are included by using an appropriate model strain field. Within
the TB-model, the strain-effects can be artifically switched off to investigate
the infuence of strain on the bound electronic states and, in particular, their
spatial orientation. The theoretical results for spherical nanocrystals are
compared with data from tunneling spectroscopy and optical experiments.
Furthermore the influence of the spin-orbit coupling is investigated
The Timing of Daily Demand for Goods and Services - Multivariate Probit Estimates and Microsimulation Results for an Aged Population with German Time Use Diary Data
The Timing of Daily Demand for Goods and Services – Multivariate Probit Estimates and Microsimulation Results for an Aged Population with German Time Use Diary Data
Though consumption research provides a broad spectrum of theoretical and empirical founded results, studies based on a daily focus are missing. Knowledge about the individual timing of daily demand for goods and services, opens – beyond a genuine contribution to consumption research – interesting societal and macro economic as well as individual personal and firm perspectives: it is important for an efficient timely coordination of supply and demand in the timing perspective as well as for a targeted economic, social and societal policy for a better support of the every day coordination of life. Last not least, the individual daily public and private living situations will be visible, which are of particular importance for the social togetherness in family and society. Our study contributes to the timing of daily consumption for goods and services with an empirical founded microanalysis on the basis of more than 37.000 individual time use diaries of the nationwide Time Budget Survey of the German Federal Statistical Office 2001/02. We describe the individual timing of daily demand for goods and services for important socio-demographic groups like for women and men, the economic situation with income poverty and daily working hour arrangements. The multivariate microeconometric explanation of the daily demand for goods and services is based on a latent utility maximizing approach over a day. We estimate an eight equation Multivariate/Simultaneous Probit Model, which allows the decision for multiple consumption activities in more than one time period a day. The estimates quantify effects on the timing of daily demand by individual socio-economic variables, which encompasses, personal, household, regional characteristics as well as daily working hour arrangements within a flexible labour market.
The question about individual effects of an aged society on the timing of daily demand for goods and services is analyzed with our microsimulation model ServSim and a population forecast for 2020 by the German Federal Statistical Office. Main result: There are significant differences in explaining the timing of daily demand for goods compared to services on the one hand and in particular for different daily time periods.
The conclusion: without the timing aspects an important and significant dimension for understanding individual consumption behaviour and their impacts on other individual living conditions would be missing
Perturbation strength and the global structure of qap fitness landscapes
We study the effect of increasing the perturbation strength on the global structure of QAP fitness landscapes induced by Iterated Local Search (ILS). The global structure is captured with Local Optima Networks. Our analysis concentrates on the number, characteristics and distribution of funnels in the landscape, and how they change with increasing perturbation strengths. Well-known QAP instance types are considered. Our results confirm the multi-funnel structure of QAP fitness landscapes and clearly explain, visually and quantitatively, why ILS with large perturbation strengths produces better results. Moreover, we found striking differences between randomly generated and real-world instances, which warns about using synthetic benchmarks for (manual or automatic) algorithm design and tuning
The parallel tales of microvascular angina and heart failure with preserved ejection fraction: a paradigm shift.
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Non-Gaussian errors of baryonic acoustic oscillations
We revisit the uncertainty in baryon acoustic oscillation (BAO) forecasts and
data analyses. In particular, we study how much the uncertainties on both the
measured mean dilation scale and the associated error bar are affected by the
non-Gaussianity of the non-linear density field. We examine two possible
impacts of non-Gaussian analysis: (1) we derive the distance estimators from
Gaussian theory, but use 1000 N-Body simulations to measure the actual errors,
and compare this to the Gaussian prediction, and (2) we compute new optimal
estimators, which requires the inverse of the non-Gaussian covariance matrix of
the matter power spectrum. Obtaining an accurate and precise inversion is
challenging, and we opted for a noise reduction technique applied on the
covariance matrices. By measuring the bootstrap error on the inverted matrix,
this work quantifies for the first time the significance of the non-Gaussian
error corrections on the BAO dilation scale. We find that the variance (error
squared) on distance measurements can deviate by up to 12% between both
estimators, an effect that requires a large number of simulations to be
resolved. We next apply a reconstruction algorithm to recover some of the BAO
signal that had been smeared by non-linear evolution, and we rerun the
analysis. We find that after reconstruction, the rms error on the distance
measurement improves by a factor of ~1.7 at low redshift (consistent with
previous results), and the variance ({\sigma}^2) shows a change of up to 18%
between optimal and sub-optimal cases (note, however, that these discrepancies
may depend in detail on the procedure used to isolate the BAO signal). We
finally discuss the impact of this work on current data analyses.Comment: 13 pages, 11 figures, MNRAS accepte
Start-up success of freelancers New microeconometric evidence from the German Socio-Economic Panel
If certain start-up characteristics will indicate a business success, knowing such characteristics
could generate more successful start-ups and more efficient start-up counseling. Our study
will contribut e to this by quantifying individual success determinants of freelance start-ups.
The data base for the microeconometric analyses of the survival of the first three years is a
revised German Socio-Economic Panel (SOEP) for 1992 until 2002, which allows to
incorporate institutional, personal and family/household socio-economic variables. We
describe and discuss the datawork to achieve compatible information over time within a
revised GSOEP and present microeconometric rare events logit, logit and probit results.
The start-up success measured as the probability to survive the first three years is first of all
influenced by an active labour force participation with its acquired skills and working
experiences just before the start-up period (rank 1), followed by a non-university degree as
the highest general human capital indicator (rank 2), a general (non-linear) experience
indicated by age (rank 3) and the business related background (rank 4) as the type of liberal
profession in the group of the liberal medical professions and the liberal technical and
scientific professions
Floods and climate: emerging perspectives for flood risk assessment and management
Flood estimation and flood management have traditionally been the domain of
hydrologists, water resources engineers and statisticians, and disciplinary
approaches abound. Dominant views have been shaped; one example is the
catchment perspective: floods are formed and influenced by the interaction
of local, catchment-specific characteristics, such as meteorology,
topography and geology. These traditional views have been beneficial, but
they have a narrow framing. In this paper we contrast traditional views with
broader perspectives that are emerging from an improved understanding of the
climatic context of floods. We come to the following conclusions: (1) extending the traditional
system boundaries (local catchment, recent decades, hydrological/hydraulic
processes) opens up exciting possibilities for better understanding and
improved tools for flood risk assessment and management. (2) Statistical
approaches in flood estimation need to be complemented by the search for the
causal mechanisms and dominant processes in the atmosphere, catchment and
river system that leave their fingerprints on flood characteristics.
(3) Natural climate variability leads to time-varying flood characteristics, and
this variation may be partially quantifiable and predictable, with the
perspective of dynamic, climate-informed flood risk management.
(4) Efforts are needed to fully account for factors that contribute to changes
in all three risk components (hazard, exposure, vulnerability) and to
better understand the interactions between society and floods. (5) Given the
global scale and societal importance, we call for the organization of an
international multidisciplinary collaboration and data-sharing initiative to
further understand the links between climate and flooding and to advance
flood research
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