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Selection-Based Learning: The Coevolution Of Internal And External Selection In High-Velocity Environments
To understand the effects of selection on firm-level learning, this study synthesizes two contrasting views of evolution. Internal selection theorists view managers in multiproduct firms as the primary agents of evolutionary change because they decide whether individual products and technologies are retained or eliminated. In contrast, external selection theorists contend that the environment drives evolution because it determines whether entire firms live or die. Though these theories differ, they describe tightly interwoven processes. In assessing the coevolution of internal and external selection among personal computer manufacturers across a 20-year period, we found that (1) firms learned cumulatively and adaptively from internal and partial external selection, the latter occurring when the environment killed part but not all of a firm; (2) internal and partial external selection coevolved, as each affected the other's future rate and the odds of firm failure; (3) partial external selection had a greater effect on future outcomes than internal selection; and (4) the lessons gleaned from prior selection were reflected in a firm's ability to develop new products, making that an important mediator between past and future selection events.Managemen
Conversion Efficiencies of Heteronuclear Feshbach Molecules
We study the conversion efficiency of heteronuclear Feshbach molecules in
population imbalanced atomic gases formed by ramping the magnetic field
adiabatically. We extend the recent work [J. E. Williams et al., New J. Phys.,
8, 150 (2006)] on the theory of Feshbach molecule formations to various
combinations of quantum statistics of each atomic component. A simple
calculation for a harmonically trapped ideal gas is in good agreement with the
recent experiment [S. B. Papp and C. E. Wieman, Phys. Rev. Lett., 97, 180404
(2006)] without any fitting parameters. We also give the conversion efficiency
as an explicit function of initial peak phase space density of the majority
species for population imbalanced gases. In the low-density region where
Bose-Einstein condensation does not appear, the conversion efficiency is a
monotonic function of the initial peak phase space density, but independent of
statistics of a minority component. The quantum statistics of majority atoms
has a significant effect on the conversion efficiency. In addition,
Bose-Einstein condensation of an atomic component is the key element
determining the maximum conversion efficiency.Comment: 46 pages, 32 figure
Pair extended coupled cluster doubles
The accurate and efficient description of strongly correlated systems remains
an important challenge for computational methods. Doubly occupied configuration
interaction (DOCI), in which all electrons are paired and no correlations which
break these pairs are permitted, can in many cases provide an accurate account
of strong correlations, albeit at combinatorial computational cost. Recently,
there has been significant interest in a method we refer to as pair coupled
cluster doubles (pCCD), a variant of coupled cluster doubles in which the
electrons are paired. This is simply because pCCD provides energies nearly
identical to those of DOCI, but at mean-field computational cost (disregarding
the cost of the two-electron integral transformation). Here, we introduce the
more complete pair extended coupled cluster doubles (pECCD) approach which,
like pCCD, has mean-field cost and reproduces DOCI energetically. We show that
unlike pCCD, pECCD also reproduces the DOCI wave function with high accuracy.
Moreoever, pECCD yields sensible albeit inexact results even for attractive
interactions where pCCD breaks down.Comment: submitted manuscrip
Contemporary practice education: exploring student perceptions of an industrial radiography placement for final year diagnostic radiography students.
Introduction: There is a paucity of evidence in diagnostic radiography evaluating a career path into industrial imaging despite several higher education institutes stating this route as a career option on graduation. The link between a career in industrial radiography and diagnostic routes is unknown although there are anecdotal examples of individuals transferring between the two. Successfully obtaining a first post job following graduation in diagnostic radiography can be challenging in the current financial climate. A partnership was formed with an energy sector company that offered non-destructive testing/non-destructive evaluation (NDT/NDE) employing industrial radiographic technicians. Method: As an initial pilot, 5 (. n = 5) final year diagnostic radiography students visited an industrial radiography site and underwent theoretical and practical training. Following this placement they engaged in a focus group and the student perceptions/responses were explored and recorded. Results: Common themes were identified and categorised via a thematic analysis. These were; radiation safety, physics and technology, widening access, graduate attributes/transferable skill sets and working conditions. Conclusion: Student discussion focussed around the benefits of working conditions in healthcare, the value of technology, safety and physics education in alternative placements and the transferability of skills into other/industrial sectors (e.g. NDT/NDE). Contemporary practice placements are a useful pedagogical approach to develop complex conceptual theoretical constructs, such as radiation physics. An in depth evaluation between the two industries skill sets is postulated. Additionally, this could offer alternative/emerging roles to interested diagnostic radiographers potentially meeting the skill shortage in industrial radiography
Negative Differential Resistance in the Scanning Tunneling Spectroscopy of Organic molecules
The conductance-voltage spectrum of molecular nanostructures measured by
scanning tunneling spectroscopy (STS) is generally assumed to reflect the local
density of states of the molecule. This excludes the possibility of observing
negative differential resistance (NDR). We report here the observation of NDR
in the scanning tunneling microscope (STM) current-voltage (I-V)
characteristics of self-assembled monolayer (SAM) of 4-p-Terphenylthiol
molecules on gold substrate measured using a platinum probe. We argue that the
NDR arises from narrow structures in the local density of states at the tip
apex atom and show that depending on the electrostatic potential profile across
the system, NDR could be observed in one or both bias directions.Comment: 13 Pages, 4 figures, submitted to Phys. Rev. B Rapid Communicatio
Synthesis of neutral nickel catalysts for ethylene polymerization – the influence of ligand size on catalyst stability
A facile synthesis of nickel salicylaldimine complexes with labile dissociating ligands is described. In addition to producing highly active ethylene polymerization catalysts, important insights into the effect of ligand size on catalyst stability and information on the mechanism of polymerization are provided
Comment on 'Algebraic perturbation theory for polar fluids: A model for the dielectric constant'
Kalikmanov [Phys. Rev. E 59, 4085 (1999)] proposed a perturbation theory method to calculate the dielectric constant of dipolar hard sphere fluids using an infinitely long cylindrical container to avoid the depolarization. We demonstrate that while the method is very helpful, his theory appears to be incomplete because of the incorrect calculation of the corresponding three-body integrals. It is shown that with the correct consideration of these terms the theory is consistent with the results of earlier work in low-density limit, and at high densities the method yields the equation of Tani et al. [Mol. Phys. 48, 863 (1983)] for the dipolar hard sphere fluid dielectric constant.published_or_final_versio
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