23,244 research outputs found
Critical comparison of electrode models in density functional theory based quantum transport calculations
We study the performance of two different electrode models in quantum
transport calculations based on density functional theory: Parametrized Bethe
lattices and quasi-one dimensional wires or nanowires. A detailed account of
implementation details in both cases is given. From the systematic study of
nanocontacts made of representative metallic elements, we can conclude that
parametrized electrode models represent an excellent compromise between
computational cost and electronic structure definition as long as the aim is to
compare with experiments where the precise atomic structure of the electrodes
is not relevant or defined with precision. The results obtained using
parametrized Bethe lattices are essentially similar to the ones obtained with
quasi one dimensional electrodes for large enough sections of these, adding a
natural smearing to the transmission curves that mimics the true nature of
polycrystalline electrodes. The latter are more demanding from the
computational point of view, but present the advantage of expanding the range
of applicability of transport calculations to situations where the electrodes
have a well-defined atomic structure, as is case for carbon nanotubes, graphene
nanoribbons or semiconducting nanowires. All the analysis is done with the help
of codes developed by the authors which can be found in the quantum transport
toolbox Alacant and are publicly available.Comment: 17 pages, 12 figure
Lab notebooks as scientific communication: investigating development from undergraduate courses to graduate research
In experimental physics, lab notebooks play an essential role in the research
process. For all of the ubiquity of lab notebooks, little formal attention has
been paid to addressing what is considered `best practice' for scientific
documentation and how researchers come to learn these practices in experimental
physics. Using interviews with practicing researchers, namely physics graduate
students, we explore the different experiences researchers had in learning how
to effectively use a notebook for scientific documentation. We find that very
few of those interviewed thought that their undergraduate lab classes
successfully taught them the benefit of maintaining a lab notebook. Most
described training in lab notebook use as either ineffective or outright
missing from their undergraduate lab course experience. Furthermore, a large
majority of those interviewed explained that they did not receive any formal
training in maintaining a lab notebook during their graduate school experience
and received little to no feedback from their advisors on these records. Many
of the interviewees describe learning the purpose of, and how to maintain,
these kinds of lab records only after having a period of trial and error,
having already started doing research in their graduate program. Despite the
central role of scientific documentation in the research enterprise, these
physics graduate students did not gain skills in documentation through formal
instruction, but rather through informal hands-on practice.Comment: 10 page
Instructor perspectives on iteration during upper-division optics lab activities
Although developing proficiency with modeling is a nationally endorsed
learning outcome for upper-division undergraduate physics lab courses, no
corresponding research-based assessments exist. Our longterm goal is to develop
assessments of students' modeling ability that are relevant across multiple
upper-division lab contexts. To this end, we interviewed 19 instructors from 16
institutions about optics lab activities that incorporate photodiodes.
Interviews focused on how those activities were designed to engage students in
some aspects of modeling. We find that, according to many interviewees,
iteration is an important aspect of modeling. In addition, interviewees
described four distinct types of iteration: revising apparatuses, revising
models, revising data-taking procedures, and repeating data collection using
existing apparatuses and procedures. We provide examples of each type of
iteration, and discuss implications for the development of future modeling
assessments.Comment: 4 pages, 1 figure; under revie
Student ownership of projects in an upper-division optics laboratory course: A multiple case study of successful experiences
We investigate students' sense of ownership of multiweek final projects in an
upper-division optics lab course. Using a multiple case study approach, we
describe three student projects in detail. Within-case analyses focused on
identifying key issues in each project, and constructing chronological
descriptions of those events. Cross-case analysis focused on identifying
emergent themes with respect to five dimensions of project ownership: student
agency, instructor mentorship, peer collaboration, interest and value, and
affective responses. Our within- and cross-case analyses yielded three major
findings. First, coupling division of labor with collective brainstorming can
help balance student agency, instructor mentorship, and peer collaboration.
Second, students' interest in the project and perceptions of its value can
increase over time; initial student interest in the project topic is not a
necessary condition for student ownership of the project. Third, student
ownership is characterized by a wide range of emotions that fluctuate as
students alternate between extended periods of struggle and moments of success
while working on their projects. These findings not only extend the literature
on student ownership into a new educational domain---namely, upper-division
physics labs---they also have concrete implications for the design of
experimental physics projects in courses for which student ownership is a
desired learning outcome. We describe the course and projects in sufficient
detail that others can adapt our results to their particular contexts.Comment: 22 pages, 3 tables, submitted to Phys. Rev. PE
Evaluating the impact of active labor programs : results of cross country studies in Europe and Central Asia
The objective of this cross country study was to determine if there was any significant difference those individuals who participated in active labor programs (ALP) and similar individuals who did not participate in the programs (the comparison group), with regard to agreed outcome measures of program success (e.g., employment, wage levels) in four countries: the Czech Republic, Poland, Hungary, and Turkey. The study evaluated five different ALPs across the four countries for several categories of program participants, grouped by demographic characteristics and geographic location. The active labor programs studies comprised: retraining, public works or temporary community employment, wage subsidy, self-employment initiatives, and general employment services. All five ALPs evaluated were found to have a significant positive net impact for some population subgroups, a general finding supported by the 1996 OECD review that concluded that there are ALPs that work for most groups of individuals. However, the study found the impact was not significant for some sub-groups, and for some ALPs the impact was negative. The final chapter summarizesthe general trends in findings on the various outcome measures across demographic and regional subgroups.Environmental Economics&Policies,Poverty Impact Evaluation,ICT Policy and Strategies,Health Monitoring&Evaluation,Poverty Monitoring&Analysis
The density of critical percolation clusters touching the boundaries of strips and squares
We consider the density of two-dimensional critical percolation clusters,
constrained to touch one or both boundaries, in infinite strips, half-infinite
strips, and squares, as well as several related quantities for the infinite
strip. Our theoretical results follow from conformal field theory, and are
compared with high-precision numerical simulation. For example, we show that
the density of clusters touching both boundaries of an infinite strip of unit
width (i.e. crossing clusters) is proportional to .
We also determine numerically contours for the density of clusters crossing
squares and long rectangles with open boundaries on the sides, and compare with
theory for the density along an edge.Comment: 11 pages, 6 figures. Minor revision
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