23 research outputs found
Electronic Cooling via Interlayer Coulomb Coupling in Multilayer Epitaxial Graphene
In van der Waals bonded or rotationally disordered multilayer stacks of
two-dimensional (2D) materials, the electronic states remain tightly confined
within individual 2D layers. As a result, electron-phonon interactions occur
primarily within layers and interlayer electrical conductivities are low. In
addition, strong covalent in-plane intralayer bonding combined with weak van
der Waals interlayer bonding results in weak phonon-mediated thermal coupling
between the layers. We demonstrate here, however, that Coulomb interactions
between electrons in different layers of multilayer epitaxial graphene provide
an important mechanism for interlayer thermal transport even though all
electronic states are strongly confined within individual 2D layers. This
effect is manifested in the relaxation dynamics of hot carriers in ultrafast
time-resolved terahertz spectroscopy. We develop a theory of interlayer Coulomb
coupling containing no free parameters that accounts for the experimentally
observed trends in hot-carrier dynamics as temperature and the number of layers
is varied.Comment: 54 pages, 15 figures, uses documentclass{achemso}, M.T.M. and J.R.T.
contributed equally to this wor
One- and two-photon ionization cross sections of the laser excited 6s6p^1P_1 state of barium
Stimulated by a recent measurement of coherent control in photoionization of
atomic barium, we have calculated one- and two-photon ionization cross sections
of the aligned 6s6p^1P_1 state of barium in the energy range between the
5d_{3/2} and 5d_{5/2} states of Ba^+. We have also measured these
photionization spectra in the same energy region, driving the one- or
two-photon processes with the second or first harmonic of a tunable dye laser,
respectively. Our calculations employ the eigenchannel R-matrix method and
multichannel quantum defect theory to calculate the rich array of autoionizing
resonances in this energy range. The non-resonant two-photon process is
described using lowest-order perturbation theory for the photon-atom
interactions, with a discretized intermediate state one-electron continuum. The
calculations provide an absolute normalization for the experiment, and they
accurately reproduce the rich resonance structures in both the one and
two-photon cross sections, and confirm other aspects of experimental
observations. These results demonstrate the ability of these computationally
inexpensive methods to reproduce the experimental observables in one- and
two-photon ionization of heavy alkaline earths, and they lay the groundwork for
future studies of the phase-controlled interference between one-photon and
two-photon ionization processes.Comment: 10 pages, 9 figures, submitted to Phys.Rev.
Models of classroom assessment for course-based research experiences
Course-based research pedagogy involves positioning students as contributors to authentic research projects as part of an engaging educational experience that promotes their learning and persistence in science. To develop a model for assessing and grading students engaged in this type of learning experience, the assessment aims and practices of a community of experienced course-based research instructors were collected and analyzed. This approach defines four aims of course-based research assessment—(1) Assessing Laboratory Work and Scientific Thinking; (2) Evaluating Mastery of Concepts, Quantitative Thinking and Skills; (3) Appraising Forms of Scientific Communication; and (4) Metacognition of Learning—along with a set of practices for each aim. These aims and practices of assessment were then integrated with previously developed models of course-based research instruction to reveal an assessment program in which instructors provide extensive feedback to support productive student engagement in research while grading those aspects of research that are necessary for the student to succeed. Assessment conducted in this way delicately balances the need to facilitate students’ ongoing research with the requirement of a final grade without undercutting the important aims of a CRE education
Dispersion forces between weakly disordered Van der Waals crystals
We describe a many-body theory for interlayer dispersion forces between
weakly disordered atomically thin crystals and numerically investigate the role
of disorder for different layer-separation distances and for different
densities of induced electrons and holes. In contrast to the common wisdom that
disorder tends to enhance the importance of Coulomb interactions in Fermi
liquids, we find that short range disorder tends to {\it weaken} interlayer
dispersion forces. This is in line with previous findings that suggest that
transitioning from metallic to insulating propagation weakens interlayer
dispersion forces. We demonstrate that disorder alters the scaling laws of
dispersion forces and we comment on the role of the maximally crossed
vertex-correction diagrams responsible for logarithmic divergences in the
resistivity of two-dimensional metals.Comment: 10 pages, 8 figures. Added references and revised interpretation of
result
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Perceptions as the crucial link? The mediating role of neighborhood perceptions in the relationship between the neighborhood context and neighborhood cohesion.
This study examines the effects of neighborhood racial in-group size, economic deprivation and the prevalence of crime on neighborhood cohesion among U.S. whites. We explore to what extent residents' perceptions of their neighborhood mediate these macro-micro relationships. We use a recent individual-level data set, the American Social Fabric Study (2012/2013), enriched with contextual-level data from the U.S. Census Bureau (2010) and employ multi-level structural equation models. We show that the racial in-group size is positively related to neighborhood cohesion and that neighborhood cohesion is lower in communities with a high crime rate. Individuals' perceptions of the racial in-group size partly mediate the relationship between the objective racial in-group size and neighborhood cohesion. Residents' perceptions of unsafety from crime also appear to be a mediating factor, not only for the objective crime rate but also for the objective racial in-group size. This is in line with our idea that racial stereotypes link racial minorities to crime whereby neighborhoods with a large non-white population are perceived to be more unsafe. Residents of the same neighborhood differ in how they perceive the degree of economic decay of the neighborhood and this causes them to evaluate neighborhood cohesion differently, however perceptions of neighborhood economic decay do not explain the link between the objective neighborhood context and neighborhood cohesion
Perceptions as the crucial link? The mediating role of neighborhood perceptions in the relationship between the neighborhood context and neighborhood cohesion.
This study examines the effects of neighborhood racial in-group size, economic deprivation and the prevalence of crime on neighborhood cohesion among U.S. whites. We explore to what extent residents' perceptions of their neighborhood mediate these macro-micro relationships. We use a recent individual-level data set, the American Social Fabric Study (2012/2013), enriched with contextual-level data from the U.S. Census Bureau (2010) and employ multi-level structural equation models. We show that the racial in-group size is positively related to neighborhood cohesion and that neighborhood cohesion is lower in communities with a high crime rate. Individuals' perceptions of the racial in-group size partly mediate the relationship between the objective racial in-group size and neighborhood cohesion. Residents' perceptions of unsafety from crime also appear to be a mediating factor, not only for the objective crime rate but also for the objective racial in-group size. This is in line with our idea that racial stereotypes link racial minorities to crime whereby neighborhoods with a large non-white population are perceived to be more unsafe. Residents of the same neighborhood differ in how they perceive the degree of economic decay of the neighborhood and this causes them to evaluate neighborhood cohesion differently, however perceptions of neighborhood economic decay do not explain the link between the objective neighborhood context and neighborhood cohesion
Establishment of the Box-Ironbark ecological thinning trial in north central Victoria
An ecological thinning trial was established in 2003 in north-central Victoria as part of the development of an ecological management strategy to support the newly created Box-Ironbark Parks and Reserves System. The objective of the trial was to restore diversity of habitat structure to declining Box-Ironbark forests and woodlands. Three ecological thinning techniques were designed around several principles: reducing total basal-area of trees and retaining levels of patchiness whilst retaining large trees. Thinning treatments were implemented in 30 ha plots at four conservation reserves south of Bendigo, Victoria. A range of ecosystem components were monitored before and after thinning. A woody-debris removal treatment was also set-up at a 1 ha scale within thinning treatments. Prior to thinning, plots were dominated by high numbers of coppice regenerated trees with few of the trees sampled considered large, resulting in low numbers of tree hollows and low loadings of coarse woody debris. It is anticipated that the establishment of the ecological thinning trial (Phase I), is the beginning of long-term monitoring, as effects of thinning on key habitat values may not be apparent for up to 50 years or more. The vision for restoration of Box-Ironbark forests and woodlands is one of a mosaic landscape with a greater diversity of habitat types including open areas and greater numbers of larger, hollow-bearing trees. This paper summarises the experimental design and the techniques adopted in Phase I of this project during 2003-2008