2,669 research outputs found
Conceptualizing Democracy as Preparation for Teaching for Democracy
In this essay, a broad spectrum of the work of influential educational scholars was examined in order to identify crucial components of teaching for democracy. Synthesizing the literature with their experiences as middle level teachers and teacher educators, the authors determined those conceptions that would be most fruitful for moving in-service teachers to enact the more “muscular” concepts that foster civic participation and social justice. This collaboration resulted in the identification of four democratic practices as a foundation for designing a course on teaching for democracy. These included amplification of the voices of historically marginalized people, recognition that those in power must work to meet the needs of those without power, recognition of the advantages of diversity even at the potential expense of efficiency, and collaboration in order to teach for democracy
A Call for Self-Study in Middle Level Teacher Education
To promote dialogue and in response to calls for rigorous, large-scale, empirical studies as the standard that will move the field of middle level education forward, a collaborative of middle level teacher researchers submit three counterpoints to the appeals for consideration by the research community: 1) the power of the insights the authors’ gained from using the alternative research method of self-study; 2) the authenticity of using alternative research methods that mirror the uniqueness of a field predicated on the distinctiveness of educating diverse young adolescents; and 3) a reframing of “generalizability” from a “results” perspective to one of generalizability of the process that self-study methodology offers
Early Thermal Evolution of Planetesimals and its Impact on Processing and Dating of Meteoritic Material
Radioisotopic ages for meteorites and their components provide constraints on
the evolution of small bodies: timescales of accretion, thermal and aqueous
metamorphism, differentiation, cooling and impact metamorphism. Realising that
the decay heat of short-lived nuclides (e.g. 26Al, 60Fe), was the main heat
source driving differentiation and metamorphism, thermal modeling of small
bodies is of utmost importance to set individual meteorite age data into the
general context of the thermal evolution of their parent bodies, and to derive
general conclusions about the nature of planetary building blocks in the early
solar system. As a general result, modelling easily explains that iron
meteorites are older than chondrites, as early formed planetesimals experienced
a higher concentration of short-lived nuclides and more severe heating.
However, core formation processes may also extend to 10 Ma after formation of
Calcium-Aluminum-rich inclusions (CAIs). A general effect of the porous nature
of the starting material is that relatively small bodies (< few km) will also
differentiate if they form within 2 Ma after CAIs. A particular interesting
feature to be explored is the possibility that some chondrites may derive from
the outer undifferentiated layers of asteroids that are differentiated in their
interiors. This could explain the presence of remnant magnetization in some
chondrites due to a planetary magnetic field.Comment: 24 pages, 9 figures, Accepted for publication as a chapter in
Protostars and Planets VI, University of Arizona Press (2014), eds. H.
Beuther, R. Klessen, C. Dullemond, Th. Hennin
Manipulation of the graphene surface potential by ion irradiation
We show that the work function of exfoliated single layer graphene can be
modified by irradiation with swift (E_{kin}=92 MeV) heavy ions under glancing
angles of incidence. Upon ion impact individual surface tracks are created in
graphene on SiC. Due to the very localized energy deposition characteristic for
ions in this energy range, the surface area which is structurally altered is
limited to ~ 0.01 mum^2 per track. Kelvin probe force microscopy reveals that
those surface tracks consist of electronically modified material and that a few
tracks suffice to shift the surface potential of the whole single layer flake
by ~ 400 meV. Thus, the irradiation turns the initially n-doped graphene into
p-doped graphene with a hole density of 8.5 x 10^{12} holes/cm^2. This doping
effect persists even after heating the irradiated samples to 500{\deg}C.
Therefore, this charge transfer is not due to adsorbates but must instead be
attributed to implanted atoms. The method presented here opens up a new way to
efficiently manipulate the charge carrier concentration of graphene.Comment: 6 pages, 4 figure
Systematic errors in Gaussian Quantum Monte Carlo and a systematic study of the symmetry projection method
Gaussian Quantum Monte Carlo (GQMC) is a stochastic phase space method for
fermions with positive weights. In the example of the Hubbard model close to
half filling it fails to reproduce all the symmetries of the ground state
leading to systematic errors at low temperatures. In a previous work [Phys.
Rev. B {\bf 72}, 224518 (2005)] we proposed to restore the broken symmetries by
projecting the density matrix obtained from the simulation onto the ground
state symmetry sector. For ground state properties, the accuracy of this method
depends on a {\it large overlap} between the GQMC and exact density matrices.
Thus, the method is not rigorously exact. We present the limits of the approach
by a systematic study of the method for 2 and 3 leg Hubbard ladders for
different fillings and on-site repulsion strengths. We show several indications
that the systematic errors stem from non-vanishing boundary terms in the
partial integration step in the derivation of the Fokker-Planck equation.
Checking for spiking trajectories and slow decaying probability distributions
provides an important test of the reliability of the results. Possible
solutions to avoid boundary terms are discussed. Furthermore we compare results
obtained from two different sampling methods: Reconfiguration of walkers and
the Metropolis algorithm.Comment: 11 pages, 14 figures, revised version, new titl
Systematic errors in Gaussian Quantum Monte Carlo and a systematic study of the symmetry projection method
Gaussian Quantum Monte Carlo (GQMC) is a stochastic phase space method for
fermions with positive weights. In the example of the Hubbard model close to
half filling it fails to reproduce all the symmetries of the ground state
leading to systematic errors at low temperatures. In a previous work [Phys.
Rev. B {\bf 72}, 224518 (2005)] we proposed to restore the broken symmetries by
projecting the density matrix obtained from the simulation onto the ground
state symmetry sector. For ground state properties, the accuracy of this method
depends on a {\it large overlap} between the GQMC and exact density matrices.
Thus, the method is not rigorously exact. We present the limits of the approach
by a systematic study of the method for 2 and 3 leg Hubbard ladders for
different fillings and on-site repulsion strengths. We show several indications
that the systematic errors stem from non-vanishing boundary terms in the
partial integration step in the derivation of the Fokker-Planck equation.
Checking for spiking trajectories and slow decaying probability distributions
provides an important test of the reliability of the results. Possible
solutions to avoid boundary terms are discussed. Furthermore we compare results
obtained from two different sampling methods: Reconfiguration of walkers and
the Metropolis algorithm.Comment: 11 pages, 14 figures, revised version, new titl
Vector chiral order in frustrated spin chains
By means of a numerical analysis using a non-Abelian symmetry realization of
the density matrix renormalization group, we study the behavior of vector
chirality correlations in isotropic frustrated chains of spin S=1 and S=1/2,
subject to a strong external magnetic field. It is shown that the field induces
a phase with spontaneously broken chiral symmetry, in line with earlier
theoretical predictions. We present results on the field dependence of the
order parameter and the critical exponents.Comment: 8 pages, 9 figure
Radioactive Probes of the Supernova-Contaminated Solar Nebula: Evidence that the Sun was Born in a Cluster
We construct a simple model for radioisotopic enrichment of the protosolar
nebula by injection from a nearby supernova, based on the inverse square law
for ejecta dispersion. We find that the presolar radioisotopes abundances
(i.e., in solar masses) demand a nearby supernova: its distance can be no
larger than 66 times the size of the protosolar nebula, at a 90% confidence
level, assuming 1 solar mass of protosolar material. The relevant size of the
nebula depends on its state of evolution at the time of radioactivity
injection. In one scenario, a collection of low-mass stars, including our sun,
formed in a group or cluster with an intermediate- to high-mass star that ended
its life as a supernova while our sun was still a protostar, a starless core,
or perhaps a diffuse cloud. Using recent observations of protostars to estimate
the size of the protosolar nebula constrains the distance of the supernova at
0.02 to 1.6 pc. The supernova distance limit is consistent with the scales of
low-mass stars formation around one or more massive stars, but it is closer
than expected were the sun formed in an isolated, solitary state. Consequently,
if any presolar radioactivities originated via supernova injection, we must
conclude that our sun was a member of such a group or cluster that has since
dispersed, and thus that solar system formation should be understood in this
context. In addition, we show that the timescale from explosion to the creation
of small bodies was on the order of 1.8 Myr (formal 90% confidence range of 0
to 2.2 Myr), and thus the temporal choreography from supernova ejecta to
meteorites is important. Finally, we can not distinguish between progenitor
masses from 15 to 25 solar masses in the nucleosynthesis models; however, the
20 solar mass model is somewhat preferred.Comment: ApJ accepted, 19 pages, 3 figure
Thermal history modeling of the H chondrite parent body
The cooling histories of individual meteorites can be empirically
reconstructed by using ages from different radioisotopic chronometers with
distinct closure temperatures. For a group of meteorites derived from a single
parent body such data permit the reconstruction of the cooling history and
properties of that body. Particularly suited are H chondrites because precise
radiometric ages over a wide range of closure temperatures are available. A
thermal evolution model for the H chondrite parent body is constructed by using
all H chondrites for which at least three different radiometric ages are
available. Several key parameters determining the thermal evolution of the H
chondrite parent body and the unknown burial depths of the H chondrites are
varied until an optimal fit is obtained. The fit is performed by an 'evolution
algorithm'. Empirical data for eight samples are used for which radiometric
ages are available for at least three different closure temperatures. A set of
parameters for the H chondrite parent body is found that yields excellent
agreement (within error bounds) between the thermal evolution model and
empirical data of six of the examined eight chondrites. The new thermal model
constrains the radius and formation time of the H chondrite parent body
(possibly (6) Hebe), the initial burial depths of the individual H chondrites,
the average surface temperature of the body, the average initial porosity of
the material the body accreted from, and the initial 60Fe content of the H
chondrite parent body.Comment: 16 pages, 7 figure
Graphene on Si(111)7x7
We demonstrate that it is possible to mechanically exfoliate graphene under
ultra high vacuum conditions on the atomically well defined surface of single
crystalline silicon. The flakes are several hundred nanometers in lateral size
and their optical contrast is very faint in agreement with calculated data.
Single layer graphene is investigated by Raman mapping. The G and 2D peaks are
shifted and narrowed compared to undoped graphene. With spatially resolved
Kelvin probe measurements we show that this is due to p-type doping with hole
densities of n_h \simeq 6x10^{12} cm^{-2}. The in vacuo preparation technique
presented here should open up new possibilities to influence the properties of
graphene by introducing adsorbates in a controlled way.Comment: 8 pages, 7 figure
- …