462 research outputs found
Running Water Cuts Deep: An Action Research Study of At-Risk Students in a Rural High School
As a high school teacher, I have formulated a standing comeback for the complaints of my students who whine, “That’s not fair!” My retort is the very cliche, “Life’s not fair”. I tell my students they might as well get used to things that are unfair because they simply cannot go through the rest of their lives whining about it. If life is indeed unfair, students who choose to drop out of school are reducing the chance that they will experience their fair portion of “The good life”. While it is true that the dropout rate in the United States has been steadily decreasing over the last several decades, the fact remains that a student who chooses to drop out of school has a rough road ahead. Why should these at-risk students make their lives any more difficult in such an unfair world? What follows is a description of action research conducted in an attempt to understand at-risk high school students. I am especially interested in the reasons they feel alienated from and sometimes drop out of high school, and what the school could do to lessen the likelihood of these students dropping out
Anomalous structure in the single particle spectrum of the fractional quantum Hall effect
The two-dimensional electron system (2DES) is a unique laboratory for the
physics of interacting particles. Application of a large magnetic field
produces massively degenerate quantum levels known as Landau levels. Within a
Landau level the kinetic energy of the electrons is suppressed, and
electron-electron interactions set the only energy scale. Coulomb interactions
break the degeneracy of the Landau levels and can cause the electrons to order
into complex ground states. In the high energy single particle spectrum of this
system, we observe salient and unexpected structure that extends across a wide
range of Landau level filling fractions. The structure appears only when the
2DES is cooled to very low temperature, indicating that it arises from delicate
ground state correlations. We characterize this structure by its evolution with
changing electron density and applied magnetic field. We present two possible
models for understanding these observations. Some of the energies of the
features agree qualitatively with what might be expected for composite
Fermions, which have proven effective for interpreting other experiments in
this regime. At the same time, a simple model with electrons localized on
ordered lattice sites also generates structure similar to those observed in the
experiment. Neither of these models alone is sufficient to explain the
observations across the entire range of densities measured. The discovery of
this unexpected prominent structure in the single particle spectrum of an
otherwise thoroughly studied system suggests that there exist core features of
the 2DES that have yet to be understood.Comment: 15 pages, 10 figure
High Resolution Spectroscopy of Two-Dimensional Electron Systems
Spectroscopic methods involving the sudden injection or ejection of electrons
in materials are a powerful probe of electronic structure and interactions.
These techniques, such as photoemission and tunneling, yield measurements of
the "single particle" density of states (SPDOS) spectrum of a system. The SPDOS
is proportional to the probability of successfully injecting or ejecting an
electron in these experiments. It is equal to the number of electronic states
in the system able to accept an injected electron as a function of its energy
and is among the most fundamental and directly calculable quantities in
theories of highly interacting systems. However, the two-dimensional electron
system (2DES), host to remarkable correlated electron states such as the
fractional quantum Hall effect, has proven difficult to probe
spectroscopically. Here we present an improved version of time domain
capacitance spectroscopy (TDCS) that now allows us to measure the SPDOS of a
2DES with unprecedented fidelity and resolution. Using TDCS, we perform
measurements of a cold 2DES, providing the first direct measurements of the
single-particle exchange-enhanced spin gap and single particle lifetimes in the
quantum Hall system, as well as the first observations of exchange splitting of
Landau levels not at the Fermi surface. The measurements reveal the difficult
to reach and beautiful structure present in this highly correlated system far
from the Fermi surface.Comment: There are formatting and minor textual differences between this
version and the published version in Nature (follow the DOI link below
HI, FRB, what's your z: The first FRB host galaxy redshift from radio observations
Identification and follow up observations of the host galaxies of fast radio
bursts (FRBs) not only help us understand the environments in which the FRB
progenitors reside, but also provide a unique way of probing the cosmological
parameters using the dispersion measures of FRBs and distances to their origin.
A fundamental requirement is an accurate distance measurement to the FRB host
galaxy, but for some sources viewed through the Galactic plane, optical/NIR
spectroscopic redshifts are extremely difficult to obtain due to dust
extinction. Here we report the first radio-based spectroscopic redshift
measurement for an FRB host galaxy, through detection of its neutral hydrogen
(HI) 21-cm emission using MeerKAT observations. We obtain an HI-based redshift
of z = 0.0357 for the host galaxy of FRB 20230718A, an apparently non-repeating
FRB detected in the CRAFT survey and localized at a Galactic latitude of -0.367
deg. Our observations also reveal that the FRB host galaxy is interacting with
a nearby companion, which is evident from the detection of an HI bridge
connecting the two galaxies. A subsequent optical spectroscopic observation
confirmed an FRB host galaxy redshift of 0.0359 +- 0.0004. This result
demonstrates the value of HI to obtain redshifts of FRBs at low Galactic
latitudes and redshifts. Such nearby FRBs whose dispersion measures are
dominated by the Milky Way can be used to characterise these components and
thus better calibrate the remaining cosmological contribution to dispersion for
more distant FRBs that provide a strong lever arm to examine the Macquart
relation between cosmological DM and redshift.Comment: 13 pages, 3 figures. Accepted to ApJ Letter
A valley-spin qubit in a carbon nanotube
Although electron spins in III-V semiconductor quantum dots have shown great
promise as qubits, a major challenge is the unavoidable hyperfine decoherence
in these materials. In group IV semiconductors, the dominant nuclear species
are spinless, allowing for qubit coherence times that have been extended up to
seconds in diamond and silicon. Carbon nanotubes are a particularly attractive
host material, because the spin-orbit interaction with the valley degree of
freedom allows for electrical manipulation of the qubit. In this work, we
realise such a qubit in a nanotube double quantum dot. The qubit is encoded in
two valley-spin states, with coherent manipulation via electrically driven spin
resonance (EDSR) mediated by a bend in the nanotube. Readout is performed by
measuring the current in Pauli blockade. Arbitrary qubit rotations are
demonstrated, and the coherence time is measured via Hahn echo. Although the
measured decoherence time is only 65 ns in our current device, this work offers
the possibility of creating a qubit for which hyperfine interaction can be
virtually eliminated
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