1,943 research outputs found
The Effect of Providing Breakfast on Student Performance: Evidence from an In-Class Breakfast Program
In response to low take-up, many public schools have experimented with moving breakfast from the cafeteria to the classroom. We examine whether such a program increases performance as measured by standardized test scores, grades and attendance rates. We exploit quasi-random timing of program implementation that allows for a difference-in-differences identification strategy. Our main identification assumption is that schools where the program was introduced earlier would have evolved similarly to those where the program was introduced later. We find that in-class breakfast increases both math and reading achievement by about one-tenth of a standard deviation relative to providing breakfast in the cafeteria. Moreover, we find that these effects are most pronounced for low performing, free-lunch eligible, Hispanic, and low BMI students. We also find some improvements in attendance for high achieving students but no impact on grades.
Negative frequency tuning of a carbon nanotube nano-electromechanical resonator
A suspended, doubly clamped single wall carbon nanotube is characterized as
driven nano-electromechanical resonator at cryogenic temperatures.
Electronically, the carbon nanotube displays small bandgap behaviour with
Coulomb blockade oscillations in electron conduction and transparent contacts
in hole conduction. We observe the driven mechanical resonance in dc-transport,
including multiple higher harmonic responses. The data shows a distinct
negative frequency tuning at finite applied gate voltage, enabling us to
electrostatically decrease the resonance frequency to 75% of its maximum value.
This is consistently explained via electrostatic softening of the mechanical
mode.Comment: 4 pages, 4 figures; submitted for the IWEPNM 2013 conference
proceeding
Baryons and baryonic matter in the large Nc and heavy quark limits
This paper explores properties of baryons and finite density baryonic matter
in an artificial world in which Nc, the number of colors, is large and the
quarks of all species are degenerate and much larger than {\Lambda}_QCD. It has
long been known that in large Nc QCD, baryons composed entirely of heavy quarks
are accurately described in the mean-field approximation. However, the detailed
properties of baryons in the combined large Nc and heavy quark limits have not
been fully explored. Here some basic properties of baryons are computed using a
variational approach. At leading order in both the large Nc and heavy quark
expansions the baryon mass is computed explicitly as is the baryon form factor.
Baryonic matter, the analog of nuclear matter in this artificial world, should
also be well described in the mean-field approximation. In the special case
where all baryons have an identical spin flavor structure, it is shown that in
the formal heavy quark and large Nc limit interactions between baryons are
strictly repulsive at low densities. The energy per baryon is computed in this
limit and found to be exponentially small. It is shown that when the
restriction to baryons with an identical spin-flavor structure is dropped, a
phase of baryonic matter exists with a density of 2Nf times that for the
restricted case but with the same energy (where Nf is the number of degenerate
flavors). It is shown that this phase is at least metastable.Comment: 19 page
Exclusive channels in semi-inclusive production of pions and kaons
We investigate the role of exclusive channels in semi-inclusive
electroproduction of pions and kaons. Using the QCD factorization theorem for
hard exclusive processes we evaluate the cross sections for exclusive
pseudoscalar and vector meson production in terms of generalized parton
distributions and meson distribution amplitudes. We investigate the
uncertainties arising from the modeling of the nonperturbative input
quantities. Combining these results with available experimental data, we
compare the cross sections for exclusive channels to that obtained from quark
fragmentation in semi-inclusive deep inelastic scattering. We find that rho^0
production is the only exclusive channel with significant contributions to
semi-inclusive pion production at large z and moderate Q^2. The corresponding
contribution to kaon production from the decay of exclusively produced phi and
K^* is rather small.Comment: 33 pages, 18 figure
Nanometer Scale Mapping of the Density of States in an Inhomogeneous Superconductor
Using high speed scanning tunneling spectroscopy, we perform a full mapping
of the quasiparticle density of states (DOS) in single crystals of
BiPbSrCaCuO(2212). The measurements carried out at 5 K showed a complex spatial
pattern of important variations of the local DOS on the nanometer scale.
Superconducting areas are co-existing with regions of a smooth and larger
gap-like DOS structure. The superconducting regions are found to have a minimum
size of about 3 nm. The role of Pb-introduced substitutional disorder in the
observed spatial variations of the local DOS is discussed.Comment: 4 page Letter with 3 figures (2 color figures
In-plane optical spectral weight transfer in optimally doped BiSrCaCuO
We examine the redistribution of the in-plane optical spectral weight in the
normal and superconducting state in tri-layer \bbb (Bi2223) near optimal doping
( = 110 K) on a single crystal via infrared reflectivity and spectroscopic
ellipsometry. We report the temperature dependence of the low-frequency
integrated spectral weight for different values of the cutoff
energy . Two different model-independent analyses consistently show
that for = 1 eV, which is below the charge transfer gap,
increases below , implying the lowering of the kinetic
energy of the holes. This is opposite to the BCS scenario, but it follows the
same trend observed in the bi-layer compound \bb (Bi2212). The size of this
effect is larger in Bi2223 than in Bi2212, approximately scaling with the
critical temperature. In the normal state, the temperature dependence of
is close to up to 300 K
Deformed Skyrme Crystals
The Skyrme crystal, a solution of the Skyrme model, is the lowest
energy-per-charge configuration of skyrmions seen so far. Our numerical
investigations show that, as the period in various space directions is changed,
one obtains various other configurations, such as a double square wall, and
parallel vortex-like solutions. We also show that there is a sudden "phase
transition" between a Skyrme crystal and the charge 4 skyrmion with cubic
symmetry as the period is gradually increased in all three space directions.Comment: 13 pages, 6 figures. To be published in JHE
Engineering ultralong spin coherence in two-dimensional hole systems at low temperatures
For the realisation of scalable solid-state quantum-bit systems, spins in
semiconductor quantum dots are promising candidates. A key requirement for
quantum logic operations is a sufficiently long coherence time of the spin
system. Recently, hole spins in III-V-based quantum dots were discussed as
alternatives to electron spins, since the hole spin, in contrast to the
electron spin, is not affected by contact hyperfine interaction with the
nuclear spins. Here, we report a breakthrough in the spin coherence times of
hole ensembles, confined in so called natural quantum dots, in narrow
GaAs/AlGaAs quantum wells at temperatures below 500 mK. Consistently,
time-resolved Faraday rotation and resonant spin amplification techniques
deliver hole-spin coherence times, which approach in the low magnetic field
limit values above 70 ns. The optical initialisation of the hole spin
polarisation, as well as the interconnected electron and hole spin dynamics in
our samples are well reproduced using a rate equation model.Comment: 16 pages, 6 figure
A scalable halftoning coprocessor architecture
Exact-angle superscreen dithering requires large dither tiles. Since storing precomputed screen elements for each intensity level would require too much memory, dithering must be executed on the fly at halftoning time. For this purpose a dithering coprocessor is presented which generates halftoned images at high speed. The proposed hardware architecture is based on a pipelined and scalable design which speeds up halftoning by a factor of twenty compared with modern RISC software-based solutions. We describe the architecture of the coprocessor and show to what extent it can be scaled for improving performances. The proposed coprocessor could find applications in digital color copiers which need to print scanned color images at high spee
Gate control of low-temperature spin dynamics in two-dimensional hole systems
We have investigated spin and carrier dynamics of resident holes in
high-mobility two-dimensional hole systems in GaAs/AlGaAs
single quantum wells at temperatures down to 400 mK. Time-resolved Faraday and
Kerr rotation, as well as time-resolved photoluminescence spectroscopy are
utilized in our study. We observe long-lived hole spin dynamics that are
strongly temperature dependent, indicating that in-plane localization is
crucial for hole spin coherence. By applying a gate voltage, we are able to
tune the observed hole g factor by more than 50 percent. Calculations of the
hole g tensor as a function of the applied bias show excellent agreement with
our experimental findings.Comment: 8 pages, 7 figure
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