851 research outputs found
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Addressing School Dropout in Texas: A Summary for Administrators and Policymakers of "Dropout Prevention: A Practice Guide"
This booklet summarizes the information from: Dropout Prevention: A Practice Guide, available on the Institute of Education Sciences web site at http://ies.ed.gov/ncee/wwc.Educatio
Measurement of Magnetization Dynamics in Single-Molecule Magnets Induced by Pulsed Millimeter-Wave Radiation
We describe an experiment aimed at measuring the spin dynamics of the Fe8
single-molecule magnet in the presence of pulsed microwave radiation. In
earlier work, heating was observed after a 0.2-ms pulse of intense radiation,
indicating that the spin system and the lattice were out of thermal equilibrium
at millisecond time scale [Bal et al., Europhys. Lett. 71, 110 (2005)]. In the
current work, an inductive pick-up loop is used to probe the photon-induced
magnetization dynamics between only two levels of the spin system at much
shorter time scales (from ns to us). The relaxation time for the magnetization,
induced by a pulse of radiation, is found to be on the order of 10 us.Comment: 3 RevTeX pages, including 3 eps figures. The paper will appear in the
Journal of Applied Physics as MMM'05 conference proceeding
Non-equilibrium Magnetization Dynamics in the Fe_8 Single-Molecule Magnet Induced by High-Intensity Microwave Radiation
Resonant microwave radiation applied to a single crystal of the molecular
magnet Fe_8 induces dramatic changes in the sample's magnetization. Transitions
between excited states are found even though at the nominal system temperature
these levels have negligible population. We find evidence that the sample heats
significantly when the resonance condition is met. In addition, heating is
observed after a short pulse of intense radiation has been turned off,
indicating that the spin system is out of equilibrium with the lattice.Comment: Version to appear in Europhysics Letters. Minor changes and updated
reference
Experimental Upper Bound on Superradiance Emission from Mn12 Acetate
We used a Josephson junction as a radiation detector to look for evidence of
the emission of electromagnetic radiation during magnetization avalanches in a
crystal assembly of Mn_12-Acetate. The crystal assembly exhibits avalanches at
several magnetic fields in the temperature range from 1.8 to 2.6 K with
durations of the order of 1 ms. Although a recent study shows evidence of
electromagnetic radiation bursts during these avalanches [J. Tejada, et al.,
Appl. Phys. Lett. {\bf 84}, 2373 (2004)], we were unable to detect any
significant radiation at well-defined frequencies. A control experiment with
external radiation pulses allows us to determine that the energy released as
radiation during an avalanche is less than 1 part in 10^4 of the total energy
released. In addition, our avalanche data indicates that the magnetization
reversal process does not occur uniformly throughout the sample.Comment: 4 RevTeX pages, 3 eps figure
Radiation- and Phonon-Bottleneck-Induced Tunneling in the Fe8 Single-Molecule Magnet
We measure magnetization changes in a single crystal of the single-molecule
magnet Fe8 when exposed to intense, short (<20 s) pulses of microwave
radiation resonant with the m = 10 to 9 transition. We find that radiation
induces a phonon bottleneck in the system with a time scale of ~5 s. The
phonon bottleneck, in turn, drives the spin dynamics, allowing observation of
thermally assisted resonant tunneling between spin states at the 100-ns time
scale. Detailed numerical simulations quantitatively reproduce the data and
yield a spin-phonon relaxation time of T1 ~ 40 ns.Comment: 6 RevTeX pages, including 4 EPS figures, version accepted for
publicatio
Abrupt Transition between Thermally-Activated Relaxation and Quantum Tunneling in a Molecular Magnet
We report Hall sensor measurements of the magnetic relaxation of Mn
acetate as a function of magnetic field applied along the easy axis of
magnetization. Data taken at a series of closely-spaced temperatures between
0.24 K and 1.4 K provide strong new evidence for an abrupt ``first-order''
transition between thermally-assisted relaxation and magnetic decay via quantum
tunneling.Comment: 4 pages, including 7 figure
Choice of reference levels of education matter in the ORU earnings equation?
This paper examines whether the results of the earnings equation developed in the over-education/required education/under-education (ORU) literature are sensitive to whether the usual or reference levels of education are measured using the Realized Matches or Worker Self-Assessment methods. The analyses are conducted for all male native-born and immigrant workers in the US, by level of skill, and by occupation. While point estimates differ, particularly when earnings equations are estimated for the smaller samples of sub-groups of the workforce, the general findings are robust to this measurement issue. Thus, the answers provided to the typical research questions in the ORU literature on the productivity of schooling are independent of the measure of the usual or reference level of education used in the analyses
Dynamic Participation in Interdistrict Open Enrollment
Interdistrict open enrollment is the nation’s largest and most widespread school choice program, but our knowledge of these programs is limited. Drawing on 5 years of student-level data from the universe of public school attendees in Colorado, we perform a three-stage analysis to examine the dynamics of student participation in the state’s interdistrict open enrollment program. First, we explore the characteristics of students who open enroll in a defined baseline year. Second, we analyze the characteristics of students who continue to participate in the program in subsequent years. Finally, we examine the characteristics of students who—conditional on not open enrolling in the defined-baseline year—choose to participate in the program in one or more subsequent years.Yeshttps://us.sagepub.com/en-us/nam/manuscript-submission-guideline
Linewidth of single photon transitions in Mn-acetate
We use time-domain terahertz spectroscopy to measure the position and
linewidth of single photon transitions in Mn-acetate. This linewidth is
compared to the linewidth measured in tunneling experiments. We conclude that
local magnetic fields (due to dipole or hyperfine interactions) cannot be
responsible for the observed linewidth, and suggest that the linewidth is due
to variations in the anisotropy constants for different clusters. We also
calculate a lower limit on the dipole field distribution that would be expected
due to random orientations of clusters and find that collective effects must
narrow this distribution in tunneling measurements.Comment: 5 pages, accepted to Physical Review
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