551,951 research outputs found
Two-probe study of hot carriers in reduced graphene oxide
The energy relaxation of carriers in reduced graphene oxide thin films is
studied using optical pump-probe spectroscopy with two probes of different
colors. We measure the time difference between peaks of the carrier density at
each probing energy by measuring a time-resolved differential transmission and
find that the carrier density at the lower probing energy peaks later than that
at the higher probing energy. Also, we find that the peak time for the lower
probing energy shifts from about 92 to 37 fs after the higher probing energy
peak as the carrier density is increased from 1.5E12 to 3E13 per square
centimeter, while no noticeable shift is observed in that for the higher
probing energy. Assuming the carriers rapidly thermalize after excitation, this
indicates that the optical phonon emission time decreases from about 50 to
about 20 fs and the energy relaxation rate increases from 4 to 10 meV/fs. The
observed density dependence is inconsistent with the phonon bottleneck effect.Comment: 10 pages, 4 figure
Optimal Relay Selection with Non-negligible Probing Time
In this paper an optimal relay selection algorithm with non-negligible
probing time is proposed and analyzed for cooperative wireless networks. Relay
selection has been introduced to solve the degraded bandwidth efficiency
problem in cooperative communication. Yet complete information of relay
channels often remain unavailable for complex networks which renders the
optimal selection strategies impossible for transmission source without probing
the relay channels. Particularly when the number of relay candidate is large,
even though probing all relay channels guarantees the finding of the best
relays at any time instant, the degradation of bandwidth efficiency due to
non-negligible probing times, which was often neglected in past literature, is
also significant. In this work, a stopping rule based relay selection strategy
is determined for the source node to decide when to stop the probing process
and choose one of the probed relays to cooperate with under wireless channels'
stochastic uncertainties. This relay selection strategy is further shown to
have a simple threshold structure. At the meantime, full diversity order and
high bandwidth efficiency can be achieved simultaneously. Both analytical and
simulation results are provided to verify the claims.Comment: 8 pages. ICC 201
Exploring the Relationship between K-8 Prospective Teachers’ Algebraic Thinking Proficiency and the Questions They Pose during Diagnostic Algebraic Thinking Interviews
In this study, we explored the relationship between prospective teachers’ algebraic thinking and the questions they posed during one-on-one diagnostic interviews that focused on investigating the algebraic thinking of middle school students. To do so, we evaluated prospective teachers’ algebraic thinking proficiency across 125 algebra-based tasks and we analyzed the characteristics of questions they posed during the interviews. We found that prospective teachers with lower algebraic thinking proficiency did not ask any probing questions. Instead, they either posed questions that simply accepted and affirmed student responses or posed questions that guided the students toward an answer without probing student thinking. In contrast, prospective teachers with higher algebraic thinking proficiency were able to pose probing questions to investigate student thinking or help students clarify their thinking. However, less than half of their questions were of this probing type. These results suggest that prospective teachers’ algebraic thinking proficiency is related to the types of questions they ask to explore the algebraic thinking of students. Implications for mathematics teacher education are discussed
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