362 research outputs found
The RHESSI Microflare Height Distribution
We present the first in-depth statistical survey of flare source heights
observed by RHESSI. Flares were found using a flare-finding algorithm designed
to search the 6-10 keV count-rate when RHESSI's full sensitivity was available
in order to find the smallest events (Christe et al., 2008). Between March 2002
and March 2007, a total of 25,006 events were found. Source locations were
determined in the 4-10 keV, 10-15 keV, and 15-30 keV energy ranges for each
event. In order to extract the height distribution from the observed projected
source positions, a forward-fit model was developed with an assumed source
height distribution where height is measured from the photosphere. We find that
the best flare height distribution is given by g(h) \propto exp(-h/{\lambda})
where {\lambda} = 6.1\pm0.3 Mm is the scale height. A power-law height
distribution with a negative power-law index, {\gamma} = 3.1 \pm 0.1 is also
consistent with the data. Interpreted as thermal loop top sources, these
heights are compared to loops generated by a potential field model (PFSS). The
measured flare heights distribution are found to be much steeper than the
potential field loop height distribution which may be a signature of the flare
energization process
Cyclic oxidation of yttrium/ytterbium disilicate environmental barrier coatings
Please click Additional Files below to see the full abstract
Mechanically induced current and quantum evaporation from Luttinger liquids
We investigate transport through a tunnelling junction between an
uncorrelated metallic lead and a Luttinger liquid when the latter is subjected
to a time dependent perturbation. The tunnelling current as well as the
electron energy distribution function are found to be strongly affected by the
perturbation due to generation of harmonics in the density oscillations. Using
a semiconducting lead instead of a metallic one results in electrons being
injected into the lead even without applied voltage. Some applications to
carbon nanotubes are discussed.Comment: 7 pages, 2 figures (eps files
A Combined Experimental and Individual-Differences Investigation into Mind Wandering During a Video Lecture
A combined experimental–correlational study with a diverse sample (N = 182) from two research sites tested a set of five a priori hypotheses about mind wandering and learning, using a realistic video lecture on introductory statistics. Specifically, the study examined whether students’ vulnerability to mind wandering during the lecture would predict learning from, and situational interest in, the video, and also whether longhand note-taking would help reduce mind wandering, at least for some students. Half the subjects took notes during the video, and all were subsequently tested on lecture content without notes. Regression and mediation analyses indicated that: (a) several individual-differences variables (e.g., pretest score, prior math interest, classroom media multitasking habits) uniquely predicted in-lecture mind wandering frequency; (b) although the note-taking manipulation did not reduce mind wandering at the group level, note-taking still reduced mind wandering for some individuals (i.e., those with lower prior knowledge and those who took notes of high quality and quantity); (c) mind wandering uniquely predicted both learning (posttest) and situational interest outcomes above and beyond all other individual-differences variables; (d) moreover, mind wandering significantly mediated the effects of several individual differences; and, finally, (e) not all types of mind wandering were problematic—in fact, off-task reflections about lecture-related topics positively predicted learning. These results, which were generally robust across the two sites, suggest that educationally focused cognitive research may benefit from considering attentional processes during learning as well as cognitive and noncognitive individual differences that affect attention and learning
Decoherence of electron spin qubits in Si-based quantum computers
Direct phonon spin-lattice relaxation of an electron qubit bound by a donor
impurity or quantum dot in SiGe heterostructures is investigated. The aim is to
evaluate the importance of decoherence from this mechanism in several important
solid-state quantum computer designs operating at low temperatures. We
calculate the relaxation rate as a function of [100] uniaxial strain,
temperature, magnetic field, and silicon/germanium content for Si:P bound
electrons. The quantum dot potential is much smoother, leading to smaller
splittings of the valley degeneracies. We have estimated these splittings in
order to obtain upper bounds for the relaxation rate. In general, we find that
the relaxation rate is strongly decreased by uniaxial compressive strain in a
SiGe-Si-SiGe quantum well, making this strain an important positive design
feature. Ge in high concentrations (particularly over 85%) increases the rate,
making Si-rich materials preferable. We conclude that SiGe bound electron
qubits must meet certain conditions to minimize decoherence but that
spin-phonon relaxation does not rule out the solid-state implementation of
error-tolerant quantum computing.Comment: 8 figures. To appear in PRB-July 2002. Revisions include: some
references added/corrected, several typos fixed, a few things clarified.
Nothing dramati
Single Spin Measurement using Single Electron Transistors to Probe Two Electron Systems
We present a method for measuring single spins embedded in a solid by probing
two electron systems with a single electron transistor (SET). Restrictions
imposed by the Pauli Principle on allowed two electron states mean that the
spin state of such systems has a profound impact on the orbital states
(positions) of the electrons, a parameter which SET's are extremely well suited
to measure. We focus on a particular system capable of being fabricated with
current technology: a Te double donor in Si adjacent to a Si/SiO2 interface and
lying directly beneath the SET island electrode, and we outline a measurement
strategy capable of resolving single electron and nuclear spins in this system.
We discuss the limitations of the measurement imposed by spin scattering
arising from fluctuations emanating from the SET and from lattice phonons. We
conclude that measurement of single spins, a necessary requirement for several
proposed quantum computer architectures, is feasible in Si using this strategy.Comment: 22 Pages, 8 Figures; revised version contains updated references and
small textual changes. Submitted to Phys. Rev.
Fast Non-Adiabatic Two Qubit Gates for the Kane Quantum Computer
In this paper we apply the canonical decomposition of two qubit unitaries to
find pulse schemes to control the proposed Kane quantum computer. We explicitly
find pulse sequences for the CNOT, swap, square root of swap and controlled Z
rotations. We analyze the speed and fidelity of these gates, both of which
compare favorably to existing schemes. The pulse sequences presented in this
paper are theoretically faster, higher fidelity, and simpler than existing
schemes. Any two qubit gate may be easily found and implemented using similar
pulse sequences. Numerical simulation is used to verify the accuracy of each
pulse scheme
The universe formation by a space reduction cascade with random initial parameters
In this paper we discuss the creation of our universe using the idea of extra
dimensions. The initial, multidimensional Lagrangian contains only metric
tensor. We have found many sets of the numerical values of the Lagrangian
parameters corresponding to the observed low-energy physics of our universe.
Different initial parameters can lead to the same values of fundamental
constants by the appropriate choice of a dimensional reduction cascade. This
result diminishes the significance of the search for the 'unique' initial
Lagrangian. We also have obtained a large number of low-energy vacua, which is
known as a 'landscape' in the string theory.Comment: 17 pages, 1 figur
Individual differences in task-unrelated thought in university classrooms
This study investigated what academic traits, attitudes, and habits predict individual differences in task-unrelated thought (TUT) during lectures, and whether this TUT propensity mediates associations between academic individual differences and course outcomes (final grade and situational interest evoked by material). Undergraduates (N = 851) from ten psychology classes at two US universities responded to thought probes presented during two early-course lectures; they also indicated sitting in the front, middle, or back of the classroom. At each probe, students categorized their thought content, such as indicating on-task thought or TUT. Students also completed online, academic-self-report questionnaires at the beginning of the course and a situational interest questionnaire at the end. Average TUT rate was 24% but individuals’ rates varied widely (SD = 18%). TUT rates also increased substantially from the front to back of the classroom, and modestly from the first to second half of class periods. Multiple-group analyses (with ten classroom groups) indicated that: (a) classroom media-multitasking habits, initial interest in the course topic, and everyday propensity for mind-wandering and boredom accounted for unique variance in TUT rate (beyond other predictors); (b) TUT rate accounted for unique (modest) variance in course grades and situational interest; and (c) classroom media multitasking and propensity for mind-wandering and boredom had indirect associations with course grades via TUT rate, and these predictor variables, along with initial interest, had indirect associations with end-of-term situational interest via TUT rate. Some academic traits and behaviors predict course outcomes in part because they predict off-task thought during class
Svestka's Research: Then and Now
Zdenek Svestka's research work influenced many fields of solar physics,
especially in the area of flare research. In this article I take five of the
areas that particularly interested him and assess them in a "then and now"
style. His insights in each case were quite sound, although of course in the
modern era we have learned things that he could not readily have envisioned.
His own views about his research life have been published recently in this
journal, to which he contributed so much, and his memoir contains much
additional scientific and personal information (Svestka, 2010).Comment: Invited review for "Solar and Stellar Flares," a conference in honour
of Prof. Zden\v{e}k \v{S}vestka, Prague, June 23-27, 2014. This is a
contribution to a Topical Issue in Solar Physics, based on the presentations
at this meeting (Editors Lyndsay Fletcher and Petr Heinzel
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