1,532,906 research outputs found
The Development of Instruments for Assessment of Instructional Practices in Standards-Based Teaching
We provide a description and rationale for the development of two instruments: 1) a classroom observation protocol; and, 2) a teacher interview protocol—designed to document the impact of reform-based professional development with undergraduate mathematics and science faculty, and its impact on the resultant preparation of teachers. Constructed upon review of the research on teaching and standards documents in mathematics and science, these instruments form the basis for data collection in a three-year longitudinal study of teaching practice among early career teachers as well as undergraduate college faculty. In addition, we suggest further applications of the observation protocol beyond the original purpose of our research study
A Tidal Flare Candidate in Abell 1795
As part of our ongoing archival X-ray survey of galaxy clusters for tidal
flares, we present evidence of an X-ray transient source within 1 arcmin of the
core of Abell 1795. The extreme variability (a factor of nearly 50), luminosity
(> 2 x 10^42 erg s^{-1}), long duration (> 5 years) and supersoft X-ray
spectrum (< 0.1 keV) are characteristic signatures of a stellar tidal
disruption event according to theoretical predictions and to existing X-ray
observations, implying a massive >~10^5 M_sun black hole at the centre of that
galaxy. The large number of X-ray source counts (~700) and long temporal
baseline (~12 years with Chandra and XMM-Newton) make this one of the
best-sampled examples of any tidal flare candidate to date. The transient may
be the same EUV source originally found contaminating the diffuse ICM
observations of Bowyer et al. (1999), which would make it the only tidal flare
candidate with reported EUV observations and implies an early source luminosity
1-2 orders of magnitude greater. If the host galaxy is a cluster member then it
must be a dwarf galaxy, an order of magnitude less massive than the quiescent
galaxy Henize 2-10 which hosts a massive black hole that is difficult to
reconcile with its low mass. The unusual faintness of the host galaxy may be
explained by tidal stripping in the cluster core.Comment: Accepted by MNRAS 2013 July 23. 27 pages, 10 figure
Evolution of SU(4) Transport Regimes in Carbon Nanotube Quantum Dots
We study the evolution of conductance regimes in carbon nanotubes with doubly
degenerate orbitals (``shells'') by controlling the contact transparency within
the same sample. For sufficiently open contacts, Kondo behavior is observed for
1, 2, and 3 electrons in the topmost shell. As the contacts are opened more,
the sample enters the ``mixed valence'' regime, where different charge states
are strongly hybridized by electron tunneling. Here, the conductance as a
function of gate voltage shows pronounced modulations with a period of four
electrons, and all single-electron features are washed away at low temperature.
We successfully describe this behavior by a simple formula with no fitting
parameters. Finally, we find a surprisingly small energy scale that controls
the temperature evolution of conductance and the tunneling density of states in
the mixed valence regime.Comment: 4 pages + supplementary info. The second part of the original
submission is now split off as a separate paper (0709.1288
Approximating Steady States in Equilibrium and Nonequilibrium Condensates
We obtain approximations for the time-independent Gross-Pitaevskii (GP) and
complex GP equation in two and three spatial dimensions by generalizing the
divergence-free WKB method. The results include an explicit expression of a
uniformly valid approximation for the condensate density of an ultracold Bose
gas confined in a harmonic trap that extends into the classically forbidden
region. This provides an accurate approximation of the condensate density that
includes healing effects at leading order that are missing in the widely
adopted Thomas-Fermi approximation. The results presented herein allow us to
formulate useful approximations to a range of experimental systems including
the equilibrium properties of a finite temperature Bose gas and the
steady-state properties of a 2D nonequilibrium condensate. Comparisons between
our asymptotic and numerical results for the conservative and
forced-dissipative forms of the GP equations as applied to these systems show
excellent agreement between the two sets of solutions thereby illustrating the
accuracy of these approximations.Comment: 5 pages, 1 figur
Microstructure Controlled Shear Band Pattern Formation and Enhanced Plasticity of Bulk Metallic Glasses Containing in situ Formed Ductile Phase Dendrite Dispersions
Results are presented for a ductile metal reinforced bulk metallic glass matrix composite based on glass forming compositions in the Zr-Ti-Cu-Ni-Be system. Primary dendrite growth and solute partitioning in the molten state yields a microstructure consisting of a ductile crystalline Ti-Zr-Nb β phase, with bcc structure, in a Zr-Ti-Nb-Cu-Ni-Be bulk metallic glass matrix. Under unconstrained mechanical loading organized shear band patterns develop throughout the sample. This results in a dramatic increase in the plastic strain to failure, impact resistance, and toughness of the metallic glass
Large supercooled liquid region and phase separation in the Zr–Ti–Ni–Cu–Be bulk metallic glasses
Results of calorimetric, differential thermal analysis, and structural measurements are presented for a series of bulk metallic glass forming compositions in the Zr-Ti-Cu-Ni-Be alloy system. The calorimetric data for five alloys, prepared along the tie line between phase separating and nonphase separating compositions, show that the transition from phase separating to nonphase separating behavior is smooth. The bulk glasses near the center of the tie line exhibit large supercooled liquid regions: Delta T approximate to 135 K, the largest known for a bulk metallic glass
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