4,739 research outputs found
Anomalous high energy dispersion in photoemission spectra from insulating cuprates
Angle resolved photoelectron spectroscopic measurements have been performed
on an insulating cuprate Ca_2CuO_2Cl_2. High resolution data taken along the
\Gamma to (pi,pi) cut show an additional dispersive feature that merges with
the known dispersion of the lowest binding energy feature, which follows the
usual strongly renormalized dispersion of ~0.35 eV. This higher energy part
reveals a dispersion that is very close to the unrenormalized band predicted by
band theory. A transfer of spectral weight from the low energy feature to the
high energy feature is observed as the \Gamma point is approached. By comparing
with theoretical calculations the high energy feature observed here
demonstrates that the incoherent portion of the spectral function has
significant structure in momentum space due to the presence of various energy
scales.Comment: 5 pages, 3 figure
Gauss Sums and Quantum Mechanics
By adapting Feynman's sum over paths method to a quantum mechanical system
whose phase space is a torus, a new proof of the Landsberg-Schaar identity for
quadratic Gauss sums is given. In contrast to existing non-elementary proofs,
which use infinite sums and a limiting process or contour integration, only
finite sums are involved. The toroidal nature of the classical phase space
leads to discrete position and momentum, and hence discrete time. The
corresponding `path integrals' are finite sums whose normalisations are derived
and which are shown to intertwine cyclicity and discreteness to give a finite
version of Kelvin's method of images.Comment: 14 pages, LaTe
Theory of Thermal Conductivity in High-Tc Superconductors below Tc: Comparison between Hole-Doped and Electron-Doped Systems
In hole-doped high-Tc superconductors, thermal conductivity increases
drastically just below Tc, which has been considered as a hallmark of a nodal
gap. In contrast, such a coherence peak in thermal conductivity is not visible
in electron-doped compounds, which may indicate a full-gap state such as a
(d+is)-wave state. To settle this problem, we study the thermal conductivity in
the Hubbard model using the fluctuation-exchange (FLEX) approximation, which
predicts that the nodal d-wave state is realized in both hole-doped and
electron-doped compounds. The contrasting behavior of thermal conductivity in
both compounds originates from the differences in the hot/cold spot structure.
In general, a prominent coherence peak in thermal conductivity appears in
line-node superconductors only when the cold spot exists on the nodal line.Comment: 5 pages, to be published in J. Phys. Soc. Jpn. Vol.76 No.
The Three-Phase Reading Comprehension Intervention (3-RCI): A Support for Intermediate-Grade Word Callers
This article describes results of a reading comprehension intervention for students with adequate decoding but poor comprehension skills. Five teachers and 25 students in grades 3-5 from two rural public schools participated in this naturalistic experimental research study. Teachers met with identified students in a small group setting for 30 intervention sessions. The intervention involved explicit teaching and gradual release of instruction in three phases: metacognitive strategies, comprehension strategies, and peer-led discussions. To measure growth in reading comprehension, the Qualitative Reading Inventory-3 (Leslie & Caldwell, 2001) was administered as the pre- and posttest and analyzed through t-test comparisons. Interactive teaching is characterized by a dynamic flow of instruction with a powerful use of questioning used as a tool to assist students in understanding what they read. Recitative teaching is marked by static interactions that did not change across treatment intervention. Subsequently, the groups receiving the interactive instruction were compared to those receiving recitative instruction, and growth in reading comprehension for each group was compared. While all students gained in reading comprehension, students in the interactive teaching groups gained more in reading comprehension than those in the recitative teaching groups. Instructional implications of this research are presented and discussed, providing suggestions for teaching reading comprehension
Photoevaporation of Circumstellar Disks due to External FUV Radiation in Stellar Aggregates
When stars form in small groups (N = 100 - 500 members), their circumstellar
disks are exposed to little EUV radiation but a great deal of FUV radiation
from massive stars in the group. This paper calculates mass loss rates for
circumstellar disks exposed to external FUV radiation. Previous work treated
large disks and/or intense radiation fields in which the disk radius exceeds
the critical radius (supercritical disks) where the sound speed in the FUV
heated layer exceeds the escape speed. This paper shows that significant mass
loss still takes place for subcritical systems. Some of the gas extends beyond
the disk edge (above the disk surface) to larger distances where the
temperature is higher, the escape speed is lower, and an outflow develops. The
evaporation rate is a sensitive function of the stellar mass and disk radius,
which determine the escape speed, and the external FUV flux, which determines
the temperature structure of the flow. Disks around red dwarfs are readily
evaporated and shrink to disk radii of 15 AU on short time scales (10 Myr) when
exposed to moderate FUV fields with = 3000. Although disks around solar
type stars are more durable, these disks shrink to 15 AU in 10 Myr for intense
FUV radiation fields with = 30,000; such fields exist in the central 0.7
pc of a cluster with N = 4000 stars. If our solar system formed in the presence
of such strong FUV radiation fields, this mechanism could explain why Neptune
and Uranus in our solar system are gas poor, whereas Jupiter and Saturn are gas
rich. This mechanism for photoevaporation can also limit the production of
Kuiper belt objects and can suppress giant planet formation in sufficiently
large clusters, such as the Hyades, especially for disks associated with low
mass stars.Comment: 49 pages including 12 figures; accepted to Ap
Superconducting Gap Anisotropy in NdCeCuO: Results from Photoemission
We have performed angle resolved photoelectron spectroscopy on the electron
doped cuprate superconductor NdCeCuO. A comparison of the
leading edge midpoints between the superconducting and normal states reveals a
small, but finite shift of 1.5-2 meV near the (,0) position, but no
observable shift along the zone diagonal near (/2,/2). This is
interpreted as evidence for an anisotropic superconducting gap in the electron
doped materials, which is consistent with the presence of d-wave
superconducting order in this cuprate superconductor.Comment: 5 pages, 4 figures, RevTex, to be published in Phys. Rev. Let
Electronic States in the Antiferromagnetic Phase of Electron-Doped High-Tc Cuprates
We investigate the electronic states in the antiferromagnetic (AF) phase of
electron-doped cuprates by using numerically exact diagonalization technique
for a t-t'-t''-J model. When AF correlation develops with decreasing
temperature, a gaplike behavior emerges in the optical conductivity.
Simultaneously, the coherent motion of carriers due to the same sublattice
hoppings is enhanced. We propose that the phase is characterized as an AF state
with small Fermi surface around the momentum k=(\pi,0) and (0,\pi). This is a
remarkable contrast to the behavior of hole-doped cuprates.Comment: RevTeX, 5 pages, 4 figures, to appear in Phys. Rev. B Brief Report
Global variation of the dust-to-gas ratio in evolving protoplanetary discs
Recent theories suggest planetesimal formation via streaming and/or
gravitational instabilities may be triggered by localized enhancements in the
dust-to-gas ratio, and one hypothesis is that sufficient enhancements may be
produced in the pile-up of small solid particles inspiralling under aerodynamic
drag from the large mass reservoir in the outer disc. Studies of particle
pile-up in static gas discs have provided partial support for this hypothesis.
Here, we study the radial and temporal evolution of the dust-to-gas ratio in
turbulent discs, that evolve under the action of viscosity and
photoevaporation. We find that particle pile-ups do not generically occur
within evolving discs, particularly if the introduction of large grains is
restricted to the inner, dense regions of a disc. Instead, radial drift results
in depletion of solids from the outer disc, while the inner disc maintains a
dust-to-gas ratio that is within a factor of ~2 of the initial value. We
attribute this result to the short time-scales for turbulent diffusion and
radial advection (with the mean gas flow) in the inner disc. We show that the
qualitative evolution of the dust-to-gas ratio depends only weakly upon the
parameters of the disc model (the disc mass, size, viscosity, and value of the
Schmidt number), and discuss the implications for planetesimal formation via
collective instabilities. Our results suggest that in discs where there is a
significant level of midplane turbulence and accretion, planetesimal formation
would need to be possible in the absence of large-scale enhancements. Instead,
trapping and concentration of particles within local turbulent structures may
be required as a first stage of planetesimal formation.Comment: Accepted by Monthly Notices of the Royal Astronomical Society on the
8th of March 2012. 20 pages. 16 figure
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