24,792 research outputs found
Structural changes at the semiconductor-insulator phase transition in the single layered La0.5Sr1.5MnO4 perovskite
The semiconductor-insulator phase transition of the single-layer manganite
La0.5Sr1.5MnO4 has been studied by means of high resolution synchrotron x-ray
powder diffraction and resonant x-ray scattering at the Mn K edge. We conclude
that a concomitant structural transition from tetragonal I4/mmm to orthorhombic
Cmcm phases drives this electronic transition. A detailed symmetry-mode
analysis reveals that condensation of three soft modes -Delta_2(B2u), X1+(B2u)
and X1+(A)- acting on the oxygen atoms accounts for the structural
transformation. The Delta_2 mode leads to a pseudo Jahn-Teller distortion (in
the orthorhombic bc-plane only) on one Mn site (Mn1) whereas the two X1+ modes
produce an overall contraction of the other Mn site (Mn2) and expansion of the
Mn1 one. The X1+ modes are responsible for the tetragonal superlattice
(1/2,1/2,0)-type reflections in agreement with a checkerboard ordering of two
different Mn sites. A strong enhancement of the scattered intensity has been
observed for these superlattice reflections close to the Mn K edge, which could
be ascribed to some degree of charge disproportion between the two Mn sites of
about 0.15 electrons. We also found that the local geometrical anisotropy of
the Mn1 atoms and its ordering originated by the condensed Delta_2 mode alone
perfectly explains the resonant scattering of forbidden (1/4,1/4,0)-type
reflections without invoking any orbital ordering.Comment: 3 tables and 10 figures; accepted in Phys. Rev.
Cosmic Microwave Background temperature and polarization anisotropies from the large-N limit of global defects
We determine the full C_l spectra and correlation functions of the
temperature and polarization anisotropies in the CMB, generated by a source
modeled by the large N limit of spontaneously broken global O(N)-theories. We
point out a problem in the standard approach of treating the radiation-matter
transition by interpolating the eigenvectors of the unequal-time correlators of
the source energy-momentum tensor. This affects the CMB predictions from all
type of cosmic defects. We propose a method to overcome this difficulty, and
find that in the large-N global model that we study, differences in the final
CMB power spectra amplitudes reach up to 25%, when compared to implementations
of the eigenvector interpolation technique. We discuss as well how to optimally
search for the contribution in the CMB from active sources such as cosmic
defects, in experiments like Planck, COrE and PRISM.Comment: 16+4 pages, 13 figures (Version 2: minor changes to match published
version in PRD
The local B-polarization of the CMB: a very sensitive probe of cosmic defects
We present a new and especially powerful signature of cosmic strings and
other topological or non-topological defects in the polarization of the cosmic
microwave background (CMB). We show that even if defects contribute 1% or less
in the CMB temperature anisotropy spectrum, their signature in the local
-polarization correlation function at angular scales of tens of arc
minutes is much larger than that due to gravitational waves from inflation,
even if the latter contribute with a ratio as big as to the
temperature anisotropies. We show that when going from non-local to local
-polarization, the ratio of the defect signal-to-noise with respect
to the inflationary value increases by about an order of magnitude. Proposed
B-polarization experiments, with a good sensitivity on arcminute scales, may
either detect a contribution from topological defects produced after inflation
or place stringent limits on them. Even Planck should be able to improve
present constraints on defect models by at least an order of magnitude, to the
level of \ep <10^{-7}. A future full-sky experiment like CMBpol, with
polarization sensitivities of the order of K-arcmin, will be able to
constrain the defect parameter \ep=Gv^2 to a few , depending
on the defect model.Comment: Version Published in Physics Letters
Change in the North Atlantic circulation associated with the mid-Pleistocene transition
The southwestern Iberian margin is highly sensitive to changes in the distribution of North Atlantic currents and to the position of oceanic fronts. In this work, the evolution of oceanographic parameters from 812 to 530 ka (MIS20-MIS14) is studied based on the analysis of planktonic foraminifer assemblages from site IODP-U1385 (37 degrees 34.285' N, 10 degrees 7.562' W; 2585m b.s.l.). By comparing the obtained results with published records from other North Atlantic sites between 41 and 55 degrees N, basin-wide paleoceano-graphic conditions are reconstructed. Variations of assemblages dwelling in different water masses indicate a major change in the general North Atlantic circulation during MIS16, coinciding with the definite establishment of the 100 ky cyclicity associated with the mid-Pleistocene transition. At the surface, this change consisted in the redistribution of water masses, with the subsequent thermal variation, and occurred linked to the northwestward migration of the Arctic Front (AF), and the increase in the North Atlantic Deep Water (NADW) formation with respect to previous glacials. During glacials prior to MIS16, the NADW formation was very weak, which drastically slowed down the surface circulation; the AF was at a southerly position and the North Atlantic Current (NAC) diverted southeastwards, developing steep south-north, and east-west, thermal gradients and blocking the arrival of warm water, with associated moisture, to high latitudes. During MIS16, the increase in the meridional overturning circulation, in combination with the northwestward AF shift, allowed the arrival of the NAC to subpolar latitudes, multiplying the moisture availability for ice-sheet growth, which could have worked as a positive feedback to prolong the glacials towards 100 ky cycles.info:eu-repo/semantics/publishedVersio
Statistical Error in Particle Simulations of Hydrodynamic Phenomena
We present predictions for the statistical error due to finite sampling in
the presence of thermal fluctuations in molecular simulation algorithms.
Specifically, we establish how these errors depend on Mach number, Knudsen
number, number of particles, etc. Expressions for the common hydrodynamic
variables of interest such as flow velocity, temperature, density, pressure,
shear stress and heat flux are derived using equilibrium statistical mechanics.
Both volume-averaged and surface-averaged quantities are considered.
Comparisons between theory and computations using direct simulation Monte Carlo
for dilute gases, and molecular dynamics for dense fluids, show that the use of
equilibrium theory provides accurate results.Comment: 24 pages postscript (including 16 figures
Surface plasmon enhanced absorption and suppressed transmission in periodic arrays of graphene ribbons
Resonance diffraction in the periodic array of graphene micro-ribbons is
theoretically studied following a recent experiment [L. Ju et al, Nature
Nanotech. 6, 630 (2011)]. Systematic studies over a wide range of parameters
are presented. It is shown that a much richer resonant picture would be
observable for higher relaxation times of charge carriers: more resonances
appear and transmission can be totally suppressed. The comparison with the
absorption cross-section of a single ribbon shows that the resonant features of
the periodic array are associated with leaky plasmonic modes. The
longest-wavelength resonance provides the highest visibility of the
transmission dip and has the strongest spectral shift and broadening with
respect to the single-ribbon resonance, due to collective effects.Comment: 5 pages, 3 figure
Lower Cretaceous (Hauterivian-Albian) ammonite biostratigraphy in the Maestrat Basin (E Spain)
Peer reviewedPublisher PD
Statistical Error in Particle Simulations of Fluid Flow and Heat Transfer
We present predictions for the statistical error due to finite sampling in the presence of thermal fluctuations in molecular simulation algorithms. Specifically, we present predictions for the error dependence on hydrodynamic parameters and the number of samples taken. Expressions for the common hydrodynamic variables of interest such as flow velocity, temperature, density, pressure, shear stress and heat flux are derived using equilibrium statistical mechanics. Both volume-averaged and surface-averaged quantities are considered. Comparisons between theory and computations using direct simulation Monte Carlo for dilute gases, and molecular dynamics for dense fluids, show that the use of equilibrium theory provides accurate results.Singapore-MIT Alliance (SMA
Misalignment of the microquasar V4641 Sgr (SAX J1819.3--2525)
In the microquasar V4641 Sgr the spin of the black hole is thought to be
misaligned with the binary orbital axis. The accretion disc aligns with the
black hole spin by the Lense-Thirring effect near to the black hole and further
out becomes aligned with the binary orbital axis. The inclination of the radio
jets and the Fe line profile have both been used to determine the
inclination of the inner accretion disc but the measurements are inconsistent.
Using a steady state analytical warped disc model for V4641 Sgr we find that
the inner disc region is flat and aligned with the black hole up to about . Thus if both the radio jet and fluorescent emission originates in
the same inner region then the measurements of the inner disc inclination
should be the same.Comment: Accepted for publication in MNRA
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One class, two modes of participation: Fully integrating online students into residential classes via web conferencing
What if online and residential students could have an expanded choice of electives, more scheduling options, and greater opportunities for networking with a larger group of peers by offering courses that online and residential students can take together, at the same time?
At Columbia Universityâs School of Social Work, in our Masterâs of Science in Social Work program, we are piloting a new type of hybrid course that uses Zoom to bring online students into residential courses. The pilot is an opportunity to figure out how to set up, design, and teach this type of course, and in this poster, we propose to share our lessons learned while we prepared for our courses during the fall semester and have begun to teach during the spring semester.
While it is early to evaluate the pilotâs outcomes, we have already learned a great deal about how to plan for technical logistics and how to design engaging class sessions, and the first week of class included breakout discussions in groups that combined online and residential students, as well as whole-class discussions in which both types of students participated equally.
We propose to share our work towards the goal of creating one cohesive classroom community rather than two parallel communities or a situation in which the online students simply watched the residential students through a virtual window. We will share our lessons learned around logistics, and considerations for designing lessons plans and assignments that fully integrate the online and residential students, including small-group breakout activities, group presentations, lectures, guest speakers, whole-class discussions, homework assignments, and potentially field trips off campus.
This poster will draw from the presentersâ experiences teaching and preparing to teach two Spring 2018 pilot seminar courses: Staff Development, Training, and Coaching, and Managing NGOs
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