3,192 research outputs found
Spontaneous Octahedral Tilting in the Cubic Inorganic Caesium Halide Perovskites CsSnX and CsPbX (X = F, Cl, Br, I)
The local crystal structures of many perovskite-structured materials deviate
from the average space group symmetry. We demonstrate, from lattice-dynamics
calculations based on quantum chemical force constants, that all the
caesium-lead and caesium-tin halide perovskites exhibit vibrational
instabilities associated with octahedral titling in their high-temperature
cubic phase. Anharmonic double-well potentials are found for zone-boundary
phonon modes in all compounds with barriers ranging from 108 to 512 meV. The
well depth is correlated with the tolerance factor and the chemistry of the
composition, but is not proportional to the imaginary harmonic phonon
frequency. We provide quantitative insights into the thermodynamic driving
forces and distinguish between dynamic and static disorder based on the
potential-energy landscape. A positive band gap deformation (spectral
blueshift) accompanies the structural distortion, with implications for
understanding the performance of these materials in applications areas
including solar cells and light-emitting diodes
Non-Gaussian Covariance of the Matter Power Spectrum in the Effective Field Theory of Large Scale Structure
We compute the non-Gaussian contribution to the covariance of the matter
power spectrum at one-loop order in Standard Perturbation Theory (SPT), and
using the framework of the effective field theory (EFT) of large scale
structure (LSS). The complete one-loop contributions are evaluated for the
first time, including the leading EFT corrections that involve seven
independent operators, of which four appear in the power spectrum and
bispectrum. We compare the non-Gaussian part of the one-loop covariance
computed with both SPT and EFT of LSS to two separate simulations. In one
simulation, we find that the one-loop prediction from SPT reproduces the
simulation well to 0.25 h/Mpc, while in the other simulation
we find a substantial improvement of EFT of LSS (with one free parameter) over
SPT, more than doubling the range of where the theory accurately reproduces
the simulation. The disagreement between these two simulations points to
unaccounted for systematics, highlighting the need for improved numerical and
analytic understanding of the covariance.Comment: v2 - 10+9 pages, 6 figures; minor changes + data analysis and
conclusions updated. Version accepted for publication in PR
Direct Observation of Dynamic Symmetry Breaking above Room Temperature in Methylammonium Lead Iodide Perovskite
Lead halide perovskites such as methylammonium lead triiodide (MAPI) have
outstanding optical and electronic properties for photovoltaic applications,
yet a full understanding of how this solution processable material works so
well is currently missing. Previous research has revealed that MAPI possesses
multiple forms of static disorder regardless of preparation method, which is
surprising in light of its excellent performance. Using high energy resolution
inelastic X-ray (HERIX) scattering, we measure phonon dispersions in MAPI and
find direct evidence for another form of disorder in single crystals: large
amplitude anharmonic zone-edge rotational instabilities of the PbI_6 octahedra
that persist to room temperature and above, left over from structural phase
transitions that take place tens to hundreds of degrees below. Phonon
calculations show that the orientations of the methylammonium couple strongly
and cooperatively to these modes. The result is a non-centrosymmetric,
instantaneous local structure, which we observe in atomic pair distribution
function (PDF) measurements. This local symmetry breaking is unobservable by
Bragg diffraction, but can explain key material properties such as the
structural phase sequence, ultra low thermal transport, and large minority
charge carrier lifetimes despite moderate carrier mobility.Comment: 30 pages, 11 figure
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Broadcasting Flu Messages – Citywide Transmission and Community Reception: An Evaluation of Ready New York’s pandemic influenza outreach campaign
Public health risk communication is a central feature of New York City’s pandemic flu preparedness plan. Particularly in the early stages of a pandemic, before effective therapeutic measures are available, non-pharmaceutical interventions such as social distancing, personal protective hygiene, and voluntary isolation are critical strategies for suppressing the spread of a novel viral strain. New York City health and emergency management officials have decided to use the city’s risk communication structure – the Office of Emergency Management’s Ready New York outreach and dissemination capacities – as one of the primary means to communicate pandemic flu health messages. In October 2008, NCDP contracted with Public Health Solutions and the Office of Emergency Management (OEM) to evaluate the reach and effectiveness of its planned Ready NY Pandemic Flu community outreach campaign
Running Electroweak Couplings as a Probe of New Physics
The energy dependence of the electroweak gauge couplings has not been
measured above the weak scale. We propose that percent-level measurements of
the energy dependence of can be performed now at the LHC and at
future higher energy hadron colliders. These measurements can be used to set
limits on new particles with electroweak quantum numbers without relying on any
assumptions about their decay properties. The shape of the high invariant mass
spectrum of Drell-Yan, , constrains , and the shape of the high transverse
mass distribution of constrains
. We use existing data to perform the first fits to
above the weak scale. Percent-level measurements are possible
because of high precision in theoretical predictions and existing experimental
measurements. We show that the LHC already has the reach to improve upon
electroweak precision tests for new particles that dominantly couple through
their electroweak charges. The 14 TeV LHC is sensitive to the predicted
Standard Model (SM) running of , and can show that
decreases with energy at significance. A future 100 TeV
proton-proton collider will have significant reach to measure running weak
couplings, with sensitivity to the SM running of at and
sensitivity to winos with masses up to 1.3 TeV at .Comment: 34 pages + appendices; v2: references and minor changes to text
added, version to appear in JHE
Lattice dynamics and vibrational spectra of the orthorhombic, tetragonal and cubic phases of methylammonium lead iodide
The hybrid halide perovskite CH3NH3PbI3 exhibits a complex structural
behaviour, with successive transitions between orthorhombic, tetragonal and
cubic polymorphs at ca. 165 K and 327 K. Herein we report first-principles
lattice dynamics (phonon spectrum) for each phase of CH3NH3PbI3. The
equilibrium structures compare well to solutions of temperature-dependent
powder neutron diffraction. By following the normal modes we calculate infrared
and Raman intensities of the vibrations, and compare them to the measurement of
a single crystal where the Raman laser is controlled to avoid degradation of
the sample. Despite a clear separation in energy between low frequency modes
associated with the inorganic PbI3 network and high-frequency modes of the
organic CH3NH3+ cation, significant coupling between them is found, which
emphasises the interplay between molecular orientation and the corner-sharing
octahedral networks in the structural transformations. Soft modes are found at
the boundary of the Brillouin zone of the cubic phase, consistent with
displacive instabilities and anharmonicity involving tilting of the PbI6
octahedra around room temperature.Comment: 9 pages, 4 figure
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