1,205 research outputs found
Lattice dynamics of quasi-2D perovskites from first-principles
We present the vibrational properties and phonon dispersion for quasi-2D
hybrid organic-inorganic perovskites (BA)CsPbI,
(HA)CsPbI, (BA)(MA)PbI, and (HA)(MA)PbI
calculated from first principles. Given the highly complex nature of these
compounds, we first perform careful benchmarking and convergence testing to
identify suitable parameters to describe their structural features and
vibrational properties. We find that the inclusion of van der Waals corrections
on top of generalized gradient approximation (GGA) exchange-correlation
functionals provides the best agreement for the equilibrium structure relative
to experimental data. We then compute vibrational properties under the harmonic
approximation. We find that stringent energy cut-offs are required to obtain
well-converged phonon properties, and once converged, the harmonic
approximation can capture key physics for such a large, hybrid
inorganic-organic system with vastly different atom types, masses, and
interatomic interactions. We discuss the obtained phonon modes and dispersion
behavior in the context of known properties for bulk 3D perovskites and ligand
molecular crystals. While many vibrational properties are inherited from the
parent systems, we also observe unique coupled vibrations that cannot be
associated with vibrations of the pure constituent perovskite and ligand
subphases. Dispersive low energy phonon branches primarily occur in the
in-plane direction and within the perovskite subphase, and arise from bending
and breathing modes of the equatorial Pb-I network within the perovskite
octahedral plane
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Dissecting the regulatory strategies of NF-kB RelA target genes in the inflammatory response reveals differential transactivation logics
Nuclear factor κB (NF-κB) RelA is the potent transcriptional activator of inflammatory response genes. We stringently defined a list of direct RelA target genes by integrating physical (chromatin immunoprecipitation sequencing [ChIP-seq]) and functional (RNA sequencing [RNA-seq] in knockouts) datasets. We then dissected each gene’s regulatory strategy by testing RelA variants in a primary-cell genetic-complementation assay. All endogenous target genes require RelA to make DNA-base-specific contacts, and none are activatable by the DNA binding domain alone. However, endogenous target genes differ widely in how they employ the two transactivation domains. Through model-aided analysis of the dynamic time-course data, we reveal the gene-specific synergy and redundancy of TA1 and TA2. Given that post-translational modifications control TA1 activity and intrinsic affinity for coactivators determines TA2 activity, the differential TA logics suggests context-dependent versus context-independent control of endogenous RelA-target genes. Although some inflammatory initiators appear to require co-stimulatory TA1 activation, inflammatory resolvers are a part of the NF-κB RelA core response
A method for measuring the Neel relaxation time in a frozen ferrofluid
We report a novel method of determining the average Neel relaxation time and
its temperature dependence by calculating derivatives of the measured time
dependence of temperature for a frozen ferrofluid exposed to an alternating
magnetic field. The ferrofluid, composed of dextran-coated Fe3O4 nanoparticles
(diameter 13.7 nm +/- 4.7 nm), was synthesized via wet chemical precipitation
and characterized by x-ray diffraction and transmission electron microscopy. An
alternating magnetic field of constant amplitude (H0 = 20 kA/m) driven at
frequencies of 171 kHz, 232 kHz and 343 kHz was used to determine the
temperature dependent magnetic energy absorption rate in the temperature range
from 160 K to 210 K. We found that the specific absorption rate of the
ferrofluid decreased monotonically with temperature over this range at the
given frequencies. From these measured data, we determined the temperature
dependence of the Neel relaxation time and estimate a room-temperature
magnetocrystalline anisotropy constant of 40 kJ/m3, in agreement with
previously published results
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