672 research outputs found
Point group symmetry of cadmium arsenide thin films determined by convergent beam electron diffraction
Cadmium arsenide (Cd3As2) is one of the first materials to be discovered to
belong to the class of three-dimensional topological semimetals. Reported room
temperature crystal structures of Cd3As2 reported differ subtly in the way the
Cd vacancies are arranged within its antifluorite-derived structure, which
determines if an inversion center is present and if Cd3As2 is a Dirac or Weyl
semimetal. Here, we apply convergent beam electron diffraction (CBED) to
determine the point group of Cd3As2 thin films grown by molecular beam epitaxy.
Using CBED patterns from multiple zone axes, high-angle annular dark-field
images acquired in scanning transmission electron microscopy, and Bloch wave
simulations, we show that Cd3As2 belongs to the tetragonal 4/mmm point group,
which is centrosymmetric. The results show that CBED can distinguish very
subtle differences in the crystal structure of a topological semimetal, a
capability that will be useful for designing materials and thin film
heterostructures with topological states that depend on the presence of certain
crystal symmetries.Comment: Accepted for publication in Physical Review Material
A Large Effective Phonon Magnetic Moment in a Dirac Semimetal
We investigated the magnetoterahertz response of the Dirac semimetal
CdAs and observed a particularly low frequency optical phonon, as well
as a very prominent and field sensitive cyclotron resonance. As the cyclotron
frequency is tuned with field to pass through the phonon, the phonon become
circularly polarized as shown by a notable splitting in their response to
right- and left-hand polarized light. This splitting can be expressed as an
effective phonon magnetic moment that is approximately 2.7 times the Bohr
magneton, which is almost four orders of magnitude larger than ab initio
calculations predict for phonon magnetic moments in nonmagnetic insulators.
This exceedingly large value is due to the coupling of the phonons to the
cyclotron motion and is controlled directly by the electron-phonon coupling
constant. This field tunable circular-polarization selective coupling provides
new functionality for nonlinear optics to create light-induced topological
phases in Dirac semimetals.Comment: 15 pages for main text and SI; To appear in Nano Letters (2020
Analytical study of the effect of recombination on evolution via DNA shuffling
We investigate a multi-locus evolutionary model which is based on the DNA
shuffling protocol widely applied in \textit{in vitro} directed evolution. This
model incorporates selection, recombination and point mutations. The simplicity
of the model allows us to obtain a full analytical treatment of both its
dynamical and equilibrium properties, for the case of an infinite population.
We also briefly discuss finite population size corrections
Thermodynamic theory of epitaxial ferroelectric thin films with dense domain structures
A Landau-Ginsburg-Devonshire-type nonlinear phenomenological theory is
presented, which enables the thermodynamic description of dense laminar
polydomain states in epitaxial ferroelectric thin films. The theory explicitly
takes into account the mechanical substrate effect on the polarizations and
lattice strains in dissimilar elastic domains (twins). Numerical calculations
are performed for PbTiO3 and BaTiO3 films grown on (001)-oriented cubic
substrates. The "misfit strain-temperature" phase diagrams are developed for
these films, showing stability ranges of various possible polydomain and
single-domain states. Three types of polarization instabilities are revealed
for polydomain epitaxial ferroelectric films, which may lead to the formation
of new polydomain states forbidden in bulk crystals. The total dielectric and
piezoelectric small-signal responses of polydomain films are calculated,
resulting from both the volume and domain-wall contributions. For BaTiO3 films,
strong dielectric anomalies are predicted at room temperature near special
values of the misfit strain.Comment: 19 pages, 8 figure
Genetically engineered minipigs model the major clinical features of human neurofibromatosis type 1.
Neurofibromatosis Type 1 (NF1) is a genetic disease caused by mutations in Neurofibromin 1 (NF1). NF1 patients present with a variety of clinical manifestations and are predisposed to cancer development. Many NF1 animal models have been developed, yet none display the spectrum of disease seen in patients and the translational impact of these models has been limited. We describe a minipig model that exhibits clinical hallmarks of NF1, including café au lait macules, neurofibromas, and optic pathway glioma. Spontaneous loss of heterozygosity is observed in this model, a phenomenon also described in NF1 patients. Oral administration of a mitogen-activated protein kinase/extracellular signal-regulated kinase inhibitor suppresses Ras signaling. To our knowledge, this model provides an unprecedented opportunity to study the complex biology and natural history of NF1 and could prove indispensable for development of imaging methods, biomarkers, and evaluation of safety and efficacy of NF1-targeted therapies
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