12 research outputs found
Feshbach resonance of heavy exciton-polaritons
We study interactions between polaritons formed by hybridization of excitons
in a two-dimensional (2D) semiconductor with surface optical phonons or
plasmons. These quasiparticles have a high effective mass and can bind into
bipolaritons near a Feshbach-like scattering resonance. We analyze the phase
diagram of a many-body condensate of heavy polaritons and bipolaritons and
calculate their absorption and luminescence spectra, which can be measured
experimentally.Comment: 11 pages, 7 figure
Local Structure and Bonding of Carbon Nanothreads Probed by High-Resolution Transmission Electron Microscopy
Carbon nanothreads are a new one-dimensional sp^3-bonded nanomaterial of CH stoichiometry synthesized from benzene at high pressure and room temperature by slow solid-state polymerization. The resulting threads assume crystalline packing hundreds of micrometers across. We show high-resolution electron microscopy (HREM) images of hexagonal arrays of well-aligned thread columns that traverse the 80–100 nm thickness of the prepared sample. Diffuse scattering in electron diffraction reveals that nanothreads are packed with axial and/or azimuthal disregistry between them. Layer lines in diffraction from annealed nanothreads provide the first evidence of translational order along their length, indicating that this solid-state reaction proceeds with some regularity. HREM also reveals bends and defects in nanothread crystals that can contribute to the broadening of their diffraction spots, and electron energy-loss spectroscopy confirms them to be primarily sp^3-hybridized, with less than 27% sp^2 carbon, most likely associated with partially saturated “degree-4” threads
Human embryonic genome activation initiates at the one-cell stage.
In human embryos, the initiation of transcription (embryonic genome activation [EGA]) occurs by the eight-cell stage, but its exact timing and profile are unclear. To address this, we profiled gene expression at depth in human metaphase II oocytes and bipronuclear (2PN) one-cell embryos. High-resolution single-cell RNA sequencing revealed previously inaccessible oocyte-to-embryo gene expression changes. This confirmed transcript depletion following fertilization (maternal RNA degradation) but also uncovered low-magnitude upregulation of hundreds of spliced transcripts. Gene expression analysis predicted embryonic processes including cell-cycle progression and chromosome maintenance as well as transcriptional activators that included cancer-associated gene regulators. Transcription was disrupted in abnormal monopronuclear (1PN) and tripronuclear (3PN) one-cell embryos. These findings indicate that human embryonic transcription initiates at the one-cell stage, sooner than previously thought. The pattern of gene upregulation promises to illuminate processes involved at the onset of human development, with implications for epigenetic inheritance, stem-cell-derived embryos, and cancer
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A program of successive gene expression in mouse one-cell embryos
At the moment of union in fertilization, sperm and oocyte are transcriptionally silent. The ensuing onset of embryonic transcription (embryonic genome activation [EGA]) is critical for development, yet its timing and profile remain elusive in any vertebrate species. We here dissect transcription during EGA by high-resolution single-cell RNA sequencing of precisely synchronized mouse one-cell embryos. This reveals a program of embryonic gene expression (immediate EGA [iEGA]) initiating within 4 h of fertilization. Expression during iEGA produces canonically spliced transcripts, occurs substantially from the maternal genome, and is mostly downregulated at the two-cell stage. Transcribed genes predict regulation by transcription factors (TFs) associated with cancer, including c-Myc. Blocking c-Myc or other predicted regulatory TF activities disrupts iEGA and induces acute developmental arrest. These findings illuminate intracellular mechanisms that regulate the onset of mammalian development and hold promise for the study of cancer
Constraining Carbon Nanothread Structures by Experimental and Calculated Nuclear Magnetic Resonance Spectra
A one-dimensional
(1D) sp<sup>3</sup> carbon nanomaterial with
high lateral packing order, known as carbon nanothreads, has recently
been synthesized by slowly compressing and decompressing crystalline
solid benzene at high pressure. The atomic structure of an individual
nanothread has not yet been determined experimentally. We have calculated
the <sup>13</sup>C nuclear magnetic resonance (NMR) chemical shifts,
chemical shielding tensors, and anisotropies of several axially ordered
and disordered partially saturated and fully saturated nanothreads
within density functional theory and systematically compared the results
with experimental solid-state NMR data to assist in identifying the
structures of the synthesized nanothreads. In the fully saturated
threads, every carbon atom in each progenitor benzene molecule has
bonded to a neighboring molecule (i.e., 6 bonds per molecule, a so-called
“degree-6” nanothread), while the partially saturated
threads examined retain a single double bond per benzene ring (“degree-4”).
The most-parsimonious theoretical fit to the experimental 1D solid-state
NMR spectrum, constrained by the measured chemical shift anisotropies
and key features of two-dimensional NMR spectra, suggests a certain
combination of degree-4 and degree-6 nanothreads as plausible components
of this 1D sp<sup>3</sup> carbon nanomaterial, with intriguing hints
of a [4 + 2] cycloaddition pathway toward nanothread formation from
benzene columns in the progenitor molecular crystal, based on the
presence of nanothreads IV-7, IV-8, and square polymer in the minimal
fit
Constraining Carbon Nanothread Structures by Experimental and Calculated Nuclear Magnetic Resonance Spectra
A one-dimensional
(1D) sp<sup>3</sup> carbon nanomaterial with
high lateral packing order, known as carbon nanothreads, has recently
been synthesized by slowly compressing and decompressing crystalline
solid benzene at high pressure. The atomic structure of an individual
nanothread has not yet been determined experimentally. We have calculated
the <sup>13</sup>C nuclear magnetic resonance (NMR) chemical shifts,
chemical shielding tensors, and anisotropies of several axially ordered
and disordered partially saturated and fully saturated nanothreads
within density functional theory and systematically compared the results
with experimental solid-state NMR data to assist in identifying the
structures of the synthesized nanothreads. In the fully saturated
threads, every carbon atom in each progenitor benzene molecule has
bonded to a neighboring molecule (i.e., 6 bonds per molecule, a so-called
“degree-6” nanothread), while the partially saturated
threads examined retain a single double bond per benzene ring (“degree-4”).
The most-parsimonious theoretical fit to the experimental 1D solid-state
NMR spectrum, constrained by the measured chemical shift anisotropies
and key features of two-dimensional NMR spectra, suggests a certain
combination of degree-4 and degree-6 nanothreads as plausible components
of this 1D sp<sup>3</sup> carbon nanomaterial, with intriguing hints
of a [4 + 2] cycloaddition pathway toward nanothread formation from
benzene columns in the progenitor molecular crystal, based on the
presence of nanothreads IV-7, IV-8, and square polymer in the minimal
fit
Constraining Carbon Nanothread Structures by Experimental and Calculated Nuclear Magnetic Resonance Spectra
A one-dimensional
(1D) sp<sup>3</sup> carbon nanomaterial with
high lateral packing order, known as carbon nanothreads, has recently
been synthesized by slowly compressing and decompressing crystalline
solid benzene at high pressure. The atomic structure of an individual
nanothread has not yet been determined experimentally. We have calculated
the <sup>13</sup>C nuclear magnetic resonance (NMR) chemical shifts,
chemical shielding tensors, and anisotropies of several axially ordered
and disordered partially saturated and fully saturated nanothreads
within density functional theory and systematically compared the results
with experimental solid-state NMR data to assist in identifying the
structures of the synthesized nanothreads. In the fully saturated
threads, every carbon atom in each progenitor benzene molecule has
bonded to a neighboring molecule (i.e., 6 bonds per molecule, a so-called
“degree-6” nanothread), while the partially saturated
threads examined retain a single double bond per benzene ring (“degree-4”).
The most-parsimonious theoretical fit to the experimental 1D solid-state
NMR spectrum, constrained by the measured chemical shift anisotropies
and key features of two-dimensional NMR spectra, suggests a certain
combination of degree-4 and degree-6 nanothreads as plausible components
of this 1D sp<sup>3</sup> carbon nanomaterial, with intriguing hints
of a [4 + 2] cycloaddition pathway toward nanothread formation from
benzene columns in the progenitor molecular crystal, based on the
presence of nanothreads IV-7, IV-8, and square polymer in the minimal
fit
Constraining Carbon Nanothread Structures by Experimental and Calculated Nuclear Magnetic Resonance Spectra
A one-dimensional
(1D) sp<sup>3</sup> carbon nanomaterial with
high lateral packing order, known as carbon nanothreads, has recently
been synthesized by slowly compressing and decompressing crystalline
solid benzene at high pressure. The atomic structure of an individual
nanothread has not yet been determined experimentally. We have calculated
the <sup>13</sup>C nuclear magnetic resonance (NMR) chemical shifts,
chemical shielding tensors, and anisotropies of several axially ordered
and disordered partially saturated and fully saturated nanothreads
within density functional theory and systematically compared the results
with experimental solid-state NMR data to assist in identifying the
structures of the synthesized nanothreads. In the fully saturated
threads, every carbon atom in each progenitor benzene molecule has
bonded to a neighboring molecule (i.e., 6 bonds per molecule, a so-called
“degree-6” nanothread), while the partially saturated
threads examined retain a single double bond per benzene ring (“degree-4”).
The most-parsimonious theoretical fit to the experimental 1D solid-state
NMR spectrum, constrained by the measured chemical shift anisotropies
and key features of two-dimensional NMR spectra, suggests a certain
combination of degree-4 and degree-6 nanothreads as plausible components
of this 1D sp<sup>3</sup> carbon nanomaterial, with intriguing hints
of a [4 + 2] cycloaddition pathway toward nanothread formation from
benzene columns in the progenitor molecular crystal, based on the
presence of nanothreads IV-7, IV-8, and square polymer in the minimal
fit