44 research outputs found
Manipulation of graphene's dynamic ripples by local harmonic out-of-plane excitation
With use of carefully designed molecular dynamics simulations, we demonstrate
tuning of dynamic ripples in free-standing graphene by applying a local
out-of-plane sinusoidal excitation. Depending on the boundary conditions and
external modulation, we show control of the local dynamic morphology, including
flattening and stable rippling patterns. In addition to studying the dynamic
response of atomically thin layers to external time-varying excitation, our
results open intriguing possibilities for modulating their properties via local
dynamic morphology control
Low Energy Electron Point Projection Microscopy of Suspended Graphene, the Ultimate "Microscope Slide"
Point Projection Microscopy (PPM) is used to image suspended graphene using
low-energy electrons (100-200eV). Because of the low energies used, the
graphene is neither damaged or contaminated by the electron beam. The
transparency of graphene is measured to be 74%, equivalent to electron
transmission through a sheet as thick as twice the covalent radius of
sp^2-bonded carbon. Also observed is rippling in the structure of the suspended
graphene, with a wavelength of approximately 26 nm. The interference of the
electron beam due to the diffraction off the edge of a graphene knife edge is
observed and used to calculate a virtual source size of 4.7 +/- 0.6 Angstroms
for the electron emitter. It is demonstrated that graphene can be used as both
anode and substrate in PPM in order to avoid distortions due to strong field
gradients around nano-scale objects. Graphene can be used to image objects
suspended on the sheet using PPM, and in the future, electron holography
Graphene as an electronic membrane
Experiments are finally revealing intricate facts about graphene which go
beyond the ideal picture of relativistic Dirac fermions in pristine two
dimensional (2D) space, two years after its first isolation. While observations
of rippling added another dimension to the richness of the physics of graphene,
scanning single electron transistor images displayed prevalent charge
inhomogeneity. The importance of understanding these non-ideal aspects cannot
be overstated both from the fundamental research interest since graphene is a
unique arena for their interplay, and from the device applications interest
since the quality control is a key to applications. We investigate the membrane
aspect of graphene and its impact on the electronic properties. We show that
curvature generates spatially varying electrochemical potential. Further we
show that the charge inhomogeneity in turn stabilizes ripple formation.Comment: 6 pages, 11 figures. Updated version with new results about the
re-hybridization of the electronic orbitals due to rippling of the graphene
sheet. The re-hybridization adds the next-to-nearest neighbor hopping effect
discussed in the previous version. New reference to recent STM experiments
that give support to our theor
Buckling Thin Disks and Ribbons with Non-Euclidean Metrics
I consider the problem of a thin membrane on which a metric has been
prescribed, for example by lithographically controlling the local swelling
properties of a polymer thin film. While any amount of swelling can be
accommodated locally, geometry prohibits the existence of a global strain-free
configuration. To study this geometrical frustration, I introduce a
perturbative approach. I compute the optimal shape of an annular, thin ribbon
as a function of its width. The topological constraint of closing the ribbon
determines a relationship between the mean curvature and number of wrinkles
that prevents a complete relaxation of the compression strain induced by
swelling and buckles the ribbon out of the plane. These results are then
applied to thin, buckled disks, where the expansion works surprisingly well. I
identify a critical radius above which the disk in-plane strain cannot be
relaxed completely.Comment: 6 pages, 5 figures; lengthened to clarify previously confusing
issues. To appear in EP
Early Release Science of the exoplanet WASP-39b with JWST NIRSpec PRISM
Transmission spectroscopy of exoplanets has revealed signatures of water
vapor, aerosols, and alkali metals in a few dozen exoplanet atmospheres.
However, these previous inferences with the Hubble and Spitzer Space Telescopes
were hindered by the observations' relatively narrow wavelength range and
spectral resolving power, which precluded the unambiguous identification of
other chemical speciesin particular the primary carbon-bearing molecules.
Here we report a broad-wavelength 0.5-5.5 m atmospheric transmission
spectrum of WASP-39 b, a 1200 K, roughly Saturn-mass, Jupiter-radius exoplanet,
measured with JWST NIRSpec's PRISM mode as part of the JWST Transiting
Exoplanet Community Early Release Science Team program. We robustly detect
multiple chemical species at high significance, including Na (19),
HO (33), CO (28), and CO (7). The non-detection
of CH, combined with a strong CO feature, favours atmospheric models
with a super-solar atmospheric metallicity. An unanticipated absorption feature
at 4m is best explained by SO (2.7), which could be a tracer
of atmospheric photochemistry. These observations demonstrate JWST's
sensitivity to a rich diversity of exoplanet compositions and chemical
processes.Comment: 41 pages, 4 main figures, 10 extended data figures, 4 tables. Under
review in Natur
Identification of carbon dioxide in an exoplanet atmosphere
Carbon dioxide (CO2) is a key chemical species that is found in a wide range of planetary atmospheres. In the context of exoplanets, CO2 is an indicator of the metal enrichment (that is, elements heavier than helium, also called 'metallicity')1-3, and thus the formation processes of the primary atmospheres of hot gas giants4-6. It is also one of the most promising species to detect in the secondary atmospheres of terrestrial exoplanets7-9. Previous photometric measurements of transiting planets with the Spitzer Space Telescope have given hints of the presence of CO2, but have not yielded definitive detections owing to the lack of unambiguous spectroscopic identification10-12. Here we present the detection of CO2 in the atmosphere of the gas giant exoplanet WASP-39b from transmission spectroscopy observations obtained with JWST as part of the Early Release Science programme13,14. The data used in this study span 3.0-5.5 micrometres in wavelength and show a prominent CO2 absorption feature at 4.3 micrometres (26-sigma significance). The overall spectrum is well matched by one-dimensional, ten-times solar metallicity models that assume radiative-convective-thermochemical equilibrium and have moderate cloud opacity. These models predict that the atmosphere should have water, carbon monoxide and hydrogen sulfide in addition to CO2, but little methane. Furthermore, we also tentatively detect a small absorption feature near 4.0 micrometres that is not reproduced by these models
Identification of carbon dioxide in an exoplanet atmosphere
Carbon dioxide (CO2) is a key chemical species that is found in a wide range of planetary atmospheres. In the context of exoplanets, CO2 is an indicator of the metal enrichment (that is, elements heavier than helium, also called ‘metallicity’)1–3, and thus the formation processes of the primary atmospheres of hot gas giants4–6. It is also one of the most promising species to detect in the secondary atmospheres of terrestrial exoplanets7–9. Previous photometric measurements of transiting planets with the Spitzer Space Telescope have given hints of the presence of CO2, but have not yielded definitive detections owing to the lack of unambiguous spectroscopic identification10–12. Here we present the detection of CO2 in the atmosphere of the gas giant exoplanet WASP-39b from transmission spectroscopy observations obtained with JWST as part of the Early Release Science programme13,14. The data used in this study span 3.0–5.5 micrometres in wavelength and show a prominent CO2 absorption feature at 4.3 micrometres (26-sigma significance). The overall spectrum is well matched by one-dimensional, ten-times solar metallicity models that assume radiative–convective–thermochemical equilibrium and have moderate cloud opacity. These models predict that the atmosphere should have water, carbon monoxide and hydrogen sulfide in addition to CO2, but little methane. Furthermore, we also tentatively detect a small absorption feature near 4.0 micrometres that is not reproduced by these models
Effect of Wrinkles on the Surface Area of Graphene: Toward the Design of Nanoelectronics
Finishing the euchromatic sequence of the human genome
The sequence of the human genome encodes the genetic instructions for human physiology, as well as rich information about human evolution. In 2001, the International Human Genome Sequencing Consortium reported a draft sequence of the euchromatic portion of the human genome. Since then, the international collaboration has worked to convert this draft into a genome sequence with high accuracy and nearly complete coverage. Here, we report the result of this finishing process. The current genome sequence (Build 35) contains 2.85 billion nucleotides interrupted by only 341 gaps. It covers ∼99% of the euchromatic genome and is accurate to an error rate of ∼1 event per 100,000 bases. Many of the remaining euchromatic gaps are associated with segmental duplications and will require focused work with new methods. The near-complete sequence, the first for a vertebrate, greatly improves the precision of biological analyses of the human genome including studies of gene number, birth and death. Notably, the human enome seems to encode only 20,000-25,000 protein-coding genes. The genome sequence reported here should serve as a firm foundation for biomedical research in the decades ahead
Early Release Science of the exoplanet WASP-39b with JWST NIRSpec G395H
This is the author accepted manuscript. The final version is available from Nature Research via the DOI in this recordData Availability:
The data used in this paper are associated with JWST program ERS 1366 (observation #4) and
are available from the Mikulski Archive for Space Telescopes (https://mast.stsci.edu). Science
data processing version (SDP_VER) 2022_2a generated the uncalibrated data that we
downloaded from MAST. We used JWST Calibration Pipeline software version (CAL_VER)
1.5.3 with modifications described in the text. We used calibration reference data from context
(CRDS_CTX) 0916, except as noted in the text. All the data and models presented in this
publication can be found at 10.5281/zenodo.7185300.Code Availability:
The codes used in this publication to extract, reduce and analyze the data are as follows;
STScI JWST Calibration pipeline45 (https://github.com/spacetelescope/jwst), Eureka!53
(https://eurekadocs.readthedocs.io/en/latest/), ExoTiC-JEDI47 (https://github.com/ExoTiC/ExoTiC-JEDI), juliet71 (https://juliet.readthedocs.io/en/latest/), Tiberius15,49,50,
transitspectroscopy51 (https://github.com/nespinoza/transitspectroscopy). In addition, these
made use of batman65 (http://lkreidberg.github.io/batman/docs/html/index.html), celerite86
(https://celerite.readthedocs.io/en/stable/), chromatic (https://zkbt.github.io/chromatic/),
Dynesty72 (https://dynesty.readthedocs.io/en/stable/index.html), emcee69
(https://emcee.readthedocs.io/en/stable/), exoplanet83 (https://docs.exoplanet.codes/en/latest/),
ExoTEP75–77, ExoTHETyS79 (https://github.com/ucl-exoplanets/ExoTETHyS), ExoTiCISM57 (https://github.com/Exo-TiC/ExoTiC-ISM), ExoTiC-LD58 (https://exoticld.readthedocs.io/en/latest/), george68 (https://george.readthedocs.io/en/latest/) JAX82
(https://jax.readthedocs.io/en/latest/), LMFIT70 (https://lmfit.github.io/lmfit-py/),
Pylightcurve78 (https://github.com/ucl-exoplanets/pylightcurve), Pymc3138
(https://docs.pymc.io/en/v3/index.html) and Starry84 (https://starry.readthedocs.io/en/latest/),
each of which use the standard python libraries astropy139,140, matplotlib141, numpy142,
pandas143, scipy64 and xarray144. The atmospheric models used to fit the data can be found at
ATMO[Tremblin2015,Drummond2016,Goyal2018,Goyal2020]88–91, PHOENIX92–94,
PICASO98,99 (https://natashabatalha.github.io/picaso/), Virga98,107
(https://natashabatalha.github.io/virga/), and gCMCRT115
(https://github.com/ELeeAstro/gCMCRT).Measuring the abundances of carbon and oxygen in exoplanet atmospheres is considered a crucial avenue for unlocking the formation and evolution of exoplanetary systems. Access to an exoplanet’s chemical inventory requires high precision observations, often inferred from individual molecular detections with low-resolution space-based and high-resolution ground-based facilities. Here we report the medium-resolution (R≈600) transmission spectrum of an exoplanet atmosphere between 3–5 μm covering multiple absorption features for the Saturn-mass exoplanet WASP-39b, obtained with JWST NIRSpec G395H. Our observations achieve 1.46×
photon precision, providing an average transit depth uncertainty of 221 ppm per spectroscopic bin, and present minimal impacts from systematic effects. We detect significant absorption from CO2 (28.5σ
) and H2O (21.5σ
), and identify SO2 as the source of absorption at 4.1 μ
m (4.8σ
). Best-fit atmospheric models range between 3×
and 10×
solar metallicity, with sub-solar to solar C/O ratios. These results, including the detection of SO2, underscore the importance of characterising the chemistry in exoplanet atmospheres, and showcase NIRSpec G395H as an excellent mode for time series observations over this critical wavelength range.Science and Technology Facilities Council (STFC)UKR