4,558 research outputs found
Hybrid expansions for local structural relaxations
A model is constructed in which pair potentials are combined with the cluster
expansion method in order to better describe the energetics of structurally
relaxed substitutional alloys. The effect of structural relaxations away from
the ideal crystal positions, and the effect of ordering is described by
interatomic-distance dependent pair potentials, while more subtle
configurational aspects associated with correlations of three- and more sites
are described purely within the cluster expansion formalism. Implementation of
such a hybrid expansion in the context of the cluster variation method or Monte
Carlo method gives improved ability to model phase stability in alloys from
first-principles.Comment: 8 pages, 1 figur
Tidal Venuses: Triggering a Climate Catastrophe via Tidal Heating
Traditionally stellar radiation has been the only heat source considered
capable of determining global climate on long timescales. Here we show that
terrestrial exoplanets orbiting low-mass stars may be tidally heated at high
enough levels to induce a runaway greenhouse for a long enough duration for all
the hydrogen to escape. Without hydrogen, the planet no longer has water and
cannot support life. We call these planets "Tidal Venuses," and the phenomenon
a "tidal greenhouse." Tidal effects also circularize the orbit, which decreases
tidal heating. Hence, some planets may form with large eccentricity, with its
accompanying large tidal heating, and lose their water, but eventually settle
into nearly circular orbits (i.e. with negligible tidal heating) in the
habitable zone (HZ). However, these planets are not habitable as past tidal
heating desiccated them, and hence should not be ranked highly for detailed
follow-up observations aimed at detecting biosignatures. Planets orbiting stars
with masses <0.3 solar masses may be in danger of desiccation via tidal
heating. We apply these concepts to Gl 667C c, a ~4.5 Earth-mass planet
orbiting a 0.3 solar mass star at 0.12 AU. We find that it probably did not
lose its water via tidal heating as orbital stability is unlikely for the high
eccentricities required for the tidal greenhouse. As the inner edge of the HZ
is defined by the onset of a runaway or moist greenhouse powered by radiation,
our results represent a fundamental revision to the HZ for non-circular orbits.
In the appendices we review a) the moist and runaway greenhouses, b) hydrogen
escape, c) stellar mass-radius and mass-luminosity relations, d) terrestrial
planet mass-radius relations, and e) linear tidal theories. [abridged]Comment: 59 pages, 11 figures, accepted to Astrobiology. New version includes
an appendix on the water loss timescal
Genetic analysis of wheat nitrogen use efficiency: coincidence between QTL for agronomical and physiological traits
A 4-unit-cell superstructure in optimally doped YBa2Cu3O6.92 superconductor
Using high-energy diffraction we show that a 4-unit-cell superstructure,
q0=(1/4,0,0), along the shorter Cu-Cu bonds coexists with superconductivity in
optimally doped YBCO. A complex set of anisotropic atomic displacements on
neighboring CuO chain planes, BaO planes, and CuO2 planes, respectively,
correlated over ~3-6 unit cells gives rise to diffuse superlattice peaks. Our
observations are consistent with the presence of Ortho-IV nanodomains
containing these displacements.Comment: Corrected typo in abstrac
Recommended from our members
A Phase II Basket Trial of Dual Anti-CTLA-4 and Anti-PD-1 Blockade in Rare Tumors (DART SWOG 1609) in Patients with Nonpancreatic Neuroendocrine Tumors.
PurposeImmune checkpoint blockade has improved outcomes across tumor types; little is known about the efficacy of these agents in rare tumors. We report the results of the (nonpancreatic) neuroendocrine neoplasm cohort of SWOG S1609 dual anti-CTLA-4 and anti-PD-1 blockade in rare tumors (DART).Patients and methodsWe performed a prospective, open-label, multicenter phase II clinical trial of ipilimumab plus nivolumab across multiple rare tumor cohorts, with the (nonpancreatic) neuroendocrine cohort reported here. Response assessment by grade was not prespecified. The primary endpoint was overall response rate [ORR; RECIST v1.1; complete response (CR) and partial response (PR)]; secondary endpoints included progression-free survival (PFS), overall survival (OS), stable disease >6 months, and toxicity.ResultsThirty-two eligible patients received therapy; 18 (56%) had high-grade disease. Most common primary sites were gastrointestinal (47%; N = 15) and lung (19%; N = 6). The overall ORR was 25% [95% confidence interval (CI) 13-64%; CR, 3%, N = 1; PR, 22%, N = 7]. Patients with high-grade neuroendocrine carcinoma had an ORR of 44% (8/18 patients) versus 0% in low/intermediate grade tumors (0/14 patients; P = 0.004). The 6-month PFS was 31% (95% CI, 19%-52%); median OS was 11 months (95% CI, 6-∞). The most common toxicities were hypothyroidism (31%), fatigue (28%), and nausea (28%), with alanine aminotransferase elevation (9%) as the most common grade 3/4 immune-related adverse event, and no grade 5 events.ConclusionsIpilimumab plus nivolumab demonstrated a 44% ORR in patients with nonpancreatic high-grade neuroendocrine carcinoma, with 0% ORR in low/intermediate grade disease
Multi-filament structures in relativistic self-focusing
A simple model is derived to prove the multi-filament structure of
relativistic self-focusing with ultra-intense lasers. Exact analytical
solutions describing the transverse structure of waveguide channels with
electron cavitation, for which both the relativistic and ponderomotive
nonlinearities are taken into account, are presented.Comment: 21 pages, 12 figures, submitted to Physical Review
Subcontinental heat wave triggers terrestrial and marine, multi-taxa responses
Heat waves have profoundly impacted biota globally over the past decade, especially where their ecological impacts are rapid, diverse, and broad-scale. Although usually considered in isolation for either terrestrial or marine ecosystems, heat waves can straddle ecosystems of both types at subcontinental scales, potentially impacting larger areas and taxonomic breadth than previously envisioned. Using climatic and multi-species demographic data collected in Western Australia, we show that a massive heat wave event straddling terrestrial and maritime ecosystems triggered abrupt, synchronous, and multi-trophic ecological disruptions, including mortality, demographic shifts and altered species distributions. Tree die-off and coral bleaching occurred concurrently in response to the heat wave, and were accompanied by terrestrial plant mortality, seagrass and kelp loss, population crash of an endangered terrestrial bird species, plummeting breeding success in marine penguins, and outbreaks of terrestrial wood-boring insects. These multiple taxa and trophic-level impacts spanned \u3e300,000 km2—comparable to the size of California—encompassing one terrestrial Global Biodiversity Hotspot and two marine World Heritage Areas. The subcontinental multi-taxa context documented here reveals that terrestrial and marine biotic responses to heat waves do not occur in isolation, implying that the extent of ecological vulnerability to projected increases in heat waves is underestimated
The Effect of Lattice Vibrations on Substitutional Alloy Thermodynamics
A longstanding limitation of first-principles calculations of substitutional
alloy phase diagrams is the difficulty to account for lattice vibrations. A
survey of the theoretical and experimental literature seeking to quantify the
impact of lattice vibrations on phase stability indicates that this effect can
be substantial. Typical vibrational entropy differences between phases are of
the order of 0.1 to 0.2 k_B/atom, which is comparable to the typical values of
configurational entropy differences in binary alloys (at most 0.693 k_B/atom).
This paper describes the basic formalism underlying ab initio phase diagram
calculations, along with the generalization required to account for lattice
vibrations. We overview the various techniques allowing the theoretical
calculation and the experimental determination of phonon dispersion curves and
related thermodynamic quantities, such as vibrational entropy or free energy. A
clear picture of the origin of vibrational entropy differences between phases
in an alloy system is presented that goes beyond the traditional bond counting
and volume change arguments. Vibrational entropy change can be attributed to
the changes in chemical bond stiffness associated with the changes in bond
length that take place during a phase transformation. This so-called ``bond
stiffness vs. bond length'' interpretation both summarizes the key phenomenon
driving vibrational entropy changes and provides a practical tool to model
them.Comment: Submitted to Reviews of Modern Physics 44 pages, 6 figure
Ab initio atomistic thermodynamics and statistical mechanics of surface properties and functions
Previous and present "academic" research aiming at atomic scale understanding
is mainly concerned with the study of individual molecular processes possibly
underlying materials science applications. Appealing properties of an
individual process are then frequently discussed in terms of their direct
importance for the envisioned material function, or reciprocally, the function
of materials is somehow believed to be understandable by essentially one
prominent elementary process only. What is often overlooked in this approach is
that in macroscopic systems of technological relevance typically a large number
of distinct atomic scale processes take place. Which of them are decisive for
observable system properties and functions is then not only determined by the
detailed individual properties of each process alone, but in many, if not most
cases also the interplay of all processes, i.e. how they act together, plays a
crucial role. For a "predictive materials science modeling with microscopic
understanding", a description that treats the statistical interplay of a large
number of microscopically well-described elementary processes must therefore be
applied. Modern electronic structure theory methods such as DFT have become a
standard tool for the accurate description of individual molecular processes.
Here, we discuss the present status of emerging methodologies which attempt to
achieve a (hopefully seamless) match of DFT with concepts from statistical
mechanics or thermodynamics, in order to also address the interplay of the
various molecular processes. The new quality of, and the novel insights that
can be gained by, such techniques is illustrated by how they allow the
description of crystal surfaces in contact with realistic gas-phase
environments.Comment: 24 pages including 17 figures, related publications can be found at
http://www.fhi-berlin.mpg.de/th/paper.htm
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