“No commercial company without the support of NASA and government is going to get to Mars,”
Dream on Chuck – “There’s no place like home.”
Launch Vehicle Designer and Space Architect
“No commercial company without the support of NASA and government is going to get to Mars,”
Dream on Chuck – “There’s no place like home.”
Electromagnetism on ice: classical and quantum theories of proton disorder in hexagonal water ice, Owen Benton, Olga Sikora and Nic Shannon
Water ice is simultaneously one of the most ubiquitous and most mysterious materials on Earth. The arrangement of protons in hexagonal water ice (ice Ih) is subject to strong local constraints (“the ice rules”) but remains globally disordered, leading to a large entropy even at very low temperatures. Experimental measurements have indicated that quantum tunnelling of protons persists down to the low temperature regime, possibily proceeding via loop processes which preserve the ice rules. In a related class of systems- “the spin ices” such processes are known to give rise to a quantum spin-liquid state at low temperatures, featuring fractionalized “magnetic monopole” excitations and emergent photons. In this paper, we explore the consequences of the formation of an analogous “proton liquid” state in ice Ih, via the same mechanism. We construct field theories describing both the classical and quantum regimes of proton correlations obtaining good agreement with Monte Carlo simulations in both cases. We find that the quantum liquid state possesses birefringent “photons” and additional, gapped, local fluctuations of the electric polarisation.
Shocking! Call the neighbors, wake the kids.
Mystery of Ceres’ bright spots grows, Alexandra Witze, Nature (13 April 2015), doi:10.1038/nature.2015.17313
SpaceX image taken from the drone.
Update: Another SpaceX Close But No Cigar Moment.
Probing the electron-phonon interaction in correlated systems with coherent lattice fluctuation spectroscopy, Andreas Mann, Edoardo Baldini, Antonio Tramontana, Ekaterina Pomjakushina, Kazimierz Conder, Christopher Arrell, Frank van Mourik, José Lorenzana and Fabrizio Carbone
Tailoring the properties of correlated oxides is accomplished by chemical doping, pressure, temperature or magnetic field. Photoexcitation is a valid alternative to reach out-of-equilibrium states otherwise inaccessible. Here, we quantitatively estimate the coupling between a lattice distortion and the charge-transfer excitation in (La2CuO4+δ). We photoinduce a coherent La ion vibration and monitor the response of the optical constants in a broad energy range, providing quantitative information on the electron-phonon matrix element that can be compared to theoretical models. We propose the same methodology to probe electron-electron interactions in other materials.
Giant Oscillating Thermopower at Oxide Interfaces, I. Pallecchi, F. Telesio, D. Li, A. Fête, S. Gariglio, J.-M. Triscone, A. Filippetti, P. Delugas, V. Fiorentini and D. Marré
Understanding the nature of charge carriers at the LaAlO3/SrTiO3 interface is one of the major open issues in the full comprehension of the charge confinement phenomenon in oxide heterostructures. Here, we investigate thermopower to study the electronic structure in LaAlO3/SrTiO3 at low temperature as a function of gate field. In particular, under large negative gate voltage, corresponding to the strongly depleted charge density regime, thermopower displays record-high negative values of the order of 104 – 105 µV/K, oscillating at regular intervals as a function of the gate voltage. The huge thermopower magnitude can be attributed to the phonon-drag contribution, while the oscillations map the progressive depletion and the Fermi level descent across a dense array of localized states lying at the bottom of the Ti 3d conduction band. This study is the first direct evidence of a localized Anderson tail in the two-dimensional (2D) electron liquid at the LaAlO3/SrTiO3 interface.
This just keeps getting better every day.
Coupling to real and virtual phonons in tunneling spectroscopy of superconductors, Jasmin Jandke, Patrik Hlobil, Michael Schackert, Wulf Wulfhekel and Jörg Schmalian
Fine structures in the tunneling spectra of superconductors have been widely used to identify fingerprints of the interaction responsible for Cooper pairing. Here we show that for scanning tunneling microscopy (STM) of Pb, the inclusion of inelastic tunneling processes is crucial for the proper interpretation of these fine structures. For STM the usual McMillan inversion algorithm of tunneling spectra must therefore be modified to include inelastic tunneling events, an insight that is crucial for the identification of the pairing glue in conventional and unconventional superconductors alike.
Oxygen Isotope Effects on Lattice Properties of La2-xBaxCuO4 (x = 1/8), Z. Guguchia, D. Sheptyakov, E. Pomjakushina, K. Conder, R. Khasanov, A. Shengelaya, A. Simon, A. Bussmann-Holder and H. Keller
A novel negative oxygen-isotope (16O/18O) effect (OIE) on the low-temperature tetragonal phase transition temperature TLTT was observed in La2-xBaxCuO4 (x = 1/8) using high-resolution neutron powder diffraction. The corresponding OIE exponent α(TLTT) = – 0.36(5) has the same sign as α(Tso) = -0.57(6) found for the spin-stripe order temperature Tso. The fact that the LTT transition is accompanied by charge ordering (CO) implies the presence of an OIE also for the CO temperature Tco. Furthermore, a temperature dependent shortening of the c-axis with the heavier isotope is observed. These results combined with model calculations demonstrate that anharmonic electron-lattice interactions are essential for all transitions observed in the stripe phase of cuprates.
A new type of Weyl semimetal with quadratic double Weyl fermions in SrSi2, Shin-Ming Huang, Su-Yang Xu, Ilya Belopolski, Chi-Cheng Lee, Guoqing Chang, BaoKai Wang, Nasser Alidoust, Madhab Neupane, Hao Zheng, Daniel Sanchez, Arun Bansil, Guang Bian, Hsin Lin and M. Zahid Hasan
We identify a Weyl semimetal state in an inversion breaking, stoichiometric compound strontium silicide, SrSi2, with many new and novel properties that are distinct from the TaAs family. We theoretically show that SrSi2 is a Weyl semimetal even without spin-orbit coupling and that, after the inclusion of spin-orbit coupling, two Weyl fermions stick together forming an exotic double Weyl fermion with quadratic dispersions and a higher chiral topological charge of 2. Moreover, we find that the Weyl nodes with opposite charges are located at different energies due to the absence of mirror symmetry in SrSi2, leading to a unique topological quantum response that an external magnetic field can induce a dissipationless current. Our systematic results not only identify a much-needed robust Weyl semimetal candidate but also open the door to new topological Weyl physics that is not possible in the TaAs family of materials.
I’m cracking up! I can’t take it anymore!
Highly Siderophile Elements in the Earth’s Mantle as a Clock for the Moon-forming Impact, Seth A. Jacobson, Alessandro Morbidelli, Sean N. Raymond, David P. O’Brien, Kevin J. Walsh and David C. Rubie
According to the generally accepted scenario, the last giant impact on the Earth formed the Moon and initiated the final phase of core formation by melting the Earth’s mantle. A key goal of geochemistry is to date this event, but different ages have been proposed. Some argue for an early Moon-forming event, approximately 30 million years (Myr) after the condensation of the first solids in the Solar System, whereas others claim a date later than 50 Myr (and possibly as late as around 100 My) after condensation. Here we show that a Moon-forming event at 40 Myr after condensation, or earlier, is ruled out at a 99.9 per cent confidence level. We use a large number of N-body simulations to demonstrate a relationship between the time of the last giant impact on an Earth-like planet and the amount of mass subsequently added during the era known as Late Accretion. As the last giant impact is delayed, the late-accreted mass decreases in a predictable fashion. This relationship exists within both the classical scenario and the Grand Tack scenario of terrestrial planet formation, and it holds across a wide range of disk conditions. The concentration of highly siderophile elements (HSEs) in Earth’s mantle constrains the mass of chondritic material added to Earth during Late Accretion. Using HSE abundance measurements, we determine a Moon-formation age of 95 +/- 32 Myr since condensation. The possibility exists that some late projectiles were differentiated and left an incomplete HSE record in Earth’s mantle. Even in this case, various isotopic constraints strongly suggest that the late-accreted mass did not exceed 1 per cent of Earth’s mass, and so the HSE clock still robustly limits the timing of the Moon-forming event to significantly later than 40 My after condensation.
Discovery of Weyl semimetal NbAs, Su-Yang Xu, Nasser Alidoust, Ilya Belopolski, Chenglong Zhang, Guang Bian, Tay-Rong Chang, Hao Zheng, Vladimir Strokov, Daniel S. Sanchez, Guoqing Chang, Zhujun Yuan, Daixiang Mou, Yun Wu, Lunan Huang, Chi-Cheng Lee, Shin-Ming Huang, BaoKai Wang, Arun Bansil, Horng-Tay Jeng, Titus Neupert, Adam Kaminski, Hsin Lin, Shuang Jia and M. Zahid Hasan
We report the discovery of Weyl semimetal NbAs featuring unusual Fermi arc surface states.
Three types of Fermions play a fundamental role in our understanding of nature: Dirac, Majorana, and Weyl. While Dirac fermions are known, the latter two have not been observed as any fundamental particle in high energy physics and have emerged as a much-sought-out treasure in condensed matter physics. A Weyl semimetal is a novel crystal whose low-energy electronic excitations behave as Weyl fermions. It has received worldwide interest and is believed to open the next era of condensed matter physics after graphene and three-dimensional topological insulators.
However, experimental research has been held back because Weyl semimetals are extremely rare in nature. Here, we present the experimental discovery of the Weyl semimetal state in niobium arsenide (NbAs). Utilizing the combination of soft X-ray and ultraviolet photoemission spectroscopy,
we systematically study both the surface and bulk electronic structure of NbAs. We experimentally observe both the Weyl cones and Weyl nodes in the bulk and the Fermi arcs on the surface of this system. Our ARPES data, in agreement with our previous theoretical prediction and present band structure calculations, provide conclusive evidence for the topological Weyl semimetal state in NbAs. Our discovery not only paves the way for the many fascinating topological quantum phenomena predicted in Weyl semimetals, but also establishes a new cornerstone in the correspondence between high-energy and condensed matter physics.
Ok then, the revolution will be televised in quantum holography.
Hasan is really tearing it up.
See the original theory paper here: http://arxiv.org/abs/1501.00755
An inversion breaking Weyl semimetal state in the TaAs material class, Shin-Ming Huang, Su-Yang Xu, Ilya Belopolski, Chi-Cheng Lee, Guoqing Chang, BaoKai Wang, Nasser Alidoust, Guang Bian, Madhab Neupane, Arun Bansil, Hsin Lin and M. Zahid Hasan
The recent discoveries of Dirac fermions in graphene and on the surface of topological insulators have ignited worldwide interest in physics and materials science. A Weyl semimetal is an unusual crystal where electrons also behave as massless quasi-particles but interestingly they are not Dirac fermions. These massless particles, Weyl fermions, were originally considered in massless quantum electrodynamics but have not been observed as a fundamental particle in nature. A Weyl semimetal provides a condensed matter realization of Weyl fermions, leading to unique transport properties with novel device applications. Such a semimetal is also a topologically non-trivial metallic phase of matter extending the classification of topological phases beyond insulators. The signature of a Weyl semimetal in real materials is the existence of unusual Fermi arc surface states, which can be viewed as half of a surface Dirac cone in a topological insulator. Here, we identify the first Weyl semimetal in a class of stoichiometric materials, which break crystalline inversion symmetry, including TaAs, TaP, NbAs and NbP. Our first-principles calculations on TaAs reveal the spin-polarized Weyl cones and Fermi arc surface states in this compound. We also observe pairs of Weyl points with the same chiral charge which project onto the same point in the surface Brillouin zone, giving rise to multiple Fermi arcs connecting to a given Weyl point. Our results show that TaAs is the first topological semimetal identified which does not depend on fine-tuning of chemical composition or magnetic order, greatly facilitating an exploration of Weyl physics in real materials.
The Need for Speed in Near-Earth Asteroid Characterization, J. L. Galache, C. L. Beeson, K. K. McLeod, M. Elvis, Accepted for Publication in Planetary & Space Science
We have used Minor Planet Center data and tools to explore the discovery circumstances and properties of the currently known population of over 10,000 NEAs, and to quantify the challenges for follow-up from ground-based telescopes. The increasing rate of discovery has grown to ~1,000/year as surveys have become more sensitive, by 1mag every ~7.5 years. However, discoveries of large (H =< 22) NEAs have remained stable at ~365/year over the past decade, at which rate the 2005 Congressional mandate to find 90% of 140m NEAs will not be met before 2030. Meanwhile, characterization is falling farther behind: Fewer than 10% of NEAs are well characterized in terms of size, rotation periods, and spectra, and at current rates of follow-up it will take about a century to determine them even for the known population. Over 60% of NEAs have an orbital uncertainty parameter, U >= 4, making reacquisition more than a year following discovery difficult; for H > 22 this fraction is over 90%. We argue that rapid follow-up will be essential to characterize newly-discovered NEAs. Most new NEAs are found within 0.5mag of peak brightness and fade quickly, typically by 0.5/3.5/5mag after 1/4/6 weeks. About 80% have synodic periods of <3 years that bring them close to Earth several times a decade. However, follow-up observations on subsequent apparitions will be near impossible for the bulk of new discoveries, as these will be H > 22 NEAs that tend to return 100 times fainter. We show that for characterization to keep pace with discovery would require: Visible spectroscopy within days with a dedicated >2m telescope; long-arc astrometry, used also for phase curves, with a >4m telescope; and fast-cadence (
= 4m telescope. For the already-known large (H =< 22) NEAs, subsequent-apparition spectroscopy, astrometry, and photometry could be done with 1-2m telescopes.
The Case for a Hot Archean Climate and its Implications to the History of the Biosphere, David W. Schwartzman
The case for a much warmer climate on the early Earth than now is presented. The oxygen isotope record in sedimentary chert and the compelling case for a near constant isotopic oxygen composition of seawater over geologic time support thermophilic surface temperatures prevailing in the Archean, with some support for hot conditions lasting until about 1.5 billion years ago, aside from lower temperatures including glacial episodes at 2.1-2.4 Ga and possibly an earlier one at 2.9 Ga. Other evidence includes the following: 1) Melting temperatures of proteins resurrected from sequences inferred from robust molecular phylogenies give paleotemperatures at emergence consistent with a very warm early climate. 2) High atmospheric pCO2 levels in the Archean are consistent with high climatic temperatures near the triple point of primary iron minerals in banded iron formations, the formation of Mn-bicarbonate clusters leading to oxygenic photosynthesis and generally higher weathering intensities on land. These higher weathering intensities would not have occurred if seafloor weathering dominated the carbon sink, pulling down the temperature, hence this empirical evidence supports a hot climate and high carbon dioxide levels. 3) The inferred viscosity of seawater at 2.7 Ga is consistent with a hot Archean climate. 5) A cold Archean is hard to explain taking into account the higher outgassing rates of carbon dioxide, significantly smaller land areas and weaker biotic enhancement of weathering than present in the context of the long-term carbon cycle, taking into account the fainter Archean sun in climate modeling. This evidence points to an important conclusion regarding biological evolution, namely to the critical role of a temperature constraint holding back the emergence of major organismal groups, starting with phototrophs, culminating with metazoans in the latest Precambrian.
Relation between the strength and dimensionality of defect-free carbon crystals, Sergey Kotrechko, Andrey Timoshevskii, Eugene Kolyvoshko and Yuriy Matviychuk
On the basis of ab-initio simulations, the value of strength of interatomic bonds in one-, two- and three-dimensional carbon crystals is obtained. It is shown that decreasing in dimensionality of crystal gives rise to nearly linear increase in strength of atomic bonds. It is ascertained that growth of strength of the crystal with a decrease in it dimensionality is due to both a reduction in coordination number of atom and increase in the angle between the directions of atomic bonds. Based on these data, it is substantiated that the one-dimensional crystals have maximum strength, and strength of carbyne is the absolute upper limit of strength of materials.
The Mythical Unobtanium.
Pressure-induced Lifshitz transition in black phosphorus, Z. J. Xiang, G. J. Ye, C. Shang, B. Lei, N. Z. Wang, K. S. Yang, D. Y. Liu, F. B. Meng, X. G. Luo, L. J. Zou, Z. Sun, Y. B. Zhang and X.H. Chen,
In a semimetal, both electron and hole carriers contribute to the density of states at the Fermi level. The small band overlaps and multi-band effects give rise to many novel electronic properties, such as relativistic Dirac fermions with linear dispersion, titanic magnetoresistance and unconventional superconductivity. Black phosphorus has recently emerged as an exceptional semiconductor with high carrier mobility and a direct, tunable bandgap. Of particular importance is the search for exotic electronic states in black phosphorus, which may amplify the material’s potential beyond semiconductor devices. Here we show that a moderate hydrostatic pressure effectively suppresses the band gap and induces a Lifshitz transition from semiconductor to semimetal in black phosphorus; a colossal magnetoresistance is observed in the semimetallic phase. Quantum oscillations in high magnetic field reveal the complex Fermi surface topology of the semimetallic black phosphorus. In particular, a Dirac-like fermion emerges at around 1.2 GPa, which is continuously tuned by external pressure. The observed semi-metallic behavior greatly enriches black phosphorus’s material property, and sets the stage for the exploration of novel electronic states in this material. Moreover, these interesting behaviors make phosphorene a good candidate for the realization of a new two-dimensional relativistic electron system, other than graphene.
Phosphorus is the new bismuth!
Rockets Shake And Rattle, So SpaceX Rolls Homegrown CFD, Timothy Prickett Morgan, The Platform (27 March 2015)
The Platform is a new publication, which formally launched February 23, 2015, in partnership with The Register. It will offer in-depth coverage of high-end computing at large enterprises, supercomputing centers, hyperscale data centers, and public clouds.
One Satellite Data Set is Underestimating Global Warming
Removing Diurnal Cycle Contamination in Satellite-Derived Tropospheric Temperatures: Understanding Tropical Tropospheric Trend Discrepancies, Stephen Po-Chedley, Tyler J. Thorsen, and Qiang Fu, J. Climate, 28, 2274–2290 (March 2015)
Independent research teams have constructed long-term tropical time series of the temperature of the middle troposphere (TMT) using satellite Microwave Sounding Unit (MSU) and Advanced MSU (AMSU) measurements. Despite careful efforts to homogenize the MSU/AMSU measurements, tropical TMT trends beginning in 1979 disagree by more than a factor of 3. Previous studies suggest that the discrepancy in tropical TMT trends is caused by differences in both the NOAA-9 warm target factor and diurnal drift corrections. This work introduces a new observationally based method for removing biases related to satellite diurnal drift. Over land, the derived diurnal correction is similar to a general circulation model (GCM) diurnal cycle. Over ocean, the diurnal corrections have a negligible effect on TMT trends, indicating that oceanic biases are small. It is demonstrated that this method is effective at removing biases between coorbiting satellites and biases between nodes of individual satellites. Using a homogenized TMT dataset, the ratio of tropical tropospheric temperature trends relative to surface temperature trends is in accord with the ratio from GCMs. It is shown that bias corrections for diurnal drift based on a GCM produce tropical trends very similar to those from the observationally based correction, with a trend difference smaller than 0.02 K decade−1. Differences between various TMT datasets are explored further. Large differences in tropical TMT trends between this work and that of the University of Alabama in Huntsville (UAH) are attributed to differences in the treatment of the NOAA-9 target factor and the diurnal cycle correction.
Roy Spencer and John Christy, that’s what’s funny.
Some splainin’ will be goin on.
Answer : Zero.
Wiedemann-Franz law in the underdoped cuprate superconductor YBa2Cu3Oy, G. Grissonnanche, F. Laliberte, S. Dufour-Beausejour, M. Matusiak, S. Badoux, F. F. Tafti, B. Michon, A. Riopel, O. Cyr-Choiniere, J. C. Baglo, B. J. Ramshaw, R. Liang, D. A. Bonn, W. N. Hardy, S. Kramer, D. LeBoeuf, D. Graf, N. Doiron-Leyraud and L. Taillefer
The recent detection of charge-density modulations in YBa2Cu3Oy and other cuprate superconductors raises new questions about the normal state of underdoped cuprates. In one class of theories, the modulations are intertwined with pairing in a dual state, expected to persist up to high magnetic fields as a vortex liquid. In support of such a state, specific heat and magnetisation data on YBa2Cu3Oy have been interpreted in terms of a vortex liquid persisting above the vortex-melting field Hvs at T = 0. Here we report high-field measurements of the electrical and thermal Hall conductivities in YBa2Cu3O6.54 that allow us to probe the Wiedemann-Franz law, a sensitive test of the presence of superconductivity in a metal. In the T = 0 limit, we find that the law is satisfied for fields immediately above Hvs. This rules out the existence of a vortex liquid and it places strict constraints on the nature of the normal state in underdoped cuprates.
Well I guess it’s back to the drawing boards again.
Volume loss from Antarctic ice shelves is accelerating, Fernando S. Paolo, Helen A. Fricker and Laurie Padman, Science (26 March 2015), DOI: 10.1126/science.aaa0940
The floating ice shelves surrounding the Antarctic Ice Sheet restrain the grounded ice-sheet flow. Thinning of an ice shelf reduces this effect, leading to an increase in ice discharge to the ocean. Using eighteen years of continuous satellite radar altimeter observations we have computed decadal-scale changes in ice-shelf thickness around the Antarctic continent. Overall, average ice-shelf volume change accelerated from negligible loss at 25 ± 64 km3 per year for 1994-2003 to rapid loss of 310 ± 74 km3 per year for 2003-2012. West Antarctic losses increased by 70% in the last decade, and earlier volume gain by East Antarctic ice shelves ceased. In the Amundsen and Bellingshausen regions, some ice shelves have lost up to 18% of their thickness in less than two decades.
The Antarctic ice sheets can’t be far behind.
This result has been out there for a while now.
Quasiparticle mass enhancement approaching optimal doping in a high-Tc superconductor, B. J. Ramshaw, S. E. Sebastian, R. D. McDonald, James Day, B. S. Tan, Z. Zhu, J. B. Betts, Ruixing Liang, D. A. Bonn, W. N. Hardy and N. Harrison, Science (26 March 2015), DOI: 10.1126/science.aaa4990
In the quest for superconductors with higher transition temperatures (Tc), one emerging motif is that electronic interactions favorable for superconductivity can be enhanced by fluctuations of a broken-symmetry phase. Recent experiments have suggested the existence of the requisite broken symmetry phase in the high-Tc cuprates, but the impact of such a phase on the ground-state electronic interactions has remained unclear. We use magnetic fields exceeding 90 Tesla to access the underlying metallic state of the cuprate YBa2Cu3O6+δ over a wide range of doping, and observe magnetic quantum oscillations that reveal a strong enhancement of the quasiparticle effective mass toward optimal doping. This mass enhancement results from increasing electronic interactions approaching optimal doping, and suggests a quantum-critical point at a hole doping of pcrit ≈ 0.18.
Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation, Stefan Rahmstorf, Jason E. Box, Georg Feulner, Michael E. Mann, Alexander Robinson, Scott Rutherford and Erik J. Schaffernicht, Nature Climate Change (23 March 2015), doi:10.1038/nclimate2554
Possible changes in Atlantic meridional overturning circulation (AMOC) provide a key source of uncertainty regarding future climate change. Maps of temperature trends over the twentieth century show a conspicuous region of cooling in the northern Atlantic. Here we present multiple lines of evidence suggesting that this cooling may be due to a reduction in the AMOC over the twentieth century and particularly after 1970. Since 1990 the AMOC seems to have partly recovered. This time evolution is consistently suggested by an AMOC index based on sea surface temperatures, by the hemispheric temperature difference, by coral-based proxies and by oceanic measurements. We discuss a possible contribution of the melting of the Greenland Ice Sheet to the slowdown. Using a multi-proxy temperature reconstruction for the AMOC index suggests that the AMOC weakness after 1975 is an unprecedented event in the past millennium (p > 0.99). Further melting of Greenland in the coming decades could contribute to further weakening of the AMOC.
Damn, it’s cold here. I mean hot. Conservation of energy is weird that way.
Growth of asteroids, planetary embryos and Kuiper belt objects by chondrule accretion, Anders Johansen, Mordecai-Mark Mac Low, Pedro Lacerda and Martin Bizzarro, Accepted for Publication in Science Advances (A New AAAS Journal)
Chondrules are millimeter-sized spherules that dominate primitive meteorites (chondrites) originating from the asteroid belt. The incorporation of chondrules into asteroidal bodies must be an important step in planet formation, but the mechanism is not understood. We show that the main growth of asteroids can result from gas-drag-assisted accretion of chondrules. The largest planetesimals of a population with a characteristic radius of 100 km undergo run-away accretion of chondrules within ~3 Myr, forming planetary embryos up to Mars sizes along with smaller asteroids whose size distribution matches that of main belt asteroids. The aerodynamical accretion leads to size-sorting of chondrules consistent with chondrites. Accretion of mm-sized chondrules and ice particles drives the growth of planetesimals beyond the ice line as well, but the growth time increases above the disk life time outside of 25 AU. The contribution of direct planetesimal accretion to the growth of both asteroids and Kuiper belt objects is minor. In contrast, planetesimal accretion and chondrule accretion play more equal roles for the formation of Moon-sized embryos in the terrestrial planet formation region. These embryos are isolated from each other and accrete planetesimals only at a low rate. However, the continued accretion of chondrules destabilizes the oligarchic configuration and leads to the formation of Mars-sized embryos and terrestrial planets by a combination of direct chondrule accretion and giant impacts.
On the rotation rates and axis ratios of the smallest known near-Earth asteroids – the archetypes of the Asteroid Redirect Mission targets, Patrick Hatch and Paul Wiegert, Accepted by Planetary and Space Science
NASA’s Asteroid Redirect Mission (ARM) has been proposed with the aim to capture a small asteroid a few meters in size and redirect it into an orbit around the Moon. There it can be investigated at leisure by astronauts aboard an Orion or other spacecraft. The target for the mission has not yet been selected, and there are very few potential targets currently known. Though sufficiently small near-Earth asteroids (NEAs) are thought to be numerous, they are also difficult to detect and characterize with current observational facilities. Here we collect the most up-to-date information on the smallest known near-Earth asteroids to outline the properties of these small NEAs as currently understood, in order to examine what the eventual ARM target might be like. Observational biases certainly mean that our sample is not an ideal representation of the true population of small NEAs. However our sample is representative of the eventual target list for the ARM mission, which will be compiled under very similar observational constraints unless dramatic changes are made to the way near-Earth asteroids are searched for and studied.
We find that the typical rotation period is 40 minutes. The mean and median axis ratios were 1.43 and 1.29. Rotation rates much faster than the spin barrier are seen, reaching below 30 seconds, and implying that most of these bodies are monoliths. Non-principal axis rotation is uncommon. Axial ratios often reach values as high as two, though no undisputed results reach above three. We find little correlation of axis ratio with size. The most common spectral type in the sample of small NEAs is S-type (> 90%), with only a handful of C and X types known.
This result basically kills the NASA ARM Asteroid Redirect Mission as originally designed.
So … let’s get a big boulder. A giant barbecue briquette.
Square ice in graphene nanocapillaries, G. Algara-Siller, O. Lehtinen, F. C. Wang, R. R. Nair, U. Kaiser, H. A. Wu, A. K. Geim and I. V. Grigorieva, Nature, 519, 443–445 (26 March 2015), doi:10.1038/nature14295
Bulk water exists in many forms, including liquid, vapour and numerous crystalline and amorphous phases of ice, with hexagonal ice being responsible for the fascinating variety of snowflakes. Much less noticeable but equally ubiquitous is water adsorbed at interfaces and confined in microscopic pores. Such low-dimensional water determines aspects of various phenomena in materials science, geology, biology, tribology and nanotechnology. Theory suggests many possible phases for adsorbed and confined water, but it has proved challenging to assess its crystal structure experimentally. Here we report high-resolution electron microscopy imaging of water locked between two graphene sheets, an archetypal example of hydrophobic confinement. The observations show that the nanoconfined water at room temperature forms ‘square ice’—a phase having symmetry qualitatively different from the conventional tetrahedral geometry of hydrogen bonding between water molecules. Square ice has a high packing density with a lattice constant of 2.83 Å and can assemble in bilayer and trilayer crystallites. Molecular dynamics simulations indicate that square ice should be present inside hydrophobic nanochannels independently of their exact atomic nature.
Somebody dug it up. Not me, though.
Jupiter’s decisive role in the inner Solar System’s early evolution, Konstantin Batygin and Greg Laughlin, PNAS (23 March 2015), doi: 10.1073/pnas.1423252112
The Solar System is an unusual member of the galactic planetary census in that it lacks planets that reside in close proximity to the Sun. In this work, we propose that the primordial nebula-driven process responsible for retention of Jupiter and Saturn at large orbital radii and sculpting Mars’ low mass is also responsible for clearing out the Solar System’s innermost region. Cumulatively, our results place the Solar System and the mechanisms that shaped its unique orbital architecture into a broader, extrasolar context.
The statistics of extrasolar planetary systems indicate that the default mode of planet formation generates planets with orbital periods shorter than 100 days and masses substantially exceeding that of the Earth. When viewed in this context, the Solar System is unusual. Here, we present simulations which show that a popular formation scenario for Jupiter and Saturn, in which Jupiter migrates inward from a > 5 astronomical units (AU) to a ≈ 1.5 AU before reversing direction, can explain the low overall mass of the Solar System’s terrestrial planets, as well as the absence of planets with a < 0.4 AU. Jupiter’s inward migration entrained s ≳ 10−100 km planetesimals into low-order mean motion resonances, shepherding and exciting their orbits. The resulting collisional cascade generated a planetesimal disk that, evolving under gas drag, would have driven any preexisting short-period planets into the Sun. In this scenario, the Solar System’s terrestrial planets formed from gas-starved mass-depleted debris that remained after the primary period of dynamical evolution.
Habitability of waterworlds: runaway greenhouses, atmospheric expansion and multiple climate states of pure water atmospheres, Colin Goldblatt, Accepted for Publication in Astrobiology
There are four different stable climate states for pure water atmospheres, as might exist on so-called “waterworlds”. I map these as a function of solar constant for planets ranging in size from Mars size to 10 Earth-mass. The states are: globally ice covered (Ts < 245 K), cold and damp (270 K < Ts < 290 K), hot and moist (350 K < Ts < 550 K) and very hot and dry (Ts < 900 K). No stable climate exists for 290 K < Ts < 350 K or 550 K < Ts < 900 K. The union of hot moist and cold damp climates describe the liquid water habitable zone, the width and location of which depends on planet mass. At each solar constant, two or three different climate states are stable. This is a consequence of strong non-linearities in both thermal emission and the net absorption of sunlight.
Across the range of planet sizes, I account for the atmospheres expanding to high altitudes as they warm. The emitting and absorbing surfaces (optical depth of unity) move to high altitude, making their area larger than the planet surface, so more thermal radiation is emitted and more sunlight absorbed (the former dominates). The atmospheres of small planets expand more due to weaker gravity: the effective runaway greenhouse threshold is about 35 Wm-2 higher for Mars, 10 Wm-2 higher for Earth or Venus but only a few Wm-2 higher for a 10 Earth-mass planet. There is an underlying (expansion neglected) trend of increasing runaway greenhouse threshold with planetary size (40 Wm-2 higher for a 10 Earth-mass planet than for Mars). Summing these opposing trends means that Venus-size (or slightly smaller) planets are most susceptible to a runaway greenhouse.
The habitable zone for pure water atmospheres is very narrow, with an insolation range of 0.07 times the solar constant. A wider habitable zone requires background gas and greenhouse gas; N2 and CO2 on Earth, which are biologically controlled. Thus, habitability depends on inhabitance.
It’s gotta be tough running a space program on these planets.
Interplay between density and superconducting quantum critical fluctuations, S. Caprara, N. Bergeal, J. Lesueur and M. Grilli
We consider the case of a density-driven metal-superconductor transition in the proximity of an electronic phase separation. In particular we investigate the interplay between superconducting fluctuations and density fluctuations, which become quantum critical when the electronic phase separation vanishes at zero temperature into a quantum critical point. In this situation the critical dynamical density fluctuations strongly affect the dynamics of the Cooper pair fluctuations, which acquire a more singular character with a z=3 dynamical critical index. This gives rise to a scenario that possibly rules the disappearance of superconductivity when the electron density is reduced by elecrostatic gating at the LaAlO3/SrTiO3 interface.
These crazy Italians just won’t quit!
Electronic phase transitions of bismuth under strain from relativistic self-consistent GW calculations, Irene Aguilera, Christoph Friedrich and Stefan Blügel
We present quasiparticle self-consistent GW (QSGW) calculations of semimetallic bulk Bi. We go beyond the conventional QSGW method by including the spin-orbit coupling throughout the self-consistency cycle. This approach improves the description of the electron and the hole pockets considerably with respect to standard density functional theory (DFT), leading to excellent agreement with experiment. We employ this relativistic QSGW approach to conduct a study of the semimetal-to-semiconductor and the trivial-to-topological transitions that Bi experiences under strain. DFT predicts that an unphysically large strain is needed for such transitions. We show, by means of the relativistic QSGW description of the electronic structure, that an in-plane tensile strain of only 0.3% and a compressive strain of 0.4% are sufficient to cause the semimetal-to-semiconductor and the trivial-to-topological phase transitions, respectively. Thus, the required strain moves into a regime that is likely to be realizable in experiment, which opens up the possibility to explore bulklike topological behavior of pure Bi.
This should give you a hint of what is yet to come with bismuth.
The nontrivial electronic structure of Bi/Sb honeycombs on SiC(0001), Chia-Hsiu Hsu, Zhi-Quan Huang, Feng-Chuan Chuang, Chien-Cheng Kuo, Yu-Tzu Liu, Hsin Lin and Arun Bansil, New J. Phys., 17, 025005 (February 2015), doi: 10.1088/1367-2630/17/2/025005
We discuss two-dimensional (2D) topological insulators (TIs) based on planar Bi/Sb honeycombs on a SiC(0001) substrate using first-principles computations. The Bi/Sb planar honeycombs on SiC(0001) are shown to support a nontrivial band gap as large as 0.56 eV, which harbors a Dirac cone lying within the band gap. Effects of hydrogen atoms placed on either just one side or on both sides of the planar honeycombs are examined. The hydrogenated honeycombs are found to exhibit topologically protected edge states for zigzag as well as armchair edges, with a wide band gap of 1.03 and 0.41 eV in bismuth and antimony films, respectively. Our findings pave the way for using planar bismuth and antimony honeycombs as potential new 2D-TI platforms for room-temperature applications.
Whoo hoo, Whoop de doo.
Dawn will wreck your Ceres theories.
Try saying that five times really fast.
Using the inclinations of Kepler systems to prioritize new Titius–Bode-based exoplanet predictions, Using the inclinations of Kepler systems to prioritize new Titius–Bode-based exoplanet predictions, Timothy Bovaird, Charles H. Lineweaver and Steffen K. Jacobsen, MNRAS, 448 (4): 3608-3627 (April 21, 2015) doi: 10.1093/mnras/stv221
We analyse a sample of multiple-exoplanet systems which contain at least three transiting planets detected by the Kepler mission (‘Kepler multiples’). We use a generalized Titius–Bode relation to predict the periods of 228 additional planets in 151 of these Kepler multiples. These Titius–Bode-based predictions suggest that there are, on average, 2 ± 1 planets in the habitable zone of each star. We estimate the inclination of the invariable plane for each system and prioritize our planet predictions by their geometric probability to transit. We highlight a short list of 77 predicted planets in 40 systems with a high geometric probability to transit, resulting in an expected detection rate of ∼15 percent, ∼3 times higher than the detection rate of our previous Titius–Bode-based predictions.
Let’s see, carbon emissions and climate change, religious nutjobs the world over, rampant banking corruption, police fascism, war and drug cartels, debt and default, all problems that he either created himself or has done absolutely nothing about, and yet he thinks that innovating ourselves out of these really bad situations that are no fault of stoners, using a ubiquitous fast growing versatile weed, should still be prosecuted. Fuck you, dude. I just can’t wait until 2017.
First solid-state cooling below 100K, Seth Melgaard, Denis Seletskiy, Alexander Albrecht and Mansoor Sheik-Bahae, SPIE Newsroom (13 March 2013)
Advances in material purity and laser light absorption offer new possibilities for vibration-free cryogenic cooling.