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Layer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limit

Nature - Wed, 06/07/2017 - 00:00

Layer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limit

Nature 546, 7657 (2017). doi:10.1038/nature22391

Authors: Bevin Huang, Genevieve Clark, Efrén Navarro-Moratalla, Dahlia R. Klein, Ran Cheng, Kyle L. Seyler, Ding Zhong, Emma Schmidgall, Michael A. McGuire, David H. Cobden, Wang Yao, Di Xiao, Pablo Jarillo-Herrero & Xiaodong Xu

Since the discovery of graphene, the family of two-dimensional materials has grown, displaying a broad range of electronic properties. Recent additions include semiconductors with spin–valley coupling, Ising superconductors that can be tuned into a quantum metal, possible Mott insulators with tunable charge-density waves, and topological semimetals with edge transport. However, no two-dimensional crystal with intrinsic magnetism has yet been discovered; such a crystal would be useful in many technologies from sensing to data storage. Theoretically, magnetic order is prohibited in the two-dimensional isotropic Heisenberg model at finite temperatures by the Mermin–Wagner theorem. Magnetic anisotropy removes this restriction, however, and enables, for instance, the occurrence of two-dimensional Ising ferromagnetism. Here we use magneto-optical Kerr effect microscopy to demonstrate that monolayer chromium triiodide (CrI3) is an Ising ferromagnet with out-of-plane spin orientation. Its Curie temperature of 45 kelvin is only slightly lower than that of the bulk crystal, 61 kelvin, which is consistent with a weak interlayer coupling. Moreover, our studies suggest a layer-dependent magnetic phase, highlighting thickness-dependent physical properties typical of van der Waals crystals. Remarkably, bilayer CrI3 displays suppressed magnetization with a metamagnetic effect, whereas in trilayer CrI3 the interlayer ferromagnetism observed in the bulk crystal is restored. This work creates opportunities for studying magnetism by harnessing the unusual features of atomically thin materials, such as electrical control for realizing magnetoelectronics, and van der Waals engineering to produce interface phenomena.

Categories: Literature

Microresonator-based solitons for massively parallel coherent optical communications

Nature - Wed, 06/07/2017 - 00:00

Microresonator-based solitons for massively parallel coherent optical communications

Nature 546, 7657 (2017). doi:10.1038/nature22387

Authors: Pablo Marin-Palomo, Juned N. Kemal, Maxim Karpov, Arne Kordts, Joerg Pfeifle, Martin H. P. Pfeiffer, Philipp Trocha, Stefan Wolf, Victor Brasch, Miles H. Anderson, Ralf Rosenberger, Kovendhan Vijayan, Wolfgang Freude, Tobias J. Kippenberg & Christian Koos

Solitons are waveforms that preserve their shape while propagating, as a result of a balance of dispersion and nonlinearity. Soliton-based data transmission schemes were investigated in the 1980s and showed promise as a way of overcoming the limitations imposed by dispersion of optical fibres. However, these approaches were later abandoned in favour of wavelength-division multiplexing schemes, which are easier to implement and offer improved scalability to higher data rates. Here we show that solitons could make a comeback in optical communications, not as a competitor but as a key element of massively parallel wavelength-division multiplexing. Instead of encoding data on the soliton pulse train itself, we use continuous-wave tones of the associated frequency comb as carriers for communication. Dissipative Kerr solitons (DKSs) (solitons that rely on a double balance of parametric gain and cavity loss, as well as dispersion and nonlinearity) are generated as continuously circulating pulses in an integrated silicon nitride microresonator via four-photon interactions mediated by the Kerr nonlinearity, leading to low-noise, spectrally smooth, broadband optical frequency combs. We use two interleaved DKS frequency combs to transmit a data stream of more than 50 terabits per second on 179 individual optical carriers that span the entire telecommunication C and L bands (centred around infrared telecommunication wavelengths of 1.55 micrometres). We also demonstrate coherent detection of a wavelength-division multiplexing data stream by using a pair of DKS frequency combs—one as a multi-wavelength light source at the transmitter and the other as the corresponding local oscillator at the receiver. This approach exploits the scalability of microresonator-based DKS frequency comb sources for massively parallel optical communications at both the transmitter and the receiver. Our results demonstrate the potential of these sources to replace the arrays of continuous-wave lasers that are currently used in high-speed communications. In combination with advanced spatial multiplexing schemes and highly integrated silicon photonic circuits, DKS frequency combs could bring chip-scale petabit-per-second transceivers into reach.

Categories: Literature

Accelerated discovery of two crystal structure types in a complex inorganic phase field

Nature - Wed, 06/07/2017 - 00:00

Accelerated discovery of two crystal structure types in a complex inorganic phase field

Nature 546, 7657 (2017). doi:10.1038/nature22374

Authors: C. Collins, M. S. Dyer, M. J. Pitcher, G. F. S. Whitehead, M. Zanella, P. Mandal, J. B. Claridge, G. R. Darling & M. J. Rosseinsky

The discovery of new materials is hampered by the lack of efficient approaches to the exploration of both the large number of possible elemental compositions for such materials, and of the candidate structures at each composition. For example, the discovery of inorganic extended solid structures has relied on knowledge of crystal chemistry coupled with time-consuming materials synthesis with systematically varied elemental ratios. Computational methods have been developed to guide synthesis by predicting structures at specific compositions and predicting compositions for known crystal structures, with notable successes. However, the challenge of finding qualitatively new, experimentally realizable compounds, with crystal structures where the unit cell and the atom positions within it differ from known structures, remains for compositionally complex systems. Many valuable properties arise from substitution into known crystal structures, but materials discovery using this approach alone risks both missing best-in-class performance and attempting design with incomplete knowledge. Here we report the experimental discovery of two structure types by computational identification of the region of a complex inorganic phase field that contains them. This is achieved by computing probe structures that capture the chemical and structural diversity of the system and whose energies can be ranked against combinations of currently known materials. Subsequent experimental exploration of the lowest-energy regions of the computed phase diagram affords two materials with previously unreported crystal structures featuring unusual structural motifs. This approach will accelerate the systematic discovery of new materials in complex compositional spaces by efficiently guiding synthesis and enhancing the predictive power of the computational tools through expansion of the knowledge base underpinning them.

Categories: Literature

New fossils from Jebel Irhoud, Morocco and the pan-African origin of Homo sapiens

Nature - Wed, 06/07/2017 - 00:00

New fossils from Jebel Irhoud, Morocco and the pan-African origin of Homo sapiens

Nature 546, 7657 (2017). doi:10.1038/nature22336

Authors: Jean-Jacques Hublin, Abdelouahed Ben-Ncer, Shara E. Bailey, Sarah E. Freidline, Simon Neubauer, Matthew M. Skinner, Inga Bergmann, Adeline Le Cabec, Stefano Benazzi, Katerina Harvati & Philipp Gunz

Fossil evidence points to an African origin of Homo sapiens from a group called either H. heidelbergensis or H. rhodesiensis. However, the exact place and time of emergence of H. sapiens remain obscure because the fossil record is scarce and the chronological age of many key specimens remains uncertain. In particular, it is unclear whether the present day ‘modern’ morphology rapidly emerged approximately 200 thousand years ago (ka) among earlier representatives of H. sapiens or evolved gradually over the last 400 thousand years. Here we report newly discovered human fossils from Jebel Irhoud, Morocco, and interpret the affinities of the hominins from this site with other archaic and recent human groups. We identified a mosaic of features including facial, mandibular and dental morphology that aligns the Jebel Irhoud material with early or recent anatomically modern humans and more primitive neurocranial and endocranial morphology. In combination with an age of 315 ± 34 thousand years (as determined by thermoluminescence dating), this evidence makes Jebel Irhoud the oldest and richest African Middle Stone Age hominin site that documents early stages of the H. sapiens clade in which key features of modern morphology were established. Furthermore, it shows that the evolutionary processes behind the emergence of H. sapiens involved the whole African continent.

Categories: Literature

The age of the hominin fossils from Jebel Irhoud, Morocco, and the origins of the Middle Stone Age

Nature - Wed, 06/07/2017 - 00:00

The age of the hominin fossils from Jebel Irhoud, Morocco, and the origins of the Middle Stone Age

Nature 546, 7657 (2017). doi:10.1038/nature22335

Authors: Daniel Richter, Rainer Grün, Renaud Joannes-Boyau, Teresa E. Steele, Fethi Amani, Mathieu Rué, Paul Fernandes, Jean-Paul Raynal, Denis Geraads, Abdelouahed Ben-Ncer, Jean-Jacques Hublin & Shannon P. McPherron

The timing and location of the emergence of our species and of associated behavioural changes are crucial for our understanding of human evolution. The earliest fossil attributed to a modern form of Homo sapiens comes from eastern Africa and is approximately 195 thousand years old, therefore the emergence of modern human biology is commonly placed at around 200 thousand years ago. The earliest Middle Stone Age assemblages come from eastern and southern Africa but date much earlier. Here we report the ages, determined by thermoluminescence dating, of fire-heated flint artefacts obtained from new excavations at the Middle Stone Age site of Jebel Irhoud, Morocco, which are directly associated with newly discovered remains of H. sapiens. A weighted average age places these Middle Stone Age artefacts and fossils at 315 ± 34 thousand years ago. Support is obtained through the recalculated uranium series with electron spin resonance date of 286 ± 32 thousand years ago for a tooth from the Irhoud 3 hominin mandible. These ages are also consistent with the faunal and microfaunal assemblages and almost double the previous age estimates for the lower part of the deposits. The north African site of Jebel Irhoud contains one of the earliest directly dated Middle Stone Age assemblages, and its associated human remains are the oldest reported for H. sapiens. The emergence of our species and of the Middle Stone Age appear to be close in time, and these data suggest a larger scale, potentially pan-African, origin for both.

Categories: Literature

Corrigendum: Black hole growth in the early Universe is self-regulated and largely hidden from view

Nature - Wed, 06/07/2017 - 00:00

Corrigendum: Black hole growth in the early Universe is self-regulated and largely hidden from view

Nature 546, 7657 (2017). doi:10.1038/nature22810

Authors: Ezequiel Treister, Kevin Schawinski, Marta Volonteri, Priyamvada Natarajan & Eric Gawiser

Nature474, 356–358 (2011); doi:10.1038/nature10103Subsequent analysis with updated methodology by us and others has not confirmed the detection of the population described in this Letter. We suspect, as described in later work by our group,

Categories: Literature

Biodiversity goes beyond species counts

Nature - Tue, 06/06/2017 - 13:00

Biodiversity goes beyond species counts

Nature 546, 7656 (2017). http://www.nature.com/doifinder/10.1038/546022a

Author: Rachel Cernansky

Ecologists are increasingly looking at how richness of traits — rather than number of species — helps set the health of ecosystems.

Categories: Literature

Taxonomy anarchy hampers conservation

Nature - Tue, 06/06/2017 - 13:00

Taxonomy anarchy hampers conservation

Nature 546, 7656 (2017). doi:10.1038/546025a

Authors: Stephen T. Garnett & Les Christidis

The classification of complex organisms is in chaos. Stephen T. Garnett and Les Christidis propose a solution.

Categories: Literature

Gabon Sets Plan for Africa's Largest Marine Reserve

Yale Environment 360 - Tue, 06/06/2017 - 09:43

The African nation of Gabon has announced the creation of the continent's largest network of marine protected areas, covering 20,500 square miles.

Read more on E360 →

Categories: Environmental News

NASA’s dark-energy probe faces cost crisis

Nature - Tue, 06/06/2017 - 00:00

NASA’s dark-energy probe faces cost crisis

Nature 546, 7657 (2017). http://www.nature.com/doifinder/10.1038/546195a

Author: Alexandra Witze

Space agency takes a hard look at plans for its next big space observatory.

Categories: Literature

AI summit aims to help world’s poorest

Nature - Tue, 06/06/2017 - 00:00

AI summit aims to help world’s poorest

Nature 546, 7657 (2017). http://www.nature.com/doifinder/10.1038/546196a

Author: Declan Butler

United Nations meeting hopes to focus artificial intelligence on sustainable development goals.

Categories: Literature

Romania’s science reforms prompt boycott

Nature - Tue, 06/06/2017 - 00:00

Romania’s science reforms prompt boycott

Nature 546, 7657 (2017). http://www.nature.com/doifinder/10.1038/546197a

Author: Inga Vesper

Researchers refuse to sit on evaluation panels after government bans international participation.

Categories: Literature

Deadline June 9 for Entries to Fourth Annual Yale Environment 360 Video Contest

Yale Environment 360 - Mon, 06/05/2017 - 15:00

The fourth annual Yale Environment 360 Video Contest is now accepting entries. The contest honors the year's best environmental videos. Submissions must focus on an environmental issue or theme, have not been widely viewed online, and be a maximum of 20 minutes in length. Videos that are funded by an organization or company and are primarily about that organization or company are not eligible.

Read more on E360 →

Categories: Environmental News

China’s Air Pollution Sharply Limits CO2 Uptake by Plants on Large Scale, Study Shows

Yale Environment 360 - Mon, 06/05/2017 - 08:38

The exceptionally high levels of surface ozone, aerosol particles, and other air pollutants in China are damaging plants and interfering with their ability to absorb carbon dioxide, according to a new study.

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Categories: Environmental News

A giant planet undergoing extreme-ultraviolet irradiation by its hot massive-star host

Nature - Mon, 06/05/2017 - 00:00

A giant planet undergoing extreme-ultraviolet irradiation by its hot massive-star host

Nature 546, 7659 (2017). doi:10.1038/nature22392

Authors: B. Scott Gaudi, Keivan G. Stassun, Karen A. Collins, Thomas G. Beatty, George Zhou, David W. Latham, Allyson Bieryla, Jason D. Eastman, Robert J. Siverd, Justin R. Crepp, Erica J. Gonzales, Daniel J. Stevens, Lars A. Buchhave, Joshua Pepper, Marshall C. Johnson, Knicole D. Colon, Eric L. N. Jensen, Joseph E. Rodriguez, Valerio Bozza, Sebastiano Calchi Novati, Giuseppe D’Ago, Mary T. Dumont, Tyler Ellis, Clement Gaillard, Hannah Jang-Condell, David H. Kasper, Akihiko Fukui, Joao Gregorio, Ayaka Ito, John F. Kielkopf, Mark Manner, Kyle Matt, Norio Narita, Thomas E. Oberst, Phillip A. Reed, Gaetano Scarpetta, Denice C. Stephens, Rex R. Yeigh, Roberto Zambelli, B. J. Fulton, Andrew W. Howard, David J. James, Matthew Penny, Daniel Bayliss, Ivan A. Curtis, D. L. DePoy, Gilbert A. Esquerdo, Andrew Gould, Michael D. Joner, Rudolf B. Kuhn, Jonathan Labadie-Bartz, Michael B. Lund, Jennifer L. Marshall, Kim K. McLeod, Richard W. Pogge, Howard Relles, Christopher Stockdale, T. G. Tan, Mark Trueblood & Patricia Trueblood

The amount of ultraviolet irradiation and ablation experienced by a planet depends strongly on the temperature of its host star. Of the thousands of extrasolar planets now known, only six have been found that transit hot, A-type stars (with temperatures of 7,300–10,000 kelvin), and no planets are known to transit the even hotter B-type stars. For example, WASP-33 is an A-type star with a temperature of about 7,430 kelvin, which hosts the hottest known transiting planet, WASP-33b (ref. 1); the planet is itself as hot as a red dwarf star of type M (ref. 2). WASP-33b displays a large heat differential between its dayside and nightside, and is highly inflated–traits that have been linked to high insolation. However, even at the temperature of its dayside, its atmosphere probably resembles the molecule-dominated atmospheres of other planets and, given the level of ultraviolet irradiation it experiences, its atmosphere is unlikely to be substantially ablated over the lifetime of its star. Here we report observations of the bright star HD 195689 (also known as KELT-9), which reveal a close-in (orbital period of about 1.48 days) transiting giant planet, KELT-9b. At approximately 10,170 kelvin, the host star is at the dividing line between stars of type A and B, and we measure the dayside temperature of KELT-9b to be about 4,600 kelvin. This is as hot as stars of stellar type K4 (ref. 5). The molecules in K stars are entirely dissociated, and so the primary sources of opacity in the dayside atmosphere of KELT-9b are probably atomic metals. Furthermore, KELT-9b receives 700 times more extreme-ultraviolet radiation (that is, with wavelengths shorter than 91.2 nanometres) than WASP-33b, leading to a predicted range of mass-loss rates that could leave the planet largely stripped of its envelope during the main-sequence lifetime of the host star.

Categories: Literature

Six decades of struggle over the pill

Nature - Mon, 06/05/2017 - 00:00

Six decades of struggle over the pill

Nature 546, 7657 (2017). doi:10.1038/546185a

A notable anniversary highlights the progress and benefits of contraceptives, but also the continuing battle for access to them.

Categories: Literature

We need a science of philanthropy

Nature - Mon, 06/05/2017 - 00:00

We need a science of philanthropy

Nature 546, 7657 (2017). http://www.nature.com/doifinder/10.1038/546187a

Author: Caroline Fiennes

Billions of dollars are being donated without strong evidence about which ways of giving are effective, says Caroline Fiennes.

Categories: Literature

America Heads to the Exit: What Trump Got Wrong about Paris

Yale Environment 360 - Fri, 06/02/2017 - 11:19

In announcing his decision to withdraw from the Paris accord, President Trump cast it as a matter of putting America first. But the move is a colossal mistake that will push the U.S. to the sidelines as China and others take the lead on climate policy and renewable energy.

Read more on E360 →

Categories: Environmental News

Waving Banner of Economic Nationalism, Trump Withdraws U.S. from Paris Climate Accord

Yale Environment 360 - Thu, 06/01/2017 - 15:03

Saying that the Paris climate agreement is an onerous infringement on U.S sovereignty and “puts our country at a very, very big economic disadvantage,” President Donald Trump withdrew today from the accord.

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Categories: Environmental News

A Call for a Hippocratic Oath on Protecting the World's Oceans

Yale Environment 360 - Thu, 06/01/2017 - 04:30

In a Yale Environment 360 interview, scientist Nathan Bennett explains why he and other marine experts are calling for a a code of conduct for ocean conservation to ensure that local communities benefit from newly created marine reserves.

Read more on E360 →

Categories: Environmental News

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