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  • Inequivalent Berry Phases for the Bulk Polarization
    Phys. Rev. X (IF 12.789) Pub Date : 2018-06-15
    Haruki Watanabe and Masaki Oshikawa
    更新日期:2018-06-16
  • Dynamically Encircling Exceptional Points:In situControl of Encircling Loops and the Role of the Starting Point
    Phys. Rev. X (IF 12.789) Pub Date : 2018-06-15
    Xu-Lin Zhang, Shubo Wang, Bo Hou, and C. T. Chan
    更新日期:2018-06-16
  • Fracton models on general three-dimensional manifolds
    Phys. Rev. X (IF 12.789) Pub Date : 
    Wilbur Shirley, Kevin Slagle, Zhenghan Wang, and Xie Chen

    Fracton models, a collection of exotic gapped lattice Hamiltonians recently discovered in three spatial dimensions, contain some `topological' features: they support fractional bulk excitations (dubbed fractons), and a ground state degeneracy that is robust to local perturbations. However, because previous fracton models have only been defined and analyzed on a cubic lattice with periodic boundary conditions, it is unclear to what extent a notion of topology is applicable. In this paper, we demonstrate that the $X$-cube model, a prototypical type-I fracton model, can be defined on general three-dimensional manifolds. Our construction revolves around the notion of a singular compact total foliation of the spatial manifold, which constructs a lattice from intersecting stacks of parallel surfaces called leaves. We find that the ground state degeneracy depends on the topology of the leaves and the pattern of leaf intersections. We further show that such a dependence can be understood from a renormalization group transformation for the X-cube model, wherein the system size can be changed by adding or removing 2D layers of topological states. Our results lead to an improved definition of fracton phase and bring to the fore the topological nature of fracton orders.

    更新日期:2018-06-16
  • Translationally invariant non-Fermi liquid metals with critical Fermi-surfaces: Solvable models
    Phys. Rev. X (IF 12.789) Pub Date : 
    Debanjan Chowdhury, Yochai Werman, Erez Berg, and T. Senthil

    We construct examples of translationally invariant solvable models of strongly-correlated metals, composed of lattices of Sachdev-Ye-Kitaev dots with identical local interactions. These models display crossovers as a function of temperature into regimes with local quantum criticality and marginal-Fermi liquid behavior. In the marginal Fermi liquid regime, the dc resistivity increases linearly with temperature over a broad range of temperatures. By generalizing the form of interactions, we also construct examples of non-Fermi liquids with critical Fermi-surfaces. The self-energy has a singular frequency dependence, but lacks momentum dependence, reminiscent of a dynamical mean field theory-like behavior but in dimensions $dless than \infty$. In the low temperature and strong-coupling limit, a heavy Fermi liquid is formed. The critical Fermi-surface in the non-Fermi liquid regime gives rise to quantum oscillations in the magnetization as a function of an external magnetic field in the absence of quasiparticle excitations. We discuss the implications of these results for local quantum criticality and for fundamental bounds on relaxation rates. Drawing on the lessons from these models, we formulate conjectures on coarse grained descriptions of a class of intermediate scale non-fermi liquid behavior in generic correlated metals.

    更新日期:2018-06-16
  • $^{7}$Be($n,p$)$^{7}$Li reaction and the cosmological lithium problem: Measurement of the cross section in a wide energy range at n\_TOF at CERN
    Phys. Rev. Lett. (IF 8.462) Pub Date : 
    L. Damone et al.

    We report on the measurement of the $^{7}$Be($n, p$)$^{7}$Li cross section from thermal to approximately 325 keV neutron energy, performed in the high-flux experimental area (EAR2) of the n\_TOF facility at CERN. This reaction plays a key role in the lithium yield of the Big Bang Nucleosynthesis (BBN) for standard cosmology. The only two previous time-of-flight measurements performed on this reaction did not cover the energy window of interest for BBN, and showed a large discrepancy between each other. The measurement was performed with a Si-telescope, and a high-purity sample produced by implantation of a $^{7}$Be ion beam at the ISOLDE facility at CERN. While a significantly higher cross section is found at low-energy, relative to current evaluations, in the region of BBN interest the present results are consistent with the values inferred from the time-reversal $^{7}$Li($p, n$)$^{7}$Be reaction, thus yielding only a relatively minor improvement on the so-called Cosmological Lithium Problem (CLiP). The relevance of these results on the near-threshold neutron production in the p+$^{7}$Li reaction is also discussed.

    更新日期:2018-06-16
  • Importance of isobar density distributions on the chiral magnetic effect search
    Phys. Rev. Lett. (IF 8.462) Pub Date : 
    Hao-jie Xu, Xiaobao Wang, Hanlin Li, Jie Zhao, Zi-Wei Lin, Caiwan Shen, and Fuqiang Wang

    Under the approximate chiral symmetry restoration, quark interactions with topological gluon fields in quantum chromodynamics can induce chirality imbalance and parity violation in local domains. An electric charge separation (\cs) could be generated along the direction of a strong magnetic field ($\Bbf$), a phenomenon called the chiral magnetic effect (\cme). \csmeasurements by azimuthal correlators are contaminated by major backgrounds from elliptic flow anisotropy ($\vv$). Isobaric \Ru+\Ruand \Zr+\Zrcollisions have been proposed to identify the \cme(expected to differ between the two systems) out of the backgrounds (to be almost the same). We show, by using the density functional theory calculations of the proton and neutron distributions, that these expectations may not hold as originally anticipated because the two systems may have sizable differences in eccentricity and $\vv$.

    更新日期:2018-06-16
  • Nonreciprocal current in noncentrosymmetric Rashba superconductors
    Phys. Rev. Lett. (IF 8.462) Pub Date : 
    Ryohei Wakatsuki and Naoto Nagaosa

    We study theoretically the nonreciprocal charge transport in two-dimensional noncentrosymmetric superconductors with Rashba spin--orbit interaction. The resistivity $R$ depends on the current $I$ linearly under the external magnetic field $B$, i.e., $R = R_0 \left(1+\gamma B I \right)$, which is called the magnetochiral anisotropy (MCA). It is found that the coefficient $\gamma$ is gigantically enhanced by the superconducting fluctuation with the components of both spin singlet and triplet pairings, compared with that in the normal state. This finding offers a method to quantitatively estimate the ratio of the pairing interactions between the singlet and triplet channels including its sign.

    更新日期:2018-06-16
  • Light-cone spreading of perturbations and the butterfly effect in a classical spin chain
    Phys. Rev. Lett. (IF 8.462) Pub Date : 
    Avijit Das, Saurish Chakrabarty, Abhishek Dhar, Anupam Kundu, David A. Huse, Roderich Moessner, Samriddhi Sankar Ray, and Subhro Bhattacharjee

    We find that the effects of a localised perturbation in a chaotic classical many-body system--the classical Heisenberg chain at infinite temperature--spread ballistically with a finite speed even when the local spin dynamics is diffusive. We study two complementary aspects of this butterfly effect: the rapid growth of the perturbation, and its simultaneous ballistic (light-cone) spread, as characterised by the Lyapunov exponents and the butterfly speed respectively. We connect this to recent studies of the out-of-time-ordered commutators (OTOC), which have been proposed as an indicator of chaos in a quantum system. We provide a straightforward identification of the OTOC with a natural correlator in our system and demonstrate that many of its interesting qualitative features are present in the classical system. Finally, by analysing the scaling forms, we relate the growth, spread and propagation of the perturbation with the growth of one-dimensional interfaces described by the Kardar-Parisi-Zhang (KPZ) equation.

    更新日期:2018-06-16
  • Spin-torque-biased magnetic strip: Nonequilibrium phase diagram and relation to long Josephson junctions
    Phys. Rev. Lett. (IF 8.462) Pub Date : 
    Daniel Hill, Se Kwon Kim, and Yaroslav Tserkovnyak

    Spin-torque-biased magnetic dynamics in an easy-plane ferromagnet (EPF) is theoretically studied in the presence of a weak in-plane anisotropy. While this anisotropy spoils U(1) symmetry thereby quenching the conventional spin superfluidity, we show that the system instead realizes a close analog of a long Josephson junction (LJJ) model. The traditional magnetic-field and electric-current controls of the latter map respectively onto the symmetric and antisymmetric combinations of the out-of-plane spin torques applied at the ends of the magnetic strip. This suggests an alternative route towards realizations of superfluid-like transport phenomena in insulating magnetic systems. We study spin-torque-biased phase diagram, providing analytical solution for static multidomain phases in the EPF. We adapt an existing self-consistency method for the LJJ to develop an approximate solution for the EPF dynamics. The LJJ-EPF mapping has potential for producing applications with superconductor-based circuit functionality at elevated temperatures. The results apply equally to antiferromagnets with suitable effective free energy in terms of the N\'{e}el order instead of magnetization.

    更新日期:2018-06-16
  • Reversible Aggregation and Colloidal Cluster Morphology: The Importance of the Extended Law of Corresponding States
    Phys. Rev. Lett. (IF 8.462) Pub Date : 2018-06-15
    Néstor E. Valadez-Pérez, Yun Liu, and Ramón Castañeda-Priego
    更新日期:2018-06-15
  • ZNBerry Phases in Symmetry Protected Topological Phases
    Phys. Rev. Lett. (IF 8.462) Pub Date : 2018-06-15
    Toshikaze Kariyado, Takahiro Morimoto, and Yasuhiro Hatsugai
    更新日期:2018-06-15
  • Fragility of Charge Order Near an Antiferromagnetic Quantum Critical Point
    Phys. Rev. Lett. (IF 8.462) Pub Date : 2018-06-15
    Xiaoyu Wang, Yuxuan Wang, Yoni Schattner, Erez Berg, and Rafael M. Fernandes
    更新日期:2018-06-15
  • Order Out of Chaos: Slowly Reversing Mean Flows Emerge from Turbulently Generated Internal Waves
    Phys. Rev. Lett. (IF 8.462) Pub Date : 2018-06-15
    Louis-Alexandre Couston, Daniel Lecoanet, Benjamin Favier, and Michael Le Bars
    更新日期:2018-06-15
  • Extreme Small-Scale Clustering of Droplets in Turbulence Driven by Hydrodynamic Interactions
    Phys. Rev. Lett. (IF 8.462) Pub Date : 2018-06-15
    M. A. Yavuz, R. P. J. Kunnen, G. J. F. van Heijst, and H. J. H. Clercx
    更新日期:2018-06-15
  • Origins of Extreme Liquid Repellency on Structured, Flat, and Lubricated Hydrophobic Surfaces
    Phys. Rev. Lett. (IF 8.462) Pub Date : 2018-06-15
    Dan Daniel, Jaakko V. I. Timonen, Ruoping Li, Seneca J. Velling, Michael J. Kreder, Adam Tetreault, and Joanna Aizenberg
    更新日期:2018-06-15
  • 更新日期:2018-06-15
  • Electromagnetic Helicity in Complex Media
    Phys. Rev. Lett. (IF 8.462) Pub Date : 2018-06-15
    F. Alpeggiani, K. Y. Bliokh, F. Nori, and L. Kuipers
    更新日期:2018-06-15
  • Sensing Coherent Dynamics of Electronic Spin Clusters in Solids
    Phys. Rev. Lett. (IF 8.462) Pub Date : 2018-06-15
    E. L. Rosenfeld, L. M. Pham, M. D. Lukin, and R. L. Walsworth
    更新日期:2018-06-15
  • Chaotic-Integrable Transition in the Sachdev-Ye-Kitaev Model
    Phys. Rev. Lett. (IF 8.462) Pub Date : 2018-06-15
    Antonio M. García-García, Bruno Loureiro, Aurelio Romero-Bermúdez, and Masaki Tezuka
    更新日期:2018-06-15
  • Multi-PeV Signals from a New Astrophysical Neutrino Flux beyond the Glashow Resonance
    Phys. Rev. Lett. (IF 8.462) Pub Date : 2018-06-15
    Matthew D. Kistler and Ranjan Laha
    更新日期:2018-06-15
  • Exact Confirmation of 1D Nonlinear Fluctuating Hydrodynamics for a Two-Species Exclusion Process
    Phys. Rev. Lett. (IF 8.462) Pub Date : 2018-06-15
    Zeying Chen, Jan de Gier, Iori Hiki, and Tomohiro Sasamoto
    更新日期:2018-06-15
  • Latent Space Purification via Neural Density Operators
    Phys. Rev. Lett. (IF 8.462) Pub Date : 2018-06-15
    Giacomo Torlai and Roger G. Melko
    更新日期:2018-06-15
  • Totally Destructive Many-Particle Interference
    Phys. Rev. Lett. (IF 8.462) Pub Date : 2018-06-15
    Christoph Dittel, Gabriel Dufour, Mattia Walschaers, Gregor Weihs, Andreas Buchleitner, and Robert Keil
    更新日期:2018-06-15
  • Nuclear-spin comagnetometer based on a liquid of identical molecules
    Phys. Rev. Lett. (IF 8.462) Pub Date : 
    Teng Wu, John W. Blanchard, Derek F. Jackson Kimball, Min Jiang, and Dmitry Budker

    Atomic comagnetometers are used in searches for anomalous spin-dependent interactions. Magnetic field gradients are one of the major sources of systematic errors in such experiments. Here we describe a comagnetometer based on the nuclear spins within an ensemble of identical molecules. The dependence of the measured spin-precession frequency ratio on the first-order magnetic field gradient is suppressed by over an order of magnitude compared to a comagnetometer based on overlapping ensembles of different molecules. Our single-species comagnetometer is capable of measuring the hypothetical spin-dependent gravitational energy of nuclei at the $10^{-17}$ eV level, comparable to the most stringent existing constraints. Combined with techniques for enhancing the signal such as parahydrogen-induced polarization, this method of comagnetometry offers the potential to improve constraints on spin-gravity coupling of nucleons by several orders of magnitude.

    更新日期:2018-06-15
  • Hybrid sound modes in one-dimensional quantum liquids
    Phys. Rev. Lett. (IF 8.462) Pub Date : 
    K. A. Matveev and A. V. Andreev

    We study sound in a single-channel one-dimensional quantum liquid. In contrast to classical fluids, instead of a single sound mode we find two modes of density oscillations. The speeds at which these two sound modes propagate are nearly equal, with the difference that scales linearly with the small temperature of the system. The two sound modes emerge as hybrids of the first and second sounds, and combine oscillations of both density and entropy of the liquid.

    更新日期:2018-06-15
  • HIV—No time for complacency
    Science (IF 37.205) Pub Date : 2018-06-15
    Quarraisha Abdool Karim, Salim S. Abdool Karim

    Today, the global HIV epidemic is widely viewed as triumph over tragedy. This stands in stark contrast to the first two decades of the epidemic, when AIDS was synonymous with suffering and death. But have we turned the tide on HIV sufficiently to warrant directing our attention and investments elsewhere?

    更新日期:2018-06-15
  • News at a glance
    Science (IF 37.205) Pub Date : 2018-06-15
    American Association for the Advancement of Science

    In science news around the world, the European Commission proposes a 7-year, €94.1 billion research budget that would provide a 22% increase from the current 7-year program; but universities complain that basic research's share of the spending total would remain flat, whereas the portion for innovation would rise. Prominent cancer biologist Inder Verma resigns from the Salk Institute for Biological Studies in San Diego, California, after it investigated allegations that he sexually harassed women. Spain gets its first science minister in 7 years. NASA's new administrator, Jim Bridenstine, says he supports climate missions. And researchers report that some of Africa's oldest baobab trees are dying for unknown reasons.

    更新日期:2018-06-15
  • Seaweed masses assault Caribbean islands
    Science (IF 37.205) Pub Date : 2018-06-15
    Katie Langin

    In 2011, massive rafts of Sargassum—a brown seaweed—washed up on beaches across the Caribbean, trapping sea turtles and filling the air with the stench of rotting eggs. The deluge was the first of its kind, but since then it has become a regular phenomenon. At first, scientists thought the seaweed drifted down from the North Atlantic's Sargasso Sea, where most Sargassum is found. But satellite imagery and data on ocean currents told a different story, revealing a new source in the tropical Atlantic Ocean. Now, scientists are scrambling to figure out what has caused the mysterious seaweed blooms.

    更新日期:2018-06-15
  • Quantum physics could get big boost from U.S. Congress
    Science (IF 37.205) Pub Date : 2018-06-15
    Gabriel Popkin

    U.S. officials are backing an emerging effort to better organize and boost funding for quantum research, which could reshape computing, sensors, and communications. In the coming weeks, the science committee of the House of Representatives is expected to introduce legislation calling for a new, 10-year-long National Quantum Initiative. The White House and science agencies are also calling for a larger role for the federal government in quantum science. A yearlong push by a coalition of academic researchers and technology firms helped trigger this flurry of activity. Proponents argue that the United States needs a better plan for harvesting the potential fruits of quantum research—and for keeping up with global competitors.

    更新日期:2018-06-15
  • Report details persistent hostility to women in science
    Science (IF 37.205) Pub Date : 2018-06-15
    Meredith Wadman

    A groundbreaking report on sexual harassment from the National Academies of Sciences, Engineering, and Medicine, released on 12 June, says that deep-seated, cultural change is needed to curb widespread sexual harassment of women in science. And it lays responsibility for making that change largely at the feet of leaders at academic institutions. The report says sexual harassment continues to hobble careers or even drive women from their fields—despite protective laws that have been in place for decades. To bend this curve, it says, universities need to take active measures—like transparently reporting the number of complaints they get and investigations they have underway—and insulating students by having committees advise them, to prevent them from falling under the power of a single, harassing mentor. Two years in the making, the report describes pervasive and damaging "gender harassment"—belittling behaviors intended to make women feel they don't belong, including sexist comments and demeaning jokes. In large surveys conducted at 36 campuses by two big university systems—the University of Texas and Pennsylvania State University—up to 50% of female students said they had experienced this kind of harassment, with medical students affected at the highest rates.

    更新日期:2018-06-15
  • Seafloor fiber optic cables can listen for earthquakes
    Science (IF 37.205) Pub Date : 2018-06-15
    Eric Hand

    Some 70% of Earth's surface is covered by water, and yet nearly all earthquake detectors are on land. Aside from some expensive battery-powered sensors dropped to the sea floor and later retrieved, and a few arrays of near-shore detectors connected to land, seismologists have no way of monitoring the quakes that ripple through the sea floor and sometimes create tsunamis. Now, a technique described online in Science this week promises to take advantage of more than 1 million kilometers of fiber optic cables that crisscross the ocean floors and carry the world's internet and telecom traffic. By looking for tiny changes in an optical signal running along the cable, scientists can detect and potentially locate earthquakes. The technique requires little more than lasers at each end of the cable and access to a small portion of the cable's bandwidth. Crucially, it requires no modification to the cable itself and does not interfere with its everyday use.

    更新日期:2018-06-15
  • Stricter Chinese student visas raise alarm
    Science (IF 37.205) Pub Date : 2018-06-15
    Jeffrey Mervis

    Reversing yet another policy of the previous administration, the U.S. Department of State this week began to apply tougher restrictions on some Chinese graduate students. The new policy shortens from 5 years to 1 year the duration of visas for those planning to study aviation, robotics, and advanced manufacturing. It will make it harder for the affected Chinese students to attend international conferences and to work collaboratively with scientists abroad, say U.S. higher education officials. It may also curtail periodic visits back home. Although the ostensible reason for the change is to improve national security, U.S. university officials see it as one more assault on graduate education and the free flow of scientific knowledge. When added to other policies by the current administration that affect non-U.S. citizens, the change may give talented foreign students one more reason to pursue advanced degrees in countries with lower barriers to entry.

    更新日期:2018-06-15
  • Far from over
    Science (IF 37.205) Pub Date : 2018-06-15
    Jon Cohen

    Powerful tools exist now to stop AIDS and HIV transmission, which has led to an international push to end AIDS by 2030. But not everywhere is making progress toward that ambitious goal. A unique package of graphics looks at HIV/AIDS around the world using five different metrics: How many people are living with HIV? What is the rate of new infection? What percentage of infected people are receiving antiretroviral drugs, which both stave off disease and prevent transmission? How many infected people have progressed to AIDS and how many have died from it? And how many children are infected by their mothers? By these gauges, Nigeria, Russia, and Florida stand out from their neighbors and, in some cases, the entire world. None of these three locales has high numbers on every one of these measures. But each ranks first—an unenviable distinction—in at least one of the five metrics assessed by total cases, rates, or proportions.

    更新日期:2018-06-15
  • The mother of all challenges
    Science (IF 37.205) Pub Date : 2018-06-15
    Jon Cohen

    Mother-to-child transmission (MTCT) of HIV is relatively easy to stop with antiretroviral drugs—and some countries, including the United States, have nearly eliminated it. But Nigeria has 37,000 of the 160,000 cases of MTCT that occurred in the world in 2016. The central problem is that an estimated 40% of pregnant Nigerian women never visit formal health care facilities and so do not receive HIV tests. Instead, they give birth at home with traditional birth attendants. The country has launched novel programs to test more pregnant women, including one that works with churches and another that focuses on traditional birth attendants, and then help those who test positive receive the treatment that can both prolong their lives and prevent the virus from being transmitted to their babies.

    更新日期:2018-06-15
  • Building TRUST in an LGBTQ-hostile country
    Science (IF 37.205) Pub Date : 2018-06-15
    Jon Cohen

    Nigerian law makes providing services to men who have sex with men (MSM) a punishable offense. But a clinic in Abuja, run by the Institute for Human Virology, Nigeria, bills itself as "MSM friendly" and dodges the legal hurdles to test and treat this community for HIV and other sexually transmitted infections. Studies run by the clinic, which has another outpost in Lagos, have found that a startling 45% of them test positive for HIV. Official Nigerian statistics by and large ignore this community, and a recent survey of undergraduates found that 40% of them believed that health care workers should not provide services to them.

    更新日期:2018-06-15
  • Babies who dodge HIV may not be unscathed
    Science (IF 37.205) Pub Date : 2018-06-15
    Jon Cohen

    Children born to HIV-positive mothers who do not become infected themselves may still suffer some health problems, such as delayed growth and immune abnormalities. A research project in Benin City run by the Institute for Human Virology, Nigeria, is attempting to assess whether exposure to the virus—or to antiretroviral drugs taken by the mother and baby that thwart transmission—may cause harm. It's difficult to separate out the many confounding variables that could lead to problems in HIV-negative babies, including the compromised health of the mothers. But the study hopes to clarify these issues by comparing 300 exposed and unexposed infants and their mothers during their first 2 years of life.

    更新日期:2018-06-15
  • Dark nights, bright stars
    Science (IF 37.205) Pub Date : 2018-06-15
    Jon Cohen

    At a time when HIV epidemics in Western Europe are shrinking, Russia's is growing by about 10% a year. Russia, in fact, between 2010 and 2015 accounted for more than 80% of all the new infections in Eastern Europe and Central Asia. Not only has the country turned away assistance from The Global Fund to Fight AIDS, Tuberculosis and Malaria, it also has refused to adopt scientifically proven strategies to slow spread between people who inject drugs, the community that kicked off the Russian epidemic. Only about one in three of the estimated 1 million Russians living with HIV have access to antiretroviral drugs, which means that the country doesn't receive much benefit on a population level from what's known as treatment as prevention. But there are signs in St. Petersburg that its unusually progressive response—by Russian standards—is beginning to turn things around.

    更新日期:2018-06-15
  • Status symbol
    Science (IF 37.205) Pub Date : 2018-06-15
    Jon Cohen

    Pavel Lobkov, a well-known anchor on the news station TV Rain, in 2015 took an action that few prominent Russians have dared to take: He went public with the fact that he is living with HIV. Stigma and discrimination against HIV-infected people still runs high in Russia, leading many to avoid getting tested and, in turn, preventing them from receiving the treatment they need both to prevent AIDS and to lower the chance of them transmitting the virus to others. Lobkov says many people reached out to him after he went public with his HIV status because he demonstrated that you could lead a normal, healthy life even if you're infected.

    更新日期:2018-06-15
  • Poster couple
    Science (IF 37.205) Pub Date : 2018-06-15
    Jon Cohen

    Tatiana Vinogradova, deputy director of St. Petersburg's AIDS center, is married to an HIV-positive man, Andrey Skvortsov. Many couples of course have "discordant" HIV status, but because Vinogradova is a leader in the city's response to the epidemic and Skvortsov is a well-known patient advocate, they agreed to appear on a poster that told their story, in the hopes of convincing others that there's no reason to fear HIV-infected people. They also want to reduce bias and fear in the medical community, as many doctors, dentists, and other professionals still shy away from offering care to people who are living with HIV.

    更新日期:2018-06-15
  • The pill exchange
    Science (IF 37.205) Pub Date : 2018-06-15
    Jon Cohen

    Alexander Chebin of Yekaterinburg and Yulia Vereschagina of St. Petersburg live more than 1000 kilometers away from each other, but they work together sharing antiretroviral drugs, which they then distribute to people in need. Their "reserve pharmacy" is an underground network that collects the drugs from people who switch regimens or die, and distributes them to people who have trouble accessing the life-saving medication because of bureaucratic hurdles or the occasional "stock-outs" that occur in official government pharmacies. People find them through social media websites, and they distribute the drugs free of charge.

    更新日期:2018-06-15
  • The loyal opposition
    Science (IF 37.205) Pub Date : 2018-06-15
    Jon Cohen

    Russian government officials who speak critically about their leadership often find themselves out of a job—or worse. Vadim Pokrovsky, who heads the Federal Center for the Prevention and Control of the Spread of AIDS in Moscow, blasts his government for its lackluster response to HIV/AIDS and even says Ministry of Health officials routinely play down the epidemic's severity. Pokrovsky says the Russian Orthodox Church holds too much sway, and he thinks the federal government needs to provide much more substantial funding if it hopes to slow the rampant spread of HIV and help more infected people. And for whatever reason, the government tolerates this lone official voice of dissent.

    更新日期:2018-06-15
  • The Sunshine State's dark cloud
    Science (IF 37.205) Pub Date : 2018-06-15
    Jon Cohen

    Miami in 2016 was in first place on the list that ranks U.S. cities by new HIV diagnoses. And three other cities in Florida were also on the list. Florida also has more cases of AIDS than any state. The problem is complex, and it differs in the cities and rural areas. Miami has a terrifically diverse community with many immigrants from other countries that cannot be reached with a one-size-fits-all message or outreach efforts. Rural Florida is in the heart of the conservative Bible Belt in the deep South, which is famously homophobic and looks askance at sexual education. New efforts are underway to try to improve the situation in Florida by squarely looking its shortcomings in the eye and confronting them one by one.

    更新日期:2018-06-15
  • A tool for finding rare marine species
    Science (IF 37.205) Pub Date : 2018-06-15
    Ellen K. Pikitch

    By their very nature, rare marine species are challenging to study. They often elude detection by established research methods because of their scarcity and patchy distribution. Their small populations demand the use of sampling techniques that minimize the risks of injury or death and that conserve the habitats and ecological communities that support them. At the same time, ever more once-common marine species are becoming rare as threats such as overfishing, invasive species, and environmental destruction reduce their abundance (1). Analysis of environmental DNA (eDNA) shows promise as a rapid, safe, sensitive, and cost-effective means for detecting and studying rare marine species.

    更新日期:2018-06-15
  • Crystallizing a memory
    Science (IF 37.205) Pub Date : 2018-06-15
    Steve Ramirez

    What is the physical basis of memory? What does it take to retrieve a memory in the brain? What would it take to activate or erase memories? In the early 20th century, the German zoologist Richard Semon coined the term “engram” to denote the physical manifestation of a memory in the brain (1). Two decades later, Canadian psychologist Donald Hebb posited a physiological correlate for learning and recollection: The process of learning strengthens the connections, or synapses, between neurons, which leads to the development of brain-wide cell assemblies that undergo changes in their structural and functional connectivity (2). The coordinated activity of these assemblies—called ensembles, traces, or engrams—that occurs during learning (memory formation) is thought to be reengaged during recall and thereby forms a stable neuronal correlate of memory (2). As subsequent memories are formed, the dynamics of these assemblies evolve and provide preexisting scaffolds to influence how the brain processes the variety of memories an organism forms. Studies by Abdou et al. (3) on page 1227 of this issue and by Choi et al. (4) develop new technologies to visualize discrete engrams in the brain and modulate them in a synapse-specific manner to understand memory strength and memory restoration from an amnestic state. This improved understanding could eventually be translated to modulate memories to alleviate maladaptive memory states.

    更新日期:2018-06-15
  • Scaling of human brain size
    Science (IF 37.205) Pub Date : 2018-06-15
    David C. Van Essen

    What makes humans unique as a species and as individuals? Our uniqueness stems from language, tool use, reasoning, and other cognitive abilities that are largely mediated by specialized regions of the cerebral cortex. These regions of higher cognitive function have expanded disproportionately during human evolution (compared with nonhuman primates) and during postnatal maturation, when cortical surface area expands threefold between infancy and adulthood (1). Our uniqueness as individuals reflects countless differences in brain structure, function, and connectivity. One basic anatomical difference between similarly aged individuals is a more than 1.5-fold variation in total brain size (and total cortical volume) (2). On page 1222 of this issue, Reardon et al. (3) bring this aspect of individual variability under the umbrella of “differential scaling” by showing that human brains of different sizes do not scale uniformly across all regions. Rather, larger brains show greater expansion in regions associated with higher cognition and less expansion in regions associated with sensory, motor, and limbic (emotion- and affect-related) functions.

    更新日期:2018-06-15
  • Animals feel safer from humans in the dark
    Science (IF 37.205) Pub Date : 2018-06-15
    Ana Benítez-López

    About 75% of Earth's land surface is currently modified by human activities (1). The expanding footprint of human activities is not only causing the loss of habitat and biodiversity but also affecting the dynamics of wildlife populations. Researchers have long examined human-induced spatial shifts in the distribution of wildlife, but temporal adjustments in animal activity have received less attention. On page 1232 of this issue, Gaynor et al. (2) present a comprehensive meta-analysis to quantify the increase in wildlife nocturnality due to human disturbance.

    更新日期:2018-06-15
  • Facing your fears
    Science (IF 37.205) Pub Date : 2018-06-15
    Paul W. Frankland, Sheena A. Josselyn

    Remembering traumatic fearful events is adaptive. However, treating no-longer-threatening situations as dangerous may be maladaptive and lead to anxiety disorders, including phobias and posttraumatic stress disorder. Central to many forms of therapy designed to tackle these anxiety disorders is the idea that to overcome fear, one needs to face it. For instance, cognitive behavioral therapy and exposure therapy allow patients to confront the objects or situations that provoke their anxiety in the controlled environment of the therapist's office. With repeated exposures, the patients' anxiety levels gradually decline, and the objects or situations that they once feared no longer trouble them. On page 1239 of this issue Khalaf et al. (1) provide a neural mechanism in mice for “facing one's fears.” These findings may inform the development of more effective forms of treatment for anxiety disorders.

    更新日期:2018-06-15
  • Understanding spatial environments from images
    Science (IF 37.205) Pub Date : 2018-06-15
    Matthias Zwicker

    The ability to understand spatial environments based on visual perception arguably is a key function of the cognitive system of many animals, including mammalians and others. A common presumption about artificial intelligence is that its goal is to build machines with a similar capacity of “understanding.” The research community in artificial intelligence, however, has settled on a more pragmatic approach. Instead of attempting to model or quantify understanding directly, the objective is to construct machines that merely solve tasks that seem to require understanding. Understanding can only be measured indirectly, for example, by analyzing the ability of a system to generalize the solving of new tasks, which is sometimes called transfer learning (1). Transfer learning is particularly appealing in an unsupervised setting, which means that the objective of the original task is defined in terms of the input data itself, without requiring additional, task-specific information (see the figure). On page 1204 of this issue, Eslami et al. (2) present an important step toward building machines that learn to understand spatial environments using unsupervised transfer learning. Remarkably, they develop a system that relies only on inputs from its own image sensors, and that learns autonomously and without human supervision.

    更新日期:2018-06-15
  • Preparing ocean governance for species on the move
    Science (IF 37.205) Pub Date : 2018-06-15
    Malin L. Pinsky, Gabriel Reygondeau, Richard Caddell, Juliano Palacios-Abrantes, Jessica Spijkers, William W. L. Cheung

    The ocean is a critical source of nutrition for billions of people, with potential to yield further food, profits, and employment in the future (1). But fisheries face a serious new challenge as climate change drives the ocean to conditions not experienced historically. Local, national, regional, and international fisheries are substantially underprepared for geographic shifts in marine animals driven by climate change over the coming decades. Fish and other animals have already shifted into new territory at a rate averaging 70 km per decade (2), and these shifts are expected to continue or accelerate (3). We show here that many species will likely shift across national and other political boundaries in the coming decades, creating the potential for conflict over newly shared resources.

    更新日期:2018-06-15
  • Asperger's chilling complicity
    Science (IF 37.205) Pub Date : 2018-06-15
    Meredith Wadman

    With the growing prominence of Asperger's syndrome has come increased scrutiny of the man behind it. But until recently, historians writing in English have either skirted Asperger's closeness to the Nazi regime or depicted him as a compassionate physician who used his medical position to rescue disabled children otherwise bound for destruction. The 2016 book In a Different Key: The Story of Autism, by John Donvan and Caren Zucker, began to dismantle this portrait of Asperger. In her new book, Asperger's Children, Edith Sheffer finishes the job.

    更新日期:2018-06-15
  • Platforms of power
    Science (IF 37.205) Pub Date : 2018-06-15
    Anna Lauren Hoffmann

    In Custodians of the Internet, Tarleton Gillespie goes beyond a mere account of the tools and practices employed by social media companies to address problems of harassment, obscenity, and hate speech on their platforms. He also aims to capture just what is at stake in debates over online expression—from the consequences of being "censored" online to the fate of social and democratic norms in the face of easily manipulable news feeds.

    更新日期:2018-06-15
  • Neural scene representation and rendering
    Science (IF 37.205) Pub Date : 2018-06-15
    S. M. Ali Eslami, Danilo Jimenez Rezende, Frederic Besse, Fabio Viola, Ari S. Morcos, Marta Garnelo, Avraham Ruderman, Andrei A. Rusu, Ivo Danihelka, Karol Gregor, David P. Reichert, Lars Buesing, Theophane Weber, Oriol Vinyals, Dan Rosenbaum, Neil Rabinowitz, Helen King, Chloe Hillier, Matt Botvinick, Daan Wierstra, Koray Kavukcuoglu, Demis Hassabis

    Scene representation—the process of converting visual sensory data into concise descriptions—is a requirement for intelligent behavior. Recent work has shown that neural networks excel at this task when provided with large, labeled datasets. However, removing the reliance on human labeling remains an important open problem. To this end, we introduce the Generative Query Network (GQN), a framework within which machines learn to represent scenes using only their own sensors. The GQN takes as input images of a scene taken from different viewpoints, constructs an internal representation, and uses this representation to predict the appearance of that scene from previously unobserved viewpoints. The GQN demonstrates representation learning without human labels or domain knowledge, paving the way toward machines that autonomously learn to understand the world around them.

    更新日期:2018-06-15
  • Photochemistry beyond the red limit in chlorophyll f–containing photosystems
    Science (IF 37.205) Pub Date : 2018-06-15
    Dennis J. Nürnberg, Jennifer Morton, Stefano Santabarbara, Alison Telfer, Pierre Joliot, Laura A. Antonaru, Alexander V. Ruban, Tanai Cardona, Elmars Krausz, Alain Boussac, Andrea Fantuzzi, A. William Rutherford

    Photosystems I and II convert solar energy into the chemical energy that powers life. Chlorophyll a photochemistry, using red light (680 to 700 nm), is near universal and is considered to define the energy “red limit” of oxygenic photosynthesis. We present biophysical studies on the photosystems from a cyanobacterium grown in far-red light (750 nm). The few long-wavelength chlorophylls present are well resolved from each other and from the majority pigment, chlorophyll a. Charge separation in photosystem I and II uses chlorophyll f at 745 nm and chlorophyll f (or d) at 727 nm, respectively. Each photosystem has a few even longer-wavelength chlorophylls f that collect light and pass excitation energy uphill to the photochemically active pigments. These photosystems function beyond the red limit using far-red pigments in only a few key positions.

    更新日期:2018-06-15
  • Giant tunneling magnetoresistance in spin-filter van der Waals heterostructures
    Science (IF 37.205) Pub Date : 2018-06-15
    Tiancheng Song, Xinghan Cai, Matisse Wei-Yuan Tu, Xiaoou Zhang, Bevin Huang, Nathan P. Wilson, Kyle L. Seyler, Lin Zhu, Takashi Taniguchi, Kenji Watanabe, Michael A. McGuire, David H. Cobden, Di Xiao, Wang Yao, Xiaodong Xu

    Magnetic multilayer devices that exploit magnetoresistance are the backbone of magnetic sensing and data storage technologies. Here, we report multiple-spin-filter magnetic tunnel junctions (sf-MTJs) based on van der Waals (vdW) heterostructures in which atomically thin chromium triiodide (CrI3) acts as a spin-filter tunnel barrier sandwiched between graphene contacts. We demonstrate tunneling magnetoresistance that is drastically enhanced with increasing CrI3 layer thickness, reaching a record 19,000% for magnetic multilayer structures using four-layer sf-MTJs at low temperatures. Using magnetic circular dichroism measurements, we attribute these effects to the intrinsic layer-by-layer antiferromagnetic ordering of the atomically thin CrI3. Our work reveals the possibility to push magnetic information storage to the atomically thin limit and highlights CrI3 as a superlative magnetic tunnel barrier for vdW heterostructure spintronic devices.

    更新日期:2018-06-15
  • Probing magnetism in 2D van der Waals crystalline insulators via electron tunneling
    Science (IF 37.205) Pub Date : 2018-06-15
    D. R. Klein, D. MacNeill, J. L. Lado, D. Soriano, E. Navarro-Moratalla, K. Watanabe, T. Taniguchi, S. Manni, P. Canfield, J. Fernández-Rossier, P. Jarillo-Herrero

    Magnetic insulators are a key resource for next-generation spintronic and topological devices. The family of layered metal halides promises varied magnetic states, including ultrathin insulating multiferroics, spin liquids, and ferromagnets, but device-oriented characterization methods are needed to unlock their potential. Here, we report tunneling through the layered magnetic insulator CrI3 as a function of temperature and applied magnetic field. We electrically detect the magnetic ground state and interlayer coupling and observe a field-induced metamagnetic transition. The metamagnetic transition results in magnetoresistances of 95, 300, and 550% for bilayer, trilayer, and tetralayer CrI3 barriers, respectively. We further measure inelastic tunneling spectra for our junctions, unveiling a rich spectrum consistent with collective magnetic excitations (magnons) in CrI3.

    更新日期:2018-06-15
  • Normative brain size variation and brain shape diversity in humans
    Science (IF 37.205) Pub Date : 2018-06-15
    P. K. Reardon, Jakob Seidlitz, Simon Vandekar, Siyuan Liu, Raihaan Patel, Min Tae M. Park, Aaron Alexander-Bloch, Liv S. Clasen, Jonathan D. Blumenthal, Francois M. Lalonde, Jay N. Giedd, Ruben C. Gur, Raquel E. Gur, Jason P. Lerch, M. Mallar Chakravarty, Theodore D. Satterthwaite, Russell T. Shinohara, Armin Raznahan

    Brain size variation over primate evolution and human development is associated with shifts in the proportions of different brain regions. Individual brain size can vary almost twofold among typically developing humans, but the consequences of this for brain organization remain poorly understood. Using in vivo neuroimaging data from more than 3000 individuals, we find that larger human brains show greater areal expansion in distributed frontoparietal cortical networks and related subcortical regions than in limbic, sensory, and motor systems. This areal redistribution recapitulates cortical remodeling across evolution, manifests by early childhood in humans, and is linked to multiple markers of heightened metabolic cost and neuronal connectivity. Thus, human brain shape is systematically coupled to naturally occurring variations in brain size through a scaling map that integrates spatiotemporally diverse aspects of neurobiology.

    更新日期:2018-06-15
  • Synapse-specific representation of the identity of overlapping memory engrams
    Science (IF 37.205) Pub Date : 2018-06-15
    Kareem Abdou, Mohammad Shehata, Kiriko Choko, Hirofumi Nishizono, Mina Matsuo, Shin-ichi Muramatsu, Kaoru Inokuchi

    Memories are integrated into interconnected networks; nevertheless, each memory has its own identity. How the brain defines specific memory identity out of intermingled memories stored in a shared cell ensemble has remained elusive. We found that after complete retrograde amnesia of auditory fear conditioning in mice, optogenetic stimulation of the auditory inputs to the lateral amygdala failed to induce memory recall, implying that the memory engram no longer existed in that circuit. Complete amnesia of a given fear memory did not affect another linked fear memory encoded in the shared ensemble. Optogenetic potentiation or depotentiation of the plasticity at synapses specific to one memory affected the recall of only that memory. Thus, the sharing of engram cells underlies the linkage between memories, whereas synapse-specific plasticity guarantees the identity and storage of individual memories.

    更新日期:2018-06-15
  • The influence of human disturbance on wildlife nocturnality
    Science (IF 37.205) Pub Date : 2018-06-15
    Kaitlyn M. Gaynor, Cheryl E. Hojnowski, Neil H. Carter, Justin S. Brashares

    Rapid expansion of human activity has driven well-documented shifts in the spatial distribution of wildlife, but the cumulative effect of human disturbance on the temporal dynamics of animals has not been quantified. We examined anthropogenic effects on mammal diel activity patterns, conducting a meta-analysis of 76 studies of 62 species from six continents. Our global study revealed a strong effect of humans on daily patterns of wildlife activity. Animals increased their nocturnality by an average factor of 1.36 in response to human disturbance. This finding was consistent across continents, habitats, taxa, and human activities. As the global human footprint expands, temporal avoidance of humans may facilitate human-wildlife coexistence. However, such responses can result in marked shifts away from natural patterns of activity, with consequences for fitness, population persistence, community interactions, and evolution.

    更新日期:2018-06-15
  • Missing enzymes in the biosynthesis of the anticancer drug vinblastine in Madagascar periwinkle
    Science (IF 37.205) Pub Date : 2018-06-15
    Lorenzo Caputi, Jakob Franke, Scott C. Farrow, Khoa Chung, Richard M. E. Payne, Trinh-Don Nguyen, Thu-Thuy T. Dang, Inês Soares Teto Carqueijeiro, Konstantinos Koudounas, Thomas Dugé de Bernonville, Belinda Ameyaw, D. Marc Jones, Ivo Jose Curcino Vieira, Vincent Courdavault, Sarah E. O’Connor

    Vinblastine, a potent anticancer drug, is produced by Catharanthus roseus (Madagascar periwinkle) in small quantities, and heterologous reconstitution of vinblastine biosynthesis could provide an additional source of this drug. However, the chemistry underlying vinblastine synthesis makes identification of the biosynthetic genes challenging. Here we identify the two missing enzymes necessary for vinblastine biosynthesis in this plant: an oxidase and a reductase that isomerize stemmadenine acetate into dihydroprecondylocarpine acetate, which is then deacetoxylated and cyclized to either catharanthine or tabersonine via two hydrolases characterized herein. The pathways show how plants create chemical diversity and also enable development of heterologous platforms for generation of stemmadenine-derived bioactive compounds.

    更新日期:2018-06-15
  • Reactivation of recall-induced neurons contributes to remote fear memory attenuation
    Science (IF 37.205) Pub Date : 2018-06-15
    Ossama Khalaf, Siegfried Resch, Lucie Dixsaut, Victoire Gorden, Liliane Glauser, Johannes Gräff

    Whether fear attenuation is mediated by inhibition of the original memory trace of fear with a new memory trace of safety or by updating of the original fear trace toward safety has been a long-standing question in neuroscience and psychology alike. In particular, which of the two scenarios underlies the attenuation of remote (month-old) fear memories is completely unknown, despite the impetus to better understand this process against the backdrop of enduring traumata. We found—chemogenetically and in an engram-specific manner—that effective remote fear attenuation is accompanied by the reactivation of memory recall–induced neurons in the dentate gyrus and that the continued activity of these neurons is critical for fear reduction. This suggests that the original memory trace of fear actively contributes to remote fear attenuation.

    更新日期:2018-06-15
Some contents have been Reproduced with permission of the American Chemical Society.
Some contents have been Reproduced by permission of The Royal Society of Chemistry.
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