Physical sciences

  • Letter |

    The most distant quasars known, at redshifts z ≈ 6, generally have the same properties as lower-redshift quasars, implying that although the Universe was young at z ≈ 6, such quasars are still evolved objects. One z ≈ 6 quasar was shown to have no detectable emission from hot dust, but it was not clear whether it was an outlier. Now, a second quasar without hot-dust emission has been discovered in a sample of 21 z ≈ 6 quasars. Moreover, hot-dust abundance in these quasars builds up as the central black hole grows.

    • Linhua Jiang
    • , Xiaohui Fan
    •  & Fabian Walter
  • Letter |

    Of the more than 400 known exoplanets, about 70 transit their central star, most in small orbits (with periods of around 1 day, for instance). Here, observations are reported of the transit of CoRoT-9b, which orbits with a period of 95.274 days, on a low eccentricity, around a solar-like star. Its relatively large periastron distance yields a 'temperate' photospheric temperature estimated to be between 250 and 430 K, and its interior composition is inferred to be consistent with those of Jupiter and Saturn.

    • H. J. Deeg
    • , C. Moutou
    •  & G. Wuchterl
  • Article |

    Quantum mechanics provides an accurate description of a wide variety of physical systems but it is very challenging to prove that it also applies to macroscopic (classical) mechanical systems. This is because it has been impossible to cool a mechanical mode to its quantum ground state, in which all classical noise is eliminated. Recently, various mechanical devices have been cooled to a near-ground state, but this paper demonstrates the milestone result of a piezoelectric resonator with a mechanical mode cooled to its quantum ground state.

    • A. D. O’Connell
    • , M. Hofheinz
    •  & A. N. Cleland
  • Letter |

    An insulator does not conduct electricity, and so cannot in general be used to transmit an electrical signal. But an insulator's electrons possess spin in addition to charge, and so can transmit a signal in the form of a spin wave. Here a hybrid metal–insulator–metal structure is reported, in which an electrical signal in one metal layer is directly converted to a spin wave in the insulating layer; this wave is then transmitted to the second metal layer, where the signal can be directly recovered as an electrical voltage.

    • Y. Kajiwara
    • , K. Harii
    •  & E. Saitoh
  • Letter |

    Although general relativity underlies modern cosmology, its applicability on cosmological length scales has yet to be stringently tested. Now, at a length scale of tens of megaparsecs, the quantity EG, which combines measures of large-scale gravitational lensing, galaxy clustering, and the growth rate of structure, has been measured to be 0.39±0.06, in agreement with the general relativistic prediction of about 0.4.

    • Reinabelle Reyes
    • , Rachel Mandelbaum
    •  & Robert E. Smith
  • Letter |

    When a shape memory polymer is deformed at a temperature defined by a specific phase transition, the deformed shape is fixed upon cooling, but the original shape can be recovered on reheating. Here the perfluorosulphonic acid ionomer Nafion is shown to exhibit at least four different shapes as a result of its broad reversible phase transition.

    • Tao Xie
  • Letter |

    The presence of gaseous chlorine atom precursors within the troposphere was thought only to occur in marine areas but now nitryl chloride has been found at a distance of 1,400 km from the nearest coastline. A model study shows that the amount of nitryl chloride production in the continental USA alone is similar to previous global estimates for marine regions. A significant fraction of tropospheric chlorine atoms may arise directly from anthropogenic pollutants.

    • Joel A. Thornton
    • , James P. Kercher
    •  & Steven S. Brown
  • Letter |

    Measuring the oscillations of a star can allow the various mixing processes in its interior to be disentangled, through the signature they leave on period spacings in the gravity mode spectrum. Here numerous gravity modes in a young star of about seven solar masses are reported: the mean period spacing enables the extent of the convective core to be determined, and the clear periodic deviation from the mean constrains the location of the chemical transition zone — at about 10 per cent of the radius.

    • Pieter Degroote
    • , Conny Aerts
    •  & Eric Michel
  • Review Article |

    • T. D. Ladd
    • , F. Jelezko
    •  & J. L. O’Brien
  • Letter |

    Current models indicate that the Milky Way's stellar halo was assembled from many smaller systems, and recent studies claimed that the true Galactic building blocks must have been vastly different from the surviving dwarfs. But the overall abundance pattern of elements in S1020549, the most iron-poor star in the Sculptor dwarf galaxies, is now found to follow that seen in low-metallicity halo stars, indicating that the systems destroyed to form the halo billions of years ago were not fundamentally different from the progenitors of present-day dwarfs.

    • Anna Frebel
    • , Evan N. Kirby
    •  & Joshua D. Simon
  • Letter |

    To integrate microchips with optical communications a photodetector is required to mediate the optical and electronic signals. Although germanium photodetectors are compatible with silicon their performance is impaired by poor intrinsic noise. Here the noise is reduced by nanometre engineering of optical and electrical fields to produce a compact and efficient photodetector.

    • Solomon Assefa
    • , Fengnian Xia
    •  & Yurii A. Vlasov
  • Letter |

    The phenomenon of superconductivity continues to intrigue, and several new superconducting materials have been discovered in recent years — but in the case of organic superconductors, no new material system with a high superconducting transition temperature has been identified in the past decade. Now it has been shown that the introduction of potassium into crystals of organic molecule picene can yield superconductivity at temperatures as high as 18 K.

    • Ryoji Mitsuhashi
    • , Yuta Suzuki
    •  & Yoshihiro Kubozono
  • Letter |

    The addition of shear orthogonal to the tension-loading plane of crack propagation generates an instability that results in three-dimensional helical crack propagation, atomically rough surfaces and a fracture pattern resembling a series of lance shapes. Here numerical simulations reveal a new law that governs crack propagation in space for materials subject to general stress conditions.

    • Antonio J. Pons
    •  & Alain Karma
  • Letter |

    WASP-12b is a planet of 1.4 Jupiter masses that orbits at a mean distance of only 3.1 stellar radii from its star; its orbital period is 1.1 days, and its radius (1.79 times that of Jupiter) is unexpectedly large. An analysis of its properties now reveals that the planet is losing mass to its host star at a rate of 10−7 Jupiter masses per year, and that dissipation of the star's tidal perturbation in the planet's convective envelope provides the energy source for its large volume.

    • Shu-lin Li
    • , N. Miller
    •  & Jonathan J. Fortney
  • Letter |

    In principle, it is possible to simulate some astrophysical phenomena inside the highly controlled environment of an atomic physics laboratory: previous work on the thermodynamics of a two-component Fermi gas (a system suited for such studies) led to thermodynamic quantities averaged over the trap. Now a general experimental method is reported that yields the equation of state of a uniform gas, providing new physical insights and enabling a detailed comparison with existing theories.

    • S. Nascimbène
    • , N. Navon
    •  & C. Salomon
  • Letter |

    Many technological materials are intentionally 'doped' with foreign elements to impart new and desirable properties, a classic example being the doping of semiconductors to tune their electronic behaviour. Here lanthanide doping is used to control the growth of nanocrystals, allowing for simultaneous tuning of the size, crystallographic phase and optical properties of the hybrid material.

    • Feng Wang
    • , Yu Han
    •  & Xiaogang Liu
  • Letter |

    One of the central predictions of general relativity is that a clock in a gravitational potential well runs more slowly than a similar clock outside the well. This effect, known as gravitational redshift, has been measured using clocks on a tower, an aircraft and a rocket, but here, laboratory experiments based on quantum interference of atoms are shown to produce a much more precise measurement.

    • Holger Müller
    • , Achim Peters
    •  & Steven Chu
  • Letter |

    Type Ia supernovae are thought to be associated with the thermonuclear explosions of white dwarf stars, but the nuclear runaway that leads to the explosion could occur through two different pathways with different X-ray signatures. The X-ray flux from six nearby elliptical galaxies and galaxy bulges is now observed to reveal that it is a factor of about 30–50 less than predicted by the accretion scenario, where a white dwarf accumulates material from a companion star.

    • Marat Gilfanov
    •  & Ákos Bogdán
  • Letter |

    It is widely accepted that strong and variable radiation detected over all accessible energy bands in a number of active galaxies arises from a relativistic, Doppler-boosted jet pointing close to our line of sight. However, the size of the emitting zone and the location of this region relative to the central supermassive black hole are poorly understood. Here, the coincidence of a γ-ray flare with a dramatic change of optical polarization angle is reported, providing evidence for co-spatiality of optical and γ-ray emission regions and indicating a highly ordered jet magnetic field.

    • A. A. Abdo
    • , M. Ackermann
    •  & M. Sikora
  • Letter |

    For the first billion years or so of the Earth's history, there may have been whole-mantle convection, but after this period differentiation of the Earth's mantle has been controlled by solid-state convection. Many trace elements — known as 'incompatible elements' — preferentially partition into low-density melts and are concentrated into the crust, but half of these incompatible elements should be hidden in the Earth's interior. It is now suggested that a by-product of whole-mantle convection is deep and hot melting, resulting in the generation of dense liquids that sank into the lower mantle.

    • Cin-Ty A. Lee
    • , Peter Luffi
    •  & John Hernlund
  • Letter |

    T lymphocytes, which are an integral part of most adaptive immune responses, recognize foreign antigens through the binding of antigenic peptide–major histocompatibility complex (pMHC) molecules on other cells to specific T-cell antigen receptors (TCRs). Using single-molecule microscopy and fluorescence resonance energy transfer, the kinetics of TCR–pMHC binding are now measured in situ, revealing accelerated kinetics and increased affinity when compared with solution measurements.

    • Johannes B. Huppa
    • , Markus Axmann
    •  & Mark M. Davis
  • Letter |

    Stars form from cold molecular interstellar gas, which is relatively rare in the local Universe, such that galaxies like the Milky Way form only a few new stars per year. However, typical massive galaxies in the distant Universe formed stars much more rapidly, suggesting that young galaxies were more rich in molecular gas. The results of a survey of molecular gas in samples of typical massive star-forming galaxies when the Universe was 40% and 24% of its current age now reveal that distant star-forming galaxies were indeed gas rich.

    • L. J. Tacconi
    • , R. Genzel
    •  & B. Weiner
  • Letter |

    The difference between the mass of an atom and the sum of its building blocks (the binding energy) is a manifestation of Einstein's famous relation E = mc2. Superheavy elements have been observed, but our present knowledge of the binding energy of these nuclides is based only on the detection of their decay products, although they represent the gateway to the predicted 'island of stability'. Here, direct mass measurements of trans-uranium nuclides are reported, providing reliable anchor points en route to the island of stability.

    • M. Block
    • , D. Ackermann
    •  & C. Weber
  • Letter |

    Ferroelectrics are electro-active materials that can store and switch their polarity, sense temperature changes, interchange electric and mechanical functions, and manipulate light. Subtle changes in the topology of certain chemical bonds have long been identified as a possible route for achieving ferroelectricity in organic molecular crystals. Ferroelectricity above room temperature is now demonstrated by applying an electric field to coherently align the molecular polarities in crystalline croconic acid.

    • Sachio Horiuchi
    • , Yusuke Tokunaga
    •  & Yoshinori Tokura
  • Letter |

    Infrared spectroscopy can probe the conditions and compositions of exoplanet atmospheres. Previous results relied on space-based telescopes that do not provide spectroscopic capability in the 2.4–5.2 μm spectral region. Here, ground-based observations of the dayside emission spectrum for HD 189733b are reported between 2.0–2.4 μm and 3.1–4.1 μm; an unexpected feature at around 3.25 μm is found that is difficult to explain with models that assume local thermodynamic equilibrium conditions, and is assigned to methane.

    • Mark R. Swain
    • , Pieter Deroo
    •  & Thomas Henning
  • Letter |

    Many plants and animals make use of biological surfaces with structural features at the micro- and nanometre-scale that control the interaction with water. The appearance of dew drops on spider webs is an illustration of how they are one such material capable of efficiently collecting water from air. The water-collecting ability of the capture silk of the Uloborus walckenaerius spider is now shown to be the result of a unique fibre structure that forms after wetting.

    • Yongmei Zheng
    • , Hao Bai
    •  & Lei Jiang
  • Letter |

    The primary sequence of a protein defines its free-energy landscape and thus determines the rate constants of folding and unfolding, with theory suggesting that roughness in the energy landscape leads to slower folding. However, obtaining experimental descriptions of this landscape is challenging. Landscape roughness is now shown to be responsible for the slower folding and unfolding times observed in the R16 and R17 domains of α-spectrin relative to the similar R15 domain.

    • Beth G. Wensley
    • , Sarah Batey
    •  & Jane Clarke
  • Letter |

    Long duration γ-ray bursts mark the explosive death of some massive stars and are a rare sub-class of type Ibc supernovae. To date, central-engine-driven supernovae have been discovered exclusively through their γ-ray emission, yet it is expected that a larger population goes undetected. The discovery of luminous radio emission from the seemingly ordinary type Ibc supernova SN 2009bb, which requires a substantial relativistic outflow powered by a central engine, is now reported.

    • A. M. Soderberg
    • , S. Chakraborti
    •  & M. A. P. Torres
  • Letter |

    In the study of high-transition-temperature (high-Tc) copper oxide superconductors, a fundamental question is what symmetries are broken when the pseudogap phase sets in below a temperature T*. A large in-plane anisotropy of the Nernst effect is now observed in a high-Tc copper oxide superconductor that sets in precisely at T* throughout the doping phase diagram. It is concluded that the pseudogap phase is an electronic state that strongly breaks four-fold rotational symmetry.

    • R. Daou
    • , J. Chang
    •  & Louis Taillefer
  • Letter |

    The transformation of petroleum-derived feedstocks into useful chemicals often requires controllable cleavage of C–H or C–C bonds. There are many examples of achieving this through the oxidative addition of C–H bonds to metal centres, but analogous transformations of C–C bonds are rare. Here, using a tungsten centre and exploiting the formation of an unusual chelating ligand, a strong C–C bond is cleaved; other metal centres with suitable ancillary ligands could perform the same function.

    • Aaron Sattler
    •  & Gerard Parkin
  • Letter |

    Type Ic supernovae have drawn attention since 1998 owing to their sparse association with long duration γ-ray bursts (GRBs). Although the GRB central engine generates ultra-relativistic jets, no relativistic outflows have yet been found in type Ib/c supernovae explosions. Here, radio observations reveal a mildly relativistic expansion in a nearby type Ic supernova, SN 2007gr.

    • Z. Paragi
    • , G. B. Taylor
    •  & B. Paczyński
  • Letter |

    In the search to reduce our dependency on fossil-fuel energy, new plastic materials that are less dependent on petroleum are being developed, with water-based gels — hydrogels — representing one possible solution. Here, a mixture of water, 3% clay and a tiny amount of a special organic binder is shown to form a transparent hydrogel that can be moulded into shape-persistent, free-standing objects and that rapidly and completely self-heals when damaged.

    • Qigang Wang
    • , Justin L. Mynar
    •  & Takuzo Aida
  • Letter |

    Although deformation twinning in crystals controls the mechanical behaviour of many materials, its size-dependence has not been explored. Using micro-compression and in situ nano-compression experiments, the stress required for deformation twinning is now found to increase drastically with decreasing sample size of a titanium alloy single crystal, until the sample size is reduced to one micrometre; below this point, deformation twinning is replaced by dislocation plasticity.

    • Qian Yu
    • , Zhi-Wei Shan
    •  & Evan Ma
  • Letter |

    Telescopic measurements of asteroids' colours rarely match laboratory reflectance spectra of meteorites owing to a 'space weathering' process that rapidly reddens asteroid surfaces. 'Unweathered' asteroids, however, with spectra matching ordinary chondrite meteorites, are seen only among small bodies with orbits that cross inside the orbits of Mars and Earth. Such unweathered asteroids are now shown to have experienced orbital intersections closer than the Earth–Moon distance within the past half-million years.

    • Richard P. Binzel
    • , Alessandro Morbidelli
    •  & Alan T. Tokunaga
  • Letter |

    The close binary Algol system contains a radio-bright KIV sub-giant star in a very close and rapid orbit with a main sequence B8 star. Evidence points to the existence of an extended, complex coronal magnetosphere originating at the cooler K subgiant, but the detailed morphology of the subgiant's corona and its possible interaction with its companion are unknown. Multi-epoch radio imaging of the Algol system now reveals a large coronal loop suggestive of a persistent asymmetric magnetic field structure aligned between the two stars.

    • W. M. Peterson
    • , R. L. Mutel
    •  & W. M. Goss
  • Letter |

    The properties of 'dwarf' galaxies have long challenged the cold dark matter (CDM) model of galaxy formation, as the properties of most observed dwarf galaxies contrast with models based on the dominance of CDM. Here, hydrodynamical simulations (assuming the presence of CDM) are reported in which the analogues of dwarf galaxies — bulgeless and with shallow central dark-matter profiles — arise naturally.

    • F. Governato
    • , C. Brook
    •  & P. Madau
  • Letter |

    Phytochromes regulate numerous photoresponses in plants and microorganisms through their ability to photointerconvert between a red-light-absorbing, ground state (Pf) and a far-red-light-absorbing, photoactivated state (Pfr). The structures of several phytochromes as Pf have been determined previously; here, the three-dimensional solution structure of the bilin-binding domain as Pfr is described. The results shed light on the structural basis for photoconversion to the activated Pfr form.

    • Andrew T. Ulijasz
    • , Gabriel Cornilescu
    •  & Richard D. Vierstra
  • Letter |

    The amplitude of the magnetic field near the Galactic Centre has been uncertain by two orders of magnitude for several decades. A compilation of previous data now reveals a downward break in the region's non-thermal radio spectrum; this requires that the Galactic Centre field be at least 50 microgauss on 400 parsec scales, with evidence supporting a field of 100 microgauss. This would imply that over 10% of the Galaxy's magnetic energy is contained in only around 0.05% (or less) of its volume.

    • Roland M. Crocker
    • , David I. Jones
    •  & Raymond J. Protheroe
  • Letter |

    From earthquakes to hard drives, frictional motion and its strength are involved in a wide range of phenomena. The strength of an interface that divides two sliding bodies is determined by both the real contact area and the contacts' shear strength. By continuous measurements of the concurrent local evolution of the real contact area and the corresponding interface motion from the first microseconds when contact detachment occurs, frictional strength is now characterized from short to long timescales.

    • Oded Ben-David
    • , Shmuel M. Rubinstein
    •  & Jay Fineberg
  • Letter |

    The Dirac equation successfully merges quantum mechanics with special relativity. It predicts some peculiar effects such as 'Zitterbewegung', an unexpected quivering motion of a free relativistic quantum particle. This and other predicted phenomena are key fundamental examples for understanding relativistic quantum effects, but are difficult to observe in real particles. Here, using a single trapped ion set to behave as a free relativistic quantum particle, a quantum simulation of the one-dimensional Dirac equation is demonstrated.

    • R. Gerritsma
    • , G. Kirchmair
    •  & C. F. Roos