Volume 5 Issue 6, June 2009

Volume 5 Issue 6

From a map, created using traction microscopy, of the physical forces at play in a sheet of canine kidney cells as the colony expands, it is clear that the cells many rows from the front do most of the work. This is contrary to the current belief that sheets of cells move through traction forces exerted by the cells at the leading edge. Letter p426; News & Views p377 Cover design by David Shand

Editorials

  • Editorial |

    Nature Physics now requires a statement of authors' contributions to a paper.

Commentary

  • Commentary |

    With the increasingly urgent need to find solutions to the impending energy crisis, there is growing interest within the fusion community in revisiting the concept of the fusion–fission hybrid reactor. But how soon could such reactors be realized, and could they meet the challenges of the coming century?

    • Jeffrey P. Freidberg
    •  & Andrew C. Kadak

Thesis

Books and Arts

Research Highlights

News and Views

  • News & Views |

    The formation of complex organs, tissue repair and metastasis all require a coordinated regulation of the shape and movement of groups of cells. The mechanical means of communication between cells is crucial to understanding collective cell motions — so how can cells transmit physical forces within cell sheets?

    • Benoit Ladoux
  • News & Views |

    Spin–orbit coupling in some materials leads to the formation of surface states that are topologically protected from scattering. Theory and experiments have found an important new family of such materials.

    • Joel Moore
  • News & Views |

    Stirring a two-dimensional quantum fluid at just the right frequency causes the particles to develop strong quantum correlations. This could reveal much about the nature of phase transitions.

    • Jacob A. Dunningham
  • News & Views |

    A new approach to lasers that promises optical emission with a spectral linewidth of just 1 mHz could lead to even more accurate and stable atomic clocks.

    • Uwe Sterr
    •  & Christian Lisdat
  • News & Views |

    A demonstration that Cooper pairs mediate a non-local coherent coupling between carriers in two normal metal electrodes connected to a superconductor could lead to novel types of superconducting quantum interference devices for studying cross-correlations.

    • Matthias Eschrig
  • News & Views |

    The publication of a potentially testable quantum field theory that can accommodate gravity is causing excitement — but it comes at the expense of Lorentz invariance.

    • Matt Visser
  • News & Views |

    Is superconductivity in the iron arsenides conventional? The large isotope effect on both the magnetic and superconducting transitions may indicate that magnetic fluctuations are involved in the superconducting pairing.

    • D. G. Hinks

Letters

  • Letter |

    An experiment distributing entangled photons over 144 km significantly raises the bar on distance, channel loss and transmission time—encouraging news with regard to future long-distance quantum-communication networks.

    • Alessandro Fedrizzi
    • , Rupert Ursin
    • , Thomas Herbst
    • , Matteo Nespoli
    • , Robert Prevedel
    • , Thomas Scheidl
    • , Felix Tiefenbacher
    • , Thomas Jennewein
    •  & Anton Zeilinger
  • Letter |

    The separation between two electrons bound in a Cooper pair in a conventional superconductor can extend up to several hundred nanometres. A new study shows that these long-range interactions can reach beyond the confines of a superconductor itself to coherently couple electrons in two normal metals either side of the superconductor.

    • P. Cadden-Zimansky
    • , J. Wei
    •  & V. Chandrasekhar
  • Letter |

    Topological insulators are exotic states of matter that show quantum-Hall-like behaviour in the absence of a magnetic field. Surface states in such systems are protected against scattering and are thought to provide an avenue for the realization of fault-tolerant quantum computing. Experiments now reveal the observation of such a topological state of matter in Bi2Se3, a naturally occurring stoichiometric material with a simple surface-state structure and a bulk energy gap larger than kBT at room temperature.

    • Y. Xia
    • , D. Qian
    • , D. Hsieh
    • , L. Wray
    • , A. Pal
    • , H. Lin
    • , A. Bansil
    • , D. Grauer
    • , Y. S. Hor
    • , R. J. Cava
    •  & M. Z. Hasan
  • Letter |

    A recent experimental study of the fractional quantum Hall state—a prototypical system exhibiting strong collective quantum behaviour—provided evidence for the existence of unexpected collective modes at a filling factor of 1/3. Fully microscopic calculations now explain these modes as arising from collective excitations within the composite fermion theory.

    • Dwipesh Majumder
    • , Sudhansu S. Mandal
    •  & Jainendra K. Jain
  • Letter |

    Interacting nuclear spins on a crystalline lattice are commonly believed to be well described within a thermodynamic framework that uses the concept of spin temperature. Demagnetization experiments now challenge this belief, showing that in general the spin-temperature concept fails to describe a nuclear-spin ensemble in a quantum dot when strong quadrupolar interactions are induced by strain.

    • P. Maletinsky
    • , M. Kroner
    •  & A. Imamoglu
  • Letter |

    A method for tomographic imaging of molecular orbitals—based on the alignment of molecules in the laboratory frame and linearly polarized laser fields—has now been extended to atoms, which cannot be naturally aligned.

    • D. Shafir
    • , Y. Mairesse
    • , D. M. Villeneuve
    • , P. B. Corkum
    •  & N. Dudovich
  • Letter |

    The Kadowaki–Woods ratio attempts to relate the temperature dependence of a metal to its heat capacity. However, as it takes different values for different classes of metals it is not universal. By including effects related to carrier density and spatial dimensionality, a much more universal ratio, which describes the properties of many different systems, has been achieved.

    • A. C. Jacko
    • , J. O. Fjærestad
    •  & B. J. Powell
  • Letter |

    It has been thought that sheets of cells move by traction forces exerted by the cells at the leading edge of the sheet. Using traction microscopy to create a map of physical forces, it is now shown that in fact it is cells many rows from the front that do most of the work.

    • Xavier Trepat
    • , Michael R. Wasserman
    • , Thomas E. Angelini
    • , Emil Millet
    • , David A. Weitz
    • , James P. Butler
    •  & Jeffrey J. Fredberg

Articles

  • Article |

    A potentially general mechanism for symmetry breaking in mesoscopic quantum systems is revealed in a theoretical study, which shows how, in a rotating Bose–Einstein condensate, the symmetry properties of the true many-body state are related to those of its mean-field approximation.

    • D. Dagnino
    • , N. Barberán
    • , M. Lewenstein
    •  & J. Dalibard
  • Article |

    First-principles calculations predict that Bi2Se3, Bi2Te3 and Sb2Te3 are topological insulators—three-dimensional semiconductors with unusual surface states generated by spin–orbit coupling—whose surface states are described by a single gapless Dirac cone. The calculations further predict that Bi2Se3 has a non-trivial energy gap larger than the energy scale kBT at room temperature.

    • Haijun Zhang
    • , Chao-Xing Liu
    • , Xiao-Liang Qi
    • , Xi Dai
    • , Zhong Fang
    •  & Shou-Cheng Zhang
  • Article |

    The distributions of the sizes of cities or earthquakes, for example, follow a power law, but in physical systems different distributions of critical properties are usually seen. A scaling argument provides a practical rule to relate the type of distribution to an experimental quantity.

    • Steven T. Bramwell