Every year, a committee of experts sits down with a tough job to do: from among all ICREA publications, they must find a handful that stand out from all the others. This is indeed a challenge. The debates are sometimes heated and always difficult but, in the end, a shortlist of 24 publications is produced. No prize is awarded, and the only additional acknowledge is the honour of being chosen and highlighted by ICREA. Each piece has something unique about it, whether it be a particularly elegant solution, the huge impact it has in the media or the sheer fascination it generates as a truly new idea. For whatever the reason, these are the best of the best and, as such, we are proud to share them here.


Format: yyyy
  • New ideas and discoveries on gamma-ray binaries (2012)

    Torres, Diego F. (CSIC - ICE)

    view details

    New ideas and discoveries on gamma-ray binaries

    Among the many X-ray binaries that are known in the sky, only a handful emit up to gamma-ray energies. 2012 marks the year in which one such system was discovered first in GeV gamma rays, using a mathematical algorithm applied to all sources detected by the Large Area Telescope onboard the satellite Fermi. This discovery was featured in Science, as the first of a possible population to follow [1].
    What is the physical nature of these gamma-ray binaries? Growing consensus mount as to that the compact object in these gamma-ray binaries is pulsar. One of the key pieces of the puzzle in this sense was the discovery of two very short X-ray flares lasting less than 1 second, coming from the system LS I +61 303, and with luminosities orders of magnitude larger than its usual emission at all wavelengths. Only magnetars behave this way. If some gamma-ray binaries are magnetar-composed, they would likely flip-flop between states along the orbital motion, traversing a variety of conditions that may explain the varied phenomenology at all frequencies [2].

    Another 2012 discovery in this topic has been that the X-ray emission of the aforementioned system is modulated at long scales (>4 years), similar to -but dephased from- its radio emission. Hints for a TeV modulation in similar timescales were also already apparent. These two facts can provide clues as to finally disentangling the nature of these objects [3].

  • Cells ride on stress waves (2012)

    Trepat, Xavier (IBEC)

    view details

    Cells ride on stress waves

    When an organism develops its shape or heals wounds, or when tumours metastasize, cells undergo massive collective movements. Despite decades of research, the mechanisms underpinning these movements remain poorly understood. It has been argued that chemical stimuli and chemical gradients alone cannot explain this form of cellular migration, and some evidence has implicated a role for mechanotransduction -- the perception and reaction to a mechanical force. Using new technologies developed in our own group, we discovered that cells transmit forces from their leading edge, creating a stress wave that propagates through the mass of expanding tissue.

    To study collective cell migration in a controlled manner we used soft lithography techniques. Using these techniques we were able to produce polydimethylsiloxane membranes with a rectangular opening that was placed on top of a polyacrylamide gel coated with collagen I. We then grew Madin-Darby Canine Kidney cells to confluence in the rectangular opening and, as the membrane was removed, cells spontaneously migrated. To measure the forces driving such migration we used monolayer stress microscopy, a technique developed in our own group.

    Cells at the edges of the epithelial monolayer were the first to migrate and to generate forces. Surprisingly, these forces were transmitted from cell to cell through intercellular junctions in a wave-like manner. The wave velocity was ultraslow, roughly one millimetre per day. As such, these waves are certainly among the slowest mechanical waves ever described.

    To study the origin of these mechanical waves we built a mathematical model that captured the observed phenomenology using two purely mechanical assumptions: that a cell acquires a motile phenotype only when an adjacent cell creates space or pulls on the shared inter-cellular junction; and that the cell contains a strain threshold that, once exceeded, results in the cytoskeleton first reinforcing its stiffness and then breaking down (fluidizing). These findings indicate that although chemical stimuli and/or gene expression could be involved in epithelial monolayer migration, these are not necessary for the generation of mechanical waves.

    This study brings us one step closer to understanding how cells migrate, and thus a stage nearer to understanding the dynamics of tumour cells and the physical mechanisms they use to break away and metastasize.

  • The elusive magnon drag observed. A 50-year quest to isolate this thermoelectric effect is now over (2012)

    Valenzuela, Sergio O. (ICN2)

    view details

    The elusive magnon drag observed. A 50-year quest to
    isolate this thermoelectric effect is now over

    As electrons move past atoms in a solid, their charge distorts the nearby lattice and can create a wave. Reciprocally, a wave in the lattice affects the electrons motion, in analogy to a wave in the sea that pushes a surfer riding it. This interaction results in a thermoelectric effect that was first observed during the 1950´s and has come to be known as phonon-drag, because it can be quantified from the flow of lattice-wave quanta (phonons) that occurs over the temperature gradient. Soon after the discovery of the phonon drag, an analogous phenomenon was predicted in magnetic materials: the so-called magnon drag. In ferromagnets, the intrinsic magnetic moment or spin of the electrons arrange in an organized fashion, maintaining a parallel orientation. If a distortion in the preferred spin orientation occurs, a spin wave is created that could affect electron motion, that is, a flow of magnons (spin-wave quanta) could also drag the electrons. Despite the similarities with phonon drag, the observation of the magnon drag has been elusive, and only a few indirect indications of its existence have been reported over the years. The main reason being the presence of other thermoelectric effects, most notably the phonon drag, that make it difficult to discriminate its contribution to the thermopower. ICREA Research Professor Sergio O. Valenzuela and his group at ICN, Physics and Engineering of Nanodevices, used a unique device geometry to discriminate the magnon drag from other thermoelectric effects. As reported in the journal Nature Materials, the device resembles a thermopile formed by a large number of pairs of ferromagnetic wires placed between a hot and a cold source and connected thermally in parallel and electrically in series. By controlling the relative orientation of the magnetization in pairs of wires, the magnon drag can be studied independently of the electron and phonon drag thermoelectric effects. The work is very timely as thermoelectric effects in spin- electronics (spintronics) are gathering increasing attention as a means of managing heat in nanoscale structures and of controlling spin information by using heat flow. Measurements as a function of temperature reveal the effect on magnon drag following a variation of magnon and phonon populations. This information is crucial to understand the physics of thermal spin transport. It both provides invaluable opportunities to gather knowledge about electron-magnon interactions and may be beneficial for energy conversion applications and for the search of nov

  • Seeing light by electrons (2012)

    van Hulst, Niek F. (ICFO)

    view details

    Seeing light by electrons

    "Fiat Lux - he saw the light and it was good.......". Indeed light is at the basis of our sensations and observations. Yet, oddly enough, light has its limits in seeing how light and matter interact. Specifically, the behaviour of light at the nanoscale is hard to discern by light microscopy. Here we show that electrons are an effective alternative to image light at the true nanoscale. Exciting by electrons, while still detecting light, we have probed the inside of photonic crystals and mapped how light behaves with a spatial resolution of 30 nanometer, far below the wavelength of light.

    Photonic crystals are nanostructures in which two materials with different refractive index are arranged in a regular pattern, giving rise to exotic optical properties. Natural photonic crystals can be found in certain species of butterflies, birds and beetles as well as in opal gemstones where they give rise to beautiful iridescent colors. Due to major advances in nanofabrication techniques it has become possible to fabricate artificial photonic crystals with optical properties that can be accurately engineered. These structures can be used to make high-quality nanoscale optical waveguides and cavities, which are important in telecommunication and sensing applications.

    We constructed 2-dimensional photonic crystals by etching a hexagonal pattern of holes in a very thin silicon nitride membrane. The photonic crystal inhibits light propagation for certain colors of light, which leads to strong reflection of those colors. By leaving out one hole a very small cavity can be defined where the surrounding crystal acts as a mirror for the light, making it possible to strongly confine light within such a "crystal defect cavity".

    Using the electron approach we can now see the finest details of photonic crystals that were simply inaccessible before. Seeing where the various colours are trapped in the cavity provides direct insight into the light - matter interaction on the nanoscale. Such understanding is crucial for the development of enhanced optical devices such as bio-sensors for healthcare and more efficient solar cells and displays.

  • Modernity - understanding the present (2012)

    Wagner, Peter (UB)

    view details

    Modernity - understanding the present

    We live in a modern age, but what does `modern' mean and how can a reflection on `modernity' help us to understand the world today? These are the questions that Peter Wagner sets out to answer in this concise and accessible book.

    Wagner begins by returning to the question of modernity's western origins and its claims to open up a new and better era in the history of humanity. Modernity's claims and expectations have become more prevalent and widely shared, but in the course of their realization and diffusion they have also been radically transformed. In an acute and engaging analysis, Wagner examines, among others, the following key issues:

    * Modernity was based on the hope for freedom and reason, but it created the institutions of contemporary capitalism and democracy. How does the freedom of the citizen relate to the freedom of the buyer and seller today? And what does disaffection with capitalism and democracy entail for the sustainability of modernity?

    * Rather than a single model of modernity, there is now a plurality of forms of modern socio-political organization. What does this entail for our idea of progress and our hope that the future world can be better than the present one?

    * All nuance and broadening notwithstanding, our concept of modernity is in some way inextricably tied to the history of Europe and the West. How can we compare different forms of modernity in a `symmetric', non-biased or non-Eurocentric way? How can we develop a world-sociology of modernity?

    This book is the first major publication from the European Research Council-funded research project "Trajectories of modernity: comparing non-European and European varieties" (TRAMOD), which is based in the Department of Sociological Theory, Philosophy of Law and Methodology of the Social Sciences at the University of Barcelona and of which Peter Wagner is the Principal Investigator. The project TRAMOD aims at advancing the analysis of the contemporary plurality of self-understandings and related institutional structures of societies and polities in the current global context. It analyzes these self-understandings against the background of the historical trajectories of those societies. The analysis of the existing multiple forms of modernity is the major challenge to current social and political theory and comparative-historical and political sociology. It requires a conceptual and empirical analysis of that which is common to different forms of modernity and that which varies between them.

  • Europe's Oldest Cave Art (2012)

    Zilhão, João (UB)

    view details

    Europe's Oldest Cave Art

    Dating rock art has traditionally been made by indirect methods, such as stylistical comparison, or, when buried by sediments that have an archeological content or can themselves be dated, stratigraphic context. Over the last two decades, radiocarbon has been applied to obtain direct dates, namely to black paintings containing organic pigments (mostly charcoal), but most rock art comes in the form of engravings or paintings made with mineral pigments (black manganese, yellow or red ochre). Moreover, curatorial concerns limit the size of such radiocarbon samples, which makes it difficult to deal with decontamination, and an offset may exist between the sample's date and the time of painting (e.g., fossil bone collected from the floor of a cave could have been used many thousands of years later as fuel for the production of the charcoal used for pigment).

    Technical developments in the Uranium-series dating of speleothems, allowing age measurements for samples as small as 10 mg, make it possible to circumvent the problems with radiocarbon and obtain secure minimum dates for Paleolithic art covered by thin calcite films. We applied the technique to a number of sites in Northern Spain, including the World Heritage properties of El Castillo, Altamira and Tito Bustillo. The age of the calcite covering red disks at El Castillo places their execution at a time before 41,000 years ago, possibly significantly earlier. Ages in excess of 37,000 years have also been obtained for calcite-covered hand stencils in the same panel. These results show that the walls of European caves were being decorated many millennia before the time indicated by traditional methods. Additionally, the great antiquity of this cave art implies a strong probability that the first cave painters were the Neanderthals, with attendant implications for current debates concerning their behavioral and cognitive capabilities and evolutionary status -- as a different species altogether or, as suggested by the recent genetic and fossil evidence, simply as a geographical variant of early Homo sapiens.