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  the most outstanding publications of the year 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.


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  • Ultracold Atoms in Optical Lattices Simulating quantum many-body systems By Maciej Lewenstein, Anna Sanpera and Verónica Ahufinger (2012)

    Sanpera Trigueros, Anna (UAB)

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    Ultracold Atoms in Optical Lattices
    Simulating quantum many-body systems
    By Maciej Lewenstein, Anna Sanpera and Verónica Ahufinger

    First comprehensive book on ultracold gases in optical lattices
    First book on quantum simulators
    Broad range of topics covered
    Interdisciplinary character
    Covers both theoretical and experimental aspects

    Quantum computers, though not yet available on the market, will revolutionize the future of information processing. Quantum computers for special purposes like quantum simulators are already within reach. The physics of ultracold atoms, ions and molecules offer unprecedented possibilities of control of quantum many body systems and novel possibilities of applications to quantum information processing and quantum metrology. Particularly fascinating is the possibility of using ultracold atoms in lattices to simulate condensed matter or even high energy physics.

    This book provides a complete and comprehensive overview of ultracold lattice gases as quantum simulators. It opens up an interdisciplinary field involving atomic, molecular and optical physics, quantum optics, quantum information, condensed matter and high energy physics. The book includes some introductory chapters on basic concepts and methods, and then focuses on the physics of spinor, dipolar, disordered, and frustrated lattice gases. It reviews in detail the physics of artificial lattice gauge fields with ultracold gases. The last part of the book covers simulators of quantum computers. After a brief course in quantum information theory, the implementations of quantum computation with ultracold gases are discussed, as well as our current understanding of condensed matter from a quantum information perspective.

    Readership: Graduate students and researchers in atomic, molecular, and optical physics, quantum optics, quantum information, condensed matter physics, and quantum field theory.

  • USP15: a Promising Novel Therapeutic Target in Cancer (2012)

    Seoane Suárez, Joan (VHIO)

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    USP15: a Promising Novel Therapeutic Target in Cancer

    In March 2012, Dr. Joan Seoane´s group at the Vall d'Hebron Institute of Oncology (VHIO) published a study in Nature Medicine identifying USP15 as a critical protein in cancer that, thanks to its molecular characteristics, shows enormous therapeutic promise.

    USP15 as a "Biological Thermostat" at the core of a TGFß chain reaction, Dr. Seoane's team unmasked the USP15 enzyme as the activator of the TGFß chain reaction. In normal tissue, USP15 acts by controlling and correcting the TGFß activity in the same way that a thermostat regulates temperature. If the TGFß activity is high, USP15 reduces; and if it is low, USP15 increases the TGFß activity. USP15 therefore achieves optimal TGFß activity.

    Protein stability is regulated through the elimination or aggregation of ubiquitins, small proteins that establish which molecules need to be eliminated. This process is regulated by deubiquitinating enzymes (DUBs) and ubiquitin ligases such as USP15 and Smurf2, respectively, which determine the correct level of a protein under certain physiological conditions. The group found that in this orchestrated manner USP15 controls and adapts the TGFß receptor stability and, therefore, the activity of the pathway.

    The problem arises when, in some tumors, the USP15 gene is amplified due to genetic mutations and the enzyme is overproduced. The thermostat breaks down and is therefore only sensing the "cold" resulting in the (overheating) overactivation of the TGFß pathway. In this context, TGFß acts as an oncogenic factor promoting tumor progression. Hence, the amplification of the USP15 gene promotes tumorigenesis through the hyperactivation of the TGFß signal.

    USP15 as a therapeutic target in cancer Seoane's lab used a model of human glioblastoma that reproduces in mice the tumor from patients undergoing surgery at the Vall d'Hebron Hospital. Inactivation of USP15 resulted in a decrease in oncogenesis indicating that USP15 is a critical oncogenic factor in tumors with USP15 gene amplification. This opens a novel avenue of therapeutic strategies against such a dismal disease. Since USP15 is an enzyme, small organic molecules have been designed to inhibit its catalytic activity being putative therapeutic compounds. The group has proved the efficacy of these USP15 inhibitors in preclinical models of glioblastoma with promising results.

    Remarkably, USP15 is not only an oncogenic factor in glioblastomas since the USP15 gene amplification has also been found activated in other types of cancer such as breast or ovarian cancer

  • Phonons in slow motion (2012)

    Sotomayor Torres, Clivia Marfa (ICN2)

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    Phonons in slow motion

    A main issue in information technology affecting heterogeneous integration and autonomous systems is the fact that they need advanced thermal management and, in the autonomous case, their own energy source (energy harvesters). In general, in energy conservation and generation and in efficient energy transformation, among others, the control of heat dissipation is of paramount importance. From data centres to basic transistors with a sub 20 nm gate width, the issue is thermal management involving heat transport in the plane and across interfaces, all of which is exacerbated as dimensions become smaller.
    Our work addresses the physics underpinning heat transport by focusing on the changes in the dispersion relations of phonons in freestanding membranes down to 8 nm thickness. We have observed and simulated the strong modifications of the dispersion of phonons in model structures, namely freestanding ultra-thin crystalline silicon membranes, which happen to be the materials at the centre of information and communication technologies.
    Our results show that to simulate, and eventually design, new components and circuits, thermal conductance/conductivity and other properties affected by the density of states, including the spectral and geometric dependencies of phonon properties, must be taken into account. Although our studies are only on Si, they are equally applicable to other thin film materials and nanostructures and even more so to multilayer structures, since the acoustic impedance and contrast changes with type, shape, geometry and thickness of each layer.
    This is probably the first report of Lamb waves in the sub-THz regime, which will be of interest to the acoustics community and some sectors of mechanical engineering.
    Our results impact directly not only heat dissipation issues in nanoelectronics but also the physics of nano-scale thermoelectricity. Both are phonon-dependent and naturally affected by temperature and frequency.
    All sensors using nanostructures which require low power electronics and energy harvesting to be really "autonomous", sooner or later, hit upon the issue of thermal management. Our results will help in the modelling and design of nanotechnology-enabled sensors, especially to advance knowledge on lattice vibrations and heat transport in nanosystems.
    The simulations, theories and experiments are intricate and involved knowledge from mechanical and electrical engineering, solid state physics, statistical mechanics, inorganic chemistry and even biophysics. For example, artificial

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

    Torres, Diego F. (CSIC - ICE)

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    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)

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    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)

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    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