Highlights

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.

LIST OF SCIENTIFIC HIGHLIGHTS

Format: yyyy
  • Magneto-electric effect in the simplest copper oxide revealed (2016)

    Skumryev, Vassil (UAB)

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    Magneto-electric effect in the simplest copper oxide revealed

    Magnetism and ferroelectricity are essential to many areas of technology and the quest for multiferroic materials, where these two phenomena coexist, is of immense industrial and fundamental importance. Magnetically-induced ferroelectrics (i.e., multiferroics) constitute an exciting new paradigm in the design of functional materials by intimately coupling magnetic and polar orders. Apart from being so far the only known binary multiferroic compound, the cupric oxide CuO has a much higher transition temperature into the multiferroic state, 230 K, than any other known material in which the electric polarization is induced by spontaneous magnetic order. However, until now no magneto-electric effect has been observed as direct crosstalk between bulk magnetization and electric polarization counterparts, prompting to label CuO as “material with persistent multiferroicity without magneto-electric effects”.

    We have demonstrated that sufficiently high magnetic fields of up to 50 tesla are able to suppress the helical modulation of the magnetic moments in the multiferroic phase and dramatically affect the electric polarization. Furthermore, just below the spontaneous magnetic transition from commensurate (paraelectric) to incommensurate (ferroelectric) magnetic structures at 213 K, even modest magnetic fields can induce a transition into magnetic structure compatible with ferroelectricity and then suppress it at higher fields, thus causing remarkable polarization changes. The synergic use of number of experimental techniques at large scale European facilities (the Eur. Magnetic Field Laboratory; Laue Langevin Inst.) allowed tracing the magnetoelectric phase diagram of CuO, identifying new phase transitions and magnetic structures.

    This study, conceived and coordinated by ICREA researcher Vassil Skumryev, was carried out in collaboration with scientists from Russia, France, Germany, Brazil and Bulgaria. It adds substantial new knowledge, unveiling important new features of this prominent simple oxide with distinct position among the multiferroic materials and of general interest for material scientists and physicists.

  • DOES DISORDER MATTER TO PROPAGATING VIBRATIONS? (2016)

    Sotomayor Torres, Clivia Marfa (ICN2)

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    DOES DISORDER MATTER TO PROPAGATING VIBRATIONS?

    Phonons are lattice vibration arising from collective atomic motion and one of the questions popping up is whether the associated physical properties can be explained by the particle picture or by the wave nature of these excitations. Why does this matter? It matters since it is intimately related to energy dissipation, rendering inadequate attempts to transmit useful signals, from few GHz to 100s of THz impacting, e.g., information processing.

    We investigated propagating phonons in two-dimensional silicon crystals with various positional disorder of otherwise periodically positioned holes, to ascertain to what degree disorder matters in their propagation, since they carry most of the heat in semiconductors materials, underpinning today’s (opto)electronics and nanoelectronics. We used a state-of-the-art pump-and-probe spectroscopy (see fig. 1) and a home-built two-laser Raman thermometry (2LRT) set up to extract the thermal conductivity. We measure at room temperature and simulate by finite element methods. We found that disorder lowers phonon energies and facilitates their in-plane propagation becoming impervious to disorder.  Surprisingly, disorder was found to be unaffected by disorder. We thus propose a practical criterion for predicting phonon coherence as a function of roughness and disorder: (i) phonon coherence is unaffected if the roughness R is smaller than 1/25 of the phonon wavelength and (ii) phonon coherence is destroyed if R is greater than 1/10 of the phonon wavelength.

    Our results have repercussions well beyond our own research in nano-scale thermal transport and thermoelectricity. They provide a hint to the factors affecting vibrational energy transport in condensed matter with variations in the nm-scale, the length scale of acoustic phonons relevant to electronics but also to biology.

  • Increase of protein abundance induces formation of cytotoxic assemblies (2016)

    Tartaglia, Gian Gaetano (CRG)
    Lehner, Ben (CRG)

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    Increase of protein abundance induces formation of cytotoxic assemblies

    From birth to degradation, RNA is bound to proteins. When the abundance of specific proteins increases, RNA is sequestered in large assemblies or granules. We investigated the physico-chemical principles promoting formation of granules and observed toxicity when the assemblies are formed in non-physiologial conditions.  Indeed, trapping molecules in granules prevents RNA from translation, which perturbes cell functions and impairs general homeostasis. Our work sheds light on the molecular mechanisms behind  processes that lead to devastating human diseases.

     

  • Three first-timers in Galactic high-energy astrophysics (2016)

    Torres, Diego F. (CSIC - ICE)

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    Three first-timers in Galactic high-energy astrophysics

    As time goes by, continuing revolutions of the largest gamma-ray astronomy satellite ever flown, Fermi, allows for deeper and deeper studies of the sky. Joined by mature ground-based observational facilities at even higher gamma-ray frequencies, like MAGIC at TeV energies (TeV = 1e12 eV), as well as by a plethora of concurrent observations at lower energies (from radio to optical to X-rays), significant discoveries are driving theoretical research.

    This year has seen the announcement of the most energetic light ever observed from a pulsar, for the first time with energies of more than trillion electron volts, about a thousand times larger than previously observed, arriving at the detector concurrently with the pulsar period. These photons are thus coming from the close proximity of the rotating neutron star, but exactly how and where they are generated is unknown. No theory to date can cope with such measurements.

    Another first-timer this year has been the detection, at similarly such high energies, of a years-long recurrent variability. Every 4.2 years, the TeV emission from a gamma-ray binary has been seen to oscillate, in an effect theoretically predicted a few years before (Torres, D. F., et al. 2012, ApJ, 744, 106). Much is yet to learn from the recurrence of such oscillation and how can, perhaps, be driven by oscillations in the surrounding circumstellar disk of the companion.

    Finally, the spatial connection of the archetypical, accreting millisecond pulsar with a gamma-ray source has been also noted for the first time. If this connections proves real, for instance via a detection of gamma-ray pulsations, it would imply rotationally-powered activity in quiescence mode, showing evidence of a transition to a rotation-powered radio pulsar state in X-ray quiescence, whilst it is observed as an accreting pulsar when it has a disk.

     

  • Cells move en masse towards rigid tissues   (2016)

    Trepat, Xavier (IBEC)

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    Cells move en masse towards rigid tissues  

    We discovered that several types of cells are attracted to the most rigid areas of tissues. The discovery questions the traditional view that cell movement is guided primarily by variations in the chemical concentration of proteins and ions.

    In 2000, researchers at Boston University and the University of Massachusetts first proposed that the stiffness of a tissue could guide the movement of isolated cells. However, subsequent studies showed that this experimental mechanism was very inefficient. Here we found that when cells cooperate with each other, they are able to respond to variations in tissue stiffness much more efficiently than when they are isolated.

    It is an example of what is often called collective intelligence: a group can carry out a task that their isolated individuals are unable to perform. The key is not in any property of the individual, but in their interaction with their peers. In this case, the interaction is physical, cells transmit information between them by means of forces.

    To reach our conclusions, we developed new techniques to create biomaterials with variations in stiffness, and used these to observe which cell groups preferentially moved to the more rigid areas. The larger the group, the more efficient the movement; and individual cells were unable to find their way to the most rigid areas.

    We developed a theory explaining the phenomenon, which we named collective durotaxis. In the theory, each cell applies a force to its environment that allows it to measure the surrounding stiffness. But cells need to physically interact with each other to transmit this information collectively in order to move.

    Tumors are more rigid than their surroundings, so collective durotaxis might contribute to explain the mechanisms by which tumor cells move to initiate the metastatic process. Similarly, scars are also more rigid than their surrounding tissues. As such, collective durotaxis might also explain how cells move to heal wounds. 

  • The puzzle of spin relaxation in graphene about to be solved: a tool to achieve full control of the spin dynamics (2016)

    Valenzuela, Sergio O. (ICN2)
    Roche, Stephan (ICN2)

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    The puzzle of spin relaxation in graphene about to be solved: a tool to achieve full control of the spin dynamics

    Spin electronics, or spintronics, relies on the spin of the electrons, rather than their charge, to transport, manipulate and store information in an electronic device. Modern spintronic technologies, including magnetic sensors and magnetic memories, rely on the non-volatility (storage of information) provided by ferromagnetic materials. In order to unlock its full potential, spintronics still requires a suitable template to transport and manipulate the spins, which would enable the implementation of novel spin-logic architectures with very low power requirements. Graphene and engineered graphene are amongst the most promising candidates to fill this gap. Spins are expected to be conserved over long distances in pristine graphene, and could in principle be manipulated within graphene regions that are modified by the proximity of a ferromagnetic insulator or a material with large spin-orbit interaction.

    Such advances require full understanding and control of the behaviour of the spins in graphene. Nevertheless, after 10 years of intense research, even the basic process leading to the loss of spin information in pristine graphene remains largely debated. This is a fascinating puzzle rooted in the properties of this unique material. Indeed, graphene is now believed to support a number of spin relaxation mechanisms with no equivalent in any previously studied system, even though these mechanisms have yet to be established experimentally. We have developed and demonstrated a novel approach to solve this puzzle based on the determination of the spin relaxation anisotropy. Graphene is a two-dimensional system and the spin relaxation anisotropy quantifies the difference between the relaxation rates of spins oriented in- or out-of- the plane of graphene. We show that its magnitude provides direct evidence of the spin relaxation mechanisms at play. Future work will focus on modifying graphene to achieve full control of its spin dynamics properties.