Cada año, un comité de expertos debe acometer una ardua tarea: de entre todas las publicaciones de ICREA, debe escoger unas cuantas que destaquen del resto. Es todo un reto: a veces los debates se acaloran, y siempre son difíciles, pero acaba saliendo una lista de 24 publicaciones. No se concede ningún premio, y el único reconocimiento adicional es el honor de ser resaltado en la web de ICREA. Cada publicación tiene algo especial, ya sea una solución especialmente elegante, un éxito espectacular en los medios de comunicación o la simple fascinación por una idea del todo nueva. Independientemente de la razón, se trata de los mejores de los mejores y, como tales, nos complace compartirlos aquí.


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  • 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.

  • Single molecules show their colours (2016)

    van Hulst, Niek F. (ICFO)

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    Single molecules show their colours

    Molecules are composed of atoms and electrons; their arrangement determines the energy states and spectroscopy, i.e. the response of a molecule to incident colours of light. A specific molecule has a corresponding specific response, which underlies the essence of optical spectroscopy. Yet in the real world, in a liquid or solid host, molecules do change their conformation depending on the local “nano”-environment. As a result, even for chemically identical molecules, each molecule has a shifted absorption spectrum, and in a typical single molecule experiments an unknown fraction is easily missed. Capturing all molecules is challenging as they fluctuate, blink and bleach: one needs to keep track of all colours at the same time.

    To this end we have developed a novel approach based on interferometric white light excitation combined with confocal fluorescence detection, revealing the specific colour of each molecule individually and bringing single molecule excitation spectroscopy side-by-side to single molecule emission spectroscopy. The interferometric approach, exploiting broad-band femtosecond pulses and rapid delay line scanning, is resilient against blinking and bleaching as the entire excitation spectrum is probed at once. Unprecedented spectral heterogeneities of single molecules, with individual excitation spectra shifted in wavelength by more than 100 nm are revealed. Conventional narrow-band excitation techniques would be incapable to capture the whole extent of the spectral distribution and would meagerly miss out on molecules detected by the broad-band scheme.

    The new femtosecond single molecule excitation spectroscopy addresses the ultrafast dynamics in the electronic excited state, giving access to the interaction between individual molecules and their environment as well as biomolecules in more complex systems. Equally, the technique will prove useful to follow slow-occurring chemical reactions in time through changes in the molecular excitation spectrum both in solution and on the single-molecule level.


  • Coherence: distilled, diluted, weighed and measured (2016)

    Winter, Andreas (UAB)
    Lewenstein, Maciej Andrzej (ICFO)

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    Coherence: distilled, diluted, weighed and measured

    Every true quantum experiment relies on interference effects, which are the physical manifestation of the superposition principle of quantum mechanics. In our work [WY], for the first time, we are able to derive universal measures of how strong the coherence of superposition is in any given quantum state. We do this by viewing superposition as a resource, and consider how it can be manipulated under so-called 'incoherent' operations [BCP].

    Our main findings are simple formulas for how much pure coherence is contained in a given quantum state, by answering two fundamental questions: How efficiently can one transform the state into pure coherence (distillation)? And how efficient is the reverse process (formation)? It turns out that any quantum state that is not itself incoherent, however noisy it is, has some bit of pure coherence that can be extracted from it. At the same time, distillation and formation are in general not inverses to each other. Rather, there is irreversible loss of coherence in any cyclic process of first forming a noisy state from pure coherence and then distilling that coherence back to pure.

    Traditionally, the degree of coherence is linked to the visibility of interference fringes in characteristic standardized experiments: double or multi-slit setups, interferometers, etc. In contrast, our approach quantifies the strength of coherence not with respect to a certain standard task, but by the performance at the best experiment tailored to the specific resource state.

    In subsequent work [SCR+], we made a fruitful comparison between the resource of coherence, and another major manifestation of quantumness, entanglement, which intuitively is the natural form of quantum coherence as correlation. We did this by studying their interplay and mutual tradeoff in the basic quantum information task of "state merging", a fundamental primitive protocol unifying data compression and error correction. We found various bounds on the tradeoff, suggesting that entanglement can be much more powerful than mere coherence.

  • Ocean warming and acidification on calcareous phytoplankton alter its ability to sequester atmospheric CO2   (2016)

    Ziveri, Patrizia (UAB)

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    Ocean warming and acidification on calcareous phytoplankton alter its ability to sequester atmospheric CO2  

    More than a quarter of the anthropogenic CO2 emissions are absorbed by the ocean, changing the ocean chemistry and resulting in ‘ocean acidification’. Anthropogenic CO2 emission leads also to a rise in global average temperatures, including sea surface temperature at an unprecedented pace. The risks posed by warming and acidification are expected to become more acute in the next decades, as CO2 emissions into the atmosphere are accelerating.

    Coccolithophores are a very abundant marine calcifying phytoplankton group that plays a major role in biogeochemical cycles and in the regulation of climate. These tiny algae measuring a few thousands of a millimeter, form the base of the marine trophic web, and through calcification and photosynthesis contribute to the  regulation of the atmospheric and oceanic CO2 levels. The effects of acidification - and in particular warming - are rarely considered for the organism itself, and there is very little knowledge on how warming and acidification combined may affect their adaptation and evolution.

    Culture experiments were conducted on Mediterranean Sea and North Pacific Ocean strains of Emiliania huxleyi, the most abundant coccolitophore species. A main aim was to detect the effects of temperature, and secondarily of acidification, on the coccolithophore calcification and sinking rates.

    Using scanning electron microscopy, an increase in malformed and incomplete coccoliths in a warmer and more acidified ocean was shown. This will hamper the evolutionary success of these calcifiers and their role in regulating atmospheric carbon. In addition, using a novel approach to calculate sinking rate from cell-architecture,  the studies showed that a warmer and more acidic ocean will lead to an increase in their cell sinking rate. The faster sinking of this group of calcifying phytoplankton can have an impact on their survival.