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.


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  • Flexoelectricity via coordinate transformations (2013)

    Stengel, Massimiliano (CSIC - ICMAB)

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    Flexoelectricity via coordinate transformations

    Flexoelectricity describes the electric polarization that is linearly induced by a strain gradient, and is being intensely investigated as a tantalizing new route to converting mechanical stimulation into electrical signals and vice versa.Contrary to its close cousin, piezoelectricity (the polarization response to a uniform strain), flexoelectricity is a universalproperty of all insulators regardless of crystal symmetry, and therefore appears highly attractive as a cost-effective andenvironment-friendly (piezoelectrics are typically based on lead, a toxic element) alternative to the former. Strain gradientscan be easily generated by bending a sample or by applying pressure by means of a local probe, and naturally arise when certain topological defects, such as dislocations or ferroelastic domain walls, are present in the bulk material. Particularly at the nanoscale, it is becoming increasingly clear that understanding the fundamentals of strain-gradient effects is crucially important, either for avoiding their sometimes deleterious impact (e.g. in ferroelectric memories, or in foldable electronic devices), or for harnessing the exciting new functionalities that they provide.While several breakthough experiments have been reported in the past few years, progress on the theoretical front has been comparatively slow, especially in the context of first-principles electronic-structure theory. The main difficulty with calculating the flexoelectric response of a material is the inherent breakdown of translational periodicity that a strain gradient entails, which at first sight questions the very applicability of traditional plane-wave pseudopotential methods.Here I show how these obstacles can be overcome by combining density-functional perturbation theory with generalized coordinate transformations of space. In particular, by writing the equations of electrostatics in a fully covariant form, I derive the full microscopic response (in terms of electronic charge density, polarization and atomic displacements) of a crystal or nanostructure to an arbitrary deformation field. This methodological advance sets the stage for attacking an essentially endless variety of curvature-related phenomena with full ab initio power; here I address, in full generality, the surface contributions to the flexoelectric response of a finite sample.  Inspiration for solving this important materials science problem has come from the apparently unrelated field of transformation

  • Swings between rotation and accretion power in a binary millisecond pulsar [astro-ph/1305.3884] (2013)

    Torres, Diego F. (CSIC - ICE)

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    Swings between rotation and accretion power in a binary millisecond pulsar [astro-ph/1305.3884]

    Pulsars are the highly magnetised, spinning remnants of massive stars and are primarily observed as pulsating sources of radio waves. The radio emission is powered by the rotating magnetic field and focused in two beams stemming from the magnetic poles. As the pulsar rotates, the effect is similar to that of a rotating lighthouse beacon, resulting in distant observers seeing regular pulses of radio waves.The emission mechanism of pulsars transforms kinetic rotational energy into radiation, and as this energy is radiated over time, the rotation is slowed down. Whilst pulsars spin rapidly at birth, they tend to rotate more slowly – with periods of up to a few seconds – as they age. For this reason, astronomers in the 1980s were puzzled by the discovery of millisecond pulsars – old but extremely quickly rotating pulsars with periods of a few thousandths of a second.  For the first time, astronomers have caught a pulsar in a crucial transitional phase that explains the origin of the mysterious millisecond pulsars. This ends a quest that has been going on the last 30 years.These pulsars spin much faster than expected for their old age, and astronomers believe their rotation receives a boost as they accrete matter in a binary system. This year, astronomer have found the first pulsar swinging back and forth between accretion-powered X-ray emission and rotation-driven radio emission, bringing conclusive evidence for their 'rejuvenation'. The discovery was made possible by the coordinated efforts of ESA's two missions that scan the high-energy sky: INTEGRAL and XMM-Newton, working together with Chandra X-ray Observatory, Green Bank Telescope, the Parkes radio telescope, and the Westerbork Synthesis Radio Telescope.(Adapted from the European Space Agency Press Release.)

  • A bit of Quantum in Photosynthesis? (2013)

    van Hulst, Niek F. (ICFO)

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    A bit of Quantum in Photosynthesis?

    Plants, bacteria and algae collect sunlight to store its energy and synthesize high energy molecular species to power life. This photosynthetic process involves light harvesting complexes, sophisticated molecular constructs which act as antennas to capture light. Surprisingly almost every visible photon is captured and transferred to makes its energy to work. What hidden mechanism does nature use to transfer energy so efficiently?
    Nature has arranged the photosynthetic systems rather cleverly: the reaction center is surrounded by many antenna complexes, which all capture photons, funnelling the energy from one site to the next, to finally reach the reaction centre. Such dense network of antennas explains the high capture efficiency. Yet what guarantees that the diffusing photon energy does actually reach its target? In recent experimental and theoretical studies it was found that the photon-hopping picture is a good model, however it is not the full story. To surprise of many researchers, coherences were observed in the energy transfer, which could only be explained by concepts of Quantum Mechanics!
    Quantum Mechanics, normally associated to physics at low temperatures, would not quite be expected in warm and wet soft living entities. Yet even at room temperature quantum phenomena do occur at the ultrafast femtosecond timescale and can even persist when protected against environmental disorder. Exactly such persistent coherence is observed in photosynthesis. The new field is coined “Quantum Biology” and could be a revolution in Science, if the bit of Quantum would actually have a biological role.
    To address such important question it is essential to track the femtosecond energy hops amidst the complex network, best at the level of single sites. This is exactly what we have done. We developed a unique experimental technique, pushing ultrafast spectroscopy to the single-molecule limit: with dedicated femtosecond light flashes we capture a high-speed series of ‘pictures’ of the states of individual antenna complexes after light absorption. We indeed found persistent coherence, a genuine quantum effect of superposition of states. A surprising discovery was that transport pathways within a single antenna complex can vary over time, while maintaining coherence at each path.
    Fascinating questions remain. Did quantum transport outcompete other mechanisms during evolution to achieve such extraordinary efficiencies in photosynthesis? Are there other biological processes

  • Experimental estimation of the dimension of classical and quantum systems (2012)

    Acín Dal Maschio, Antonio (ICFO)

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    Experimental estimation of the dimension of classical and quantum systems

    Dimensionality is one of the most basic and essential concepts in science, inherent to any theory aiming at explaining and predicting experimental observations. In building up a theoretical model, one makes some general and plausible assumptions about the nature and the behavior of the system under study. The dimension of this system, that is, the number of relevant and independent parameters needed to describe it, represents one of these initial assumptions. In general, the failure of a theoretical model in predicting experimental data does not necessarily imply that the assumption on the dimensionality is incorrect, since there might exist a different model assuming the same dimension that is able to reproduce the observed data. A natural question is whether this approach can be reversed and whether the dimension of an unknown system, classical or quantum, can be estimated experimentally. That is, is the standard initial assumption on the dimension unavoidable? If not, what can be said about the dimension of an unknown system only from the observed measurement data and without making any assumption about the detailed functioning of the devices used in the experiment? The concept of a dimension witness answers this question, as it allows bounding the dimension of an unknown system only from measurement statistics. In a recent work, we report the first experimental demonstration of a dimension witnesses. We use photon pairs entangled in polarization and orbital angular momentum to generate ensembles of classical and quantum states of dimensions up to four. We then use a dimension witness to certify their dimensionality as well as their quantum nature. Proving that the dimension of an unknown system is an experimentally accessible quantity is a fundamental result. Besides its fundamental interest, our work opens new avenues in quantum information science, where dimension represents a powerful resource, especially for device-independent estimation of quantum systems and quantum communications.

  • Redirecting the design of high temperature superconductors by using a "Gruyère cheese"-type nanostructure. (2012)

    Arbiol Cobos, Jordi (CSIC - ICMAB)

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    Redirecting the design of high temperature superconductors by using a "Gruyère cheese"-type nanostructure.

    A new and easy mechanism for the design of high temperature superconducting materials has been developed. The technique is based on the alteration of the superconducting material, creating special regions in its structure, where superconductivity breaks. These regions could resemble the holes in Swiss cheese at the nanoscale. Superconducting materials are capable of carrying electric currents up to 100 times higher than copper thanks to their quantum mechanical properties. For this, the material must exhibit nanometric dimensions where quantum coherence is broken and the superconducting magnetic vortices are stored. Until now, these regions were obtained by creating defects and overlapping in the structure of a non-conductive secondary phase that created the voids. The present work shows that by creating a strained structure of the crystal lattice at the nanoscale extraordinary electrical currents, governed by a new physical mechanism, are generated. The main advantage of this mechanism is that it allows designing a new generation of high-temperature superconductors able to provide unsuspected benefits for the most demanding applications.
    Up to date high temperature superconductors are the most efficient. This entails a great advantage from the practical point of view because the used cooling temperature has a ten-times lower cost than their low temperature counterparts. Thanks to the high current volume produced by superconducting materials, they are able to generate magnetic fields much higher than conventional metals. The large particle accelerators like CERN in Geneva (Switzerland) and the large fusion reactor ITER in Marseille (France) are nowadays based on low-temperature superconductors. With these new nanotechnology superconductors the barriers of magnetic fields accessible to humanity will be broken. In addition, the mechanism developed significantly reduces the operating costs of the magnets.
    Applications that can take advantages of this discovery cover engines and power generators for boats, wind power or different industries, and cables and current limiters to achieve a smarter and safer electrical power network. It is estimated that this nascent industry will have a global market estimated at over 3,000 million euros annually in ten years. Superconducting systems will be more efficient, lighter and safer, generating an environmental friendlier electrical system. Given this increase in efficiency, the development of a new superconductor technology will result in significant savings in energy and, th

  • Worldrecord broadband and coherent light source (2012)

    Biegert, Jens (ICFO)

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    Worldrecord broadband and coherent light source

    An energy and coherent supercontinuum light source was demonstrated by the group of Prof. Biegert from ICFO with theoretical support from Ecole Polytechnique in France and Heriot-Watt University in the UK. The results are published in Nature Communications and highlight a source spanning a continuous wavelength range between 450 nm to 4500 nm.

    Supercontinuum light sources are similar to a very intense rainbow of light, containing a continuum of wavelengths, finding widespread applications in optical spectroscopy, optical coherence tomography, optical microscopy, frequency metrology, fluorescence lifetime imaging, optical communications, gas sensing and many others. In fluorescence microcopy, for instance, alleviates a supercontinuum light source the microscopist from using many individual laser sources or to tune a laser to excite a specific fluorescing dye.
    A supercontinua is generated during the nonlinear propagation of intense laser light in optical fibers, gases or solid state materials. The standing issue with such processes had been the increasing loss of coherence and energy with increasing bandwidth which was now resolved.