Tà Physikà ... le cose naturali
Tà Physikà ... le cose naturali

Scienza

Chi saranno i Nobel 2017? (Mon, 25 Sep 2017)
Lunedì 2 ottobre, con l'annuncio del premio per la fisiologia o medicina, inizierà la settimana dei premi Nobel 2017. Come ogni anno sono arrivate puntuali le previsioni formulate dagli analisti dell'agenzia Thomson Reuters. Ecco l'elenco dei possibili vincitori.
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I grandi telescopi usano solo specchi di vetro? (Mon, 25 Sep 2017)
No, anzi ormai si usano altri materiali.
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Sei mutazioni genetiche che ti danno i superpoteri (Mon, 25 Sep 2017)
Veloci come Flash, resistenti agli urti e quasi immuni al richiamo del sonno: sono gli "effetti collaterali" di alcune varianti genetiche, che regalano qualità inaspettate.
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Un asteroide-cometa tra Marte e Giove (Sat, 23 Sep 2017)
Il telescopio spaziale Hubble ha scoperto due oggetti che ruotano uno attorno all’altro. Si trovano nella Fascia principale degli asteroidi, ma a tutti gli effetti sono delle comete. 
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I dinosauri erbivori non erano del tutto vegetariani (Sat, 23 Sep 2017)
Negli escrementi fossili di adrosauridi vissuti 75 milioni di anni fa, sono state trovate tracce di granchi e gamberi: forse, supplementi di proteine occasionali.
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A caccia di asteroidi con una flotta di nano-satelliti (Fri, 22 Sep 2017)
L'idea di un gruppo di ricercatori finnici: inviare 50 micro-sonde verso la fascia degli asteroidi per studiarne almeno 300, con costi estremamente contenuti.
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I raggi cosmici hanno un'origine extragalattica (Fri, 22 Sep 2017)
Dopo 12 anni di ricerche e supposizioni, finalmente la conferma che le particelle più energetiche dell'Universo provengono da una regione esterna alla Via Lattea.
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L'Equinozio d'autunno è oggi (Fri, 22 Sep 2017)
Inizia oggi la stagione dai colori più caldi dell'anno: ma che cosa significa esattamente, dal punto di vista astronomico? Davvero notte e dì avranno la stessa durata? E perché "l'inverno sta arrivando"? L'equinozio autunnale spiegato a domande e risposte.
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Un vaccino contro la carie dentaria (Fri, 22 Sep 2017)
È allo studio di scienziati cinesi e ha dato, nei topi, risultati incoraggianti. Non servirà a fare a meno dei dentisti, ma a incrementare la protezione nelle aree più difficilmente raggiungibili.
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I record e i non-record della stagione degli uragani del 2017 (Thu, 21 Sep 2017)
L’ondata di uragani che si è propagata nell’Oceano Atlantico ha segnato vari record, ma è la più drammatica di sempre?
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Le Scienze > Rss

La complessa fisica della stappatura dello champagne (mer, 20 set 2017 11:22:00 +0200)
La rimozione del tappo da una bottiglia di champagne è seguita dall'uscita di un pennacchio di anidride carbonica e goccioline di vapore acqueo, ma a seconda della temperatura di ...
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ICTP: La Medaglia Dirac 2017 ai pionieri dei computer quantistici (mer, 09 ago 2017 10:23:00 +0200)
Comunicato stampa - Proclamati ieri i vincitori del prestigioso premio conferito dall’ICTP di Trieste. Si tratta di tre scienziati protagonisti dello sviluppo della ...
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L'orbita relativistica della stella al centro della Via Lattea (mer, 09 ago 2017 01:42:00 +0200)
La prima misurazione dell'orbita di una stella che ruota vorticosamente attorno al buco nero supermassiccio al centro della nostra galassia sembra indicare una sottile deviazione dalla ...
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CNAO: Adroterapia più precisa contro il cancro (gio, 27 lug 2017 17:56:00 +0200)
Comunicato stampa - Il CNAO, Centro Nazionale di Adroterapia Oncologica, e il centro tedesco di ricerca GSI hanno avviato un progetto per rendere ancora più precisa ...
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La strana topologia che sta cambiando il volto della fisica (sab, 22 lug 2017 00:45:00 +0200)
Molti fisici hanno iniziato da alcuni anni ad applicare allo studio dei materiali concetti derivati dalla topologia, una branca della matematica che si occupa delle forme degli oggetti e ...
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La corsa alla supremazia dei computer quantistici (sab, 08 lug 2017 01:57:00 +0200)
Due tecnologie per questi calcolatori avveniristici potrebbero essere sul punto di superare anche i più potenti computer digitali entro un anno o poco di più, tuttavia le ...
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Sempre più vicini a ridefinire il chilogrammo (mar, 04 lug 2017 01:49:00 +0200)
I ricercatori del NIST hanno misurato con notevole precisione la costante di Planck, uno dei parametri fondamentali della fisica. Il nuovo valore, insieme ad altri, permetterà di ...
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Einstein VS Podolsky (lun, 03 lug 2017 07:43:00 +0200)
Il 15 maggio 1935 la rivista scientifica statunitense «Physical Review» pubblicò un articolo firmato da Albert Einstein, Boris Podolsky e Nathan Rosen ...
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Le due forme dell'acqua liquida (mar, 27 giu 2017 09:30:00 +0200)
A temperature molto vicine a quella di congelamento l'acqua oscilla fra due fasi che si differenziano tra loro per densità e viscosità. La scoperta conferma ...
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Una ricarica a distanza per dispositivi wireless (gio, 15 giu 2017 01:54:00 +0200)
Realizzato un nuovo e più efficiente sistema di ricarica senza fili per batterie che funziona con elevata efficienza su diverse distanze, fino a un metro. Ancora allo stadio di ...
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Repubblica.it > Scienze

Iraq, scoperta l'antica città perduta di Alessandro Magno (Mon, 25 Sep 2017)
A più di duemila anni dalla morte del grande condottiero, droni e scavi per riportare alla luce Qalatga Darband, avamposto greco fondato nel 331 a.C. in...
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Spazio, scoperta una "doppia cometa" fatta da due asteroidi, oggetto mai visto nel Sistema solare (Wed, 20 Sep 2017)
288P è stato Osservato nel 2016, è formato da due distinti corpi che ruotano l'uno attorno all'altro. Ma avvicinandosi al Sole sviluppano insieme una chioma e...
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Premio Balzan, riconoscimenti per scoperte su memoria collettiva e terapia anti cancro (Tue, 12 Sep 2017)
Nell'edizione 2017 non figurano italiani tra i vincitori. All'indiana Bina Agarwal meriti per lo studio di genere. A ognuno 660mila euro, la metà dei quali da...
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Tornata sulla Terra Peggy Whitson, l'astronauta dei record (Sun, 03 Sep 2017)
Con il collega della Nasa, Jack Fischer, e il comandante russo Fyodor Yurchikhin, l'americana è arrivata nella notte in Kazakistan a bordo della Soyuz. Quattro...
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La cacciatrice della malaria. Un’italiana nel cuore dell’Africa. “La chiave è nel Dna” (Fri, 01 Sep 2017)
La sfida della veterinaria Giovanna Carpi, ricercatrice a Baltimora: battere l’animale più pericoloso del mondo, la zanzara. Sul campo
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Nature Physics&Science

Light-field-driven currents in graphene
The ability to steer electrons using the strong electromagnetic field of light has opened up the possibility of controlling electron dynamics on the sub-femtosecond (less than 10−15 seconds) timescale. In dielectrics and semiconductors, various light-field-driven effects have been explored, including high-harmonic generation, sub-optical-cycle interband population transfer and the non-perturbative change of the transient polarizability. In contrast, much less is known about light-field-driven electron dynamics in narrow-bandgap systems or in conductors, in which screening due to free carriers or light absorption hinders the application of strong optical fields. Graphene is a promising platform with which to achieve light-field-driven control of electrons in a conducting material, because of its broadband and ultrafast optical response, weak screening and high damage threshold. Here we show that a current induced in monolayer graphene by two-cycle laser pulses is sensitive to the electric-field waveform, that is, to the exact shape of the optical carrier field of the pulse, which is controlled by the carrier-envelope phase, with a precision on the attosecond (10−18 seconds) timescale. Such a current, dependent on the carrier-envelope phase, shows a striking reversal of the direction of the current as a function of the driving field amplitude at about two volts per nanometre. This reversal indicates a transition of light–matter interaction from the weak-field (photon-driven) regime to the strong-field (light-field-driven) regime, where the intraband dynamics influence interband transitions. We show that in this strong-field regime the electron dynamics are governed by sub-optical-cycle Landau–Zener–Stückelberg interference, composed of coherent repeated Landau–Zener transitions on the femtosecond timescale. Furthermore, the influence of this sub-optical-cycle interference can be controlled with the laser polarization state. These coherent electron dynamics in graphene take place on a hitherto unexplored timescale, faster than electron–electron scattering (tens of femtoseconds) and electron–phonon scattering (hundreds of femtoseconds). We expect these results to have direct ramifications for band-structure tomography and light-field-driven petahertz electronics.
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Layer-by-layer assembly of two-dimensional materials into wafer-scale heterostructures
High-performance semiconductor films with vertical compositions that are designed to atomic-scale precision provide the foundation for modern integrated circuitry and novel materials discovery. One approach to realizing such films is sequential layer-by-layer assembly, whereby atomically thin two-dimensional building blocks are vertically stacked, and held together by van der Waals interactions. With this approach, graphene and transition-metal dichalcogenides—which represent one- and three-atom-thick two-dimensional building blocks, respectively—have been used to realize previously inaccessible heterostructures with interesting physical properties. However, no large-scale assembly method exists at present that maintains the intrinsic properties of these two-dimensional building blocks while producing pristine interlayer interfaces, thus limiting the layer-by-layer assembly method to small-scale proof-of-concept demonstrations. Here we report the generation of wafer-scale semiconductor films with a very high level of spatial uniformity and pristine interfaces. The vertical composition and properties of these films are designed at the atomic scale using layer-by-layer assembly of two-dimensional building blocks under vacuum. We fabricate several large-scale, high-quality heterostructure films and devices, including superlattice films with vertical compositions designed layer-by-layer, batch-fabricated tunnel device arrays with resistances that can be tuned over four orders of magnitude, band-engineered heterostructure tunnel diodes, and millimetre-scale ultrathin membranes and windows. The stacked films are detachable, suspendable and compatible with water or plastic surfaces, which will enable their integration with advanced optical and mechanical systems.
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Nanotechnology: A molecular assembler
The idea of nanometre-scale machines that can assemble molecules has long been thought of as the stuff of science fiction. Such a machine has now been built — and might herald a new model for organic synthesis. See Letter p.374
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Materials science: Long-lived electrodes for plastic batteries
Organic materials are potential substitutes for the costly transition-metal oxides used in battery electrodes, but their stability is often poor. A polymer design that uses intermolecular interactions solves this problem.
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Metallurgy: No more tears for metal 3D printing
3D printing could revolutionize manufacturing processes involving metals, but few industrially useful alloys are compatible with the technique. A method has been developed that might open up the 3D printing of all metals. See Letter p.365
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A binary main-belt comet
Asteroids are primitive Solar System bodies that evolve both collisionally and through disruptions arising from rapid rotation. These processes can lead to the formation of binary asteroids and to the release of dust, both directly and, in some cases, through uncovering frozen volatiles. In a subset of the asteroids called main-belt comets, the sublimation of excavated volatiles causes transient comet-like activity. Torques exerted by sublimation measurably influence the spin rates of active comets and might lead to the splitting of bilobate comet nuclei. The kilometre-sized main-belt asteroid 288P (300163) showed activity for several months around its perihelion 2011 (ref. 11), suspected to be sustained by the sublimation of water ice and supported by rapid rotation, while at least one component rotates slowly with a period of 16 hours (ref. 14). The object 288P is part of a young family of at least 11 asteroids that formed from a precursor about 10 kilometres in diameter during a shattering collision 7.5 million years ago. Here we report that 288P is a binary main-belt comet. It is different from the known asteroid binaries in its combination of wide separation, near-equal component size, high eccentricity and comet-like activity. The observations also provide strong support for sublimation as the driver of activity in 288P and show that sublimation torques may play an important part in binary orbit evolution.
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Tunable interacting composite fermion phases in a half-filled bilayer-graphene Landau level
Non-Abelian anyons are a type of quasiparticle with the potential to encode quantum information in topological qubits protected from decoherence. Experimental systems that are predicted to harbour non-Abelian anyons include p-wave superfluids, superconducting systems with strong spin–orbit coupling, and paired states of interacting composite fermions that emerge at even denominators in the fractional quantum Hall (FQH) regime. Although even-denominator FQH states have been observed in several two-dimensional systems, small energy gaps and limited tunability have stymied definitive experimental probes of their non-Abelian nature. Here we report the observation of robust even-denominator FQH phases at half-integer Landau-level filling in van der Waals heterostructures consisting of dual-gated, hexagonal-boron-nitride-encapsulated bilayer graphene. The measured energy gap is three times larger than observed previously. We compare these FQH phases with numerical and theoretical models while simultaneously controlling the carrier density, layer polarization and magnetic field, and find evidence for the paired Pfaffian phase that is predicted to host non-Abelian anyons. Electric-field-controlled level crossings between states with different Landau-level indices reveal a cascade of FQH phase transitions, including a continuous phase transition between the even-denominator FQH state and a compressible composite fermion liquid. Our results establish graphene as a pristine and tunable experimental platform for studying the interplay between topology and quantum criticality, and for detecting non-Abelian qubits.
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3D printing of high-strength aluminium alloys
Metal-based additive manufacturing, or three-dimensional (3D) printing, is a potentially disruptive technology across multiple industries, including the aerospace, biomedical and automotive industries. Building up metal components layer by layer increases design freedom and manufacturing flexibility, thereby enabling complex geometries, increased product customization and shorter time to market, while eliminating traditional economy-of-scale constraints. However, currently only a few alloys, the most relevant being AlSi10Mg, TiAl6V4, CoCr and Inconel 718, can be reliably printed; the vast majority of the more than 5,500 alloys in use today cannot be additively manufactured because the melting and solidification dynamics during the printing process lead to intolerable microstructures with large columnar grains and periodic cracks. Here we demonstrate that these issues can be resolved by introducing nanoparticles of nucleants that control solidification during additive manufacturing. We selected the nucleants on the basis of crystallographic information and assembled them onto 7075 and 6061 series aluminium alloy powders. After functionalization with the nucleants, we found that these high-strength aluminium alloys, which were previously incompatible with additive manufacturing, could be processed successfully using selective laser melting. Crack-free, equiaxed (that is, with grains roughly equal in length, width and height), fine-grained microstructures were achieved, resulting in material strengths comparable to that of wrought material. Our approach to metal-based additive manufacturing is applicable to a wide range of alloys and can be implemented using a range of additive machines. It thus provides a foundation for broad industrial applicability, including where electron-beam melting or directed-energy-deposition techniques are used instead of selective laser melting, and will enable additive manufacturing of other alloy systems, such as non-weldable nickel superalloys and intermetallics. Furthermore, this technology could be used in conventional processing such as in joining, casting and injection moulding, in which solidification cracking and hot tearing are also common issues.
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Stereodivergent synthesis with a programmable molecular machine
It has been convincingly argued that molecular machines that manipulate individual atoms, or highly reactive clusters of atoms, with Ångström precision are unlikely to be realized. However, biological molecular machines routinely position rather less reactive substrates in order to direct chemical reaction sequences, from sequence-specific synthesis by the ribosome to polyketide synthases, where tethered molecules are passed from active site to active site in multi-enzyme complexes. Artificial molecular machines have been developed for tasks that include sequence-specific oligomer synthesis and the switching of product chirality, a photo-responsive host molecule has been described that is able to mechanically twist a bound molecular guest, and molecular fragments have been selectively transported in either direction between sites on a molecular platform through a ratchet mechanism. Here we detail an artificial molecular machine that moves a substrate between different activating sites to achieve different product outcomes from chemical synthesis. This molecular robot can be programmed to stereoselectively produce, in a sequential one-pot operation, an excess of any one of four possible diastereoisomers from the addition of a thiol and an alkene to an α,β-unsaturated aldehyde in a tandem reaction process. The stereodivergent synthesis includes diastereoisomers that cannot be selectively synthesized through conventional iminium–enamine organocatalysis. We anticipate that future generations of programmable molecular machines may have significant roles in chemical synthesis and molecular manufacturing.
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Patchy particles made by colloidal fusion
Patches on the surfaces of colloidal particles provide directional information that enables the self-assembly of the particles into higher-order structures. Although computational tools can make quantitative predictions and can generate design rules that link the patch motif of a particle to its internal microstructure and to the emergent properties of the self-assembled materials, the experimental realization of model systems of particles with surface patches (or ‘patchy’ particles) remains a challenge. Synthetic patchy colloidal particles are often poor geometric approximations of the digital building blocks used in simulations and can only rarely be manufactured in sufficiently high yields to be routinely used as experimental model systems. Here we introduce a method, which we refer to as colloidal fusion, for fabricating functional patchy particles in a tunable and scalable manner. Using coordination dynamics and wetting forces, we engineer hybrid liquid–solid clusters that evolve into particles with a range of patchy surface morphologies on addition of a plasticizer. We are able to predict and control the evolutionary pathway by considering surface-energy minimization, leading to two main branches of product: first, spherical particles with liquid surface patches, capable of forming curable bonds with neighbouring particles to assemble robust supracolloidal structures; and second, particles with a faceted liquid compartment, which can be cured and purified to yield colloidal polyhedra. These findings outline a scalable strategy for the synthesis of patchy particles, first by designing their surface patterns by computer simulation, and then by recreating them in the laboratory with high fidelity.
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"Non leggete per contraddire e confutare, né per credere e accettar per concesso, non per trovare argomento di ciarle e di conversazione ma per pesare e valutare".

(F. Bacon)

"Ci sono due modi di vivere la vita. Uno é pensare che niente é un miracolo. L'altro é pensare che ogni cosa é un miracolo".

(Albert Einstein)

IN EVIDENZA ...

SMART, smartphone e tablet nella didattica ...

(24 gennaio 2017) ... attività di formazione organizzata da AIF Brescia, col patrocinio dell'Università Cattolica .... Vai alla notizia

PROBLEMI, ESPERIENZE ED ESPERIMENTI

(28 novembre 2016) ...  L'Università Cattolica, in collaborazione con la sezione AIF di Brescia.... Vai alla notizia