ALBERT

Beschreibung: Nature Physics 11, 167 (2015). doi:10.1038/nphys3202 Authors: Seiji Armstrong, Meng Wang, Run Yan Teh, Qihuang Gong, Qiongyi He, Jiri Janousek, Hans-Albert Bachor, Margaret D. Reid & Ping Koy Lam Einstein, Podolsky and Rosen (EPR) pointed out in their famous paradox that two quantum-entangled particles can have perfectly correlated positions and momenta. Such correlations give evidence for the nonlocality of quantum mechanics and form the basis for quantum cryptography and teleportation. EPR steering is the nonlocality associated with the EPR paradox and has traditionally been investigated between only two parties. Using optical networks and efficient detection, we present experimental observations of multiparty EPR steering and of the genuine entanglement of three intense optical beams. We entangle the quadrature phase amplitudes of distinct fields, in analogy to the position–momentum entanglement of the original paradox. Our experiments complement tests of quantum mechanics that have entangled small systems or have demonstrated tripartite inseparability. Our methods establish principles for the development of multiparty quantum communication protocols with asymmetric observers, and can be extended to qubits, whether photonic, atomic, superconducting, or otherwise.

Beschreibung: Nature Physics 11, 173 (2015). doi:10.1038/nphys3178 Authors: C. M. Huntington, F. Fiuza, J. S. Ross, A. B. Zylstra, R. P. Drake, D. H. Froula, G. Gregori, N. L. Kugland, C. C. Kuranz, M. C. Levy, C. K. Li, J. Meinecke, T. Morita, R. Petrasso, C. Plechaty, B. A. Remington, D. D. Ryutov, Y. Sakawa, A. Spitkovsky, H. Takabe & H.-S. Park Collisionless shocks can be produced as a result of strong magnetic fields in a plasma flow, and therefore are common in many astrophysical systems. The Weibel instability is one candidate mechanism for the generation of sufficiently strong fields to create a collisionless shock. Despite their crucial role in astrophysical systems, observation of the magnetic fields produced by Weibel instabilities in experiments has been challenging. Using a proton probe to directly image electromagnetic fields, we present evidence of Weibel-generated magnetic fields that grow in opposing, initially unmagnetized plasma flows from laser-driven laboratory experiments. Three-dimensional particle-in-cell simulations reveal that the instability efficiently extracts energy from the plasma flows, and that the self-generated magnetic energy reaches a few percent of the total energy in the system. This result demonstrates an experimental platform suitable for the investigation of a wide range of astrophysical phenomena, including collisionless shock formation in supernova remnants, large-scale magnetic field amplification, and the radiation signature from gamma-ray bursts.

Beschreibung: Nature Physics 11, 141 (2015). doi:10.1038/nphys3203 Authors: Ajit Srivastava, Meinrad Sidler, Adrien V. Allain, Dominik S. Lembke, Andras Kis & A. Imamoğlu A monolayer of a transition metal dichalcogenide such as WSe2 is a two-dimensional direct-bandgap valley-semiconductor having an effective honeycomb lattice structure with broken inversion symmetry. The inequivalent valleys in the Brillouin zone could be selectively addressed using circularly polarized light fields, suggesting the possibility for magneto-optical measurement and manipulation of the valley pseudospin degree of freedom . Here we report such experiments that demonstrate the valley Zeeman effect—strongly anisotropic lifting of the degeneracy of the valley pseudospin degree of freedom using an external magnetic field. The valley-splitting measured using the exciton transition deviates appreciably from values calculated using a three-band tight-binding model for an independent electron–hole pair at ±K valleys. We show, on the other hand, that a theoretical model taking into account the strongly bound nature of the exciton yields an excellent agreement with the experimentally observed splitting. In contrast to the exciton, the trion transition exhibits an unexpectedly large valley Zeeman effect that cannot be understood within the same framework, hinting at a different contribution to the trion magnetic moment. Our results raise the possibility of controlling the valley degree of freedom using magnetic fields in monolayer transition metal dichalcogenides or observing topological states of photons strongly coupled to elementary optical excitations in a microcavity.

Beschreibung: Nature Physics 11, 183 (2015). doi:10.1038/nphys3194 Authors: Maryam Nikkhou, Miha Škarabot, Simon Čopar, Miha Ravnik, Slobodan Žumer & Igor Muševič Creating, imaging, and transforming the topological charge in a superconductor, a superfluid, a system of cold atoms, or a soft ferromagnet is a difficult—if not impossible—task because of the shortness of the length scales and lack of control. The length scale and softness of defects in liquid crystals allow the easy observation of charges, but it is difficult to control charge creation. Here we demonstrate full control over the creation, manipulation and analysis of topological charges that are pinned to a microfibre in a nematic liquid crystal. Oppositely charged pairs are created through the Kibble–Zurek mechanism by applying a laser-induced local temperature quench in the presence of symmetry-breaking boundaries. The pairs are long-lived, oppositely charged rings or points that either attract and annihilate, or form a long-lived, charge-neutral loop made of two segments with a fractional topological charge.

Beschreibung: Nature Physics 11, 188 (2015). doi:10.1038/nphys3227 Authors: Daniel Sherman, Uwe S. Pracht, Boris Gorshunov, Shachaf Poran, John Jesudasan, Madhavi Chand, Pratap Raychaudhuri, Mason Swanson, Nandini Trivedi, Assa Auerbach, Marc Scheffler, Aviad Frydman & Martin Dressel

Beschreibung: Nature Physics 11, 199 (2015). doi:10.1038/nphys3243 Author: Robert W. Style, Rostislav Boltyanskiy, Benjamin Allen, Katharine E. Jensen, Henry P. Foote, John S. Wettlaufer & Eric R. Dufresne

Beschreibung: Nature Physics 11, 157 (2015). doi:10.1038/nphys3214 Authors: T. M. Weber, M. Höning, T. Niederprüm, T. Manthey, O. Thomas, V. Guarrera, M. Fleischhauer, G. Barontini & H. Ott The control of strongly interacting many-body systems enables the creation of tailored quantum matter with complex properties. Atomic ensembles that are optically driven to a Rydberg state provide many examples for this: atom–atom entanglement, many-body Rabi oscillations, strong photon–photon interaction and spatial pair correlations. In its most basic form Rydberg quantum matter consists of an isolated ensemble of strongly interacting atoms spatially confined to the blockade volume—a superatom. Here we demonstrate the controlled creation and characterization of an isolated mesoscopic superatom by means of accurate density engineering and excitation to Rydberg p-states. Its variable size allows the investigation of the transition from effective two-level physics to many-body phenomena. By monitoring continuous laser-induced ionization we observe a strongly anti-bunched ion emission under blockade conditions and extremely bunched ion emission under off-resonant excitation. Our measurements provide insights into both excitation statistics and dynamics. We anticipate applications in quantum optics and quantum information as well as many-body physics experiments.

Beschreibung: Nature Physics 11, 200 (2015). doi:10.1038/nphys3258 Author: David G. Blake It's no game.

Beschreibung: Nature Physics 11, 153 (2015). doi:10.1038/nphys3185 Authors: Jayson Paulose, Bryan Gin-ge Chen & Vincenzo Vitelli Mechanical metamaterials are artificial structures with unusual properties, such as negative Poisson ratio, bistability or tunable vibrational properties, that originate in the geometry of their unit cell. Often at the heart of such unusual behaviour is a soft mode: a motion that does not significantly stretch or compress the links between constituent elements. When activated by motors or external fields, soft modes become the building blocks of robots and smart materials. Here, we demonstrate the existence of topological soft modes that can be positioned at desired locations in a metamaterial while being robust against a wide range of structural deformations or changes in material parameters. These protected modes, localized at dislocations in deformed kagome and square lattices, are the mechanical analogue of topological states bound to defects in electronic systems. We create physical realizations of the topological modes in prototypes of kagome lattices built out of rigid triangular plates. We show mathematically that they originate from the interplay between two Berry phases: the Burgers vector of the dislocation and the topological polarization of the lattice. Our work paves the way towards engineering topologically protected nanomechanical structures for molecular robotics or information storage and read-out.

Beschreibung: Nature Physics 11, 162 (2015). doi:10.1038/nphys3171 Authors: M. Aidelsburger, M. Lohse, C. Schweizer, M. Atala, J. T. Barreiro, S. Nascimbène, N. R. Cooper, I. Bloch & N. Goldman Sixty years ago, Karplus and Luttinger pointed out that quantum particles moving on a lattice could acquire an anomalous transverse velocity in response to a force, providing an explanation for the unusual Hall effect in ferromagnetic metals. A striking manifestation of this transverse transport was then revealed in the quantum Hall effect where the plateaux depicted by the Hall conductivity were attributed to a topological invariant characterizing the Bloch bands: the Chern number. Until now, topological transport associated with non-zero Chern numbers has only been observed in electronic systems. Here we use the transverse deflection of an atomic cloud in response to an optical gradient to measure the Chern number of artificially generated Hofstadter bands. These topological bands are very flat and thus constitute good candidates for the realization of fractional Chern insulators. Combining these deflection measurements with the determination of the band populations, we obtain an experimental value for the Chern number of the lowest band νexp = 0.99(5). This first Chern-number measurement in a non-electronic system is facilitated by an all-optical artificial gauge field scheme, generating uniform flux in optical superlattices.

Beschreibung: Nature Physics 11, 148 (2015). doi:10.1038/nphys3201 Authors: G. Aivazian, Zhirui Gong, Aaron M. Jones, Rui-Lin Chu, J. Yan, D. G. Mandrus, Chuanwei Zhang, David Cobden, Wang Yao & X. Xu Local energy extrema of the bands in momentum space, or valleys, can endow electrons in solids with pseudospin in addition to real spin. In transition metal dichalcogenides this valley pseudospin, like real spin, is associated with a magnetic moment that underlies the valley-dependent circular dichroism that allows optical generation of valley polarization, intervalley quantum coherence and the valley Hall effect. However, magnetic manipulation of valley pseudospin via this magnetic moment, analogous to what is possible with real spin, has not been shown before. Here we report observation of the valley Zeeman splitting and magnetic tuning of polarization and coherence of the excitonic valley pseudospin, by performing polarization-resolved magneto-photoluminescence on monolayer WSe2. Our measurements reveal both the atomic orbital and lattice contributions to the valley orbital magnetic moment; demonstrate the deviation of the band edges in the valleys from an exact massive Dirac fermion model; and reveal a striking difference between the magnetic responses of neutral and charged valley excitons that is explained by renormalization of the excitonic spectrum due to strong exchange interactions.

Beschreibung: Nature Physics 11, 177 (2015). doi:10.1038/nphys3187 Authors: M. P. Allan, Kyungmin Lee, A. W. Rost, M. H. Fischer, F. Massee, K. Kihou, C-H. Lee, A. Iyo, H. Eisaki, T-M. Chuang, J. C. Davis & Eun-Ah Kim Cooper pairing in the iron-based high-Tc superconductors is often conjectured to involve bosonic fluctuations. Among the candidates are antiferromagnetic spin fluctuations and d-orbital fluctuations amplified by phonons. Any such electron–boson interaction should alter the electron’s ‘self-energy’, and then become detectable through consequent modifications in the energy dependence of the electron’s momentum and lifetime. Here we introduce a novel theoretical/experimental approach aimed at uniquely identifying the relevant fluctuations of iron-based superconductors by measuring effects of their self-energy. We use innovative quasiparticle interference (QPI) imaging techniques in LiFeAs to reveal strongly momentum-space anisotropic self-energy signatures that are focused along the Fe–Fe (interband scattering) direction, where the spin fluctuations of LiFeAs are concentrated. These effects coincide in energy with perturbations to the density of states N(ω) usually associated with the Cooper pairing interaction. We show that all the measured phenomena comprise the predicted QPI ‘fingerprint’ of a self-energy due to antiferromagnetic spin fluctuations, thereby distinguishing them as the predominant electron–boson interaction.

Beschreibung: Nature Physics 11, 193 (2015). doi:10.1038/nphys3197 Authors: Shuoming An, Jing-Ning Zhang, Mark Um, Dingshun Lv, Yao Lu, Junhua Zhang, Zhang-Qi Yin, H. T. Quan & Kihwan Kim

Beschreibung: Nature Physics 11, 517 (2015). doi:10.1038/nphys3405 The general theory of relativity, tested time and time again, is a cornerstone of modern physics — but marrying it with quantum mechanics remains a major challenge.

Beschreibung: Nature Physics 11, 518 (2015). doi:10.1038/nphys3317 Author: Milena Wazeck The history of the fierce opposition met by Einstein's theory of relativity in the 1920s teaches us that public controversies about science are not necessarily settled by sound scientific reasoning.

Beschreibung: Nature Physics 11, 520 (2015). doi:10.1038/nphys3388 Author: Mark Buchanan

Beschreibung: Nature Physics 11, 522 (2015). doi:10.1038/nphys3391 Author: Iulia Georgescu & Federico Levi

Beschreibung: Nature Physics 11, 523 (2015). doi:10.1038/nphys3397 Author: May Chiao

Beschreibung: Nature Physics 11, 523 (2015). doi:10.1038/nphys3398 Author: Bart Verberck

Beschreibung: Nature Physics 11, 523 (2015). doi:10.1038/nphys3399 Author: Luke Fleet

Beschreibung: Nature Physics 11, 523 (2015). doi:10.1038/nphys3401 Author: Abigail Klopper

Beschreibung: Nature Physics 11, 524 (2015). doi:10.1038/nphys3381 Author: Frank Verstraete Quantum many-body systems are often so complex as to be intractable. An algorithm that finds the ground state of any one-dimensional quantum system has now been devised, proving that the many-body problem is tractable for quantum spin chains.

Beschreibung: Nature Physics 11, 525 (2015). doi:10.1038/nphys3379 Author: Laurent Sanchez-Palencia The Anderson transition point between localization and diffusion — the mobility edge — has now been directly measured in an ultracold-atom experiment.

Beschreibung: Nature Physics 11, 554 (2015). doi:10.1038/nphys3339 Authors: G. Semeghini, M. Landini, P. Castilho, S. Roy, G. Spagnolli, A. Trenkwalder, M. Fattori, M. Inguscio & G. Modugno

Beschreibung: Nature Physics 11, 551 (2015). doi:10.1038/nphys3348 Authors: Sergei Fabrika, Yoshihiro Ueda, Alexander Vinokurov, Olga Sholukhova & Megumi Shidatsu The black hole mass and accretion rate in ultraluminous X-ray sources (ULXs) in external galaxies, whose X-ray luminosities exceed those of the brightest black holes in our Galaxy by hundreds and thousands of times, is an unsolved problem. Here we report that all ULXs ever spectroscopically observed have almost the same optical spectra, apparently of WNL type (late nitrogen Wolf–Rayet stars) or LBV (luminous blue variables) in their hot state, which are very scarce stellar objects. We show that the spectra do not originate from WNL/LBV-type donors but from very hot winds from the accretion disks with nearly normal hydrogen content, which have similar physical conditions to the stellar winds from these stars. The optical spectra are similar to that of SS 433, the only known supercritical accretor in our Galaxy, although the ULX spectra indicate a higher wind temperature. Our results suggest that ULXs with X-ray luminosities of ∼1040 erg s−1 must constitute a homogeneous class of objects, which most likely have supercritical accretion disks.

Beschreibung: Nature Physics 11, 543 (2015). doi:10.1038/nphys3371 Authors: J-X. Yin, Zheng Wu, J-H. Wang, Z-Y. Ye, Jing Gong, X-Y. Hou, Lei Shan, Ang Li, X-J. Liang, X-X. Wu, Jian Li, C-S. Ting, Z-Q. Wang, J-P. Hu, P-H. Hor, H. Ding & S. H. Pan In superconductors, electrons are paired and condensed into the ground state. An impurity can break the electron pairs into quasiparticles with energy states inside the superconducting gap. The characteristics of such in-gap states reflect accordingly the properties of the superconducting ground state. A zero-energy in-gap state is particularly noteworthy, because it can be the consequence of non-trivial pairing symmetry or topology. Here we use scanning tunnelling microscopy/spectroscopy to demonstrate that an isotropic zero-energy bound state with a decay length of ∼10 Å emerges at each interstitial iron impurity in superconducting Fe(Te,Se). More noticeably, this zero-energy bound state is robust against a magnetic field up to 8 T, as well as perturbations by neighbouring impurities. Such a spectroscopic feature has no natural explanation in terms of impurity states in superconductors with s-wave symmetry, but bears all the characteristics of the Majorana bound state proposed for topological superconductors, indicating that the superconducting state and the scattering mechanism of the interstitial iron impurities in Fe(Te,Se) are highly unconventional.

Beschreibung: Nature Physics 11, 576 (2015). doi:10.1038/nphys3355 Authors: Gyung-Min Choi, Chul-Hyun Moon, Byoung-Chul Min, Kyung-Jin Lee & David G. Cahill

Beschreibung: Nature Physics 11, 597 (2015). doi:10.1038/nphys3374 Author: Filippo Radicchi

Beschreibung: Nature Physics 11, 570 (2015). doi:10.1038/nphys3356 Authors: Can Onur Avci, Kevin Garello, Abhijit Ghosh, Mihai Gabureac, Santos F. Alvarado & Pietro Gambardella

Beschreibung: Nature Physics 11, 582 (2015). doi:10.1038/nphys3357 Authors: Atsuhiko Miyata, Anatolie Mitioglu, Paulina Plochocka, Oliver Portugall, Jacob Tse-Wei Wang, Samuel D. Stranks, Henry J. Snaith & Robin J. Nicholas

Beschreibung: Nature Physics 11, 588 (2015). doi:10.1038/nphys3341 Authors: Georgios Katsikis, James S. Cybulski & Manu Prakash

Beschreibung: Nature Physics 11, 604 (2015). doi:10.1038/nphys3407 Author: Martin Hayes A voyage into the unknown.

Beschreibung: Nature Physics 11, 789 (2015). doi:10.1038/nphys3516 Looking back at a decade of Nature Physics.

Beschreibung: Nature Physics 11, 791 (2015). doi:10.1038/nphys3494 Authors: Roberta Sinatra, Pierre Deville, Michael Szell, Dashun Wang & Albert-László Barabási An analysis of Web of Science data spanning more than 100 years reveals the rapid growth and increasing multidisciplinarity of physics — as well its internal map of subdisciplines.

Beschreibung: Nature Physics 11, 798 (2015). doi:10.1038/nphys3504 Author: Mark Buchanan

Beschreibung: Nature Physics 11, 799 (2015). doi:10.1038/nphys3500 Author: Jorge Cham Jorge Cham reflects on the most important physics discoveries of the past decade.

Beschreibung: Nature Physics 11, 800 (2015). doi:10.1038/nphys3510 Author: Iulia Georgescu

Beschreibung: Nature Physics 11, 800 (2015). doi:10.1038/nphys3511 Author: Andrea Taroni

Beschreibung: Nature Physics 11, 800 (2015). doi:10.1038/nphys3512 Author: May Chiao

Beschreibung: Nature Physics 11, 800 (2015). doi:10.1038/nphys3513 Author: Luke Fleet

Beschreibung: Nature Physics 11, 800 (2015). doi:10.1038/nphys3514 Author: Bart Verberck

Beschreibung: Nature Physics 11, 801 (2015). doi:10.1038/nphys3447 Author: Tomoki Ozawa For ultracold atoms experiencing a synthetic magnetic field in an optical lattice, it is possible to observe the translational symmetry-breaking pattern determined by the chosen gauge.

Beschreibung: Nature Physics 11, 806 (2015). doi:10.1038/nphys3480 Author: Daniele Faccio Nonlocal, nonlinear interactions of optical beams can be described by the Newton–Schrödinger equation for quantum gravity, offering an analogue for studying gravitational phenomena.

Beschreibung: Nature Physics 11, 802 (2015). doi:10.1038/nphys3455 Author: Nicolas Vandewalle Crushing a brittle porous medium such as a box of cereal causes the grains to break up and rearrange themselves. A lattice spring model based on simple physical assumptions gives rise to behaviours that are complex enough to reproduce diverse compaction patterns.

Beschreibung: Nature Physics 11, 808 (2015). doi:10.1038/nphys3501 Authors: E. Gull & A. J. Millis When Nature Physics celebrated 20 years of high-temperature superconductors, numerical approaches were on the periphery. Since then, new ideas implemented in new algorithms are leading to new insights.

Beschreibung: Nature Physics 11, 805 (2015). doi:10.1038/nphys3499 Author: Jonathan Oppenheim

Beschreibung: Nature Physics 11, 825 (2015). doi:10.1038/nphys3418 Authors: Hans T. Nembach, Justin M. Shaw, Mathias Weiler, Emilie Jué & Thomas J. Silva Proposals for novel spin-orbitronic logic and memory devices are often predicated on assumptions as to how materials with large spin–orbit coupling interact with ferromagnets when in contact. Such interactions give rise to a host of novel phenomena, such as spin–orbit torques, chiral spin structures and chiral spin torques. These chiral properties are related to the antisymmetric exchange, also referred to as the interfacial Dzyaloshinskii–Moriya interaction (DMI; refs , ). For numerous phenomena, the relative strengths of the symmetric Heisenberg exchange and the DMI are of great importance. Here, we use optical spin-wave spectroscopy (Brillouin light scattering) to directly determine the volume-averaged DMI vector D for a series of Ni80Fe20/Pt thin films, and then compare the nearest-neighbour DMI coupling energy with an independently measured value of the Heisenberg exchange for each sample. We show that the dependence on Ni80Fe20 thickness of both the microscopic symmetric and antisymmetric exchange are nearly identical, consistent with the notion that the fundamentals of the DMI and Heisenberg exchange essentially share the same underlying physics, albeit with different symmetries, as was originally proposed by Moriya for superexchange in magnetic oxides, and by Fert and Levy for RKKY coupling in metallic spin glasses. Indeed, our result demonstrates the generality of the original DMI theory, insofar as the proportionality of the symmetric and antisymmetric exchange is robust with regard to the details of spin coupling for the material system in question. Although of significant fundamental importance, this result also leads us to a deeper understanding of DMI and how it could be optimized for spin-orbitronic applications.

Beschreibung: Nature Physics 11, 803 (2015). doi:10.1038/nphys3448 Authors: Shahid M. Khan & Justin E. Molloy Real-time tracking of self-propelled biomolecules provides insight into the physical rules governing self-organization in complex living systems — including evidence to suggest that their alignment requires multiple simultaneous interactions.

Beschreibung: Nature Physics 11, 830 (2015). doi:10.1038/nphys3419 Authors: Luyi Yang, Nikolai A. Sinitsyn, Weibing Chen, Jiangtan Yuan, Jing Zhang, Jun Lou & Scott A. Crooker The recently discovered monolayer transition metal dichalcogenides (TMDCs) provide a fertile playground to explore new coupled spin–valley physics. Although robust spin and valley degrees of freedom are inferred from polarized photoluminescence (PL) experiments, PL timescales are necessarily constrained by short-lived (3–100 ps) electron–hole recombination. Direct probes of spin/valley polarization dynamics of resident carriers in electron (or hole)-doped TMDCs, which may persist long after recombination ceases, are at an early stage. Here we directly measure the coupled spin–valley dynamics in electron-doped MoS2 and WS2 monolayers using optical Kerr spectroscopy, and reveal very long electron spin lifetimes, exceeding 3 ns at 5 K (two to three orders of magnitude longer than typical exciton recombination times). In contrast with conventional III–V or II–VI semiconductors, spin relaxation accelerates rapidly in small transverse magnetic fields. Supported by a model of coupled spin–valley dynamics, these results indicate a novel mechanism of itinerant electron spin dephasing in the rapidly fluctuating internal spin–orbit field in TMDCs, driven by fast inter-valley scattering. Additionally, a long-lived spin coherence is observed at lower energies, commensurate with localized states. These studies provide insight into the physics underpinning spin and valley dynamics of resident electrons in atomically thin TMDCs.

Beschreibung: Nature Physics 11, 815 (2015). doi:10.1038/nphys3436 Authors: O. Pedatzur, G. Orenstein, V. Serbinenko, H. Soifer, B. D. Bruner, A. J. Uzan, D. S. Brambila, A. G. Harvey, L. Torlina, F. Morales, O. Smirnova & N. Dudovich Attosecond physics offers new insights into ultrafast quantum phenomena involving electron dynamics on the fastest measurable timescales. The rapid progress in this field enables us to re-visit one of the most fundamental strong-field phenomena: field-induced tunnel ionization. In this work, we employ high-harmonic generation to probe the electron wavefunction during field-induced tunnelling through a potential barrier. By using a combination of strong and weak driving laser fields, we modulate the atomic potential barrier on optical subcycle timescales. This induces a temporal interferometer between attosecond bursts originating from consecutive laser half-cycles. Our study provides direct insight into the basic properties of field-induced tunnelling, following the evolution of the electronic wavefunction within a temporal window of approximately 200 attoseconds.

Beschreibung: Nature Physics 11, 807 (2015). doi:10.1038/nphys3445 Author: Eli Barkai We're well versed on the first-passage time for a random process, but the time required to cover more than one site in a system is a different problem altogether. It turns out that the two measures have more in common than we thought.

Beschreibung: Nature Physics 11, 844 (2015). doi:10.1038/nphys3413 Authors: Marie Chupeau, Olivier Bénichou & Raphaël Voituriez How long must one undertake a random search to visit all sites of a given domain? This time, known as the cover time, is a key observable to quantify the efficiency of exhaustive searches, which require a complete exploration of an area and not only the discovery of a single target. Examples range from immune-system cells chasing pathogens to animals harvesting resources, from robotic exploration for cleaning or demining to the task of improving search algorithms. Despite its broad relevance, the cover time has remained elusive and so far explicit results have been scarce and mostly limited to regular random walks. Here we determine the full distribution of the cover time for a broad range of random search processes, including Lévy strategies, intermittent strategies, persistent random walks and random walks on complex networks, and reveal its universal features. We show that for all these examples the mean cover time can be minimized, and that the corresponding optimal strategies also minimize the mean search time for a single target, unambiguously pointing towards their robustness.

Beschreibung: Nature Physics 11, 839 (2015). doi:10.1038/nphys3423 Authors: Ryo Suzuki, Christoph A. Weber, Erwin Frey & Andreas R. Bausch From the self-organization of the cytoskeleton to the synchronous motion of bird flocks, living matter has the extraordinary ability to behave in a concerted manner. The Boltzmann equation for self-propelled particles is frequently used in silico to link a system’s meso- or macroscopic behaviour to the microscopic dynamics of its constituents. But so far such studies have relied on an assumption of simplified binary collisions owing to a lack of experimental data suggesting otherwise. We report here experimentally determined binary-collision statistics by studying a recently introduced molecular system, the high-density actomyosin motility assay. We demonstrate that the alignment induced by binary collisions is too weak to account for the observed ordering transition. The transition density for polar pattern formation decreases quadratically with filament length, indicating that multi-filament collisions drive the observed ordering phenomenon and that a gas-like picture cannot explain the transition of the system to polar order. Our findings demonstrate that the unique properties of biological active-matter systems require a description that goes well beyond that developed in the framework of kinetic theories.

Beschreibung: Nature Physics 11, 811 (2015). doi:10.1038/nphys3432 Authors: The measurement of the mass differences for systems bound by the strong force has reached a very high precision with protons and anti-protons. The extension of such measurement from (anti-)baryons to (anti-)nuclei allows one to probe any difference in the interactions between nucleons and anti-nucleons encoded in the (anti-)nuclei masses. This force is a remnant of the underlying strong interaction among quarks and gluons and can be described by effective theories, but cannot yet be directly derived from quantum chromodynamics. Here we report a measurement of the difference between the ratios of the mass and charge of deuterons (d) and anti-deuterons (nphys3432-m1gif121712), and 3He and nphys3432-m2gif1802212 nuclei carried out with the ALICE (A Large Ion Collider Experiment) detector in Pb–Pb collisions at a centre-of-mass energy per nucleon pair of 2.76 TeV. Our direct measurement of the mass-over-charge differences confirms CPT invariance to an unprecedented precision in the sector of light nuclei. This fundamental symmetry of nature, which exchanges particles with anti-particles, implies that all physics laws are the same under the simultaneous reversal of charge(s) (charge conjugation C), reflection of spatial coordinates (parity transformation P) and time inversion (T).

Beschreibung: Nature Physics 11, 859 (2015). doi:10.1038/nphys3421 Authors: Colin J. Kennedy, William Cody Burton, Woo Chang Chung & Wolfgang Ketterle

Beschreibung: Nature Physics 11, 820 (2015). doi:10.1038/nphys3411 Authors: A. Barfuss, J. Teissier, E. Neu, A. Nunnenkamp & P. Maletinsky Quantum devices for sensing and computing applications require coherent quantum systems, which can be manipulated in fast and robust ways. Such quantum control is typically achieved using external electromagnetic fields, which drive the system’s orbital, charge or spin degrees of freedom. However, most existing approaches require complex and unwieldy gate structures, and with few exceptions are limited to the regime of weak coherent driving. Here, we present a novel approach to coherently drive a single electronic spin using internal strain fields in an integrated quantum device. Specifically, we employ time-varying strain in a diamond cantilever to induce long-lasting, coherent oscillations of an embedded nitrogen–vacancy (NV) centre spin. We perform direct spectroscopy of the phonon-dressed states emerging from this drive and observe hallmarks of the sought-after strong-driving regime, where the spin rotation frequency exceeds the spin splitting. Furthermore, we employ our continuous strain driving to significantly enhance the NV’s spin coherence time. Our room-temperature experiments thereby constitute an important step towards strain-driven, integrated quantum devices and open new perspectives to investigate unexplored regimes of strongly driven multilevel systems and exotic spin dynamics in hybrid spin-oscillator devices.

Beschreibung: Nature Physics 11, 835 (2015). doi:10.1038/nphys3424 Authors: François Guillard, Pouya Golshan, Luming Shen, Julio R. Valdes & Itai Einav Brittle porous media exhibit a variety of irreversible patterns during densification, including stationary and moving compaction bands in rocks, foams, cereal packs and snow. We have recently found moving compaction bands in cereal packs; similar bands have been detected in snow. However, the question of generality remains: under what conditions can brittle porous media disclose other densification patterns? Here, using a new heuristic lattice spring model undergoing repeated crushing events, we first predict the possible emergence of new types of dynamic compaction; we then discover and confirm these new patterns experimentally in compressed cereal packs. In total, we distinguish three observed compaction patterns: short-lived erratic compaction bands, multiple oscillatory propagating compaction bands reminiscent of critical phenomena near phase transitions, and diffused irreversible densification. The manifestation of these three different patterns is mapped in a phase diagram using two dimensionless groups that represent fabric collapse and external dissipation.

Beschreibung: Nature Physics 11, 848 (2015). doi:10.1038/nphys3402 Authors: Marco Tulio Angulo, Yang-Yu Liu & Jean-Jacques Slotine The microscopic principles organizing dynamic units in complex networks—from proteins to power generators—can be understood in terms of network ‘motifs’: small interconnection patterns that appear much more frequently in real networks than expected in random networks. When considered as small subgraphs isolated from a large network, these motifs are more robust to parameter variations, easier to synchronize than other possible subgraphs, and can provide specific functionalities. But one can isolate these subgraphs only by assuming, for example, a significant separation of timescales, and the origin of network motifs and their functionalities when embedded in larger networks remain unclear. Here we show that most motifs emerge from interconnection patterns that best exploit the intrinsic stability characteristics at different scales of interconnection, from simple nodes to whole modules. This functionality suggests an efficient mechanism to stably build complex systems by recursively interconnecting nodes and modules as motifs. We present direct evidence of this mechanism in several biological networks.

Beschreibung: Nature Physics 11, 853 (2015). doi:10.1038/nphys3414 Authors: Ognyan Oreshkov & Nicolas J. Cerf

Beschreibung: Nature Physics 11, 879 (2015). doi:10.1038/nphys3489 Author: M. J. Bhaseen, Benjamin Doyon, Andrew Lucas & Koenraad Schalm

Beschreibung: Nature Physics 11, 865 (2015). doi:10.1038/nphys3416 Authors: Yejun Feng, Jasper van Wezel, Jiyang Wang, Felix Flicker, D. M. Silevitch, P. B. Littlewood & T. F. Rosenbaum

Beschreibung: Nature Physics 11, 878 (2015). doi:10.1038/nphys3488 Author: Hannah Clevenson, Matthew E. Trusheim, Carson Teale, Tim Schröder, Danielle Braje & Dirk Englund

Beschreibung: Nature Physics 11, 880 (2015). doi:10.1038/nphys3521 Author: Lyssa L. Martin A song of freedom.

Beschreibung: Nature Physics 11, 879 (2015). doi:10.1038/nphys3493 Author: L. X. Yang, Z. K. Liu, Y. Sun, H. Peng, H. F. Yang, T. Zhang, B. Zhou, Y. Zhang, Y. F. Guo, M. Rahn, D. Prabhakaran, Z. Hussain, S.-K. Mo, C. Felser, B. Yan & Y. L. Chen

Beschreibung: Nature Physics 11, 872 (2015). doi:10.1038/nphys3451 Authors: Rivka Bekenstein, Ran Schley, Maor Mutzafi, Carmel Rotschild & Mordechai Segev

Beschreibung: Nature Physics 11, 891 (2015). doi:10.1038/nphys3505 Author: Iulia Georgescu

Beschreibung: Nature Physics 11, 936 (2015). doi:10.1038/nphys3457 Authors: Weiran Cai, Li Chen, Fakhteh Ghanbarnejad & Peter Grassberger The spreading of contagions can exhibit a percolation transition, which separates transitory prevalence from outbreaks that reach a finite fraction of the population. Such transitions are commonly believed to be continuous, but empirical studies have shown more violent spreading modes when the participating agents are not limited to one type. Striking examples include the co-epidemic of the Spanish flu and pneumonia that occurred in 1918 (refs , ), and, more recently, the concurrent prevalence of HIV/AIDS and a host of diseases. It remains unclear to what extent an outbreak in the presence of interacting pathogens differs from that due to an ordinary single-agent process. Here we study a mechanistic model for understanding contagion processes involving inter-agent cooperation. Our stochastic simulations reveal the possible emergence of a massive avalanche-like outbreak right at the threshold, which is manifested as a discontinuous phase transition. Such an abrupt change arises only if the underlying network topology supports a bottleneck for cascaded mutual infections. Surprisingly, all these discontinuous transitions are accompanied by non-trivial critical behaviours, presenting a rare case of hybrid transition. The findings may imply the origin of catastrophic occurrences in many realistic systems, from co-epidemics to financial contagions.

Beschreibung: Nature Physics 11, 978 (2015). doi:10.1038/nphys3524 Author: Daniel Hauser, Seunghyun Lee, Fabio Carelli, Steen Spieler, Olga Lakhmanskaya, Eric S. Endres, Sunil S. Kumar, Franco Gianturco & Roland Wester

Beschreibung: Nature Physics 11, 981 (2015). doi:10.1038/nphys3598 South Korea's march from fast follower to first mover in science and technology.

Beschreibung: Nature Physics 11, 984 (2015). doi:10.1038/nphys3586 Author: Mark Buchanan

Beschreibung: Nature Physics 11, 987 (2015). doi:10.1038/nphys3599 Author: Iulia Georgescu

Beschreibung: Nature Physics 11, 985 (2015). doi:10.1038/nphys3588 Author: Abigail Klopper

Beschreibung: Nature Physics 11, 982 (2015). doi:10.1038/nphys3589 Author: Wolfgang Ketterle Bose–Einstein condensation in atomic gases was first observed in 1995. As we look back at the past 20 years of this thriving field, it's clear that there is much to celebrate.

Beschreibung: Nature Physics 11, 989 (2015). doi:10.1038/nphys3587 Authors: François Amet & Gleb Finkelstein Electric fields can controllably break the inversion symmetry of bilayer graphene, which can be harnessed to generate pure valley currents.

Beschreibung: Nature Physics 11, 993 (2015). doi:10.1038/nphys3591 Author: Friedrich-Karl Thielemann Deep-sea sediments reveal the production sites of the heaviest chemical elements in the Universe to be neutron star mergers — rare events that eject large amounts of mass — and not core-collapse supernovae.

Beschreibung: Nature Physics 11, 995 (2015). doi:10.1038/nphys3590 Author: Nigel Goldenfeld

Beschreibung: Nature Physics 11, 990 (2015). doi:10.1038/nphys3542 Author: Joseph P. Heremans Anharmonicity is a property of lattice vibrations governing how they interact and how well they conduct heat. Experiments on tin selenide, the most efficient thermoelectric material known, now provide a link between anharmonicity and electronic orbitals.

Beschreibung: Nature Physics 11, 997 (2015). doi:10.1038/nphys3595 Author: Andrea Taroni

Beschreibung: Nature Physics 11, 992 (2015). doi:10.1038/nphys3496 Author: André Estevez-Torres Cells exploit chemical waves to map the space around them, but their dynamics is difficult to replicate. Using a set of genes to generate a travelling front of protein concentration outside a living cell constitutes a remarkable achievement.

Beschreibung: Nature Physics 11, 987 (2015). doi:10.1038/nphys3601 Author: May Chiao

Beschreibung: Nature Physics 11, 996 (2015). doi:10.1038/nphys3575 Author: Ellen Zweibel Two observational studies published in Nature Physics provided early evidence for the mechanisms of magnetic reconnection in three dimensions and in a turbulent medium.

Beschreibung: Nature Physics 11, 1009 (2015). doi:10.1038/nphys3491 Authors: D. S. Lobser, A. E. S. Barentine, E. A. Cornell & H. J. Lewandowski Boltzmann noticed that his transport equation predicts special cases in which gases never reach thermal equilibrium. One example is the monopole breathe mode of atoms confined in a perfectly isotropic three-dimensional (3D) harmonic potential. Such a complete absence of damping had not been observed in nature, and this anomaly weakened Boltzmann’s then-controversial claim to have established a microscopic, atomistic basis for thermodynamics. Only recently has non-damping of a monopole mode in lower-dimensional systems been reported in cold-atom experiments performed in highly elongated trap geometries. The difficulty in generating a sufficiently spherical harmonic confinement for cold atoms has so far prevented the observation of Boltzmann’s fully 3D, isotropic case. Here, thanks to a new magnetic trap capable of producing near-spherical harmonic confinement for cold atoms, we report a long-delayed vindication for Boltzmann: the observation of a 3D monopole mode for which the collisional contribution to damping vanishes.

Beschreibung: Nature Physics 11, 1013 (2015). doi:10.1038/nphys3508 Authors: Gerbold C. Ménard, Sébastien Guissart, Christophe Brun, Stéphane Pons, Vasily S. Stolyarov, François Debontridder, Matthieu V. Leclerc, Etienne Janod, Laurent Cario, Dimitri Roditchev, Pascal Simon & Tristan Cren The quantum coupling of fully different degrees of freedom is a challenging path towards new functionalities for quantum electronics. Here we show that the localized classical spin of a magnetic atom immersed in a superconductor with a two-dimensional electronic band structure gives rise to a long-range coherent magnetic quantum state. We experimentally evidence coherent bound states with spatially oscillating particle–hole asymmetry extending tens of nanometres from individual iron atoms embedded in a 2H–NbSe2 crystal. We theoretically elucidate how reduced dimensionality enhances the spatial extent of these bound states and describe their energy and spatial structure. These spatially extended magnetic states could be used as building blocks for coupling coherently distant magnetic atoms in new topological superconducting phases.

Beschreibung: Nature Physics 11, 1027 (2015). doi:10.1038/nphys3485 Authors: Mengqiao Sui, Guorui Chen, Liguo Ma, Wen-Yu Shan, Dai Tian, Kenji Watanabe, Takashi Taniguchi, Xiaofeng Jin, Wang Yao, Di Xiao & Yuanbo Zhang Valley pseudospin, the quantum degree of freedom characterizing the degenerate valleys in energy bands, is a distinct feature of two-dimensional Dirac materials. Similar to spin, the valley pseudospin is spanned by a time-reversal pair of states, although the two valley pseudospin states transform to each other under spatial inversion. The breaking of inversion symmetry induces various valley-contrasted physical properties; for instance, valley-dependent topological transport is of both scientific and technological interest. Bilayer graphene is a unique system whose intrinsic inversion symmetry can be controllably broken by a perpendicular electric field, offering a rare possibility for continuously tunable topological valley transport. We used a perpendicular gate electric field to break the inversion symmetry in bilayer graphene, and a giant nonlocal response was observed as a result of the topological transport of the valley pseudospin. We further showed that the valley transport is fully tunable by external gates, and that the nonlocal signal persists up to room temperature and over long distances. These observations challenge the current understanding of topological valley transport in a gapped system, and the robust topological transport may lead to future valleytronic applications.

Beschreibung: Nature Physics 11, 1032 (2015). doi:10.1038/nphys3551 Authors: Y. Shimazaki, M. Yamamoto, I. V. Borzenets, K. Watanabe, T. Taniguchi & S. Tarucha The field of ‘Valleytronics’ has recently been attracting growing interest as a promising concept for the next generation electronics, because non-dissipative pure valley currents with no accompanying net charge flow can be manipulated for computational use, akin to pure spin currents. Valley is a quantum number defined in an electronic system whose energy bands contain energetically degenerate but non-equivalent local minima (conduction band) or maxima (valence band) due to a certain crystal structure. Specifically, spatial inversion symmetry broken two-dimensional honeycomb lattice systems exhibiting Berry curvature is a subset of possible systems that enable optical, magnetic and electrical control of the valley degree of freedom. Here we use dual-gated bilayer graphene to electrically induce and control broken inversion symmetry (or Berry curvature) as well as the carrier density for generating and detecting the pure valley current. In the insulating regime, at zero-magnetic field, we observe a large nonlocal resistance that scales cubically with the local resistivity, which is evidence of pure valley current.

Beschreibung: Nature Physics 11, 987 (2015). doi:10.1038/nphys3600 Author: Abigail Klopper

Beschreibung: Nature Physics 11, 1037 (2015). doi:10.1038/nphys3469 Authors: Alexandra M. Tayar, Eyal Karzbrun, Vincent Noireaux & Roy H. Bar-Ziv Living systems employ front propagation and spatiotemporal patterns encoded in biochemical reactions for communication, self-organization and computation. Emulating such dynamics in minimal systems is important for understanding physical principles in living cells and in vitro. Here, we report a one-dimensional array of DNA compartments in a silicon chip as a coupled system of artificial cells, offering the means to implement reaction–diffusion dynamics by integrated genetic circuits and chip geometry. Using a bistable circuit we programmed a front of protein synthesis propagating in the array as a cascade of signal amplification and short-range diffusion. The front velocity is maximal at a saddle-node bifurcation from a bistable regime with travelling fronts to a monostable regime that is spatially homogeneous. Near the bifurcation the system exhibits large variability between compartments, providing a possible mechanism for population diversity. This demonstrates that on-chip integrated gene circuits are dynamical systems driving spatiotemporal patterns, cellular variability and symmetry breaking.

Beschreibung: Nature Physics 11, 1042 (2015). doi:10.1038/nphys3574 Authors: Kenta Hotokezaka, Tsvi Piran & Michael Paul The origin of heavy elements produced through rapid neutron capture (‘r-process’) by seed nuclei is one of the current nucleosynthesis mysteries. Core collapse supernovae (cc-SNe; ref. ) and compact binary mergers are considered as possible sites. The first produces small amounts of material at a high event rate whereas the latter produces large amounts in rare events. Radioactive elements with the right lifetime can break the degeneracy between high-rate/low-yield and low-rate/high-yield scenarios. Among radioactive elements, most interesting is 244Pu (half-life of 81 million years), for which both the current accumulation of live 244Pu particles accreted via interstellar particles in the Earth’s deep-sea floor and the Early Solar System (ESS) abundances have been measured. Interestingly, the estimated 244Pu abundance in the current interstellar medium inferred from deep-sea measurements is significantly lower than that corresponding to the ESS measurements. Here we show that both the current and ESS abundances of 244Pu are naturally explained within the low-rate/high-yield scenario. The inferred event rate remarkably agrees with compact binary merger rates estimated from Galactic neutron star binaries and from short gamma-ray bursts. Furthermore, the ejected mass of r-process elements per event agrees with both theoretical and observational macronova/kilonova estimates.

Beschreibung: Nature Physics 11, 1057 (2015). doi:10.1038/nphys3507 Authors: M. T. Greenaway, E. E. Vdovin, A. Mishchenko, O. Makarovsky, A. Patanè, J. R. Wallbank, Y. Cao, A. V. Kretinin, M. J. Zhu, S. V. Morozov, V. I. Fal’ko, K. S. Novoselov, A. K. Geim, T. M. Fromhold & L. Eaves

Beschreibung: Nature Physics 11, 1063 (2015). doi:10.1038/nphys3492 Authors: C. W. Li, J. Hong, A. F. May, D. Bansal, S. Chi, T. Hong, G. Ehlers & O. Delaire

Beschreibung: Nature Physics 11, 1070 (2015). doi:10.1038/nphys3468 Authors: Martin Lilienblum, Thomas Lottermoser, Sebastian Manz, Sverre M. Selbach, Andres Cano & Manfred Fiebig

Beschreibung: Nature Physics 11, 988 (2015). doi:10.1038/nphys3522 Authors: David Guéry-Odelin & Emmanuel Trizac An experiment with cold atoms confined in an isotropic three-dimensional harmonic potential confirms the long-predicted non-damping oscillations of the breathing mode.

Beschreibung: Nature Physics 11, 1074 (2015). doi:10.1038/nphys3471 Authors: Dapeng Bi, J. H. Lopez, J. M. Schwarz & M. Lisa Manning

Beschreibung: Nature Physics 11, 1079 (2015). doi:10.1038/nphys3577 Authors: Catalin Miron, Christophe Nicolas, Oksana Travnikova, Paul Morin, Yuping Sun, Faris Gel'mukhanov, Nobuhiro Kosugi & Victor Kimberg

Beschreibung: Nature Physics 11, 1005 (2015). doi:10.1038/nphys3470 Authors: Alexander Bechtold, Dominik Rauch, Fuxiang Li, Tobias Simmet, Per-Lennart Ardelt, Armin Regler, Kai Müller, Nikolai A. Sinitsyn & Jonathan J. Finley The control of solid-state qubits requires a detailed understanding of the decoherence mechanisms. Despite considerable progress in uncovering the qubit dynamics in strong magnetic fields, decoherence at very low magnetic fields remains puzzling, and the role of quadrupole coupling of nuclear spins is poorly understood. For spin qubits in semiconductor quantum dots, phenomenological models of decoherence include two basic types of spin relaxation: fast dephasing due to static but randomly distributed hyperfine fields (∼2 ns) and a much slower process (〉1 μs) of irreversible monotonic relaxation due either to nuclear spin co-flips or other complex many-body interaction effects. Here we show that this is an oversimplification; the spin qubit relaxation is determined by three rather than two distinct stages. The additional stage corresponds to the effect of coherent precession processes that occur in the nuclear spin bath itself, leading to a relatively fast but incomplete non-monotonic relaxation at intermediate timescales (∼750 ns).

Beschreibung: Nature Physics 11, 1017 (2015). doi:10.1038/nphys3461 Authors: A. P. Higginbotham, S. M. Albrecht, G. Kiršanskas, W. Chang, F. Kuemmeth, P. Krogstrup, T. S. Jespersen, J. Nygård, K. Flensberg & C. M. Marcus Quasiparticle excitations can compromise the performance of superconducting devices, causing high-frequency dissipation, decoherence in Josephson qubits, and braiding errors in proposed Majorana-based topological quantum computers. Quasiparticle dynamics have been studied in detail in metallic superconductors but remain relatively unexplored in semiconductor–superconductor structures, which are now being intensely pursued in the context of topological superconductivity. To this end, we use a system comprising a gate-confined semiconductor nanowire with an epitaxially grown superconductor layer, yielding an isolated, proximitized nanowire segment. We identify bound states in the semiconductor by means of bias spectroscopy, determine the characteristic temperatures and magnetic fields for quasiparticle excitations, and extract a parity lifetime (poisoning time) of the bound state in the semiconductor exceeding 10 ms.

Beschreibung: Nature Physics 11, 987 (2015). doi:10.1038/nphys3603 Author: Bart Verberck

Beschreibung: Nature Physics 11, 987 (2015). doi:10.1038/nphys3602 Author: Luke Fleet

Beschreibung: Nature Physics 11, 1045 (2015). doi:10.1038/nphys3484 Authors: I.-C. Hoi, A. F. Kockum, L. Tornberg, A. Pourkabirian, G. Johansson, P. Delsing & C. M. Wilson

Beschreibung: Nature Physics 11, 1050 (2015). doi:10.1038/nphys3478 Authors: Eriko Kaminishi, Takashi Mori, Tatsuhiko N. Ikeda & Masahito Ueda