In the last 30 days

• Open/close Encoding many channels on the same frequency through radio vorticity: first experimental test

  Fabrizio Tamburini et al 2012 New J. Phys. 14 033001 Tag this article

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  We have shown experimentally, in a real-world setting, that it is possible to use two beams of incoherent radio waves, transmitted on the same frequency but encoded in two different orbital angular momentum states, to simultaneously transmit two independent radio channels. This novel radio technique allows the implementation of, in principle, an infinite number of channels in a given, fixed bandwidth, even without using polarization, multiport or dense coding techniques. This paves the way for innovative techniques in radio science and entirely new paradigms in radio communication protocols that might offer a solution to the problem of radio-band congestion.

• Open/close Evidence for large electric polarization from collinear magnetism in TmMnO₃

  V Yu Pomjakushin et al 2009 New J. Phys. 11 043019 Tag this article

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  There has been tremendous research activity in the field of magneto-electric (ME) multiferroics after Kimura et al (2003 Nature  426 55) showed that antiferromagnetic and ferroelectric orders coexist in orthorhombically distorted perovskite TbMnO ₃ and are strongly coupled. It is now generally accepted that ferroelectricity in TbMnO ₃ is induced by magnetic long-range order that breaks the symmetry of the crystal and creates a polar axis (Kenzelmann et al 2005 Phys. Rev. Lett.  95 087206). One remaining key question is whether magnetic order can induce ferroelectric polarization that is as large as that of technologically useful materials. We show that ferroelectricity in orthorhombic (o) TmMnO ₃ is induced by collinear magnetic order, and that the lower limit for its electric polarization is larger than in previously investigated orthorhombic heavy rare-earth manganites. The temperature dependence of the lattice constants provides further evidence of large spin–lattice coupling effects. Our experiments suggest that the ferroelectric polarization in the orthorhombic perovskites with commensurate magnetic ground states could pass the 1 μC cm ^-2 threshold, as predicted by theory (Sergienko et al 2006 Phys. Rev. Lett.  97 227204; Picozzi et al 2007 Phys. Rev. Lett.  99 227201).

• Open/close Traffic jams without bottlenecks—experimental evidence for the physical mechanism of the formation of a jam

  Yuki Sugiyama et al 2008 New J. Phys. 10 033001 Tag this article

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  A traffic jam on a highway is a very familiar phenomenon. From the physical viewpoint, the system of vehicular flow is a non-equilibrium system of interacting particles (vehicles). The collective effect of the many-particle system induces the instability of a free flow state caused by the enhancement of fluctuations, and the transition to a jamming state occurs spontaneously if the average vehicle density exceeds a certain critical value. Thus, a bottleneck is only a trigger and not the essential origin of a traffic jam. In this paper, we present the first experimental evidence that the emergence of a traffic jam is a collective phenomenon like 'dynamical' phase transitions and pattern formation in a non-equilibrium system. We have performed an experiment on a circuit to show the emergence of a jam with no bottleneck. In the initial condition, all the vehicles are moving, homogeneously distributed on the circular road, with the same velocity. The average density of the vehicles is prepared for the onset of the instability. Even a tiny fluctuation grows larger and then the homogeneous movement cannot be maintained. Finally, a jam cluster appears and propagates backward like a solitary wave with the same speed as that of a jam cluster on a highway.

• Open/close Enhanced control of light and sound trajectories with three-dimensional gradient index lenses

  T M Chang et al 2012 New J. Phys. 14 035011 Tag this article

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  We numerically study the focusing and bending effects of light and sound waves through heterogeneous isotropic cylindrical and spherical devices. We first point out that transformation optics and acoustics show that the control of light requires spatially varying anisotropic permittivity and permeability, while the control of sound is achieved via spatially anisotropic density and isotropic compressibility. Moreover, homogenization theory applied to electromagnetic and acoustic periodic structures leads to such artificial (although not spatially varying) anisotropic permittivity, permeability and density. We stress that homogenization is thus a natural mathematical tool for the design of structured metamaterials. To illustrate the two-step geometric transform-homogenization approach, we consider the design of cylindrical and spherical electromagnetic and acoustic lenses displaying some artificial anisotropy along their optical axis (direction of periodicity of the structural elements). Applications are sought in the design of Eaton and Luneburg lenses bending light at angles ranging from 90° to 360°, or mimicking a Schwartzchild metric, i.e. a black hole. All of these spherical metamaterials are characterized by a refractive index varying inversely with the radius which is approximated by concentric layers of homogeneous material. We finally propose some structured cylindrical metamaterials consisting of infinitely conducting or rigid toroidal channels in a homogeneous bulk material focusing light or sound waves. The functionality of these metamaterials is demonstrated via full-wave three-dimensional computations using nodal elements in the context of acoustics, and finite edge-elements in electromagnetics.

• Open/close Al₂O₃-films on Ni₃Al(111): a template for nanostructured cluster growth

  C Becker et al 2002 New J. Phys. 4 75 Tag this article

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  In scanning tunnelling microscope images of thin Al ₂O ₃-films grown on Ni ₃Al(111) at 1000 K two super-lattices with periodicities of 2.6 and 4.5 nm, respectively, can be identified. These well-ordered nanostructures can be used as nucleation centres for metal particle growth. It can be shown that both nanostructures act as a template for the fabrication of ordered assemblies of metal clusters by mere physical vapour deposition. The degree of ordering of these nanostructures is largely dependent on the metal deposited. Here we report on the growth of Cu, Ag, Au, Mn, and V clusters on the Al ₂O ₃-films. The best results as far as ordering of the clusters is concerned was reached for V deposition at 550 K, which resulted in a nearly perfect hexagonal array of clusters with a spacing of 2.6 nm.

• Open/close Flexible metamaterials at visible wavelengths

  Andrea Di Falco et al 2010 New J. Phys. 12 113006 Tag this article

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  We report on the fabrication and characterization of plasmonic structures on flexible substrates (Metaflex) and demonstrate the optical properties of a single layer of Metaflex. The layer exhibits a plasmonic resonance in the visible region around 620 nm. We show experimental and numerical results for both nano-antennas and fishnet geometries. We anticipate the use of Metaflex as a building block for flexible metamaterials in the visible range.

• Open/close A nanostructural investigation of glassy gelatin oligomers: molecular organization and interactions with low molecular weight diluents

  M Roussenova et al 2012 New J. Phys. 14 035016 Tag this article

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  The effects of low molecular weight diluents (namely water and glycerol) on the nanostructure and thermodynamic state of low water content gelatin matrices are explored systematically by combining positron annihilation lifetime spectroscopy (PALS) with calorimetric measurements. Bovine gelatin matrices with a variation in the glycerol content (0–10 wt.%) are equilibrated in a range of water activities ( a _w = 0.11–0.68, T = 298 K). Both water and glycerol reduce the glass transition temperature, T _g, and the temperature of dissociation of the ordered triple helical segments, T _m, while having no significant effect on the level of re-naturation of the gelatin matrices. Our PALS measurements show that over the concentration range studied, glycerol acts as a packing enhancer and in the glassy state it causes a nonlinear decrease in the average hole size, v _h, of the gelatin matrices. Finally, we report complex changes in v _h for the gelatin matrices as a function of the increasing level of hydration. At low water contents ( Q _w ∼ 0.01–0.10), water acts as a plasticizer, causing a systematic increase in v _h. Conversely, for water contents higher than Q _w ∼ 0.10, v _h is found to decrease, as small clusters of water begin to form between the polypeptide chains.

• Open/close Ultrahigh-Q mechanical oscillators through optical trapping

  D E Chang et al 2012 New J. Phys. 14 045002 Tag this article

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  Rapid advances are being made toward optically cooling a single mode of a micro-mechanical system to its quantum ground state and observing the quantum behavior at macroscopic scales. Reaching this regime in room-temperature environments requires a stringent condition on the mechanical quality factor Q _m and frequency f _m, Q _m f _m ≳  k _B T _bath/ h, which so far has been marginally satisfied only in a small number of systems. Here we propose and analyze a new class of systems that should enable one to obtain unprecedented Q-frequency products. The technique is based on the use of optical forces to ‘trap’ and stiffen the motion of a tethered mechanical structure, thereby freeing the resulting mechanical frequencies and decoherence rates from the underlying material properties.

• Open/close Creating electromagnetic cavities using transformation optics

  V Ginis et al 2012 New J. Phys. 14 033007 Tag this article

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  We investigate the potential of transformation optics for the design of novel electromagnetic cavities. First, we determine the dispersion relation of bound modes in a device performing an arbitrary radial coordinate transformation and we discuss a number of such cavity structures. Subsequently, we generalize our study to media that implement azimuthal transformations, and show that such transformations can manipulate the azimuthal mode number. Finally, we discuss how the combination of radial and azimuthal coordinate transformations allows for perfect confinement of subwavelength modes inside a cavity consisting of right-handed materials only.

• Open/close Experimental verification of three-dimensional plasmonic cloaking in free-space

  D Rainwater et al 2012 New J. Phys. 14 013054 Tag this article

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  We report the experimental verification of metamaterial cloaking for a 3D object in free space. We apply the plasmonic cloaking technique, based on scattering cancellation, to suppress microwave scattering from a finite-length dielectric cylinder. We verify that scattering suppression is obtained all around the object in the near- and far-field and for different incidence angles, validating our measurements with analytical results and full-wave simulations. Our near-field and far-field measurements confirm that realistic and robust plasmonic metamaterial cloaks may be realized for elongated 3D objects with moderate transverse cross-section at microwave frequencies.