Research Highlights

    Linear behaviour from nonlinear transport

    The exponential nature of electrons tunnelling across a barrier is an inherently nonlinear process, representing the ultimate quantum limit to the downsizing of devices, such as the transistor. Fortunately, this hasnt stopped Petra Schmidt and co-workers in their search for a linear transmission voltage at such length scales (Appl. Phys. Lett. 88, 013502; 2006).

    True, the simplest square-well potential barrier leads to an exponential electron transmission. However, when the symmetry of the barrier profile is reduced – by adding steps, for example – other solutions involving power laws become possible. Schmidt et al. solved the Schrödinger and Poisson equations numerically for a 20 nm structure, and found that the addition of potential steps of differing height resulted in a superposition of broad resonances (due to elastic scattering) that linearize the transmission voltage, at least in a given bias-voltage window of 0–0.25 volts. Such a linear, or ohmic, resistive element would be stable against fabrication variations of ±2 monolayers or ±1% Al doping levels in AlxGa1–xAs.

    String theory

    At the centre of the Galaxy, there is a source of positrons. No one knows how they are generated, but their annihilation with electrons to produce 511-keV photons (pictured) was picked up by the Compton Gamma-Ray Observatory, and has been confirmed by the International Gamma-Ray Astrophysics Laboratory (INTEGRAL). Possible sources for the positrons include neutron stars, black holes, supernovae, hypernovae and gamma-ray bursts — although it's not clear that any of these possibilities could account in detail for the signal seen. On the other hand, it could be something to do with dark matter. But Francesc Ferrer and Tanmay Vachaspati have another suggestion: superconducting cosmic strings (Phys. Rev. Lett. 95, 261302; 2005).

    Credit: W. PURCELL (NWU) ET AL., OSSE, COMPTON OBSERVATORY, NASA.

    The movement of a tangle of superconducting strings would cut through the magnetic field in the Milky Way, generating current that could include positrons. What is especially tantalizing is that the strings in Ferrer and Vachaspati's proposal are 'light', at an energy scale of around 1 TeV — exactly the scale of the physics set to be explored by CERN's Large Hadron Collider from 2007. Higher-resolution observations from INTEGRAL should also help to pin down the source of the positrons.

    Evolution of a laser plume

    The ability of lasers to cut almost any material into almost any shape could hold great promise for fabricating structures at the micrometre scale. But to avoid melting such delicate structures, only very short laser pulses may be used, severely limiting the speed with which they can be formed. One way around this is to shorten the time between pulses, but this too is limited by the fact that the plasma plume of material ejected by each pulse absorbs the light of subsequent pulses if they arrive too soon.

    To better understand the processes involved, Jens König and colleagues have collected timeresolved images and optical transmission measurements of the plumes generated at a metal surface by a femtosecond pulsed laser (Opt. Express 13, 10597–10607; 2005). As well as providing new insight into how these plumes evolve, the work enables the maximum useful pulse repetition rate to be estimated.

    Bound to the usual format

    Data can be efficiently encoded using changes in light reflectivity, as experienced each time we listen to a CD. Sebastian A. Lange and colleagues now report that the same principles could be harnessed to densely pack — and conveniently unpack — information about biomolecular binding events on a disc (Angew. Chem. Int. Edn 45, 270–273; 2006).

    Lange et al. use conventional plastic discs and print onto them antibodies that can capture a specific protein. The antibody spots are comparable in size to the pits moulded into a CD carrying audio or other data. Once a protein is captured, its signature is enhanced by staining it with a silver particle, leading to high reflection of laser light shone on the spot using a standard CD reading head. If no binding occurs, however, the reflectivity remains low.

    In their initial experiments, the reading head was mounted on the stage of a microscope, but Lange et al. hope that eventually a CD player can be used to detect the reflection from metal-enhanced proteins arranged in CD format, providing a handy high-density immunoassay technique.

    Narrow spread

    When the beam from an ultrahigh-intensity laser hits a thin metal target, the electric field produced in the resulting plasma can be a million times greater than those in a conventional particle accelerator. Hence, laser-based accelerators could bring high-energy particle beams to the benchtop, and at a fraction of the usual cost.

    However, for many of the potential uses of such benchtop devices — in proton therapy for the treatment of cancer, for example — the spread in energy of the beams they produce has been far too broad. Two groups report that the solution to this problem lies in the design of the target (Nature 439, 441–444, 445–448; 2006).

    Previously, laser-based accelerators relied on the presence of contaminants on the surface of a metal foil as a source of protons and light ions. But by paying greater attention to the makeup of this target, the groups demonstrate a better than fourfold decrease in the energy spread of a laser-driven beam of protons and carbon-ions respectively.

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    Research Highlights. Nature Phys 2, 66 (2006). https://doi.org/10.1038/nphys232

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