Shear afterglow

Cosmological fireworks, better known as γ-ray bursts (GRBs), begin with a fiery GRB, followed by an afterglow of less-energetic photons. In the popular 'fireball shock' model, a collapsing star releases jets of relativistic material that produce γ-rays, and later interact with the interstellar medium to generate the afterglow. But different jet geometries are possible, and two in particular — called uniform or structured jets — are compatible with data.

Frank M. Rieger and Patrick Duffy have examined the shear acceleration of particles by these jets: particles gain energy by colliding with magnetic-field irregularities moving systematically in a shear velocity flow (Astrophys. J. 632, L21–L24; 2005). They analysed constraints on the efficiency of particle acceleration; for a structured jet with a weak (and/or rapidly decaying) magnetic field, electrons and protons can reach ultrahigh energies — 1018 eV for electrons and 1021 eV for protons. However, for high jet magnetic fields, only protons can be accelerated efficiently. Thus shear acceleration, as well as the standard shock acceleration, could produce the lingering afterglow, and the highest energy cosmic-ray protons observed.

Happily frustrated


In so-called Kagomé antiferromagnets, the triangular symmetry (similar to that pictured below) of the two-dimensional lattice is incompatible with lowering the interaction energy — the spins are geometrically frustrated. Such systems, in particular the case of spin-½, have attracted interest in the context of the 'resonating valence bond' state, a proposed unusual ground state consisting of spin-paired singlets. Many of the predictions made, however, have eluded experimental verification, simply because no suitable practical material has been available.

Matthew P. Shores and colleagues have succeeded in synthesizing a mineral that has the desired properties (J. Am. Chem. Soc. 127, 13462–13463; 2005). The material features structurally perfect Kagomé layers based on spin-½ Cu(II) ions. Although similar systems containing ions with higher spin have been made before, Shores et al. had to develop a new protocol to prepare the present copper-ion Kagomé antiferromagnet. The product — answering to the name of herbertsmithite, Zn[Cu3(OH)6Cl2] — is shown to display very strong spin frustration and could serve as the long-sought test bed for studying spin liquid phases.

State of fusion

Credit: EFDA-JET

In the 1950s, the promise of controlled nuclear fusion to provide an effectively limitless source of safe, clean energy seemed just around the corner. Fifty years on, although there is still some way to go to realize the dream, the latest status report on fusion research compiled by the International Fusion Research Council (Nucl. Fusion 45, A1–A28; 2005) provides good reason for continued optimism.

Nuclear fusion power relies on the energy released when two light atomic nuclei fuse together to form heavier nuclei. The most promising reaction for this purpose is between deuterium and tritium nuclei. But to ensure that this reaction proceeds at a rate that is self-sustaining, and that produces enough excess energy to drive a steam turbine for power generation, requires a high-density deuterium–tritium plasma to be heated to temperatures in excess of 200 million kelvin. Although technically challenging, facilities such as the Joint European Torus (above) achieve such temperatures regularly, have produced up to 16 MW of fusion power over periods of several seconds, and are steadily approaching the fusion 'break-even' point.

Guiding light

In laser wakefield acceleration, a short laser pulse is used to generate a wave inside a plasma, then particles (such as electrons) 'surf' the electric field of the wave's wake and undergo immense acceleration over very short intervals of space and time. The performance of such an accelerator could be greatly improved by guiding the laser pulse over a larger distance. C. G. R. Geddes and colleagues show that this is possible using a plasma channel as the guide (Phys. Rev. Lett. 95, 145002; 2005).

The guiding process involves two laser pulses. Firstly, a powerful 'ignitor' pulse generates a plasma inside a dense jet of hydrogen gas. By heating the gas, the plasma expands hydrodynamically and a plasma channel opens up. This channel then acts as the guide for a second 'drive' pulse, which is focused and pushed through the plasma. Without the guiding channel, the drive pulse would soon suffer diffraction. But, by tailoring the shape of the channel, the laser can be confined and driven over more than ten diffraction lengths.

Nanoelectronics disentangled

No matter how densely packed the devices of future nanocircuits become, they will always need to pass the signals they process back up to the macroscopic world. To this end, Robert Beckman and colleagues describe, in Science Express (, an approach that can unpick individual signals from a multitude travelling in a dense array of silicon nanowires, by using a clever arrangement of micrometre-scale electrical contacts.

Instead of trying to make electrical contact to individual wires within a nanowire array — an approach that is never likely to be commercially feasible — the authors construct a device that controls the conductivity of individual wires using the same mechanism that controls the conductivity of a field-effect transistor. The device consists of a set of five gate-electrodes each of which controls a different sequence of sixteen bundles of silicon nanowires all connected at one end to a single drain electrode. By applying the correct sequence of voltages to these electrodes, all but one of these bundles can be switched off, enabling the signal in the remaining bundle to be addressed.