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New clue for planet hunters

Planetary systems are thought to condense from clouds of gas and small, solid particles known as interplanetary dust. Planets form near the star, where the material is most dense. Further from the star the material coalesces into a vast band of small, icy bodies that in our Solar System is known as the Kuiper belt.

Dust left over at the time of planetary formation is thought to disappear into deep space, so any dust present in the Solar System today must have been produced more recently. Landgraf and co-workers have analysed data from the NASA space probes Pioneer 10 and 11 and conclude that this dust results from collisions between objects in the Kuiper belt. Since the presence of such dust around a star depends upon the existence of a band similar to the Kuiper belt, a dust band is evidence of planetary formation.

Using the measurements of dust flux made by the Pioneer spacecraft, the researchers calculated that there is one particle in every 50 cubic kilometres of the Solar System’s dust ring. Although this density seems very low, it cannot be explained merely by the shedding of dust from comets travelling through the Solar System. Near Earth, comets give off significant amounts of dust, but beyond Saturn they freeze and shed little material.

The researchers also used data from ESA’s Ulysses spacecraft, which has been orbiting the poles of the Sun for more than 10 years, to rule out the possibility that the dust grains originated from outside the Solar System. The Ulysses data show that such interstellar dust grains are considerably smaller than interplanetary dust grains.

Bright infrared emissions that could be produced by such dust rings have already been observed around other stars. Future missions – such as ESA’s Herschel space telescope – will hunt for similar emissions from many more distant stars. Other probes will then make detailed measurements of the chemical compositions of these stars and look for signs of life on Earth-like planets.

“If we see a similar dust ring around a mature star, we’ll know it must have asteroids or comets,” says Landgraf. “If we see gaps in the dust ring, it will probably have planets which are sweeping away the dust as they orbit.”

The results of the study are to appear in the Astrophysical Journal.

Holograms help build 3D nanostructures

The light field of an interference pattern can induce an electric dipole in certain atoms, and this draws the atoms to the most intense regions of electric field in the pattern – that is, the bright fringes. This effect is widely used to build two-dimensional structures, but the process becomes cumbersome in three dimensions because many lasers are needed to create the interference patterns.

In contrast, the new technique can create a three-dimensional interference pattern using just one laser. To do this, Meschede and co-workers first ‘imprinted’ a crystal with a hologram, by shining three lasers into the crystal and heating it for three hours. When a ‘read-out’ laser was subsequently shone into the crystal, the interference pattern that created the hologram was reconstructed outside the crystal.

The team then fired a caesium atom laser through the interference pattern towards a specially prepared gold substrate behind it. A simple nano-structure then built up on this surface according to the geometry of the interference pattern.

Meschede’s team demonstrated its principle with three lasers, but such crystals could store holograms created by up to a thousand lasers, which will allow more elaborate structures to be built. The crystals can also hold many holograms at the same time, each one created by light of different wavelengths. Every hologram acts as a stencil for different atoms, which means that a single crystal could be used to make a variety of structures.

“The beautiful thing about these holographic crystals is that you can use one laser to reconstruct many holograms”, says Meschede.

Although the regular spacing of interference patterns means that their technique can only be used to make periodic structures, Meschede and colleagues are optimistic that it could become an integral part of the semiconductor industry. Lithography is widely used in the industry to make two-dimensional circuit boards, but a convenient method for creating three-dimensional nanoscale structures has so far been elusive.

Micro-jets make designer droplets

Coated droplets are widely used in industry and research: volatile compounds can be isolated from their environment by a protective shell, and drugs can be coated with substances that enable them to target certain receptors in the body. The technique can also be used to encapsulate large biological ions so that they can be studied with mass spectroscopy.

These droplets can be formed when a ‘coaxial’ jet of two non-mixing liquids breaks up. To create such a jet, Loscertales and co-workers constructed two concentric needles that pump out a narrow stream of liquid within a wider stream. The needles were vertical, with their nozzles pointing downwards.

The team attached electrodes to give the inner needle a variable potential and the outer needle a potential of several kilovolts with respect to a grounded target beneath the nozzles of the needles. The device works by creating electrical stresses at the boundary between the two liquids, and this means that the inner liquid must be an electrical conductor and the outer liquid must be an insulator.

As the liquids emerge from the needles and flow downward towards the target, the voltage applied to the inner needle is adjusted to control the diameters of the inner and outer jets, which determines the geometry of the droplets that form. The exact voltage depends on the viscosity and conductivity of the liquids.

Loscertales and colleagues demonstrated their technique – which can produce droplets with diameters from 0.15 to 10 micrometres – with water and olive oil, and other solvents and polymer solutions. Polymers are widely used as coating layers, and some polymers are hardened with ultraviolet light to form a rigid shell around the droplet core.

Loscertales and colleagues are optimistic that their technique will prove to be more versatile than other techniques, which can only control the diameter of the droplet and the thickness of the coating over narrow size ranges.

Spacecraft join forces at Jupiter

In the six years it has orbited Jupiter, Galileo has studied the planet’s atmosphere, satellites and surrounding magnetosphere. Launched in 1997, Cassini-Huygens is heading for Saturn where it will make similar observations.

To boost its energy for the journey to Saturn, Cassini-Huygens performed a fly-by of Jupiter in January 2001, when the planet’s magnetosphere was unusually large. This enabled the craft to make direct observations of Jupiter’s large magnetic field. One team found that variations in the polar aurorae on Jupiter arose from shock waves propagating out from the Sun (D Gurnett et al p985). This process is similar to that on Earth, where the solar wind can cause magnetic storms.

Another group has shown that it is valid to use the Earth’s magnetosphere as a model for Jupiter’s magnetosphere (W Kurth et al p991). The changing magnetic field of Earth has previously been measured by several space-based experiments, and the conjunction of the Cassini-Huygens and Galileo spacecraft has allowed the astronomers to obtain a dynamic picture of Jupiter’s changing magnetosphere for the first time.

Synchrotron radiation emitted by Jupiter arises from electrons spiralling in its magnetic field, and has previously been observed with Earth-based telescopes and on previous missions. But Cassini-Huygens detected synchrotron emission with a frequency of 13.8 GHz on its fly-by, which suggests that ultra-relativistic electrons in Jupiter’s magnetosphere have energies of 50 MeV – some 30 MeV more energetic than previous studies had indicated (S Bolton et al p987).

This discovery challenges existing theories of how charged particles are accelerated in the magnetospheres of planets, and the researchers speculate that electrons in Jupiter’s radiation belts may be accelerated by processes similar to those on Earth. New theories were needed to explain the existence of relativistic electrons in the Van Allen radiation belts – two doughnut-shaped regions of charged particles above the equator.

Other achievements made by the Cassini-Huygens and Galileo missions include the detection of a hot wind of neutral particles originating from the Jovian moon Io, and the discovery of ultraviolet emission from Jupiter’s atmosphere caused by the magnetic effect of the moons Ganymede and Europa.

The physics of personal income

Many attempts have been made to model the distribution of incomes in a society, but until now no formula had successfully described all salary levels and periods in history. The economist Pareto proposed in 1897 that income distribution followed a simple power law – that is, the number of people earning a certain wage falls as that wage rises. This law is characterized by the ‘Pareto index’, which is small if incomes are distributed unevenly across the population, and large if the spread is more equal.

But Pareto’s theory only holds for the top 1% of earners. The economist Gibrat later found that the incomes of the remaining 99% of earners follow a log-normal distribution – that is, the logarithms of the incomes have a normal, symmetrical distribution. This relationship is defined by the ‘Gibrat index’, which is also small for uneven income distributions.

In order to combine these formulas, Souma analysed the salary data of over 80% of the working Japanese population. Employing techniques commonly used to model ‘many-body’ systems in condensed matter physics, he has devised a formula – consisting of a log-normal curve with a power-law tail – that successfully describes the income distribution of the whole population.

The income details analysed by Souma dated from 1887 to 1998, which enabled him to study how the distribution evolved. Comparing his work with an earlier American study, Souma notes that the indices in the new formula are almost identical in Japan and the US.

Souma does not attempt to explain why income follows the observed distribution, but he believes that the new model is a fundamental law of economics that could describe income distribution in all societies at any point in history.

Prizes reward geophysics and optoelectronics

McKenzie proposed in the 1960s that the Earth’s crust consisted of rigid, mobile ‘plates’ – a theory now central to geology – and went on to study how these plates interact to produce earthquakes and volcanoes. McKenzie’s later investigations of the effect of Earth’s gravity on the structure of its crust were extended when he collaborated with NASA scientists to study the surface of Venus from gravity data collected by the Magellan mission in the 1990s. He also identified the mechanisms behind certain surface features on Mars. McKenzie will receive the prize from the King of Sweden on 18 September in Stockholm.

Rank certificate

The Rank Prize goes to three teams who each share around £40 000 for their contributions to optoelectronics. Ken Hill’s team at the Communications Research Centre in Ottawa, Canada, Brian Garside’s group at McMaster University, Canada, and Gerald Meltz, William Morey and colleagues at the United Technologies Research Center in Connecticut, US, are recognized for the invention of fibre Bragg gratings.

Bragg gratings are widely used in telecommunications systems to increase the capacity of optical fibres. Signals with different wavelengths can travel through an optical fibre simultaneously, and this multiplies the capacity of the fibre by the number of wavelengths used. Bragg gratings are used to shift the wavelengths of transmitted signals to maximize the capacity of the fibre, and also to recover the original signals at the receiving end.

James Fujimoto and Eric Swanson of the Massachusetts Institute of Technology, and Carmen Puliafito of the New England Eye Center in Boston, receive the award for the development of optical coherence tomography. This technique – which works by analysing light scattered from biological tissue – is already used to diagnose eye disorders, and could soon be used to detect cancer and image the gastrointestinal tract.

Kenichi Iga of the Tokyo Institute of Technology and Robert Burnham and Donald Scifres of Xerox Corporation, are rewarded for the development of vertical cavity surface emitting lasers, or VCSELs. Amplified light is emitted from a VCSEL through a specially engineered reflector on top of the laser cavity, in contrast to conventional lasers, which emit light from the side of the cavity. VCSELs are more compact and easier to manufacture then ‘edge-emitting’ lasers and are widely used in telecommunications and optical data storage.

The Rank Prizes will be awarded on 25 February at the Royal Society of Medicine in London.

Crystals give clues to ancient cosmetics

The main ingredients of early cosmetics were lead sulphide and lead carbonate, the most common naturally occurring lead compounds. Lead sulphide – or ‘galena’ – is grey and relatively soft with a cubic crystal structure. Lead carbonate – or ‘cerussite’ – is white, harder than galena, and has an orthorhombic crystal structure.

In a study funded partly by cosmetics giant L’Oréal, Ungár and colleagues analysed 39 ancient lead-based cosmetic powders from the Louvre museum in Paris. Lead compounds strongly absorb X-rays, so the researchers used the powerful X-ray source at the European Synchrotron Radiation Facility in Grenoble, France, for their diffraction studies.

When X-rays enter a crystal, they are diffracted by the crystal lattice. The position of the peaks in the diffraction pattern reveals the spacing of the atoms in the crystal. Since the crystals in a powder are randomly oriented, X-ray diffraction leads to a wide range of diffraction angles. Ungár’s team were able to analyse these ‘peak profiles’ to reveal strain in the crystal lattices, which depends on the level of defects, such as dislocations.

By fitting their data to established models of crystallography, Ungár and co-workers also calculated the size of the crystallites, their size distribution and the density of dislocations in the samples. The team then compared these results with tests carried out on natural galena and cerussite, which they had processed in the lab.

Ungár and colleagues found that the ancient cosmetics were either dull or shiny, and that these were mixed in different proportions to vary the colour of the make-up. The dull powders had small crystals – between 130 and 240 nanometres across – with a high density of dislocations. Comparison with the artificial specimens suggested that these powders were ground for up to an hour. These fine powders were black with a matt texture.

The shiny powders contained crystals between 400 and 550 nanometres across, with a much lower dislocation density. This suggests that they were gently crushed then sieved to retain the larger crystallites, which had a high reflectivity.

Ungár’s team also found some evidence that ancient Egyptians heated lead compounds to create different coloured cosmetics. Lead sulphide oxidises upon heating, turning first yellow and then blue, but the samples were too small to confirm that this method was used.

“The Egyptians’ processes were not terribly sophisticated”, Ungár told PhysicsWeb. “They used grinding, sieving, and probably heating and annealing, but not at very high temperatures.”

Semiconductor laser makes broadband debut

Conventional lasers emit radiation at a single wavelength, so applications that require many wavelengths often need several lasers. Existing broadband lasers – which are usually based on titanium-sapphire – are not always suitable because they only emit pulses of radiation. Some of these devices are also bulky because their spectra are artificially broadened by external gadgets.

The ‘quantum cascade’ devised by Gmachl’s group consists of 36 semiconductor layers. Each layer emits a different range of wavelengths, and these add up to produce a smooth spectrum of wavelengths from 6 to 8 micrometres. The light is intense enough for the device to act as a laser, and each layer is transparent to the light emitted by other layers. The layers consist of indium gallium arsenide ‘quantum wells’ separated by aluminium indium arsenide barriers.

A quantum well is a layer of semiconducting material embedded in a semiconductor with a larger bandgap. Charge carriers such as electrons can then be trapped in the well, where they can only occupy certain quantized energy states, similar to the electrons in an atom. When an electron that has been excited into a higher energy state returns to its ground state, it emits light with a frequency corresponding to the energy difference between these states. Unlike atoms, quantum wells can be engineered to emit different wavelengths.

“After we calculated the layer structure, we used molecular beam epitaxy to create the device,” Gmachl told PhysicsWeb, “and it worked on the first attempt!”

Gmachl and colleagues believe that their compact laser technique could lead to single light sources for many applications across the spectrum. The micrometre-sized device is mounted on a chip just millimetres across, and they plan to adapt it to make broadband lasers that operate at fibre-optic wavelengths – around 1.3 micrometres – and in the visible spectrum.

Acoustics sound out strained steel

Metal components that are under repeated strain can break without warning. Until now, scientists have found it difficult to predict when such damage will occur because it does not depend on the age of the metal or the applied stress. Some existing tests are also unsuitable for real-life systems because a sample of the component needs to be removed for analysis.

But the technique devised by Ogi and colleagues – known as electromagnetic acoustic resonance – allows the lifetime of components to be tested in situ. The team placed coils around a carbon steel rod 14 millimetres in diameter to generate an oscillating magnetic field around it. Bending stresses of between 140 and 490 MPa were then applied to the rod as sound waves were sent through it.

Ogi’s team found that these waves made the oscillations in the magnetic field interfere constructively with each other, and these disturbances induced an electrical signal in the coils. This allowed the sound absorption to be measured without touching the rod, which would otherwise lead to loss of acoustic energy.

When the steel rod had been stressed for a certain length of time, the researchers noticed a dip in this signal, which showed that the rod was absorbing more acoustic energy. Scanning tunnelling micrographs of an identically stressed rod showed that this drop coincided with the appearance of a large number of dislocations. The replica rod also returned to its original state after heat treatment, which is a signature of dislocation damage in a metal.

After the dip was observed, Ogi and colleagues continued to bend the rod until it broke. They found that – depending on the experimental set-up – the dip occurred at either 85% or 72% of the total lifetime of the rod. These fractions remained the same regardless of the carbon content of the rod or the stress applied to it.

Dislocations reduce the energy of an acoustic wave as it propagates because they vibrate ‘anharmonically’. Since these defects are a common feature of metals, Ogi and colleagues believe that their technique will be suitable for a wide range of materials.

Spider-Man spins a web of contacts

‘Collaborative networks’ – such as the pattern of links between scientists who have worked together – are good candidates for study because they tend to be clear-cut: the links between people are well defined, the dates of the associations are clear, and the details are often logged.

Alberich’s team realised that the relationships between the characters in Marvel comics – there are dozens of titles in total – formed an artificial collaborative network. Characters from one comic frequently appear in another, and Alberich and colleagues viewed a ‘collaboration’ as each time two characters turn up together. They analysed around 96 000 appearances by 6500 characters in 13 000 issues. The data were gathered by the Marvel Chronology Project.

Most real collaborative networks are ‘scale-free’, that is the number of people with links to others falls as the number of links grows. Alberich’s team found that the ‘Marvel Universe’ was also scale-free – most characters have appeared with an ‘average’ number of other characters, but fewer are associated with many. This relationship has a cut-off point: with 1625 appearances, Spider-Man is the most ‘connected’ character.

“Every fan believes that the Marvel Universe is a real place”, team member Francesc Rossello told PhysicsWeb. “Now we have shown that this perception has a mathematical basis”.

But the Marvel Universe lacks the ‘clustering’ effect found in real networks, in which two people are more likely to be connected if they are both linked to a third individual. Alberich and co-workers attribute this to the writers, who had to distribute super-heroes evenly among the comics.

The investigation – which was funded by the Spanish government – concludes that although the Marvel Universe successfully mimics many aspects of human networks, it cannot disguise its artificial origins. Alberich and colleagues now plan to study the evolution of the Marvel Universe, in order to establish the factors that lead to differences between social networks and completely random networks.

Alberich’s team has submitted its study to the journal Social Networks.

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