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Plutonium is also a superconductor

Sarrao and co-workers found that the transition temperature (T_c) in the plutonium compound – the temperature at which the electrical resistance of a superconducting material drops to zero – is an order of magnitude higher than the highest seen in the heavy fermion systems (compounds based on uranium and cerium). The material also has a large critical current, which would be of technological importance if it were not for the hazardous radioactive properties of plutonium. This critical current comes from pinning centres due to defects in the material, created by radiation induced “self-damage”.

The team observed the superconductivity in measurements of magnetic susceptibility and specific heat. Further measurements on temperature-dependent magnetic susceptibility and electrical resistivity over a wide range of temperatures suggest that the degree of localization of the 5f electrons lies between that of compounds based on cerium and those based on uranium.

Plutonium is an actinide element located at the transition where the 5f electrons go from being delocalized to localized, which makes it one of the most complex materials known. The researchers believe that the superconductivity in plutonium comes directly from its anomalous electronic properties and that it is an intermediate addition, in terms of T_c, to the two other new classes of “magnetically mediated” superconductors – the heavy- fermion materials, which have T_cs of about 1 K and the copper oxides, which have T_cs of about 100 K.

The team hopes that future research will unearth superconductivity in other transuranic compounds with lower toxicity. “Experience tells us that where one superconductor is found, others are usually nearby, so there are many other related compounds to explore,” Sarrao told PhysicsWeb.

Sunspots come into focus

The Sun’s surface is constantly changing with turbulent convection and magnetic fields interacting to produce fine-scale structures, such as penumbral filaments. Although solar telescopes have greatly improved in the last decade, it is now clear that observations at higher spatial resolutions are needed.

Scharmer and co-workers used the recently installed Swedish 1-m Solar Telescope in the Canary Islands to examine part of the largest spot in the active region “10030” of the Sun. The new telescope has a spatial resolution of 0.12 arcsecs, which is about a factor of two better than previous instruments.

The researchers noticed that many of the penumbral filaments, which are between 150 and 180 km wide, have dark cores themselves and are surrounded in turn by dark umbrae. They also found that these dark cores, which are as yet unresolved, branch into several different cores that can be as little as 90 km across but up to 100 km long.

Power from terahertz beams

The terahertz region lies in the far-infrared region of the electromagnetic between about 300 GHz and 20 THz. All objects emit terahertz electromagnetic waves as “black-body radiation” but the total intensity emitted at all frequencies is less than one millionth of a watt per square centimetre.

The past decade however has seen a significant advance in the production of coherent broad-band terahertz beams. A common method of producing such a beam is to generate an electric field inside a high-resistance semiconductor. Typically, the average power of a beam generated by this method is less than 10^-6 W. The beam can be used for high-resolution spectroscopy and some imaging techniques but it is of limited use, being of such low power.

Now Carr and co-workers have devised a new process in which bunches of electrons travel at nearly the speed of light, inside an accelerator at the Jefferson Laboratory in Virginia. They use a strong magnetic field to accelerate the electron bunches which emit a pulse of electromagnetic radiation lasting 500 femtoseconds. The pulse has a power peak of about 106 W and a peak frequency of 0.6 terahertz, although the detectable radiation continues up to several terahertz. When the electrons are generated at the maximum rate of 37 million each second, the average power reaches about 20 W. This is a 100,000 times higher than the power produced in previous terahertz beams.

“Of course producing and measuring the light is just a first step,” Larry Carr, a member of the group, told PhysicsWeb. “Most applications depend on coherent detection, so we need to develop this.” The Jefferson laboratory is already planning space for a terahertz laboratory in their Free Electron Laser facility.

Carr thinks it will be difficult to predict the most important applications for such a beam, although the team would like to use the large peak power to study advanced materials and devices, chemical reactions and biological processes. It could also be used for “full-field, real-time image capture” – in other words terahertz “movies”.

Magnetic meteorites

The magnetic properties of carbon-60 compounds have intrigued physicists since they were first reported in 1991 and researchers have recently discovered weak magnetic behaviour in polymerised rhomohedral carbon-60. Ferromagnetism has previously been observed in other carbon-based ferromagnets, but only at very low temperatures. However, the weakness of the effect makes it difficult to determine the origins of the magnetism – it could be intrinsic or it might be caused by minute concentrations of iron-rich impurities in the samples.

Coey and co-workers characterised the magnetism associated with the ferromagnetic phases in their samples using Mossbauer spectroscopy, chemical analysis and a combination of scanning electron microscopy and X-ray diffraction analysis. From the Mossbauer results, they determined the concentration of the ferromagnetic minerals in each of the graphitic samples and calculated their combined contribution to the magnetization.

The observed magnetization, however, significantly exceeded the magnetization that was due to these magnetic phases. The researchers attribute this difference to the graphite. They calculate the average room temperature magnetization of carbon to be 23.1 Am² kg^-1, which corresponds to 0.05 Bohr magnetons per atom. By comparison the figure for iron is 2.2 Bohr magnetons per atom.

The results raise the question of the origin of the ferromagnetism. It could be that meteoritic graphite differs from its terrestrial counterpart because of the way it was formed or changes it underwent when it landed on Earth. The shock of this impact could produce defects, which are known to increase the magnetic susceptibility of graphite. Another possibility is that the dispersed nanocrystalline ferromagnetic phases induce a magnetic moment in the graphite. The researchers suggest a “magnetic proximity” effect induced at the border between the graphite and the magnetic materials as a possible explanation.

Whatever its origin, the implications of ferromagnetic carbon are likely to be far-reaching. This material could, for example, be used as a high-temperature ferromagnetic semiconductor or in “spintronic” applications.

Evidence for dark energy gets stronger

If the actual energy density in the universe is less than a certain value, the critical density, then the universe will continue to expand forever. If the actual energy density is more than the critical density, the universe will eventually stop expanding and begin to collapse under the influence of gravity. Astronomers believe that the actual energy density is equal to the critical density, and this has been confirmed by measurements on the cosmic microwave background — the radiation left over from the big bang.

However, ordinary matter and “dark matter” – matter that we cannot see but which nevertheless exerts a gravitational influence on stars and galaxies – can only account for about a third of the critical density. “Dark energy” is the name given to the remaining two thirds of the energy density – yet no one knows what it is.

Dark energy is unlike gravity in that it repels matter and therefore causes the expansion of the universe to accelerate. The first evidence for dark energy came from supernovae observations in 1998 and further evidence arrived earlier this year from a survey of 250,000 galaxies. The latest evidence comes from observations of gravitational lensing.

Gravitational lensing occurs when the light from a distant object, such as a quasar, is bent by the gravitational field of another object on its journey to Earth. Chae and co-workers used three major radio telescopes, the Very Large Array in New Mexico, the National Radio Astronomy Facility in the UK and the Very Long Baseline Array in the US, to search for the gravitational lensing of radio waves from distant quasars.

Chae and co-workers then combined the CLASS statistics with the latest results from galaxy surveys. They noticed that the effective lens size of galaxies was smaller than previously thought. For this result to be compatible with measurements from galaxy surveys requires the presence of a large amount of dark energy in the universe.

The group now plans to examine the individual gravitational lenses in more detail and a much larger radio survey is also under discussion.

Easy escape for Martian meteorites

Previous computer models supposed that meteorites were launched from large craters. These models were, however, unable to account for the relatively large number of small and young Martian meteorites found on Earth. Now, Head and colleagues have shown that the velocities of smaller lumps of ejected material can overcome the escape velocity of Mars. “Previous analytical solutions used less-sophisticated equations of state than used in our computer code, ” explains Head, “In each case, the diameter of the minimum sized source crater was overestimated.”

The results also show that regions of Mars’s surface covered by a relatively thicker layer of material require larger – and thus less frequent – impacts to eject rocks into space. As older regions are more likely to be covered with such material, the researchers think that the Martian meteorites we find on Earth are likely to come from younger rock. This agrees well with the age and size ranges of terrestrial Martian meteorite samples studied.

The age distribution of the Martian meteorites should remain unchanged as more samples are discovered. Owing to the slow discovery rate of one sample every two years or so, the group will only have enough new samples to confirm the results in about ten years time.

This work could help meteoriticists shed light on where life on Earth originated. “Besides estimating the total mass of potentially biological material ejected from Mars, we calculate directly the acceleration history of the ejected fragments,” says Head. Bacterial spores, for example, from Mars could survive the immense acceleration of meteorite ejection and might successfully make the journey to Earth.

Nanowires within nanowires

Semiconductor nanowires are one-dimensional structures, with unique electrical and optical properties, that are used as building blocks in nanoscale devices. Their low dimensionality means that they exhibit quantum confinement effects. For example, narrowing the wire’s diameter increases its band gap, compared to the bulk material.

Researchers have examined various ways of growing semiconductor nanowires, including laser ablation, chemical vapour deposition (CVD) and template-assisted growth. While laser ablation and template-assisted approaches provide bulk quantities of semiconductor nanowires, they do not provide much control over the composition, size or crystallographic direction of the nanowire.

What Lauhon and co-workers have done is to grow core-shell and core-multishell nanowire heterostructures using a CVD method that provides increased control over the structure’s composition. Using their technique, the nanowires are grown by gradually building up thin, uniform shells around a nanometre-sized cluster of gold atoms. The nanowires had boron-doped silicon shells surrounding intrinsic silicon, as well as silicon wrapped around a silicon oxide core. They also investigated the growth of crystalline germanium-silicon and silicon-germanium core-shell heterostructures.

The Harvard group has already used the approach to prepare new devices called nanowire field-effect transistors. Working with researchers from Intel, the team also plans to integrate these transistors with conventional semiconductor processing to produce advanced hybrid devices, “This is quite exciting since it may lead to real-world use of a very basic discovery,” says Charles Lieber, the leader of the group.

The researchers will now look at other materials and believe that the general ideas outlined in their work are applicable to many materials. “One particularly interesting application that we are pursuing is core-shell-shell structures in III-V semiconductor materials such as gallium nitride,” Lieber told PhysicsWeb. “These structures can give rise to very interesting photonic devices.”

X-rays probe neutron stars

Neutron stars are extremely dense objects that are formed by the collapse of massive stars. They are typically only about 10 kilometres in diameter, but are at least 40% heavier than the Sun, which means that their core density is several times that of the density of an atomic nucleus. However, it is possible that neutron stars could also contain other particles such as strange quarks, pions or kaons.

It is relatively straightforward to determine the mass of a star but it is much more difficult to measure the radius – especially if the star is only kilometres across and more than 10¹⁶ kilometres away. One approach is to use is high-resolution spectroscopy to measure the redshift caused by the strong gravitational field at the surface of the star. This redshift depends on the ratio of the star’s mass to its radius. However, the magnetic field around a star can greatly modify the lines in the spectrum, making them difficult to interpret.

To overcome this problem, the researchers studied EXO0748-676, a low-mass neutron star that has a weak surface magnetic field (about 10⁷-10⁹ gauss). Although this field is immense compared to the Earth’s (1 gauss), it is too weak to have a significant effect on atomic spectra.

The NASA-Columbia-SRON research group observed the star during a series of 28 X-ray bursts. They found three strong spectral lines of iron and oxygen and identified three sets of redshifts, all with a value of 0.35. This value is in the range expected for a star made of normal neutron matter. In addition, the results do not agree with the models for strange-matter stars.

The group now plans to perform more detailed calculations in order to better quantify and interpret their results. “The next step will be to calculate the properties of the neutron star atmosphere so that we can fully utilise the information available in our spectra,” Jean Cottam of the NASA Goddard Space Flight Center told PhysicsWeb. “We have only begun to extract all the information available in our data, but the next steps will require more theoretical work.”

New director at ICTP

Sreenivasan is an experimentalist and is currently a professor of physics at the University of Maryland, where he also directs the Institute for Physical Science and Technology. He received his doctorate in aerospace engineering at the Indian Institute of Science in Bangalore and spent two years as a post-doctoral research fellow in Australia before moving to Johns Hopkins University in Baltimore. He moved from Yale University to Maryland last January.

Sreenivasan, who is 55, has published over 150 papers in the fields of complex fluids, turbulence, combustion, cryogenic helium and nonlinear dynamics. He will begin his tenure at the ICTP in March 2003.

World record for silicon light-emission

Researchers at ST’s Corporate Technology R&D Organisation in Catania, Sicily, carried out the work. They implanted ions of rare-earth metals such as erbium and cerium into a layer of silicon rich oxide (silicon dioxide enriched with silicon nanocrystals 1-2 nanometres in diameter). The frequency of the light emitted by the silicon depended on which metal was chosen.

“The ability to combine optical and electronic processing on the same chip presents enormous opportunities for ST to be the first to develop many new types of semiconductor products,” says GianGuido Rizzotto, director of Corporate Technology R&D. Rizzotto adds that the company should soon be commercialising its technology as it is compatible with existing production methods and equipment.

ST, the world’s third largest semiconductor manufacturer, is using its technique to improve power control devices, such as power supplies and solid-state relays. In these devices the power circuit needs to be electrically isolated from the control circuit because it experiences much higher voltages, but at present can only be isolated using bulky and expensive external components. In contrast, ST builds the two circuits onto the same chip and places silicon dioxide – an insulator – between them. The circuits then communicate with one another through light-emitters and detectors integrated into the silicon.

ST also plans to use its technology in fibre-optic communications and in optical data-transmission systems for advanced CMOS circuits, where signals are distributed through the chip at light speed.

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