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Acoustics map out the brass section

The main body of a wind instrument is connected to its mouthpiece by a ‘leadpipe’ that has an inner radius of about 7 mm, so even a small defect can significantly change the sound of the instrument. Manufacturers have traditionally used calipers to measure the inner radius of the leadpipe, but this method only reveals its dimensions at the points of measurement and does not show the whole profile.

In contrast, the technique developed by Buick and colleagues maps the entire inner profile of the leadpipe. When an acoustic pulse travels though a tube, it is partially reflected and partially transmitted wherever the radius of the tube changes. Pulse reflectometry – which has also been used in medicine to study the windpipe – exploits this effect.

Buick’s team sent an acoustic pulse into a leadpipe and picked up the reflected sound waves with a microphone. Accounting for the energy absorbed by the walls of the leadpipe, they used a Fourier technique to convert this signal into a series of peaks corresponding to changes in its diameter. The complete profile of the leadpipe can then be calculated from the heights of these peaks and the time intervals between them.

The technique can measure the dimensions of a leadpipe to an accuracy of 0.03 mm, and the team established that the results were reliable by conducting the measurements ten times on a single leadpipe. The researchers say that this consistency is the main strength of the technique because manufacturers and players are more interested in the relative shapes of different leadpipes than in their actual dimensions.

According to Buick, the technique will be particularly useful for the leadpipes of cornets, which are bent by hand. “This curve makes it hard to detect defects with traditional measuring methods,” he says. “The next step will be to use the technique on whole instruments like trombones and French horns.”

Cooperation to answer cosmic questions

NASA, the Department of Energy and the National Science Foundation currently fund most US research in physics and astronomy, with each agency traditionally supporting well-defined areas. But the report says that this practice could hinder the cross-disciplinary research needed to answer the eleven questions. It recommends that the three bodies work together to set up an interagency initiative on the physics of the universe to support the new scientific ventures it proposes.

Among these projects is an unmanned space mission to study the polarization of radiation left over from the big bang. Fluctuations in this ‘cosmic microwave background’ reveal the conditions in the early universe that led to the structure we see in the universe today. Previous studies of the microwave background – such as the Boomerang experiment – have been conducted from balloons high in the Earth’s atmosphere.

The report also says that a new underground laboratory should be built to conduct experiments on elementary particles including neutrinos, whose signals are swamped by more energetic particles at ground level. The properties of such particles could shed light on the nature of ‘dark matter’, a substance proposed to explain why galaxies seem to contain more matter than we can detect. A consortium of US physicists recently named the now-closed Homestake gold mine in South Dakota as a promising site for a new underground lab.

The other projects four projects recommended by the report should:

* determine the properties of ‘dark energy’, a kind of ‘repulsive gravity’ proposed to explain why the expansion of the universe is accelerating

* use space to probe the basic laws of physics, including tests of Einstein’s theories and the detection of ‘gravitational waves’

* determine the origin of the highest-energy gamma rays, neutrinos and cosmic rays

* study high-energy-density physics in devices such as accelerators to establish the laws that govern extreme astrophysical environments

Physicists scoop cool prize

Donnelly, who is based at the University of Oregon in the US, is recognised for his studies of fluid dynamics at low temperatures, and in particular his investigations of turbulence in superfluid helium – an unusual liquid phase of helium that exists below 2 kelvin.

Goldman, of the University of Minnesota in the US, receives the award for his contribution to the physics of superconductors. He discovered so-called gapless collective modes, and studied transitions from superconducting to insulating states in thin films.

Hardy, who works at the University of British Columbia in Canada, is rewarded for his research into atomic and solid hydrogen, and his studies of electron pairing mechanisms in the high-temperature superconductor YBCO.

The London prize recognizes outstanding experimental and theoretical contributions to low-temperature physics. It is supported partly by a bequest from Nobel Prize winner John Bardeen, who helped devise the BCS theory of high-temperature superconductivity.

New results back neutrino oscillation

Neutrinos have no charge and very little mass, and come in three ‘flavours’ – electron, muon and tau. Solar scientists believe that nuclear reactions in the Sun create electron neutrinos, but they had been puzzled by a shortfall in the number of electron neutrinos detected by experiments on Earth. Last year’s results showed that this shortfall – known as the solar neutrino problem – arises because some of the electron neutrinos change into muon and tau neutrinos as they travel to Earth.

“These new results show in a clear, simple and accurate way that solar neutrinos change their type,” says project director Art McDonald of Queen’s Univeristy in Canada. “The total number of neutrinos we observe is in excellent agreement with calculations of the nuclear reactions powering the Sun”.

The 99.9% certainty of the 2001 results was surpassed because the new data were collected in a single experiment at the Sudbury Neutrino Observatory (SNO). In contrast, the earlier discovery was a joint effort by SNO and the SuperKamiokande collaboration in Japan. “Whenever a conclusion relies on two experiments, it is twice as hard to be certain that you understand what is going on,” says Dave Wark of the University of Sussex, UK spokesman for the SNO experiment.

Many physicists believe that last year’s results confirmed the discovery made in 1998 by the SuperKamiokande collaboration that neutrinos – previously thought to have no mass – in fact have a small mass. But Wark says that the new findings open up more “revolutionary” explanations for neutrino oscillation. The SNO team has submitted its results to the journal Physical Review Letters.

Photonic fibres weave smart fabrics

Man-made fibres are robust and versatile, and cheap to produce in large quantities. In contrast, optical devices known as dielectric mirrors are fragile and expensive. But Hart and colleagues have combined the resilience of synthetic fibres with the properties of an optical device known as a dielectric mirror to create a fibre that could be woven into fabrics to make radiation shields and filters.

Dielectric mirrors are made of alternating layers of two materials with different refractive indices. These layers give the mirror a ‘photonic band gap’, so that it reflects light in a certain range of wavelengths, but absorbs light outside this range. Dielectric mirrors are widely used in optical communications systems to reflect light of selected wavelengths.

Hart’s team made the fibres from arsenic triselenide, a glass with a refractive index of 2.8, and poly(ether sulphone), a polymer with a refractive index of 1.6. The researchers deposited a layer of glass on each side of a sheet of polymer, and rolled the glass-coated polymer sheet around a polymer core several times. This produced a fibre ‘preform’ with 21 alternating layers of glass and polymer, which was then drawn out into fibres with diameters ranging from 175 to 500 micrometres.

The diameter of a fibre determines the thickness of its dielectric layers, and therefore its reflectivity characteristics. This allowed the researchers to make fibres with various photonic band gaps. Hart’s team found that these band gaps arose in the infrared portion of the spectrum at the positions predicted by their calculations. This shows that the fibres contained few impurities, were uniform, and kept their optical properties at the high temperatures used in the drawing process.

The fibres are also sensitive to light approaching from any angle, in contrast with some dielectric mirrors that only respond to light that hits their surfaces at right angles.

Hart and colleagues hope that their fibres will be suitable for a wide range of applications. They even suggest that customized fibres could also be woven into textiles, acting as ‘bar codes’ that could be read to show that fabrics are authentic.

Spintronics gets serious

Conventional electronic devices only exploit the charge of electrons. But physicists believe that much more powerful devices could be built if the ‘spin’ of electrons – which can be either +1/2 or -1/2 – could be controlled too. When the spins of electrons are aligned – or ‘polarized’ – by a magnetic field, the resistance they experience as they travel through a conductor is different to that of unpolarized electrons. This effect can be studied in a ‘spin valve’, in which a conductor is sandwiched between two ferromagnetic electrodes.

With no external magnetic field, the electrodes are magnetized in the same direction – or ‘parallel’ – and the electrons travelling through the conductor are polarized. If an external magnetic field is switched on and increased until the magnetization of one electrode flips, the electrodes become ‘anti-parallel’ and the electrons are depolarized. But when the magnetic field becomes strong enough to flip the magnetization of the second electrode, the electrodes return to their parallel state.

Hendrik Schön of Bell Labs and colleagues used a spin valve to show that the electrons flowing through a single molecule of benzene-1,2-dithiolate can be polarized (J Schön et al 2002 Science to appear). His team placed the organic molecule between parallel nickel electrodes and applied a magnetic field. As they increased the field, the magnetization of the electrodes became anti-parallel and the current flowing through the molecule fell to half its previous value. But when the magnetic field became strong enough to return the electrodes to their parallel state, the current returned to its earlier level.

Bart van Wees and colleagues at the University of Groningen studied the conduction of electrons in a piece of aluminium sandwiched between cobalt electrodes (F Jedema et al 2002 Nature 416 713). As they increased the external magnetic field they found that the output voltage of the spin valve – the voltage across the detecting electrode and the aluminium conductor – changed from positive to negative, and back again. These reversals took place as the electrons first became unpolarized, and then returned to their polarized state.

A similar effect has been seen before (M Johnson and R Silsbee 1995 Phys. Rev. Lett. 55 1790), but the new device was around a thousand times smaller and allowed van Wees and colleagues to generate a sufficiently strong signal to observe changes in the sign of the output voltage.

Despite his team’s achievement, team member Friso Jedema warns that it will be difficult to make integrated spintronic devices that are controlled by magnetic fields. “An external magnetic field could not offer individual control over each spin transistor on a chip,” he told PhysicsWeb. “We see our results as a step forwards in the study and control of spin dynamics.”

Both groups demonstrated their breakthroughs at temperatures of just a few degrees kelvin to achieve the greatest effects, but the phenomena can be seen clearly at room temperature.

Solutions to the skills gap

The review makes 37 recommendations on how to increase the supply of trained scientists and engineers throughout the education system. It says that the problem starts in schools, where pupils should be encouraged into science through more inspiring curricula, better equipped labs and improved careers advice. It says that science teachers should be paid more, and calls for better training to help new teachers in areas outside their speciality.

The review also proposes a scheme that would pay undergraduates and postgraduates to help school science teachers in practical classes. “Rather than stacking supermarket shelves, students would be able to develop their transferable skills,” Roberts told PhysicsWeb. “The scheme would expose youngsters to the possibilities of a career in science and engineering, and may even encourage more students to become teachers themselves.”

At universities, the review calls for new “entry support courses” that would help students bridge the gap between A-level and degrees. It calls for the government to refurbish undergraduate teaching labs and says that degrees should be updated to make graduates more attractive to employers. It wants the government to make it easier for science students to access hardship funds because they have less time than other students to supplement their income with part-time jobs.

The review also contains proposals for improving PhD degrees. It says that PhD grants should be increased to the tax-free equivalent of the average graduate starting salary in the UK – currently just over £12 000. It says that the research councils should fund students for three-and-a-half years to encourage them to tackle innovative, rather than “safe”, projects. The review also wants universities to ensure that all PhD projects “test and develop the creativity prized by employers”.

Finally, the review says that universities should ensure that all post-docs have a career development plan and receive at least two weeks careers training a year. It calls for at least 200 five-year academic fellowships to be set up by the research councils to give post-docs a clear path to permanent academic jobs. Employers, meanwhile, should make R&D careers better paid and more interesting, for example, by allowing staff to take part-time PhDs or make links with local universities.

NMR could help clear landmines

Manufacturers of landmines deliberately use little metal in their devices to make them hard to find with conventional detectors. Physicists originally tackled this problem with a technique based on ‘nuclear quadrupole resonance’, which detects the nitrogen that most explosives contain. When the explosive is placed in an electric field, the spins of the nitrogen nuclei line up, and emit a characteristic signal when the field is switched off.

But the strength of this signal depends on the crystal structure of the explosive, and TNT – or trinitrotoluene – which is widely used in mines, only produces a weak signal. This means that the method can spot anti-tank mines, which contain around five kilograms of TNT, but not anti-personnel mines, which contain around a hundred times less.

To solve the problem, Nolte and colleagues developed a device based on a nuclear effect known as cross-relaxation. The researchers use a strong magnetic field to align – or ‘polarize’ – the spins of the hydrogen nuclei in a sample of explosive. The magnetic field is then reduced until the hydrogen nuclei fall to an energy level that matches that of the nitrogen nuclei in the sample. At this point, the hydrogen nuclei ‘transfer’ some of their polarization to the nitrogen nuclei.

The magnetic field is then increased to its original level and the researchers measure the polarization of the hydrogen nuclei. This reveals the number of nitrogen nuclei present in the sample, and the method is sensitive enough to detect as little as half a gram of TNT.

Nolte and co-workers stress that their technique has only been demonstrated in the lab, and say that it will be difficult to build a practical device. But they are very optimistic about the potential of their method. “The technique is providing us with stunningly accurate results, which we hope will one day save many lives,” says team member Alexei Privalov.

Super-sensor seeks out water

Many irrigation systems automatically water crops at regular intervals, but these systems can be inefficient in very dry climates because they do not account for unexpected downpours or unusually hot spells. Humidity detectors would allow such systems to maintain a certain level of moisture in the soil, but conventional devices are unsuitable for arid regions because they are only sensitive to high levels of humidity. They also tend to be expensive.

In an attempt to solve this problem, Juan Bisquert and co-workers focused on indium-doped tin oxide (ITO), a ‘degenerate’ semiconductor with conductance characteristics similar to that of a metal. This material is widely used as a conducting substrate in the semiconductor industry and glass sheets coated with ITO are readily available.

Bisquert and colleagues cut a centimetre-sized square from such a sheet, and removed a strip of the ITO layer. This left two ITO regions – to which they attached electrodes – separated by a channel 500 micrometres wide. The device was then buried in dry soil, so that soil particles filled the channel, and connected to a power supply. The researchers then recorded the resistance of the soil as they added water to it, and as it dried out.

As the team varied the soil humidity between 5% and 10%, they found that the resistance changed by two orders of magnitude, showing that the device is very sensitive to low levels of water. The researchers were able to reproduce their results adequately, even when they adjusted the acidity of the soil to simulate the effects of chemical fertilizers.

Although the sensitivity of the device arises from the electrical properties of ITO, it works on the same principles as other humidity detectors: when water molecules adsorb onto the surface of the sensor, some of them split into H₃O⁺ and OH^– ions. Conduction takes place when a H₃O⁺ ion donates a proton to a neighbouring water molecule, which releases another proton, and so on. Conductivity therefore increases with rising moisture levels.

Bisquert and colleagues are pleased with their device, but caution that it has only been demonstrated in the lab. “We now need to check that the readings correlate well with the water needs of crops over a period of time,” Bisquert told PhysicsWeb. “This will be the crucial step, because fabrication is straightforward.”

Condensed matter escapes the lab

The space between stars in galaxies – the so-called interstellar medium – is filled with dust and gas, and the interactions between these particles are crucial in the formation of stars. To better understand the events that lead to star birth, Martin McCoustra and colleagues investigated the key role that ice plays in these interactions, which typically take place at temperatures of around 10 kelvin.

Astronomers believe that most interstellar dust grains are coated with ice, which is composed of water, carbon monoxide and other compounds. When these particles cluster together, gravitational energy is thought to be converted into heat, causing the ice to evaporate. Using a technique known as temperature-programmed desorption, McCoustra and colleagues measured the rate that different molecules evaporated from an icy surface as it was heated up.

The researchers had expected to find that the water ice and the carbon monoxide ice formed distinct layers. But they discovered that these ices become mixed when the grains are heated slightly. “When you grow water films at very low temperatures, they act like a sponge, and carbon monoxide deposited on top of these films can flow into the cavities,” explains McCoustra. The team will now incorporate their results into models of star formation.

Geophysicists know from our planet’s magnetic field that iron exists at the centre of the Earth, but the exact composition of the core is unknown. Other elements are thought to be present because seismic waves – which move at well-defined speeds through different materials – travel through the Earth more slowly than they travel through iron.

By studying the abundance of various elements in the solar system, Mike Gillan and Dario Alfe chose likely candidates for these additional elements – silicon, sulphur and oxygen. Taking into account the quantum mechanical properties of these elements, Gillan and Alfe used Monte Carlo simulations to compute the density of the core – which has a solid centre and a liquid outer layer – for a wide range of compositions. They also estimated the temperature of the core for the different compositions and pressures to within 400 kelvin – an improvement on the accuracy of earlier experiments.

“We found that there must be oxygen in the core, otherwise you can’t reproduce the seismic observations at all,” says Gillan. He and Alfe calculated that at least 8% of the core must consist of oxygen, and about 8% of it consists of a mixture of sulphur and silicon.

Both groups were speaking at the Institute of Physics Condensed Matter and Materials Physics Conference, which was held jointly with the 19th General Conference of the Condensed Matter Division of the European Physical Society.

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