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Displays scoop Descartes prize

One award, worth €700 000, has been given to a team led by Richard Friend at Cambridge University in the UK for developing technology that could replace conventional glass and silicon-backed displays with much more flexible plastic substrates. The work originated with the chance discovery by Friend’s group in 1989 that certain semiconducting polymers can emit light. It was soon realized that these materials could be coated onto plastic to create flexible displays, but early devices were not efficient and lasted for little more than a few hours.

The various partners in Friend’s team – including Philips, Cambridge Display Technology and three other firms in Belgium, Germany and Sweden – have helped to commercialize the technology and develop longer-lasting and more efficient polymers. Last year Philips launched the first product containing a polymer light-emitting display, which appeared on its top-of-the-range “Spectra” shaver. Although such displays are still based on glass and silicon, the research could eventually lead to roll-up televisions and computer screens.

The remaining €300 000 of the prize goes to a team led by Veronique Dehant of the Royal Observatory of Belgium, in association with researchers from eight other countries. They have produced a highly accurate computer model that can predict future variations in the Earth’s axis caused by the pull of the Sun and the Moon. The model will help to improve the accuracy of global satellite positioning and navigation systems from 2 metres to just 2-3 centimetres and increase the reliability of geophysical measurements.

New look for micro fuel cells

Previously the power sources for micromachines have only worked at high temperatures and have emitted toxic by-products. Moreover, the devices contain high-speed moving parts that must be operated within tight limits. Now, Zhang and co-workers have fabricated a micro fuel cell that uses methanol. This material has a high energy density and it is easier to store in a miniature device than the hydrogen gas traditionally used in fuel cells.

The UCLA-Penn State team first made a proton exchange membrane assembly by sandwiching a solid-state electrolyte layer between a cathode and an anode. Then they integrated this electrode assembly into a silicon-based micro fuel cell that contained channels 750 microns wide and 400 microns deep.

An aqueous solution of methanol is fed into the anode via the microchannels, where it generates electrons and protons. The electrons flow through an external circuit to deliver current while the protons migrate through the exchange membrane to the cathode. The protons then combine with electrons from the circuit and oxygen in the air to produce water as a harmless by-product at the cathode. The higher the methanol concentration, the more protons it can supply and the higher the current produced. Carbon dioxide is also produced at the anode

Using a 1 molar solution, the researchers observed a power density of 47 milliwatts per square centimetre at a temperature of 60 degrees centigrade, which they say is among the highest densities ever achieved in a micro fuel cell. At room temperature, the density was 14.3 milliwatts per square centimetre. Furthermore, the design of the new cell also means that moving parts can be eliminated.

The team now plans to improve the performance of the exchange membranes and hopes to further increase the power density of the device by making a three-dimensional fuel cell. “We have already increased operating times of the cell by using 8 molar methanol without loss of performance,” team member David Yen told PhysicsWeb.

Condensates enter new era

Atoms behave very differently at temperatures near absolute zero depending on the value of their intrinsic angular momentum or spin. Spin is measured in units of the Planck constant divided by 2π. Bosons have spins with integer values in these units, while fermions have spins of 1/2, 3/2, 5/2 and so on. A molecule made of two fermionic atoms will be a boson because it will have an integer value of spin.

All fermions must obey the Pauli exclusion principle, which means that they cannot occupy the same quantum state. However, there are no such restrictions on bosons, so they can all collapse into the same quantum ground state. This process, known as Bose-Einstein condensation, is at the heart of superconductivity – the flow of electric current without any resistance.

Since electrons are fermions they must form Cooper pairs – named after Leon Cooper of the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity – to allow them to collapse into a Bose condensate. If physicists were able to mimic this process in a gas of fermionic atoms, it should be possible to learn a great deal more about superconductivity.

Rudolf Grimm and colleagues at the University of Innsbruck in Austria started with a gas of fermionic lithium-6 atoms in an optical trap and cooled them in a magnetic field to produce a condensate containing over 100 000 lithium molecules (S Jochim et al. 2003 Sciencexpress 1093280). The condensate lasted for more than 20 seconds. Meanwhile, Deborah Jin and co-workers at the JILA laboratory in Boulder, Colorado, performed a similar feat with potassium-40 atoms (M Greiner et al. 2003 Nature to be published; arxiv.org/abs/cond-mat/0311172).

The atoms in a molecule are strongly bound together, whereas the particles in a Cooper pair are only weakly bound and can be quite far apart. By using magnetic fields to control the interactions between fermionic atoms – which is difficult to do with electrons – physicists hope to explore a wide range of novel phenomena between these two extremes.

Nanowires drop out of fluid research

There is a widely accepted universal rule which states that drops and bubbles always break away from a nozzle in the same way, regardless of the liquid or gas. As the drop forms, it is attached to the nozzle by a thin segment of liquid or gas. This segment grows thinner before breaking at a single point, allowing the drop to fall away from the nozzle.

Now, Osman Basaran at the University of Purdue and colleagues have discovered an exception to this rule. While studying how liquid drops and gas bubbles are formed by nozzles – such as those in inkjet printers – the researchers found that for a nozzle immersed in a viscous liquid such as silicone oil, water drops formed differently to the way they would in air. The drops formed much more slowly and the segment of liquid between the growing drop and the tip of the nozzle grew longer than it would in air, before cutting away from the nozzle at two points rather than one.

“In this special case, this region doesn’t shrink to a point and break off like it ordinarily would,” said Basaran. “Mathematically we say that it ‘remembers’ its initial state, which is very unusual.”

The liquid separated from the nozzle at both the place where the drop formed and at a point nearer to the nozzle. This left a drop of liquid, along with an extremely thin liquid thread. “This thin thread forms so slowly that you have enough time to solidify it into a filament or wire,” said Basaran. By adding a prepolymer to the water and then exposing the liquid to light to bring about photopolymerization, the scientists were able to create solid fibres less than 100 nanometres wide.

The researchers calculated that the length and width of the fibre depends on the viscosity of the fluid outside relative to the viscosity of the fluid inside the drop – the greater the difference in viscosity, the thinner and longer the filaments.

“One can make every drop very tiny – as small as pico- and even femto-litres – and identical, even if you were to produce a trillion drops in a row from the same nozzle,” said Basaran. “The drops have great potential in advanced applications. These include microreactors to make ceramic particles, microcapsules for controlled-release applications in medicine, and to produce nanostructures.”

New particle turns up in Japan

Mesons are particles that contain a quark and an antiquark that are held together by the strong nuclear force. Since there are six different “flavours” of quark – up, down, strange, charm, bottom and top – it is possible to form a large number of different mesons.

The Belle team measured the decay of B-mesons – mesons that contain a bottom quark – produced in electron-positron collisions at the KEK B-factory in Japan. The team plotted the number of candidate events for B mesons against mass and observed a significant spike in the distribution at 0.775 GeV. This corresponds to a mass of nearly 3872 MeV. The particle decayed almost immediately into other, longer lived particles.

The KEK team says that the mass of this new meson is higher than theoretical predictions. Moreover, the way in which it decays also differs from theory. One possibility is that current models of the strong force need to be modified. Alternatively it could be that X(3872) is the first example of a “molecular state” meson that contains two quarks and two antiquarks.

Until recently particle physicists had only ever detected particles that contain two or three quarks. However, in the past year evidence has emerged for another four-quark particle known as the Ds(2317) and a five-quark particle known as the pentaquark.

Nanotubes join the army

Carbon nanotubes are rolled up sheets of graphite just nanometres in diameter that have very high mechanical strength and novel electronic properties. Recently, researchers found that individual semiconducting single-walled nanotubes show a large change in electrical resistance when exposed to certain gases. This property could be exploited in chemical sensors.

Snow and colleagues first grew an interconnected network of single-walled nanotubes in a tube furnace, and then patterned them into an array of sensor electrodes using optical lithography and metal lift-off techniques. The NRL team made its detector by coating the inner surface of a chemi-resistor flow cell – a quartz tube about 50 mm long and 3mm across – with the nanotube sensor material.

To test the device, Snow and co-workers exposed the tubes to DMMP (a chemical similar to the nerve agent Sarin), ammonia, water vapour and various hydrocarbons using air as the carrier gas. They observed a large increase in the resistance of the sensor as it adsorbed DMMP, but little or no change in resistance when it was exposed to water vapour or hydrocarbons. According to the team, this is because chemicals such as DMMP are strong electron donors and therefore reduce the hole density in the semiconducting nanotubes. This leads to an increase in their resistance. In contrast, water vapour and hydrocarbons do not possess these charge transfer properties.

The nerve gas detector is sensitive to one part in a billion of DMMP. The team now hopes to improve the device’s ability to distinguish between different chemicals by incorporating chemo-selective polymers into the sensors. Preliminary demonstrations with a hydrogen-bonding compound have shown that the device can effectively separate out signals from DMMP and ammonia.

Data storage made easy

Researchers use polymers to make electronic devices such as diodes and transistors, but they have paid little attention to exploiting polymers as memory devices. Now, Forrest and colleagues have made a ‘write-once read-many-times’ (WORM) device by layering a conductive plastic called ‘PEDOT’ onto the surface of a thin-film silicon diode that has been deposited on a flexible metal foil. The data can only be written once because the write process causes permanent physical changes in the material. However, the data can be read and re-read indefinitely.

The Princeton-HP team discovered that PEDOT – which is used as a coating for photographic film and in video displays – conducts electricity at low voltages but becomes permanently non-conducting at higher voltages (of around 10 volts). It thus acts a fuse or circuit breaker.

In a memory device, data needs to be written as a string of ‘1s’ and ‘0’s. The new memory element would consist of a grid of circuits in which all the connections contained a PEDOT fuse. A large applied voltage would result in the fuse being blown and would close the circuit. This would be a ‘0’. At lower voltages, the fuse would remain intact, leaving the circuit open and would act as a ‘1’.

The researchers say that their technique could be used to make memory blocks as small as 1 square millimetre that were capable of storing 1 megabit of information. Although this is 100 times lower than the best magnetic memories, the device could find applications in ultra-low cost data archiving. Moreover, the memory would contain no moving parts – such as the laser and motor drives found in conventional magnetic and optical writers.

The team now hopes to turn their invention into a commercially viable product. “This could take as little as five years,” says Forrest.

Ups and downs for big projects

The European Space Agency (ESA) has decided to abandon Eddington – a mission to search for Earth-like planets outside the solar system – and to cancel the lander that was originally part of the BepiColombo mission to Mercury. The only new mission to be approved for the agency’s “Cosmic Vision” plan is a pathfinder mission that will be a forerunner to the Laser Interferometer Space Antenna (LISA) – the world’s first space-based gravitational wave observatory. The pathfinder mission is due to be launched in 2008.

ESA’s current financial problems were caused, in part, by the grounding of the Ariane-5 rocket in January this year and the subsequent delays in the launches of Rosetta and Smart-1. The space agency has been given a temporary loan of €100 million but this must be paid back by the end of 2006. ESA admits that these decisions are “hard to take scientifically” and that they “reflect financial conditions rather than the ambitions of the scientific community”.

The decisions were taken by ESA’s Science Programme Committee and a last-minute campaign by more than 400 space scientists across Europe to save Eddington failed. Ian Roxburgh of Queen Mary University London, co-ordinating scientist for the mission, hopes that there is still time to change the agency’s mind. “We hope to persuade the ESA Council to provide the additional funds to permit Eddington to be flown” he told PhysicsWeb. The Council meets on December 4.

ESA announced the changes to Cosmic Vision at the end of last week – a few days before the European Commission presented a “Space Action Plan” calling for substantial additional spending in this sector. “If we do not act now Europe runs the risk of decline as a space power,” said EU research boss Philippe Busquin. “The action plan will help us move ahead and put Europe’s scientific talents, technologies and entrepreneurial skills to work for Europe and its citizens.”

Meanwhile, the Department of Energy in the US has whittled down a list of 53 proposed new research facilities and upgrades to a prioritized list of 28. The highest priority in the near-term is the International Thermonuclear Experimental Reactor (ITER) – a collaboration between the US, EU, Japan, Russia, China and Korea to build a next-generation fusion reactor. A project to increase computing power available to DOE researchers by a factor of 100 is second highest priority. Three physics projects – a space-based probe to study dark energy, the most powerful X-ray source in the world and an accelerator for rare isotopes – tie for third priority, along with a facility to mass-produce and characterize proteins.

The highest priority for the medium term is the linear collider – the machine that particle physicists want to build to follow on from the Large Hadron Collider. Two projects share top priority in the far-term: an upgrade to the National Synchrotron Light Source and a Super Neutrino Beam, which would outperform current neutrino beams by a factor of 10.

Inclusion in the plan does not guarantee that facilities will be built. However, the head of the DOE, Spencer Abraham, was enthusiastic. “These facilities will revolutionize science – and society,” he said. “Our goal is to keep the United States at the scientific forefront.”

Mesons violate Bell’s inequality

Experiments to test Bell’s inequality involve measuring the properties of pairs of particles that are space-like separated in the sense of special relativity: in other words, there is no time for a light signal to travel between them within the duration of the experiment. In a typical Bell’s inequality experiment the polarizations of a pair of photons are measured as the relative angle between the axes of polarizers making the measurements is varied.

Quantum mechanics predicts that “non-local” correlations can exist between the particles. This means that if one photon is polarized in, say, the vertical direction, the other will always be polarized in the horizontal direction, no matter how far away it is. However, some physicists argue that this cannot be true and that quantum particles must have local values – known as “hidden variables” – that we cannot measure.

Bell and others showed that it was possible to distinguish between quantum mechanics and these hidden-variable theories in a certain type of experiment that measure a parameter known as S. Put simply, the local theories predict that S will always be less than two, whereas the quantum prediction is S = 2√2. When S is greater than two, Bell’s inequality is said to be violated.

Apollo Go of the National Central University in Taiwan and co-workers in the Belle collaboration performed the experiment at the KEK B-factory. At this accelerator beams of electrons and positrons are collided to produce pairs of B mesons and their antiparticles, which then decay into lighter particles. The meson pairs behave like photon pairs, but instead of analyzing correlations between directions of polarization, the Belle team study particle-antiparticle correlations using a technique known as “flavour tagging”. Go and colleagues calculated that S = 2.725, with error bars that mean that the inequality is violated by three standard deviations.

“If quantum mechanics is the fundamental description of nature, then non-local correlations should be found with any quantum number,” Go told PhysicsWeb. “In this experiment we are testing a quantum number that has never been tested before. Moreover, the particle-antiparticle quantum number is a very fundamental quantity in particle physics and the results might have implications in this area – I am waiting for comments from other particle theorists!”

The team now plans to study particle-antiparticle correlations in more detail and probe the boundary between classical and quantum mechanics.

Mixed messages from the edge of the solar system

The limits of the solar system lie between about 85 and 120 astronomical units away, where one astronomical unit (AU) is about 150 million kilometres or the distance between the Earth and the Sun. At these distances the solar wind – a supersonic plasma of charged particles from the Sun – collides with the interstellar plasma of outer space to create a “termination shock”. Theory predicts that the speed of the solar wind will decrease abruptly in this region, and that there will also be an increase in the strength of the magnetic field. The shock should also accelerate particles from the interstellar medium to high energies.

Tom Krimigis of the Johns Hopkins University in Maryland and colleagues analyzed energetic particles – such as protons and electrons – in the 85 AU region with the LECP instrument on Voyager 1 (S Krimigis et al. 2003 Nature 425 45). In mid 2002, they observed that the intensity of these particles increased by a factor of about 100 over a period of about 7 months. Moreover, they calculated that the solar wind decreased from supersonic (above 300 kilometres per second) to subsonic (less than 50 kilometres per second) speeds during this time.

However, Frank McDonald of the University of Maryland and colleagues argue that cosmic-ray measurements from another Voyager instrument show that Voyager has only reached a precursor region and not the actual termination shock itself (F McDonald et al. 2003 Nature 425 48). Their argument is based on the energy distribution of the cosmic rays and measurements of the magnetic field. “The absence of any significant increase in magnetic field strength means that we have not entered the shock area,” McDonald told PhysicsWeb. “We have seen the foothills but not the big mountain yet.”

Which explanation is correct? “I tend to agree with Krimigis and colleagues that their data can most readily be explained if the termination shock had been crossed,” writes Len Fisk of the University of Michigan in the issue of Nature in which the two papers appear. Another possibility is that the termination shock is moving and that Voyager 1 passed through it, only for the shock to overtake the spacecraft again. Both teams are now analyzing further data.

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