In a paper entitled “A Microsoft Word Font for Anti-Matter”, Don Summers and colleagues explain how they have developed a font that allows users “to directly add overlined English and the most used overlined Greek characters to Microsoft Word documents on Apple Macintosh computers.” To make anti-matter a user simply types shift 5: for instance, shift 5 followed by B will produce an anti-B meson, while shift 5 followed by option shift z will produce an anti-Lambda particle. The font, called LinguistA, is available in 12 and 24 point type. Now all that is needed is something to make overlining easy on the Web.
EPS announces prize winners
The five prize-winners pioneered experiments on single crystals of large molecules with magnetic properties – such as manganese acetate and the iron-based Fe8 spin cluster. By studying the quantum tunnelling of magnetization in such large systems, it was possible to probe the boundary between quantum and classical mechanics. The official citation states that the prize has been awarded “for developing the field of quantum dynamics of nanomagnets, including the discovery of quantum tunnelling and interference in dynamics of magnetisation.”
The EPS has also announced several other prizes. Eddy Lingeman of the NIKHEF laboratory in the Netherlands receives the Gero Thomas Memorial Medal for outstanding contributions to the EPS. The Joint EPS/BPU Medal in Environmental Physics will be shared by Ivar Isaksen of the Centre for International Climate and Environmental Research in Oslo, Norway, Guy Brasseur of the Max Planck Institute for Meteorology in Hamburg, and Wei-Chyung Wang of the State University of New York at Albany. Isaksen, Brasseur and Wang share the prize for outstanding contributions to environmental physics
The High Energy Particle Physics Public Outreach Prize is awarded to Michael Kobel of Bonn University in Germany for his work in bringing high-energy particle physics into schools in Germany, while the EPS Public Understanding of Physics Prize goes to Rafel Carreras of CERN. James Phillip Elliott of Sussex University in the UK and Francesco Iachello of Yale University in the US share the Lise Meitner Prize for their innovative applications of group theoretical methods to the understanding of atomic nuclei.
The EPS quantum electronics and optics division also announced its awards recently. Serge Haroche of the Ecole Normale Supérieure and the Collège de France won the division’s prize for fundamental research, while Wilson Sibbett of St Andrews University in Scotland received the prize for applied research. The winners of the Fresnel prize for young physicists were Rüdiger Paschotta of ETH-Zurich in Switzerland (applied) and Carlo Sirtori of Thales in France (fundamental).
Magnesium diboride: mind the gaps
Superconductivity occurs in a material when electrons bind together to form Cooper pairs, which can travel through the materials without any resistance. Electrons normally repel each other but in low-temperature superconductors they can overcome their mutual repulsion by interacting with lattice vibrations known as phonons. (The binding mechanism in high-temperature superconductors is still a mystery). The energy gap is essentially the energy needed to break the pairs apart: it also determines the thermodynamic properties of the material and is directly related to the superconducting transition temperature.
Most superconductors have just one energy gap but experiments suggested that magnesium diboride might have more than one. Now Steven Louie, Marvin Cohen and co-workers at the University of California at Berkeley and the Lawrence Berkeley National Laboratory have performed ab initio calculations which strongly suggest that magnesium diboride does indeed have two energy gaps. The gaps correspond to transition temperatures of 15 K and 45 K, and combine to give an overall transition temperature of 39 K. Their model can also explain the unusual variation of specific heat capacity with temperature that has been measured in magnesium diboride.
Magnesium diboride is a metal with a layered structure in which the boron atoms form hexagonal layers and the magnesium atoms are located in between the boron layers, above and below the centres of the hexagons. The unusual properties of the material are thought to result from strong interactions between phonons and electron orbitals in the boron layer. The Berkeley team predicts that other layered materials based on boron, carbon and nitrogen should also display similar or higher transition temperatures.
Prize opportunity for women in condensed matter
The awards are organized by L’Oréal and the United Nations Educational, Scientific, Educational and Cultural Organization (UNESCO) as part of their women in science programme. “This partnership is a concrete expression of our firm conviction that science is the source of all progress and that the contribution of women is vital to its future,” says Lindsay Owen-Jones, chairman and chief executive of L’Oréal.
The programme has three aims: to encourage the participation of more women at all levels in the world of science; to work to reduce women’s present unequal access to the research professions; and to recognize the contributions of outstanding women in scientific research.
The winners will be selected by a jury chaired by Pierre-Gilles de Gennes, who won the Nobel Prize for Physics in 1991 for his work on the theory of soft condensed matter. The deadline for nominations is the end of August.
Water vapour supplies new climate clues
Water vapour absorbs radiation at many wavelengths, but it absorbs infrared and ultraviolet radiation – which are strongly emitted by the Sun – particularly well. The different energy transitions in molecules of water vapour allow it to absorb radiation of certain wavelengths, and when light is shone through this vapour and dispersed using a prism, this absorption shows up as dark lines in the resulting spectrum.
Many researchers have determined the positions and intensities of these absorption lines by measuring the electric dipole moment of molecules of water vapour. This quantity describes how charge is spread through the molecule, and determines how it absorbs radiation. But the electric dipole moment is tricky to measure using conventional spectroscopy, and the best calculations based on previous results cannot fully explain the observed absorption of solar radiation by water vapour in the atmosphere.
Now Rizzo and colleagues hope that their new results will help to resolve this discrepancy, and provide a benchmark against which to measure earlier calculations. In the first experiment of its kind, the team has measured the electric dipole moment of molecules of water vapour in highly excited states – the states they are in when they absorb solar radiation.
The electric dipole moment is directly linked to the ‘Stark effect’, in which the absorption lines of atoms or molecules split into several thinner lines when an electric field is switched on. Exploiting this effect, Rizzo and colleagues applied an electric field to a collection of water molecules and excited them with a laser pulse.
Stark splitting created a series of lines with very similar wavelengths, so Rizzo’s team used a ‘quantum beat’ method – analogous to the beat effect in acoustics – to measure their wavelengths. This allowed them to calculate the electric dipole moment extremely accurately. “This technique is quite novel and provides stringent tests of calculated dipole moments,” Rizzo told PhysicsWeb.
Rizzo and colleagues hope that their results will help scientists to determine exactly how solar radiation is absorbed in the atmosphere, and lead to a better understanding of energy transport in the atmosphere and its effect on the climate. The findings could also help astronomers to observe sunspots and to understand how light is absorbed by clouds of water vapour in the atmospheres of distant stars.
Electrons probe single atoms
The resolution of electron microscopes is superior to that of optical microscopes because high-energy electrons have a much shorter wavelength than light. In the 1970s Albert Crewe at Chicago University and colleagues invented the scanning transmission electron microscope, which builds up an image by scanning an electron beam across a sample and measuring the electrons that are reflected and emitted back from the sample. Crewe’s team successfully imaged single uranium atoms with the device.
Researchers have since improved on this technique, but the blurring – or aberration – of magnetic lenses has limited the resolution of electron microscopes to about 50 times the wavelength of the electrons used. For electrons with energies between 100 and 200 keV, this resolution is about 0.2 nm, which is slightly larger than the typical distance between atoms.
Now Batson and colleagues have reduced this figure to less than 0.1 nm by using electrons with energies of 120 keV – electrons with energies greater than this cannot be used because they would damage the material under study. This means that the resolution of the electron beam is good enough to investigate the environment of individual atoms.
“This advance means that the electron probe is smaller than the inter-atomic distances,” says Batson. “We can now look into the bulk of a material to locate atoms which might be out of place or which might be of the wrong kind.”
Batson and co-workers used a television camera to record ‘electron shadow images’ from a scanning transmission electron microscope and analysed these images with computer software to calculate – and correct for – the aberration of the microscope’s lenses. Using this arrangement they imaged gold atoms on a carbon substrate, observing single atoms and groups of atoms, both stationary and in motion. The IBM-Nion R&D team also demonstrated the technique on an alloy of germanium and silicon (see figure). The researchers say that this technique could also be used to image individual dopant atoms in semiconductors.
Earlier this year, Paul Voyles of Bell Labs and colleagues used an electron microscope to observe dopant atoms, but the resolution of their microscope – 0.16 nm – exceeded the inter-atomic distance in their sample.
“We believe aberration-correction technology combined with the inherent power of scanning transmission electron microscopes opens the way for smaller and smarter instruments to image and analyse materials at sub-0.1 nm resolution,” says Batson.
Correction: The original caption for the figure in this article was incorrect and was corrected on 27 September 2002.
Cosmologists collect 2002 Dirac medal
The ‘big bang’ theory of the 1960s stated that the Universe expanded rapidly shortly after its birth. But it did not explain how the Universe came to have its relatively smooth structure that is demonstrated by the uniformity of the cosmic microwave background. Based on the results of particle-physics experiments, Guth of the Massachusetts Institute of Technology was the first to propose the ‘inflation’ mechanism to explain how such uniformity could arise.
Linde of Stanford University and Steinhardt of Princeton University developed this idea further, successfully accounting for the existence of objects such as stars and galaxies in the otherwise uniform Universe. Experiments such as COBE and BOOMERANG have supported these predictions with their measurements of the cosmic microwave background, and have helped to make the theory of inflation a cornerstone of modern cosmology.
Guth, Linde and Steinhardt are recognized on the centenary of Dirac’s birth, an event that is celebrated today at the University of Bristol – in his home city – with a series of talks about his work and its impact on modern technology. The Edinburgh Fringe Festival is also staging special performances of Into the Antiworld, a ‘physical theatre’ show inspired by Dirac’s revolutionary prediction of the existence of antimatter.
Lasers lick dentists’ drills
Lasers are widely used in medicine to remove soft biological tissue, and scientists are keen to develop a laser-based tool to replace dentists’ drills. But hard dental material can only be removed by very powerful lasers.
Such high powers have been achieved in previous studies by using laser pulses lasting several picoseconds, or 10-12 seconds. These ‘heat ablation’ techniques proved unsuccessful because they were too hard to control – strong thermal shocks led to uneven removal of material and made the teeth crack.
But lasers have now been developed that emit more powerful bursts of radiation that last just tens of femtoseconds, or 10-15 seconds. Using such pulses, Rode’s team has successfully removed dental enamel without the excessive heating that can damage healthy tissue. In contrast with earlier attempts, the new technique removes matter by a process known as electrostatic ablation.
The team used two titanium-sapphire lasers that emitted infrared pulses with a frequency of 1 kHz. One laser emitted pulses lasting 95 fs with an average power of 0.5-0.6 W, and the other one emitted pulses lasting 150 fs with an average power of 0.8-1.0 W. Healthy human teeth – donated for medical research – were used in the study.
Rode and co-workers found that these laser pulses were powerful enough to eject electrons from atoms on the surface of the teeth, ionzing the atoms and molecules in the dental enamel. This ionization created a local electric field strong enough to remove the ions from the enamel altogether. Since the laser pulses are shorter than the characteristic heat conduction time of dental matter, there is no time for heat ablation – and its damaging effects – to occur.
The researchers admit that their technique removes enamel around a hundred times more slowly than mechanical drills, but they say that decayed dental material – which is softer than the healthy teeth used in their experiment – could be removed ten times faster than this.
Scientists push for laser links in space
Satellites and space stations will soon be gathering more data than they can transmit back to the Earth. If current trends continue, within five years only 0.3% of all collected data could be sent home.
“Our opinion is that in the long term, optical communications will be more efficient than radio telemetry,” said Andrew McGrath from the Anglo-Australian Observatory. “To deploy a deep space optical communications link on a 15-year timescale, the enabling technologies must be pushed for today.”
The root of the problem for radio-telemetry devices is bandwidth. Transmitting over more frequencies – that is, increasing the bandwidth – boosts the amount of information a signal can carry. But as the carrier frequency increases, atmospheric absorption becomes a problem.
With next generation spacecraft capable of collecting gigabits of information every second, the range of radio waves that penetrate the atmosphere is not enough to carry all the information home.
Lasers could be the answer. According to McGrath, more research is required to decide on ideal wavelengths and technologies. “We could take advantage of established technologies at 1300 and 1550 nm,” he says. “But shorter wavelengths offer high efficiencies in the long term.”
In November 2001, the European Space Agency established the world’s first optical data link between two orbiting satellites using a laser beam as a signal carrier. This test had a data rate of 50 megabytes per second.
The researchers claim that data transfer rates could be increased by several orders of magnitude without basic changes in principle. But this will require increased signal power, a suitable transmitter and adequate onboard memory. Well-separated mountaintop receiving stations around the globe will also be required. Six stations will guarantee cloud-free operation, says McGrath.
“Work toward a near-infrared telemetry system carries little risk and will pay for itself in the efficiency with which data can be gathered and transmitted,” the team conclude in their paper. “However, progress will only come about with the allocation of sufficient resources by NASA and ESA and the attention of the scientific community.”
At around 350m US dollars, the total cost is estimated to be comparable to that of a medium-sized space mission. Hawthorn and colleagues hope to win 10m Australian dollars to fund their research over the next five years.
Semiconductors stride ahead
Skin cancer changes the thickness of the layers of skin it affects, so measurements of this thickness can be used to spot tumours. Different layers of skin contain different levels of moisture, so skin thickness can be determined by measuring the amount of radiation each layer absorbs. Terahertz radiation – which has a wavelength of about a tenth of a millimetre – is ideal for this technique because it is absorbed by water and is much safer than X-rays.
Now Michael Johnston and colleagues from the universities of Cambridge and Sheffield have developed a semiconductor emitter that produces 20 times more terahertz radiation than existing devices, and could help tumours to be spotted more quickly. To achieve this, the researchers increased the emission of a semiconductor chip by attaching a prism to it.
In the US, a light source based on an atom or ion trapped in a semiconductor ‘cage’ has been developed by Rameshwar Bhargava’s team at Nanocrystals Technology, New York. These ‘quantum confined atoms’ are trapped in crystals just billionths of a metre across. When the trapped atoms are stimulated by another laser, they can emit as much light as phosphorescent particles a thousand times larger. Bhargava and colleagues are optimistic that their research will lead to advances in optoelectronics and X-ray imaging.
Meanwhile, Lars Samuelson and co-workers at Lund University in Sweden have created some of the first nanowires made from layers of different semiconductors. Using this layer structure – on which conventional electronic devices are based – the team has successfully made a ‘resonant tunnelling’ device from a nanowire.
“Our group is, to the best of my knowledge, the only one that has made functional electronic devices and investigated their electronic properties,” says Samuelson. The researchers hope that their growth techniques will allow them to build a range of electronic components using nanowires.
Semiconductors could also play a key role in the development of a quantum computer. In theory, such a machine could outperform conventional computers because it could carry out many processes at once. But a practical machine would require many identical quantum bits – or ‘qubits’ – that can take on two values at once.
“In a quantum gate, a controllable interaction is introduced between the qubits so that when the bits are addressed to perform a calculation, they are not affected individually like in a classical system, but all at once,” says team member Manfred Bayer.
One candidate for such a system is a pair of linked quantum dots. A quantum dot is a tiny region of semiconductor embedded in a different semiconductor. This dot can trap a single electron, which can have one of two spin states. Now a team led by Gerhard Ortner of the University of Dortmund in Germany has connected two such dots and shown that the spin states of the two electrons were linked.
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