The fascination of physics, for me, lies in its supremely exotic nature. Although it is a subject grounded in experimental test and rigorous verification, it nonetheless spans an awesome range of weird and wonderful concepts. The power of physics to describe the real world is persuasive, but it is its power to describe other worlds beyond our normal experience – worlds of antimatter and of hyperspace, of dilated time and infinite possibility – that is most potent.
At least, that’s how I see it. And I’m far from alone. According to a recent survey by the UK’s Particle Physics and Astronomy Research Council, it is precisely these areas of so-called “sexy physics” – cosmology, chaos, elementary particles and the like – that attract most undergraduate physics students to the subject.
So we arrive at university with these delirious visions in our heads of learning about quarks, quanta and quasars; about black holes and dark matter, white dwarfs and red giants; about superstrings, supernovae and the mystery of Schrödinger’s cat. But, of course, what our first-year physics lectures actually cover are things like oscillations, wave mechanics, magnetic and dielectric properties of matter, the kinetic theory of gases, the laws of thermodynamics and so on.
Now, these topics are obviously all essential components of an introductory course in physics – it would be impossible to go on to study, for example, quantum mechanics without first having done wave equations – and I am not for a moment suggesting that there is anything wrong with teaching them. The problem is that while these things are not exactly boring, they are not really all that exciting either, especially when compared with the glamorous topics we had in mind.
This clash between the quite mundane reality of first-year university physics and the glorious wonders we students had anticipated can have major repercussions. To use an astrophysical analogy, it is rather like a star about to go supernova: the actual course content is the inner core, while our great expectation of the subject is the unstable outer envelope – collapsing rapidly onto the core and generating a shock wave that can have catastrophic consequences.
Dull and dreary
I know of many students who started doing physics but then changed courses, so disillusioned had they become with the subject. I should stress that these people were not just “dossers”, who dropped out because they found the going too tough. They were bright students who could have made a contribution to the discipline. But the disappointment they experienced on seeing that the physics course did not match up to their expectations made them susceptible to the lure of other subjects, which seemed to promise the intellectual thrills they were seeking.
This is a great loss for physics. The intelligence and enthusiasm of these students were available for the subject to harness, yet these valuable commodities were thrown away by the solid but dull content of the first-year syllabus.
Less drastic than the drop-out cases, but almost as serious, is the number of remaining students who are drained of all of their enthusiasm for physics by their first-year experiences. Enthusiasm is a bit like virginity – once it’s been lost, it’s difficult to get back. These students are doomed to drag themselves through the rest of their degree courses without ever again really appreciating those imaginative qualities of physics that had once so attracted them.
This whole scenario of dropping out or just switching off seems even more unfortunate when one considers that the expectations-versus-reality conflict is largely a phenomenon specific to the first year of courses.
I chose to persevere with physics, despite the reservations induced by the culture clash I have just described, and I have found that in subsequent years the situation improved immensely. The second and third years generally offer a wider choice of modules along with deeper content, so I have at last been able to study those fascinating fields that attracted me to physics in the first place. The experimental side of the subject also improves over the duration of a degree course. There is quite a rapid progression from the basic (and often boring) practicals at introductory level to the research-orientated projects in the advanced stages. This too helps stimulate the interest of the students who stick with physics despite the initial setbacks.
Mind-bending changes
So a physics degree definitely does contain the exciting elements we had anticipated. The trouble is, there’s little or no indication of this in the first year. There is no doubt that this has to change. But how?
The most immediate thing that can be done is to simply place more emphasis on those topics already in the syllabus that do have the potential to be inspiring. A good instance of this is special relativity. Einstein’s theory is a truly mind-bending concept, which requires us to completely abandon our common-sense notions of time and space. But precisely because it is so challenging, this aspect of the theory is often merely glossed over, in favour of a more mathematical treatment.
Yet this is neglecting the very thing that makes the theory so exciting. Surely students would be stimulated more if they were given the chance to ponder such strange concepts as time dilation and the twin paradox? Quantum mechanics could similarly be treated in a more exciting manner, by describing and comparing the many different physical interpretations of what the theory “really means”. In this way, the true mystique of the subject could be revealed.
Physics at the frontier
To properly solve the problem, however, more radical action is needed. I suggest setting up a new course of “frontier physics”, which would be taken along with the main first-year physics courses, and which would contribute perhaps 10-20% of the yearly assessment. As the name implies, frontier physics would cover areas at the cutting edge of modern physics – the topics that have been demonstrated to particularly attract students to the discipline. The emphasis throughout would be on understanding the physical principles involved, rather than any detailed mathematics.
The course would be taught as a combination of lectures – some of which could be given by visiting experts in the field, since such events usually generate plenty of interest – and prescribed reading. Many excellent popular science books are now available that describe these areas of physics very well and do not require the reader to have too much mathematical knowledge. These books would be perfect background for this kind of course.
Students would then choose from this course a couple of topics on which to write short essays. This would give them the opportunity to learn more about the things that particularly interest them, while at the same time developing invaluable research and communication skills. Frontier physics would, I believe, go a long way towards bridging the gap between what first-year physics students expect and what they actually get.
However, this is just one idea as to what I, as a student, would like to have done in my first year. There are probably other ways of tackling the problem. But the important point is to recognize that there is indeed a problem, which must be solved if our subject is to continue to attract bright, keen students. We have to find some way of putting the “fizz” back into physics.
1 Operation of a conventional transistor The metal-oxide-semiconductor field-effect transistor (MOSFET) is the basic switching and amplification device of digital electronics. The current between the source and drain electrodes is controlled by the gate voltage, (a) When the gate voltage is zero, no conduction electrons are present in the channel, (b) When the gate is at a positive voltage, electrons from the source and drain accumulate in the area of the channel close to the gate, (c) As the gate voltage is increased further, the number of electrons in the channel increases until saturation is reached. The potential seen by the electrons is also shown along a line going from the gate to the channel. With no gate voltage, electrons in the channel experience a potential that is higher than the bias potential, shown by the dashed line (d). As the gate voltage increases, the potential in the channel gradually lowers and electrons accumulate there (e-f).
The invention of the transistor by John Bardeen and William Shockley in 1948 triggered a new era in electronics. Originally designed simply to emulate the vacuum tube, scientists soon found that this solid-state device could offer much more. The great potential of the transistor for speed, miniaturization and reliability has been fully exploited since well controlled materials such as pure single-crystal silicon became available. According to the latest “road-map” for the microelectronics industry, microchips containing one billion transistors and operating with a clock cycle of a billionth of a second will be on the market just a few years into the new millennium.
As transistors continue to shrink, a question naturally arises: will the quantum nature of electrons and atoms become important in determining how the devices are built? In other words, what will happen when a transistor is reduced to the size of a few atoms or a single molecule?
Researchers seeking to answer these questions have devised the so-called single-electron tunnelling transistor – a device that exploits the quantum effect of tunnelling to control and measure the movement of single electrons. Experiments have shown that charge does not flow continuously in these devices but in a quantized way. Indeed, single-electron transistors are so sensitive to charge that they can be used as extremely precise electrometers.
At the beginning
The most common transistor in today’s microchips is the metal-oxide-semiconductor field-effect transistor (MOSFET). Its operation is surprisingly simple: not much quantum mechanics is required to understand how it works, even though the size of a typical device is now just a few thousand atoms placed side by side.
Two conducting electrodes, called the source and drain, are connected together by a channel of material in which the density of free electrons can be varied – in practice a semiconductor (figure 1). A voltage is applied to the semiconducting channel through the “gate”, a third electrode that is separated from the channel by a thin insulating layer. When the gate voltage is zero, the channel does not contain any conduction electrons and is insulating. But as the voltage is increased, the electric field at the gate attracts electrons from the source and drain, and the channel becomes conducting.
2 Amplification in a field-effect transistor The current in a field-effect transistor varies with gate voltage and with the bias voltage between the source and drain. For a fixed bias voltage, the current is turned on when the gate voltage is positive, and turned off when the gate voltage is negative.
This field effect leads to an amplification mechanism in which the gate voltage can control the current flowing between the source and drain when a bias voltage is applied across these two electrodes (figure 2). The source-drain current is determined by the conductance of the channel, which in turn depends on two factors: the density of the conduction electrons and their mobility. The mobility of electrons depends on how often the electrons collide with crystal imperfections, and is essentially independent of the gate voltage. In contrast, the density of electrons is controlled directly by the gate voltage.
The transistor therefore works like a tap controlling the flow of water between two tanks, where the opening of the tap is set by the pressure of the water in a third tank. The difference is that electrons in the channel behave as a compressible fluid with a local density that depends strongly on the electric potential at that point. In other words, the electric field produced by the gate does not generate a “hard wall” for electrons inside the channel, but a smoothly varying potential that is modified by the presence of electrons (figure 1d-f ).
Note that we have made no reference to the wave-like properties of electrons, nor to the fact that the channel is made from individual atoms. The only quantum property that comes into play is the Pauli exclusion principle, which dictates that each possible state in the channel can be occupied by only one electron. This means that only a certain number of electrons can accumulate in the channel, and this sets a limit on the current flow.
However, the quantum properties of electrons and atoms will play a more important role as transistors are made smaller. For example, the wave-like nature of electrons will influence the way in which they travel through the channel. When the width of the channel becomes comparable to the wavelength of electrons (around 100 nm), electron propagation becomes more sensitive to the atomic disorder in the device, which is created in the fabrication process. This will pose a major problem if the reduction in size is not accompanied by an improvement in the atomic structure of the fabricated devices.
The technological constraints of moving towards the atomic scale may force us to adopt a new physical principle for achieving the transistor’s function. Alternatively, a new principle might be found that can provide functions that are not possible with current devices.
Towards single-electron devices
In 1985 Dmitri Averin and Konstantin Likharev, then working at the University of Moscow, proposed the idea of a new three-terminal device called a single-electron tunnelling (SET) transistor. Two years later Theodore Fulton and Gerald Dolan at Bell Labs in the US fabricated such a device and demonstrated how it operates.
Unlike field-effect transistors, single-electron devices are based on an intrinsically quantum phenomenon: the tunnel effect. This is observed when two metallic electrodes are separated by an insulating barrier about 1 nm thick – in other words, just 10 atoms in a row. Electrons at the Fermi energy can “tunnel” through the insulator, even though in classical terms their energy would be too low to overcome the potential barrier.
The electrical behaviour of the tunnel junction depends on how effectively the barrier transmits electron waves, which decreases exponentially with its thickness, and on the number of electron-wave modes that impinge on the barrier, which is given by the area of the tunnel junction divided by the square of the electron wavelength. A single-electron transistor exploits the fact that the transfer of charge through the barrier becomes quantized when the junction is made sufficiently resistive.
3 An electron in a box (a) When a capacitor is charged through a resistor, the charge on the capacitor is proportional to the applied voltage and shows no sign of quantization, (b) When a tunnel junction replaces the resistor, a conducting island is formed between the junction and the capacitor plate. In this case the average charge on the island increases in steps as the voltage is increased (c). The steps are sharper for more resistive barriers and at lower temperatures.
This quantization process is shown particularly clearly in a simple system known as a single-electron box (figure 3). If a voltage source charges a capacitor, Cg, through an ordinary resistor, the charge on the capacitor is strictly proportional to the voltage and shows no sign of charge quantization. But if the resistance is replaced by a tunnel junction, the metallic area between the capacitor plate and one side of the junction forms a conducting “island” surrounded by insulating materials. In this case the transfer of charge onto the island becomes quantized as the voltage increases, leading to the so-called Coulomb staircase (figure 3c). This effect was first observed by Philippe Lafarge and collaborators in our laboratory in 1991.
This Coulomb staircase is only seen under certain conditions. Firstly, the energy of the electrons due to thermal fluctuations must be significantly smaller than the Coulomb energy, which is the energy needed to transfer a single electron onto the island when the applied voltage is zero. This Coulomb energy is given by e2/2C, where e is the charge of an electron and C is the total capacitance of the gate capacitor, Cg, and the tunnel junctions. Secondly, the tunnel effect itself should be weak enough to prevent the charge of the tunnelling electrons from becoming delocalized over the two electrodes of the junction, as happens in chemical bonds. According to recent work by theorists at the universities of Freiburg and Karlsruhe in Germany, this means that the conductance of the tunnel junction should be much less than the quantum of conductance, 2e2/h, where h is Planck’s constant.
When both these conditions are met, the steps observed in the charge are somewhat analogous to the quantization of charge on oil droplets observed by Millikan in 1911. In a single-electron box, however, the charge on the island is not random but is controlled by the applied voltage. As the temperature or the conductance of the barrier is increased, the steps become rounded and eventually merge into the straight line typical of an ordinary resistor.
A single-electron transistor
The SET transistor can be viewed as an electron box that has two separate junctions for the entrance and exit of single electrons (figure 4). It can also be viewed as a field-effect transistor in which the channel is replaced by two tunnel junctions forming a metallic island. The voltage applied to the gate electrode affects the amount of energy needed to change the number of electrons on the island.
4 Principle of the SET transistor Like a MOSFET. the single-electron tunnelling (SET)transistor consists of a gate electrode that electrostatically influences electrons travelling between the source and drain electrodes. However, the electrons in the SET transistor need to cross two tunnel junctions that form an isolated conducting electrode called the island. Electrons passing through the island charge and discharge it, and the relative energies of systems containing 0 or 1 extra electrons depends on the gate voltage. At a low source-drain voltage, a current will only flow through the SET transistor if these two charge configurations have the same energy.
The SET transistor comes in two versions that have been nicknamed “metallic” and “semiconducting”. These names are slightly misleading, however, since the principle of both devices is based on the use of insulating tunnel barriers to separate conducting electrodes.
In the original metallic version fabricated by Fulton and Dolan, a metallic material such as a thin aluminium film is used to make all of the electrodes. The metal is first evaporated through a shadow mask to form the source, drain and gate electrodes. The tunnel junctions are then formed by introducing oxygen into the chamber so that the metal becomes coated by a thin layer of its natural oxide. Finally, a second layer of the metal – shifted from the first by rotating the sample – is evaporated to form the island.
In the semiconducting versions, the source, drain and island are usually obtained by “cutting” regions in a two-dimensional electron gas formed at the interface between two layers of semiconductors such as gallium aluminium arsenide and gallium arsenide. In this case the conducting regions are defined by metallic electrodes patterned on the top semiconducting layer. Negative voltages applied to these electrodes deplete the electron gas just beneath them, and the depleted regions can be made sufficiently narrow to allow tunnelling between the source, island and drain. Moreover, the electrode that shapes the island can be used as the gate electrode.
In this semiconducting version of the SET, the island is often referred to as a quantum dot, since the electrons in the dot are confined in all three directions. In the last few years researchers at the Delft University of Technology in the Netherlands and at NTT in Japan have shown that quantum dots can behave like artificial atoms. Indeed, it has been possible to construct a new periodic table that describes dots containing different numbers of electrons (see “Quantum dots” by Leo Kouwenhoven and Charles Marcus Physics World June pp35-39).
Operation of a SET transistor
So how does a SET transistor work? The key point is that charge passes through the island in quantized units. For an electron to hop onto the island, its energy must equal the Coulomb energy e2/2C. When both the gate and bias voltages are zero, electrons do not have enough energy to enter the island and current does not flow. As the bias voltage between the source and drain is increased, an electron can pass through the island when the energy in the system reaches the Coulomb energy. This effect is known as the Coulomb blockade, and the critical voltage needed to transfer an electron onto the island, equal to e/C, is called the Coulomb gap voltage.
Now imagine that the bias voltage is kept below the Coulomb gap voltage. If the gate voltage is increased, the energy of the initial system (with no electrons on the island) gradually increases, while the energy of the system with one excess electron on the island gradually decreases. At the gate voltage corresponding to the point of maximum slope on the Coulomb staircase, both of these configurations equally qualify as the lowest energy states of the system. This lifts the Coulomb blockade, allowing electrons to tunnel into and out of the island.
5 Counting electrons with a single-electron transistor The current flowing in a single-electron transistor increases with the bias voltage between the source and drain, and varies periodically with the gate voltage. For low bias voltages, current flows when the charge on the gate capacitor is a half-integer multiple of e, but is suppressed for integer multiples of e. Each time an electron is added to the gate, an electron tunnels into the island, which sets the field in the gate capacitor back to its initial value. Peaks in the conductance are observed for half-integer multiples of e, and minima are seen at integer multiples of e. For bias voltages larger than e/C, conduction occurs independently of the gate voltage.
The Coulomb blockade is lifted when the gate capacitance is charged with exactly minus half an electron, which is not as surprising as it may seem. The island is surrounded by insulators, which means that the charge on it must be quantized in units of e, but the gate is a metallic electrode connected to a plentiful supply of electrons. The charge on the gate capacitor merely represents a displacement of electrons relative to a background of positive ions.
If we further increase the gate voltage so that the gate capacitor becomes charged with –e, the island again has only one stable configuration separated from the next-lowest-energy states by the Coulomb energy. The Coulomb blockade is set up again, but the island now contains a single excess electron. The conductance of the SET transistor therefore oscillates between minima for gate charges that are integer multiples of e, and maxima for half-integer multiples of e (figure 5).
Accurate measures of charge
Such a rapid variation in conductance makes the single-electron transistor an ideal device for high-precision electrometry. In this type of application the SET has two gate electrodes, and the bias voltage is kept close to the Coulomb blockade voltage to enhance the sensitivity of the current to changes in the gate voltage.
The voltage of the first gate is initially tuned to a point where the variation in current reaches a maximum. By adjusting the gate voltage around this point, the device can measure the charge of a capacitor-like system connected to the second gate electrode. A fraction of this measured charge is shared by the second gate capacitor, and a variation in charge of ¼e is enough to change the current by about half the maximum current that can flow through the transistor at the Coulomb blockade voltage. The variation in current can be as large as 10 billion electrons per second, which means that these devices can achieve a charge sensitivity that outperforms other instruments by several orders of magnitude. Indeed, a collaboration between researchers at Yale University in the US and Chalmers University in Gothenburg, Sweden, recently showed that charge variations smaller than 10-5e can be detected in a measurement period of just one second and with a bandwidth of several hundred megahertz.
SET transistors have already been used in mesoscopic physics experiments that have required extreme charge sensitivity. For example, earlier this year Robert Westervelt and co-workers at Harvard University in the US used this type of device to measure the rounding of steps observed in a Coulomb staircase.
Electrometers based on SET transistors could also be used to measure the delicate quantum superpositions of charge states in a superconducting island connected by a tunnel junction to a superconductor. Such an island can accommodate not only several charge states corresponding to different numbers of Cooper pairs, but also coherent quantum superpositions of these states. Superconducting islands could therefore provide a means for implementing the quantum bits needed for a quantum computer (see “Fundamentals of quantum information” by Anton Zeilinger Physics World March pp35-40). The feasibility of the idea has been shown by experiments at Delft University, the State University of New York at Stony Brook, the PTB Laboratory in Germany, the NEC Laboratory in Japan and in our lab.
As we have seen, the charge sensitivity of the SET is ultimately linked to the fact that electrons traverse the island one at a time. In 1990 Bart Geerligs, Valerie Anderegg, Peter Holweg and Hans Mooij at Delft, together with Hugues Pothier, Daniel Esteve, Cristian Urbina and one of us (MD) at CEA Saclay showed that electrons can be counted one by one by creating devices that combine several SET transistors. And in 1996 John Martinis and colleagues at the National Institute for Standards and Technology in Boulder, Colorado, showed that a device called the electron pump can count electrons with an accuracy of 15 parts in a billion. The same group is now attempting to measure the charge of the electron with an accuracy better than 1 part per 10 million by combining an electron pump with a specially calibrated capacitor. Other metrology labs are aiming to use arrays of single-electron transistors to establish a standard for electric current.
The precision with which electrons can be counted is ultimately limited by the quantum delocalization of charge that occurs when the tunnel-junction conductance becomes comparable with the conductance quantum, 2e2/h. However, the current through a SET transistor increases with the conductance of the junctions, so it is important to understand how the single-electron effects and Coulomb blockade disappear when the tunnel conductance is increased beyond 2e2/h. In 1991 Konstantin Matveev, now at Duke University in North Carolina, drew a parallel between the suppression of the Coulomb blockade and the Kondo effect, in which magnetic impurities in metals are screened by conduction electrons. Experiments by Philippe Joyez and co-workers in our lab, and by Leonid Kuzmin and collaborators at Chalmers University have confirmed that quantum fluctuations in the charge of the tunnelling electrons reduce the Coulomb energy.
Towards room temperature
Until recently single-electron transistors had to be kept at temperatures of a few hundred millikelvin to maintain the thermal energy of the electrons below the Coulomb energy of the device. Most early devices had Coulomb energies of a few hundred microelectronvolts because they were fabricated using conventional electron-beam lithography, and the size and capacitance of the island were relatively large. For a SET transistor to work at room temperature, the capacitance of the island must be less than 10-17 F and therefore its size must be smaller than 10 nm.
This year two experiments have demonstrated that SET transistors can work at room temperature. Lei Zhuang and Lingjie Guo of the University of Minnesota, and Stephen Chou of Princeton University in the US fabricated a SET transistor in a similar way to a field-effect transistor with a channel just 16 nm wide. The fabrication process generated variations in the channel that act as tunnel junctions defining several different islands, and the behaviour of the device is dominated by the smallest island.
As expected from theory, the conductance of the device shows a series of peaks as a function of gate voltage. The Coulomb energy of the device is around 100 meV, which is large enough to reveal single-electron effects at room temperature. However, the Coulomb energy is too small to provide a large Coulomb gap, so the transistor is not sensitive enough to be used as an electrometer. Interestingly, the wavelength of electrons is comparable with the size of the dot, which means that their confinement energy makes a significant contribution to the Coulomb energy.
Meanwhile, Jun-ichi Shirakashi and colleagues at the Electrotechnical Laboratory and the Tokyo Institute of Technology fabricated a metallic SET from a very thin layer of niobium. Tunnel junctions were created by oxidizing areas of the niobium with the tip of a scanning electron microscope. The tip had almost atomic resolution, allowing the researchers to make very narrow oxide barriers between the island and the source and drain, and a wider barrier between the island and gate.
The team measured the current through the device as a function of both bias and gate voltages, and the results closely match theoretical predictions. The Coulomb energy of the device is 250 meV, which means that single-electron effects can readily be seen at room temperature. However, tunnel barriers fabricated in this way are highly resistive, which means that the current is about 100 times smaller than in devices operating at low temperatures. This problem also limits the usefulness of the device for electrometry.
Perspectives on the future
Researchers have long considered whether SET transistors could be used for digital electronics. Although the current varies periodically with gate voltage – in contrast to the threshold behaviour of the field-effect transistor – a SET could still form a compact and efficient memory device. However, even the latest SET transistors suffer from “offset charges”, which means that the gate voltage needed to achieve maximum current varies randomly from device to device. Such fluctuations make it impossible to build complex circuits.
One way to overcome this problem might be to combine the island, two tunnel junctions and the gate capacitor that comprise a single-electron transistor in a single molecule – after all, the intrinsically quantum behaviour of a SET transistor should not be affected at the molecular scale. In principle, the reproducibility of such futuristic transistors would be determined by chemistry, and not by the accuracy of the fabrication process. Last year a team led by Cees Dekker at Delft and Richard Smalley at Rice University in the US made a crucial step in this direction by observing Coulomb blockade in an island consisting of a single carbon nanotube.
It is not yet clear whether electronics based on individual molecules and single-electron effects will replace conventional circuits based on scaled-down versions of field-effect transistors. Only one thing is certain: if the pace of miniaturization continues unabated, the quantum properties of electrons will become crucial in determining the design of electronic devices before the end of the next decade.
Alcock and co-workers have been searching for MACHOs – massive compact halo objects – with a technique known as gravitational lensing. In this the gravitational field of a heavy object acts like a lens when it passes in front of a background star, magnifying the star’s light. Researchers have so far found 15 such events using background stars in a small neighbouring galaxy called the Large Magellanic Cloud (LMC). However, some astronomers claim that the heavy objects are actually stars in the LMC rather than MACHOs in our galaxy. They point out the unless the microlensing event occurs in a binary star system, it is impossible to determine how far the object is from Earth.
Alcock’s team claims that the objects they discovered have masses at least half the mass of the Sun, which is heavier than expected – suggesting, according to Gerry Gilmore of Cambridge University in the UK, that they are in fact stars in the LMC. Gilmore also points out that objects cannot be normal stars in the Milky Way, otherwise the Hubble Space Telescope would have seen them. However, he agrees that more events have been seen than would be expected if there were no MACHOs in the Milky Way, but adds that there are still problems associated with some of the candidates.
This view is shared by Alcock: “The favoured candidates are dim shrunken stars called ‘white dwarfs’, but there are problems with those.” A white dwarf is an earth-sized remnant of a star after it has ended its hydrogen-burning life. For the galaxy to be made mainly of white dwarfs in the past, there would have had to have been a large population of stars in a narrow mass range and astronomers see no evidence of this. Moreover, as Alcock points out: “When white dwarfs form there’s a lot of associated chemical ‘pollution’ in space, and we’re not seeing that.”
Another favourite dark matter candidate – failed stars called brown dwarfs with masses between those of Jupiter and the Sun – have turned out to be unexpectedly scarce. In the past it was thought that brown dwarfs were too faint to be detected. Improvements in astronomical technology now mean that this is no longer the case, but only a small number have been detected. “We expected dozens and dozens, ” says Chris Tinney of the Anglo-Australian Observatory, “but only twenty have been found in four years.”
Another possibility is that the lensing events are due to black holes that formed microseconds after the big bang. However, Michael Rowan-Robinson of Imperial College, London, thinks that this interpretation is unlikely and that it has been driven by the difficulty of explaining how white dwarfs can account for a large fraction of the halo. There is also a dispute about what fraction of the Galaxy’s dark matter can be accounted for by MACHOs. Rowan-Robinson, points out that estimates range from 0 to 100%, and that other teams have used the same data as that presented by the MACHO collaboration to derive a figure of 20%, rather than 50%.
According to Gilmore, the microlensing events suggest two possibilities: “Does this imply that 90 percent of the mass of the Universe is in a population of primordial black holes, which for some coincidental reason happen to weigh roughly as much as the Sun? Or does it mean that the Large Magellanic Cloud is somewhat thicker along the line of sight than was assumed? The first of these ‘explains’ the dark matter problem. The second does not, but favours a particle physics answer.”
And Ken Freeman of Mount Stromlo Observatory cautions that even if we solve the dark matter problem for our Galaxy, there is a separate problem for the Universe. Indeed, as much as 99 percent of the universe could be ‘dark’. “There is a great deal of dark matter out there”, says Freeman, “and conventional cosmology still tells us that most of it is not ordinary matter.”
Gilbert Collins and colleagues used the NOVA laser at the Lawrence Livermore National Laboratory in California to carry out the experiments (Science281 1178). The deuterium was placed in a small copper cell with a plunger fitted at one end. When the laser hits the plunger, it sends a shock wave through the chamber. By shining a beam of X-rays through two windows in the cell, Collins and co-workers were able to measure the shock speed – and hence the pressure and density of the deuterium – in the few nanoseconds before the sample flew apart. Another laser measures the reflectivity of the deuterium, which increases when the molecular fluid changes into a metallic fluid.
Collins and colleagues discovered that deuterium compresses more easily than predicted by theory. “A lot of their data disagrees with every [theoretical] model available” writes David Ceperley, a theorist at the University of Illinois, in an accompanying article. The data might be able to explain why Saturn is so warm, and why Jupiter has such a strong magnetic field.
Speculation on the origin of high energy cosmic rays has existed for some time. The best candidates are massive supernova explosions or the decay of superheavy particles near the beginning of the universe. The AGASA results show that the energy spectrum extends beyond 1020 eV, but they have been unable to identify sources for any of the six highest-energy rays they have detected – although they do know that the sources must be within 50 megaparsecs of the solar system, which is “nearby” in astrophysical terms.
The team says that the results lead “highest energy cosmic-ray physics into a much more exciting stage” and that these mysteries might be solved by a new generation of cosmic-ray experiments such as the Telescope Array, the High Resolution Fly’s Eye and the Auger project.
Signals produced by the satellites are over a 100 times stronger than the radio sources studied by astronomers. Under the new agreement, European radio astronomers will get 24 hours of ‘unpolluted’ observation time from 1 January 2006. However, they will have to negotiate for ‘restricted’ observation times with Iridium until that date. The CRAF has also agreed to help to try and find ways of improving Iridium’s satellite transmissions to reduce leakage from the network.
Earlier this year, the Arecibo radio telescope in Puerto Rico signed a separate time- sharing agreement with Iridium, which was seen by many in the radio astronomy community as too favourable to the company. European governments, on the other hand, refused to grant operating licences to Iridium – which goes operational next month – until they had reached an agreement with the CRAF.
Despite this agreement, interference from satellites remains an increasing threat to astronomy. “This is not an isolated problem, ” says Jim Cohen, chairman of CRAF. “The number of cases of interference to radio astronomy from satellites is growing steadily. Unless the protection of radio astronomy is taken into account early in the design of new satellite systems, our science could face a difficult future.”
Previously satellite measurements have suggested that the atmosphere was cooling at 0.05 Kelvin per decade, while ground-based instruments showed an increase of 0.13 Kelvin per decade. However, as polar satellites orbit the Earth, they suffer atmospheric drag. The thickness and height of the Earth’s atmosphere is dependent on the sun’s solar activity. When solar activity is at a maximum, ultraviolet rays from the sun heat the upper atmosphere. This causes the atmosphere to expand outwards, which increases the drag experienced by satellites. Wentz and Schabel calculated that this caused the orbits of polar satellites operated by the National Oceanic and Atmospheric Administration (NOAA) in the US to decay by 1.2 km per year over a 17 year period.
They also discovered that the Microwave Sounding Unit (MSU) used to measure the troposphere temperature profile is very sensitive to height fluctuations. By developing a correction factor for the instrument, they were able to re-evaluate the satellite data. They found that the instrument now measured a temperature rise of 0.07 Kelvin per decade instead of the previously reported fall. This new figure is similar to results predicted by computer climate models and explains why the satellites had seen increased cooling in the troposphere during periods of increased solar activity in 1979-83 and 1989-92.
The technique uses an iron-silica catalyst created by coating a quartz plate with an iron-silica solution. Once the solution dries it cracks into a film 30-50 microns thick. After the substrate is heated in a vacuum and then in a hydrogen-nitrogen atmosphere to make sure that it becomes chemically unreactive, the film breaks into large quantities of iron-silica nanoparticles. The researchers noticed that the nanotubes became longer if the acetylene was passed over the catalyst for a long period. By increasing the length of time still further, they hope to exceed their new record. Nearly all the nanotubes produced by this method grow in parallel arrays from the catalyst surface, making them easy to remove.
Meanwhile, a team of French and Chinese scientists has constructed nanotubes that can act as’coaxial cables’ (Science281 973). Zhang and his colleagues achieved their breakthrough with nanotubes 50 micrometers long and a few tens of nanometers in diameter. They managed to form a nanotube with three ‘jackets’ each with different conducting properties. Such behaviour will make nanotubes prime candidates for the developing nanoelectronic components. They found that if they used a selection of boron nitride, carbon, silicon oxide and lithium nitride compounds, the nanotubes form inside each other in a series of 3 layers. A core layer of silicon carbide (semiconducting); an intermediate layer of amorphous silicon oxide (insulating); and a shell of boron nitride and carbon (semiconducting). The authors speculate that by adjusting the quantities of the starting compounds, it may be possible to build self- organising nanocables with different components inside them.
Most automobile vibration testing is sinusoidal in nature, in which a regular pattern of rapid, slow and zero vibrations vary in a predictable pattern to mimic driving conditions. Hu’s random vibration testing is more realistic. It uses several random processes to mimic different driving routes and styles – and hence mimic the stresses on the car. Although manufacturers have increased the speed of their vibration testing to accelerate the testing process, such tests do not consider the effect of a variable load on the car. Adding random vibrations to the accelerated test compensates for this deficiency.
Hu’s new method also allows the user to specify the frequency range of the tests. This allows researchers to study the individual behaviour of components inside a car, and how they interact in a specific frequency range – just in case certain frequencies cause the structure to fail.
“This is the best news I’ve heard since we lost contact with SOHO” says Roger Bonnet, ESA’s director of science. “We should just hope that the damage sustained by SOHO’s enforced period of deep freeze does not affect the scientific payload too much.”
SOHO is a $1 billion joint venture between the European Space Agency (ESA) and NASA to observe the Sun in unprecedented detail. Scientists were hoping to extend SOHO’s life until 2003 to cover an upcoming period of maximum solar activity. However, contact with the craft was lost on June 25 when a programming error caused the craft to spin out of control.
