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Home » Page 1796

Magnetism on the move

Many phenomena in condensed-matter physics are explained in terms of the “atomic washboard” energy potential that was introduced by Rudolph Peierls in 1940. This potential contains a series of peaks and troughs in three dimensions, with the peaks coinciding with the positions of the atomic planes in the crystal. However, the Peierls potential had never been probed directly until the latest experiments by Andre Geim and co-workers at the Centre for Mesoscience and Nanotechnology at Manchester University in the UK and the Institute for Microelectronics Technology in Chernogolokva, Russia.

The team studied the movement of the domain walls that separate regions of different magnetic polarization (e.g., north and south) in thin films of yttrium-iron garnet. These domain walls tend to be many atomic layers thick at room temperature. However, at the cryogenic temperatures used in the latest experiments they had thicknesses of only 11 nanometres – which is only about six times larger than the largest spacing between the planes in the yttrium-iron crystal.

Geim and colleagues used submicron Hall probes made from two-dimensional electron gases to follow the movement of the domain walls. These devices are extremely sensitive to small changes in magnetic flux, such as those caused by a small movement of a domain wall, and can be used to measure changes in the position of the wall. They found that the domain walls can become trapped between crystalline planes, and that they move through the crystal in a series of discrete jumps. The size of the smallest jump is equal to the magnetic period of the crystal in the direction of that the domain wall is moving in (about 1.75 nanometres).

However, theorists have predicted that the domain wall should also be able to move while it is trapped in a valley of the Peierls potential. By measuring the AC magnetic susceptibility of the system, the UK—Russia team found that it did indeed move by an average of 0.5 angstroms, although the details of this motion are not yet fully understood. The team has also detected atomic-sized “kinks” in the domain walls.

“The Peierls potential is a textbook phenomenon but it has eluded direct experimental detection for many decades until now,” Geim told PhysicsWeb. “A few years ago no one, including myself, would have believed that this was possible,” he says. In addition to opening up new avenues of fundamental research in condensed matter physics, the results could also lead to the development of new magnetic materials.

New doubts on dark energy

In February, NASA unveiled the first detailed full sky map of the cosmic microwave background – the microwave “echo” of the big bang. The data, which were collected by the Wilkinson Microwave Anisotropy Probe satellite (WMAP), supported the currently popular “concordance model” of the universe. This model predicts that the universe is made up of 5% ordinary matter, 25% undetectable “dark matter”, and 70% dark energy. Although the nature of dark energy is not yet known, galaxies in a universe with such a low density of matter should have stopped growing early in the history of the universe. They should therefore appear the same today as they did then.

David Lumb and colleagues at the Space Research Technology Centre in the Netherlands (ESTEC) have now measured eight distant galaxy clusters using the European Space Agency’s X-ray observatory, XMM-Newton. These clusters – the furthest of which is about 10 billion light years away – provide a picture of the universe as it was around seven billion years ago. Lumb and co-workers surprisingly found that galaxy clusters in the distant universe emit more X-rays than those in the near universe.

Moreover, a second group of physicists, led by Alain Blanchard at the Observatoire Midi-Pyrénées have analysed the data to demonstrate that the universe is a high-density environment that contains more matter than commonly assumed (arxiv.org/abs/astro-ph/0311381). “To account for these results you have to have a lot of matter in the universe – and that leaves little room for dark energy,” said Blanchard. The data, if confirmed, could have important implications for the concordance model and other fundamental assumptions about the nature of the universe.

Another superconductor shows up

Recent advances in superconductivity – the complete loss of electrical resistance in certain metals when cooled to low temperatures – continues to hold surprises. Three years ago, researchers discovered surface superconductivity in carbon-60 and, in 2001, bulk superconductivity in magnesium diboride. They have since found that certain elements, including boron and lithium, become superconductors under extreme pressures and that an alloy of plutonium containing cobalt and gallium can also superconduct.

Two years ago Zenji Hiroi and colleagues discovered the first pyrochlore superconductor – a class of material that usually has the general formula A2B2O7, where A and B are positive metal ions. Their material – Cd2Re2O7 – lost all electrical resistance when cooled to 1K. They then realised that replacing the rhenium with osmium would create a metal that becomes insulating at 225 K. It seemed that the number of d-electrons on the cations – two for rhenium and three for osmium – has a big effect on the properties of the material, as do electron correlations near the metal-insulator transition. This inspired them to create the new material KOs2O6, which is the second pyrochlore superconductor. It is a “defect pyrochlore”, having only six oxygen atoms and one A-type ion.

Hiroi and co-workers synthesized KOs2O6 from potassium oxide and osmium oxide. X-ray diffraction measurements showed that the compound crystallizes into the pyrochlore structure and the researchers observed superconductivity in measurements of resistivity and magnetic susceptibility. The team now hopes to perform measurements on single crystals of the material to further investigate its properties.

Radio waves tackle dehydration

It is normal for people to lose between 2 and 4% of their total body weight during intense physical activity because of the dehydration caused by sweating. However, severe dehydration – losing more than 8% of total body weight – can be fatal. Being able to measure hydration levels is therefore important when looking after, for example, malnourished children, the elderly or athletes. Although methods that rely on taking blood samples are highly accurate, they can be impractical in emergencies. Other methods, such as those that measure the electromagnetic impedance of the body, are not sensitive enough.

Shapiro and co-workers weighed 12 young male volunteers –average age 24, average weight 71.5 kg – before and after 30 and 60 minutes of exercise. At the same time, they observed how radio waves of different frequencies were absorbed by the subjects using a small radio-frequency absorption device placed on their wrists (see figure).

They found that the average loss in body weight was about 0.78 kg after 30 minutes of exercise, and 1.59 kg after 60 minutes. These results indicate levels of dehydration that are between 1 to 2.5% of total body weight. More importantly, they observed a definite correlation between weight loss and the absorption of radio waves by the subjects.

The Israeli team now hopes to repeat the experiment with female volunteers and people from different age groups to further establish the reproducibility and validity of the method.

Switching light on and off

Light travels at a speed of 300 million metres per second in vacuum, but in recent years physicists have managed to slow laser pulses down to speeds of metres per second in atomic gases, and to even stop light completely. Such experiments rely on a process known as electromagnetically induced transparency. However, in these experiments the gas only stores the signature of the laser pulse; it does not contain the actual photons.

Now, Mikhail Lukin and colleagues at Harvard University, the Harvard-Smithsonian Center for Astrophysics and the Lebedev Institute in Moscow have demonstrated a new method that does indeed store the photons at rest in a gas. First, they fire a short “signal” laser pulse into a hot gas of rubidium atoms that is also being illuminated by a strong “control” beam. The signal pulse is slowed down when it enters the gas, and a holographic imprint of the pulse is stored in the rubidium atoms when the control beam is turned off. Such experiments have been performed before, and the pulse is generally recreated by turning on the control laser again.

However, the Harvard-Moscow approach is different because it relies on two control beams travelling in opposite directions. In addition to recreating the signal pulse, the control beams also produce an interference pattern that, in the words of team member Michal Bajcsy, “makes the atoms in the gas behave like tiny mirrors”. The photons in the recreated signal pulse therefore bounce backwards and forwards between these “mirrors”, which means that the overall pulse essentially remains frozen in space. The pulse can be re-released by switching off one of the control beams.

“Earlier experiments on the storage of light stored only the ‘signature’ of the light pulses in a process somewhat similar to creating a hologram,” Bajcsy told PhysicsWeb, “so there were no signal photons present in the medium when the light was being stored. Our experiment, on the other hand, ‘traps’ actual signal photons inside the rubidium vapour in such a way that the overall signal pulse does not travel.”

Indium nitride springs a surprise

Almost all semiconductors have a near-surface region that is depleted of electrons. This means that any metal contacts to the semiconductor tend to have a high resistance, and that the electrons have to tunnel through a large Schottky barrier. This makes the fabrication of hybrid devices difficult. If semiconductors with an excess of electrons at their surface could be manufactured, it could be possible to overcome this problem.

Recently, indium nitride attracted attention when researchers found that its band gap is 0.7-0.8 eV, rather than 1.8-2.1 eV as previously believed. This means that alloys of indium nitride – such as indium gallium nitride – have band gaps that span the entire visible spectrum from the near infrared to the ultraviolet. Such materials could be used in a wide range of optoelectronics devices such as light-emitting diodes and solar cells.

Chris McConville and colleagues at Warwick University in the UK and Cornell University in the US analyzed the electron density profile of indium nitride using high-resolution electron spectroscopy and computer modelling. They studied samples that had been grown by molecular beam epitaxy and whose surfaces had been cleaned with hydrogen. The researchers found clear evidence for a layer of accumulated electrons on the indium nitride surface, and their results indicate that metal-semiconductor contacts would have low resistance and no Schottky barrier.

Potential applications for indium nitride include magnetoelectronic devices and semiconductor-superconductor hybrids, such as Josephson-junction field-effect transistors. “Coupling superconductors to semiconductors is problematic when the surface of the semiconductor is depleted of electrons,” said McConville. The team now plans to study the electronic properties of indium gallium nitride using the same technique.

Radar reveals asteroid force

The basic idea behind the Yarkovsky effect is that the surface of an asteroid is heated by the Sun during the day, and then cools off during the night. This means that the asteroid tends to emit more heat from its “afternoon side”, and less from the opposite side. The recoil from the afternoon side is therefore larger as well, and the overall effect is that a tiny, non-gravitational force acts on the asteroid.

The acceleration caused by this force is also tiny, but over millions of years its effect could become large enough to displace an asteroid from its natural orbit around the Sun. This might be enough to push the asteroid inwards from the “main belt” – which lies between Mars and Jupiter – and towards the Earth.

Steven Chesley of the Jet Propulsion Lab (JPL) in California and colleagues at JPL, Charles University in Prague, the Arecibo Observatory in Puerto Rico and the University of California at Los Angeles have now studied how the orbit of asteroid 6489 “Golevka” has changed over a period of 12 years. They analysed radar data taken using telescopes at Arecibo and the Goldstone Observatory in California, and found that the Yarkovsky effect has changed Golevka’s orbit by about 15 kilometres since 1991. Chesley and co-workers also calculated that the asteroid is just 530 metres in diameter, weighs about 210 billion kilograms and has a bulk density of 2.7 grams per cubic centimetre.

“Never before has the mass of a small solitary asteroid been measured,” Chesley told PhysicsWeb. “This allows us to determine the object’s density, which provides clues to its internal structure. In this case it suggests that the asteroid has a heavily fractured interior.”

The US-Czech team now plans to study other asteroids and hopes that its new method will eventually become routine when tracking these objects. The astronomers also believe that measuring the strength of the Yarkovsky acceleration is the only way to determine the mass and density of small (sub-kilometre) asteroids from Earth.

As far apart as ever


Take intellectual property, for example. Decades of angst about British inventions being commercialized abroad – liquid crystal displays, for instance – and more recent calls for universities to be more entrepreneurial have resulted in stand-offs between businesses and university researchers as they attempt to deal with each other.

This is one of the issues that has come to light in a review of business-university collaboration that is being carried out by Richard Lambert, a former editor of the Financial Times, for the UK Treasury. “A number of businesses reported that some universities had become too aggressive in their valuation of, and negotiations over, intellectual property,” Lambert writes in an interim report. “For their part, universities reported that business had become accustomed to receiving university intellectual property for free in the past and therefore had unrealistic expectations.”

As Lita Nelsen, director of the technology-licensing office at the Massachusetts Institute of Technology, pointed out recently, income from technology transfer is highly unpredictable – she likened it to a lottery – and universities should not rely on it as a major source of income. This is an area where the government could take action: universities should be provided with resources for technology transfer, but should not be expected to rely on revenue from such ventures to support their core missions of teaching and research. If a company pays for the research, it is entitled to expect most of the profits that ultimately result from the research.

On the other hand, if an academic comes up with money-spinning idea on their own, they can license the technology from a position of strength or form a spin-off company. Recent research shows that more and more academics are doing just this, and that the number of spin-offs as a proportion of spending on research is higher in the UK than in the US.

If Lambert’s interim findings are any guide, his final report – which should be published before the end of the year – is going to be more critical of the business world than the academic sector. “While the research output of British universities compares favourably with that of many other developed countries,” he writes, “relatively few British companies and companies based in Britain are research-led.” However, if British firms continue to consistently spend less on R&D as a proportion of turnover than their main competitors – and, remarkably, continue to claim that it is difficult to find out “who does what?” in universities – the anxiety about business-university interactions is going to continue.

Clear message for causality

Ever since Einstein stated that nothing can travel faster than light, physicists have delighted in finding exceptions. One after another, observations of such “superluminal” propagation have been made. However, while some image or pattern- such as the motion of a spotlight projected on a distant wall – might have appeared to travel faster than light, it seemed that there was no way to use the superluminal effect to transmit energy or information.

In recent years, the superluminal propagation of light pulses through certain media has led to renewed controversy. In 1995, for example, Günther Nimtz of the University of Cologne encoded Mozart’s 40th Symphony on a microwave beam, which he claimed to have transmitted at a speed faster than light. Others maintain that such a violation of Einstein’s speed limit would wreak havoc on our most fundamental ideas about causality, allowing an effect to precede its cause. Relativity teaches us that sending a signal faster than light would be equivalent to sending it backwards in time.

Obeying the speed limit
Obeying the speed limit

Now an experiment by Michael Stenner and Daniel Gauthier at Duke University in North Carolina and Mark Neifeld at the University of Arizona promises to shed fresh light on this century-old conundrum. The Duke-Arizona team has attempted to directly measure the “speed of information” by sending a message through a superluminal medium (M Stenner et al. 2003 Nature 425 695). However, the work could merely fan the flames of the dispute over faster-than-light propagation.

The speed of light

Special relativity tells us that no matter how much kinetic energy a massive object gains, its speed will never exceed the speed of light in a vacuum, c. This, at least, is generally accepted. However, the issue for light – which is an electromagnetic wave composed of massless photons – has never been quite as clear-cut. Special relativity says that massless particles always travel exactly at c, but one of the first things we are taught about light is that it travels slower in glass or water than it does in a vacuum.

This delay is due to the absorption and re-emission of photons by particles in the medium. When light hits an atom, the electrons begin vibrating at the optical frequency, and the electromagnetic fields that they re-radiate interfere with the original fields. Whether this interference delays or advances the phase of the waves depends on the properties of the atom and the frequency of the wave.

The fact that light might travel faster than it does in a vacuum caused great bewilderment at the beginning of the 20th century. But the issue was eventually clarified by Arnold Sommerfeld and Léon Brillouin, who showed that the “phase velocity” – the speed of the individual ripples on an idealized wave that extends to infinity in both directions – does not directly describe the motion of the energy in a light pulse. Instead, Sommerfeld and Brillouin realized that the propagation of a photon should be described by the speed at which the peak of localized packets of ripples moves – the “group velocity”. Even when the phase velocity is faster than light, the group velocity should be slower.

Despite this careful redefinition of the velocity of a wave, there are, in fact, still exotic situations where the group velocity can exceed c. However, these situations are characterized by a great deal of absorption and distortion, which led Sommerfeld and Brillouin to argue that the group velocity itself loses any meaning in such regions. They showed that any abrupt change in the pulse shape – which the person receiving the signal would not expect, and which would therefore carry new information – would travel not at this superluminal group velocity but only at c.

The speed of information

Experiments in the 1980s, however, showed that it was possible for the peak of a wave packet to arrive sooner than it would had it travelled at c. Most researchers agreed that the peaks of such smooth, predictable pulses carried no new information, since one could foresee the arrival of the peak from the shape of the earlier portion of the pulse. The modern version of Einstein’s law that no signal can travel faster than light was therefore left intact. Some researchers, on the other hand, pointed out that these “superluminal” pulses could well be used to trigger a practical detection system earlier than pulses that had travelled at c.

One obstacle to resolving the issue was that all known examples of superluminality involved the loss of most of the incident pulse, either through absorption or reflection. This loss degrades the quality of any signal, leaving open the argument that it could still take longer to accumulate information than in the case of slower, but lossless, transmission through the vacuum.

However, 10 years ago Raymond Chiao of the University of California at Berkeley proposed that under the right conditions an optical pulse might pass through a transparent (lossless) medium faster than light. Together, we suggested an experiment in which this could be observed. The idea was to pump a sample of atoms into an inverted state in which their optical properties are essentially opposite to those in normal matter, and then to tune the signal probe to a particular frequency that allowed it to propagate superluminally.

In 2000 Lijun Wang and co-workers at NEC in Princeton demonstrated this effect, proving that a pulse peak could exit a small vapour cell even before it should have had time to enter (see “No thing goes faster than light” and “Taming light with cold atoms”). This rekindled the embers of controversy, and much theoretical work has followed. However, until now no experiment has come any closer to closing the book on this issue.

The work of Stenner and colleagues extends the NEC work in two ways. First the researchers devised an improved way to pump a sample of potassium vapour into the special superluminal state, which allowed them to increase the relative size of the effect by a factor of five. Instead of an advance of only 1/50 of the original pulse width, as Wang and co-workers had achieved, their pulse leaped an astounding 1/10 of a pulse width ahead of one travelling at c.

Second, not content to work with infinitely smooth, predictable pulses, the researchers encoded a realistic “signal” on top of their light beams (see figure). This signal can represent either a “1” or a “0”, and the goal is to find the earliest moment at which a receiver would be able to determine whether a “0” or a “1” had been sent. Stenner and co-workers found that although the smooth pulse arrives noticeably earlier through the superluminal medium, the instant at which the “1s” and “0s” begin to differ does not seem to be accelerated. In fact, carrying out a very careful analysis of signal and noise, and eliminating nearly all spurious delays that the equipment itself might introduce, the team found that “new” information actually arrives somewhat slower than light.

New information

This result will be welcomed by mainstream physicists, who believe that Einstein’s speed limit will always be respected. But it is worth pointing out that in this experiment, as in any real-world situation, the detection of information has to be defined statistically in terms of how long it takes to reach a certain level of confidence about the content of the message. In reality, there is always some delay between the decision to send a message and the point at which the first photon leaves the transmitter. Similarly, it takes a finite time after the arrival of this first photon before any level of confidence can be achieved.

It is therefore easy to mistake a reduction in these latency times for true faster-than-light propagation. For example, a system that filters out some of the noise would make it easier to extract the message quickly. While the current experiment has done an excellent job of eliminating spurious effects, it would be easy to construct similar experiments that erroneously indicate superluminal information transfer. Furthermore, one could argue that such information transfer could have occurred but still be masked from view by mere technical noise.

Perhaps more important is the continuing uncertainty about how to truly pin down, even in theory, where this “new information” resides. Clearly, a signal cannot be spread out over all of history in a perfectly smooth pulse. On the other hand, truly discontinuous pulses are never observed in the real world and they are unpalatable even in theory. While theorists focus on geometric points where something unpredictable happens, experimentalists counter that no energy is contained in an idealized point, and that no information can be obtained until at least one photon is detected. Experiments like this force us – in this information-dominated age – to come to terms with the fact that we still do not truly know how to describe a task as simple as saying “yes” or “no”.

Newton and his non-Newtonian mind

Within the small world that is the history of science, biographies are about the only type of book that appeals to a wider readership. Books on the giants of science – especially Newton, Darwin and Einstein – are published in such numbers that they have almost become entire industries in their own right. Here, then, is another output of the Newton industry, but the product is not like most others.

Physicists will know James Gleick from his insightful books on chaos theory (Chaos) and Richard Feynman (Genius), and they will expect his portrait of Newton to be no less interesting. They will not be disappointed. The slim volume – a mere 190 pages of text – combines in a remarkable way a beautifully composed description of Newton and his time with a precise account of his pioneering contributions to natural philosophy.

It goes without saying that Gleick includes the essential moments of Newton’s life and career. He tells us about Newton’s fatherless upbringing, his early studies in Cambridge, and how he came to hit upon the idea of a force of gravity extending indefinitely from the Earth. (And, yes, the legend of the falling apple is there, too, only to be dismissed as vulgar and unnecessary.)

Gleick describes Newton’s invention of the reflecting telescope, his protracted controversies with Hooke, how Halley persuaded him to write Principia, and his nervous breakdown in 1692-93. Interesting chapters are devoted to Newton’s life after he went to London to become Master of the Mint and President of the Royal Society, and when he became involved, if only off the record, in the famous dispute with Leibniz over the invention of calculus.

Although Gleick does provide an intimate portrait of Newton’s life and personality, this is not where I see the focus and originality of his book. What distinguishes it from so many other popular works on Newton is its emphasis on his science and philosophy of nature. It is no easy matter to explain to non-scientists in an engaging way what Newton thought about optical phenomena, how he arrived at his law of universal gravitation, or why he felt alchemy to be so attractive. And it is an even more difficult task if the explanations are grounded in contemporary texts and avoid anachronisms and easy rationalizations. It is an art that Gleick masters admirably.

The reader is brought to understand, or at least to glimpse, the technical and conceptual problems that Newton wrestled with and how he, in most cases, solved them. As Gleick repeatedly points out, Newton’s thinking was – indeed had to be – constrained by a language that was inappropriate to express what he wanted to convey. Terms such as “force”, “mass” and “quantity of motion” (or what we now call momentum) did not exist, and so Newton had to construct them himself. Although “space” and “time” did exist, Newton gave them radically new meanings; in the process he created a vocabulary with a special meaning to natural philosophers. Of course, not all of Newton’s words and concepts were easily accepted. “Force” in particular was considered an occult, non-scientific quantity by both Cartesians and Leibnizians alike. So although the Cartesian philosopher and theologian Nicolas Malebranche found Opticks to be interesting, he thought that “Mr Newton is no physicist”.

During the Romantic era, Newton and the Newtonian conception of the universe came under attack from poets and artists such as Keats, Wordsworth and Blake. To them, and to like minds in German-speaking Europe (such as Goethe), Newton’s science was cold, deterministic and reductionistic. It had, they claimed, done away with the mysteries and spiritual dimensions of nature, turning it into a machine to be understood only by abstract mathematics.

Yet, as Gleick makes it clear, Newton was by no means a “Newtonian”. His universe had little similarity to a perfect machine, a fact for which Leibniz criticized him: would God have created something that was less than perfect? As Newton realized that there were chaotic, non-repeatable elements in the motion of the planets, so he denied that the world system could operate without the constant intervention of God.

The non- or pre-Newtonian elements in Newton’s philosophical project are even more pronounced in his works in optics and alchemy. Here, spirits, souls and powers abounded in a vision of nature that was more vitalistic than mechanistic. Had Blake and his fellow Romantics known about Newton’s obsession with chymistry, they might have evaluated him differently. They did not, but they did know about Opticks, where spirits and active, life-generating principles occur too, so their view of him was even more misguided.

Newton even suggested what later generations of physicists would call “anti-entropic processes” – active principles of a non-mechanical nature that would secure an eternal and vital universe. Without such principles, he wrote, “the Earth, Planets, Comets, Sun, and all things in them, would grow cold and freeze, and become inactive Masses; and all Putrefaction, Generation, Vegetation and Life would cease”.

Although Gleick’s account of Newton is at times critical, it is generally sympathetic. He stresses, for example, how enormously successful Newton was in establishing a physical world picture that we still share, more or less. However, on this point I think Gleick exaggerates. Whatever the genius of Newton, he had no idea of the two key concepts that would complete the classical world picture in the 19th century – the field concept and the notion of energy as a conserved quantity. It seems to me that Gleick goes too far when he suggests that the field concept can be traced back to Newton, and also that Newton “suspected” a unity between light and matter – something like an anticipation of Einstein’s equivalence of energy and mass.

Gleick’s Isaac Newton succeeds admirably in uniting readability with scholarship. It is intellectually stimulating and written in a formidable style that instantly carries the reader back to Newton’s time. Although the book is presumably aimed at general readers, in places it is quite demanding. It also has much to offer to readers who already possess a knowledge of physical science and its history.

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