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Home » Page 1883

Muon result could jeopardise Standard Model

The Standard Model describes how three of the four fundamental forces – the strong, weak and electromagnetic forces, but not gravity – affect subatomic particles. The anomalous results came from an experiment to determine how much the ‘spin’ on a muon moves or ‘precesses’ when placed in a magnetic field. The physicists injected an intense beam of muons into a powerful magnetic field and measured the so-called g-2 (‘g minus 2’) value. The g-factor of a particle relates its magnetic moment to its intrinsic angular momentum or spin. The g-factor of both the muon and its lighter sibling, the electron, are slightly larger than 2 due to various ‘radiative corrections’. Precise measurements of g-2 are powerful tests of theory.

The team found that the new value differed from the Standard Model by 2.6 standard deviations – in contrast with values previously obtained less rigorous experiments that exactly matched the predictions of the theory. “We are 99% sure that the present Standard Model calculations cannot describe our data”, said Gerry Bunce, project manager of the experiment.

The results can be interpreted in three ways. Firstly, the Standard Model is fundamentally correct but need to be extended: the team could, for instance, have glimpsed evidence of supersymmetry. This theory states that every particle has a companion particle called its superpartner. “Many people believe that the discovery of supersymmetry may be just around the corner”, said team member Lee Roberts, “We may have opened the first tiny window to that world”. Secondly, it is still statistically possible that the value is consistent with theory. Thirdly, the model may be incomplete or entirely wrong.

The collaboration of physicists from the US, Russia, Japan and Germany has collected muon g-2 data since 1997, and the surprise result comes from data collected between 1997 and 1999. “When we analyse the data from the year 2000, we’ll halve the level of error”, said William Morse, a member of the Brookhaven team. The final analysis is expected within a year and data from accelerators in China and Russia should reduce the error even further.

A new spin on magnets

When an energetic electron with a certain spin is injected into a ferromagnet – such as iron, cobalt or nickel – the spin on the electron partially aligns itself with the magnetic field. This means the magnet must be exerting a force on the spin. In turn, the electron exerts a force on the magnet to conserve angular momentum. The effect of a single electron on the magnetic field would be tiny, but Weber’s team found that a pulse of energetic electrons with identical spins packs a considerable punch.

The researchers injected electrons into a ferromagnet and checked their spins just femtoseconds – millionths of a billionth of a second – later. By measuring the degree to which the spins lined up with the direction of the magnetic field, they deduced how far the magnetic field vector moved – that is, an equal and opposite amount to the movement of the electron. In short, the electrons change the magnetization.

Weber’s team found that this so-called spin-transfer effect can produce an effective magnetic flux density as much as 1 tesla – ten times the value needed to switch the magnetization. The new technique also pinpoints the magnetic field to a very small area – the size of just a few atoms – which is an advantage in miniature devices. Existing devices rely on magnetic fields that decay slowly with distance – and this makes it hard to switch a single domain in a magnetic recording medium without disturbing its neighbours. The new method is particularly promising for high-speed recording because the pulse of electrons must be injected into the magnet in a tenth of a nanosecond, otherwise magnetic relaxation blocks the switching action.

The experimental system uses a beam of free electrons, but the next goal is to use electrons with lower energies in a more practical device. “Next, we aim to integrate the spin-polarized electron source, the ferromagnetic film and the spin analyser in a layered-film structure,” Weber told PhysicsWeb.

Physicists J C Slonczewski and L Berger independently discovered the ‘spin-transfer’ effect in 1996. Slonczewski’s patent – number 5695864 – on any future devices that exploit the effect is registered with the US Patent Office.

Uranium reveals the age of the universe

Cayrel and colleagues used the Very Large Telescope at the European Southern Observatory in Chile to measure the spectra of a very old star – known as CS31082-001 – near the edge of the Milky Way. Astronomers know that the star formed in the very early universe because it contains so little metal. Metals were scarce at this stage in the evolution of the universe because very few supernova, which create metals, had yet exploded. Indeed, the traces of uranium-238 in the star’s atmosphere could have come from just one supernova. The uranium-238 absorption lines are relatively easy to detect in metal-poor stars because they are not obscured by the strong absorption lines of other metals.

Astronomers know approximately how much of each element was originally present in the star and they can compare this with the abundances they see in the spectra today. They can then calculate – using the half-lives of the radioactive elements – how long ago the star was born. This is known as cosmochronometry.

Cayrel’s team has now used this technique to date the universe from the new uranium-238 signal from CS31082-001. The half-life of uranium-238 is only 4.5 billion years, so it has had time to decay to about an eighth of its original abundance. Taking into account uncertainties about the initial abundance of elements, Cayrel’s and co-workers have obtained an estimate of 12.5 billion – or 12.5 x 109 – years old, plus or minus just 3 billion years. This is three times more accurate than the previous best estimate, which was based on absorption lines of thorium-232. Thorium-232 has a half-life of 14 billion years – similar to our current best guess of the age of the universe, and this means it can only have decayed by about half. Accounting for our assumptions about the initial composition of stars, thorium-232 can only provide a rough estimate – 4 to 5 billion years each way – of the age of the universe.

Cayrel and colleagues hope that they will soon be able to date the universe even more precisely by measuring the relative line intensities more accurately. The possible range of ages for the universe will also narrow as we learn more about the ratios in which elements are created in stars. ‘We are already planning new surveys to discover other metal-poor stars in which we can measure the abundances of uranium and thorium,’ said Timothy Beers, a member of the research team based at Michigan State University. ‘In the next few years, we expect to find perhaps 10 or 20 of these stars.’

Other methods of dating the universe – such as measuring how quickly galaxies recede from us – are less reliable because they are based on untested assumptions about the evolution of the universe.

Neutron flashes may forecast earthquakes

Volodichev and Panasjuk measured levels of neutron emission in the Pamir mountains in Tajikistan – a seismically active region where the Indian and Eurasian tectonic plates meet – and found that they peaked at twelve-hour intervals. The researchers noticed that the variation in neutron flux coincided with the daily fluctuations of the moon’s gravitational pull. This prompted them to study the neutron bursts when these tidal forces are at their greatest – that is, when the Sun, Earth and moon line up – at the time of a new moon or a full moon. Volodichev and Panasjuk found that the neutron flow during these periods was around twelve times higher than the background level. This led them to believe that the tidal stress on the Earth’s crust opens up fissures through which radioactive gases and particles can escape. The radioactive material quickly decays, emitting alpha particles that contain neutrons.

To back their theory, the researchers analysed data collected over 28 years from the Pacific ‘ring of fire’ – a region of intense earthquake activity. They found that the most severe earthquakes took place around the time of a new moon or a full moon. “Our work suggests that neutron flashes and an increase in seismic activity are closely related, and are bound by tidal forces”, said Volodichev. “The prediction of earthquakes from neutron bursts is still in the development stage but it is very promising”.

Chirality crops up in nuclear physics

The nucleons – neutrons and protons – that make up a nucleus ‘pair up’ in a structure analogous to the electrons in an atom. Certain nuclei have an odd number of neutrons and protons, leaving two ‘spare’ nucleons orbiting the ‘core’ independently. If these nucleons orbit the same axis that the core spins around, the whole system is highly symmetric – and mirror images could not exist. But when the nucleus becomes ‘potato-shaped’ – it has a different diameter in all three directions – the nucleons split up and move in orbits that are perpendicular to each other and to the spinning core. The resulting three vectors of angular momentum can be oriented with respect to each other in two distinct ways – corresponding to two possible resultant spins for the nucleus – and Starosta’s team believes this is the key to the chiral nuclei.

To test the theory, Starosta and colleagues created samples of caesium, lanthanum, praseodymium and promethium – which all have the ‘spare nucleon’ structure – using heavy-ion induced nuclear reactions. The nuclei are in a wide range of excited energy states and emit gamma rays as they fall to lower energy states. Starosta’s team noticed that the nuclei emitted pairs of gamma rays with slightly different energies but the same amount of angular momentum. The best explanation for this tiny discrepancy is that left- and right-handed versions of the same nuclei produced the signals. The energies were found to be too similar to originate from completely different decay processes. “The role of chirality may also be important in other many-body systems”, Reiner Kruecken, a team member, told PhysicsWeb, “and we hope our work will inspire such investigations”.

Metal superconductors reach new high

The previous highest temperature for a metallic superconductor – niobium tin – was 20 K. Existing superconducting wires and tapes operate at around 16 K and rely on cumbersome liquid helium cooling systems. But a practical device such as a wire or tape made from the new compound – although still some way off – could be cooled using electrical refrigerators. “This is a very exciting discovery”, Colin Gough of Birmingham University told PhysicsWeb.

The Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity explains the behaviour of low-temperature superconductors very successfully, but has proved to be inadequate for high-temperature materials. “The challenge now is to establish whether superconductivity in the new compound is governed by the BCS theory, which is unlikely given its relatively high transition temperature, or if its origins are more obscure”, David Cardwell of Cambridge University told PhysicsWeb. The outcome may shed light on the mechanisms that underlie resistance-free current flow in other high-temperature superconductors.

Magnesium diboride is relatively easy to make – Gough joked that you can even buy it at the chemists – and physicists speculate that it may be possible to raise the superconducting transition temperature by lacing the compound with other elements. “We need to establish the nature of the charge carriers before thinking about which elements to use”, said Cardwell. “But high temperature superconductors – which are oxide in nature – contain predominantly copper, so this might be a reasonable place to start”.

American physicists John Hulm and Bernd Matthias tested many combinations of transition metals in their search for new superconductors in the early 1950s – with much success – but apparently overlooked magnesium diboride.

Twisters turn up the current in superconductors

Bhattacharya and colleagues studied the superconductor niobium diselenide, which – like other so-called type-II superconductors – displays this anomalous current peak. They applied a magnetic field to a crystal of the material and used a technique called scanning Hall probe microscopy to measure the varying magnetic response all over its surface. The team converted these measurements – taken every 5 millikelvin as the sample was cooled through the superconducting transition – into a series of ‘magnetic maps’ of the niobium diselenide crystal. “Our work provides a direct visualization of the dynamics of the peak effect”, Marchevsky told PhysicsWeb.

Under the influence of a magnetic field, wandering vortices can develop in the superconducting current, hindering the flow of current. But the vortices are less disruptive if they are static. Bhattacharya and colleagues found that their sequence of maps revealed two distinct and changing regions. These regions corresponded to two different behaviours of these vortices – a stable phase and a mobile phase – which have different current densities. In the stable phase, the vortices are strongly ‘pinned’ to defects in the crystal and are mostly static. In the mobile phase, the vortices are weakly ‘pinned’ and can move more easily.

These two vortex phases compete for space and occupy distinct regions of the crystal as it is cooled through its transition. Bhattacharya’s team believes that this interaction is the key to understanding the anomalous peak in type-II superconductors. “The complex dynamics of the phase mixture appears to be the primary cause of many unexplained anomalies in the peak region over the years”, said Marchevsky. Their results may also shed light on the second peak observed in the current profile of many high-temperature copper oxide superconductors.

Black holes go supersonic

Now Luis Garay, James Anglin, Ignacio Cirac and Peter Zoller at the University of Innsbruck in Austria have proposed a realistic way to make an artificial “sonic” black hole in a tabletop experiment (L J Garay et al. 2000 Phys. Rev. Lett. 85 4643).

In the February issue of Physics World, Ulf Leonhardt of the School of Physics and Astronomy, University of St Andrews, UK, explains how the simulated black holes work.

Enraptured by the stars

I remember arguing long and hard with a friend at a campsite in deepest Bavaria about how many stars one can see in the night sky with the unaided human eye. The answer, according to this charming new book by James Kaler, is about 8000 – so my original estimate was about right all along and my friend’s, as I recall, was way over the mark.

The Little Book of Stars by James Kaler, a professor of astronomy at the University of Illinois at Urbana-Champaign, is everything a popular-science book should be. It is thankfully short – about 180 small-format pages. It is a delight to read and is packed with accurate yet easy-to-digest scientific information. There are also none of the distracting thumbnail portraits of influential scientists that other popular-science books like to include to dilute the science.

Here the focus is purely and simply on the stars themselves: what they are made of, where they come from, how the live and die, how to study them, and how they affect our lives. Along the way we are also given a clear and lucid grounding in basic physics, including radioactivity, atomic structure, the gas laws, fundamental forces and so on.

Almost every page is beautifully written, with text that sometimes verges on the poetic. “New stars will someday form, carrying with them the ashes of the Sun – and perhaps the ashes of planets too,” concludes Kaler in the epilogue. Perhaps someday someone on a planet orbiting another star will tread on ground that is partially made of Earth. So it has gone and will continue to go for unimaginable time.”

Silicon lasers start to take shape

Devices made from silicon dominate the microelectronics industry, so silicon should be the material of choice for anyone designing new electronic devices that will be integrated with microelectronic circuits. This also applies to the optoelectronic devices that act as gateways between the electronic realm and the worlds of photonics and optical communications. Most optoelectronic components – such as waveguides and modulators – can be made from silicon, but a completely silicon-based system has remained elusive because there is a crucial missing link: a silicon light source.

A silicon laser made using “conventional” silicon-manufacturing technologies would be a disruptive new technology that could have a massive impact on the future of the semiconductor, IT and telecommunications industries. However, the electronic structure of silicon means that it is not good at amplifying light – a key characteristic of any laser medium. Now Lorenzo Pavesi of the University of Trento in Italy, and colleagues at Trento and the University of Catania, have taken a significant step towards a silicon laser by demonstrating light amplification in silicon (L Pavesi et al. 2000 Nature 408 440). However, several technical challenges must be overcome before this breakthrough can be transformed into a practical silicon laser.

Making the connection

Microelectronic systems have now reached the stage where their performance depends on the connections between the different chips and devices, rather than on the chips and devices themselves. Industry experts believe that within a decade the same problem will apply at the level of single chips. As semiconductor systems get ever smaller, the metal tracks currently used to connect the different components on a single chip will suffer increasingly from problems such as lack of speed and unacceptable levels of power dissipation.

Optical connections are an attractive alternative because they promise to eliminate these potential problems. However, the practical implementation of optical connections presents a major challenge, and the key problem has always been the lack of a semiconductor laser that is fully compatible with silicon microelectronics.

The electronic structure of silicon does not allow it to emit light readily. In a so-called direct-band-gap semiconductor, such as gallium arsenide, a photon is emitted when an electron from the conduction band falls into the valence band and “recombines” with a positive hole. The wavelength of the photon is determined by the energy difference or “band gap” between the conduction and valence bands.

Silicon, however, has an “indirect” band gap: this means that the minimum in the energy of the conduction band and the maximum in the energy of the valence band occur at different momenta. Therefore, electrons and holes can only recombine if a lattice vibration – known as a phonon – with the correct momentum is available. This makes the emission of a photon much less likely.

One way to force silicon to emit light is to illuminate it with a separate light source: this excites electrons from the valence band into the conduction band, from where they fall back down to the valence band – sometimes emitting photons in the process. This phenomenon is known as photoluminescence. Bulk silicon has a photoluminescence efficiency of much less than 0.01%. Moreover, the light is emitted at only one wavelength in the infrared part of the spectrum.

In 1990 Leigh Canham at the DRA Malvern laboratory in the UK showed that, under certain conditions, silicon can emit light efficiently, even in the visible part of the spectrum. This was first achieved using porous silicon, which has a band structure that is slightly different from bulk silicon. Today we know that very efficient (i.e. above 1%) photoluminescence can be achieved in many materials if they incorporate quantum dots or wires made of silicon.

Measuring just a few nanometres, these structures are known as nanocrystals and they have unusual electronic properties because their band gaps are wider than those of the materials from which they are made. In particular, electrons and holes become “localized” inside the nanocrystals, which increases the chance that they will recombine and emit photons. Competing recombination processes that do not result in the emission of light can also be suppressed by passivating the surface.


Lorenzo Pavesi

Now Pavesi and colleagues have produced optical gain in silicon nanocrystals made by implanting silicon in thin silicon-dioxide layers grown on silicon wafers. The nanocrystals, which measured about 3 nm across, are embedded in a crude waveguide. A laser then excites the structure, generating electrons and holes in the nanocrystals that recombine to emit light with a wavelength of about 800 nm, which lies between the visible and infrared regions of the spectrum. They reported amplification factors or “net modal gains” of 100 per centimetre in their structures. This means that spontaneous emission in the device is amplified by a factor of [l/(g – a)](e(g – a)l – 1), where l is the distance over which amplification occurs and g – a is the net modal gain.

In a different experiment, the “material gain” – the gain or amplification that would be measured if all of the beam passed through the gain region – reached 10 000 per centimetre. This is comparable with the best results for indium arsenide and other quantum-dot systems. This totally unexpected result is due to the very large concentration of nanocrystals, which more than offsets the smaller gain cross-section of each nanocrystal.

The researchers propose that the recombination involves silicon-oxygen double bonds located at the interface between the silicon nanocrystals and the oxide matrix. In their three-level model, pumping takes place from the valence band to the conduction band within the nanocrystals. The electron is then captured by the silicon-oxygen double bond, and emission occurs when the electron in the double bond recombines with the hole that is still in the valence band of the nanocrystal. Whether this model is correct remains to be seen.

Towards a practical device

So how can we exploit this phenomenon to make a working silicon laser? A laser consists of an optical-gain material sandwiched between two mirrors to form an “optical cavity”. The mirrors direct the emitted light back into the cavity to produce more light and, ultimately, create an intense, coherent beam. To achieve laser action, the optical gain must be high enough to outweigh the losses from the cavity.

How soon will we be able to make a silicon laser? First, Pavesi’s findings must be reproduced, and other groups are likely to take up the challenge soon. Next we must establish the conditions for maximum optical gain and optimize the device. It is also important to realize that Pavesi’s group achieved optical gain in silicon by exciting it with light, whereas any practical device will need to be driven electrically. The final hurdle in the race for a silicon laser could well be the challenge of finding a way to inject electrons or holes into silicon nanocrystals using an electrical current – and so far that has not been easy.

* A group at the University of Illinois claims to have seen evidence for the stimulated emission of blue light from individual silicon nanoparticles (see Physics World January p7, print version only)

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