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The social physicist

What made you decide to apply statistical physics to social problems?

As a physicist, I have studied many different types of physical system using statistics, and in particular I’ve had a longstanding interest in how magnets become ordered at low temperatures. This ordering is akin to reaching consensus in a socially interacting population, where political preference is analogous to the orientation of a magnetic moment. I realized that there are in fact lots of societal phenomena that can be analysed using statistics, with the interactions between individuals being analogous to the particle interactions in a physical system. The human world provides such a rich laboratory that I can see statistical physics almost everywhere that I look.

But what exactly allows you to make this link between physical and societal phenomena?

I try to understand the collective behaviour of entire populations by postulating just a few simple interactions between individuals

Statistical physics allows you to study the macroscopic properties of a system of many interacting particles without having to track the behaviour of every single particle. For example, to understand the properties of air in a room you don’t have to care about the motion of every single molecule in that room, even though in principle you could do this. You might instead want to know how the temperature in the room can vary, or what determines wind velocity through the room — these kinds of things can be described using statistical physics. The crucial idea here is one of emergence — that from a few simple rules governing the interaction of individual particles you can end up with collective behaviour that is not described by the rules themselves. So in the context of society, I try to understand the collective behaviour of entire populations by postulating just a few simple interactions between individuals and calculating the outcome of millions of such interactions using computer modelling.

How have you used this approach to model the way people vote?

Part of my work is based on the so-called voter model: an idealized model that assumes individuals cannot make up their own minds about how to vote but simply mimic what their neighbours are doing. This involves placing voters, as if they were simply points, on the nodes of a regular lattice. So using a square lattice, for example, everyone has four neighbours with whom to interact. The model evolves by picking any person on the lattice at random and then assigning that person the voting preference of their nearest neighbour. Depending on exactly how you use the model, the initial state may be random or structured.

How does this simple voter model evolve?

When you run the model, you are guaranteed that the system will eventually reach consensus. In other words, given a choice of two parties to vote for, A or B, the whole population will end up voting for either party A or party B. This model is “conservative” in the sense that by running the system many times it will sometimes end up in state A and at other times state B, but the ratio of these two states exactly matches the initial configuration of the system. So, taking the US for example, if we set our system up so that initially 55% intended to vote Republican and 45% sided with the Democrats, then 55% of time the system will reach a Republican consensus, while 45% of the time it will end up with everyone voting Democrat. In other words, on average no one changes their mind.

But in reality surely there are some people who do think for themselves?

Just a few zealots can completely screw up the system

Indeed. To make the model more sophisticated we have looked at what happens when you make some of the voters “zealots”, which means that they never change their mind, and, at the other extreme, others “vacillators”, who find it very hard to make their mind up and need to hear the opinions of several other voters before changing their intention. In fact, it turns out that the introduction of these types of voter is fundamental to the outcome of the system. Just a few zealots, for example, can completely screw up the system. For one thing, a few zealots on either side of the party divide make it impossible to ever reach consensus because by definition they will never change their mind. In fact, just a handful of zealots in an infinite system causes the system to end up in a more or less 50:50 split between the two parties regardless of the initial distribution of voting intentions.

Do you plan to test your models against real election data?

Up to now we have not been able to say too much about real voting because we have been looking at models that we can solve analytically. But in the near future we hope to do some reverse engineering, which means that we will look at past election results and infer how people came to their decision — in other words, we will work out what fraction of people were likely to have been easily swayed and how many never changed their opinion etc.

Could your research be of practical help to people who set the rules for elections?

Statistical physics cannot be used as a predictive tool, but rather it is a descriptive tool

That’s a tricky question. If I were to study the distribution of people’s heights and find, for example, that it was a Gaussian curve peaking at a figure of 5’10”, then this information might be useful for designing doors on subway trains so that people don’t hit their heads too often. With voting, we can hopefully say something about the distribution of certain kinds of election result for certain types of election. For example, the model might be able to tell us what the chances are that a landslide victory will occur. However, it is very important to emphasize that statistical physics cannot be used as a predictive tool, but rather it is a descriptive tool.

What other systems have you investigated using applied statistical mechanics?

I have used it, for example, to look at the class structure in society; to assess the importance of scientific papers; and to study the relationship between climate change and record-breaking temperatures. And recently I have also investigated the statistics of baseball by trying to identify patterns of winning and losing. We have built a model to predict on average how many wins the first-placed team will record, how many the second-placed team will achieve, and so on, and we have found that our predictions very closely match end-of-season data averaged over more than a century. We can also use this same theory to successfully predict the distribution of teams’ winning and losing streaks.

But is there a danger that you can take this reasoning too far? Can’t statistics only tell us so much?

Certainly in baseball people have gone crazy in trying to quantify the game. Score is the most basic statistic, but then people have introduced many secondary and tertiary statistics, such as an individual’s batting performance in a specific type of game situation. But statistical-physics ideas are best suited to the simplest and most fundamental measures, such as the number of wins and losses, or the score.

Have you collaborated with social scientists?

I’ve had discussions with various sociologists and pure statisticians. But these discussions can be difficult because of the technical jargon on both sides and also the fundamental question of what turns people on. The notion of emergence in complex systems is fundamentally important to me as a statistical physicist, whereas some sociologists couldn’t care less about that. Often they are interested in concrete issues such as who is going to win the next election or whether the minimum wage should be increased. I am, however, still trying to make connections with these people.

LHC set for July start up

Engineers at CERN are making the final touches to the Large Hadron Collider (LHC) — the biggest experiment in particle physics — and expect to have it running in the first half of July. Although the start-up schedule of the European particle accelerator has slipped by over a month since the last official announcement, there appear to be only minor problems left to resolve.

“It has been some time since we’ve been in this kind of position with this kind of research facility,” says James Gillies, a spokesman for CERN. “There’s real confidence that we’ll be collecting new data this year. It’s a very big time for us.”

When all is done and dusted, the LHC will have cost around $6.3bn to build. Some 6000 superconducting magnets will whip proton beams in opposite directions around a 27 km-long ring and smash them together at energies bordering on 14 TeV. The impacts will generate a hoard of new particles, possibly including the highly anticipated Higgs particle and so-called supersymmetric particles. But regardless of what is or is not detected, it is almost certain that the LHC will provide a window onto new physics.

Until recently, the official line from CERN was that the first proton beams would be injected into the ring in May, despite status reports from the LHC website suggesting otherwise. According to Gillies, previous problems have now compelled CERN to set back the start up to the first half of July. An official date will be announced sometime after mid-June, the earliest time that all the magnets can be cooled to their operating temperature of below 2 K.

There’s real confidence that we’ll be collecting new data this year. It’s a very big time for us James Gillies, CERN spokesman

Latent problems

The main problem that has dogged the LHC start-up schedule of late erupted with a bang this time last year, when one of the “quadrupole” magnets used to focus and manipulate the proton beams failed during preliminary tests. Fermilab, the US laboratory who manufactured the magnets, was quick to accept responsibility, but it soon became apparent that all similar magnets would have to be redesigned and replaced. CERN is still reeling from this overhaul, having had to delay the cooling of magnets and skip the low-energy test runs that were due to take place before winter.

There have since been other, less serious problems. Towards the end of last year CERN found that certain “copper fingers” used to ensure electrical continuity between magnets had buckled when the magnets were warmed up. Presently, LHC engineers are having a few difficulties with leaky plumbing of liquid helium, which is used to cool the magnets. “Superfluid helium has no viscosity, so it can find any cracks,” explains Gillies.

Even though proton beams will not enter the LHC before July, by May 21 the beams will be running through two of CERN’s existing particle accelerators, which are serving as preliminary accelerator stages. The Proton Synchrotron, built in the late 1950s, will speed the protons up to 25 GeV and feed into the Super Proton Synchrotron, built in the 1970s, to get them up to 450 GeV.

Wide media coverage

On the day when the LHC is ready to have its proton beams injected, onlookers can expect wide media coverage. According to Gillies, they will inject the first beam in one direction at 9:30am (central-Europe time) to tie in with a live broadcast from BBC Radio 4’s Today Programme. Visuals will show the beam’s progress while CERN scientists analyse it. Every 10 minutes, they will be able to send in another beam. “Hopefully we’ll get one circulating all the way round by the end of the day,” says Gillies. Once they have understood the circulation in one direction, the scientists will begin experimenting with counter-circulating beams. “Then we’ll ramp up the energies,” he adds.

In light of the huge public interest in the LHC, CERN is holding an open day for the accelerator on April 6. From April 2–7 the lab is also allowing US high-school students to visit and report back their experiences via blogs and videos.

It appears that the pangs of excitement are beginning to be felt at CERN. Still, few of those involved are counting their chickens just yet. “We have to cool the whole machine down first,” says LHC project leader Lyn Evans. “I hope that that can be achieved by mid-June so we can start taking data in July.”

Quantum logic gate is miniaturized

Researchers in the UK have taken a small but important step towards the creation of practical quantum computers by creating the first logic gates on a silicon chip that can process individual photons. The chip, which measures several millimetres across, reproduces an earlier version of the gate that occupied several square metres of space on an optical bench.

Individual photons of light show great promise as quantum bits of information (qubits) in a quantum computer because they can travel great distances through optical fibres or even air without losing their quantum nature. One reason for this is that individual photons of light do not normally interact with each other. However, this makes it hard to create devices for processing quantum information such as logic gates, which rely on the interaction of two or more photons.

Target and control

In 2003 Jeremy O’Brien at the University of Bristol and colleagues in Australia built the first controlled NOT (CNOT) quantum logic gate for single photons. A CNOT gate has two inputs – “target” and “control” – and is considered a fundamental building block of any quantum computer. If no photon enters the control input, which corresponds to a “0”, then the logical state of the target (“0” if no photon enters or a “1” if a photon enters) remains unchanged. However, if the control is set to 1 (i.e. a photon enters) the state of the target is flipped from 1 to 0 and vice versa.

However, O’Brien’s first gate was made using conventional optical components such as mirrors and beam splitters and was spread out over a laboratory bench – not very practical for making a quantum computer, which would require thousands of such gates.

Silica waveguides

Now, O’Brien along with Alberto Politi, and colleagues at the University of Bristol have built hundreds of versions of the same gate in a millimetre-sized piece of silicon (Sciencexpress). Instead of mirrors and beam splitters, they used coupled waveguides – micrometre-wide channels of transparent silica that can be created in silicon using a well-established industrial process.

A controlled NOT gate

Each gate contains six parallel waveguides that are normally separated by tens of micrometres of silicon so that the photons do not interact. However, at certain points along the device, pairs of waveguides are brought very close together so they are separated by a distance on a par with the wavelength of the photons (about 800 nm).

At these points, some of the light can leak from one waveguide to the other in a process called evanescence. This effect is widely used in commercial optical communications devices that split a beam of light into two beams.

However, if two photons travelling in separate waveguides come close together at the same time they experience quantum interference. At this point the two photons become entangled — a feature of quantum mechanics that allows particles to share a much closer relationship than classical physics allows. The CNOT gate works by performing a series of three such entangling processes on the target and control photons as they move through the chip.

Although the devices were shown to be very good at entangling photons, O’Brien told physicsworld.com that they have an inherent success rate of 1/9, which means that nearly 90% of the input photons are not processed correctly. In principle, these incorrect results could be rejected by using verification qubits – other photons that interact with the target and control qubits via additional coupled waveguides — and determine whether the CNOT process was successful.

Verification schemes

The Bristol team are currently working on implementing verification schemes and also exploring how photon sources and detectors – which are currently off-chip – could be integrated within the chip.

O’Brien described the gates as “a very important step [towards practical quantum computers], but a very small step”.

NASA science director quits

The chief executive of NASA’s science mission directorate has decided to quit the agency after less than a year in the post. Alan Stern will be replaced temporarily by Edward Weiler, the director of NASA’s Goddard Space Flight Centre. Stern, who declined to comment, has not yet publicly revealed the reasons behind his decision.

In a statement released yesterday, NASA administrator Michael Griffin lauded the achievements of Stern, in particular his contributions as principal investigator for the New Horizons mission to Pluto and being a member of the NASA advisory council. “While I deeply regret his decision to leave NASA, I understand his reasons for doing so, and wish him all the best in his future endeavours,” the statement read.

Stern was appointed the role in April last year after his predecessor, Mary Cleave, retired. At the time Stern vowed to make the best of NASA’s festering budget problems, but in certain areas they have gotten worse. At the end of last year US Congress let support for basic and applied research remain static at $3.4bn, which — bolstered by mounting research and development costs for the forthcoming Mars Science Laboratory — have forced the agency to cut back costs for current Mars rovers. In February this year the Bush administration requested the science budget slip by almost $265m.

Many contributions

Edward Weiler

Stern, 50, received his PhD in astrophysics and planetary atmospheres from the University of Colorado at Boulder in 1989. Since then he has contributed widely to planetary physics, writing over 175 papers, 40 popular articles and two books, The US Space Program After Challenger (1987) and Pluto and Charon: Ice Worlds on the Ragged Edge of the Solar System (2005). Pluto has been one of his primary areas of study, and in 2006 remarked that he would still refer to it as the “ninth planet” in spite of the International Astronomical Union’s decision to reclassify it as a dwarf planet.

It is understood that Stern will remain involved with his present science missions, including being principal investigator for New Horizons, which is expected to reach Pluto in 2015.

Weiler, who has served as the director at Goddard since 2004, will serve as interim science director. He has a PhD in astrophysics from Northwestern University, and has had responsibilities including being chief scientist for the Hubble space telescope between 1979 and 1998, and being director of the Astronomical Search for Origins programme.

Graphene makes for better optical displays

Graphene may be just one atom thick, but the wonder material has yet another application to add to its mounting stack of potential applications. According to the same group of researchers that first fabricated the 2D sheets of carbon nearly four years ago, graphene has the ideal optical properties to form the transparent electrodes in liquid crystal displays (LCDs). The researchers have also developed a technique that overcomes the traditional problems with manufacturing sizable quantities of graphene.

LCDs typically contain an array of many “cells”, each of which consists of a thin layer of liquid crystal sandwiched between a pair of polarizers crossed at 90° to each other. Light entering from behind a cell gets polarized in one direction when it passes through the first polarizer, so when it reaches the second it cannot get through. This makes the cell appear dark. To make the cell bright, the light must pass through the second polarizer, which requires the intervening liquid crystal to rotate the light’s polarization.

To do this, an electric field is applied across the polarizers and this twists the orientation of the long molecules in the liquid crystal. The polarization of the light is guided along the twist of the molecules, and by the time it reaches the second polarizer it has rotated through 90° so that it can pass.

Of course, the electric field has to be applied using electrodes, and these have to be both transparent and good electrical conductors. For such qualities engineers usually turn to indium tin oxide (ITO). However, this material has its drawbacks: indium is rare and therefore expensive; and ITO can release both indium and oxygen ions, which prevent the liquid crystal from aligning correctly. Now, a team including Andre Geim and Kostya Novoselov from the University of Manchester in the UK and Sergey Morozov from the Institute for Microelectronics Technology in Chernogolovka in Russia have found that graphene is generally more transparent than ITO, but with seemingly no drawbacks (arXiv:0803.3031).

Many applications

Graphene comprises a rippled sheet of carbon just one atom thick, rather like a single layer from a crystal of graphite. Indeed, graphene is often fabricated by ripping a layer off a thin piece of graphite with sticky tape, a process known as micromechanical cleavage (or the “Scotch tape method”). Since Geim and colleagues discovered graphene in 2004, researchers have found no end of desirable properties for the material — it can be an excellent electrical and thermal conductor, an equally good semiconductor, and a sensitive mass detector.

A benefit of using graphene for LCD electrodes is that, unlike ITO, it is stable. This prevents it from releasing ions into an “alignment layer”, which is sometimes applied onto LCD electrodes to encourage the liquid-crystal molecules to align properly. Such stray ions can reduce the effectiveness of the alignment layer, causing undesirable “image sticking”. Perhaps more importantly, however, graphene trumps ITO for transparency. Geim’s team used micromechanical cleavage to deposit flakes of graphene onto a glass slide, which they put under an optical microscope. They found that graphene had an optical transmission of about 98%, significantly higher than the 82–85% of standard ITO.

What is doubly impressive about graphene is that it can achieve high optical transmission with a corresponding sheet resistance (a 2D measure of resistivity) of just 6 kΩ. With an added alignment layer of polyvinyl alcohol, which has the side effect of reducing resistance, this figure drops to 400 Ω. Further chemical doping can reduce the sheet resistance to 50 Ω. ITO, on the other hand, has to trade resistance for transparency. Indeed, if an ITO electrode is made thin enough to rival the transparency of graphene, its sheet resistance skyrockets.

Faster production

The one downside with graphene is that, in the past, it has been difficult to produce. Micromechanical cleavage can only produce a few flakes at a time, and is therefore unlikely to ever be employed commercially. However, Geim’s team have developed a new production technique that can reap larger quantities.

They begin by placing crystals of graphite in a bath of dimethylformamide (DMF) and then sonicate it with ultrasound for over three hours. Graphite is hydrophobic which means it tends to clump together in water, but in DMF the sonication allows it to “dissolve” into flakes. Next, the researchers centrifuge the mixture for 10 minutes to remove thick flakes from the monolayer flakes of graphene, which they subsequently spray onto a glass slide. Finally, they anneal the slides for two hours at 250 °C amid hydrogen and argon gas. Although the thickness is not consistent over the slide — it varies between one and four layers of graphene — the optical properties match those of graphene produced by micromechanical cleavage.

Geim’s team are not planning to commercialize graphene electrodes themselves. Novoselov told physicsworld.com that they have had “interest” from the LCD industry, although he could not name the companies to which he was referring. The team will shortly be publishing more fundamental results on graphene’s optical properties.

Putting a new spin on nanotubes

A team of physicists has uncovered two unexpected twists in the tale of how spin-polarized electrons are transported along carbon nanotubes. By trapping just one electron in a tiny length of nanotube, the team found that the motion of the electron has a significant effect on the direction of its spin — and that this effect is very different for “holes”, which are positively charged particles.

Both of these results show that carbon nanotubes (CNTs) — sheets of carbon one atom thick that have been rolled up to create tiny cylinders — are unlikely to be any good at transporting spin-polarized electrons far enough to make them useful in “spintronic devices”, which exploit both the charge and the spin of electrons. However, the new work suggests that CNTs could be better at manipulating electron spin in spintronic devices — or as quantum bits (qubits) in quantum computers.

Spin-orbit coupling

CNTs have a number of desirable electronic properties such as high conductivity, and researchers have already used them to create prototype devices such as diodes and transistors. Many physicists had also believed that CNTs would be very good at transporting spin-polarized electrons, making them ideal for use in spintronic devices and qubits.

Conventional conductors like copper, in contrast, are bad at conducting spin-polarized electrons because of “spin-orbit coupling”, in which the spin of the electron is deflected by an effective magnetic field that is generated by the electron’s orbital motion relative to the material’s atomic nuclei.

But as the strength of this effect is proportional to the fourth power of the number of protons in the nucleus, it should be almost negligible in carbon, which has an atomic number of six, compared to copper with atomic number 29. Carbon nanotubes should therefore be great materials for ferrying spin-polarized electrons around because their spins will not flip very often from up to down, say, as they move. Indeed, it has already been shown that spin-polarized electrons can travel hundred of micrometres in silicon, which also has a low atomic number of 14.

This view had been supported by several independent studies of electrons in CNT “quantum dots”, tiny lengths of nanotube containing just handfuls of electrons. While some deviations from the expected behaviour was seen, these were explained in terms of interactions between the electrons, not spin-orbit coupling.

One electron per dot

Now, Shahal Ilani, Ferdinand Kuemmeth and colleagues at Cornell University in the US have shown that the spins are in fact strongly coupled to the orbital motions of electrons in CNT, by using a nanotube quantum dot containing just one electron (Nature 452 448).

Their quantum dot was a length of CNT about 500 nm long which had metal electrodes at each end (the source and drain). Two gate electrodes were placed under the CNT, one at the source end and one at the gate end. By adjusting voltages applied to the two gates, the team were able to trap a single electron within a 200 nm portion of the CNT.

The energy levels of the electron were then measured by scanning a second voltage between the source and drain. At certain voltages the electron will tunnel in and out of the dot and these voltages can be used to determine the energy levels of the electron in the CNT.

A single electron in a CNT is expected to have four possible combinations of spin up/down and clockwise and counter-clockwise orbital motion around the tube’s circumference. If there was no spin-orbit coupling, all four states should have the exact same energy.

Instead, the team discovered that the energies of these states were split in two according to the relative orientations of the spin and the orbital motion of the electron around the tube. The higher energy level corresponded to the spin and orbital magnetic moments pointing in opposite directions and the lower level to spin and orbital moments in the same direction. The energy gap between the two levels is about 0.37 meV — which is several orders of magnitude greater than expected in a flat sheet of carbon.

Four-way splitting

The team then repeated the measurement while applying an external magnetic field along the length of the CNT. This further split the energy levels into four, corresponding to the relative orientations of the spin and orbital magnetic moments to the applied field — exactly as expected from strong spin-orbit coupling.

While this unexpectedly strong interaction means that free spins might not be the best way to realize quantum bits in CNTs, Ilani believes that if properly exploited, this coupling could yield new types of robust quantum bits in which the spin and orbital degrees of freedom are entangled. He further notes that this coupling adds a new tool that was so far missing in carbon based system — the ability to control the spin state of an electron by intentionally manipulating its orbits.

Holes versus electrons

According to Kuemmeth, the experiment yielded another unexpected result — that spin-orbit coupling affects electrons and holes differently in CNTs. In semiconductor physics, a “hole” is the absence of an electron, which behaves like a positively charged particle. In most semiconductors, holes have different physical properties than electrons, but physicists had thought in CNTs, apart from opposite charge, holes and electrons in NTs should have the exact same physical properties.

However, by measuring the energy states of a single hole in a the same CNT dot, the Cornell team discovered that the spin-orbit interaction causes anti-parallel alignment of spin and orbital moments for a hole, the exact opposite than it does for an electron.

Entangled photons could sharpen your view

Noise is a lethal enemy of quantum information systems and even the slightest amount of it could prevent a quantum computer from working. But now, Seth Lloyd from the Massachusetts Institute of Technology (MIT) in the US has proposed a way to exploit this sensitivity to noise to create a novel quantum imaging system. He believes that the system could offer an exponential improvement in signal-to-noise over conventional optical imaging techniques, although he admits that implementing it in practice will not be easy.

Conventional optical imaging systems such as a microscope work by shining light onto an object and detecting the light that is reflected back. This is a simple process as long as the system isn’t operating in a noisy environment in which photons from random sources get mixed up with the reflected light.

Discriminating detector

While a conventional detector has no way of discriminating between a photon of reflected light and a similar photon of noise, Lloyd believes that the quantum principle of entanglement could be harnessed to filter out the noise.

Lloyd’s system involves creating pairs of photons that are entangled quantum states. Entanglement is a feature of quantum mechanics that allows particles to share a much closer relationship than classical physics allows. An important feature of entanglement is that the photons retain a “memory” of being created as a pair.

In Lloyd’s scheme, one entangled photon (called the signal) is directed at the object of interest, while the other (the ancilla) is retained at the imaging device for future reference (arXiv: 0803.2022). If a signal photon reflects from the object and returns to the imager, it can be compared to the ancilla, which retains a memory of its entangled partner. If the partnership is verified the photon is used to build an image of the object. However, if the ancilla has no memory of the photon, it is rejected as noise.

According to Lloyd’s calculations, an imaging device using his scheme would have a signal-to-noise ratio that is 2^e times that of a conventional system. Where 2^e is a measure of the degree of entanglement of the two photons in terms of the number of modes of the electromagnetic field that are entangled between the two photons.

Photonic crystal

The challenge, however, is how to compare the signal and ancilla photons. In principle, Lloyd believes that this could be allowing the two photons to recombine to create a single high-energy photon by firing them into a photonic crystal — a special material that contains regularly alternating regions with high and low refractive indices.

This is the reverse of the “down-conversion” technique that is commonly used to make entangled photons in the first place. According to Lloyd, the two photons are more likely to recombine to make a single higher-energy photon, which can be detected, if they retain a memory of entanglement.

But this requires a system that can put both reflected signal and ancilla at the same place at the same time, which Lloyd admits is no mean feat. However, he points out that there is no reason why it couldn’t be done — and that it should be relatively easy to achieve compared to other quantum-information processes that physicists are currently trying to develop.

Lloyd told physicsworld.com that a possible application of the system is enhancing the performance of optical communications systems.

Nanocrystals could help recover waste heat

Researchers in the US have come up with a new and simple way to boost the performance of a common thermoelectric material — bismuth antimony telluride — which had defied attempts at improvement for over 50 years.

Thermoelectric materials convert heat directly into electricity and could be used to boost the efficiency of conventional coal and nuclear power stations by recovering heat that is normally wasted. The materials could also improve the effectiveness of solar cells and could be used to cool computer chips and other devices.

To be used in such ways, however, a thermoelectric material must be good at conducting electricity but poor at conducting heat. This requirement is expressed in the thermoelectric figure of merit ZT, which should be greater than 1.0. The ZT of bismuth antimony telluride — one of the most common thermoelectric materials — has remained stubbornly at around 1.0 for more than half a century.

Now, Zhifeng Ren and Gang Chen and colleagues at Boston College and the Massachusetts Institute of Technology have made a significant breakthrough by milling the material into a fine powder that contained nanoparticles measuring about 20 nm across (Sciencexpress DOI: 10.1126/science.1156446). Next, they hot-pressed the powder into nanocrystalline ingots.

Improved ZT

The researchers found that the thermoelectric figure of merit (ZT) for the ingots increased to 1.2 at room temperature (from a value of 1.0 previously). Moreover, they found that the ZT peaks at 1.4 at 100 °C.

This might not sound like a big improvement, but the researchers describe it as a “significant step” towards creating materials that are useful for cooling and power generation.

Electrical transport measurements on the ingots, together with microstructure and modelling, showed that the ZT improves thanks to the low thermal conductivity caused by increased phonon scattering at grain boundaries and defects in the material. However, the electrical conductivity of the material is not affected signficantly by grain boundaries and defects.

Promising for applications

The high ZT in the temperature range 25–250 °C makes these materials promising for cooling and waste heat recovery applications, say the researchers. Potential applications include converting the heat of car exhausts into electricity, for example. “Other applications include efficient thermoelectric cooling, such as air conditioning and refrigeration, and solar thermoelectricity,” explained Ren and Chen.

The researchers have also built a prototype cooling device to confirm the properties.The team now plans to make efficient coolers and power generators using the improved materials. “At the same time, we will apply the approach to other promising thermoelectric materials,” they stated.

The company GMZ Energy Inc, a Massachusetts-based start-up, is now mass-producing the materials.

Iconic UK telescope faces closure

Today is the last chance for researchers in the UK to have their say on whether the e-MERLIN network of radio telescopes, which includes the iconic Lovell telescope at Jodrell Bank in northern England, should close.

If funding is withdrawn that will seriously threaten Jodrell Bank Phil Diamond, Jodrell Bank Centre for Astrophysics

The Science and Technology Facilities Council (STFC) has spent the last few weeks consulting the scientific community about the fate of the network after its biennial ‘programmatic’ review of funding, published earlier this month, deemed it low priority. That consultation will end today and a panel of experts will then consider comments from the community and make recommendations to the STFC about whether e-MERLIN should close. Funding for e-MERLIN could be withdrawn from April next year.

£80m shortfall

The programmatic review took place in the wake of an £80m shortfall in the budget of the STFC, which led to the UK pulling out of a number of international projects such as the International Linear Colider. However, astronomers involved in the e-MERLIN network were shocked to see the project deemed low priority as it had recently gone through a £8m upgrade.

“If funding is withdrawn that will seriously threaten Jodrell Bank” says Phil Diamond, director of the Jodrell Bank Centre for Astrophysics. This withdrawal would save the STFC around £2.7m per year in operational costs. Jodrell Bank Observatory, operated by the University of Manchester, celebrated its 50th anniversary last year.

The STFC says it expects fund all of the “high” and “medium-high” priority projects, which include the Advanced LIGO project to hunt for gravitational waves, the Compact Muon Solenoid at the Large Hadron Colider to detect traces of the Higgs boson and the SCUBA 2 camera for sub-millimetre astronomy that hopes to see the early stages of galaxy formation.

Mission to Mercury

The STFC also says that it will fund a “significant part” of the low priority projects, which includes the BepiColombo mission to Mercury planned for 2013 and the Gemini telescopes in Hawaii and Chile. The council has already decided to continue as a full member in Gemini despite having threatened to pull out in the wake of the funding crisis.

Indeed, Diamond is confident that some factors that were overlooked in the peer-review process that decided e-MERLIN’s low priority status will now help to reverse the STFC’s recommendation. “We hope to do well in the consultation process, the public response in support has been amazing,” says Diamond, referring to a string of articles in the UK media on the threat to Jodrell Bank.

Mesons could offer new clue in antimatter mystery

“Where has all the antimatter in the universe gone?” is one of the great unanswered questions in cosmology. Now, an unexpected difference in the behaviour of two types of exotic particle seen at an accelerator in Japan could help physicists understand why the universe is dominated by matter.

According to conventional cosmological models, equal amounts of matter and antimatter should have been created in the aftermath of the Big Bang. Matter and antimatter should have then annihilated, leaving only photons. However, the clear domination of matter in our visible universe indicates that our understanding of the physics of the early universe is incomplete.

One possible explanation for the dearth of antimatter was proposed by Makoto Kobayashi and Toshihide Maskawa in 1973. They suggested that the weak nuclear force — responsible for some types of radioactive decay — could act differently on matter and antimatter. This is asymmetry is thought contribute to what physicists refer to as charge–parity (CP) violation.

BaBar and Belle

The first observations of this Kobayashi–Masakawa (KM) asymmetry were reported 2002 and came out of studies of the decays of K and B mesons — short-lived sub-atomic particles consisting of quark–antiquark pairings and created by smashing electrons and positrons together. The measurements were made at both the Belle experiment at the KEK-B accelerator in Japan and at the BaBar experiment at the Stanford Linear Accelerator in the US.

However the asymmetry seen in these experiments is much too small to explain the elimination of antimatter from the universe.

Now, Belle scientists may have found the first inklings of where the rest of the asymmetry may be coming from — having observed an unexpected difference between the time it takes for charged and neutral versions of the B-meson to decay (Nature 452 332). The measurement was made over six years and involved the observation of some 535 million B mesons, which decayed into lighter subatomic particles called K and π mesons.

New physics

“This difference could be an indication of a new source of CP violation that is needed to explain the matter-dominated universe,” says Paoti Chang, a member of the Belle collaboration at National Taiwan University.

The result could be due to new types of short-lived unknown particles that are created as an intermediate step in the decay process, exacerbating the matter–antimatter imbalance. Alternatively, the effect could also be caused by interactions linked to another fundamental force — the strong nuclear force. However, if the latter is true, Chang points out that this may indicate a breakdown in our theoretical understanding of B meson decays.

“To understand whether new physics is indeed involved, study of CP violations from other modes of decay is needed,” says Chang. “Current experimental measurements on CP violation for these candidates are not precise enough, and much more data are needed.”

The researchers will be hoping future experiments such as the upcoming Large Hadron Collider and the Super B factory upgrade at KEK-B will help them find the reasons for the missing antimatter.