Skip to main content

Reset your password

Please enter the e-mail address you used to register to reset your password

Note: The verification e-mail to change your password should arrive immediately. However, in some cases it takes longer. Don't forget to check your spam folder.

If you haven't received the e-mail in 24 hours, please contact customerservices@ioppublishing.org

Registration complete

Thank you for registering with Physics World
If you'd like to change your details at any time, please visit My account

Close
Home » Page 1322

Dirac seen in a new light

By Matin Durrani

It’s probably because he was born and raised in Bristol, UK – the city where Physics World is based – that my colleagues and I perhaps give a disproportionate amount of coverage to Paul Dirac compared with other great theoretical physicists of the 20th century.

PW-2012-08-08-Dirac-art.jpg

But although Dirac did his most famous work at the University of Cambridge, where he was Lucasian professor for more than 35 years, it is nevertheless true to say that his approach to science was forged by his educational experiences in Bristol, as Graham Farmelo’s classic 2009 biography makes clear.

Dirac studied for two separate degrees in engineering and mathematics at the University of Bristol and before that gained a wealth of practical experience, particularly in the art of technical drawing, when he was a pupil at Merchant Venturers’ Technical College – an institution that was the forerunner of today’s Cotham School.

Given that 8 August is the day on which Dirac was born back in 1902, I thought today an appropriate moment to mention an interesting new artwork (see right, click to enlarge) that is currently on show at Cotham School.

Created by Eric Hardy, the work is an alternative version of the traditional end-of-year school photograph and consists of a pixelated image of Dirac himself. All the pixels, however, have been replaced by photos taken in 2010 – when Hardy was still at Cotham School – of fellow pupils, teachers and other members of staff.

“As such it connects the past to the present, the individual to the collective,” says Hardy’s father Tim.

The original artwork, which is printed on a canvas about 100 × 90 cm in size, was on display at the school in May when its other great former pupil – the University of Edinburgh theorist Peter Higgs – paid a visit.

If you can’t make out Dirac in the image, try scrunching up your eyeballs.

And talking of Dirac, don’t forget that today is also the day that the International Centre for Theoretical Physics in Trieste awards its annual Dirac prize, which this year went to Duncan Haldane, Charles Kane and Shoucheng Zhang for their work on a new class of exotic materials called “topological insulators”.

X-ray spectroscopy detects single atoms

Researchers in Japan are the first to have succeeded in detecting single atoms using X-ray spectroscopy. Although a difficult technique, the work is an important step forward in studying and characterizing nanoscale structures and devices using X-rays.

Previous work in this field has largely focused on using electron energy-loss spectroscopy (EELS) to detect single lanthanide metal atoms and light atoms like carbon. However, EELS can only be applied to certain elements thanks to the high-energy beams used in this method that can damage samples. Nobel metals, such as gold and platinum, are also difficult to detect with high sensitivity using EELS – a major drawback when it comes to investigating meteorites, catalytic clusters or anticancer drugs, where only a very small number of noble metals are looked at in any given sample.

While energy-dispersive X-ray spectroscopy (EDX) is a good way to chemically characterize a wide range of materials, researchers have been reluctant to use the technique to detect single atoms because of the difficulties involved in obtaining good photoemission spectra. Kazu Suenaga of the Nanotube Research Center at AIST in Tsukuba and colleagues at JEOL Ltd in Akishima and Kyushu University in Fukuoka are now saying that they have successfully used EDX to sense single atoms of erbium thanks to advanced excitation and detection apparatus.

Metallofullerene peapods

Suenaga and colleagues studied metallofullerene peapods in their experiments and, in particular, erbium peapods (Er@C82) – so-called because the atoms are lined up in rows like peas in a pod. Each peapod is made up of a single erbium atom inside a carbon-82 cage, supported in a carbon nanotube. The advantage of looking at such a sample is that the structure is well ordered, with each metal atom separated from its neighbour by around 1 nm. The atoms can thus easily be distinguished in the resulting X-ray spectra.

The team obtained its results by using a finely focused electron beam (down to a few angstroms) to excite single Er atoms in an electron microscope so that they emitted X-ray photons. A newly developed large-sized (around 100 mm2) silicon drift detector was also employed to collect as many X-rays as possible from the sample.

“X-rays are typically emitted in all directions, so a normal-sized detector misses a lot of them and only a few per cent can be collected,” says Suenaga. “Our new large SSD greatly improves on collecting efficiency – by at least several times,” he told physicsworld.com.

“Being able to perform X-ray spectroscopy on single atoms in this way will be of great help in nano-optics research,” he adds.

The research is published in Nature Photonics.

What is fracking?

In less than 100 seconds, James Verdon describes how to squeeze out hard-to-reach shale gas.

Physicists see hints of Majorana fermions

Evidence for the existence of “Majorana fermions” – theoretically proposed particles that are also their own anti-particles – could be seen in the behaviour of a novel Josephson junction. That is the view of physicists at Stanford University in the US, who have examined the properties of a Josephson junction that incorporates material called a “topological insulator” sandwiched between two superconducting contacts. The researchers found significant deviations from what is seen in conventional Josephson junctions – differences that they believe could be explained in terms of Majorana-like quasiparticles.

First predicted by the Italian physicist Ettore Majorana in 1937 – shortly before he mysteriously disappeared aged just 31 – Majorana fermions are interesting not just because they are their own antiparticles but also because they should be resistant to environmental noise. Majorana fermions, in other words, could be used to store and transmit quantum information without being perturbed by the outside world, which is the bane of anyone trying to build a practical quantum computer.

Although definite proof of the existence of Majorana fermions has not yet been obtained, theorists have calculated that particle-like excitations, or quasiparticles, which look like Majorana fermions could exist at the interface where a topological insulator – a material that only conducts electricity on its surface – is placed next to an ordinary superconductor. These quasiparticles are called “zero-energy modes” because they lie along the Fermi energy of the material.

In the case of a Josephson junction containing a topological insulator as the “weak link” between two superconductors, there are actually two superconductor–topological insulator interfaces back-to-back, and the Majoranas are expected to couple to each other and depart from zero energy. However, if a tiny magnetic field – even as small as half a superconducting flux quantum – is applied to the junction, the two Majorana modes decouple and both reside at zero energy.

The weakest link

David Goldhaber-Gordon and colleagues at Stanford have now studied such junctions and have found some bizarre behaviour, which they have tried to explain in terms of Majorana fermions. When experimentalists plot a graph of the superconducting current flowing across a Josephson junction against the value of an applied magnetic field, they usually see a distinct “magnetic diffraction pattern” (MDP). Normally, the MDP has a strong central peak, but in topological-insulator Josephson junctions, Goldhaber-Gordon and colleagues saw a much more complicated MDP with several unexpected peaks. Indeed, the first minimum occurs at about one-fifth of the magnetic field strength that is expected in a conventional Josephson junction.

According to Goldhaber-Gordon, this more complicated structure could be related to the zero-energy Majorana modes that are expected to occur at specific values of magnetic flux. However, to explain the observed diffraction pattern, Goldhaber-Gordon points out that three – rather than one – zero-energy modes are required. One of these modes could be associated with a Majorana fermion, whereas the other two could be associated with other conventional fermions – something that Goldhaber-Gordon says has been suggested by some theorists.

Smaller critical currents

Another atypical feature seen by the team is the value of the device’s critical current (above which it no longer superconducts) multiplied by its resistance in the normal, non-superconducting state. This product is usually proportional to the superconducting energy gap, but the team measured a value that is much smaller than expected. The value was also found to be inversely proportional to the width of the Josephson-junction device – that is, the distance across the device perpendicular to the flow of the supercurrent.

Building on a theoretical description published in 2008 by Charles Kane and Liang Fu at the University of Pennsylvania in the US, Goldhaber-Gordon and colleagues assume that the Majorana fermions are confined to a 1D wire that runs along the width of the Josephson junction. The result is a series of quantized energy levels that are inversely proportional to the width of the device. The team speculates that the gap between these energy levels provides a new and smaller energy scale above which superconductivity ceases to occur – explaining the smaller measured values.

Although the team analysed its results in the context of Majorana fermions, Goldhaber-Gordon stresses that his team are still only at the early stages of exploring the behaviour of junctions between superconductors and topological insulators. “Many aspects of the materials and junctions are not yet well understood,” he says. “We welcome ideas for the explanation of these data, whether they are Majorana-related, or not.”

The research is described in Physical Review Letters.

How do you hunt for planets that are outside our solar system?

In less than 100 seconds, Zoe Leinhardt explains how astronomers are discovering alien worlds.

Martin Fleischmann: 1927–2012

By Hamish Johnston

In the autumn of 1989 I was doing what many physicists were also doing at the time – I was trying to get deuterium atoms to fuse together in a solid after hearing about the work of Martin Fleischmann and Stanley Pons. Working at the University of Utah, the pair used electrolysis to “load” metal electrodes with deuterium and claimed to have seen excess heat and particles that could be interpreted as by-products of nuclear fusion. This process was dubbed “cold fusion” and was touted in the popular press as a solution to the world’s energy problems – if only it was…

Fusion normally occurs at extremely high temperatures and therefore it was very difficult to understand how the nuclei could overcome the considerable electrostatic repulsion in order to fuse. A popular explanation at the time was that the positive charges of deuterium nuclei within a solid such as palladium were screened by the negatively charged electrons in the metal, thereby allowing two nuclei to get close enough to fuse.

Like the hundreds of others worldwide, my little experiment found no evidence for cold fusion. With the exception of a few diehard enthusiasts, interest in cold fusion has since withered. Indeed, for physicists of my generation, the cold-fusion saga was a public embarrassment and an example of “bad science” – so much so that even legitimate investigations into its possibility are still viewed by many with scorn.

Fleischmann died on Friday at the age of 85 in England, where he had arrived from his native Czechoslovakia in 1938. I find it sad to think that things could have been so very different for him – and humanity – if he had indeed discovered cold fusion.

What is a polymer?

In less than 100 seconds, Peter Barham describes the science of molecular chains.

Curiosity is winched down to Mars

Early image of Mars from Curiosity


An early view of Mars from Curiosity. (Courtesy: NASA/JPL-Caltech)



By Hamish Johnston

It cost about one billion dollars and took hundreds of scientists and engineers more than eight years to build – and earlier today NASA’s Curiosity rover landed on Mars and is now sending back images.

The above photograph is one of first sent back by Curiosity. It was taken through a wide-angle lens on the left “eye” of a stereo pair of “hazard-avoidance” cameras on the rover. The object on the right of the image is one of the rover’s wheels.

The image is in black and white and is taken at a relatively low resolution. Larger colour images from higher-resolution cameras should be beamed back to Earth later this week when Curiosity’s mast is deployed.

Because it is five times heavier than NASA’s previous Martian rovers, Curiosity was winched down to the surface of the planet by a retro-rocket-firing “sky crane”.

“Ambitious, audacious and unconventional” is how one NASA scientist described the landing.

Curiosity landed on target in a huge crater, where it will look for evidence that the local area has – or ever had – conditions that could support life. The rover will be able to travel up to 200 m per day and the mission is expected to last one Martian year or 687 Earth days.

During that time it will use a suite of scientific instruments to study the Martian soil and atmosphere. Curiosity’s alpha-particle X-ray spectrometer was built by a team led by the physicist Ralf Gellert of the University of Guelph in Canada – my alma mater. Guelph physicists have a long history of using X-ray spectroscopy to study everything from precious works of art to Martian rocks and it was learning about this work many years ago that first got me interested in the practical applications of physics.

Physicists unveil plans for ‘LEP3’ collider at CERN

A group of physicists from Switzerland, Japan, Russia, US and the UK has proposed using the tunnel that currently houses the Large Hadron Collider (LHC) at the CERN particle-physics lab near Geneva for a dedicated machine to study the Higgs boson. The facility, dubbed LEP3, is named after CERN’s previous accelerator, the Large Electron–Positron Collider (LEP), which used to exist in the LHC tunnel before being shut down in 2000. In a preliminary study submitted to the European Strategy Preparatory Group, LEP3’s backers say that the machine could be constructed within the next 10 years.

The plans for LEP3 come just weeks after physicists working at CERN reported that they had discovered a new particle that bears a striking resemblance to a Higgs boson, as described by the Standard Model of particle physics. The ATLAS experiment measured its mass at around 125 GeV and the CMS experiment at 126 GeV.

LEP3 would operate at 240 GeV and comprise two separate accelerator rings that would smash electrons and positrons rather than protons and protons, as with the LHC. In their study, the 20 authors call the concept for LEP3 “highly interesting” and that it deserves more detailed study. “Now is the right moment to get this on the table,” says theorist John Ellis from Kings College London in the UK, who is an author of the preliminary study and hopes that it will trigger debate among physicists as to how to study the new boson in detail.

Tunnel vision

LEP3 is designed to be installed in the LHC tunnel and serve the two LHC’s general-purpose detectors – ATLAS and CMS. If LEP3 is to be built, it will have to fight off two rival proposals for a future collider to study the Higgs – the International Linear Collider (ILC) and the Compact Linear Collider (CLIC). But Ellis says that one advantage of LEP3 is that the tunnel to house it is already built and the collider would use the existing infrastructure, such as cryogenics equipment, thus making LEP3 more cost-effective. LEP3 would also use conventional methods to accelerate particles rather than the accelerating superconducting cavities that will be employed by the ILC.

Which collider is built to succeed the LHC will depend on what the LHC discovers in the next couple of years after it has run at its full design energy of 14 TeV. If it turns out that the LHC finds only the Higgs, then Ellis says there would be a strong case for LEP3. But if more particles are discovered by the LHC – such as supersymmetric particles – it would make sense to consider the other two proposals. “LEP3 could be a more secure option than the ILC if only a Higgs is discovered,” Ellis told physicsworld.com. “But, of course, it would be foolish to choose anything now, given that the LHC has not hit full energy yet.”

CERN plans to run the LHC into the 2030s after it has undergone a major upgrade in energy and luminosity in the coming decade. However, Ellis thinks that it may even be possible for the LHC and LEP3 to cohabit for a short time. “It would not be ideal, but it could be something to think about,” says Ellis. “If the LHC does not discover anything beyond the Higgs, then would you keep running it for years?”

“Little scope”

Yet some disagree that LEP3 represents the best way to study the Higgs, adding that a decision would have to be made between building LEP3 and running the high-luminosity upgrade to the LHC in the 2020s. “They both have an excellent physics case, but somehow LEP3 presents less chance of a huge breakthrough,” says one leading CERN researcher who prefers not to be named. “[The LHC upgrade] has precision measurements as well as discovery reach to offer.”

That view is shared by linear-collider director Lyn Evans, who told physicsworld.com that he thinks it is unlikely that the proposal for LEP3 will get very far. “The first job is to fully exploit the LHC and all its upgrades,” says Evans, who led the construction of the LHC. “This is at least a 20 year programme of work, so I think that it is very unlikely that the LHC will be ripped out and replaced by a very modest machine with little scope apart from studying the Higgs.”

Graphene logic for the real world

Researchers in Italy and the US have created the first integrated graphene logic gates that work in air and at room temperature. The work represents an important milestone in the development of graphene-based logic, says the team.

The devices are also the first graphene logic gates to operate with matched-voltage input and output digital signals, explains team leader Roman Sordan of the Politecnico di Milano at Como. Such an operation is the main prerequisite for practical use of this type of gate. “Moreover, our gates are integrated on a type of graphene that can easily be grown over large areas, thus paving the way for mass production of such carbon-based electronic devices,” he says.

To continue making more-powerful computers in the future, electronics devices must be able to perform simple logic tasks at ever-faster speeds. Conventional silicon chips are limited by the speed at which electrons travel in the material, a parameter known as carrier mobility. Graphene – a sheet of carbon atoms just one atom thick – is often touted as being the silicon of the future because it could overcome this problem thanks to its unique electronic properties, which include very high electron and hole mobility.

First graphene inverter

Sordan and colleagues made the first ever graphene integrated circuit back in 2009, when they fabricated a complementary inverter – the main building block of modern digital electronics. Although fully functioning, this device was not suited for real-world applications because it was made using exfoliated graphene in a process that cannot be scaled up to industrial levels. What is more, the device did not operate with matched-voltage input and output signals. “Without such signal matching, logic gates cannot be ‘cascaded’ – that is, one logic gate is incapable of triggering its neighbour – and so complex logic functions cannot be realized,” explains Sordan.

Since this early work, other research groups have been working on producing signal matching in graphene inverters. However, even the best devices only appear to work at very low temperatures and are based on exfoliated graphene samples.

CVD graphene to the rescue

Sordan and colleagues have now made inverters from graphene grown on wafers by chemical vapour deposition – a process that is amenable to mass production. The devices are capable of digital-signal matching and also operate at room temperature and in air. But that is not all. “We have also demonstrated the highest voltage gain of 5.3 reported so far in CVD graphene under ambient conditions, which is instrumental in matching digital signals,” says Sordan. “In 2009 we only had a gain of 0.04 – and that was in exfoliated graphene devices.”

The researchers achieved their feat in a self-aligned device design similar to a graphene amplifier that they had made previously. “We have not only demonstrated signal matching in our new experiments, but have cascaded graphene logic gates into more complex circuits too,” adds Sordan. “Such results have never been seen at any temperature until now – cryogenic or otherwise.”

And if that was not enough, the team says that it has also succeeded in identifying the parameters needed for cascading.

Doubts put to rest?

“There have been some doubts in the scientific community in the past about using graphene logic gates in real-life applications,” Sordan says. “We now show that graphene logic gates can operate in everyday, ambient conditions and that these gates might also be used in a plethora of applications.” Indeed, the gates already produced have larger voltage swings than emitter-coupled logic (ECL) gates – the fastest logic family that exists today. ECL gates are used for digital signal processing at extremely high frequencies of above 100 GHz – a range that is currently inaccessible to conventional state-of-the-art CMOS technology.

One drawback of the technology that the team is working to overcome is that the graphene gates cannot yet be used for low-power applications because the power dissipation remains too high.

The team, which includes researchers from Eric Pop’s group at the University of Illinois at Urbana-Champaign in the US, describes its work in Nano Letters.

Copyright © 2026 by IOP Publishing Ltd and individual contributors