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Home » Page 1335

Physicists peek through opaque materials

A team of researchers in the Netherlands has developed a new imaging technique that can see through opaque materials – despite the fact that such materials scatter virtually all of the light passing through. Although scientists have previously developed other methods for seeing behind opaque screens, the new scheme does not involve making initial measurements on both sides of the screen. The technique could therefore be used as a non-invasive medical imaging technique to diagnose diseases that lurk under the skin.

Developed by Allard Mosk and colleagues at the MESA+ Institute at the University of Twente, the new imaging system takes advantage of the “speckle pattern” that occurs whenever laser light is fired at a disordered material. Caused by light interfering after being scattered from randomly located molecules or structures in the medium, a speckle pattern may appear random but it actually contains important information about the incident light. In particular, when the angle with which the light strikes the medium is tweaked slightly, the speckle pattern stays more or less the same but shifts by a small angle – called the memory effect.

Speckles stay the same

Rather than simply looking at light that has reflected off an object of interest, Mosk’s team instead examined the light emitted by a hidden object that is fluorescent. This fluorescence is excited by light from a green diode laser that is fired at an opaque screen that obscures the object. The light emerges as speckles on the other side of the screen and this creates a speckled pattern on the object. The bright parts of the pattern create more fluorescent light than the dark regions – and therefore the total intensity of the fluorescent light is proportional to the integral of the speckle pattern over the image area of the object.

If the angle of incidence of the laser is changed, the memory effect causes the speckle pattern to move across the object. The new technique involves changing the angle of incidence while keeping the laser aimed at the same part of the screen. “During the scan we measure the amount of fluorescent light that comes back through the screen,” explains Mosk. Thanks to the memory effect, the team is able to use mathematical manipulation to separate the “autocorrelation” of the speckle pattern – the similarity of successive patterns – and the autocorrelation of the fluorescent light coming from the object. More mathematics is then used to convert this autocorrelation into an image of the object.

Cell-sized object

The team first studied a pi-shaped fluorescent object that measured about 50 μm across (see figure). This size was chosen because it corresponds to that of a typical human cell. The object was placed about 6 mm behind a ground-glass diffuser. To show that the technique also works on biological systems, the team obtained an image of a naturally fluorescent cell from a sample of lily-of-the-valley that was placed between two diffusing screens.

Mosk points out that the technique works best when a relatively thin but high-diffusive screen is used – rather than a thick screen made of less-diffusive material. In the latter case a different technique called optical coherence tomography can obtain better images and he believes that a hybrid of the two methods could be developed to address a wider range of screening materials.

Another possible drawback of the technique is that it only works if the object being studied emits light when illuminated by a laser. While some biological materials are naturally fluorescent, others would have to be labelled with fluorescent molecules. If this is not possible, the technique could work using other light-emitting processes such as nonlinear conversion, Raman scattering or photoacoustics, according to Mosk. “However none of these signals are as strong and as sensitive as fluorescence,” he adds.

Mosk also points out that the technique will not work if the opaque screen itself is strongly fluorescent. “We are planning to use longer-wavelength lasers to avoid this,” he says.

Sylvain Gigan at the Institut Langevin-ESPCI ParisTech says the work is an important breakthrough, even though it is not yet ready for practical biological imaging. “It opens really interesting avenues and I am sure that the idea will have long-lasting consequences and will eventually help develop new imaging techniques,” he says.

The research is described in Nature.

Bill Foster regains US Congress seat

The former Fermilab physicist Bill Foster has regained his seat in the US House of Representatives, doubling the number of physicists in Congress from one to two. When the House convenes in early January, Foster will join fellow Democrat Rush Holt, a one-time assistant director of the Princeton Plasma Physics Laboratory, who has represented a constituency in New Jersey for the last 14 years.

Foster first won his Congressional seat representing the state of Illinois in a 2008 by-election, joining Holt and Michigan Republican Vern Ehlers to make a trio of physicists in Congress. Foster retained the seat in the general election later that year but lost it two years later when a Republican surge defeated large numbers of Democratic representatives. And when Ehlers retired in 2010, Holt was left as the sole physicist in the US Congress.

In this week’s election Foster stood in a different constituency, where the boundaries had been redrawn as a result of the 2010 census. He easily beat his Republican rival Judy Biggert, who had served the constituency for 14 years, winning 57.6% of the vote against Biggert’s 42.4%. “Our nation faces tough problems – more than a decade in the making. And there is no doubt that compromise will be required to resolve these problems,” Foster said during his victory speech.

The campaign was notable for its emphasis on science, as the constituency includes the Argonne National Laboratory and lies close to Fermilab. Moreover, Biggert had been a member of the House of Representative’s science committee, which has direct influence on government funding, including support of science. During the election, Foster noted that he would like to serve on the appropriations committee, adding that the proposed budget that Biggert and other House Republicans had approved would cut government support of non-military science by up to 30%.

Devoted to politics

Foster, 56, brings a strong résumé to his backing of science. He earned a bachelor’s degree in physics from the University of Wisconsin in 1975. While still at school he founded, together with his brother, a company that now makes more than half of the theatre-lighting equipment in the US. After earning his PhD in 1984 from Harvard University, he began what was to become a 22-year stint at Fermilab, managing several multimillion-dollar projects for accelerator construction and research.

As his first task, he designed and built components of the Tevatron’s CDF detector, which discovered the top quark in 1994. In the early 1990s he led a team responsible for designing an integrated circuit that ratcheted up the speed and accuracy of measuring particle collisions. Foster left Fermilab in 2006 to devote himself to politics. Indeed, as Congress’s informal physics caucus, Foster and Holt can expect to be consulted by their colleagues on science issues.

Nanotechnology: the view from the community

To outsiders, the world of nanotechnology can seem like an alien land, conjuring up images of matter behaving in ways that are contrary to our everyday experiences. At Physics World we were keen to find out if people on the inside view this world with the same sense of wonder and intrigue. We caught up with researchers attending a recent nanotechnology conference in Bristol, UK, and asked them the following question: “What does the term nanotechnology mean to you?”

We were also keen to find out about the new technologies that could emerge from this research field in the coming years. So, we also asked the same scientists this question: “What do you think is the single most exciting potential application of nanoscience?”

The world’s quietest building – a video tour

The Centre for Nanoscience and Quantum Information (NSQI) opened its doors to researchers in 2009. Located at the University of Bristol in the UK, the centre is home to a collection of laboratories that offers the lowest vibration and acoustic noise levels of any research facility in the world. Physics World journalist James Dacey visits the NSQI to find out how its researchers manage to keep the place so peaceful and quiet.

The NSQI was created primarily for researchers working in the fields of nanoscience and quantum information. Dacey meets some of the scientists based at the NSQI to find out about their research. They include Jeremy O’Brien, who is developing the practical knowledge needed to create quantum computers that could drastically outperform any existing computer.

Starting from scratch

Some people decide to do a PhD because they think it will help them get a job. I thought so too, and in a way, I was right. My PhD did indeed lead me to my current job but, more importantly, it gave me the opportunity to become an entrepreneur and create jobs for other people.

I am originally from Iran and I earned my undergraduate degree in applied physics at Tehran’s Sharif University of Technology, followed by a Master’s degree from Shahid Beheshti University in the same city. Then I decided to come to the US to get my PhD, and I chose the University of Louisville (UofL) in Kentucky thanks to Robert W Cohn, who offered me a research assistantship and became my thesis adviser in the electrical engineering department.

At the start of my PhD, I often asked myself whether I wanted to become a professor or a researcher in a large established company such as IBM or Intel. I was particularly unsure whether switching between physics and electrical engineering would help or hurt my chances of finding a job in academia. I still do not know how my career would have worked out if I had not decided to found my own firm, NaugaNeedles.

An accidental discovery

My research project at UofL involved working on direct patterning of liquid metals, such as gallium, using atomic force microscopy (AFM). One day in 2004 I found by accident that gallium interacts with metal films, such as gold and silver, at room temperature in a way that produces interesting self-assembled nanostructures. For example, when a molten gallium droplet was placed on a silver film, I observed very long needle-like structures form immediately inside the gallium droplet in ambient conditions.

After talking to my adviser, we decided it might be possible to grow these “nano-needles” selectively at the end of AFM probes. Our idea was to coat AFM probes with silver film, move them deep into a small gallium droplet, and then pull away from the droplet to grow freestanding nanoneedles. I prepared 15 probes coated with silver film and tried to grow needles on them. After investigating the finished samples using a scanning-electron microscope, I found that only one of them had worked. Still, the fact that I had reproduced the effect was promising, so when I showed the result to Cohn, he gave me two thumbs up.

At the same time, I began thinking about the commercial potential of the technology, especially for fabrication of specialized AFM probes. Currently, most AFM probes have a conical silicon tip, but although these probes are cheap (just $10–20 each), they do not last very long – after a few scans they become dull and useless. Another problem with commercial silicon probes is that they are made from pure “intrinsic” silicon and therefore do not conduct unless coated by the user. In addition, their conical tips cannot image deep micro- and nano-structures. A conical AFM probe with a conductive nanoneedle grown at its tip would, in contrast, be electrically conductive, capable of imaging deep trenches and longer-lasting than conventional probes. Finally, the simple geometry of these probes would make them useful for quantitative force microscopy measurements.

Considering all these advantages, I thought that my devices could make a successful business. But a few hours later, reality began to set in. How could I manufacture such a product? I was living in a foreign country on a student visa. I had $500 in my bank account. I knew that starting a business costs a lot of money, and I did not have much knowledge or experience of the business world. All of these things kept me from taking immediate action on my idea. Nonetheless, between 2004 and 2006, I continued working on the technology and its applications, and they ended up forming the main part of my PhD dissertation.

When I graduated in July 2006, my wife was still a PhD student at UofL, so I started working as a postdoc there in order to stay in the area until she finished her degree. I looked for opportunities outside academia, too, but I soon realized that my chances of finding a job in the Louisville region as a PhD graduate in nanotechnology were close to zero. On the positive side, the lack of job opportunities at someone else’s company encouraged me to start my own. I told myself, “I got a PhD not to get a job, but to create jobs,” and I signed up for a day-long “business boot camp” at UofL to help me get started.

I thought my devices could make a successful business

Learning the ropes

At the boot camp, I learned about several funding opportunities, including one programme called Small Business Innovation Research (SBIR) that gives US government grants to small firms, and another “matching fund” initiative sponsored by the state of Kentucky. I also realized that there are “angel investors” who invest money in start-ups and various sources of venture-capital funding.

All of this made me realize that having $500 in savings was not going to be a problem, as long as I had a promising technology. That is when I took my first step towards establishing a company: I paid a $45 fee to register NaugaNeedles LLC with Kentucky’s secretary of state. So far, so good, I thought; I had a company now, and it only cost me 45 bucks! My next step was to ask my PhD adviser if he would be interested in joining NaugaNeedles as the first member of its board of advisers. He accepted, and so did a few others, including the president of Zyvex Labs, an established nanotechnology firm based in Texas.

Now I had a company and a board of advisers. What next? The immediate answer, I realized, was “nothing” – I couldn’t do anything if I had no cash. So my next step was to apply for a $25,000 grant from the state of Kentucky via a “concept pool fund” aimed at early-stage start-up firms. I obtained this funding in November 2007, and it allowed me to make a website for NaugaNeedles, prepare evaluation samples for potential customers using facilities at UofL, and develop a business plan. This small fund was instrumental to NaugaNeedles’ success, because it helped me find a few customers who were ready to pay for my company’s product.

During this early phase, I began collaborating with a physicist, Ron Reifenberger, and a mechanical engineer, Arvind Raman, at Purdue University in the neighbouring state of Indiana. As a result of this collaboration, in 2009 NaugaNeedles received funding via the US National Institutes of Health. This NIH grant allowed us to demonstrate that nanoneedles could be used as “nanocantilevers” for mass sensing, as well as measuring interaction forces at the piconewton (10–12 N) level.

Things really started to take off after my business plan won a prestigious $120,000 cash award from the Louisville-based Vogt Invention and Innovation Fund. I used this funding to get an additional loan of $120,000, and the combination of the two allowed me to rent manufacturing space, and build or buy the equipment that I needed to establish NaugaNeedles’ current facility. By June 2009 the facility was ready and in-house production began.

At about the same time, I applied for an entrepreneurship fellowship with a philanthropic organization, the Kauffman Foundation. I was selected along with 12 other top scientific researchers for a year-long programme that aimed to teach us how to take promising research forward to commercialization. Each of us had a business mentor and attended intensive workshops where we had the opportunity to network and learn from each other and from entrepreneurship experts. This fellowship allowed me to leave my job at UofL and become the first full-time employee of NaugaNeedles in October 2009.

Reaching maturity

One key question that start-up founders have to address is when to bring in new people and employees. For NaugaNeedles, this point came in mid-2008 – almost a year after the company was officially established, but also a year before we got our own manufacturing facility. By this time, I had realized that a lot of investors, both private and governmental, view the team as one of the most important aspects of a company; if you are a “one-man show”, no-one is going to take you seriously. After talking with several colleagues from UofL and old friends from other institutions, I expanded the company to include two additional employees: my partners and co-founders, Amir Birjandi and David Mudd, who are NaugaNeedles’ chief financial officer and director of operations, respectively.

Today, in addition to the three co-founders and the members of the advisory board, NaugaNeedles has six full-time and four part-time employees. We have launched several categories of products including high-aspect-ratio NeedleProbes (used for both conductive AFM scanning and imaging deep trenches) and exposed-end NeedleProbes, which are specialized for conductive AFM scanning under liquid media. Since its founding, the company has raised more than $2.5m in funds from various sources, and our production facility is capable of making more than 100 NeedleProbes per day – the equivalent of about $3m worth of products per year.

The lesson I take from this is that entrepreneurship is not as scary as it sounds. My main advice is that if you believe there is a business opportunity in your research, go after it! It is not as hard as you might think to start a business, even if all you have is $500 and one great idea.

Animal magic

By Matin Durrani

PWNov12cover-200.jpg

If you’re lucky enough to be a member of the Institute of Physics, you’ll have had access for almost a week now to the November 2012 issue of Physics World – either in print or through our digital issue, which you can access online or via our apps for smartphone and tablet devices.

But as the November issue is a special issue devoted to “animal physics”, we felt we wanted to share the issue more widely because we know how much everyone loves animals. So from today we’re making the issue available in free downloadable PDF form.

Of course, the PDF doesn’t have all the goodies of the digital issue, which this month includes some fantastic videos and audio of animals in action. But nevertheless the PDF is packed with a series of fascinating photos and features on a selection of animals all of which have some interesting physics involved in their daily lives.

So you can read how mosquitoes survive collisions with raindrops, find out why a certain species of hornet has an in-built solar cell, and discover why lions – strange as it may seem – roar like babies cry. You can also examine the age-old question of why zebras have stripes and ask whether cats and dogs drink in the same way.

Plus there is a series of seven fabulous images each devoted to a particular animal with some amazing physics powers. Download the PDF now.

Remember that if you want to read Physics World every month you can join the Institute of Physics as an IOPimember quickly and easily online by visiting the Institute’s website. IOPimember includes an annual digital subscription to Physics World.

And while I’m on animals, don’t forget to register – if you haven’t already – for our free online lecture on animal physics, which will be given by David Hu from the “laboratory for biolocomotion” at Georgia Institute of Technology at 3.00 p.m GMT on Thursday 8 November. You can register via this link.

Quantum corrections could boost combustion models

Hydrogen’s ability to burn and explode more readily than expected at high pressures is down to quantum mechanics – that is the conclusion of a new study by physicists in Russia, who claim that their research could lead to fewer hydrogen explosions at nuclear power plants and to the safer industrial production of hydrogen and other gaseous fuels.

The potential for hydrogen explosions at nuclear reactors was graphically illustrated in March last year at the Fukushima Daiichi power plant in Japan. Three of the reactors at the plant melted down after the enormous tsunami that struck the north-east of the country knocked out the reactors’ cooling systems. The zirconium alloy cladding around the nuclear fuel in the reactors’ cores was then exposed by falling water levels, which reacted with steam to produce hydrogen. Once vented into the surrounding containment vessels, the built-up hydrogen then reacted with oxygen in the air and caused an explosion that blew the vessels apart. Such explosions are dangerous because they can potentially release radioactivity into the environment.

However, it is not known under precisely what conditions such hydrogen explosions occur. The problem lies with determining the time needed for hydrogen gas at a certain temperature and pressure to ignite, as well as the threshold temperature required for that gas to detonate – detonation occurs when combustion reactions propagate more quickly than sound. Theoretical predictions of these quantities do not agree with experimental results, and this disagreement increases as temperature decreases and pressure increases. At temperatures of 700–800 K, the calculated ignition delays can be up to 1000 times longer than those measured in reality.

Uncertain energies

Scientists have previously tried to explain these discrepancies by arguing that the calculations ignored the effect of certain impurities within the gas or, more simply, that the measurements themselves were inaccurate. But these discrepancies also occur for other gaseous fuels. In the latest research, Vladimir Fortov and colleagues at the Joint Institute for High Temperatures in Moscow, together with two physicists at the Troitsk Institute for Innovation and Thermonuclear Research, located outside the Russian capital, provide a general explanation for the mismatches by incorporating quantum corrections into existing combustion models.

In their new study, Fortov and co-workers considered the classical Maxwell Boltzmann distribution, which describes the spectrum of molecular velocities in a gas at a certain temperature. Theorists have recently shown that quantum-mechanical effects can significantly alter the shape of the higher-energy end of this distribution for gases at relatively low temperatures and high pressures. The modification is a result of Heisenberg’s uncertainty principle. The uncertainty lies in the energy of colliding particles increasing the probability of reactions taking place between them, as long as the density of those particles is high enough.

While this modification is insignificant for general thermodynamical phenomena governed by the overwhelming majority of molecules with kinetic energies close to the mean, it is relevant for processes in which the high-energy “tail” of the distribution plays an important role. One such process is nuclear fusion in dense, cool plasmas. The Russian group has now shown that it also holds sway in the chemical reactions involved in combustion, given that the energy needed to activate such reactions is far higher than that of thermal molecules’ mean energy. The researchers found that by introducing quantum corrections, they obtained a close agreement between theory and experiment for pressurized argon gas containing 4% molecular hydrogen and 2% molecular oxygen. They also found a good agreement when analysing the detonation of acetylene.

Optimized geometry for safety

According to Fortov, these calculations could help improve safety at nuclear plants by allowing engineers to study the full range of conditions across which hydrogen detonation could occur. As such, he says, it should be possible to reduce the chances of hydrogen explosions by optimizing reactor geometry or by knowing how best to position devices known as “hydrogen recombiners” that eliminate hydrogen from containment vessels by combining it with oxygen to form water vapour. Fortov also claims that his group’s work could allow industry to produce and store hydrogen more safely and also to better handle acetylene, which is widely used as a fuel and as a chemical building block, and can ignite and detonate without an oxidant.

However, Tony Roulstone, a nuclear engineer at the University of Cambridge, doubts whether the latest work will have significant practical benefits for nuclear power production. He says that designers of reactor containment vessels assume that hydrogen will always burn if it exceeds the “experimentally determined hydrogen flammability limit”. He reasons that it might be possible to reduce the pressure that the containment structure is designed to withstand, if there were better ways of calculating when and at what rate the hydrogen is burnt. But he points out that, in practice, such a reduction may not be possible given that reactor containments are also designed to resist large aircraft crashes. “It is these crashes that are likely to be the controlling case,” he says.

Keith Ross of Salford University, meanwhile, is doubtful that the quantum corrections will have any significant impact on the implementation of hydrogen storage. He says that he assumes any calculations by safety engineers are simply based on the shorter ignition time revealed by experiment, although he does add that “there is always the potential advantage of using a theoretical experiment to assess a range of experimental situations”.

The research is published in Physical Review Letters.

Highly charged ions could make better atomic clock

A new atomic clock that promises to be accurate to within 40 ms over the age of the universe has been proposed by physicists in the US and Australia. Based on a bismuth atom that has been stripped of 25 of its electrons, the clock could be used to look for variations in the fine-structure constant – according to its designers. The discovery of such variations could lead to a new unified theory of physics.

Today, the best clocks use an atomic transition as a time standard to measure time to an accuracy of about one part in 1017 – and physicists are keen on building even better timekeepers. A clock with an accuracy of one part in 1019 could help reveal minuscule changes in the values of fundamental physical constants such as the fine-structure constant. This parameter characterizes the strength of the electromagnetic interaction and detecting possible variations could help solve the biggest mystery of physics – how to formulate a single unified theory that describes the four fundamental forces: gravity, electromagnetism and the strong and weak nuclear forces.

In March 2012, a team led by Corey Campbell at the Georgia Institute of Technology argued that the required level of precision could be attained by using a particular nuclear transition in the thorium-229 ion with a charge of 3+. However a practical clock is unlikely because there are two problems with the scheme. The first problem is that the exact frequency of the thorium-229 transition is extremely difficult to calculate and therefore a great deal of time and effort could be spent in the lab just searching for the transition. The second problem is that thorium-229 is radioactive, making it difficult to work with.

Highly charged solution

Now three of Campbell’s collaborators have proposed what could be a more practical way to reach this level of precision using highly charged ions. Andrei Derevianko of the University of Nevada in Reno and Vladimir Dzuba and Victor Flambaum of the University of New South Wales in Sydney, looked at traditional atomic clocks to see if their errors could be brought down to the level of the nuclear clock.

Today, the most accurate current atomic clocks use aluminium ions (Al+) in an electromagnetic trap. However, stray fields can exist in the trap and these perturb the energy levels of the ion – reducing the performance of the clock. Derevianko and colleagues reasoned that as more and more electrons were stripped away from the ion the remaining electrons would be pulled closer to the nucleus and stray fields would have a less detrimental effect on performance.

The researchers calculated, therefore, that observing a specific electronic transition in a bismuth-209 ion (209Bi25+) would allow them to reach the required accuracy level. Like the aforementioned nuclear transition, this transition has not yet been observed. Derevianko explains, however, that, in stark contrast to the nuclear-structure calculations required to locate the nuclear transition, electronic-structure calculations are far more reliable and so the transition’s location can be predicted with much greater accuracy. Furthermore, bismuth-209 has a half-life of greater than 10 billion billion years – so can be considered non-radioactive.

New technology, new challenges

While such a clock would be difficult to build, the trio argue that it should be possible. “It is much harder to trap and cool highly charged ions,” explains Flambaum, “It is a new technique that has just started to appear. But people do this – it is not like our proposal just appeared out of the blue. It is just a new technology that requires new installations. It was much easier to work with neutral atoms or singly ionized atoms so of course people started from this, but now the time has come to search for other opportunities.”

Helen Margolis, an atomic-clock expert at the National Physical Laboratory in Teddington, is intrigued by the proposal but believes that it will pose numerous challenges before it can be experimentally implemented. “People are very clever at dreaming up new ways to do things,” she says, “but working on highly charged ions of this type is certainly not easy and they would need to do a lot of things that have never been done before.”

In particular, she does not share Derevianko’s confidence that the researchers’ calculations can reliably predict the location of the relevant transition. “It is true that for the highly charged ion clock the calculations of where these transitions occur are probably better,” she says, “but they are still not accurate enough to make the search for these transitions easy.”

The research is published in Physical Review Letters.

Earth’s magnetic shield behaves like a sieve

The Earth’s magnetic field is more permeable than previously thought, according to researchers analysing data from the European Space Agency’s Cluster mission. The findings have implications for modelling the dangers posed by space weather and could also help us better understand the magnetic environments around Jupiter and Saturn.

The Cluster mission, launched in 2000, comprises four identical satellites flying in a tetrahedral formation in close proximity to Earth. With highly elliptical orbits, the satellites are able to sweep in and out of the Earth’s magnetic environment, building up a 3D picture of interactions between the solar wind and our planet. The solar wind is a stream of charged particles from the outer layers of the Sun blowing into the solar system. The Earth’s magnetic field is thought to form a protective barrier against it.

It is well known, however, that if the magnetic field of the incoming solar wind has the opposite orientation to the Earth’s magnetic field, then the field lines can break and join up again in a process known as “magnetic reconnection”. This process allows the plasma from the solar wind to breach the boundary of the Earth’s magnetic field – the magnetopause – where it can then potentially reach our planet.

Swirling vortices

In 2004 data from Cluster revealed that this mismatch in magnetic orientation was not a hard and fast rule – 40,000 km-long swirls of plasma were spotted along the magnetopause, creating gateways into the magnetosphere even when the two magnetic fields were aligned. By 2006, researchers had concluded that these vortices were likely caused by Kelvin–Helmholtz waves (KHWs), occurring when two mediums are flowing on either side of a boundary at different velocities. An earthly example is wind blowing over the boundary between the air and the ocean. In space, the boundary is the magnetopause, with the decelerated plasma on the Earth-side travelling slower than the solar-wind plasma beyond.

Once created, the amplitude of these instabilities can build up, tangling the magnetic field lines and triggering magnetic reconnection despite the field lines being aligned. This phenomenon was only thought to happen under special conditions, however. “We thought [it] was restricted to areas around the Earth’s equator,” Arnaud Masson, one of the scientists working on the Cluster mission, told physicsworld.com. Now, new analysis of Cluster data, initially obtained in 2003, shows the same thing happening at much higher latitudes, and at a wider range of magnetic-field alignments. “It seems that no matter what the orientation of magnetic fields, the same effect can occur,” explains Masson. “It appears that it happens all the time, rather than just in special circumstances.”

Modelling space weather

Knowing the range of conditions under which the solar wind can penetrate the Earth’s magnetic defences plays an important role in modelling space weather – a catalogue of effects including disruption to GPS navigation caused by interaction with the solar wind. “You have to know where the doors are open in our protective shield,” explains Masson. Chris Arridge, of University College London, agrees. “It seems there are a lot more holes in the Earth’s magnetic sieve than we thought,” he says. “If we want to develop the ability to predict space-weather effects, then it is important to know the full range of ways that energy, mass and momentum can get into the system.”

Arridge, a researcher on the interaction between the solar wind and the outer planets of the solar system, believes this line of research could also help us understand Jupiter and Saturn. “The exact role of KHWs in the magnetospheres of the giant planets is a hot topic,” he reveals. Understanding the Earth’s KHW mechanisms will help us understand Jupiter and Saturn’s magnetic environments and vice versa.”

The research is published in Journal of Geophysical Research.

Improved diamond-anvil cell allows higher pressures than ever before

An international team of researchers has developed a technique that allows higher-than-ever-before static pressures to be generated in the laboratory, using a newly designed diamond-anvil cell. The team was able to create 640 gigapascals (GPa) of pressure – 50% more pressure than previously demonstrated and 150% more pressure than can be achieved by most typical high-pressure experiments.

At pressures vastly greater than those found on the surface of the Earth, matter can behave in strange ways. Oxygen can become superconductive, while metals can turn into insulators. In a 2007 experiment, sodium was found to turn transparent when squeezed by a pressure of 200 GPa – two million times the pressure at the surface of the Earth. Theorists have predicted, and there have been several unconfirmed observations, that hydrogen can become metallic.

Up the pressure

The diamond-anvil cell is the tool of choice to generate extreme pressures as it squeezes a sample between two tiny, gem-grade diamond crystals – diamond being one of the hardest substances known. Unfortunately, hard as diamonds are, they always fail eventually. This makes it challenging to achieve static pressures above about 250 GPa, and almost impossible above 420 GPa.

The only way to achieve super-high pressures in the laboratory, until now, has been to bombard a sample with shock waves, which compress it suddenly to generate pressures of many hundreds of gigapascals. But there are two problems with this technique: first, it grants only nanoseconds of observation time; and second, it generates a lot of heat as well as very high pressure, which can make it difficult to disentangle the effects of the two as solids are often turned into liquids. This is problematic for geophysicists trying to study the reactions at the centre of the Earth, where the pressure is stable at above 350 GPa, and even more so for those studying the gas giants, for example, where the internal pressures are well above this. A tool to generate static pressures in these ranges would be useful in the geophysicist’s arsenal.

Diamond clusters

To try to develop such a tool, Leonid Dubrovinsky and Natalia Dubrovinskaia of the University of Bayreuth in Germany, together with colleagues from Belgium and the US, took a hard look at the existing diamond-anvil cell. When diamonds eventually fracture, they do so along one of the cleavage planes running through the material. Based on previous work, the researchers knew that a diamond made up of numerous small crystals would not have these well-defined cleavage planes.

The researchers fabricated nanocrystalline diamond hemispheres, about 12–20 μm diameter, from tiny carbon balls at 2200 K and 20 GPa pressure using a newly developed technique. They then made a two-stage diamond anvil. On the outside of the press, they arranged two flat, gem-grade diamond plates. Inside these they placed their nanocystalline hemispheres. The flat edges were placed against the plates, with the curved edges, giving the smallest contact area and the largest pressure, placed against the sample. After some optimization, the researchers were able to generate static pressures of up to 640 GPa of the nanocrystalline diamond on the sample, allowing them to measure the equations of state of rhenium and gold at these extraordinary pressures.

Increasing stability

Dubrovinskaia believes that by implementation of computer control it should be possible to improve the stability of the pressure in the device. “By increasing the stability, we should be able to reach higher pressures,” she says. “That is one side – the other aspect is improvement of the materials themselves. The strength of nanocrystalline diamonds, like any polycrystalline material, is different depending on the size of nanoparticles. So if we play with particles with different sizes and different shapes of nanocrystals, it may play a significant role.”

Katsuya Shimizu, an expert in the study of matter at high pressures from the University of Osaka, Japan, who led the team that demonstrated superconductivity in oxygen, believes the double-stage diamond-anvil technique proposed by the researchers, which requires a tiny amount of the sample to be placed between two “semi-balls”, may limit its applicability. Nevertheless, he believes that “some researchers will perform experiments following this technique”.

The researchers conclude that “achieving 1 TPa in static-compression experiments with double-stage diamond-anvil cells is a viable goal”. The pressure in the cores of gas giants is about 700 GPa, so if Dubrovinskaia and colleagues further perfect their technique, astronomers may soon have a tool to study the conditions within these planets.

The research is published in Nature Communications.

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