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Polaron melting heralds colossal resistance

Nearly 15 years ago, two groups working independently discovered that the electrical resistance of manganese oxides called manganites dropped by several orders of magnitude when the materials were exposed to a magnetic field. Dubbed colossal magnetoresistance (CMR), the effect is much different than giant magnetoresistance, which is relatively well understood and involves changes in resistance of several tens of percent. However, very little progress has been made in understanding CMR and the related effect of colossal electroresistance (CER), which occurs when some manganites are exposed to external electric fields.

Physicists had suspected that CMR and CER are related to polarons, which are charged quasiparticles that arise when conduction electrons and vibrating atoms (phonons) in a solid interact. In the absence of an external electric or magnetic field, electron-phonon interactions are thought to be strong enough to localize the polarons in an ordered “solid” that gives the material a high resistance.

However, an applied field is thought to weaken the electron-phonon interaction, allowing the polarons to move around much like a liquid. In some cases, the polarons strongly delocalize and move so freely that resemble the conduction electrons in a metal, explaining the 10¹⁰ drop in resistance seen in some manganites.

Now, a team including Christian Jooss at the University of Goettingen in Germany and Yimei Zhu from the Brookhaven National Laboratory in the US has confirmed the role of polaron “melting” in colossal resistance by watching the resistance of a manganite sample drop as its polarons transformed from solid to liquid.

The team used the tip of a scanning tunnelling microscope (STM) to apply an electric field to a small region of the sample and also to measure the resistance in that region. The tip was brought into contact with the sample and several volts was applied to create an electric field. The resistance of the sample could be determined by measuring the current flowing between the tip and the sample. At the same time a transmission electron microscope (TEM) was used to perform electron-diffraction measurements which confirmed that solid polarons in that region turned to liquid when a sufficiently large field was applied.

Although their experiments focussed on CER, the team are confident that their findings with help further the understanding of CMR as well. The researchers also believe that their work could make it possible for CER to be used to create resistive -random access memory (RRAM) devices, which are currently being developed by several chip makers. In a RRAM, data are stored in bits that can be switched between a very high resistance state and a very low resistance state. This simplifies the circuit design, shrinking the size of individual memory bits and allowing the data to be read and written hundreds of times faster than conventional memories.

Dreaming of fusion

You’ve had many jobs, including being a scientific advisor on the International Thermonuclear Experimental Reactor (ITER) and on the Joint European Torus, the largest fusion experiment currently in operation. What’s the most important role you’ve had?

After the fall of the Soviet Union I was part of an international team that helped convert Russian weapons scientists to peaceful activities. The challenge was to prevent them from being “brain drained” to unstable countries by finding them jobs in the civil environment. It was extremely challenging and political from a managerial point of view, which really prepared me for the job I have today.

One example was when – with help from a friend at CERN – we restructured a company that was making crystals for laser-guided weapons to make crystals that will detect the Higgs boson in the CMS experiment at the Large Hadron Collider.

It’s not easy to work with former military scientists. They would say “Well, it’s fine to convert my activities, but I am not going to build a washing machine!” It wouldn’t have been challenging enough for them.

Is the project over now?

No, but it is in the process of some restructuring. The situation in Russia today is completely different from ten years ago.

What first interested you in fusion?

After I graduated from the Ecole Normale Supérieure in 1980 I became concerned about the consumption of oil and the need to develop alternative energy systems. Originally I was attracted to solar power, but when I was looking to start a PhD I came to meet very good people that were dealing with fusion. Its difficulty and complexity engaged me right from the start.

What are your challenges as director of Fusion for Energy?

I want to create an organization that is efficient, effective and economic. And I want it to be respected by the ITER community, the equivalent non-European national agencies and the network of laboratories and industries in Europe. Fusion for Energy is the largest agency because Europe is the largest contributor – we provide about 50% of ITER’s money, components and personnel.

But Fusion for Energy is not limited to ITER – we’ve also established a partnership with Japan on “Broader Approach” activities. These will help to construct a future prototype-material test facility, a tokamak and a computing centre. In the longer term, Fusion for Energy will then help to organize the construction of the first demonstration or “DEMO” fusion power plant some years from now. For me, ITER will be an excellent benchmark to prepare for the future construction of other fusion reactors that will eventually go on the market worldwide.

If we’re successful we will have accumulated an enormous amount of experience

What is the current status of ITER?

So far we’ve cleared the Cadarache site and are drawing up the contract that will allow us to start excavating, and we’re preparing a specification for the tokamak and other buildings. Because the design of the facility had to remain somewhat generic until an agreement was reached on where to locate the facility two years ago, ITER is now completing a review to adapt it to the particulars of the Cadarache site.

One of the pioneers of tokamak research, a Soviet physicist called Lev Artsimovich, once said: “Fusion will be here when society needs it.” So how far away are we from a commercial fusion reactor?

Well, ITER will put out ten times more power than it uses, so it will bring the reality of commercial fusion closer. Importantly, though, ITER is also going one step ahead in terms of scientific cooperation. For example, a poloidal field coil – one of the coils that shape the magnetic field – will have components made in China, Russia and Europe that are assembled on-site. So if the EU gets the components made on time but the Chinese do not, we will have a problem. If we’re successful – and I wouldn’t be doing this if I didn’t think we would be – we will have accumulated an enormous amount of experience.

If current climate data are anything to go by, it looks as though we’re going to need a clean source of energy soon. Does fusion fit the bill?

We need to have large, localized sources of energy. A fusion power plant delivering, say, 2 GW is good because you can draw 2 GW from a single point. But if we try to get the same output from solar panels, for example, it becomes much more difficult. I’m not saying it’s impossible, but it becomes more complex to draw as much energy as you need from a network of low-output sources instantaneously.

But given that fusion is a nascent technology, do you think scientists still have a responsibility to look at interim measures to tackle climate change?

Yes. I am not saying: “believe in fusion, we’ll be there tomorrow!” Climate change is taking place now. How do we mitigate the impact of it? Not by pouring more CO₂ into the atmosphere as we are doing today. Every bit of technology, of behaviour, that society can put together so that we put less CO₂ into the atmosphere should be used. If the problem is serious, everyone’s help is needed.

Modified gravity fails at long distances

To write down a successful “theory of everything”, physicists need to know why gravity is many orders of magnitude weaker than the other three fundamental forces of nature. One explanation is that gravity leaks into extra dimensions, leaving us to feel a mere fraction of its attractive force.

Most theories predict that these dimensions would be no larger than a millimetre or so. But some say extra dimensions could be non-compact, meaning that odd gravitational effects would occur at astronomical distances. In the past, such long-range effects have also been put forward to explain the odd trajectories of NASA’s disused Pioneer 10 and 11 probes, which appear to be being affected by a small additional force as they leave the Solar System.

Iorio’s study, however, discredits ideas of long-range effects. He has examined a general class of theories that add “Yukawa” modifications to Newton’s inverse-square law of gravitation, which allow gravity’s attractive force to fall more or less rapidly with distance. He has then calculated how much several different modifications would affect the Newtonian time-varying “perihelion” – the closest point in an orbit to the Sun – of Earth and Mercury.

Physicists can already simulate how the planets’ perihelia change over time to a reasonable accuracy using Newton’s and Einstein’s theories of gravity, but have always had to make small corrections to make them fit observational data. Iorio has checked whether the sizes of his calculated effects from different Yukawa modifications are small enough to fit within these corrections.

He found that the only modifications compatible were those that operated on a range less than a tenth of the distance from the Earth to the Sun, or 0.1 AU. However, he warns that more evidence from other perihelia corrections would be needed before Yukawa modifications with a range greater than 0.1 AU can be ruled-out with certainty.

Orfeu Bertolami, a theoretical physicist from the Instituto Superior Técnico in Portugal, seconds Iorio’s caution. “I am not sure that the study of the perihelia of some planets is sufficient to draw any definite conclusion,” he told physicsworld.com. “A new contribution for the gravitational force affects the whole orbit of the planets and not just the perihelia motion.”

Team claims first silicon spinFET

The spinFET was made by Biqin Huang and Ian Appelbaum of the University of Delaware, along with Douwe Monsma of Cambridge Nanotech in Massachusetts, who earlier this year worked out a way to transport a spin-polarized current of electrons 10 µm through a piece of silicon. Their new device builds on this work, taking advantage of the fact that the direction of the spin-polarization can be rotated as the electrons move through the silicon by applying a magnetic field.

The device was set up so that the current entering the silicon was spin-polarized in one direction – “up”, for example. As the electrons move through the silicon, they are exposed to the magnetic field for a relatively long time and therefore experience significant spin rotation. However, if an electric field is applied along the direction of travel, the electrons move more quickly, spend less time in the magnetic field and are rotated less.

Once the current has traversed the silicon, it goes through a spin filter, which only passes the portion of the current that is still polarized in the up direction. As a result, if no electric field is applied, very little current is detected at the collector. However, if a voltage is applied along the silicon to create an electric field, more current is passed through to the collector. In one experiment, the current at the collector increased by a factor of about seven when the voltage is increased from zero to about 3 V.

The device is similar to a conventional FET because it uses an electric field to control an output current. While it is not the first spinFET – that honour goes to Christian Schönenberger and colleagues at the University of Basel, Switzerland, who built a carbon nanotube-based device two years ago — Appelbaum told physicsworld.com that theirs is the first spinFET made from silicon. This is significant because silicon-based spintronics should be compatible with today’s commercial chip-making processes.

Indeed, the spintronics pioneer David Awschalom of the University of California, Santa Barbara told physicsworld.com that the silicon spinFET is “an important step driving the transition from research to practical applications in spintronics”.

However, Appelbaum remains cautious, citing two significant challenges that remain before a commercial spinFET becomes a reality. Their device is based on the “ballistic” transport of electrons through thin magnetic films, which results in very small output currents on the order of tens of picoamps. Also, their device must be operated a very low temperatures – about 85 K – which would be impractical for a commercial device. “This [temperature] can easily be increased substantially”, says Appelbaum, “but design changes are most likely necessary for demonstration of spin transport at room temperature”.

Ralph Alpher: 1921 – 2007

Born into a Jewish family in Washington DC on 3 February 1921, Ralph Alpher studied at George Washington University. It was here that he met Gamow, who took him on as a PhD student. Together Alpher and Gamow began calculating the relative abundance of elements that would be produced in a hot Big Bang.

The pair assumed that the early universe was very hot and full of neutrons. Nuclei then formed by capturing neutrons one at a time, with the occasional nucleus decaying to produce a heavier nucleus plus an electron and a neutrino. Their calculations correctly showed that the abundance of elements in the universe should decrease with atomic mass.

However, this early version of “Big Bang nucleosynthesis” could not explain the origin of all the chemical elements as Alpher and Gamow had hoped — we now know that elements heavier than lithium are produced in the interior of stars. Nevertheless, their calculations did mark the start of cosmology as a branch of physics by providing estimates for nuclear abundances that could be checked with experiment.

Alpher and Gamow reported their calculations in a paper published in 1948 (Phys. Rev. 73 803). Gamow famously invited the physicist Hans Bethe to be a co-author so that the paper was written by “Alpher, Bethe, Gamow” as a pun on the first three letters of the Greek alphabet. Bethe, however, had contributed almost nothing to the work.

Several months later, Alpher and Robert Herman from Johns Hopkins University published a separate paper predicting that the radiation left over from the Big Bang would have a temperature of 5K. Arno Penzias and Robert Wilson of Bell Labs later shared the 1978 Nobel Prize for Physics for discovering this cosmic microwave background, which has a temperature of 2.7K.

However, Alpher’s contribution went largely unrecognized partly because he left cosmology and joined General Electric’s research centre in Schenectady in New York in 1955. Alpher later moved to Union College in 1986, where he was emeritus professor. He was, however, awarded the US National Medal of Science in 2005. Alpher and Herman also wrote a book about their early work entitled Genesis of the Big Bang in 2001.

Light collapses step-by-step

Quantum mechanics says the world is innately random: an isolated system will remain in a fuzzy “superposition” of all possible states until we measure it, at which point it will collapse with a certain probability into just one state. Recently physicists have discovered that this collapse can be witnessed step-by-step if a series of special quantum non-demolition (QND) measurements are performed. Now, Serge Haroche and colleagues from the Ecole Normale Supérieure have developed a new QND technique to see, for the first time, the step-by-step collapse of a coherent light field.

Their system is a box lined with superconducting mirrors, which can keep the photons from a coherent microwave field bouncing around inside for a fraction of a second. Because of the Heisenberg uncertainty principle, which states that two “conjugate” properties cannot be known simultaneously with precision, the microwave field is coherent at the expense of having a well-defined number of photons. This means that, before measurement, the system is in a superposition of several different photon numbers — in this case between zero and seven.

To watch this superposition collapse the researchers used a method they pioneered in 1999, in which individual atoms are sent in to interact with the field’s photons without destroying any of them. Electrons in these atoms oscillate between two excited states, and the rate of the oscillation is governed by the photon number. For each measurement of an atom’s oscillation rate, therefore, the researchers get an answer for how many photons are in the box.

For the first few measurements the answers are evenly distributed from zero to seven. This means that not enough information has been gathered to ascertain the number of photons, since the system is still in a superposition. But after many measurements the cumulative distribution of answers begins to centre on a particular number, revealing that the system is collapsing into a well-defined state. And as predicted by the probabilistic nature of quantum mechanics, if the whole measurement procedure is performed again, this particular number could well be different.

“All previous methods for counting discrete photons have been destructive,” Haroche told physicsworld.com. “The photon, like the marathon soldier, was dying delivering its message. Our experiment demonstrates that counting is not a curse…this was predicted by quantum mechanics but never demonstrated.”

Ionic winds could chill computers

Most portable computers are cooled by a fan which blows air across its hot components. However, this method is reaching the limit of its effectiveness, in part because the air right next to the surface of a chip tends to remain stationary and insulates the hot surface from the cooling breeze of the fan.

Researchers at Purdue University in Indiana and Intel have worked out a way around this problem by using a stream of ions to get this layer of stationary air moving. The team built an “ionic wind engine” that fires ionized air at a hot spot on the surface of a heated glass plate, which was designed to mimic the backside of a computer chip.

The engine consists of two electrodes — with the negative electrode located on the plate and the positive electrode a few millimetres away from the plate. A high voltage across the electrodes creates ions in the air above the hot spot in a process called coronal discharge. A current of positively-charged ions then flows towards the hot spot, causing the normally stationary air to move.

In one experiment, the team found that when the ion engine was turned on, a conventional fan was able to cool the hot spot from about 60 to 35 degrees Celsius. But without the ion engine, the fan could only cool the hot spot to 55 degrees.

According to the researchers, the ions increase the heat transfer coefficient of the cooling system – its ability to transfer heat from a solid to the air — by 2.5 times. According to Purdue’s Suresh Garimella, previous attempts at improving the performance of fan-cooling systems had only managed a 50% improvement.

Although the ionic wind engine is too large (about 4 mm across) and requires too high a voltage (more than 4000 V) to be of any practical use, the team is now working on miniaturizing the generator by a factor of about 1000 so it can be used to cool a real chip. The team have already worked out a way to create a tiny coronal discharge using diamond-coated electrodes separated by about 10 µm and operating at tens of volts. The next step, according to Garimella, is to make the technology rugged enough for use in portable equipment.

The team have applied for patents on the technology, which Garimella claims could be used in computers within three years.

Garimella and colleagues are no the only ones interested in cooling chips using ionic wind. Earlier this year, US-based Kronos Advanced Technologies claimed to have worked out a way to replace the conventional cooling fan in a computer with a coronal discharge generator that uses ionic wind to push air across a microprocessor.

Physicists hit the rippled road

Unpaved roads around the world are plagued by surface ripples — called washboards — that are several centimetres high and formed under the rolling wheels of cars, buses and lorries. Unfortunately, numerous attempts by road engineers to find out how washboards could be avoided have met with little success. Indeed, the only practical way of dealing with these ripples is using heavy machinery to smooth-out road surfaces on a regular basis, which can be a very costly exercise in the remote regions where unpaved roads are usually found.

Now Jim McElwaine of Cambridge University, Nicolas Taberlet of the École Normale Supérieure de Lyon and Stephen Morris of the University of Toronto have devised a very simple experiment to study ripple formation. The researchers placed a flat bed of sand on a round table, which could be rotated at a constant rate of about 0.6 revolutions per second. A rubber wheel could be lowered onto the moving sand, where it could move up and down according to the level of the sand (see Ripples in the sand).

The researchers found that the wheel typically caused a small single ripple to form at one location in the sand after about ten or so rotations of the table. New ripples then grew rapidly from the single ripple and spread until the entire path of the wheel was covered in ripples.

The experiment was repeated using a different type of sand and again using long-grain rice. To their surprise, the researchers discovered that changing materials had little effect on ripple formation – suggesting that washboards cannot be avoided by using a specific type of material in road construction.

Indeed, the experiments and related computer simulations revealed that ripple formation is governed only by the speed of the wheel, its weight per unit width and the density of the granular material. Ripples were not seen when the wheel was kept below a critical speed – about 8 km/h for a car – leading the team to conclude that at higher vehicular speeds a flat road is unstable and will quickly become rippled. The study also suggests that heavier wheels will produce smaller ripples because their greater mass inhibits the vertical motion required to make a washboard.

According to McElwaine, ripple formation begins because no road surface is perfectly flat. When a wheel encounters a random hump in the road, it rises up and falls back down, pushing a bit of material out of its way to create a trough and a second hump. Subsequent wheels continue this process and a pattern of regular humps and troughs emerges.

McElwaine believes that the emergence of ripples could be delayed by making make the road as smooth as possible. Indeed, in their experiments, it took hours for ripples to emerge if the sand bed started off being extremely smooth.

Another way of avoiding ripples, according to McElwaine, is the development of active vehicle suspension systems that prevent the wheels from going up and down at frequencies that correspond to ripple creation – something that the team are currently investigating.

Several movies of the experiment can be viewed online.

‘Cosmic train wreck’ stumps dark-matter physicists

Most physicists think dark matter exists because large structures in the universe appear to be held together by the gravitational attraction of much more mass than we can see through telescopes. One way to test theories of dark matter is to study cluster mergers, which are collisions between galaxy clusters after they have steadily gravitated towards each other. Cluster mergers are also a testing ground for alternative theories of gravitation, such as modified Newtonian dynamics (MOND), that eschew the possibility of dark matter altogether.

Observations of the Abell 520 cluster by Andisheh Mahdavi and colleagues at the University of Victoria, together with Peter Capak from the California Institute of Technology, however, seem to be inexplicable using either dark-matter or alternative-gravity theories.

The researchers used data taken from the Canada-France-Hawaii telescope and the Subaru telescope in Hawaii, along with data from the Chandra X-ray telescope, to see how gravity in the Abell 520 cluster acted as a lens to bend light passing through it on the light’s journey to Earth. Using this “gravitational lensing” technique, they mapped the distribution of the three components of the cluster: galaxies, prevalent hot gas and dark matter.

Mahdavi and colleagues discovered a core of dark matter and hot gas, with a bound group of galaxies separated to one side. This goes against accepted “collisionless” dark-matter theories because both the galaxies and the dark matter should have remained unimpeded in the collision – in other words, they should be in the same place. Although the observations could be explained by using a “collisional” dark matter theory, this would not simultaneously be able to explain other cluster mergers, such as the Bullet Cluster, that are already described well by the collisionless theories.

The researchers also say that they could not account for the observations using MOND. However, Hong-Sheng Zhao – a physicist from St Andrews University in Scotland who was part of a group that explained the dynamics of the Bullet Cluster using a relativistic alternative-gravity theory called TeVeS – told physicsworld.com that this might be because current simulations of MOND tend to ignore a subtle time-dependent effect of the gravity field. By including this effect in future simulations, he says, both the Bullet Cluster and the Abell 520 cluster could have the chance to be explained with an alternative-gravity theory. “Right now it is very curious,” he said.

Helices swirl in space-dust simulations

Vadim Tsytovich and colleagues at the Russian Academy of Science along with researchers at the Max Planck Institute for Extraterrestrial Physics and the University of Sydney simulated the behaviour of mixtures of inorganic interstellar dust in a plasma, which is an extremely hot gas of charged particles. Such plasmas are common in space and can even occur naturally on Earth at the point of a lightning strike.

Dust particles in a plasma are themselves charged, which leads to electrostatic interactions between the particles. The researchers assumed that — at certain separations — two dust particles would be attracted to one another, while at other separations the particles would repel. A computer program was then used to simulate how a large number of dust particles would interact within a plasma.

The simulations suggested that under conditions commonly found in space, the dust particles first form a cylindrical structure that sometimes evolved into helical structures. Along some spirals, the radius of the helix was seen to change abruptly from one value to another and then back again, providing a mechanism for storing information in terms of the length and radius of a section of a spiral.

In some simulations, a spiral would divide into two, effectively reproducing itself. In other simulations, two spirals induced structural changes in each other, and some spirals even appeared to evolve with time into more robust structures.

While many scientists would balk at calling such structures life – if they indeed exist in the first place — Tsytovich has no doubt. “These complex, self-organized plasma structures exhibit all the necessary properties to qualify as candidates for inorganic living matter,” he said. “They are autonomous, they reproduce and they evolve”. The team has also suggested that such inorganic life could have been a precursor to organic life here on Earth.

While these specific simulations have not been verified in the laboratory, experiments by other researchers on other dust-plasma systems have revealed the emergence of simple helical structures.

As for finding inorganic life in dust clouds surrounding nearby stars, the researchers say this could be done by looking for changes in infrared light from distant astronomical objects as it passes though a cloud of spirals – a measurement that could in principle be done with NASA’s Spitzer space telescope.

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