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Home » Page 1450

Holography puts electrons in a twist

 

Physicists have used holography to reliably do to electrons what they can already do to photons – twist the particle wavefronts to create beams with vortices at their centre. The researchers say that such vortex electron beams could reveal the magnetic properties of materials at the atomic scale and be used to build structures atom by atom.

Since at least the early 1990s, physicists have been able to produce light beams in which the photons not only have spin angular momentum – in other words, the wave is circularly polarized – but also orbital angular momentum. This means that the wavefront is made to spiral around the direction of propagation, generating vortices around the centre of the beam where the intensity of the wave is zero and its phase is undefined. These beams have been used to drive microscopic motors and have also served as “optical tweezers” that capture particles such as biological cells in their vortex and then move them around.

Earlier this year Masaya Uchida and Akira Tonomura of the RIKEN Institute in Japan showed how to produce vortex beams of electrons. They did this by directing electron beams around a tiny spiral-staircase like structure made from extremely thin slices of graphite. However, the technique is difficult to reproduce because it involves a painstaking search for the staircases within the graphite, rather than systematically producing such structures. The structures are also vulnerable to contamination and damage.

No object required

Jo Verbeeck of the University of Antwerp in Belgium and colleagues from Antwerp and the Technical University in Vienna say they have overcome this problem by instead making electron vortex beams using holograms. The technique is similar to that used to make optical holograms, in which light is reflected off the object and then made to interfere with a reference beam, creating a fringe pattern on a photographic plate that when illuminated by the original reference beam generates a three-dimensional image of the object. The crucial difference in this case, however, is that no object is required.

Computer software is used to calculate the fringe pattern that would be created by the interference of the object beam – the vortex beam – with a reference beam from a standard electron microscope. Then they use a focused beam of ions to carve out the pattern calculated by the computer in a thin foil mask made from platinum. Shining the reference beam through the mask then generates the vortex beam.

“I don’t see a good reason why this couldn’t have been done 10 years ago,” says Verbeeck. “I’ve been walking around with these ideas for three or four years, and it was only about six months ago that I actually sat down and thought about how to put them into practice. We then produced the beams the first time we tried.”

Easy to make

Verbeeck points out that the graphite slices produced by Uchida and Tonomura must have a thickness of the order of nanometres, a precision dictated by the need to directly manipulate the phase of the electron beam. In contrast, he says, the features in his group’s fringe patterns need be no smaller than microns, with the upper limit in the case determined by the need to keep a certain minimum distance between the vortex beams that emerge from the mask. Such micron resolution is relatively easily achieved using ion beams, as illustrated by the very close match between the shape of the image of the vortex beams obtained by the group experimentally with that calculated in the simulations beforehand (see figure above). “Anyone with the right equipment can make these masks and reproduce the experiment” he claims.

One of the main applications of this research, says Verbeeck, is likely to be the investigation of magnetism at very small scales. He and his colleagues have shown that electron vortex beams could provide information about magnetic materials because they generate very slightly different spectra of these materials depending on whether the wavefront of the beams spiral in a clockwise or anti-clockwise sense. And, because electrons typically have a much smaller wavelength than photons, these beams could potentially be used to study a material’s magnetism atom by atom, allowing, for example, the development of improved spintronic devices. According to Verbeeck, electron vortex beams might also be used to move single atoms and molecules around and permit the assembly of objects such as nanoscale motors from the bottom up.

Uchida agrees that the latest research represents a “very practical method” to make electron vortex beams, which he says will accelerate the use of these beams in areas of basic research such as quantum mechanics and particle physics. Franco Nori of the University of Michigan in the US and RIKEN in Japan also believes that the masks produced by Verbeeck’s team “open an avenue” for the practical use of electron vortex beams. He says they may be particularly useful in condensed-matter physics, as well as electron microscopy, but cautions that it is often difficult to predict the applications of a particular piece of research.

The work is reported in Nature 467 301.

Physicists crack the 'water-enhanced Brazil nut effect'

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Computer simulation showing a time sequence of the water-enhanced effect

By Hamish Johnston

First there was the “Brazil nut effect”, then the “reverse Brazil nut effect” – and now physicists in the UK have cracked the “water-enhanced Brazil nut effect”.

For those of you not familiar with these phenomena, grab a can of mixed nuts and give it a good shake – but make sure the top of the can is always pointing upwards.

When you open the can, you’ll be amazed to find that most of the Brazil nuts have risen to the top – and the smaller peanuts and hazel nuts will be hiding at the bottom.

That’s great if, like me, Brazil nuts are your favourite.

If not, you might prefer the reverse Brazil nut effect, whereby the larger bodies sink to the bottom.

Physicists have been puzzled by this effect for at least 35 years and now they have one more variant to worry about thanks to Michael Swift and team at the UK’s University of Nottingham.

The researchers placed a steel “Brazil nut” (radius 3.5 mm) in a water-filled box and then topped it up with a thick layer of glass beads (radius 1 mm). The box is then vibrated vertically under the watchful eye of a high-speed camera. The experiment is then repeated in a dry box.

They found that the water makes the Brazil nut rise much faster that in the dry situation. To understand why, the team did a series of computer simulations.

The simulations suggest that when the beads are thrown upwards during the vibration cycle, the Brazil nut travels further because its motion is less affected by fluid drag. But when the beads fall back, the Brazil nut cannot drop to its former height because beads have filled the space beneath it.

paste.jpg

You can read all about it in this paper in Europhysics Letters.

That brings me to another food-related effect that I first spotted years ago – the “anomalous curry paste effect”.

Take a jar of Patak’s curry paste (other brands are available), scoop out a few spoonfuls, replace the lid tightly and then place in a cupboard at room temperature for a few weeks. You will find that oil from the paste somehow escapes through the lid, and flows down the sides of the jar to make a messy red ring on the shelf.

Repeat the experiment with the paste in the refrigerator and the oil stays in the jar.

If you can explain why, there’s an IgNobel prize waiting for you at Harvard!

SANE overcomes nanofabrication grand challenges

The first nanofabrication technique that overcomes the three “grand challenges in nanofabrication” has been unveiled by researchers in the US. The technique, dubbed solvent-assisted nanoscale embossing (SANE), could be used to make cheap large-area nanoscale patterns in applications like plasmonics, solar cells and data storage.

At present, scientists use electron-beam lithography or focused ion beam milling to create patterns at nanometre length scales. However, such techniques are not practical for mass production because they must start from scratch each time and the patterns cannot be made over large areas. Moulding, imprint lithography and soft lithography can mass produce patterns, but these methods are limited to the fixed features on the mould or the original master template.

Teri Odom and colleagues at Northwestern University have now developed SANE as a way around these problems. The technique meets three grand challenges in nanofabrication: it can alter the density of nano-arrays; reduce critical feature sizes; and configure different lattice symmetries. Key to SANE is that all three of these challenges can be met starting from the same master pattern, says Odom.

Unlimited master and mould patterns

The researchers showed that they could tune array densities from 300% above to 75% below the density of a master pattern. SANE can also be used to decrease feature sizes while keeping array spacing constant by applying different solvents during the procedure. And most importantly, the technique can be used to make an unlimited supply of new master and mould patterns.

SANE is also cheap: no other existing nanopatterning method can prototype arbitrary patterns with small separations and implement them over 6" wafers for less than $100, explains Odom.

To demonstrate their technique, the Northwestern researchers started with a 6″ diameter polyurethane master with 180 nm features separated by 400 nm. Next, they cast an elastomer (poly(dimethylsiloxane), or PDMS) against the master to create PDMS moulds. The moulds were first wet with solvent and then placed in contact with a photoresist-coated substrate.

Higher array densities

To create patterns with higher array densities, the team heated a photoresist placed on “shrink film” in an oven at 115 °C. After 20 minutes, the thermoplastic substrate shrank by a third and the lattice spacing also decreased by this amount. This technique thus easily produces a new master covering an area of 10 cm2 with a density that is twice as large as the original pattern’s.

Heating the substrate for 40 minutes reduces the size of the shrink film even more – by 50% – and the array spacing by the same amount. This produces a master with a density three times higher than the original. Such high-density arrays from lower density masters cannot be made using conventional soft lithography.

Lower array densities

To create arrays with lower densities than the original, the researchers simply stretched the substrate after heating it. For example, uniformly stretching the film in two perpendicular directions produces a master with spacings that are up to twice as large as the original pattern’s and densities 75% lower.

The technique could come in useful for anyone needing access to cheap large-area nanoscale patterns because it is simple, low-cost and high-throughput, says Odom. “It can be done in any laboratory and biologists, chemists and physicists not familiar with nanopatterning could now use SANE for research on the nanoscale.”

Plasmonics – a new branch of photonics that exploits surface polariton plasmons (SPPs) could also benefit. SPPs are quasiparticles that arise from the interaction of light with the electrons that oscillate at a metal’s surface and depend on the density of metallic patterns.

“The solar energy community might be interested too, especially those who want to exploit light absorption efficiencies from different densities of materials,” Odom says. “Data storage material researchers may also want to know more about our technique for increasing the storage capacities of optical and magnetic storage.”

The work was published in Nano Letters.

The future’s bright for DESY

How do you convert a major particle physics lab into a leading centre for materials science, chemistry and biology? That was the leading question confronting Helmut Dosch when he was appointed director of DESY in 2009 – the first condensed-matter physicist ever to hold that post.

In this exclusive video interview, Dosch explains why DESY, which is located in Hamburg, Germany, is making the transition from colliding particles to developing world-class “photon science” facilities that are used by scientists across a wide range of disciplines.

We will have a high-speed camera for the nanoworld 

Helmut Dosch, director of DESY

Dosch talks about the key role that the lab is playing in building the €1.2 billion European X-ray Free Electron Laser (XFEL), which Dosch describes as “a high-speed camera for the nanoworld”. At the heart of the laser is an electron accelerator that will run 3.4 km from DESY to an experimental hall on the outskirts of Hamburg.

The European XFEL uses superconducting cavity technology that was developed at DESY – and the same technology is now being used in preliminary designs for the International Linear Collider (ILC). The ILC will be the next big thing in particle physics after CERN’s Large Hadron Collider, and Dosch explains how DESY’s involvement in the project will keep the lab at the cutting edge of accelerator technology.

While he admits that it’s unlikely that the ILC will be built in Hamburg, there are plenty of options for boosting DESY’s photon-science capability. One discussed by Dosch is the possible conversion of the dormant HERA accelerator ring into a light source.

In a separate interview filmed in the experimental hall of the FLASH free electron laser, DESY’s director of photon science, Edgar Weckert, explains how that facility is informing the development of the European XFEL.

Weckert explains how FLASH was used to make aluminium momentarily transparent to light and other photon-science highlights at the facility. Indeed, FLASH is so popular that DESY has to turn down most of the requests it gets from scientists for time on the instrument. An enviable problem that soon could be relieved with the building of FLASH II, according to Weckert.

  • For readers interested in finding out more about the research at DESY, Journal of Physics B: Atomic, Molecular and Optical Physics has just published a special issue entitled Intense X-ray Science: the First Five Years of FLASH. All papers in the issue are free to download until March 2011.

Shining a light on FLASH

As director of photon science at DESY, Edgar Weckert is responsible for an impressive array of scientific equipment, including the FLASH free electron laser.

In this interview filmed in FLASH’s vast experimental hall, Weckert describes some of the laser’s greatest accomplishments, including how it was used to make aluminium momentarily transparent to light. FLASH has become so popular with scientists that DESY has to turn down a significant number of beam-time requests. But photon scientists mustn’t despair, according to Weckert, because plans are well under way for FLASH II, which will double the capacity of the existing facility.

Weckert also explains how FLASH is informing the development of the European XFEL and gives an update on DESY’s PETRA III light source, which has been up and running for about a year.

  • For readers interested in finding out more about the research at DESY, Journal of Physics B: Atomic, Molecular and Optical Physics has just published a special issue entitled Intense X-ray Science: the First Five Years of FLASH. All papers in the issue are free to download until March 2011.

New videos shine a light on DESY

By Hamish Johnston

How do you convert a major particle physics lab into a leading centre for materials science, chemistry and biology?  That was the leading question confronting Helmut Dosch when he was appointed director of DESY in 2009 – the first condensed-matter physicist ever to hold that post.

In this exclusive video interview, Dosch explains why DESY, which is located in Hamburg, Germany, is making the transition from colliding particles to developing world-class “photon science” facilities that are used by scientists across a wide range of disciplines.

Dosch talks about the key role that the lab is playing in building the €1.2 billion European X-ray Free Electron Laser (XFEL), which Dosch describes as “a high-speed camera for the nanoworld”. At the heart of the laser is an electron accelerator that will run 3.4 km from DESY to an experimental hall on the outskirts of Hamburg.

The European XFEL uses superconducting cavity technology that was developed at DESY – and the same technology is now being used in preliminary designs for the International Linear Collider (ILC). The ILC will be the next big thing in particle physics after CERN’s Large Hadron Collider, and Dosch explains how DESY’s involvement in the project will keep the lab at the cutting edge of accelerator technology.

While he admits that it’s unlikely that the ILC will be built in Hamburg, there are plenty of options for boosting DESY’s photon-science capability. One discussed by Dosch is the possible conversion of the dormant HERA accelerator ring into a light source.

In a separate interview filmed in the experimental hall of the FLASH free electron laser, DESY’s director of photon science, Edgar Weckert, explains how that facility is informing the development of the European XFEL.

Weckert explains how FLASH was used to make aluminium momentarily transparent to light and other photon-science highlights at the facility. Indeed, FLASH is so popular that DESY has to turn down most of the requests it gets from scientists for time on the instrument. An enviable problem that soon could be relieved with the building of FLASH II, according to Weckert.

If you want to know more about the research at DESY, Journal of Physics B: Atomic, Molecular and Optical Physics has just published a special issue entitled Intense X-ray Science: the First Five Years of FLASH. All papers in the issue are free to download until March 2011.

Digesting Stephen Hawking's new book

grand design.jpg

By James Dacey

I’m a big fan of the Guardian‘s Digested Read in which columnist John Crace takes a newly released book and condenses it into short humorous parody of the original work, often relying on our intimate knowledge of the celebrity author in question. This week, it was the turn of Stephen Hawking and his recent release The Grand Design, which was making headlines across the globe before it was even released last week.

I have picked out a couple of bits that really made me chortle:

“Quantum theories can be formulated in many ways, but the most intuitive is the description of it as a system that has not just one history but every possible history. Let me explain. This book may look unique. But really it’s almost identical to at least three other books in which I have tried and failed to explain cutting-edge astrophysics to the scientifically illiterate…

“But I suppose we should start with Democritus’s theory of the atom, God, scientific determinism and effective theory because that’s pretty much what I’ve done in the past, but I can’t help feeling I’m wasting my time. Though that feeling may not be correct, as there are many different pictures of reality. In other words, there is no theory-independent concept of reality; rather there is only model-dependent realism, where our four-dimensional world may be shadows on the boundaries of 11-dimensional space–time. Sod it. I was right first time. I have lost you already. So there’s probably no point you reading the next bit about quarks and pi mesons.”

The parody is written in good spirit, but I couldn’t help but smile in recognition when I read Crace’s closing footnote: “Er…thanks Stephen, that’s lovely. If you could just end with something you haven’t written before to create a few headlines, then we’re done. How about God doesn’t exist? Lovely job. Let’s do it all again in a couple of years.”

Read the full digested read on the Guardian website.

Logic circuit takes the heat

Researchers in the US are the first to make tiny electromechanical logic circuits that operate at temperatures as high as 500 °C. The circuits contain two switches made from silicon carbide and operate as a logical NOT gate. The team believes that its nanoelectromechanical system (NEMS) could be used in microcontrollers embedded in hot machinery such as jet engines or oil-drilling rigs.

Modern technology is increasingly reliant on embedded computer control systems, but some equipment is simply far too hot for conventional silicon electronics to function. The problem is that normal computer chips do not work above about 300 °C because heat causes transistor junctions to degrade – and also because thermally excited electrons alter the electronic properties of the semiconductor.

Designers have got round this problem by making high-temperature microcontrollers from silicon carbide (SiC), which is much more resistant to heat damage than plain silicon. In addition, the amount of energy needed to thermally excite an electron is much greater in SiC. Unfortunately, transistors made from SiC tend to be large, slow and power hungry and only work at high voltages.

Switching cantilever

Now, Mehran Mehregany and colleagues at Case Western Reserve University in Cleveland, Ohio, have shown that superior high-temperature microcontrollers can be made from tiny mechanical switches just a few hundred nanometres in size.

To build its device, the team coated a silicon wafer with a thin layer of silicon oxide and then a 400 nm thick layer of SiC. The researchers then used electron beam lithography to make a simple switch comprising two SiC electrodes (the gate and drain) that are spanned by a SiC cantilever beam (the source). The SiC switch was released from the wafer by using a chemical to etch away the silicon oxide.

When a voltage was applied between the gate and source, the electrostatic force was found to pull the beam into contact with the drain (but not the gate). This allows current to flow between source and drain, making the device a NEMS field-effect transistor. Mehregany and colleagues were then able to make a NOT logic gate by combining two such switches.

The team operated the device at 500 °C at a frequency of 500 kHz and with logical input voltages of ±6 V. While this voltage is much higher than silicon logic devices – which work at 3 V or less – the NEMS logic of ±6 V is in line with other high-temperature devices, according to the team. And because the switching voltage is not an intrinsic property of the device – as it is in a semiconductor – the device’s voltage could in principle be further reduced by making the component switches smaller.

No lower limit

The team was able to operate a typical switch for about 21 billion cycles at room temperature before the cantilever beam fractured. At 500 °C, however, the switches only lasted about 2 billion cycles. The team also found that at this temperature, there was a tiny ball of SiC at one end of the fracture. This is puzzling because SiC normally sublimates at 1800 °C.

Case physicist Te-Hao Lee told physicsworld.com that the ball fracture could be caused by a temperature-related electrical spike that occurred during the switching operation. “By refining the design of the switch, it is possible to reduce the bending stress on the switch and as a result, improve the switch reliability and lifetime significantly,” he says.

Gigahertz operation

Lee believes that these and other improvements could result in devices that could deliver a trillion cycles of reliable operation at gigahertz speeds – which would exceed the speed and lifetime requirements of a typical microcontroller circuit.

In addition to boosting the performance of individual switches, the team is working on integrating the devices into more complex circuit elements such as an adder or register. It is also looking at the problem of how to package the devices for high-temperature applications.

The work is reported in Science 329 1316.

Chu’s right about bouncing atoms?

By Hamish Johnston

UPDATE: Chu and colleagues have uploaded a second preprint related to the gravitational redshift debate.

Steven Chu and Claude Cohen-Tannoudji shared the 1997 Nobel Prize for Physics (along with William Phillips) for their work on the laser cooling and trapping of atoms.

Now the two Nobel Laureates find themselves on opposing sides of a “preprint battle” over the re-interpretation of an experiment done in 1998. The experiment involved using vertical laser pulses to bounce atoms up and down in order to study the interference patterns that occur when different atomic trajectories meet.

In February 2010, Chu (who is now US energy secretary) along with Holger Mueller and Achim Peters published new calculations showing that the experiment confirms gravitational redshift to a few parts in a billion.

A result of Einstein’s general theory of relativity, gravitational redshift is the stretching of the wavelength of a particle as it moves away from a massive object such as the Earth. Other experiments have confirmed this aspect of general relativity to much greater precision, but these involved macroscopic objects. Any deviation in redshift for a quantum particle such as an atom could point towards a unified theory of gravity and quantum mechanics – the Holy Grail of physics.

But about 10 days ago, Cohen-Tannoudji and colleagues uploaded a paper to the arXiv preprint server in which they argue that Chu and colleagues have not measured gravitational redshift after all.

As far as I can tell, Cohen-Tannoudji and colleagues argue that Chu and company made a mistake in their calculation of the expected interference caused by a deviation from gravitational redshift. When the correct calculation is done, they say, a deviation from gravitational redshift has zero effect on what was measured in 1998.

Now, Chu and colleagues have hit back with their own preprint. It argues that Cohen-Tannoudji and team made their calculations using mathematics that assumes gravitational redshift cannot be violated – essentially precluding its violation!

Stay tuned for round three.

Searching the Sun for dark matter

Physicists have so far failed to find direct evidence for the existence of dark matter – the non-luminous substance believed to make up some 23% of the mass-energy content of the universe – at least to the satisfaction of everyone working in the field. But now physicists in Portugal and the UK suggest that such evidence could be found in precision measurements of the neutrinos given off by the Sun.

Although astronomical observations provide indirect evidence for dark matter, experiments on Earth have yet to secure direct and definitive proof that this form of matter exists. Ilidio Lopes of the Technical University of Lisbon and the University of Évora in Portugal together Joseph Silk of the University of Oxford argue that proof might lurk within the Sun. This is because the huge gravitational field of our star is expected to suck in weakly interacting massive particles (WIMPs), which are a leading candidate for dark matter.

Once trapped inside the core of the Sun, WIMPs should collide with protons, gain energy slightly and gradually remove heat outwards from the centre of the star. As such, the radial distribution of temperature across the Sun would be different in the absence or presence of dark matter.

Taking the Sun’s temperature

The pair’s plan for identifying this altered temperature distribution relies on detecting neutrinos that are generated by a number of different fusion reactions within the Sun. The crucial point is that different reactions take place at different distances from the centre of the Sun – the reaction producing boron-8, for example, takes place at just 4% of the solar radius whereas that involving the production of nitrogen-13 occurs at 16% of the solar radius. Since the strength of these reactions depends strongly on temperature, and this dependence varies from reaction to reaction, the presence of solar dark matter would lead to a well defined change in the relative fluxes of the neutrinos produced in the different reactions. The detection of dark matter would therefore fall to experiments set up to measure neutrino fluxes, with the fluxes from the different fusion reactions distinguishable on the basis of the distinctive energy spectra of the neutrinos in each case.

The two physicists used computer simulations to calculate, given certain assumptions about the properties of WIMPs – including their mass, strength of interaction with baryons, and likelihood of mutual annihilation – that the presence of dark matter could increase the flux of neutrinos produced in the boron-8 reactions by as much as 30% and would decrease the flux from the basic proton–proton reaction by around 2%. These results back up a similar analysis carried out earlier this year by Marco Taoso of the University of Valencia in Spain and colleagues, who used slightly different theoretical assumptions and computer codes.

Lopes and Silk say that if they are lucky and WIMPs do have the characteristics assumed in their analysis then the effect of dark matter on the strength of the relative neutrino fluxes would be striking enough to permit a direct detection of dark matter in upgrades to existing solar neutrino detectors. But they claim that their approach could in any case reduce the number of candidate dark-matter particles put forward by particle physicists and cosmologists.

Nothing in the data yet

Gianpaolo Bellini, spokesperson of the Borexino solar-neutrino detector at the Gran Sasso laboratory in Italy, points out that measurements by Borexino and other similar experiments are consistent with the current standard solar model, which does not take into account possible contributions from dark matter. He says that there is a significant margin of error on this agreement between measurements and model, potentially leaving room for some variation in neutrino flux along the lines laid out by Lopes and Silk. But he says he would need to see a more detailed presentation of their argument before really judging the feasibility of their approach.

Dave Wark of Imperial College in London describes the research as “extremely interesting” but agrees with Bellini that more information is needed to make a detailed critique. In particular, he says that further work is required to establish that any apparent solar cooling is due to dark matter rather than to misunderstandings of the Sun’s internal properties. He adds that changes to the parameters governing neutrino oscillation might also mimic or hide the effects of dark matter.

The work is described published online in Sciencexpress.

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