New ways to tune electrical conductivity revealed by electron interaction

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The difference between anelectrical conductorand an insulator is thatelectronsin the latter cannot move freely through the crystal. This is because insulators have a gap in their energy spectrum that electrons cannot overcome. Hiroshi Watanabe, Tomonori Shirakawa and Seiji Yunoki from the RIKEN Advanced Science Institute in Wako and the Japan Science and Technology Agency have now uncovered how the electronic gap in Sr2IrO4arises. Other RIKEN scientists had shown previously that the compound is aninsulator.

Sr₂IrO₄is a member of the oxygen-containing compounds based on transition metals that have high atomic numbers. In these transition metals, the electrons of elements such as nickel, copper or cobalt strongly interact with each other, which results in effects such assuperconductivityor magnetism.

In compounds made from the heavier transition metals, the outermost electrons circle the atoms in the so-called‘5d electron shell’, which is relatively distant from the core. For electrons that occupy this shell there is an unusually strong interaction between their magnetic property, called spin, and the orbital motion around the atomic nucleus. The energy of this spin–orbit interaction is as large as the electron’s energy of motion or the energy arising from the electrostatic interaction between the electrons. This has dramatic consequences on their electronic properties, according to Yunoki, who led the research team.“Literally anything can happen in 5d electron systems because of the subtle balance of those three fundamental energy scales.”

How this energetic interplay modifies the electron conducting behavior in Sr2IrO4 became evident from the researchers’ calculations. The strong spin–orbit interaction in Sr₂IrO₄shifts some of the electronic states to higher energies, which is sufficiently strong to create an energy gap in the electronic states.

Furthermore, the calculations reveal an intriguing connection to the family of high-temperature superconductors that have a similar gap in their electronic states. In these compounds, superconductivity is achieved through a small addition of atoms introducing an electron surplus. The researchers are now investigating the possibility that this could also be the case here.“It would have an enormous impact if one can make Sr₂IrO₄superconducting,” says Yunoki.“We hope that our theoretical calculations will be of help to experimentalists.”

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The 'mad' Egyptian scholar who proved Aristotle wrong

January'sPhysics Worldfeatures a fanciful re-imagining of the 10-year period in the life of the medieval Muslim polymath, written by Los Angeles-based science writer Jennifer Ouellette.

The feature covers the time when al-Haytham -- banished from society and deprived of books -- came up with his revolutionary theories about the form and passage of light.

Ouellette brings detail to the skeletal plot of al-Haytham's life, from the awe and intimidation felt when he was summoned by the Caliph to use his engineering prowess to overcome the annual flooding of the Nile, to his fear of punishment when he realised he had failed in his task.

Al-Haytham was only able to escape a death sentence from the notoriously brutal Caliph by pretending he had gone mad. The Caliph instead incarcerated Al-Haytham, imprisoning him under house arrest to a cell. Confined and alone, it was here that Al-Haytham carried out the work that was to make him famous.

In 11th-century Egypt, Aristotle's ancient thought that visible objects and our own eyes emit rays of light to enable our vision still held.

Ouellette imagines al-Haytham lying alone in his darkened room questioning why the objects in the room are not emitting light and asking 'Is it possible that the ancients were mistaken?'

The question providing the crux, al-Haytham was spurred into experimental action with the candles and copper in his bare room to conclude that there is no mysterious"form"that all objects emit; rather there are sources of primary light that are reflected by other objects.

As Ouellette writes,"This is a work of fiction– a fanciful re-imagining of a 10-year period in the life of Ibn al-Haytham, considered by many historians to be the father of modern optics. Living at the height of the golden age of Arabic science, al-Haytham developed an early version of the scientific method 200 years before scholars in Western Europe."

Released from prison after the Caliph's death, Al-Haytham (AD 965-1040) went on to make contributions to astronomy, mathematics, engineering and medicine, as well as physics. But it his seven-volume Book of Optics, which he wrote while imprisoned, that remain his most famous contributions to science, covering visual perception, psychology and physical optics.

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Newly developed cloak hides underwater objects from sonar

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Led by mechanical science and engineering professor Nicholas Fang, Illinois researchers have demonstrated an acoustic cloak, a technology that renders underwater objects invisible to sonar and other ultrasound waves.

"We are not talking about science fiction. We are talking about controlling sound waves by bending and twisting them in a designer space,"said Fang, who also is affiliated with the Beckman Institute for Advanced Science and Technology."This is certainly not some trick Harry Potter is playing with."

While materials that can wrap sound around an object rather than reflecting or absorbing it have been theoretically possible for a few years, realization of the concept has been a challenge. In a paper accepted for publication in the journalPhysical Review Letters, Fang's team describe their working prototype, capable of hiding an object from a broad range of sound waves.

The cloak is made of metamaterial, a class ofartificial materialsthat have enhanced properties as a result of their carefully engineered structure. Fang's team designed a two-dimensional cylindrical cloak made of 16 concentric rings of acoustic circuits structured to guide sound waves. Each ring has a differentindex of refraction, meaning that sound waves vary their speed from the outer rings to the inner ones.

"Basically what you are looking at is an array of cavities that are connected by channels. The sound is going to propagate inside those channels, and the cavities are designed to slow the waves down,"Fang said."As you go further inside the rings, sound waves gain faster and faster speed."

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Illinois professor Nick Fang developed a two-dimensional acoustic cloak that makes objects in the center invisible to sonar and other ultrasound waves. Credit: L. Brian Stauffer

Since speeding up requires energy, the sound waves instead propagate around the cloak's outer rings, guided by the channels in the circuits. The specially structured acoustic circuits actually bend thesound wavesto wrap them around the outer layers of the cloak.

The researchers tested their cloak's ability to hide a steel cylinder. They submerged the cylinder in a tank with an ultrasound source on one side and a sensor array on the other, then placed the cylinder inside the cloak and watched it disappear from their sonar.

Curious to see if the hidden object's structure played a role in the cloaking phenomenon, the researchers conducted trials with other objects of various shapes and densities.

"The structure of what you're trying to hide doesn't matter,"Fang said."The effect is similar. After we placed the cloaked structure around the object we wanted to hide, the scattering or shadow effect was greatly reduced."

An advantage of the acoustic cloak is its ability to cover a broad range of sound wavelengths. The cloak offers acoustic invisibility toultrasound wavesfrom 40 to 80 KHz, although with modification could theoretically be tuned to cover tens of megahertz.

"This is not just a single wavelength effect. You don't have an invisible cloak that's showing up just by switching the frequencies slightly,"Fang said."The geometry is not theoretically scaled with wavelengths. The nice thing about the circuit element approach is that you can scale the channels down while maintaining the same wave propagation technology."

Next, the researchers plan to explore how the cloaking technology could influence applications from military stealth to soundproofing to health care. For example, ultrasound and other acoustic imaging techniques are common in medical practice, but many things in the body can cause interference and mar the image. A metamaterial bandage or shield could effectively hide a troublesome area so the scanner could focus on the region of interest.

The cloaking technology also may affect nonlinear acoustic phenomena. One problem plaguing fast-moving underwater objects is cavitation, or the formation and implosion of bubbles. Fang and his group believe that they could harness their cloak's abilities to balance energy in cavitation-causing areas, such as the vortex around a propeller.

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Device controls, measures spin current injected into semiconductor material

In contrast to electronics technology which led industrial development in the 20th century and is based on the flow of electron charge (electric current), this technology is an achievement which leads the way in spintronics which is based on the other basic attribute of an electron, itsspin. This technology is expected to contribute to significant energy conservation and increased functionality in social infrastructure, quantum computing and new directions in scientific development. The results of this development will be published in the 24th December 2010 edition ofScience.

Since the development of the transistor in the 1940s, the operation of electronic devices which contributed to the advancement of the electronics industry have utilized physical principles to electrically manipulate and measure the charge of electrons (electric current). Meanwhile, on the other hand, the electron has another basic attribute, its elementary magnetic moment so-called spin. The application of spintronics based on the manipulation of the spin of an electron is highly expected to open the way to new low-power consuming electronics, hybrid electric-magnetic systems and devices with completely new functionalities. The theory of electrically controlling and measuring the spin of an electron was proposed 20 years ago in the area of spin-transistors. However, many fundamental and critical issues in spintronics such as spin-injection, generation of pure spin-current, spin-manipulation and spin observation needed to be achieved to verify this theory. Until the present time, there have been no demonstration to manipulate spin current in the same way as electrical current or the measurement thereof.

In response to this need,Hitachiand international research team measured separately an up and down spin (Spin-Hall Effect) at an extremely low temperature of -269°C in a gallium-arsenide semiconductor, a non-magnetic material in 2005. Further in 2009, using the same gallium arsenide semiconductor at a temperature of -53°C, the team measured the flow of spin polarized current over a distance of a few microns (Spin-injection Hall effect). In the current development, the up or down spin was controlled by a gate voltage, and the successful ON/OFF operation as a transistor have been verified. In this experiment, a circularly polarized light was used to generate pure spin current in the semiconductor. If we can develop spin-injection technology for ferromagnetic material, the spintronics device which was proposed as a theory by Supriyo Datta&Biswajit A. Das in 1990, will be realized. Further, realizing a solid device which can control and detect the polarization of the light, a new dimension of light polarization can be employed as information in future optical communication to open the way for even larger capacity information transmission systems, or in new analytical systems to which use the polarization of light to study the characteristics of biological or molecular material.

The device consists of a planar photodiode with a pn-junction diode and a n-type channel which forms the Hall Bar. By shining light on the diode, photo-excited electrons generated by the photovoltaic effect are injected into the device. The degree of circular polarization of the incident light is used to generate the spin-polarized electrons. The injected spin precede as a spin-current (Spin-injection Hall effect). At this point, if a p-type electrode is formed above the n-type channel and a voltage is applied, according to quantum relativistic effects, the precession of the spins are controlled by the input gate-electrode voltages. These effects are also responsible for the onset of transverse electrical voltages in the device, which represent the output signal, dependent on the local orientation of precessing electron spins.

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Potential 4-D imaging technique modeled by UNL physicists

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How electrons rearrange when atoms or molecules come together is the essence of chemistry, and the ability to manipulate those rearrangements is the goal of the emerging sphere of nanotechnology. A fuller understanding could lead to enormous scientific and technological breakthroughs.

Unfortunately, significant problems confront scientists and engineers in attacking the question. Not only are atoms and molecules very small, requiring highly specialized equipment to"see"them, everything at theatomic levelhappens very, very fast. For example, an electron goes around the nucleus of an atom once every 150 attoseconds -- that's around one 10 millionth of a billionth of a second, far too quick for the human eye or any existing equipment to detect.

Scientists have a good understanding for simple chemical combinations, but not for complex entities likebiological molecules. And nanotechnologists essentially operate in the blind in the sense that they are not yet able to watch while the nanostructures they construct are being assembled. They only know if they have succeeded after the fact.

Nothing exists to allow scientists and engineers to see those electronic processes, but University of Nebraska-Lincoln physics graduate student Hua-Chieh Shao and his adviser,theoretical physicistAnthony Starace, have modeled a four-dimensional imaging technique that could lead to a breakthrough. They report their findings this week in the online edition ofPhysical Review Letters. Their paper will appear in the Dec. 31 print edition of the journal. The research was supported in part by funding from the National Science Foundation and the Nebraska Research Initiative.

Starace said he and Shao were intrigued by research that his UNL colleagues Herman Batelaan and Kees Uiterwaal were doing with Nobel laureate Ahmed Zewail of Cal Tech on what Zewail terms four-dimensional imaging -- imaging electronic processes in both space and time with very energetic, but extremely short, pulses of electrons.

"We got excited about this idea and decided to do what I think are among the first calculations, if not the first, to show what you would see,"said Starace, University Professor and George Holmes Professor of Physics.

Deriving analytic formulas as far as possible and then writing computer programs to evaluate the formulas and analyze the results, Shao and Starace determined what would happen if electron pulses of 110 attoseconds were fired at targets excited by a laser field. Their first target was the hydrogen atom, with one proton and one electron. Their second target was the tritium molecule, a hydrogen isotope with one electron and two nuclei.

In both instances, they found patterns in the distribution of scattered electrons that could be used to track changes in the target charge distribution over time -- in other words, how the position of the electrons was changing.

"These are sort of benchmark results showing this new technology that Zewail has called four-dimensional imaging,"Starace said"We've shown among the first results in a realistic calculation that one could see for both an atom and a molecule. We made some assumptions (in the tritium calculations), but it's a fairly realistic picture of how electrons move in molecules."

It remains for experimentalists to develop the technique.

"Herman Batelaan and Kees Uiterwaal are working in collaboration with Zewail to develop the short electron pulses and our newest faculty member, Martin Centurion, has been talking with us about what experiments he may be able to do that would provide evidence of time-dependent electron motion. He does experiments with short pulses of electrons, but much longer than the pulses which we considered here,"Starace said.

"I hope this will give added incentive to pursue this technology because we've shown that it can provide information that no other technique can provide. So now it's up to experimentalists to develop the tools. Scientifically, there's nothing stopping the development of the tools except money and manpower, as usual."

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Membrane protein structure can be seen using new X-ray free-electron lasers

About 70% of drugs on the market today depend on the activity ofmembrane proteins, which are complexmoleculesthat form the membranes of thecellsin our body.

A major problem for the design of new pharmaceuticals, often known as the“membrane protein problem”, is that they do not form the crystals needed to enable further investigation of the structure to design better drugs.

A major international effort is being mounted to determine the structures of membrane proteins using XFELs - large facilities that create such a bright beam of X-rays it is possible to see the X-ray light bouncing off a single molecule without forming a crystal.

Professor Keith Nugent, Laureate Professor and ARC Federation Fellow and Director of the Australian Research Council Centre of Excellence for Coherent X-ray Science (CXS) at the University of Melbourne said a key problem was that the light from an XFEL was so bright a molecule would start to disintegrate in less than one thousandth of a millionth of a millionth of a second.

In a paper published today in the journalNature Physics, Professor Nugent and Associate Professor Harry Quiney from the ARC Centre of Excellence for Coherent X-ray Science (CXS) have developed a method by which the damage from the XFEL pulse may be included in the data analysis.

Associate Professor Quiney, also of the School of Physics at the University of Melbourne, said results showed that high-resolution molecular structures may be obtained from X-ray scattering data using a few-femtosecond pulse from an XFEL, even if the interaction resulted in significant electronic damage to the target.

“This result has far-reaching implications for the future development of structural biology, because it removes a significant obstacle to the practical realisation of the molecular microscope using XFEL sources,” he said.

It also provides important insights into the complex, turbulent and poorly-understood interactions that are driven by the interaction of an XFEL pulse with an atom, molecule or solid.

Their approach uses sophisticated molecular physics and careful data analysis to determine a new approach to measuring molecular structure.

Although still at the theoretical and computation level when put into practice this discovery will remove a major road-block in the path to solving the membrane protein problem.

This year, CXS signed an agreement with Japanese colleagues and will host the 4th Asia-Oceania Workshop on Science with X-ray Free Electron Lasers in 2011.

Professor Nugent said this was an extremely exciting time for X-ray science.

“My colleagues and I are convinced that our recent work is a critically important step forward,” he said.

“We are very much looking forward to working with our Japanese colleagues in the coming years”.

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Dutch researchers build affordable alternative to mega-laser X-FEL

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It's one of the few remaining 'holy grails' of science: a system that allows you to observe the extremely high-speed molecular processes at anatomic scale. You could call it an ultra-fast video microscope. Instead of visible light this kind of system uses X-rays or electrons, because it requires radiation with a wavelength of less than a nanometer. The X-rays or electrons have to be emitted in ultra short pulses, so that the exposure time is extremely short. However these pulses are not easy to generate. An X FEL uses X-ray pulses for this purpose, generated by accelerating electrons in an accelerator of a kilometer, or longer. These electrons are then converted into X-rays. An installation of this kind is very costly, uses large amounts of energy and needs a whole team to operate it. A European X-FEL, which will cost a billion euro, is currently under construction in Hamburg (Germany).

TU/e doctoral candidate ir. Thijs van Oudheusden has developed a machine that in many respects can compete with this billion-euro facility, based on ideas from his co supervisor dr.ir. Jom Luiten. The essence of their 'poor man's X-FEL' is that it uses electrons instead of X-rays."Why convert electrons into X-rays if you can use the electrons themselves?", asks Van Oudheusden."As well as that you only need to give the electrons a low energy, so you can accelerate them in just a centimeter. That's why the whole system fits on a tabletop."

The physical barrier that Van Oudheusden had to overcome is that the electrons in electron bunches repel each other. This causes the electron bunches to expand, making them longer than the desired 100 femtoseconds (1 femtosecond is 10-15 second), which in turn would make the 'video microscope' too slow. Jom Luiten thought of a solution to prevent the undesired expansion. The key was to create bunches of exactly the right shape, so they can be controlled and focused by means of electrical fields into bunches of the desired type and length. All with a number ofelectrons(1 million) that is sufficient to create a diffraction pattern in just a single shot.

Supervisor prof.dr. Marnix van der Wiel believes that half to three-quarters of the kind of research that can be done on an X-FEL can also be done with the 'poor man's X_FEL'. But this doesn't immediately mean that the latter is automatically a lot cheaper in relation to the scientific output that can be generated with it."The X-FEL at Stanford works non-stop, all year round, and is used by thousands of research groups over several decades. So if you're allocated time on the system you have to take all your equipment to the USA, where you have to stick to a very strict schedule. Our finding is a good alternative for people who want to have the freedom to do research in their own labs. As far as the costs are concerned, it depends on the user if our system will turn out to be cheaper on a per publication basis."

TU/e spin-off AccTec BV intends to build the machine developed by Van Oudheusden and Luiten and to sell it to scientific users. AccTec expects the total price to be below half a million euro.

Thijs van Oudheusden gained his PhD on 13 December with his doctoral thesis entitled 'Electron source for sub-relativistic single-shot femtosecond diffraction'.

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