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Electron microscope breaks half-Angstrom barrier

A TEM works by focusing a beam of electrons through a thin sample and capturing the image on a detector on the other side. Such instruments are much better at looking at very tiny objects than an optical microscope because the wavelengths of electrons are much shorter than that of light. A STEM is similar to a TEM except that the electron source can be scanned across the sample, which allows individual atoms to be imaged and identified.

However, even electrons struggle to resolve very small objects and physicists have had great difficulties getting the resolution of electron microscopes below one Angstrom, which is smaller than the distance between individual atoms in a solid. One barrier to sub-Angstrom resolution is spherical aberration, which is the unavoidable blurring of images by the cylindrical lenses used to focus the electrons.

The TEAM instrument is based on FEI’s Titan 80-300 S/TEM microscope, which has been available commercially since 2005. TEAM reached half-Angstrom resolution using new aberration-correction technologies designed by CEOS and built into the microscope’s probe, sample stage and the region between the sample and the electron detector. These technologies were integrated with an aberration correction system that was already used on the electron lenses of the microscope.

The Titan was already the best in the world for TEM, having been able to study samples with a resolution of 0.7 Angstrom. The previous record for a STEM was 0.63 Angstrom set by a competitor’s microscope. While the move to half an Angstrom may not seem like a significant improvement, it had taken the partners in the collaboration three years to get there from 0.7 Angstrom.

According to Dominique Hubert, who is general manager of FEI’s NanoResearch division, reaching 0.5 Angstom was particularly challenging because it was achieved in an instrument that can do both TEM and STEM. This, according to Hubert, required the researchers to overcome significant challenges in designing an instrument that is optimized to perform both types of microscopy.

TEAM was developed in FEI’s research and development lab in Oregon and will be installed later this year at the National Center for Electron Microscopy at Lawrence Berkeley National Laboratory. It could be in use by the third quarter of 2008 to study how atoms combine to form materials and how the growth of crystals and other materials is affected by external factors. Hubert physicsworld.com that technology developed for TEAM will eventually be used in commercial Titan microscopes.

The researchers behind TEAM now want to correct for chromatic aberration, which is caused by electrons of different energies being focused to slightly different points by the microscope.

Experiment finds graphene’s missing pi

Graphene is the darling of nanotechnologists because it is tough, easy to make and a very good conductor of both heat and electricity. The fact that it is one atom thick also makes it an ideal system for exploring the often bizarre properties of “two-dimensional” electrons.

Perhaps the most curious property of graphene is that it appears to behave like both a metal and a semiconductor. If electrodes are placed at either end of a sheet and a gate voltage is applied across the surface, the electrical conductance along the sheet will be different for different values of the gate voltage — just like a semiconductor. But unlike a semiconductor the conductance does not go to zero when the gate voltage drops below a certain value — something that you would expect of a metal. In the past when physicists have tried to measure this minimum conductance, however, they have found that it is a factor of pi (about 3.14) greater than predicted by theory.

While some worried that the theory could be wrong, others began to wonder if the minimum value was also related to the size and shape of the graphene sheet. Now, Chun Ning Lau and colleagues at the University of California at Riverside have showed this to be the case.

The team measured the minimum conductance of 14 different graphene rectangles with widths and lengths in the 300 to 8000 nm range. For samples with lengths smaller than 500 nm, the team discovered that the minimum conductance approached the theoretical value when the width of the rectangle was more than twice its length. However, when the width became any smaller than this, the conductance rose beyond the theoretical value. The team also found that in sheets longer than about 3000 nm, the conductance was always greater than the theoretical value, even when the width was greater than twice the length.

Lau told physicsworld.com that the experiment shows that the theory only applies to very small pieces of graphene, and that the minimum conductance is dependent upon the shape of the sheet.

According to Carlo Beenakker, a theoretical physicist at Leiden University in the Netherlands who studies graphene, Lau’s data agree with theoretical predictions regarding the relationship between conductance and the width and length of the sample. He told physicsworld.com that Lau’s work “closes a chapter on graphene.”

Selene blasts off for the Moon

45 minutes after launch, JAXA confirmed that the $484m spacecraft separated from its rocket and then orbited Earth twice before starting its journey to the Moon. Once there, it will separate into a main orbiter, which will observe the Moon from a 100-km circular orbit for one year, and a small “VRAD” satellite that will measure the Moon’s gravitational field from an 800-km elliptical orbit. A third small satellite will assist VRAD’s measurements from a distant 2400-km elliptical orbit and relay data from the main orbiter to Earth.

In total, Selene has 15 different observation missions. These range from recording the different elements and minerals on the Moon’s surface using spectrometers and infrared imagers, to mapping the larger topographical structure using a stereo camera, radar and a laser altimeter. The mission will also investigate environmental properties of the Moon such as its magnetic field, and see how Earth’s ionosphere and magnetosphere look from the Moon’s perspective. Finally, Selene will use a high-definition camera to make a film of the Earth rising from the Moon’s horizon.

Reach for the Moon

JAXA hopes that the spacecraft will put Japan at the front of lunar exploration and possibly even help plan for a manned moon base around 2030. However, the agency will have to shake memories of several failed space missions. Earlier this year JAXA’s Lunar-A spacecraft was scrapped because of worries that an aging mothership would jeopardize the mission. In 2005 the Hayabusa spacecraft, which was designed to fetch rock samples from an asteroid, failed because of thruster problems. Prior to that, JAXA had to destroy a rocket carrying spy satellites when it veered off course after lift-off.

Still, if all goes to plan, Selene will carry out the most wide-ranging study of the moon since NASA’s Apollo programme of the 1960s and 1970s and will keep Japan ahead of China and India, which plan to launch lunar missions over the next year.

Stringy ‘filaments’ could have produced first stars

As yet undiscovered, dark matter is thought to exist because galaxies seem to be held together by the gravitational attraction of much more mass than we can see through telescopes. Dark-matter particles could be “hot”, meaning that they are light and fast, although simulations suggest that hot dark matter would not explain how cosmic structure formed after the Big Bang. Most physicists therefore think “cold” or slower-moving particles are more likely because they do a better job of accounting for the universe’s evolution. But cold dark matter appears to be at odds with the observed densities of certain sub-galactic structures.

In the last few years, however, some researchers have latched onto the possibility of warm dark matter, which would still get the universe’s large-scale evolution right but would agree better with observations of smaller-scale structures. Now Liang Gao from Durham University and Tom Theuns from the University of Antwerp have performed numerical simulations on early star formation that give further credence to warm dark matter.

According to cold dark-matter theory, the first stars formed from “minihalos” inside huge isolated clouds of gas and dark matter — unlike stars in our present era, which form in molecular gases inside galaxies. In Gao and Theuns’s simulation, a generic warm dark-matter particle would change this picture so that the minihalos are replaced with trailing “filaments” of accumulated gas. These filaments would have been massive — about 9,000 light years long or a quarter the size of the Milky Way.

The researchers say that the filaments could well have produced a large number of low-mass stars that could exist to this day, and over time may have collapsed to seed the supermassive black holes that we appear to detect at the centres of most large galaxies.

Matter-antimatter molecule makes its debut

The Standard Model of particle physics says that every particle has an antimatter counterpart – the electron, for example, is paired with the positively charged positron. Although electrons and positrons annihilate each other, they can bind together temporarily to create a positronium atom, which resembles a hydrogen atom. In theory, two positronium atoms could join to form a dipositronium molecule. However, physicists had found it hard to make detectable quantities of dipositronium because it is very difficult to get enough atoms in the same place to react and form molecules.

Now, David Cassidy and Allen Mills of the University of California at Riverside have managed to collect and react enough positronium to confirm that dipositronium exists. The pair used a special positron trap developed by Clifford Surko and colleagues at the University of California at San Diego to collect positrons from the decay of sodium-22.

When about 20 million positrons were accumulated, the contents of the trap were focused onto a small spot on a piece of porous silica. The positrons made their way into the pores, where they reacted with electrons to form positronium. Some of these atoms stick to the surfaces of the silica, where they combine to form dipositronium. The surface plays a crucial role in encouraging the dipositronium to form because it stabilizes the molecules by absorbing energy that is given off when the molecule is formed.

The presence of dipositronium was confirmed by keeping an eye on electron-positron annihilation in the silica. Positronium atoms exist in two different quantum states depending on the relative orientation of the electron and positron spins. The “para” state only lasts about 125 ps before annihilating, while the “ortho” state hangs on for more than 1000-times longer (142 ns) before annihilating. Dipositronium is formed when two ortho atoms come together, but there is nothing to stop the two positrons in the molecule from exchanging their electron partners and creating para atoms. As a result, ortho atoms in molecules don’t last as long as free ortho atoms.

By monitoring the gamma rays that are given off during annihilation, researchers saw a reduction in the overall lifetime of positronium in the silica, which they interpreted as evidence for the formation of dipositronium. According to Cassidy, this was confirmed by heating the silica, which prevents positronium from sticking and reduces the number of dipositronium molecules that can be created. When this was done, the lifetime of the positronium increased.

Cassidy told physicsworld.com that he and Mills are now working on creating a Bose-Einstein condensate (BEC) of positronium, in which all the molecules settle into the same quantum state. Calculations suggest that BEC could be made by boosting the density of positronium by a factor of 1000 and cooling it to about 15 K. Cassidy says that this could be done by accumulating more positrons in the trap and then firing a more intense beam at the silica. Improvements to the silica itself could also help, he says.

If the density were increased by another factor of 1000, the BEC could be used to create an annihilation gamma-ray laser. In such a device the positron/electron pairs could be made to annihilate in a cascade, which would produce a stream of coherent gamma-ray photons resembling laser light. Annihilation gamma rays have a very short wavelength, which means that such a laser could someday be used to study objects as small as atomic nuclei.

Earth could survive a red-giant Sun

When the Sun becomes a red giant it will steadily lose mass and affect the orbits of the planets, making it hard to predict what will happen to them. Scientists think it is likely that Mercury and Venus will evaporate as the Sun’s surface expands outwards, but the fate of Earth is less certain.

A study by Roberto Silvotti from the Astronomical Observatory of Capodimonte in Naples, Italy and colleagues from Europe, the US, Israel and Taiwan suggests that planets orbiting close to a star — within twice the distance from the Sun to Earth, or 2 AU — can survive the red-giant phase. They have analysed observations of V 391 Pegasi, a star that ceased to be a red giant some 100 million years ago when, unusually, it blew away its outer “envelope” of remaining hydrogen. In its current form as a rare “B-type subdwarf” V 391 Pegasi pulsates as it fuses helium into carbon in its core.

Over seven years Silvotti and colleagues monitored the maxima of these pulsations, which occur on average every six minutes, by recording the flux of light coming from the star. But they found that every 3.2 years the maxima are shifted five seconds early or late, indicating that the star must be wobbling as a result of the gravitational pull of a low-mass companion with this orbit period. With 97% certainty, they calculated, this companion is a large planet roughly 10 billion years old — the first known to orbit a post-red-giant star.

The current of orbit of the planet lies at 1.7 AU, but the researchers estimate that before the red-giant phase when the star had more mass the orbit would have been closer, probably around 1 AU. Likewise, Earth’s orbit is expected to increase from 1 AU to roughly 1.5 AU when the Sun eventually sheds mass while turning into a red giant.

Despite the similarities, Silvotti says that the discovery does not necessarily mean that Earth will avoid assimilation like the planet orbiting V 391 Pegasi. But he hopes that the discovery will be the first of many that will enable physicists to more accurately forecast Earth’s fate.

Even if Earth does escape, however, humans will have to invest in some effective Sun protection — the researchers think that V 391 Pegasi’s planet has temperatures in the region of 200°C.

Thundercloud “accelerator” fires gamma-ray beam

Physicists have known for over a decade that 10 – 20 MeV gamma rays are produced in millisecond bursts during electrical storms. These bursts are believed to occur when high voltages in a thundercloud accelerate electrons to energies up to about 35 MeV. These electrons are slowed down by colliding with atoms in the air and as a result give off bremsstrahlung — gamma rays that are created when an electron is deflected off its course by an atom.

Much longer bursts lasting up to several minutes have also been seen, but these events seem to be much rarer than their shorter counterparts. Physicists have yet to work out where in the sky the longer pulses are coming from – if they are indeed coming from the sky. The energy distribution of the pulses and whether the pulses contain any other types of radiation such as charged particles was also unclear.

Now Harafumi Tsuchiya of the Cosmic Ray Laboratory of Japan’s RIKEN research institute and colleagues have used a new bank of direction-sensitive detectors they installed at a nuclear power plant to detect a 40-s gamma ray burst during a very intense thunderstorm on 6 January, 2007. Their system at the Kashiwazaki-Kariwa plant on the coast of the Sea of Japan was designed to measure the energy distribution, composition and source of thundercloud pulses.

By analysing the energy distribution of the pulse, the team was able to say that the pulse was made of bremsstrahlung gamma rays. The directionality of their detectors allowed the team to confirm that the pulse came from the storm and because such gamma rays can only travel short distances in the atmosphere, the team could also conclude that the pulse was created a kilometre or less from the detectors.

Since the gamma rays arrived about a minute before the first lightning strike, Tsuchiya believes that the pulse was probably created while electrical energy was building up in the thundercloud, rather than when energy is being discharged as lightning. He adds that the process is likely to begin with a cosmic ray passing through the cloud and ionizing the air to produce electrons, which are accelerated to towards the bottom of the cloud, which has a positive charge. These electrons ionize other atoms on the way, creating a stream of high-energy electrons.

Tsuchiya says that bremsstrahlung at MeV energies would be focused into a beam that only illuminates a small area on the ground, which could explain why so few long-duration pulses have been seen. The team plan to verify this by placing many radiation detectors over a wider area.

David Smith, a physicist at the University of California at Santa Clara and an expert on atmospheric gamma ray pulses, agrees that the pulse was made in a thundercloud accelerator. “The spectrum looks just right for bremsstrahlung”, he says. According to Smith, the millions of volts required to produce MeV electrons could be sustained in a cloud for seconds or even minutes as long as the stream of electrons does not become so intense that it causes an avalanche-like electric breakdown of the type that could lead to lightning. Smith is now designing an airborne experiment to test whether lightning flashes are preceded by gamma-ray pulses.

Lensless X-ray microscope fits in the lab

X-rays are desirable for microscopy because their short wavelength enables high-resolution images, but unlike electron microscopes they can be used on thick samples. Unfortunately lenses for X-rays are tricky to make, and as a result there has been a lot of research into creating lensless microscopes, which use a computer algorithm to generate images from a sample’s diffraction patterns. However, these microscopes rely on coherent X-rays, which normally are only obtainable from large accelerator facilities such as free-electron lasers.

Now Henry Kapteyn, Margaret Murnane and others from the University of Colorado, together with colleagues from the University of California in Los Angeles and Lawrence Berkeley National Laboratory, have shown that lensless X-ray imaging can be done in the lab using a process called high-harmonic generation. This makes use of a compact source that can produce coherent light, but with a longer wavelength than the desired X-rays. The light is shone into a gas-filled tube where atoms absorb bunches of photons, and then spit out single X-ray photons with a much shorter wavelength.

Tabletop imaging

The group used an infrared laser with a wavelength of 780 nm as the light source, and after high-harmonic generation ended up with a coherent X-ray source with a wavelength of 29 nm. They found that these “soft” X-rays could image objects with a resolution of 214 nm. This is not quite as fine as the 62 nm resolution recently demonstrated at the large FLASH free-electron laser in Hamburg, Germany, but that fact that the imaging can be performed in any lab could make lensless X-ray microscopy feasible for many researchers. Richard Sandberg, one of the researchers, told physicsworld.com that his group are currently improving their device to have a higher spatial resolution.

Ideal materials make for perfect invisibility

First proposed theoretically in May last year and realized for microwaves five months later, the invisibility cloak has captured the imagination of the public and defence agencies alike. The idea is that metamaterials — artificial materials with exotic electromagnetic properties — can be designed to guide radiation around an object, like water flowing around a smooth stone.

A perfect cloak would need to make the light passing through its interior catch up with the light passing around it to prevent any scattering. Physicists think the only way to do this would be to make the phase velocity of light on the inner lining infinite by using a metamaterial with infinite values of permittivity and permeability, which is impossible. Still, because it is difficult to deal with infinite values numerically, there has been uncertainty over whether a metamaterial with even these ideal parameters would be able to render an object perfectly invisible, and hence how much of a deviation from the ideal parameters would be acceptable in a real device.

Min Qiu and co-workers from the Royal Institute of Technology have got around this problem using an analytical approach. They began by considering a cylindrical cloak, and examined the equations that relate the electric field of the light to the radius of the metamaterial layer. To prevent infinite values occurring at the innermost layer, they added a small perturbation to this radius, but then calculated what happened when they shrunk this perturbation closer to zero.

The team found that the light’s scattering did gradually disappear as the perturbation was reduced. However, they also found that just a tiny perturbation — for example 10^-99 of the inner radius — would still produce significant scattering. To an observer, claims Qiu, this scattering would appear as a thin line at the centre of the cloak that would gradually become more blurred as the perturbation was increased.

Ulf Leonhardt — one of the physicists who came up with the proposal for invisibility cloaks last year — told physicsworld.com that he thinks the theoretical sensitivity to perturbations is very interesting. But he pointed out that the experimental device demonstrated last year by researchers at Duke University in the US worked very well, suggesting that the sensitivity might not be that important in real devices.

Sound causes colossal drop in resistance

Physicists know that the electrical resistance of certain manganese oxides called manganites can drop by as much as ten orders of magnitude when the materials are exposed to a magnetic field. While a full explanation of why this colossal magnetoresistance (CMR) occurs has evaded researchers, physicists have suspected for some time that it is related to interactions between electrons and phonons.

Now, an international team led by Andrea Cavalleri at Oxford University has performed an experiment that provides further insight into the role of phonons in CMR. The team fired a short terahertz (THz) laser pulse at a manganite sample while monitoring its electrical resistance by measuring the current flowing through it. When the energy of the laser is tuned to a specific phonon frequency, the resistance of sample drops dramatically for about 5 ns before returning to its original value.

According to Cavalleri, the pulse – which is about 300 fs in duration — is long enough to create phonons at a specific frequency (about 17 THz). However, it is short enough to avoid exciting electrons and other phonons at other frequencies. This allowed the team to conclude that the drop in resistance was caused exclusively by interactions between 17 THz phonons and electrons in their equilibrium state. The pulse was also short enough to ensure that the electrons did not heat up, which means that CMR does not necessarily require the electrons to be “hot”.

By exciting only 17 THz phonons and not heating the sample the team has managed to avoid the “chicken and egg” problem, which normally makes it very difficult to study materials such as manganite. In such materials the electrons interact with each other via phonons and if an experiment excites both the electrons and phonons it can be impossible to determine, for example, what is a cause of CMR and what is an effect of CMR.

The “chicken and egg” problem also affects those studying cuprate high-temperature superconductors and Cavalleri and colleagues now plan to use the technique to gain a better understanding of the role of electron-phonon interactions in these materials.

Cavalleri told physicsworld.com that there could be practical applications for colossal phonoresistance – particularly because it works at room temperature. It could be used, for example, to make THz radiation detectors and other THz optoelectronic devices. He also believes that the technique could be used to change the magnetic properties of certain materials using a THz laser pulse.

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