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Bomb builder turned critic passes away

Born in New Jersey in 1915, Morrison caught polio as a child which left him partly handicapped for the rest of his life. He obtained his first degree from the California Institute of Technology and his PhD from the University of California at Berkeley, where his supervisor was Robert Oppenheimer, who went on to lead the Manhattan Project to build the first atomic bomb at Los Alamos during the second world war. Morrison was one of a small team of physicists who travelled to the island of Tinian to assemble the bomb that was dropped on Hiroshima, and later was part of the team that surveyed the devastation caused by the bomb.

In 1946 Morrison left Los Alamos to join Hans Bethe — who died last month aged 98 — at Cornell University, where his interests shifted from nuclear physics to astrophysics and cosmology, and where he also became a vocal critic of nuclear weapons. In 1959 he wrote a paper with Giuseppe Cocconi that is widely credited with starting the search for extraterrestrial intelligence (SETI).

In 1964 Morrison moved to MIT where he also became a noted popularizer of science through various books, the film Powers of Ten, and his book reviews in Scientific American.

“Phil was a great physicist. His field was astrophysics, but he was interested in all of physics,” said Marc Kastner, head of the physics department at MIT “Despite his physical disabilities, he would come to seminars on many different subjects and always had insightful comments. He was also deeply committed to education, both at the undergraduate level and for younger students, and was spectacular at explaining physics to the public.”

“The world has lost one of the major voices of social conscience in science,” said Charles Weiner, emeritus professor of the history of science at MIT. “For more than 50 years, since his involvement in the development of the first atomic bomb, Philip Morrison has been a leading participant in the efforts to control and eliminate nuclear weapons.”

Supercomputing “Grid” passes latest test

This is the first time that such high rates of data transfer have been maintained between so many sites and over such a long period of time. The total amount of data transmitted during the challenge — some 500 terabytes — would take about 250 years to download using a typical 512 kilobit per second household broadband connection. The exercise was the second of four “service challenges” that will be performed before data starts to flow from the four detectors at the collider — ALICE, ATLAS, CMS and LHCb.

“This service challenge is a key step on the way to managing the torrents of data anticipated from the LHC,” says Jamie Shiers, manager of the service challenges at CERN. “When the LHC starts it will be the most data-intensive physics instrument on the planet, producing more than 1500 megabytes of data every second for over a decade.”

CERN plans to use a worldwide supercomputing “Grid” — similar to the World Wide Web but much more powerful — of computing centres to handle the data from the collider, which will then be analyzed by more than 6000 scientists all over the world.

The labs that participated in the latest challenge were: the Karlsruhe research centre in Germany, CCIN2P3 in France, INFN-CNAF in Italy, SARA/NIKHEF in the Netherlands, the Rutherford Appleton Laboratory in the UK, and Brookhaven and Fermilab in the US.

“This is the highest ever end-to-end data transfer across a computing grid,” Fermilab supercomputing project manager Lothar Bauerdick told PhysicsWeb. “Today’s result shows that the LHC project can be a truly international collaboration. However, more work on software and network research is still needed to reach higher rates of data transfer.”

The next service challenge will take place this summer and will involve connecting more computing centres over a three-month period.

Superlens breakthrough

Conventional, positive-refractive-index lenses create images by capturing the light waves emitted by an object and then bending them. However, objects also emit “evanescent” waves that contain a lot of information at very small scales about the object. These waves are much harder to measure because they decay exponentially and never reach the image plane — a threshold in optics known as the diffraction limit.

In 2000, John Pendry of Imperial College in London suggested that a material with a negative refractive index — that is, one that bends light in the opposite direction to an ordinary material — could capture and “refocus” these evanescent waves. This idea of a perfect lens or “superlens” came over 30 years after Russian physicist Victor Veselago first speculated that negative index materials could exist. In such a superlens, electromagnetic waves that reach the surface of a negative refraction lens excite a collective movement of surface waves, such as electric oscillations — also known as “surface plasmons”. This process enhances and recovers the evanescent waves.

In 2003, Zhang’s group showed that optical evanescent waves could indeed be enhanced as they passed through a silver superlens. Now they have taken this work one step further and have imaged objects as small as 40-nm across with their superlens, which is just 35-nm thick (see figure). In contrast, current optical microscopes can only resolve objects down to around 400-nm, which is about one tenth the diameter of a red blood cell.

“Our work provides a new imaging method that can beat the optical diffraction limit and that has tremendous potential to revolutionize a wide range of technologies,” says Zhang. These include detailed biomedical imaging in real-time and in vivo, optical lithography to make higher density electronic circuits and faster fibre-optic communications.

“This paper represents a very critical step forward,” David Smith of Duke University in the US told PhysicsWeb. “It provides confirmation of Pendry’s original conjecture that a negative refractive element can focus near-fields and demonstrates clearly that evanescent refocusing occurs to create an image.”

“The work is a remarkable accomplishment,” says Pendry. “Although superlensing has previously been demonstrated at microwave frequencies, this is the first true super resolution at optical frequencies — where the greatest rewards in terms of applications are to be had. I am extremely pleased with this result.”

Negative refraction goes optical

The existence of materials with a negative refractive index was first predicted by the Russian physicist Victor Veselago in 1967. He speculated that materials in which the electric permittivity and the magnetic permeability are both less than zero would refract light in the opposite direction to conventional materials. In 2000, John Pendry, a theorist at Imperial College in London, showed that such materials could also behave as perfect lenses.

Although negative-refractive-index materials do not occur naturally, several groups have successfully made them. But so far these materials have only operated at microwave frequencies.

The new negative-refraction material made by the Purdue team consists of an array of closely spaced pairs of parallel gold nanorods, measuring 2-mm by 2-mm (figure 1). Two rods form a “tuning fork”, which has a pronounced resonance at a certain frequency of light. This resonance occurs for both the electric and magnetic components of light and it can result in negative refraction at frequencies higher than the resonance frequency — an observation that agrees with previous calculations performed by the group.

The Purdue team

“This work is an imaginative solution to obtaining both a negative electric and negative magnetic response — and hence a negative refractive index — at optical frequencies in a material,” Pendry told PhysicsWeb. “It exploits the two resonances, symmetric and asymmetric, of two parallel rods to achieve the effect. Further work needs to be done to apply the ideas to make a working device, but the approach is promising.”

A transmission medium with a negative index of refraction would enable a flat planar lens to focus light to a precision that is smaller than the wavelength of light itself, says the team. Such a superlens could therefore overcome the so-called diffraction limit – the fact that the resolution of an object can be no smaller than half a wavelength of the light used to illuminate it.

“Portable and versatile, the new lens would have the potential to revolutionize the market for most technology areas where light is used,” says team member Alexander Kildishev. “These include optical recording (for enhanced DVDs), nanofabrication and optical lithography, enhanced sensing, such as in biomedical sensors and implants.”

The team now plans on fabricating new optical materials and building its superlens.

Quark-gluon plasma goes liquid

A quark-gluon plasma (QGP) is believed to have existed before the universe cooled and free quarks and gluons combined into protons and neutrons, which then bound together to form light nuclei. Physicists at the CERN laboratory in Geneva claimed to have created a QGP in 2000 but the results were inconclusive because the plasma existed only fleetingly. Then in 2003, RHIC scientists said they had come closer than ever before to creating a QGP.

RHIC uses accelerators to increase the energies of gold atoms up to 100 billion electron volts inside a 4-kilometre ring and then collides them together. When a gold nucleus collides with another gold nucleus the constituent protons and neutrons are thought to melt together to form a QGP.

The new results indicate that some of the observations at RHIC agree with theoretical predictions for a QGP. But many theoretical physicists believe that the QGP should be a gas, whereas the matter formed at RHIC appears to behave more like an almost “perfect” liquid.

The new matter created at RHIC could in fact be a form of the QGP but just different from what has been theorized says Sam Aronson, a Brookhaven director. More detailed measurements are now underway at RHIC to resolve this question.

According to Ulrich Heinz, a theoretical physicist at Ohio State University in Columbus, the prediction that the QGP is a gas is not based on solid theory but is more a qualitative statement based on “folklore” that few physicists have really challenged. “I think this is the most important nuclear physics result in recent years and have been saying for the last two years that RHIC has produced a QGP,” he told PhysicsWeb. “That the QGP is an almost ideal liquid instead is extremely interesting — and definitely not expected by everyone — but it is in no way inconsistent with previous theoretical calculations.”

Johann Rafelski, a nuclear physicist at Arizona University in the US, agrees. “The RHIC experiment has much improved since the last results were published in 2003. Furthermore, the signatures are much clearer and not in conflict with earlier results at RHIC and CERN,” he says.

The teams will publish their results in the journal Nuclear Physics A, and in a special 350-page Brookhaven report.

Unnatural look for nickel

Iron, cobalt and nickel are the most ferromagnetic elements in the periodic table and naturally adopt different structures: body-centred cubic (bcc) for iron, hexagonal close packed (hcp) for cobalt, and face-centred cubic (fcc) for nickel. Iron and cobalt can exist naturally in an fcc structure at high temperatures, and these phases have also been made in the laboratory at room temperature. However, it has not been possible to make nickel with a bcc structure until now.

Jin and co-workers grew their bcc nickel, which was less than 3.5 nanometres thick, on top of a gallium arsenide substrate using molecular beam epitaxy. They found that it had a magnetic moment of about 0.52 Bohr magnetons per atom and a Curie temperature — the temperature below which it becomes ferromagnetic — of 456 K.

To its surprise, the team found that bcc and fcc nickel are completely different in some respects. The bcc phase, for instance, has a positive magnetic anisotropy, while the fcc phase as a negative value. This results in the inherent magnetic fields of the materials pointing in different directions in an applied magnetic field. In this regard bcc nickel has more in common with iron than with naturally occurring nickel.

“Our work shows that one can essentially make ‘new’ materials from ‘old’ elements, since fcc nickel is certainly an old element,” says Jin. “Moreover, bcc nickel has very different magnetic properties compared to fcc nickel.” The team now hopes to make other novel crystallographic structures from different elements and alloys.

And then there were two

CP violation is responsible for the difference between matter and antimatter in the Standard Model. CP violation means that the laws of physics change slightly when a particle is replaced by its antiparticle and all three directions in space are reversed. CP violation was first detected in kaons in 1964. However, it was not observed in another system until Belle and a similar experiment at Stanford called BaBar observed it in B-meson decays in 2001.

The amount of CP violation in the Standard Model is proportional to the area of the unitarity triangle. The base of this triangle is one unit long, so physicists need to measure the values of two more angles or lengths to calculate the area. However, the more angles and lengths they can measure, the better they can test and explore the model.

A B-meson contains a bottom quark and either an anti-up or anti-down quark, and it can decay into a variety of other particles, as can an anti-B-meson. By counting the numbers of particular combinations of particles produced in these decays, it is possible to determine the sizes of the various angles and the lengths of the different sides in the triangle.

The Belle scientists measured rare events in which a charged B-meson decayed into a neutral D-meson and a charged kaon. The neutral D-meson then decayed into a “three-body” state consisting of a positive pion, a negative pion and a neutral kaon. From the slight differences in the decays of the B-mesons and the anti-B-mesons, they extracted a value of 68°, plus or minus 15°, for the angle known as ø₃ (which is also known as gamma) The angle ø₁ (beta) has already been measured by both BaBar and Belle with a precision of several degrees, but the precise value of ø₂ (alpha) has not been obtained yet.

Previously it was thought that it would not be possible to measure ø₃ because of the statistically small number of events produced in experiments like Belle. However, the Dalitz analysis technique is several times more sensitive than other methods, which allows ø₃ to be constrained with reasonable precision. Although the new result implies that CP violation has been seen in the experiment, Anton Poluektov of the Belle collaboration says the team is reluctant to state that it has directly observed CP violation because of the low number of events.

“This particular result is just a first try of the promising method of ø₃ measurement,” he says. “Although currently our result will not have any theoretical impact, in the future, when more data will be available, a precise measurement of ø₃ — as well as the other angles and sides of the unitarity triangle — would allow to check the consistency of the Standard Model, and to search for effects which can be explained by the extensions of the model, like supersymmetry and grand unified theories.”

New transistor breaks speed record

The new device is a so-called bipolar transistor, which is very different from the more well-known field-effect transistor. In it, electrons are injected from the “emitter” terminal, travel towards the “base” and are then received by the “collector”, an arrangement that allows the device to work faster than a field-effect transistor.

Hafez and Feng have previously built a high-frequency bipolar transistor, but this earlier work focused on reducing the time it takes electrons to pass through the device by minimizing the device’s vertical thickness. Their new research further increases electron speeds through the device by slightly varying, or “grading”, the composition of the semiconductor layers. This, say the researchers, lowers the band gap in selected areas of the transistor and makes it easier for electrons to travel across the device.

The two physicists have shown their transistor can operate at a frequency of 604 gigahertz, a new record. However, according to Hafez, what is more important is that they have developed a technology that could be used to build transistors operating in the terahertz range. “Projections from our earlier high-frequency devices indicated that in order to create a transistor with a cutoff frequency of 1 terahertz, the devices would have to operate above 10,000 degrees C,” he says. “By introducing the grading into the layer structure of the device, we have been able to lower the potential operating temperature for a terahertz transistor to within an acceptable range.”

Devices operating at terahertz frequencies (the far infrared) could be used in communications applications or as sensors to detect toxic gases. They could also be used for medical imaging, since the radiation is long enough to penetrate skin and image what lies underneath.

The researchers’ next step is to show that their devices can be assembled into circuits.

Nickel nuclei yield magic finding

With 50 neutrons and 28 protons nickel-78 has an extremely large neutron excess compared to naturally occuring nickel isotopes. Nickel-78 is said to be a “doubly-magic” nucleus because it has closed shells of both protons and neutrons. It is one of only ten such nucleides that can be formed in nature. It is also very difficult to produce experimentally. Although physicists at the GSI lab in Darmstadt, Germany, had produced three nickel-78 nuclei before, they were not able to measure their properties.

Working at the National Superconducting Cyclotron Laboratory at Michigan State University, Hosmer and colleagues first fired a beam of stable krypton into a beryllium target. The krypton fragments to produce many exotic neutron-rich isotopes. About twice a day, one out of ten billion attempts per second produces a nickel-78 nucleus. This isotope was separated out and its decay half-life measured to be about 110 milliseconds, which is about four times shorter than nuclear theories predict.

According to some models, the decay of nickel-78 is part of the so-called “rapid neutron capture process” or r-process, which is thought to produce about half the elements heavier than iron in the universe. The r-process is the main source of elements such as gold, platinum and uranium and may take place in supernova explosions. More importantly, nickel-78 is one of the main bottlenecks in this process and acts like a valve for the build-up of heavier elements. The new shorter half-life for the isotope means that the r-process could be creating gold and other heavy elements much faster than previously thought. The finding could force existing models for the synthesis of heavy elements in the universe to be modified.

“Our result provides an important benchmark for nuclear theories attempting to extend our knowledge into the unknown domain of exotic neutron-rich nuclei,” says team member Hendrik Schatz. “It is also crucial for the ongoing quest of finding the origin of the heavy elements in nature — one of the most important unanswered questions in nuclear astrophysics today.”

Photonic crystals come under the microscope

Photonic crystals are materials in which a periodic variation of the dielectric constant results in a photonic band gap. Photons with wavelengths or energies in this gap cannot travel through the crystal. However, by introducing defects into photonic crystals it is possible to build “waveguides” that can channel light along certain paths (figure 1).

Existing methods to study photonic band structures compare the shapes of pulses leaving the crystal with the shapes of those entering. However, such techniques cannot identify exactly where the changes take place inside the device, and therefore provide only limited information about the band structure.

Figure 2

The near-field scanning optical microscope developed by Kuipers and colleagues employs a fibre-optic probe to measure variations in the intensity of the light waves that are produced on the surface of the crystal by a femtosecond laser pulse as it travels through a waveguide (figure 2).

“This is the first comprehensive determination of a photonic band structure,” Kuipers told PhysicsWeb. “Moreover, we actually see how the shape of the light waves is changed as they are forced to comply with the periodicity of the photonic crystal.”

The technique also allows the directions of the phase and group velocities of the light to be determined, and shows that under certain conditions the phase velocity can be negative while the group velocity is positive. The team now plan to study an array of phenomena – including negative refraction – with the technique.

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