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Bubble fusion makes controversial return

In sonoluminescence, the bubbles in a liquid emit light when they are forced to expand and collapse by sound waves. Physicists believe that the pressures and temperatures inside the collapsing bubbles could be high enough to initiate nuclear reactions. If achieved, such “bubble fusion” could lead to a new clean energy source.

In a controversial paper published in Science in March 2002 Taleyarkhan and co-workers described how they had used high-energy neutrons to create tiny bubbles of gas in a beaker of acetone in which the hydrogen atoms had been replaced by deuterium (D). Taleyarkhan, who was then based at the Oak Ridge National Laboratory, claimed that the temperature inside the collapsing bubbles was in excess of a million degrees – high enough for two deuterium nuclei to undergo a fusion reaction (Science 295 1868). DD fusion reactions can produce a helium-3 nucleus plus a neutron, or a tritium nucleus and a proton.

However, the results were questioned by many researchers in the field. Now, Taleyarkhan says his team has repeated the experiment with more sensitive detectors. “A fair amount of very substantial new work has been conducted,” he said in a press release issued by Purdue. “And this time I made a conscious decision to involve as many individuals as possible – top scientists and physicists from around the world and experts in neutron science.”

As before, the team claims to detect tritium as well as neutrons with the characteristic energy for DD fusion reactions. Moreover, the fusion products are not observed in experiments with ordinary acetone. Taleyarkhan says that chances of the result being due a phenomenon other than fusion have been reduced from 1 in 100 to 1 in 10¹¹.

Michael Saltmarsh of Oak Ridge says he is “intrigued but sceptical” about the new work. “Unlike their Science paper, most of the background notes and supporting information seem to be correct but there are still some puzzling inconsistencies,” he told PhysicsWeb. “In particular, the estimated neutron detection efficiency is still an order of magnitude too low. While better than the Science article, the difference would produce a mismatch between the reported neutron and tritium yields.”

“Thermonuclear sono-fusion may not be impossible,” says Willy Moss of the Lawrence Livermore National Lab, “but more tests need to done. Personally, I would like the results to be real, but I believe that the nature of these claims requires absolute proof.”

“When a startling new discovery is announced, it is the responsibility of the authors to lay things clear,” adds Aaron Galonsky of Michigan State University. “Taleyarkhan and co-workers have not done that well enough for me to be able to say whether they have seen nuclear fusion in a bottle of acetone. With two million 14 MeV neutrons per second injected into the room where the experiment was performed, there are opportunities for error in detecting the much rarer, lower-energy sonoluminescent neutrons.”

Making atoms cooler

Conventional laser cooling of atoms relies on photons exciting the electrons inside the atom. When the electron falls back into the ground state, the atom spontaneously emits a photon. Since this photon is emitted in a random direction, the overall effect of the absorption and emission process is to reduce the momentum of the atom – that is, to “cool” it – in the direction of the laser beam. This random emission removes heat from the system and so cools the atoms down.

The cavity-cooling technique developed by Gerhard Rempe and colleagues at the Max Planck Institute for Quantum Optics in Garching is different. First, it avoids spontaneous emission. Second, the atoms and the photons in the cavity are strongly coupled.

Rempe and co-workers start by trapping single atoms of rubidium in a tiny optical cavity between two mirrors. Next, they use a probe laser to excite the cavity, rather than the atom. Photons from the laser then escape from the cavity with slightly more energy than they had when they entered.

The strong coupling between the atom and the cavity means that the extra energy carried away by the scattered laser photons comes from the kinetic energy of the atom, which causes it to slow down or become cooler. Moreover, exciting the cavity – rather than an atom – ensures that the internal quantum state of the atom does not change, which could make it possible to process quantum information stored in atoms.

“Our cooling method allows us to localize and hold an atom in the cavity for longer times,” team member Pepijn Pinkse told PhysicsWeb. “This will allow us to perform detailed studies that were not possible before.”

Galaxy breaks distance record

When hot hydrogen cooled to form galaxies in the early universe, intense far-ultraviolet radiation was emitted as electrons in the atoms fell from excited states to the ground state. This so-called Lyman alpha emission is a tell-tale signature of galaxy formation and has a characteristic wavelength of 121.6 nanometres. However, this radiation has been red-shifted to longer wavelengths by the expansion of the universe, and astronomers can calculate the age of a galaxy by measuring the red-shift of its Lyman alpha line.

Roser Pelló of the Observatoire Midi- Pyrénées in France, Daniel Schaerer of the Geneva Observatory in Switzerland and colleagues took images of the galaxy cluster Abell 1835 with the ISAAC instrument on the Very Large Telescope in Chile. This cluster behaves as a gravitational lens and allows astronomers to study galaxies otherwise too faint to be seen.

The French-Swiss team detected a weak but clear feature in the spectrum of Abell 1835 IR1916 that they believe is the Lyman alpha line red-shifted to a wavelength of 1337 nanometres in the near-infrared region of the spectrum. This corresponds to a red-shift of 10, which is significantly higher than the previous record of 6.6.

Moreover, Pelló, Schaerer and co-workers calculated that the galaxy is undergoing a period of intense star formation and that it has produced some 10 million solar masses worth of stars. The team says that these stars – which may have been the “building blocks” of today’s large galaxies – could have provided the first light sources that put an end to the dark ages in the early universe.

Zinc-oxide nanorings circle new applications

“This is the first report on the growth of freestanding, single-crystal, complete nanorings, demonstrating the possibility of synthesizing extreme structures and offering a new nanostructure that was not previously thought possible,” said team leader Zhong Lin Wang. “The growth mechanism is a spontaneous self-coiling process – the ‘slinky’ growth mechanism – which is fundamentally a new crystal growth process. It sets the foundation for understanding the formation of polar-surface-induced nanostructures.”

Wang and colleagues made the nanorings by a solid-vapour technique, from powders of zinc oxide, indium oxide and lithium carbonate in a horizontal tube furnace. Heating the materials to 1400° C in argon caused material to deposit on a silicon substrate. Around 20 to 40% of this material was made up of zinc-oxide nanorings with diameters of 1 to 4 microns and shells around 10 to 30 nanometres thick.

“Nanorings are made up of fine nanobelts that are rolled up as coils layer-by-layer, with as many as a hundred loops,” said Wang.

Zinc oxide has a wurtzite crystal structure, as do materials such as gallium nitride, aluminium nitride, indium nitride and zinc sulphide. Wang reckons that the results received from zinc oxide should impact the growth of nanostructures for the entire wurtzite family.

“The [zinc oxide] structure can be used for fabricating piezoelectric-based fluid pumps and switches for biotechnology,” said Wang. “The near-term application will be in situ, real-time monitoring of blood pressure and blood flow rate.” The nanorings could also measure stress at the scale of a single cell. For applications outside biotechnology, the scientists now plan to integrate the piezoelectric nanorings into micro-electromechanical and nano-electromechanical systems, as well as using them to investigate fundamental physical phenomena.

Revealing the secrets of planetary formation

Direct images of proto-planetary disks – features that are thought to be produced by comets and asteroids – are very rare. Astronomers produced the first such image 20 years ago around the star beta-Pictoris. This star weighed 2.5 times the mass of the Sun. AU Microscopii, on the other hand, weighs just half a solar mass. It is only the fourth star ever to be imaged in this way and it is the closest.

“Since 85% of all stars in the galaxy are low mass stars like AU Microscopii, the dust disc around this star may reveal the most clues for how the majority of planet systems evolve,” Kalas told PhysicsWeb.

Kalas and colleagues made their discovery using the University of Hawaii’s 2.2-meter telescope. They observed an excess of infra red in the spectrum of AU Microscopii, a tell-tale sign that the star is surrounded by a dusty disk. To then obtain an image of the disk they blocked out the glare of the star using a device known as a coronagraph. They found that the disk extended out to about 210 astronomical units (AUs) from the star (1 AU is equal to the average distance between the Earth and the Sun).

In a separate experiment that is to be reported in the Astrophysical Journal, Kalas’ group calculated that the disk has a hole in it that extends out to 17 AU from the star. According to the researchers, this may indicate the presence of a planet close to the star. To try and confirm that this is indeed the case, they will now collect sharper images of the star using the Hubble and Spitzer space telescopes.

AU Microscopii is some 33 light years away and about 12 million years old. In comparison, the Sun is roughly 4.6 billion years old.

Spintronics goes organic

A spin valve consists of a thin layer of metal or insulator sandwiched between two ferromagnetic electrodes. The spin of the electrons passing through the device can be flipped by an external magnetic field, which changes the resistance of the two ferromagnetic layers. This effect, known as magnetoresistance, has already been used to make highly sensitive magnetic-recording devices and memory chips.

Extending these spin-dependent effects to semiconductor materials has, however, proved difficult. Shi and co-workers have now built a spin valve with a 100 nanometre thick organic semiconductor made from aluminium and hydroxyquinoline. The semiconductor was sandwiched between a layer of cobalt and an alloy of lanthanum, strontium and magnesium (see figure).

To test their device, the Utah team first calculated the current that flowed through the semiconductor when the two electrodes were magnetized in the same direction – or parallel – and then in opposite directions – or anti-parallel. Shi and colleagues found that that the current increased by as much as 40% when the magnetization of the electrodes was switched from anti-parallel to parallel. This constitutes giant magnetoresistance.

At present, the device only works at low temperatures – between about –260°C to about –40°C – but Shi’s team says that the experiment is “a proof of concept that sets the stage for more practical applications”. The long-term aim is to make the device work at room temperature. The group believes that organic semiconductors have many advantages over conventional semiconductors, such as those made from silicon. They are simpler to make, are flexible and their resistance can be tuned by doping.

Mass detection enters attogram regime

The nanoelectromechanical device used by Craighead and colleagues consists of an oscillating cantilever made from a small wafer of silicon 4 microns long and 500 nanometres wide. When a small particle is absorbed onto the wafer, it alters the frequency at which the wafer vibrates. The team was able to monitor this change by measuring laser light reflected off the wafer, which then allowed the mass of the particle to be calculated.

Using this technique, the physicists were able to “weigh” small gold dots, and coatings of molecules on the dots, with masses in the attogram range (see figure). Last year, researchers at the Oak Ridge National Lab in the US reported on being able to detect particles on the femtogram mass scale using a similar technique.

The Cornell researchers predict that their cantilever will be able to measure masses as small as 0.39 attograms. They would now like to improve on this by further reducing the size of the device. This could lead to a sensitivity in the zeptogram range (10^-21), they speculate, and allow them to detect viruses.

In 2001, Craighead’s group measured the mass of a single bacterium – which is around 1 picogram (10^-12grams) – with its oscillating cantilever.

Closing in on a theory of superconductivity

Superconductivity is the complete absence of electrical resistance in a material and is observed in certain materials when they are cooled to below their superconducting transition temperature. Physicists agree that superconductivity relies on getting electrons to overcome their mutual Coulomb repulsion and form “Cooper pairs”. In the Bardeen-Cooper-Schrieffer (BCS) theory of low-temperature superconductivity, the electrons are held together as a result of their interactions with phonons – lattice vibrations – in the material.

However, the BCS theory can not explain the behaviour of high-temperature superconductors – which were discovered in 1986 and can have transition temperatures as high as 138 Kelvin. Now, Thomas Timusk of McMaster and colleagues have measured the optical self-energy spectrum of a bismuth-based cuprate superconductor known as Bi-2212. The McMaster-Brookhaven team found that the spectrum was broad and featureless above 88 Kelvin, the superconducting transition temperature of the material. However, a sharp peak appeared in the spectrum below this temperature (see figure).

According to the team, the results rule out theories based on phonons and the creation of a magnetic resonance state in the material, but they are consistent with the “magnetic-pairing” model of high-temperature superconductivity. “The results show that the magnetic resonance that has been the subject of a large number of recent papers does not by itself cause high-temperature superconductivity,” says Timusk. “Moreover, a hypothetical phonon at the same energy as the magnetic resonance cannot be the cause either.”

Timusk adds that the team’s approach could be used to analyse excitations in a wide range of other high-temperature superconductors and “exotic” materials.

Looking at decoherence

Researchers have seen quantum interference effects in electrons, atoms and small molecules but never in macroscopic objects. In 1999, the Vienna group observed wave properties in carbon-60 and carbon-70 molecules. With a diameter of about 1 nanometre, these were the biggest objects to have shown quantum interference at the time. Since then, the team has observed wave properties in larger molecules such as tetraphenylporphyrin. This molecule, which is present in chlorophyll and haemoglobin, has a diameter of about 2 nanometres.

Arndt and co-workers first sent a beam of carbon-70 molecules through a laser system that heated them to around 5000 Kelvin. As the molecules then cooled down by emitting photons, they were passed through an interferometer that contained three sets of diffraction gratings. The first grating produced a coherent beam of molecules, the second created the interference pattern, and the third imaged this pattern. The slits in the gratings were about 500 nanometres wide and the grating had a period of around 1000 nanometres.

The Vienna physicists found that below about 1000 Kelvin, a high-quality interference fringe pattern characteristic of quantum behaviour could be seen. However, these patterns gradually vanished as the molecules were heated and started to emit thermal radiation. Since these photons could, in principle, be detected to reveal which slit the molecule has passed through, the wave-like quantum behaviour of the molecules disappears. The group now hopes to observe the effect of decoherence in even larger molecules, such as proteins.

Packing them in

Mathematicians and physicists have long been interested in how objects pack together. In the sixteenth century, Johannes Kepler predicted that the densest possible arrangement of spheres would fill about 74% of the total available space. However, it was not until 1998 that this arrangement was mathematically proven to be an ordered face-centred cubic lattice. Random or amorphous packing fractions are lower, and fill a maximum of only 64% of the total available space.

Now, Chaikin and colleagues have shown that spheroids – like M&Ms – can randomly pack more densely to fill between 68 to 71% of the total available space. Moreover, they found that cigar-shaped ellipsoids could be randomly packed with a density of almost 74%.

The Princeton-Cornell team determined the packing fractions of M&Ms in different shaped containers and then compared these results to those obtained with spherical ball bearings measuring about 3 millimetres across. The team used magnetic resonance imaging (MRI) to confirm that the sweets had not arranged themselves in an ordered “crystalline” structure.

Shape effects

Chaikin and colleagues believe that the higher packing densities are possible because spheroid and ellipsoid particles are free to move in the container. Spherical particles can only rotate but non-spherical particles can also move sideways to settle into more stable positions. Furthermore, the team found that this movement was related to the number of contacts created between neighbouring particles. This number is higher for spheroids than spheres.

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