Page 1893 – Physics World

Home » Page 1893

Astronomers find middleweight black hole

The black hole is around 600 light years from the centre of galaxy M82. “This black hole might eventually sink to the centre of the galaxy where it may grow to become a supermassive black hole”, says Hironori Matsumoto of the Massachusetts Institute of Technology, lead author on one of three papers about the discovery.

The Chandra team compared new high-resolution images with optical, radio and infrared maps to establish that the X-rays were coming from a single strong source. Eight months of observations revealed that the signal intensity was fluctuating slowly over a period of months, and also flickering with a 600-second oscillation. “This flickering is similar to the well-studied characteristics of black holes swallowing gas from a nearby star or cloud”, explains Philip Kaaret of the Harvard-Smithsonian Center for Astrophysics. “Explanations other than a massive black hole for this object are implausible”.

The next challenge for astronomers is to establish how the newly discovered black hole formed. “This opens a whole new field of research”, says Martin Ward of the University of Leicester in the UK. “Nobody was sure that such black holes existed, especially outside the centres of galaxies”. Some scientists believe that it could be a ‘cannibal’ that has reached its present size by swallowing many other stars. Astronomers may also need to modify current theories of stellar evolution in the light of the discovery.

CERN chases the Higgs

“It was a difficult decision,” admitted Roger Cashmore, CERN’s director of research to Physics Web. “We wanted to give the experiments the opportunity to better test the effect without compromising the LHC schedule.” If the experiments continue to measure the same effects, the extra data will reduce the likelihood that the observation is a statistical fluctuation from 6 in 1000 to 1 in 10 000.

Over the next five weeks the experiments will focus solely on collecting as much data as possible at a centre-of-mass-energy of 206.6 GeV. The gamble will cost CERN about an extra SwF 7 million (about £2.7 m), says Cashmore. However, the decision sets the scene for a nail-biting end to the accelerator’s illustrious career.

SOLEIL synchrotron for Saclay

The intense X-rays from SOLEIL will be used for a range of experiments in physics, chemistry and biology. The machine is expected to come on-line in 2005. According to Schwartzenberg, the French government will pay no more than 20% of the total costs, leaving regional and local authorities to contribute 75%, with the remaining funds coming from the UK, Spain, Belgium and Portugal. France is already a partner in DIAMOND, a similar third-generation synchrotron that will be built at the Rutherford Appleton Laboratory in the UK.

Superconductivity: getting warmer

The discovery of high-temperature superconductivity in 1986 raised hopes that loss-free superconducting power cables would be able to operate at liquid nitrogen temperatures. Previously, superconductors had to be cooled with liquid helium, which is much more expensive. But researchers soon found that the new polycrystalline superconductors could support only extremely low current densities at this temperature. This is a result of the large degree of misalignment between the individual crystals in the superconductors. Further studies suggested that oxygen depletion at these large-angle boundaries results in a lack of charge carriers, which leads to the drop in current. This prompted Hammerl and co-workers to investigate the possibility of repopulating or “doping” the boundary regions with charge carriers to stem the loss of current. “We observed the first enhancements of the critical current just a few days after we had the idea”, Jochen Mannhart, one of Hammerl’s colleagues, told PhysicsWeb.

The team interleaved layers of superconducting yttrium barium copper oxide (YBCO) with layers of calcium-doped YBCO, each 25 nm thick. Calcium ions are similar in size to yttrium ions but have a smaller positive charge. When these calcium ions take the place of yttrium ions in YBaCuO, they act as ‘positive holes’, charge carriers opposite in polarity to electrons. The group found that the holes diffused preferentially into grain boundaries in the adjacent undoped layers and increased the inter-grain current flow by between three and six times. “It would be particularly exciting to measure the calcium concentration around the grain boundaries and determine its electronic structure”, explained Mannhart, “and we have plans underway to do that”.

Mannhart and colleagues believe that their techniques could be transferred to the manufacture of high-temperature superconductors, but emphasise that although the results are highly encouraging, an industrial process has yet to be devised for large-scale fabrication.

Ulysses probes the solar maximum

“Solar activity has been increasing for the past three or four years, and now Ulysses has started observing much more disturbed conditions at increasing solar latitudes,” explains Andre Balogh of Imperial College, a principal scientific investigator on the mission. “The passes over the poles of the Sun will show a different kind of interplanetary environment from the one we observed in the mid-1990s. Instead of coronal holes covering the polar regions, hot and active regions are generating a surprising amount of disturbance over the whole Sun now.” Ulysses provides this unique perspective because it is the only spacecraft to have flown in an orbit outside the ecliptic, the plane in which the planets orbit the sun.

The high-level solar winds are currently chaotic and blustery, in contrast to the fast – 750 km s^-1 – but steady conditions encountered in 1994. The last passage over the south pole revealed a surprisingly high level of fluctuation in the magnetic field that restricts the influx of cosmic rays into our solar system. It also showed that the boundary between the fast solar winds and the more irregular equatorial winds was unexpectedly distinct.

Ulysses, a joint ESA/NASA mission, will approach the equator early next year and continue its journey over the northern polar region, completing its second orbit of the Sun in 2004. “By then, Ulysses will have gathered the only set of observations above the solar poles covering more than a complete 11-year solar cycle”, says Balogh. “In the absence of follow-up missions, Ulysses will continue to be the benchmark for our understanding of the heliosphere [the region of space dominated by the Sun and its solar wind] for another generation.”

Bad vibrations from acoustic lenses

Davis’ team developed a three-dimensional model to pinpoint possible underground focusing structures after analysing aftershock data and the local geological structure. The north-dipping Santa Monica fault and the basement of the Los Angeles basin form a convex structure, about three kilometres below the Earth’s surface. The group believes that seismic waves passed through this structure and converged at many different points on the surface because of irregularities in the ‘lens’ structure. “It is possible we could use the discovery to decide which buildings should be reinforced first in earthquake ‘retrofitting'”, Davis told PhysicsWeb.

The lens theory is supported by the observation that the amplification effect increased with frequency. Many of the buildings destroyed in Santa Monica had resonant frequencies between 5 and 15 Hz, and Davis and colleagues found that the acoustic lenses would have selectively amplified these wavelengths. The pattern also shifts with changing angle of incidence: the amplification recorded 20° away from the critical angle was three times smaller than the maximum. So waves from other earthquakes will not be focused to the same extent as those from the Northridge event if they pass through the lens at a different angle.

Davis and his colleagues have conducted further studies in which they detonated explosives underground to simulate shockwaves from the Northridge earthquake. “Once all of the seismic sounding has been completed, a more extensive computation will be carried out”, said Davis, “but although the optics theories will serve as a check on the more detailed computations, I think the current work is about as far as they will be applicable”.

Breaking through the quantum limit

Andrey Geim of the Universities of Nijmegen and Manchester, and co-workers from Russia, Belgium and the UK, measured the amount of flux entering micron-sized aluminium discs, where edge effects are important. The measurements were made using a so-called ballistic Hall magnetometer. The samples were first cooled to 0.5 K in the absence of a magnetic field. As the field was increased, changes in the magnetic flux led to changes in the Hall voltage, which can easily be measured. The technique has a resolution of a tiny fraction of a flux quantum.

The team found that the magnetic flux increased in steps as individual vortices penetrated the disc. On closer examination, they noted that the height of some of the steps was as small as 0.001f. In other cases the jumps were negative, indicating that magnetic flux may be expelled.

Geim and co-workers repeated the experiments using discs with slightly different shapes and found that the amount of flux carried by the vortices depends crucially on the roughness of the edge of the disk. This effect arises due to changes in the structure of vortices near the edge. The team concludes that the effect will be important in the majority of experiments with thin films.

First evidence for the Brain Drain

Sharp Pierson and Cotgreave of the Save British Science Society analysed the publishing records of 252 scientists who gained their doctoral degrees in the UK. They compared the quality of the scientists’ early published research with their current country of residence, using the number of citations a paper received as a measure of quality. Scientists who remained in the UK were found to have published similar numbers of papers to those currently in the US, but the mean number of citations per article was significantly higher for scientists who now live in the US.

The result is not conclusive proof that the best British scientists are emigrating to the US, emphasise Sharp Pierson and Cotgreave. They point out that citation level is an unreliable indicator of quality, and that there are many reasons why scientists may not publish their work. Nevertheless, the study provides the first quantitative support for the brain drain, and shows that the UK Government’s significant new investment in British science has come at a good time.

Nanoscientists go on a roll

Whereas the carbon atoms in a graphite sheet are arranged in hexagons, the fullerene form of carbon includes both pentagons and hexagons. The first fullerene to be discovered, carbon-60, contains 12 pentagons and 20 hexagons and is shaped like a football. Now Paul McEuen’s group at the Lawrence Berkeley National Laboratory and the University of California at Berkeley has fabricated a single-molecule transistor based on a carbon-60 molecule bouncing between gold electrodes (H Park et al. 2000 Nature 407 57).

The transistor uses a process known as vibration-assisted tunnelling. For current to flow through the transistor, electrons must tunnel across the gaps between the carbon-60 molecule and the electrodes. To tunnel onto the molecule, an electron in the source electrode must have precisely the correct energy to occupy the lowest energy level in the molecule. However, if the electron has extra energy that is equivalent to the vibrational energy of carbon-60, it can still tunnel onto the molecule by using this surplus energy to set the molecule in motion.

The Berkeley team believe that, under certain conditions, a single electron is transferred between the electrodes during each cycle of the carbon-60 oscillation. Since the frequency of the vibration is quantized, it follows that the current flowing through the transistor is also under tight control. The device, in effect, acts like an “electron turnstile”. This control could be used to measure electric currents with extreme accuracy.

Carbon nanotubes are made by rolling up sheets of graphite to create hollow cylinders of pure carbon. These nanotubes have remarkable properties: they are incredibly strong and can behave either as semiconductors or conductors. Now Alan Johnson of the University of Pennsylvania and co-workers have discovered that nanotubes also conduct heat extremely well (J Hone et al. 2000 Science 289 1730). The heat is conducted by low-energy phonons, which can be thought of as quanta of sound waves. The electrical properties of nanotubes have been extensively investigated and found to be consistent with the electrons being confined to the one dimension of the tube. However, similar confinement of phonons has not been observed until now.

The results also show that the remarkable heat conduction properties of nanotubes extend to larger “bundles” of tubes. The bonds between individual nanotubes are extremely weak, so the phonons do not scatter into adjacent tubes. Unfortunately, this weak bonding between tubes also means that “ropes” formed from the nanotubes are likely to be relatively weak.

Finally, Horst Prinzbach and co-workers at Albert-Ludwigs-University in Freiburg, Germany and Boston College in the US have produced minute quantities of the smallest possible fullerene, carbon-20 (H Prinzbach et al. 2000 Nature 407 60). The molecule, which consists of 12 pentagonal rings, was previously thought to be too unstable to exist. Prinzbach and co-workers have created both the fullerene form of carbon-20 and a bowl-shaped isomer which may improve our understanding of how fullerenes form.

Sonic crystals make the sound barrier

The crystals themselves are constructed from lead balls 1 cm in diameter, coated with a 2.5-mm layer of silicone rubber, and placed inside an epoxy matrix. The strong periodic variation in density creates spectral gaps that prevent the transmission of waves. This is analogous to the attenuation of electromagnetic waves in photonic crystals. Liu’s team placed a sound source near the crystal and compared the amplitudes of sound waves at the surface of the crystal and at the centre of the crystal. They found distinct gaps in the range of frequencies transmitted through the crystal. The missing frequencies are absorbed by certain oscillations of the coated spheres, which are like vibrations in molecular crystals.

But to attenuate sound, the lattice spacing inside the crystal must usually be of the same order as the sound’s wavelength, and for environmental noise the crystals would need to be metres across. Liu’s team overcame this problem by creating disordered composites of the crystals. The local resonant properties of the disordered composite give it a negative elastic constant so that the absorption of sound increases exponentially with the thickness of the material.

The size and geometry of the crystals can be tuned to absorb different wavelengths. Future development of the sonic crystals to extend their frequency range may lead to applications in seismic wave reflection and ultrasonics.

Browse articles by content type