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Home » Page 1869

Nuclear symmetry enters a new phase

The point at which a phase transition occurs is called the critical point and is dear to every physicist’s heart. The fascination with systems at the critical point stems from the fact that a small change in one parameter, such as temperature, can trigger spectacular changes in the other properties. Indeed, it is this discontinuity in the properties at the critical point that makes phase transitions so challenging to describe.

Phase transitions arguably play an even more important role in the world of quantum mechanics. In the case of nuclei, the numbers of neutrons, N, and protons, Z, act as a control parameter in the same way that temperature affects water. Physicists can study variations in the shape of the nucleus as a function of N and Z, paying particular interest to those regions where the shape changes most rapidly. These regions are the equivalent of the critical points in a standard phase diagram.

In many examples in macroscopic and condensed-matter physics, it is easy to draw a phase diagram because huge numbers of particles participate. But nuclei contain at most a few hundred nucleons – and this makes the analysis of nuclear phase transitions more difficult.

Now Franceso Iachello at Yale University in the US has proposed a method based on nuclear symmetries that might lead to a framework to deal with nuclei at, or around, critical points. Moreover, Rick Casten and Victor Zamfir, also at Yale, have shown convincing evidence that the properties predicted by Iachello’s model are approximately observed in a samarium isotope (Phys. Rev. Lett. 2001 87 052502; 052503).

In the August issue of Physics World, Piet Van Isacker of the GANIL laboratory, France, explores the new method for modelling the often complicated transitions in nuclear phase diagrams.

Web server celebrates a decade of hits

When two physicists from the University of California at Santa Barbara posted a paper called “Exact black string solutions in three dimensions” on a Web site at Los Alamos on 14 August 1991, a revolution in physics began. Since then, the pre-print server at the Los Alamos National Laboratory (LANL) has allowed researchers in physics and related disciplines to post and retrieve papers far faster than any other publishing method.

“It has had an enormous impact on the accessibility of the physics literature in terms of both reading other people’s papers and having your own papers read,” says Heath O’Connell, manager of the SPIRES database at the Stanford Linear Accelerator Laboratory. “It also paved the way for on-line journals.”

The physics community will mark the tenth anniversary of the “e-print” server this month during a meeting on strings in Aspen, Colorado, where Paul Ginsparg, a theoretical physicist at LANL, originally had the idea for xxx.lanl.gov. After the meeting, however, Ginsparg will move the server, now named arXiv.org (pronounced ar-chi-v), in a new direction. He will take up the post of professor of physics and computer science at Cornell University, where he earned his PhD 20 years ago. The server will be rehoused in the university itself.

“My hope is to see the research in communication resources represented by the arXiv continue to evolve and continue to serve as a model for other disciplines,” Ginsparg told Physics World. “In physics research, I’m hoping to pick up where I left off.” His non-arXiv work includes the chiral symmetry of the lattice and string theory.

Cornell regards the move as a double coup. “Ginsparg is an important hire because he is an excellent theoretical physicist and because his arXiv is transforming the way physicists interact with each other and with their scientific literature,” says Peter Lepage, head of physics.

Instant appeal

The arXiv is a fully automated electronic archive and distribution server for research papers. Authors can submit their papers to the site via the Web interface, by e-mail or by ftp. They can also update their submissions as they receive comments. Users access the papers through the Web or by e-mail.

The system has grown significantly since it began. Originally, recalls Ginsparg, “it ran on a NeXTstation computer in a colleague’s office with a 25 MHz processor and a 400 Mb hard drive”. It had a constituency of about 100 scientists, most of them particle physicists who were already used to exchanging paper pre-prints by post. Ginsparg anticipated 100 submissions per year.

Today, after several upgrades, the system serves the bulk of the physics community worldwide. Some two million visitors check the site each week and Ginsparg expects to receive 35 000 submissions from about 100 different countries this year. The scope of the site has also broadened, with the number of papers on condensed-matter physics rapidly overtaking that on particle physics.

“It has been an essential tool for superconductivity,” says David Cardwell from Cambridge University in the UK. “In my opinion it has enabled more coherent and efficient progress to be made by reducing repetition of experiments and by establishing correlation at an earlier stage.”

This year’s activity in superconductors based on magnesium-diboride compounds illustrates that value. “When we wrote our first papers on MgB2 we would submit them to Physical Review Letters, post them on LANL, and then e-mail copies of the file to friends, colleagues, and/or researchers in the field,” recalls Paul Canfield from Iowa State University. “The LANL server rapidly became the focus of MgB2 research. By the time the first paper was published there had been scores of papers posted on the server. The fact that the research community could exchange information so rapidly allowed for explosive growth of knowledge.”

Access all areas

The global nature of the interest illustrates another advantage of the server. “Primarily it has levelled the playing field, both geographically and hierarchically from graduates students up,” says Ginsparg. “There’s no privileged loop of researchers with advanced access to new material. It gives researchers everywhere the ability to decide when they want to go fully public with new results and makes those results simultaneously accessible to all interested.”

One might imagine that the arXiv would have succeeded at the expense of physics journals. In fact, says Ginsparg, “in physics this set of resources was established long before any of the publishers had discovered the Internet. It would have been a mistake for any publisher to alienate the established physicist clientele.” And while the American Physical Society (APS) initially worried about arXiv’s impact on its publishing, Ginsparg continues, “it’s clear now that we’re all trying to figure out how best to port physics communications infrastructure to the new technology”.

Conventional publishers have the obvious advantage of peer review. The arXiv, by contrast, simply screens e-mail domains. “We require a recognized affiliation,” says Ginsparg. “If someone from Caltech tried to submit a paper on perpetual motion, it would appear.” Iowa’s Canfield warns about this “slightly dark side” to the server’s use. “Several groups posted magnesium-diboride papers that were highly suspect, if not clearly wrong, that were then removed within several days for a variety of claimed errors,” he says. “This simply means that people who read the server have to be extra cautious about what to believe.”

Moving east

The arXiv will experience that future in its new location. Rumours in the physics community suggest that Ginsparg left LANL because it had not given him and the arXiv enough support. However, he sees the move as necessary to continue the server’s evolution. “The LANL research environment was essential to starting it and it wouldn’t have been possible had I been a university faculty member with too many other obligations,” he explains. “[Los Alamos] was a good place to incubate the project. But now it has achieved a level of maturity that makes it possible to institutionalize in a new and more appropriate setting.”

Although LANL will continue to provide some costs and services to arXiv, the site will lose a contribution from the US Department of Energy to its $300 000 annual cost. However, the server will continue to receive support from the National Science Foundation. Cornell University is providing some money and will also seek private funds.

As to future development, Ginsparg anticipates plenty of cross-fertilization with other projects at Cornell. “The Cornell group is strongly committed to electronic self-archiving,” says Stanford’s O’Connell. “Giving Ginsparg this official position should lay to rest any fears about the archive’s future.”

Positive ions pull together

Zajfman and colleagues injected around 10 000 argon ions into an ‘electrostatic trap’ – a tube 40 cm long with ring-shaped electrodes around both ends. These electrodes act as ‘mirrors’, and their voltages can be tuned to propel the argon ions up and down the tube. The ions travel at slightly different speeds when they are injected into the tube, and this velocity spread becomes more pronounced as they travel back and forth between the mirrors.

At certain voltages, however, the team noticed that the ions all travelled at the same speed, and stayed tightly packed in a cloud. ‘Our first reaction was, “that’s impossible”‘, says Zajfman. ‘But then we had to scratch our heads and try to understand it’.

Zajfman and colleagues believe that this effect is due to ‘Coulomb repulsion’, which usually pushes ions apart when they get too close to each other. They suggest that this repulsion makes the ions bounce off one another when they collide within the cloud, and the energetic ions impart energy to the less energetic ions during this process. After a short time, the momentum is distributed evenly throughout the cloud and the ions all travel at the same speed. The overall lifetime of the cloud is determined by the number of neutral atoms that leave the electrostatic trap. These neutral atoms are produced when ions collide with residual gas atoms in the tube.

Zajfman and colleagues are optimistic that their technique could lead to a mass spectrometer with significantly better resolution than existing devices. Mass spectrometry measures the masses of ions by studying their motion as they are injected into an electric field. But a sample of ions only stays together for short time inside the mass spectrometer, and this limits the resolution of the technique.

Quantum pulses pinpoint position

Conventional positioning techniques measure the time it takes for laser pulses to travel between a reference point and the location in question. The accuracy of this method relies on how precisely the arrival of the pulses can be timed. But in practice, light pulses from a laser consist of a range of frequencies, and the wider this range, the less accurately the arrival of the pulse can be timed. This is because different wavelengths travel at slightly different speeds.

Lloyd and colleagues realised that the use of entangled pulses would allow them to side-step this limit. In general, quantum entanglement allows correlation between particles that are much stronger than those allowed in classical physics. In the MIT experiment, the frequencies of all the photons in the entangled pulse are intrinsically linked. This means that the arrival times of the photons are bunched together, which allows the time of arrival to be determined more precisely.

However, this relationship makes the quantum technique fragile, because all of the information is lost if a single photon does not reach the detector. In contrast, conventional techniques measure the arrival of individual photons. Lloyd’s team claims to have addressed this problem by using ‘partially entangled’ photons that are less sensitive to the loss of photons, but these results are unpublished. Nevertheless, Lloyd and colleagues assert that their ‘fully entangled’ technique is still a significant improvement on conventional methods, and could also be used to synchronise clocks more precisely than ever before.

“Quantum positioning is unlikely to replace global positioning systems in the near future”, Lloyd told PhysicsWeb, “but as techniques for making these funky quantum pulses improve, quantum positioning systems are likely to be used where high accuracy and low power are important, for example, satellite positioning”.

DIAMOND boss appointed

Materlik, 56, brings wide experience to the job, having helped to build the HASYLAB facility and been its director from 1986 to 1993. He has also been involved in the European Synchrotron Radiation Facility from its inception in 1978 and has served on the advisory boards for synchrotrons around the world, including Spring8 in Japan and the Advanced Photon Source in the US.

“It’s a great project and I’m really enthusiastic about building it,” says Materlik. “My priority will be to deliver a source that is better than the competition and has an outstanding beam that can be used for top-class experiments.” He also wants to encourage links between the different synchrotron groups who will use the machine and to develop a strong synergy between DIAMOND and the other facilities at the Rutherford Appleton Laboratory, which include the ISIS neutron source and the Central Laser Facility. Materlik is also a member of the science directorate of the DESY particle-physics lab and professor of physics at Hamburg University.

DIAMOND will consist of a 3 GeV electron storage ring some 178 metres in diameter. Electrons fed into the ultra-high vacuum ring will be forced through complex ‘insertion devices’ and ‘bending magnets’ to generate X-rays that will be used for experiments in physics, chemistry and biology. The facility – expected to open in September 2006 – will include an initial seven beamlines with space for a further 37.

Astronomy meets angels in a divine comedy

Cotterill and Harkness had not planned to base the show on astronomy. Their initial idea involved the angels, who come to live on Earth after losing their jobs in heaven. But although the angels are wise to the laws of the cosmos, they are mystified by human behaviour. “We thought an astronomer would be the perfect contrast – a human being fascinated by cosmology”, Harkness told PhysicsWeb.

Writing around the time of the 1999 eclipse, solar astronomy appealed to Cotterill and Harkness because of its wide popularity. Brown suggested that the astronomer in the play could be engaged in the high-profile search for neutrino oscillations at the Sudbury Neutrino Observatory in Canada. Despite the subject matter, Harkness emphasises that the comedy – which was adapted for radio following the play’s success at the Edinburgh Fringe Festival – is accessible to all. “As non-physicists ourselves, it was very interesting to learn about the experiment”, she says.

Ironically, it was announced that neutrino oscillations had been detected just two days after the radio show was recorded. Astronomers had previously observed too few neutrinos arriving from the Sun, but their detectors could not detect all three types of neutrino effectively. The fact that neutrinos can change from one type to another – or ‘oscillate’ – en route to Earth explains this shortfall.

“We were still very excited when we heard that they had solved the solar neutrino problem”, says Harkness. “But I suppose that’s the occupational hazard in writing scientific comedy”.

Crystals could make super semiconductors

Most semiconductor devices consist of an insulating layer sandwiched between a semiconductor and a metal conductor. This is the well-known MOS – metal-oxide-semiconductor – structure. When a voltage is applied across the device to contacts on the semiconductor and metal layers, positive holes move from the semiconductor into the insulator, while electrons travel in the opposite direction. This produces a narrow region of charges at the boundary in which current can flow.

But imperfections are common in the amorphous insulators, such as silicon dioxide, used in most semiconductor devices. This leads to an uneven distribution of charge at the interface and reduces efficiency. To overcome this problem, McKee and co-workers developed crystalline materials made from various combinations of barium, strontium, titanium and oxygen that were almost defect-free. In contact with silicon or germanium layers, these produce a far more uniform region of charge. “The barium-titanium-oxygen-germanium structure may well be electrically perfect”, says McKee.

Scientists have long searched for a way to reduce the number of imperfections in the insulating layers of MOS structures, but McKee’s team is the first to experiment with a crystalline structure. “To our knowledge, this is the first demonstration of charge inversion for a gate oxide on germanium”, he says.

The breakthrough should also allow physicists to control semiconductor interfaces on the atomic scale – a feat necessary for fledgling technologies such as ferroelectric lithography and quantum computing.

Fluid dynamics slips up

Most fluid-dynamics simulations based on the non-slip boundary condition produce good results on large scales. However, previous experiments with capillary tubes have hinted that the molecules in contact with the vessel wall do move, and that this effect may become important at small scales. To test this hypothesis, Craig and colleagues studied a solution of sugar and water. This is a ‘perfect’ or Newtonian liquid, which means that its viscosity increases as a greater shear force is applied.

The team immersed a mica sheet and a silica sphere 20 micrometres in diameter in the solution. A very fine spring connected the sphere to an atomic force microscope, which measured the force on the sphere as the mica surface was pushed towards it. When the distance between the sphere and the surface was less than the radius of the sphere, Craig and colleagues found that the motion of molecules in contact with the surface and with the sphere became important.

Craig’s team modified an existing relationship to account for the effect of this slip on the force experienced by the sphere. This allowed them to calculate that the molecules at the solid-fluid interface shifted by up to 20 nanometres as the liquid flowed through the progressively narrower channel between the sphere and the surface. Craig and co-workers found that this ‘slip distance’ is related to the viscosity of the fluid and the velocity of the sphere.

This discovery helps to explain why red blood cells can squeeze through narrow capillaries without placing too much pressure on the capillary walls. Ink-jet printers and other micro- and nanomachines should also benefit from the improved understanding of how fluids behave in closed systems.

“The occurrence of slip in aqueous Newtonian fluids considerably complicates flow predictions in confined systems”, says Craig. “But this is the price that must be paid for accurate modelling”.

Tiny black hole lurks in neighbouring galaxy

Merritt and colleagues studied images of the spiral galaxy M33 that were taken by the Hubble Space Telescope, and had a resolution ten times better than previous Earth-based observations. The team measured the velocities of stars circulating in the nucleus of M33. In other galaxies, the gravity of the central black hole makes the stars closest to it move very rapidly. But Merritt’s team found that stars close to the centre of M33 were moving slowly.

Merritt and colleague Laura Ferrarese recently discovered how the mass of the central black hole in a galaxy relates to the velocities of the stars in orbit around it. This allowed them to calculate that the black hole in M33 must be less than 3000 times the mass of the Sun, even accounting for the orbital eccentricities and gravitational interactions of the stars. This means the black hole at the centre of M33 is of ‘intermediate’ mass at most.

Astronomers previously thought that all galaxies were powered by super-massive black holes, and believed that large and small black holes evolved differently. But the search for the black hole at the centre of M33 suggests that these ideas may be flawed.

“The upper limit on the mass is still consistent with the relation that we discovered a year ago, and implies that small black holes – if they exist – formed in much the same way as very massive ones”, says Ferrarese.

Bose-Einstein condensation bursts out

A Bose-Einstein condensate is a collection of gas atoms – cooled to a few millikelvin above absolute zero – that all occupy the same quantum state. This means that a single wave function describes the behaviour of all the atoms, and the quantum properties of the atoms can be seen in the macroscopic behaviour of the condensate.

Donley and co-workers used a magnetic field to control the attractive and repulsive forces between rubidium-85 atoms in a condensate. Such interatomic forces determine the properties – electrical, magnetic and mechanical – of all types of matter. The interactions between atoms in a Bose-Einstein condensate depend on the density of the condensate – which is typically ten thousand times less dense than air – and the ‘scattering length’. Rubidium-85 is unusual because its scattering length can be adjusted by an external magnetic field. A positive scattering length corresponds to repulsive forces between the atoms, while a negative value results in mutual attraction.

When Donley and colleagues suddenly made the attractive forces very strong, they found that the condensate imploded. But during the collapse, a proportion of the atoms exploded out of the condensate. Donley and colleagues found that these bursts of energetic atoms were ‘spin-polarized’, that is, they all had the same quantum mechanical ‘spin’.

Some of the atoms in the condensate did not take part in the collapse but remained in their original state for around a second after the implosion. The team calculated that the number of atoms in this state was related to the scattering length at which the implosion took place, and the initial number of atoms in the condensate. Donley’s team also forced jets of atoms from the imploding condensate by suddenly changing the scattering length before the collapse was complete.

Current theories of Bose-Einstein condensation cannot account for many of these observations. “We hope our work will generate new theoretical ideas to explain our data”, says team leader Carl Wieman. “This will provide a deeper understanding of Bose-Einstein condensation and quantum physics in general”.

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