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Latest superconductor has an iron constitution

Magnetism has long been thought to be incompatible with superconductivity. According to the Bardeen-Cooper-Schreiffer theory of superconductivity, electrons with opposite spins pair up and are propelled through the superconductor by vibrations of the crystal lattice known as phonons. But magnetism changes the spin on the electrons and stops them pairing up. Even a very small number of magnetic impurities can inhibit the formation of pairs. In order to superconduct, iron – which is strongly ferromagnetic – must therefore overcome this effect.

Iron has a body-centred cubic structure under ambient conditions, which gives rise to its ferromagnetism. Under high pressures, however, it transforms to hexagonal close packing structure, in which ferromagnetism is not thought to exist. This raises the possibility that iron could become a superconductor, as first predicted in 1979.

To investigate, Shimizu and colleagues placed an iron chip just tens of microns across and about 100 microns long inside a pressure cell made of diamond. The sample was compressed and then chilled with a helium refrigerator. The team measured the resistivity of the chip between 10 kelvin and 30 millikelvin for a range of pressures.

The resistivity of the iron sample fell sharply when the pressure reached 15 gigapascals – over ten thousand times greater than atmospheric pressure – close to absolute zero. Shimizu and co-workers found that this transition temperature rose to 2 kelvin at a pressure of 21 gigapascals, but dropped again at higher pressures.

SQUID measurements of magnetic flux also revealed that the iron expelled magnetic flux from its interior. This phenomenon – known as the Meissner effect – is a characteristic of diamagnetic superconductors, and suggests that iron may be weakly magnetic under extreme pressure.

Elsewhere, recent studies have shown that some ‘organic metals’ can superconduct inside a strong magnetic field under certain circumstances. Together with these results, the findings of Shimizu and colleagues will help physicists to understand the mechanisms that underpin superconductivity. Insights into how pressure affects the structure of iron could also shed light on the properties of the highly compressed iron that makes up the Earth’s core.

Optical clock is on the dot

The second is currently defined as the time it takes for the radiation corresponding to a certain energy gap in a caesium atom to complete 9192 631 770 oscillations. This technique clearly depends on our ability to count the oscillations reliably. Existing atomic clocks are accurate to one part in 10¹⁵, but optical signals are desirable because they could provide an even more accurate definition of the second. A higher-frequency signal oscillates more often – in effect, the ‘pendulum’ swings more quickly – but it is much harder to count these oscillations.

In the past, a complicated ‘frequency chain’ was needed to relate optical signal to microwave signals, whose oscillations are easier to count. But the set-up was cumbersome and difficult to operate. Last year Udem and colleagues developed a ‘frequency comb’ that replaces the frequency chain with a femtosecond laser and a photonic fibre – an optical fibre with an array of tiny holes running along its core.

“Our optical clock promises accuracy that would be difficult to achieve with current atomic clocks based on microwave transitions”, say Udem and colleagues. “There is little dispute that accurate microwave atomic clocks have greatly improved navigation and communication. It is likely that optical clocks of the future will have a similar impact.”

Superconductivity: boron goes it alone

Hemley and colleagues attached platinum electrodes to a flake of boron around 40 microns across and 2 microns thick. The flake was then sandwiched between two diamonds, which compressed the boron until it became metallic. Boron is a non-metallic semiconductor under normal conditions, but its resistance falls and it becomes completely metallic when compressed to 175 gigapascals at room temperature.

Next, the team cooled the metallic boron and measured its conductivity for a range of pressures. At 160 gigapascals, they found that boron is a superconductor up to 6 kelvin. At 250 gigapascals, it exhibited superconductivity up to 11 kelvin – the first time that conductivity has been measured under such high pressures.

This makes boron unusual: in other metals, this transition temperature – the temperature below which superconductivity takes place – falls with increasing pressure. According to the Bardeen-Cooper-Schreiffer theory of superconductivity, vibrations of the crystal lattice – or ‘phonons’ – enable paired electrons to flow in some superconductors. Theorists previously thought that high pressure increased the frequency of these phonons, reducing their ability to aid electron flow. Hemley and colleagues speculate that this effect is outweighed in boron by other complex interactions within the crystal.

The underlying mechanism of boron’s superconductivity arises from its crystal structure under high pressure. Distinct steps were visible in the conductivity measurements as the pressure increased, which are known to correspond to shifts in the crystal structure. But although a boron crystal has the structure of an icosahedron – a polyhedron with twenty faces – under ambient conditions, Hemley and colleagues are unsure of its structure when it is highly compressed.

Interest in the superconducting properties of boron has been high since the discovery earlier this year of the metal superconductor magnesium diboride. Theory has also long predicted that compressed low-mass elements would support resistance-free current flow. Scientists even believe that hydrogen will become a superconductor under sufficiently high pressures.

“These experiments are very difficult”, Hemley told PhysicsWeb, “and it is a measure of the recent advances in high-pressure physics that we can now perform electrical conductivity measurements under these conditions”.

Moon-count rises on Saturn

Gladman’s team inspected the entire stable portion of space around Saturn in which satellites are able to orbit. This is known as the Hill sphere, and is the region around any planet where its own gravitational forces outweigh the Sun’s tidal forces. The powerful optics based on charge-coupled devices could detect very faint objects of just 23rd magnitude – over a hundred million times dimmer than a bright star. Gladman and co-workers are confident that they have detected all the moons orbiting Saturn that are greater than about four kilometres in diameter.

Satellites of the giant planets in the solar system fall into two categories: ‘regular’ moons with circular orbits in the planet’s plane of rotation, and ‘irregular moons’ with highly elliptical and inclined orbits. Saturn’s new moons join Phoebe – the only previously known irregular satellite orbiting the planet – in the second group.

Different formation processes are thought to have given rise to the two distinct groups of satellites. Regular moons probably formed long ago from the disks of gas and dust circulating around planets. But it is likely that the irregular moons were wandering objects later captured by the gravitational fields of the giant planets.

Gladman and colleagues calculated the orbits of Saturn’s new moons – which range from six to 32 kilometres in diameter – and found that they were sharply inclined to the planet’s plane of rotation. Together with their sizes, this strongly suggests that the satellites are fragments left over from collisions between larger moons, according to the group.

Satellites are visible because they reflect sunlight, and this makes it easier to spot satellites that are orbiting planets closer to the Sun. Gladman and co-workers believe a similar system of irregular satellites could be orbiting Neptune, but speculate that they could be between ten and a hundred times dimmer.

Saturn’s new moons were tracked for six months during 2000 and 2001 from several ground-based observatories, including the European Southern Observatory telescope and the Canada-France-Hawaii telescope.

Relativity passes the pulsar test

Pulsars are rapidly spinning neutron stars that earn their name from the beams of radiation they emit, which appear as pulses to a stationary observer. Binary pulsars – systems in which a pulsar orbits another object – emit very regular pulses of radiation because their orbital and rotation periods are extremely regular. These properties make them excellent tools for probing the effects of general relativity.

At 450 light years from Earth, PSR J0437-4715 is the closest known binary pulsar, in which a pulsar orbits a white dwarf. Its proximity allows astronomers to measure its radio output from different angles, due to its motion relative to the Earth. Using this geometrical method, van Straten and colleagues calculated the orbit of the pulsar in three-dimensions. This also revealed the centre-of-mass of the system, from which the team calculated the masses of the pulsar and the white dwarf.

Next, van Straten and colleagues studied variations in the pattern of the radio pulses arriving on Earth. If the pulses emitted throughout the orbital period of the pulsar travelled through equivalent regions of space on their journey to Earth, they should all arrive at equal intervals.

But after eliminating geometrical effects, van Straten’s team found that the pulses took longer to arrive when the plane of the binary system was in their line of sight. Conversely, when viewing the plane ‘face on’, the pulses were not delayed. This phenomenon arises because when the plane is ‘edge on’, the signal from the pulsar passes through a region of space that is distorted by the gravity of the white dwarf. This distortion means that the signal takes a longer route to Earth, and therefore arrives later. This is the Shapiro delay.

General relativity also states that binary systems should gradually slow down, and emit the excess rotational energy as gravitational waves. This predicted increase in orbital period has been observed in previous experiments, but attempts to detect gravitational waves have so far failed. The length of the observed Shapiro delay was consistent with the amount of energy the binary system should be losing through gravitational waves.

“To our knowledge, this verification of the predicted space-time distortion is the first confirmation – outside the solar system – in which the orbital inclination was determined independently of general relativity”, say the authors.

The study also means that pulsar PSR J0437-4715 has the most accurately known location of any astronomical object. Now this system is known to exhibit the Shapiro delay, it is likely to be the subject of many more cosmological studies.

X-rays light up chemical reactions

Most chemical reactions take place within just a few picoseconds – that is, 10^-12 seconds. Several techniques are already available for studying reactions between gases or solutions, but the dynamics of reactions at surfaces is poorly understood. Previous attempts to observe such reactions have produced ‘before’ and ‘after’ pictures, but the advent of laser pulses only tens of femtoseconds (10^-15 seconds) in duration has enabled Murnane and co-workers to monitor what happens between the initial and final states.

Murnane’s team chilled a platinum crystal to around 78 kelvin, at which temperature oxygen molecules bond with the crystal surface by accepting an electron from the platinum. This process – known as chemisorption – results in an even layer of oxygen atoms on the platinum surface. The team then excited the surface with infrared lasers, which excites electrons in the platinum, and these electrons then migrate into the oxygen-platinum bond.

To measure how this process affects the bond, the surface was bombarded with pulses of ‘soft x-rays’. These pulses – which lasted for less than 10 femtoseconds – ejected electrons from the valence band of the oxygen-platinum complex. The energies of these electrons reveal the structure of the complex. “Photoelectron spectroscopy has proven useful for studying surface physics and chemistry because the electrons are emitted from only the first few atomic layers”, fellow team leader Henry Kapteyn told PhysicsWeb.

The short x-ray pulses allowed Murnane and co-workers to capture several photoemission spectra as the reaction proceeded. The reaction took around half a picosecond. Their spectra form a ‘movie’ that shows the oxygen molecules twisting on the platinum surface when the extra electrons seep into the bonds. The molecules flip back to their original positions within a few picoseconds.

“Surface chemistry is technologically very important – for example, there are plenty of oxygen molecules on the platinum granules inside an automotive catalytic converter”, says Kapteyn. According to Kapteyn, the next step is to induce a reaction between two different molecules on a surface, such as creating carbon dioxide from oxygen and carbon monoxide. “Following the dynamics of such a reaction will help us to understand how to control them”, he says.

Antiproton weighs in

The ASACUSA – atomic spectroscopy and collisions using slow antiprotons – experiment measures the mass of antiprotons by monitoring the decay of ‘antiprotonic’ helium, in which an electron is replaced by an antiproton. The Antiproton Decelerator fires a beam of antiprotons into a chamber of helium gas, which is cooled to about 6 kelvin. Most of the antiprotons annihilate as soon as they enter the helium, but a tiny fraction of them produce about half a million ‘antiprotonic’ helium atoms.

As soon as the antiprotonic helium atoms are created, lasers excite them into a higher energy state from which the antiprotons annihilate, releasing energetic particles. These particles leave easily detectable trails of light known as Cherenkov radiation. The laser frequency at which this radiation appears reveals the mass and charge of the antiproton.

“We show that if there is any difference between the charges and masses of the proton and the antiproton, it can’t be more than about six parts in a hundred million”, group member John Eades told PhysicsWeb. “That’s pretty tiny, but we haven’t finished yet – it is just a challenge to do even better in future experiments”. The mass of a proton is 1.007276 atomic mass units.

The Standard Model of particle physics dictates that particles must have the same mass as their antiparticles. Any discrepancy in their masses would force theorists to rethink the model. However, a difference between particle and antiparticle masses could account for the excess of matter in the universe today. “The standard model is not in trouble”, emphasises Eades, “but our new job is to reduce the limit on any possible difference still further”.

BaBar claims matter-antimatter first

Matter-antimatter asymmetry is normally expressed as a parameter sin 2 beta. If there was no asymmetry, then sin 2 beta should be zero. In a paper submitted to Physical Review Letters on 5 July, the BaBar team report that sin 2 beta = 0.59, with error bars of 0.14. There is only a 3 in 100 000 chance that the effect is due to statistical fluctuations. The Standard Model of particle physics predicts sin 2 beta = 0.72.

Earlier this year BaBar and the rival Belle experiment in Japan reported values of sin 2 beta that were both consistent with zero and the standard model. Both experiments measure the decay of B-mesons produced in collisions between electron and positron beams. The BaBar collaboration, which is based at the Stanford Linear Accelerator Center in California, involves over 600 physicists from nine countries. CP violation was first observed indirectly in the decay of neutral kaons in 1964.

High-energy and particle physics win prizes

Donald Perkins of the University of Oxford in the UK wins the 2001 High-Energy and Particle Physics prize, for his studies of neutrinos and their use as a probe of the quark structure of nucleons.

Arnulf Quadt of Bonn University in Germany is named as the 2001 Young Particle Physicist for his investigations into proton structure. In the US, cosmological studies linking gravitation to the properties of particles earns Steven Gubser of the California Institute of Technology the Gribov medal, which is named after Vladimir Gribov, a founder of modern particle physics.

The European Physical Society’s Outreach Prize is awarded to Christine Sutton of the University of Oxford and Erik Johansson of the University of Stockholm for successfully raising the profile of high-energy and particle physics – in the public, professional and educational arenas – through innovative use of both electronic and traditional media.

Single electrons flick the switch

Conventional transistors require millions of electrons to operate, so a single-electron version would enable electronic circuitry to occupy just a fraction of its present size. Transistors have three terminals: the source, drain and gate electrodes. The gate controls the electron density in the central region of the transistor, which is usually a semiconductor. If the electron density is high, current flows from the source to the drain. If it is low, the current is switched off.

Using an atomic force microscope, Dekker and colleagues made a kink at each end of a carbon nanotube, which was 25 nanometres long and one nanometre in diameter. Carbon nanotubes are rolled up sheets of graphite that can conduct electricity. When the team connected electrodes to the kinks, however, they found that the tube no longer conducted because the buckled regions blocked the current. At room temperature, the tube had a resistance of around 500 000 ohms.

But when a bias voltage was applied across the electrodes, the tube started to conduct again. Dekker and colleagues could also manipulate the conductance of the tube by applying a voltage directly to the section of the tube between the kinks. This region corresponds to the semiconductor portion of a conventional transistor. This ‘gate voltage’ was applied via the silicon substrate on which the nanotube rests.

Dekker and colleagues also found that the conductance of the device rose and fell repeatedly as the bias and gate voltages increased. This is due to the ‘quantised’ nature of the process – the current can flow when a single electrons tunnels through a kinked barrier into the central region. An electron needs a particular amount of energy to do this – known as the Coulomb energy – and this is provided by voltages that correspond to exact multiples of the charge on an electron.

“We’ve added yet another important piece to the toolbox for molecular electronics”, says Dekker. “The next step is to think about how to combine these elements into complex circuits”.

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