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Home » Page 1694

Laser pioneer dies

Maiman was born in Los Angeles, California, on 11 July 1927. After supporting himself in college by repairing radios and other electrical appliances, and then serving in the Navy, Maiman earned a degree in engineering physics from the University of Colorado in 1949. He then went on to Stanford University in California where he completed a masters degree in electrical engineering and a PhD in physics under the guidance of soon-to-be Nobel-prize winner Willis Lamb.

Maiman received his doctorate in 1955, two years after Townes had built the first maser at Columbia University in New York. Townes then attempted to develop an optical equivalent of the maser with his brother-in-law Arthur Schawlow at Bell Labs in New Jersey. Townes and Schawlow, however, were primarily interested in developing the laser for spectroscopic studies and therefore attempted to build a continuous – rather than a pulsed – device, which meant they could not use ruby as the lasing medium.

Maiman’s breakthrough occurred after he had joined Hughes Research Laboratories, the Californian aerospace company owned by eccentric billionaire Howard Hughes. Maiman’s initial assignment at the company was to build a much slimmed-down version of Townes’ device, which he did in the form of a ruby maser weighing just 2 kg. Then, on 16 May, 1960, he succeeded in generating a laser beam from a ruby rod that he had fitted into a coiled flash lamp placed inside an aluminium cylinder. He had initially attempted to illuminate the ruby using a movie projector lamp but switched to a photographic flash on the suggestion of his student assistant Charles Asawa.

Following his demonstration, Maiman submitted a paper to Physical Review Letters (PRL), but the then-editor Sam Goudsmit rejected it on the basis that it was just the latest in a long line of articles on masers. Maiman did succeed in publishing a brief version of the paper in Nature but it was Townes and colleagues at Bell Labs who became known as the inventors of the laser after they published a paper about their own, subsequently constructed, ruby laser in PRL. Indeed, Townes went on to share the 1964 Nobel Prize in Physics, having been awarded a patent in 1960 for his proposal of the laser with Schawlow two years’ earlier.

Although the primacy of Maiman’s achievement is now widely acknowledged he struggled for many years to gain recognition (inventor Gordon Gould also fought to have his role in the development of the laser acknowledged). Within a year of his ground-breaking work, Maiman left Hughes to set up his own laser manufacturing company, Korad Corporation. After selling Korad to Union Carbide he joined the aerospace company TRW in 1976. In later life, Maiman did receive belated recognition for his development of the laser, including the Wolf prize in 1984, the year in which he was also added to the US National Inventors Hall of Fame.

Hunt for fifth force focuses on Bullet Cluster

According to our tried-and-tested laws of gravity, galaxies do not have nearly enough visible matter to stop themselves flying apart as they rotate. This is the main reason why physicists think they must contain an extra component of “dark matter”, which as the name suggests has remained hidden despite our best attempts to observe it directly. This could be because its interactions with ordinary matter are so weak that it has passed under the detection threshold of all current laboratory experiments here on Earth. On the other hand, it could interact via some long-range force different to the four forces that appear in our established Standard Model, meaning that it would only ever be revealed indirectly by studying large objects such as galaxies.

Glennys Farrar and Rachel Rosen of New York University in the US think that evidence for such a “fifth force” could be found by studying a collision between two individual galaxy clusters some three billion light-years away. This collision, which has been collectively nicknamed the Bullet Cluster, consists of a smaller “bullet” galaxy cluster, which has passed through an exceptionally large galaxy cluster.

The trick to their reasoning is to place plausible limitations on what the initial approach speed and mass distribution of the bullet could have been, given what astronomers already know about the dynamics of other galaxy clusters. Then, by recording how fast the bullet is moving away from the larger cluster, they would be able to deduce whether the acceleration is the value that would be produced by gravity alone acting on dark matter.

Farrar and Rosen have already tested their theory using x-ray data taken by the Chandra satellite, which suggest the speed of the “shock front” of the bullet to be 4740 km/s. This speed is too high to have been produced by gravity alone, implying that gravity acting on the dark matter in the Bullet Cluster is being augmented by a fifth force between 0.4 and 1.2 times as strong. Such a fifth force, if it does exist, would be proof that the Standard Model is incomplete — possibly requiring extensions such as supersymmetry that include additional force-carrying particles.

However, the physicists now think that the speed of the bullet given by the Chandra data may not be quite right. In particular, Farrar believes that speed recorded was of the shock front relative to the surrounding gas, which is falling into the cluster. This means the data produced an estimate of the overall speed of the collision too high by some 1500 km/s (arXiv.org/astro-ph/0703232).

Farrar told Physics Web that it is too soon to know whether this will rule out the existence of a fifth force, as she will now have to reanalyse the Bullet Cluster using the updated measurements. Nevertheless, Douglas Clowe, a cosmologist from Ohio University in the US who has spent much time searching for evidence of dark matter in the Bullet Cluster, says that the test for the fifth force laid out by Farrar and Rosen will still have importance in the future. “Whether a fifth force is [ultimately] measured or we put a limit on how strong it could be will help in our continuing search for what dark matter actually is – but it is unlikely to be the final piece that identifies it.”

Chilling out with polaritons

Solids can be cooled with light if they absorb a photon of light at one energy and then re-radiate a photon of higher energy. As long as slightly more energy goes out than in, the temperature of the material will drop. This effect, known as anti-Stokes photoluminescence, has been used since the mid-1990s to cool various glasses doped with ytterbium and other rare-Earth elements.

However, trying to laser-cool a semiconductor is far harder because the absorbed photon creates an electron-hole pair that only occassionally recombines to create a higher-energy photon. Instead, recombination usually results in heat being transferred to the surrounding lattice. Even if reradiation occurs, there is a very good chance that the new photon will be reabsorbed by the semiconductor, further increasing the chances of heating.

Khurgin’s solution is to place a small block of metal, such as silver, about 10 nm away from the semiconductor. This is done to take advantage of the cooling effects of surface plasmon polaritons (SPPs), which exist on metallic surfaces. SPPs are quantum oscillations that arise from the interaction of light with the metal’s conduction electrons.

While SPPs are normally found on the surfaces of metals, Khurgin has calculated that if the metal and semiconductor surfaces are separated by a very narrow gap, the SPPs could be created within the semiconductor material by the recombination of electron-hole pairs (see figure). Khurgin has also calculated that nearly all these SPPs would exit the semiconductor and deposit 99.9% of their energy in the metal – thus cooling the semiconductor.

Khurgin says that if silver was used as the metal and gallium nitride as the semiconductor, each SPP should remove nearly three times as much energy as a photon. He predicts that the device could achieve cooling efficiencies of about 3%, which is enough for practical applications. According to Khurgin, the technique could allow the semiconductors in electronic devices to be cooled directly, rather than employing an external refrigerator. This could be particularly important for those designing compact infrared detectors for use on Earth observation satellites or in portable night-vision systems.

Speckles expose magnet’s noisy secrets

Ferromagnets such as iron can contain tiny “domains”, in which all the individual magnetic moments point in the same direction. In an unmagnetized piece of iron, the magnetic moments of the individual domains can point in different directions, but by applying a magnetic field the domains can be made to point in the same direction. This gives the material a bulk magnetic moment – or magnetization — that persists even if the field is removed.

It has been known for almost a century that thermal energy can cause random movement of the boundaries between domains – called domain walls. This “noise” can be measured by detecting small jumps in the magnetization using a coil of wire placed near the ferromagnet and it can provide information about a material’s magnetic properties.

However, measuring similar fluctuations in antiferromagnets had not been possible. The problem is that these materials, in which the magnetic moments of neighbouring atoms point in opposite directions, have no bulk magnetization. It is therefore impossible to detect the fluctuations using conventional magnetic probes such as a coil.

Now, Oleg Shpyrko and colleagues at Argonne National Laboratory near Chicago have worked out a clever way to eavesdrop on the antiferromagnetic domain walls in chromium using a beam of coherent X-rays from Argonne’s Advanced Photon Source.

Working with physicists from the University of Chicago and University College London, the team exploited the fact that antiferromagnetism in chromium arises from its conduction electrons rather than the atoms themselves. The electrons exist in “spin density waves” (SDWs), in which the spin density of the electrons alternates in direction, and the amplitude of the magnetization varies sinusoidally with position. Although the X-rays could not probe the SDWs directly, each SDW is accompanied by a commensurate electron charge density wave (CDW), the presence of which can be detected using a technique called X-ray photon correlation spectroscopy (XPCS).

The team scattered a coherent beam of X-rays from a chromium sample. This produced an interference pattern called a speckle that was captured by a CCD camera over a period of several hours. The precise appearance of the speckle pattern is related to the arrangement of CDWs in a tiny portion of the sample. By watching how the speckle changed over time, Shpyrko and colleagues were able to observe changes in the antiferromagnetic domains over distances a small as 1 µm.

Shpyrko told Physics Web that the group were somewhat surprised to see domain fluctuations occurring on a timescale of about one hour at temperatures as low as 4 K. Physicists usually regard domain walls as relatively large structures that require a significant amount of thermal energy to move. The Argonne result, however, suggested that at very low temperatures the walls are moving thanks to quantum mechanical tunnelling.

Antiferromagnetic materials are currently used in read heads for magnetic storage devices and show promise for use in spintronic devices, which could make use of both the spin and charge of the electron to process information. However, any future technologies that rely on the precise location of antiferromagnetic domains could be affected by this tunnelling. A solution, according to Shpyrko, is the introduction of defects or impurities in the antiferromagnetic materials, which tend to fix the domain locations.

The researchers are now turning their attention to the study of other magnetic materials including those that can contain both ferromagnetic and antiferromagnetic domains. Shpyrko also believes that the technique could be used to study quantum phase transitions in antiferromagnets.

Molten core solves mystery of Mercury’s magnetic field

Despite its 400 °C surface temperatures, physicists originally predicted that Mercury’s small mass – about 5% that of the Earth – would have allowed its core to cool down enough to solidify long ago. But their predictions became much less certain in the 1970s after NASA’s Mariner 10 spacecraft flew by the planet and detected a small internal dipole magnetic field. Although some claimed that the field could have been a fossil of an earlier one “frozen” into the crust, others maintained that this was very unlikely, and that dipole magnetic fields in terrestrial planets are normally a result of the convection of molten iron producing a dynamo.

Now Jean-Luc Margot of Cornell University and physicists from other US and Russian institutions have used two previously-untried techniques to settle the dispute. The first technique required the measurement of the small oscillation in the rate at which Mercury spins on its axis, which on average is three rotations for every two 88-day orbits around the Sun. The second involved tracing how the “speckles” in radar images returned from the planet rotate as the planet spins. By combining data from both of these, they could calculate the periodic variations in Mercury’s spin. Then, because the Sun’s gravitational field affects the spin of planets differently depending on their composition, these variations would tell whether Mercury is solid throughout or has a detached, molten core.

Margot’s team took measurements over five years from three telescopes – the National Science Foundation’s Robert C Byrd Green Bank Telescope in West Virginia, the Arecibo Observatory in Puerto Rico and NASA/Jet Propulsion Laboratory antennas in California. These, together with previous estimations of the tilt of the spin axis and components of the gravitational field made by Mariner 10, enabled them to determine the periodic variations in Mercury’s spin rate with an accuracy of one part in 100,000. They found that the variations were relatively large – characteristic of a planet with a molten core. This means that a lighter element, such as sulfur, must have alloyed with the iron in the core to lower the melting temperature and hence prevent it from solidifying.

The discovery also means that Mercury’s magnetic field is almost certainly due to dynamo action. However, at just 1% the strength of Earth’s, the field detected by Mariner 10 is too small to be have been produced by a completely molten core. Therefore the question still remains as to how deep into the core the molten iron goes, which is only likely to be answered when NASA’s MESSENGER begins three flybys within 200 km of the planet next January. “MESSENGER carries a very good magnetometer and is magnetically clean,” Sean Solomon, principal investigator on the MESSENGER mission, told Physics Web. “A lot of the theoretical models of Mercury’s [core composition] depend on the geometry of the field. Not only will we get a very good measure of the dipole strength, we will also measure many of the shorter wavelength components of the field, which will give this geometry.”

Diamond maker focuses on particle detectors

The diamonds used in detectors must be very pure but such stones are rare in nature and are difficult to grow in the lab. Having honed the technique of chemical vapour deposition, however, Diamond Detectors says it can produce high-quality synthesized diamonds, and its detectors have already been installed in CERN’s Large Hadron Collider, which is scheduled to start up later this year.

Silicon detectors are often used in high energy physics to detect incoming charged particles or radiation. When a charged particle enters a silicon detector, its energy ionizes nearby atoms, creating electron-hole pairs. Because silicon is a semiconductor, these pairs are then free to move, and so they can be attracted towards electrodes surrounding the silicon to produce a measurable signal. Pure diamond, however, can also behave as a semiconductor, and if it is used in a detector it can detect particles and radiation from UV to X-rays with much less noise. In addition, unlike silicon, which must be cooled using liquid nitrogen, diamond can operate stably at temperatures well over 100 °C, making it ideal for monitoring alpha, beta or neutron radiation from hot nuclear reactors.

Since the 1980s Element Six (E6), which is named after carbon’s atomic number, has been developing chemical vapour deposition (CVD) in order to produce synthetic diamonds. To make diamond via CVD, gases that contain carbon, such as methane, are passed at low pressure over a substrate, which is heated so that the gases break down and form diamond. E6 has recently refined this technique by altering growth parameters so that the diamond doesn’t grow too quickly, enabling large crystals with very few imperfections to be consistently produced. Today Diamond Detectors will take over the development role and package the diamond into detectors.

“We have been selling to quite a few companies, which is why it seemed like a good time to get the business started and make it stand on its own two feet,” Chris Wart, the technical manager of Diamond Detectors, told Physics Web. The firm’s customers include CERN, which has worked with E6 since 2002 and is now using the diamond detectors in the large ATLAS detector in its forthcoming Large Hadron Collider. Its detectors are also used at the Diamond Light Source synchrotron facility, which opened recently in the UK.

Diamond Detectors are also planning to create “body compatible” detectors for use in radiotherapy. These instruments will try to take advantage of the fact that the density of carbon in diamond is similar to that in soft tissue. “The applications of diamond are so diverse,” Wart added. “We are getting the diamond synthesis right, and then we’re investing in other ‘portfolio’ companies who can get on with the market channels and expertise.”

Once a physicist: Burnaby Q Orbax


How did you first become interested in physics?

I got into physics for two reasons: Doctor Who and my personal quest to learn how to throw energy balls. After a few years of my undergraduate degree, I had to resign myself to the fact that a long scarf was about as close as I was going to get to my Gallifrean mentor; and I decided to take up fire-breathing in order to achieve the latter goal.

What is your background in physics?

I studied at the University of Guelph, in Ontario, Canada. I completed a Bachelors degree in physics and mathematics in 2000 and a Masters degree in polymer physics in 2002. I then spent two and a half years studying for a PhD before running away to join the circus.

How did you develop your passion for entertainment?

I have always been interested in TV, theatre and film; and I had an obsession with monsters. The world of the freak show became the natural evolution of everything I was good at. I create and build our props (which vary from a giant working mousetrap to robotic pig clowns), paint our banners, build our freak museum (using taxidermy and modern special-effects techniques), write our scripts and design the stunts we perform.

How did your career progress after you graduated?

During my PhD studies I suffered an accident on stage that resulted in third-degree burns to 11% of my body (my head and shoulders). I was in hospital for 10 days, before being home-nursed for two months. It was then that I had to work out whether to pursue the freak show full time or give it up and finish my degree. I chose the former, deciding that 20 years on I would be more upset that I didn’t take a chance on a dream. By that time I had already established a small fan base, and using direct marketing to colleges and universities we’ve created quite a niche for the show.

What is involved in your act?

The show combines elements of vaudeville, slapstick comedy, sideshow stunts and terrible jokes. We have a guy who eats bugs, a woman who walks up a ladder of swords and a guy who sets mousetraps on his face. I personally perform the human blockhead (a stunt involving hammering nails or sticking forks up the nose), walk on broken glass, escape a straitjacket and get cinder blocks broken over me.

How does your physics education help you in your career as an entertainer?

My education has made it possible for me to figure out how to perform the dangerous stunts we do without getting hurt. All of our stunts involve exploiting simple principles of mechanics. It has also helped in the design of my evil weather machine – however that will not be complete until 2008.

How to become a European champion

Anyone who has tried to apply to the European Union for money for science will know what is involved: big collaborations, a focus on applied research and lots of bureaucracy. But that is all set to change with the arrival of the European Research Council (ERC), which was launched with great fanfare in Berlin in February at a meeting attended by 300 scientists plus German Chancellor (and former physicist) Angela Merkel.

The ERC, which has a total budget of €7.5bn for the next seven years, is designed to support fundamental research in all fields of science and technology, including social sciences and the humanities. With large grants going directly from the European Union (EU) to individual researchers for the first time, the new research council is being billed as the European equivalent of the National Science Foundation in the US. The ERC aims to help research in Europe compete with that in the US and Japan by introducing pan-European competition for grants and boosting investment in research and development. Currently, Europe spends only 1.9% of its Gross Domestic Product on R&D, compared with 2.7% in the US and 3.2% in Japan, but the EU wants Europe’s total investment to rise to 3% by 2010.

Scientists have long dreamed of a pan- European research council but a political head of steam only began to build in 2002. It has taken five years to hammer out the details of how the ERC will be funded and governed, and how its money will be allocated. The funding for the ERC will come from the EU’s €53bn Seventh Framework Programme; but unlike previous framework programmes, the ERC will emphasize fundamental over applied research and individuals over large collaborations. Furthermore, decisions about what to fund will be made by scientists, not bureaucrats. The ERC’s strategy is set by a council of 22 scientists headed by biologist Fotis Kafatos from Imperial College London together with Daniel Esteve, a physicist working on quantum electronics at CEA Saclay in France as a vice-chair.

Starting out

There will be two types of ERC grants: “starting grants” are aimed at researchers who completed their PhD between 2 and 9 years ago (or up to a maximum of 12 years for women who have taken a career break); while advanced grants are intended for more established researchers. The starting independent researcher grant, to give it its full name, is the first to get under way, with the deadline for the first set of research proposals at the end of April having just passed. But do not worry if you feel that you have missed the boat this time: there will be another call for starting-grant proposals next year.

The starting grant is intended to support researchers who are planning to set up or are already leading their first research team. A key theme is independence – the ERC wants to allow outstanding researchers to take decisions about the direction of their team’s research and their budget independent from their university. This significantly extends the idea of the European Young Investigator (EURYI) awards, which the European Science Foundation has offered since 2003. These grants are worth about €1m over five years, but only 20 or so researchers each year are able to benefit from them.

Funding from the ERC is available to researchers from anywhere in the world, as long as they plan to work within an EU institution. The ERC thus hopes to attract the best new talent from around the globe, as well as tempting back some of the many European researchers who have moved elsewhere.

Rewarding excellence

In the first call for proposals about 200 grants are available with funding per grant of €100,000– €400,000 per year for five years. There is a two-stage application process: first an outline proposal is submitted; then, for the successful applicants, a full proposal and an interview are required. Applications must be submitted online, in English, by a single principal investigator in association with a host institution. Several thousand applications are expected from the first call, which will be whittled down to those from about 500 researchers who will be asked for their more detailed proposal by September.

Each proposal will be evaluated by one of 20 panels of about a dozen experts in fields ranging from genetics to economics. An encouraging sign for the fledgling ERC is that 95% of the scientists asked to join these panels accepted the invitation. Physicists are most likely to send their proposals to one of three specialist panels: “fundamental constituents of matter” (chaired by Massimo Inguscio of the European Laboratory for Non-Linear Spectroscopy in Florence, Italy); “condensed matter in physics and chemistry” (headed by Robert Blinc of the Jozef Stefan Institute in Slovenia); and “universe science” (led by Catherine Cesarsky, director-general of the European Southern Observatory).

The ERC insists that the sole criterion for selection is scientific excellence, both of the principal investigator and the proposed project. This will mean abandoning the long-established principle of juste retour, whereby EU member states receive an amount of funding proportional to how much money they put in. Specific points for assessment listed by the ERC are the quality of the principal investigator’s research output (based on their five most important papers), the groundbreaking nature of the project being proposed and the potential for the principal investigator to become independent. Physical sciences will get 45% of the money, life sciences 40% and social sciences and the humanities 15%, but within these broad categories, proposals from different fields will have to fight it out based on scientific merit.

Champions League

Getting started with the ERC
Getting started with the ERC

As the deadline for the second stage of the starting-grant applications arrives in the autumn, the first call for proposals for advanced grants will be issued. Full details have not yet been released, but there will be about 300 grants, each of €100,000 to €500,000 per year for five years. Although these grants are targeted at researchers who have already established themselves as independent, world-class leaders, scientists at any stage of their career may apply. From 2008 there will be annual calls for both starting and advanced grants with about a third of the money going to the former, and the rest to the latter.

As well as providing scientists with hefty sums of money, the awards are sure to confer a good deal of prestige. After all, successful researchers will have won out against thousands of others across the continent. These are still early days for the ERC, but it may soon become, in the words of Merkel herself, “a Champions League for research” where the cream of Europe’s scientific talent can compete for big money.

Casimir force could drive tiny ratchets

The mysterious attraction between two flat, neutral surfaces was first predicted in 1948 by the Dutch physicist Henrik Casimir. It is a purely quantum effect arising from fluctuations of electromagnetic fields, which exert a radiation pressure on the surfaces that is, on average, stronger on the outer than the inner surfaces. The overall Casimir force is therefore weaker in the gap between the surfaces than elsewhere, drawing the surfaces together. Although the Casimir force decays quickly with distance, it becomes very important for surfaces separated by distances of several micrometres. As a result, the force is of great interest to those trying to built micrometre-sized machines.

More recently, physicists have discovered that if both surfaces are corrugated, rather than smooth, there will also be a lateral Casimir force acting on the plates. If the plates are held at a fixed distance, this lateral force tends to cause the plates to slide across one another until the corrugations are aligned such that the potential energy is a minimum. In Emig’s ratchet, one plate has a symmetric corrugation — peaks with the same slope on either side — while the other plate has an asymmetric corrugation — peaks with a gradual slope on one side and a steep slope on the other (see figure “Corrugations”). According to Emig, this asymmetry allows the plates to slide easily over each other in one direction, but makes it more difficult for the plates to slip back in the opposite direction.

Emig’s ratchet could be actuated by causing one of the plates to vibrate relative to the other. This would cause the other plate to slide from one position of minimum energy to the next along the easy direction. Emig has calculated that the velocity at which the ratchet moves is proportional to the frequency of vibration – with oscillations in the kilohertz range causing a velocity of about 5 mm/s. Such a ratchet would have silicon plates a few micrometres thick with corrugation heights of 10 nm and a corrugation period of 1 µm.

Unlike electrostatic microratchets — which have to be made from conducting materials — the Casimir ratchets could be made from electrical insulators and do not require any electrical contacts or external electric fields. According to Emig, this means that the ratchet could operate in real-world environments such as in air or even with a liquid between the plates.

Chris Binns of the UK’s University of Leicester is familiar with Emig’s proposal and is currently building devices to explore how the lateral Casimir force can be exploited in micromachines. “We have already designed a machine that uses patterned surfaces to demonstrate the lateral force”, said Binns. “Making the pattern asymmetric should not be difficult.”

Supersolids reliant on disorder, say physicists

Supersolidity was first predicted in 1969 by Russian theorists Alexander Andreev and Ilya Liftshitz, who said that at temperatures close to absolute zero any vacancies left in the lattices of solids made from bosonic atoms would all collapse into the same ground state, becoming what is known as a Bose-Einstein condensate (BEC). In the supersolid state, vacancies would behave as a coherent entity, moving throughout the rest of the solid effortlessly like a superfluid. However, only very weakly-bound elements such as helium would be able to become a supersolid, because their structure would be disturbed sufficiently by quantum “zero point” energy to still have vacancies.

Evidence for supersolidity was first found by Moses Chan and Eun-Seong Kim of Pennsylvania State University in the US in early 2004. They looked for changes in the rotational inertia of a sample of helium-4 contained in a disc of porous glass that was supported inside a torsion oscillator. Below a temperature of 175 mK, they found that the inertia of a sample of solid helium-4 suddenly dropped, implying that 2% of the sample had condensed into a supersolid that remained at rest in the lab frame. Later that year, they convinced themselves of their result by performing the same experiment with bulk solid helium, and found a similar drop in inertia.

Three other groups have since confirmed Kim and Chan’s findings. But one of them – John Reppy and Sophie Rittner of Cornell University in the US – also found that by slowly heating a bulk helium-4 sample and cooling it down again they could make the supersolid phase disappear completely (see related story: “Supersolid’s existence remains in dispute”). Because such “annealing” is expected to reduce the levels of imperfection in the solid, the Cornell physicists thought that the observed supersolid phase might only occur when the crystal structure has some disorder. If this conclusion were correct, samples of helium-4 containing many imperfections should therefore show large amounts of supersolidity.

Now, Reppy and Rittner have performed experiments on such samples. They used a torsion oscillator similar to Kim and Chan, but have confined the bulk helium-4 in a different geometry that allowed them to maximize disorder by rapid cooling. When the sample was slowly annealed over 14 hours to give a very ordered crystal, they found that just 6% of the solid demonstrated supersolidity. However, they could get 20% or more of the solid to become a supersolid by “quench” cooling the sample in 90 seconds, thereby introducing significant disorder.

According to Reppy and Rittner, this result makes it clear that disorder must play a role in superfluidity, and so shows that the 1969 theory by Andreev and Liftshitz is not the whole story as other theorists had recently suspected. Nevertheless, they say they are puzzled by Kim and Chan’s original samples, which contained the helium in a porous glass, because they should have had even more disorder – and thus should have demonstrated an even larger proportion of supersolidity than the 2% observed. “The truth is, it’s not understood… the experiments are way ahead of the theory,” Reppy told Physics Web. “But that’s what makes it so fascinating.”

Comprehensive coverage of the supersolids debate can be found in the article “The quantum solid that defies expectation”, which appears in this month’s Physics World.

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