Page 1562 – Physics World

Supersolid saga continues

The first convincing evidence for supersolidity came in 2004, when US physicists Moses Chan and Eun-Song Kim were monitoring the rotation of a sample of helium-4 supported inside a torsion oscillator. As they reduced the temperature below 230 mK, they noticed that the oscillations sped up slightly, and concluded that 1% of the sample had become a supersolid and was therefore staying still in the lab frame.

At first, this was thought to be proof of a phase transition predicted in the late 1960s, which suggested that close to absolute zero any lattice vacancies present in a sample would collapse into the same quantum state, becoming a so-called Bose-Einstein condensate. In this supersolid phase, the vacancies would be able to move throughout the rest of the solid effortlessly like a superfluid.

More recent calculations, however, have shown that there would not be enough vacancy condensation at low temperature to give supersolid signal as large as 1%.

One alternative explanation is that the signal actually comes from supersolidity involving tiny “grain boundaries”, but this was ruled-out experimentally last month by Chan, who told Physics Web that he suspected the signal could be originating in dislocations within the crystal lattice.

Now, Massimo Boninsegni from the University of Alberta and collaborators from the US and Switzerland have backed-up this suspicion by simulating a “screw” dislocation in a microscopic helium-4 crystal. Screw dislocations form when a break in the crystal lattice causes atoms to stack-up in a structure akin to a spiral staircase, and for complex helium-4 crystals have been too difficult to simulate accurately in the past.

Using a new computer algorithm Boninsegni’s group found that, as they reduced the temperature parameter towards absolute zero, the core of the screw dislocation acted like a tube through which some of the atoms could flow freely – essentially in one dimension – as a superfluid. “This is an important aspect experimentally, as a one-dimensional system has distinct physical signatures that can be probed by measurement,” Boninsegni said.

The researchers say that a network of these superfluid cores could produce a supersolid signal in a real sample, although it would not produce one as high as the 1% originally recorded by Kim and Chan. This signal strength, they suggest, might be due to auxiliary sources, such as “pockets” of superfluid.

Partnership to boost fuel-cell research

Fuel cells are typically made up of an anode and a cathode, separated by a membrane that conducts protons but not electrons. When a fuel, such as hydrogen, is supplied to the anode, it is broken into its constituent protons and electrons. The protons are attracted to the cathode, while the electrons – unable to travel through the membrane – are harnessed in an adjacent circuit to produce electrical energy.

Fuel-cell technologies could bring important environmental benefits because they are much cleaner than traditional sources of power such as internal combustion engines. At worst, fuel cells release small amounts of carbon dioxide, but often the only by-product is water.

In this agreement, physicists at Oak Ridge National Laboratory (ORNL), which is owned by the US Department of Energy, will use their skills in the imaging of solids and surfaces to develop metal and carbon bipolar plates for the anodes and cathodes. Jülich researchers will share their materials and fabrication techniques.

The researchers will concentrate on two types of fuel cells: proton-exchange membrane fuel cells (PEMFCs) and direct-methanol fuel cells (DMFCs), both of which are already used to some extent commercially. Because of their relatively high output, PEMFCs are primarily being developed for the automotive industry. DMFCs can currently only produce small amounts of power, albeit over a long period of time, which makes them better suited to portable devices such as mobile phones, digital cameras or laptops.

“This agreement underscores the Department of Energy’s commitment to the [US] president’s hydrogen fuel initiative,” said Tim Armstrong, the manager of ORNL’s fuel cells program.

Liquid jets bounce along

The photographs were taken by Matthew Thrasher and colleagues at the University of Texas at Austin, who built a rotating oil bath into which a stream of oil was dropped under the watchful eye of a video camera. The photographs reveal that when the stream strikes the surface of the bath, it slides along the surface on a thin layer of air. Furthermore, the force of impact creates a bowl-shaped indentation in the surface of the liquid that acts as a ramp, launching the stream back into the air (see figure “Bouncing jet”). The stream then arcs over the surface and plunges back into the liquid, sometimes emerging in a second arc.

Skimming

Bouncing was observed in a number of different silicone oils with viscosities ranging from 56 to 560 times that of water. The arc became smaller when the bath was rotated more quickly until the jet no longer lifted off the surface, but rather skimmed its surface (see figure “Skimming”)

Thrasher told Physics Web that a crucial requirement for bouncing is that the layer of air supporting the jet must not break into air bubbles, which would disrupt the stream. He adds that an understanding of why some liquids bounce while others bubble could help improve the metal casting process — in which molten material is poured into a mould and bubbles must be carefully avoided because they weaken the solid metal. Conversely, plunging jets are often used to introduce air bubbles into liquids – a familiar example being bath bubbles that form under a running tap. Therefore Thrasher believes that those designing liquid aeration systems should understand how to avoid the bouncing of fluids.

Thrasher came up with the idea for the experiment when he was pouring oil from one container to another and noticed that the stream of poured oil would sometimes bounce across the surface of the container. In their paper, the researchers suggest several simple experiments for observing bouncing jets in the classroom or even at home.

Nanostructures help mosquitoes walk on water

Scientists know pond skaters stay afloat because of a special “hairy” nanostructure on their legs that holds pockets of air. These prevent the legs from ever getting wet, thus allowing the insects to be propped up on water’s surface tension. Flies, on the other hand, are adept at clinging vertically or upside down to smooth surfaces, using either bristles that produce attractive forces or claws on their feet.

To see if mosquitoes exploit a similar nanostructure to pond skaters for landing on water, a group led by Chengwei Wu from the Dalian University of Technology in China used a scanning electron microscope (SEM) to examine the six legs, between 5 and 7 mm in length, of mosquitoes caught in their local area.

In addition to claws for clinging to surfaces, they found that the legs were covered in a profusion of scales, of which each was adorned with six to twelve randomly-spaced ridges with a thickness of about 200 nm. These ridges were linked by many smaller “ribs” with a thickness of 100 nm.

Wu’s team thought that, like pond skaters, air would be contained in the gaps created between these ridges and ribs to make the legs stay afloat in water. To see if this was right, they connected the severed leg from a mosquito to a moveable steel needle, and positioned it over a container of water sitting on a force balance. They then inclined the leg to a 30° angle – roughly the angle that would be used for landing – and pushed it against the water’s surface.

Surprisingly, the researchers found that the leg resisted with a force of up to 600 µN – roughly 23 times the mosquito’s body weight – before breaking the surface. In contrast, a pond skater can only receive a force equal to 15 times its body weight through its legs. A common fly, the researchers later found, could barely support its own body weight at all. This more refined water-repelling nanostructure could provide greater stability when mosquitoes land on the surface of water.

To check whether the ability was not simply a result of a low-density – that is, more buoyant – material, Wu’s team repeated the experiment with the leg pitched vertically, which would prevent any air from being trapped. As expected, the force from the leg was heavily reduced, almost by a factor of 50.

Mosquitoes do not have the best reputation. Although in developed nations their damage tends to be limited to itchy bites, in the third world they cause millions of deaths each year by spreading malaria. “Most of the previous studies on mosquitoes were focused on the harmful sides that they bring to human beings,” said the researchers. They added that among the special skills of the mosquitoes, the water-repelling ability “might be the most surprising.”

Dark-energy teams win cosmology prize

Each team presented its findings in two key papers, published early in 1998. Perlmutter, who is at the Lawrence Berkeley National Laboratory and the University of California at Berkeley, was leader of the Supernova Cosmology Project. Brian Schmidt, who headed the High-Z Supernova Search, is based at the Australian National University in Canberra. Perlmutter and Schmidt take home $125,000 in prize money each, while the other team members will share the remaining $250,000.

Brian Schmidt

Both teams discovered the accelerating expansion of the universe by studying distant type Ia supernovae. These exploding stars are believed to all have the same luminosity, which means that their brightness can be used to determine how far away they are. However, Perlmutter and Schmidt’s teams were surprised to find that the light from these supernovae was fainter than expected for a given expansion velocity, which indicated that the supernovae were further away than expected.

Since the 1998 breakthrough, the accelerating expansion of the universe has been confirmed by independent studies including those looking at the cosmic microwave background and the distribution of thousands of galaxies. These and other studies have strengthened the argument for the existence of dark energy, which some physicists believe accounts for nearly three-quarters of the mass of the universe. However, dark energy has yet to be observed directly, either the laboratory or the cosmos.

The prize citation describes the discovery as “a crazy result that was hard to accept”. It also commends both teams for developing “new techniques that use supernovae exploding within distant galaxies to measure precise distances across a large fraction of the observable universe”. The Gruber prize has been awarded annually since 2000 by the Peter and Patricia Gruber Foundation, which is based in the US Virgin Islands.

First “heat transistor” unveiled

The transistor is based on a design for a single-electron refrigerator that was unveiled earlier this year by Jukka Pekola and colleagues at the Helsinki University of Technology and NEST in Pisa, Italy (see Refrigerator cools one electron at a time).

Heat is transported by electrons as they tunnel though junctions between a metal and a superconductor (see figure). Only one electron at a time can squeeze through a tiny junction because mutual repulsion between electrons prevents multiple tunnelling. This orderly process ensures that only the hottest electrons can leave the metal, thereby causing heat to flow from the metal to the superconductor.

The researchers found that the heat flux though the transistor was a function of the temperature of the metal electrode, leading them to suggest that the device could be used as an accurate thermometer for measuring temperatures in the range of hundreds of millikelvin. Because the device allows for the precise study of the heat transported by single electrons, it could also help physicists to design better conventional electronic refrigerators, which also use electrons to transfer heat.

Pekola told Physics Web that while the heat transistor could someday be used to control the temperature of electronic devices, its very small cooling power rules out most practical applications.

Steel balls make a splash in sand

Two years ago, Detlef Lohse and a team of physicists from the University of Twente devised a mechanism for crater formation by dropping metal balls in beds of sand. They found that the impact first throws sand outwards in a crown-shaped “splash”, leaving the ball to penetrate further into the surface and create a void. The pressure of the sand then forces the grains to fill the void, causing a vigorous jet of sand to shoot up from the centre (see related story: “Extraterrestrial impact created in the lab”).

The striking conclusion for the researchers was that this mechanism is very similar to what would happen in a liquid. This meant that, in certain circumstances, the well-established equations for fluid dynamics could potentially be used to describe poorly-understood granular systems such as sand. Now, however, Lohse and co-workers have gone one step further and found that the ambient pressure of the air above the sand is related to the height of the jet and the depth of penetration of the ball.

In a new set of experiments, they dropped 1.4 cm steel balls into a bed of sand 40 cm deep. But this time they put the entire apparatus inside a container that could be partially evacuated with an air pump, and attached a taut thread with markers to the balls so that the penetration depth could be read off easily.

With more air evacuated from the container, and hence less pressure, they discovered that the jet was less vigorous, and that the ball did not penetrate as deep. The researchers claim that this ties in with their existing mechanism: a lower pressure means there is less air surrounding the ball and the sand grains, thus increasing drag and making the sand behave less like a fluid. The net result is a shallower penetration, and a shorter jet. “It reveals the importance of air in fine granular matter,” Lohse told Physics Web.

Gabriel Caballero, a co-author of the study, said that the research could eventually have applications ranging from the design of space probes that have to land on other planets, to how powdered chemicals are mixed in pharmaceuticals. The team are now looking to investigate how the splash of the sand creates the ensuing crater.

Astronomers discover the power behind supernovae

Type Ia supernovae are thought to be produced when the gravitational pull of a white dwarf star draws in enough material from its surroundings to begin nuclear fusion on a large scale, exploding the star into a fleeting object as bright as a billion Suns.

Astronomers had thought these relatively-common events all gave off the same amount of light. As a result, they have been used as “standard candles” for judging distances across the cosmos.

In the last decade, however, astronomers have noticed that there are actually small fluctuations in the brightness of these supernovae, which could affect the reliability of distance estimates. To predict where these deviations arise from, and so make allowances for them, we require a better understanding of what supplies a white dwarf with the extra material.

Now, Ferdinando Patat of the European Southern Observatory (ESO) in Germany together with co-workers from Europe, Japan and the US has used a new technique to find that one type Ia supernova collects its material from a nearby “donor” star in the red giant phase, thus confirming a popular theory of supernovae formation.

The measurement exploited the fact that donor stars expel material in all directions – some goes to fuel the supernova, while some comes between the supernova and our viewpoint on Earth. As light from the supernova travels to a telescope on Earth, this latter material absorbs certain wavelengths, leaving gaps or “absorption lines” in the observed spectrum.

The denser the material present, the stronger the absorption lines will be. However, the absorption is also affected by the physical condition of the supernova, which changes with the amount of material it acquires from the donor star. This two-way relationship causes variations in the strength of the absorption lines over time.

Patat’s team used the ESO’s Very Large Telescope (VLT) in Chile to search for the strength variation in the sodium absorption lines of the supernova 2006X over four different periods in its lifetime. Because of the VLT’s eight-metre diameter and the number of observations, they were able to calculate the variations in the density of the sodium material in unprecedented detail. “This had never been attempted before,” Patat told Physics Web. “It was commonly believed that no variations should be seen in the inter-stellar absorptions.”

In addition to variations in density, the researchers calculated variations in the speed of the expelled sodium by looking for the blue Doppler shift in the wavelength of the absorption lines – an effect that arises because the waves appear “compressed” when they are travelling towards an observer.

Patat’s team claim that both these measurements point to a donor star in the red giant phase, which is known for expelling material in patchy “shells”. “The red giant steadily loses material, but has some hiccups, so to speak,” Patat said. This goes against another theory of type Ia supernovae that involved two white dwarfs merging.

The researchers’ finding will help astronomers to use these standard candles more accurately, but Patat points out that it does not rule-out other type Ia supernova theories yet. “Supernova 2006X is only one object,” he added. “We will need much more data to tell if this is the rule rather than the exception.”

Water found on distant planet

Astronomers have so far discovered almost 250 planets beyond our solar system. Most of these “exoplanets” are gaseous giants similar to Jupiter, which itself contains large quantities of water. Researchers had therefore suspected that water also exists on planets of a similar size outside our solar system.

Now Giovanna Tinetti of the European Space Agency and University College London, along with colleagues in France, Taiwan, Spain and the US, has studied exoplanet HD 189733b, a gas giant that crosses between the Earth and its companion star every 2.2 days. As it does so, some of the light from the star is absorbed by the exoplanet, causing the star to dim.

To search for water, the astronomers paid close attention to three wavelengths of infrared light – 3.6, 5.8 and 8.0µm – as the edge of the exoplanet’s atmosphere crossed the star. They discovered that the atmosphere absorbed less light at 3.6 µm than it did at the longer wavelengths. According to Tinetti, this could only occur if the atmosphere contains a significant amount of water vapour.

While it is unlikely that life exists on HD 189733b or any similar exoplanet, the research supports the idea that significant quantities of water could exist in other planetary systems. According to Tinetti the technique could someday be used to search for Earth-like exoplanets with water – which she describes as “the ‘holy grail’ for today’s planet hunters”. However, such studies would be very difficult to do using existing telescopes, which are not capable of performing such measurements on Earth-sized exoplanets. As a result the quest will probably have to wait for NASA’s James Webb Space Telescope, which should launch in 2013.

US chooses site for underground lab

Homestake, which contains over 600km of tunnels, operated as a gold mine from 1876 until 2001. Scientific experiments have been carried out there before — in 1965 it became home to the world’s first solar neutrino detector, which was set up by the late Raymond Davis of the Brookhaven National Laboratory. Davis went on to share the 2002 Nobel prize in physics for this work.

The proposal for the new lab involves experiments at two levels — one at about 1500m below the surface and another at around 2200m down. The intermediate level, which is where Davis set up his experiment, will involve the modification of an existing scientific site and the setting up of a number of new experimental chambers, while the deeper level will involve the conversion of caverns, boreholes and other structures previously used by the miners.

Many different types of experiment could be built at Homestake, including a number dedicated to studying the elusive neutrino. For example, physicists could build detectors to study an extremely rare nuclear process known as neutrinoless double beta decay, which, if real, would mean the neutrino is its own antiparticle and which would also permit researchers to work out the absolute mass of the neutrino. The mine might also host detectors to study the properties of neutrinos that have been sent some 1500km through the Earth from Fermilab near Chicago.

Other physics-based experiments at Homestake could include studies of proton decay or nuclear astrophysics, or lead to the development of next-generation gravitational wave detectors. Scientists working at the lab would also be able to study the Earth’s crust, examine life forms that live in conditions of extreme heat and pressure, as well as improve technologies for sequestering greenhouse gases underground.

The proposal for the new lab has been put forward by a multi-institutional collaboration of researchers headed by Kevin Lesko, a physicist at the University of California Berkeley. It was chosen ahead of three other proposals by a 22-member panel of experts appointed by the NSF, and marks the culmination of a drawn-out selection process. Homestake was originally put forward as a potential host several years ago, but disagreements between Barrick Gold Corporation, the Toronto-based company that owns the mine, and the South Dakota state government and the NSF have held up progress. In 2003, Barrick turned off the pumps that were preventing the mine from flooding, prompting fears that the site would be unfit for experiments.

The Homestake collaboration is now set to receive some $5m per year for up to three years from the NSF for design work. Funding to actually build and operate the lab will need further approval and ultimately require the go ahead from Congress. If approval is forthcoming for the current design, the Homestake facility would be the largest and deepest underground lab in the world, surpassing existing labs in Italy, Japan and Canada. It is to be called the Sanford Underground Science and Engineering Laboratory in recognition of a gift of $70m provided by the bank owner Denny Sanford.

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