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Nanocrystals could help recover waste heat

Researchers in the US have come up with a new and simple way to boost the performance of a common thermoelectric material — bismuth antimony telluride — which had defied attempts at improvement for over 50 years.

Thermoelectric materials convert heat directly into electricity and could be used to boost the efficiency of conventional coal and nuclear power stations by recovering heat that is normally wasted. The materials could also improve the effectiveness of solar cells and could be used to cool computer chips and other devices.

To be used in such ways, however, a thermoelectric material must be good at conducting electricity but poor at conducting heat. This requirement is expressed in the thermoelectric figure of merit ZT, which should be greater than 1.0. The ZT of bismuth antimony telluride — one of the most common thermoelectric materials — has remained stubbornly at around 1.0 for more than half a century.

Now, Zhifeng Ren and Gang Chen and colleagues at Boston College and the Massachusetts Institute of Technology have made a significant breakthrough by milling the material into a fine powder that contained nanoparticles measuring about 20 nm across (Sciencexpress DOI: 10.1126/science.1156446). Next, they hot-pressed the powder into nanocrystalline ingots.

Improved ZT

The researchers found that the thermoelectric figure of merit (ZT) for the ingots increased to 1.2 at room temperature (from a value of 1.0 previously). Moreover, they found that the ZT peaks at 1.4 at 100 °C.

This might not sound like a big improvement, but the researchers describe it as a “significant step” towards creating materials that are useful for cooling and power generation.

Electrical transport measurements on the ingots, together with microstructure and modelling, showed that the ZT improves thanks to the low thermal conductivity caused by increased phonon scattering at grain boundaries and defects in the material. However, the electrical conductivity of the material is not affected signficantly by grain boundaries and defects.

Promising for applications

The high ZT in the temperature range 25–250 °C makes these materials promising for cooling and waste heat recovery applications, say the researchers. Potential applications include converting the heat of car exhausts into electricity, for example. “Other applications include efficient thermoelectric cooling, such as air conditioning and refrigeration, and solar thermoelectricity,” explained Ren and Chen.

The researchers have also built a prototype cooling device to confirm the properties.The team now plans to make efficient coolers and power generators using the improved materials. “At the same time, we will apply the approach to other promising thermoelectric materials,” they stated.

The company GMZ Energy Inc, a Massachusetts-based start-up, is now mass-producing the materials.

Iconic UK telescope faces closure

Today is the last chance for researchers in the UK to have their say on whether the e-MERLIN network of radio telescopes, which includes the iconic Lovell telescope at Jodrell Bank in northern England, should close.

If funding is withdrawn that will seriously threaten Jodrell Bank Phil Diamond, Jodrell Bank Centre for Astrophysics

The Science and Technology Facilities Council (STFC) has spent the last few weeks consulting the scientific community about the fate of the network after its biennial ‘programmatic’ review of funding, published earlier this month, deemed it low priority. That consultation will end today and a panel of experts will then consider comments from the community and make recommendations to the STFC about whether e-MERLIN should close. Funding for e-MERLIN could be withdrawn from April next year.

£80m shortfall

The programmatic review took place in the wake of an £80m shortfall in the budget of the STFC, which led to the UK pulling out of a number of international projects such as the International Linear Colider. However, astronomers involved in the e-MERLIN network were shocked to see the project deemed low priority as it had recently gone through a £8m upgrade.

“If funding is withdrawn that will seriously threaten Jodrell Bank” says Phil Diamond, director of the Jodrell Bank Centre for Astrophysics. This withdrawal would save the STFC around £2.7m per year in operational costs. Jodrell Bank Observatory, operated by the University of Manchester, celebrated its 50th anniversary last year.

The STFC says it expects fund all of the “high” and “medium-high” priority projects, which include the Advanced LIGO project to hunt for gravitational waves, the Compact Muon Solenoid at the Large Hadron Colider to detect traces of the Higgs boson and the SCUBA 2 camera for sub-millimetre astronomy that hopes to see the early stages of galaxy formation.

Mission to Mercury

The STFC also says that it will fund a “significant part” of the low priority projects, which includes the BepiColombo mission to Mercury planned for 2013 and the Gemini telescopes in Hawaii and Chile. The council has already decided to continue as a full member in Gemini despite having threatened to pull out in the wake of the funding crisis.

Indeed, Diamond is confident that some factors that were overlooked in the peer-review process that decided e-MERLIN’s low priority status will now help to reverse the STFC’s recommendation. “We hope to do well in the consultation process, the public response in support has been amazing,” says Diamond, referring to a string of articles in the UK media on the threat to Jodrell Bank.

Mesons could offer new clue in antimatter mystery

“Where has all the antimatter in the universe gone?” is one of the great unanswered questions in cosmology. Now, an unexpected difference in the behaviour of two types of exotic particle seen at an accelerator in Japan could help physicists understand why the universe is dominated by matter.

According to conventional cosmological models, equal amounts of matter and antimatter should have been created in the aftermath of the Big Bang. Matter and antimatter should have then annihilated, leaving only photons. However, the clear domination of matter in our visible universe indicates that our understanding of the physics of the early universe is incomplete.

One possible explanation for the dearth of antimatter was proposed by Makoto Kobayashi and Toshihide Maskawa in 1973. They suggested that the weak nuclear force — responsible for some types of radioactive decay — could act differently on matter and antimatter. This is asymmetry is thought contribute to what physicists refer to as charge–parity (CP) violation.

BaBar and Belle

The first observations of this Kobayashi–Masakawa (KM) asymmetry were reported 2002 and came out of studies of the decays of K and B mesons — short-lived sub-atomic particles consisting of quark–antiquark pairings and created by smashing electrons and positrons together. The measurements were made at both the Belle experiment at the KEK-B accelerator in Japan and at the BaBar experiment at the Stanford Linear Accelerator in the US.

However the asymmetry seen in these experiments is much too small to explain the elimination of antimatter from the universe.

Now, Belle scientists may have found the first inklings of where the rest of the asymmetry may be coming from — having observed an unexpected difference between the time it takes for charged and neutral versions of the B-meson to decay (Nature 452 332). The measurement was made over six years and involved the observation of some 535 million B mesons, which decayed into lighter subatomic particles called K and π mesons.

New physics

“This difference could be an indication of a new source of CP violation that is needed to explain the matter-dominated universe,” says Paoti Chang, a member of the Belle collaboration at National Taiwan University.

The result could be due to new types of short-lived unknown particles that are created as an intermediate step in the decay process, exacerbating the matter–antimatter imbalance. Alternatively, the effect could also be caused by interactions linked to another fundamental force — the strong nuclear force. However, if the latter is true, Chang points out that this may indicate a breakdown in our theoretical understanding of B meson decays.

“To understand whether new physics is indeed involved, study of CP violations from other modes of decay is needed,” says Chang. “Current experimental measurements on CP violation for these candidates are not precise enough, and much more data are needed.”

The researchers will be hoping future experiments such as the upcoming Large Hadron Collider and the Super B factory upgrade at KEK-B will help them find the reasons for the missing antimatter.

Single photons make the trek from space

A team of Italian and Austrian scientists has shown it is possible to send single photons from a satellite to a receiving station on Earth. The work, carried out using the Matera Laser Ranging Observatory in southern Italy, paves the way for global quantum cryptography and more rigorous tests of quantum mechanics.

Quantum cryptography exploits the laws of quantum mechanics to create keys for encoding and decoding messages. These keys are strings of 1s and 0s, which are represented by the quantum states of individual subatomic particles, such as the polarization of photons. In principle, quantum keys are uncrackable — this is because a measurement of a quantum system in general alters the state of that system. In other words, an eavesdropper situated between a sender and a receiver cannot intercept and identify a key without corrupting it.

Quantum cryptographic systems are already available commercially and have been used, for example, to make bank transfers. Indeed, physicists have shown how to transmit quantum keys over distances of more than 100 km by sending single photons either along optical fibres or via telescopes. Extending this range significantly is difficult, however — in optical fibres photon scattering causes unacceptably high losses, and telescopes are subject to atmospheric turbulence, which can distort a photon beam.

High expectations

Now, Paolo Villoresi of the University of Padova, along with other scientists in Italy and a group lead by Anton Zeilinger at the University of Vienna in Austria, have shown how to overcome these limitations by extending quantum cryptography into space (The New Journal of Physics 10 033038 ).

The Matera Laser Ranging Observatory is usually used to measure variations in the Earth’s gravity and motion, by measuring the time it takes for laser pulses to return to the observatory having been reflected off a passing satellite. Villoresi and colleagues employ the same basic technique but make the beam deliberately weak so less than one photon from each pulse returns to Earth. Transmitting individual photons is crucial for realizing quantum cryptography to prevent an eavesdropper siphoning off excess photons without altering the key.

By bouncing the beam off the Japanese Ajisai satellite, which orbits at an altitude of about 1500 km, the researchers calculate that they receive an average of just 0.4 photons per pulse (after taking into account losses such as the inefficiency of their photon detector). Crucially, by precisely calculating when each pulse is to return to the observatory (accounting for the changing position of the satellite), they are able to show that these detected photons are those transmitted by the telescope and not stray photons from background sources. “Not only have we shown that it is possible to detect single photons from a satellite, we have also demonstrated that we can do this using existing technology,” says Villoresi. “We are very happy about that.”

This work not only opens up the possibility of a network of satellites sending quantum encrypted messages around the world, but it should allow scientists to carry out experiments in fundamental quantum physics. Among these is the Bell inequality, which is a way of testing “non-local” correlations between particles.

The team’s next step is to demonstrate that it can send an actual quantum key from a dedicated satellite. The researchers hope to obtain funding from the Italian Space Agency in order to make an instrument that can transmit polarized single photons, which they can then piggyback onto a suitable satellite. The instrument could be operational by around 2011.

Iron-based high-Tc superconductor is a first

For more than 20 years, physicists have been unable to explain exactly how and why high-temperature superconductivity only seems to occur in a special group of mostly copper-based compounds. Now, scientists in Japan have discovered a completely new type of high-temperature superconductor based on iron that could provide physicists with new ways of studying the phenomenon — and shed new light on an important mystery of condensed-matter physics.

Superconductivity is the complete absence of electrical resistance in a material and is observed in many materials when they are cooled to below their superconducting transition temperature (Tc). Superconductivity relies on getting electrons to overcome their mutual electrical repulsion and form “Cooper pairs” that then travel unheeded through the material. In the Bardeen–Cooper–Schrieffer (BCS) theory of low-temperature superconductivity, the electrons are held together as a result of their interactions with lattice vibrations (called phonons) in the material.

However, the BCS theory can not explain the behaviour of high-temperature superconductors, discovered in 1986, which have transition temperatures as high as 138 K. These “cuprates” consist of parallel planes of copper oxide in which the copper atoms lie on a square lattice and where the charge is carried by “holes” sitting on oxygen sites. Each copper atom has an unpaired electron, and hence a magnetic moment or “spin”, and some researchers believe that it is the coupling between these spins that gives rise to superconductivity in these materials.

Superconductor below 26 K

Now, Hideo Hosono of the Tokyo Institute of Technology and colleagues have discovered the first iron-based high-temperature superconductor, which has no electrical resistance at temperatures below 26 K (J. Am. Chem. Soc. 130 3296).

The crystalline material is called LaOFeAs comprises layers of lanthanum and oxygen sandwiched between layers of iron and arsenic — and is doped with fluoride ions. The researchers expect that the Tc of 26 K could be further increased by modifying the material by applying pressure, for example.

Not phonon-mediated

Preliminary studies of the material suggest that its superconductivity is not phonon-mediated, as expected from the classic “BCS theory”, but could be similar to that thought to exist in the high-temperature cuprates.

“One would think superconductivity to be phonon-mediated in this material — like in low-temperature superconductors,” said Kristjan Haule, a theorectical physicist at Rutgers University in the US, whose team is also working on understanding this material. “However, we performed density functional calculations that suggest that the Tc would be around 1 K at most if phonons were indeed responsible.”

Haule’s team have calculated that the undoped LaOFeAs compound should be a very bad metal at low temperatures and is almost an insulator (arXiv: 0803.1279). “This is another strong indication that the superconductivity is not mediated by phonons, which requires a good metallic state with coherent carriers,” Haule told physicsworld.com.

Weak-coupling theories

Indeed, this bad metal state resembles lightly-doped high-temperature superconductors, he explained. According to the team, this means that weak-coupling theories — such as spin fluctuations — suggested in the past (and which failed) to describe cuprates might now be useful explaining superconductivity in LaOFeAs compounds. Preliminary experimental results from Hosono’s group appear to agree with these findings.

The new superconductor is also proof that superconductivity is not limited to copper oxides and a few other compounds based on uranium, cerium and plutonium. Although superconductivity is destroyed by high magnetic fields, the discovery shows that it can even exist in a strongly magnetic material like iron when the iron is surrounded by other suitable atoms (in this case, arsenic). Moreover, effects related to the orbital properties of electrons, usually neglected in cuprates, can play an important role too.

Haule believes that the new class of superconductors might be technologically important but much more research is still needed before this can be said with any certainty.

Arthur C Clarke dies at 90

Arthur C Clarke, the veteran science fiction writer, has died in Sri Lanka aged 90. Aside from the total of nigh-on 100 books to his name, Clarke will be remembered for his foresight for scientific endeavour, including the idea of positioning satellites 35,800 km above the equator to achieve a “geostationary” orbit suitable for radio communications.

Clarke was born the son of a farmer and post-office worker on 16 December 1917 in the seaside town of Minehead in south-west England. His interest in science started young with a fondness for stories by Jules Verne and H G Wells, and the popular US science fiction magazine, Astounding Stories of Super-Science.

In 1941, during World War II, he enlisted in the Royal Air Force (RAF) where he would stay as a specialist in radar for five years. It was while in service that he wrote the paper, published in the UK journal Wireless World, that put forward the possibility of geostationary satellites for relaying radio communications between ground-based stations, an ability that would not be realized for nearly two decades. This wasn’t the only prophecy of his that turned out to be true — around the same time he predicted that man would reach the moon by the turn of the millennium. However, his hunch in 1999 that the first years of the millennium would also bring commercially available cold fusion may now seem optimistic.

After leaving the RAF, Clarke went to King’s College, London, receiving a first-class honours degree in physics and mathematics in 1948. During his studies he wrote what was to be his first published science fiction novel, Prelude to Space. In 1949, he spent a brief period as an assistant editor for the journal Physics Abstracts.

For the next half century Clarke wrote some of the world’s most famous science fiction works, including 2001: A Space Odyssey in 1968 and Rendezvous with Rama in 1972. The former novel, which started life 17 years earlier as a short story entitled The Sentinel, he wrote while collaborating with the filmmaker Stanley Kubrick in making the film of the same name. In fact, it was the film version of 2001: A Space Odyssey — famous for its portrayal of the “HAL 9000” computer that develops a deadly, humanlike power complex — that cemented Clarke’s science fiction fame.

In 1956, in the wake of a failed marriage, Clarke moved to Colombo in Sri Lanka, then known as Ceylon. Back at the seaside, though now some 7000 km from his first home, he could pursue his newfound hobby of diving, for him the closest experience to being in the weightlessness of outer space. However, in 1962 he suffered a bout of polio, and in 1984 he developed post-polio syndrome which would eventually confine him to a wheelchair.

Knighted in 1998, and with book sales thought to top $25m, Clarke has won the acclaim of non-scientists and scientists alike, including the likes of fellow mathematical physicist Roger Penrose. To fans, he will likely be immortalized in his Three Laws, penned in 1962 for Profiles of the Future:

  • “When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong.”
  • “The only way of discovering the limits of the possible is to venture a little way past them into the impossible.”
  • “Any sufficiently advanced technology is indistinguishable from magic.”

Supersymmetry could be seen in ultracold atoms

Physicists in China and the US have proposed a way of using a mixture of ultracold atoms to study supersymmetry — a concept usually associated with particle physics.

The team has calculated that a “Goldstino-like” excitation should occur in mixtures of bosons and fermions as a result of supersymmetry breaking. While the researchers admit that it not currently possible to detect the excitation, they believe that advances in ultracold-atom experiments could make it possible to study supersymmetry in atomic mixtures. This, they claim, could lead to a better understanding of the fundamental properties of sub-atomic particles.

Superpartners for all

All known sub-atomic particles are either fermions (having half-integer values of spin) or bosons (having integer values). For some reason there seems to be more types of fermion in the universe than types of boson. This doesn’t sit well with some physicists who believe that the universe should posses supersymmetry, in which every fermion has a corresponding “superpartner” that is a boson — and vice versa. One attractive consequence of supersymmetry is that it could help unify the electroweak and strong forces of nature into one “electrostrong” force — something that physicists have been struggling to do for decades.

While physicists have never actually seen a superpartner, some are hopeful that they could reveal themselves at the high collision energies that will soon be possible at CERN’s Large Hadron Collider.

Breaking supersymmetry

Atoms are also either bosons or fermions — and this has inspired Yue Yu of the Chinese Academy of Sciences in Beijing and Kun Yang of Florida State University to propose a way to use mixtures of both types of atoms to simulate how supersymmetry is broken (Phys Rev Lett 100 090404). They began with equations that describe a system containing large numbers of one type of fermion and one type of boson. Supersymmetry is introduced using mathematical operators that convert a fermion into a boson and vice versa.

Yu and Yang calculated that the lowest energy state of such a system contains no fermions or exactly one fermion — in other words, there is a spontaneous breaking of supersymmetry that transforms nearly all the fermions into bosons.

This occurs because all of the bosons in the ensemble are able to occupy the lowest quantum energy state, whereas quantum mechanics allows only one fermion per state — forcing fermions to occupy higher energy states. As a result, the energy of the system increases if a boson is converted into a fermion, but decreases if the opposite occurs.

Chemical potential

Yu and Yang also studied a more complicated system in which a “chemical potential difference” was introduced to offset the bias towards bosons, which made it energetically favourable for more than one fermion to exist.

In both cases they were able to show that the breaking of supersymmetry led to a collective excitation of the system called a Goldstone mode. This excitation behaves just like a fermion particle, which is dubbed a “Goldstino”. Similar Goldstino’s are also expected to arise in particle physics as a result of supersymmetry breaking.

Sharp peak

What’s more, Yu and Yang have calculated that the Goldstino should result in a sharp peak in the energy spectrum of a single fermion that is mixed in with a large number of bosons — something that in principle could be seen in a collection of ultracold atoms that are held in an optical trap. They say the peak could be identified because it would remain sharp at temperatures above absolute zero, unlike other features in the energy spectrum that would broaden with increasing temperature.

While there is currently no way to observe the energy spectrum of a single fermion in a cold bosonic gas, the team are hopeful that new techniques such as stimulated Raman spectroscopy may be used in the future to detect Goldstinos.

While Yu and Yang have shown that it may be possible to simulate the effects of supersymmetry breaking in a cold atomic gas, Andrew Cohen a particle physicist at Boston University in the US cautions against taking such analogies too literally. He told physicsworld.com that several kinds of supersymmetry are possible “So the kind of supersymmetry that might manifest itself in condensed-matter physics isn’t necessarily the same as that in [particle physics]”.

In addition, Cohen points out that even if the cold atoms had the same supersymmetry as that believed to play a role in particle physics, understanding the cold atoms might not necessarily shed any light on questions of particles physics. He pointed out that while some processes in particle physics occur with rotational symmetry, there is little that particle physicists can learn from studying bowling balls, for example, which also have rotational symmetry.

Alpha Centauri might harbour an ‘Earth’

Earth-like rocky planets could be hiding just a few light years away in our closest stellar neighbours. That’s the bold claim by a team of American astronomers at the University of California, Santa Cruz, who also argue that dedicated study of the three-star Alpha Centauri system some four light-years away using existing astronomical techniques could potentially unveil terrestrial exoplanets in as little as five years.

The possible existence of planets beyond our Solar System, and hence the existence of extraterrestrial life, has been the subject of speculation for much of recorded history. Greek philosophers such as Thales argued for a universe full of other planets, and in 1600 the Catholic Church even burnt the Italian astronomer Giordano Bruno — a contemporary of Galileo — at the stake for publishing a book arguing for an infinite universe in which every star has a solar system and in which intelligent life populates the planets.

However, it took until 1993 for astronomers to discover the first so-called exoplanet, thanks to the difficulty in detecting a planet’s reflected light amid the glare of its parent star. Since then over 200 exoplanets have been discovered, many by examining the “wobble” in a star caused by the shifting centre of gravity of an orbiting planet, a technique known as Doppler detection. Nevertheless, only a handful of the exoplanets discovered are believed to have a density high enough to be rocky, and only one of these — found in mid 2007 by the ESO’s 3.6-m telescope at La Silla, Chile — has presented even the possibility of having liquid water.

In five years we could detect entire neighbouring planetary systems Javiera Guedes, University of California, Santa Cruz

Seeing our neighbours

Now, Javiera Guedes and her colleagues argue that the same technique could be applied to our nearest neighbour — the Alpha Centauri star system — to find Earth-like planets. According to them, computer simulations reveal that terrestrial planets are likely to have formed around Alpha Centauri B and could even be within the star’s “habitable zone” where liquid water can exist on a planet’s surface (Astrophys. J. in publication).

“We simulated a proto-planetary disk around Alpha Centauri B over 200 million years and in all simulations were able to form planets of 1-2 Earth masses,” explains Guedes. “Forty percent of these Earths lie in the so-called habitable zone of the star. If these planets do exist, we can observe them using modest resources, such as a one-metre telescope.”

The researchers believe Alpha Centauri B is among the best candidates for finding terrestrial planets thanks to its brightness and position in the sky, which gives a long observational window each year from the southern hemisphere. However, detecting small, rocky planets the size of Earth is challenging because of the relatively small wobbles they induce in their host stars. Up to five years of dedicated observations may be needed to detect any around Alpha Centauri B.

“With tens of thousands of observations, we might hope to actually detect small rocky planets analogous to Earth and Mars,” says Debra Fischer, a team member at San Francisco State University. “That would be a huge breakthrough for understanding how frequently rocky planets form, and for understanding planet formation in binary star systems.”

The researchers have received some initial funding and will be starting an observational program in mid-May to monitor Alpha Centauri A and B intensively using the 1.5-meter telescope at the Cerro Tololo Inter-American Observatory in Chile. “In five years we could detect not only these Earths, but entire neighbouring planetary systems,” says Guedes.

The four horsemen

horsemen.jpg

This is a slide from a talk given by David Bader of Lawrence Livermore on behalf of Brian Soden of the University of Miami. It shows the four main feedback mechanisms that are believed to play a role in climate change.

They are the temperature and water content of the atmosphere; ice and snow cover; and cloud cover. Physicists are fairly certain that the first three are have a positive-feedback effect — that is they tend to increase the rate of global warming — but they are not so sure about clouds.

In particular, the effect of stratocumulus clouds on climate has been very difficult to understand. The problem is that these puffy clouds are very thin and turbulent, making it hard to understand the physics of how they participate in the transfer of radiation into and out of the atmosphere.

According to Bader “uncertainty of cloud feedback is the primary cause of uncertainties in climate models”. That sounds like a challenge to the physics community.

Weathering the storm

“You can flood a city, but you can’t drown a university”, says Greg Seab, a physicist at the University of New Orleans who was speaking at a press conference on the impact of Katrina on local physics departments.

Although the university was above the high water mark when Katrina flooded much of the city in September 2005, the campus was without electricity for six months. Indeed, the power only came on three days before the campus was scheduled to reopen in 2006.

But instead of cowering in the dark, the University re-invented itself online. Just a month after the disaster, faculty were delivering lectures and course work to 7000 students. However, one third of the university’s faculty eventually left after Katrina — something that Seab blames in part on “abysmal support from the state [of Louisiana].

The Xavier University campus suffered a direct blow, with many of its lecture halls underwater. The institute managed to reopen in January 2007, extending its academic year until August. Repairs have so far cost the university $50 million according to physicist Murty Akundi. 75% of students returned that January and Akundi says that enrolment is expected to be back to 80% of pre-Katrina levels by September 2008.

On a more cheerful note, David Hoagland of the University of Mass. at Amherst explained how he received a call from a colleague at New Orleans’s Tulane University asking if he could move his entire research group to Amherst. Hoagland said yes and the team were up and running in a month — and apparently “flourished with no scientific loss”.

I naturally assumed that these were theorists — but no, these intrepid experimentalists managed to clone their Tulane lab using borrowed equipment, much of it coming from scientific equipment makers. Where there is a will, there is a way!

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