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Home » Page 1629

Surprise return

Every so often a new scientific discovery is made that catches researchers by surprise. That is exactly what happened earlier this year when scientists in Japan reported a new high-temperature superconductor containing a layer of iron and arsenic sandwiched between layers of lanthanum and oxygen. The material, known as an “iron oxypnictide”, was found to carry electric current without resistance when cooled below a transition temperature (Tc) of about 26 K. By tweaking the new superconductor’s composition, other researchers had, within weeks, boosted the Tc of the oxypnictides to as high as 55 K (see “Rebirth of the hot”).

Over 100 papers have so far been written about these materials. One reason for this interest is that we now know that the cuprates, discovered in 1986, are no longer the only type of high-temperature superconductor. And if there are two classes of such materials, there may well be others. Moreover, the behaviour of the oxypnictides could shed much-needed light on why the cuprates superconduct — a riddle that has left theorists stumped.

Researchers are also fascinated because the doping, composition and structure of these new materials can be almost endlessly modified. With suitable alchemical tinkering, their Tc could possibly be boosted to above the all-important temperature of liquid nitrogen (77 K). That could allow such materials to be used in commercial applications such as lossless electricity transmission lines — provided they can be fashioned cheaply into wires, that is. More fundamentally, researchers are mystified as to why the presence of iron, which is magnetic, does not destroy the superconductivity of the oxypnictides; magnetic fields are usually the death knell for supercurrents.

Cynics will say that we have been here before. There was, after all, similar excitement after the discovery in 2001 that magnesium diboride (MgB2) could superconduct, yet interest faded fast after its Tc stalled at about 39 K. Moreover, the oxypnictides, which contain arsenic, can be dangerous. But iron oxypnictides are different from MgB2: they have a higher Tc, their behaviour is more mysterious, and their chemical structure can be varied. The challenge now is for experimentalists to grow good-quality single crystals of the oxypnictides, so that their physical properties can be measured more accurately, while theorists should try to explain why the materials superconduct. It is highly unlikely that the oxypnictides will ever superconduct at room temperature, but many more surprises are sure to lie in store.

Obama takes a stand on science

In his campaign to become president of the US, Barack Obama has outlined a plan to boost funding for scientific research and base policy decisions on advice that is “expert and uncoloured by ideology”.

Obama, who accepted the role of Democratic nominee for the US presidency last week, revealed his stance on science and technology on Saturday in response to questions put forward by the ScienceDebate 2008 organization. According to the ScienceDebate team, John McCain, the expected Republican nominee, has said he will answer the same questions in due course.

Although Obama says US innovation “is still the envy of the world”, he admits that the country faces “unprecedented challenges”, such as competing with China’s growing presence in the technology market. Expanding on promises made previously, he says he will double funds allocated for basic research over the next decade as well as provide more support for high-risk, high-payoff research portfolios.

The route to economic growth

The same is true for cash research into defence, which Obama will “put on a path” to double. Recalling the drive to stimulate education in maths and science after the Soviet Union launched Sputnik in 1957, he recognizes the benefit that national-security and space programmes can have on economic growth and innovation. “Our nation is again hearing a threatening ‘ping’ in the distance,” he says, “this time not from a single satellite in space but instead from threats that range from asymmetric conflicts to cyber attacks, biological terror and nuclear proliferation.”

To act on his view that humans are affecting the Earth’s climate, the Democratic nominee insists that the US should “take a leadership role” in designing technologies that reduce greenhouse emissions by 80% below 1990 levels by 2050. He says this can be achieved in part by investing $150 billion over a decade on clean energy research, development and deployment.

Given an Obama administration, scientists could expect their views to be aired by an “impressive team of science advisors”. These would apparently include Nobel laureates, though it is not known who. Obama also plans to introduce the nation’s first “chief technology officer” to oversee the technologies, infrastructure and practices used across all agencies. Moreover, he vows to “restore the science integrity” of the government by only releasing publications that “are not distorted by the ideological biases of political appointees”. This promise will be backed up by bolstering support for whistleblowers who want to publicize instances of malpractice.

Hopes for live debate

In a written statement, Matthew Chapman, president of Science Debate 2008, said: “We are grateful for Senator Obama’s detailed responses and look forward to receiving the same from Senator McCain.” He added that he still hopes the two candidates will agree to attend a televised debate on science policy, which was the original motive of the organization.

ScienceDebate 2008 was formed towards the end of last year by a group of six people who wanted science policy to be debated by the presidential candidates in the run up to the November election. Since then the organizers have gathered the signatures of some 37,000 supporters including university presidents, the representatives of scientific institutions and Nobel laureates.

• You can read Barack Obama’s full responses to the questions posed by ScienceDebate 2008 here.

Memory device could store data using heat

Heat has long been regarded as useless or even harmful in electronic circuits. But some researchers think that it might be possible to build computers that process phonons — pulses of vibration that carry heat — rather than conventional electrons.

Physicists in Singapore and China have now taken a step towards such thermal computation or “phononics” by devising a model for storing thermal information. Although their scheme has yet to be tested experimentally, the researchers claim that bits of information could be read out without destroying the stored data (arXiv:0808.3311v1).

In a conventional electronic circuit, the states “0” and “1” are usually defined by standard voltages. In thermal circuits, however, the states are defined by two arbitrary temperatures. In-line with the second law of thermodynamics, a temperature drop leads to a heat current flowing from a hot to a cold area. Generally, the larger the temperature drop the larger the heat current, which is known as positive differential thermal resistance.

These currents are carried by phonons, which are difficult to control because as they are bundles of energy that have no electrical charge and therefore cannot be manipulated using electromagnetic fields.

Missing memories

Researchers have already managed to build a thermal diode and have even shown that it could be possible to build thermal transistors and logic gates — all standard components for functional thermal devices. But memory is required to store the output after performing logical operations.

Now, Baowen Li from the National University of Singapore and Lei Wang from Renmin University of China in Beijing have devised a theoretical model for such thermal memory. Their model takes into account a key element in thermal logic gates — yet to be demonstrated experimentally — by generating a “negative differential thermal resistance” (NDTR). An NDTR means that a large temperature drop leads to a small heat current and a small temperature drop leads to a large heat current.

In their model of thermal memory, Li and Wang considered two heat baths, held at a constant temperature, each sitting at the end of a rod. The other, free ends of the rod do not touch each other, but are nevertheless weakly coupled so that there is an NDTR between them. The final component of their model is a “particle” that sits at the end of one the rods, near the gap between the pair.

Reading and writing

Li and Wang then consider what happens when an object — connected to its own heat bath — cools this particle down to an arbitrary temperature, dubbed “0”. This is what they call the “writing” process.

To “read” out the temperature of the particle they use another object — dubbed the “reader” — which is set at a temperature halfway between “0” and another temperature, defined as “1”. The particle then warms up when this reader is brought into contact with it, which causes a large heat current to flow from the particle, down the rod to the heat bath.

However, there is only a small heat flow in the other rod as it is connected via NDTR. In other words, the current in the second rod minus the current in the first rod is negative. This draws heat away from the particle, which cools back down to “0”. As the reader is in contact with the particle, the reader also moves into the “0” state. In other words, it has read out the original “0” state of the particle.

In a similar manner, Li and Weng also showed that if the particle is prepared in the “1” state which is hotter than the reader, then it can also be read out without the state being destroyed.

Data cannot be stored for a long time before the heat leaks away. Li calculates that thermal memory will have to be refreshed every 100 μs if the rods were made of carbon nanotubes. This is much more frequent than electronic DRAM currently used in computers today that require refreshing every 64  ms.

Not so instant recall

The speed of thermal memory is a key issue that needs further investigation Baowen Li National University of Singapore

Another difficulty with thermal memory is the slow access times. “The big difference is between the speed of electromagnetic waves and phonons,” Li told physicsworld.com. Phonons travel at speeds around 1000  ms-1, hundreds of thousands of times slower than electromagnetic waves. “The speed of thermal memory is a key issue that needs further investigation,” he says. Once and if, NTDR is experimentally realized, Li and colleagues are confident that thermal memory will be the next step towards thermal computers.

DAMA results go through the looking glass

By Jon Cartwright

The debate as to whether the DAMA/LIBRA team has detected dark matter, as it claimed in April, will no doubt persist until fresh data can say either way. But in the meantime, Robert Foot, a physicist from the University of Melbourne, suggests an alternative interpretation: “mirror matter”.

I’ll take a step back for a moment in case you aren’t familiar with the story. (Alternatively, you can see Physics World’s feature DAMA/LIBRA is an underground experiment based at the Gran Sasso laboratory in Italy. It looks for dark-matter particles known as WIMPs (weakly interacting massive particles) — a class favoured by theorists for the mysterious substance — by monitoring for flashes that occur when the particles collide with nuclei in 250 kg of sodium-iodide detectors. The idea is that the frequency of flashes should modulate over the year as the Earth changes its speed through our galaxy’s “halo” of dark matter: in June, when the Earth’s orbit takes us faster through the halo, one would expect to see more flashes; in December, when we are moving slower, one would expect to see fewer.

(more…)

Bubble-fusion researcher loses professorship

Purdue University in the US has announced how it will reprimand Rusi Taleyarkhan after an internal committee ruled in July that he is guilty of scientific misconduct. The committee has also denied an appeal from Taleyarkhan about the misconduct verdict.

Taleyarkhan, a nuclear engineer who claimed the discovery of “bubble fusion” in 2002, will lose his title of Al Bement Jr Professor of Nuclear Engineering and will not be able to be thesis adviser to graduate students for at least three years.

Taleyarkhan retains his position as a member of the Purdue University graduate faculty, but with the reduced rank of “special graduate faculty”.

In July the university concluded that he had cited a paper by researchers in his own lab as if it were an independent confirmation of his alleged discovery of bubble fusion.

Proportional punishment

“In considering the sanctions to impose, I have been guided by the principle that the sanctions should address and be proportional to the specific findings of the research misconduct,” Purdue provost Randy Woodson wrote in a letter to Taleyarkhan that outlined the disciplinary actions.

Woodson added that the university will review Taleyarkhan’s conduct after three years, to determine whether he can apply for reinstatement as a full faculty member.

Taleyarkhan believes that the decision is unreasonable. “The sanctions are unfair and egregious in their severity,” he told physicsworld.com. He pointed out that a previous Purdue committee had exonerated him of misconduct charges in 2006, and that the latest committee absolved him of most charges of research misconduct.

Political motivation

The university started its latest investigation, involving “new allegations” of falsifying the research record, he said, “following political pressure from Congress motivated by articles in Nature.” Overall, he continued, “the university system has failed miserably and taken the expedient way out.”

What will happen next? Taleyarkhan, who has initiated a civil lawsuit against the university, does not entirely discount further legal action. “As a faculty member and a US citizen,” he said, “I have a right to appeal the findings along with seeking redress from the courts of the United States for the extensive damage caused to me and several others.”

Legal options

His lawyer, Indianapolis attorney John Lewis, said that “given the way Purdue administrators have handled this matter, Dr. Taleyarkhan has many options in the judicial system.” However, Lewis continued, “after years of fighting and being overwhelmingly successful against his detractors, he may not want a further part in this aspect.” Rather he may choose to focus on his teaching and research.”

Taleyarkhan argues that the affair has not damaged the credibility of his research. “The matter . . . will not affect bubble fusion research,” he declared, “as the final two allegations have nothing to do with the science which, as a consequence of this overall ordeal has been further vetted and strengthened in terms of its credibility.”

Six-year saga

The controversy began in 2002 when Taleyarkhan, who was then working at the Oak Ridge National Laboratory in Tennessee, co-authored a paper in Science in which he reported firing a barrage of ultrasound waves into a liquid mixture of benzene and acetone (Science 295 1868). He claimed that bubbles of gas, which emit flashes of light when the sound waves force them to expand and collapse, could reach such high temperatures and pressures that during this process fusion reactions are initiated.

Several groups, however, failed to replicate the research, while two other Purdue engineers — Lefteri Tsoukalas and Tatjana Jevremovic — complained that Taleyarkhan had tried to prevent them publishing their negative results. An internal investigation last year cleared Taleyarkhan of that charge, but when critics argued that the panel had not taken their views into account, Purdue began a second investigation. Although the committee completed its work in April 2008, Purdue did not issue the report until July when the Office of Naval Research, which funded Taleyarkhan’s research, had accepted it.

Quantum repeater demonstrated

An international team of physicists has taken an important step on the road to global quantum communication by demonstrating the basic principle of a quantum repeater. The breakthrough, which marks the first time that two atom clouds have been entangled remotely, could someday be used to counteract decay in quantum signals.

Quantum communication provides a means to transmit information that is fundamentally secure. It requires two parties to be entangled over a quantum channel, over which a “key” for decoding encrypting information can be established. Because this key becomes corrupt as soon as it is used once, the intended receiver can always tell if the key has been intercepted by an eavesdropper.

Although quantum communication has been used already over distances of up to 100 km or so, it is difficult to create entanglement over larger distances because of signal degradation. In classical communication the simple remedy would be to amplify the signal periodically, but this is impossible for quantum keys because of the “no cloning theorem” which precludes a quantum signal from being copied.

Building up entanglement

The answer is the quantum repeater — and now Jian-Wei Pan, Yu-Ao Chen and colleagues from the University of Heidelberg, the University of Science and Technology of China the Vienna University of Technology have demonstrated a crude version of such a device. The idea is that the quantum channel is split up into segments, each of which is easier to entangle. Once the segments are entangled individually they can then be entangled together via the same process.

Pan and colleagues have shown how to entangle one of the segments. They begin with two ultra-cold clouds of rubidium atoms, each of which they entangle with a single photon. The two photons are then sent towards each other via fibre-optic channel. At the middle the photons cross at a beam splitter, and finally the researchers perform a measurement on them to entangle the atom clouds (Nature 454 1098).

The team states in its report in Nature that it has only managed the entanglement of a fibre-optic channel 300 m long, which was limited by a 10 µs lifetime of the atom cloud. However, Chen told physicsworld.com that they have since increased the atom-cloud lifetime to 1 ms, thereby opening the door to a quantum channel 100 km long.

The researchers are presently trying to increase the efficiency from 10% towards unity so that they can couple several of the entanglement segments together.

Gamma-ray telescope renamed after Fermi

NASA’s Gamma-ray Large Area Space Telescope has been successfully calibrated and has begun to map gamma-ray sources throughout the universe. The international mission — which was launched into Earth-orbit in June — has also been renamed the Fermi Gamma-ray Space Telescope in honour of the Italian-American physicist Enrico Fermi.

NASA marked the occasion yesterday by releasing the first data from the satellite: a gamma-ray image of the entire sky taken over four days by Fermi’s Large Area Telecsope (LAT). According to the agency, an image with comparable resolution taken by Fermi’s predecessor (the Compton Gamma-ray Observatory) took several years to obtain.

Working fine

The four-tonne observatory was built by researchers in the US, France, Germany, Italy, Japan and Sweden. It also contains the GLAST Burst Monitor (GBM) to detect transient sources such gamma-ray bursts and solar flares.

Ronaldo Bellazzini of Italy’s University of Pisa — which was involved in building the LAT — told physicsworld.com that the telescope is “working as expected” and has already made several new discoveries regarding pulsars, active galactic nuclei and gamma-ray bursts. “These results will be published shortly”, he said.

However, Bellazzini added that astrophysicists will have to wait a little longer before Fermi begins to deliver meaningful data on one of its prime objectives — shedding light on the nature of dark matter. Certain dark matter particles, called Weakly Interacting Massive Particles, or WIMPs, could annihilate to produce gamma rays, which would show up as tiny gamma-ray signals from the dark-matter haloes that surround galaxies. “It will take about one year to understand the subtleties of the instrument and gather the data,” said Bellazzini.

Proud in Pisa

Bellazzini is particularly pleased with the telescope’s new name because Fermi was a student at the University of Pisa. “Fermi worked at the frontier of particle physics and astrophysics,” he said, citing Fermi’s pioneering work on the acceleration of cosmic rays.

Born in Rome on 29 September, 1901, Fermi won the Nobel Prize for Physics in 1938 for his work on nuclear reactions. That same year he left Italy for the US, where he played an important role in the Manhattan Project. Fermi died in Chicago on 28 November, 1954.

Cold atoms explode like cloverleafs

Physicists in Germany have created spectacular, cloverleaf-shaped explosions in a gas of ultracold atoms trapped by magnetic fields. The cloverleaf shapes were formed by finely tuning the magnetic interactions between the atoms, which had formed a Bose-Einstein condensate (BEC) and so were all in the same quantum state. Although such “bosenovas” have been seen before, they have previously always been the same shape in all directions.

Bosenovas were first created about 10 years ago by adjusting the magnetic-field strength between the atoms in a BEC so that the short-range “van der Waals” forces between the atoms are attractive, rather than repulsive. This causes the BEC to collapse in on itself much like a dying star. It then explodes like a tiny supernova and throws off many of its constituent atoms.

Physicists believe that the explosion occurs when the atoms are close enough for short-range interactions to affect groups of three atoms (“three-body” interactions) rather than just pairs. Until now the attractive forces between atoms were isotropic, which meant that the explosions ejected atoms equally in all directions.

Preferred directions

Now, however, Tilman Pfau and colleagues at the University of Stuttgart have created the first bosenovas in which the attractive forces between atoms are dipolar and therefore depend on the relative orientation of the atoms. This caused clover-leaf shaped explosions, which the team says reflect the underlying symmetry of the attractive forces (Phys Rev Lett 101 080401).

The team used a BEC of chromium-52 atoms, which have large magnetic dipole moments. The magnetic forces between the atoms are attractive and are normally much weaker than the van der Waals forces. However, the team adjusted the magnetic field so that the van der Waals forces were near zero, allowing the magnetic forces between atoms to take over.

According to Pfau, this caused the BEC to contract until the atoms were close enough for three-body interactions to cause an explosion. In this process, some of the atoms are thrown out of the BEC, while the remaining BEC expands outwards with a distinctive cloverleaf pattern. Such a pattern is expected for an expanding BEC with dipolar interactions in the presence of a magnetic field, said Pfau.

A gentler kind of implosion

Because the implosion was driven by the much weaker magnetic interactions, it was much more “gentle” than implosions caused by van der Waals interactions, according to Pfau. This he said, made it easier for Masahito Ueda and colleagues at the University of Tokyo to use current theories of BECs to describe the bosenova process — something that had proved difficult in the past.

Computer simulations by Ueda’s team suggest that the BEC collapse involves the formation of two “vortex rings” that spin in opposite directions. The physicists are now keen to see if they can create stable vortices by switching the repulsive van der Waals interactions back on before the BEC explodes.

Pfau believes that the insights gained into how to create and control chromium-52 BECs could someday be technologically relevant. Chromium is already used in a number of nanotechnologies and chromium-52 BECs could form the basis of “atom lasers” that could deposit tiny amounts of chromium to an extremely high degree of spatial precision.

Simulating magnetism

On a more fundamental level, the dipole interactions in the BEC are the same as those found in magnetic materials and Pfau believes that the system could be used as a “quantum simulator” to study magnetism.

Dave DeMille of Yale University agrees. He told physicsworld.com that such BECs could allow physicists to make “a new, deep connection between ultracold atom experiments and … many interesting magnetic systems in real-world materials”.

LHC kicks in both directions

LHCb.jpg
(Credit: Olaf Behrendt)

By Jon Cartwright

Could it be — touch wood — that the Large Hadron Collider (LHC) will make it to the official 10 September start-up date without any further hiccups?

On Friday scientists at the European laboratory CERN were able to tick off two more items on the accelerator’s commissioning list. First, they managed to feed a bunch of protons from the transfer line of the Super Proton Synchrotron (SPS) into the LHC and then steer it some three kilometres round the beam pipe in a counter-clockwise direction. Second, a detector at LHCb — one of the four main experiments at the LHC — got the first taste of collision debris.

(more…)

Bush honours US atomic physicist

The pioneering atomic physicist David Wineland is one of eight US researchers to be awarded the 2007 National Medal of Science. Wineland, a physicist at the National Institute of Standards and Technology (NIST), was cited for his “outstanding leadership in developing the science of laser cooling and manipulation of ions”. He will receive the award from President George W Bush in a ceremony in Washington, D.C., on 29 September.

The National Medals, which are administered by the National Science Foundation (NSF) and awarded each year, were established by the US Congress in 1959 to honour scientific research that “enhances understanding of the world and leads to innovations and technologies that give the US a global economic edge”.

Based in Boulder, Colorado, Wineland is credited for his work on “applications in extremely precise measurements and standards, quantum computing, and fundamental tests of quantum mechanics, and for his major impact on the international scientific community through training scientists and outstanding publications.”

Helped isolate a single electron

Wineland’s long record of breakthroughs began in 1973, when as a postdoctoral researcher with Hans Dehmelt at the University of Washington he helped isolate a single electron using a Penning trap. Five years later, Wineland — by then in charge of his own group at NIST in Boulder — succeeded in laser cooling magnesium ions to below 40 K.

This early work on trapped ions was aimed at developing alternatives to the caesium beam clock, which then formed the basis of international time and frequency standards. However, the ion cooling and trapping techniques Wineland developed had a much wider impact, sparking work on neutral atom laser cooling, which formed the foundations of both the 1997 and 2001 Nobel Prizes.

Later, Wineland helped launch the field of experimental quantum computing by demonstrating the first two-bit “controlled-NOT” quantum logic gate. Recent work has focused on overcoming barriers to turning such few-qubit systems into a full-scale quantum computer.

“Wineland is an outstanding scientist who has made revolutionary contributions to the development and application of laser cooling and trapping,” said Katherine Gebbie, head of NIST’s Physics Laboratory. “We are thrilled by this well-deserved recognition for him”.

‘Invented ion trap quantum computing’

There was also praise from Winfried Hensinger, senior lecturer and head of the Ion Quantum Technology group at the University of Sussex in the UK. “Dave Wineland really invented ion trap quantum computing, which has been the most successful experimental method used to date,” he said. Hensinger described the award as “well overdue” and noted that Wineland “stands out in that he’s immersed in all the results that come out of his group – he always knows exactly what’s going on in the lab”.

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