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A new type of superfluidity?

By James Dacey

Have you come across new research, utterly failed to realise its significance, then the penny drops shortly afterwards? Well it happened to me this week. On Monday, I stumbled across a new research paper in Nature about the behaviour of quasi-particles in a semiconductor and quickly dismissed it as niche physics. However – a little sniffing around the edges and a few phone-calls-to-experts later – I’m beginning to realise the significance this paper may hold for our understanding of Bose-Einstein condensates and superfluidity.

The research in a nutshell: a group of physicists led by Alberto Amo of Madrid’s Autonomous University have observed polaritons — quasiparticles merging photons with excitons — travelling without resistance in a semiconductor microcavity; thus behaving like a superfluid; thus potentially being the first Bose-Einstein condensate in a system out-of-equilibrium.

But I think the research still needs some historical context…

Bose-Einstein condensation was first predicted back in 1925 when Einstein — building on the work of Satyendra Nath Bose — predicted that when weakly interacting atoms are cold enough they drop into their ground state and the individual waveforms merge to create a single quantum state.

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'Academic squatter' banned from campus

By Hamish Johnston

Thanks to blogger Chad Orzel for pointing out this story from the old country.

According to an article in the Ottawa Sun, the University of Ottawa has suspended physics professor Denis Rancourt and banned him from campus — apparently because of his attempts at changing how a physics course is taught.

Writing in 2007, Rancourt explained: “In response to twenty years of observing classes that both delivered soulless material and served mainly to prepare students to be obedient and indoctrinated employees, I felt I had to do something more than give a ‘better’ physics course.”

Things seem to have come to a head last May, when The Sun says Rancourt tried to give everyone in his Physics and the Environment class an A+. As far as I can tell, this was an act of what Rancourt calls “academic squatting…where one openly takes an existing course and does with it something different”.

You can read more about academic squatting and Rancourt’s other views on his blog.

Rancourt encourages academics to use the freedom of tenure to take hold of the courses they teach and change them.

“Tenure – use it or lose it,” he writes in a 2007 blog entry on academic squatting

It looks like this has become more than just rhetoric for Rancourt and the university brass.

Did an Englishman beat Galileo to the moon?

One of Harriot’s maps (Courtesy: Lord Egremont)

By Hamish Johnston

Yes, at least according to the BBC, which is running a news story about Thomas Harriot, who apparently used a telescope to draw maps of the moon about four months before Galileo famously turned his own telescope skywards — making Harriot the father of modern astronomy, not Galileo.

The proof will soon be on display at the West Sussex Record Office in Chichester, where the documents are stored.

Oxford University science historian Allan Chapman told the BBC “Thomas Harriot was not only the first person ever to draw an astronomical body with a telescope on 26 July 1609, he rapidly developed to become an absolutely superb lunar cartographer”.

Astronomer Sir Patrick Moore told the BBC, “Harriot was first… and his map of the moon is better than Galileo’s.”

Harriot was a wealthy businessman who apparently did not feel the need to publicize his findings.

The good news for organizers of the International Year of Astronomy 2009 — which is celebrating the 400th anniversary of the first time a telescope was used in astronomy — is that Harriot also got started in 1609, so they still have the right year!

Sand pushes Martian rocks into place

Can you spot the pattern in this photo of Martian rocks? If not, then don’t feel bad: according to researchers in the US and Canada the very lack of a pattern means that there must be some mechanism driving the rocks that way. Studying this process, in which built-up sand levers rocks away from one another, could help geophysicists to understand how climate has steadily transformed the surface of the red planet.

“Rather than look at patterns on the Martian surface, we were looking at a pronounced absence of any patterns at some sites,” Andrew Leier of the University of Calgary, Canada, told physicsworld.com. “We had noticed similar features in desert settings on Earth and tried to understand how this distribution came about.”

The photograph above and others like it come from NASA’s Spirit Rover, which touched down on the red planet at the beginning of 2004. When other geophysicists first saw the pictures, there was an idea that the even distribution could have emerged during intense bouts of wind in order to minimize air drag. However, there has been little lab evidence to suggest that an even distribution would improve aerodynamics and, moreover, shifting just small rocks would take winds speeds of hundreds of kilometres per hour.

“We’re not saying that the idea is wrong,” says group leader Jon Pelletier of the University of Arizona in Tuscon, US, “just that there is a simpler explanation.”

Mounds of dust

Pelletier and colleagues sketched out their mechanism while performing experiments in a wind tunnel containing a Mars-like surface. They found that the wind throws up dust and sand, which gradually accumulates behind a rock. When the mound becomes big enough, the rock rolls forwards.

Computer simulations of the dust build-up behind rocks

But the effect is not be the same for all rocks. Those within a group are better shielded from the wind, and the mound forms to one side, so that the rock falls sideways. In this way the rocks “repel” one another, and the group spreads out (Geology 37 55).

“You could probably see it happen if you were looking for it, say, in a desert during an extreme windstorm,” says Pelletier. But, he adds, the growth of plants and the contraction of the surface due to moisture would often disguise the effect before it has a chance to alter the rock distribution visibly.

After the wind tunnel experiments, Pelletier’s group designed a computer simulation to study the mechanism in more detail (see figure). Based on a well-known model of the migration of sand dunes, it focused on three key aspects: how airflow generates a shearing force on the rocks’ surface; how this force affects the movement of sand; and the critical angle when the rocks topple.

The researchers found that the critical angle greatly affects the rock migration, indicating that larger rocks or those that are more angular will not so readily demonstrate the effect.

The rocky road

Pelletier explains that although his group’s work does not tell us anything “fundamental” about the wind in Martian climate, it does show how Mars’s landscape has evolved and how rocks of different shapes and sizes co-exist on a surface.

“Mars is a nice place to isolate the effect,” he says, adding that his type of work has only been possible in the few years since Spirit landed. “We spend a great deal of effort trying to get images of rocks on sand, and I just want to understand what’s going on.”

Construction set to start on $900m light source

Construction of a major new synchrotron-radiation facility has been approved by the US Department of Energy (DOE). The National Synchrotron Light Source II (NSLS-II), which will be built at Brookhaven National Laboratory, will come online in 2015 and cost $912m. It will have 58 beamlines, which will provide researchers with radiation from the far infrared (1 mm) to hard X-rays (10 pm) for a range of experiments in everything from condensed-matter physics to biomedicine.

A third-generation synchrotron source, the NSLS-II will replace Brookhaven’s existing NSLS light source, which has been operating for 22 years. The new facility will, however, be 10,000 times brighter and have 10 times higher X-ray flux than the present synchrotron.

NSLS-II will consist of a linear accelerator that will fire electrons to energies of 200 MeV. The electrons will be fed into a booster ring that will increase their energy to 3.0 GeV. The electrons will then be sent to a 791 m circumference storage ring that contains magnetic devices called “undulators”, which forces the electrons to follow a sinusoidal path and emit X-rays.

Once complete,only the SPring-8 synchrotron source in Hyogo, Japan, the Advanced Photon Source in Argonne, US, the European Synchrotron Radiation Facility in Grenoble, France, and the Shanghai Synchrotron Radiation Facility in China will have a higher storage ring energy (8, 7, 6 and 3.5 GeV respectively).

Unification could be ripe for the picking

Physicists in the US have proposed a new way of testing the veracity of Lorentz symmetry, a fundamental tenet of Einstein’s theory of relativity. They believe that careful observation could reveal tiny differences in how a body falls to the Earth, depending on the time of year at which the measurement is made. If verified experimentally, such observations would indicate breakdowns in Einstein’s theory and provide important clues in the search for a theory that unifies quantum mechanics and gravity.

Lorentz symmetry says that the laws of physics remain the same for any two objects that are travelling at a constant velocity, or rotated, relative to one another. For example, an apple dropped from the height of 1 m in a moving train will take the same time to hit the floor as an apple dropped 1 m at the side of the tracks.

It is possible, however, for a moving body to experience different physics than a stationary particle — if the apples were electrically charged and the train was moving through a stationary magnetic field, for example, the two apples would feel different forces as they dropped.

Preferred directions

What do apples and fields have to do with unifying the theories of quantum mechanics and gravity? Some researchers believe that a route to unification called the Standard Model Extension could involve a background of quantum fields that introduce preferred directions in space–time — much like the stationary magnetic field along the railway.

Such fields could interact with particles, leading to minuscule but theoretically measurable violations of Lorentz symmetry. However, increasingly sensitive experiments have failed to detect any evidence of violations to date.

This has not deterred Alan Kostelecky and Jay Tasson of the University of Indiana, who argue that violations of Lorentz symmetry have yet to be spotted because physicists are not doing the right sorts of experiments (Phys Rev Lett 102 010402).

According to the duo, their new class of violations is very different from those proposed before. In their theory, violation occurs when new fields are distorted by gravity and this can only be detected in experiments involving gravity.

A simple violation could arise from a field represented by an arrow that points in the same direction at every point in space. In non-gravitational experiments, the corresponding arrows will all point the same way making the violation undetectable as all particles are feeling an identical effect. However, a massive body like the Earth curves space–time in its vicinity and so distorts the distribution of arrows. This changes the gravitational properties of objects according to their motion and can produce detectable effects, such as how an object falls to the Earth.

Evaded detection thus far

These violations would not have registered in previous tests of Lorentz symmetry, which don’t involve gravity and instead examine other aspects of relativity such as the constancy of the speed of light and time dilation. “Our key finding is that comparatively large relativity violations could exist in nature and nonetheless have evaded detection to date,” Kostelecky told physicsworld.com. Indeed, he believes that such gravity-related violations “could be as much as 30 orders of magnitude greater than the relativity violations already excluded by sensitive non-gravitational tests”.

“We also show that certain existing and future experiments could observe these effects,” he said. “One striking effect results from the revolution of the Earth around the Sun,” says Kostelecky. “For example, an apple may fall to the Earth at different rates according to the season.” Practical experiments could include dropping weights from a tower or balloon.

If discovered, the relativity violations would represent a profound change in our understanding of fundamental physics and could offer important clues to the nature of the underlying unified theory. “It can be viewed as analogous to Einstein’s discovery that Newton’s theory is not exact,” said Kostelecky. “It is surprising and delightful that comparatively large relativity violations could still be awaiting discovery despite a century of precision testing.”

Exciting premise

Physicists have welcomed the research. Abhay Ashtekar at Penn State University in the US finds the work imaginative. “Though approximations are made in the way gravity is treated within their calculations, the basic premise is still quite exciting,” he said.

Others agree. “This work points to subtle and non-trivial effects which may occur when gravity is not neglected and as experiments measuring such effects need to be highly sensitive, it is quite possible that these interactions — though present — may have so far gone undetected,” says Parampreet Singh at the Perimeter Institute for Theoretical Physics in Canada. “Irrespective of the outcome of future experiments, I find the idea exciting as these experiments can potentially teach us a lot about space–time symmetries and allowed matter-gravity interactions.”

Martin Bojowald at Penn State University also points to the potentially wide-ranging implications. “If such an effect is indeed found, it will have profound importance for the construction of theories such as quantum gravity, which produce these effects but at the same time do not give rise to other potential violations that are already tightly constrained,” he said.

LHC to hit the stage

By James Dacey

First there was the LHC rap then the media bonanza for the big September switch-on; also playing their part were the harbingers of doom – foretelling apocalypse from Geneva’s ‘black hole machine’.

Now CERN’s (in)famous experiment is about to get even more dramatic as it provides the fictional setting for a new theatre production.

The Gentlemen’s Tea Drinking Society is produced by Ransom Theatre Company who bill it as “a fast and funny exploration of science, friendship, sexuality and the end of everything as four men face the truth on one fateful night”.

The play was written by Richard Dormer who made a name in 2003 with his internationally-acclaimed portrayal of the talented-yet-troubled snooker legend Alex “Hurricane” Higgins. It also contains an original score by Belfast born DJ David Holmes who produced the music for Ocean’s Twelve and Out of Sight.

Dormer and Co haven’t revealed much about the plot other than it centres around four men in a room, one of whom is a physicist harbouring a very big secret – he’s found the Higgs boson.

This is not the first time physics has taken to the stage. Famous examples include Tom Stoppard’s Arcadia (1993) — a look at the life of Byron which incorporated ideas from thermodynamics and chaos theory; and Michael Fryan’s Copenhagen (1998) — a play built around a 1941 conversation between Neils Bohr and Werner Heisenberg about the nature of the quantum world.

More recently American composer John Adams created an opera based on The Bomb and its creation at the Manhattan project. Dr Atomic premiered in 2005 and finally comes to London this February.

The Gentlemen’s Tea-Drinking Society launches on 4 February at Belfasts’s Old Museum before going on tour across Ireland until 10th March. Later in the year it will appear in Glasgow and London.

Was Einstein an atheist?

By James Dacey

I’m still finding my feet here at Physics World so it seemed wise to try and sneak a fairly inconspicuous first post on the blog. So here’s a little story involving two rarely-discussed, uncontroversial topics: British public transport and the religious views of some bloke named Einstein.

If you’ve been in a major city in England, Scotland or Wales this week, you may have noticed a slogan with a difference on the side a bus. “There’s probably no God. Now stop worrying and enjoy your life” has been printed onto 800 buses in the UK’s first ever atheist advertisement campaign.

Campaign organisers – whose financial backers include Richard Dawkins – say the campaign is, “a response to a series of evangelical Christian adverts running on buses in June 2008, which featured the URL of a website saying all non-Christians were going to hell.”

Touché. It’s a free country. What’s this got to do with physicists anyway?

Well, the campaign enters its second phase on Monday which – according to the press release – will involve quotes from “famous atheists” hitting the London underground. Included is Einstein’s quote:

“I do not believe in a personal God and have never denied this but have expressed it clearly”

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Experiment resolves century-old optics mystery

Since the early 20th century physicists have known that light carries momentum, but the way that this momentum changes as light passes through different media is much less clear. Two rival theories of the time predicted precisely the opposite effect for light incident on a dielectric — one suggesting that it pushes the surface in the direction light is travelling, the other suggesting that it drags the surface backwards towards the source of light. After 100 years of conflicting experimental results, a team of experimentalists from China believes it has finally found a resolution.

Weilong She and his colleagues from Sun Yat-Sen University have studied the effect of light at the interface of air and a silica filament and they find that light exerts a push force on the surface (Phys Rev Lett 101 243601) “This paper is a beautiful piece of work and may become one of the classic papers on the momentum of light,” said Ulf Leonhardt, a researcher in transformation optics at the University of St Andrews, UK.

The authors suggest that this finding could now pave the way for new applications, such as highly efficient fusion using laser ‘compression’.

100 year riddle

Hermann Minkowski had proposed in 1908 that light momentum is proportional to a material’s refractive index. The following year another German theorist, Max Abraham, proposed the opposite — momentum is inversely proportional to a material’s refractive index.

This paper is a beautiful piece of work and may become one of the classic papers on the momentum of light Ulf Leonhardt, University of St Andrews

It was suggested that this debate should be resolved experimentally but it proved to be notoriously difficult to record the momentum of light in a dielectric. In the 1970s it seemed as though the mystery was finally solved using a simple experiment involving an air–water interface. Conservation of momentum inferred that, if Abraham was right, the water surface would compress slightly as light rays pass through, but if Minkowski was correct it would bulge. A bulge was witnessed and Minkowski was declared the victor.

Unfortunately, later the same year further analysis showed the bulge to be the result of an unrelated optical effect; the debate was once again thrown open.

21st century makeover

She and colleagues have now finally overcome these difficulties by replacing the water surface with a nanometre silica filament. “We report direct observation of a push force on the end face of the silica filament exerted by the outgoing light,” said She. Given this result, Abraham has been declared the new winner and light momenta is inversely proportional to the refractive index of the material it is travelling through. “The experiment represents a modern form of a beautifully simple idea,” said Leonhardt.

One application that may spring from this knowledge is a more precise technique for laser-induced inertially-confined fusion: a method of producing fusion energy by compressing a fuel capsule made to high density. A series of incoherent laser beams incident on a transparent dielectric ball in a vacuum would cause it to shrink under pressure to achieve nuclear fusion.

Mansud Mansuripur from the University of Arizona recognizes the potential of radiation pressure for inertially confined fusion, but he warns that She and colleagues have only considered electromagnetic pressure without taking account of mechanical forces. “A correct accounting for the deformation of the silica filament in the reported experiments would have required a complete balancing of the momenta,” he said.

Iron-based superconductors really are different

Neutron-scattering experiments have revealed an important clue as to why a recently discovered family of iron-based materials are superconducting at relatively high temperatures. The measurements were made by researchers in the US and UK and show that the “superconducting energy gap” in such materials is different from that found in either conventional or cuprate superconductors. This suggests that a new mechanism of superconductivity is at work.

Superconductivity occurs when a material is cooled below a certain temperature and its conduction electrons form pairs that can flow without any resistance. The superconducting energy gap, which is an important physical property of a superconductor, is the energy required to break one of these pairs. Indeed, measuring the precise nature of the gap is crucial for understanding the physics of superconductivity.

In a conventional, low-temperature superconductor, such as lead, physicists know that the gap is perfectly symmetrical with respect to the direction of the momentum of the electrons. Such superconductors can be described using BCS theory, developed in 1957 by John Bardeen, Leon Cooper and Robert Schrieffer. The theory shows that electron pairs are created as a result of interactions between electrons and vibrating atoms in the material.

However, in other materials, such as the high-temperature (high-Tc) cuprate superconductors, the gap is not perfectly symmetric, but rather has distinct lobes described as “d-wave” symmetry. As BCS theory requires a symmetric gap, it therefore cannot be used to explain the behaviour of high-temperature superconductors. While there seems to be a strong relationship between gaps with unusual symmetry and high-Tc superconductivity, physicists are still trying to understand the pairing mechanism in such materials.

Symmetric or not?

To complicate matters even further, the new family of iron-arsenide-based high-Tc superconductors that physicists discovered last year do not appear to fit either the BCS or cuprate models. When the energy gaps of these materials are measured using the standard technique of angle-resolved photoemission spectroscopy (ARPES), the gaps appear to be symmetric — but BCS theory is unable to explain why they are superconductors at relatively high temperatures.

Some physicists have realized that it could be possible for a superconductor to have a gap that is perfectly symmetric in terms of its magnitude, but with a positive phase for some electrons and a negative phase for others. A gap with such “S±” symmetry could be related to iron arsenide’s high-Tc behaviour.

Now, Ray Osborn and colleagues at Argonne and Oak Ridge National Labs and Northwestern University in the US and the Rutherford Appleton Lab (RAL) in the UK have used inelastic neutron scattering to find the first experimental evidence for S± symmetry in an iron arsenide superconductor containing some barium and potassium (Ba_0.6K_0.4 Fe₂As₂).

Merlin reveals magnetic excitation

The team used the Merlin spectrometer at the ISIS neutron scattering facility at the Rutherford Appleton Laboratory in the UK (Nature 456 930). Pulses of neutrons are fired at the material and the energy and momentum distributions of the scattered neutrons are measured. The team found that the neutrons were causing a magnetic excitation in the material at a unique momentum and energy — which they believe can only occur if the energy gap has S± symmetry. According to Osborn, this symmetry had not been detected before because ARPES is not sensitive to the phase of the gap.

Osborn told physicsworld.com that the S± symmetry has “profound consequences for the nature of the pairing interaction”. Unlike the BCS interaction, which is attractive over short distances, Osborn says that S± symmetry implies that the pairing mechanism in their superconductor is repulsive at short distances and attractive at longer distances. “That’s important information for theorists”, he said, but cautioned: “it doesn’t say what the precise mechanism actually is”.

According to Igor Mazin at the Naval Research Lab in Washington, the discovery means that it is very likely that the mechanism causing superconductivity in iron-arsenide high–Tc materials is different from that in either the cuprates or the conventional BCS materials.

Mazin also pointed out that the S± symmetry also means that it is likely that magnetism is involved in the pairing interaction and that a better understanding of the iron arsenide materials will be the key to understanding their superconductivity.

Osborn and colleagues are already planning to do further neutron scattering experiments on single-crystal (rather than polycrystalline) samples, which should reveal more information about the S± gap.

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A new type of superfluidity?

'Academic squatter' banned from campus

Did an Englishman beat Galileo to the moon?