By Hamish Johnston
If you can’t get enough of Richard Feynman, the BBC has released the second part of its television tribute to the late Nobel laureate.
The fantastic Mr Feynman
Solar ‘sandwich’ could cover a variety of surfaces
Ultrathin and flexible solar cells could be one step closer thanks to an international team of researchers that has made photovoltaics from 2D crystals called semiconducting transition-metal dichalcogenides (TMDCs). These devices could someday coat any surface exposed to sunlight to produce electricity – something that has proved difficult to do with current solar-cell technologies, which are thick, heavy and brittle.
TMDCs are layered materials with each layer comprising three atomic planes. The outer two planes are triangular lattices of atoms from the chalcogen group of elements – sulphur, selenium, tellurium – that sandwich an inner triangular lattice of transition-metal atoms. TDMCs show great promise for solar-cell applications because electrons within the material interact exceptionally strongly with light. This means that even though these materials are just a few atoms thick, a large portion of the absorbed photons can be used to produce electric current.
“If this technology is developed further, it has a chance of becoming a game-changer in the area of solar energy,” says team member Antonio Helio Castro Neto of the National University of Singapore. The group also included Andre Geim and Kostya S Novoselov from the University of Manchester in the UK and researchers in Portugal, South Korea and Germany.
“Hi-tech sandwich”
Castro Neto and colleagues made a heterostructure, or “hi-tech sandwich” as they call it, containing atomically thin materials, each with a well-defined role. The first component, boron nitride, is a one-atom-thick transparent insulator and it encapsulates the entire ensemble. As such, it is described as the “bread” in the sandwich. Next is graphene, a 2D sheet of conducting carbon that plays the role of the “lettuce”. It is used to collect the electrons produced by the TMDCs (which are the “meat” in the sandwich).
“We also employed gold nanoparticles, which you can think of as the ‘pepper’,” explains Castro Neto. “Although not strictly needed, these particles do ‘spice’ things up by increasing the amount of light absorbed by the structure thanks to a phenomenon called plasmonics.”
The various materials in the sandwich structure all have different electronic properties. “Separately, they are not particularly good for photovoltaic applications but put them all together in a certain combination and you get a very ‘juicy’ photovoltaic device,” says Castro Neto.
As a case in point, the researchers succeeded in fabricating extremely efficient flexible devices with a photoresponsivity above 0.1 A/W, which is equivalent to an external quantum efficiency of above 30%. The latter means that more than 30% of the light that falls on the cell is converted to an electric current – and it is an important parameter that determines the overall efficiency of how a device converts solar energy to electrical energy.
Extremely sensitive electrons
According to the team, it is so-called Van Hove singularities in the TMDCs that allow for enhanced light–matter interactions. These are named after the Belgian physicist Léon van Hove, who in 1953 discovered that electrons travelling freely through certain crystals could come to a standstill at specific wave frequencies and wavelengths. The electrons essentially “freeze” and their speed reduces to zero. “In this standstill state, the electrons become extremely sensitive to any kind of external stimulus,” says Castro Neto. This results in improved photon absorption and more electron–hole pairs (or excitons) being created in the device. These excitons are responsible for producing electricity – when the electrons and holes separate.
“When light with the right frequency then hits electrons in this unique Van Hove state, they respond massively. This is what we observed in our experiments and we exploited the singularity to boost light absorption and create electric current,” says Castro Neto.
The researchers say that they are now busy looking for materials with stronger Van Hove singularities so that they can increase light absorption even further and improve overall quantum efficiencies. “We are also keen to produce such materials artificially,” reveals Castro Neto. “At the moment, we extract the atomically thin layers from 3D crystals, but we know that there are ways to grow 2D layers artificially and this will be fundamental for technological applications.”
“This is a field that is very much in its infancy,” he adds. “If the graphene field is young – less than 10 years – then this new one is even younger. There is much to do and explore,” he says.
The research is described in Science.
Why humour matters: physics’ tradition of thoughtful joke-telling
If you only laugh at jokes, then you’re not taking them seriously enough. Jokes have functions. “I don’t suffer from insanity – I enjoy every minute of it!” (That’s releasing stress.) “Mixed emotions is watching your mother-in-law drive off the cliff in your new Mercedes.” (That’s releasing feelings.) “Schrödinger’s cat walks into a bar – and doesn’t.” (That’s dumbing down a complex image.)
I’m not dissing such humour. Joke-telling helps us to cope. If we can laugh at something, we don’t have to feel, engage or understand it. Physics, however, has a tradition of thoughtful joke-telling that uses humorous tales inquisitively – to deepen appreciation of a person, of the world, or of both.
Jokes revealing personal characteristics have covered everything from Albert Einstein’s childlike personality and Niels Bohr’s mystique to Wolfgang Pauli’s famously brutal putdowns. Abraham Pais, for example, heard Paul Dirac enthusiastically tell (on several different occasions) a joke about a priest who is newly appointed to serve a village and is doing the rounds to get to know his parishioners. Calling on one modest home, the priest notices that the woman’s house is full of children, and asks how many children she’s got. “10 – five pairs of twins”, comes the reply. “You mean you always had twins?” asks the astonished priest. “No, Father, sometimes we had nothing.” As Pais put it: “Precision at that level had an immense appeal to Dirac.”
Too serious not to laugh
Among the jokes that reveal aspects of the world is what – or so I’m told – was the favourite piece of humour of the mathematical physicist John von Neumann. It takes place in the main square of Budapest, where a man cries out “The emperor is an idiot! The emperor is an idiot!” He is promptly arrested and carried to prison by two police officers. As they drag him away, the man begins to defend himself: “This is a mistake! I didn’t mean our emperor, I meant the Prussian emperor!” The police officers are having none of it. As the jail door clinks shut they go: “You can’t fool us! We know which emperor is the idiot!”
As for jokes that reveal both character and situation, I would include the one about the rabbinical student who goes to hear a series of three speeches by a famous and revered rabbi. When his friends ask the student what he thought about the speeches, he replies: “The first talk was brilliant, clear and simple. I understood every word. The second was even better, deep and subtle. I didn’t understand much, but the rabbi understood all of it. The third was by far the finest, a great and unforgettable experience. I understood nothing and the rabbi didn’t understand much either.” Okay it might not be a side-splitter – but according to Pais it was one of Bohr’s favourites.
Indeed, humour was never far away at Bohr’s own Institute for Theoretical Physics in Copenhagen, as Paul Halpern – a physicist from the University of the Sciences in Philadelphia – pointed out last year (Physics in Perspective 14 279). The institute hosted, for example, annual skits – the best known of which was the Blegdamsvej Faust, named after the street in front of the institute. This play was mentioned in George Gamow’s book Thirty Years That Shook Physics and was the subject of Gino Segrè’s 2007 book Faust in Copenhagen (November 2007 pp44–45).
In his article, Halpern also discusses the lesser-known Journal of Jocular Physics, compiled for Bohr’s 50th, 60th and 70th birthdays. A recurrent topic in its three volumes is complementarity – Bohr’s name for one of the most mysterious aspects of quantum behaviour – which, Halpern argues, often shares with humour itself “absurd aspects of contradictions”.
In trying to explain why members of the Copenhagen institute liked humour, Halpern says it was because jokes quite simply “spiced [up] breaks from calculation”. But are there deeper reasons? As Pais, quoting Bohr, once put it, “Some subjects are so serious that one can only joke about them.” (Having said that, I am not sure what Bohr would have said after the 2005 controversy surrounding publication, by the Danish newspaper Jyllands-Posten, of cartoons mocking the prophet Muhammad.)
The critical point
Jokes today tend to be short, visual and readily digestible. Type “science jokes” into Google and you’re apt to find witticisms like the one about two pieces of coal (clearly a mother and child) and a diamond, with a speech bubble from the mother saying: “Your dad’s been under a lot of pressure lately!”. That kind of joke will get a sure-fire laugh, but it passes in and out of your mind quickly without altering anything.
Once upon a time, however, science jokes were more discursive and more interesting – even in publications. In Pais’s 1988 book Inward Bound, he mentions a “Note on the quantum theory of absolute zero”, which is a paragraph-long parody of numerological attempts to derive the fine-structure constant (1/137) from the temperature of absolute zero (–273 °C). It was concocted in 1931 by three postdocs at the Cavendish Laboratory, Cambridge (Guido Beck, Hans Bethe and Wolfgang Riezler), who managed to sneak it past the editors of Naturwissenschaften (19 37); the editors were not amused when they eventually found out and published a correction.
Pais called this story “arguably the best physics joke ever to slip by an editor of a first-rate physics journal”. Really? Still the best in a physics journal? (Alan Sokal’s 1996 parody “Transgressing the boundaries: towards a transformative hermeneutics of quantum gravity” was good, but it targeted a social-science – not a science – journal.) For that matter, why are the best-known examples of thoughtful humour from the era of quantum mechanics, and not from the age of string theory, dark matter and M-branes? Has big science killed deep jokes?
Please send me your suggestions of more recent humorous stories – peer-reviewed or not – to the e-mail below and I will discuss them in a future column. (But no ha-ha jokes please!)
Getting to the bottom of foamy physics
Researchers in the US have created a new mathematical model to describe the complex evolution of foamy bubbles – something that has proved fiendishly difficult to model thanks to the hugely varying length and time scales involved. Their computed results closely match theoretical models as well as lab-based observations of foamy bubbles. The team hopes the underlying equations could have a variety of applications, including helping to make better metal and plastic foams, developing lightweight crash-absorbent materials and also to model a number of biological processes such as the growth of cell clusters.
Heady maths
Foams are all around us: from the froth on a cappuccino or beer to the soapy suds in a bubble bath. However, scientists have found it difficult to describe exactly how such clusters of bubbles coalesce, grow and change shape over time – before they ultimately go pop. An early attempt at understanding the structure of soapy foams is encapsulated in “Plateau’s laws” – formulated by 19th-century Belgian physicist Joseph Plateau. Then Lord Kelvin developed his theory of an “ideal foam” of equal-sized bubbles in 1887, an accurate version of which was finally made in the lab in 2012 by a team at Trinity College, Dublin. But a more general set of equations describing bubbles on varying length and time scales remained elusive, until now. The challenge is to create mathematical models that describe how interfaces between bubbles move and how they “meet” in complicated phases.
Key phases
Now, James Sethian and Robert Saye of the University of California, Berkeley have separated the various processes that determine a foam’s evolution according to the different length and time scales at which they occur – and have created a model for bulk foam dynamics. The researchers say that the model accurately describes how fluid moves within a bubble and how the individual cells form and how their junctions (or borders) are rearranged as individual bubbles within the foam burst.
To do this, Sethian and Saye identified three distinct regimes or phases of foam evolution. “We identified and separated the three phases – the drainage of liquid from a bubble’s membrane, the rupture of the drained bubble and the macroscopic rearrangement of the bubbles within the foam – to simulate the system,” explains Sethian.
The first set of equations describes how the liquid drains from a bubble wall, thanks to gravity, so that the wall eventually becomes so thin that it ruptures. The next set of equations explains the liquid flow at the junctions between bubble membranes; while the third set considers how the entire foam rearranges to move closer to equilibrium, a motion that happens on a macroscopic scale.
Beach bubbles
Sethian and Saye tested their formulae on bubble clusters of different sizes and found that they could accurately predict the interactions of gases and liquids in these foamy materials. They also developed a fourth set of equations that allowed them to simulate a movie that shows how light would reflect off a small foam sample as its bubbles rearrange. The researchers picked a beach scene for the simulation, so that they could “visualize and see how well the model captures what you would see in real life, while still accurately showing how the light would reflect”, as Sethian explains.
These processes are all influenced by a variety of factors, including viscosity, surface tension, gravity and other terms of fluid dynamics. Some of these factors can be modified in the current model, but others, such as evaporation, that are currently not included can be added quite easily, according to the researchers.
Sethian points out that it took the team five days to solve the full set of equations of motion using a supercomputer to get the most refined solution of the algorithms. He says that the entire mathematical formulation and codes will be available to anyone who is interested in running similar simulations at whatever scales they wish, for any applications, including industrial ones.
While a large part of the aim of this work was to develop a fundamental model, the researchers claim that it could have other applications. When it comes to biological modelling, Sethian says the equations could help to understand highly complex systems, such as cell cluster growth, that may go from being organized to unorganized systems. According to him, the models might help “to better understand how cells group together and aggregate…and to study the kind of physical forces involved – such as adhesion between cell boundaries, fluid dynamics, etc – as well as the mechanisms involved in how cell cluster grow from clusters of 5 to 10 cells to those of hundreds to thousands of cells”.
Take a look at the video below of a collapsing soap-bubble cluster, shown with thin-film interference and computed using Sethian and Saye’s multiscale model.
The research is published in Science.
Research galore in Singapore
By James Dacey
Despite its modest size, the city-state of Singapore is clearly an ambitious nation, boasting a leading financial centre and one of the world’s busiest ports. During a recent visit to Boston I met a man called Lim Tze Min who works for a government agency called Contact Singapore, which exists to try and attract skilled people to live and work in Singapore. I wanted to know why a physicist might consider relocating to the country. Listen to our conversation here.
Tze Min talks about research facilities including the Centre for Quantum Technologies (CQT), the founding director of which is the Polish-born physicist Artur Ekert, who is also affiliated with the University of Oxford in the UK. According to Tze Min, one of the major bonuses of being a researcher in Singapore is the small amount of bureaucracy invovled, which allows scientists to get on with just doing the science. Give it a listen and decide for yourself whether it sounds like a place where you could imagine yourself working.
Hawking’s academic boycott divides opinion
By James Dacey
Do you agree with the principle of academic boycotts?
Yes
No
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This question has arisen after it was revealed yesterday that Stephen Hawking will be boycotting a prominent conference in Jerusalem in protest against the policies of the Israeli government. The British cosmologist and science communicator had been set to talk at the Israeli Presidential Conference: Facing Tomorrow, which will take place in June and which will feature a string of high-profile speakers, including Bill Clinton and Tony Blair. But the University of Cambridge has confirmed that Hawking, who is director of research at its Centre for Theoretical Cosmology, has pulled out of the conference for political reasons. Physics World has spoken to a university spokesperson who confirmed that Hawking has sent a letter to the conference organizers to explain his decision not to take part. The UK newspaper the Guardian has today published what it says is the full text of this letter, dated 3 May.
“I accepted the invitation to the Presidential Conference with the intention that this would not only allow me to express my opinion on the prospects for a peace settlement but also because it would allow me to lecture on the West Bank. However, I have received a number of e-mails from Palestinian academics. They are unanimous that I should respect the boycott. In view of this, I must withdraw from the conference. Had I attended, I would have stated my opinion that the policy of the present Israeli government is likely to lead to disaster.”
Cold-atom random laser simulates stellar clouds
Physicists in France are the first to create a random laser in a cloud of cold atoms under laboratory conditions. The effect was first seen decades ago in stellar clouds and the team believes that its set-up could provide basic insights into the conditions necessary for random lasing. This could improve our understanding of astrophysics and even lead to practical applications of the phenomenon here on Earth.
A conventional laser usually comprises a gain medium (solid, liquid or gas) that is sandwiched between two mirrors. Light bounces back and forth many times in this optical cavity, stimulating the emission of more light and creating a coherent field of light. In a random laser there are no mirrors and the light simply bounces around between particles located at random positions in the gain medium. This light can stimulate the emission of light from the medium just as in a conventional laser. However, because of the random paths taken by the light, a laser beam is not produced. Instead, coherent light is emitted in all directions.
Random lasing was first proposed in the 1960s to explain why certain specific emission lines in some clouds of stellar gas are more intense than theoretically predicted. On Earth, random lasers have been made using liquid suspensions and solid powders. In these lasers, the light-scattering particles are classical objects such as grains of zinc oxide – whereas astronomers believe that atoms do the scattering in stellar-gas lasers.
Forbidden transition
Robin Kaiser and colleagues at the CNRS Non-linear Institute of Nice made their laser with a cloud of cooled rubidium-85 atoms confined to a magneto-optical trap. They used a pump laser to create a population inversion between two hyperfine levels of the same electronic orbital and a second, tunable laser to stimulate emission back to the lower level. The key to creating a random laser is to ensure that some of this emitted light will be scattered by the atoms, rather than being reabsorbed by them. This was done by adjusting the wavelength of the tunable laser such that the emitted light corresponded to that of a forbidden transition in the atom. The researchers found that, when the laser light had exactly the same frequency as the forbidden transition, there was a bump in the intensity of the laser output – a sign that it was being boosted by random lasing.
This is the first demonstration of a random laser in the laboratory in which photons are scattered by atoms, as happens in stellar gases. Whereas the effect of radiation pressure on a grain of zinc oxide is insignificant, the scattering of a photon will cause measurable recoil in an atom. Whether or not this and other effects of random lasing have macroscopic effects in astrophysics remains an open question, however. “It’s possible in principle that, if you put more stimulated emission into the radiation, the sign of the radiation pressure could be inverted,” Kaiser explains, “so we could have attractive components of the radiation pressure.” He points to the team’s observation of an oscillating cloud of cold atoms bears similarities to Cepheid variables, an oscillating equilibrium between gravity and radiation pressure. So signs of negative radiation pressure should be accessible to experimental observations.
Kaiser hopes that such ideas might be testable in the laboratory with a gaseous random laser. He points out, however, that significant differences between the team’s laboratory device and a stellar-gas laser remain. The atoms in their magneto-optical trap are cooled to about 50 µK, whereas the atoms in a stellar gas are hot. The team pumped the random laser with another laser, while a stellar gas is pumped by broad spectrum light from a star. Kaiser hopes to optimize the team’s system to resemble more closely the conditions in a stellar environment. “I want to interact more with people from astrophysics to find out which schemes are more or less realistic,” he says.
Quantum effects
Diederik Wiersma, an expert on disordered photonics at the University of Florence, is impressed by the work. He believes that, in addition to radiation pressure, it may be possible to study other phenomena, such as the effect of photon scattering from several atoms in an entangled state and quantum interference between atoms and photons. “You might have access to knowledge about which path the photon took,” he explains, “and if you have that, quantum mechanics tells you it will not behave as a wave anymore.”
Further into the future, Wiersma suggests that the system may aid understanding of quantum computing. “There have been proposals about a possible quantum Internet,” he says, “where you could use optical connections to connect matter at different locations in a single quantum state.”
The research is published in Nature Physics.
What is quantum gravity?
In less than 100 seconds, Leron Borsten explains that general relativity and quantum mechanics are very successful in their own domains, but the jury is still out on how to unify these two great theories of physics.
Watch more from our 100 Second Science video series.
Nuclear physics goes pear-shaped
An international team of physicists has found the best evidence yet that some heavy nuclei are not spherical or ellipsoidal – but “pear-shaped”. The researchers found clear signs of this lopsidedness in two particular nuclei – radon-220 and radium-224 – that were created by smashing protons into a uranium carbide target at the REX-ISOLDE facility at CERN. As well as providing new information about the forces that bind protons and neutrons together in nuclei, the discovery could also help shed light on physics beyond the Standard Model of particle physics.
Physicists have known for decades that nuclei can occur in different shapes beyond simple spheres. In most cases, these non-spherical nuclei look either like a rugby or American football, or like a discus – both these shapes having axial and reflection symmetries. Any departure from a sphere is usually described by a “quadrupole deformation”, driven by underlying “quadrupole” interactions between the nucleons (protons and neutrons) in the nucleus. However, physicists have also suspected that there are even more subtle “octupole” interactions between nucleons, which would be expected to cause some nuclei to be pear-shaped – or even resemble a pyramid.
Although there was some limited evidence for pear-shaped nuclei in experiments carried out on radium-226 and neodynium-148 in the 1990s, neither study was conclusive. What Peter Butler of the University of Liverpool and colleagues in Belgium, Finland, Germany, Poland, Spain, Switzerland, the UK and the US have now done is to find strong evidence for octupole transitions in radon-220 and radium-224. These transitions are a sign that the nuclei are lopsided and appear in the spectrum of gamma rays these nuclei emit as they decay from an excited state.
Smoking gun
After the radon and radium nuclei were created at REX-ISOLDE’s uranium-carbide target, they were then accelerated and passed through thin targets of nickel, tin and cadmium. As they travel through these targets, some radon and radium nuclei come close to a target nucleus such that its intense electric field excites the radon or radium – an experimental technique called “coulomb excitation”. The nuclei were then fired through CERN’s MINIBALL detector, which can detect gamma rays emitted in all directions from the nuclei.
By looking at the energy and spatial distributions of the gamma rays, the team could conclude that both nuclei are pear-shaped. In particular, the team found that a certain octupole transition was much more common than expected if the nuclei were not pear-shaped – indeed, in the case of radium it was about 30 times stronger than expected. “This ‘enhanced’ [transition] could be considered a smoking gun for a static octupole deformation of the nuclear matter in such nuclei,” says Paddy Regan of the University of Surrey, who was not involved in the research.
Enhanced electric dipoles
According to Butler, the new results provide some insight into the efficacy of several models that attempt to describe the structure of large nuclei such as radon and radium – something that continues to be very hard to achieve. But beyond nuclear physics, the study of pear-shaped nuclei could also shed light on why there is far more matter than antimatter in the universe. This is because atoms that have a pear-shaped nucleus are expected to be a good place to look for violation of time (T) and charge–parity (CP) symmetries beyond that allowed by the Standard Model.
These symmetries forbid an atom from having a permanent electric-dipole moment (EDM). So far, physicists have not found evidence for an atom with an EDM but Butler reckons that the experimental signature of EDM would be enhanced by a factor of about 1000 in an atom with a pear-shaped nucleus. Measuring CP and T violation beyond the Standard Model could explain why most of the antimatter created in the Big Bang has since vanished – and why the universe is dominated by matter.
Once the accelerators are up and running again at CERN in 2015, the team plans to study other nuclei that are expected to be pear-shaped.
The study is described in Nature.
What is cosmic inflation?
In less than 100 seconds, Andrew Jaffe explains why cosmologists believe that the universe underwent a period of vast and rapid growth when it was just fractions of a second old.
Watch more from our 100 Second Science video series.
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