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Home » Page 1226

Scattered neutrons could mimic DAMA-LIBRA’s ‘dark matter’ modulation

For the last 16 years, researchers at the DAMA/LIBRA experiment in Italy have seen a controversial annual oscillation in the signal from their dark-matter detector. This type of variation would be seen if the Milky Way galaxy was wreathed in a “halo” of dark matter. But apart from the CoGENT dark-matter experiment in the US, no other dark-matter searches have seen a similar effect. Now, a physicist at Durham University in the UK has proposed an alternative source for the modulation in the form of neutrons, which are knocked out of atoms by muons and neutrinos scattering in the rock or shielding material around DAMA/LIBRA.

The most recent cosmic microwave background (CMB) data from the Planck mission reveal a universe that is composed of 26.8% dark matter and 68.3% dark energy, with less than 5% of “normal” visible matter, such as galaxies and gas clouds. Dark matter is thought to interact weakly with ordinary matter, making dark-matter particles extremely difficult to detect. The DAMA detector – located deep underground at the Gran Sasso National Laboratory – reported the first signs of dark matter in 1998, and further data over the years has cemented the result at a statistical significance at 9.3σ – well beyond the 5σ that usually signifies a discovery in particle physics.

Summer high

The modulation, which peaks in the month of May, is thought to come about as the solar system sweeps through a dark-matter halo enveloping the Milky Way. The peak occurs in the northern-hemisphere summer because the tangential velocity of the Earth as it orbits the Sun is in the same direction as the motion of the solar system at that time of the year. This means that the number of collisions detected by DAMA should be at a maximum in summer and then drop in the winter.

Now, however, Jonathan Davis of Durham University has developed a new model to explain the DAMA/LIBRA signal without invoking dark matter. Rather, he shows that neutrons scattering in the detector could easily produce an annual signal. The neutrons are released when solar neutrinos and atmospheric muons scatter in the shielding material or the rock that envelops the DAMA/LIBRA set-up.

Scattered signal

The muons come from cosmic rays decaying in the atmosphere and their rate varies across the year, peaking around 21 June. The solar neutrinos’ rate also varies annually, but instead they peak around 4 January. “When combined, this means that the neutrons from both of these sources also have a rate that varies annually but peaks somewhere in between the two and can match the DAMA phase that is in late May,” explains Davis. “There is an annual peak because the interference between the muons and neutrinos is not perfect. So, they don’t exactly cancel…the idea is that because both of the constituent signals peak at different times, when they add up there is some cancellation but this is not total,” he says, further explaining that it is the remnant signal after the cancellation that peaks around late May, just like the DAMA data.

Muon mimic

While the idea of muons mimicking the DAMA signal is not new, the timing of muons in isolation does not match the DAMA data and the idea was dismissed. But Davis’s model solves this problem by adding the effect of solar neutrinos. Davis told physicsworld.com that it is currently unclear how important a role the lead shielding surrounding the experiment plays in neutron production; however, it is likely to be significant. Lead is particularly good at producing neutrons from neutrinos and muons. “Indeed, the cross-section – which gives you the interaction rate – for neutron production from neutrinos and muons is particularly high. Also the lead shielding is very close to the DAMA detector,” he says. He also points out that neutrons produced in this way have a spectrum that tends to peak at low energy, similar to what one would expect from dark matter, meaning that “the signals can be easily confused”.

Davis acknowledges the fact that neutrinos – often referred to as the ghosts of matter – are known particularly for their ability to not interact with matter as they pass through it. But he says that as the DAMA detector is particularly sensitive to low-energy recoils, it will pick up the neutrons produced by these neutrinos. “Also, most other experiments, such as LUX, have more advanced shielding than DAMA, so they would be able to stop the neutrons before they get to the detector,” he says. He also states that other neutrino experiments do see the modulation he considers – he points to papers from the Borexino and SuperKamiokande experiments, which measure the modulation caused by neutrinos precisely. “However, these are directly down to neutrino scattering, not neutrons from neutrinos. The phase should be the same though,” he says.

Other experiments?

When it comes to the CoGeNT experiment, which also sees the same type of early modulation, Davis is intrigued. “In principle, the model would be the same, however, since CoGeNT is in a different lab to DAMA, the phase of the signal would be different. CoGeNT has had a lot of trouble recently with surface event backgrounds, so we will have to wait and see as it is not clear what it is seeing,” he cautions. Other dark-matter experiments, he says, have not seen the signal, probably thanks to a combination of shielding and thresholds. Because most of the more recent experiments employ more effective neutron shields than DAMA, the neutrons, which make up the DAMA signal, would not reach detectors such as CDMS, LUX or XENON100. Also, DAMA is more sensitive to low-energy recoils than most experiments, and so might be more susceptible to the muon/neutrino signal than other experiments.

To check for the accuracy of Davis’s model, the DAMA/LIBRA collaboration could study in more detail how the phase of its signal changes with the energy of the events. According to Davis, this has been studied before, and DAMA found that the phase does change with energy – something that you would not expect from standard dark matter but that is explained by his model. Also, with the increasing number of data that DAMA will collect in the coming years, the researchers will be able to look for “an additional mode with a period of 11 years, which would be expected if the signal is due to muons (it comes from solar activity), but not for dark matter” Davis says.

Davis is also keen to emphasize the importance of future dark-matter experiments – such as DM-Ice, KIMS, and ANAIS – which are looking to replicate DAMA. “My model gives them something they can test as a comparison with dark matter,” he says. “This is especially interesting for DM-Ice because it will be in Antarctica, so the muons will have the opposite phase.”

A preprint of the research is available on the arXiv server.

UPDATE: The paper has now been published in Physical Review Letters.

A century of general relativity

It doesn’t seem that long since “Einstein Year”, the worldwide celebration held in 2005 to commemorate the great physicist’s extraordinary scientific output a century earlier. 2015 will mark another important Einstein anniversary: the centenary of the presentation of his general theory of relativity.

Among physicists, this theory is regarded as Einstein’s greatest achievement, a towering scientific theory that remains unsurpassed in terms of its originality, elegance and predictive power. By replacing Newton’s force of gravity with a warping of space–time, Einstein transformed our view of space, time, force and gravitation, a revolution that continues to deliver astonishing insights into the world of the very large.

The general theory stands alongside quantum theory as one of the great pillars of 20th-century physics, but where quantum theory had a long and difficult birth, with multiple modifications and many “parents”, general relativity sprang from the mind of one man and has remained virtually unchanged ever since. Over the years, the theory has provided the framework for almost all of our knowledge of the universe, from the “Big Bang” model of the evolution of the universe to our understanding of black holes.

In The Perfect Theory, Pedro Ferreira provides a timely, expert and highly readable history of general relativity. An astrophysicist at the University of Oxford, Ferreira is renowned for his work on the problem of galaxy formation and his research into alternative theories of gravity. His biography of the general theory is affectionate and meticulous, although the narrative is that of a physicist rather than a mathematician or relativist. This approach is neatly summed up in the book’s excellent prologue, where Ferreira writes that “The reward for harnessing Albert Einstein’s general theory of relativity is nothing less than the key to understanding the universe, the origin of time, and the evolution of all the stars and galaxies in the cosmos.” At the same time, the book is firmly aimed at a public audience and is a welcome addition to popular books on the topic such as Jean Eisenstaedt’s The Curious History of Relativity or God’s Equation by Amir Aczel.

From a cosmologist’s point of view, the story of general relativity can be usefully divided into five distinct periods. The first era saw the formulation of the theory and its initial application to the universe as a whole, resulting in the “static” cosmic models of Einstein and Willem de Sitter. In the second epoch, time-varying models of the cosmos were proposed by Alexander Friedmann and Georges Lemaître; such models were further explored by Einstein, De Sitter, Lemaître, Howard Percy Robertson, Richard Tolman and Arthur Eddington in the wake of Edwin Hubble’s observations of the recession of the galaxies in 1929. Little theoretical progress was made in relativity during the third period (1940–1960), but this era did see the proposal of a hot, radiation-dominated infant universe by George Gamow, Ralph Alpher and Robert Herman, and the rise of a rival “steady-state” cosmology proposed by Fred Hoyle, Hermann Bondi and Tommy Gold. Next came the “golden decade” of 1963–1973, which saw the discovery of radio-galaxies, quasars, pulsars and the cosmic microwave background, and parallel progress in theoretical work on singularities. This period was followed by the modern era of precision measurements of the cosmic microwave background and the emergence of theories such as cosmic inflation and dark energy.

Ferreira covers each of these periods in an engaging, conversational way. He does not skimp on detail in most instances, yet the lively narrative holds the reader’s attention throughout. I particularly enjoyed the section on the renaissance of general relativity, from John Wheeler’s famous presentation “The issue of the final state” at the 1963 Texas Symposium on Relativistic Astrophysics to the furious efforts of the world’s top relativists at Cambridge, Princeton and Moscow to crack the problem of black holes in the 1970s. Another unusual section is the description of attempts by some theorists to reinstate the cosmological constant before the discovery of the universe’s accelerating expansion in 1998.

That said, there is some unevenness in the level of detail in the narrative, no doubt owing to considerations of length. For example, there is surprisingly little discussion about the plethora of dynamic cosmic models that were proposed in the early 1930s, almost no details are given of the pioneering work of the Gamow group in the 1940s and very little information is presented on modern measurements of the cosmic microwave background by the COBE, WMAP or Planck satellites. On the other hand, the author does present an intriguing chapter on alternative theories of gravity that have recently come to the fore, and expertly conveys the excitement of future experiments that “could confirm or refute the fundamental tenets of general relativity”.

With regard to audience, the book will be an enjoyable read for physicists in any field. Some physics teachers and students might be disappointed by the complete absence of equations and diagrams, and wonder what the mathematical machinery of general relativity looks like – in this respect, the story is less satisfying than the author’s earlier book The State of the Universe. Mathematically inclined readers might also be disappointed that there is very little description of the theoretical development of general relativity by key players such as Hermann Weyl and Cornelius Lanczos in the 1920s, or John Lighton Synge and William McCrea in the 1950s and 1960s. On the other hand, the book is very approachable for a lay audience, despite the level of historical detail.

Historians of science may notice some minor historical errors. For example, it is known from Einstein’s travel diaries that his conversion to the expanding universe was influenced by discussions with Tolman (not Hubble, as stated). It is also known that Einstein first formally embraced the expanding universe and banished the cosmological constant in the Friedmann–Einstein model of 1931 (not the Einstein–de Sitter model of 1932 as stated). The existence of a universal background radiation was first predicted by Alpher and Herman (not Gamow). Finally, it is not made clear that Alan Guth’s proposal of cosmic inflation addressed a theoretical puzzle concerning spatial flatness, rather than an observational problem. Useful historical notes are given for each chapter at the end of the book, but they are easy to miss because they are not flagged in the main text.

The above are minor criticisms. In this book, the story of general relativity is told with clarity and authority, and the narrative speeds along at a cracking pace. I particularly admired how the book opens with Eddington’s 1919 address to the Royal Society in which he announced the observation of a warping of space by our Sun – an experiment that was carried out on the island of Principe during an eclipse – and closes with a description of the author’s visit to the island 90 years later to lay a plaque in honour of that landmark experiment. All in all, this is a masterful, well written and timely addition to the literature on the greatest theory of them all.

  • 2014 Little, Brown/Houghton Mifflin Harcourt £20/$28hb 304pp

Portugal slashes funding for physics research

At least half of all Portugal’s scientific research units will receive only a limited amount of cash during the next five years from the country’s main funding agency, the Science and Technology Foundation (FCT). An evaluation process carried out by the agency in collaboration with the European Science Foundation (ESF) graded 322 proposals in science with six grades – “exceptional”, “excellent”, “very good”, “good”, “fair” or “poor”.

The process resulted in 71 out of 322 proposals being ranked “poor”, and those will receive no funding, while 83 were ranked as “good” or “fair”, and they will now get a maximum of €40,000 per year from 2015 to 2020 – for the majority this will be a substantial cut in funding. The remaining 52% were graded as being “exceptional”, “excellent” or “very good”, and they will now compete for a total of €50m in funding per year – about the same amount as in the previous evaluation process five years ago – in a second round of evaluation this autumn, that could, however, see more proposals downgraded.

Bibliometric evaluation

The FCT carries out evaluations of the country’s research every five years. While in the previous evaluation 16% of proposals were denied funding, this evaluation round was carried out for the first time in collaboration with the ESF, with the FCT also asking the publisher Elsevier to give bibliometric data about the researchers involved.

Physics in Portugal is being badly damaged
Carlos Fiolhais, Coimbra University

The results of the first round have been met with outrage by the Portuguese scientific community. “Physics in Portugal is being badly damaged,” says Carlos Fiolhais, a physicist at Coimbra University. “The government is trying to shut down very active physics research units.” In a statement, the Physics Society of Portugal also expressed concern, stating that “the majority of units in the centre and north of Portugal are going to be eliminated, or heavily constrained”.

Critics also point out the mismatch between the evaluation and the actual performance of the units. For example, the Center for Nuclear Physics and the Center of Physics and Technological Research, both based in Lisbon, have the highest numbers of papers and citations per researcher in physics in the country, yet they have not progressed to the second round.

“We were graded ‘excellent’ in the previous evaluation and our bibliometric indexes have improved since then, but still we have been graded ‘good’ now,” says Nuno Miguel Reis Peres, the director of the Center of Physics at the universities of Minho and Oporto. This now means that the institute’s cash from the FCT will fall from €380,000 to just €40,000 per year.

The FCT and the ESF have defended the quality of the evaluation. “The bibliometric output is only relevant to part of the evaluation. The strategic research plans proposed also had to be convincing to the panels,” Nicholas Walter, a senior science officer at the ESF who reviewed the FCT’s process, told physicsworld.com.

Prioritizing excellence

Indeed, the Portuguese government insists that there have been no budget cuts, with the exercise only a matter of prioritizing excellence. “I think it is a deliberate effort to redirect funding to areas that the FCT and the government feel are going to be competitive, and where innovation is likely to occur,” says biophysicist Alex Quintanilha, who is a member of the European commission’s Scientific Advisory Panel. “Social sciences, humanities and certain basic sciences are less important, in their view.”

While Quintanilha says that evaluation is necessary, he is concerned that the panellists evaluating the units were not experts in the same field. However, FCT spokesperson Ana Godinho maintains that each application was reviewed “by at least two area-specific experts” before the proposal was sent to the panels.

Rebuilding Tesla Tower

Artist’s impression of the revamped Tesla Tower. (Global Energy Transmission)

Two Russian physicists have turned to the fundraising website Indiegogo in the hope of raising a cool $800,000 to build a Tesla Tower.

Leonid and Sergey Plekhanov – graduates of Moscow Institute of Physics and Technology but now working in industry – want to reconstruct the famous Wardenclyffe Tower that was built by the inventor and engineer Nicola Tesla to find a commercial application for long-distance wireless energy transmission.

(more…)

Dark-matter searches get US government approval

Two key US federal funding agencies – the Department of Energy’s Office of High Energy Physics and the National Science Foundation’s Physics Division – have revealed the three “second generation” direct-detection dark-matter experiments that they will support. The agencies’ programme will include the Super Cryogenic Dark Matter Search-SNOLAB (SuperCDMS), the LUX-ZEPLIN (LZ) experiment and the next iteration of the Axion Dark Matter eXperiment (ADMX-Gen2).

“We are pleased to announce that the joint DOE/NSF second-generation programme will include the LZ and SuperCDMS-SNOLAB experiments with their collective sensitivity to both low- and high-mass WIMPS, and ADMX-Gen2 to search for axions. It will also include a programme of R&D to test and develop technologies for future experiments, consistent with the recent P5 recommendations,” says a joint statement from the two agencies. The P5 recommendations refer to the Particle Physics Project Prioritization Panel’s (P5) 2014 report, which was released in May this year. The P5 advisory panel considered what high-energy particle-physics experiments and collaborations the US government should fund across the next five years, in the light of declining spending on particle physics in the US.

Narrowing the field

Programme directors at both the DOE and the NSF have been looking into which of the many dark-matter-detector design bids they have received would give the best results. After narrowing it down to five experiments last year, the final three were chosen based on the advice of an external panel of experts.

Second-generation dark-matter experiments are experiments that will reach sensitivities that are at least 10 times better than current detectors can achieve. The ADMX-Gen2 will hunt for a dark-matter candidate particle known as an axion.

Both the LZ and the SuperCDMS will look for a type of dark-matter particle called WIMPs – weakly interacting massive particles – across a range of masses. SuperCDMS, which will be underground at SNOLAB in Ontario, Canada, will be particularly good at looking for light WIMPs with masses lower than 10 GeV.

The LZ is the union of the UK-based ZEPLIN programme, which has run three experiments at the Boulby mine during the past decade, and the current most sensitive dark-matter detector – the US-based LUX experiment. Like most of today’s dark-matter experiments, LUX is currently nestled deep in a subterranean cavern at the Sanford Underground Research Facility in South Dakota in the US, to shield it from any background sources – if all goes to plan, LZ will occupy the same space in a few years.

Unprecedented sensitivity

“While the SuperCDMS is wonderful at low masses, the LZ has unprecedented sensitivity across the scale,” says Chamkaur Ghag, from University College London, who was involved in ZEPLIN and now works on both LUX and LZ, and is delighted by the news of the US backing. “None of the competing experiments will be able to match it at that timescale,” he says.

Ghag told physicsworld.com that dark-matter searches must sweep from heavy to light particles because the masses of WIMPs are currently unknown. The technology developed for the current LUX detector was initially pioneered in the UK, “so it is wonderful to see the progress it has now made”, he says. Ghag explains that the UK also carried out a similar consolidation process last year, and the LZ detector was once more chosen to be the project of choice. He points out that the added US backing will further boost the already advanced design of LZ, saying that there is major UK involvement in the project. Three out of 10 main “work packages” that include building the cryostat that will hold the experiment, developing the internal “eyes” of the detectors or its photomultiplier tubes and screening all of the materials that will be used in constructing the detector, will be co-lead by researchers in the UK, who will also work on other parts of the project.

“This announcement by the US funding agencies DOE and NSF means that the LZ dark-matter experiment is on its way to becoming an approved project in the US. In the UK we are about to submit a proposal to the Science and Technology Facilities Council (STFC) for a three-year construction programme,” says Henrique Araujo, from Imperial College London, who is the principal investigator for LZ in the UK. “LZ uses extremely sensitive liquid-xenon detector technology to search for the very rare and extremely faint interactions of dark-matter particles – that we believe make up most of the mass of the universe.” With all three detectors gearing up to being up and running in the next few years, it could be only a matter of time before the dark-matter debate will be settled once and for all.

Hydrogel matrix makes superhydrophobic surface

A superhydrophobic thin film that can be coated onto virtually any substrate has been synthesized by an international team of researchers. The material, produced using a 3D nanotextured hydrogel matrix, is strong, very flexible and optically transparent. It might be used as a waterproof coating in applications such as self-cleaning windows, antifouling surfaces, and as a filter and sponge to separate oil from water after an industrial oil spill.

Superhydrophobic surfaces efficiently repel water in a phenomenon that is also known as the “lotus effect”. Now, a team led by Guihua Yu and Yi Shi from Nanjing University in China, along with colleagues at the University of Texas at Austin in the US, has made a new type of superhydrophobic surface comprising a 3D silica nanostructure replicated from a hydrogel template. The resulting hybrid coating consists of 3D interconnected nanofibres with uniform diameters of about 100 nm. Its morphology is like that of the lower surface of a lotus leaf, which contains micron-sized bumps that, in turn, are covered with nanoscale hair-like tubes. The nanofibres trap air under any water drops falling on them, creating a surface that repels water.

Stretched to their limit

The films produced by these inherently 3D nanotextured hydrogel templates remain superhydrophobic, even when stretched to their limit – and after more than 5000 stretching cycles at 100% strain. This is a first, because most superhydrophobic surfaces made to date lose their hydrophobic properties when exposed to a strain of more than 30%.

The films can be coated onto virtually any substrate, including metals, cement, wood, fabrics and plastics, thanks to their good wettability. They are also optically transparent (letting through 98% of light falling on them). They might come in handy as screen filters and sponges for separating oil from water, says Yu, because they can absorb up to 40 times their weight in oil.

The researchers made their superhydrophobic films using a polyaniline (PAni) hydrogel template. First, they mixed three precursor solutions together: an aqueous solution of oxidative initiator; an aqueous solution of aniline monomer and phytic acid; and tetraethoxysilane in isopropanol. The polyaniline hydrogel polymerizes and gels out fairly fast, and forms a 3D structure within three minutes.

Thanks to the high-acidic, high-water-content hydrogel matrix, the silica layer preferentially coats onto the PAni nanostructured template. Next, the silica layer is chemically modified, or “silanized” by depositing trichloro(octadecyl)silane onto the template to produce a superhydrophobic surface. The overall process is simple and can be scaled up to produce large amounts of superhydrophobic film, team member Lijia Pan told physicsworld.com.

The Texas–Nanjing researchers say that they are now looking at making super-oleophobic (oil-repelling) surfaces using the same hydrogel matrix template but a different version of their process.

The research is published in Nano Letters.

Molecular footballs, knotty headphones and a naming contest for exoplanets

Artist impression of an explanet
Is this exoplanet more of a Joanne than a Derek? You could soon be casting your vote to name 305 explanets. (Courtesy: IAU/M Kornmesser/N Risinger)

With the final two matches of the FIFA World Cup to look forward to this weekend, I thought I would sneak one more football-related story into the Red Folder. Over on the arXiv blog, there is a nice commentary about the topological nature of World Cup balls through the ages. Why? Well, two chemists in Taiwan have worked out a way to create a carbon-based molecule with the same shape as the football currently being used in the tournament in Brazil. Called the Brazuca, the ball is made from six panels that each have a four-leafed clover shape. Together, they form a structure with octahedral symmetry.

(more…)

Gluons get in on proton spin

For a quarter of a century, physicists have faced a paradox regarding the net spin of protons and neutrons – the spin of their constituent quarks accounts for only a small fraction of their overall spin. Now, new research carried out by physicists in Argentina and Germany who have analysed data produced by the Relativistic Heavy Ion Collider (RHIC), suggests that the missing spin might come from gluons that hold quarks together.

Misplaced spins?

Spin, an intrinsic angular momentum, is a property of both protons and neutrons (collectively known as nucleons). Until the 1980s, physicists had assumed that the spin-1/2 of both the neutron and the proton was simply the sum of the spin-1/2 of their three constituent quarks – with two quarks spinning in the opposite direction to the third. But a series of experiments found that the quark spins contributed only a small fraction to the nucleon spins, leading to what was known as the “spin crisis”. Those experiments involved firing spin-polarized beams of electrons or muons at targets containing spin-polarized nucleons. The idea was to compare the deflection of the particles in the beam when their spin axis was pointed in the same direction as the beam with those in the opposite direction. The results of these scattering experiments showed that no more than about 25% of nucleon spin comes from the constituent quarks, meaning that physicists could not determine where protons and neutrons get their net spin.

One possibility lay with gluons that hold quarks together and are exchanged by quarks in strong-force interactions. As the experiments studying quark spin cannot measure the properties of gluons, which do not interact electromagnetically, researchers turned to RHIC. Situated at the Brookhaven National Laboratory near New York, it collides two beams of protons – the gluon from one proton can interact with the quark in another via the strong force.

Gyrating gluons

In the latest work, a group of theorists – Daniel de Florian, from the Aires University in Argentina, and colleagues – analysed several years’ worth of collision data from RHIC’s STAR and PHENIX experiments. De Florian and colleagues have now studied data collected up until 2009, and have compared those data with a theoretical model they have developed that predicts the likely spin direction of gluons carrying a certain fraction of the momentum involved in the proton collisions.

The researchers discovered, in contrast to a null result they obtained using fewer data five years ago, that gluon spin does tend to line up with that of the protons, rather than against it. In fact, they estimate that gluons could supply as much as half of a proton’s spin. “This is the first evidence that suggests gluons could make a significant contribution to proton spin,” says team member Werner Vogelsang of Tübingen University in Germany, who adds that, on theoretical grounds, gluons ought to supply the same amount of spin to neutrons.

Dizzy orbits

Vogelsang cautions that he and his colleagues cannot be sure of their result because they have not yet analysed the possible spin contribution of gluons with low momenta. Doing so, he says, will require data from higher-energy collisions at RHIC, where proton energies have recently been increased from 100 to 250 GeV, and potentially from a new generation of very-high-energy electron–proton colliders. These advanced machines might also allow physicists to study another possible source of nucleon spin – the orbital, as opposed to spin, angular momentum of quarks and gluons – an analysis that requires the measurement of extremely rare collision outcomes.

Robert Jaffe of the Massachusetts Institute of Technology in the US praises De Florian and co-workers for their “fine work”, saying that their research is an “important step” in understanding what makes up a proton’s spin. He adds that it makes it even more important for physicists to understand why the three-quark model of the proton works so well in describing properties such as the magnetic moment and yet falls so far short in the case of spin.

The research is published in Physical Review Letters.

Plasmons excite hot carriers

The first complete theory of how plasmons produce “hot carriers” has been developed by researchers in the US. The new model could help make this process of producing carriers more efficient, which would be good news for enhancing solar-energy conversion in photovoltaic devices, making better photocatalysts and for applications like water splitting to produce hydrogen, to name but a few.

Plasmons are quantized collective oscillations of conduction electrons on the surface of metallic nanostructures that interact strongly with light. Such enhanced interaction allows them to concentrate light into subwavelength volumes, well below the diffraction limit of light. The phenomenon could be put to good use in a range of technologies, such as light detection and modulation, optical communications, photovoltaics and spectroscopy.

Surface plasmons only live for a short while, after which they either decay radiatively by emitting a photon or non-radiatively by generating electron–hole (charge carrier) pairs, explains team leader Peter Nordlander from Rice University. In the non-radiative case, hot charge carriers are produced. These carriers are electrons and holes that have been excited by photons with high energies.

Capturing hot-carrier energy

In bulk materials, hot carriers quickly cool in a matter of picoseconds, releasing phonons (vibrations of the crystal lattice, or heat). Indeed, such wasted heat can account for up to 50% of the energy losses in present-day solar cells. If the energy of hot carriers could be captured before it converts into wasted heat, solar-to-electric power-conversion efficiencies might be greatly increased.

Hot carriers can also induce chemical reactions – that would otherwise be too energetically demanding – in molecules near the surface of plasmonic nanostructures. Such reactions might help in water splitting, for example. Here, water is separated into oxygen and hydrogen using sunlight, which is a clean and renewable way to produce energy. They might also be used to transfer electrons into molecules or structures nearby – and so act as dopants.

Simple model

To fully exploit these carriers for such applications, researchers need to understand the physical processes behind plasmon-induced hot-carrier generation. Nordlander’s team has now developed a simple model that describes how plasmons produce hot carriers in spherical silver nanoparticles and nanoshells. The model describes the conduction electrons in the metal as free particles and then analyses how plasmons excite hot carriers using Fermi’s golden rule – a way to calculate how a quantum system transitions from one state into another following a perturbation.

The model allows the researchers to calculate how many hot carriers are produced as a function of the light frequency used to excite the metal, as well as the rate at which they are produced. The spectral profile obtained is, to all intents and purposes, the “plasmonic spectrum” of the material.

Particle size and hot-carrier lifetimes

“Our analyses reveal that particle size and hot-carrier lifetimes are central for determining both the production rate and the energies of the hot carriers,” says Nordlander. “Larger particles and shorter lifetimes produce more carriers with lower energies and smaller particles produce fewer carriers, but with higher energies.”

The team says that it has also succeeded in characterizing how efficient the hot-carrier generation process is, thanks to a figure of merit that measures how many high-energy carriers are produced per plasmon.

“Our results could help provide strategies for making the hot-carrier generation process more efficient,” says team member Alejandro Manjavacas. “Indeed, we are now busy developing another theory for how hot carriers are produced in transition-metal particles and a third one that describes how the hot carriers evolve over time.” Identifying the timescales involved in carrier decay will be another essential element for optimizing the carrier-generation process, he adds.

The results are published in ACS Nano.

Physics World tackles the valley of death

Many academics believe that they have an idea in them that could lead to a nifty new technology – and make them some cash in the process. But there is a world of difference between discussing an idea in the departmental common room and actually launching a new product to fit into an unexploited niche in the market. One of the biggest challenges that start-up companies face is known as the valley of death, which we have illustrated for you here with this quirky animation.

The voice you hear is that of Stan Reiss, who works for the international venture capitalist firm Matrix Partners. He explains how the valley of death is a metaphor for the financial challenges faced by a spin-off company in the early stages of its development. In this phase, the firm may have a prototype for a product but it might not have the income or the capital to comfortably survive and grow. Often, the company simply runs out of money and falls by the wayside. “There’s a lot of dead bones and skeletons at the end of that valley,” says Reiss.

This video is part of a series we produced following a recent visit to the Boston area of the US, which is a hotbed of academic spin-offs thanks to a glut of world-leading universities and numerous sources of investment. We published a video profile of a company called MC10, which is developing flexible-electronics products that can conform to clothing and skin. We also featured the lab of Joanna Aizenberg at Harvard University, which is creating new materials inspired by biological materials and processes.

We also published a couple of video interviews with professionals involved in the financial aspects of developing spin-off companies. There is an in-depth interview with Stan Reiss, who explains what venture capitalists do on a day-to-day basis. Reiss also discusses the types of thing he is looking for when deciding whether to invest in a science-based spin-off. Then there is an interview with Leon Sandler, who works at the MIT Deshpande Center for Technological Innovation. Sandler explains how the centre was founded at the Massachusetts Institute of Technology (MIT) in order to support the commercialization of technology developed at the university. He talks about the types of innovation the centre nurtures and the different forms of support that it can provide.

If these videos have piqued your interest in the art of commercializing physics, then there will be plenty more for you to feast upon in the near future. In November we will be publishing a special issue of Physics World exploring the different themes involved in the process of taking physics research from the lab to the marketplace. You can also find out more about how research is commercialized, specifically in the materials-science field, via the TMR+ blog. It is published by IOP Publishing, which also publishes Physics World.

You never know, once you have ingested all these great stories, you may feel sufficiently nourished to have a go at tackling the dreaded valley of death yourself. If you do, then good luck with your journey. Just don’t forget to pack the November issue of Physics World!

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