Skip to main content

Reset your password

Please enter the e-mail address you used to register to reset your password

Note: The verification e-mail to change your password should arrive immediately. However, in some cases it takes longer. Don't forget to check your spam folder.

If you haven't received the e-mail in 24 hours, please contact customerservices@ioppublishing.org

Registration complete

Thank you for registering with Physics World
If you'd like to change your details at any time, please visit My account

Close
Home » Page 999

Graphene bubbles measure shear forces

The force needed to slide sheets of graphene across each other has been measured using a new technique that involves blowing air bubbles made of the material. Developed by Zhong Zhang of the National Center for Nanoscience and Technology in Beijing and colleagues in China and the US, the technique was also used to measure the force needed to slide graphene across a surface of silicon dioxide.

Graphene is a layer of carbon just one atom thick that has a wide range of potentially useful electronic and mechanical properties. Developing practical graphene-based devices will require an understanding of how well graphene layers stick to each other, and also how they stick to popular substrates such as silicon dioxide. This stickiness is expressed as the shear resistance – the minimum force required to slide one layer over another. This quantity is not well known for graphene because it is extremely difficult to measure for a material just one atom thick.

Tiny bubbles

Now, Zhang and colleagues have adapted a technique called the blister test for use on graphene. One of their measurements involves a single layer of graphene on a silicon-dioxide substrate with micron-sized holes in it. The air pressure in the holes is increased, causing the graphene sheet to form tiny bubbles in the regions above the holes. The size of each bubble is monitored using atomic force microscopy (see figure). As the bubble pushes up, the surrounding graphene on the substrate is pulled and stretched towards the hole – creating a zone where shear occurs. Raman spectroscopy is used to measure the stretching of graphene’s carbon bonds in this “shear zone” as the size of the bubble increases. A similar measurement is made on two layers of graphene on the substrate, which has an additional graphene-on-graphene shear zone that can be analysed.

Small resistance

The team found that the shear resistance between layers of graphene was relatively small at 40 kPa. The value for graphene on silicon dioxide was about 40 times greater at 1.64 MPa. Writing in Physical Review Letters, Zhang and colleagues say that their technique could be used to study the interfaces between other 2D materials.

Proton tomography offers better preparation for therapy

Physicists in the US have shown that protons themselves can be used to provide the complex 3D images essential for tailoring proton therapy to individual patients. Researchers in the US built a prototype detector to carry out “proton computed tomography” (proton CT), and found that in around 6 min it could generate maps of proton stopping power – energy lost per unit distance in a material – that were more accurate and required a much lower radiation dose than existing techniques.

Proton therapy is becoming an increasingly popular tool to treat cancer because it can be used to target tumours very precisely. Unlike most other types of radiation, protons deposit a large fraction of their energy at the point where they stop in the body. By tuning the beam so that protons stop where the tumour is located, therapy can be made more effective and safer.

To deliver the radiation to a particular point in a person’s body, medical physicists must first establish the extent to which protons will be slowed down by intermediate layers of tissue. This is currently done by placing the patient in an X-ray CT scanner and creating a 3D plot of X-ray absorption within the relevant part of their body. But the accuracy of this technique is limited by a mismatch between X-ray absorption and proton energy loss within particular types of tissue.

Direct measurement

Proton CT would overcome this problem by measuring proton energy loss directly. Working scanners would employ the same source of protons used to carry out the therapy, but would require a different set-up – a higher-energy but lower-intensity beam and detectors that could be moved relative to the patient to get a 360° view. And as with X-ray CT, the procedure would be carried out ahead of treatment.

The idea of proton CT has been around ever since computer tomography was first proposed in the early 1960s. But the need for a proton beam makes it more expensive than X-ray CT and it also requires a more complex detector as well as more intense computation to transform measurements into an image, according to Robert Johnson, a particle physicist at the University of California, Santa Cruz.

In the latest work, Johnson and colleagues at Santa Cruz, Loma Linda University, Northern Illinois University, University of Wollongong and Baylor University set out to build a prototype proton CT scanner that was large enough to image a fake (phantom) human head and quick enough to be usable clinically. The system uses a couple of spare silicon detectors from NASA’s Fermi Gamma-ray Space Telescope, which Johnson had previously worked on. The idea was to place these detectors in the path of a proton beam, with one positioned in front of the head in question and the other behind, to plot the trajectory of the protons through the head.

Specific trajectories

By subtracting the known energy of the particle beam from the energy dumped in a series of plastic scintillators placed behind the second silicon detector, the researchers could work out how much energy a head absorbs along specific trajectories. By then rotating the head very slightly and repeating the process, they could build up a detailed 3D image of the head’s proton stopping power.

The team tested its device in the proton beam at the Northwestern Medicine Chicago Proton Center using phantom heads made from various materials with different stopping powers. Recording the trajectory and energy of more than a million protons a second using very fast electronics, within about six minutes they were able to generate an image with stopping powers generally within 1% of their known values.

Johnson says he does not have precise figures for the accuracy of proton stopping powers obtained using X-ray CT. But he thinks that proton CT is probably better suited to certain kinds of tissue. “If the tissues are all soft and fairly uniform, then X-ray CT will likely be just as accurate as proton CT,” he says. “If there are layers of dense material, then I think we can do better, but that is what we are currently trying to prove.”

Lower doses

The researchers also found they could significantly reduce the dose needed to do the scanning. Whereas a typical X-ray scan of the head requires between 30–50 mGy, they needed just 1.4 mGy using protons.

Johnson and collaborators are now applying for grants to develop a more compact detector with even faster electronics, to reduce the necessary exposure time to about a minute. Six minutes, he reckons, “isn’t bad”, but is not ideal. Having been in an MRI scanner for longer than six minutes, he says the duration “is doable but not the most pleasant thing”.

The American group is one of several around the world that are developing proton CT, with others located in Italy, Japan and the UK. Leader of the British group, Nigel Allinson of the University of Lincoln, says that Johnson and colleagues “certainly set a standard for others to follow”, but notes that technical success will need to be followed by clinical trials as well as assessments of cost and the extent to which the technique “fits with radiotherapy workflows”.

“Still a long way”

Likewise, Mohammad Naimuddin of the University of Delhi cautions that “there is still a long way” before proton CT can be used clinically. Naimuddin, who has collaborated with some members of Johnson’s group, says that the latest work represents a “significant improvement compared to past efforts”, but argues that better modelling of proton scattering off tissue is needed to improve the technique’s resolution. “In the current paper the spatial resolution of the phantom image is still not comparable to X-ray CT,” he says.

The research is reported on the arXiv server.

Music and science: a harmonious or discordant duo?

Featured in the podcast is UK recording artist Hannah Peel, along with a track from her 2016 album Awake But Always Dreaming. Peel talks about how that record was inspired by witnessing her grandma’s struggle with dementia and how music helped the pair to communicate when memory began to fail. Peel says her forthcoming album, Mary Casio: Journey to Cassiopeia, is a journey from the mind into space, influenced by a visit to an Alzheimer’s research lab at University College London.

Glester recorded the podcast at the Cheltenham Science Festival 2017, where he met academics with a variety of interests and opinions about music. On the one hand, polymath Raymond Tallis believes that scientific data about people’s physical response to music is of limited value to understanding why we appreciate music. On the other hand, physicist and “rock doctor” Mark Lewney speaks about the useful role acoustics can play a role in designing guitars with the tonal properties desired by musicians. Researcher and science popularizer Alice Roberts speaks about the possible evolutionary functions of music, while violinist Jenny Glester speaks about her experiences taking music into healthcare settings.

Snapping the Milky Way, art inspired by SLAC blueprints, doppelganger magazine covers

Top tip: use a remote shutter control. (Courtesy: Clifton Cameras)

By Hamish Johnston

If you are lucky enough to live somewhere with dark skies, you know that the Milky Way is a truly majestic sight. But how exactly would you go about capturing its magnificence with a camera? UK-based Clifton Cameras has put together an infographic with a few helpful hints. The image above is an excerpt and you can view the entire infographic here.

(more…)

Formation of other-worldly ‘hot ice’ observed for first time

The transformation of water freezing into an exotic type of ice has been directly observed for the first time by researchers in the US.

Most of the ice on Earth has a hexagonal crystal structure, but water can transform into more than 15 types of ice, each with a different molecular arrangement. These rare frozen phases require non-atmospheric pressures and controlled temperature environments to form, so can only be produced on Earth in laboratory experiments.

One exotic type is ice VII – a cubic crystal phase that can form at high pressure and high temperature. It is thought that this “hot ice” could be found on the ocean floor of Saturn’s moon Titan and other watery exoplanets. Back on Earth, however, it is difficult to create and maintain ice VII in a lab. Previous studies have attempted to “shock freeze” water using lasers to create pressure changes, but they have not been able to measure its rapid formation or characterize its structure.

Popular topic

“There have been a tremendous number of studies on ice because everyone wants to understand its behaviour,” says team member Wendy Mao from Stanford University. “What our new study demonstrates, and which hasn’t been done before, is the ability to see the ice structure form in real time.”

To create ice VII, the team fire an intense laser at a sample of water sandwiched between a diamond platelet coated with gold and a quartz platelet. The laser light vapourizes the diamond, generating a huge pressure shock 50,000 imes greater than that of Earth’s atmosphere at sea level. The force triggers the phase change to ice VII, with the transformation happening in only 6 ns.

Mao and colleagues recorded the fast phase change with femtosecond-long X-ray pulses generated by the X-ray Free Electron Laser at SLAC’s Linac Coherent Light Source. The X-rays are diffracted by the transforming water, allowing the researchers to characterize the changing molecular structure.

Disorder to order

“These experiments with water are the first of their kind, allowing us to witness a fundamental disorder-to-order transition in one of the most abundant molecules in the universe,” says team member Arianna Gleason. “Learning about the icy interiors [of icy satellites and planets] will help us understand how the worlds in our solar system formed and how at least one of them, so far as we know, came to have all the necessary characteristics for life.”

The work is presented in Physical Review Letters.

Hidden stars affect exoplanet measurements

A significant number of known exoplanets could have larger diameters than previously thought, according to two astronomers in the US. These exoplanets orbit stars that are in a binary system with another star – and it is light from this companion star that has thrown off previous measurements. The work suggests that some known exoplanets are less dense than previously thought, which means that they resemble Jupiter, rather than Earth.

Many of the exoplanets discovered by the Kepler space telescope and other instruments orbit stars in binary systems. It can be difficult to differentiate between the two stars in such systems – which can appear as a single point of light. This is a problem if the exoplanet is studied using the transit method whereby the diameter of the planet relative to that of its star is determined from how much starlight it blocks when it passes between Earth and the star. Astronomers can inadvertently be measuring extra light from the companion star, which means that the measurement yields a smaller diameter for the exoplanet than its actual value.

Large errors

If the exoplanet orbits the brighter of the two stars in a binary system, then the measurement error is small. If the binary stars are the same brightness, the error is about 40% – and can be even larger if the exoplanet orbits the dimmer of the two stars.

The diameter is then used to calculate the density of the exoplanet, which determines whether it is a dense, rocky body like Earth or a gaseous planet like Jupiter. The diameter of the planet is also used to work out how closely the exoplanet orbits its star, which determines whether it lies within the habitable zone where liquid water and life could exist.

Elise Furlan of Caltech and Steve Howell of NASA Ames Research Centre looked into the extent of this problem by looking at Kepler data from 50 exoplanets that had already had their masses and diameters calculated. All of the exoplanets’ stars are in binary systems, but this had only been previously accounted for in seven cases.

Unknown orbit

Furlan and Howell worked out that 35 of the exoplanets orbited the larger star in their binary systems – which meant that their calculated sizes did not change significantly. For the other 15, they were unable to determine which star the exoplanet orbited. Five of these are in systems with stars of similar brightness, suggesting the exoplanets are 40% bigger than previously thought.

“Our understanding of how many planets are small like Earth, and how many are big like Jupiter, may change as we gain more information about the stars they orbit,” says Furlan. “You really have to know the star well to get a good handle on the properties of its planets.”

Howell adds: “In further studies, we want to make sure we are observing the type and size of planet we believe we are.” He adds: “In the big picture, knowing which planets are small and rocky will help us understand how likely we are to find planets the size of our own elsewhere in the galaxy.”

The research will be described in the Astronomical Journal and a preprint is available on arXiv.

Type-II Dirac fermions spotted in two different materials

The first experimental evidence of a quasiparticle known as a type-II Dirac fermion has been found by three independent research groups – one based in South Korea and two in China. Two of the groups found signs of the quasiparticle in the crystalline material palladium ditelluride. This could mean that the material is a topological superconductor – a hypothetical material with unique properties that could be useful as components in the proposed technology known as a topological quantum computer. The third group found evidence for type-II Dirac fermions in a similar material called platinum ditelluride.

Dirac fermions are subatomic particles with half-integer spin that are not their own antiparticles. Electrons in solids can also exhibit particle-like collective behaviour that can be described in terms of Dirac-fermion quasiparticles, which obey the same physics as their subatomic counterparts. These quasiparticles can exist as a so-called topological phase of matter with unique properties that condensed-matter physicists think could eventually be useful in quantum computing.

A type-II Dirac fermion is a special type of Dirac fermion that has a specific electronic band structure resembling a tilted cone. Prior theoretical calculations suggested that they could be lurking in palladium ditelluride, says Han-Jin Noh of Chonnam National University, who is a member of the South Korean group. To confirm this, his team used a technique called angle-resolved photoemission spectroscopy (ARPES), in which high-energy photons strike the material from different directions, causing the material to emit electrons. The researchers measure the energy and momenta of the emitted electrons and use that data to map out the material’s electronic band structure.

Telltale sign

ARPES measurements were also carried out on platinum ditelluride by Mingzhe Yan from Tsinghua University. Both teams found that the conduction and valence bands meet at a single point called a Dirac or Weyl point. This point is the telltale sign that the materials could harbour Dirac fermions – and in this case, type-II Dirac fermions, because of the specific geometries of the materials’ band structures.

Meanwhile, a group that included Xiangang Wan of Nanjing University in China performed a different type of measurement on palladium ditelluride. The researchers placed the material in a magnetic field and measured its resistivity, which oscillated back and forth. These Shubnikov–de Haas oscillations are also a consequence of the Dirac-point geometry in the material’s electronic band structure, Wan says.

Noh says that the evidence for Dirac fermions in palladium ditelluride is exciting, particularly because the material is also a superconductor below 1.7 K. Since it is a superconductor and can host topological states, it could be an exotic new material known as a topological superconductor. “We still don’t know whether it is a topological superconductor, but we expect it might be,” Wan says.

Anyone for anyons?

Topological superconductors have different properties compared with regular superconductors, says Alexey Soluyanov of ETH Zürich in Switzerland, who was not involved in the research. They could host another sought-after quasiparticle known as an anyon. Anyons could be used in topological quantum computers, a proposed type of quantum computer that relies on topological states of matter that should be more stable than the quantum computers currently being built.

But these technologies are all speculative, and now the teams need to show that the palladium ditelluride actually is a topological superconductor. The groups are also focused on understanding the basic science of these quasiparticles and exotic states of matter. Noh’s group plans to introduce impurities into the material to make its exotic properties easier to access in experiments. “Academically, they are really interesting things,” Noh says. “They’re really something new in the condensed-matter physics world.”

The research is described in Physical Review Letters, Physical Review B and arXiv.

Study reveals ‘cash for papers’ rewards in China

Chinese researchers were paid an average of $43,000 in 2016 for each paper published in Nature and Science, according to a study by information specialists in China and Canada.

Wei Quan from Wuhan University, Bikun Chen at Nanjing University of Science and Technology and Fei Shu at McGill University analysed 168 “cash-per-publication” policies at 100 Chinese universities from 1999 to 2016. The researchers found that Chinese universities offer cash rewards ranging from $30 to $165,000 for papers that are published in journals indexed by Web of Science, with the average amount increasing over the past decade. The largest payouts were for Nature and Science papers, with the average award increasing 67% from $26,212 in 2008 to $43,783 in 2016.

Negotiable awards

They also note that in some cases the amount of cash available for such papers was “negotiable”. Payments for publication in other journals were significantly lower, with the average cash award for a paper published in the Proceedings of National Academy of Sciences $3513 in 2016, increasing slightly from $3156 in 2008.

Physics World’s latest special report on China is out now

By Michael Banks

The pace of change in China can be bewildering – and science is no exception. With every year that goes by, the country publishes more papers, spends more cash on research and opens up yet more world-class facilities.

PWChina17cover-200This is the third Physics World special report on physics in China – following publications last year and in 2011.

Most of this year’s report was based on an action-packed schedule of visits and interviews earlier this year at institutes and labs in Shanghai and Beijing.

Speaking to researchers during my travels, it became clear that China is reaping particular benefits from its 1000 Talents programme, which seeks to persuade top Chinese  researchers who have spent time abroad to return home. Such scientists are bringing huge experience back and using it to put China at the forefront of many fields of research.

China is also showing a growing appetite to attract foreign scientists who have not worked there before. Getting overseas researchers to move to China is not always easy, so one solution has been for China to encourage Western institutions to branch out into the country. The Kavli Foundation, for example, has just opened a new Kavli Institute for Theoretical Sciences in Beijing, which aims to have about a third of its faculty from outside China.

(more…)

Why abseiling spiders don’t spin out of control

Spiders don’t spin when hanging from their webs because their silk dissipates energy by partially deforming when twisted. This is the conclusion of Dabiao Liu from Queen Mary University of London in the UK and colleagues, who have investigated the unusual ability with the hope it can be mimicked in synthetic fibres.

Spiders produce various types of silk, each with different properties for specific purposes. For the outer rim and spokes of their webs, they use dragline silk, which has incredible strength comparable to high-grade steel. Spiders also use this silk to lower themselves from heights. But, rather unusually, spiders do not spin uncontrollably on their threads as they descend. “Spider silk is very different from other, more conventional materials,” says Liu. “We find that the dragline from the web hardly twists, so we want to know why.”

The team tested the silk of two species of golden silk orb weavers using a torsion pendulum experiment. They suspended washers from dragline silk in an isolated environment, thereby removing any external disturbances. Using a turntable, they twisted the silk and recorded the behaviour with a high-speed camera.

Liu and colleagues discovered the silk deforms slightly when twisted, while carbon-fibre and metal wires do not, and therefore causes the washers to spin. This process dissipates more than 75% of the potential energy, rapidly slowing oscillations. When twisting stops, the silk partially snaps back.

The researchers suggest the unusual property is due to the silk’s structure – a core of multiple fibrils surrounded by a “skin”. The fibrils are made of amino acids, some of which are organized in sheets and others in unstructured looping chains. Liu and team believe twisting causes the sheets to stretch, and warps the hydrogen bonds linking the chains. While the sheets return to their original shape when twisting stops, the chains remain partially deformed.

This unique ability to glide gracefully from heights helps spiders avoid startling prey or attracting predators. If scientists can harness the property in synthetic fibres, they could have many applications, such as violin strings, helicopter rescue ladders and parachute cords. Liu and team’s study is presented in Applied Physical Letters.

Copyright © 2026 by IOP Publishing Ltd and individual contributors