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‘Magic-angle’ graphene doubles up

Two years ago a team of researchers at the Massachusetts Institute of Technology (MIT) in the US kicked off the field of “twistronics” by discovering that two layers of graphene offset by a small angle could support an array of insulating and superconducting electron states. This novel electronic platform, dubbed “magic-angle” graphene, heralded the beginning of a fundamentally new approach to device engineering. Now the researchers, again led by Pablo Jarillo-Herrero, have extended the “magic” twist to another graphene system: twisted bilayer-bilayer graphene, made from misaligned stacks of bilayer sheets of atom-thick carbon instead of monolayer ones. The system, which can be tuned by applying an electric field, could be used to investigate the strong electron-electron interactions that lead to phenomena such as high-temperature superconductivity and correlated quantum phases.

Jarillo-Herrero and colleagues made the first magic-angle graphene by stacking two sheets of 2D carbon on top of each other to form a moiré lattice. When they twisted these sheets so that the misalignment angle between them was 1.1°, they observed two unexpected effects. The first was that by applying a voltage, they could electrically tune the system so that it became a correlated (“Mott”) insulator. This transition to a Mott insulator occurs when electrons become localized in the moiré lattice – meaning that a material that would usually conduct electricity can no longer do so because of the strong repulsion between electrons.

The second effect they discovered was that, by further applying a small electric field (and thus adding a few extra charge carriers) to this insulator, they could tune the graphene superlattice so that it became a superconductor at 1.7 K. Both the insulating and the superconducting effects disappeared at slightly larger or smaller angle twists.

The start of twistronics

These results kick-started the field of twistronics, in which the weak coupling between different layers of 2D materials is used to manipulate their electronic properties simply by varying the angle between the layers. Following this discovery, researchers have also reported on superconductivity and Mott insulation in other similar systems, including moiré superlattices of three graphene layers on 2D boron nitride and twisted four layers of graphene.

The twisted bilayer-bilayer graphene (TBBG) studied in the latest work is conceptually similar to twisted bilayer graphene, Jarillo-Herrero explains, except that it involves four layers of graphene instead of two. In this system, the top and bottom layers remain aligned with respect to each other, and only the interface between the two middle layers is twisted.

Flat electronic bands

At twist angles of around 1 to 1.5°, TBBG possesses “flat” electronic bands in its energy spectrum where the kinetic energy of electrons is strongly suppressed. In a simple non-interacting description, this means that an electron is “dispersionless” – that is, no matter how much energy is pumped into it, it will not budge. “Correlated electron behaviour appears when we add interactions, however, and we indeed found correlated insulator states near these angles in our experiments,” study lead author Yuan Cao says. These states are highly sensitive to both the twist angle and the application of an electric field, he adds.

Flat bands also occur in twisted bilayer graphene and are thought to be involved in producing superconductivity in these systems, too. The difference, Jarillo-Herrero tells Physics World, is that in TBBG, “we can switch the correlated insulator states on and off by simply applying an electric field. What is more, these correlated states appear to have a distinct spin-polarized ground state compared to twisted bilayer graphene – as we found in their peculiar response to magnetic fields.”

These findings, which are detailed in Nature, make TBBG a new and handy platform for studying strongly correlated physics in external electromagnetic fields, he says. Once understood, this physics could be exploited to engineer the next generation of high-temperature superconductor and correlated quantum materials.

“Our work on TBBG demonstrates yet again how rich the behaviour of a seemingly simple superlattice made only of carbon can be,” Jarillo-Herrero adds. “Extending the electrical field tunability to other configurations of twisted graphene systems and indeed other twisted material platforms will possibly allow us to uncover further exotic quantum phases of matter in the future.”

Solar geoengineering could cause unwanted changes in climate, new modelling suggests

Using aerosols to reflect sunlight and cool the planet will weaken storm tracks in the temperate latitudes in both hemispheres, an international team of scientists warn. Their modelling suggests that while such solar geoengineering schemes could reduce the severity of winter storms, they would also stagnate weather systems in the summer. This could lead to more intense heat waves, increases in air pollution, and changes in ocean circulation.

Solar geoengineering involves cooling the Earth by reflecting incoming sunlight and is seen by some scientists as a way of mitigating the effects of global warming. One popular strategy involves placing reflective aerosols in the stratosphere – using aircraft, balloons or blimps – to block sunlight.

But the effects of solar geoengineering are unknown. It would not work as simply as cooling the planet and therefore returning Earth’s climate to pre-industrial levels. Climate under solar geoengineering would be different, as there would still be marked increases in atmospheric carbon dioxide levels.

Extratropical storm tracks

Charles Gertler, a graduate student in the Department of Earth, Atmospheric and Planetary Sciences at the Massachusetts Institute of Technology, in the US, and colleagues were interested in how injecting aerosols into the atmosphere would impact the pole‐to‐equator temperature gradient in both hemispheres, and the effect that could have on extratropical storm tracks. These are regions in the mid and high latitudes with heightened incidences of storms known as extratropical cyclones, which play a significant role in determining the day-to-day weather conditions in many parts of the world.

“About half the world’s population lives in the extratropical regions where storm tracks dominate weather,” Gertler explains. He adds, “Storm tracks feed off of meridional temperature gradients, and storm tracks are interesting because they help us to understand weather extremes.”

The team used various climate models to explore the effects of solar geoengineering on storm tracks. First, they analysed simulations from experiment G1 of the Geoengineering Model Intercomparison Project which provides solar radiation management schemes for researchers to use with climate models.

Balancing warming

In the G1 scenario solar radiation is reduced to balance warming caused by a quadrupling of carbon dioxide concentrations, relative to pre-industrial levels. This was run for 50 years and compared with a model that kept carbon dioxide at pre-industrial level and one that simulated a quadrupling of carbon dioxide concentrations, to provide a baseline and a global warming scenario, respectively.

The team ran two other climate simulations. The first, known as ‘half G1’, aims to model a scenario half-way between the G1 geoengineering simulation and a future where carbon dioxide concentrations quadruple. In the other model aerosols are injected into the stratosphere at four different latitudes controlled by a feedback algorithm.

Their results, described in Geophysical Research Letters, show that reflecting solar radiation to counteract global warming would weaken storm tracks in both the northern and southern hemispheres. These effects are driven by changes in mean temperature and humidity at different latitudes that reduce the pole‐to‐equator temperature gradient in both hemispheres. Essentially, reducing incoming solar radiation cools the equator while the poles continue to warm.

“Novel changes in climate”

“Our results show that solar geoengineering will not simply reverse climate change,” Gertler explains. “Instead, it has the potential itself to induce novel changes in climate.”

In the Northern hemisphere storm tracks are also predicted to weaken with climate change. The latest work suggests that this would occur at a similar magnitude as with solar geoengineering. In the southern hemisphere, however, global warming is expected to increase the intensity of the storm tracks and shift them south. With solar geoengineering these storm track would weaken, with some of the models indicating that there may also be a poleward shift in these systems.

“A weakened storm track, in both hemispheres, would mean weaker winter storms but also lead to more stagnant weather, which could affect heat waves,” Gertler says. “Across all seasons, this could affect ventilation of air pollution. It also may contribute to a weakening of the hydrological cycle, with regional reductions in rainfall. These are not good changes, compared to a baseline climate that we are used to.” In the southern hemisphere changes in storm track intensity could impact wind‐driven ocean circulations and affect the stability of Antarctic ice sheets, the researchers warn.

“This work highlights that solar geoengineering is not reversing climate change, but is substituting one unprecedented climate state for another,” Gertler says.

Scanning tunnelling microscopy images intramolecular details

Scanning tunnelling microscopy (STM) is routinely employed to identify individual molecules, but it cannot usually resolve their internal structure. An interdisciplinary team of researchers at the universities of Warwick and Cardiff in the UK has now shown that a variation of high-resolution STM can in fact deliver information about the positions of atoms and bonds within molecules. The technique, which can also determine the types of intermolecular bonds present, could be used in a wide range of fields, including materials science, biochemistry, and pharmaceuticals development and testing.

Invented in 1981, STM is a powerful way of visualizing the nano-world all the way down to its elementary constituents – that is, individual molecules and atoms, explains team leader Giovanni Costantini of Warwick. It works by exploiting the tiny current which, thanks to quantum tunnelling, flows between a very sharp metallic tip and the surface of a sample less than 1 nm away. The intensity of this tunnelling is typically fairly uniform across the molecules in the sample, however, which means that STM cannot usually resolve the internal structure of individual molecules. This is a serious limitation for researchers trying to determine the precise chemical structure of an unknown molecule or ascertain the way molecules interact with one another.

Sharper and chemically more defined

The STM tip in Costantini and colleagues’ new work is equipped with a single carbon monoxide molecule at its end. This molecule is held at liquid helium temperatures of about 1 K, which makes the tip even sharper than usual and chemically much better defined. “We are able to bring the tip extremely close to the molecule being analysed, at a distance where a very strong repulsion (the Pauli repulsion) between the electrons in the molecule and in the last atom of the tip becomes significant,” explains Costantini. “Under these conditions, the current measured in STM depends on the exact location of the tip within the molecule. The resulting images can thus show up intramolecular details, such as the position of the constituent atoms and the bonds between them.”

The Br and O atoms in 3,9-Br2PXX can be held together either by hydrogen or halogen bonds when arranged into supramolecular arrays on a substrate. The H bonds come from weak C-H-O interactions, while the halogen bonds come from so-called “sigma-holes” on the Br atoms.

Halogen bonding is the intermolecular interaction

Costantini and colleagues say their technique allows them to establish that halogen bonding is in fact the intermolecular interaction holding the 3,9-Br2PXX molecules together. Such a result would not be possible using standard STM since the lower resolution images it produces do not distinguish between these two types of bonds.

The researchers also determined that the samples analysed contained impurities. These come from the chemical synthesis process and are often present in such small amounts that they cannot be detected using classical spectroscopic analytical techniques. The HR-STM technique might thus also be used to identify contamination in pharmaceutical compounds with the aim of making them purer in the future, they say.

Once they established the position of the atoms and the type of intermolecular bonds between the 3,9-Br2PXX molecules, Costantini and colleagues could then identify which functional groups of one molecule were interacting with those of another molecule. They could also see how far apart the molecules were and how the supramolecular assemblies were oriented with respect to each other. The main findings, explains Costantini, are that the positive sigma-holes of the Br atoms of one molecule face straight onto the negative O atoms of an adjacent molecule, and that these two non-covalently bound atoms are separated by a distance smaller than the sum of each atom’s van der Waals radius (which is a measure for the size of an atom that is not ionically or covalently bound).

“These two observations perfectly match the definition of a halogen bond as given by the International Union of Pure and Applied Chemistry (IUPAC),” he tells Physics World. “The structural information we obtained from our high-resolution molecular imaging also allowed us to perform several (density functional theory) calculations (in collaboration with Gabriele Sosso’s group at Warwick) that revealed a number of electronic features recognized by IUPAC as further identifying traits of halogen bonding.”

A very fundamental study

While a very fundamental study, the researchers hope it will be valuable for future work on understanding (bio)molecular recognition and designing novel materials by exploiting non-conventional intermolecular interactions. Indeed, halogen bonding has recently been found to play an important role in the structure of biological macromolecules and in the folding of protein-ligand and DNA structures, says Costantini.

The team, who report their work in Nature Communications, now plan to extend their HR-STM technique to study other types of intermolecular interactions, such as pnictogen and chalcogen bonding, and to untangle the interplay between halogen and hydrogen bonding when assembling small molecules for biochemical and materials applications.

“The long-term goal is to take what, at present, is still a niche experimental technique, developed and used in the rather limited research areas of surface- and nano-science, and to transform it into a new and potentially paradigm-shifting analytical tool for the much wider fields of synthetic and biological chemistry, materials science and pharmaceuticals development,” they say.

As an aside, Costantini adds that the three corresponding authors on the study – himself, Sosso, and Davide Bonifazi (formerly at Cardiff, now at the University of Vienna in Austria) – are all originally from Italy. “This is a direct testimony to the significant brain-drain that Italy has experienced in the last 10-20 years and which, unfortunately, today is stronger than ever,” he says. James Lawrence, the paper’s first author and a former PhD student in his group, is now a post-doc at the Donostia International Physics Center in San Sebastián, Spain.

Toilets could be flush with coronavirus, masks struggle to contain repeated coughs

Here is another very good reason to wash your hands after using the facilities – and you might also want to wear a mask. Yun-Yun Li, Ji-Xiang Wang and Xi Chen of Southeast University in Nanjing, China have published a paper called “Can a toilet promote virus transmission? From a fluid dynamics perspective”. The answer, at least according to their computer simulation, is an emphatic “yes”.

They found that as water pours into the toilet bowl it creates vortices that continue upward into the air carrying droplets to a height of nearly 1 m. These droplets are so small they float in the air for more than 1 min and could be inhaled or settle onto surfaces – say the researchers.

They point out that their discovery could be significant for controlling the spread of the SARS-CoV-2, which is known to transmit via faecal matter. One suggestion from the researchers is that public toilets could be designed so they will not flush unless the lid is closed.

In other virus news, Talib Dbouk and Dimitris Drikakis at the University of Nicosia in Cyprus have created a computer model of what happens when a mask wearer coughs repeatedly. In “On respiratory droplets and face masks” they report that repeated coughs reduce a mask’s efficiency at preventing the spread of fluid – letting many more droplets through.

XENON1T may have detected something very interesting, or maybe not

Earlier this week we received a curious embargoed press release from the XENON collaboration about a preprint that they have since posted to the arXiv server. The team has measured an excess of detection events in the XENON1T dark-matter detector, which ran for two years deep under a mountain at the Gran Sasso National Laboratory in Italy.

The team is very explicitly not claiming this to be the first-ever direct detection of dark matter, a mysterious substance that comprises most of mass in the universe. Instead, they are suggesting that the excess could be caused by axions from the Sun – hypothetical particles whose existence is not predicted by the Standard Model of particle physics.

Another suggestion made by the team is that the excess is caused by the unexpected detection of neutrinos. This could mean that our current understanding of neutrinos is deficient – they could have larger magnetic moments than currently predicted by the Standard Model, for example – which would also be interesting.

Tiny amount of tritium

A more mundane explanation is that the excess is related to the radioactive decay of an extremely tiny amount of tritium that managed to get into the detector, which comprises 2 ton of ultra-pure xenon.

But the physics is not why we thought the press release was curious.  One thing is that the preprint has not yet been peer-reviewed by external experts prior to publication – although I have no doubt that this will be done in due course and a paper will be published. The second thing is that from a statistical point of view, the excess could very well be a fluctuation rather than a real thing. Normally a statistical significance of 5σ or greater is required to claim a discovery in particle physics, whereas values of 3.5σ or less are quoted for the axion, neutrino and tritium hypotheses.

As a result, we passed on rushing out a news story about this – so perhaps we missed the boat on all the excitement. And who knows, maybe a successor to XENON1T will discover solar axions.

Artificial intelligence spots unusual feature at Earth’s core-mantle boundary

An artificial intelligence algorithm originally developed for astrophysics has revealed a previously unknown feature on the core-mantle boundary deep inside the Earth. The algorithm allowed researchers in the US to find patterns in seemingly unconnected seismic data from different sources and the team believes that the new technique could glean insights from other types of geophysical datasets in future.

The motion of tectonic plates continues to shape the surface of the Earth and this activity is monitored constantly by a worldwide network of seismometers. Studying how seismic signals travel through the different layers of the Earth has informed much of our knowledge of the structure of Earth’s interior. In the early 20th century, for example, researchers inferred that the Earth has a liquid outer core because shear waves from an earthquake were not detected by seismometers on the opposite side of the globe – because a liquid cannot shear.

As seismology has grown more sophisticated and more monitoring stations have been installed, researchers have discovered much more about Earth’s interior, such as physical stratification and plumes of rising hot material within the mantle and mysterious “ultra-low velocity zones” on the core-mantle boundary, where waves travel much more slowly than expected. “They’re inferred from the data but we don’t really know what these things are,” says seismologist Doyeon Kim of University of Maryland, College Park, who led the latest research.

Ever-growing volume of data

Researchers normally make such deductions from patterns of seismological signals that clearly stand out to a human observer. However, the ever-growing volume of seismic data from across the world may contain patterns that humans cannot perceive. Kim and colleagues therefore worked with astrophysicists at Johns Hopkins University in nearby Baltimore to apply a computer algorithm called the Sequencer developed by astrophysicists. Last year, the algorithm revealed a previously unknown relationship between the masses of supermassive black holes and the properties of their host galaxies.

In a paper published in Science, the team describes how the Sequencer systematically sifted through thousands of seismic signals of diffracted waves, measuring the value of a specific quantity called the Wasserstein metric. Then it played a mathematical game of join the dots – finding the shortest path between all the data points. When there was clearly one optimal path, this showed a trend in the data.

When the researchers focused on signals beneath the northern Pacific Ocean, two regions stood out as strong generators of diffracted, delayed waves called postcursors, which are produced when seismic waves interact with anomalous structures. The first is beneath Hawaii. This was known to host a seismic wave anomaly, but data from the Sequencer — as well as additional analysis — shored up the hypothesis that it may result from a mantle plume and helped to localize it more precisely.

“Mega ultralow-velocity zone”

The second notable anomaly is, they believe, a previously unknown “mega ultralow-velocity zone” beneath the remote Marquesas Islands in French Polynesia. Scientists already know of some similar zones — beneath Iceland and Samoa, for example – that have been associated with areas of highly unusual geochemical compositions. This has led to suggestions these features may harbour material predating the giant impact on Earth that is thought to have formed the Moon. The new discovery beneath Marquesas, suggests Kim, could potentially test this hypothesis.

Kim explains the work is part of a trend in many sciences towards the use of artificial intelligence to optimize search strategies and find patterns in data that elude humans. “Broadly speaking in seismology and geophysics in general, we use supervised machine learning to search for ‘labelled’ objects such as earthquake signals from seismic recordings,” he explains, “However, where it gets interesting is in unsupervised learning, where we don’t know what we’re looking for in our datasets. That’s where this algorithm fits – you’re looking at datasets as points in a high-dimensional space and looking for patterns and clusters.” He says a forthcoming paper by the Johns Hopkins researchers describes how the Sequencer was used to sort samples of another type of earthquake wave called surface waves.

“This is very innovative, and represents a direction we need to go in seismology,” says earth scientist Edward Garnero of Arizona State University in the US. “Methods like this may help us to find things we may not even have been looking for – and apparently they did.” He is cautious, however, about the assumption that the delay in the postcursor signals came solely from the core-mantle boundary, wondering whether mantle heterogeneity elsewhere might have played a role. Ultimately, he says: “In seismic modelling, we are always up against the issue of knowing if our favoured solution model is [both] unique and the real earth.”

Physicists propose how to entangle macroscopic objects using pulses of light

Physicists at Imperial College London, UK, and Stockholm University, Sweden, have proposed a new way of creating entanglement between mechanical motion and an optical field, and also between two mechanical oscillators, using short pulses of light. Here research students Jack Clarke and Paulo Sahium along with group director Michael Vanner from the Quantum Measurement Lab at Imperial explain the work.

The research is reported in full in New Journal of Physics, published by IOP Publishing – which also publishes Physics World.

What was the motivation for the research?

Quantum entanglement is one of the most intriguing aspects of physics and allows objects to be more strongly correlated than is allowed classically. Albert Einstein called this perplexing behaviour “spooky action at a distance” and many objections to this radical idea were raised in the early days of quantum mechanics. Fast forward 80 years, and quantum entanglement is a now a well-established phenomenon and is routinely generated between objects such as photons, atoms and molecules.

Making an entangled state of something bigger allows us to test quantum physics on a macroscopic scale and paves the way for the development of powerful new quantum technologies, such as sensing and quantum networking. In this work, we propose a new technique to create such quantum states by using quantum optomechanics. In our scheme, light bounces off two tiny mechanical oscillators causing them to move, creating an entangled state in their motion. 

What did you do in the work?

It is a very exciting time in quantum optomechanics at present, as early signatures of quantum motion of mechanical oscillators are now being observed. In this work, we theoretically proposed and analysed a powerful new approach to create entanglement between mechanical motion and an optical field, and also between two mechanical oscillators, using short pulses of light.  

With this work, we have laid the foundation for future experiments in this area by showing how to both generate and, importantly, verify the entanglement. To make this a more useful study, we also dived into the details and computed the effects of optical loss and mechanical quantum decoherence on the quantum states. We are very encouraged by these results, as our schemes offer many advantages including being able to operate with weak optomechanical coupling, and being resilient to optical loss and mechanical decoherence.  

What was the most interesting or important finding?

Our research is the first theoretical proposal for preparing and verifying mechanical and optomechanical entanglement in the emerging field of pulsed optomechanics. This highlights a route to observe quantum entanglement in a regime where it hasn’t been seen before. 

More specifically, there are two main regimes in quantum optomechanics. One is the “resolved-sideband regime”, where light circulates inside an optical cavity for a timescale that is long compared to the mechanical period. The other is the “unresolved-sideband regime”, which allows for rapid pulsed interactions over a timescale much shorter than the mechanical period. Optomechanical entanglement has not yet been observed in this latter regime and our work makes key steps in this direction. One of the most important results from our work is that in the pulsed regime creating such entanglement looks experimentally accessible, and is robust to effects that often hinder its observation. 

Why is this research significant?

Entanglement can be exploited to develop new quantum technologies such as quantum networks and quantum-enhanced force sensors. Studying it also allows us to address very fundamental questions, such as “what are the limits of quantum theory?” To these ends, a key current goal of research is to create and observe entanglement on larger and larger mass scales. Quantum optomechanics has significant potential to contribute to both areas, which helps to make it such an exciting field. This research project makes a significant step in these directions, as it opens a rich new avenue for studies of optomechanical entanglement in the pulsed regime. 

What do you plan to do next?

In our research group – the Quantum Measurement Lab – we pursue a combination of experimental and theoretical quantum science to advance our understanding of the foundations of physics and to leverage this understanding to develop new quantum technologies.  

We will build on our research in three key ways. First, our results have sparked new theoretical questions on optomechanical entanglement creation and its verification, which we have begun investigating. Second, it’s a very exciting time for our group at present as our new lab in London has completed a refurbishment phase and we are eager to build experiments in these directions once the current coronavirus pandemic is over. Third, we hope our research in pulsed optomechanics inspires other experimental groups around the world and that we will engage with our global research community to initiate new collaborative projects.

The full results are reported in New Journal of Physics.

CERN approves further work on Future Circular Collider – but delays final decision

The CERN Council has today approved an update to the European Strategy for Particle Physics that recommends further work on a huge 100 km collider – dubbed the Future Circular Collider (FCC) – that would be built in Geneva. But with no formal decision having been made to go ahead with the FCC, the strategy also calls for Europe to back a Japanese-led linear collider if it receives the go-ahead from the Japanese government.

The report, released this morning, sets out a plan for the future of particle physics in Europe to the mid-2020s and beyond. It especially concerns planning the next collider that would succeed the Large Hadron Collider, which first switched on in 2008. The 27 km-circumference LHC has been smashing protons together at energies up to 13 TeV in the hunt for new particles and in 2012 physicists announced they had discovered the Higgs boson with a mass of 125 GeV.

The LHC is currently undergoing a major £1.1bn “high luminosity” upgrade – dubbed HL-LHC – that will increase the collider’s luminosity by a factor of 10 over the original machine. The strategy indicates that the completion and exploitation of the HL-LHC should remain “the focal point of European particle physics”.

The strategy update, however, gives the green light for further study into the FCC, which would cost around £20bn. In January 2019, CERN released a four-volume conceptual design report for the FCC, which first called for the construction of a 100 km underground tunnel that would house an electron–positron collider (FCC-ee). The FCC-ee would focus on creating a million Higgs particles to allow physicists to study its properties with an accuracy an order of magnitude better that what is possible today with the LHC.

Once the physics programme for the FCC-ee is complete, the same tunnel could then be used to house a proton-proton collider, dubbed FCC-hh. The FCC-hh would use the LHC and its pre-injector accelerators to feed the collider that could reach a top energy of 100 TeV – seven times greater than the LHC. CERN will now carry out a more detailed costing of the FCC as well as continue research and development into the magnet technology that will be required for such a machine at higher energies.

Eyes on Japan

The strategy also approves European participation in the ¥800bn ($7.5bn) International Linear Collider (ILC) if it receives support from the Japanese government. First mooted over a decade ago, the ILC would accelerate and smash together electrons with positrons at 125 GeV in a 20 km tunnel to study the Higgs boson and other particles in precise detail.

In March 2019, officials in Japan said that their government has formally “expressed an interest” in the particle smasher but did not decide whether to host the machine. The final go-ahead will only be given if enough international support and funding can be found to construct the machine and there is a consensus within the Japanese scientific community that the project is worth pursuing.

The European strategy is to prepare a Higgs factory, followed by a future hadron collider.

Halina Abramowicz

Yet backing the FCC does not contradict supporting the ILC as the two could be complementary. If the ILC is given the green light, then CERN could opt to bypass the FCC-ee and build the FCC-hh after the LHC programme is complete. “The European strategy is to prepare a Higgs factory, followed by a future hadron collider with sensitivity to energy scales an order of magnitude higher than those at the LHC,” noted Halina Abramowicz, who is secretary of the European particle physics strategy update.

“What this [decision] is about is prioritizing the R&D for what comes after the LHC, it’s not proposing a new machine right now,” physicist John Butterworth from Univeristy College London, who sits on the European strategy group, told BBC Radio 4. “It’s looking at what we have learned, looking at the technologies that we think we might need, where should we be prioritizing our resources for what we might want to do after the LHC”.

End of CLIC?

However, the decision to prioritise the FCC and back the ILC if Japan gives the go-ahead,  puts the CERN-led Compact Linear Collider – another linear collider proposal – as a “plan B”. CLIC is not as technologically mature as the ILC, but could run at higher energies. It is now only likely to go ahead if the FCC turns out to be too costly and the ILC is not given the go ahead by Japan.

An electron-positron collider Higgs factory as highest priority for our field is clearly the way forward

Philip Burrows

Philip Burrows from the University of Oxford, who is CLIC’s spokesperson, says he congratulates the European Strategy group on their “careful deliberations and clear recommendations”. “An electron–positron collider Higgs factory as highest priority for our field is clearly the way forward,” Burrows told Physics World. “The options for CLIC as a CERN-based Higgs factory, and subsequent energy-frontier exploration collider, are clearly articulated, and the door is opened wide for Europe to make major contributions to ILC should Japan go ahead and realize ILC as a global project. Either way we can plan to realise a linear collider Higgs factory.”

Particle theorist John Ellis from King’s College London told Physics World that he is “happy” with the proposal to support a Higgs factory and prepare for a high-energy collider at CERN. “CLIC is de-emphasized, though not explicitly dropped,” he says, adding that the ILC is mentioned “only as something with which the European particle physics community would wish to collaborate, without any commitment by CERN”.

CLIC is de-emphasized, though not explicitly dropped.

John Ellis, King's College London

The FCC, CLIC and the ILC are not the only proposals for a future high-energy collider. Physicists in China unveiled the conceptual design for its own 100 km tunnel in September 2018. It would first house an electron–positron machine before hosting a proton–proton collider operating at 100 TeV. If it gets the go-ahead, construction of the Chinese collider could start before the the FCC.

The European strategy was originally due to be announced in May, but this was postponed due to the COVID-19 pandemic.

MR-guided focused ultrasound treats psychiatric disorders

MR-guided focused ultrasound (MRgFUS) is a novel surgical technique that can be used for incision-free ablative neurosurgery. Two Phase I clinical trials conducted at Sunnybrook Research Institute have demonstrated that MRgFUS appears to be safe and effective for patients with treatment-resistant obsessive compulsive disorders (OCD) and major depressive disorder (MDD). Of 12 study participants who underwent MRgFUS bilateral anterior capsulotomy, half reported an improvement in their quality-of-life.

Anterior capsulotomy, an established neurosurgical procedure, creates a lesion in a brain pathway known to be involved in OCD and MDD. This region, the anterior limb of the internal capsule (ALIC), connects key areas of the brain involved in anxiety and regulation of emotions. The lesion, which is usually created using radiofrequency (RF) ablation or stereotactic radiosurgery (SRS), interrupts the ALIC fibres to help reset abnormal communication in the brain.

Treatment with MRgFUS does not require the invasive surgery needed for RF ablation, and does not expose a patient to the high radiation doses of SRS. Performed inside an MRI scanner, MRgFUS uses MR imaging to guide multiple high-powered ultrasound beams to a small target, to achieve controlled heating and ablation.

For the procedure, patients wore a helmet fixed over a rigid headframe. The helmet contains 1024 ultrasound elements that target brain regions with millimetre accuracy. The team performed three low-powered test sonications, which raised the temperature of the targeted region to 40–45°C, to align the sonication focus and confirm targeting accuracy. They then performed higher powered sonications to raise temperatures to over 53°C and create lesions. These were repeated based on the temperature rise, estimated thermal dosing and patient tolerance. They used T2-weighted MRI and real-time thermography to confirm the accuracy of the lesion targeting.

The treatments, performed at Sunnybrook Health Sciences Center, took three to four hours. The researchers treated 16 patients in total, but the procedure was not successful for four of them due to insufficient MRgFUS heating at the target site.

Writing in Molecular Psychiatry, the researchers report interim six-month post-treatment findings from their 12 month trial. They found that patients with OCD responded better to MRgFUS capsulotomy than those with MDD, with 66% and 33% of patients, respectively, achieving a significant treatment response.

None of the patients experienced any serious treatment-related adverse effects, reports principal investigator Nir Lipsman, director of Sunnybrook’s Harquail Centre for Neuromodulation. Seven patients reported swelling at the site of the headframe pins and headaches for several days, and one patient experienced mild headaches for several months.

Four of the six OCD patients experienced a more than 35% reduction in their Yale-Brown Obsessive Compulsive Scale (YBOCS), and two patients with MDD had a greater than 50% reduction in their Hamilton Depression Rating Scale (HAMD17). Improvements in YBOCS and HAMD17 scores also were generally accompanied by measurable improvements in mood, anxiety and quality-of-life. No negative cognitive or behavioural effects were experienced by nine patients who underwent neuropsychological testing.

The studies also showed that MRgFUS capsulotomy lesions led to significant metabolic and functional alterations in brain circuits governing affective processing, and that pre-operative functional connectivity – measured using resting-state functional MRI – could differentiate treatment responders from non-responders.

“These findings suggest that a focal lesion in bilateral ALIC can have widespread effects on glucose utilization, supporting the notion that OCD and depression are indeed circuit conditions in the brain, and that influencing key nodes in that circuit can have brain-wide effects,” comments Lipsman.

“Our study supports the importance of continued research into focused ultrasound as a safe and innovative treatment approach for patients with difficult to treat OCD and depression,” he adds. “Patients with these debilitating neuropsychiatric conditions have tried conventional treatments without success and are in need of novel treatment options.”

Lipsman tells Physics World that following these initial trials, the next steps will be to expand to larger patient populations and more comprehensively study both the safety and efficacy of MRgFUS in treatment-resistant psychiatric disorders. “More patients, in more centres, and with comprehensive study of responses prior to and after treatment will help inform what role MRgFUS may play in the care of these patients,” he says.

Metasurface laser produces super-twisted light

A new metasurface laser can produce light in any desired angular momentum state, including highly chiral or “twisted” light capable of manipulating physical objects. According to its developers at the University of the Witwatersrand (Wits) in South Africa and Harvard University in the US, this tunable, high-angular momentum light source could also be used to encode information in optical communications.

The angular momentum of light is the sum of two independent components: spin angular momentum (SAM) and orbital angular momentum (OAM). SAM is associated with circularly polarized light and arises when the electric and magnetic field vectors of light rotate over the course of a wavelength. Because SAM can have only two values – right or left circular polarization – its applications are relatively limited. OAM, on the other hand, results from the rotation of a light wave’s phase, and can take on any value. This variability makes OAM useful for a wider range of applications, including “optical spanners” – devices that trap and rotate tiny particles using light – and transferring data through optical fibres without crosstalk (multiplexing), to name but two examples.

Challenges of producing OAM states

The flexible nature of OAM means that a beam of light can, in principle, carry an unlimited amount of angular momentum. In practice, however, dialling up a desired OAM state is far from easy, explains study co-leader Andrew Forbes from Wits’ School of Physics. While various techniques exist, their efficiency – the proportion of light converted into the desired state – is limited. Alternatively, a device known as a q-plate can transform SAM into OAM with up to 100% efficiency, but it only works with pure right or left circularly polarized light. Because real light beams often have intermediate (elliptical) polarization, this is a significant drawback, since adding a fixed amount of OAM to one spin state and an equal and opposite amount to the other produces a net angular momentum of zero.

New forms of chiral light and the highest AM

The new device overcomes these obstacles by incorporating a metasurface – an artificially engineered nanostructure that interacts with light in unusual ways – into the laser cavity. The design of this metasurface builds on the Harvard group’s previous work and consists of rectangular amorphous pillars of TiO2 just 600 nm high. These nanopillars are separated by distances shorter than the wavelength of light being modulated, and they act like optical antennas – introducing spatially varying phase delays in the light rays that pass through them and moulding the light beam according to the desired profile. “In our experiment, we pass light through the metasurface many times, giving it a new twist in its phase each time we do so, while controlling the polarization of the light at the same time,” Forbes explains.

The result is a device that produces two output beams with OAM values that differ by as much as 90 units, resulting in a large non-zero total angular momentum. According to Forbes, this is the first laser that can produce such highly chiral light in any desired angular momentum state. “One of our demonstrations was a laser beam with OAMs of 10 and 100 in the same beam (with horizontal and vertical polarizations respectively),” he tells Physics World. “The prior record was just +10 and -10 (and therefore zero total AM).”

According to Federico Capasso, the study’s other co-leader and a professor of applied physics at Harvard, the use of metasurfaces was “the determining factor” in achieving a record-high optical angular momentum, L, of 100. “Alternative technologies such as q-plates and spatial light modulators (SLMs) have not even come close to these values of L,” he says. “What is more, the design limitations and fabrication constraints of those technologies can’t give the arbitrary wavefront control provided by metasurfaces, of which the non-symmetric vector vortex beams described in this paper are an outstanding example.”

Dramatically reducing light losses

According to Harvard’s Yao-Wei Huang, who constructed the metasurface used in the laser, the new design “demonstrates the highly effective coupling between arbitrary spin (a linearly, circularly, or any elliptically polarized state) and orbit (symmetric or non-symmetric helicity) of light in a compact planar structure”. Forbes adds that the device can couple non-symmetric OAM to linearly polarized states, rather than being limited to symmetric OAM and circularly polarized states, as q-plates are. “This may seem like a minor technical detail, but it means we can halve the number of elements inside the laser, so dramatically reducing light losses and allowing us to reach OAM values of 100 (a x10 advance over the prior state-of-the-art from such lasers),” he explains.

According to Forbes, another interesting feature is that the beams carrying 10 and 100 units of angular momentum are significantly different in size when they come out of the laser. When they travel around the cavity, however, they converge to a similar shape and size, where they experience optical gain. This allows for a coherent mode – a tell-tale sign of lasing – even though the actual beams appear spatially separated.

“We can use this type of light to optically drive gears in situations where physical mechanical systems would not work, such as in microfluidic systems to drive flow,” he explains. “Such systems could be used to make miniature lab-on-a-chip devices in which medicine would be performed on a single chip rather than in large experimental apparatus in the lab.”

The laser, which is described in Nature Photonics, could also be made bigger by increasing the size of the metasurface and the gain volume to produce a high-power bulk device. “In both these cases, the lasing mode wouldn’t require any intra-cavity elements other than the metasurface itself,” Forbes says.

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