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From the lab to the courtroom

Not many physicists carry a gun to defend themselves against attackers provoked by their research, but that’s exactly what Wilmer Souder once felt the need to do. Since 1911 he worked at the US National Bureau of Standards (NBS) in Washington, DC (today it is the National Institute of Standards and Technology (NIST)), eventually developing forensic techniques that convicted criminals. Souder was not the only forensic physicist in that era. John H Fisher, another ex-NBS physicist, invented a device essential for forensic firearms identification. Both of their contributions were important in major criminal trials and made a sizable impact on the justice system.

Fisher worked at the independent Bureau of Forensic Ballistics, established in 1925, where he invented the helixometer to peer inside the barrel of a firearm without sawing it in half lengthwise. His patent shows the device’s optical arrangement and graduated angular scale that allowed an investigator to examine defects in the barrel and find the pitch of its rifling – the internal spiral groove that imparts a stabilizing spin to a bullet. These features leave unique marks on bullets fired from a given weapon. Along with the double microscope for side-by-side comparison of bullets, invented at that same bureau, the helixometer made it possible to link a bullet from a crime scene to a specific weapon.

Souder’s forensic work was not well known until 2014, when Kristen Frederick-Frost, curator of the NIST Museum, found a forgotten trove of Souder’s old notebooks. She joined forces with John Butler at NIST, whose own work on DNA analysis has contributed to forensic science, and who compiled much of Souder’s work from his notebooks.

Souder earned his physics PhD in 1916, from the University of Chicago. One of his teachers was Albert Michelson, who won the 1907 Nobel Prize for Physics for the precise interferometric measurements crucial to the 1887 Michelson–Morley experiment. Souder’s PhD adviser was experimentalist Robert Millikan, who would earn the 1923 Nobel Prize for Physics, for research on the photoelectric effect and the charge on the electron. Souder published two papers with Millikan, and his dissertation about the photoelectric effect, in Physical Review.

Initially, Souder studied dental materials at NBS, to help the US Army develop treatments for soldiers – a research award in dentistry is now named after him. But another pressing need soon arose, thanks to growing criminal activity in the 1920s. Much of this was fuelled by Prohibition, the era from 1920 to 1933 when the US banned alcoholic beverages, and criminal gangs fought viciously to control illegal bootlegging. Souder’s notebooks show that he responded by providing forensic analysis of handwriting, typewriting and bullets on more than 800 criminal cases for the Department of Justice, the Treasury Department and other agencies. As the NIST researchers discovered, this resulted in an appreciative note from FBI director J Edgar Hoover, and a gun carry permit for Souder (seen above) that was justified protection for a witness in criminal trials.

These pioneering forensic approaches played roles in major cases. The historical research at NIST showed for the first time that Souder was involved in a sensational 1935 “trial of the century”. It found Bruno Hauptmann guilty of kidnapping and killing the 20-month-old son of Charles Lindbergh, famous for the first solo flight across the Atlantic Ocean in 1927. Souder’s study of the ransom notes in the case, with that of other handwriting experts, provided much of the evidence that put Hauptmann in the electric chair.

In 1932 Wilmer Souder was already calling for standards to be established for forensics equipment

Weapons identification was likewise essential in another world-famous trial. In 1921, two Italian-born anarchists, Nicola Sacco and Bartolomeo Vanzetti, were convicted of shooting and killing two men during an armed robbery in Massachusetts. The verdict was widely condemned as having been unjustly influenced by the prevailing anti-radical sentiment in the US. At a final review of the case in 1927, Calvin Goddard, head of the Bureau of Forensic Ballistics, testified that the helixometer and the comparison microscope unequivocally showed that one fatal bullet and a cartridge case came from Sacco’s pistol. Sacco and Vanzetti were executed but controversy continued, although modern bullet analysis has confirmed Goddard’s result.

These early forensic methods remain valuable, but forensic science in the US has lost some of its lustre. Reviews in 2009 and 2016 found that much of forensic practice has developed without the scientific rigour that would make it truly reliable in deciding guilt or innocence. The reviews called for improvements in forensic science, some of which are under way (see October 2019 p43). Souder, well-trained in scientific exactness, would have applauded these recommendations. The NIST researchers found that in 1932 he was already calling for standards to be established for forensics equipment, for precise forensic data and its detailed recording, and for stringent testing to qualify forensics experts.

Souder was also well aware of the difficulties in presenting scientific evidence to judges and juries who lacked scientific training. He used oversized aluminium models of bullets to illustrate ballistic methods, and in 1954, writing in Science, described how to be an effective scientific witness in court. The article ended with Souder’s rallying cry for the value of good forensic science that still resonates: “Justice is sometimes pictured as blindfolded. However, scientific evidence usually pierces the mask.”

Readers are invited to submit their own Lateral Thoughts. Articles should be 900–950 words, and can be e-mailed to pwld@ioppublishing.org

Valence proton could play a key role in oxygen neutron drip line anomaly

Adding a single proton to a doubly magic isotope of oxygen is enough to significantly alter its properties, an international team of physicists has discovered. Led by Tsz Leung Tang at the University of Tokyo, the researchers made the unexpected discovery after removing a proton from a neutron-rich isotope of fluorine. Their work could lead to a better understanding of the complex interactions that take place between protons and neutrons within atomic nuclei.

Basic information about how protons and neutrons interact within a nucleus can be gleaned from a nuclide chart, which plots the numbers of protons in an isotope against the number of neutrons. The “neutron drip line” in such a plot shows the maximum number of neutrons an isotope of each element can contain.

One particularly striking feature of this boundary is the sharp jump in neutrons between neighbouring oxygen and fluorine, which has one more proton than oxygen. An oxygen nucleus (containing eight protons) can contain up to 16 neutrons, however fluorine can contain as many as 22 neutrons. The reasons behind this jump are poorly understood, but researchers believe it is related to oxygen-24’s “doubly magic” nucleus, which contains extremely stable filled “shells” of protons and neutrons.

Valence and core

To explore the jump in more detail, Tang’s team prepared a beam of the isotope fluorine-25 at the Radioactive Isotope Beam Factory near Tokyo – which is run jointly by Japan’s national research institute RIKEN and the University of Tokyo. Fluorine-25 contains one more proton than oxygen-24 and can be thought of as an oxygen-24 core plus a single valence proton.

This latest research involved colliding fluorine-25 nuclei with a target to remove a proton. Using the SHARAQ detector, Tang and colleagues measured correlations between the motions of the collision products and found that around 65% of the resulting oxygen-24 nuclei were in an excited state. This is contrary to current theory, which predicts that the oxygen-24 core of fluorine-25 should exist in its lowest energy state.

This suggests that the addition of a single valence proton to oxygen-24 has a profound effect on the doubly magic core. Indeed, Tang’s team concluded fluorine-25’s excited core is likely responsible for the neutron drip line’s dramatic jump – although the reasons why such significant changes can be driven by a single proton remain a mystery.

The team now aims to uncover the physical mechanisms in future experiments. If successful, future experiments could lead to significant improvements to our understanding of the processes that occur inside atomic nuclei – and also provide new insights into the mysterious properties of neutron-rich astronomical features, including supernovae and neutron stars.

The research is described in Physical Review Letters.

3D ultrafast ultrasound quantifies coronary blood flow

LAD imaging — Left: 3D power-Doppler volumes overlaid on anatomical volumes of the myocardium in the left-anterior descending artery. Right: Absolute flow velocities estimated by 4D ultrafast ultrasound flow imaging. (Courtesy: *Phys. Med. Biol.* 65 105013)

Decreased blood supply to the heart muscles, known as cardiac ischemia, can lead to chest pain or even heart attack. Cases of suspected ischemia are currently investigated using invasive coronary angiography (ICA), which provides both anatomical and functional assessment of the coronary vessel. ICA, however, is an invasive procedure that involves relatively rare – but potentially serious – risks for patients.

In the peripheral arteries, non-invasive, non-ionizing Doppler ultrasound imaging is used instead of angiography. But for cardiac applications, Doppler imaging is difficult, because of the rapid motion of the myocardium and the insufficient definition of conventional ultrasound.

To overcome this challenge, researchers at the French research unit Physics for Medicine (INSERM, ESPCI, CNRS, PSL University) recently introduced a method called ultrafast Doppler coronary angiography (UDCA), which uses 2D ultrafast ultrasound to visualize coronary vessels as small as 100 µm in a beating heart. They have now extended their UDCA approach to three dimensions, enabling 3D imaging and quantification of coronary blood flow in a single heartbeat.

“2D UDCA can quantify relative changes of coronary flow, for example between rest and stress states, but it cannot quantify the absolute coronary flow velocity,” explains co-senior author Mathieu Pernot. “With 3D UDCA, it’s a completely different story, as it provides an enormous amount of data at a very high volumetric rate. These data contains all the tissue and flow motion information that allow absolute flow velocity to be measured accurately in a few tens of milliseconds.”

In vivo evaluation

To assess their new 3D UDCA technique, Pernot and colleagues performed coronary volumetric blood flow imaging in vivo in open-chest swine experiments. They placed a 32×32 element ultrasound matrix-array probe on the animal’s left-ventricle in the region perfused by the left anterior descending (LAD) artery.

The team designed an ultrafast (1000 volumes/s) ultrasound sequence that images the coronary vasculature in 3D using power-Doppler imaging. They also employed vector Doppler analysis (4D ultrafast ultrasound flow imaging) to assess the absolute flow velocities. To estimate flow rates, they first used the 3D power-Doppler volumes to delineate the coronary vessel on 32 successive 2D slices. For each slice, they computed the flow rate by integrating the flux (rate of flow per unit area) over the cross-sectional area of the vessel. Finally, they averaged the flow rate over the different slices.

In their first set of experiments, the researchers imaged a small portion of the LAD artery in the hearts of five animals. They used 3D UDCA to assess coronary flows throughout the diastolic phase, in which the heart relaxes after contraction, in a single heartbeat.

Arterial flow visualization was most efficient when myocardial tissue motion was small – at early-diastole before myocardial relaxation and at end-diastole. During the mid-diastole phase, rapid tissue motion prevented accurate signal reconstruction. The researchers also used vector Doppler analysis to assess absolute blood flow velocity. They observed a maximal velocity of approximately 15 cm/s in the middle of the artery, decreasing towards the edges.

Next, the team examined reactive hyperaemia (the increase in blood flow following arterial occlusion) in five animals, after occluding the LAD artery for up to 90 s. The maximal flow velocity increased from about 12 cm/s to more than 20 cm/s during reactive hyperaemia, which corresponded well with the observed flow rate increase from about 70 to 120 ml/min.

The researchers also evaluated whether 3D UDCA can visualize a coronary stenosis (narrowing of the arteries) in three animals. They used an inflatable pneumatic cuff occluder positioned around the artery to create 30%, 50% and 70% narrowing of the proximal LAD artery.

LAD stenosis — Power-Doppler volumes overlaid on anatomical B-mode volumes show the induced stenosis. 4D ultrafast ultrasound flow imaging (right panel) shows the induced flow acceleration at the centre of the stenosis. (Courtesy: *Phys. Med. Biol.* 65 105013)

Overlaying power-Doppler volumes on anatomic volumes of the myocardium revealed a reduced signal in the stenosis region, demonstrating the reduction in the epicardial diameter. Vector Doppler analysis revealed a significant flow acceleration in the centre of the stenosis, with maximal velocity of approximately 20 cm/s.

Comparing flow rates estimated by 3D UDCA with measurements from a gold-standard, invasive coronary flowmeter (placed close to the ultrasound probe) revealed good agreement during baseline, reactive hyperaemia and coronary stenosis.

Clinical potential

Writing in Physics in Medicine & Biology, the researchers conclude that 3D UDCA could have major potential as a new non-invasive tool to measure coronary flow at the patient’s bedside.

“We envision several important clinical applications for diagnosis and management of coronary artery diseases,” says Pernot. “One could be estimation of the coronary flow reserve, an important parameter for clinical decision making in coronary intervention that’s currently obtained by catheterization under ionizing imaging modalities. Because of its high sensitivity, 3D UDCA could also be used to diagnose coronary microvascular disease, which is challenging with current imaging modalities.”

The team is now working to translate 3D UDCA for human use. They note that the open-chest configuration used in this proof-of-concept study provided optimal imaging conditions, while clinical translation will require more challenging trans-thoracic or trans-oesophageal imaging. Another limitation is the small region that can be imaged, which restricts the field-of-view to a small part of a large coronary artery. “We are currently developing new approaches that could image the coronary vasculature of the entire heart,” Pernot tells Physics World.

Explorer versus salesman

**Sense and superiority** Stephen Wolfram in July 2008. (Courtesy: Stephen Wolfram’s PR team/Stephen Faust)

I need to start this review by saying that I loved the premise of this collection of essays by the physicist-turned-computer scientist Stephen Wolfram, who is chief executive of the Wolfram Group. Promising “surprising and engaging intellectual adventures”, the cover blurb of Adventures of a Computational Explorer teases “science consulting for a Hollywood movie, solving problems of AI ethics, hunting for the source of an unusual polyhedron, communicating with extraterrestrials” and even “finding the fundamental theory of physics and exploring the digits of pi”. What fun!

From supporting the production of the 2016 film Arrival by exploring how alien spacecraft might work, to considering how humanity might best leave behind a message for other civilizations (one option being the Wolfram computational language, of course), the opening chapters are perfectly pitched for the general reader. They’re all written in Wolfram’s compelling and, at its best, charmingly avuncular style. Some later chapters also deliver well on the advertised concept – covering topics such as computationally analysing the Facebook data of consenting Wolfram customers, and playfully imagining what kind of tech might be made by combining four current buzzwords (to form “Quantum Neural Blockchain AI”) .

Bright ideas and their architects

Different zonohedra shapes created by mathematician Russell Towle

Unfortunately, though, a key flaw of the book is that it lacks cohesion – perhaps as a result of being a compendium of seemingly loosely edited pre-existing essays. Repetition abounds, and I have no idea who the target audience for Adventures of a Computational Explorer is, with the popular accessibility of the initial chapters giving way to those that presume existing knowledge of specialist acronyms such as QCD (quantum chromodynamics), UDP (user datagram protocol) and TCP (transmission control protocol). A quick intro chapter to some of Wolfram’s key themes (computational irreducibility, cellular automata and the main Wolfram Group products) would also have provided a welcome explainer – especially given that, for Wolfram, the latter are the solution to almost all issues. If you must write a book that serves as stealth advertising for your software suites, you might as well explain what they each do, clearly, and in the first instance.

These issues might have been avoided with a stronger editorial hand – but one can imagine why this wasn’t delivered by the publisher, Wolfram Media. A more involved editor might also have reined in Wolfram’s predilection not only for promoting his products but also himself. This tendency is exhibited to such an extent that I eventually found it thoroughly off-putting. One might forgive the odd indulgence, but not multiple chapters devoted to, for example, his particular approaches to work and preferred methods of file organization.

At one point, Wolfram recapitulates his life through the lens of technology and artefacts from his considerable personal archive, beginning with a glowing elementary school report from 1967 – making for rather nauseating reading. For the reader not put off by this particular display of self-indulgence, the following chapter – “Things I learned in kindergarten” – goes further, relating tales of a six-year-old Wolfram, presumably still in knee-high socks, outsmarting adults and already realizing “obvious” things that his peers were simply incapable of.

This sentiment of superlativeness is Adventures of a Computational Explorer’s most unappealing leitmotif. Wolfram “independently came up with” data hashing functions at the age of 13 (p321); became a “card-carrying physicist” as a mere teenager; and gathered “what is probably one of the world’s largest collections of personal data” (p351). Wolfram also claims that when it comes to conceptualizing networks to represent physical space, many other physicists “haven’t quite reached the level of abstractness that [he is] at” (p29), and adds that his idiomatic ideas on how fundamental physics works just “aren’t yet mainstream” (p24).

If you must write a book that serves as stealth advertising for your software, you might as well explain what they each do clearly

The Wolfram language, meanwhile, is said to provide “a compressed representation…of the core content of our civilization” (p58), while mobile-phone-jingle-generating Wolfram Tones has “surpassed our species in musical output” (p159). While one has no doubt that many, if not all, of these assertions are accurate – Wolfram is clearly extraordinarily talented and accomplished – they need not all be expressed.

Opening the book of revelations

In other sections, Wolfram’s unabashed braggadocio takes on a more personal tone. The sixth chapter, originally written in 2016, pauses to take a seemingly random and unprofessional swipe at noted theoretical physicist Richard Feynman who, according to Wolfram, “came to a bunch of the meetings I had to discuss the design of the SMP [Symbolic Manipulation Program], offering various ideas – which I had to admit I considered hacky”, he writes. A later chapter reveals that Feynman was an examiner during Wolfram’s thesis defence in 1979, and the pair apparently had what Wolfram (euphemistically?) calls a “rather spirited discussion”. One can’t help but wonder if there isn’t a little grudge there. The rest of that chapter, meanwhile, is devoted to the task of demonstrating that Wolfram’s record of being the youngest person to graduate from the California University of Technology had not been superseded by a close contender.

This, really, is the crux of the problem with Adventures of a Computational Explorer. There was a great book to be cooked up here, but the meat of it has been completely drowned by the sauce of Wolfram’s unbridled and embarrassing self-promotion. Give this one a miss.

2019 Wolfram Media, 430pp £16hb

Thermodynamic study of D-Wave processor could lead to better quantum calculations

Why is important to study the thermodynamics of D-Wave quantum processors?

Michele Campisi: Quantum technology has just entered the “noisy intermediate-scale quantum” (NISQ) era. This is characterized by the ability to fabricate hardware with hundreds or even thousands (in the case of D-Wave) of qubits, but also the inability to control the qubits with the degree of accuracy and fidelity that is necessary to accomplish fault-tolerant quantum computations.

One of the main obstacles on the path to fault-tolerant quantum computations is noise – for example, the thermal noise that results from the interaction of the qubits with the substrate on which they are patterned. Understanding and quantifying the thermal phenomena that accompany the operation of a NISQ device is therefore crucial in the present stage of their development.

What is reverse annealing?

Lorenzo Buffoni: In a generic annealing process, you slowly drive a quantum device so as to change in time the Hamiltonian that describes its dynamics. In forward annealing you start from some Hamiltonian, call it H_x, and end-up at some other Hamiltonian, say H_z, that does not commute with H_x. The presence of non-commuting terms during the evolution results in purely quantum phenomena, such as quantum tunnelling.

In reverse annealing you start with H_z; then turn on an external control field to introduce an H_x term in the Hamilitonian (thus causing quantum dynamics); and then you go back to H_z. Such reverse annealing protocols have been recently added to the set of operations that can be run on D-Wave because they can be used to perform local searches in the neighbourhood of a solution encoded in an eigenstate of H_z. We chose to do reverse annealing because it is the only choice for which the D-Wave interface gives you full freedom to prepare any eigenstate of your initial Hamiltonian (H_z, in our case) and that was a crucial requirement for our experiments.

How did you perform your experiments using D-Wave’s Leap service?

LB: With D-Wave’s Leap service, virtually everyone can access one of the quantum annealers that D-Wave hosts locally via a cloud service. The devices can be programmed using D-Wave’s own APIs, which are easy to embed in a Python script. The APIs are well documented, and D-Wave provides a variety of examples and demos to get started.

Once you have programmed your own experiment, the script running on your computer automatically connects to the selected D-Wave quantum annealer, runs your programs on it, and gives the program output back to your computer, all in a matter of seconds. For those interested in the implementation details of our experiments and/or willing to try out this service, we have open-sourced our code.

You conclude that the D-Wave system acts as a “thermal accelerator”, what do you mean by this?

LB: In order to characterize the device from a thermodynamic point of view we have prepared it in a hot (high-temperature) state. Then we have studied how much energy it exchanges in the form of work — exchanged with the external control electrical fields– and in the form of heat, which is exchanged with the cold chip substrate.

Thermodynamics allows only four possible ways for this to occur:

The device gives away energy both to the work source and to the cold thermal source: that is what standard heat engines do
The device receives energy both from the work source and from the cold substrate: in that case it would function as a refrigerator
Both the device and the cold substrate gain energy from the work source, so the device operates as a “heater”
The hot device loses energy to the cold substrate, while the work source spends energy to accelerate that natural energy flow, hence the expression “thermal accelerator”

MC: It is important to stress that how to precisely quantify the actual heat and work involved during the operation of the D-Wave processor is a challenging, yet unsolved, problem. However, by using recent results in non-equilibrium thermodynamics, we were able to put quantitative bounds on the heat and work, and that was sufficient to tell us that thermal acceleration was occurring. Due to its generality, our method can be used in the thermodynamic study of other quantum devices as well.

Does you research have any implications for how D-Wave systems are used to solve problems?

MC: Our work suggests that in the specific case of D-Wave, thermal noise might indeed be beneficial. In quantum annealing you want the processor to follow a path towards a specific target state. It is very much the same as having to walk from A to B while holding a pendulum and you want to reach B without setting the pendulum into oscillation. The D-wave strategy is to achieve that by walking very slowly.

Imagine however that you are now walking through a very viscous fluid, you spend now more energy to walk through it, but it is easier to prevent the pendulum from oscillating, because oscillations are quickly dumped. That is indeed what we have observed in our experiments. We have thus learned that in designing optimal annealing paths, it might be useful to go through the more “viscous” regions, you pay an energetic cost to traverse them, but you might be able to do that faster, and more reliably. Hopefully, our work will trigger further investigation in this new direction.

Buffoni and Campisi describe their research in Quantum Science and Technology.

‘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.

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