Guest post by David Collomb, PhD student at the university of Bath
Category Archives: Uncategorized
Interactions: Chen Fang and the Materiae database
Post by Anastasiia Novikova.
In theory, many ordinary materials can have exotic topological phases. But how can we find them? In 2018 a research group from the National Laboratory for Condensed Matter Physics in Beijing scanned 39519 materials to predict which phases of the already-known compounds might exhibit topological properties. These materials were summarised into an interactive database Materiae, where you can browse compounds containing particular elements, check if they have any topological phases and visualise their band structure.
We asked Prof. Chen Fang — one of the team members who worked on Materiae along with Prof. Hongming Weng — to give us more details of the project, which has now been published in Nature.
When did the database start? What were the main challenges of this project? What goals do you have for the future?
The database has been online since 23 July 2018; it appeared simultaneously with the posting of the corresponding paper on arXiv. By now there have been over 10000 unique visitors (1=ip*day). The most difficult part is, naturally, the calculation that was done to obtain the topological properties of about 30000 materials. The theory, the underlying work was accomplished back in late 2017 (arXiv:1711.11049 and 1711.11050), but even so, it was an effort to implement the fully automated algorithm shown in the flowchart. Currently we have the band structures plotted for topological materials only, and in the future we will add the band structure plots for all materials, topological and non-topological.
Using your algorithm, you scanned 39519 materials. How much time did the whole calculation process take?
We didn’t track the CPU hours used on this, but if we count the time spent on debugging small bugs now and then, it took us about three to four months in total for the bulk results to come out.
You mention that 8056 materials from your database are actually topological. How many of these materials were experimentally studied?
All materials have been reportedly synthesized in literature, but most of them were not studied from a “topological perspective”, but were studied for superconductivity or ferroelectricity, for example. I think at most few hundreds of these materials have been studied for potential topological properties.
What is the most “underestimated” material?
One example is Tl2Nb2O7. Oxides are seldom considered as topological materials in literature, yet our database registers it as a topological semi-metal. Surprised by this result, we further looked into this material, and realized that the mixed-valence nature of Tl ion is the origin of the nontrivial topology.
Another is Ba3Cd2As4. The layered structure made us expect it to be a weak topological insulator, but our database shows it to be a new type of topological crystalline insulator (having so-called C2-anomalous surface states). Shortly after the prediction, experimental groups have started synthesizing this material.
We expect the study of certain materials, like the ones above, may be “revived” by what we show in the database.
The database contains only non-magnetic materials. Is it possible to envision a similar type of database for magnetic materials?
The entire prediction is based on first-principle calculation, but magnetism is notoriously difficult to predict/include in any first-principle calculations. Therefore, while some theoretical work on the mapping between symmetry data and topological data has been out there for a while (arXiv:1707.01903), I do not think a similar material database can be obtained in near future because of the inherent difficulty of DFT mentioned above.
Interactions: Anastasiia Novikova
Anastasiia Novikova will join Nature Reviews Physics in January after a PhD at Synchrotron SOLEIL and a postdoc at CEA Saclay in France.
What made you want to be a physicist?
I was always curious to understand natural phenomena, and physics seemed to explain how almost everything worked in the Universe. Besides, I enjoyed the scientific approach used in physics: experiment and demonstration.
If you weren’t a physicist, what would you like to be (and why)?
If not a physicist, I would definitely be an artist. As a child, I was passionate about drawing and painting (and I still am). Shapes and colours of nature were always hypnotizing me.
Which historical figure would you most like to have dinner with — and why?
I have a whole list of historical figures but the one I would really like to meet is Richard Feynman. To me, he is a person remarkable for his manner of popularizing physics and capturing the audience. The first thing I would ask him: “What is your secret? “
Which is the development that you would really like to see in the next 10 years?
I would like to see the development of Artificial Intelligence in the domain of Genetics to help us understand such issues as genetic disorders.
What’s your favourite particle?
While studying the Physical Chemistry module at Pierre and Marie Curie University, I was fascinated with how the electronic structure of a compound could influence its colour. In this regard, my favourite particle is, definitely, the electron.
What would your dream conference be like?
The conference I dream of would be dedicated to the greatest discoveries of all time. And being imaginary, it would be organized by the pioneers, with, for example, Isaac Newton giving a Welcome speech.
What is physics? Challenges and opportunities when working at the interface with other disciplines.
Post by Stefanie Reichert, Nature Physics
This year’s Berlin Science Week kicked off with a diverse programme. Among many events, visitors could discuss the connection between art and astronomy or learn how new technologies can be inspired by nature, or participate in a panel discussion at the Springer Nature office. The panellists set out to find an answer on how we define physics today, and to map out the boundaries with other related areas such as chemistry or biology.
Meet the panellists in our interviews from the run-up to the event: Abigail Klopper, Alba Diz-Muñoz, Cosima Schuster, Magdalena Skipper Beatriz Roldán Cuenya.
Heuristics for better figures
Post by Jesse L. Silverberg
Here’s the tldr: (1) Images = Information, (2) Colour communicates meaning, (3) Understand the limits of visual communication, (4) Move through colour space deliberately to reduce complexity, (5) Combine #3 and #4 to pick your colours wisely.
Long before I thought about studying physics, I saw myself on the path to becoming a graphic designer. I enrolled in a graphic design program at a nearby college, had a well-stocked supply of brushes, pencils, and Bristol board, and even generated a portfolio of nearly 100 compositions before taking my first course. I ultimately left design school when I recognized the differences between “art for the sake of art” vs “art for the sake of selling a product,” but that’s a story for another day. In my year studying graphic design, I practiced and learned a set of skills that became extremely useful during my PhD. What I eventually came to discover was that when I was designing scientific figures, I felt confident that I was making rational design choices, such as visually distinct colours to represent categorical variables and thought-out colour gradients to represent continuous quantities. This blog post is about those design skills and is intended for my fellow researchers who never had the opportunity to learn the language of design. My hope is that I can serve as a useful translator to convey some of the practical ideas that designers routinely employ with respect to visual communication, and explain how they can be used in service of articulating a clearer scientific message. Continue reading
University drops test scores from graduate-admissions criteria
PhD students have led a successful push for greater inclusivity of under-represented groups in science, technology, engineering and maths.
{credit}Cody Anthony Hernandez{/credit}
Above, GRIT co-founders Cody Hernandez, Christina Roman, and Mat Perez-Neut, PhD students at the University of Chicago in Illinois, take a break.
By Kendall Powell
How advances in active noise cancellation unlocked a new form of waves
Post by Romain Fleury, commissioned by David Abergel.
The original paper in Nature Physics can be read here.
{credit}Credit: Jamani Caillet, EPFL{/credit}
Imagine you are playing the popular Nintendo game Mario Kart, and as you try to win the race one of the other players suddenly drives into the worst possible item box you can imagine: it covers the road in front of you with a very, very large number of banana peels, making it extremely unlikely for you to avoid these obstacles. If waves could have feelings, this is probably what they would think when a scientist tries to transmit them through a strongly localized disordered medium.
Yet, imagine now that you have the possibility to install some sort of magic boosters, or conveyor belts, that auto-pilot your kart seamlessly through these obstacles, while maintaining your precious velocity. This is certainly not possible in the game, but for our team of physicists and engineers, it made perfect sense to try this for waves in disordered media.
In our recent Nature Physics article, we have used acoustic boosters, or relays, to guide sound through a very nasty series of obstacles, and turned an Anderson-localized opaque medium into a perfectly transparent one by doping it with gain and loss. Interestingly, these acoustic boosters were made possible by recent advances in active noise control devices, similar to the ones you may use in your noise cancellation headphones during your next flight. Here is the story of how this idea came to life. Continue reading
Frequency scanning optoelectronic oscillator
Post by Ming Li, commissioned by Heather Partner
The original paper in Nature Communications can be read here.
Radar and microwave communication systems have been invented many decades ago, but are still a growing area of research. For example, it is important for modern communication systems to be able to create microwave signals with fast-varying frequencies, called chirps. Optoelectronic oscillators are one way to produce ultra-low-noise microwave signals, but using them to produce a fast-varying signal with high quality is difficult, because a cavity-like component is used to reduce noise within these oscillators, and when the frequency is changed it takes time for a new low-noise frequency signal to build up in the cavity. In work published last month, we showed it is possible to have many frequencies oscillating in the system at once, so that the frequency can be changed rapidly without waiting for this build-up time. These simultaneous oscillations, all with locked phases, are made possible in a scheme known as Fourier-domain mode locking, which was previously applied to optical signals in lasers, but in this work is applied to microwaves using an optoelectronic oscillator .
An optoelectronic oscillator is like a laser, except that it has an optoelectronic cavity rather than a pure optical cavity. Although frequency-tunable optoelectronic oscillators have been widely studied, it is still a challenge to achieve continuous frequency scanning. Following the demonstration of frequency scanning lasers based on the Fourier domain mode locking technique in recent years, we wondered if it would be appropriate to extend this mode-locking principle to an optoelectronic oscillator.
In order to apply this technique to an optoelectronic oscillator, we needed a filter that could scan the selected frequency very rapidly — faster than what is made possible by most electrical schemes — so we decided to employ a microwave photonics solution that could perform faster tuning than electrical solutions.
One of the research interests of our group is semiconductor lasers. It is known that the lasing frequency of certain kinds of semiconductor lasers can be tuned by changing the driving current in a fast way. Fortunately, the passband of a microwave photonics filter based on phase-modulation to intensity-modulation conversion is related to the lasing frequency of the signal laser. Thus we achieved a fast frequency scanning microwave photonics filter by sweeping the frequency of the signal laser. Continuous frequency scanning microwave waveforms with very large time-bandwidth product are generated based on a Fourier domain mode locked optoelectronic oscillator. We run simulations that show that a Fourier domain mode locked optoelectronic oscillator oscillates in the same way as a conventional single-mode optoelectronic oscillator that uses the same optical and electronic components, except that the energy is shared by the many oscillation modes.
The employment of a Fourier domain mode locking technique in an optoelectronic oscillator provides an effective solution to generate frequency scanning microwave signals with large time-bandwidth product, which can find applications in radar and communication systems.
Ming Li
Reference: Hao T. et al, Breaking the limitation of mode building time in an optoelectronic oscillator. Nat. Commun. 9, 1839 (2018)
Built on instability
Post by Daniel Rayneau-Kirkhope and Marcelo Azevedo Dias
Built-in motion
From hierarchical architectures to complex composites, nature’s inventive use of geometry yields remarkable functionality from some rather unremarkable construction materials. This same control of geometry alongside a mastery of mechanics is used to transform elastic ‘failure’ into a crucial ingredient in the inner working of plants and organisms. Nature employs elastic instability so that large-scale motions can be triggered by the smallest and most specific stimuli. The Venus flytrap is perhaps the best-known example of this design philosophy — swelling induces an elastic instability that allows its leaves to snap between two stable configurations [1]. Using this snap-through behaviour, the plant moves quickly to capture its prey, allowing for the slow process of digestion to begin. Bacteria exhibit another beautiful example of this design paradigm, whereby their flagella, which are used to create thrust, buckle into a secondary configuration allowing the bacteria to control direction [2].
It is only recently that designers have started to use loss of structural stability in a similar manner. From merely being a mode of failure, buckling has become an increasingly well-trodden route to introducing novel functionality in the design of man-made structures and materials on many different length scales. This transition in perspective has been encapsulated as a move from ‘buckliphobia’ to ‘buckliphilia’ [3].
A powerful example of this paradigm is the use of buckling to turn simple geometries into mechanical machines: work in Physical Review Letters recently demonstrated that the buckling-unbuckling transitions in a hollow spherical shell can be used to create thrust in spherical swimmers [5]. It is well known that a spherical shell will buckle into a new geometry when the internal and external pressures are sufficiently different; as this deformation is elastic, the structure can return to its initial configuration when the pressure differential is removed. It was found that the asymmetry of geometries in the process of buckling and unbuckling allows for a net thrust to be created by cycling through these geometries while the structure is immersed in liquid. Continue reading
Plato, superheroes and a visit to the abattoir
Post by Malte Gather, commissioned by Nina Meinzer
The original paper in Nature Communications can be read here.
2010, Boston. The 50th anniversary of the laser. A device that was originally famously proclaimed as “a solution in search of a problem” and that became the solution to so many crucial problems of modern society and science. What better way to pay tribute to the laser than to bring it quite literally to life? With this in mind, my former supervisor Andy Yun and I got to work on turning living cells into tiny lasers, using a green fluorescent protein produced by the cells themselves as optical gain medium. When our paper came out, it inspired many people in … well … unexpected ways: “Sharks with frickin’ lasers attached to their heads”, “bacterial infection that shoots lasers in your body”, “superheroes shooting laser beams from their eyes”. The first two seemed rather undesirable, let alone the red-tape involved in any study aimed at their realization. But superheroes with laser eyes? Back in ancient Greece, Plato even believed vision itself was mediated by “eye beams” that scan our environment. So could laser beams emitted from the eye have a more peaceful application than what is suggested in the comic books of modern times?
Fast forward to 2017, St Andrews, Scotland. Working with Professors Ifor Samuel and Graham Turnbull, our jointly supervised PhD student Markus Karl develops an ultra-thin organic semiconductor laser. He strips all non-essential components and ends up with a 200 nm-thick membrane that contains only gain medium and resonator; the pump is supplied externally by optical excitation. To fabricate these devices, Markus uses a carrier substrate and a sacrificial intermediate layer. In the final step of the fabrication, the membrane floats off the substrate and rises to the surface of a water bath. What now? Another solution in search of a problem?
We soon find that we can pick up the membranes with another substrate, or fish for them with a little net. Then we find that our membranes work like stickers, stickers that can turn any object into a laser. Ifor suggests to put them on banknotes as a new security feature. A membrane laser on every banknote in the United Kingdom would probably make our membrane lasers the world’s most numerous type of laser.
But what about Plato and the superheroes? Our lasers are not only among the world’s thinnest, they also have very low lasing thresholds. But how low? Say, compared to laser safety standards? It takes a bit of courage but eventually my student and I go to see the department laser safety officer to ask how their intensity – and more importantly the intensity of the optical pump – compare to permissible levels for intentional ocular exposure. In other words, could we use our membranes to shoot laser beams from one’s eyes without blinding ourselves? We check, twice, three times, four times, but the answers seems to be that it should be safe, with about a ten-fold margin before reaching maximum permissible exposure levels.
We refrained from testing our lasers on the human eye – at least for now. Instead, the last part of our study involved a trip to an abattoir near Edinburgh to buy some cow’s eyes. (Ophthalmology research often uses them as model for the human eye.) A few hours later a cow’s eyeball in a Petri dish shoots a green laser beam across our optics lab. In the future, we hope to use such lasers as an authentication and access control feature, complementing a biometric iris scan. For now, we are left with the slightly weird image of a zombie supercow shooting laser beams from its large blank eyes…
Malte Gather
Reference: Nature Commun. 9, 1525 (2018) doi:10.1038/s41467-018-03874-w