All That Matters

Sixty years ago this month Nature published the famous paper by Watson and Crick solving the structure of DNA. At the time many researchers pursued this goal, made difficult by the complexity of the DNA itself. A key contribution to the solution of the puzzle was the x-ray diffraction data provided by Rosalind Franklin. Indeed, without x-ray diffraction experiments this discovery would have been almost impossible at the time.

Rosalind Franklin’s x-ray diffraction image of DNA. (c) Nature Magazine. Franklin, R. & Gosling, R. G. Nature 171, 740-741 (1953) – doi:10.1038/171740a0

The way x-ray crystallography works is that a beam of x-rays is directed at a crystal, where the x-rays bounce off the atoms. Because the atoms in a crystal are periodically arranged, the x-rays form complex but regular patterns (such as the one seen for DNA). A detailed analysis of these patterns enables the precise determination of the crystal structure.

To this day such experiments aren’t easy. They require relatively large crystals and typically are done at major facilities such as electron synchrotrons. The synthesis of the crystals for these experiments can often be very difficult.

Yasuhide Inokuma, Makoto Fujita and colleagues from the University of Tokyo in Japan  and the University of Jyväskylä in Finland have now developed a clever method that does away with many limitations of x-ray crystallography. Their method works with tiny amounts of material, only about a half to 5 micrograms are enough. This is around a millionth of a gram – truly tiny. The difference between a microgram and a gram is the same as that between a gram and a metric ton. In addition, another major advance of their method is that the target molecules don’t even need to be in a crystalline state. [...]

There is a lot of buzz in the physics community about a new topological insulator: samarium hexaboride, SmB₆. The reason why any major discovery about topological insulators seems to be big news is that these materials have some unique electrical characteristics that make them not only very interesting from a fundamental point of view but also for electronic applications.

Topological insulators are electrically insulating in their interior, but at the surface they do conduct current. Moreover, the surface currents are topologically protected (hence the name), which means that the electrons that carry those currents don’t veer off the track easily and maintain their properties over long distance. Although a number of topological insulator compounds are known, the problem so far has been that it has been difficult to fabricate these with sufficient purity such that the interior was indeed insulating. This has been a problem, as the electrical current inside the materials just overwhelms the surface properties. [...]

For the past weeks this blog has been more quiet than usual. Mostly, I was busy with a number of projects, including the co-organisation of a Nature Conference – ‘Frontiers in Electronic Materials: Correlation Effects and Memristive Phenomena‘. The conference took place in Aachen/Germany, and was organized in collaboration with the Jülich-Aachen Research Alliance (JARA) formed by the RWTH Aachen University and the Helmholtz research centre in Jülich – who did a great job in getting this meeting off the ground. Rainer Waser in particular dedicated a tremendous amount of work to the conference. And with close to 600 attendees, the popularity of the conference certainly exceeded all our expectations.

I do not intend to summarize all the interesting talks at the conference here. Instead, I like to focus on two aspects that I think contributed in particular to the success of the conference, and that could be of interest also to those that couldn’t attend the meeting. They’re related to the scope of the conference and its organisation. [...]

Symphonies are some of the most complex musical pieces. They involve different instruments, each with their own unique sound, and each instruments section playing their own tunes. Yet, what are symphonies in comparison to the complexity of life? Proteins for example, they are made of a limited number of building blocks, amino acids, but take highly complex shapes and assume a broad range of functions in the body.

Still, there is a commonality underlying such complex systems, in many cases they are hierarchical, which means they’re made of different objects on different scales – instruments playing tunes, amino acids forming proteins and so on. As David Spivak, Markus Buehler and others from MIT have described in a recent paper, a mathematical approach, known as category theory, can be used as a versatile tool that is capable of modelling complex systems by using the underlying rules governing a structure’s components. This is a very powerful approach and there is a lot to be gained by using this mechanism in materials science, to describe biomolecules or other hierarchical materials. Moreover, their approach makes it easy to connect different complex system. To put it crudely, understanding a Beethoven symphony may also provide insights into the properties of a protein, because category theory helps us links various complex systems.

Photo by Wayne Dixon via flickr.

To understand how this works, let’s take a look at an example provided by Buehler and colleagues – spider webs. These are made of individual fibres, consisting of smaller fibrils. The fibrils are made of a nanocomposite of crystal-like structures connected by flexible links. These structures are in turn made of various amino acids.

The complex structural hierarchy of spider silk (and other systems) is of course well-known. The problem researchers face is, however, that knowing the individual components of a material doesn’t necessarily mean that the properties of  the full system are known. For example, even though the molecular composition of a protein may be known, predicting its three-dimensional shape is notoriously difficult. It is the behaviour of structural elements in the context of their use that can be so difficult to understand. And this is where category theory is useful. [...]

The study of materials is one of the major areas of science, with legions of researchers in physics, chemistry and materials science working on this topic. Condensed matter physics is one of the largest research areas in physics. Yet, it makes me often uneasy how the benefits of materials science are promoted. It is all too often about applications, and not about fundamental physics. How materials such as graphene might revolutionize electronics. And how new physical concepts could be used to develop materials for energy applications: solar cells, batteries and so on. In classical materials science it’s often about tougher materials, such as enhanced steels, and less about the fundamental insights they are based on. Of course, applications are an important aspect in the study of materials. But does this mean that too often fundamental insights are neglected in favour of a material’s commercial potential?

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Understanding the properties of something chaotic such as a bowl of spaghetti may seem a daunting task. But that’s what Garry Rumbles from the National Renewable Energy Laboratory in the USA, Natalie Stingelin from Imperial College London in the UK, and coworkers are trying to do. With success. They study polymers – long spaghetti-like molecules made of repeating atomic subunits – and have now uncovered how the microstructure of these polymers controls the behaviour of optically generated electrical charges in such a tangled molecular web, with important implications for the design of electronic devices.

The physical properties of polymers depend a lot on the length of the molecules as a whole, the atomic make-up of their structural units and the physical interactions between the individual strings. That’s why polymers come in so many forms, from hard plastics to stretchable synthetic rubbers. And what Stingelin and Rumbles now show is that also their electronic properties depend not only the chemical make-up of the polymers, but also the details of their structure and their molecular weight. This has dramatic consequences for the search of new polymers for various optical and electronic applications, says Stingelin. “Are there otherwise wonderful polymers out there that were cast aside because their creators tested the wrong molecular weight? We think it’s quite possible.”

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