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. [...]

A scanning tunnelling microscope image of a single-atom transistor during fabrication. The pink colours represent the areas where a single phosphorus atom (centre) as well as phosphorus source and drain contacts will be placed. The gate contacts that control the transistor action from the side are not visible here. Credit: Martin Fuechsle

When Gordon Moore made his observation in 1965 that the number of transistors integrated on a single silicon chip is doubling roughly every two years, the only logical end point for such a trend would be a transistor made from a single atom. This point has now been reached. Writing in Nature Nanotechnology, Michelle Simmons from the University of New South Wales in Sydney and colleagues report a single-atom transistor, the world’s smallest, on a silicon chip. The transistor is based on current flowing through a single atom of phosphorus embedded in a silicon wafer. [...]

A beautifully looking graphics, isn’t it? But there is a major caveat. As its creators would agree, this image is only a very crude depiction of reality and shouldn’t be used for any scientific purpose… (c) LANES, EPFL

Nanotechnology is a wonderful science that has pushed functional devices to sizes not far away from the size of atoms. So small that if you want to image such structures, even a conventional electron microscope wouldn’t get you far. There is no way to directly see what is going on. This is a common problem. Take condensed matter physics – it is impossible to directly visualize the various interactions and events taking place inside a crystal. Or photonics, where complex light fields interact with tiny nanostructures in ways that can be really difficult to visualize, especially in real-time.

So, no wonder that artificial graphics often serve to illustrate a scientific concept or a certain device. And with the prevalence of advanced computer graphics programs such illustrations are becoming more and more fancy. In my opinion, this is a dangerous trend, because such graphics can distort the underlying science they try to depict. [...]

We are all familiar with the basic ways in which light interacts with matter, when light absorption  causes atoms to move and creates heat, or when light gets absorbed by the outer electrons of atoms so that they move into energetically excited states, which is how electricity in solar cells is created. Common to both examples is that light is mainly used as an energy source, and it is easy to visualize. When scientists draw such light interactions into the energy diagram of say a molecule, they often draw little wavy arrows from one energy state to another.

But that’s the boring stuff. Far more interesting is that light can also strongly couple to matter, but without getting absorbed. The example I am discussing here is when the interaction between light and a molecule is so strong that it profoundly alters the molecule’s energy states themselves, and not merely lifts electrons from one state to another. In particular, what Thomas Ebbesen, Tal Schwartz, James Hutchison and colleagues at the University of Strasbourg have now shown is that such interactions could find exciting new applications: to control energy levels of molecules, and in this way to influence the kinetics of  chemical reactions in a new way that creates many new possibilities.

Strong coupling of light and matter. Light confined between two mirrors can strongly interact between matter that is also between the mirrors and has a matching energy level. The strong light-matter coupling then causes a splitting of the matching energy level into two separate states.

To see how this looks in practice it is necessary to understand what the strong coupling between light and molecules means. First of all, to achieve the necessary strong coupling, it is necessary to create a strong feedback mechanism between light and matter. This can be done by squeezing the light field between two closely spaced mirrors, with the desired molecules in-between. In addition, the energy levels of the light field between the mirrors and one of the energy levels of the molecule need to match up. If all these conditions are fulfilled, then the energy state in question is split into two separated states (see figure). This is called Rabi splitting. The stronger the coupling, the larger the energy separation between the two states. Because of the beauty of quantum mechanics this doesn’t even require light to be present, the mirrors are enough. [...]