Shrinking magnetic storage devices

January 15, 2012 by Joerg Heber 1 Comment

Information stored by a chain of magnetic atoms. Left: an STM tip measures the magnetic state of the iron atoms. Right: through increasing the current between tip and atoms the magnetic states can be switched. Peaks become valleys and vice versa. (c) Science Magazine

I now finally got the time to follow-up on last week’s paper in Science by Andreas Heinrich‘s group at IBM on magnetic storage elements that are only a few atoms in size. There have been a few misconceptions in some of the news reports with some being plainly wrong (‘smallest storage device ever made’), and many didn’t mention much about the scientific principles behind this study, although these are quite interesting. One of the better reports appeared in the New York Times, albeit again without going much into details. So I hope I can still add something useful with this blog post.

And actually, we’ve come across Andreas Heinrich’s previous research before, he does very innovative research with scanning tunneling microscopes (STM). In this latest Science paper he has now explored the limits of magnetic storage devices. Magnetism is of course the basis for storage such as magnetic hard drives. The problem in increasing the storage density in any magnetic storage device is that the magnetic regions begin to interfere with each other as they become smaller and are integrated closer together, because magnetic states on the order of just a couple of atoms are not very stable. Read more…

Condensed Matter Physics, Nanotechnology
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The cloak that hides events in time

January 4, 2012 by Joerg Heber 1 Comment

The temporal cloak. A light beam of a single colour is directed at a split-time lens (STL) that converts it into different colours. As the beam then propagates through an optical fibre, the blue light components travel faster than the red ones (the vertical axis shows time), so that eventually a brief gap is formed in the travelling beam during which there is no light present. Therefore, any even taking place during that temporal gap will be concealed from the beam. Afterwards, a reverse process restores the original beam so that an observer does not notice the cloaking device. Reprinted by permission from Macmillan Publishers Ltd. Nature (2012). doi:10.1038/nature10695

Devices that conceal objects from an observer are called cloaks. Conceptually, the idea of cloaking devices has its roots in science fiction, but such devices have indeed been demonstrated in the past few years. These cloaks are based on tiny structures that are able to bend light on predetermined paths as it passes through the structure. This is like a lens, but consisting of manmade materials, and much more versatile and powerful.

A very different type of cloak has now been published in Nature by Alexander Gaeta and colleagues from Cornell University. Following earlier theoretical proposals, they have now demonstrated the first temporal cloak where events are hidden in time, not in space so that an event is concealed from a light beam travelling through the same space for a certain amount of time. To understand the difference of a temporal to a spatial cloak, Robert Boyd and Zhimin Shi from the University of Rochester make a very good comparison in their News and Views article on the paper: Read more…

Photonics
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The Beethoven connection

December 12, 2011 by Joerg Heber 0 Comments

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. Read more…

Biomaterials, Chemistry, Condensed Matter Physics, Materials Science, Nanotechnology

Whither organic solar cells?

December 2, 2011 by Joerg Heber 8 Comments

This week I am attending the Materials Research Society Fall meeting in Boston, where there is a big focus on energy. Catalysis, fuel cells, batteries, solar cells, solar fuel, you name it. And I had a discussion with some researchers from the inorganic solar cell community, who asked me what is with the organic solar cells? There is a lot of university research in this area they said, but at industrial trade shows in comparison you don’t see as many start-ups working on organic solar. Eight19 is an exception to this that comes to mind.

And as we’ve discussed, the problem is basically efficiency. There have been a lot of advances in inorganics recently, with single films now easily reaching efficiencies above 20%. A thin film GaAs solar cell this year achieved a record efficiency of 28.2%! These highly efficient cells are only about 1 micrometre thick(!), which means they are also quite flexible and bendable. And what’s more, fabrication is also very cheap. To make a thin-film solar cell doesn’t even waste an expensive wafer any more, there are techniques to remove the devices from the substrate and to reuse the wafer for the fabrication of the next cell.

In contrast, organic solar cells are much less efficient, less than half what those record breakers achieve – whether it is dye-sensitized cells or polymer-based ones. In the official, verified solar cell efficiency tables (reference below), GaAs as said achieves 28.2%, silicon thin films 19.1%, silicon crystals 25%, CIGS (of Solyndra fame) 19.6%. On the other hand, dye-sensitized solar cells achieve 10.9% and organic polymers 8.3%. And if you’re wondering, the absolute record is held by the more expensive so-called inorganic multijunction cells at 43.5%, but for concentrated light, not normal light.

But such huge differences in efficiency are known. Typically, the argument made in favour of organic solar cells is cost. But is that so? As explained, the latest generation of inorganic thin-film cells are very cheap to make as well. Moreover, one of the most expensive parts of solar cells are the panels that hold the cells, as well as installation. Assuming that these costs are half of the costs of solar modules (a not unreasonable approximation), fabricating organic solar cells that even would be only 10% to 20% the cost of inorganic ones will cut the cost per panel by 40% to 45%. Yet, with efficiencies of less than half of the inorganic ones, you need twice the amount of panels, so it won’t come cheaper. Read more…

Chemistry, Clean Energy, Condensed Matter Physics

Electrons out of balance

November 27, 2011 by Joerg Heber 1 Comment

Permanent electric polarization. Left: the electron density of the rubidium Rydberg electron in one of the atoms of the molecule. The asymmetry is hardly visible. Right: The same electron density, but with the density of the Rydberg atom on its own subtracted. The difference clearly shows an asymmetric distribution of electrons roughly at the position of the second rubidium atom, causing an electric polarization. (c) 2011 Science Magazine

Apply an electric field to a material, and its positive and negative charges will separate, creating an electric polarization. This is the fundamental effect behind capacitors used in electronics as well as in ferroelectrics used in some computer memories. In the latter case, to achieve a permanent electric polarization, the positive and negative charges need to be shifted permanently. This is the case if in a crystal all positive and all negative ions in a crystal are shifted in the same way with respect to each other.

The separation of positive and negative ions in a crystal can lead to a permanent electric polarization. A similar effect has now been achieved with electrons.

In a paper in Science from last week, researchers from the group of Tilman Pfau at the University of Stuttgart in Germany with colleagues from a number of other institutions have now demonstrated an entirely new way of achieving permanent electric polarization – namely by using electrons and not ions. This effect is remarkable, because electrons usually are much more mobile than atoms. Normally, any charge imbalance in the electron distribution of a material or molecule is easily neutralized simply by shifting electrons in the molecule around. At the same time, looking far ahead, such electron-based effects could lead to applications where the electric polarization needs to switch ultrafast.

However, the molecule studied by the researchers is quite different to usual molecules. It is formed by two rubidium atoms, which means that normally it should not show any electric polarization, simply because both atoms in the molecule are identical and for symmetry reasons no positive or negative ions would form in the first place.

But although they are both rubidium atoms, here there is a crucial difference in the electronic states. One of the atoms is in its energetic ground state, while the other is a so-called Rydberg atom, which means that its outermost electron is excited into a very high energy state and circulates the atom’s nucleus at a large distance. Rydberg atoms are huge in comparison. Here, the rubidium atom is roughly about 50 nanometres in size, corresponding to about 1,000 times the size of a oxygen molecule - and is larger also than the transistors in modern computer chips. Read more…

Atomic Physics, Condensed Matter Physics
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Science doesn’t shoot from the hip

November 21, 2011 by Joerg Heber 0 Comments

The young Max Planck, when completing his high school degree, asked a professor of physics at the University of Munich, Philipp von Jolly, whether he should study physics. He got the famous answer that this wouldn’t make much sense, because physics is an almost fully mature science with not much to discover. (If you happen to speak German, it is worth reading the original text, reprinted in this biography of Max Planck.)

Of course, luckily Planck ignored this advice and went on to make some of the most profound discoveries in modern physics. And well, if you think we are in a similarly dull situation in physics at present, the past few weeks would have certainly disproved this, because a couple of intriguing, unpublished (in the academic sense) research findings have appeared widely in the news: neutrinos that continue to appear to be faster than the speed of light, a completely new view on wavefunctions in quantum mechanics, and it seems also that there isn’t much hiding space left for the Higgs boson, if it exists.

Arthur Eddington's 1919 photograph of the sun during a total eclipse. The position of the stars appearing behind the sun verified Einstein's theory of relativity. Photo via Wikimedia.

Those discoveries all come with the promise of significant changes to our understanding of physics, and we’ve seen some exposure in the news (and the occasional hype, too). This is perhaps not surprising. The neutrino experiment questions the theory of relativity. The absence of the Higgs boson on the other hand would open the question again about the different masses of particles. And the new view of wavefunctions seems to add further to the arguments whether the wavefunctions in quantum mechanics are purely an expression of probability to find an object in a certain physical state, or are a representation of actual reality. The paper now rules out the possibility that wavefunctions are probabilistic states, but still having an underlying reality. Instead, there are two interpretations left. One can either fall back to the argument that there is no underlying reality in quantum mechanics and wavefunctions simply are nothing but probabilistic. Or, the second option is that wavefunctions are an expression of actual reality, abandoning the probabilistic interpretation. Not surprisingly, for this reason the paper got lots of headlines. Most people my colleagues at Nature spoke to were quite enthusiastic, whereas Scott Aaronson didn’t seem to see that much of a surprise. Matt Leifer has an informative, quite detailed description of the paper on his blog. Read more…

Quantum physics, Science Communication
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