All That Matters

Heike Kamerlingh Onnes (photo from Museum Boerhaave)

Today marks the 100th anniversary of superconductivity by Heike Kamerlingh Onnes. In a superconductor, the electrons flow without any electrical resistance.

Apart from their fundamental scientific interest, superconductors are used to make powerful electromagnets, for example for MRI and NMR machines in medical diagnostics. Other promising applications include power transmission cables with low losses, highly sensitive devices to measure magnetic fields and so on.

Working in his lab at Leiden University, on 8 April 1911 he experimented with the electrical resistance of mercury at low temperatures. In his notebook he noted that at 3 K (-270°C), ‘Kwik nagenoeg nul’, mercury’s resistance drops to ‘practically zero’.

This discovery at such low temperatures was only made possible by Kamerlingh Onnes previous achievement of liquifying helium at 4.22 K. this provided the means to cool samples down to even lower temperatures. For this breakthrough in cryogenics, Kamerlingh Onnes received the 1913 Nobel prize in physics.

When superconductivity was discovered, it certainly was a puzzling observation at the time. Some scientists believed that at low temperatures electrical resistance would shoot up towards infinity, whereas others thought that it would gradually go down, which is what indeed happens for many materials. However, superconductivity is not simply a new form of electrical resistance – it is a thermodynamic state in its own right, and its unique properties can’t be explained by classical physics alone. Indeed, it was not until 1957, when Bardeen, Cooper and Schrieffer provided the quantum-theory that explains superconductivity of materials such as mercury.

However, that’s not where research into superconductivity stops. In 1987, the so-called high-temperature superconductors were discovered. Their superconducting temperatures are so high that cooling with helium isn’t even necessary. Interestingly, mercury (Hg) plays a key role there as well: the superconductor with the highest known temperature at normal pressures (135 K) is HgBa₂Ca₂Cu₃O_x!

The origin of superconductivity in these new superconductors is different to the classical superconductors, and remains not fully understood. This makes Kamerlingh Onnes discovery all the more relevant to this day.

Further reading:

it seems this nice article is free access:

van Delft, D., & Kes, P. (2010). The discovery of superconductivity Physics Today, 63 (9) DOI: 10.1063/1.3490499

This post was chosen as an Editor’s Selection for ResearchBlogging.org

Optical switching might make computer hard drives faster. Photo by pobre.ch via flickr.

Magnetism remains the most developed way to store digital information. The giga and terabytes of computer hard drives as well as the magnetic stripes that still are used for credit cards or hotel room keys, all function with the help of magnetic fields. There, the direction of the magnetic fields, up or down, expresses the digital 0s and 1s that make up the computer bits and bytes.

As the amount of data we store on hard drives continues to increase, it is of course desirable that read and write speeds follow that trend. As far as writing data is concerned, however, switching the magnetisation is not that easy as all the individual magnetic fields of the majority of atoms that make up a bit, their so-called magnetic moment, has to be reversed. Given that these magnetic moments are interconnected through magnetic forces, such reversals aren’t very fast.

Modern hard drives manage to write about 1 billion bits per second. That’s a nanosecond per bit. In the lab, switching speeds are even faster, achieving hundreds of picoseconds to nanoseconds. But while this sounds like a pretty fast process, it is orders of magnitude slower than many other electronic processes in a crystal. Yet, magnets needn’t be that slow. What we have considered so far is switching magnetization by an external magnetic field, such as that generated in the write head of a hard drive. This isn’t the only possibility. If ultrashort optical laser pulses are used instead, magnetism can be switched a hundred times faster, on the order of a picosecond.

How does this work? In a paper published in advance on the Nature website this week, Ilie Radu, Theo Rasing from Radboud University in Nijmegen and others have investigated the details of the optical switching proceeds for a particular class of magnets, antiferromagnets.

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X-ray data of protein crystals obtained from over 15,000 single snapshots. Credit: Thomas White, DESY

When you go to the doctor for an X-ray, the nurse or doctor briefly disappear behind a screen, presses a button for a brief moment, and you’re all set. It seems an X-ray takes about a second but the actual exposure times is much faster. Milliseconds more likely.

Such speeds seem like almost an eternity compared to what is achieved by a new generation of X-ray sources that have begun to become operational: free-electron X-ray lasers. The first of these big machines is the LCLS at Stanford University, which achieves laser pulses shorter than 70 femtoseconds (100 femtoseconds = 1/10 of a trillionth of a second). The beam intensities of these lasers are ten billion times brighter than the sun. And all this with a potential imaging precision down to the atomic scale. In other words, if you like to take things to the extreme, these lasers are for you.

In one of the first studies to make use of the LCLS X-ray free-electron laser, two research collaborations now present first experiments on biological samples in this week’s Nature.

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