Sunday, 6 April 2008

Resistance is futile

If you mentioned to people what they remember about 1986, what would they say? You may get answers such as the challenger space shuttle disaster which killed all seven crew; Argentina winning the World cup with the help of Diego Maradona or maybe the year when the European flag was adopted by the European Union. Probably one answer you wouldn't get (unless you happened to ask a physicist) is the discovery of high temperature superconductivity.

Superconductivity is one of those weird effects in nature: if you cool a metal such as lead or tin to low temperatures then all of a sudden its resistance will fall to zero, meaning that below this temperature, a current flowing through a wire of this material will incur no resistance and therefore persist indefinitely.

The discovery by Bednorz and Müller of superconductivity at 30 K was important for a number of reasons, one was that the standard model of superconductivity didn't allow superconducting transition temperatures this high (lead and tin are 7 and 4 K respectively) thus leading to a potential new mechanism for superconductivity. Two, it opened up a variety of related materials which pumped up the superconducting transition to 138 K at standard pressure in 1995 (applying an external pressure makes it go even higher). The discovery was so important that even by the next year Bednorz and Müller were awarded the Nobel prize in physics.

These materials, known collectively as the cuprates, are a double edged sword. One they enable very high superconducting transition temperatures to be achieved by chemical substitution and/or doping, but this delicate parameter space has meant getting a coherent experimental and theoretical picture of high temperature superconductivity has become cloudy.

A few years after the discovery, there were around 8000 papers per year being churned out, that fell to around 5000 in 2005 (still a high number nonetheless). Indeed, someone new to the field has to either spend a decade reading every publish paper or only select papers in the top journals. Grants were being pulled on research in high temperature superconductivity, researchers were thinking about other topics, a theory, and new materials seemed elusive.

but maybe that's about to change..

At the end of February a group of researchers in Tokyo reported a new Iron based superconductor at 26 K. Now, 26 K is still far less than the record, but already a few theoretical papers came out predicting that this was not a standard 'low' temperature superconductor - another high temperature superconductor has hit the scene.

Not only did it take a few days later before the first experimental paper on arxiv came out, by today there has already been around 20 papers on this material on arxiv. With chemical substitution the superconducting transition temperature has already increased to 52 K in a related material.

Are there similarities between the cuprates? Probably in more sense that one. The ability to substitute elements and also to tweak the amount of oxygen or other halogens means the parameter space is large. The 'first to the finish line' approach in superconductivity research in terms of finding a new superconductor with an even higher superconducting transition temperature is likely to make a splurge of papers to come out shortly.

Maybe this material gives us a chance to find out the mechanism for high temperature superconductivity or puts the elusive room temperature superconductor in sight. An explanation will need experimentalists to produce careful measurements that give theorists a clear view.