Science & Environment

Superconductors got hot 25 years ago

Maglev train in Japan
Image caption Superconductivity gave rise to magnetically levitated trains

Superconductivity is a hundred years old this month, and a way to make it accessible turned 25 this week. But just how it does what it does remains a mystery even now.

Essentially, it is the property - exhibited by certain materials, often at low temperatures - to channel electrical current with zero resistance and very little power loss.

Imagine hitting a cue ball with a snooker cue and it never slowing down, carrying on across the baize for years, or forever.

In essence, that was the promise of superconductivity a century ago when the phenomenon was first discovered by Dutch physicist Kamerlingh Onnes.

The charge-carrying electrons travelling through the wires of your computer right now are bounced and slowed down by the vibrations of the atoms in the wires, and by impurities in the metal that makes them up.

That is largely why computers get hot, and why long-distance power lines are inefficient - that bouncing gets turned into useless heat.

With superconductivity, those tortuous movements are removed; electrons career through a material unimpeded because of the lack of electrical resistance.

However, for decades after its discovery, the effect was only seen at temperatures within 23 degrees of absolute zero - temperatures that were available only in the most advanced labs.

So it was that superconductivity remained a textbook curiosity, with an ever-diminishing effort to make it happen at higher temperatures.

But then, exactly 25 years ago, a pair of researchers at IBM's Zurich research labs announced they had discovered a material that started to become superconducting at 35 degrees above absolute zero.

Model role

There are not many occasions in history that a physics discovery makes a splash like the paper by Georg Bednorz and Alex Mueller.

The year following the discovery, thousands of academic papers had been written on the topic of superconducting copper oxides, and the discovery garnered the pair the very next year's Nobel prize in physics - making it the shortest-ever time between a discovery and recognition with a Nobel.

Image caption The era of overhead power lines may end, thanks to superconducting cables

"People still argue that the model we applied for our work was completely wrong," said Dr Bednorz.

"I'm not so sure about that - but if a model is suited to lead people into the right direction, then the model has played its role," he told BBC News.

Resistance-free wires would be much more efficient, so thoughts immediately ran to the idea of long-distance transmission of electrical power.

"We were very euphoric about that," Dr Bednorz said. "Alex and myself started talking about transmission of electricity and high-power transmission lines and we were immediately warned by some of our colleagues who said... 'don't talk too much about this, you'll be regarded as too optimistic and promising too much'."

The principal problem then was that the team's copper oxide materials were brittle ceramics, not amenable to being turned into wires.

But the push to make use of the first big superconductivity discovery in half a century proved too much, despite initial hopes.

Big push

"Some of those people who warned us, who were very sceptical, changed their minds completely," Dr Bednorz recalled.

"A couple of months later I heard from one of these colleagues that he was leaving IBM and joining a startup company producing superconducting wires."

For the most part, though, the ease of cooling the materials to their useful temperatures only made existing applications more accessible.

Superconductivity has other effects such as the ability to exclude the force of another magnet, causing it to levitate. Cooled superconducting magnets also produce very strong magnetic fields.

Magnetically levitated trains had been demonstrated before 1986, as had the superconducting magnets that now form the basis for magnetic resonance imaging (MRI).

Superconducting magnetic propulsion was considered, and a ship was built in Japan in 1991 - but the technology has gone no further.

Maglev trains have been built and decommissioned around the world in the intervening years, and currently only Japan has a significant rail line that uses the technology.

But the delay in widespread use, Dr Bednorz said, was coming to an end, if only because the energy-efficient nature of so-called High-Temperature Superconductivity (HTS) materials made them a compelling choice.

"I think with the dilemma we are facing at present with the energy crisis... we have to save energy at every corner of our planet," he said.

"If we can reduce losses in transporting energy from A to B, if we can produce more efficient machinery, we need to do it."

Generation gap

With three of his colleagues at the Massachusetts Institute of Technology (MIT) in Cambridge, US, Greg Yurek co-founded American Superconductor in April of 1987 - just a year after Bednorz and Mueller's landmark paper.

The company's first task was to, as Professor Yurek put it, "find ways to bend the unbendable wire" for application first of all in power transmission.

So - where are they?

Image caption Thin strips of superconducting wire can vastly outperform copper cables

Until now, short runs of these superconducting cables have featured in demonstration projects, the first of which was switched on a decade ago in the US city of Detroit, and international efforts, have shown similar small-scale successes.

"You always think you can do it faster but if you look in the history of advances, it takes about a generation to get from lab discovery to marketable option," Professor Yurek told BBC News.

He noted that the optical fibres that criss-cross the globe carrying data and even transistors experienced the same lag between discovery and widespread use. He too, though, suggests that the green credentials of the approach mark it for wider adoption.

Over a 1000-km run, superconducting cables can carry far more power than cables of a comparable size. Even taking into account the energy required to cool them, they lose just a third as much energy in transmission.

Just as in the case of optical fibres, the technology has been quietly refined over the years and in some senses is ready to be deployed. This will be costly at first, but with tangible benefits that the wider market may just not yet be convinced of.

What would almost certainly push things along would be a material that becomes superconducting at much higher temperatures - radically simplifying the cooling that wires, motors and magnets in current HTS approaches require.

But the truth is, 100 years after its discovery, there is still no consensus view on how superconductors really work at a microscopic level.

"Who knows whether there's yet another class of materials with a higher [temperature to superconduct]? Whether that is a development that takes place in the next couple of years or never I can't predict," Dr Bednorz said.

"But as long as we don't have a really solid theory as for what's causing these materials to get into the superconducting state, we cannot predict anything."

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