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A pellet of yttrium barium cuprate, when exposed to a styrofoam cup filled with liquid nitrogen, superconducts and levitates a neodymium magnet. Scientists have finally figured out how these superconductors and others work, a mystery that has long eluded them.  (Source: coronene.com)
With a newfound technical understanding, the skies are clear to develop truly super superconductors

There has been a number of advances in the field of superconducting recently.  Superconductors, materials that conduct electricity with no resistance below a certain temperature (the critical temperature, Tc), have a variety of incredible applications.  For example, superconductors could offer electricity transmission with no losses from power plants, saving the country's money and fuel, could power faster computers, and could make ultra-efficient motors.

Over the last few years, many breakthroughs in the field of conductivity have been achieved.  The first superinsulators have been created and magnetism-immune superconductors have also been made.  The greatest goal of superconductor research -- to achieve room temperature superconductivity -- still remains unattained, but thanks to new cuprate (copper and oxygen) superconductors we're a lot closer.

However, one critical problem was that superconductor behavior in these cuprate superconductors were not well understood -- until now.  Researchers at U.S. Department of Energy's Brookhaven National Laboratory along with partners from Cornell University, Tokyo University, the University of California, Berkeley, and the University of Colorado have finally developed a cohesive explanation for superconductor behavior.

To gain their insight, the researchers used "quasiparticle interference imaging" with a scanning tunneling microscope to look at Cooper pairs of electrons.  Cooper pairs are paired electrons in superconductors that allow for the phenomena to occur.

The puzzling phenomena, which the scientists solved, was that in normal superconductors raising the binding energy, to hold these pairs together raises the critical temperature closer to room temperature.  However, in cuprate superconductors, which have higher starting temperatures, raising the binding energy actually lowers the Tc, the opposite of the desired result.

Researchers determined that this is due to a "quantum traffic jam" effect.  Normally cuprates are stuck in a jammed stated known as the Mott insulating state, named after the late Sir Neville Mott of Cambridge, UK.  To create cuprate superconductors, electrons are removed from cuprates, leaving holes.  Cooper pairs can then start to flow into these holes, allowing for superconduction, akin to a couple cars exiting the highway during rush hour starting traffic moving.

However, the critical discovery the researchers made was that increasing the binding energy also increased the "Mottness" of cuprate superconductors.  Thus, raising the temperature only made the traffic jam worse, lowering the critical temperature.  Seamus Davis of Brookhaven National Laboratory and Cornell University, lead author on the paper describes, "It has been a frustrating and embarrassing problem to explain why this is the case."

Now the dilemma becomes applying this new knowledge to new superconductors.  Traditional superconductors have low Mottness, allowing for binding energies to be used to raise their critical temperature.  However, they start at very low critical temperatures, so the temperature can only be raised so high, typically well below the starting critical temperatures of cuprate superconductors.  Cuprate superconductors start high, but can't get any lower.

A new hope is that superconductors comprised of arsenic and iron, instead of copper and oxygen, might have less Mottness, but be able to make gains from raising binding energies, but also enjoy higher starting temperatures.  Mr. Davis states, "We need to look for materials with such strong pairing but which don't exhibit this Mottness or 'quantum traffic-jam' effect.  Our hope is that (iron/arsenic superconductors) will have less 'traffic-jam' effect while having stronger electron pairing."

If such a superconductor can be created and tuned, room temperature superconductivity may finally be achieved, and affordable, bringing great economic and scientific gains.  For now, the breakthrough represents perhaps the greatest advance in understanding superconductivity yet.





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