University scientists study superconductors

Japan’s Maglev Shinkansen trains, which have achieved record speeds of up to 361 miles per hour, literally float above their tracks, thanks to powerful superconductors, which scientists believe may also one day be used to revolutionize power transmission and electric car motors.

In the two decades since high-temperature superconductivity was discovered, however, scientists have failed to understand the phenomenon thoroughly enough to fully harness its tremendous potential. A research group led by University physicists has now found that this failure may be due to a pervasive misconception in the scientific world about the very nature of high-temperature superconductivity.

While many scientists believe that the electrons in high-temperature superconductors are bound together in pairs by some type of external “glue” to resist their natural repulsion, the University group found evidence that, paradoxically, this repulsion may itself be the cause of the electron binding needed to form superconductors.

What are superconductors?

Superconducting materials conduct electricity without dissipating energy at either very high or very low temperatures. The crucial feature of superconductors is that their electrons are bound together in pairs in spite of natural repulsion.

The discovery of low-temperature superconductors dates back to 1911. By the 1950s, scientists had established a widely accepted theory to describe how they work. This theory of superconductivity stated that negatively charged electrons in a superconductor distorted a lattice of positively charged ions in such a way that the lattice created an attractive force between pairs of repellent electrons.

“It’s like two people lying on a waterbed,” explained physics professor Ali Yazdani, who led the research team. “One person creates a depression on the bed, and the other person’s body is drawn to sink into it, even if they’re kind of repelled by each other.”

Since 1986, when the first high-temperature superconductors were discovered, many scientists have clung to the idea that all superconductors have some sort of “glue” — like the ion lattice — binds together electrons, Yazdani said.

The controversial data uncovered by his research group, however, suggests that no “glue” substance is responsible for the electron binding in high-temperature superconductivity.

In search of the missing glue

Yazdani’s group, which included postdoctoral fellow Abhay Pasupathy and graduate students Kenjiro Gomes and Aakash Pushp, initially aimed to verify the evidence of the existence of a glue that had been reported by other scientists by experimenting with a superconducting material made of strontium, bismuth, calcium and copper oxide.

Studying high-temperature superconductors presents a wide array of challenges, Yazdani explained, especially because the chemical structure of the superconducting materials is not uniform.

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“Different parts of superconducting materials can have atoms in completely different arrangements,” Yazdani said. “Most previous studies have averaged results over the entire sample, but with the microscopes we used, we were actually able to study how superconductivity forms in the material on the scale of atoms, so we could observe how the superconductivity strength varies throughout the sample.”

Using special scanning tunneling microscopes, the researchers were shocked to find that though they could in fact find signatures of a glue substance, these signatures did not correlate with whether the superconductivity was weaker or stronger in certain areas. Thus, while there did seem to be a glue of some sort, it did not appear to be responsible for the electron binding needed for superconductivity.

“We were astounded,” Yazdani said of the discovery. “But I said, look, we can’t ever publish this, we don’t have evidence for any other binding force besides the glue.”

“So then we started looking for some other observable data that could tell us why superconductivity was stronger or weaker in certain parts of the material,” he said.

What they ultimately discovered was a counterintuitive correlation between the strength of the superconductivity and the strength of the electron repulsion in certain regions of the superconductor. Areas of the material in which electrons most strongly repelled other electrons at room temperature, at which superconduction does not occur, were the same areas that, at high temperatures, displayed the most effective attractive electron bindings and the strongest superconductivity.

How does strong electron repulsion cause strong attraction at high temperatures? The repulsion causes the electrons to alter the shape of their orbitals, Yazdani said, so that they do not orbit in the same space simultaneously.

“The repulsion seems to drive the electrons to adjust their orbitals and seek paired space in order to avoid further repulsion,” Yazdani said. “The conductivity is a direct consequence of the repulsion.”

The future of superconductivity

While the researchers’ results may dishearten scientists committed to finding the glue that permits superconduction, they also bring the scientific world one step closer to thoroughly understanding the increasingly important phenomenon of high-temperature superconductivity, which scientists believe has innumerable applications to modern technology.

“Superconductivity can have ground breaking applications which can revolutionize pretty much anything and everything in the world of gadgets,” Pushp said in an e-mail. “The first huge impact will be in the transmission lines that carry electricity from the power stations to our homes — about 40% of energy gets wasted just reaching our homes.”

This energy loss is due to power dissipation in the copper wires that transmit electricity. Superconducting wires, by contrast, dissipate no power and could therefore be a much more efficient form of electricity transmission.

Superconductivity could also allow more power to be carried on smaller wires, Yazdani said. “In the future you could have power generation happening at plants far away from cities, and when you wanted to bring this power into very dense urban areas, you could do it very efficiently and limit the number of cables coming into these urban areas.”

The use of superconducting materials for improving power transmission is particularly important given the “astronomically growing” need for electricity in the modern world, Yazdani added.

Other potential applications of superconductors include improved power storage devices and MRI scanning technology as well as Maglev — short for magnetic levitation — trains that can operate faster, more quietly and more efficiently than their grounded counterparts.