Researchers find magnetic link to high-temperature superconductivity

February 24, 2011 by Lori Ann White, SLAC National Accelerator Laboratory
The sharp, magnetized tip of the probe used in this study induces an answering field in the superconductor in a phenomenon called the Meissner Effect. The strength of the response at each point gave SIMES researchers new information about the superconducting state of the pnictide. (Image courtesy Lan Luan)

Researchers from the Stanford Institute for Materials and Energy Science, a joint SLAC-Stanford institute, have seen strong indications of a relationship between the superconductive and magnetic properties of high-temperature superconductors -- a relationship long suspected but difficult to investigate experimentally. Any step toward a real understanding of high temperature superconductors is a big step right now. Today's superconductors need extreme cold to keep conducting electricity with 100 percent efficiency, but extreme cold is not cheap. If current research leads to room temperature superconductors, superconducting technologies such as loss-less power lines and levitating high-speed trains will be economically as well as technically feasible.

A paper in last week's explains how the researchers, led by Kathryn Moler, attacked the mystery by employing a new technique to investigate a suspected connection between magnetism and superconductivity in called iron pnictides. The researchers "doped" the pnictide crystals (in this case barium iron arsenide) with varying amounts of cobalt, replacing some of the iron. Then they subjected the crystals to miniscule magnetic probes to see how they would react. They took readings mere microns apart across the face of each crystal.

"This gave us information about how many superconducting electrons were actually in a ," explained SIMES researcher Lan Luan, first author on the paper, who recently defended her doctoral thesis based on this research. Previous experiments had been able to capture data only across the bulk of a material, Luan explained, but the "spot measurements" her team took gave a more detailed look into the electronic and superconductive behavior of the pnictide.

The researchers saw three trends emerge which could be explained by an interrelationship between the magnetic and superconductive properties of the doped barium iron arsenide crystals. The first trend showed fewer electrons available to act as superconducting electrons in the underdoped crystals, which had too little cobalt for optimal superconductivity, and the overdoped crystals, which had too much. Underdoping and overdoping reduce the number of charge carriers, or electrons available to become superconducting electrons.

"The magnetic phase and the superconducting phase competed over electrons," Luan said.

The second trend revealed a connection between the amount of cobalt in the crystal and the amount of energy needed to disrupt the superconducting state—to kick electrons out of the smooth flow of charge. Again, the effects of the doping could combine with naturally occurring to explain the results.

The third trend showed that superconducting properties appeared more quickly in underdoped and optimally doped crystals as the temperature decreased, as if superconductivity suddenly won out over magnetism in the continuing battle for electrons.

"Unconventional superconductivity is a field full of controversy," Luan said. What controversy can't touch is the data. "This is the first quantitative data set across the superconducting dome"—from underdoped to optimally-doped to overdoped barium iron arsenide. Even if her group's exact interpretation falls, the data remain.

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not rated yet Feb 24, 2011
As it seems that magnetism and conductivity compete for the electrons at higher temperatures I wonder if sheathing the conductor with mumetal or permalloy has been considered? I'm just thinking that by shunting the magnetism internally it just might reduce the availability of electrons for the magnetic phase as described in the article.
1 / 5 (2) Feb 25, 2011
To be 100% efficient superconductors need to be extreme cold. But do we need 100% efficiency? I mean, if it can work at 50% efficiency and be better than copper then we should replace copper right away.

Have they stopped to think about it?
5 / 5 (1) Feb 25, 2011
rjsc2000 : good idea, but at higher temperatures the superconducting wire has resistances on the order or more than copper. In fact, they are designed such that strands are enveloped in a copper (or other similar conductor) such that in the event of a quench the current is dumped into the copper so as not to burn up the previously superconducting strands, prevent damage and allow the possibility of reusing the wire.

The change in resistivity at low temperatures is a sudden drop with nothing in between - it's a quantum effect.

So it's all or nothing.
1 / 5 (1) Feb 25, 2011
We already have a Room Temperature Superconductivity invention with a prototype already tested -

Certain people have kept it from being utilized, that's all.
not rated yet Feb 28, 2011
And where is the link to an article?

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