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In this issue:
Researchers Find Magnetic Link to High-temperature Superconductivity
Word of the Week: Meissner Effect

SLAC Today

Friday - February 18, 2011

Researchers Find Magnetic Link to High-temperature Superconductivity

(Photo)
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 Physical Review Letters 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 high-temperature superconductors 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 superconducting state," 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.  Read more...

(Image - Meissner Effect illustration)
The left image shows an externally-applied magnetic field (arrows) penetrating a superconductor that is still above its critical temperature (too warm to superconduct). The right image shows the same superconductor, this time cooled below its critical temperature, expelling the magnetic field. (Image: public domain/Wikimedia Commons.)

Word of the Week: Meissner Effect

One of the most striking characteristics of a superconductor—aside from the fact it conducts electricity without resistance—is its ability to almost completely expel an external magnetic field. This ability is due to the Meissner Effect, named after German physicist Walther Meissner, in which the superconductor responds to an external magnetic field by setting up an electric current near its surface. The surface current generates its own equal and opposite magnetic field—thus canceling the applied magnetic field in the interior of the superconductor—and the very nature of a superconductor ensures that the current continues to flow indefinitely.

The magnetic probe used in a recent SIMES study ("SIMES Researchers Find Magnetic Link to High-temperature Superconductivity," above) measured pinpoint Meissner Effects across the face of the superconducting iron pnictide crystal.

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