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November 18, 2010

Simon Fraser University detect weak magnetic field and possible key to puzzle of room temperature superconductivity

Simon Fraser University physicist Jeff Sonier and scientists at TRIUMF have discovered something that they think may severely hinder the creation of room-temperature (37 degrees Celsius) superconductors. There is a weak magnetism in a certain type of lanthanum-based copper oxide material, which is the closest known warm-temperature superconductor.

Sonier says, “Understanding what destroys superconductivity during high chemical doping could provide a vital clue about the microscopic mechanism responsible for high-temperature superconductivity. Knowledge of this would be a monumental step toward making a room-temperature superconductor.”

PNAS - Direct search for a ferromagnetic phase in a heavily overdoped nonsuperconducting copper oxide

20 page arxiv version of the paper



The doping of charge carriers into the CuO2 planes of copper oxide Mott insulators causes a gradual destruction of antiferromagnetism and the emergence of high-temperature superconductivity. Optimal superconductivity is achieved at a doping concentration p beyond which further increases in doping cause a weakening and eventual disappearance of superconductivity. A potential explanation for this demise is that ferromagnetic fluctuations compete with superconductivity in the overdoped regime. In this case, a ferromagnetic phase at very low temperatures is predicted to exist beyond the doping concentration at which superconductivity disappears. Here we report on a direct examination of this scenario in overdoped La2-xSrxCuO4 using the technique of muon spin relaxation. We detect the onset of static magnetic moments of electronic origin at low temperature in the heavily overdoped nonsuperconducting region. However, the magnetism does not exist in a commensurate long-range ordered state. Instead it appears as a dilute concentration of static magnetic moments. This finding places severe restrictions on the form of ferromagnetism that may exist in the overdoped regime. Although an extrinsic impurity cannot be absolutely ruled out as the source of the magnetism that does occur, the results presented here lend support to electronic band calculations that predict the occurrence of weak localized ferromagnetism at high doping.


Adding charge carriers (electric charge carrying particle) is known as chemical doping. With increased chemical doping the operational temperature of a cuprate superconductor rises to a certain point and then collapses.

Until this latest research, scientists could only speculate on whether a competing magnetic phase might exist during high chemical doping and ultimately destroy their superconductivity.

Sonier and his colleagues used a subatomic particle, called a muon, to microscopically probe the magnetic nature of a cuprate. This led them to discover that a strange kind of magnetism appears to accompany the destruction of superconductivity during high chemical doping.

The scientists are now trying to figure out the origin of the magnetism and whether it actually competes with superconductivity.

1 page of supplemental material

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