Picking Apart Superconductivity

Puzzle pieces.Precise alignment of hundreds of tiny superconductor crystals led to new spin observations.

If you want to start a fight in a roomful of physicists, ask them how high-temperature superconductors work--a question that hasn't been resolved since their discovery 15 years ago. Now, results of a laborious experiment may have solved part of this longstanding puzzle by showing that all such superconductors have a fundamental fingerprint of magnetic spin. In some theories, spin can help electrons sneak through superconductors without electrical resistance.

The first trace of this particular spin showed up in 1991, when researchers shot beams of neutrons onto high-temperature superconductors. Oddly, the scattering of neutrons, which have a small magnetic spin and can reveal what the material's electrons are up to, suggested the electron spins were all vibrating in the same way, a property called resonance. But the resonance was seen only in high-temperature superconductors whose crystal structure had two or more layers of copper oxide. Some physicists believed that meant spin resonances were a red herring that they could ignore.

Now the herring is back, and it's real. A collaboration between the Max Planck Institute for Solid State Research in Stuttgart, Germany, two labs from France's Atomic Energy Agency, and the Institute of Solid State Physics in Chernogolovka, Russia, reports online in the 24 January Science Express that the spin resonance occurs in a single-layer high-temperature superconductor. Because the material they studied, a thallium-barium-copper oxide compound, hasn't been grown in crystals big enough for neutron scattering, the scientists had to devise a painstaking technical workaround. By aligning hundreds of small crystals to behave like one large crystal, they could detect the resonance peak, says co-author Bernhard Keimer of Max Planck.

"Now it's clear that this resonance is the rule rather than the exception," says Michael Norman, a theorist at Argonne National Laboratory in Illinois. Less clear is how theory will accommodate the new observations. "This is where the real debates start," Norman says, "and it's a mine field." Physicists are stepping lightly, because each theory has a different idea of what makes the high-temperature superconductors tick--and no theorist is going to yield ground easily.

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